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Biogas Purification and Upgrading Technologies
Abstract
The fact that most countries do not promote the use of biogas as energy vector via tax incentives entails the need for an optimization of biogas upgrading technologies in order to support a cost-competitive utilization of this renewable energy source. Nowadays, the contaminants present in biogas such as CO2, H2S, H2O, N2, O2, siloxanes, and halocarbons are removed through the implementation of costly and environmentally unfriendly upgrading processes. Conventional biogas upgrading is based on physical/chemical technologies leading to CH4 purities of 88–98% and removal efficiencies of higher than 99% for H2S, halocarbons, and siloxanes. Unfortunately, their high energy and chemical demands limit the environmental and economic sustainability of these conventional biogas upgrading technologies. In this sense, biological processes have emerged in the past decade as an economic and environmentally friendly alternative to conventional biogas upgrading technologies. Thus, biotechnologies such as microalgae-based CO2 fixation, H2-assisted litoautotrophic CO2 bioconversion to CH4, enzymatic CO2 dissolution or fermentative CO2 reduction have been consistently shown to result in CO2 removals of 80–100% with CH4 purities of 88–100%, while allowing the valorization of CO2 into bioproducts of commercial interest (therefore preventing its release to the atmosphere). Similarly, H2S removals > 99% are consistently achieved in aerobic and anoxic biotrickling filters, algal-bacterial photobioreactors, and digesters under microaerobic conditions. In addition, recent investigations have shown the potential biodegradability of siloxanes and halocarbons under both aerobic and anaerobic conditions. This chapter constitutes a state of the art comparison of physical/chemical and biological technologies for the removal of CO2, H2S, halocarbons, and siloxanes from biogas.
... 99%). Unfortunately, typical high energy and chemical demands affect environmental and economic sustainability [81]. Conventional biogas purification and upgrading can be divided into sorption and separation processes; they are shown in Tables 4 and 5. In-situ microaeration Adding O 2 or air directly in digester. ...
... Modern developments in biogas purification and upgrading techniques have been reviewed by different authors [14,81,103]. Rodero et al. [81] stated that technologies which use aerobic and anoxic biotrickling filters, algal-bacterial photobioreactors and digesters under microaerobic conditions, successfully remove H 2 S (>99%). ...
... Modern developments in biogas purification and upgrading techniques have been reviewed by different authors [14,81,103]. Rodero et al. [81] stated that technologies which use aerobic and anoxic biotrickling filters, algal-bacterial photobioreactors and digesters under microaerobic conditions, successfully remove H 2 S (>99%). They also made a comparison of various modern technologies for the biogas purification of H 2 S, halocarbons, and siloxanes. ...
In line with the low-carbon strategy, the EU is expected to be climate-neutral by 2050, which would require a significant increase in renewable energy production. Produced biogas is directly used to produce electricity and heat, or it can be upgraded to reach the “renewable natural gas”, i.e., biomethane. This paper reviews the applied production technology and current state of biogas and biomethane production in Europe. Germany, UK, Italy and France are the leaders in biogas production in Europe. Biogas from AD processes is most represented in total biogas production (84%). Germany is deserving for the majority (52%) of AD biogas in the EU, while landfill gas production is well represented in the UK (43%). Biogas from sewage sludge is poorly presented by less than 5% in total biogas quantities produced in the EU. Biomethane facilities will reach a production of 32 TWh in 2020 in Europe. There are currently 18 countries producing biomethane (Germany and France with highest share). Most of the European plants use agricultural substrate (28%), while the second position refers to energy crop feedstock (25%). Sewage sludge facilities participate with 14% in the EU, mostly applied in Sweden. Membrane separation is the most used upgrading technology, applied at around 35% of biomethane plants. High energy prices today, and even higher in the future, give space for the wider acceptance of biomethane use.
... Conventional biogas upgrading techniques generally focus on CO 2 removal due to its relatively high concentration in raw biogas. However, raw biogas containing H 2 S (even in trace amounts) must also be treated before its end-use application, as H 2 S is toxic and corrosive (Noorain et al., 2019;Pokorna and Zabranska, 2015;Rodero et al., 2018). NH 3, siloxanes and volatile organic compounds (VOCs) are other unwanted pollutants in raw biogas, usually present in much lower concentrations than H 2 S. For example, NH 3 induces corrosion in engines and pipelines during combustion (Khan et al., 2021;Rodero et al., 2018), siloxanes (e. g. octamethylcyclotetrasiloxane) in silicium deposits during combustion, which form microcrystalline quartz deposits on engine surfaces resulting in abrasion, overheating, and malfunctioning of engines and valves (García et al., 2021;Yang and Corsolini, 2019), whereas combustion of VOCs such as benzene and other aromatic compounds give an unpleasant smell to biogas, and are potentially toxic to human health (Carriero et al., 2018). ...
... However, raw biogas containing H 2 S (even in trace amounts) must also be treated before its end-use application, as H 2 S is toxic and corrosive (Noorain et al., 2019;Pokorna and Zabranska, 2015;Rodero et al., 2018). NH 3, siloxanes and volatile organic compounds (VOCs) are other unwanted pollutants in raw biogas, usually present in much lower concentrations than H 2 S. For example, NH 3 induces corrosion in engines and pipelines during combustion (Khan et al., 2021;Rodero et al., 2018), siloxanes (e. g. octamethylcyclotetrasiloxane) in silicium deposits during combustion, which form microcrystalline quartz deposits on engine surfaces resulting in abrasion, overheating, and malfunctioning of engines and valves (García et al., 2021;Yang and Corsolini, 2019), whereas combustion of VOCs such as benzene and other aromatic compounds give an unpleasant smell to biogas, and are potentially toxic to human health (Carriero et al., 2018). Hence, comprehensive biogas purification is required prior to its application, i.e. to generate electricity or to upgrade to biomethane quality as green alternative fuel to natural gas (Mulu et al., 2021;Rodero et al., 2018;Yang and Corsolini, 2019). ...
... NH 3, siloxanes and volatile organic compounds (VOCs) are other unwanted pollutants in raw biogas, usually present in much lower concentrations than H 2 S. For example, NH 3 induces corrosion in engines and pipelines during combustion (Khan et al., 2021;Rodero et al., 2018), siloxanes (e. g. octamethylcyclotetrasiloxane) in silicium deposits during combustion, which form microcrystalline quartz deposits on engine surfaces resulting in abrasion, overheating, and malfunctioning of engines and valves (García et al., 2021;Yang and Corsolini, 2019), whereas combustion of VOCs such as benzene and other aromatic compounds give an unpleasant smell to biogas, and are potentially toxic to human health (Carriero et al., 2018). Hence, comprehensive biogas purification is required prior to its application, i.e. to generate electricity or to upgrade to biomethane quality as green alternative fuel to natural gas (Mulu et al., 2021;Rodero et al., 2018;Yang and Corsolini, 2019). ...
Raw biogas generated in the anaerobic digestion (AD) process contains several undesired constituents such as H2S, CO2, NH3, siloxanes and VOCs. These gases affect the direct application of biogas, and are a prime concern in biogas utilization processes. Conventional physico-chemical biogas purification methods are energy-intensive and expensive. To promote sustainable development and environmental friendly technologies, biological biogas purification technologies can be applied. This review describes biological technologies for both upstream and downstream processing in terms of pollutant removal mechanisms and efficiency, bioreactor configurations and different operating conditions. Limitations of the biological approaches and their future scope are also highlighted. A conceptual framework Driver-Pressure-Stress-Impact-Response (DPSIR) and Strengths-Weaknesses-Opportunities-Threats (SWOT) analysis have been applied to analyse the present situation and future scope of biological biogas clean-up technologies.
... The solubility of all components increases when pressure is higher, and CO2 is more soluble than CH4 according to Henry's Law. Therefore, the process is based on the higher aqueous solubility of CO2 compared to CH4 that is 26 times lower at 25 °C [11]. ...
... Biogas upgrading with water scrubbing is an efficient technology with low chemical requirement and high methane recovery (up to 98%). The investment cost is around EUR 2500 per Nm 3 /h at design flow rates in plants with capacities of 500-1000 Nm 3 /h [11]. In addition, high amounts of water and energy consumption are needed [101]. ...
... Chemical scrubber is, together with membrane separation, the most common technology in biogas upgrading facilities, but it requires higher capital costs (around EUR 3200 per Nm 3 /h at design flow rates in capacities under 1000 Nm 3 /h) [11]. The energy consumption for raw biogas is between 0.05-0.15 ...
The present work reviews the role of biogas as advanced biofuel in the renewable energy system, summarizing the main raw materials used for biogas production and the most common technologies for biogas upgrading and delving into emerging biological methanation processes. In addition, it provides a description of current European legislative framework and the potential biomethane business models as well as the main biogas production issues to be addressed to fully deploy these upgrading technologies. Biomethane could be competitive due to negative or zero waste feedstock prices, and competitive to fossil fuels in the transport sector and power generation if upgrading technologies become cheaper and environmentally sustainable.
... The latter, known as chemolithotrophic denitrifying SOB, display an advantage due to the fact that BTF or bioreactor aeration cycles are not necessary. In this bacterial group, NO3 − denitrification can be completely carried out up to N2 or incompletely up to NO2 − [55,56]. Thus, when N/S > 1.6 is given, anoxic SOB oxidize the H2S and reduce the NO3 − completely down to SO4 2− and N2, respectively, while at N/S < 0.4 such bacteria only perform oxidoreduction down to S 0 and NO2 − [18,55]. ...
... In this bacterial group, NO3 − denitrification can be completely carried out up to N2 or incompletely up to NO2 − [55,56]. Thus, when N/S > 1.6 is given, anoxic SOB oxidize the H2S and reduce the NO3 − completely down to SO4 2− and N2, respectively, while at N/S < 0.4 such bacteria only perform oxidoreduction down to S 0 and NO2 − [18,55]. Added to the actual behaviour of the EC and the RE with respect to the LR, this strongly suggests that BTFs should be microbiologically studied to maintain a strain or bacterial community that is efficient in terms of the parameters described above. ...
... The latter, known as chemolithotrophic denitrifying SOB, display an advantage due to the fact that BTF or bioreactor aeration cycles are not necessary. In this bacterial group, NO 3 − denitrification can be completely carried out up to N 2 or incompletely up to NO 2 − [55,56]. Thus, when N/S > 1.6 is given, anoxic SOB oxidize the H 2 S and reduce the NO 3 − completely down to SO 4 2− and N 2 , respectively, while at N/S < 0.4 such bacteria only perform oxidoreduction down to S 0 and NO 2 − [18,55]. ...
The agriculture and livestock industry generate waste used in anaerobic digestion to produce biogas containing methane (CH4), useful in the generation of electricity and heat. However, although biogas is mainly composed of CH4 (~65%) and CO2 (~34%), among the 1% of other compounds present is hydrogen sulphide (H2S) which deteriorates engines and power generation fuel cells that use biogas, generating a foul smell and contaminating the environment. As a solution to this, anoxic biofiltration, specifically with biotrickling filters (BTFs), stands out in terms of the elimination of H2S as it is cost-effective, efficient, and more environmentally friendly than chemical solutions. Research on the topic is uneven in terms of presenting performance markers, underestimating many microbiological indicators. Research from the last decade was analyzed (2010–2020), demonstrating that only 56% of the reviewed publications did not report microbiological analysis related to sulphur oxidising bacteria (SOB), the most important microbial group in desulphurisation BTFs. This exposes fundamental deficiencies within this type of research and difficulties in comparing performance between research works. In this review, traditional and microbiological performance markers of anoxic biofiltration to remove H2S are described. Additionally, an analysis to assess the efficiency of anoxic BTFs for biogas desulphurisation is proposed in order to have a complete and uniform assessment for research in this field.
... Therefore, MABs are capable to convert biogas into biomethane (CH 4 concentration > 95 %), which complies in many cases with international regulations for injection in natural gas grids [5,7]. The most common setup of pilotand industrial-scale MABs for biogas desulfurization and upgrading include a photobioreactor (commonly operated as a raceway-type algal pond) coupled to a bubble column for H 2 S and CO 2 absorption [1,8]. In this configuration, the liquid phase containing the microalgae-bacteria consortium is continuously recirculated from the photobioreactor to the bubble column. ...
... g N m − 3 d − 1 using real centrate as nutrient medium for biogas purification and upgrading. Rodero et al. [8] coupling the treatment of real centrate with biogas purification in a pilot HRAP, achieved a maximum total nitrogen removal of ~9 g N m − 3 d − 1 . Likewise, Bahr et al. [46] coupling the treatment of centrate with photosynthetic biogas upgrading, reported total nitrogen removal rates of up to 17 g N m − 3 d − 1 . ...
Biogas desulfurization is in many cases the main purification step required for energy generation. This is the case of wastewater treatment plants, waste management facilities or intensive livestock farms where the produced biogas is burned in situ to generate thermal and electrical power. The potential of compact H2S/CO2 absorption units coupled to a photobioreactor for biogas desulfurization was investigated in the present study. Two absorption unit configurations, bubble column and airlift, were evaluated in terms of H2S and CO2 removal at biogas retention times of 10 and 30 min, and liquid-to-biogas flowrate ratios of 1/1 and 4/1. Complete H2S removal was achieved in both absorption unit configurations regardless of the biogas retention time or liquid-to-biogas flowrate ratio applied. These results confirmed the feasibility of setting biogas retention times as short as 10 min, resulting in absorption units up to 49-fold smaller than that required for biogas upgrading up to biomethane. Biogas calorific value increases from 14.7±4.0 to 19.3±7.2% were observed in the airlift configuration, while increases from 5.5±2.2 to 25.2±4.0% were recorded in the bubble column. N-NO3⁻ removal rates ranging from 16±4 to 25±5 g m⁻³ d⁻¹ were recorded under the experimental conditions tested, no significant differences in nitrogen removal being observed between the airlift and bubble column configurations. The significant removal of CO2 that can be achieved at short biogas retention times certainly constitutes a motivation for implementing photobioreactors for biogas desulfurization, broadening the application niches of microalgae-bacteria systems in the context of renewable energy production.
... A porous absorbent with desired characters including high specific surface area, linear adsorption isotherm, non-hazardous, and stable is selected. Some absorbents with such characters are activated alumina, activated carbon, zeolite, polymeric sorbents, or silica gel (Rodero et al., 2018) Physicochemical methods are efficient in the removal of around 99% for H 2 S, halocarbons, and siloxanes with about 88%-98% CH 4 purity. But due to high energy and chemical requirement, the cost of these processes are high, therefore, people are bound to look for alternatives in biological processes (Rodero et al., 2018). ...
... Some absorbents with such characters are activated alumina, activated carbon, zeolite, polymeric sorbents, or silica gel (Rodero et al., 2018) Physicochemical methods are efficient in the removal of around 99% for H 2 S, halocarbons, and siloxanes with about 88%-98% CH 4 purity. But due to high energy and chemical requirement, the cost of these processes are high, therefore, people are bound to look for alternatives in biological processes (Rodero et al., 2018). Biological processes include the utilization of enzymes and microorganisms including prokaryotes and microeukaryotes for the removal of undesired compounds. ...
Second-generation biofuels produced from lignocellulose biomass (LCB) have the potential to compete with fossil fuels, provided that appropriate cost-effective and efficient technologies are used in the production process. As agriculture is practiced all over the world and millions of tons of biomass are produced every year, the production of biofuel from such wastes could be an environmentally friendly approach to meet the increasing energy demand. Biofuel production is quite a challenging process involving several technologies and methods that need to be employed for maximum conversion of complex lignocellulose into functional biofuel. This review provides insight into the various strategies, including upstream and downstream, involved in the production of biofuels from agriculturally produced LCB. For decades, numerous researches have been conducted worldwide to find the best strategies for biofuel production. This has led to several recent advances that help to achieve maximum biofuel production in an environmentally friendly and cost-effective system. To complete the overview, the analysis of the impact of biofuel production has also been discussed in this review.
... It is predominantly present in waste gas streams released from industries, such as pulp and paper manufacturing, rayon production, biogas production, and crude petroleum refineries. 1,2 Physico-chemical and biological treatment processes are commonly employed to treat H 2 S. 3 Although physico-chemical technologies (e.g., water scrubbing or chemical scrubbing) are widely applied, the generation of secondary pollution and the need for large amounts of chemicals and energy limit their application. 4 On the other hand, biological processes (e.g., biofiltration and bioscrubbing) are environment friendly and easy to operate under a wide range of conditions that can significantly reduce the energy requirement and generate ecologically safe end products. ...
... 4 On the other hand, biological processes (e.g., biofiltration and bioscrubbing) are environment friendly and easy to operate under a wide range of conditions that can significantly reduce the energy requirement and generate ecologically safe end products. 4,5 Several bioreactor configurations, such as the biofilter, 6,7 biotrickling filter, 2,4,[8][9][10][11][12] and bioscrubber, 5,13 are frequently used for biological H 2 S removal. The bioreactors are operated under either aerobic or anoxic conditions, where sulphur-oxidizing bacteria convert sulphide (S 2− ) into elemental sulphur (S 0 ) and sulphate (SO 4 2− ) as end products during the bioconversion process. ...
BACKGROUND
Hydrogen sulphide (H2S) must be treated at its emission source to avoid health risks, odour, and corrosion. Conventional physico‐chemical H2S removal technologies (for example, membrane contactors or chemical scrubbers) have several limitations, such as requiring high amounts of absorption chemicals and energy. In contrast, biological H2S removal technologies are environment friendly, easy to operate, and less expensive due to their low energy requirements. In this study, the feasibility of a porous hydrophilic polyethersulfone hollow fibre membrane bioreactor (HFMB) was tested for the biological removal of gas‐phase H2S by employing three lab‐scale reactors (two biotic and one abiotic). The HFMBs were operated at ~20 °C for ~3 months, employing different H2S inlet loading rates (ILR) and an empty bed residence time of 187 s.
RESULTS
Biotic performance of the HFMBs demonstrated that the removal efficiency (RE) varied between the different inocula and was in the range of 80–100% for the applied H2S ILR of ~5.0–7.5 g m⁻³ h⁻¹. The RE reached a constant value of ~100% in both biotic reactors at an ILR of ~17.0 g m⁻³ h⁻¹ when using acclimatized inoculum. The biotic HFMBs demonstrated ~5–9 times higher H2S flux and ~ 20–26 times higher mass transfer compared to the abiotic control. Surface morphology revealed attached microbial growth on the outer surface of the membranes, while the high throughput sequencing confirmed the richness of H2S oxidizing microbial communities on the shell side.
CONCLUSION
The obtained results confirm that the HFMB configuration is suitable for biological treatment of H2S laden waste gas. © 2021 Society of Chemical Industry (SCI).
... [228][229][230]). Traditional biogas upgrading mostly uses chemical or physical approaches that can purify methane as much as 88-98% with around 99% removal of CO 2 , H 2 S, halocarbons, and siloxanes [231]. Despite its good efficiency, physicochemical approaches are energy intensive and not eco-friendly that has resulted in a growing research interest on biological biogas upgrading. ...
... Despite its good efficiency, physicochemical approaches are energy intensive and not eco-friendly that has resulted in a growing research interest on biological biogas upgrading. Biological approaches may include CO 2 fixation by microalgae during their cultivation, H 2 -aided conversion of CO 2 into CH 4 by the litho-autotrophs, enzymatic removal of CO 2 etc., which exhibited competitive efficiency by 80-100% of CO 2 and as much as 99% of H 2 S removal [231]. ...
Production of biogas from microalgae has been receiving significant attention since 1950s, as microalgae are capable of rapidly growing in the non-arable land area and accumulating high quantity of digestible macromolecules in the biomass. Nevertheless, commercial production of microalgae-based biogas is still in its immature stage for some existing technoeconomic challenges, as exemplified by the requirement of a costly and energy consuming step in biomass harvesting, recalcitrance of the components, low biomass loading rate, and interferences of various operational factors. Real-world research and developments are therefore necessary for achieving a state-of-the-art technology to produce microalgal biogas. In this context, extensive research efforts have been devoted since decades as attempts to improve efficiency and sustainability of this bioprocess. This paper presents a critical and comprehensive review on the key issues and perspectives of microalgal biogas in terms of current knowledge and future developments. Over the contemporary reviews published on this topical area, present paper distinctly covers almost all the relevant aspects, which might draw a complete picture on microalgal biogas and provide necessary perspectives for conducting further research efforts. Specifically, this paper discusses potentials of microalgae as the biogas feedstocks, screening and selection approaches of potent strains, technological aspects of microalgal biogas production, reactor design for anaerobic digestion (AD), operational conditions affecting AD, strategies for improving strains and biogas yield, biogas upgrading, kinetics, modelling and economics, life cycle assessment (LCA), and challenges coupled with further research opportunities.
... Nowadays, only physicochemical processes such as cryogenic separation, chemical scrubbing, pressure swing adsorption, organic solvent scrubbing, membrane separation and water scrubbing are available at commercial scale for biogas upgrading (Aghel et al., 2022;Mulu et al., 2021). However, these technologies entail a high energy demand and ecological impacts (Rodero et al., 2018). ...
The effect of carbon-coated zero-valent iron nanoparticles (NPs) on the upgrading of biogas based on bacterial-algal symbiosis was assessed in an indoor pilot scale algal open pond interconnected to a biogas purification column. The addition of NPs at 70 mg L−1 stimulated photosynthetic activity, resulting in an enhanced concentration of biomass from 1.56 to 3.26 g VSS L−1. The presence of NPs in the culture broth increased CO2 removal from 86% to 92% at low IC concentrations (≤600 mg L−1) and a decrease in the content of O2 and N2 in the upgraded biomethane. This entailed an increase of the CH4 concentration in the upgraded biomethane from 83% to 91%. However, the higher biomass productivity resulted in a gradual depletion of the IC concentration concomitant with a reduction of the buffer capacity which requires a further optimization in the operational conditions to maintain the beneficial effects of NPs.
... In fact, several review papers in the literature address this issue. As an example, Rodero et al. [57] presented a review of biogas' purification-and-upgrading technologies. In addition, Khan et al. [58] focused on the available technologies for direct biomethane use, and Angelidaki et al. [59] provided an analysis of the currently available technologies and investigated the emerging ones. ...
Energy production from biogas can play a pivotal role in many European countries, and specifically in Italy, for three main reasons: (i) fossil fuels are scarce, (ii) imports cover large shares of internal demand, and (iii) electricity and heat production from biogas is already a consolidated business. Nonetheless, in Italy, current legislation and incentive policies on electricity generation from biogas are causing a stagnation of the entire sector, which may lead to the shutting down of many in-operation plants in the years 2027–2028 and the consequent loss of 573 MWel over a total of 1400 MWel. This work aims to investigate the potential of revamping biogas power plants in prolonging operation until the end of the plants’ useful life, regardless of the implementation of a new government’s incentive schemes. Based on the time-series analysis of electricity prices in Italy and a case study representative of the vast set of in-operation power plants, our findings show that 700 plants will likely shut down between 2027 and 2028 unless the government adequately rewards electricity produced and fed into the grid via incentive schemes. In detail, our results show that the investment to revamp the plant exhibits a highly negative Net Present Value.
... This is the case of FAKA®, SAG®, BGAK®, GES®, TSA®, SWOPT®, HELASORP®, and TCR® technologies based on adsorption, absorption and gas chilling processes, which are currently commercialized by Siloxa Engineering, Applied Filter Technology (AFT), PpTek, Parker Hannifin, Jenbacher, Pioneer Air Systems, among others [36]. However, temperatures and pressures of up to 400 °C and 27 atm [37][38][39][40][41], as well as the use of chemicals required in physical-chemical technologies, increase VMS treatment cost and the CO 2 footprint of these approaches as a result of the intensive energy required [42][43][44]. Such high operating costs and environmental impacts of physical-chemical technologies have been the central motivation for developing biological alternatives for VMS removal from biogas [8,45]. ...
Volatile methyl siloxanes (VMS) are biogas pollutants generated from the metabolism of polydimethylsiloxanes (PDMS) in anaerobic digestion processes. In the past, VMS were considered an issue only for biogas produced in landfills. However, the widespread presence of VMS in different types of biogas has been demonstrated in recent years as a consequence of the intensive use of PDMS in the formulation of personal care products, industrial lubricants, glues, paints, and detergents. Burning biogas (or biomethane) laden with VMS leads the formation of silicate (SiO2) deposits, resulting in abrasion and severe lubrication issues, which finally produce irreversible damage to energy production devices. The present work provides a comprehensive review of the VMS concentrations recently reported in several types of biogas, as well as the physical–chemical technologies available on a commercial scale for the removal of VMS. Alternative biological processes for VMS removal are also described, including the most recent advances in microbial degradation mechanisms, bioreactor configurations, and operation modes. Critical research niches and challenges towards the consolidation of biotechnologies as efficient and cost-effective VMS treatment systems are identified and critically discussed.
... Biological technologies can remove 80−100% CO 2 and up to 99% H 2 S [22]. The use of CO 2 by microalgae offers a new method for upgrading biogas. ...
The production of biogas from microalgae biomass is an attractive alternative in the biofuel industry. The cultivation of these microorganisms can be carried out with wastewater and industrial effluents, which contributes to reducing costs and negative impacts on the environment. Microalgal biomass is an ideal substrate for the nutrient demands of bacteria that act in anaerobic digestion. With microalgae technology, both raw biomass and residual biomass can be used for biogas production. Thus, it is possible to maximize the use of biomass by extracting various compounds and generating appropriate residual biomass for obtaining biogas. Moreover, the microalgal biorefinery concept encourages the production of this biofuel by suggesting the application of biomass in different industrial sectors. In this context, this chapter presents the biotechnological strategies, recovery, and pretreatments of microalgal biomass to maximize the production of biogas. Future trends inserted in the biorefinery context are also addressed for the sustainable production of biogas from microalgae.
... Hence, the technical and economic feasibility of the entire production process must be fully demonstrated before any attempt is made to bring it up to commercial scale. Currently, several technologies are available for biogas refining and their application depends on the biogas source, as well as the final use aimed thereof (Rodero et al., 2018). Table 1 presents an overview of biogas refining technologies, highlighting their main advantages and disadvantages. ...
Biogas production from organic wastes and its subsequent conversion to sustainable drop-in fuels may be a cost-effective way to reduce greenhouse gas emissions and improve the carbon footprint of transportation. In this overview, biogas refining, refined biogas conversion to syngas, Fischer-Tropsch synthesis, and syncrude upgrade to drop-in fuels are discussed with emphasis on sustainable aviation fuels, which are seen as mandatory to curb the impact of aviation emissions on climate change. In this pathway for next generation sustainable fuels, biogas refining must eliminate contaminants that are detrimental to reforming, catalytic reforming must be optimized to produce syngas with an acceptable H2/CO ratio, and Fischer-Tropsch catalysts with higher selectivity and productivity must be identified and optimized.
... Various biogas purification technologies have been available such as; water-scrubbing (CH 4 % > 96), solvent-scrubbing (CH 4 % > 96-98.5), chemical-scrubbing (CH 4 % > 99), PSA -pressure swing adsorption (CH 4 % > 96-98), membrane separation (CH 4 % > 96-98), and cryogenic separation (CH 4 % > 97) [63,64]. Among these technologies, PSA is being widely used technology for biogas purification and the same was considered for largescale biogas plant design. ...
Worldwide methane production by anaerobic digestion (AD) from organic waste has been expanded for greenhouse gas emission reduction by replacing fossil-fuel needs to facilitate sustainable energy supply. India also launched Sustainable Alternative Towards Affordable Transportation (SATAT) policy to promote compressed biogas (CBG) production through anaerobic digestion of organic waste. However, it is vital to understand productivity, raw material availability, and raw material quality to produce biogas at profit margin lucrative for business. The present work depicts biogas production from 11 organic feedstocks from community, industrial, and agricultural waste through 25 m³ demo-scale biogas plant. All materials were analysed for moisture, total solids, organic matter, carbon, nitrogen, C:N (carbon:nitrogen) ratio, and volatile solids, to optimize process parameters. Average 423 Nm³ biogas yield with 57% methane was obtained with average hydraulic retention time (HRT) of 60 days. The design, operation, and cost–benefit analysis of a 5 tons per day (TPD) CBG plant were carried out by integrating two digesters with pressure swing technology (PSA) for biogas purification, which resulted in 14% IRR and 3.6-year payback period after 10 years of operation. The 1500-tons/year CBG production can replace gasoline demand of 2070 kL/year while reducing CO2 emission of 41,218.6 tons/year through replacing gasoline and synthetic fertilizer (urea) by organic biogas slurry as by-product and preventing open dumping of organic waste.
... It has been reported that carbon dioxide, nitrogen, and oxygen are the main constituents along with methane in biogas [7,10]. Rodero et al. reported that the production of biogas in closed digesters offers higher methane concentration and accordingly lower concentrations of oxygen and nitrogen than landfill-derived biogas [11]. ...
Biogas upgrading is an important technique for biomethane production, a renewable source of energy with low carbon footprint. The presence of even small concentrations of impurities in biogas influences its properties at different levels. The effect of the individual component species on the biogas upgrading performance has not been fully evaluated and closely understood. In this work, biogas component species, including the typical binary biogas mixture (CH4/CO2) and one impurity with maximum concentration, flowing over silica gel bed are modelled and analyzed to study their effects on the biogas upgrading process. The model is built using Aspen Adsorption™ and Aspen Hysys version 10 and validated using experimental data reported previously. The effect of each component species is reported, and the deviation from the typical binary biogas mixture is marked. From our analyses, the presence of all the component species causes an adverse implication on the biogas upgrading performance. The presence of 20% nitrogen, for example, brings about the highest increase in viscosity and density of the biogas mixture, resulting in the highest friction factor between gas molecules and bed surface. Ten percent of moisture content causes the highest pressure drop during adsorption. Interestingly, 5% oxygen, 3% hydrogen, and 1% ammonia enhance the recovery of methane by 1.48%, 0.93%, and 0.56%, respectively. This study can be used to predict the biogas mixing behavior during the biomethane upgrading using pressure swing adsorption. The proposed model possesses a great competence for making prediction of biomethane upgrading performance of PSA process with impressive ability to disclose the influence of the common impurities on upgrading performance and biogas properties.
Graphical abstract
... Hydrogenotrophic CO2 removal 1) Chemically store renewable energy as CH 4 by H 2 production (via water electrolysis) and the subsequent upgrading of biogas 2) Use the heat from methanation and the O 2 produced during electrolysis used for microaerobic removal of H 2 S. Power-to-gas approach exhibits low operating costs Rittmann et al. (2013) and Rodero et al. (2018 Gübitz et al. (2015), and Xia et al. (2016). ...
The world is mired with the rising energy demand, soaring up of fossil fuels, and increased environmental concern. This quest of searching and developing eco-friendly alternatives put forward many reasonable renewable energy processes. Anaerobic digestion producing biogas is one of the well-established processes for simultaneous generation of renewable energy and treatment of organic wastes. Biogas consisting of CH 4 along with other gases like CO 2 , H 2 O, N 2 , etc. has increased the interest of utilizing it as a substitute to natural gas. However, the constituents other than CH 4 decrease its calorific value and thus energy efficiency. The purification of biogas is required to achieve maximum energy efficiency by removing these impurities. It has exaggerated the interest of researchers to explore and upgrade the biogas technologies. This chapter focuses on biogas developments around the globe and discusses the biogas upgrading technologies along with current trends and issues in its production. Apart from this, it gives an insight on the role of nanotechnology and biotechnology in biogas enhancement.
... Although there are many techniques used to treat or purify biogas in physical, chemical and biological ways. A traditional physical/chemical route is chosen, which can purify methane up to 88-98%, with about 99% removal efficiency of H 2 S, halocarbons and siloxanes [20]. The Pressure Swing Adsorption (PSA) technique is used by compressing the biogas to a pressure between 4-10 bar then fed to a vessel containing a solid porous adsorbent with a high surface area that retain CO 2 then can be injected into the HRAPs and the purified CH 4 is recovered at the top of the vessel to be used as a fuel. ...
... BU by fixation of CO 2 and H 2 in the presence of redox intermediates to CH 4 is still undeveloped, but is gaining interest in the context of renewable energy utilization (Alvarez-Gutierrez et al., 2016;Angelidaki et al., 2019). Commercial biogas upgrading systems using conventional processes can perform only the separation of CH 4 from CO 2 and requires additional integration of individual processes to increase the efficiency of CH 4 conversion and also avoiding the carbon emissions (Baena- Moreno et al., 2020;Xu et al., 2018;Rodero et al., 2018;Vrbova and Ciahotny, 2017;Yuan et al., 2013). The lower density of H 2 requires higher storage capacities, while ...
Biogas upgradation (BU) is an alternative route for carbon dioxide (CO2) sequestration for increasing biogas utilization/reuse in integrating processes for the commercialization of technologies. This chapter critically discusses the current state-of-the-art of BU in the global context with the specific concepts required for research and development (R&D) applications for centralized and decentralized applications for commercialization. It also discusses future perspectives that essentially help in overcoming the challenges faced at the R&D stage in terms of biogas production and reuse to achieve energy-intensive processes. Anaerobic digestion (AD) is a potential bioconversion process utilized for treating diverse waste/wastewaters for energy and product generation. Apart from its efficiency, AD also has few limitations that need to be overcome for recovering maximum methane (CH4). The integration of electrochemical processes with interference of electrodes as electron acceptors in microbial processes could beneficially help in direct utilization of electrical energy for BU by converting CO2 to CH4 to achieve higher process efficiency. It influences the overall CH4 yield, while providing new insights to counteract the operational instability of bioprocesses like AD through applied potential by altering the in situ microbial reactions. The integration of the centralized and decentralized biogas upgrading units in bioprocess units helps to improve the sustainability of the individual processes in a circular economy system. The industrial perspective for commercialization of BU as a technology reveals significant applications for decreasing the gaseous carbon footprints for enhanced biobased product generation using a biological route.
... EM triggers and regulates the microbial electron flux with electrode placement and applied potential creating a synergistic redox microenvironment towards CH 4 formation Rodero et al., 2018;Chen and Liu, 2017). Electrodes as well as few intermediate electron acceptors like granular activated carbon, biochar or magnetic field could act as redox shuttlers which influence the microbial electrogenic activity towards decreased losses thereby increasing the CH 4 recovery. ...
Global need for transforming from fossil-based to bio-based economy is constantly emerging for the production of low-carbon/renewable energy/products. Microbial fuel cell (MFC) catalysed by bio-electrochemical process gained significant attention initially for its unique potential to generate energy. Diversification of MFC is an emerging trend in the context of prioritising/enhancing product output while exploring the mechanism specificity of individual processes. Bioelectrochemical treatment system (BET), microbial electrosynthesis system (MES), bioelectrochemical system (BES), electro-fermentation (EF), microbial desalination (MDC), microbial electrolysis cell (MEC) and electro-methanogenesis (EM) are the diversified MFC systems that are being researched actively. Owing to its broad diversification, MFC domain is increasing its potential credibility. Microbial catalyzed electrochemical reactions are the key which directly/indirectly are proportionally linked to electrometabolic activity of microorganisms towards final output. This review intends to holistically document the mechanism, applications and current trends of MFC diversifications towards multi-faced applications.
... However, knowledge of both q max and K S is of paramount relevance for the design and optimization of microalgal-based processes where the biological reaction rather than the CO 2 transfer is expected to be the limiting step. This is particularly true for biogas enrichment and flue gas purification processes based on microalgae [14,18,20,46]. ...
... adsorption on activated carbon or metal ions-based in situ precipitation) and a high energy input (0.2-0.7 kWh/ m 3 biogas ), with the associated increase in operational costs. Thus, the high energy and chemical requirements of conventional biogas upgrading processes, among other factors such as the cost of acquisition of the organic substrate and the type of digestion process, limit the costeffective use of biomethane as a renewable substitute of natural gas (Rodero et al., 2018a). On the other hand, biological technologies such as biofiltration or in situ microaerobic anaerobic digestion for H 2 S removal followed by hydrogenotrophic biogas upgrading (power to gas) for CO 2 bioconversion into CH 4 entail the need of a two-stage process and can be only applied in locations with a sustained surplus of renewable electricity (Angelidaki et al., 2018;Muñoz et al., 2015a). ...
The performance of photosynthetic biogas upgrading coupled to wastewater treatment was evaluated in an outdoors high rate algal pond (HRAP) interconnected to an absorption column at semi-industrial scale. The influence of biogas flowrate (274, 370 and 459 L h⁻¹), liquid to biogas ratio (L/G = 1.2, 2.1 and 3.5), type of wastewater (domestic versus centrate) and hydraulic retention time in the HRAP (HRT) on the quality of the biomethane produced was assessed. The highest CO2 and H2S removal efficiencies (REs) were recorded at the largest L/G due to the higher biogas-liquid mass transfer at increasing liquid flowrates. No significant influence of the biogas flowrate on process performance was observed, while the type of wastewater was identified as a key operational parameter. CO2 and H2S-REs of 99% and 100% at a L/Gmax = 3.5 were recorded using centrate. The maximum CH4 content in the biomethane (90%) was limited by N2 and O2 desorption.
As a result of the rapid pace of urbanization, industrialization, and technology‐driven industry, the world's energy demand is increasing on a daily basis. For many decades, it has been seen and reported that fossil‐fuel‐based energy sources are the primary energy source. However, due to the non‐renewable nature of these traditional energy sources, they will be depleted in the near future. Furthermore, the continual decline in fossil‐fuel‐based oil production, the low number of finds of newer oil reserves, the instability of oil‐producing countries, and the skyrocketing pricing of these fuels render these fuels unsustainable for use in the future. Earlier findings on bioethanol production were made utilizing food‐based carbon sources such as rice, sugar cane, maize, barley, and other grains. However, increasing energy production at the expense of food crops can cause a food crisis. Rice straw is produced as a by‐product of rice production during harvest, and it is a rich source of lingo‐cellulosic material because of its high lignin concentration. It has become increasingly popular over the last decade to burn rice straw in open fields as a post‐harvest management tool for paddy fields; however, it contributes to air pollution as a result of this practice. According to the International Rice Research Institute, the annual production of rice straw (Parali) ranges between 800–1000 million tons, and it has the capacity to produce approximately 205 billion liters of bioethanol per year. After being treated for the physical, chemical, and biological components, lingo‐cellulosic waste created from paddy fields can be used as a sustainable source of bio‐ethanol production as an alternative. The following article includes thorough information on recent breakthroughs in the pre‐treatment of lingo‐cellulosic materials, sustainable bio‐ethanol production from parali, bioethanol yields, properties, and possibilities of bioethanol as an alternative to fossil fuels.
Biogas has received an increasing attention in the past decade as a renewable and sustainable energy vector that can be used for the generation of household heat, steam, and electricity in industries or as a natural gas substitute after upgrading. The raw biogas has multiple impurities at varying concentrations that limit the widespread use of this renewable energy vector. In this context, biogas upgrading is a mandatory step prior to biomethane injection into natural gas grids or use as a vehicle fuel according to international regulations. Physical/chemical biogas upgrading technologies are available at commercial scale, but exhibit high operating costs and environmental impacts. The integration of biogas upgrading and digestate treatment in algal-bacterial photobioreactors represents a promising technology platform that has attracted an increased attention in the last years and it is under validation in pilot systems. This combined technology is based on the symbiosis between microalgae and heterotrophic/chemoautotrophic bacteria cultured at high pH for the simultaneous removal of CO2, H2S, and volatile organic compounds from biogas at low energy costs and low environmental impacts. In addition, the economic and environmental sustainability of this platform technology can be enhanced by using wastewaters as a low-cost water and nutrient source. Life cycle assessment of photosynthetic biogas upgrading coupled with digestate treatment is crucial to estimate the energy balance and environmental impacts of the whole system. The aim of this chapter was to review the recent advances and sustainability of pilot and real-scale photosynthetic biogas upgrading systems and high-value algal biomass production.
For a cleaner and healthier planet, industries must decarbonize and move away from ‘take-make-waste’ linear economies to circular economies, in ways that maximize value and minimize negative environmental impact. Anaerobic digestion (AD) is a well-known technology used to process domestic and industrial wastes to produce biogas (mainly CH4) but with large volumes of high moisture content digestate as by-product The digestate is both difficult and expensive to manage, often requiring more than half the operating cost of treatment plants. Hydrothermal gasification (HTG) can convert the recalcitrant digestate into renewable gases including CH4. So, for high carbon conversion a hybrid AD and HTG technology is an attractive solution. In the hybrid process, AD can be used to treat the wet biomass (an existing practice), and the liquid AD digestate can then be processed by HTG to optimize CH4 yield. This paper reviews the pros and cons of the AD and HTG processes, examines the valorization of co-products, and assesses the potential of the hybrid process with additional information from other AD-thermochemical process hybrids that have widely reported economic feasibilities.
Water systems need to become more locally robust and sustainable in view of increased population demands and supply uncertainties. Decentralized treatment is often assumed to have the potential to improve the technical, environmental, and economic performance of current technologies. The techno-economic feasibility of implementing independent building-scale decentralized systems combining rainwater harvesting, potable water production, and wastewater treatment and recycling was assessed for six main types of buildings ranging from single-family dwellings to high-rise buildings. Five different treatment layouts were evaluated under five different climatic conditions for each type of building. The layouts considered varying levels of source separation (i.e., black, grey, yellow, brown, and combined wastewater) using the corresponding toilet types (vacuum, urine-diverting, and conventional) and the appropriate pipes and pumping requirements. Our results indicate that the proposed layouts could satisfy 100% of the water demand for the three smallest buildings in all but the aridest climate conditions. For the three larger buildings, rainwater would offset annual water needs by approximately 74 to 100%. A comprehensive economic analysis considering CapEx and OpEx indicated that the cost of installing on-site water harvesting and recycling systems would increase the overall construction cost of multi-family buildings by around 6% and single-family dwellings by about 12%, with relatively low space requirements. For buildings or combined water systems with more than 300 people, the estimated total price of on-site water provision (including harvesting, treatment, recycling, and monitoring) ranged from $1.5/m³ to $2.7/m,³ which is considerably less than the typical tariffs collected by utilities in the United States and Western Europe. Where buildings can avoid the need to connect to centralized supplies for potable water and sewage disposal, water costs could be even lower. Urine-diversion has the potential to yield the least expensive solution but is the least well developed and had higher uncertainty in the cost analysis. More mature layouts (e.g., membrane bioreactors) exhibited less cost uncertainty and were economically competitive. Our analysis indicates that existing technologies can be used to create economically viable systems that greatly reduce demands on centralized utilities and, under some conditions, eliminate the need for centralized water supply or sewage collection.
Biofiltration is a recent pollution control technique that involves the removal of odor, volatile organic compounds, and other contaminants from waste gas or water. Lately, this technique has gained a lot of popularity globally due to its simple and low cost technique, easy operation, high removal efficiency, less energy requirement, and the residual products not requiring any further treatment or disposal. The process makes use of microorganisms that are grown on various natural and synthetic media such as sand, crushed rock, river gravel, some form of plastic or ceramic material shaped as small beads, and rings. The polluted air or water is allowed to pass through the filter media where the microbes degrade the contaminants into simpler and less toxic compounds which are utilized as energy and food source by the microbes themselves for their growth and development. This technique has been widely used to remove contaminants from the polluted water and air with removal efficiency of more than 90%. The present chapter aims to provide fresh impetus to the process of biofiltration, factors affecting it, biofilter media, applications, and comparisons with other pollution controlling technologies.
This study showed the impact of an anoxic desulfurization process on the CH4 and CO2 content of the purified biogas. The desulfurization system was implemented in an anoxic bioscrubber, which supported H2S removal efficiencies ranging from 92 ± 8 to 97 ± 6% for H2S loading rates between 28.3 and 37.8 g S mliquid⁻³h⁻¹. CO2 absorption during the desulfurization process mediated a slight reduction in the CO2 concentration from 39.5% in the raw biogas to 36.5–38.2% in the purified biogas. Such reduction in the CO2 concentration was proportional to the increase of CH4 concentration recorded, passing from 60.0% in the raw biogas to 61.7–63.5% in the purified biogas. Bacterial community characterization by means of 16S rRNA high-throughput sequencing revealed the consistent presence of both methanotrophs (Methylibium, Methylomonas and Methylosinus genera) and methanol-oxidizing denitrifiers (Simplicispira and Methyloversatilis genera), which suggested that methane oxidation indeed occurred. The quantification of a potential CH4 uptake in the desulfurization process was hindered by the CO2 absorption observed, however, strategies for quantifying biological CH4 consumption in anoxic desulfurization systems are proposed and discussed, based on the results herein obtained.
Biogas is a renewable energy source with numerous final applications for the production of heat, power, and useful chemicals. The raw biogas coming from the anaerobic digestion of biomass contains a series of contaminants that can cause significant processing, engineering, and environmental and health problems. Biogas sweetening technologies are used to purify the fuel from these substances or to upgrade it into biomethane of high purity. The present chapter provides an overview of the most important biogas sweetening technologies that are currently used throughout the world. Initially, the technologies used for the purification of biogas from water vapors, hydrogen sulfide and sulfur containing compound, siloxanes, volatile organic compounds, ammonia, nitrogen, and oxygen will be presented. Then, the analysis will cover the technologies used for the removal of carbon dioxide that upgrade biogas into biomethane.
Three innovative operational strategies were successfully evaluated to improve the quality of biomethane in an outdoors pilot scale photobioreactor interconnected to an external absorption unit: i) the use of a greenhouse during winter conditions, ii) a direct CO2 stripping in the photobioreactor via air stripping during winter conditions and iii) the use of digestate as make-up water during summer conditions. CO2 concentrations in the biomethane ranged from 0.4% to 6.1% using the greenhouse, from 0.3% to 2.6% when air was injected in the photobioreactor and from 0.4% to 0.9% using digestate as make up water. H2S was completely removed under all strategies tested. On the other hand, CH4 concentrations in biomethane ranged from 89.5% to 98.2%, from 93.0% to 98.2% and from 96.3% to 97.9%, when implementing strategies i), ii) and iii), respectively. The greenhouse was capable of maintaining microalgae productivities of 7.5 g m⁻² d⁻¹ during continental weather conditions, while mechanical CO2 stripping increased the pH in order to support an effective CO2 and H2S removal. Finally, the high evaporation rates during summer conditions allowed maintaining high inorganic carbon concentrations in the cultivation broth using centrate, which provided a cost-effective biogas upgrading.
The future demands of transportation and industrial sectors necessitate groundbreaking
research towards a sustainable energy source. Biogas from anaerobic digestion is a well-
studied research concept for the last 2-3 decades. The biogas technology mainly faded with the composition of unwanted contaminants (mainly CO 2 , H 2 S) in the production stream and utilized cumbersome, energy- worn removal technologies. The photosynthetic microalgae sequester's CO 2 further utilizes for algal biomass (source of different value-added products) growth and brings simultaneous biogas upgradation under mild conditions. The present chapter put forth the positive attributes of photosynthetic microalgae-based biogas upgradation towards the enhanced CO 2 removal from the biogas by meeting the natural grid system's standards. More emphasis has been given towards photosynthetic biogas upgradation process set up with possible integration with wastewater treatment and biomass production, the existing photobioreactors, influential process variables towards the better biogas upgradation, and discussed the prospects of the photosynthetic biogas upgradation technology.
Biogas is a valuable renewable energy and also a secondary energy carrier produced from biodegradable organic materials via anaerobic digestion. It can be used as a fuel or as starting material for the production of chemicals, hydrogen and/or synthesis gas etc. The main constituents of biogas are methane (CH4) and carbon dioxide (CO2), with various quantities of contaminants, such as ammonia (NH3), water vapour (H2O), hydrogen sulfide (H2S), methyl siloxanes, nitrogen (N2), oxygen (O2), halogenated volatile organic compounds (VOCs), carbon monoxide (CO) and hydrocarbons. These contaminants presence and quantities depend largely on the biogas source, which could be anaerobic digestion of many substrates and landfill decompositions. The removal of these contaminants especially H2S and CO2 will significantly improve the quality of the biogas for its further uses. In parallel, biogas upgrading market is facing challenges in term of operating costs and energy consumption. The selection of appropriate technology depends on the specific biogas requirements, site specific, local circumstances and is case sensitive. This paper reviews the present state-of-the-art of biogas cleaning and upgrading technologies, including its composition, upgrading efficiency, methane recovery and loss. In addition, biogas production, utilization and the corresponding requirements on gas quality for grid injection and vehicle usage are investigated. Based on the results of comparisons of various technologies, recommendations are made on further research on the appropriate low cost technologies, especially using solid waste as low cost materials for biogas purification and upgrading.
We studied the feasibility of the microaerobic process, in comparison with the traditional chemical absorption process (NaOH), on H2S removal in order to improve the biogas quality. The experiment consisted of two systems: R1, biogas from an anaerobic reactor was washed in a NaOH solution, and R2, headspace microaeration with atmospheric air in a former anaerobic reactor. The microaeration used for low sulfate concentration wastewater did not affect the anaerobic digestion, but even increased system stability. Methane production in the R2 was 14 % lower compared to R1, due to biogas dilution by the atmospheric air used. The presence of oxygen in the biogas reveals that not all the oxygen was consumed for sulfide oxidation in the liquid phase indicating mass transfer limitations. The reactor was able to rapidly recover its capacity on H2S removal after an operational failure. Bacterial and archaeal richness shifted due to changes in operational parameters, which match with the system functioning. Finally, the microaerobic system seems to be more advantageous for both technical and economical reasons, in which the payback of microaerobic process for H2S removal was 4.7 months.
Both biogas desulfurization and wastewater denitrification can be achieved simultaneously, when nitrate/nitrite is used as the electron acceptor for H2S oxidation. The main objective of this study was to investigate the influence of the molar ratio of sulfide/nitrate (S/N) on biogas desulfurization performance in a biotrickling filter (BTF) and a biobubble column (BBC). The results show that with the decrease of the S/N ratios from 3.6 to 0.7, the removal efficiencies of H2S increased from about 66 to 100 %, while the removal of nitrate decreased from 100 to 70 % in the two bioreactors. The BTF has a better and more stable desulfurization performance than the BBC does, which could be attributed to their different gas-liquid contacting modes. With the increase of the S/N ratios from 1.0 to 2.5 in the BTFs, the removal of H2S in biogas was affected slightly, while the percentages of the produced sulfate decreased evidently. In addition, different supplying methods of nitrate wastewater, i.e., intermittent and continuous, did not affect the removal of H2S significantly, while the intermittent addition of nitrate wastewater increased the percentages of sulfate and denitrification performance.
A pilot high rate algal pond (HRAP) interconnected to an external CO2–H2S absorption column via settled broth recirculation was used to simultaneously treat a synthetic digestate and to upgrade biogas to a bio-methane with sufficient quality to be injected into natural gas grids. An innovative HRAP operational strategy with biomass recirculation based on the control of algal-bacterial biomass productivity (2.2, 4.4 and 7.5 g m− 2 d− 1) via settled biomass wastage was evaluated in order to enhance nutrient recovery from digestate at a constant hydraulic retention time. The influence of the recycling liquid to biogas (L/G) ratio on the quality of the upgraded biogas was assessed. The bio-methane composition under a L/G ratio of 1 (0.4 ± 0.1% CO2, 0.03 ± 0.04% O2, 2.4 ± 0.2% N2 and 97.2 ± 0.2% CH4) complied with the technical specifications of most European bio-methane legislations regardless of the biomass productivity established. The HRAP operational strategy applied allowed increasing the N and P recovery from 19 and 22% to 83 and 100%, respectively, when the biomass productivity was increased from 2.2 to 7.5 g m− 2 d− 1. Finally, the dynamics of microalgae and bacteria population structure were characterized by morphological identification and Denaturing Gradient Gel Electrophoresis analysis.
The present research was conducted to simultaneously optimize biogas upgrading and carbon and nutrient
removal from centrates in a 180-L high-rate algal pond interconnected to an external CO2 absorption unit. Different biogas
and centrate supply strategies were assessed to increase biomass lipid content. Results showed 99 % CO2 removal efficiencies from simulated biogas at liquid recirculation rates in
the absorption column of 9.9 m3 m−2 h−1, concomitant with
nitrogen and phosphorus removal efficiencies of 100 and
82 %, respectively, using a 1:70 diluted centrate at a hydraulic
retention time of 7 days. The lipid content of the harvested
algal–bacterial biomass remained low (2.9–11.2 %) regardless
of the operational conditions, with no particular trend over
time. The good settling characteristics of the algal–bacterial
flocs resulted in harvesting efficiencies over 95 %, which represents a cost-effective alternative for algal biomass reutilization compared to conventional physical–chemical techniques.
Finally, high microalgae biodiversity was found regardless of
the operational conditions.
High sulfide concentrations in biogas are a major problem associated with the anaerobic treatment of sulfate-rich substrates. It causes the corrosion of concrete and steel, compromises the functions of cogeneration units, produces the emissions of unpleasant odors, and is toxic to humans. Microaeration, i.e. the dosing of small amounts of air (oxygen) into an anaerobic digester, is a highly efficient, simple and economically feasible technique for hydrogen sulfide removal from biogas. Due to microaeration, sulfide is oxidized to elemental sulfur by the action of sulfide oxidizing bacteria. This process takes place directly in the digester. This paper reviews the most important aspects and recent developments of microaeration technology. It describes the basic principles (microbiology, chemistry) of microaeration and the key technological factors influencing microaeration. Other aspects such as process economy, mathematical modelling and control strategies are discussed as well. Besides its advantages, the limitations of microaeration such as partial oxidation of soluble substrate, clogging the walls and pipes with elemental sulfur or toxicity to methanogens are pointed out as well. An integrated mathematical model describing microaeration has not been developed so far and remains an important research gap.
Novel operational strategies to reduce the O2 concentration in the upgraded biogas were evaluated in a 180L algal-bacterial photobioreactor interconnected to a 2.5L external absorption column during the simultaneous treatment of diluted anaerobically digested or raw vinasse and biogas upgrading. The lowest biomethane O2 levels (0.7±0.2%) were recorded when raw vinasse was fed directly into the absorption column, which resulted in CO2 and H2S removals from biogas of 72±1% and 100±0%, respectively. Process operation at a Hydraulic Retention Time (HRT) of 7d under the above configuration also supported the maximum total carbon, nitrogen and phosphorus removals of 72±4%, 74±3% and 78±5%, respectively. Biomass productivity ranged from 11.4±1.8 to 13.5±2.2gm-2d-1 during microalgae cultivation in diluted anaerobically digested vinasse, while this productivity increased to 16.9±0.7gm-2d-1 when feeding diluted raw vinasse. The good settling characteristics of the algal-bacterial flocs resulted in an average harvesting efficiency of 98.6±0.5% at a HRT in the settler of 23min, regardless of the treated vinasse. The morphological and molecular characterization of the microbial communities showed a high microalgae diversity and bacterial species richness, regardless of the operational conditions (Shannon-Wiener indices ranging from 2.8 to 3.3).
The lack of tax incentives for biomethane use requires the optimization of both biogas production and upgrading in order to allow the full exploitation of this renewable energy source. The large number of biomethane contaminants present in biogas (CO2, H2S, H2O, N2, O2, methyl siloxanes, halocarbons) has resulted in complex sequences of upgrading processes based on conventional physical/chemical technologies capable of providing CH4 purities of 88–98 % and H2S, halocarbons and methyl siloxane removals >99 %. Unfortunately, the high consumption of energy and chemicals limits nowadays the environmental and economic sustainability of conventional biogas upgrading technologies. In this context, biotechnologies can offer a low cost and environmentally friendly alternative to physical/chemical biogas upgrading. Thus, biotechnologies such as H2-based chemoautrophic CO2 bioconversion to CH4, microalgae-based CO2 fixation, enzymatic CO2 dissolution, fermentative CO2 reduction and digestion with in situ CO2 desorption have consistently shown CO2 removals of 80–100 % and CH4 purities of 88–100 %, while allowing the conversion of CO2 into valuable bio-products and even a simultaneous H2S removal. Likewise, H2S removals >99 % are typically reported in aerobic and anoxic biotrickling filters, algal-bacterial photobioreactors and digesters under microaerophilic conditions. Even, methyl siloxanes and halocarbons are potentially subject to aerobic and anaerobic biodegradation. However, despite these promising results, most biotechnologies still require further optimization and scale-up in order to compete with their physical/chemical counterparts. This review critically presents and discusses the state of the art of biogas upgrading technologies with special emphasis on biotechnologies for CO2, H2S, siloxane and halocarbon removal.
Production of biogas is based on anaerobic digestion of different renewable raw materials including human, animal, agricultural, industrial, and municipal wastes. In addition to methane content, biogas contains carbon dioxide along with water vapor, hydrogen sulfide, ammonia, and depending on the raw materials siloxane can be present. Thus, different purification and upgrading strategies are necessary in order to enhance the methane content; this review presents some of the upgrading technologies for practical removal of major contaminants in biogas. Recent development in membrane technology with high selectivity and permeability could serve as a boost in search for the most efficient biogas upgrading process capable of meeting the requirements for its use in vehicle fuel as well as incorporation in the natural gas grid. © 2015 American Institute of Chemical Engineers Environ Prog, 2015
The Power-to-Gas (PtG) process chain could play a significant role in the future energy system. Renewable electric energy can be transformed into storable methane via electrolysis and subsequent methanation.
This article compares the available electrolysis and methanation technologies with respect to the stringent requirements of the PtG chain such as low CAPEX, high efficiency, and high flexibility.
Three water electrolysis technologies are considered: alkaline electrolysis, PEM electrolysis, and solid oxide electrolysis. Alkaline electrolysis is currently the cheapest technology; however, in the future PEM electrolysis could be better suited for the PtG process chain. Solid oxide electrolysis could also be an option in future, especially if heat sources are available.
Several different reactor concepts can be used for the methanation reaction. For catalytic methanation, typically fixed-bed reactors are used; however, novel reactor concepts such as three-phase methanation and micro reactors are currently under development. Another approach is the biochemical conversion. The bioprocess takes place in aqueous solutions and close to ambient temperatures.
Finally, the whole process chain is discussed. Critical aspects of the PtG process are the availability of CO2 sources, the dynamic behaviour of the individual process steps, and especially the economics as well as the efficiency.
BACKGROUND
Biological oxidation in biotrickling filters of high H2S loads contained in biogas streams still requires further study to reduce elemental sulfur accumulation due to limited gas-liquid oxygen mass transfer inside biotrickling filters bed. Reduction of elemental sulfur accumulation may be improved by regulating the main manipulated variables related to oxygen mass transfer efficiency during biological hydrogen sulfide removal in biotrickling filters.RESULTSTrickling liquid velocity and co-current were selected as the most appropriate variable and flow pattern configuration to manipulate compared to manipulate air supply regulation and counter-current flow pattern in order to improve gas-liquid oxygen mass transfer in abiotic conditions. Then, trickling liquid velocity influence on the performance of a lab-scale biotrickling filter treating high loads of H2S on a biogas mimics and operated in co-current flow at neutral pH and packed with plastic pall rings was investigated.CONCLUSIONS
Effect of trickling liquid velocity modulation between 4.4 and 18.9 m h−1 in biotrickling filters performance was compared with operation without trickling liquid velocity regulation. Resulting in an improvement of 10% on the elimination capacity and most importantly, a 9% increase in the product selectivity to sulfate at a loding rate of 283.8 g S-H2S m−3 h−1.Concentration profiles along the biotrickling filter height evidenced that trickling liquid velocity regulation progressively lead to a better dissolved oxygen distribution and, thus, enhanced overall biotrickling filter performance.
A new type of anaerobic trickle-bed reactor was used for biocatalytic methanation of hydrogen and carbon dioxide under mesophilic temperatures and ambient pressure in a continuous process. The conversion of gaseous substrates through immobilized hydrogenotrophic methanogenic archaea in a biofilm is a unique feature of this type of reactor. Due to the formation of a three-phase system on the carrier surface and operation as a plug flow reactor without gas recirculation, a complete reaction could be observed. With a methane concentration higher than [Formula: see text] , the product gas exhibits a very high quality. A specific methane production of [Formula: see text] was achieved at a hydraulic loading rate of [Formula: see text] . The relation between trickle flow through the reactor and productivity could be shown. An application for methane enrichment in combination with biogas facilities as a source of carbon dioxide has also been positively proven.
tBiotrickling filters for biogas desulfurization still must prove their stability and robustness in the longrun under extreme conditions. Long-term desulfurization of high loads of H2S under acidic pH wasstudied in a lab-scale aerobic biotrickling filter packed with metallic Pall rings. Reference operating con-ditions at steady-state corresponded to an empty bed residence time (EBRT) of 130 s, H2S loading rate of52 g S–H2S m−3h−1and pH 2.50–2.75. The EBRT reduction showed that the critical EBRT was 75 s and themaximum EC 100 g S–H2S m−3h−1. Stepwise increases of the inlet H2S concentration up to 10,000 ppmvlead to a maximum EC of 220 g S–H2S m−3h−1. The H2S removal profile along the filter bed indicated thatthe first third of the filter bed was responsible for 70–80% of the total H2S removal. The oxidation rateof solid sulfur accumulated inside the bioreactor during periodical H2S starvation episodes was verifiedunder acidic operating conditions. The performance under acidic pH was comparable to that under neu-tral pH in terms of H2S removal capacity. However, bioleaching of the metallic packing used as supportand chemical precipitation of sulfide/sulfur salts occurred.
The use of biogas for grid injection or vehicle fuel requires purification steps to obtain biomethane, process normally called biogas upgrading. The use of microalgae cultures has been proposed as a new alternative for CO2 removal from biogas. Full-scale systems for biogas upgrading using microalgae should be able to deal with natural existing day/night photoperiods. This research evaluated the effect of a light/dark photoperiod on the operation of a photosynthetic biogas upgrading system, at lab-scale conditions. A system based on an open-photobioreactor connected to a mass transfer column was used for that purpose. Using a continuous biogas flow, an upgraded biogas with a CO2 concentration between 2 and 4.5% was obtained throughout light and dark periods. O2 concentrations below 1% in final biogas were observed. Mass balances showed that CO2 desorption was the main process behind CO2 removal. CO2 removal during the dark phase was possible, under the tested conditions, as a result of inorganic carbon desorption from the photobioreactor and accumulation in the liquid phase.
Biogas upgrading is envisioned as a key process for clean energy production. The current study evaluates the efficiency of different reactor configurations for ex-situ biogas upgrading and enhancement, in which externally provided hydrogen and carbon dioxide were biologically converted to methane by the action of hydrogenotrophic methanogens. The methane content in the output gas of the most efficient configuration was >98%, allowing its exploitation as substitute to natural gas. Additionally, use of digestate from biogas plants as a cost efficient method to provide all the necessary nutrients for microbial growth was successful. High-throughput 16S rRNA sequencing revealed that the microbial community was resided by novel phylotypes belonging to the uncultured order MBA08 and to Bacteroidales. Moreover, only hydrogenotrophic methanogens were identified belonging to Methanothermobacter and Methanoculleus genera. Methanothermobacter thermautotrophicus was the predominant methanogen in the biofilm formed on top of the diffuser surface in the bubble column reactor.
The potential of an algal-bacterial system consisting of a high rate algal pond (HRAP) interconnected to an absorption column (AC) via recirculation of the cultivation broth for the upgrading of biogas and digestate was investigated. The influence of the gas-liquid flow configuration in the AC on the photosynthetic biogas upgrading process was assessed. AC operation in a co-current configuration enabled to maintain a biomass productivity of 15 g m⁻² d⁻¹, while during counter-current operation biomass productivity decreased to 8.7 ± 0.5 g m⁻² d⁻¹ as a result of trace metal limitation. A bio-methane composition complying with most international regulatory limits for injection into natural gas grids was obtained regardless of the gas-liquid flow configuration. Furthermore, the influence of the recycling liquid to biogas flowrate (L/G) ratio on bio-methane quality was assessed under both operational configurations obtaining the best composition at an L/G ratio of 0.5 and co-current flow operation.
Power to Gas (PtG) processes have appeared in the last years as a long-term solution for renewable electricity surplus storage through methane production. These promising techniques will play a significant role in the future energy storage scenario since it addresses two crucial issues: electrical grid stability in scenarios with high share of renewable sources and decarbonisation of high energy density fuels for transportation. There is a large number of pathways for the transformation of energy from renewable sources into gaseous or liquid fuels through the combination with residual carbon dioxide. The high energy density of these synthetic fuels allows a share of the original renewable energy to be stored in the long-term. The first objective of this review is to thoroughly gather and classify all these energy storage techniques to define in a clear manner the framework which includes the Power to Gas technologies. Once the boundaries of these PtG processes have been evidenced, the second objective of the work is to detail worldwide existing projects which deal with this technology. Basic information such as main objectives, location and launching date is presented together with a qualitative description of the plant, technical data, budget and project partners. A timeline has been built for every project to be able of tracking the evolution of research lines of different companies and institutions.
The purpose of this laboratory pilot scale study at the Wastewater Technology Centre (WTC), Environment Canada, Burlington, ON was to investigate the anaerobic biological removal of H2S from biogas under real-time operating conditions. Biogas produced in a 538 litre pilot anaerobic digester was continuously fed into a 12 litre biotrickling filter containing plastic fibres as packing bed media. The process was monitored for several months. The biogas flowrate and H2S concentration ranged between 10 to 70 L/h and 1,000 to 4,000 ppmv respectively over the course of the test period. Nitrate-rich wastewater from a pilot scale sequencing batch reactor effluent was used as the nutritive solution for the biotrickling filter. The paper presents the influence of several operational parameters such as biogas flowrate, hydrogen sulphide concentration and composition of nutrient solution on process performance. To date, our results show H2S removal rates up to 100% without adverse effects on the methane concentration of the biogas. No system deterioration was observed over long term operation. This non-conventional technology is very promising and could be considered for full scale applications.
BACKGROUND
Biological technologies for biogas upgrading such as anoxic biotrickling filters (BTF) and algal-bacterial photobioreactors (PBR) constitute a cost-effective, environmentally friendly alternative to conventional physical-chemical methods. Nonetheless, there is a lack of studies comparing their performance when treating real biogas. RESULTSA H2S removal efficiency >98% was consistently recorded in both systems, with elimination capacities up to 26 and 12.4 g S-H2S m(-3) h(-1) in the BTF and PBR, respectively, at empty bed residence times (EBRTs) of approximate to 30 min. Both bioreactors demonstrated a high robustness towards fluctuations in biogas composition and flowrate, maintaining nearly complete desulfurization despite the variations in sulfur load and EBRT. The BTF also showed an immediate recovery from a 15 days operational shut-down, and the ability to utilize the nutrients from nitrate-supplemented digestate during biogas desulfurization. In addition, the PBR supported an average CO2 removal of 23.011.8%, increasing to 62% at higher pH of 8.1, with a carbon fixation rate of 285 mg CO2 L-1 d(-1). CONCLUSIONS
The BTF was confirmed as a robust technology for the desulfurization of real biogas, even when supplemented with effluent from the digester. High H2S removal was also achieved in the PBR, the CO2 fixation capacity of microalgae enhancing biogas purification. (c) 2015 Society of Chemical Industry
This study proposes an innovative setup composed by two stage reactors to achieve biogas upgrading coupling the CO2 in the biogas with external H2 and subsequent conversion into CH4 by hydrogenotrophic methanogenesis. In this configuration, the biogas produced in the first reactor was transferred to the second one, where H2 was injected. This configuration was tested at both mesophilic and thermophilic conditions. After H2 addition, the produced biogas was upgraded to average CH4 content of 89% in the mesophilic reactor and 85% in the thermophilic. At thermophilic conditions, a higher efficiency of CH4 production and CO2 conversion was recorded. The consequent increase of pH did not inhibit the process indicating adaptation of microorganisms to higher pH levels. The effects of H2 on the microbial community were studied using high-throughput Illumina random sequences and full-length 16S rRNA genes extracted from the total sequences. The relative abundance of archaeal community markedly increased upon H2 addition with Methanoculleus as dominant genus. The increase of hydrogenotrophic methanogens and syntrophic Desulfovibrio and the decrease of aceticlastic methanogens indicate a H2-mediated shift towards the hydrogenotrophic pathway enhancing biogas upgrading. Moreover, Thermoanaerobacteraceae were likely involved in syntrophic acetate oxidation with hydrogenotrophic methanogen in absence of aceticlastic methanogenesis.
Microbiological biogas upgrading could become a promising technology for production of methane (CH4). This is, storage of irregular generated electricity results in a need to store electricity generated at peak times for use at non-peak times, which could be achieved in an intermediate step by electrolysis of water to molecular hydrogen (H2). Microbiological biogas upgrading can be performed by contacting carbon dioxide (CO2), H2 and hydrogenotrophic methanogenic Archaea either in situ in an anaerobic digester, or ex situ in a separate bioreactor. In situ microbiological biogas upgrading is indicated to require thorough bioprocess development, because only low volumetric CH4 production rates and low CH4 fermentation offgas content have been achieved. Higher volumetric production rates are shown for the ex situ microbiological biogas upgrading compared to in situ microbiological biogas upgrading. However, the ex situ microbiological biogas upgrading currently suffers from H2 gas liquid mass transfer limitation, which results in low volumetric CH4 productivity compared to pure H2/CO2 conversion to CH4. If waste gas utilization from biological and industrial sources can be shown without reduction in volumetric CH4 productivity, as well as if the aim of a single stage conversion to a CH4 fermentation offgas content exceeding 95 vol% can be demonstrated, ex situ microbiological biogas upgrading with pure or enrichment cultures of methanogens could become a promising future technology for almost CO2-neutral biomethane production.
The simultaneous capture of CO2 from biogas and removal of carbon and nutrients from diluted centrates in a 180 L high-rate algal pond (HRAP) interconnected to a 2.5 L absorption column were evaluated using a C, N and P mass balance approach. The experimental set-up was operated indoors at 75 μE/m(2)·s for 24 h/d at 20 days of hydraulic retention time for 2 months of steady state, and supported a C-CO2 removal in the absorption column of 55 ± 6%. Carbon fixation into biomass only accounted for 9 ± 2% of the total C input, which explains the low biomass productivity recorded in the HRAP. In this context, the low impinging light intensity along with the high turbulence in the culture broth entailed a C stripping as CO2 of 49 ± 5% of the total carbon input. Nitrification was the main NH4(+) removal mechanism and accounted for 47 ± 2% of the inlet N-NH4(+), while N removal as biomass represented 14 ± 2% of the total nitrogen input. A luxury P uptake was recorded, which resulted in a P-PO4(-3) biomass content over structural requirements (2.5 ± 0.1%). Phosphorus assimilation corresponded to a 77 ± 2% of the inlet dissolved P-PO4(-3) removed.
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In this study, the potential of a pilot hollow-fiber membrane bioreactor for the conversion of H2 and CO2 to CH4 was evaluated. The system transformed 95% of H2 and CO2 fed at a maximum loading rate of 40.2m3 H_2/m3_R d and produced 0.22 m3 of CH4 per m3 of H2 fed at thermophilic conditions. H2 mass transfer to the liquid phase was identified as the limiting step for the conversion, and kLa values of 430 h-1 were reached in the bioreactor by sparging gas through the membrane module. A simulation showed that the bioreactor could upgrade biogas at a rate of 25m3/m3_R d, increasing the CH4 concentration from 60 to 95%v. This proof-of-concept study verified that gas sparging through a membrane module can efficiently transfer H2 from gas to liquid phase and that the conversion of H2 and CO2 to biomethane is feasible on a pilot scale at noteworthy load rates.
Biological removal of hydrogen sulfide in biogas is an increasingly adopted alternative to the conventional physicochemical processes, because of its economic and environmental benefits. In this study, a microaerobic biofiltration system packed with polypropylene carrier was used to investigate the removal of high concentrations of H2S contained in biogas from an anaerobic digester. The results show that H2S in biogas was removed completely under different inlet concentrations of H2S from 2065 ± 234 to 7818 ± 131 ppmv, and the elimination capacity of H2S in the filter achieved about 122 g H2S/m(3)/h. It was observed that the content of CH4 in biogas increased after the biogas biodesulfurization process, which was beneficial for the further utilization of biogas. The elemental sulfur and sulfate were the main sulfur species of H2S degradation, and elemental sulfur was dominant (about 80 %) under high inlet H2S concentration. The results of terminal restriction fragment length polymorphism (T-RFLP) and fluorescence in situ hybridization (FISH) show that the population of sulfide-oxidizing bacteria (SOB) species in the filter changed with different concentrations of H2S. The microaerobic biofiltration system allows the potential use of biogas and the recovery of elemental sulfur resource simultaneously.
The influence of biogas flow rate (0, 0.3, 0.6, and 1.2 m(3) m(-2) h(-1)) on the elemental and macromolecular composition of the algal-bacterial biomass produced from biogas upgrading in a 180 L photobioreactor interconnected to a 2.5 L external bubbled absorption column was investigated using diluted anaerobically digested vinasse as cultivation medium. The influence of the external liquid recirculation/biogas ratio (0.5 < L/G < 67) on the removal of CO2 and H2S, and on the concentrations of O2 and N2 in the upgraded biogas, was also evaluated. A L/G ratio of 10 was considered optimum to support CO2 and H2S removals of 80% and 100%, respectively, at all biogas flow rates tested. Biomass productivity increased at increasing biogas flow rate, with a maximum of 12 ± 1 g m(-2) d(-1) at 1.2 m(3) m(-2) h(-1), while the C, N, and P biomass content remained constant at 49 ± 2%, 9 ± 0%, and 1 ± 0%, respectively, over the 175 days of experimentation. The high carbohydrate contents (60-76%), inversely correlated to biogas flow rates, would allow the production of ≈100 L of ethanol per 1000 m(3) of biogas upgraded under a biorefinery process approach.
Several obstacles and limitations currently prevent the industrial exploitation of microalgae for feed, food and biofuel production. Photobioreactors (closed systems for algae cultivation) suffer from high-energy expenditures for mixing and cooling, while cultures in large-scale open ponds, which have a more favorable net energy ratio, are unstable ecosystems in which maintaining selected strains for long periods is difficult. Techniques for supplying nutrients and CO2, for mixing and for harvesting and processing the biomass in an energy-efficient manner are still under study and development. Despite these impediments and although microalgae are not superior to higher plants in terms of photosynthetic efficiency and productivity, microalgal cultures remain one of the most attractive sources of feed, food and next-generation biofuels since microalgae can be grown in saline or seawater on nonarable lands, can use fertilizers with an almost 100% efficiency, are able to attain much higher oil and protein yields than traditional crops and, being endowed with high growth rates, are easier to be improved via genetic and metabolic engineering.
The most harmful biogas contaminant for energy conversion equipment such as fuel cells is hydrogen sulphide (H2S); thus efficient and cost-effective treatment systems for this compound should be designed and developed. A pilot-scale biotrickling filter (BTF) working in acidic media (pH = 1.5-2) was operated for raw sewage biogas desulphurisation. Its operational performance as a function of two key important process parameters (temperature and retention time) was evaluated through short-term experimentation; showing that H2S removal efficiencies greater than 90% can be obtained at temperatures of 30 C, retention times of 80-85 s and H2S Loading Rates of 210 gH(2)S/(m(bed)(3) h). The system was afterwards operated for 924 h and showed an average elimination capacity of 169 gH(2)S/(m(bed)(3) h) at an average removal efficiency of 84%. The unit proved to be reversible to the effect of operation disruptions (lack of temperature control, limitations on oxygen supply), which were introduced to simulate possible system miss functioning or operational failures. Nevertheless, partial oxidation to elemental sulphur (S-(s)) accounted for 70% of H2S removal progressively increasing the pressure drop over the column; reducing the availability of the treatment line and eventually leading to fuel cell shutdowns. More efficient systems for oxygen supply and solids removal are the key factors to be addressed for a sustainable deployment of BTF technology in waste water treatment plants (WWTP).
A microalgal growth model has been developed based upon experiments using three species – (Cylindrotheca closterium, Nannochloropsis gaditana and Phaeodactylum tricornutum) – at three different levels of carbon dioxide (provided by aeration with ambient air, ambient air enriched with 0.5% carbon dioxide, and ambient air enriched with 1% carbon dioxide). We used a two-step growth model for phytoplankton comprising first uptake of the nutrients (C, N and P) and then growth based on the intracellular concentrations of the three nutrients. In addition, the model considered the fraction of cellular carbon that is lipid. The model did not require calibration, as the parameters were based on literature values, allometric principles and direct measurements. The validation of the model gave acceptable results. Based upon both the model itself and experimental results for P. tricornutum, it can be concluded that higher content of organic carbon and lipid would not be obtained by further addition of carbon dioxide beyond 0.5–1%. The production of organic carbon by this species with addition of 1% carbon dioxide could be about 90 mg/l in 24 h, which is consistent with other production results from the literature.
Canadian Biosystems Engineering/Le génie des biosystèmes au Canada. 53:8.1-8.18. One of the major barriers to the use of biogas as an alternative renewable energy source is the presence of siloxanes. At combustion temperatures, siloxanes are converted to silicon dioxide (SiO 2), which forms deposits on the combustion surfaces (pistons and cylinders) of gas processing equipment, thus reducing engine life and increasing overall operational and maintenance costs. This paper presents key multidisciplinary information with respect to siloxane removal, aimed at evaluating various treatment options and identifying future research needs. Current removal methods are typically based on the adsorption process, while others employ gas-liquid absorption and refrigeration/condensation. Otherwise these methods are appropriate for siloxane removal with regard to siloxane physical-chemical characteristics (for example, low solubility in water, high solubility in organic solvents, volatility, chemical resistance) their application in practice is cost-limited. Adsorption and absorption methods become inappropriate when applied to moisture-rich biogas, and are often used in combination with a pre-treatment stage. Development of more cost-effective technologies, such as membrane separation and biofiltration is in progress, and initial findings suggest that these methods could represent an attractive alternative.
The removal of hydrogen sulfide from biogas by microaeration was studied in Up-flow Anaerobic Sludge Blanket (UASB) reactors treating synthetic brewery wastewater. A fully anaerobic UASB reactor served as a control while air was dosed into a microaerobic UASB reactor (UMSB). After a year of operation, sulfur balance was described in both reactors. In UASB, sulfur was mainly presented in the effluent as sulfide (49%) and in biogas as hydrogen sulfide (34%). In UMSB, 74% of sulfur was detected in the effluent (41% being sulfide and 33% being elemental sulfur), 10% accumulated in headspace as elemental sulfur and 9% escaped in biogas as hydrogen sulfide. The efficiency of hydrogen sulfide removal in UMSB was on average 73%. Microaeration did not cause any decrease in COD removal or methanogenic activity in UMSB and the elemental sulfur produced by microaeration did not accumulate in granular sludge.
One option to utilize excess electric energy is its conversion to hydrogen and the subsequent methanation. An alternative to the classical chemical Sabatier process is the biological methanation (methanogenesis) within biogas plants. In conventional biogas plants methane and carbon dioxide is produced. The latter can be directly converted to methane by feeding hydrogen into the reactor, since hydrogenotrophic bacteria are present.
In the present contribution, a comprehensive simulation study with respect to stationary operating conditions and disturbances is presented. It reveals two qualitative different limitations, namely a biological limit (appr. at 4mH23/mCO23 corresponds to 4.2mH2,STP3/mliq3/d) as well as a transfer limit. A parameter region for a safe operation was defined. The temporary operation with stationary unfeasible conditions was analysed and thereby three qualitatively different disturbances can be distinguished. In one of these the operation for several days is possible. On the basis of these results, a controller was proposed and tested that meets the demands on the conversion of hydrogen and also prevents the washout of the microbial community due to hydrogen overload.
BACKGROUND: Biological conversion of CO2 to useful carbonic compounds such as methane is a potentially attractive technology for reducing its concentration in the atmosphere. One of the advantages of this technology over chemical conversion is that it requires much lower energy for reduction of CO2. In this article, biological conversion of CO2 to CH4 using hydrogenotrophic methanogens was examined in a fixed bed reactor inoculated with anaerobic mixed culture from the anaerobic digestor of a sewage treatment plant.
RESULTS: Methane formation commenced on the first day of operation of the fixed bed reactor. CO2 fed to the reactor was reduced with H2 by hydrogenotrophic methanogens. The feed ratio of CO2 to H2 is an important factor in determining the conversion rate of CO2. When the feed ratio is 4, methane is produced at the expected rate according to the chemical equation. The CO2 conversion rate was 100% when the gas retention time was 3.8 h in the fixed bed reactor.
CONCLUSIONS: The results show that the fixed bed reactor employing hydrogentrophic methanogens has the potential to be effective in converting CO2 to CH4 with a conversion rate of 100% at 3.8 h retention time. Copyright
The biological conversion of H2 and CO2 into CH4, using methanogenic archaea is an interesting technology for CO2 conversion, energy storage and biogas upgrading. For an industrial application of this process however, the optimization of the volumetric productivity and the product quality is an important issue. Since the reactants in this fermentation process are, unlike in most microbial fermentations, solely gasses, the gas liquid mass transfer is supposed to play an important role on the way to a higher volumetric productivity.
This work aimed at investigating the effects of the gassing rate, the reactor pressure, as well as reactor design issues on the performance of Methanothermobactermarburgensis by using continuous cultures. Our results show that biological methanogenesis with M. marburgensis is gas limited. Maximum physiological capacity is not reached yet. The gassing rate influenced mainly the volumetric methane production rate (MER), the reactor pressure influenced mainly the offgas quality. Based on this information, we demonstrated how a combination of increased gas flow rate and increased reactor pressure can be used to reach high volumetric productivity at high offgas quality. Maximum MER was 950 mmol L−1 h−1 at a CH4 concentration of 60 Vol.-%, maximum CH4 concentration reached was 85 Vol.-% at a MER of 255 mmol L−1 h−1.
The reactor design currently limits further increase in gas flow rate and reactor pressure. Therefore Interdisciplinary bridges from bioprocessing to chemical reactor design must be followed in the future to boot this promising bioprocess to gain biomethane via CO2 fixation.
Removing volatile methyl siloxanes (VMSs) from biogas remains a longstanding challenge in the field of biological process due to their low bioavailability and biodegradation. To address this issue, a lab-scale aerobic biotrickling filter, packed with porous lava and inoculated with an effective strain of Pseudomonas aeruginosa, was developed and its performance for octamethylcyclotetrasiloxane (D4, selected as a model VMS) removal from an aerobic synthetic gas was monitored. The biotrickling filter exhibited a relatively high removal efficiency over 74% at empty bed residence time of 13.2min. Rhamnolipids, biosurfactants produced by P. aeruginosa, were identified in the liquid phase of the biotrickling filter by HPLC-MS and ATR-FTIR, and they were found to be the main factor of improving D4 removal. Moreover, dimethylsilanediol, methanol, silicic acid in the liquid phase and carbon dioxide in the gas phase, as the biodegradation products of D4, were determined by GC-MS, silicic acid analysis and non-dispersive infrared analysis. To our knowledge, it is the first time to report the existence of methanol in the D4 degradation products. Finally, a metabolic pathway for D4 degradation by P. aeruginosa was proposed based on our results.
Highlights
► High removal efficiencies are found in BTFs for hydrogen sulfide removal operating at low (<2) pH. ► Elemental sulfur in the BTF can be oxidized by stopping the biogas feed. ► A jet-venturi based device gives an improvement in oxygen mass transfer in BTFs.
This paper presents the potentials of using biogas production and hydrogen sulphide concentration as the parameters to regulate the oxygen supply to microaerobic reactors in order to control the biogas sulphide content. Research was carried out in two identical bioreactors of 200 L at 35 °C and 19 d of hydraulic retention time. The feed consisted of mixed sludge from a municipal wastewater treatment plant with variable organic and sulphur load. The oxygen flow rate was automatically adjusted according to the biogas sulphide content (which ranged from 0.62 to 0.24%v/v) by a feedback Proportional-Integral-Derivative controller. The target hydrogen sulphide concentration (0.01%v/v) was achieved in 4.0–5.5 h. The micro-oxygenation level reached was considered to be the optimum in the short-medium term, since it kept the removal efficiency above 99% and minimised the oxygen concentration in the biogas during the days following the controller application. Specifically, the average biogas oxygen content was 0.09%v/v. Subsequently, biogas production was used as the parameter to regulate the oxygen supply. When the biogas sulphide content was around 0.33 and 0.50%v/v, approximately 3.5 and 5.0 NL of oxygen were supplied per N m3 of biogas (respectively). An average sulphide removal efficiency of 99%, and oxygen concentrations in the biogas of less than 0.08%v/v were achieved. Biogas production could be employed to develop precise control strategies during microaerobic digestion under variable organic load and steady sulphur load. Under unstable sulphur load, biogas sulphide content should be used instead.