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Measurement and Prediction of Mass Transfer Coefficients for Syngas Constituents in a Hollow Fiber Reactor
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This paper presents the results from biomass gasification tests in a pilot-scale (6.5-m tall × 0.1-m diameter) air-blown circulating fluidized bed gasifier, and compares them with model predictions. The operating temperature was maintained in the range 700–850°C, while the sawdust feed rate varied from 16 to 45kg/h. Temperature, air ratio, suspension density, fly ash re-injection and steam injection were found to influence the composition and heating value of the product gas. Tar yield from the biomass gasification decreased exponentially with increasing operating temperature for the range studied. A non-stoichiometric equilibrium model based on direct minimization of Gibbs free energy was developed to predict the performance of the gasifier. Experimental evidence indicated that the pilot gasifier deviated from chemical equilibrium due to kinetic limitations. A phenomenological model adapted from the pure equilibrium model, incorporating experimental results regarding unconverted carbon and methane to account for non-equilibrium factors, predicts product gas compositions, heating value and cold gas efficiency in good agreement with the experimental data.
Naturally occurring catalytic substances are employed in biomass steam-gasification processes to enhance the yield of fuel gas and reduce its tar content by cracking and reforming the high molecular weight organic components. Calcined dolomite is widely used for this purpose; it exhibits good catalytic activity under the operating conditions of the gasifier. However, due to its poor mechanical strength, it gives rise to a large production of fines in a fluidised-bed environment. This work reports an investigation into the catalytic behaviour of olivine, a common, naturally occurring mineral containing magnesium, iron oxides and silica: iron is known to play a positive role in tar decomposition reactions. The gasification runs, performed with a laboratory scale, biomass gasification unit, show that the olivine activity is close to that exhibited by dolomite under comparable operating conditions. Olivine has the additional advantage, however, that its resistance to attrition in the fluidised bed is much greater, similar to that of sand. Parametric sensitivity studies of a gasification process, utilising olivine as the fluidised-bed inventory, indicate an optimum gasification temperature of just above 800°C, and little influence of the steam/biomass ratio in the range 0.5–1.
A new hollow-fiber membrane remediation system has recently been developed to passively supply groundwater with dissolved hydrogen (H2) to stimulate the biodegradation of chlorinated solvents. Understanding the mass transfer behavior of membranes under conditions of creeping flow is critical for the design of such systems. Therefore, the objectives of this research were to evaluate the gas transfer behavior of hollow-fiber membranes under conditions typical of groundwater flow and to assess the effect of membrane configuration on gas transfer performance. Membrane gas transfer was evaluated using laboratory-scale glass columns operated at low flow velocities (8.6-12,973 cm/d). H2 was supplied to the inside of the membrane fibers while water flowed on the outside and normal to the fibers (i.e. cross-flow). Membrane configuration (single fiber and fabric) and membrane spacing for the fabric modules did not affect gas transfer performance. Therefore, the results from all of the experiments were combined to obtain the following dimensionless Sherwood number (Sh) correlation expressed as a function of Reynolds number (Re) and Schmidt number (Sc): Sh = 0.824Re(0.39)Sc(0.33) (0.0004<Re<0.6). This correlation is useful for predicting the rate of transfer of any gas from clean membranes to flowing water at low Re. This correlation provides a basis for estimating the membrane surface area requirements for groundwater remediation as illustrated by a simple example.
The development of freshwater multispecies biofilms at solid-liquid interfaces occurs both in quiescent waters and under conditions of high shear rates. However, the influence of hydrodynamic shear rates on bacterial biofilm diversity is poorly understood. We hypothesized that different shear rates would significantly influence biofilm diversity and alter the relative proportions of coaggregating and autoaggregating community isolates. In order to study this hypothesis, freshwater biofilms were developed at five shear rates (<0.1 to 305 S(-1)) in a rotating concentric cylinder reactor fed with untreated potable water. Eubacterial diversity was assessed by denaturing gradient gel electrophoresis (DGGE) and culturing on R2A agar. Fifty morphologically distinct biofilm strains and 16 planktonic strains were isolated by culturing and identified by partial 16S rRNA gene sequencing, and their relatedness was determined by the construction of a neighbor-joining phylogenetic tree. Phylogenetic and DGGE analyses showed an inverse relationship between shear rate and bacterial diversity. An in vitro aggregation assay was used to assess the relative proportions of coaggregating and autoaggregating species from each biofilm. The highest proportion of autoaggregating bacteria was present at high shear rates (198 to 305 S(-1)). The intermediate shear rate (122 S(-1)) selected for the highest proportion of coaggregating bacteria (47%, or 17 of a possible 36 coaggregation interactions). Under static conditions (<0.1 S(-1)), 41 (33%) of a possible 125 coaggregation interactions were positive. Few coaggregation (3.3%) or autoaggregation (25%) interactions occurred between the 16 planktonic strains. In conclusion, these data show that shear rates affect biofilm diversity as well as the relative proportions of aggregating bacteria.
We introduce a procedure for determining shear forces at the balance between attachment and detachment of bacteria under flow.
This procedure can be applied to derive adhesion forces in weak-adherence systems, such as polymer brush coatings, which are
currently at the center of attention for their control of bacterial adhesion and biofilm formation.
This study investigated the effects of electrolytes (CaCl2, K2HPO4, MgSO4, NaCl, and NH4Cl) on CO mass transfer and ethanol production in a HFMBR. The hollow fiber membranes (HFM) were found to generate tiny gas bubbles; the bubble coalescence was significantly suppressed in electrolyte solution. The volumetric gas-liquid mass transfer coefficients (kLa) increased up to 414% compared to the control. Saturated CO (C∗) decreased as electrolyte concentrations increased. Overall, the maximum mass transfer rate (Rmax) in electrolyte solution ranged from 106% to 339% of the value obtained in water. The electrolyte toxicity on cell growth was tested using Clostridium autoethanogenum. Most electrolytes, except for MgSO4, inhibited cell growth. The HFMBR operation using a medium containing 1% MgSO4 achieved 119% ethanol production compared to that without electrolytes. Finally, a kinetic simulation using the parameters got from the 1% MgSO4 medium predicted a higher ethanol production compared to the control.
A practical guidance on the use of Henry's law in engineering calculations and how to account for the temperature dependence of the Henry's constant, which is crucial for accurate process design, has been presented. As Henry's law is used in many different disciplines, many different usage and conventions have been development. Proper care should be taken to avoid the use of a Henry's constant in a manner that does not match the way in which the original value was defined, which leads to serious errors. Those who report Henry's constant in the literature, both from original measurements or in compiling data from others, should be very clear about definitions. Those who use Henry's constant should clearly understand how the numbers they are using were defined. In estimation of temperature dependence of Henry's constant, if units of volumetric concentration are used, an additional temperature dependence is introduced by the variation of fluid-phase density with temperature.
Syngas fermentation to biofuel is often plagued by mass transfer limitations of sparingly soluble gases, namely carbon monoxide (CO) and hydrogen (H2), in the aqueous phase. In this study, the volumetric mass transfer coefficients (kLa) for H2 and CO were examined in a gas-lift reactor coupled with a 20 μm bulb diffuser. Furthermore, a correlation between the myoglobin-protein bioassay (liquid samples) and the head space gas analysis via gas chromatography (GC) for CO was developed for the same reactor configuration. The highest kLa values of 97.2 and 129.6 h−1 were observed for H2 and CO, respectively at gas flow rates of 5.0 L min−1. The kLa values determined using GC equipped with a thermal conductivity detector (GC-TCD) and myoglobin-protein bioassay methods for different CO gas flow rates were highly correlated with correlation (R2) factors of 0.99 (without microorganisms) and 0.987 (with Clostridium carboxidivorans culture media). This study confirms that the myoglobin-protein bioassay, which is a much simpler, faster and cheaper method compared with GC analysis, can be used as a reliable method for determining the volumetric mass transfer coefficient of CO in syngas fermentation studies.
This study proposed a submerged hollow fibre membrane bioreactor (HFMBR) system capable of achieving high carbon monoxide (CO) mass transfer for applications in microbial synthesis gas conversion systems. Hydrophobic polyvinylidene fluoride (PVDF) membrane fibres were used to fabricate a membrane module, which was used for pressurising CO in water phase. Pressure through the hollow fibre lumen (P) and membrane surface area per unit working volume of the liquid (AS/VL) were used as controllable parameters to determine gas-liquid volumetric mass transfer coefficient (kLa) values. We found a kLa of 135.72h(-1) when P was 93.76kPa and AS/VL was fixed at 27.5m(-1). A higher kLa of 155.16h(-1) was achieved by increasing AS/VL to 62.5m(-1) at a lower P of 37.23kPa. Practicality of HFMBR to support microbial growth and organic product formation was assessed by CO/CO2 fermentation using Eubacterium limosum KIST612.
Gasification followed by syngas fermentation is a unique hybrid process for converting lignocellulosic biomass into fuels and chemicals. Current syngas fermentation faces several challenges with low gas-liquid mass transfer being one of the major bottlenecks. The aim of this work is to evaluate the performance of hollow fiber membrane biofilm reactor (HFM-BR) as a reactor configuration for syngas fermentation. The volumetric mass transfer coefficient (KLa) of the HFM-BR was determined at abiotic conditions within a wide range of gas velocity/flowrate passing through the hollow fiber lumen and liquid velocity/flowrate passing through the membrane module shell. The KLa values of the HFM-BR were higher than most reactor configurations such as stir tank reactors and bubble columns. A continuous syngas fermentation of Clostridium carboxidivorans P7 was implemented in the HFM-BR system at different operational conditions, including the syngas flow rate, liquid recirculation between the module and reservoir, and the dilution rate. It was found that the syngas fermentation performance such as syngas utilization efficiency, ethanol concentration and productivity, and ratio of ethanol to acetic acid depended not only on the mass transfer efficiency but also the characteristics of biofilm attached on the membrane module (biofouling or abrading of the biofilm). The HFM-BR results in a highest ethanol concentration of 23.93 g/L with an ethanol to acetic acid ratio of 4.79. Collectively, the research shows the HFM-BR is an efficient reactor system for syngas fermentation with high mass transfer.
Syngas fermentation to fuels is a technology on the verge of commercialization. Low cost of fermentation medium is important for process feasibility. The use of corn steep liquor (CSL) instead of yeast extract (YE) in Alkalibaculum bacchi strain CP15 bottle fermentations reduced the medium cost by 27% and produced 78% more ethanol. When continuous fermentation was performed in a 7-L fermentor, 6g/L ethanol was obtained in the YE and YE-free media. When CSL medium was used in continuous fermentation, the maximum produced concentrations of ethanol, n-propanol and n-butanol were 8g/L, 6g/L and 1g/L, respectively. n-Propanol and n-butanol were not typical products of strain CP15. A 16S rRNA gene-based survey revealed a mixed culture in the fermentor dominated by A. bacchi strain CP15 (56%) and Clostridium propionicum (34%). The mixed culture presents an opportunity for higher alcohols production from syngas.
Syngas fermentation is a promising technology for sustainable production of fuels and chemicals. Gas–liquid mass transfer of syngas, however, is regarded as a limiting step of the fermentation process. The authors designed an innovative external hollow fiber membrane (HFM) diffuser to remove this hurdle. In this study, the gas–liquid mass transfer of carbon monoxide, the major component of syngas, was optimized by implementing three operational factors, membrane surface area per working volume (A/v), water velocity (VL), and specific gas flow rate (Vg). The maximum observed CO mass transfer coefficient (KLa) of 385.0 1/h in water, which is higher than that yielded by previous CO transfer methods, was achieved at an A/v of 0.56 1/cm, a VL of 2.20 cm/s, and a Vg of 1.02 1/min. At these conditions, the gas void fraction rate, the syngas supply rate per working volume, was lower than all reported values as well. The high volumetric mass transfer coefficient at low gas supply rate of the HFM diffuser would make syngas fermentation a feasible alternative industrial process. A three-factor quadratic model and a dimensionless model with high correlation coefficients were developed from the experimental data for a process scale-up. These two models verified that the membrane surface area is the most significant design factor with respect to the KLa. Three screen analyses also indicated that the membrane surface area had the highest positive impact on the KLa. As a result, the external HFM diffuser appears to be a feasible technology that can considerably increase the yield of syngas fermentation to fuels and chemicals.
At temperatures between 10 and 60°C the diffusion coefficients of helium, hydrogen, oxygen and nitrogen in water have been determined from the permeability of a stagnant liquid layer in the quasi-steady state (SLL method). With this method we actually measure the difference in diffusive flow between two gases through a horizontal stagnant liquid layer between gas-permeable membranes. If oxygen is one of the gases, we are able to determine the diffusion coefficient of oxygen with a maximum experimental error of 3.5% and that of the other gases within 5%. Results are given and compared with experimental values stated in the literature.
In this study, a dual distributor type fluidized bed gasifier was used for the air gasification of rice husk in view of producing fuel gas. The effects of varying fluidization velocity (0.22, 0.28 and 0.33 m s−1) and equivalence ratio (0.25, 0.30 and 0.35) on the gasifier performance were discussed. The steady state temperature varied between 665 and 830°C. The fluidization velocity of 0.22 m s−1 and equivalence ratio of 0.25 appeared to be the optimum conditions with respect to the quality of gas. The mole fractions of the combustible components reached their maximum values at these conditions with a typical gas composition of 4% H2, 5% hydrocarbons (CH4, C2H2, C2H4 and C2H6), 15% CO2, 20% CO and 57% N2. The higher heating value of the gas obtained at these fluidization velocity and equivalence ratio (3.09–5.03 MJ Nm−3) compared very well with published data from air-blown biomass gasifiers of similar scale of operation. The gas yield and carbon conversion were in the range of 1.30 to 1.98 Nm3 kg−1 and 55.0 to 81.0%, respectively.
Trickle-bed reactor (TBR), hollow fiber membrane reactor (HFR) and stirred tank reactor (STR) can be used in fermentation of sparingly soluble gasses such as CO and H(2) to produce biofuels and bio-based chemicals. Gas fermenting reactors must provide high mass transfer capabilities that match the kinetic requirements of the microorganisms used. The present study compared the volumetric mass transfer coefficient (K(tot)A/V(L)) of three reactor types; the TBR with 3mm and 6mm beads, five different modules of HFRs, and the STR. The analysis was performed using O(2) as the gaseous mass transfer agent. The non-porous polydimethylsiloxane (PDMS) HFR provided the highest K(tot)A/V(L) (1062h(-1)), followed by the TBR with 6mm beads (421h(-1)), and then the STR (114h(-1)). The mass transfer characteristics in each reactor were affected by agitation speed, and gas and liquid flow rates. Furthermore, issues regarding the comparison of mass transfer coefficients are discussed.
Acid gases such as H[sub 2]S and CO[sub 2] are generally removed from natural gas, biogas, synthetic natural gas, and other process gas streams by means of absorption into aqueous alkanolamine solutions. A key parameter needed to model this diffusion with chemical reaction process in the liquid phase is the diffusion coefficient. A wetted-sphere absorption apparatus was used to measure the liquid-phase diffusion coefficients for hydrogen sulfide, carbon dioxide, and nitrous oxide over the temperature range 293--368 K. The experimental results obtained in this work are compared with values in the literature and with predictions from the Wilke-Chang equation. The data presented here extend the temperature range of reported diffusivities for these gases in water.
Diffusion coefficients for dissolved neon, krypton, xenon, carbon monoxide and nitric oxide have been measured in water at 10°, 20°, 30°, 40°, 50° and 60°C by following the rate of collapse of small bubbles in gas-free water. The temperature dependence of the binary diffusion coefficients is expressed in terms of an Arrhenius-type exponential relationship. The sensitivity of aqueous diffusivities to solute molecular weight, solute molecule size and solute—solvent interactions is discussed.RésuméLes coefficients de diffusion pour le néon, le krypton, le xénon, l'oxyde de carbone et l'oxyde nitrique dissous, ont été mesurés dans de l'eau à 10°, 20°, 30°, 40°, 50° et 60°C, en suivant le taux d'écrasement de petites bulles dans de l'eau non gazeuse. La dépendance de température des coefficients de diffusion est exprimée selon les termes d'une relation exponentielle du type Arrhenius. On discute de la sensibilité des diffusivités aqueuses au poids moléculaire d'un solute, à la taille des molécules d'un soluté et aux inter-réactions soluté-colvent.ZusammenfassungDie Diffusionskoeffizienten für gelöstes Neon, Krypton, Xenon, Kohlenmonoxyd und Stickstoffoxyd wurden in Wasser bei 10°, 20°, 30°, 40°, 50° und 60°C aufgrund der Zusammenbruchsgeschwindigkeit von kleinen Bläschen in gasfreiem Wasser gemessen. Die Temperaturabhängigkeit der binären Diffusionskoeffizienten wird im Sinne einer Exponentialbeziehung nach Arrhenius zum Ausdruck gebracht. Die Empfindlichkeit des Diffusionsvermögen in wässriger Lösung gegenüber dem Molekulargewicht und der Molekulargrösse des gelösten Stoffes, sowie gegenüber der gegenseitigen Wirkung von gelöstem Stoff und Lösungsmittel, wird erörtert.
Separation and Purification Technology j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s e p p u r a b s t r a c t The objective of this work was to characterize the main mass transfer resistance for CO 2 capture in the gas–liquid membrane contacting process by both physical and chemical absorption conditions. The char-acterization was performed based on the resistance-in-series model as well as the Wilson-plot method. In addition, a multistage cascade model, which is able to predict the time for the system to reach a steady-state condition, was developed to describe CO 2 absorption in the membrane contacting process. The cascade model was numerically solved by using the MATLAB program. It was found that the main mass transfer resistance of the physical absorption (using pure water as an absorbent) and the chemical absorption (using 2 M NaOH as an absorbent) was in the liquid phase and in the membrane, respectively. The membrane mass transfer resistance in the case of physical absorption presented approximately 36% of the total resistances at a liquid velocity of 2.13 m/s. For the chemical absorption condition applied, the membrane mass transfer resistance occupied around 99% of the total resistance. The results of simulation by the cascade model agreed well with the experimental results when the overall mass transfer coefficient obtained form the experiment was employed. The model can potentially be used with various operating conditions including the liquid velocity, gas concentration, and reactive absorbent used.
Abstract Ethanol is a high performance fuel in internal combustion engines. It is a liquid, which is advantageous in terms of storage, delivery, and infrastructural compatability. Ethanol burns relatively cleanly, especially as the amount of gasoline with which it is blended decreases. Evaporative and toxicity-weighted air toxics emissions are consistently lower for ethanol than for gasoline. It is likely that vehicles can be configured so that exhaust emissions of priority pollutants are very low for ethanol-burning engines, although the same can probably be said for most other fuels under consideration. Recent work suggests that ethanol may be more compatible with fuel cell-powered vehicles than has generally been assumed. Research and development-driven advances have clear potential to lower the price of cellulosic ethanol to a level competitive with bulk fuels. Process areas with particular potential for large cost reductions include biological processing (with consolidated bioprocessing particularly notable in this context), pretreatment, and incorporation of an advanced power cycle for cogeneration of electricity from process residues. The cellulosic ethanol fuel cycle has a high thermodynamic efficiency (useful energy/high heating value = from 50% to over 65% on a first law basis, depending on the configuration), and a decidedly positive net energy balance (ratio of useful energy output to energy input). Cellulosic ethanol is one of the most promising technogical options available to reduce transportation sector greenhouse gas emissions. It may well be possible to develop biomass-based energy on a very large scale in the United States with acceptable and in some cases positive environmental impacts. To do so will however require responsible management and increased understanding of relevant technological and natural systems. The potential biomass resource is large, but so is demand for transportation fuels as well as other uses. The following hypotheses are offered as tentative hypotheses pertaining to biomass supply and demand in the United States: There will probably not be enough suitable land available to meet transportation demand if total vehicle miles traveled increase relative to current levels, and vehicle efficiency and animal protein utilization are unchanged. There probably is enough suitable land to meet transportation demand, even given some increase in vehicle miles traveled, given large but probably possible increases in vehicle efficiency, or large but probably possible decreases in reliance on animal protein, or a combination of less aggressive changes in both of these factors. The policy debate concerning fuel ethanol has tended to ignore cellulosic ethanol. It is suggested that an appropriate policy objective is to foster a transition to cellulosic feedstocks at a pace such that opportunities for ethanol producers and the farmers that supply them are expanded rather than contracted.
Clostridium carboxidivorans P7 is one of three microbial catalysts capable of fermenting synthesis gas (mainly CO, CO(2) , and H(2) ) to produce the liquid biofuels ethanol and butanol. Gasification of feedstocks to produce synthesis gas (syngas), followed by microbial conversion to solvents, greatly expands the diversity of suitable feedstocks that can be used for biofuel production beyond commonly used food and energy crops to include agricultural, industrial, and municipal waste streams. C. carboxidivorans P7 uses a variation of the classic Wood-Ljungdahl pathway, identified through genome sequence-enabled approaches but only limited direct metabolic analyses. As a result, little is known about gene expression and enzyme activities during solvent production. In this study, we measured cell growth, gene expression, enzyme activity, and product formation in autotrophic batch cultures continuously fed a synthetic syngas mixture. These cultures exhibited an initial phase of growth, followed by acidogenesis that resulted in a reduction in pH. After cessation of growth, solventogenesis occurred, pH increased and maximum concentrations of acetate (41 mM), butyrate (1.4 mM), ethanol (61 mM), and butanol (7.1 mM) were achieved. Enzyme activities were highest during the growth phase, but expression of carbon monoxide dehydrogenase (CODH), Fe-only hydrogenases and two tandem bi-functional acetaldehyde/alcohol dehydrogenases were highest during specific stages of solventogenesis. Several amino acid substitutions between the tandem acetaldehyde/alcohol dehydrogenases and the differential expression of their genes suggest that they may have different roles during solvent formation. The data presented here provide a link between the expression of key enzymes, their measured activities and solvent production by C. carboxidivorans P7. This research also identifies potential targets for metabolic engineering efforts designed to produce higher amounts of ethanol or butanol from syngas. Biotechnol. Bioeng. 2012; 109: 2720-2728. © 2012 Wiley Periodicals, Inc.
In this study, the volumetric mass transfer coefficients (Ka) for CO were examined in a composite hollow fiber (CHF) membrane bioreactor. The mass transfer experiments were conducted at various inlet gas pressures (from 5 to 30psig (34.5-206.8kPa(g))) and recirculation flow rates (300, 600, 900, 1200 and 1500mL/min) through CHF module. The highest Ka value of 946.61/h was observed at a recirculation rate of 1500mL/min and at an inlet gas pressure of 30psig(206.8kPa(g)). The findings of this study confirm that the use of CHF membranes is effective and improves the efficiency CO mass transfer into the aqueous phase.
The paper deals with fluidized bed gasification of a biomass for producing a syngas with optimized hydrogen yield thanks to in-bed catalysis. Four different bed materials have been adopted: inert quartzite as reference case, olivine and dolomite as natural catalysts, and Ni-alumina as artificial catalyst. The gasification tests have been carried out at steady state in a pilot-scale bubbling fluidized bed, under operating conditions typical for gasification as reported in the paper. The gas analyses have been performed with dedicated instrumentation, like continuous analyzers and gas chromatograph, and adopting a standard protocol for tar sampling and characterization. The influence of the catalytic materials on the concentration of stable gases (e.g. H2, CO2, CO, CH4 and light hydrocarbons) as well as on the efficiency of tar conversion has been studied. In particular the artificial catalyst has the largest effectiveness in enhancing the H2 yield as well as in tar reduction. The catalyst gives rise to an elutriation rate significantly lower than that observed for dolomite at comparable U/Umf ratio, denoting a better mechanical resistance. A stable activity of the nickel–alumina catalyst has been observed for the whole duration of reaction tests suggesting that no deactivation phenomena occurred, due to coke deposition or morphological modifications of the particles.
A literature review on gasification of lignocellulosic biomass in various types of fluidized bed gasifiers is presented. The effect of several process parameters such as catalytic bed material, bed temperature and gasifying agent on the performance of the gasifier and quality of the producer gas is discussed. Based on the priorities of researchers, the optimum values of various desired outputs in the gasification process including improved producer gas composition, enhanced LHV, less tar and char content, high gas yield and enhanced carbon conversion and cold gas efficiency have been reported. The characteristics and performance of different fluidized bed gasifiers were assessed and the obtained results from the literature have been extensively reviewed. Survey of literature revealed that several industrial biomass gasification plants using fluidized beds are currently conducting in various countries. However, more research and development of technology should be devoted to this field to enhance the economical feasibility of this process for future exploitations.
Experiments involving the co-gasification of residual biomass/poor coal blends and gasification of individual feedstocks used in the blends were performed in a bench scale, continuous fluidized-bed working at atmospheric pressure. Two types of blends were prepared, mixing pine chips (from Valcabadillo, Spain) with black coal, a low-grade coal from Escatrón, Spain, and Sabero coal, a refuse coal from Sabero, Spain, in the ratio range of 0/100–100/0. Experimental tests were carried out using as a gasification agent mixtures of air and steam with dew points of 74–85°C at gasification temperatures of 840–910°C and superficial fluidized gas velocities of 0.7–1.4 m/s. Feasibility studies were very positive, showing that blending effectively improved the performance of fluidized-bed co-gasification of the low-grade coal, and the possibility of converting the refuse coal to a low-Btu fuel gas. This study indicates that a blend ratio with no less than 20% pine chips for the low-grade coal and 40% pine chips for the refuse coal are the most appropriate. The dry product gas low heating value augments with increasing blend ratio from 3700 to 4560 kJ/N m3 for pine chips/low-grade coal, and from 4000 to 4750 kJ/N m3 for pine chips/refuse coal. Dry product gas yield rises with the increase of the blend ratio from 1.80 to 3.20 N m3/kg (pine chips/low-grade coal), and from 0.75 to 1.75 N m3/kg (pine chips/refuse coal), respectively. About 50% co-gasification process overall thermal efficiency can be achieved for the two types of blend.
In gas–liquid membrane contacting, it is important to know the gas permeance of microporous hydrophobic membranes used in such a system. Gas permeance of carbon dioxide from a CO2–N2 mixture having a low CO2 concentration into an aqueous KOH solution through flat microporous (Celgard 2400, Saint-Gobain R128-10)/nonporous poly(1-trimethysilyl-1-propyne) (PTMSP) membranes is therefore studied at zero net total pressure difference (ΔP = 0). Pure gas permeance data of CO2 through the same membranes for positive ΔP and gas–gas system are extrapolated to zero mean pressure () to find also gas permeance. Conventional theoretical estimates of the liquid film resistance for such systems are compared with the experimental results for the liquid film resistance; they were found to be considerably higher than that estimated from the theory based on a liquid film having a fast chemical reaction. The membrane resistance obtained by subtraction of the experimental liquid film resistance from the total resistance of the system appears to predict the CO2 permeance for the thicker PTMSP film measured under positive ΔP quite well. However, this method leads to higher estimates of membrane resistance for thinner PTMSP films, Celgard 2400 and other supposedly highly permeable porous substrates compared to those based on the data obtained by extrapolation to . There appears to be an upper limit of permeance which may be determined correctly in such experimental measurements based on ΔP = 0. This upper limit is considerably higher than what has been achieved by earlier investigators. Several factors potentially contributing to this discrepancy have been pointed out.
During the past decades considerably large efforts have been made to optimize the production of lignocellulose derived fuel ethanol production in order to develop a process configuration which is economically feasible and competitive with gasoline. One of the process alternatives uses cellulase enzymes for the conversion of cellulose content of lignocellulosic biomass to fermentable glucose. Due to the relatively similar process conditions in the enzymatic hydrolysis and ethanol fermentation, the option of carrying out these two-steps together in one vessel exists. The application of simultaneous saccharification and fermentation (SSF) for the conversion of lignocellulosics to alcohol would result in a more cost-effective process. In the present study various lignocellulosic substrates, i.e. Solka Floc, OCC waste cardboard, and paper sludge, were examined in SSF experiments for the production of ethanol. Two yeast strains were compared, a commercially available baker’s yeast and a thermotolerant Kluyveromyces marxianus, in two types of SSF experiments, i.e. isothermal SSF and SSF with temperature profiling. The results showed that OCC waste and paper sludge could be used as substrates for ethanol production in SSF. There was no significant difference observed between Saccharomyces cerevisiae and K. marxianus when the results of SSF were compared. The ethanol yields were in the range of 0.31–0.34 g/g for both strains used. SSF resulted in higher ethanol yields compared to non-isothermal SSF (NSSF; SSF with temperature profiling).
Potential application of monolith reactors in a biological process was investigated experimentally. A possible problem when using monolith reactors in biological applications is clogging due to biofilm formation. An interesting phenomenon is the pattern in which biofilms develop inside the monolith channels. Rather unexpectedly at a first glance, it was repeatedly observed that biofilm formation started in the middle of a side of the square-section monolith channels, instead of colonizing first the low-shear areas in the corners. To explain this biofilm formation pattern, a two-dimensional mechanistic model based on substrate diffusion and consumption accompanied by microbial growth and detachment was developed in this study. Simulation results suggest that the unexpected biofilm patterns are generated by the balance between biofilm growth and biofilm detachment due to shear stress induced erosion. In the early stages, the biofilm growth in the corners is strongly limited by the external resistance to substrate transfer. As time passes and the biofilm grows in thickness, mechanical forces due to passing gas bubbles will lead to a more regular biofilm shape, including the channel corners.
Bagasse and four banagrass fuels with different inorganic fractions were gasified in a bench-scale fluidized bed at a nominal equivalence ratio of 0.3, reactor temperature of 800°C and atmospheric pressure. The gasifier output stream was characterized for permanent gas species, ammonia, condensable hydrocarbon species, char content and composition, and gas-phase inorganic species concentrations. Gas-phase concentrations of K, Na and Ca exceeded combustion turbine fuel specifications. Si, Fe, P, and Cl were also present in the gas phase. Significant amounts of inorganic fuel constituents were retained in the fluidized bed, dispersed over the surface of the bed particles.
Bioconversion of syngas/waste gas components to produce ethanol appears to be a promising alternative compared to the existing chemical techniques. Recently, several laboratory-scale studies have demonstrated the use of acetogens that have the ability to convert various syngas components (CO, CO2, and H-2) to multicarbon compounds, such as acetate, butyrate, butanol, lactate, and ethanol, in which ethanol is often produced as a minor end-product. This bioconversion process has several advantages, such as its high specificity, the fact that it does not require a highly specific H-2/CO ratio, and that biocatalysts are less susceptible to metal poisoning. Furthermore, this process occurs under mild temperature and pressure and does not require any costly pre-treatment of the feed gas or costly metal catalysts, making the process superior over the conventional chemical catalytic conversion process. The main challenge faced for commercializing this technology is the poor aqueous solubility of the gaseous substrates (mainly CO and H-2). In this paper, a critical review of CO-rich gas fermentation to produce ethanol has been analyzed systematically and published results have been compared. Special emphasis has been given to understand the microbial aspects of the conversion process, by highlighting the role of different micro-organisms used, pathways, and parameters affecting the bioconversion. An analysis of the process fundamentals of various bioreactors used for the biological conversion of CO-rich gases, mainly syngas to ethanol, has been made and reported in this paper. Various challenges faced by the syngas fermentation process for commercialization and future research requirements are also discussed. (C) 2011 Society of Chemical Industry and John Wiley & Sons, Ltd
Biomass is a promising sustainable energy source. A tar-free fuel gas can be obtained in a properly designed biomass gasification process. In the current study, a tar-free biomass gasification process by air was proposed. This concept was demonstrated on a lab-scale fluidized bed using sawdust under autothermic conditions. This lab-scale model gasifier combined two individual regions of pyrolysis, gasification, and combustion of biomass in one reactor, in which the primary air stream and the biomass feedstock were introduced into the gasifier from the bottom and the top of the gasifier respectively to prevent the biomass pyrolysis product from burning out. The biomass was initially pyrolyzed and the produced char was partially gasified in the upper reduction region of the reactor, and further, char residue was combusted at the bottom region of the reactor in an oxidization atmosphere. An assisting fuel gas and second air were injected into the upper region of the reactor to maintain elevated temperature. The tar in the flue gas entered the upper region of the reactor and was decomposed under the elevated temperature and certain residence time. This study indicated that under the optimum operating conditions, a fuel gas could be produced with a production rate of about 3.0 Nm3/kg biomass and heating value of about 5000 kJ/Nm3. The concentration of hydrogen, carbon monoxide and methane in the fuel gas produced were 9.27
Ethanol production from syngas using three moderately alkaliphilic strains of a novel genus and species Alkalibaculum bacchi CP11(T), CP13 and CP15 was investigated in 250 ml bottle fermentations containing 100ml of yeast extract medium at 37 °C and pH 8.0. Two commercial syngas mixtures (Syngas I: 20% CO, 15% CO(2), 5% H(2), 60% N(2)) and (Syngas II: 40% CO, 30% CO(2), 30% H(2)) were used. Syngas I and Syngas II represent gasified biomass and coal, respectively. The maximum ethanol concentration (1.7 g l(-1)) and yield from CO (76%) were obtained with strain CP15 and Syngas II after 360 h. CP15 produced over twofold more ethanol with Syngas I compared to strains CP11(T) and CP13. In addition, CP15 produced 18% and 71% more ethanol using Syngas II compared to strains CP11(T) and CP13, respectively. These results show that CP15 is the most promising for ethanol production because of its higher growth and ethanol production rates and yield compared to CP11(T) and CP13.
Production of ethanol from fermentation of CO has received much attention in the last few years with several companies proposing to use CO fermentation in their ethanol production processes. The genomes of two CO fermenters, Clostridium ljungdahlii and Clostridium carboxidivorans, have recently been sequenced. The genetic information obtained from this sequencing is aiding molecular biologists who are enhancing ethanol and butanol production by genetic manipulation. Several studies have optimized media for CO fermentation, which has resulted in enhanced ethanol production. Also, new reactor designs involving the use of hollow fiber membranes have reduced mass transfer barriers that have hampered previous CO fermentation efforts.
Lignocellulosic biomass such as agri-residues, agri-processing by-products, and energy crops do not compete with food and feed, and is considered to be the ideal renewable feedstocks for biofuel production. Gasification of biomass produces synthesis gas (syngas), a mixture primarily consisting of CO and H(2). The produced syngas can be converted to ethanol by anaerobic microbial catalysts especially acetogenic bacteria such as various clostridia species.One of the major drawbacks associated with syngas fermentation is the mass transfer limitation of these sparingly soluble gases in the aqueous phase. One way of addressing this issue is the improvement in reactor design to achieve a higher volumetric mass transfer coefficient (k(L)a). In this study, different reactor configurations such as a column diffuser, a 20-μm bulb diffuser, gas sparger, gas sparger with mechanical mixing, air-lift reactor combined with a 20-μm bulb diffuser, air-lift reactor combined with a single gas entry point, and a submerged composite hollow fiber membrane (CHFM) module were employed to examine the k(L) a values. The k(L) a values reported in this study ranged from 0.4 to 91.08 h(-1). The highest k(L) a of 91.08 h(-1) was obtained in the air-lift reactor combined with a 20-μm bulb diffuser, whereas the reactor with the CHFM showed the lowest k(L) a of 0.4 h(-1). By considering both the k(L) a value and the statistical significance of each configuration, the air-lift reactor combined with a 20-μm bulb diffuser was found to be the ideal reactor configuration for carbon monoxide mass transfer in an aqueous phase.
The effect of steam gasification conditions on products properties was investigated in a bubbling fluidized bed reactor, using larch wood as the starting material. For bed material effect, calcined limestone and calcined waste concrete gave high content of H(2) and CO(2), while silica sand provided the high content of CO. At 650 degrees C, calcined limestone proved to be most effective for tar adsorption and showed high ability to adsorb CO(2) in bed. At 750 degrees C it could not capture CO(2) but still gave the highest cold gas efficiency (% LHV) of 79.61%. Steam gasification gave higher amount of gas product and higher H(2)/CO ratio than those obtained with N(2) pyrolysis. The combined use of calcined limestone and calcined waste concrete with equal proportion contributed relatively the same gas composition, gas yield and cold gas efficiency as those of calcined limestone, but showed less attrition, sintering, and agglomeration propensities similar to the use of calcined waste concrete alone.
Integrated energy systems in China -The cold Northeastern region experience
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Membrane Gas Exchange, Using Hollow Fiber Membranes to Separate Gases from Liquid and Gaseous. Streams
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