Article

Thermal Depolymerization of Lignin

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  • Middle East University
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Abstract

Lignin can be considered as a clean source of renewable energy, which may be a substitute to fossil fuel and thus reduce some of the environmental pollution. This paper presents a review of the thermal process of depolymerization of lignin; which results in production of oil and gaseous fuels from such important renewable energy source. Also, thermal hydro-cracking of lignin enhances the liquefaction of other solid fossil fuels such as coal, by producing intermediates which then react further with coal producing lower molecular weight material, which is more desirable.

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... Lignin depolymerization faces challenges across various methods. Thermal depolymerization, while simple, suffers from high energy needs and low selectivity (TRL 4-5) that leads to the formation of unwanted by-products like char and tar [72,124]. These issues complicate product separation and purification, limiting its industrial applications to pilotscale demonstrations. ...
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Lignin, the earth’s second-most abundant biopolymer after cellulose, has long been relegated to low-value byproducts in the pulp and paper industry. However, recent advancements in valorization are transforming lignin into a sustainable and versatile feedstock for producing high-value biofuels, bioplastics, and specialty chemicals. This review explores the conversion of lignin’s complex structure, composed of syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units, into value-added products. We critically assess various biochemical and analytical techniques employed for comprehensive lignin characterization. Additionally, we explore strategies for lignin upgrading and functionalization to enhance its suitability for advanced biomaterials. The review emphasizes key areas of lignin valorization, including catalytic depolymerization methods, along with the associated challenges and advancements. We discuss its potential as a feedstock for diverse products such as biofuels, bioplastics, carbon fibers, adhesives, and phenolic compounds. Furthermore, the review briefly explores lignin’s inherent properties as a UV protectant and antioxidant, alongside its potential for incorporation into polymer blends and composites. By presenting recent advancements and case studies from the literature, this review highlights the significant economic and environmental benefits of lignin valorization, including waste reduction, lower greenhouse gas emissions, and decreased reliance on non-renewable resources. Finally, we address future perspectives and challenges associated with achieving large-scale, techno-economically feasible, and environmentally sustainable lignin valorization.
... Formerly, lignin was considered as an impediment to producing cellulose of higher quality; however, with the advent of the energy and environmental crisis, the demand for biomass-based products has increased dramatically, and thus lignin must be fully utilized. Lignin can be depolymerized via thermal, chemical, or biological approaches (Akash 2016). The biological approach presents the advantage of simultaneous depolymerization and the production of end-use products from lignin LCB (Salvachúa et al. 2015). ...
Chapter
Lignin is one of the most abundant macromolecules on Earth. It is a complex fraction in biomass composed of various aromatic building blocks which are cross‐linked together with different carbon and ether linkages. Processes including sulfite, soda, kraft, and organosolv are commonly used for lignin separation from lignocellulosic biomass at industrial scale. Lignin has broad scope for valorization to aromatics, polymers, and other value‐added materials. However, in spite of the attractiveness of lignin as a natural source for a wide range of products, there are various technologic barriers which limit its ubiquitous use at industrial scale. Therefore, there is a need to develop economically viable, eco‐friendly and sustainable technologies for the effective valorization of lignin. The present chapter discusses the potential of lignin for various high‐value products. In addition, the chemistry of lignin, various approaches available for its processing, and the economic and environmental concerns associated with lignin valorization are discussed.
Chapter
Lignin is a polyphenolic organic polymer, a constituent of plant support tissue that represents a relevant economic and environmental opportunity for being the second most abundant polymer on earth. The objective of the following research was to describe lignin conversion through biological and chemical routes. Lignin can be extracted from biomass via the sulfite or kraft process (it represents 90% of the pulping process nowadays). The chemical removal of lignin from black liquor is performed using two approaches: LignoBoost and LignoForce. The biological approach presents the advantage of simultaneous depolymerization and production of end‐use products using lignin as feedstock. Enzymes that break lignin are currently produced by fungi and bacteria and cover especially various peroxidase and phenoloxidase laccases. After lignin depolymerization, the resulting aromatic compounds are “biologically funneled” up to a central intermediate, which is metabolized and thus produces, on the way, various end‐use products.
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A variety of protic ionic liquids (PILs) were utilized to extract lignin from black liquor (BL). The PILs and a control IL, [Emim][Ac], were evaluated to extract lignin under different conditions. The impact of temperature, time and the IL,PIL:BL were the factors evaluated for extracting lignin. [Eth][Ac] was successful in extracting 75.3 ± 3.4% of the lignin (dry-basis) from BL at 95 °C for a PIL:BL(20:1)(w:w) and a mixing time of 4.5 h. The lignin extracted (13.4 ± 0.5%) was lowest for [Pyrr][La] under the same conditions. The characterization of the extracted lignin by FTIR confirmed that the observed peaks for the extracted lignin were in agreement with the peaks for a standard lignin. The conditions for the lignin extracted from BL using [Eth][Ac] were optimized in a three-factor, three-level Box-Behnken design (BBD). The factors (levels) of optimization parameters included time (0.5 h, 4.0 h, 7.5 h), temperature (60 °C, 95 °C, 130 °C), and the PIL:BL (10:1, 15:1, 20:1)(w:w). The BBD data was employed to develop a quadratic prediction model for lignin extraction as a function of the parameters affecting the process. A comparison of the model prediction with the experimental data confirmed the model predicted values were correlated with the experimental results. The optimum lignin extraction of 70.0% predicted by the model was 5.0% less than the experimental value, 75.0 ± 2.9%. The experimental quantity of lignin extracted with [Emim][Ac] under the same conditions was 77.6 ± 2.1%. The study established that the lignin extraction using PIL is comparable with the acidification-based commercial extraction, such as LignoBoost® and LignoForce® for lignin extraction from BL.
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Mechanochemical treatment of Indulin AT kraft lignin by ball-milling with KOH and toluene produces significant carbonyl functionality, among other changes. The chemical reactivity of the lignin is increased, resulting in greater lignin degradation from porphyrin oxidation followed by Baeyer-Villiger oxidation. The mechanochemical treatment produces a level of lignin oxidation that is similar to that produced by porphyrin-catalyzed oxidation. Combining mechanochemical treatment with porphyrin oxidation has a synergistic positive effect on the depolymerization of lignin, as demonstrated by a significantly higher yield of monomers. The methyl ester of vanillic acid was obtained as the main monomeric product (after methylation), along with of methyl 5-carbomethoxyvanillate, in a 10% yield, which is among the highest reported from kraft lignin
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A systematic research about the liquefaction of alkali lignin in supercritical ethanol using ZSM-5 zeolite catalysts was presented, which includes the synergistic effect of temperature, catalytic content, reaction time on product yield as well as the distribution. FTIR and GC-MS analysis were carried out to evaluate compositions of bio-oil and solid residue. The results showed that under the moderate condition, the maximum conversion and yield of bio-oil were 80.56wt% and 64.97wt%, respectively. With the help of ZSM-5 catalyst, lignin could be successfully converted into aromatic compounds. It exhibited that at 300°C,1h, adding 10% of catalytic content can obtain the highest selectivity of the aromatic compounds, which was 81.35%.
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Previous work in our laboratories has demonstrated that addition of lignin to coal during liquefaction significantly increases the depolymerization of coal and enhances the quality of the liquid products. It is believed that thermolysis of the lignin results in the formation of phenoxy and other reactive radicals at temperatures too low for significant thermolysis of the coal matrix; such radicals are effective and active intermediates that depolymerize coal by cleaving methylene bridges. It has been reported that alkali is also effective for extraction of liquids from coal. The work presented here combines these two reactive agents by utilizing the black liquor waste stream from the Kraft pulping process for coal depolymerization. That waste stream contains large amounts of lignin and sodium hydroxide, as well as other components. To permit comparative evaluations of the extent of coal depolymerization by coprocessing coal and black liquor, reference runs were performed with tetralin alone, sodium hydroxide in tetralin, and lignin in tetralin. Results indicated that the sodium hydroxide-tetralin system resulted in almost 67% conversion at 375 degrees C, 1 hour. The black liquor system exhibited a lower conversion of 60%, indicating some inhibition of the depolymerization reactions by components in the black liquor.
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The state of the art of hydrolysis-fermentation technologies to produce ethanol from lignocellulosic biomass, as well as developing technologies, is evaluated. Promising conversion concepts for the short-, middle- and long-term are defined. Their technical performance was analysed, and results were used for economic evaluations. The current available technology, which is based on dilute acid hydrolysis, has about 35% efficiency (HHV) from biomass to ethanol. The overall efficiency, with electricity co-produced from the not fermentable lignin, is about 60%. Improvements in pre-treatment and advances in biotechnology, especially through process combinations can bring the ethanol efficiency to 48% and the overall process efficiency to 68%. We estimate current investment costs at 2.1 k€/kWHHV (at 400 MWHHV input, i.e. a nominal 2000 tonne dry/day input). A future technology in a 5 times larger plant (2 GWHHV) could have investments of 900 k€/kWHHV. A combined effect of higher hydrolysis-fermentation efficiency, lower specific capital investments, increase of scale and cheaper biomass feedstock costs (from 3 to 2 €/GJHHV), could bring the ethanol production costs from 22 €/GJHHV in the next 5 years, to 13 €/GJ over the 10–15 year time scale, and down to 8.7 €/GJ in 20 or more years.
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The global annual potential bioethanol production from the major crops, corn, barley, oat, rice, wheat, sorghum, and sugar cane, is estimated. To avoid conflicts between human food use and industrial use of crops, only the wasted crop, which is defined as crop lost in distribution, is considered as feedstock. Lignocellulosic biomass such as crop residues and sugar cane bagasse are included in feedstock for producing bioethanol as well. There are about of dry wasted crops in the world that could potentially produce of bioethanol. About of dry lignocellulosic biomass from these seven crops is also available for conversion to bioethanol. Lignocellulosic biomass could produce up to of bioethanol. Thus, the total potential bioethanol production from crop residues and wasted crops is , about 16 times higher than the current world ethanol production. The potential bioethanol production could replace of gasoline (32% of the global gasoline consumption) when bioethanol is used in E85 fuel for a midsize passenger vehicle. Furthermore, lignin-rich fermentation residue, which is the coproduct of bioethanol made from crop residues and sugar cane bagasse, can potentially generate both of electricity (about 3.6% of world electricity production) and of steam. Asia is the largest potential producer of bioethanol from crop residues and wasted crops, and could produce up to of bioethanol. Rice straw, wheat straw, and corn stover are the most favorable bioethanol feedstocks in Asia. The next highest potential region is Europe ( of bioethanol), in which most bioethanol comes from wheat straw. Corn stover is the main feedstock in North America, from which about of bioethanol can potentially be produced. Globally rice straw can produce of bioethanol, which is the largest amount from single biomass feedstock. The next highest potential feedstock is wheat straw, which can produce of bioethanol. This paper is intended to give some perspective on the size of the bioethanol feedstock resource, globally and by region, and to summarize relevant data that we believe others will find useful, for example, those who are interested in producing biobased products such as lactic acid, rather than ethanol, from crops and wastes. The paper does not attempt to indicate how much, if any, of this waste material could actually be converted to bioethanol.
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Oils have been produced thermochemically from the following lignocellulosic materials by applying constant hydrogenolytic conditions in an aqueous phase with palladium as catalyst : spruce and birch wood, spruce and birch holocellulose, cellulose, pine bark, spruce and bagasse organosolv lignins, and birch Willstätter lignin. The liquid products were separated into water-, acetone- and dichloromethane-soluble (oil) fractions. The highest oil yield was obtained from spruce organosolv lignin (64%) and the lowest from carbohydrates and pine bark (20–31%). The oils were separated into neutral, strongly acidic and weakly acidic fractions by a liquid/liquid extraction procedure. Sequential elution by solvent chromatography was used for additional comparative characterization of the oils. Calorific values of the feedstocks and oils were calculated based upon the data of elemental analysis.
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This article presents some insight into energy consumption in the commercial and public service sector (CAPSS) in Jordan. In this sector, space- and water-heating is dependent particularly upon the combustion of fossil fuels. Which thereby contribute significantly to air pollution and the build-up of carbon dioxide in the atmosphere. The results of a recent survey were used to evaluate the energy demand of the commercial and public service buildings. Diesel fuel, LPG and kerosene are mainly used for space heating, with diesel being the most popular fuel followed by LPG. Unvented combustion appliances, i.e. portable kerosene and LPG heaters, are still employed in this sector in order to provide space heating in unclassified hotels, some clinics and health centres as well as retail shops. These stoves, usually, produce high levels of combustion by-products that often exceed acceptable limits especially in a closed space. Consequently, the indoor air quality is degraded and may cause unnecessary exposure to toxic gases such as carbon monoxide and unburned hydrocarbons. Electricity consumption is relatively high due to the excessive lighting and heavy use of air-conditioning and ventilation systems during the dry and hot summer. It is estimated that about 15% of the annual consumption in CAPSS can be reduced annually with little investment. Consequently the corresponding annual CO2 emissions reduction is approximately 1%, i.e. 160×103 tons, of the present total greenhouse gas emissions in Jordan.
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Biofuel produced from lignocellulosic materials, so-called second generation bioethanol shows energetic, economic and environmental advantages in comparison to bioethanol from starch or sugar. However, physical and chemical barriers caused by the close association of the main components of lignocellulosic biomass, hinder the hydrolysis of cellulose and hemicellulose to fermentable sugars. The main goal of pretreatment is to increase the enzyme accessibility improving digestibility of cellulose. Each pretreatment has a specific effect on the cellulose, hemicellulose and lignin fraction thus, different pretreatment methods and conditions should be chosen according to the process configuration selected for the subsequent hydrolysis and fermentation steps. This paper reviews the most interesting technologies for ethanol production from lignocellulose and it points out several key properties that should be targeted for low-cost and advanced pretreatment processes.
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PAHs are aromatic hydrocarbons with two or more fused benzene rings with natural as well as anthropogenic sources. They are widely distributed environmental contaminants that have detrimental biological effects, toxicity, mutagenecity and carcinogenicity. Due to their ubiquitous occurrence, recalcitrance, bioaccumulation potential and carcinogenic activity, the PAHs have gathered significant environmental concern. Although PAH may undergo adsorption, volatilization, photolysis, and chemical degradation, microbial degradation is the major degradation process. PAH degradation depends on the environmental conditions, number and type of the microorganisms, nature and chemical structure of the chemical compound being degraded. They are biodegraded/biotransformed into less complex metabolites, and through mineralization into inorganic minerals, H(2)O, CO(2) (aerobic) or CH(4) (anaerobic) and rate of biodegradation depends on pH, temperature, oxygen, microbial population, degree of acclimation, accessibility of nutrients, chemical structure of the compound, cellular transport properties, and chemical partitioning in growth medium. A number of bacterial species are known to degrade PAHs and most of them are isolated from contaminated soil or sediments. Pseudomonas aeruginosa, Pseudomons fluoresens, Mycobacterium spp., Haemophilus spp., Rhodococcus spp., Paenibacillus spp. are some of the commonly studied PAH-degrading bacteria. Lignolytic fungi too have the property of PAH degradation. Phanerochaete chrysosporium, Bjerkandera adusta, and Pleurotus ostreatus are the common PAH-degrading fungi. Enzymes involved in the degradation of PAHs are oxygenase, dehydrogenase and lignolytic enzymes. Fungal lignolytic enzymes are lignin peroxidase, laccase, and manganese peroxidase. They are extracellular and catalyze radical formation by oxidation to destabilize bonds in a molecule. The biodegradation of PAHs has been observed under both aerobic and anaerobic conditions and the rate can be enhanced by physical/chemical pretreatment of contaminated soil. Addition of biosurfactant-producing bacteria and light oils can increase the bioavailability of PAHs and metabolic potential of the bacterial community. The supplementation of contaminated soils with compost materials can also enhance biodegradation without long-term accumulation of extractable polar and more available intermediates. Wetlands, too, have found an application in PAH removal from wastewater. The intensive biological activities in such an ecosystem lead to a high rate of autotrophic and heterotrophic processes. Aquatic weeds Typha spp. and Scirpus lacustris have been used in horizontal-vertical macrophyte based wetlands to treat PAHs. An integrated approach of physical, chemical, and biological degradation may be adopted to get synergistically enhanced removal rates and to treat/remediate the contaminated sites in an ecologically favorable process.
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Lignocellulosic biomass can be utilized to produce ethanol, a promising alternative energy source for the limited crude oil. There are mainly two processes involved in the conversion: hydrolysis of cellulose in the lignocellulosic biomass to produce reducing sugars, and fermentation of the sugars to ethanol. The cost of ethanol production from lignocellulosic materials is relatively high based on current technologies, and the main challenges are the low yield and high cost of the hydrolysis process. Considerable research efforts have been made to improve the hydrolysis of lignocellulosic materials. Pretreatment of lignocellulosic materials to remove lignin and hemicellulose can significantly enhance the hydrolysis of cellulose. Optimization of the cellulase enzymes and the enzyme loading can also improve the hydrolysis. Simultaneous saccharification and fermentation effectively removes glucose, which is an inhibitor to cellulase activity, thus increasing the yield and rate of cellulose hydrolysis.
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In this article, the technical feasibility of various low-cost adsorbents for heavy metal removal from contaminated water has been reviewed. Instead of using commercial activated carbon, researchers have worked on inexpensive materials, such as chitosan, zeolites, and other adsorbents, which have high adsorption capacity and are locally available. The results of their removal performance are compared to that of activated carbon and are presented in this study. It is evident from our literature survey of about 100 papers that low-cost adsorbents have demonstrated outstanding removal capabilities for certain metal ions as compared to activated carbon. Adsorbents that stand out for high adsorption capacities are chitosan (815, 273, 250 mg/g of Hg(2+), Cr(6+), and Cd(2+), respectively), zeolites (175 and 137 mg/g of Pb(2+) and Cd(2+), respectively), waste slurry (1030, 560, 540 mg/g of Pb(2+), Hg(2+), and Cr(6+), respectively), and lignin (1865 mg/g of Pb(2+)). These adsorbents are suitable for inorganic effluent treatment containing the metal ions mentioned previously. It is important to note that the adsorption capacities of the adsorbents presented in this paper vary, depending on the characteristics of the individual adsorbent, the extent of chemical modifications, and the concentration of adsorbate.
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Biologically mediated processes seem promising for energy conversion, in particular for the conversion of lignocellulosic biomass into fuels. Although processes featuring a step dedicated to the production of cellulase enzymes have been the focus of most research efforts to date, consolidated bioprocessing (CBP)--featuring cellulase production, cellulose hydrolysis and fermentation in one step--is an alternative approach with outstanding potential. Progress in developing CBP-enabling microorganisms is being made through two strategies: engineering naturally occurring cellulolytic microorganisms to improve product-related properties, such as yield and titer, and engineering non-cellulolytic organisms that exhibit high product yields and titers to express a heterologous cellulase system enabling cellulose utilization. Recent studies of the fundamental principles of microbial cellulose utilization support the feasibility of CBP.
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Present work deals with the biotechnological production of fuel ethanol from different raw materials. The different technologies for producing fuel ethanol from sucrose-containing feedstocks (mainly sugar cane), starchy materials and lignocellulosic biomass are described along with the major research trends for improving them. The complexity of the biomass processing is recognized through the analysis of the different stages involved in the conversion of lignocellulosic complex into fermentable sugars. The features of fermentation processes for the three groups of studied feedstocks are discussed. Comparative indexes for the three major types of feedstocks for fuel ethanol production are presented. Finally, some concluding considerations on current research and future tendencies in the production of fuel ethanol regarding the pretreatment and biological conversion of the feedstocks are presented.
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Lignocellulosic biomass represents a rather unused source for biogas and ethanol production. Many factors, like lignin content, crystallinity of cellulose, and particle size, limit the digestibility of the hemicellulose and cellulose present in the lignocellulosic biomass. Pretreatments have as a goal to improve the digestibility of the lignocellulosic biomass. Each pretreatment has its own effect(s) on the cellulose, hemicellulose and lignin; the three main components of lignocellulosic biomass. This paper reviews the different effect(s) of several pretreatments on the three main parts of the lignocellulosic biomass to improve its digestibility. Steam pretreatment, lime pretreatment, liquid hot water pretreatments and ammonia based pretreatments are concluded to be pretreatments with high potentials. The main effects are dissolving hemicellulose and alteration of lignin structure, providing an improved accessibility of the cellulose for hydrolytic enzymes.