Lignin Char Research Articles

Unravelling the kinetics and thermodynamics of CO2 gasification of lignin char: an isoconversional and multi-methodological analysis

Unravelling the kinetics and thermodynamics of CO2 gasification of lignin char: an isoconversional and multi-methodological analysis

Green and Atom Economical Route to High Compressive Strength Lignin Oil-Sulfur Composites

Lignin is the most abundant natural source of aromatics but remains underutilized. Elemental sulfur is a plentiful by-product of fossil fuel refining. Herein we report a strategy for preparing a durable composite by the one-pot reaction of elemental sulfur and lignin oil comprising lower molecular weight lignin derivatives. A lignin oil-sulfur composite (LOS90) was prepared by reacting 10 wt. % lignin oil with 90 wt. % elemental sulfur. The composite could be remelted and reshaped over several cycles without loss of properties. Results from the study showed that LOS90 has properties competitive with or exceeding values for commercial ordinary Portland cement and brick formulations. For example, LOS90 displayed impressive compressive strength (22.1 MPa) and flexural strength (5.7 MPa). LOS90 is prepared entirely from waste materials with 98.5% atom economy of composite synthesis, a low E factor of 0.057, and lignin char as the only waste product of the process for its preparation. These results suggest the potential applications of lignin and waste sulfur in the continuous efforts to develop more recyclable and sustainable materials.

Mechanistic investigation of char growth from lignin monomers during biomass utilisation

Char is formed as a significant product from the pyrolysis of biomass, and it is well reported that lignin is the greatest contributor to the production of char. The structures of lignin chars are well studied in literature; however, an elucidation on the lignin char formation mechanism is lacking and this is essential to achieve precise control of biomass thermal conversion. In this work, the char growth process from β-O-4 linkage derived lignin monomers was investigated using density functional theory (DFT) and validated by slow pyrolysis experiments and Nuclear Magnetic Resonance (NMR) analysis. It was shown that char forms in a two-step process, beginning with the aggregation of two mono-aromatic species to yield tricyclic species, which then participate in further ring forming reactions to yield larger aromatic clusters. The side products generated involve hydrogen, water, methanol, and formaldehyde. Insights from the proposed mechanism are important for guiding future research on char formation during biomass thermochemical conversion processes.

Fast-pyrolysis lignin-biochar as an excellent precursor for high-performance capacitors

Lignin-based activated carbons (LAC) were produced using thermochemical pretreatment and chemical activation with KOH. Fast pyrolysis at 550 °C in a CO2/N2 atmosphere resulted in lignin char (LC) with a more developed porous structure than slow pyrolysis. The effect of activation conditions (KOH usage, temperature, and duration) on surface/physicochemical properties and electrochemical characteristics of the resulting LAC was fully studied. Using fast pyrolysis lignin char as a precursor and optimized activation conditions (wKOH:wLC = 2, 800 °C, and 2 h), the resulting LAC featured a large surface area of 2149.5 m2 g−1, a total pore volume of 0.88 m3 g−1, and high capacitance of 300 F g−1 at 0.5 A g−1 in a 6 mol L−1 KOH electrolyte. The LAC-based symmetric supercapacitor could offer superior energy density (19.15 W h kg−1 at 250 W kg−1 power density) and a stable lifetime (98.2% of original capacity after 10,000 charge-discharge cycles). The excellent capacitor performance of LAC was attributed to its microporous-mesoporous structure developed from fast pyrolysis and subsequent chemical activation.

Interaction between coal and lignin briquettes in co-carbonization

The utilization of bio-based side streams in metallurgical coke making promotes two major factors in the mitigation of climate impact in the steel industry. Circular economy as the waste material from biorefinery industry is utilized as a raw material in the steel industry, and mitigation of the production of fossil-based CO2 emissions. In this work, lignin from the hydrolysis process was used in a briquetted form as part of the raw material blend in metallurgical coke making. For the experiments and analyses, lignin briquettes were pyrolyzed at 450, 600 and 1200 °C, while one sample was left non-pyrolyzed. In the co-carbonization of briquetted lignin, lignin chars and bituminous coal, the focus was to evaluate the interaction between char and coal in the carbonization. This was studied by thermogravimetric analysis (TGA), optical dilatometry, and light optical microscopy. The results suggested that the interaction between the coal and lignin reduced when the pyrolysis temperature of the briquettes, prior to co-carbonization, was elevated. This was due to the decrease of overlapping of the pyrolysis rates of chars and coking coal. Combined with the dilation and shrinking behaviour of the chars, presented in this paper, separate char and coke structures were formed in the final coke in co-carbonization.

Elucidating the synergistic interaction and reaction pathway between the individual lignocellulosic components during flash pyrolysis

• Homogeneous volatile-volatile interaction is overwhelming in flash pyrolysis. • Oxygen-bearing radicals induced volatile reactions between aliphatics and aromatics. • Lignin char is the most catalytically active for secondary reactions of volatiles. • Cellulose char is the most active in trapping heavy molecules derived from lignin. • The interaction between cellulose/xylan and lignin promotes the overall conversion. In this study, systematic experiments have been conducted to elucidate the synergistic interaction during the flash pyrolysis of lignocellulosic biomass. Through the use of Pyroprobe coupled with GC-FID and TCD, and the testing of individual components, their derivatives, model compounds and blends, the interaction of biomass components was confirmed to be affected by both heterogeneous char-volatile and homogeneous volatile-volatile interactions. The latter interaction was overwhelming in contributing to an enhanced bio-oil yield by the transferring of oxygen-bearing radicals from cellulose/xylan-bound hydroxyl to the volatiles such as p-cresol from lignin, and vice versa, by an induction effect of the lignin-derived volatiles on the release/transformation of cellulose/xylan volatiles. Alternatively, the heterogeneous char-volatile interaction was crucial in improving the non-condensable gas yield. The lignin char was the most catalytically active for the deoxygenation of volatiles, followed sequentially by xylan char, and cellulose char which was the most active in trapping the heavy molecules derived from lignin. The synergy of cellulose/xylan and lignin also promoted the overall conversion to complete more quickly.

Catalytic pyrolysis of biomass with char modified by cathode materials of spent lithium-ion batteries for tar reduction and syngas production

The char modified by the battery cathode material was studied for catalytic pyrolysis of rice husk (RH). Compared to the direct mixing, the addition of cathode material to char by the impregnation was more favorable to synthesize the lignin char catalyst (i.e. char 1) with hierarchical pore structure and larger specific surface area. The char 1 decreased the Ea of the RH pyrolysis from 79 kJ/mol to 66 kJ/mol. The char 1 decreased the yield of liquid product from 23.2% to 12.3% and increased the yield of gas product from 35.2% to 52.2%. The char 1 decreased the contents of acids from 20.6% to 3.6% and phenols (tar surrogates) from 29.4% to 8.0%. The increase of hydrocarbons and CO2 indicated that the char catalysts had high catalytic reactivity in converting the oxygenated components (e.g., sugars, furans, acids, phenols) into light hydrocarbons (e.g., alkanes, aromatics) and micro-molecular gases (e.g., CO2, H2).

Volatile-char interactions during biomass pyrolysis: Insight into the activity of chars derived from three major components

Char activity is a critical factor for the interactions between char and volatile, which are recognized to determine the final product distribution from biomass pyrolysis. To clarify the effect of biomass composition on char activity, the interactions between (benzyloxy)benzene as a volatile model compound and char samples obtained from cellulose, xylan, and lignin were studied. The (benzyloxy)benzene conversion increased from 0.64 % for no char to 6.31 %, 6.98 % and 53.5 % for lignin char, cellulose char, and xylan char, respectively, which were mainly derived from the contribution of surface CO groups and surface defects in the carbon structure. Especially, surface C(sp2)O groups and C(sp3)O groups on char respectively favored the breakage of C(sp3)O and C(sp2)O bonds in (benzyloxy)benzene. Despite the low activity, lignin char showed high selectivity to monomeric products due to the high content of surface CO groups, which were considered to induce the production of hydrogen radicals to stabilize the resulting intermediates. The findings of this study prove that the volatile-char interaction could be adjusted facilely by changing the hemicellulose content during feedstock pretreatments.

Effect of boron-based additives on char agglomeration and boron doped carbon microspheres structure from lignin pyrolysis

To prevent the thermal softening of lignin during the pyrolysis process, the influence of boron-based additives (boric acid, sodium borate and potassium borate) on lignin softening, pyrolysis process were investigated with metal bases (Ca(OH)2 and Mg(OH)2) as comparison, and the influence mechanism of boron based additives was explored in depth. It is found that the boron additives can effectively inhibit the agglomeration of lignin particles. For pyrolysis products, boron additive promoted the carbon-forming reaction of lignin with carbon sequestration more than 70 wt%, and inhibited the release of volatiles, especially for phenolic compounds. It also inhibited the generation of CO2 and CH4, while promoted the release of CO. The boron element in lignin char mainly exists in the form of BCO2 and B2O3 with more hydroxyl groups on the surface. Furthermore, boron-modified lignin chars are spherical with more uniform and smaller particle size compared with metal bases modified lignin chars.

Identifying Synergistic Effects between Biomass Components during Pyrolysis and Pointers Concerning Experiment Design

A review of existing data has shown that “char yield deficits” develop during the pyrolysis of lignocellulosic biomass, relative to char yields expected from pyrolyzing chemically isolated lignins and the proportion of lignin in the particular biomass. This paper describes two sets of pyrolysis experiments. The work done in a thermogravimetric (TG) balance was initiated to probe whether diminishing heating rates might reduce, or even wipe out, the “char yield deficits” identified in previous work, where a wide range of heating rates had been used. Experiments were performed at 2 °C min–¹, a lower heating rate than that has hitherto been used to investigate char deficits. The effect was confirmed at this slow heating rate, using samples of birchwood and almond shells. A parallel set of differential scanning calorimetry (DSC) experiments provided evidence that mechanisms by which biomass samples pyrolyze are distinct from those of biomass components pyrolyzing in isolation. Moreover, the observed effects could not be replicated by simply mixing the three biomass components in appropriate proportions. The “lignin char deficit” is consistent with chemical interactions between intermeshed biomass components during pyrolysis altering reaction pathways and product distributions relative to the pyrolysis of biomass components pyrolyzed in isolation. The present work also shows that sample mass loss in TG balances is affected by altering sample loading, leading to potential errors. The design of pyrolysis experiments is discussed and approaches are suggested to prevent masking of key pyrolysis phenomena, viz. synergistic effects between biomass components or onset-of-pyrolysis temperatures, through the appropriate selection of experimental parameters.

Performance of CO2 and Fe-modified lignin char on arsenic (V) removal from water

Biochar was produced by the pyrolysis of Kraft lignin at 600 °C followed by modification with CO2 at 700 and 800 °C and impregnation with FeOx. The physicochemical properties and arsenic (V) adsorption performance of biochar were evaluated. The characteristics of the lignin biochar before and after CO2 modification and FeOx impregnation were analyzed using the following methods: proximate and ultimate analysis, specific surface area (Brunauer–Emmett–Teller (BET) surface area), porosity, scanning electron microscopy and energy dispersive spectroscopy mapping, Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. The specific surface area and porosity of biochar were improved significantly after CO2 modification. However, impregnation of FeOx in CO2-modified biochar showed a 50%–60% decrease of BET surface area and porosity due to pore blocking of FeOx. The batch adsorption of arsenic (V) showed that FeOx-LC-800 (FeOx impregnation lignin char modified with CO2 at 800 °C) had the highest adsorption efficiency among the biochars tested because of its highest Fe–O intensity and large surface area. The Langmuir adsorption model was suitable for the curve fitting arsenic (V) adsorption. The theoretical equilibrium adsorption amount (qe) was calculated to be 6.8 mg/g using a pseudo-second-order kinetic model.

Experimental study on catalytic pyrolysis of lignin under char and ZSM-5 for preparation of aromatics

In order to improve yield of aromatics from catalytic pyrolysis of lignin, alkali lignin was used for rapid pyrolysis experiments under a binary catalysis system of alkali lignin char and ZSM-5. Influence of catalyst ratio and pyrolysis temperature as well as pyrolysis time on aromatics quantity and mechanism of synergistic catalysis of alkali lignin char and ZSM-5 were investigated. The results show that quantity of aromatics has increased from 17 mg/g (without char) to 33 mg/g (with char addition of 1g), which is about doubled. The optimal conditions for preparation of aromatics are under alkali lignin: alkali lignin char: ZSM-5=1:1:1 at 500°C for 10 min. Meanwhile, the mechanism analysis shows that alkali lignin char mainly plays a role in bond breaking during pyrolysis, while ZSM-5 can act as selective aromatization to obtain higher aromatics yield, whose synergistic effects result in higher aromatic yield.

A new perspective on polyethylene-promoted lignin pyrolysis with mass transfer and radical explanation

Co-pyrolysis of lignin and waste plastics, for example polyethylene (PE), has been studied, but related reports are basically on condition optimizations. This study revealed a new perspective on PE-promoted lignin pyrolysis to phenolic monomers with mass transfer and radical explanation. Lignin and PE were first pyrolyzed individually to identify pyrolysis characteristics, pyrolytic products, as well as the suitable co-pyrolysis temperature. Then, co-pyrolysis of blended lignin/PE with various ratios was investigated. Yields of lignin products reached the maximum under lignin/PE ratio of 1:1, but blended approach always inhibited the production of lignin phenols. This resulted from the poor mass transfer and interactions between lignin and PE, in which PE pyrolysates could easily escape from the particle gaps. While in layered approach, PE pyrolysates had to pass through the lignin layer which contributed to the good interactions with lignin pyrolysis intermediates, thus the yields of lignin-derived products were significantly improved. Interactions between lignin and PE (or their pyrolysates) were mainly radical quenching reactions, and X-ray photoelectron spectrum (XPS) and electron paramagnetic resonance (EPR) of pyrolytic chars were conducted to verify these interactions controlled by mass transfer. The percentage of CC (sp2) and concentration of organic stable radicals in layered lignin/PE char were both the lowest compared with those in blended lignin/PE and lignin char, indicating the stabilization of lignin-derived radicals by PE pyrolysates. Moreover, the spin concentration of radicals in the char from layered char/PE was lower than that in lignin char, which further affirmed the quenching of radicals by PE in the layered co-pyrolysis mode.

Effects of Briquetting and High Pyrolysis Temperature on Hydrolysis Lignin Char Properties and Reactivity in CO-CO2-N2 Conditions

Carbonaceous reductants for pyrometallurgical applications are usually obtained from fossil-based sources. The most important properties of the reductants greatly depend on the application and the feeding of the reductant into the process. However, the mechanical strength, calorific value, fixed carbon content, and reactivity of the reductant are the properties that usually define the applicability of the reductant for different processes. The reactivity of the biochars is usually high in comparison to metallurgical coke, which may restrict the applicability of the biochar in reduction processes. One cause of the higher reactivity is the higher surface area of the biochars, which can be suppressed with agglomeration treatment, e.g., briquetting. In this work, hydrolysis lignin was used for slow pyrolysis experiments to produce biochars. The biochars were pyrolyzed in briquetted form and in as-received form at various temperatures. The reactivity values of the biochars were tested in dynamic reactivity tests in a CO-CO2-N2 gas atmosphere at temperatures of up to 1350 °C. It was found that the yield of the hydrolysis lignin char only decreased by 3.36 wt% when the pyrolysis temperature was elevated from 600 to 1200 °C, while a decrease in yield of 4.88 wt% occurred when the pyrolysis temperature was elevated from 450 to 600 °C. The mass loss of hydrolysis lignin biochar in the reactivity experiment in CO-CO2-N2 atmosphere was significantly decreased from 79.41 wt% to 56.80 wt% when the hydrolysis lignin was briquetted before the slow pyrolysis process and the temperature of the pyrolysis process was elevated from 600 to 1200 °C. This means that the mass loss of the material was suppressed by 22.61 wt% due to the higher pyrolysis temperature and briquetting process.

Reaction of Substituted Phenols with Lignin Char: Dual Oxidative and Reductive Pathways Depending on Substituents and Conditions.

Biomass chars are known to be intrinsically redox-reactive toward some organic compounds, but the mechanisms are still unclear. To address this, a char made anoxically at 500 °C from dealkaline lignin was reacted either in the fresh state or after 180-day aging in air with p-nitrophenol (NO2-P), p-hydroxybenzaldehyde (CHO-P), phenol (H-P), or p-methoxyphenol (MeO-P). The reactions were carried out under oxic or anoxic conditions. Degradation occurred in all cases. Both oxidation and reduction products were identified, with yields dependent on the presence or absence of air during reaction or storage. They included oligomers, amines, and ring-hydroxylated compounds, among others. Exposure to air suppressed sorption, annihilated reducing sites, and provided a source of reactive oxygen species that assisted degradation. Sorption suppression was due to the incorporation of hydrophilic groups by chemisorption of oxygen, and possibly blockage of sites by products. Fresh char has comparable electron-donating and accepting capacity, whereas there is a preponderance of electron-accepting over donating capacity in aged char. Under anoxic conditions, both oxidation and reduction occurred. Under oxic conditions or after aging in air, oxidation predominated, and linear free energy relationships were found between the rate constant and the Hammett or Brown substituent electronic parameter or the standard electrode potential of the phenol. The results demonstrate that chars possess heterogeneous redox activities depending on reaction pairs, reaction conditions, and aging.

A new insight of lignin pyrolysis mechanism based on functional group evolutions of solid char

To elucidate the mechanism of biomass lignin pyrolysis, the functional group evolution in lignin chars derived at 200–600 °C was investigated using two-dimensional perturbation correlation infrared spectroscopy, associated with the properties of the volatile products. Results from 2D-PCIS pointed that the change in the alcohol groups C–OH occurs earlier than that in the carbonyl groups CO before 250 °C, corresponding to the reaction of keto–enol tautomerization: C–OH → CO, and the cleavage of bonds and remodeling reactions of the side chains of the benzene rings are synchronous. Besides, the formation of simple substituted benzene structures was earlier than complex substituted benzene structures. Decarbonylation, dehydrogenation and dealkylation were intensified during polycondensation of benzene rings. These findings are significant for achieving an in-depth understanding of lignin pyrolysis.

In-depth comparison of the physicochemical characteristics of bio-char derived from biomass pseudo components: Hemicellulose, cellulose, and lignin

Bio-char is a porous, recalcitrant, and highly aromatic carbon-rich material which can be widely used in energy, agriculture, environment, and material industry. In this study, a systematic comparison of the physicochemical characteristics of bio-char derived from three pseudo components of biomass (cellulose char (CC), hemicellulose char (HC), lignin char (LC)) was carried out at different pyrolysis temperatures (250, 350, 450, 550, 650, 750, and 850 °C). The results indicated wide variation in the physicochemical properties and quality of bio-char depending on different pyrolysis temperatures and biomass feedstock composition. Mass and energy yields of CC, HC, and LC decreased with the increase in the pyrolysis temperature, however, LC exhibited higher mass and energy yield than CC and HC because of its better thermal stability. With the increase in the pyrolysis temperature, the carbon content in CC, HC, and LC increased, while the contents of hydrogen and oxygen decreased, because the oxygen and hydrogen containing groups were easily fallen off at higher pyrolysis temperature based on the FTIR analysis. XRD analysis showed that the crystalline structure (Iα-triclinic and Iβ-monoclinic) of cellulose in CC and HC disappeared with the increase in the pyrolysis temperature above 350 °C. However, the graphite structure (002) and (101) in CC, HC, and LC increased. 13C NMR analysis indicated that the carbon structure of alkyl-C, O-alkyl-C, and carboxylic-C gradually decreased in CC, HC and LC as the pyrolysis temperature increased, while the aryl-C increased, indicating the formation of a more polyaromatic graphite-like structure at higher pyrolysis temperature. SEM results revealed that the volume of CC and LC reduced because of the significant particle agglomeration during pyrolysis process. However, the volume of HC sharply increased because of the transformation of hemicellulose into foam-like material at higher temperature. The lower values of mean absolute error strongly suggests that it is feasible to predict the properties (the contents of C, H, O, mass and energy yield) of real biomass derived bio-char based on the properties of three pseudo components derived bio-char.

A thermogravimetric analysis of lignin char combustion

Understanding the combustion behavior is the basic requirement for a new resource to be used as an alternative fuel for the industrial design of the future plants. In this article, thermogravimetric analysis (TGA) of lignin char combustion in different heating rates (5, 10 and 15 °C/min) was investigated. Extracted combustion indices showed increased weight loss rate, peak temperature and burnout temperature but no change in ignition temperature for all samples when the heating rate increased. Lignin chars containing higher volatile material illustrated higher combustibility through the low ignition and burnout temperatures. Kinetic parameters of lignin combustion were also obtained by the Coat-Redfern method in the first-order kinetic model. High combustibility of high volatile sample (L300: vol%=41) was also confirmed by its low activation energy which was 46.68 compared to 150.34 for L500 (vol%=18) and 174.37 kJ/mol for L650 (vol%=5.1). The pre-exponential factor was also measured to be 2.61E-01, 8.15E+06 and 1.21E+08 min-1for L300, L500 and L650 respectively.

Acetaldehyde removal and increased H2/CO gas yield from biomass gasification over metal-loaded Kraft lignin char catalyst

Acetaldehyde removal tests were performed to compare the catalytic activity of the Kraft lignin char (KC), KOH-treated Kraft lignin char (KKC), and activated carbon (AC) along with their impregnation with Mn in a plasma reactor. The gasification characteristics (syngas content, and H2/CO ratio) of yellow poplar were investigated using nickel catalysts supported on KC, KKC, AC, and γ-Al2O3 in a U-type quartz reactor. KKC and Mn/KKC improved significantly the surface area and contents of O and N functional groups over the raw char. In particular, Mn/KKC showed the highest acetaldehyde-removal efficiency. The catalytic activity of Ni-impregnated KC, KKC, AC, and γ-Al2O3 decreased in the order of Ni/KKC > Ni/AC > Ni/KC > Ni/γ-Al2O3 for the gas yield and Ni/γ-Al2O3 >Ni/KC > Ni/AC >Ni/KKC for the oil yield, respectively. The Ni/KKC provides a more conducive environment for gasification, resulting in larger amounts of syngas (H2 and CO) in the product gases. Moreover, Ni impregnated with char may be the most inexpensive and effective solution for achieving maximum tar reduction and syngas generation.

Slow pyrolysis of by-product lignin from wood-based ethanol production– A detailed analysis of the produced chars

Slow pyrolysis as a method of producing a high-quality energy carrier from lignin recovered from wood-based ethanol production has not been studied for co-firing or blast furnace (BF) applications up to now. This paper investigates fuel characteristics, grindability, moisture uptake and the flow properties of lignin chars derived from the slow pyrolysis of lignin at temperatures of 300, 500 and 650 °C (L300, L500 and L650 samples respectively) at a heating rate of 5 °C min−1. The lignin chars revealed a high mass and energy yield in the range of 39–73% and 53–89% respectively. Pyrolysis at 500 °C or higher, yielded lignin chars with low H/C and O/C ratios suitable for BF injection. Furthermore, the hydrophobicity of lignin was improved tremendously after pyrolysis. Pyrolysis at high temperatures increased the sphericity of the lignin char particles and caused some agglomeration in L650. Large and less spherical particles were found to be a reason for high permeability, compressibility and cohesion of L300 in contrast to L500 and L650. L300 and L500 chars demonstrated high combustibility with low ignition and burnout temperatures. Also, rheometric analysis showed that L500 has the best flow properties including low aeration energy and high flow function.