Autothermal Pyrolysis Research Articles

Evolution of heavy components in bio-oil during oxidative pyrolysis of cellulose, hemicellulose, and lignin

Biomass oxidative pyrolysis introduces restricted oxygen into the reaction zone, realizing autothermal pyrolysis to address the heat supply challenges inherent in large-scale applications. However, heavy components (＞200 Da) in bio-oil are critical precursors that lead to coke formation upon heating, which hinders the utilization of bio-oil. In this study, the conventional and oxidative pyrolysis experiments of cellulose, hemicellulose, and lignin in a fix-bed reactor were conducted at temperatures ranging from 300 °C to 800 °C, aiming to investigate the evolution of heavy components in bio-oil during biomass oxidative pyrolysis. The results showed that the addition of oxygen promoted the generation of bio-oil. Compared to conventional pyrolysis, the addition of oxygen mostly increased the yields of cellulose-oil, hemicellulose-oil, and lignin-oil by 28.21 %, 10.94 %, and 16.84 %, respectively. Further comprehensive analysis revealed that oxygen promoted the depolymerization of three components at a lower temperature range (＜ 500 °C). With increasing temperatures, oxygen enhanced the polymerization of volatiles from cellulose and lignin, where oxygen, acting as a binder, promoted the generation of phenolic compounds of heavy components in lignin-oil. Conversely, as the temperature increased, oxygen enhanced the oxidative decomposition of volatiles from hemicellulose, inhibiting the generation of heavy components in hemicellulose-oil. To sum up, this study presented a global evolution route of heavy components in bio-oil during oxidative pyrolysis of three components.

Mechanical strength characterisation of pyrolysis biochar from woody biomass

In order for biochar (which is produced via waste wood pyrolysis) to be used in sustainable energy or material production, it is crucial to determine certain properties, such as its mechanical strength. Consequently, in this paper, municipal waste wood (MWW)11Municipal waste wood. and forestry residue biomass (FRB)22Forestry residue biomass. – as a point of reference – are examined in an autothermal pyrolysis unit. The mechanical strength of the biochar is analysed via an uniaxial compression experiment. The present paper proposes an advanced method to investigate the strength of pyrolysed biochar in a representative and comprehensive manner, since no standards are available in literature. It has been found that firstly, the mechanical biochar’s strength decreases with increasing temperature, and secondly, that biochar prepared from FRB shows a higher breaking strength than biochar produced from MWW. Furthermore, an approximately linear progression between degree of pyrolysis and the mechanical strength is observable. Successful experiments on the gasification of MWW confirm the results obtained in this study. Additional allothermal pyrolysis experiments reveal higher strength values and reflect the more precise temperature adjustment. These results can be used as a point of reference in future research on alternative biogenic energy carriers and biochar products.

Oxidative fast pyrolysis of biomass in a quartz tube fluidized bed reactor: Effect of oxygen equivalence ratio

The effect of oxygen equivalence ratio (ER) on oxidative fast pyrolysis of pine sawdust was investigated in a quartz tube fluidized bed reactor. The results showed that the introduction of oxygen into fluidizing gas would mainly oxidize the biochar, thus provided the largest contribution to heat demand during biomass pyrolysis towards autothermal operation. The increase of ER from 0 to 11.15% decreased the biochar yield from 21.03 to 9.5 wt%. The enthalpy for fast pyrolysis of pine sawdust was about 1084.49 kJ/kg, and a theoretical ER to achieve autothermal pyrolysis was thus calculated as ∼3.92%. The addition of oxygen indeed changed the formation and distribution of bio-oil. With ER of 6.47%, although the bio-oil yield decreased slightly, the light compounds in bio-oil (e.g., linear alcohols/carbonyls/acids, furan compounds and cyclopentanones/cyclohexanones) were largely consumed, while the heavy compounds (e.g., phenols/aromatic hydrocarbons and anhydrosugars) were mostly retained with the formation of anhydrosugars being promoted.

Production of purified H2, heat and biochar from wood: Techno-economic and life cycle assessment of small scale units

Hydrogen is forecasted to contribute to the energy transition. This study evaluates the production of purified H2, heat and biochar from woodchips. Three pyrogasification processes are compared: gasification (O2/H2O) with or without catalytic reforming and water gas-shift, autothermal pyrolysis. Techno-economic and life cycle assessments were conducted based on detailed mass and energy balances. On the economic front, the historical market prices of hydrogen, heat and biochar are not sufficient to reach profitability. The subsidies required to reach profitability and the avoided CO2 emissions from reference steam methane reforming (SMR) process were evaluated. It is estimated between 130 and 310€/tCO2 avoided which can be reduced with higher price of hydrogen. On the environmental front, the pyrogasification processes are more attractive than SMR regarding global warming, ozone depletion layer and fossil fuel consumption potentials. Meanwhile, the acidification potential and the photochemical oxidant formation are higher than SMR.

Autothermal pyrolysis of lignocellulosic biomass: Experimental, kinetic, and thermodynamic studies

Autothermal pyrolysis that generates in situ heat is a promising way to convert lignocellulosic biomass into value-added products. Autothermal pyrolysis of pinewood under different oxygen concentrations (i.e., 3 %, 5 %, and 10 % O2) was performed in this work. The results showed oxygen addition significantly decreased the selectivity of anhydrosugars in bio-oil by 41.90% at most. Micropores formation in the char was enhanced in the autothermal pyrolysis, and the BET surface area of char obtained under 10 % O2 was 21.9045 m2·g−1. Fraser-Suzuki function was adopted to deconvolute the biomass autothermal pyrolysis process into four pseudo-reactions; namely three degradation reactions of hemicellulose, cellulose, and lignin and a combustion reaction of char residues. The activation energies of three pseudo-degradation reactions decreased after oxygen was introduced, whereas the activation energy of pseudo-combustion reaction of char increased from 69.08 to 88.34–127.47–163.13 kJ·mol−1 as oxygen content increased from 3 % to 10 %. Three major thermodynamic parameters of activation for each pseudo-reaction were also calculated. ΔS‡ results indicated that three pseudo-degradation reactions at the transition state became more ordered at higher oxygen concentration, whereas pseudo-combustion of char showed an opposite trend. The results obtained from this study provide systematic and precise fundamental information for biomass autothermal pyrolysis scale-up.

The effect of ferrous sulfate pretreatment on the optimal temperature for production of sugars during autothermal pyrolysis

Fast pyrolysis is a promising technology for producing cellulosic sugars from lignocellulosic biomass. Success in this endeavor requires measures that prevent naturally occurring alkali and alkaline earth metals (AAEM) from breaking pyranose and furanose rings in lignocellulosic biomass. This is especially critical for high ash feedstocks like corn stover and other kinds of herbaceous biomass. Pretreating corn stover with ferrous sulfate converts AAEM into thermally stable salts, which passivates the catalytic activity of these metals and dramatically improves sugar yields. AAEM passivation in combination with autothermal (partial oxidative) operation improves the prospects for intensifying the production of sugars via fast pyrolysis. We hypothesized that pretreated biomass pyrolyzed in the presence of oxygen can substantially influence the temperature dependence of pyrolysis kinetics. We found that autothermal (air-blown) pyrolysis of ferrous sulfate pretreated corn stover achieved maximum sugar yield of 15.6 wt% (biomass basis) at 450 °C, whereas the maximum phenolic oil yield of 9.2 wt% (biomass basis) was reached at 500 °C. Considering these tradeoffs in yields of these highly desirable pyrolysis products, the ideal operating temperature is between 450 and 500 °C. These results suggest that two-stage pyrolysis might allow maximum recovery of both sugars and phenolic oil.

Investigating kinetic behavior and reaction mechanism on autothermal pyrolysis of polyethylene plastic

To reduce the energy supply for pyrolysis, autothermal pyrolysis is one of the most promising approaches to upcycle plastic waste. In this work, both single-step and multi-step methods were applied to perform kinetic studies of low-density polyethylene (LDPE) pyrolysis under oxidative atmospheres (0, 5, and 10% O2 balanced by N2) by changing the heating rate from 5 to 20 K min−1. Miura integral method was then used to estimate the apparent activation energy. The major findings showed that the multi-step method using asymmetric double sigmoidal (Asym2Sig) deconvolution procedure was more appropriate to study kinetic behaviors of LDPE autothermal pyrolysis. The activation energy needed for LDPE pyrolysis under N2 was 271 kJ·mol−1, while the activation energies of the three pseudo-reactions under 5% O2 were 71, 153 and 189 kJ·mol−1, and under 10% O2 were 74, 224 and 169 kJ·mol−1, indicating that LDPE autothermal pyrolysis was more energy-saving than conventional LDPE pyrolysis. Additionally, the reaction mechanism was proposed to provide an accurate and critical guideline for commercializing this novel technical route, which is beneficial to achieving sustainable plastic waste management and mitigating plastic pollution, simultaneously.

Optimization of temperature parameters for the autothermic pyrolysis in-situ conversion process of oil shale

In this study, a temperature optimization strategy for the Huadian oil shale autothermal pyrolysis in-situ conversion process (ATS) was first proposed by systematically investigating the reaction characteristics of various semi-cokes. As the pyrolysis temperature rised, the semi-coke's calorific value was found to undergo three different stages of increasing, decreasing, and flattening, peaking at around 330 °C. Additionally, the semi-cokes formed at different temperatures exhibited similar combustion characteristics, including combustion activation energy, combustion characteristic parameters, and product release characteristics. Due to the serious pore blockage caused by the substantial generation and the ignition coking of the bitumen, the reaction characteristics of semi-cokes were dramatically decreased at about 330 °C. Finally, the relationship between in-situ heat generation and demand at various stages of ATS process was discussed, and a reasonable strategy for the screening of temperature parameters was proposed. According to this strategy, the optimal control temperature for the preheating stage was determined at 350–370 °C and at Tact (defined in 4.3.2) for the retorting zone in the reaction stage. The results of this study provide a new perspective on the theoretical foundation of the ATS process and have crucial guiding implications for practical engineering applications.

CFD–DEM modeling of autothermal pyrolysis of corn stover with a coupled particle- and reactor-scale framework

Autothermal operation of fast pyrolysis is an efficient process-intensification technique wherein exothermic oxidation reactions are used to overcome the heat-transfer bottleneck of conventional pyrolysis. The development of accurate, reliable modeling toolsets is imperative to generating a deeper understanding of biomass autothermal pyrolysis systems to support scale-up and industrial deployment. This modeling effort describes the development of single-particle and reactor models which incorporate detailed reaction schemes and simultaneous exothermic oxidation reactions. The particle-scale model was parameterized for corn stover feedstock with particle morphology, density, ash content, and biopolymer composition, all of which impact the emergent conversion characteristics during pyrolysis. Results were then used to parameterize a reactor-scale autothermal pyrolysis model, which was developed using a coarse-grained computational fluid dynamic–discrete element method. The simulation results compared well with experimental results, with the predicted bio-oil, light gas, and biochar yield within 3.0 wt% of the experimental yields. Further analyses were performed to test the influence of equivalence ratio, biomass injection position, and particle size distribution on autothermal pyrolysis. The analysis of the physio-chemical properties of the fluid and solid phase inside the reactor and at the reactor outlet help reveal important process interactions of autothermal pyrolysis.

Investigating the Impacts of Feedstock Variability on a Carbon-Negative Autothermal Pyrolysis System Using Machine Learning

Feedstock properties impact the economic feasibility and sustainability of biorefinery systems. Scientists have developed pyrolysis kinetics, process, and assessment models that estimate the costs and greenhouse gas (GHG) emissions of various biorefineries. Previous studies demonstrate that feedstock properties have a significant influence on product costs and lifecycle emissions. However, feedstock variability remains a challenge due to a large number of possible feedstock property combinations and limited public availability of feedstock composition data. Here, we demonstrate the use of machine learning (ML) models to generate large feedstock sample data from a smaller sample set for sustainability assessment of biorefinery systems. This study predicts the impact of feedstock properties on the profitability and sustainability of a lignocellulosic biomass autothermal pyrolysis (ATP) biorefinery producing sugar, phenolic oil, and biochar. Generative Adversarial Networks and Kernel Density Estimation machine learning models are used to generate 3,000 feedstock samples of diverse biochemical compositions. Techno-economic and lifecycle assessments estimated that the ATP minimum sugar selling price (MSSP) ranges between $66/metric ton (MT) and $280/MT, and the greenhouse gas (GHG) range from a net negative GHG emission(s) of −0.56 to −0.74 kg CO2e/kg lignocellulosic biomass processed. These results show the potential of ML to enhance sustainability analyses by replacing Monte Carlo-type approaches to generate large feedstock composition datasets that are representative of empirical data.

Cfd–Dem Modeling of Autothermal Pyrolysis of Corn Stover with a Coupled Particle- and Reactor-Scale Framework

Cfd–Dem Modeling of Autothermal Pyrolysis of Corn Stover with a Coupled Particle- and Reactor-Scale Framework

Production of Purified H2, Heat, and Biochar from Wood: Comparison between Gasification and Autothermal Pyrolysis Based on Advanced Process Modeling

Production of Purified H₂, Heat, and Biochar from Wood: Comparison between Gasification and Autothermal Pyrolysis Based on Advanced Process Modeling

The role of biochar in the degradation of sugars during fast pyrolysis of biomass

With appropriate pretreament, sugars can be a major product from fast pyrolysis of lignocellulosic biomass. Analytical pyrolysis of pure cellulose can produce up to 60 wt% yield of levoglucosan although yields are significantly lower in continuouos pyrolysis at larger scales. Secondary reactions of vaporized levoglucosan are thought to be responsible for this loss of sugar yield, suggesting changes in the design and operation of pyrolysis reactors to minimize these reactions. Micropyrolysis experiments were performed to better understand the mechanism of sugar degradation in the presence of biochar. A 57% loss in levoglucosan yield was observed for cellulose overlain with untreated biochar powder compared to the pure cellulose control sample. The addition of biochar derived from pyrolysis of untreated corn stover to a fluidized bed pyrolyzer reduced sugar yields from cellulose from 61.3 wt% to 21.3 wt% and 41.5–11.6 wt% for conventional and autothermal operation, respectively. The significant drop in sugar yield due to biochar interaction inspired change in feeder configuration for the fluidized bed pyrolyzer to reduce vapor-char interactions. Biomass feeding was changed from in-bed to above-bed injection, which allowed signficant devolatilization to occur above the layer of biochar that exists at the surface of the bed. By reducing secondary reactions, bio-oil and sugar yields increased by 7.9% and 14%, respectively, for autothermal pyrolysis.

The role of catalytic iron in enhancing volumetric sugar productivity during autothermal pyrolysis of woody biomass

Passivation of naturally occurring AAEM in biomass enhances sugar yields from the fast pyrolysis of biomass by preventing these metals from catalyzing the fragmentation of pyranose rings in cellulose and hemicellulose. However, because AAEM also catalyzes lignin depolymerization, its passivation can be accompanied by undesirable char agglomeration. Pretreatment of biomass with ferrous sulfate both passivates AAEM and substitutes ferrous ions as lignin depolymerization catalysts. This pretreatment has been particularly successful for high ash biomass like corn stover, but of limited value for low ash biomass like wood. This study explores the reasons for this discrepancy and offers a combined pretreatment of ferrous sulfate and ferrous acetate pretreatment to overcome char agglomeration in wood. This new pretreatment increased sugar yields from 4.4 wt% to 15.5 wt% and 5.4 wt% to 19.0 wt% for hardwood and softwood biomasses, respectively. This pretreatment produces an iron-rich biochar that catalyzes oxidation of the biochar under the oxygen-rich conditions of autothermal pyrolysis, which is preferentially consumed to provide the enthalpy for pyrolysis, preserving bio-oil as a more desirable energy product. Instead of producing carbon monoxide, which dominates oxidation of biochar from untreated biomass, the iron catalyzes oxidation to carbon dioxide, producing more energy per mole of oxygen consumed. In fact, oxygen demand to support autothermal pyrolysis of red oak and southern yellow pine was reduced 15% by the presence of iron in the biochar.

Autothermal fast pyrolysis of waste biomass for wood adhesives

Biomass fast pyrolysis was performed in a bubbling fluidized bed reactor that incorporated two crucial innovations. A fractional condensation train provided dry bio-oils with only 1% of moisture and much reduced acidity. Autothermal pyrolysis with partial oxidation enhanced dry bio-oil quality while reducing the capital and operating costs by eliminating the need for external heating. Both endothermic pyrolysis with pure N2 and autothermal pyrolysis with oxidant fluidization gases were carried out at 500 °C. Dry bio-oils of birch wood, birch bark or hydrolysis lignin, from both endothermic and autothermal pyrolysis, could substitute up to 65 % of phenol in reacting with formaldehyde to produce wood adhesives that met the international standard specifications on mechanical strength and formaldehyde emissions. Therefore, autothermal pyrolysis did not harm the bio-oil quality for phenol substitution. Dry bio-oil from autothermal pyrolysis of kraft lignin was able to replace 80 % of the phenol.

Capture and Release of Orthophosphate by Fe-Modified Biochars: Mechanisms and Environmental Applications

Biochars have been suggested to have P capture potential from effluent streams and to recycle the captured P to agricultural soils. However, most biochars have low P sorption capacity. The objective of this study was to engineer biochar for enhanced P sorption affinity. Biochar was produced from corn stover biomass pre-treated with FeSO4 (ISIB) using autothermal (air-blown) pyrolysis at 500 °C. Point of zero charge (pHZPC) shifted from 8.48 to 4.31, indicating that Fe treatment increased the dominance of acid functional groups. Batch equilibration isotherm study showed that ISIB had 11–12 times more P sorption capacity (3763 versus 46,300 mg kg–1 and 6704 versus 48,821 mg kg–1 for non-oxidized and oxidized conditions, respectively), while P desorption rate was ∼1/3 relative to the control biochar. A column leaching study also shows that ISIB was effective for removing P from simulated agricultural effluent. XRD (X-ray diffraction) and SEM-EDS (scanning electron microscopy–energy-dispersive X-ray spectrometry) analyses showed the P sorption was predominately through inner-sphere surface complexation followed by surface precipitation and that P is preferentially sorbed by hematite (α-Fe2O3) relative to magnetite (FeIII2O3 + FeIIO) or maghemite (γ-Fe2O3). This study demonstrates that ISIB can be produced by pyrolyzing corn stover with FeSO4, and the resulting ISIB is effective for adsorption and recycling of P. When loaded with P, the ISIB can potentially be used as a slow-release P fertilizer.

Investigation of the nitrogen migration characteristics in sewage sludge during chemical looping gasification

Increasing attention has been given to the control of nitrogen pollutants during the process of using sewage sludge. In this paper, the gasification characteristics and the law of nitrogen migration in the process of chemical looping gasification (CLG) of sewage sludge were explored. Copper slag calcined at 1100 °C (1100CS), FeAl (Fe2O3+Al2O3), NiFeAl (NiFe2O4+Al2O3), and NiAl (NiO + Al2O3) were used as oxygen carrier. The addition of an oxygen carrier could increase the carbon conversion rate and decrease the low heating value. NOx precursors (HCN and NH3) were the main nitrogen-containing gas pollutants (∼40%). The sum yield of char nitrogen and tar nitrogen was only ∼10%. Nitrile nitrogen, heterocyclic nitrogen and amide nitrogen were the main nitrogenous compounds in tar nitrogen. Increasing the oxidative activity of the oxygen carrier could significantly promote the oxidative transformation of N2 from the NOx precursors, tar nitrogen, char nitrogen. Additionally, the migration of fuel nitrogen includes synchronous autothermal pyrolysis (stage Ⅰ) and oxidative pyrolysis of the three-phase product under the action of [O] (stage Ⅱ). Finally, this research realizes the energy utilization of sludge and reduces the release of nitrogen pollutants.

Oxidation of phenolic compounds during autothermal pyrolysis of lignocellulose

Fast pyrolysis is traditionally defined as the rapid decomposition of organic material in the absence of oxygen to produce primarily a liquid product known as bio-oil. However, the introduction of small amounts of oxygen to the process holds prospects of internally generating the energy needed for pyrolysis. The present study investigates the partial oxidation of lignin-derived compounds during pyrolysis, which generates both carbon oxides and aromatic carbonyl compounds. Analysis of lignin derived phenolic compounds was performed to determine if the composition had changed under oxidative conditions. NMR analyses indicates aromatic carbonyls increased under oxidative conditions, with a corresponding decrease in phenolic hydroxyl groups. Model phenolic compounds were pyrolyzed to help understand the role of partial oxidation during autothermal pyrolysis of lignocellulosic biomass.

Pretreatments for the continuous production of pyrolytic sugar from lignocellulosic biomass

Fast pyrolysis of lignocellulosic biomass yields little sugar or anhydrosugars compared to pyrolysis of pure polysaccharides because naturally abundant alkali and alkaline earth metals (AAEM) in biomass catalyze the fragmentation of pyranose and furanose rings. Sugar yields can be increased dramatically by pretreating the biomass with sulfuric acid prior to pyrolysis, which passivates the catalytic activity of the metals by converting them into thermally stable salts. However, depolymerization of lignin in biomass also depends on the catalytic activity of AAEM. Thus, passivating AAEM has the unintended consequence of slowing the rate of depolymerization and volatilization of lignin, resulting in a transient melt phase that agglomerates and can foul pyrolysis reactors.To overcome this problem, various non-alkali metal sulfates were tested as replacements for sulfuric acid. Iron in the form of ferrous sulfate proved the most effective in depolymerizing lignin without fragmenting pyranose rings. Conventional nitrogen-blown pyrolysis of ferrous sulfate pretreated corn stover achieved WHSV of 4 h−1 compared to only 0.6 h−1 for acid pretreated corn stover. Autothermal (air-blown) pyrolysis of ferrous sulfate pretreated corn stover showed even more dramatic improvement, increasing WHSV from 1 h−1 to 10 h−1 compared to acid pretreated corn stover under autothermal operation. Fermentable sugar yields from the pyrolysis of corn stover increased from 0.9 wt% to 11.8 wt% on a biomass basis, a 13-fold increase as a result of the ferrous sulfate pretreatment. These advantages combine to increase volumetric sugar productivity from 62 g L−1 h−1 for conventional pyrolysis of untreated corn stover to 2041 g L−1 h−1 for autothermal pyrolysis of ferrous sulfate treated corn stover.

Conventional and autothermal pyrolysis of corn stover: Overcoming the processing challenges of high-ash agricultural residues

The high ash content of agricultural residues and other kinds of herbaceous biomass makes it a challenging feedstock for fast pyrolysis to bio-oil. Using corn stover as a representative feedstock, this study investigates fast pyrolysis of high ash, herbaceous biomass in a pilot-scale fluidized bed reactor using both conventional, nitrogen-blown and autothermal, air-blown operation. Initial efforts to pyrolyze corn stover were challenged by bed fouling, which prevented steady reactor operation. Substitution of coarser bed material allowed operation at higher superficial velocities, which promoted attrition and elutriation of recalcitrant biochar particles from the reactor. This resulted in dramatic improvement in stable reactor operation for both conventional and autothermal pyrolysis with bio-oil yields among the highest reported for pyrolysis of corn stover. The oxygen-to-biomass equivalence ratio required for autothermal operation was 6.8%. Autothermal operation also resulted in significant process intensification, increasing corn stover throughput from 7.8 kg hr−1 to 21.9 kg hr−1 for this 8.9 cm diameter reactor. Air-blown, autothermal operation did not significantly reduce bio-oil yield despite the presence of partial oxidation reactions. Carbon balances indicate carbon yields of biochar and aqueous, bio-oil light ends decreased by 18.5% and 4.7%, respectively, during autothermal pyrolysis compared to conventional pyrolysis while the more valuable, organic-rich heavy ends of the bio-oil were essentially preserved.