Density functional theory calculations and microkinetic modeling were used to study the decomposition mechanisms of the bio-oil model compound formic acid over a cobalt-stepped surface. Zero-point-energy-corrected activation barriers and reaction energies and the rate and equilibrium constants of various elementary reactions were obtained. Formic acid dissociation likely starts from dehydrogenation and dehydroxylation, with activation barriers of less than 0.5 eV. The generation of an HCOO intermediate is thermodynamically favored, but such a compound is energetically difficult to convert. COOH formation is fast and dominant at low temperatures, and it is converted rapidly after 450 K. The most favorable formic acid decomposition pathway is HCOOH → COOH → CO. Its rate-determining step is CO–OH scission, with an activation barrier of 0.66 eV and strong exothermicity of −1.19 eV.

Computational fluid dynamics (CFD) has been widely used in both scientific studies and industrial applications of reactor-scale biomass pyrolysis. In this Perspective, the state-of-the-art progress in CFD modeling of reactor-scale biomass pyrolysis was summarized and discussed. First, because of the importance of biomass pyrolysis reaction kinetics to the predictability of CFD, the commonly used pyrolysis reaction kinetics in CFD modeling of reactor-scale biomass pyrolysis were reviewed. The characteristics of each reaction kinetics were described. Then, the theoretical basis and practical applications of three main CFD modeling approaches, i.e., porous media model, multifluid model, and discrete particle model for simulating reactor-scale biomass pyrolysis were presented. The activities and progresses with respect to each CFD modeling approach for reactor-scale biomass pyrolysis were reviewed. Aspects such as experimental validation, modeling speed, and capability were discussed. Finally, the paper was concluded with comments on future directions in CFD modeling of reactor-scale biomass pyrolysis.Keywords: Biomass pyrolysis; Computational fluid dynamics; Discrete particle model; Multifluid model; Porous media model; Reactor-scale;

Co-precipitation method was selected for the preparation of Ni/Al2O3, Ni/ZrO2 and Ni/CeO2 catalysts, and their performances in methanation were investigated in this study. The structure and surface properties of these catalysts were characterized by BET, XRD, H2-TPD, TEM and H2-TPR. The results showed that the catalytic activity at low temperature followed the order: Ni/Al2O3 > Ni/ZrO2 > Ni/CeO2. Ni/Al2O3 catalyst presented the best catalytic performance with the highest CH4 selectivity of 94.5%. The characterization results indicated that the dispersion of the active component Ni was the main factor affecting the catalytic activity and the one with higher dispersion gave better performance.Download high-res image (96KB)Download full-size image

The past decades have seen increasing interest in developing pyrolysis pathways to produce biofuels and bio-based chemicals from lignocellulosic biomass. Pyrolysis is a key stage in other thermochemical conversion processes, such as combustion and gasification. Understanding the reaction mechanisms of biomass pyrolysis will facilitate the process optimization and reactor design of commercial-scale biorefineries. However, the multiscale complexity of the biomass structures and reactions involved in pyrolysis make it challenging to elucidate the mechanism. This article provides a broad review of the state-of-art biomass pyrolysis research. Considering the complexity of the biomass structure, the pyrolysis characteristics of its three major individual components (cellulose, hemicellulose and lignin) are discussed in detail. Recently developed experimental technologies, such as Py-GC–MS/FID, TG-MS/TG-FTIR, in situ spectroscopy, 2D-PCIS, isotopic labeling method, in situ EPR and PIMS have been employed for biomass pyrolysis research, including online monitoring of the evolution of key intermediate products and the qualitative and quantitative measurement of the pyrolysis products. Based on experimental results, many macroscopic kinetic modeling methods with comprehensive mechanism schemes, such as the distributed activation energy model (DAEM), isoconversional method, detailed lumped kinetic model, kinetic Monte Carlo model, have been developed to simulate the mass loss behavior during biomass pyrolysis and to predict the resulting product distribution. Combined with molecular simulations of the elemental reaction routes, an in-depth understanding of the biomass pyrolysis mechanism may be obtained. Aiming to further improve the quality of pyrolysis products, the effects of various catalytic methods and feedstock pretreatment technologies on the pyrolysis behavior are also reviewed. At last, a brief conclusion for the challenge and perspectives of biomass pyrolysis is provided.

•Effect of pyrolysis temperature on the properties of PKSC was systematicly investigated.•Higher temperature promoted content of C, the value of HHV and pH.•Higher temperature led to the reduction of the content of the surface functional group but increase of the graphitization degree.•SBET reached the maximum value of 403.99 m2/g at 650 °C.Biochar, produced from the biomass slow pyrolysis technology, has been widely applied to the industry and agriculture. This paper investigated the effect of the pyrolysis temperature (250, 350, 450, 550, 650, and 750 °C) on the chemical composition, functional group, pore structure and crystallographic structure of biochar from palm kernel shell (PKS). The basic properties of biochar was mainly tested by a serial instruments including thermogravimetric analyzer (TGA), fourier transform infrared spectrometry (FTIR), automatic adsorption analyzer, X-ray diffractometer (XRD), Raman spectra, and solid-state 13C nuclear magnetic resonance spectra (13C NMR). The results showed that as the pyrolysis temperature increased: (1) the carbon content, HHV and pH value reached their maximum values of 78.95%, 31.55 MJ kg−1 and 10.03 at 750 °C, respectively, (2) the content of the surface functional group decreased, (3) the oxygen-contained carbon structure including O-alkyl-C and carboxylic-C decreased from 11.71% and 3.72% to 1.12% and 1.11%, respectively, while the aryl-C structure increased from 65.98% to 93.18%, (4) the specific surface area (SBET) reached the maximum value of 403.99 m2 g−1 at 650 °C. This systematic study on the evolution of the basic physicochemical characteristics will provide a good reference for the high value-added application of PKSC.Download high-res image (224KB)Download full-size image

•The addition of La2O3 to Al2O3 remarkably enhanced the catalyst performance.•Ni/La-3Al performed best among Ni/xLa-yAl catalysts with the highest H2 yield.•The high performance was maintained for 30 h with deactivation less than 10%.The effect of replacement of γ-Al2O3 by La2O3 was studied on Ni catalysts for hydrogen production via acetic acid steam reforming. The La/(La + Al) weight ratio ranged from 0 to 1 in the catalyst support prepared by co-precipitation method. Over the Ni/La-3Al catalyst (the La/(La + Al) weight ratio at 0.25), the carbon conversion and hydrogen yield reached 100% and 72.72%, respectively, which was obviously higher than other catalysts at 700 °C, S/C = 1 and LHSV = 10 h−1. The effect of S/C, LHSV and stability test were studied in detail over Ni/La-3Al catalyst, whose high activity maintained for more than 30 h.

For the purpose of building a green reaction system to produce furfural (FF), the conversion of two important pentoses from hemicellulose, namely xylose and arabinose, was investigated in an aqueous reaction system including a Lewis acidic ionic liquid as a catalyst and renewable γ-valerolactone (GVL) as a co-solvent. The results showed that the introduction of GVL greatly improved the reactivity of pentose and inhibited the secondary decomposition reaction of FF compared to a pure-water reaction system. NMR analysis suggested that the composition of pentose conformers was greatly altered towards a reactive distribution. The highest FF yields were 79.76% (from xylose) and 58.70% (from arabinose), which were obtained at 140 °C. The influence of reaction parameters on pentose conversion was also studied. A comparison between different reaction conditions suggested that arabinose had less reactivity than xylose, leading to its lower conversion rate and FF yield. Furthermore, xylan and real biomass materials were tested in the proposed reaction system, and decent FF yields of up to 69.66% (from xylan) and 47.96% (from corn stalk) were obtained.

The complex composition, high degree of unsaturation, and strong corrosiveness of bio-oil make direct catalytic upgrading problematic due to coking and device corrosion, and therefore, proper pretreatment is required. In this study, catalytic hydrogenation-esterification was identified as an efficient pretreatment method for bio-oil upgrading, and the hydrogenation-esterification behavior of typical compounds found in the acid-rich fraction of bio-oil over Cu/SBA-15 catalyst was investigated. It was found that furfural, hydroxyacetone, and guaiacol could be synergistically transformed with acetic acid (AcOH) during hydrogenation-esterification. Meanwhile, a higher reaction temperature promoted AcOH conversion and increased the degree of hydrogenation. On the basis of the quantification of the main products, the reaction pathways in the presence of furfural, hydroxyacetone, or guaiacol were proposed. Finally, the hydrogenation-esterification of simulated bio-oil was performed at 300 °C. The results showed that furfural, hydroxyacetone, and guaiacol were efficiently hydrogenated, and AcOH was almost completely converted into ethanol and esters, with the fraction of acid compounds drastically declining from 25.0 wt % to 0.4 wt %. Thus, a high quality pretreated bio-oil fraction, mainly consisting of alcohols and esters and ready for subsequent upgrading, was obtained in a long-term test.Keywords: Acid-rich fraction; Bio-oil; Hydrogenation-esterification; Model compound mixture; Reaction pathway;

•Competitive degradation of crystalline and amorphous regions caused CrI change.•Cleavage of glycosidic bond and dehydration of hydroxyl occurred during torrefaction.•Am-DAEM was used to analyze the raw and torrefied cellulose pyrolysis kinetics.•Torrefaction changed cellulose pyrolytic products distribution greatly.The influence of torrefaction on cellulose structural characteristics and the resulting pyrolysis behavior was investigated in this study. Torrefaction reduced O/C ratio in cellulose and increased its high heating value. The crystallinity of cellulose increased slightly first and then decreased sharply with the increase of torrefaction temperature, which could be ascribed to competitive degradation between crystalline region and amorphous region, as indicated by 13C CP/MAS NMR analysis. Besides, the cleavage of β-1,4-glycosidic bond and the dehydration of hydroxyl were the major reactions occurring in torrefaction. Avrami-Erofeev model was found to be the most suitable kinetic reaction model for explaining the thermogravimetric weight loss during the pyrolysis of the raw and torrefied cellulose. A distributed activation energy model based on Avrami-Erofeev model was subsequently used to reveal the pyrolytic kinetics. It was found that the changes in cellulose structure influenced the kinetic parameters greatly. Torrefaction also changed pyrolytic product distribution. The yields of furfural, alicyclic ketones and anhydrosugars increased while that of 5-hydroxymethyl-furfural decreased as torrefaction temperature increased.

A mild hydrogenation of simulated bio-oil was carried out in a fixed-bed reactor. Based on the experimental results, 300 °C/4 MPa was chosen as an optimum condition for the mild hydrogenation, the simulated bio-oil was nearly completely converted. Besides, the liquid product selectivity achieved 85.0% and its (H/C)eff was significantly promoted from 1.266 to 1.554. The liquid composition was greatly improved and a notable decrease of phenols and acids contents was observed. In this case, the product activity was significantly enhanced and then the subsequent catalytic cracking was favored.

•The structure of humin by products from glucose and xylose were characterized.•Pyrolysis behavior of humins were compared using TG and Py-GC/MS.•The kinetics of humins pyrolysis were analyzed by FWO and Friedman method.•Furans and phenols were the primary volatile products from humins pyrolysis.•Increasing pyrolysis temperature favored aromatization of products.Humin, a solid by-product formed during the acid-catalyzed conversion of carbohydrate to platform chemicals, can be further utilized by pyrolysis technology. In this study, two humins derived from glucose (humin-g) and xylose (humin-x) were characterized by elemental analysis, FTIR and 13C NMR. Both humins were polymers rich in furan rings, and their carbon contents were more than 66%. Thermogravimetry and pyrolysis kinetic analyses based on Flynn-Wall-Ozawa and Friedman methods suggested that the activation energies for both humins increased as their conversion rates increased. At low conversion rates, the activation energy of humin-x was higher than that of humin-g, but this was reversed at high conversion rates. The results suggest that humin-g has better thermal stability following the removal of weak linkages at low temperature. Finally, Py-GC/MS was employed to study the product distribution from humin pyrolysis. The mechanisms for formation of typical products and the influence of temperature on product distribution are discussed.

Thermochemical conversion from biomass to higher alcohols is a promising route for manufacture of higher alcohols in an environmentally sustainable way. In this study, a production system of biomass gasification and subsequent higher alcohol synthesis was designed and simulated. Two core modules of the processes (biomass gasification and alcohol synthesis) were constructed, and the optimum reaction conditions were discussed in detail. The results showed that the lower ER (the equivalence ratio of oxygen/fuel) (ER = 0.2) could increase the CO content in the gasification product. By changing the mass ratio of steam to biomass (S/B), hydrogen-rich syngas could be obtained for higher alcohol synthesis. For the synthesis of higher alcohols from syngas, higher temperature was conducive to CO conversion but also can increase the content of byproduct such as CO2 and CH4. Higher pressure and H2/CO (1.0–2.0) were both in favor of the production of higher alcohols. Then the process simulation was based on two cases, which were mainly divided by the difference in power supply sources. In case 1, part of the electricity demand was provided by an external power source while, in case 2, the process was devised to be completely self-powered by adding in a gas and steam combined cycle power unit. An assessment of the material flow, energy consumption, energy efficiency, and exergy flow of the two cases was made according to the simulation results. The alcohol yield was 25.1 and 19.4 wt % for case 1 and case 2, respectively. And the energy efficiencies of the two cases were relatively close to each other (34.1% and 33.2%, respectively). About 33.5% exergy input was converted to alcohols in case 2, lower than case 1 (39.6%). The exergy loss in the power generation unit of case 2 was higher, resulting in a higher total exergy loss. For case 2 the electricity demand can be balanced by the system itself without any fossil fuel usage, which was quite attractive and promising from the aspect of environment protection and cost competitiveness.

•SR and SESR of acetic acid for H2 production were simulated in Aspen Plus.•Both higher hydrogen yield and purity were obtained by SESR reactor.•Higher exergy efficiency at 66.80% was found in SESR system.•Chemical reactions and temperature difference resulted in main exergy destruction.The simulation and exergetic analysis on hydrogen production from conventional steam reforming (SR) and sorption enhanced steam reforming (SESR) of acetic acid, as a major component in the aqueous bio-oil fraction, are presented in this work. The simulation of acetic acid SR and SESR systems were performed in Aspen Plus software where the main chemical reactors used in this work were Gibbs reactors based on Gibbs free energy minimization method. The equilibrium compositions of acetic acid SR and SESR reactors were investigated at different conditions including temperature (200–1000 °C) and steam to carbon molar ratio (S/C, 1–4.5). The increase of temperature and S/C favored hydrogen production in both SR and SESR processes. Higher purity and yield of hydrogen was obtained via SESR reactor than SR reactor. In addition, less CO and no coke were formed in SESR reactor, which showed the advantages of SESR over SR. The exergy efficiencies of acetic acid SR and SESR systems were 61.80% and 66.80%, respectively. The burner and reforming reactor were responsible for major exergy destruction in both SR and SESR systems. At the same time, large exergy destruction was also found in CO2 separation process in SR system. These results and the simplification of hydrogen production system show the better performance of SESR, which can be a promising way for green hydrogen production via aqueous bio-oil fraction.

Cellulose and hemicellulose were isolated directly from softwood Pinus armandii Franch by the alkali method for pyrolysis mechanism study. Structural characterization based on FTIR, 1H NMR, and 13C NMR showed that the hemicellulose was mainly composed of galactoglucomannan and arabinoxylan with abundant branches, while the cellulose was mainly composed of β-1,4-glucan with a few residual hemicellulose fragments. Pyrolysis of extracted cellulose showed high char yield due to the lower crystallinity. Hemicellulose pyrolysis showed a shoulder peak at low temperature and high char yield because of its abundant branches. A double-Gaussian distributed activation energy model was introduced for pyrolysis kinetics analysis. The devolatilization process for both cellulose and hemicellulose was dominated by a parallel decomposition reaction pathway. The distributions of their pyrolysis products were analyzed by Py-GC/MS. It is suggested that the utilization of isolated sample as model compound could give a better understanding of the biomass pyrolysis mechanism.

•Ordered crystalline structure of cellulose was destroyed during torrefaction.•Deacetylation, depolymerization and aryl ether cleavage were revealed by solid NMR.•The parallel reaction contributions to devolatilization were analyzed by 3G-DAEM.•Abundances of typical pyrolytic products largely changed after torrefaction.The influence mechanism of torrefaction on biomass pyrolysis was studied in detail. 13C CP/MAS NMR was employed to characterize the evolution of functional groups during torrefaction, indicating that hemicellulose was extensively deacetylated and depolymerized; the crystalline structure of cellulose was destroyed; and the aryl ether linkages and propyl side branches in lignin were cleaved. A distributed activation energy model with three combined Gaussian functions was introduced to analyze the pyrolysis kinetics of biomass. The mean activation energies for three parallel reactions remained unchanged after torrefaction, while the weighing factors proved that their contributions to devolatilization largely changed. Acetic acid yield significantly decreased after torrefaction, with reduction of the yields of 5-hydroxymethylfurfural and levoglucosan because of the depolymerization of carbohydrates during torrefaction. Due to the dissociation of propyl branches and the demethylation of methoxyls in lignin, pyrolysis of torrefied biomass yielded more guaiacol and catechol and less phenols with C4-propyl groups.

Bio-oil upgrading by catalytic cracking faces problems of low aromatic hydrocarbon yield and coking because of its hydrogen-lacking property. In this work, an improved hydrogenation-cocracking process was developed to achieve the appropriate hydrogen supply in two stages. Furfural was selected as the model compound of bio-oil, and methanol was the coreactant. The hydrogen supply in the hydrogenation stage was successfully regulated by hydrogenation catalysts, and the 5Ni-5Cu/SiO2 catalyst exhibited the best performance because of a suitable degree of hydrogenation of furfural over it, which favored the formation of aromatic hydrocarbons during cracking. With the optimum 5Ni-5Cu/SiO2 catalyst, hydrogenation cocracking was compared with single-stage cocracking, hydrogenation cracking, and direct cracking processes. The results confirmed that hydrogen supplements by hydrogenation pretreatment and methanol cocracking were both significant for the generation of aromatic hydrocarbons and the suppression of coke. On the basis of a proposed reaction mechanism, the corresponding hydrogen supply behaviors were revealed.

Tar contained in gasification syngas is one of the most problematic species that restricts the gasification efficiency and system availability. The catalytic decomposition of tar is a novel technology to clean the produced syngas. In this article, the interactions of cellulose and polyvinyl chloride (PVC) on the catalytic cracking of tar contained in municipal solid waste gasification syngas were experimentally studied. Results show that at the pyrolysis temperatures of 500 and 600 °C the catalytic cracking efficiency was 91–98% for tar derived from a single feedstock. When cellulose was mixed with PVC, the tar cracking efficiency decreased to 68–90% because of the interactions during the copyrolysis and decomposition reactions. Gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS) were employed to identify the main compositions of gaseous product and hydrocarbon species after cracking. The GC–MS results show that 99% of tar species have a carbon number less than 10 when the calcined dolomite catalyst was used. During the copyrolysis, PVC plays a dominant role in the formation of tar. The GC analysis shows that copyrolysis yields more CH4, C2H4, C2H6, and C2H2 and less CO than those from a single feedstock. Brunauer–Emmett–Teller, scanning electron microscopy, energy dispersive spectroscopy, and X-ray fluorescence analyses were used to characterize the catalyst before and after reaction. Results indicate that after mixing, the poisoning of catalyst by chlorine increased by 3–15 times, accounting for the low conversion ratio of tar derived from a mixture of cellulose and PVC.

Sugars are among the main compounds in bio-oil produced by biomass pyrolysis. However, they can easily form coke, resulting in fast deactivation of the catalyst and severe blockage of the reactor during catalytic upgrading of bio-oil. Molecular distillation may be performed to retain sugars and pyrolytic lignin in the heavy fraction and thus to provide a primary stage for the subsequent sugar separation and utilization. A new method involving extraction, heat treatment, and column chromatography was introduced to separate further the aqueous phase obtained from the bio-oil heavy fraction by methanol–water extraction. Thus, monophenols and sugars were coextracted through these combined separation technologies. A fraction rich in monophenols was obtained by solvent extraction. Other impurities were further removed by heat treatment and column chromatography. An alcohol sedimentation method further separated the combined sugar-rich fraction into an ethanol-soluble fraction (SF-1) and an ethanol-insoluble fraction (SF-2). Gas chromatography–mass spectrometry, Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and high-performance liquid chromatography were performed to characterize sugars in the obtained fractions. SF-1 mainly contained monosaccharides such as levoglucosan, glucose, and xylose, whereas SF-2 still had a small amount of cellobiose besides monosaccharides. The ultimate recovery rates for several identified monosaccharides were in the range of 75–86 wt %.

•A two-stage continuous hydrogenation-cracking reaction process was developed.•The reactivity and stability of reaction were improved by mild hydrogenation.•Aromatic hydrocarbons constituted more than 90% in oil phase.In order to reduce the coke formation and improve catalyst activation in the catalytic cracking of bio-oil, ethanol addition and mild hydrogenation pretreatment were introduced to improve the cracking stability. According to the analysis on the bio-oil molecular distillation, several typical compounds were selected as the representative of bio-oil distillation. Based on the two-stage fixed-bed reactor, the effect of mild hydrogenation on the cracking behavior of bio-oil model compound mixture was studied in detail. In a single-stage cracking process, the selectivity of oil phase was only 28–23 wt%, with the formation of abundant oxygenated byproducts. When mild hydrogenation was introduced before the cracking, the selectivity of oil phase increased up to 40wt% at 8 h, with an aromatic hydrocarbon content of over 90%.Download full-size image

•Model-fitting method, isoconversional method and DAEM method were combined.•Cellulose pyrolysis followed one-step Avrami-Erofeev nucleation model.•Pyrolysis of hemicellulose and lignin contained three and two stages, respectively.•Calculation method of DAEM with Am reaction model was developed for the first time.This study proposed a sequential and coupling method to determine the comprehensive kinetic models for pyrolysis of cellulose, hemicellulose and lignin. Isoconversional method was employed to find the correlation between activation energy and conversion rate. Cellulose pyrolysis could be interpreted by one-step global reaction model, while pyrolysis of hemicellulose and lignin could be divided into three stages and two stages, respectively, in which competitive parallel reactions occurred. Coats-Redfern method and Kissinger method were subsequently used to obtain the reaction model f(α). Particularly, a new mean reaction model f′(α) was introduced to deal with the complex competitive parallel reactions. It was proposed that pyrolysis of cellulose followed Avrami-Erofeev (m = 2) nucleation model, while pyrolysis of hemicellulose and lignin could be described by reaction-order model. The other kinetic parameters and the contribution of each parallel reaction to devolatilization were further analyzed based on the modified distributed activation energy models. And the detailed kinetic models for pyrolysis of biomass components were finally obtained.

5-Hydroxymethylfurfural (HMF) is a bio-based platform chemical that may be converted into various chemicals and fuels. In the present study, we developed an advanced low-boiling single-phase reaction system for producing HMF from glucose. It consists of water and 1,2-dimethoxyethane (DMOE) and uses AlCl3 as catalyst. Our results show that introduction of DMOE can substantially enhance HMF production because of the polar aprotic solvent effect provided by DMOE. Under optimal conditions, a high HMF yield (58.56%) was obtained. GC-MS of the liquid-phase products revealed that HMF and furans comprised 80% and ∼90% of the detected products. Formation of liquid-phase products, including furans, oxygenated aliphatics, cyclopenten-1-ones, and pyrans is discussed. Further study of the humins formed during glucose conversion showed the effective inhibition of humin formation by DMOE. The structure of humins was characterized by FTIR spectroscopy. Finally, HMF production from disaccharides (sucrose, maltose and cellobiose) and polysaccharide (cellulose) using the water–DMOE system resulted in good yields, demonstrating that our single-phase water–DMOE solvent system has good potential use in HMF production from glucose and complex carbohydrates.

•Coal ash shows promise as a catalyst support.•Fe and AAEM oxides in coal ash enhance catalytic activity during reforming.•A high hydrogen yield of 89.6% was achieved on the 10Ni–Fe/ZDA catalyst.The development of efficient and inexpensive catalysts will make bio-oil steam reforming a more economical and promising technology. By comparing the catalytic activities of different coal ashes based on the chemical composition, the effects of AAEM (alkali and alkaline earth metals) and Fe were confirmed. The catalytic activity of coal ash for acetic acid reforming was effectively improved by adding Fe, and superior catalysts of Ni–Fe/ZDA were prepared by further adding 5 and 10 wt% Ni. The carbon conversion of acetic acid and H2 yield over 10Ni–Fe/ZDA achieved 100% and 89.6% at 700 °C, respectively.

•Extraction of the bio-oil heavy fraction yielded 19.88 wt% pyrolytic lignins.•Two pyrolytic lignins with high molecular weights had similar structures.•Low-molecular-weight pyrolytic lignin contained abundant active functional groups.Different fractions of bio-oil could be enriched with various chemical families by molecular distillation. The distilled fraction could be upgraded by catalytic esterification, cracking, or steam reforming, whereas the heavy fraction was difficult to dispose of. In this study, we adopted the methanol–water method for primary separation of pyrolytic lignins and sugars in the heavy fraction to improve the efficiency of utilization of the heavy fraction. Ultimate analysis, gel-permeation chromatography, Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and pyrolysis–gas chromatography/mass spectrometry were employed to characterize the pyrolytic lignins obtained by methanol–water and water extraction methods. The pyrolytic lignins consisted of similar elements, and their basic structures included etherified and non-etherified syringyl and guaiacyl units. However, low-molecular-weight pyrolytic lignin from the heavy fraction differed from the high-molecular-weight pyrolytic lignin and water-extracted pyrolytic lignin in molecular weight distribution, side chains, and interunit linkages. As the low-molecular-weight pyrolytic lignin consisted of tri- to pentamers and 0.14/Ar carbonyls, it had high reactivity. Interunit linkages of the three pyrolytic lignins contained β–β′ resinol moieties, while the low-molecular-weight pyrolytic lignin had the most abundant alkyl ether linkages.Graphical abstract

Using corn stover as a raw material to produce fuel ethanol has good application prospects. The processes of ethanol synthesis from corn stover using direct and indirect thermochemical conversion methods have been developed. After the system boundaries were determined, a life cycle assessment was conducted to evaluate whether the two processes would be environmentally friendly. It was found that the energy efficiency of direct ethanol synthesis process (direct case) was lower (0.38) than that found for the indirect process (indirect case), which was 0.41. The same tendency was found with the alcohol yield. The alcohol yields in the direct case and indirect case were 23 and 30 wt %, respectively. Then, the environmental impacts including global warming potential and acid potential of the two processes were compared and discussed. Finally, the distribution of the environmental impact values for the two ethanol synthesis processes was presented. With regard to the environmental impact, the indirect case was superior to the direct case, showing good promise in future applications.

This study investigated the changes in biomass physicochemical characteristics during torrefaction and its influence on the resulting pyrolysis behavior. Torrefaction reduced biomass hemicellulose content and increased the high heating value and the mass energy density. Two-dimensional perturbation correlation analysis, based on diffuse reflectance infrared Fourier transform spectroscopy, showed that the main reactions occurring during torrefaction were dehydration, deacetylation, and cleavage of ether linkages. A distributed activation energy model with three Gaussian functions and weighting factors was used to study the pyrolysis kinetics, and it was found that as the torrefaction temperature increased, the contribution of lower activation energy parallel reactions to devolatilization decreased, while condensation became more important. The yields of acids and furans from the pyrolysis of torrefied biomass also decreased. The lignin side branches were cleaved during high temperature torrefaction, resulting in lower yields of phenols with side branches and increased production of phenols without side branches.

•The decomposition mechanism of acetic acid has been studied by DFT calculations.•Calculations have been focused on the active stepped Co surface.•Four likely parallel decomposition pathways have been identified.•Acetone formation and decomposition have also been calculated.To elucidate bio-oil catalytic reforming mechanisms, acetic acid has been selected as a model compound to carry out density functional theory calculations on its decomposition pathways over an active Co stepped surface. The adsorption energies and stable geometries of the reactant and important reaction intermediates have been obtained. Preferential adsorption on a step as opposed to a flat surface has been discerned. The activation barriers and reaction enthalpies for various elementary reactions involved in acetic acid decomposition have also been obtained. Four likely parallel decomposition pathways with activation barriers of less than 0.9 eV have been identified. The most kinetically favorable decomposition pathway is CH3COOH → CH3CO → CH2CO → CH2 → CH. The rate-determining step is CH3CO dehydrogenation, with a reaction barrier of 0.52 eV. In addition, the formation and primary decomposition pathways of the critical intermediate acetone have also been investigated.

•The structures of PL and MWL from the same biomass were characterized.•Pyrolysis behaviors of PL and MWL were compared using TG–FTIR.•Double-Gaussian DAEM was introduced to analyze the thermal reaction kinetics.•The relationship between chemical structure and pyrolysis behavior was discussed.Pyrolytic lignin (PL), the main water-insoluble fraction in bio-oil, has an obvious negative effect on the application of biomass pyrolysis technology. The structures of PL and milled wood lignin (MWL) have been characterized and compared using FTIR, 1H NMR, 13C NMR and GPC. The PL was extracted from bio-oil produced by pyrolysis of a hardwood, lauan, while the MWL was isolated directly from the same lauan. The results show that PL is composed mainly of trimers and tetramers, and its average molecular weight is about one tenth of that of MWL. The proportion of methoxy groups and ether linkages in PL were lower than that in MWL. However, PL had a larger amount of unconjugated CO functional groups and saturated aliphatic structures than MWL. Furthermore, a thermogravimetric (TG) study reveals that PL has poor thermal stability and decomposes over a lower temperature range. The double-Gaussian distributed activation energy model (DG-DAEM) is introduced to analyze the thermal reaction kinetics of PL and MWL. The apparent activation energies of PL and MWL are distributed mainly in the first Gaussian region. The evolution characteristics of typical products from the pyrolysis of PL and MWL are also discussed and compared in detail.

•New Ni/nano-Al2O3 was used for the reforming of bio-oil model compounds.•It showed high activity for phenol, acetic acid and hydroxyacetone reforming.•Long stability was achieved over the catalyst.•Possible decomposition pathways for these three organics were proposed.Catalytic reforming of three typical bio-oil model compounds, phenol, acetic acid and hydroxyacetone, has been carried out over a Ni/nano-Al2O3 catalyst. Al2O3, in the form of nano-rods of length approximately 40 nm, was selected as the catalyst support. The catalyst showed superior performance in terms of activity and stability. The conversions for phenol, acetic acid and hydroxyacetone reached 84.2%, 98.2% and 98.7%, respectively, at the reaction temperature of 700 °C. The corresponding hydrogen yields were 69%, 87% and 97.2%. The catalyst maintained its high reactivity for more than 10 h in the catalytic reforming of three model compounds. The influences of steam to carbon ratio, catalyst loading and Ni content in the catalyst on the reforming performance were also investigated. In addition, the possible decomposition pathways for phenol, acetic acid and hydroxyacetone are proposed.

•The bio-oil water-insoluble phase was multi-step separated efficiently.•The content of phenolic compounds in fraction B was up to 94.35%.•The high- and low- molecular-weight pyrolytic lignins were successfully separated.•The low-molecular-weight pyrolytic lignin contained more phenolic hydroxyl groups.To realize a high-value utilization of the water-insoluble phase of bio-oil, acid and alkaline solutions combined with organic solvents have been employed to separate the monophenols and pyrolytic lignins from this phase. The phenolic fraction B, obtained by reactive extraction of the water-insoluble phase, was found to be rich in phenolic compounds, with a high concentration of 94.35%, wherein the content of guaiacols reached 48.27%. Furthermore, Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and gel-permeation chromatography (GPC) analyses of the pyrolytic lignins showed the primary structural units to be of the guaiacol and syringol types. Polymers with molecular weight higher than 1000 dominated in the high-molecular-weight pyrolytic lignin, whereas the low-molecular-weight pyrolytic lignin contained more reactive phenolic hydroxyl groups.Graphical abstract

•Coal ash has the potential to be a catalyst support in bio-oil catalytic reforming.•Ni/Ash catalyst showed high activity in catalytic reforming reaction.•Ni/Ash catalyst exhibited superior stability in the reforming reaction.The development of a high performance and low cost catalyst is an important contribution to clean hydrogen production via the catalytic steam reforming of renewable bio-oil. Solid waste coal ash, which contains SiO2, Al2O3, Fe2O3 and many alkali and alkaline earth metal oxides, was selected as a superior support for a Ni-based catalyst. The chemical composition and textural structures of the ash and the Ni/Ash catalysts were systematically characterized. Acetic acid and phenol were selected as two typical bio-oil model compounds to test the catalyst activity and stability. The conversion of acetic acid and phenol reached as much as 98.4% and 83.5%, respectively, at 700 °C. It is shown that the performance of the Ni/Ash catalyst was comparable with other commercial Ni-based steam reforming catalysts.

In view of the severe coke formation and catalyst deactivation during crude bio-oil cracking, an innovative cracking technology based on bio-oil molecular distillation is proposed. The distilled fraction (DF) from bio-oil molecular distillation is enriched with small molecular acids and ketones and has enhanced cracking behavior compared to crude bio-oil. The influence of the reaction temperature, pressure, and the DF/ethanol ratio in the feed was studied. It was found that co-cracking of the DF and ethanol produced a well-defined gasoline phase, and both increasing the reaction temperature and adopting pressurized cracking benefited the yield and quality of this gasoline phase. Using optimum reaction temperature and pressure, co-cracking of the DF and ethanol, with different weight ratios, all generated high-quality gasoline phases. Under 400 °C and 2 MPa, co-cracking of DF and ethanol with a weight ratio of 2:3 produced a high gasoline phase yield of 25.9 wt %; the hydrocarbon content in this gasoline phase was 98.3%.

Biochar derived from the fast pyrolysis of lauan was activated to develop its pore structure and used as a catalyst support in the methanation of bio-syngas. The physicochemical properties of the support and the ruthenium (Ru)/activated biochar (ABC) catalysts used were characterized using multiple characterization techniques. The effect of Ru loading on bio-syngas methanation was investigated using a range of ABC supported Ru catalysts. CO conversion was low during bio-syngas methanation due to the low H2 content and was increased upon increasing the Ru loading. However, increasing the H2/(CO + CO2) ratio in bio-syngas by addition of H2 significantly improved the conversion of CO and CO2. The CO2 conversion was increased to 17.5% and 55%. CO conversion was 74% and 97% and the selectivity of CH4 reached 84% and 92% using a H2/(CO + CO2) ratio of 2 and 4, respectively.

Hydroxypropanone was selected as the model compound of bio-oil ketones to study its cracking behavior for bio-gasoline production. The cracking of pure hydroxypropanone generated inferior oil phase which had a content of high oxygenated compounds. The quality of oil phase was obviously improved when ethanol was adopted as the co-reactant. Furthermore, increasing cracking pressure also benefited the yield and quality of the oil phase. The conversion yield of the reactants reached 100% and the selectivity of the oil phase reached 31.9 wt.% in the co-cracking process under 400 °C and 2 MPa. The oil phase also had an outstanding quality with a hydrocarbon content of 100% and mainly contained hydrocarbons with carbon numbers ranging from 7 to 10, which are also important components in commercial gasoline. Based on the results, a co-cracking mechanism was proposed to illustrate the role of ethanol in promoting hydroxypropanone cracking.Highlights► Hydroxypropanone was co-cracked with ethanol for gasoline production. ► Catalyst deactivation was suppressed by co-cracking with ethanol. ► Oil phase was successfully produced with the highest yield of 31.9%. ► Relative content of liquid hydrocarbons in the oil phase reached 100%.

•Water-rich fraction was obtained via bio-oil molecular distillation.•Hydrogen production via bio-oil reforming without steam addition was proposed.•Organics adsorption and decomposition were performed by DFT calculations.•Reactivity on Ni(111) is: furfural > hydroxyacetone > acetic acid > phenol.A novel process for hydrogen production via bio-oil catalytic reforming without steam addition was proposed. The liquid feedstock was a distillation fraction from crude bio-oil molecular distillation. The fraction obtained was enriched with the low-molecular-weight organics (acids, aldehydes, and ketones), and contained nearly all of the water from crude bio-oil. The highest catalytic performance, with a carbon conversion of 95% and a H2 yield of 135 mg g−1 organics, was obtained by processing the distillate over Ni/Al2O3 catalyst at 700 °C. The steam involved in the reforming reaction was derived entirely from the water in the crude bio-oil. The fresh and spent catalysts were characterized by N2-physisorption, thermogravimetric analysis, and high-resolution transmission electron microscopy. To further understand the reaction mechanisms, symmetric density functional theory calculations for decomposition were performed on four model compounds in bio-oil (acetic acid, hydroxyacetone, furfural, and phenol) over the Ni(111) surface. In addition, the decomposition of H2O∗ to OH∗ and O∗ and their subsequent steam reforming reactions with carbon precursors (CH∗ and CH3C∗) were also examined.

An experimental study of the improved Fischer–Tropsch synthesis was conducted over a series of Ni-promoted Co/CNT catalysts to investigate the influence of Ni content on the synthesis of liquid hydrocarbon fuel (C5–C20). The catalysts were prepared using the impregnation method and were systematically characterized by N2 physisorption studies, X-ray diffraction, transmission electron microscopy, and hydrogen temperature-programmed reduction. The Ni promoter played a significant role in product distribution. The long-chain hydrocarbons were effectively hydrocracked because of the activity of Ni in C–C bond cleavage. A proper degree of Ni promotion could maximize the production of liquid hydrocarbon fuel. The highest selectivity for liquid hydrocarbon fuel (61.6%) and a CO conversion of 92% were obtained over the 20 wt % Co/CNT catalyst promoted by 0.5 wt % Ni.

Organosolv lignin has been selected to investigate the thermal behavior of lignin over zeolites by using a thermogravimetric analyzer coupled with a Fourier-transform infrared spectrometer (TG-FTIR). The chemical structure of this lignin has been determined by 1H NMR to obtain the distribution of main functional groups such as methoxyl groups and free aliphatic and phenolic hydroxyl groups. All three zeolite catalysts tested, HZSM-5, H-β, and USY, exerted significant influences on the dehydration reaction in the initial stage, the deoxygenation reaction of oxygenated compounds such as methanol and phenols, and the char-forming process during lignin pyrolysis in the range 30–800 °C. The dehydration reaction was enhanced in the order USY > HZSM-5 > H-β, while char formation was suppressed in the reverse order. The presence of HZSM-5 and H-β catalyzed the conversion of both oxygenated compounds and chars into the low-molecular-weight gases CO, CO2, and methane. The addition of USY clearly aided decomposition of the oxygenated compounds, but had little effect on the char degradation.Highlights► The distribution of main functional groups in Organosolv lignin is determined. ► All three zeolite catalysts, HZSM-5, H-β, and USY, have significant influences on the pyrolysis of this lignin. ► The dehydration reaction was enhanced in the order USY > HZSM-5 > H-β. ► The char formation was suppressed in the reverse order H-β > HZSM-5 > USY.

An experimental study on the catalytic steam reforming of acetic acid was initially performed over a series of co-precipitated Co–Fe unsupported catalysts at relatively low temperatures. It was found that the catalyst activity increased with increasing cobalt content, and the highest performance, with an acetic acid conversion of 100% and an H2 yield of 96% was obtained over pure cobalt catalyst at 400 °C. The catalysts have been systematically characterized by BET, XRD, and HRTEM. The results revealed that the superior activity and stability of pure cobalt catalyst can be ascribed to small particle size, coexistence of metallic cobalt and CoO, and stable H2O adsorption. Furthermore, the mechanism route of acetic acid decomposition on cobalt surface was proposed via DFT calculations.Graphical abstractHighlights► Co–Fe catalysts have high activity and stability at low temperature. ► Acetic acid and H2O adsorptions were performed by DFT calculations. ► The real effective component is cobalt rather than iron. ► H2O reforming with CHx is an essential step in the reaction. ► Acetic acid decomposition mechanism was proposed.

Steam reforming of bio-oil derived from the fast pyrolysis of biomass is an economic and renewable process for hydrogen production. The main objective of the present work has been to investigate the effects of the preparation method of Ni/Al2O3 catalysts on their performance in hydrogen production by bio-oil steam reforming. The Ni/Al2O3 catalysts were prepared by impregnation, co-precipitation, and sol–gel methods. XRD, XPS, H2-TPR, SEM, TEM, TG, and N2 physisorption measurements were performed to characterize the texture and structure of the catalysts obtained after calcination and after their subsequent use. Ethanol and bio-oil model compound were selected for steam reforming to evaluate the catalyst performance. The catalyst prepared by the co-precipitation method was found to display better performance than the other two. Under the optimized reaction conditions, an ethanol conversion of 99% and a H2 yield of 88% were obtained.

There has been much interest in the utilization of biomass-derived fuels as substitutes for fossil fuels in meeting renewable energy requirements to reduce CO2 emissions. In this study, the pyrolysis characteristics of biomass have been investigated using both a thermogravimetric analyzer coupled with a Fourier-transform infrared spectrometer (TG-FTIR) and an experimental pyrolyzer. Experiments have been conducted with the three major components of biomass, i.e. hemicellulose, cellulose, and lignin, and with four mixed biomass samples comprising different proportions of these. Product distributions in terms of char, bio-oil, and permanent gas are given, and the compositions of the bio-oil and gaseous products have been analysed by gas chromatography–mass spectrometry (GC–MS) and gas chromatography (GC). The TG results show that the thermal decomposition of levoglucosan is extended over a wider temperature range according to the interaction of hemicellulose or lignin upon the pyrolysis of cellulose; the formation of 2-furfural and acetic acid is enhanced by the presence of cellulose and lignin in the range 350–500 °C; and the amount of phenol, 2,6-dimethoxy is enhanced by the integrated influence of cellulose and hemicellulose. The components do not act independently during pyrolysis; the experimental results have shown that the interaction of cellulose and hemicellulose strongly promotes the formation of 2, 5-diethoxytetrahydrofuran and inhibits the formation of altrose and levoglucosan, while the presence of cellulose enhances the formation of hemicellulose-derived acetic acid and 2-furfural. Pyrolysis characteristics of biomass cannot be predicted through its composition in the main components.

The Cu/SiO2 catalysts were in situ synthesized by the hydrolysis of tetraethyl orthosilicate (TEOS) in one phase solution using ethanol as co-solvent or TEOS/H2O two phases solution, followed by the precipitation of copper on SiO2 by ammonia evaporation. In the hydrogenation of dimethyl oxalate, the catalyst prepared by one phase hydrolysis exhibited higher activity and ethylene glycol (EG) selectivity at lower temperature than that of two phases due to its larger BET surface area and multimodal pore distribution. At 488–503 K, the catalyst prepared in one phase solution with water/ethanol (W/E) volume ratio of 3:1 exhibited 90–95% EG selectivity, while catalyst prepared by two phase hydrolysis reached 90% EG selectivity only at 498–503 K.

Dimethyl ether (DME) has received growing attention due to its potential use as a multi-purpose fuel. A new technical route of improved two step synthesis is proposed for DME production, which is composed of methanol synthesis and methanol dehydration in a fixed-bed reactor. The influences of the operating conditions including reaction pressure, temperature, H2/CO mole ratio in the syngas and space velocity on CO conversion, selectivity and yield of DME are investigated. CO conversion and DME yield both increase monotonically with the pressure increase. The optimal reaction temperatures for the synthesis and dehydration of methanol are different. CO conversion increases at first and keeps constant when the H2/CO mole ratio is above 2. DME yield increases obviously and then decreases gradually with the space velocity increase. The optimal conditions are obtained to maximize the CO conversion and DME selectivity. The reaction temperatures of the top and bottom stage are in the range of 270–280 °C and 235–245 °C, respectively. The optimal ratio of H2/CO is above 2, and the space velocity is in the range of 1000–1300 h− 1.

In order to enhance the condensation efficiency during molecular distillation experiment, crude bio-oil was subjected to pretreatment consisting of traditional vacuum distillation. Most of the water in the crude bio-oil was removed first, and then this was used as the feed bio-oil for two molecular distillation processes. Four fractions were obtained under the operation parameters of 80 °C, 1600 Pa and 80 °C, 340 Pa. The yields of distilled fractions 1 and 2 were 26.36% (w/w) and 22.58% (w/w), respectively. The distilled fractions were rich in low molecular weight carboxylic acids and ketones, so they had better flow ability than the residual fractions. The water in the residual fractions was almost completely removed and the heating values of residual fractions 1 and 2 reached 21.29 MJ/kg and 22.34 MJ/kg. The integrated separation characteristic of bio-oil was explored by a macro-level evaluation model. It based on the distribution properties of functional groups and gave information that carboxylic acids, ketones and furans had better distillation characteristics than aldehydes, phenols and sugars. A self-defined separation factor β was used in the micro-level evaluation model to specify the separation characteristics of 15 typical compounds. It indicated that acetic acid and 2-propanone, 1-hydroxy had a high separation factor of 0.9 while 5-hydroxymethyl, furaldehyde and 1,2-benzenediol had a separation factor as low as 0.04.Graphical abstractResearch highlights▶ Thermally sensitive bio-oil was effectively separated by molecular distillation. ▶ High fraction yields were achieved without coking problems. ▶ Distilled fractions have lower viscosity and simpler chemical composition. ▶ Separation characteristic of bio-oil was studied according to evaluation models.

Cu/SiO2 catalysts were prepared by separate impregnation and deposition precipitation methods for the hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG). XRD, TEM, H2-TPR, SEM, EDS and N2 physisorption were performed to characterize the textural and structural properties of the catalysts. The results showed that Cu particles from the deposition precipitation preparation were homogeneously dispersed on the support and their sizes were found to be smaller than those from the impregnation method and the catalyst produced by the deposition precipitation method gave higher EG yields at lower reaction temperatures and lower H2/DMO mole ratio.

In this study, KDL5 molecular distillation apparatus manufactured by the UIC Corporation was adopted to separate bio-oil, which came from a bench-scale fluidized-bed fast pyrolysis reactor at a feeding rate of 1 kg/h. A maximum distillate yield of 85% was obtained without obvious coking or polymerization during the molecular distillation process. The effect of distillation temperature on physical and chemical characterization of each bio-oil fraction was investigated. Statistical calculations showed that molecular distillation was successful in the separation of bio-oil. A separation factor was proposed to reflect the ability of isolating the chemicals contained in the bio-oil using molecular distillation.

An intermediate product that was yellow, soluble, and solid was obtained in a high-radiation flash pyrolysis reactor. Under two different radiant heat fluxes, the yields tended to both increase initially until achieving a steady state, and then increase again with the progress of reaction. The compositional analysis of the yellow product was performed on high performance liquid chromatography (HPLC). It was indicated that the product mainly consisted of oligosaccharides, glucose, levoglucosan, methylglyoxal and so on. The compounds including oligosaccharides such as cellobiose and cellotriose, and monosaccharides such as glucose were regarded as active cellulose. Under the higher heat flux, the relative yield of the active cellulose increased initially, followed by a decreasing trend, and achieved a maximum mass fraction of 68% (w) in the soluble yellow product. The oligosaccharides with higher degree of polymerization (DP) were the primary components. Under the lower heat flux the yield of active cellulose was relatively lower, achieving a maximum of about 57% (w), and more saccharides with lower DP were contained. It was suggested that active cellulose was quite unstable at high temperature, and easily decomposed into saccharides with lower DP, even char, volatiles, and gaseous products. Finally an improved mechanism was proposed to describe the reaction route of formation and consequent evolution of active cellulose during cellulose pyrolysis.

•Ethanol synthesis from DME is kinetically faster than that from syngas.•Ethanol synthesis from DME is thermodynamically favored.•The pathway is DME dissociation to CH2, followed by CO insertion and hydrogenation.•Pathways for methane, acetic acid, and methanol formations were also identifiedThe reaction mechanism of ethanol synthesis from dimethyl ether (DME) and syngas was studied via density functional theory calculations. Various possible pathways for ethanol formation, and byproduct formations of methanol, acetic acid, methane, carbon dioxide, and water over an active cobalt stepped surface were calculated. The most favorable pathway for ethanol synthesis starts with the dissociation of dimethyl ether to CH2, followed by carbon monoxide insertion to form CH2CO, and then undergoes successive hydrogenations to give ethanol. CH3CHO hydrogenation to CH3CHOH becomes the rate–determining step with a reaction barrier of 1.48 eV. DME decomposition and CH2 carbonylation occur easily with low barriers of 0.61 eV and 0.48 eV, respectively. Carbon monoxide insertion into CH2 is more facile than into CH3 and CH. Hydrogenation at the carbon atom occurs prior to the oxygen atom in the order of α–carbon > carbonyl carbon > oxygen. The calculations demonstrate that ethanol synthesis via DME carbonylation and hydrogenation is thermodynamically favored and is kinetically faster than that via syngas direct synthesis. Methane and acetic acid are two dominant competing byproducts.Download full-size image

Acetic acid was selected as the model compound representing the carboxylic acids present in bio-oil. This work focuses the co-cracking of acetic acid with ethanol for bio-gasoline production. The influences of reaction temperature and pressure on the conversion of reactants as well as the selectivity and composition of the crude gasoline phase were investigated. It was found that increasing reaction temperature benefited the conversion of reactants and pressurized cracking produced a higher crude gasoline yield. At 400 °C and 1 MPa, the conversion of the reactants reached over 99% and the selectivity of the gasoline phase reached 42.79% (by mass). The gasoline phase shows outstanding quality, with a hydrocarbon content of 100%.

Physicochemical properties of bio-oil obtained from fast pyrolysis of rice husk were studied in the present work. Molecular distillation was used to separate the crude bio-oil into three fractions viz. light fraction, middle fraction and heavy fraction. Their chemical composition was analyzed by gas chromatograph and mass spectrometer (GC-MS). The thermal behavior, including evaporation and decomposition, was investigated using thermogravimetric analyzer coupled with Fourier transform infrared spectrometer (TG-FTIR). The product distribution was significantly affected by contents of cellulose, hemicellulose and lignin. The bio-oil yield was 46.36% (by mass) and the yield of gaseous products was 27% (by mass). The chemicals in the bio-oil included acids, aldehydes, ketones, alcohols, phenols, sugars, etc. The light fraction was mainly composed of acids and compounds with lower boiling point temperature, the middle and heavy fractions were consisted of phenols and levoglucosan. The thermal stability of the bio-oil was determined by the interactions and intersolubility of compounds. It was found that the thermal stability of bio-oil was better than the light fraction, but worse than the middle and heavy fractions.

Inert solvent was selected to extract corresponding extractives from different kinds of biomass. In this study, pyrolysis of the extractives on thermogravimetric analyzer coupled with Fourier transform infrared spectroscopy (TG-FTIR) was studied, and the influence of extractives on biomass pyrolysis was also discussed. The results indicate that extractives have distinct differences in composition and product distribution due to the different amount of guaiacyl and syringyl units in lignin component. Manchurian ash contains more methanol and methane caused by the decomposition of phenols at high temperature. Compared with raw biomass, extracted biomass has higher activation energy and release main products earlier; the yield of water, CO2, CO, and aldehydes increases, whereas the yield of acids and alkanes decrease.

•La2O3 was better than Al2O3 as the catalyst support due to basic feature.•Ni performed better than Co by suppressing CH4 formation.•The high performance was maintained for 38 h with less than 10% decrease.A comparative research on the catalytic activity of La2O3 and γ-Al2O3 supported Ni and Co catalysts was conducted for hydrogen production via acetic acid steam reforming. Supported on the special La2O3, Ni and Co showed higher H2 and CO2 selectivities due to the basicity and CO2 absorption characteristics compared with that on γ-Al2O3. Moreover, Ni catalysts performed better than Co by largely suppressing CH4 formation. Ni/La2O3 presented the best activity towards hydrogen production by totally converting acetic acid with a hydrogen yield of 80.9% at 700 °C. And it was found that S/C = 2.5 was essential for sufficient hydrogen yield. The high performance of Ni/La at S/C = 2.5 and T = 700 °C was stably maintained for 38 h. In addition, the decrease in activity was kept less than 10% and the final hydrogen yield was at about 73%.

In this study, a β-O-4 dimer with abundant oxygen substituents was successfully synthesized, and was used as the model compound for lignin to study the pyrolysis mechanism. Py-GC/MS (micro pyrolyzer coupled with gas chromatography/mass spectrometry) was employed to identify the distribution of pyrolytic products under temperatures of 150–850 °C. It was found that the yields of methoxylated monoaromatics underwent tendencies of first increase and then decrease at high temperature. Polyaromatics and benzofuran were only detected at high temperature. Based on experimental analysis, a detailed pyrolysis kinetic model was developed by combining the density functional theory (DFT) and the transition state theory (TST). The homolysis of Cβ-O was the most favorable route for the initial depolymerization of dimer rather than the concerted retro-ene fragmentation. For the evolution of intermediate products, the breakage of ether bond and the keto-enol tautomerization of enols were the most favorable routes, while intramolecular cyclization and group dissociation exhibited high energy barriers and low reaction rates. The methoxyl in guaiacol preferred to undergo demethylation leading to the formation of catechol.

•The bio-oil water-insoluble phase was multi-step separated efficiently.•The content of phenolic compounds in fraction B was up to 94.35%.•The high- and low- molecular-weight pyrolytic lignins were successfully separated.•The low-molecular-weight pyrolytic lignin contained more phenolic hydroxyl groups.To realize a high-value utilization of the water-insoluble phase of bio-oil, acid and alkaline solutions combined with organic solvents have been employed to separate the monophenols and pyrolytic lignins from this phase. The phenolic fraction B, obtained by reactive extraction of the water-insoluble phase, was found to be rich in phenolic compounds, with a high concentration of 94.35%, wherein the content of guaiacols reached 48.27%. Furthermore, Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and gel-permeation chromatography (GPC) analyses of the pyrolytic lignins showed the primary structural units to be of the guaiacol and syringol types. Polymers with molecular weight higher than 1000 dominated in the high-molecular-weight pyrolytic lignin, whereas the low-molecular-weight pyrolytic lignin contained more reactive phenolic hydroxyl groups.Graphical abstractDownload full-size image