The heteroatoms, such as oxygen, have great influence on tar decomposition reactivity. To investigate the effect of oxygen on biomass tar decomposition, thermal decomposition of three typical oxygenated tar compounds and toluene were conducted under inert and oxidative atmospheres in a flow tube reactor. The key roles of intrinsic oxygen of tar and extrinsic oxygen at controlled atmospheres were taken into consideration. Results show that all three tar compounds were easy to decompose at moderate temperatures accompanied by the polymerization reaction. 2-Methoxyphenol produced less naphthalene compared to anisole under an inert atmosphere. Intrinsic oxygen enhanced the reactivity of tar as well as inhibited the polymerization process. Moderate extrinsic oxygen addition could eliminate tar without sacrifice of combustible gases, such as H2 and CO. The reaction mechanism generator (RMG) method was used to build a complete detailed kinetic model to simulate biomass tar homogeneous conversion. The new detailed kinetic model could predict the conversion of model tar compounds as well as the real tar mixture.

•The new idea of catalytic partial oxidation on tar reduction by bio-char is proposed.•The synergy effect of partial oxidation and bio-char on biomass tar reduction, including the tar selective removal by char and oxygen, the influence of tar and oxygen on the char evolution (pore structure and surface functional groups).•High oxygen concentration coupling with char may lead to carbon deposition and bring down tar conversion rate at 800 °C.•The coupling of partial oxidation and char catalysis is a feasible method for tar reduction of biomass tar.In order to reveal the synergy effect of partial oxidation and bio-char on tar reduction and develop more efficient tar removal method, the tar and bio-char evolution properties were investigated on a bench-scaled fixed-bed reactor. The tar components, tar conversion rates, physical and chemical structure of bio-char after reaction at the second stage were sampled and analyzed. Results showed that at 700 °C, the coupling of char and oxygen could result in the significant improvement of tar conversion rate (89.32%) than both two separated method (85.1% and 86.14%). At 900 °C, the synergy effect could reach the highest conversion rate of 95.84%. High oxygen concentration coupling with char may lead to carbon deposition and bring down tar conversion rate at 800 °C. But a light amount of oxygen greatly promoted the formation of porosity. The reaction between tar and bio-char at high temperature (800 °C above) was in favor of toluene conversion. The coupling of char and partial oxidation benefited the elimination of larger PAHs tar compounds as well as toluene. BET analysis results showed that oxygen promoted the development of bio-char porosity at 700 °C and 900 °C under all oxygen concentrations. Slight amount of oxygen would benefit the char pore development, but high oxygen concentration (5%) would lead to the carbon deposition on char pore surface at 800 °C. FTIR results indicated that temperature and oxygen promoted the aromatic or graphitization of bio-char. The IR band peak of 1060 cm−1 showed the similar tendency with aromatic ring band peak, which meant more carbon deposition on the surface of char pore but not graphitization. The coupling of partial oxidation and char catalysis is a feasible method for tar reduction of biomass tar.

•Tar conversion efficiency of heterogeneous condition is much higher than that of the homogeneous condition.•The char catalytically promote the formation of alkyl monoaromatics and inhibit the formation of PAHs from the primary tars.•The addition of K2CO3 in chars increases the mass of toluene and styrene, decreases the mass of naphthalene in tar products.•The alkali metal elements might play a key role for the catalytic effect of the charA lab scale two staged hot rod reactor was constructed to stimulate the two staged downdraft biomass gasifier. The present study focused on the heterogeneous conversion of the pyrolysis tars over biomass char. A typical Chinese agricultural waste, the rice straw, was chosen for the raw material and the source of the biomass char. Effects of the temperature, the presence of the char and pretreatments of the char, including the water washed char and the char added with K2CO3 powder, on the pyrolysis tar removal were investigated. The products of tars were qualitatively and quantitatively analyzed. The evolution of the inner pore structure of chars with different temperatures and residence times was also investigated. The char bed condition exhibited higher tar conversion efficiency than the thermal cracking condition. The results indicated that the presence of the char could catalytically promote the formation of alkyl monoaromatics and meanwhile inhibit the formation of PAHs (polycyclic aromatic hydrocarbons) from the primary tars. The alkali metal elements might play a key role for the catalytic effect of the char. The appropriate increases in the temperature and the residence time for the char production could promote the development of the micro pores in chars.

Thermochemical conversion has drawn much attention in the treatment of municipal solid waste (MSW). However, the formation of dioxins still cannot be efficiently avoided during the process. In dioxin molecules, C–Cl is the weakest bond and the major difference from common hydrocarbons. Thus, the elimination of C–Cl is proposed as a new way for the control of dioxin. With sufficient H2, all of the Cl atoms, including those in C–Cl and those in active chlorines, should be transferred into HCl under proper conditions. To study the effect of H2 on C–Cl and find out the reaction conditions needed for C–Cl elimination, experiments of C2H4 chlorination and C2H3Cl dechlorination were carried out on a homogeneous flow reaction system. Results indicated that the chlorination of C2H4 was strengthened by the increase in the temperature. A high temperature also promoted polymerization, and C6Cl6 reached its maximum at 900 °C. The supplement of H2 was able to inhibit the chlorination of C2H4 and ensure the cracking of C–Cl in C2H3Cl. The Cl transfer process benefited from the rise of the temperature, and under a temperature above 700 °C, the formation of C–Cl can be prevented and the complete dechlorination of C2H3Cl can be achieved. A residence time of 1.0 s was also needed for the full transfer of Cl into HCl. On the basis of these results, via introducing a homogeneous conversion of the H2-contained syngas, gasification may become a promising way for the clean treatment of MSW.

The oxidative pyrolysis properties of cellulose, xylan, lignin, and rice straw were studied by thermogravimetric analysis–differential scanning calorimetry (TGA–DSC) coupled with mass spectrometry. The mass loss, reaction heat, and volatile release properties were analyzed to reveal the role of oxygen in the biomass thermal degradation process. Differential thermogravimetry (DTG) results show that the primary mass loss peak was brought forward with the increase of the oxygen concentration for all samples as well as the peak value. Oxygen improved the degradation rate of lignocellulose. The oxidative pyrolysis processes of all four types of material were accompanied by energy consumption or release, generally divided into three stages: moisture release stage, primary pyrolysis stage, and char evolution or oxidation stage. The primary degradation of cellulose under inert and 1% O2 atmospheres was distinctly endothermic. With the increase of the oxygen concentration, the endothermic peak decreased, while an exothermic peak dominated the oxidative process. Xylan and lignin showed an exothermic primary degradation peak even under an inert atmosphere at the primary pyrolysis stage, and with the increase of the oxygen concentration, the reaction heat released at the primary and char oxidation stage increased. Rice straw showed weak endothermic properties in the primary stage. Volatile compound analysis of oxidative pyrolysis indicated that oxygen promoted the yields of water and permanent gas compounds, such as CO2, CO, and CH4. The yield of condensable compounds, such as benzene, reached a maximum at a mediate oxygen concentration, and too much oxygen would lead to being combusted out completely. Diffuse reflectance infrared Fourier transform (DRIFT) spectra of three model compounds and rice straw under inert and oxidative atmospheres indicated that oxygen played a less important role at a low-temperature stage, especially for cellulose, which was kind of a uniform structure with less active function groups. Heterogenous oxidation at a relatively high temperature (>400 °C) would lead to the degradation of some weak bonds and benefited the formation of an aromatic ring.

The tar problems are the major limit for development of biomass gasification. Biomass char has been proven to be an economical and effective catalyst of tar destruction for both utilizations inside the gasifier and in downstream processes after the gasifier. In order to investigate the mechanism of catalytic cracking of tar over a biomass char bed, experimental research was performed in a bench-scale tube flow reactor, choosing rice straw char as the catalyst bed, naphthalene as the model tar compound, and argon as the inert atmosphere. The effects of temperature (700–1000 °C), tar concentration, time on stream (0–330 min), presence of syngas, and pretreatment of char (treated with deionized water or Ni(NO3)2 solution) on tar conversion were evaluated. The variation of the inner pore structure of biomass char during the process of tar removal was also investigated. Results showed that the original biomass char exhibited good catalytic activity in tar cracking and better stability compared with Ni(NO3)2 pretreated char. However, because of the naphthalene cracking reaction, soot was formed on the active sites of the inner pore surface of the char, leading to the deactivation of the char. A rapid decline in specific surface area of the char was observed from 262 to 4.6 m2/g when the test had begun to run for 5 min with high tar concentration (25 g/Nm3) and a temperature of 800 °C. The presence of syngas in the atmosphere could slow the process of deactivation of char.

Oxidative pyrolysis of pinewood was studied on a bench-scaled fixed-bed reactor. The qualitative and quantitative analysis of oxidative pyrolysis products, including permanent gases (CO, CO2, and CH4), water, char, and tar, was conducted. Two important parameters (temperature and oxygen concentration) were taken into consideration. Results showed that oxygen improved the yields of permanent gas and water but decreased the yields of char and tar. In comparison to char and water, oxidative pyrolysis had a greater effect on permanent gas and tar yields. CO and CH4 were mostly released between 300 and 400 °C, while CO2 was produced at all of the temperature investigated. CO2 was always the dominant gas in all cases. At a relatively low temperature (300 °C), the adsorption of an oxygen molecule on the reactive center and the subsequent decarbonylation reaction lead to the production of CO2. Little CO and CH4 generated when the temperature was higher than 400 °C. Gravimetric results of pyrolysis tar indicated that the tar yield decreased from 0.3321 g/g of biomass (700 °C and 0% O2) to 0.1901 g/g of biomass (700 °C and 21% O2). Gas chromatography/mass spectrometry results showed that, under an oxidative atmosphere, primary tar components tended to be converted to secondary tar. The phenols would also be converted by the partial oxidation reaction under high oxygen concentrations. Oxygen promoted the development of the pore structure when the oxygen concentration was no more than 15%. However, oxygen would restrict the further development of the char pore under ultimate conditions, resulting from the high char combustion rate at high oxygen concentrations.

Oxidative pyrolysis of pine wood was studied by thermogravimetric analysis (TGA) coupled with mass spectrometer (MS) and differential scanning calorimetry (DSC) methods. The effects of oxygen concentration on pyrolysis behavior, carbon oxide production and heat properties were investigated. Several parameters were defined to evaluate the oxygen influence. It was found that oxygen dramatically promotes the oxidative degradation and char oxidation rate. The reactivity index was found to be proportional to the oxygen concentration, which suggested that oxidative degradation reactions were under increasingly kinetic control in elevated oxygen concentration environments. Carbon oxides evolution properties were investigated. There are two releasing peaks in MS curves for oxidative condition comparing with one peak under inert condition. They are related with oxidative degradation and char oxidation, respectively. Both total amounts and rates of carbon oxides emission were found to increase with oxygen concentration. The cumulative emission ratio of CO to CO2 first decreases then increases with oxygen concentration with 10% as turning point. It may be caused by different oxygen diffusion behaviors with variable oxygen concentrations. The absolute reaction heat value of oxidative pyrolysis (−7.23 MJ kg−1, 5% O2) is much larger than that of inert condition (+0.28 MJ kg−1). Increasing of oxygen concentration results in an increase of heat emission. Comparing with pine wood low heat value, the net heat emission efficiencies under different oxygen concentrations (5%, 10%, 15%, 21%) are 39.73%, 44.84%, 68.90% and 78.41%, respectively.Highlights► Several indexes were defined to quantify the effect of oxygen on pyrolysis. ► Reactivity index shows that oxidative pyrolysis is under kinetic control. ► Emission index shows that CO/CO2 behave differently with 10% O2 as turning point. ► The absolute reaction heat is calculated considering the effect of heat capacity. ► Heat emission efficiencies under different oxygen concentration are derived.

An experimental and kinetic study of NOx reduction by reburning using syngas from updraft biomass gasification has been carried out in this paper. A 10 kW updraft gasifier close-coupled to a 20 kW entrained flow combustion reactor was the experimental system. Experiments were carried out in a temperature range of 1000–1200 °C, and the stoichiometric ratio in the reburn zone (SR2) was in the range of 0.6–1.0. Phenol (C6H5OH) was selected as a model compound for tar according to GC-MS analysis of gasification tar components. Experimental results of NO reduction by syngas with tar revealed that NO reduction efficiency of approximately 85% could be achieved at SR2 = 0.7 and reactor wall temperature = 1200 °C. In order to model the syngas reburning process with tar, a kinetic mechanism which consisted of 22 global reactions concerning tar conversion and NO reduction was set up and implemented in a two-dimensional computational fluid dynamics (CFD) program to simulate the NO reduction effect. The modeling results are discussed and compared with experimental data. It is shown that the combustion and reburning process can be reasonably modeled with the kinetic mechanism under the experimental conditions, and the effect of tar on NO reduction should not be neglected in the syngas reburning process, especially when the tar content is high.

Tar in biomass product gas is a major problem because it reduces the reliability of the equipment and enhances pollutant emission. Partial oxidation is effective for reducing tar inside gasifiers. In order to predict tar reduction with partial oxidation technology, knowledge of the fate of tar during partial oxidation process is paramount.A continuous reactor was developed for tar destruction experiment under partial oxidation environment. Tar components and non-condensable gasses were measured and analyzed. Results show that the defined “primary tar” contents share about 60% of the total tar amounts. Phenolics prevail under initial reaction, but it is easy to form PAHs (polycyclic aromatic hydrocarbons) by polymerization reactions under certain ER (actual mass flow rate of oxygen/oxygen required for complete oxidation of biomass feed stock). This process results in an increase of tertiary tar. Hydrogen's evolution curve shows a steep rise with ER, which could be an indicator for the crack of secondary tar. A model was developed for describing partial oxidation of tar. Phenol, toluene, benzene and naphthalene were chosen as model compounds. Fluid flow, chemical mechanism and heat transfer are considered in the model. The calculated total tar amounts are in qualitative agreement with experimental results.Highlights► A continuous reactor was developed for tar partial oxidation. ► Primary tar contents share about 60% of the total tar amounts. ► Phenolics prevail in the “secondary tar”, which is easy to form PAHs. ► Increase of ER results in an increase of “tertiary tar”. ► Fluid flow, chemical mechanism and heat transfer are considered in the model.

The present work studies the combustion of biomass syngas to characterize the NO-reduction by tar, benzene being selected as the representative model tar component. Experiments were carried out in a tubular flow reactor at atmospheric pressure and at different operating conditions i.e. an equivalence ratio of 0.5 to 2.5, temperatures between 1173 K and 1673 K and a reaction residence time of 50 to 100 ms. Kinetic parameters were determined from the experimental results. A simplified scheme of NO-reduction by benzene was presented and the effects of operating variables were concluded. The NO conversion is favored by an oxygen-rich condition, and the reduction efficiency falls with the rise of equivalence ratio, especially in the range of 0.5–1.0. Although high temperature enhances the reduction reactions, soot formation at high temperatures (1573 K, 1673 K) will seriously hinder the reduction of NO, which should be prevented. The ideal temperature is about 1473 K. 50 ms is found to be an insufficient residence time for the reduction. An increased contact time is of higher benefit at 1673 K where the heterogeneous reactions between NO and graphitized heavy hydrocarbons have a lower reaction rate. High pre-exponential factors and low activation energies are achieved under oxygen-rich conditions.

In order to study the mechanism of biomass tar formation and elimination in a two-stage downdraft gasifier, the nascent rice straw pyrolysis tar evolution properties under homogeneous/heterogeneous decomposition conditions have been investigated in a constructed lab-scale two-stage reactor by varying factors as temperature, concentration and reforming agents of CO2/H2O/O2, and char bed heights. The nascent tar was produced in the first stage reactor and then decomposed in the second stage with different reforming agents or char beds. In the first stage, the results showed that nascent pyrolysis tar yields increased with increasing pyrolysis temperature, tar was mainly produced during 200–400 °C, and 400–500 °C would be a proper pyrolysis temperature range in commercial operation due to little effect on tar yields in higher temperature. In the second stage, it can be observed that nascent biomass tar was converted into polycyclic aromatic hydrocarbons (PAHs) (even soot), thermally stable one ring aromatics, and noncondensable gases in homogeneous conditions with increasing temperature. Different effects were obtained in varying tar species under different homogeneous reforming agents. However, benzene, toluene, styrene, phenol, and naphthalene are the most typical compounds, accounting for 50–75% in total tar concentration at 900 °C in all decomposition conditions. Char bed can selectively reduce PAH species remarkably and increase the toluene yields. As for the three reforming agents, steam showed the highest efficiency in tar elimination, while CO2 and O2 present will induce OH, H, and O radicals formation, which increases hydrocarbon conversion. The mechanism of tar destruction in a two-stage downdraft gasifier can be concluded as follows: nascent tar yields from the pyrolysis stage will be first reformed into PAHs, thermally stable one ring aromatics and noncondensable gases in the throat region, and then PAHs species are almost completely decomposed by the char bed, which are the main troublesome tar components in syngas, and finally the syngas with low tar was obtained.

The formation and destruction of pyrolysis tar during the thermal cracking and fuel-rich oxidation have been investigated in a constructed test rig. Temperatures of 700–1100 °C and equivalence ratios (ERs) of 0–0.403 were considered, and yields of gravimetric tar, gas, water, and soot were taken into account. In inert conditions, pyrolysis tar thermal cracking was greatly enhanced with the temperature increasing. CO and CH4 increased almost linearly, and H2 increased exponentially from 700 to 1100 °C; meanwhile, oxygen-containing compounds or substituted 1-ring aromatics were converted into polycyclic aromatic hydrocarbons (PAHs). In the homogeneous reactor, the presence of oxygen induced more tar decomposition compared to inert thermal cracking. When the ER increased from 0 to 0.403 with a constant reactor temperature of 900 °C, total tar yields reduced rapidly and reached a minimum value of 0.26 wt % at an ER of 0.34; meanwhile, the mass of noncondensable gases reached a maximum value. However, the mass of combustible gases, such as H2, CO, and CH4, were sharply reduced as the ER increased from 0.34 to 0.403. Although the aromaticity index increased gradually, most aromatic compound yields increased first and then decreased with the ER increasing, except naphthalene. It is considered that a proper oxygen/fuel ratio can promote the free-radical formation and accelerate the tar destruction, but excess oxygen will burn out most combustible gases.

Agricultural biomass has some drawbacks such as high moisture content, low energy density and wide distribution and as a result, the cost of transport and storage are high. Moreover, raw biomass has poor grindability so its use in a pulverized boiler or entrained flow gasifier is difficult. Torrefaction is a mild pyrolysis process carried out at temperatures ranging from 200°C to 300°C to deal with these problems. The cotton stalk and wheat straw were torrefied in a fix-bed reactor at moderate temperatures (200°C, 230°C, 250°C, 270°C and 300°C) under N2 for 30 min. The biomass chars after torrefaction had higher energy density and improved grindability characteristics compared with raw biomass and they also showed hydrophobic characteristics. The volatiles consist of a condensable fraction and a non-condensable fraction. The former mainly contained water and tar (organic products but mainly acetic acid). The non-condensable products are typically comprised of CO2, CO and a small amount of CH4 and even trace H2. The volatiles increased with an increase in the torrefaction temperature but the solid yield and the energy yield decreased. However, the grindability and energy density of the biomass char showed great improvement. A kinetic study on the generation of the main non-condensable gases was undertaken and we conclude that the gases are formed by parallel independent first-order reactions. Characteristic kinetic parameters for the generation of each gas were determined.

Torrefaction is a main pretreatment technology for improving the properties of agricultural biomass in order to deal with such problems as high bulk volume, high moisture content and poor grindability. Two typical agricultural residues, rice straw and rape stalk were torrefied in a vertical reactor at 200 °C, 250 °C and 300 °C for 30 min, under inert atmosphere. The product distribution profiles of solid, liquid and gases were obtained. The grindability of the torrefied biomass was evaluated by the particle size distribution after being milled in a ball mill. It was found that temperature strongly affected the torrefied biomass and the type of feedstock influenced the conversion rate due to the different volatile content in raw biomass. An increase of torrefaction temperature leads to a decrease in solid bio-char yield and an increasing yield in the volatile matters including liquid and non-condensable gases. The maximum increase of the heating value of the torrefied residue compared with the raw material is 17% for the rice straw and 15% for the rape stalk, respectively. On the other hand, the torrefied residues are liable to be pulverized. A kinetic study on the generation of main non-condensable gases was accomplished, which shows that the gases are formed through parallel independent first-order reactions. The kinetic characteristic parameters for the generation of each gas were determined. A novel method which combined torrefaction with co-gasification to improve the efficiency of biomass utilization is promising.

A lab-scale two-stage reactor has been constructed for studying the release and destruction of tars in the two-stage gasifier. First, the pyrolysis characteristics of three fuel samples are investigated only using the single stage reactor. The results show that the maximum value of tar yield is: rice straw 25%, corn straw 22%, and fir sawdust 31% of the initial fuel. Then, the experimental program is extended to investigate the effect of operating conditions in the second stage of the reactor on tar removal. The effects of temperature, residence time, char particle size, char type, fuel type, and diluted air feeding to throat on tar emission has been studied. The results show that the tar decreased with increasing temperature and residence time and with decreasing char particle size. The char type has little effect on tar reduction. Tar emission with limited diluted air feeding is obviously less than that with empty second stage due to the more reactive radicals produced in oxidative conditions. The straw tars appear to have a different suite of compounds than the other two samples of derived material and presumably have different cracking pathways. The tars collected from first stage and second stage have been characterized by gas chromatography/mass spectrometry (GC/MS) and gel permeation chromatography (GPC). The results indicate that tar after pyrolysis contains a large amount of oxygenated constituents. With the increasing of reaction severity (from the empty heated second stage to heated second stage with char bed), the tar compounds reacted further (polymerized) to form larger molecular mass material. It is clear that the material characterized by GC/MS represents a very small part of the total tar. The results have shown that the tar emission from two-stage gasifier can reduce to low levels using optimized operating conditions, but complete tar removal is difficult to realize due to manipulation of operating parameters and fuel type.

A combustion instability detection method that uses the wavelet detail of combustion pressure fluctuations is put forward. To confirm this method, combustion pressure fluctuations in a stoker boiler are recorded at stable and unstable combustion with a pressure transducer. Daubechies one-order wavelet is chosen to obtain the wavelet details for comparison. It shows that the wavelet approximation indicates the general pressure change in the furnace, and the wavelet detail magnitude is consistent with the intensity of turbulence and combustion noise. The magnitude of the wavelet detail is nearly constant when the combustion is stable, however, it will fluctuate much when the combustion is unstable.