Reaction kinetics and producer gas compositions of steam gasification of coal and biomass blend chars, part 2: Mathematical modelling and model validation

In gasification of biomass, coal and blended biomass and coal, there are two steps including an initial pyrolysis process followed by gasification of solid char. The latter process is a slow process and thus dominates the whole gasification. In our previous paper (Xu et al., in press), the differences between steam gasification of biomass chars and that of coal chars have experimentally been investigated and the results show that these differences are mainly due to the difference in microstructures of these two fuels. In this work, a mathematical model of char gasification is developed based on reaction kinetics and gas transportation of both the producer gas and the gasification agent (steam). The model also includes mass conservation equations for each of the gas components and solid carbon involved in the gasification process. This has resulted in a set of highly nonlinear differential equations which have been solved using a numerical technique to predict gas production rate, gas compositions and carbon consumption rate during the gasification.The developed mathematical model is validated using experimental results reported in previous paper (Xu et al., in press), and close agreement between the simulation results and the experimental values have been observed. From the modelling, it has been confirmed that the char gasification is mainly determined by the characteristics of char matrix including the exposed surface area and the micro-pore size. The former determines intrinsic reaction rate and the latter influences the intra-particle mass transportation. Biomass char has more amorphous structure, thus the intrinsic reaction rate is enhanced. For coal char, the larger pore size enables the high transport rate of the gasification agent (water vapour) into the char particles but the resultant gases have higher resistance to transfer through compact clusters. For simulation of the blended biomass and coal, the blend properties were determined based on the blend proportion of each fuel. The close agreement between the simulation results and experimental data suggests that the approach in this work can adequately quantify the gasification kinetics and the gas composition.     

On the intrinsic reaction rate of biomass char gasification with carbon dioxide and steam

The gasification of beech wood char and oil palm shell char with carbon dioxide and steam was studied. To avoid heat and mass transport limitations during gasification, the amount of char, particle size and flow rate were varied in isothermal experiments. A rate expression of the Langmuir–Hinshelwood-type was applied to match the experimental data at different partial pressures and reaction temperatures in the intrinsic regime. Furthermore, the reactive surface area (RSA) of the biomass chars was determined as a function of the degree of conversion by the temperature-programmed desorption technique (TPD). The results show that the reaction rate is in general proportional to the RSA. The surface related reaction rates for the studied biomass chars are comparable to surface related reaction rates for coal chars at similar reaction temperatures.     

Factors affecting steam gasification rate of low rank coal char in a pressurized fluidized bed

A high-pressure bubbling fluidized bed reactor was used to study the steam gasification of coal char under pressure. Indonesian sub-bituminous coal char (Adaro) and Australian lignite char (Loy Yang) were gasified with steam in the reactor at temperatures below 1173 K and at total pressures ranging from 0.1 to 0.5 MPa. The steam gasification rates of the coal chars were determined by analysis of the gaseous products. Activation energies for the steam gasification of the chars were as high as about 250 kJ/mol, which suggests that the temperature dependence of the gasification was substantial. The apparent gasification rates under the study conditions were described by a Langmuir–Hinshelwood (L–H)-type equation. Analysis of the reaction kinetics on the basis of the L–H equation indicated that increasing steam pressure effectively increased the gasification rate.     

生物质半焦的催化气化动力学特性

采用恒温热重分析法对稻草的催化气化反应动力学进行了研究,同时研究了生物质对石油焦气化的催化作用。采用修正随机孔模型对气化反应转化速率与转化率的关系进行了拟合计算,得到生物质焦气化的活化能和指前因子。结果表明,加入催化剂后半焦的气化反应活性增大,活性顺序为：加入K+半焦> 加入Ca2+半焦> 加入Mg2+半焦> 原半焦> 酸洗后半焦,表明了生物质焦能明显提高石油焦的气化活性。不同半焦气化的活化能大小顺序为：加入K+半焦<加入Ca2+半焦<加入Mg2+半焦<原半焦<酸洗后半焦,表明了生物质半焦的加入能降低石油焦气化的活化能。     

Catalytic gasification of char from co-pyrolysis of coal and biomass

The catalytic gasification of char from co-pyrolysis of coal and wheat straw was studied. Alkali metal salts, especially potassium salts, are considered as effective catalysts for carbon gasification by steam and CO₂, while too expensive for industry application. The herbaceous type of biomass, which has a high content of potassium, may be used as an inexpensive source of catalyst by co-processing with coal. The reactivity of chars from co-pyrolysis of coal and straw was experimentally examined. The chars were prepared in a spout-entrained reactor with different ratios of coal to straw. The gasification characteristics of chars were measured by thermogravimetric analysis (TGA). The co-pyrolysis chars revealed higher gasification reactivity than that of char from coal, especially at high level of carbon conversion. The influence of the alkali in the char and the pyrolysis temperature on the reactivity of co-pyrolysis char was investigated. The experimental results show that the co-pyrolysis char prepared at 750 °C have the highest alkali concentration and reactivity.     

Comparison of steam-gasification characteristics of coal char and petroleum coke char in drop tube furnace

The steam-gasification reaction characteristics of coal and petroleum coke (PC) were studied in the drop tube fur-nace (DTF). The effects of various factors such as types of carbonaceous material, gasification temperature (1100–1400 °C) and mass ratio of steam to char (0.4:1, 0.6:1 and 1:1 separately) on gasification gas or solid products were investigated. The results showed that for al carbonaceous materials studied, H2 content exhibited the larg-est part of gasification gaseous products and CH4 had the smal est part. For the two petroleum cokes, CO2 content was higher than CO, which was similar to Zun-yi char. When the steam/char ratio was constant, the carbon con-version of both Shen-fu and PC chars increased with increasing temperature. When the gasification temperature was constant, the carbon conversions of al char samples increased with increasing steam/char ratio. For al the steam/char ratios, compared to water gas shift reaction, char-H2O and char-CO2 reaction were further from the thermodynamic equilibrium due to a much lower char gasification rate than that of water gas shift reaction rate. Therefore, kinetic effects may play a more important role in a char gasification step than thermodynamic ef-fects when the gasification reaction of char was held in DTF. The calculating method for the equilibrium shift in this study wil be a worth reference for analysis of the gaseous components in industrial gasifier. The reactivity of residual cokes decreased and the crystal layer (L002/d002) numbers of residual cokes increased with increasing gasification temperature. Therefore, L002/d002, the carbon crystallite structure parameter, can be used to evaluate the reactivity of residual cokes.     

The catalytic steam gasification of chars from various sources by K2CO3

Gasification of a char prepared from hydrocracked residuum was compared with the gasification of chars prepared from bituminous and sub-bituminous Canadian coals, wood and graphite. Each material was mixed with 10 mass per cent K₂CO₃ and pyrolyzed up to 900°C. The yield of char was inversely proportional to the amount of volatile matter in the original material. The char prepared from hydrocracked residuum was different from the others. The other chars all followed zero-order gasification kinetics. Gasification of char prepared from the residuum was first-order in the solid. The development of a liquid phase during the pyrolysis of the residuum to char may explain this difference. The gasification rate of the char. from residuum was slower than the rates with the two coal chars and the wood char, but faster than the gasification rate of graphite. A combination of transient experiments and X-ray photoelectron spectroscopic (XPS) measurements indicated that hydrogen was formed almost instantaneously when steam reacted with the char. XPS spectra at liquid nitrogen temperature indicated that during gasification the formation of carbon oxygen bonds proceeded in the following sequence: COH, CO and CO.     

Kinetics of coal char gasification at elevated pressures

The effects of temperature, pressure, steam flow rate and CO₂/H₂O ratio of gasifying agent on the pressurized gasification of Linnancang coal char were investigated. A correlation of kinetic data was developed for coal chars from coals of different ranks at 30 kg cm ⁻² and 950 °C. The catalytic effects of Ca, Na and Fe catalysts on the gasification activity, activation energy and methane recovery were studied.     

Experimental study of NO reduction over biomass char

Some biomass fuels produce more NO_x than coal on the basis of heating value, giving rise to the necessity and importance of controlling NO_x emission in biomass combustion. The present study investigated the NO reduction over biomass char in a fixed bed quartz reactor in the temperature range of 973–1173 K. The reaction rates of three biomass chars (sawdust, rice husk and corn straw) with NO were compared with Datong bituminous coal char. The results show that the reaction orders of biomass chars for NO are of fractional order and independent of temperature. Biomass chars are more active in reducing NO than coal char. The characteristics of biomass char affect NO conversion. Biomass char formed at high pyrolysis temperature, especially large in particle size, is less active in reducing NO. To some extent, increase of reaction temperature and char loading enhance NO conversion. There exists an optimum bed height for the highest NO conversion. Moreover, NO reduction over biomass char is also enhanced in the presence of CO, O₂ and SO₂.     

Steam gasification of German hard coal using alkaline catalysts: Effects of carbon burn-off and ash content

Laboratory-scale experiments were performed on chars from German hard coals with potassium carbonate addition. The steam gasification rate at 4 MPa and 700 °C as a function of the amount of catalyst added is described for a low-and high-ash char. From the burn-off behaviour the reaction order relative to carbon was determined. For the low-ash char a uniform reaction order was found but the high-ash char indicated a complex interaction of catalytic gasification, catalyst deactivation, and the development of the reacting surface.     

Reactivity and structural change of coal char during steam gasification

This study is intended to clarify the relationship among the reactivity of coal char with steam, structural change in residual carbon, and ash behavior. Steam gasification of various coal chars and demineralized chars was carried out in a fixed-bed reactor. After gasification, the reacted char was analyzed using laser raman spectroscope (LRS), and scanning electron microscope, energy dispersive X-ray spectroscope (SEM/EDX) mapping. Results of SEM images and EDX-mappings revealed that novel parallel analysis of cross correlation between EDX-mapping and LRS-mapping was found to be very effective for the comprehensive evaluation of ash behavior and carbonaceous structure. As the gasification reaction proceeds, the reactivity of the char was varied; existence of Si and Al seemed to suffocate the char reactivity.     

Gasification rate analysis of coal char with a pressurized drop tube furnace

Two coal chars were gasified with carbon dioxide or steam using a Pressurized Drop Tube Furnace (PDTF) at high temperature and pressurized conditions to simulate the inside of an air-blown two-stage entrained flow coal gasifier. Chars were produced by rapid pyrolysis of pulverized coals using a DTF in a nitrogen gas flow at 1400°C. Gasification temperatures were from 1100 to 1500°C and pressures were from 0.2 to 2 MPa. As a result, the surface area of the gasified char increased rapidly with the progress of gasification up to about six times the size of initial surface area and peaked at about 40% of char gasification. These changes of surface area and reaction rate could be described with a random pore model and a gasification reaction rate equation was derived. Reaction order was 0.73 for gasification of the coal char with carbon dioxide and 0.86 for that with steam. Activation energy was 163 kJ/mol for gasification with carbon dioxide and 214 kJ/mol for that with steam. At high temperature as the reaction rate with carbon dioxide is about 0.03 s⁻¹, the reaction rate of the coal char was controlled by pore diffusion, while that of another coal char was controlled by surface reaction where reaction order was 0.49 and activation energy was 261 kJ/mol.     

Changes in char reactivity due to char-oxygen and char-steam reactions using Victorian brown coal in a fixed-bed reactor

This study was to examine the influence of reactions of char–O2 and char–steam on the char reactivity evolution. A newly-designed fixed-bed reactor was used to conduct gasification experiments using Victorian brown coal at 800 °C. The chars prepared from the gasification experiments were then collected and subjected to reactivity characterisation (ex-situ reactivity) using TGA (thermogravimetric analyser) in air. The results indicate that the char reactivity from TGA was generally high when the char experienced intensive gasification reactions in 0.3%O2 in the fixed-bed reactor. The addition of steam into the gasification not only enhanced the char conversion sig-nificantly but also reduced the char reactivity dramatical y. The curve shapes of the char reactivity with involve-ment of steam were very different from that with O2 gasification, implying the importance of gasifying agents to char properties.     

Drastic changes in biomass char structure and reactivity upon contact with steam

An Australian cane trash biomass was pyrolysed by heating at a slow heating rate to 700-900 °C in an inert gas atmosphere. The chars were then gasified in situ with steam. Our results indicate that the gasification of char with steam, even only for 20 s when the char conversion was minimal, resulted in drastic reduction in the intrinsic reactivity of char with air at 400 °C. The decreases in the char reactivity were not mainly due to the possible volatilisation of inherent catalysts during gasification in steam. Instead, the FT-Raman spectroscopy of the chars showed that the gasification of char with steam resulted in drastic changes in char structure including the transformation of smaller ring systems (3-5 fused rings) to large ring systems (?6 fused rings). It is believed that the intermediates of char-steam reactions, especially H, penetrated deep into the char matrix to induce the ring condensation reactions.     

Gasification and co-gasification of biomass wastes: Effect of the biomass origin and the gasifier operating conditions

Air gasification of different biomass fuels, including forestry (pinus pinaster pruning) and agricultural (grapevine and olive tree pruning) wastes as well as industry wastes (sawdust and marc of grape), has been carried out in a circulating flow gasifier in order to evaluate the potential of using these types of biomass in the same equipment, thus providing higher operation flexibility and minimizing the effect of seasonal fuel supply variations. The potential of using biomass as an additional supporting fuel in coal fuelled power plants has also been evaluated through tests involving mixtures of biomass and coal–coke, the coke being a typical waste of oil companies. The effect of the main gasifier operating conditions, such as the relative biomass/air ratio and the reaction temperature, has been analysed to establish the conditions allowing higher gasification efficiency, carbon conversion and/or fuel constituents (CO, H₂ and CH₄) concentration and production. Results of the work encourage the combined use of the different biomass fuels without significant modifications in the installation, although agricultural wastes (grapevine and olive pruning) could to lead to more efficient gasification processes. These latter wastes appear as interesting fuels to generate a producer gas to be used in internal combustion engines or gas turbines (high gasification efficiency and gas yield), while sawdust could be a very adequate fuel to produce a H₂-rich gas (with interest for fuel cells) due to its highest reactivity. The influence of the reaction temperature on the gasification characteristics was not as significant as that of the biomass/air ratio, although the H₂ concentration increased with increasing temperature.     

Gasification kinetics of five coal chars with CO<Subscript>2</Subscript> at elevated pressure

For five coals, the reactivity of char-CO₂ gasification was investigated with a pressurized thermogravimetric analyzer (PTGA) in the temperature range 850-1,000 C and the total pressure range 0.5-2.0 MPa. The effect of coal rank, initial char characteristics and pressure on the reaction rate were evaluated for five coal chars. The reactivity of low lank coal char was better than that of high rank coal char. It was found that Meso/macro-pores of char markedly affect char reactivity by way of providing channels for diffusion of reactant gas into the reactive surface area. Over the range of tested pressure, the reaction rate is proportional to CO₂ partial pressure and the reaction order ranges from about 0.4 to 0.7 for five chars. Kinetic parameters, based on the shrinking particle model, were obtained for five chars.     

煤焦的水蒸汽气化反应速率解析

为得到煤的气化技术开发的基础数据,在固定床微分反应器中,于常压、动力学控制条件下,对国内外七种煤半焦进行水蒸汽气化实验.将煤的气化反应速率用水蒸汽分压及煤的未转化率的幂函数型式表示时,其结果是与水蒸汽分压及细孔表面积的一次方成正比.从理论上推导出了在反应过程中包括考虑煤的细孔结构变化的动力学方程,并且导出了含碳组分气体的生成速率与煤的气化速率的相关式.用Bhatia等人的随机细孔模型表示反应过程中的细孔表面积变化时,计算结果与实测值相当吻合.     

Catalysis of coal char gasification by alkali metal salts

A study has been made of the gasification behaviour, in carbon dioxide and steam, of a number of coal chars doped with small amounts of alkali metal carbonates. For a given additive, the magnitude of the catalytic effect increased with the rank of the parent coal. A progressive loss in catalytic activity on thermal cycling during steam gasification was associated with reaction of the alkali salts with mineral matter in the chars. The kinetic data were consistent with catalytic mechanisms involving oxidation/reduction cycles on the char substrates.     

基于简单碰撞理论的生物质焦炭气化反应的活化能

利用热重分析仪在800~1000℃及750~1000℃下分别对11种生物质原焦及6种生物质脱灰焦进行了CO2等温气化实验,用碳转化率x=0.2时的瞬时气化反应速率rc,0.2对反应速率rc进行无量纲化处理;根据简单碰撞理论,推导得出了生物质焦炭气化反应速率的表达式,求取了17种生物质焦炭气化反应的活化能;结合催化理论与简单碰撞理论建立了生物质焦炭气化反应活化能的经验预测模型. 结果表明,转化率达0.2后,各焦炭不同温度下无量纲气化反应速率曲线基本重合,表明不同温度下焦炭微观结构在转化过程中具有基本相同的演变规律. 各焦炭的活化能与催化剂所占据的活性位比例存在良好的对数关系. 忽略催化效应的影响,焦炭本征气化反应的活化能趋于某一定值,约为254.35 kJ/mol,而完全催化反应活化能约为66.02 kJ/mol.     

Effects of alkaline metal on coal gasification at pyrolysis and gasification phases

A Chinese high-rank coal was acid-washed and ion-exchanged with Na and K to prepare the H-form, Na-form and K-form coals. After pyrolysis, H-form, Na-form and K-form chars and two additional H-form chars (acid washed Na-form and K-form chars) were prepared to investigate the effects of alkaline metal (AM) on coal gasification at the pyrolysis and gasification phases. The H-form char had the highest pryolysis rate; the H-form char had a relative low gasification rate. The AM loaded coals exhibited relative low pyrolysis rate, while the corresponding chars had high gasification reactivity. Acid-washing reduced the reactivities of Na-form and K-form chars. AM inhibited the progress of graphitization of the base carbon resulting in a more reactive char of less ordered crystalline carbon structure. A kinetic model incorporating AM-catalyzed gasification and non-catalytic gasification was developed to describe the gasification rate changes in the char conversion for AM-catalyzed gasification of chars.