烟煤煤焦等温CO2气化动力学模型研究

在建立的化学反应动力学控制实验条件下利用自建固定床实验台研究了烟煤煤焦等温CO2气化反应特性。采用均相模型、未反应收缩核模型和修正体积模型计算得到气化反应活化能分别为147.7kJ/mol、102.9kJ/mol和155.5kJ/mol。利用等转化率法避开反应机理函数的选择,计算得到反应活化能为144.1～166.0kJ/mol。通过比对不同模型相关系数大小以及与等转化率法计算所得活化能范围符合程度相结合的方法,确定均相模型和修正体积模型为最佳动力学模型;根据修正体积模型中经验常数b≈1,可认为修正体积模型与均相模型为同一模型。因此确定烟煤煤焦CO2气化反应最佳动力学模型为均相反应模型。     

浑源煤焦CO_2气化反应的影响因素及动力学特性分析

研究了热解温度、热解时间以及气化温度对浑源煤焦CO2气化反应的影响,并获得了气化反应的动力学模型.结果表明：浑源煤焦的气化活性随热解温度的提高而降低;每个热解温度都对应着一个最佳热解时间,且存在最佳热解时间随温度升高而缩短的趋势;提高气化温度能够显著提高煤焦的气化反应性能,气化温度对气化反应的影响大于热解温度的影响;低温度煤焦的气化活性随气化温度的提高而增加更为剧烈;900℃及以上的高温使活性点数增加,从而使煤焦间的活性差距分布均匀;浑源煤焦的气化反应适宜用体积模型来描述,所求取的动力学参数之间存在补偿效应,其等动力学温度约为1 199.6℃.     

非等温热重分析玉米芯半焦CO_2气化特性

采用非等温热重法对玉米芯热解半焦CO2气化行为和动力学特性进行研究。结果表明:升温速率对整个气化过程有重要影响。随着升温速率的增大,完成反应所需的温度提高,反应速率增加,反应时间缩短,而且升温速率越大,反应速率的峰值越高且向高温区偏移。利用Kissinger微分法和Coats-Redferm积分法分别计算动力学参数,所得不同升温速率下的平均活化能为180.77kJ/mol;升温速率越大,活化能越小。研究发现,玉米芯热解半焦CO2非等温气化的活化能E和频率因子A之间存在动力学补偿效应,两者满足lnA=0.09384E+2.604。     

制备温度对芒草热解焦CO2气化反应动力学的影响

利用热重分析仪进行了芒草热解焦与CO_2气化反应实验研究,选取均相反应模型、颗粒反应模型和随机孔模型计算了芒草热解焦的CO_2气化反应动力学参数,探讨了3种动力学模型的适用性.为进一步探讨制备温度对热解焦CO_2气化反应的影响机理,利用扫描电镜(SEM)和Brunauer-Emmett-Teller(BET)分析了芒草热解焦的孔隙结构和表面形态.研究表明,随着制备温度的升高,热解焦表面结构被逐渐加深,表面粗糙度提高,比表面积相对增大,制备温度为600℃的热解焦具有最大的微孔容积与总孔容积之比,使得其更容易发生气化反应;制备温度为400℃时,芒草热解焦在3种模型下具有最小的平均活化能,随机孔模型对芒草热解焦实验数据拟合效果最好,其模拟的相关性系数R2均大于0.97.     

煤焦气化反应动力学研究

气流床气化技术是煤炭清洁、高效转化的重要途径和发展方向之一。利用热天平,采用等温热重法对抽样选出的煤种在800℃～1 400℃温度范围内进行了煤焦CO2气化反应动力学特性研究。研究结果表明:高温下煤焦的气化反应特性不同于低温时的反应特性,在900℃～1 000℃时气化反应逐步由化学反应控制过渡到过渡区控制,在1 100℃～1 300℃时气化从反应过渡区控制逐步到扩散区控制;不同粒径的煤粉气化反应,在相同的时间内,1 000℃时的碳转化率、气化反应速率比950℃时的碳转化率、气化反应速率高很多,950℃时的碳转化率、气化反应速率比900℃时的碳转化率、气化反应速率高。     

秸秆焦的表面特征及CO2气化反应性的研究

在700～1000℃热解温度条件下制备了稻秸秆和麦秸秆焦并进行了SEM和BET表面积测试分析,采用等温热重法研究了这些秸秆焦的CO2气化反应特性.结果表明:在较低热解温度(700℃)下,秸秆热解焦中尚含有一定量未析出的焦油;在700～1000℃范围内,随热解温度上升秸秆焦的BET表面积逐渐增加,而气化反应活性却有所下降;在800～1100℃气化温度范围内,秸秆焦的气化反应性随气化温度明显增加,两种秸秆焦的表观活化能则随热解温度稍有增加,稻秸秆和麦秸秆焦的表观活化能范围分别为183.58～196.50kJ/mol和147.27～184.01kJ/mol.     

准东煤气化动力学模型研究

混合准东煤原煤与催化剂K_2CO_3、Ca(OH)_2并制成煤样,在化学反应动力学控制条件下研究其气化反应特性,分析了煤样质量、CO_2体积流量和颗粒直径对气化过程中内、外扩散阻力的影响,获得不同反应温度下均相模型、未反应芯收缩核模型和修正体积模型的拟合曲线,利用等转化率法计算气化反应活化能,并通过催化活性指数验证了该方法计算活化能的准确性.结果表明:在转化率为0.2、0.4、0.6和0.8时对应的活化能为100.1~130.2kJ/mol,3种模型计算所得活化能分别为128.97kJ/mol、140.33kJ/mol和139.43kJ/mol;均相模型为较合适的气化反应动力学模型.     

生物质半焦CO2气化反应动力学研究

采用热天平研究生物质半焦CO2气化反应动力学特性。考察半焦粒径、热解制焦温度以及热解制焦气氛对气化反应碳转化率的影响。采用随机孔模型、未反应芯缩核模型和混合模型对生物质半焦气化反应速率随碳转化率变化的趋势进行拟合,并求出半焦气化的动力学参数,结果表明随机孔模型的拟合效果最好。     

热解条件对生物质焦气化活性的影响及等温气化动力学参数求解方法

生物质气化是生物质利用研究的一个重点。生物质气化包含生物质的热解和热解所得焦炭的气化两个过程。不同的热解条件将得到具有不同气化活性的生物质焦炭,不同热解条件制取的焦炭的动力学参数也不相同。本文主要概述了热解条件对生物质焦气化活性的影响。同时基于阿伦尼乌斯公式介绍了生物质焦等温气化动力学参数的两种获取方法,非等转化率法是通过选择动力学模型中的结构因子f(x) 来获取动力学参数,而等转化率法是通过避开选择动力学模型中的结构因子f(x) 来获取动力学参数。基于简单碰撞理论提出了获取等温气化动力学参数的新方法,对阿伦尼乌斯公式中的指数项、指前因子A提出了明确的物理意义。基于简单碰撞理论的等温求解气化动力学参数方法类似于基于阿伦尼乌斯公式的等温求解气化动力学参数方法。     

CO2气氛下生物质气化制氢的数值分析

生物质气化制氢有重要的工业应用价值,本文采用ASPEN PLUS软件数值模拟了稻壳在流化床中的气化过程。本次模拟运用吉布斯自由能最小化原理,选择RGibbs和RYield模块,采用CO2作为气化剂,计算获得了气化温度、CO2质量流量、CO2和稻壳质量比和碳转化率对产氢率的影响规律。结果表明：在CO2质量流量为200kg/h时,H2的生成率高达43%。随着CO2/B增加,CO和CO2体积分数逐渐升高,CH4体积分数下降,H2体积分数在不同的气化温度下趋于平稳（600~700℃）或下降（800~1000℃）。随着气化温度升高,碳转化率增加;随着CO2和稻壳质量比的升高,碳转化率下降。     

Assessment of the catalytic effect of various biomass ashes on CO2 gasification of tire char

The aim of this study was to determine the effect of various biomass ashes, comprising catalytically active components, on tire char reactivity during the CO₂ gasification process. Ashes from the combustion of corn cobs, beet pulp, sunflower husks and beech chips were selected for the research. Moreover, industrial fly ash from a coal-fired power plant was used as a reference. The tire char-ash blends with different ash contents (0–15 wt%) were gasified in the CO₂ atmosphere under non-isothermal conditions using dynTHERM Rubotherm thermobalance. Based on the n-order Coats and Redfern method, gasification reactivity indicators and kinetics parameters were calculated. The results showed that the addition of biomass ashes enhanced reactivity of tire char, and the magnitude of these changes depended on both the quantity and type of the additive. With the increase in the amount of added biomass ashes, the catalytic effect increased, and their efficiency can be ranked as follows: sunflower husk ash > corn cobs ash ≅ beet pulp ash > beech chips ash. In turn, reference fly ash from a power plant slightly affected the CO₂ gasification of tire char, regardless of its amount. Moreover, a statistically significant correlation between the reactivity indicator and the amount of K₂O, MgO and P₂O₅ in ashes analysed has been proved (reactivity indicator improved with an increase in these components amount). The performed analysis provides valuable information regarding the composition of catalysts characterised by high catalytic activity in the tire char gasification process.     

Kinetics of rice husk char gasification in an H2O or a CO2 atmosphere

Rice husk gasification has been attracting increasing attention in rice-producing countries, but the technology has not yet achieved optimal efficiency. Only a few studies have reported on the gasification kinetics of rice husk char, and the influence of some important parameters has not yet been investigated. This paper provides an experimental database and kinetic models of gasification of a rice husk char particle in an H₂O or a CO₂ atmosphere. A complete parametric study of rice husk char gasification was performed in a wide range of operating conditions, relevant to those that exist in industrial gasifiers. Two kinetic models were developed to predict the conversion of a particle, taking into account changes in the reactive surface. Results of this study could help researchers and engineers in the design, modeling or optimization of a new efficient rice husk gasifier.     

Experimental study on gasification and kinetic characteristics of inferior coal with high ash content under CO2 atmosphere

The effect of heating rate and particle size on gasification of one inferior coal was experimentally studied. A homogeneous reaction model was used to calculate kinetic parameters with the Freeman–Carroll method. The results show that gasification reactivity can be enhanced by reduction of coal particle size and increase of heating rate. Additionally, coal ash plays a catalytic role to a certain extent on gasification. It was also found that the reaction rate can be enhanced significantly, when increasing the ash-coal weight ratio from 1:2 to 2:1. The gasification order under CO₂ atmosphere is close to 1. There is a kinetic compensation effect between activation energy and frequency factor for the gasification of the inferior coal investigated.     

Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis

The CO₂ gasification reactivity of pine sawdust chars (PS char) obtained from the different high-temperature pyrolysis is studied based on non-isothermal thermogravimetric method. Results show that the order of gasification reactivity is PS char-1073 > PS char-1273 > PS char-1473. Under the effect of high-temperature pyrolysis, the surface structure of biomass char is gradually destroyed and the pore structure parameters of specific surface area, total pore volume and average pore diameter increase. By means of the N₂ adsorption-desorption isotherms, it is seen that biomass char has more micro- and mesoporous at higher pyrolysis temperature. Besides, the PS char-1073 mostly has rich closed cylinder pores and parallel plate pores, and the PS char-1273 and PS char-1473 have plentiful open cylinder pores and parallel plate pores. An increase of pyrolysis temperature contributes to the development of porosity and improves diffusion path, which promotes the gasification reactivity. But, its effect on the decline of active site hinders the gasification reactivity. What's more, the kinetic model of distributed activation energy model (DAEM) is applied to calculate activation energy and pre-exponential factor with the integral and differential methods. The calculation results of integral method is more accurate and precise because the differential method is more sensitive than integral method for experimental noise. There is a compensation effect in the CO₂ gasification process.     

Study on biomass gasification under various operating conditions

An Aspen Plus model of biomass gasification with different gasifying agents has been developed. Due to lack of kinetic data, the developed model is based on Gibbs free energy minimization. The main objective of this study is to study the influence of gasifying agent (pure oxygen; oxygen-enriched air and air), gasification temperature and equivalence ratio (ER) on gas composition, gas lower heating value (LHV), and energy/exergy efficiencies. The developed model was validated with experimental data which was found to be in well agreement. Increase in gasification temperature led to a significant increase in H₂ content. On the other hand, an increase in ER led to a significant reduction in H₂, CO, and CH₄ and a significant increase in CO₂. Also, a gradual downward trend of exergy efficiency (EE) was found, as ER increased from 0.15 to 0.21, while it basically kept constant as the gasification temperature was varied.     

Syngas production by gasification of aquatic biomass with CO2/O2 and simultaneous removal of H2S and COS using char obtained in the gasification

Applicability of gulfweed as feedstock for a biomass-to-liquid (BTL) process was studied for both production of gas with high syngas (CO + H₂) content via gasification of gulfweed and removal of gaseous impurities using char obtained in the gasification. Gulfweed as aqueous biomass was gasified with He/CO₂/O₂ using a downdraft fixed-bed gasifier at ambient pressure and 900 °C at equivalence ratios (ER) of 0.1–0.3. The syngas content increased while the conversion to gas on a carbon basis decreased with decreasing ER. At an ER of 0.1 and He/CO₂/O₂ = 0/85/15%, the syngas content was maximized at 67.6% and conversion to gas on a carbon basis was 94.2%. The behavior of the desulfurization using char obtained during the gasification process at ER = 0.1 and He/CO₂/O₂ = 0/85/15% was investigated using a downdraft fixed-bed reactor at 250–550 °C under 3 atmospheres (H₂S/N₂, COS/N₂, and a mixture of gases composed of CO, CO₂, H₂, N₂, CH₄, H₂S, COS, and steam). The char had a higher COS removal capacity at 350 °C than commercial activated carbon because (Ca,Mg)S crystals were formed during desulfurization. The char simultaneously removed H₂S and COS from the mixture of gases at 450 °C more efficiently than did activated carbon. These results support this novel BTL process consisting of gasification of gulfweed with CO₂/O₂ and dry gas cleaning using self-supplied bed material.     

Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char

For oxy-combustion with flue gas recirculation, elevated levels of CO₂ and steam affect the heat capacity of the gas, radiant transport, and other gas transport properties. A topic of widespread speculation has concerned the effect of gasification reactions of coal char on the char burning rate. To asses the impact of these reactions on the oxy-fuel combustion of pulverized coal char, we computed the char consumption characteristics for a range of CO₂ and H₂O reaction rate coefficients for a 100 μm coal char particle reacting in environments of varying O₂, H₂O, and CO₂ concentrations using the kinetics code SKIPPY (Surface Kinetics in Porous Particles). Results indicate that gasification reactions reduce the char particle temperature significantly (because of the reaction endothermicity) and thereby reduce the rate of char oxidation and the radiant emission from burning char particles. However, the overall effect of the combined steam and CO₂ gasification reactions is to increase the carbon consumption rate by approximately 10% in typical oxy-fuel combustion environments. The gasification reactions have a greater influence on char combustion in oxygen-enriched environments, due to the higher char combustion temperature under these conditions. In addition, the gasification reactions have increasing influence as the gas temperature increases (for a given O₂ concentration) and as the particle size increases. Gasification reactions account for roughly 20% of the carbon consumption in low oxygen conditions, and for about 30% under oxygen-enriched conditions. An increase in the carbon consumption rate and a decrease in particle temperature are also evident under conventional air-blown combustion conditions when the gasification reactions are included in the model.     

Inhibition of steam gasification of biomass char by hydrogen and tar

The influence of hydrogen and tar on the reaction rate of woody biomass char in steam gasification was investigated by varying the concentrations in a rapid-heating thermobalance reactor. It was observed that the steam gasification of biomass char can be separated into two periods. Compared with the first period, in the second period (in which the relative mass of remaining char is smaller than 0.4) the gasification rate is increased. These effects are probably due to inherent potassium catalyst. Higher hydrogen partial pressure greatly inhibits the gasification of biomass char in the first and second periods. By calculating the first-order rate constants of char gasification in the first and second periods, we found that the hydrogen inhibition on biomass char gasification is caused by the reverse oxygen exchange reaction in the first period. In the second period, dissociative hydrogen adsorption on the char is the major inhibition reaction. The influence of levoglucosan, a major tar component derived from cellulose, was also examined. We found that not only hydrogen but also vapor-phase levoglucosan and its pyrolysates inhibited the steam gasification of woody biomass char. By mixing levoglucosan with woody biomass sample, the pyrolysis of char proceeds slightly more rapidly than with woody biomass alone, and gas evolution rates of H₂ and CO₂ are larger in steam gasification.