Investigation into the kinetics of pressurized steam gasification of chars with different coal ranks

The steam gasification of coal chars derived from three different ranks of typical Chinese coals was studied in a pressurized fixed-bed differential reactor at elevated pressure (up to 2.0 MPa). Three mathematical models [volumetric model (VM), grain model (GM), and random pore model (RPM)] for the gasification kinetics of different chars were validated, through which the kinetic parameters were obtained and discussed. The results show that the evolution trend of the coal char gasification rate with carbon conversion differs from coal ranks and has little change with pressure and temperature. The pressurized gasification process of the Shenmu sub-bituminous coal char (SM char) and the Jingcheng anthracite char (JC char) can be well-predicted by the RPM, while that of the Huolinhe lignite char can be better described by the VM. The pressure has little effect on the options of the reaction kinetic models for the three chars. The kinetic parameter E is almost a constant independent of pressure, while k ₀ changes with pressure, and it seems that k ₀ would be almost a constant over 1.0 MPa for SM and JC chars. The reaction order decreases with increasing the total system pressure and differs from different coal types.     

煤焦CO_2气化反应动力学及内扩散对气化过程的影响分析

采用热重分析仪考察了气化温度(850-1 150℃)和煤焦粒径(60、505、950、1 515、2 000μm)对常压下神木煤焦气化反应的影响。在此基础上,运用体积模型、缩核模型和随机孔模型研究了煤焦常压二氧化碳气化反应动力学,分析了内扩散对煤焦气化反应的影响。结果表明,随机孔模型能够准确预测反应速率随煤焦转化率的变化。基于本征动力学数据,通过对Thiele模数、内扩散效率因子的计算,并将其与实验效率因子相比较,发现计算效率因子能够评估内扩散对初始气化反应的影响,但不能准确评估整个气化过程中内扩散对气化反应的影响。     

Kinetic models comparison for steam gasification of different nature fuel chars

The reactivity in steam of five different types of solid fuels (two coals, two types of biomass and a petcoke) has been studied. The fuel chars were obtained by pyrolysis in a fixed-bed reactor at a temperature of 1373 K for 30 min. The gasification tests were carried out by thermogravimetric analysis (TG) at different temperatures and steam concentrations. The reactivity study was conducted in the kinetically controlled regime and three representative gas-solid models, volumetric model (VM), grain model (GM) and random pore model (RPM), were applied in order to describe the reactive behaviour of the chars during steam gasification. The kinetic parameters of these models were derived and the ability of the models to predict conversion and char reactivity during gasification was assessed. The best model for describing the behaviour of the samples was the RPM. The effect of the partial pressure of steam in gasification was studied, and the reaction order with respect to steam was determined. The reactivity of the chars was compared by means of a reactivity index. Biomass exhibited a higher reactivity than coals and petcoke. However, significant differences in reactivity were observed between the two types of biomass used, which could be due to catalytic effects.     

煤焦水蒸气气化动力学模型及参数敏感性研究

在热重分析仪上对小龙潭煤焦、府谷煤焦和晋城煤焦水蒸气气化过程进行了研究。使用收缩核模型、混合模型和随机孔模型模拟了三种煤焦水蒸气气化反应过程。结果表明,混合模型总体上模拟效果最好,收缩核模型和随机孔模型对低变质程度的小龙潭煤焦气化过程模拟效果不佳,但是适用于模拟另外两种煤阶较高的煤焦气化过程。求解了三种模型的动力学参数,并分析了不同模型参数出现差异的原因。同时,采用敏感性分析法定量研究了模型中的参数发生偏差时引起模型误差的大小,并通过比较发现反应速率常数k为敏感性因素,而混合模型中反应级数n和随机孔模型中孔结构参数ψ为非敏感性因素。     

随机孔模型应用于煤焦与CO2气化的动力学研究

考察970 ℃～1 165 ℃，北宿、神府、忻州、潞安煤焦与CO2在热天平中的气化反应，用恒温法进行热重分析，考察煤种、气化温度、灰分对煤焦气化的影响。用随机孔模型模拟北宿煤反应速率与碳转化率的关系曲线，与未反应芯缩核模型和混合模型模拟结果比较。在化学控制区内，实验数据用随机孔模型拟合最佳。1 066 ℃和1 165 ℃气化数据拟合的相关系数为0.99，970 ℃拟合效果较差。随机孔模型作为简单、精度高的模型可应用于煤炭气化反应中。应用此模型计算四种煤焦反应活化能、指前因子、孔结构参数、A0等动力学参数值。同一煤种气化反应温度越高初始反应速率越大，结构参数体现了孔结构变化对反应的影响，随着温度的升高值减小。     

Kinetics of coal and char oxycombustion studied by TG–FTIR

Coal and char oxycombustion is a complex process because of very high reaction rate of oxygen with coals and chars carbon. Very important process during oxycombustion is diffusion of O₂ to surface of coal and char grain. This process can be minimized using small samples and high flow of the gas, but it is also dependent on temperature. For this reason, it is impossible to eliminate diffusion processes which cause significant impact on calculated kinetic parameters. This paper describes the results of thermogravimetric studies of oxycombustion process with evolved gas analysis by FTIR. Ultimate and proximate analysis of coal and char were made. Thermogravimetric experiments of coal and its char oxycombustion were conducted using five heating rates, namely 2.5, 5, 10, 20 and 40 K min^?1, and gas mixture composed of 20 % O₂ in CO₂. Activation energies of coal and char oxycombustion were calculated by isoconversional methods: integral Vyazovkin and differential Friedman. Activation energies for three ranges of heating rates were calculated. This paper shows influence of heating rate on calculated activation energy. The reason of this phenomenon is due to change of the mechanism of coal and char oxycombustion from the chemical kinetic control regime to mixed chemical kinetic–diffusion control regime.     

Pressurized CO<Subscript>2</Subscript>-enhanced gasification of coal

Carbon dioxide was considered as a co-gasifying agent in a coal gasification reactor. The work presented herein describes the simulation results for the process and the experimental data on coal char gasification with CO₂ addition as the rate-controlling step for the entire process. To study the potentially beneficial effect of the introduction of CO₂ into the gasification system, several simulations were conducted using the commercial process simulation software ChemCAD 6.3^®. The results of a Gibbs equilibrium reactor were evaluated. The Boudouard reaction is a critical path for the development of this process, and the kinetics were studied experimentally. Four chars derived from the pyrolysis of Polish coals of different origins were selected for the experiments. The kinetic characteristics of this system were examined using a custom-designed pressurized fixed-bed reactor. To determine the effect of pressure on the gasification rate, several preliminary studies on the gasification of coal chars were performed isothermally at the temperature of 950 °C and pressures of 1, 10, and 20 bars. In contrast to the thermodynamic calculations, the experimental data revealed that increasing the CO₂ pressure leads to a higher reaction rate for medium-rank coal chars and low-rank lignite coal char, resulting in higher efficiency for carbon monoxide production. The pressure influences the reactivity more strongly when varied from 1 to 10 bars; a further increase in pressure affects the rate almost insignificantly. The observed behavior representing the changes in carbon conversion degree during gasification is satisfactorily described by the grain model.     

水蒸气气氛煤中温催化气化动力学研究

以碳酸钾为催化剂,用热天平的等温热重法研究了四种不同变质程度煤焦常压下水蒸气催化气化反应动力学。在加和不加碳酸钾条件下,测定了温度为700~850℃煤焦的化学反应控制条件下的碳转化率与时间的关系。碳酸钾催化剂的加入对变质程度越高煤的气化催化作用越大。加碳酸钾的碳转化率与时间的关系用混合模型和修正随机孔模型可以良好的拟合关联,均相模型关联较差。利用修正随机孔模型拟合关联出了四种煤焦催化水蒸气气化反应的活化能和指前因子,活化能为90.317~167.861kJ/mol,指前因子和活化能之间具有补偿效应。     

Sorption charateristics of methane among various rank coals: impact of moisture

Enhanced coalbed methane (ECBM) in deep coal seams is being actively investigated around the world. Since the in situ coal seams are always saturated with water, methane sorption behavior on coal in the presence of water can help accurately assess the amount of recoverable methane. Thus, methane sorption isotherms have been measured on a high-rank anthracite, a low-volatile bituminous, a middle-volatile bituminous and a high-volatile bituminous coal with the manometric technology at 30 °C under six different moisture contents. The Dubinin–Astakhov (D–A) equation was used to fit the experimental sorption isotherm data. In all cases, the moisturized coals exhibited lower sorption capacity than the corresponding dry materials and moisture has a significant effect on CH₄ sorption capacity. The maximum sorption capacity, V ₀, displays a linear decline with the moisture content for the Changcun and Malan samples, but it is nonlinear for the other two coal samples. The net heat of CH₄ sorption, βE, is also reduced by the presence of water, but varies only slightly between a relatively small span of about 8.8 and 10.0 kJ mol^?1 for the dry samples studied, despite the difference in coal rank. In addition, the maximum sorption capacity of CH₄ in dry coals presents the typical “U-shape” trend with coal rank. Moisture has a greater impact on the sorption capacity in low-rank coals than that in high-rank coals. The mechanisms responsible for the effect of moisture on CH₄ sorption among various rank coals are also presented. The pore-blocking effect is the main influencing factor for high-rank anthracite, whereas, the competition sorption is dominant for low-rank coals.     

Study on the kinetics and reactivity at the ignition temperature of Jurassic coal in North Shaanxi

Thermal analysis of seven Jurassic coal samples from North Shaanxi in West China and three permo-carboniferous coal samples from East China was studied to identify ignition temperatures in the process of the oxidation and spontaneous combustion. The experiments were carried out under non-isothermal heating conditions up to 700 °C at the heating rates of 5, 10, 15, and 20 °C min^?1 in an air atmosphere. Through the FTIR spectrometer experiments, the absorbance peaks of functional groups of coal samples were analyzed at the ignition temperatures, pre-ignition of the 10 °C, post-ignition of the 10 °C at the heating rate of 10 °C min^?1. By the differential spectrum method, the changes of functional groups were discussed with the aim to determine characteristics and reactivity of the ignition temperature around. The results showed that ignition temperatures of experimental coal samples increased with the rising heating rates, and ignition temperatures of Jurassic coals were lower than that of the permo-carboniferous coal samples at the same heating rate. Apparent activation energy of experimental Jurassic coals at the ignition temperatures was calculated by Ozawa method based on the non-isothermal and differential heating rates, ranging from 80 to 105 kJ mol^?1, which were lower than that of the eastern permo-carboniferous samples. On the basis of Pearson correlation coefficient method which can signify the degree of correlations ranging from ?1 to 1, the correlation analyses were conducted between activation energy and functional groups variation within 10 °C before and after ignition temperature. It was concluded that the key functional groups of Jurassic coals in the oxidation and ignition reaction were methyl and alkyl ether within 10 °C before ignition temperature, and carboxyl and carbonyl within 10 °C after ignition temperature.     

Thermogravimetric analysis of gasification reactivity of coal chars with steam and CO2 at moderate temperatures

Comparative study on the gasification reactivity of the three types of Chinese coal chars with steam and CO₂ at 850–1050 °C was conducted by isothermal thermogravimetric analysis. The effects of coal rank, pore structure, ash behavior, and gasification temperature on the gasification reactivity of coal chars were investigated. It is found that the gasification reactivity difference between different coal chars changes with reaction degree and gasification temperature, and has no immediate connection with coal rank and initial pore structure. Ash behavior plays an important role in the char reactivity, and changes with gasification temperature and reaction degree due to the variation in the compositions and relative amount. The influence of pore structure is more noticeable during a relatively moderate reaction process. The relative reactivity ratio of steam to CO₂ gasification generally decreases with the increasing temperature, and is related with the catalytic effect of inherent minerals. The characteristic parameters of the chars were analyzed, finding that the value of half reaction specific rate is approximate to the average specific rate under the same conditions. The nth-order distributed activation energy model is proposed to describe the coal char gasification process, and the results show that the activation energy increases with the increasing carbon conversion.     

甲醇处理煤的微孔性质及反应性研究

在低于临界温度下,用甲醇处理了三种不同变质程度的煤,以研究其比表面积及微孔容积等表面特性的变化。结果表明,用甲醇处理后煤的微孔性质发生了较大变化,其变化的大小顺序是:沈北褐煤>大同烟煤>晋城无烟煤。随甲醇处理温度或干馏温度升高,煤及半焦的孔径均向小的方向偏移。煤的平均孔径为8.2—8.6A;半焦的平均孔径为6.3—7.5A。煤经甲醇处理后其半焦的反应性均比未处理的高。     

煤及煤焦微观结构特征与气化反应性

研究了不同煤化度煤及煤焦的微观结构及其对气化反应性的影响。结果表明,褐煤焦具有丰富的分支孔系统和较大的比表面积,并含有较多对气化有催化作用的可交换阳离子。无烟煤焦分支孔贫乏,比表面积很小。煤焦的总孔容、比表面积和芳核大小之间有很好的对应关系。不同煤化度煤焦气化反应性差异很大,脱矿物质后煤焦反应性差异显著减小,但是脱矿物质前后煤焦的反应性随煤化程度的变化趋势相似。     

Fast pyrolysis of coals under N2 and CO2 atmospheres

Oxyfuel combustion represents one way for cleaner energy production using coal as combustible. The comparison between the oxycombustion and the conventional air combustion process starts with the investigation of the pyrolysis step. The aim of this contribution is to evaluate the impact of N₂ (for conventional air combustion) and CO₂ (for oxy-fuel combustion) atmospheres during pyrolysis of three different coals. The experiments are conducted in a drop tube furnace over a wide temperature range 800–1400 °C and for residence time ranging between 0.2 and 1.2 s. Coal devolatilized in N₂ and CO₂ atmospheres at low temperatures (?1200 °C) and longer residence times (>?0.5 s), the char-CO₂ reaction is clearly observed, whose intensity depends on the nature of the coal. Furthermore, the volatile yields are simulated using Kobayashi’s scheme and kinetic parameters are predicted for each coal. The char gasification under CO₂ is also accounted for by the model.

     

A Model-based Phenomenological Investigation of Char Combustion Kinetics through Thermogravimetry

Five coal char samples were burnt in thermobalance with ramp heating rate of 30 K/min. The pore structure of these char samples was studied through mercury intrusion method. Combined with the kinetic theory of gases, the data of surface area was used in fitting the results. As a result, the kinetic triplet was given. The analysis showed that five char samples share almost the same intrinsic activation energy of the overall reaction. The phenomenological implication of the derived combustion rate equation was given.     

全温度段裂解提质对锡盟褐煤成浆特性的影响

通过对锡盟褐煤进行涵盖高、中、低完整温度段的裂解提质,研究了裂解过程对煤质特性、成浆浓度、浆体流变特性以及稳定性的影响。对锡盟褐煤在隔绝空气条件下裂解,测量不同浓度浆体的黏度,并对剪切速率和表观黏度进行拟合得到不同浆体不同浓度的流变特性曲线,根据国标规定方法测量浆体的实际浓度以及浆体稳定性。并从裂解过程中样品表面官能团和孔隙结构变化角度分析裂解对锡盟原煤及半焦成浆特性的作用机理。实验结果表明,裂解过程可以减少样品中的含氧官能团,降低其亲水性,有利于成浆浓度的提高,成浆浓度随裂解温度的升高呈现先增大后减小的变化趋势,但是裂解对浆体的稳定性有负面影响。随裂解温度的提高,含氧官能团的分解导致半焦的孔隙结构发生显著变化,平均孔径先减小后增大,比表面积和孔容积呈先增大后减小趋势。     

煤催化气化中非均相反应动力学的研究

以神木煤焦为研究对象,在小型加压固定床上考察了不同气化剂(水蒸气、二氧化碳、氢气)、催化剂负载量、水蒸气分压、氢气分压和一氧化碳分压对碳转化率和气化反应速率的影响。结果表明,对于非均相的催化气化反应来说,反应速率顺序为C-H2OC-CO2C-H2。H2和CO不同程度地抑制煤焦水蒸气气化反应,CO的抑制作用明显大于H2。在700℃,当添加5%的CO,碳转化率降低约50%。基于Langmuir-Hinshelwood(L-H)方程,结合随机孔模型,同时考虑催化剂负载量及气化产物分压的影响,建立了煤焦催化水蒸气气化动力学模型,模型预测反应速率常数与实验值误差在10%以内,说明建立的动力学模型可以较好地模拟煤焦的催化水蒸气气化反应过程。     

Pyrolysis and combustion model of oil sands from non-isothermal thermogravimetric analysis data

Thermogravimetric (TG) data of oil sand obtained at Engineering Research Center of Oil Shale Comprehensive Utilization were studied to evaluate the kinetic parameters for Indonesian oil sand samples. Experiments were carried out at heating rates of 5, 15, and 25 °C min^?1 in nitrogen, 10, 20, and 50 °C min^?1 in oxygen atmosphere, respectively. The extent of char combustion was found out by relating TG data for pyrolysis and combustion with the ultimate analysis. Due to distinct behavior of oil shale during pyrolysis, TG curves were divided into three separate events: moisture release, devolatilization, and evolution of fixed carbon/char, where for each event, kinetic parameters, based on Arrhenius theory, were calculated. Coats–Redfern method, Flynn–Wall–Ozawa method, and distributed activation energy model method have been used to determine the activation energies of degradation. The methods are compared with regard to their characteristics and the ease of interpretation of the thermal kinetics. Activation energies of the samples were determined by three different methods and the results are discussed.     

An understanding of the porosity of residual coal/char/ash samples from an air-blown packed bed reactor operating on inertinite-rich lump coal

The thermal treatment of coal causes a development of internal porosity of the resultant char due to the changes in the coal char pores, i.e. the opening of original closed pores, the formation of new pores, and an increase in pore size of existing and newly formed pores. Furthermore, the porosity formed during de-volatilisation causes changes in pore structural characteristics such as: density, pore size distribution, total open pore volume, porosities and average pore diameter. Much research has been conducted in this area, but was mainly focused on fine particle sizes (<1 mm) and vitrinite-rich coals, particularly from the Northern hemisphere. The objective of this study was to obtain an understanding of both the macro- and micro-porosity development within the de-volatilisation zone of a packed bed consisting of lump inertinite-rich coal (75 mm × 6 mm) from the Highveld coalfield in South Africa. This was achieved by generating samples in an air-blown packed bed reactor and conducting proximate, CO₂ reactivity, mercury intrusion porosimetry, and BET CO₂ surface area analyses on the dissected coal/char/ash samples.From mercury-intrusion porosimetry results obtained for the de-volatilisation reaction zone of the reactor, it was found that although the percentage macro-porosity and average pore diameter increased by 11% and 77% respectively (which confirms pore development), that these developments do not enlarge the surface area, and thus has no significant contribution on the reactivity of the coal/char. On the other hand, the micro-pore surface area, pore volume and pore diameter were all found to increase during de-volatilisation, resulting in an increase in the coal char reactivity. The micro-porosity is thus generally responsible for the largest internal surface area during de-volatilisation, which enables increased reactivity. The CO₂ gasification reactivity (at 1000 °C) increased from 3.8 to 4.5 h⁻¹ in the first stage of de-volatilisation, and then decreased to 3.8 h⁻¹ in a slower de-volatilisation regime. This is due to the maximum pore expansion and volatile matter evolution reached at 4.5 h⁻¹, before coalescence and pore shrinkage occur with a further increase in temperature within the slower de-volatilisation region of the reactor. During de-volatilisation there is thus both an increase and decrease in reactivity which might suggest two distinct intermediate zones within the de-volatilisation zone.     

煤高温快速热解规律研究

利用高温滴管炉在1000℃～1400℃考察了彬县烟煤在高温快速热解过程中失重的变化,同时比较了埃塞俄比亚褐煤和晋城无烟煤的热解规律。结果表明,热解失重率随温度的升高而增加,而且各种煤种的最大失重率在高温下大于工业分析的失重率。对于不同变质程度的煤种其热解特性也不相同,较低的热解温度对高阶煤的影响较小。彬县原煤经过热解后比表面积增加,且随着热解温度的提高而增大,当热解温度超过灰熔点时,总比表面积降低。通过数据回归,得到了三种煤的失重率和热解温度的关联式。