基于等转化率法的煤焦-CO_2气化动力学研究

为获得可靠的煤焦-CO_2气化反应动力学参数,采用Flunm-Wall-Ozawa(FWO)等转化率法进行煤焦-CO_2气化动力学研究。在3个不同升温速率下进行了煤焦-CO_2气化热重试验,计算不同碳转化率下的反应活化能,用主曲线法分析了气化机理模型,并采用拟合法对等转化率法的结果进行验证。结果表明,气化主反应区不同碳转化率下(α为0.2~0.8)活化能的变化较小,为(228.25±5.22)k J/mol。煤焦-CO_2气化反应为均相模型,该模型标准曲线与试验曲线重合度较好,并符合目前常用的煤气化动力学模型。拟合法计算的活化能仅与等转化率法相差0.74 k J/mol,说明等转化率法研究煤焦-CO_2动力学可行。     

高温煤焦/CO2气化反应的动力学研究

对煤气化随机孔模型的动力学控制区的假设进行了改进,建立了高温煤焦/CO2气化反应碳转化率(X)与反应时间(t)的修正随机孔模型:X=1-exp[-kt(a+bkt+k2t2)],并在950℃～1 400℃气化温度范围内,用修正随机孔模型模拟淮南慢速热解煤焦和淮南快速热解煤焦/CO2气化反应,所得表观活化能范围分别为121.99kJ/mol～153.75kJ/mol和88.57kJ/mol～121.39kJ/mol.结果表明,修正随机孔模型的拟合效果优于随机孔模型和收缩未反应芯模型的拟合效果,能很好地体现煤焦气化反应的动力学特征,且该模型适用于不同煤焦的气化反应模拟.     

煤焦燃烧特性及反应活性探究

中国褐煤资源丰富,然而由于褐煤自身特点使其应用受到了极大的限制。针对中国褐煤应用最广的途径———燃烧,借助热重分析仪对不同热解终温的褐煤半焦及热解终温为1273 K的褐煤半焦与原煤的混合燃料的燃烧特性进行了分析。并利用Coats-Redfern法进行了燃烧动力学的分析,通过求得的表观活化能表征煤焦的燃烧反应活性。研究发现：热解终温越高,煤焦的燃烧特性越差;掺混褐煤有助于提高其半焦的燃烧特性,而掺混燃料的燃烧稳定性几乎和原煤无差别,且随着掺混比例的增加,混合燃料的活化能逐渐增大,越不易点燃,掺混半焦对燃料的燃烧特性和反应活性都有影响。相同制备条件下的烟煤半焦和褐煤半焦的燃烧动力学参数尤其是活化能相差很大,可见煤焦的燃烧反应活性与煤种有关。     

低阶煤热解条件对高炉喷吹半焦燃烧性能及动力学特性的影响

采用热重分析法研究了不同热解条件下半焦的燃烧性能和动力学特征,利用Ozawa法求取动力学参数。结果表明,热解温度越低、保温时间越短时,半焦的燃烧性能越好;热解升温速率对半焦燃烧过程的反应程度影响不大;粒度越大,燃烧性能差异性越明显。热解温度对半焦燃烧性能影响较大,550℃是本研究中制备高燃烧反应性半焦的适宜热解温度。两种不同粒度原煤制得的半焦均随转化率增大,活化能减小。1~3 mm原煤在热解温度为550℃时所得半焦在燃烧过程中符合反应级数模型,化学反应为限制性环节,反应最概然机理函数为f(α)=(1–α)2。     

义马煤焦CO_2气化反应性研究

运用等温热重技术,以CO2为气化剂,在常压和1 000 ℃～1 300 ℃温度范围内,考察了义马煤焦的气化反应性.结果表明,随着气化温度的提高,义马煤焦的反应性和反应速度呈现出增加的趋势,但在灰熔融温度附近出现不同的情况.碳还原率为50%时,1 000 ℃～1 100 ℃和1 100 ℃～1 300 ℃的气化反应活化能分别为140.97 kJ/mol与20.84 kJ/mol.     

等转化率法研究蔗渣热解过程动力学

运用热重分析法在4个不同升温速率下研究了蔗渣在氮气中的热解过程,用等转化率法计算出热解的活化能(E_a)从102.40 kJ/mol逐渐增大到161.24 kJ/mol。用主曲线法判断活化能不变区域的动力学模型机理函数为1/2级简单反应,机理函数F_0.5(α)=1-(1-α)^1/2,指前因子A=(5.991±0.947)×10⁶ s^-1。     

亚麻纤维热解动力学的“model free”法和Coats-Redfern模型拟合法研究

亚麻纤维是一种潜在的气化原料,本文对亚麻纤维的热解行为进行了热重分析研究。10 mg粒径为0.60～0.85 mm的亚麻纤维颗粒在高纯氮气的保护下分别以10、20、30、50 K·min^-1的升温速率线性升温到550℃。使用“model free”方法和Coats-Redfern模型拟合方法分析亚麻纤维的热解过程,并估算出热解反应的表观活化能。本文中“model free”方法包括Friedman、Flynn-wall-Ozawa、Vyazovkin and Wight三种等转化率方法及Kissinger法。三种等转化率方法均得到活化能随着转化率的升高而升高的规律。四种“model free”方法显示亚麻纤维的活化能主要在155～175 kJ·mol^-1之间,使用模型拟合方法所获得亚麻纤维热解反应的活化能值在175 kJ·mol^-1左右,使用模型拟合方法和“model free”方法所得的活化能值接近。这些活化能值可以为亚麻纤维高效的热化学利用提供基础数据。     

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.     

热解温度对型煤半焦气化反应活性的影响

热解温度是影响煤焦气化反应活性的重要因素.以型煤和相似粒度的原煤为样品在不同热解温度下制备煤焦,研究了热解温度对煤焦/CO2反应活性的影响,并对型煤半焦与原煤半焦的气化性能进行比较.结果表明,褐煤中低温热解所得半焦在1000℃以上时具有较高的气化反应活性;在相同热解温度下,型煤半焦的反应活性稍高于原煤半焦;热解温度为650℃～750℃时半焦的反应活性最高.     

油页岩半焦燃烧反应活性分析

采用美国Perk in E lm er公司生产的Pyris1 TGA热重分析仪,对桦甸油页岩半焦进行燃烧特性试验研究,得到3种不同升温速率下的油页岩半焦燃烧特性曲线,并使用平均质量反应性指数和燃烧稳定性指数对半焦反应性加以评价。油页岩半焦燃烧分燃烧快速段、过渡段和燃烧慢速段3个阶段进行。随着升温速率的提高,在燃烧快速段,表观活化能为133.901 3—100.204 2 kJ/mol;在燃烧慢速段,表观活化能为146.317 1—211.409 3 kJ/mol。利用Coats-Redfern法确定了燃烧快速段反应级数为3,而燃烧慢速段则为5.5,从而得到油页岩半焦燃烧化学反应的动力学参数,为油页岩半焦的有效开发与经济利用提供了理论依据。     

Hydrogen production by methane cracking over different coal chars

Hydrogen production by methane cracking over a bed of different coal chars has been studied using a fixed bed reactor system operating at atmospheric pressure and 1123 K. The chars were prepared by pyrolysing four parent coals of different ranks, namely, Jincheng anthracite, Binxian bituminous coal, Xiaolongtan lignite and Shengli lignite, in nitrogen in the same fixed bed reactor operating at different pyrolysis temperatures and times. Hydrogen was the only gas-phase product detected with a GC during methane cracking. Both methane conversion and hydrogen yield decreased with increasing time on stream and pyrolysis temperature. The lower the coal rank, the greater the catalytic effect of the char. While the Shengli lignite char achieved the highest methane conversion and hydrogen yield in methane cracking amongst all chars prepared at pyrolysis temperature of 1173 K for 30 min, a higher catalytic activity was observed for the Xiaolongtan lignite char prepared at 973 K, indicating the importance of the nature of char surfaces. The catalytic activity of the coal chars were reduced by the carbon deposition. The coal chars had legible faces and sharp apertures before being subjected to methane cracking. The surfaces and pores of coal chars were covered with carbon deposits produced by methane cracking as evident in the SEM images. The results of BET surfaces areas of the coal chars revealed that the presence of micropores in the chars was not an exclusive reason for the catalytic effect of the chars in methane cracking.     

Influence of coal preoxidation and the relation between char structure and gasification potential

A study was carried out to ascertain the effects of coal preoxidation and carbonization conditions on the structure and relative gasification potential of a series of bituminous coal chars. Chars were prepared from two freshly mined bituminous coals and preoxidized samples derived from them. Carbonization conditions included a wide range of heating rate (0.2–10000K s^?1), temperature (1073–1273 K) and time (0.25–3600 s). Char properties were characterized in terms of analysis of char morphology, surface area, elemental composition, and gasification reactivity in air. Over the range of conditions used, preoxidation substantially reduced coal fluid behaviour and influenced macroscopic char properties (char morphology). Following slow heating (0.2 K s^?1), preoxidized coals yielded chars having higher total surface areas and higher reactivities toward gasification in air than did similar chars prepared from fresh coal. Following rapid heating (10000 K s^?1) and short residence times (0.25 s), chars prepared from preoxidized and fresh coals exhibited similar microstructural and chemical properties (surface area, $\text{C}\text{H}$ ratios, gasification rates). Carbonization time and temperature were found to be the critical parameters influencing char structure and gasification potential.     

Importance of carbon active sites in the gasification of coal chars

A demineralized lignite has been used in a fundamental study of the role of carbon active sites in coal char gasification. The chars were prepared in N₂ under a wide variety of conditions of heating rate (10 K min^?1 to 10⁴ K s^?1), temperature (975–1475 K) and residence time (0.3 s–1 h). Both pyrolysis residence time and temperature have a significant effect on the reactivity of chars in 0.1 MPa air, determined by isothermal thermogravimetric analysis. The chars were characterized in terms of their elemental composition, micropore volume, total and active surface area, and carbon crystallite size. Total surface area, calculated from C0₂ adsorption isotherms at 298 K, was found not to be a relevant reactivity normalization parameter. Oxygen chemisorption capacity at 375 K and 0.1 MPa air was found to be a valid index of char reactivity and, therefore, gives an indication, at least from a relative standpoint, of the concentration of carbon active sites in a char. The commonly observed deactivation of coal chars with increasing severity of pyrolysis conditions was correlated with their active surface areas. The importance of the concept of active sites in gasification reactions is illustrated for carbons of increasing purity and crystallinity including a Saran char, a graphitized carbon black and a spectroscopically pure natural graphite.     

混煤热解过程中的表面形态

以管式电炉为热解室,改变热解终温,在惰性气氛下对无烟煤与烟煤的混煤进行快速加热条件下的热解。采用低温氮气吸附方法研究混煤焦表面形态的变化规律。通过对吸附等温线的分析,表明煤焦具有连续、完整的孔隙结构,无定形孔的存在使得吸附迴线存在不闭合的状态。随着热解终温的升高,混煤焦的比表面积先增加后减小;随着烟煤掺混比例的增加,混煤焦的微孔容积和表面积也先增加后减小,A1B2混煤焦具有最大微孔容积和表面积。对煤焦孔隙的分形研究发现煤焦孔隙分形维数与微孔结构关系密切。混煤焦表面形态的变化规律体现了混煤热解的独立性以及相互作用。     

Reactivity of heat-treated coals

Lignite and two bituminous coals were pyrolysed at 1023 and 1223 K at different rates and for different heating times, producing different chars. The extent of devolatilization, the evolution of the pore structure and the differences in the ignition characteristics and reactivities of the chars during pyrolysis were examined. The kinetic data on the coals and chars produced by pyrolysis of these coals were obtained under ignition conditions. The apparent reactivity varied by two orders of magnitude among the parent coals and chars at a given temperature, whereas the intrinsic reactivity was found to vary over four orders of magnitude. Low-rank parent coal, high heat-up rates, and moderate pyrolysis time and temperature produced the most reactive chars. The values of hydrogen, volatile matter and ash content or the pore surface area could not provide an explanation for the differences in reactivity.     

Preparation of active carbons from coal Part II. Carbonisation of oxidised coal

Four coals differing in origin, volatile matter (VM) content, plastic properties and degree of preoxidation have been carbonised in nitrogen up to 1123 K. VM and oxygen contents of the chars obtained from unoxidised coals are very low. The VM content of the chars generally increases with an increase in the degree of coal preoxidation but the oxygen content increases only at lower degrees of preoxidation. While the mercury density of the chars decreases, the helium density increases with the degree of coal preoxidation and is related almost linearly to the helium densities of the oxidised coals. Preoxidation of coal also influences the pore size distribution of chars. The pore size distribution, which is more favourable to macropores in the case of chars obtained from unoxidised coal, becomes more and more in favour of micropores as the degree of coal preoxidation is enhanced. The percentage of micropores increases from 30% to more than 70% after coal preoxidation. Unoxidised coal chars adsorb an insignificant amount of nitrogen at 77 K while an appreciable amount of CO₂ is adsorbed at 273 K. The large difference between N₂ and CO₂ adsorption on chars prepared from coals with low oxidation degree becomes smaller as the degree of coal preoxidation increases. There is a linear relationship between the total pore volume of the char and that of the corresponding oxidised coal, indicating that the chars produced by carbonisation of oxidised coal retain fingerprints of the pore structure of the precursor oxidised coal.     

褐煤及其干馏半焦的微孔结构分析

用CO_2吸附法于298K下研究了大雁褐煤、黄县褐煤及其干馏半焦的微孔结构特性。用由Dubinin-Astakhov方程导出的关系式计算了所研究样品的微孔孔径分布和微孔有效表面积S_(micro),讨论了干馏温度对半焦的微孔孔容及平均当量半径的影响。     

The estimation of char reactivity from coal reflectogram

Measurements of the intrinsic reactivity of chars to oxygen are increasingly being sought as an indicator of the combustion potential of fuels. The coal reflectogram has been used to characterize the chemical properties of coal and its resultant char structure. In this study, six Australian coals varying in rank were separated using density separation technique to obtain vitrinite and inertinite rich fractions. Chars were obtained from these density fraction samples in a Drop Tube Furnace (DTF) at 1673 K. The reactivity of the chars was measured non-isothermally in a Thermal Gravimetric Analysis (TGA) in the temperature range of 573–1073 K. The results suggested that with the increase in the coal rank, the maximum reactivity of chars derived from vitrinite rich fractions decreases, while the reactivity of chars derived from inertinite rich fractions decreases with the increase in the inertinite content in samples and has no obvious relationship with rank. The kinetic parameters were derived using data from non-isothermal TGA after accounting for changing in surface area with conversion. The frequency factor is found to decrease with increasing coal FMR, defined as the summation of each reflectance value multiplied by its frequency, for a constant activation energy (E=146 kJ/mol). This suggests that the behavior of a maceral is characterized primarily by its reflectance distribution instead of the type of its parent coal.     

Reactivity study of combustion for coals and their chars in relation to volatile content

To determine the effect of volatile matter on combustion reactivity, the pyrolysis and combustion behavior of a set of four (R, C, M and K coals) coals and their chars has been investigated in a TGA (SDT Q600). The maximum reaction temperatures and maximum reaction rates of the coals and their chars with different heating rates (5–20 °C/min) were analyzed and compared as well as their weight loss rates. The volatile matter had influence on decreasing the maximum reactivity temperature of low and medium rank coals (R, C and M coals), which have relatively high volatiles (9.5–43.0%), but for high rank coal (K coal) the maximum reactivity temperature was affected by reaction surface area rather than by its volatiles (3.9%). When the maximum reaction rates of a set of four coals were compared with those of their chars, the slopes of the maximum reaction rates for the medium rank coals (C and M coals) changed largely rather than those for the high and low rank coals (R and K coals) with increasing heating rates. This means that the fluidity of C and M coals was larger than that of their chars during combustion reaction. Consequently, for C and M coals, the activation energies are lower (24.5–28.1 kcal/mol) than their chars (29.3–35.9 kcal/mol), while the activation energies of R and K coals are higher (25.0-29.4 kcal/mol) than those of their chars (24.1–28.9 kcal/mol).     

Investigation of alkali and alkaline earth coal gasification catalysts by EXAFS spectroscopy

EXAFS spectroscopy and several supplementary techniques have been used to investigate a variety of coal and polymer chars containing alkali and alkaline earth catalysts, including Ca, K and Rb. In-situ EXAFS measurements were performed on Rb-loaded chars during gasification. At 450 °C in 90% N₂-10% O₂, the Rb XANES exhibited a doublet structure similar to that of Rb₂O and Rb₂CO₃. EXAFS and other techniques show that the Ca in lignites, both naturally occurring and ion-exchanged, is essentially molecularly dispersed and bonded to carboxyl groups. Calcite-like features are induced in both the near-edge structure and radial structure functions of chars with increasing pyrolysis time and temperature. The structure of K is found to differ significantly in polymer, lignite and bituminous coal chars.