Coal volatile yield and element partition in rapid pyrolysis

The volatile yields from thirteen coals, varying in rank from lignite to anthracite, were measured under conditions of high heating rate (10⁵Ks^?1) and high temperature (1600–2400 K) representative of pulverized coal combustion and were related to coal characteristics as determined in standard laboratory tests. Positive correlations with coal reflectance and elemental composition were obtained. The distribution of coal elements between the volatiles and residual char was also investigated. The results show that hydrogen and oxygen from the coal are preferentially distributed in the volatile phase. Coal nitrogen evolution parallels the total volatiles at high temperature, whereas coal sulphur is preferentially retained in the char and no relationship between S evolution and coal rank is evident. These results are discussed in relation to the prevalent active groups in the various ranks of coal.     

非等温热重分析长焰煤热解过程与动力学特征

为提高煤热解过程中焦油的产率,用非等温热重分析方法研究了不同粒径、热解终温和升温速率条件下长焰煤的热解过程和机理,分析了20和100℃/min升温速率下长焰煤热解过程特征,并求解了热解动力学参数。结果表明,煤颗粒在2.8mm以下时,粒径对热解过程影响较小;热解终温越高,热解最终固体产物中挥发分产率越低;升温速率越快,挥发分的析出速率越快。在同一升温速率下,不同热解温度段得到的活化能呈现两头大中间小的特征,且指前因子随活化能的增大而增大。     

水煤浆挥发分热解的FG-DVC模型

采用FG-DVC模型对水煤浆不同升温速率下的热重实验数据进行处理.结果表明,在挥发分析出温度范围内,表观活化能随升温速率升高而减小,DTG曲线峰值随升温速率升高而增大,符合实验数据的规律;模拟结果与实验曲线十分接近,且n=1情况下吻合情况最好,说明用FG-DVC模型描述水煤浆挥发分析出机理比较适合.     

A single particle model for the evolution and combustion of coal volatiles

A single particle model is proposed for the evolution and combustion of coal volatiles. The analysis is divided into preignition and postignition stages. The rate limiting steps for devolatilisation are assumed to be heat transfer (both to and through the coal particle) as well as chemical reaction (overall decomposition represented by the distributed activation energy kinetics). Approximate expressions are proposed for the estimation of volatile burnout times. The model results are compared with the experimental data reported for single coal particles in stagnant as well as convective oxidizing environments. The application of the model to fluidized beds is discussed. Model predictions are also compared with the volatile burnout times in fluidized beds.     

Kinetics of lignite pyrolysis in an entrained-flow,isothermal furnace

An entrained-flow, isothermal furnace was used to study rapid pyrolysis in nitrogen of a Texas lignite. Weight losses for fixed residence times up to 0.4 s were independent of particle size over the range of mean diameter from 41 to 201 μm and increased with temperature elevation over the range 700 to 1000 °C. The maximum yield of volatiles, 66.7 wt% of the dry-ash-free coal, was particle size independent and relatively temperature independent over the range of operating conditions. The maximum yield represented a fractional increase of 1.30 over the ASTM volatile matter. This yield enhancement was associated with a reduction in the preponderence of secondary char-forming reactions of the volatiles and not directly with the increase in heating rate from 20 °C s^?1 in the ASTM test to >10⁴ °C s^?1 in the isothermal furnace. Pyrolysis was multi-stage; initial rapid devolatilization to 50% weight loss was followed by slower latter-stage devolatilization. First stage pyrolysis could be fitted by a single, first-order kinetic expression whether total weight loss was taken as just that for the first stage or for complete devolatilization. Rate constants for the two cases were 0.6 × 10³exp(? 45 kJ/mol/RT) and 0.9 × 10²exp(? 32 kJ/mol/RT) s^?1, respectively. One or more additional equations would be necessary to describe the completion of pyrolysis. It is argued that the relatively low activation energy is not necessarily indicative of physical rate control and therefore not necessarily in contradiction with the absence of particle size effects.     

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).     

神木煤显微组分热解特性和热解动力学

在高压热天平上考察了神木煤显微组分在不同热解温度、压力和升温速率下的热解行为 .并利用分布活化能模型 (DAEM)研究了显微组分的热解动力学 .结果表明 :在相同的热解条件下 ,镜质组比丝质组有较高的挥发分收率 .随热解温度升高 ,镜质组和丝质组的挥发分收率增加 .热解升温速率和压力对镜质组和丝质组的挥发分收率略有影响 .DAEM的动力学处理表明 :镜质组的归一化热解速率高于丝质组 ,热解活化能较低.     

煤中挥发分线性可加性的实验研究

根据煤的挥发分国家测试标准,对一系列单种煤及其混合煤的挥发分进行了测试,用回归分析法探讨了煤的挥发分的线性可加性,结果表明:煤的分析基挥发分Vad具有线性可加性,而干燥无灰基挥发分Vdaf是非线性可加的,这对准确指导炼焦配煤、动力配煤及煤的综合利用具有重要的理论意义和实际意义。     

Flash pyrolysis of coals: comparison of results from 1 g h−1 and 20 kg h−1 reactors

The performances of 1 g h^?1 and 20 kg h^?1 flash pyrolysers are compared for three Australian coals: Loy Yang brown coal (Victoria), Liddell bituminous coal (New South Wales), and Millmerran sub-bituminous coal (Queensland). The two reactors gave comparable yields of tar, char and C₁–C₃ hydrocarbon gases over a range of operating conditions for each particular coal. The yield of total volatile matter from Millmerran coal was similar from both reactors, as were the compositions of chars from Loy Yang coal and tars from the Liddell and Millmerran coals. For Millmerran coal, the yields of tar, C₁–C₃ gases and volatiles from the large reactor below 650 °C, were slightly lower than for the small reactor, possibly owing to a shorter retention time of Millmerran coal particles in the large-scale reactor. At a temperature near 600 °C tar yields were independent of tar concentration in the effluent gas, over a range 0.0025–0.1 kg m^?3 for Liddell coal, 0.005–0.26 kg m^?3 for Millmerran coal and 0.0045–0.09 kg m^?3 for Loy Yang coal. The tar yields from Millmerran and Liddell coals at 600 °C in the large reactor, correlate directly with the atomic $\text{H}\text{C}$ ratio of the parent coal, in the same manner as that found for a wider range of bituminous coals in the small-scale reactor.     

重质有机质热解过程中挥发物的产出与逸出气体分析研究进展

煤、重质油、生物质等重质有机质富含碳氢共价结构,其轻质化和定向化学品转化是加工利用的主要目标。热解是重质有机质加工过程中最直接、最基础的反应过程,挥发物作为重要的热解产物,其组成分布及在热解过程的演变规律解析是研究的关键和热点问题。本文综述了重质有机质热解过程中挥发物的生成过程,总结了反应类型及产物组成随温度升高的阶段性变化,并以煤、油砂、油页岩、生物质、含油污泥、市政污泥以及废橡胶为例,对比了不同种类的重质有机质热解挥发物产出的异同。针对重质有机质挥发物逸出特性分析方法,本文重点介绍了质谱、傅里叶变换红外光谱逸出气体分析技术,举例说明了各方法在重质有机质有机结构解析、热解工艺条件优化、污染物控制、催化剂设计等研究上的应用,并且就现阶段热解过程逸出气体的定量分析方法和应用进行了概述。最后,本文还就重质有机质热解逸出气体分析技术提出了建议和展望,以期为重质有机质的热解研究提供参考和借鉴。     

Mathematical models of the thermal decomposition of coal: 1. The evolution of volatile matter

To assist studies of the coking process, a series of mathematical models of coal carbonization has been developed. Models of the physical properties (e.g. specific heat, density, porosity and thermal conductivity) of coal during decomposition to coke were constructed in terms of the changes in the chemical composition and srructure. These models of physical properties were then used to develop further models of heat and mass transfer in a coke oven charge, although the models may find more general application in studies of coal-based processes. This Paper describes the model of the chemical changes during thermal decomposition. To have sufficient generality for the applications envisaged, the model describes the kinetics of the release of the main volatile matter constituents, thereby permitting the changes in the mass and composition of the solid residue to be estimated by element balances. Correlations for the parameters of the model are derived so that predictions of volatiles evolution can be made from the coal ultimate analysis and volatile matter and the heating profile.     

Nitrogen evolution during rapid hydropyrolysis of coal

The behavior of nitrogen evolution during rapid hydropyrolysis of coal has been investigated at temperatures ranging from 923 to 1123 K and hydrogen pressure up to 5 MPa using a continuous free fall pyrolyzer. Three coals have been tested in this study. The dominant nitrogen gaseous species is ammonia, together with a little amount of HCN because most of HCN is converted to NH₃ through secondary reactions. The results show that the evolution of nitrogen in coal is caused mainly by devolatilization at temperatures below 973 K, while the evolution of volatile nitrogen in char is accelerated with increasing temperature and hydrogen pressure. The mineral matter in coal act as catalysts to promote the evolution of volatile nitrogen in char to N₂ apparently at high temperatures of 1123 K, as found during pyrolysis of coal by Ohtsuka et al. A pseudo-first-order kinetic model was applied to the evolution of nitrogen in coal during rapid hydropyrolysis. The model shows the activation energy for the nitrogen evolution from coal is 36.6–58.6 kJ/mol while the rate of the nitrogen evolution depends on hydrogen pressure in the order of 0.16–0.24.     

Thermogravimetry and decomposition kinetics of British Kimmeridge Clay oil shale

The characteristics of volatile matter evolution and the kinetics of thermal decomposition of British Kimmeridge Clay oil shale have been examined by thermogravimetry. TG has provided an alternative to the Fischer assay for shale grade estimation. The following relation has been derived relating TG % volatiles yield to the shale gravimetric oil yield: oil yield (g kg^?1) = (TG volatiles, % × 5.82) ? 28.1 ± 14.5 g kg^?1. A relationship has also been established for volumetric oil yield estimation: oil yield (cm³ kg^?1) = (TG volatiles, % × 4.97) – 5.43. TG is considered to give a satisfactory estimation of shale oil yield except in certain circumstances. It is found to be less reliable for low yield shales producing <≈40 cm³ kg^?1 of oil (≈10 gal ton^?1) where oil content of the TG volatiles is low: volumetric yield estimation accuracy is affected by variations in shale oil specific gravity. First order rate constants, k = 4.82 × 10^?5s^?1 (346.3 cm³ kg^?1shale) and k = 6.78 × 10^?5s^?1 (44.6cm³ kg^?1shale) have been obtained for the devolatilization of two Kimmeridge oil shales at 280 °C using isothermal TG. Using published pre-exponential frequency factors, an activation energy of ≈57.9 kJ mol^?1 is calculated for the decomposition. Preliminary kinetic studies using temperature programmed TG suggest at least a two stage process in the thermal decomposition, with two maxima in the volatiles evolution rate at ≈450 and 325 °C being obtained for some samples. Use of published pre-exponential frequency factors gives activation energies of ≈212 and 43 kJ mol^?1 for these two stages in the decomposition.     

煤热解动力学模型的建立

通过对煤热解反应动力学分析,基于分布活化能模型DAEM,建立了集总反应动力学模型表示煤炭热解过程。确定了可以预测热解产物组成、分布与热解终温和升温速率关系的动力学方程。结果表明：随热解温度升高,各种挥发分产物析出率越来越接近最大产率;半焦C含量增加,但产率下降,H,O,N和S等元素降低。升温有利于提高半焦脱硫率、脱氮率。温度为600℃左右时,除H2外的大部分挥发分基本析出,半焦元素变化幅度减小。热解终温较低且一定时,较慢的升温速率有利于各热解挥发分最大限度析出。秦丽娜     

Graphical methods for determination of the mineral-matter-free properties of coal and the mineral matter/ash ratio

Mean values of the mineral-matter-free specific energy, volatile matter and moisture content of coal, together with the mineral matter/ash ratio and the mineral matter moisture content, can be obtained from linear regression analyses of the appropriate analytical quantities against ash yield for grouped coal samples from geographical areas. The results can be used to assess rank and quality variations and the determined mineral matter/ash ratio can be used to correct individual analyses to a mineral-matter-free basis.     

Flash pyrolysis of coals: influence of cations on the devolatilization behaviour of brown coals

The influence of cations on the pyrolysis behaviour of brown coals under flash heating conditions was investigated by means of a small fluidized-bed pyrolyser. A stream of coal particles in nitrogen was injected at rates of 1–3 g coal/h directly into a heated bed of sand fluidized by nitrogen. Yields of tar, C₁–C₃ hydrocarbons and total volatile matter from four Gelliondale brown coals and a Montana lignite were determined as a function of pyrolysis temperature. With all coals the maximum tar yield was obtained at 600 °C. Removal of cations present in the coals markedly increased the yields of tar and total volatile matter, with little effect on the yields of hydrocarbon gases. The converse was also observed in that the addition of Ca²⁺ to a cation-free coal decreased the yields of tar and total volatile matter. The extent of the reduction in tar yield at 600 °C in the presence of cations was found to be similar for all coals. After acid washing, tar yields appear to correlate with the atomic $\text{H}\text{C}$ ratios of the coals in a manner similar to that observed previously with bituminous coals.     

Effects of volatile-char interactions on the reactivity of chars from NaCl-loaded Loy Yang brown coal

In a fluidised-bed gasifier, char particles are in constant contact with the volatiles and the products from the gasification and thermal cracking of volatiles. The highly reactive nature of volatiles as well as the vulnerable structure of char from brown coal means that there are strong interactions between volatiles and char. The purpose of this study is to investigate the effects of volatile-char interactions on the reactivities of chars from a Victorian brown coal. NaCl-loaded and acid-washed Loy Yang brown coal samples were pyrolysed in a novel fluidised-bed/fixed-bed reactor at 900 °C that has been specially designed to investigate the volatile-char interactions. Char reactivity in air was measured in a thermogravimetric analyser (TGA) under conditions minimising mass transfer limitations. The oxidation of char with air in the TGA showed apparent kinetic compensation effects between the apparent activation energies and pre-exponential factors. The presence of the apparent kinetic compensation effect is a reflection of the heterogeneous structure of char having sites of a wide range of energy levels. Our results have clearly shown that volatile-char interactions can lead to drastic decreases in char reactivity due to the volatilisation of Na and the changes in char structure. The reactivities of chars from the pyrolysis of the catalyst-free H-form Loy Yang brown coal provided unequivocal evidence for the changes in char structure after volatile-char interactions. For chars from the NaCl-loaded Loy Yang brown coal, it appears that the condensation of ring structures in char as a result of volatile-char interactions could have also led to changes in the dispersion of Na catalyst.     

高硫煤泥燃烧过程中的汞排放特性

通过固定床程序升温汞脱附试验系统对所选高硫煤泥中汞排放特性进行在线监测,并利用热重分析仪对煤泥热解和燃烧特性进行研究,结合试验所得热解和燃烧特性参数,采用分布活化能模型,进行动力学分析。结果表明：煤泥的热解和燃烧过程可分为3个阶段,非等温条件下,随着升温速率增加,热解过程在高温区发生,最大失重率提升,对应峰值温度偏移,产生热滞后,利于挥发分析出;在煤泥热解过程中少量氧气的参与,抑制挥发分的析出,在7% O₂条件下综合热解特性参数值D最大。热解性能随CO₂浓度升高而得到改善;煤泥燃烧性能随升温速率的增加而得到加强,其活化能随转化率变化呈现“U”型趋势分布;煤泥中无机汞化合物主要为HgCl₂、α-HgS、HgSO₄以及硅铝酸盐类结合汞,总汞释放主要范围对应200~600℃;煤泥中汞释放量随O₂浓度增大,CO₂气氛条件下,随着CO₂浓度增加,总汞释放量逐渐增大。     

煤粉着火燃烧过程光学诊断研究发展

煤炭储量丰富,尽管新能源和可再生能源快速发展,煤炭资源在未来几十年仍将作为我国一次能源重要组成部分。同时煤炭利用带来很多环境污染问题,因此未来煤炭资源的利用逐渐向高效、低碳、低污染物排放利用方式转变。随着光学技术的不断发展,涌现出多种适用于煤粉燃烧诊断的原位非接触式光学诊断技术,极大地促进了燃烧学的发展,为煤炭清洁高效利用提供了更多试验手段。介绍了国内外煤粉着火、不同方式下燃烧特性的光学诊断研究进展,对煤粉单颗粒和煤粉颗粒流的着火燃烧过程的光学诊断研究进行总结。目前常用的煤粉燃烧光学诊断技术主要包括全光谱成像、CH*/C2*化学发光成像、平面激光诱导荧光(PLIF)、双色/三色高温计、米氏散射、激光诱导白炽光、相干反斯托克斯-拉曼光谱、激光诱导击穿光谱等多种先进的光学诊断技术,可对煤粉单颗粒、颗粒流的着火延迟、脱挥发分、挥发分燃烧、着火模式、环境因素(环境温度、氧浓度、气氛)、富氧燃烧、水-氧燃烧、煤中碱金属释放等多方面关键问题进行光学诊断研究,为煤炭清洁高效利用提供了理论和试验基础。采用OH-PLIF和三色高温计对热解半焦和神华烟煤混合燃料共燃的着火和燃烧特性进行研究。综合考虑着火延迟和混合物的燃尽率,热解半焦的最佳掺混比为20%,为热解半焦的实际工业应用提供了参考。同时采用500 Hz、5 k Hz高时间、空间分辨率的OH-PLIF技术探究煤粉颗粒流中单颗粒挥发分燃烧的发展过程和挥发分着火的时序演变过程,通过二者的结合获得煤粉颗粒流从着火到挥发分燃烧的时间特性。采用OH-PLIF技术对烟煤和褐煤煤粉颗粒流燃烧火焰的脱挥发分和挥发分燃烧行为进行探究,提出采用OH信号径向分布的相对标准偏差探究火焰稳定性的方法。相同燃烧条件下,烟煤煤粉颗粒流燃烧的稳定性高于褐煤。基于OH-PLIF和CH*化学发光诊断技术,提出一种用于探究煤粉颗粒流中颗粒挥发分燃烧振荡特性分析方法——动态模态分解方法(DMD)。随着氧浓度的增加,挥发分火焰振荡增强。颗粒的聚集可能导致煤粉挥发分燃烧的低频振荡。相反,单独或分离的颗粒燃烧会产生较大的振荡频率。但目前取得的成果还不够完善,需要继续深入开展煤粉燃烧的光学诊断试验研究,对污染物NOx的生成及排放、新型水氧燃烧技术中水蒸气作用机理等方面深入探索,开发出新型清洁煤燃烧技术,为我国煤炭资源清洁高效利用做出贡献。     

Dependence of pyrite concentration on kinetics and thermodynamics of coal pyrolysis in non-isothermal systems

Thermogravimetric investigation has been undertaken to evaluate the kinetic and thermodynamic parameters and the influence of pyrite concentration on coal pyrolysis in inert atmosphere. Weight loss up to 150 °C are 4.3 wt.%, 5.5 wt.% and 7.0 wt.% in 1%, 5% and 10% pyrite blended coals, respectively, and in the raw coal (8.2 wt.%) suggests that the presence of pyrite can influence loss of moisture and volatile matter characteristics. The essential weight loss is restricted in the temperature range of 220–650 °C for all the samples. DTA shows two peaks in that region; the first one is related with the primary carbonisation of coal and the second, which appears at a relatively higher temperature, is attributed to the decomposition of pyrite, both shifting to higher temperature with increasing pyrite concentration. Further presence of pyrite in substantial amount decreases the reactivity of coal for pyrolysis; however, its negligible amount has no significant impact on carbonisation. Kinetic study reveals that, with increase in temperature, the rate constants continuously increase; however, the trend is irregular for activation energy (range: 20.42–36.17 kJ mol⁻¹ from Arrhenius equation and 28.72–38.29 kJ mol⁻¹ from Coats and Redfern equation) and frequency factor (range: 1.35–2.11×10⁻⁴ s⁻¹) for all the samples. It has been inferred that decomposed products of pyrite rather than pyrite itself were the real catalyst for enhancing coal pyrolysis; non-stoichiometric pyrrhotite accelerates carbonisation and/or elemental sulphur stabilises free radicals through cross linking. The pyrolytic reaction in all the systems proceeds with the absorption of heat, decrease in randomness and non-spontaneous in nature. It is suggested that the carbonisation process is a diffusion controlled reaction rather than thermodynamically controlled.