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.     

Kinetics of gasification of Douglas Fir and Cottonwood chars by carbon dioxide

Arrhenius kinetic parameters have been determined for the CO₂ gasification of chars (heat treatment at 1000 °C) prepared from well-characterized samples of a hardwood, a softwood and a Montana lignite. The effects of pre-pyrolysis addition of inorganic salts of the alkali, alkaline earth and transition metal groups to the wood samples have also been determined. The reactivities of the chars of the cottonwood and lignite samples exceeded that of Douglas fir char by a factor of four to seven between 700 and 900 °C. The reactivity of the wood char was related to the inorganic content of the sample. There was very little difference in the reactivity of chars prepared from the hardwood and the softwood after treatment with similar quantities of inorganic salts. The inorganic content of the lignite char was more than five times greater than that of cottonwood char, but its reactivity was similar. The carbonates of sodium and potassium were equally effective gasification catalysts. The transition metal salts were the most effective catalysts initially, but they lost their activity well before the gasification was complete. The data indicate that treatment of wood with aqueous salts results in replacement of some of the natural minerals by ion exchange, and that these exchangeable ions play a major role in controlling reactivity of the chars.     

Gasification of woody biomass char with CO2: The catalytic effects of K and Ca species on char gasification reactivity

The effects of alkali and alkaline earth metals such as potassium (K) and calcium (Ca) on CO₂ gasification reactivity of Japanese cypress (hinoki) char under various temperatures (1123-1223 K) and CO₂ concentration (20-80 vol.%) were studied using thermal gravimetric analysis. The presence of K and Ca compounds in char improved the reactivity of hinoki char for CO₂ gasification catalytically. It was also confirmed that K and Ca compounds can be supported on char to exhibit an enhanced catalytic effect during CO₂ gasification of K-char and Ca-char. The char gasification rate increased with the increase of CO₂ concentration at higher temperatures (1173-1223 K), however at lower temperature (1123 K) the gasification rate decreased at 80% CO₂. The retardation of char gasification rate at higher CO₂ concentration is caused by the inhibition effect of CO: CO is disproportionated on alkali metal catalysts to CO₂ and carbon, and affected the CO₂ gasification rate. The dependence of char gasification rate on reaction temperature yielded a straight line in an Arrhenius-type plot which indicated that there was no significant change in the gasification mechanism in the temperature range of 1123-1223 K.     

Reaction kinetics of gasification of black liquor char

CO₂ gasification of black liquor char, prepared from kraft spent pulping liquor is studied thermogravimetrically up to 775°C. The gasification rate is described by Langmuir-Hinshelwood type kinetics and activation energy is similar to that found for alkali metal impregnated porous carbons. However the rate is about 20 times larger than that of coal char mixed with 10-20% Na₂ CO₃. CO inhibition is relatively small and the rate is first order in carbon up to 80% conversion. The behavior is explained by a fine and three-dimensional dispersion of sodium salts in the char. The rate is insensitive to pulping conditions over the range of industrial practice.     

Steam gasification of coal char using alkali and alkaline-earth metal catalysts

The catalytic effect of alkali and alkaline-earth metal salts or oxides on the gasification of Chinese Linnancang coal char was investigated at atmospheric pressure and a temperature of 800 °C. The order of catalytic activity is K₂SO₄ or K₂CO₃ Na₂CO₃ KCl NaCl CaCl₂ or CaO. The effect of amount of catalysts added on catalytic activity was studied. The distribution of K₂CO₃ or CaO catalysts on the coal char surface for different methods of catalyst loading was examined by an electron microprobe analyser. The relation between the catalytic activity and distribution of catalysts were illustrated. The loading method of K₂CO₃ has little effect on its catalytic activity but that of CaO influences the activity significantly.     

生物质炭水蒸气气化制取富氢合成气

以生物质炭为原料在上吸式固定床气化炉中进行水蒸气气化制备富氢合成气,考察了不同原料、粒径和催化剂对生物质炭水蒸气气化影响。结果表明,不同类型炭气化结果存在较大差异,其中木片炭气化结果最优,其次是玉米芯炭和稻壳炭,秸秆炭气化结果最差,木片炭产氢率最大为222.8g/kg。粒径的改变主要影响炭转化率,炭转化率随着粒径的增加呈增加趋势。通过炭吸收方式负载催化剂为有效的方法,其中在相同钾盐质量分数下,KOH催化能力较优于K₂CO₃,且气化速率为未加催化剂条件下的两倍。炭转化率随着碱液浓度的增加而增加,但浓度过高会增加灰分含量从而不利于产氢率,玉米芯炭催化气化最高产氢率为197.8g/kg,在碱质量分数为6%下获得。     

碱金属Na对黑液水煤浆焦-CO2气化特性的影响

黑液中富含大量的碱金属Na及其化合物,这些碱金属将在黑液水煤浆焦气化过程中起到催化气化的作用.为了研究黑液水煤浆焦-CO2催化气化反应特性,采用XRD、SEM和热重分析技术对黑液水煤浆焦和普通水煤浆焦CO2催化气化实验进行分析,得到了焦炭表面孔隙分布情况、煤浆焦样和气化后残渣XRD分析结果,以及等温条件下气化反应时碳转化率数据.试验结果表明:黑液水煤浆焦表面密集分布很多"斑点"和微孔,说明碱金属Na盐在焦碳表面形成了活化中心点,它们在气化过程中起到催化作用;碱金属Na使焦样表面具有更强的反应位,削弱C-C键的强度,使气化反应更容易进行;同时由于碱金属催化剂在高温气化时将与煤中矿物质反应生成惰性物质,从而可能削弱催化效果.从黑液水煤浆与普通水煤浆XRD晶相分析中可以看出碱金属Na盐主要以氯化钠、硅酸钠形式存在,气化反应后生成的晶相组成主要是霞石和微斜长石.     

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.     

Carbon nanotube growth from alkali metal salt nanoparticles

We demonstrate that alkali metal salts, including KCl, NaCl, K₂SO₄, Na₂SO₄, K₂CO₃, and Na₂CO₃, can act as catalysts for carbon nanotube (CNT) growth in chemical vapor deposition (CVD). The solution of alkali metal salt, water and ethanol was nebulized and was introduced into the CVD reactor, producing CNT with a multi-walled structure. Individual CNT are terminated with an onion-shaped carbon tip even when different alkali metal salt catalysts are used. Through observation and analysis of the catalyst particles and the resulting product, we elucidate the mechanism by which the alkali metal salt nanoparticles are served as “seeds” and provide nucleation sites for CNT growth. The ethanol decomposes to release carbon atoms into the catalyst particles, and the carbon nucleates and then begins to assemble on the surface of the catalyst particles, resulting in the CNT growth. By altering growth conditions, branched CNT and single-walled CNT also can be grown on alkali metal salt nanoparticles.     

Mechanisms of the alkali metal catalysed gasification of carbon

The catalytic effects of alkali metal salts in the gasification of carbonaceous materials by oxygen, steam and carbon dioxide are described. The most effective catalysts are generally the carbonates, oxides and hydroxides; other active salts tend to convert to these species under gasification conditions. Current theories of the mechanism of this type of catalysis are reviewed. Thermodynamic considerations, the results of thermal analysis and the magnitude of kinetic isotope effects suggest that cyclic sequences of elementary reactions are responsible for the catalytic phenomena.     

Gasification of graphite in carbon dioxide and water vapor—the catalytic effects of alkali metal salts

The catalytic effects of a series of alkali metal salts in promoting the gasification of a graphite powder by carbon dioxide and water vapor have been studied by thermogravimetry between 700 and 1100°C. Lithium salts, specifically the carbonate and hydroxide, were the most active catalysts for both reactions. Cyclic processes which may account for the observed catalytic effects were evaluated from the standpoint of thermodynamic feasibility.     

A study of the role of alkali metal salts as char gasification catalysts by Knudsen cell mass spectrometry

Knudsen cell mass spectrometry has been used to determine the gaseous species in thermodynamic equilibrium with admixtures of carbon and alkali metal salts. Both halides and carbonates were used as additives. In addition to measurements of equilibrium vapor pressures, the effect of added steam or carbon dioxide on the composition of the Knudsen cell effusate was examined. With the carbonates, the equilibrium pressure of the alkali metal over the solid phase is at least twice that of the decomposition pressure of the pure salt, but orders of magnitude below that appropriate to the carbothermic reduction of the carbonate. By way of contrast, the alkali halide-carbon admixtures exhibit no free metal vapor up to 1000 K, a temperature in the range employed for gasification. Apparently, the carbonates react with the carbon to form a stable phase with alkali metal activity substantially below unity. Inferences on the composition of such a phase and its role in promoting the carbon gasification rate are discussed.     

Kinetics of steam gasification of nascent char from rapid pyrolysis of a Victorian brown coal

Steam gasification of nascent char from rapid or slow pyrolysis of a Victorian brown coal was performed at 1073–1173 K in a novel drop-tube/fixed-bed reactor, in which steam-containing gas was forced to pass through an extremely thin bed of nascent char particles at sufficiently high velocity and large flux. The nascent char underwent parallel reactions consisting of non-catalytic gasification and catalytic one. The non-catalytic gasification followed first-order kinetics with respect to the fraction of unconverted carbon, and the rate constant was hardly influenced by operating variables such as heating rate for the pyrolysis, total pressure and even period of isothermal heating between the pyrolysis and gasification. The overall activity of inherent catalysts, alkali and alkaline earth metallic species, diminished due to volatilization and intra-particle deactivation, both of which were induced by the gasification. As a result, the catalytic gasification took place within a limited range of the char conversion up to 60–80%. The initial catalyst activity and the kinetics of activity loss largely depended on the operating variables as above and also partial pressure of steam.     

碱金属及灰分对煤焦碳微晶结构及气化反应特性的影响

通过对原煤、酸洗原煤、酸洗后负载NaOH的原煤在750~1050℃热解制得焦样,用X射线衍射技术考察了热解温度、NaOH负载量以及灰分对热解过程中煤焦微晶结构变化的影响,并运用高温高压热天平(PTGA)考察了热解后煤焦的气化反应活性。结果表明碱金属及灰分的存在可以明显减小煤焦的微晶结构参数的变化(堆垛高度Lc、微晶尺寸La、及晶层间距d002),阻碍煤焦的石墨化进程,提高煤焦的气化反应性。随着热解温度的升高,堆垛高度Lc增大显著,而微晶尺寸La和晶层间距d002变化较小。煤焦的气化反应性k0和煤焦微晶结构参数Lc、d002存在如下关系:lnk0=a(Lc/d002)+b;研究还表明用氧化还原循环机理来描述碱金属的催化作用机理是不恰当的,但碱金属Na的存在可以明显降低煤焦的石墨化程度,提高煤的活性,对煤焦的气化起到部分催化作用。     

Catalytic CO2-gasification of graphite versus coal char

The effects of alkali carbonate catalysts on the C0₂-gasification of Illinois No. 6 hvB bituminous coal char, demineralized Illinois No. 6 coal char, Pittsburgh No. 8 hvA coal char, Navajo subbituminous coal char, Reading anthracite coal char, North Dakota A lignite char and spectroscopic grade highest purity graphite are reported. Alkali carbonate salts are effective Boudouard catalysts for all these substrates, but salient differences between coal char and graphite reactivity are observed. To account for these differences, a redox mechanism based on alkali hydride intermediates is proposed.     

木炭水蒸气催化气化制取合成气

以木炭为原料,选用KOH、K₂CO₃、KHCO₃、KNO₃为催化剂,在上吸式固定床气化炉中,进行水蒸气催化气化制取合成气实验。考察了不同催化剂、催化剂用量、水蒸气流量、气化温度对木炭水蒸气气化的炭转化率、产氢率、气体组成体积分数和H₂/CO值的影响。实验通过炭吸收催化剂溶液来负载催化剂,实验结果表明：4种催化剂都可提高木炭气化效率,在浸渍相同质量分数的催化剂溶液下,催化活性顺序为KOH>K₂CO₃>KHCO₃>KNO₃。碳转化率及产氢率都随着催化剂溶液浓度的增加而增大,但浓度过高增加趋势逐渐变缓,催化剂溶液质量分数在4%~6%较为合适。增加水蒸气流量,气体产物中H₂体积分数增大,H₂/CO值增大。升高温度可促进炭气化反应,950℃时碳转化率和产氢率分别达到98.7%和145.23g/kg。实验可得到H₂/CO比1.53~4.09范围间的合成气,可用于合成甲醇、甲烷、二甲醚等燃料。     

碱金属对煤热解和气化反应速率的影响

通过对原煤、酸洗原煤、负载碱金属的酸洗原煤在800～1050℃热解制得焦样,用X射线衍射技术考察了碱金属对煤焦微晶结构的影响,在加压热天平（PTGA）上考察了煤样的热解过程,以及焦样的二氧化碳气化活性。结果表明：碱金属对煤的热解和气化阶段都有影响。在热解阶段,碱金属的存在抑制了煤焦的石墨化进程,降低了热解反应活化能,促进了热解反应的进行;在气化阶段,作为催化剂的碱金属,降低了气化反应活化能,延长了反应速率达到最大值的时间。修正的随机孔模型可以较好地描述煤焦-CO₂的气化反应过程。     

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。     

The effect of alkali metal salts promoted MgO catalysts for the oxidative coupling of methane

Catalytic activities of the alkali metal salts are discussed based on experimental observations in a fixed bed flow reactor at atmospheric condition and instrumental analysis. LiCl (30 wt%) and NaCl (30 wt%) promoted MgO catalyst showed superior activity to mono alkali metal salts promoted MgO catalysts based on the C₂ yield. This suggests that the bialkali metal salts neutralize the nonselective acid sites due to synergistic effect. Moreover, it is estimated that the active sites is O^- ions.     

煤催化气化催化剂发展现状及研究展望

煤与气化剂(如水蒸气、CO2、H2和O2)之间的气化反应最有效的催化剂主要为碱金属、碱土金属以及过渡金属的盐类,根据其组成,详细论述了煤催化气化催化剂的特性。据研究,在气化反应中碱金属催化剂如Na、K等易与煤中矿物质如Si或Al反应致使催化剂失活,同时过渡金属易被煤中S毒化,这在一定程度上制约了煤催化气化工业化进程。论述了煤催化气化催化剂的研究方向,认为开发新型高效、低廉且易回收催化剂是有必要的。