Importance of palladium dispersion in Pd/Al2O3 catalysts for complete oxidation of humid low-methane–air mixtures

The catalytic oxidation is considered as an environmental benign method for utilization of various methane-poor gas mixtures, including humid post-ventilation air of coal mines. The small crystallites of palladium phase in the Pd/Al₂O₃ catalyst decrease temperatures necessary to ignite the methane oxidation reaction and to achieve complete conversion of methane. The isotopic exchange of oxygen between the catalyst and the gas phase, the temperature-programmed reduction (TPR) with methane and the X-ray photoelectron spectroscopy studies suggest that it can result from a higher number of the Pd–PdO sites present on the catalysts with small palladium crystallites. The inhibiting effect of water vapour present in the reaction mixture increases with lower dispersion of palladium phase as well as with the water concentration in the feed. The larger palladium crystallites are more significantly affected by the presence of water. It is suggested that water vapour blocks the Pd–PdO active sites. The catalysts with small crystallites (<6.6 nm) of palladium can be successfully used for mitigation of the emission of methane from coal mine post-ventilation air and, after increasing of the methane concentration to 1–2 vol.%, for its utilization for the energy production. In the case of such catalysts even a high concentration of water vapour has the least negative influence on the catalyst activity and it will not interfere with obtaining of the 100% conversion of methane below 650 °C.     

甲烷催化燃烧反应工艺研究进展

甲烷催化燃烧是一种清洁高效的甲烷燃烧技术,在节能减排中具有重要的应用价值。从催化剂、反应工艺和过程强化等方面对近年来甲烷催化燃烧技术进行综述,重点介绍颗粒催化剂固定床反应工艺、整体式催化剂反应工艺、流化床反应工艺和吸放热耦合反应工艺研究进展。用于固定床反应器的颗粒催化剂主要为负载型贵金属催化剂和非贵金属氧化物催化剂。贵金属催化剂活性好,起燃温度低,适合低浓度甲烷的催化燃烧。非贵金属氧化物催化剂耐高温性好,适合较高浓度甲烷燃烧体系。整体式催化剂的甲烷催化燃烧反应工艺中,最常用的是蜂窝陶瓷和金属合金等整体式催化剂的多段式催化燃烧反应器的设计。设计直接采用多段式整体催化剂,催化剂的位置不同,发挥的催化作用也不同。流化床催化燃烧装置具有燃烧过程接触面积广、热容量大和换热效率高等特点,可有效避免传统的固定床催化燃烧反应工艺存在的问题,非常适合应用于低浓度甲烷的催化燃烧过程。利用甲烷催化燃烧强放热的特点,将催化燃烧产生的热量进行时间或空间的耦合,可以开发出吸-放热耦合反应工艺。其中,固定床催化反应器中的流向变换强制周期操作作为一种高效的过程强化技术,在节约反应器成本的同时,可以提高反应热量的利用率。     

A logic-based controller for the mitigation of ventilation air methane in a catalytic flow reversal reactor

The control system of a catalytic flow reversal reactor (CFRR) for the mitigation of ventilation air methane was investigated. A one-dimensional heterogeneous model with a logic-based controller was applied to simulate the CFRR. The simulation results indicated that the controller developed in this work performs well under normal conditions. Air dilution and auxiliary methane injection are effective to avoid the catalyst overheating and reaction extinction caused by prolonged rich and lean feed conditions, respectively. In contrast, the reactor is prone to lose control by adjusting the switching time solely. Air dilution exhibits the effects of two contradictory aspects on the operation of CFRR, i.e., cooling the bed and accumulating heat, though the former is in general more prominent. Lowering the reference temperature for flow reversal can decrease the bed temperature and benefit stable operation under rich methane feed condition.     

煤矿乏风瓦斯利用技术概况

叙述了国内外乏风瓦斯的利用现状及技术开发进展,以及低浓度甲烷催化燃烧催化剂的研究现状。同时对乏风瓦斯催化燃烧催化剂进行了探索性筛选实验,通过实验发现,Mn-Co/Al2O3复合氧化物催化剂对乏风瓦斯催化燃烧效果较好,可以作为重点进行研究。     

高瓦斯大采深矿井通风系统改造设计

为加强煤矿通风管理工作,防止和减少瓦斯事故,保障煤矿职工生命安全,根据《防治煤与瓦斯突出规定》,以东庞矿在高瓦斯大采深区域通风系统改造为例,在降低矿井通风阻力、增加深部区域有效风量、核实现有通风装备、增加12采区深部安全出口这几方面给予了论证,矿井通风系统改造完成后,矿井深部水平主要通风路线长度将由15 km降至8.5km,全矿井总风量将由360 m3/s提高至475 m3/s,风井主要通风机负压将由原预计的4 000Pa以上降至2 156~2 822 Pa。经校核,现有主要通风机满足通风系统改造后的矿井风量和负压的要求。     

蜂窝陶瓷蓄热体的抗热震性研究进展

本文针对高风温燃烧技术和煤矿乏风瓦斯氧化技术的蜂窝陶瓷蓄热体,系统地综述了影响蓄热体抗热震性的因素,数值模拟分析了蓄热体在热冲击下的温度场和热应力场的分布特点,总结了蜂窝陶瓷蓄热体热震损伤机理等的研究进展情况,并提出今后的研究及发展方向。     

用于煤层气开采的柔性排采系统的研究

煤层气是一种有很大利用价值的清洁能源,合理开采煤层气对优化我国能源结构具有重要的意义。有杆泵在煤层气开采中存在着能耗大、投资高和操作不便利等问题。针对现有杆泵抽油机存在的问题,研制了柔性排采系统,该系统具有结构简单、体积小、能量传递环节少、多功能、投资少等特点。     

Catalytic Combustion of Methane over Cobalt–Magnesium Oxide Solid Solution Catalysts

A series of cobalt–magnesium oxide solid solution catalysts (CoMgO) have been prepared using urea combustion methods, and characterised by X-ray diffraction (XRD) and laser Raman (LR). The catalytic activities for methane combustion have been tested in a continuous-flow microreactor. The Co content has a significant effect on the activity of the cobalt–magnesium oxide solid solution catalysts. The catalysts containing 5 and 10% Co have the lowest light-off temperature in methane combustion. In the preparation of cobalt–magnesium oxide solid solution catalysts, higher urea to metal ratio favors the formation of the catalysts with smaller crystal particles and leads to a better catalytic performance for methane combustion. Addition of lanthanum nitrate to the solution of Co and Mg nitrate depressed the formation of the cobalt–magnesium oxide solid solution and decreased the activity of the catalysts for methane combustion. The cobalt–magnesium oxide solid solution catalysts are very stable when the calcination or reaction temperature is no more than 900°C. However, the catalytic activity decreases rapidly after high temperature (>1000°C) calcination, possibly due to sintering of the catalyst and thus decrease of the surface area.     

Hydrogen production by catalytic decomposition of methane over carbon catalysts in a fluidized bed

A fluidized bed reactor made of quartz tube with an I.D. of 0.055 m and a height of 1.0 m was employed for the thermocatalytic decomposition of methane to produce CO₂ — free hydrogen. The fluidized bed was used for continuous withdrawal of the carbon products from the reactor. Two kinds of carbon catalysts — activated carbon and carbon black — were employed in order to compare their catalytic activities for the decomposition of methane in the fluidized bed. The thermocatalytic decomposition of methane was carried out in a temperature range of 800–925°C, using a methane gas velocity of 1.0–3.0 U _mf and an operating pressure of 1.0 atm. Distinctive difference was observed in the catalytic activities of two carbon catalysts. The activated carbon catalyst exhibited higher initial activity which decreased significantly with time. However, the carbon black catalyst exhibited somewhat lower initial activity compared to the activated carbon catalyst, but its activity quickly reached a quasi-steady state and was sustained over time. Surfaces of the carbon catalysts before and after the reaction were observed by SEM. The effect of various operating parameters such as the reaction temperature and the gas velocity on the reaction rate was investigated.     

Dehydrogenation and aromatization of methane under non-oxidizing conditions

The dehydrogenation and aromatization of methane on modified ZSM-5 zeolite catalysts has been studied under non-oxidizing conditions with a fixed bed continuous-flow reactor and with a temperature programmed reactor. The results show that benzene is the only hydrocarbon product of the catalytic conversion of methane at high temperature (973 K). The catalytic activity of ZSM-5 is greatly improved by incorporating a metal cation (Mo or Zn). H₂ and ethene have been directly detected in the products with a mass spectrometer during TPAR. A carbenium ion mechanism for the activation of methane is suggested.     

煤矿瓦斯气中低浓度CH4吸附富集研究

煤矿通风口处瓦斯气的CH₄浓度太低无法回收利用，只能排往大气中，既浪费能源，又污染环境。在活性炭吸附存储CH₄的基础上，对活性炭选择性吸附富集CH₄进行了初步研究。考察了多种吸附材料在常温和常压下对瓦斯气中低浓度CH₄的选择吸附能力，并关联了吸附材料结构参数和吸附性能之间的关系。实验结果表明，活性炭对低浓度CH₄有较强的吸附性能，孔径是决定活性炭能否选择性吸附CH₄的主导因素，而微孔比表面积及微孔孔容是次要因素。氧化改性不利于活性炭对CH₄的吸附，高温处理过程是获得高吸附性能活性炭的有效手段。     

Preparation and evaluation of Co/Al2O3 catalysts in the production of hydrogen from thermo-catalytic decomposition of methane: Influence of operating conditions on catalyst performance

This paper presents the synthesis of Co-doped Al₂O₃ catalysts, developed by our group, with porous structures and high surface areas. The catalytic activity of the materials obtained was evaluated in the catalytic decomposition of methane, which is an attractive method for CO/CO₂-free production of hydrogen, particularly for fuel cell applications.The catalysts were shown to be active and stable in relation to the catalytic decomposition of methane reaction. It was observed that the catalytic performance is dependent on the catalyst characteristics and on the operational conditions employed.The conversion of methane increased with metal loading, reaction temperature and N₂:CH₄ molar ratio.     

乏风瓦斯流向变换催化燃烧试验研究

为实现低浓度瓦斯气体的高转化率,满足实际工程低温排气的要求,设计制作一套新型流向变换蓄热催化燃烧反应器,并利用模拟气体进行催化燃烧实验研究。结果表明,气体流量为70 L·min^-1、燃烧反应温度控制在500 ℃和甲烷体积分数为0.2%时,甲烷催化燃烧转化率超过80%,出口气体温度低于60 ℃。该系统能满足工程低温排气要求。     

Kinetic study of ethanol reforming in a microreactor

Non-oxidative methane coupling in hybrid plasma–catalytic reactors was studied. The hybrid system which combined the gliding discharge (GD) and mobile (spouted) bed of catalyst particles revealed high efficiency in the methane conversion. Two catalysts from Pt and Pd supported by alumina ceramics were prepared and tested. The methane conversion was investigated in two GD reactors supplied by 1-phase or 3-phase circuits (50 Hz). In a homogeneous gas system and with a mobile bed of alumina–ceramic particles, mainly acetylene was produced from the CH₄ + H₂ mixture with none or a minor share of other C₂ hydrocarbons. The formation of non-volatile products (mainly soot) was also observed. In the presence of Pt and Pd catalysts, the soot formation was strongly reduced. Owing to these catalysts, ethylene and ethane became the main gaseous products replacing a major part of acetylene.     

Effects of operational parameters on emission performance and combustion efficiency in small-scale CFBCs

A well-designed CFBC can burn coal with high efficiency and within acceptable levels of gaseous emissions. In this theoretical study effects of operational parameters on combustion efficiency and the pollutants emitted have been estimated using a developed dynamic 2D (two-dimensional) model for CFBCs. Model simulations have been carried out to examine the effect of different operational parameters such as excess air and gas inlet pressure and coal particle size on bed temperature, the overall CO, NO_x and SO₂ emissions and combustion efficiency from a small-scale CFBC. It has been observed that increasing excess air ratio causes fluidized bed temperature decrease and CO emission increase. Coal particle size has more significant effect on CO emissions than the gas inlet pressure at the entrance to fluidized bed. Increasing excess air ratio leads to decreasing SO₂ and NO_x emissions. The gas inlet pressure at the entrance to fluidized bed has a more significant effect on NO_x emission than the coal particle size. Increasing excess air causes decreasing combustion efficiency. The gas inlet pressure has more pronounced effect on combustion efficiency than the coal particle size, particularly at higher excess air ratios. The developed model is also validated in terms of combustion efficiency with experimental literature data obtained from 300 kW laboratory scale test unit. The present theoretical study also confirms that CFB combustion allows clean and efficient combustion of coal.     

NO emission on pulverized coal combustion in high temperature air from circulating fluidized bed – An experimental study

High temperature air was adopted by combustion in high excess air ratio in a circulating fluidized bed. Experiments on pulverized coal combustion in high temperature air from the circulating fluidized bed were carried out in a down-fired combustor with the diameter of 220 mm and the height of 3000 mm. The NO emission decreases with increasing the residence time of pulverized coal in the reducing zone, and the NO emission increases with excess air ratio, furnace temperature, coal mean size and oxygen concentration in high temperature air. The results also revealed that the co-existing of air-staging combustion with high temperature air is very effective to reduce nitrogen oxide emission for pulverized coal combustion in the down-fired combustor.     

含氧煤层气甲烷提浓技术比较

含氧煤层气的主要成分是甲烷、氧气和氮气,为了回收利用煤层气中的甲烷,需要对煤层气进行脱氧和脱氮处理,以提高甲烷浓度.通过调研统计,甲烷提浓技术主要包括三种：含氧-低温精馏、脱氧-低温精馏和脱氧-吸附分离-液化技术.并从安全、造价和功耗三个方面对这三种提浓技术的工艺方案及特点进行分析和比较.研究结果表明：脱氧-低温精馏提浓技术的安全性高、造价较低,功耗居中.因此,含氧煤层气提浓甲烷推荐使用脱氧-低温精馏分离技术.     

煤焦中灰成分对甲烷裂解的影响

利用煤焦作为催化剂,采用小型石英管固定床为反应装置,对甲烷在煤焦、脱灰煤焦、煤灰以及石英砂床层上在温度为1123 K下的裂解反应进行了较为详细的研究。甲烷在脱灰煤焦上和新鲜的褐煤焦上的转化率和氢气收率有一定的差别。煤灰作为催化剂时,甲烷初始转化率和氢气初始收率分别为9.81%和8.14%。表明煤焦中的灰成分对甲烷裂解有一定的影响。随着反应时间的增加,甲烷的转化率和氢气的收率都逐渐降低。通过扫描电子显微镜和比表面积测定仪对反应前后的褐煤焦、脱灰煤焦进行了表征。甲烷裂解后煤焦比表面积、微孔容都明显降低,平均孔径增大。说明甲烷裂解生成的积炭堵塞煤焦的微孔。SEM照片显示甲烷裂解后积炭覆盖在煤焦的表面,使煤焦的催化活性逐渐降低。     

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.     

Catalytic coal partial gasification in an atmospheric fluidized bed

The coal partial gasification catalyzed by limestone, sodium carbonate and dolomite was studied using a bench-scale atmospheric fluidized bed in the presence of air and steam at 900 °C. The effects of limestone, sodium carbonate and dolomite on composition, heating value, gas yield of product gas and carbon conversion in the catalytic coal partial gasification have been examined. The experimental results show that the catalysts can effectively improve the gas quality, the heating value and the gas yield of product gas and carbon conversion. The catalytic effect of sodium carbonate is better than that of limestone and dolomite. The increase of limestone loading can enhance the quality of product gas, such as the content of combustible gas, the high heating value and the gas yield, during coal partial gasification. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.