脱灰对两种烟煤半焦碳结构及CO_2气化反应性的影响

Effects of de-ashing on the micro-structural transformation and CO_2 reactivity of two Chinese bituminous coal chars

利用XRD和激光Raman分析了两种典型中国烟煤脱灰前后半焦的碳结构变化,以等温热重法在常压和900～1 200℃温度范围内研究了半焦CO2气化反应性.结果表明,脱灰可以改变煤中局部大分子碳结构,导致煤焦的碳结构有序化程度降低,有利于气化剂渗入碳层内部,增加反应活性位;脱灰减少了热解气缩聚支撑点,降低了二次热缩聚发生.两个因素共同作用可提高脱灰半焦的CO2气化反应活性.神华煤灰催化成分含量较高,脱灰后催化作用消失,而脱灰引起的碳微结构变化使反应性增强,脱灰前后反应性差别不大;兖州煤灰催化作用相对较弱,而且灰分含量较高,脱灰对碳结构变化影响较大,因此脱灰后反应性明显增强.

Two Chinese bituminous coals were de-ashed by hydrochloric and hydrofluoric acids; and their gasification reactivity with CO2 were investigated using isothermal thermo-gravimetric method at constant temperatures of 900--1200 ℃, respectively. The effects of micro-structural transformation of coal chars before and after de-ashing were analyzed using X-ray diffraction （XRD） and laser Raman. Results indicate that the de-ashing treatment gives rise to the partial damage of carbon structure, and then forming more amorphous carbon during pyrolysis. Such amorphous carbon with high D band fractions of Raman spectrum demonstrate relatively less regular structure. The disordered structure allows more gasification agent （i. e. CO2） penetrate into the inner of char, which probably promotes the gasification reactivity of it. The nano-rod structure forms during the raw coal char CO2 gasification by the strong polymerization of pyrol- ysis gas. The de-ashing process is considered to diminish the support point for secondary poly- merization, which causes more decomposition. The conversion ratio and reaction rate are in- creased. As a result, the reactivity of Shenhua coal char exhibits less change after de-ashing due to the competitive effect between vanishing of catalysis of ash and micro-structural trans- formation of char; in contrast, the reactivity of Yanzhou coal char after de-ashing presents ap- parent improving because of the less catalysis of ash and more micro-structural damage.