Experimental Study Of Effects Of Calcium On Steam Gasification Of Shenhua Lignite

The optimal using of lignite is a hot topic in the field of chemical engineering science. The effect of calcium on pyrolysis and gasification process of brown coal is very important and needs more attention. Most of the research lignite in this field has been done on Australian brown coal and Indonesian coals. More research on Chinese domestic low-rank coal seems to be very critical. In this thesis, steam gasification of Shenhua lignite and the effects of calcium on it are investigated.Shenhua #6 lignite was used as raw material. The raw coal was washed using acid and then the calcium was impregnated into coal at different temperatures with different concentrations of calcium. Char supported calcium was prepared by pyrolysis of those coal samples. The reactivity of char in steam gasification was then investigated. Pyrolysis results showed that the content of calcium in the temperature range of 823-1100K did not have obvious effects on pyrolysis. Gasification results showed that gasification rate increased with the calcium content at low temperatures, but it was less influenced by calcium content which is more than 2% at high temperatures. The gasification rate increased with the calcium content at the beginning of gasification but decreased in the later period of gasification. Those results suggested that calcium could increase lignite char gasification rate, but the content of calcium in char did not play the key role in the gasification rate. The optimum experiment conditions for steam gasification are the 2% calcium content, gasification time for 30min, and gasification temperature of 1100K, respectively.XRD, SEM, BET analysis were used to characterize the char property after pyrolysis and steam gasification. XRD results showed that there would be more content of CaO in char with the increasing of gasification time, the gasification temperature or the content of calcium, which made the calcium have more time to react with water vapor. SEM showed that calcium oxide in the form of nano-particles was highly dispersed on char porous surface, thus the lignite char impregnated with calcium performed a very high reactivity. BET showed that the char surface area after gasification increased with increasing calcium content, which decreased the average pore size but improved the reactivity of char. The micro-pore volume of char increased as the average pore size decreased, which resulted in the increased calcium content, the BET surface area reduction and the decreased reactivity of chars. The BET surface area of char increased with the increasing temperature and was beneficial to the gasification while inhibiting action on the growth of micro-pore volume and surface area. The reactivity of char is higher in a short gasification time while it decreased in a longer gasification time.