Effects of CO2 on sulfur removal and its release behavior during coal pyrolysis
Introduction
Coal, main energy and chemical raw materials, plays an important role in the energy structure of China. In recent years, there is an increasing demand for clean coals [1], [2], [3]. But large-quantity use of coal in China has caused great damage to the environment [4]. The SO2 and CO2 release from coal combustion has led to a series of air pollution related problems, such as acid rain and global warming [5], [6]. Therefore, the clean coal utilization has become the urgent topic.
Sulfur in coal occurs in both inorganic and organic forms [7], [8], [9]. The inorganic sulfur is mostly pyrite and small amounts of sulfates. The organic sulfur is usually categorized as mercaptans, sulfones, aliphatic and aryl sulfides, sulfoxides, disulfides and thiophenes [6], [10]. Pyrolysis is a frequent step in various conversion processes, and it is also a simple and effective way for rational utilization of coal and environmental protection [11]. During pyrolysis, both pyrite sulfur and partial organic sulfurs can be removed, a part of sulfurs goes into the gas phase in the form of H2S, COS, SO2, etc., which can be easily recovered [12]. During pyrolysis, sulfurs and minerals or sulfurs and organic matter can interact each other, the sulfur-containing gases also can react with char and retain in the char [13], [14]. The interaction between organic matter and pyrite shows that the behavior of sulfur evolution during pyrolysis may be different in raw coal, deashed coal and pyrite-free coal [15], [16]. Therefore, the behavior of sulfur evolution during pyrolysis should be discussed separately.
Previous studies have mainly paid attention to desulfurization under inert atmospheres and hydrogen atmosphere, however desulfurization under inert atmospheres during pyrolysis is very low, and hydro-desulfurization still needs higher cost [4], [17], [18]. Semra’s study [15] shows that pyrolysis under CO2 atmosphere has higher organic sulfur removal ratio at higher temperatures. Pyrolysis under CO2 atmosphere can not only effectively improve desulfurization ratio, but reduce the release of CO2.
Due to the very complex presence of sulfur forms in coal, the mechanism of sulfur release and its transformation behavior are still not very clear under CO2 atmosphere, as well the effect of pyrite and mineral matter on them. In this study, the sulfur release and transformation behavior during pyrolysis of two Chinese raw coals, their deashed coals and pyrite-free coals were investigated by Py-MS and with Py-GC under Ar and CO2. This can provide theoretical basis for comprehensive use of coal by pyrolysis connected with pre-desulfurization.
Section snippets
Coal samples
Jiexiu (JX) and Yanzhou (YZ) raw coals, their deashed coals and pyrite-free coals were used in this study. Organic sulfur content of JX coal is the highest compared with its sulfate sulfur and pyrite, about 92.11%. Pyrite of YZ coal is higher than that in the JX coal, about 34.21%. The deashed coal and pyrite-free coal (thus depyrited dashed coal) were obtained according to the procedures described in literature [14], respectively. Their proximate and element analyses, sulfur forms and ash
Effects of CO2 on sulfur removal during pyrolysis
Table 4 shows the desulfurization ratio of different coals and their char yield during pyrolysis under Ar and CO2 atmospheres. Generally speaking, under Ar atmosphere sulfur removal ratio is related to the volatile content and the stability of organic sulfur in the coal. That is the higher the volatile content is, and the less stable the organic sulfur is, thus the higher sulfur removal ratio is. Under Ar atmosphere, for those two raw coals, the sulfur removal ratio is YZ > JX. This order is very
Conclusions
In this study, the following results could been concluded:
- (1)
The sulfur removal ratio of YZ and JX coals under CO2 atmosphere is higher than that under Ar atmosphere. Most sulfur removal of JX coal is distributed in tar under Ar atmosphere, while it is distributed in gas phase under CO2 atmosphere. The effects of ash on sulfur removal are different for different coals under both atmospheres. For YZ coal, the desulfurization ratio is deashed coal > depyrited coal > raw coal under Ar atmosphere, while
Acknowledgements
This study was financially supported by the Project of Natural Science Foundation of China (No. 21466025), the Project Natural Science Foundation of Inner Mongolia of China (No. 2013MS0205) and the State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology (2014).
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