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1997

The Effect of Acid Treatment on H2S Production during Coal Pyrolysis

Gasification Division
Energy Resources Department
Objectives
Coal pyrolysis (heat treatment) is not only common step in various conversion processes such as liquefaction, gasification and combustion, but also a kind of coal cleaning technique. Therefore, the understanding of physical and chemical transformation of sulfur in coal during pyrolysis is very important for the rational utilization of coal and also for the environmental protection. In this study, hydrogen sulfide (H2S) and carbonyl sulfide (COS) produced during pyrolysis of coal were analyzed by using GC-MS. The effect of the pre-treatment on coal using acid were demonstrated.
Results
Taiheiyo coal (TC) were treated with acid solution to eliminate the inorganic sulfur. By 7.7% HNO3 treatment at room temperature for one day, pyrite (FeS2) and inorganic sulfates in coal were dissolved and organic sulfur only remained in coal. The elemental analysis of HNO3 treated sample (TC-HNO3) indicated slight increase of nitrogen and oxygen contents. This fact suggests the nitration of coal or adsorption of nitrate ion on coal surface occurred during HNO3 treatment. On the other hand, 47% HF leaching followed by 18% HCl treatment at room temperature (TC-HF HCl) caused the complete dissolution of inorganic materials in raw coal sample without oxidation.
The effect of acid treatment on H2S production during pyrolysis of Taiheiyo coal under hydrogen flow is shown in Fig. 1. The maximum at 450°C on H2S production curve for TC-HNO3 was significantly smaller than that of raw TC. The amount of COS produced from TC-HNO3 around 450°C was increased compare to raw TC. On the other hand, the HF treatment (TC-HF HCl) caused no change in H2S production in this temperature region. Therefore, the decrease in H2S production maximum by HNO3 treatment is considered to be the effect of chemical oxidation of coal structure by nitric acid.
Above 600°C, the amount of H2 produced was considerably increased by HF treatment which gave the complete deashing (TC-HF HCl), while no change was observed in TC-HNO3 compared to raw TC. The X-ray powder pattern of pyrolyzed char from raw TC and TC-HNO3 showed the existence of gypsum. These results indicate that calcium in coal acted as an absorbent for the sulfur compounds during pyrolysis at high temperature region. From X-ray fluorescence analysis of some coal samples, it was revealed that the change in H2S production behavior caused by HF treatment was observed only for coal with low S/Ca ratio, while the effect of HNO3 treatment was not depend on inorganic compositions in raw coal.


Selected Publications
1) Hydrogen Sulfide Production from Coal during Pyrolysis, Proc. 5th China-Japan Symposium on Coal and C1 Chemistry, 1996, 43.

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