Effect Of Additives On The S-and N-Containing Gaseous Products During Pyrolysis Of High Sulfur Brown Coal

Brown coal resource is rich in China, but the features of high moisture, high volatile and low heat-value limit markedly its utilization range. So it is very important to improve the quality of brown coal through pyrolysis technique. Sulfur and nitrogen in the coal will partly release in the form of gasous products during the coal pyrolysis, which will cause environmental pollution. It is necessary to research the release of sulfur and nitrogen during coal pyrolysis. The brown coal from Ximeng mine in Inner Mongolia was used as the experimental sample. The salts of alkali metal, alkaline earth metal and transition metal were evenly mixed with raw coal and demineralized coal (the inherent mineral was removed by acid-washing). The effect of the minerals and additives added in raw coal and demineralized coal on the formation of S-containing gases and N-containing gases during temperature programmed pyrolysis was studied by the fixed-bed experimental setting. The gaseous products from coal pyrolysis were analyzed by gas chromatography and IC chromatography. The main results are shown as following: 1) The effect of inherent minerals on the release of sulfur and nitrogen during coal pyrolysisThe S-containing gases from the pyrolysis of demineralized coal are obviously higher than that of raw coal, which shows the inherent minerals in Ximeng brown coal have the desulfurization capacity and can restrain the formation and release of S-containing gases during pyrolysis. This action mainly presents the temperature range of 500-800℃. The same maximum peak temperature of H2S and COS from pyrolysis of demineralized coal and raw coal shows that the removing of minerals does not affect the sulfur and organic matrix in coal. The release amount of total N-containing gases from demineralized coal pyrolysis is also higher than that from raw coal pyrolysis, but the changes of HCN and NH3 release trend with temperature are different. The inherent minerals present the capacity of promoting HCN to NH3 in the temperature range of 550-750℃.2) The effect of alkali metal loaded in raw coal and demineralized coal on the release of sulfur and nitrogen during coal pyrolysisExcept for K2CO3 added in raw coal, the alkali metals added in raw coal and demineralized coal can prohibit the formation of S-containing gases. The additive of potassium carbonate in raw coal significantly promotes the release of S-containing gases at 300-500℃, which shows the catalysis action by potassium and carbon reaction. Alkali metals added in raw coal promote the release of N-containing gases during pyrolysis, and the role of potassium carbonate is also the most obvious. Alkali metals added in demineralized coal prohibit the release of N-containing gases, especially HCN. The results of Li2CO3 and K2CO3 promoting the release of NH3 show the secondary reaction of HCN conversion to NH3.3) The effect of alkaline earth metal loaded in raw coal and demineralized coal on the release of sulfur and nitrogen during pyrolysisWhether calcium acetate or calcium oxalate added in raw coal and demineralized coal can prohibit the release of S-containing gases during coal pyrolysis because the sulfur from coal pyrolysis can react with calcium in additives and is retained in the char. Calcium added in raw coal promotes the release of NH3 and prohibits the formation of HCN, and it is more apparent when calcium added in demineralized coal. These results show there exists the synergy reaction between inherent mineral and additives, which promotes the cracking of nitrogen group and secondary reaction of HCN conversion to NH3.4) The effect of transition metal loaded in raw coal and demineralized coal on the release of sulfur and nitrogen during coal pyrolysisIron acetate and iron oxalate added in raw coal and demineralized coal have the strong desulfurization capacity during coal pyrolysis, and this effect is bigger than that of calcium-based additives. Iron acetate and iron oxalate play a different role in the release of N-containing gases. The release amounts of NH3 and HCN for iron acetate added in raw coal and demineralized coal are higher than that of iron oxalate. Iron acetate has the strong promoting action on the transformation of nitrogen groups to NH3, but iron oxalate has the inhibition action on the formation of NH3 and HCN.5) The effect of inherent minerals and additives in coal on the release of sulfur and nitrogen during coal pyrolysisComparing the release amounts of sulfur and nitrogen during pyrolysis of raw coal and demineralized coal with additives, it can be found that the inhibition on the S-containing gases release of additives added in raw is more obvious than that in demineralized coal. This should be due to the interaction between inherent minerals and additives in coals. Contrary to the release of S-containing gases, the release amount of N-containing gases from pyrolysis of the raw coal with additives is higher than that of the demineralized coal with additives, this is mainly due to the synergy effect between minerals and additives promoting the relatively thermal stabile nitrogen groups in coal to be cracked.