SH Radical:  The Key Intermediate in Sulfur Transformation during Thermal Processing of Coal

Abstract

To uncouple the complex behavior of sulfur transformation during thermal processing of coal and to elucidate the main mechanism, typical organic and inorganic sulfur compounds impregnated on or mixed with a low-ash char are studied through temperature-programmed decomposition coupled with online mass spectrum analysis (TPDMS) and followed by temperature-programmed oxidation coupled also with online mass spectrum analysis (TPOMS) in a temperature range of up to 800 degrees C. It is evident that the cleavages of Cal-S and Car-S bonds, where the subscripts al and ar stand for aliphatic and aromatic carbon, respectively, in the organic compounds result in the formation of SH radicals, which then undergo secondary reactions with the char to form various sulfur compounds such as H2S, SO2, COS, and elemental sulfur, as well as sulfur structures in the char. H2 has the ability to stabilize the -* SH radicals and weaken the interactions between the -* SH radicals and the char. For the sulfur compounds, which do not generate the *SH radical, the only sulfur products detected are those formed directly from the decomposition of the starting sulfur compounds, H2S from FeS2 in H2 or SO2 from Fe2(SO4)3 in He, for example, and no sulfur structure is formed in the char. Minerals have significant effects on the bond cleavage temperature and the reactions of the *SH radicals with the char. It is clear that the *SH radical is a key species interacting with the char to form secondary sulfur compounds, while H2S and SO2 play no role in the sulfur transformation to the carbon structure.

Effect of coal particle size on distribution and thermal behavior of pyrite during pyrolysis

“…2(b) shows that sectioned YZ-Dem coals and YZ-Demp released SO 2 at 200-800°C. SO 2 released below 350°C primarily resulted from some unstable forms of sulfur in coal [14]. A small quantity of SO 2 is evolved between 350 and 530°C during pyrolysis of sectioned YZ-Dem coals and YZ-Demp, respectively.…”

“…During pyrolysis of coal, the decomposition of pyrite is influenced by numerous factors such as other forms of sulfur, coal rank and reaction conditions [5][6][7]11]. Previous works examined interactions between pyritic sulfur and various organic species, sulfur distribution in coke, intermediates of the secondary reactions, effect of pyrite on sulfur retention and thermal decomposition of Fe-S compounds formed after pyrolysis [2,10,[12][13][14][15]. Based on these work, it remains difficult to remove all of pyrite which is finely disseminated in coal matrix.…”

Sulfur release and transformation behaviors of sulfur-containing model compounds during pyrolysis under oxidative atmosphere

“…Only 0.8% SO 2 evolution is obtained from dibutyl sulfide decomposition, while 43.5%, 31.51%, and 67.53% SO 2 evolution is obtained from phenylsulfide, benzothiophene and 2-methylthiophene, respectively. This finding indicates that sulfur model compounds produce sulfur radicals during pyrolysis [4], some of these radicals can connect with oxygen and release SO 2 , while others can connect with each other or other radicals and transform into other sulfur compounds. But except for 2-methylthiophene, the sulfur release rate in the first instance is lower than the sulfur transformation rate, as the second peak area is bigger than the first one.…”

“…Because sulfur presence in coal is very complex, determining their release and transformation behaviors during pyrolysis is difficult. Sulfur model compounds have been used to replace coal to study sulfur release and transformation behaviors [4]; however, sulfur transformation behaviors may be properly observed only under an inert atmosphere. As dibenzothiophene is very stable and easily volatilized before decomposition, no decomposition compounds were measured in that study.…”

“…In reductive atmospheres, different sulfur functional groups have been observed to decompose to some extent over a specific temperature range as follows: alkylthiols < arylthiols < disulfides < dialkyl sulfides < alkylaryl sulfides alkylaryl sulfoxides alkylaryl sulfones < pyrite < diaryl sulfides diaryl sulfoxides < diaryl sulfones < simple thiophenic structures < troilite < inorganic sulfates and complex thiophenic structures [5][6][7]. Sulfur transformation in raw coal, model compounds, or solvent extracts from coal during pyrolysis under He and H 2 atmospheres has also been studied in detail by pyrolysis with mass spectrometer (Py-MS) [4,[8][9][10]. However, sulfur release and transformation behaviors under an oxidative atmosphere are rarely reported.…”