Transformation And Interaction Of Sulfur And Nitrogen During Pyrolysis Of Typical High-Sulfur Coals

Based on the energy consumption situation of China, coal is still the main energy resource in the foreseeable future. But it has been mined and utilized in a large-scale, resulting in the shortage of high-quality coal. Most importantly, the subsequent issues of energy security and environment protection have become increasingly prominent. Low-quality coal, such high-sulfur coal, occupies a relatively high proportion in coal reserve, but the feature of high sulfur content restrains its utilization range. In order to effectively control the emission of pollutants during utilizing the high-sulfur coal, and provide the fundamental theory for the synergistic regulation of sulfur and nitrogen-containing pollutants during further utilization of high-sulfur coals, the transformation of sulfur and nitrogen and the competition for active hydrogen in the pyrolysis of high-sulfur coals have been focused. The study mainly includes the several parts as following.(1) Four high sulfur coals (Coal A, Coal B, Coal C and Coal D) with different coal rank, sulfur content and sulfur forms were selected as the experimental samples. The essential transformation behaviors of sulfur in high sulfur coal were studied by the traditional temperature-programmed pyrolysis combining with S-XANES of modern analytical technology. The results show that there are different existing forms of sulfur in different rank Coal A and Coal D with high organic sulfur content. There are more active disulfide and sulfide in Coal A with lower coal rank, which can be decomposed and released into the gas products at below pyrolysis temperature of500℃; more complex thiophenic structure exists in the higher rank Coal D, which are difficultly decomposed even at1000℃, leading to more sulfur retain in char. During coal pyrolysis, most of nitrogen in coal retains in char, only small proportion of nitrogen transforms into HCN and NH3. The proportion of nitrogen in tar from Coal A and Coal C are higher than that of others coals, which is related to the proportion of unstable organic structure in coal matrix and the chemical surrounding of N-containing functional groups in coal.Decomposition of pyrite in coal plays an important role in the transformation of sulfur during pyrolysis of high-sulfur coal. Two step reactions were included in the process of decomposition of pyrite. Firstly, pyrite begins to decompose into FeS and active sulfur with the increase of temperature, and then FeS also transforms into active sulfur above600℃. For the transformation behavior of sulfur, except for the decomposition and release of some sulfur forms, there exists inter-conversion among the different sulfur forms. Below600℃, some unstable organic sulfur and pyrite can decompose and release into gases products. Above600℃, inter-conversions of different sulfur forms play a dominant role on the transformation behavior of sulfur. Furthermore, the maximum release temperature region of HCN and NH3locate in the higher temperature range, around650℃. The ring cleavage reaction of pyridine and pyrrole is the main pathway for the formation of HCN during coal pyrolysis, while the formation of NH3are resulting from the decomposition of quaternary-N and the secondary reactions of HCN.(2) The traditional pre-desulfurization processes were used to pretreat four high-sulfur coals and the effect of these processes on the release of sulfur and nitrogen during pyrolysis of high-sulfur coal were investigated. The removal degree of pyrite for Coal B is higher than that of Coal C by flotation because of the difference of existing forms of pyrite in these two coals. The accumulative yields of S-containing gaseous products from clean coals are higher than that of raw coals, but the release peaks of H2S and COS almost disappear above600℃for the clean coals.The change in nitrogen forms on the coal’s surface before and after flotation should be the main factor influencing the various release trends of HCN and NH3during coal pyrolysis. The formations of HCN and NH3are related to the proportion of pyrrolic-N and quaternary-N in coal, respectively. The fixing sulfur capacity of minerals is eliminated through HCl/HF pre-treated process, whilst the pore structure of coal is improved, which can promote the release of S-containing gases products. The removal of these minerals weakens the catalytic role on the secondary reaction of HCN transform into NH3(or N2), resulting in the increase of the release amount of HCN, and the decrease of NH3. The HNO3treatment can nearly remove all the pyrite in coal, resulting in the disappearance of the release peak of H2S and COS at600℃. Moreover, the residual of nitrate increases the release amount of HCN and NH3at the whole experimental temperature range.Part of sulfur was removed for Coal A and Coal D by the ultrasonic solvent extraction, and the removal of organic sulfur occupies80%of the entire removed sulfur. The solvent extraction can remove some small molecular phases in the coal and improves the pore structure of coal matrix, leading to the increase of the release of active sulfur formed by the decomposition of pyrite in Coal D. However, ultrasonic solvent extraction has a little effect on the release of H2S from Coal A. The stability of quaternary-N structure is weak and it can be partly removed by this process, resulting in the decreasing the release of NH3during pyrolysis of pre-treated coals. Step-by-step pre-treatment (flotation, HCl/HF acid-washing, HNO3acid-washing and solvent extraction) can remove most of sulfur in Coal C (total sulfur content to1.88%from5.0%), whilst the surface area, chemical structure and the sulfur and nitrogen forms on the surface of Coal C are changed. This process improves the release of S-containing gases during pyrolysis of Coal C. The release of HCN and NH3are almost unchanged, but the release amount of HCN and NH3are obvious increased after HNO3treatment.(3) By means of the release behaviors of H2S and NH3during pyrolysis of raw coal and different pretreated samples and combining with the pyrolysis results of S-and N-containing model compounds, it is convinced that there exists the competition of active hydrogen during thermal transformation of sulfur and nitrogen in coal. And the simultaneous formation of H2S and NH3in the similar temperature range is the precondition of the competition occurring during pyrolysis. The results of competing active hydrogen for H2S and NH3from rapid pyrolysis of model compounds show that the effects of three S-containing model compounds on the formation of NH3from the carbazole pyrolysis are different, which depends on the capacity of donating hydrogen from the S-containing model compounds during pyrolysis. The decomposition of benzyl-thioether and2-naphthalenethiol can provide the active hydrogen to promote the ring cleavage reaction of carbazole and increases the release of NH3, whilst decreases the release of H2S. The thermal activity of dibenzothiophene is lower than that of carbazole, so the ring cleavage reaction of dibenzothiophene can be promoted by the active hydrogen from the decomposition of carbazole during pyrolysis, resulting in the increase of the release amount of H2S and the decrease of NH3release amount.