Coal Conversion And Desulfurization During Pyrolysis

SO₂ is one of the major factors to bring about environment pollution because of coal burning, so desulfurization of coal is an important issue in clean coal research. For improving the desulfurization efficiency and tar yield of coal pyrolysis, the effect of mineral on sulfur behaviors during coal pyrolysis and hydropyrolysis was studied, and the main problems to prohibit the desulfurization of coal during pyrolysis were analyzed. To solve these problems, three new desulfurization processes concerned to coal pyrolysis were developed. The main research works and results in this paper are as follows:1. Two Chinese coals, Shenfu subbituminous coal and Huolingele lignite, were used to investigate the effect of inherent mineral matter in coal and three added mineral matter (CaO, K₂CO₃ and Al₂O₃) on reactivity and kinetic characteristics of coal pyrolysis by a thermogravimetric analyzer. The results showed that inherent mineral in coal has no evident effect on the reactivity and kinetics of coal pyrolysis. CaO, K₂CO₃ and AI₂O₃ all has a catalytic effect on the reactivity of coal pyrolysis and the effects are closely related to their own characteristic and coal type. Addition of inorganic matter in coal, the characteristic temperatures of coal pyrolysis are changed and the activation energy of pyrolysis decreases.2. The effect of mineral on conversion of coal and sulfur with different forms in Yima during pyrolysis and hydropyrolysis in a fixed bed reactor was studied. The results showed that mineral in coal has no evident effect on coal conversion, but mineral can change the primary volatile matter distribution in gas and liquid product. It was summarized that the alkaline matter in coal reacting with sulfur containing gas to form sulfide, pyrite sulfur transforming into organic sulfur and pyrite decomposing into iron sulfide are three main problems to hinder the sulfur removal from coal during pyrolysis.3. To improve the desulfurization of coal by pyrolysis, a two-step desulfurization process for high sulfur coal was investigated. Two Chinese coals, Yima and Yanzhou coal, with addition of 10wt% potassium hydroxide or sodium hydroxide were pyrolyzed under atmosphere of nitrogen or hydrogen in a fixed bed reactor at 600°C, and then the obtained chars were washed by hot water. The results indicated that the two-step desulfurization process can enhance the desulfurization of coal pyrolysis and hydropyrolysis dramatically, and the sulfur content in char decreases evidently; the desulfurization of Yima coal and Yanzhou coal reaches to 70⁸0%, the sulfur content in char is about one third of the original coals. In this process, hydrogen is unnecessary for the desulfurization efficiency is very similar between pyrolysis with N₂ and hydropyrolysis with H₂.4. The effect of partial ethylene addition in the coal pyrolysis atmosphere on desulfurizationwas studied. The results showed that ethylene as capture of H2S and other sulfur-containing products, can restrain the reaction between H2S and char, and enhance the desulfurization efficiency of coal pyrolysis. The effect of ethylene is related to the property of coal and kinds of sulfur in coal.5. A new coal pyrolysis process for obtaing high tar yield was designed in this work. In the process, methane was partially oxidated using O2 or air with Ni/Al2C>3 as catalyst, then the products of methane oxidation were directly introdued to the coal pyrolysis bed. The experimental results showed that under appropriate reaction and pyrolysis conditions, the tar yield is 1.7 to 2.3 times of that from hydropyrolysis, and char yield is equal to or higher than that of pyrolysis for all three studied coals. Moreover, this process has good effect on desulfurization for some high sulfur coals, and the desulfurization efficiency is approximate to hydropyrolysis.