Desulfurization Of Coal During Pyrolysis And Copyrolysis With Biomass

Direct combustion of coal arouses serious pollution, especially "acid rains" by SO₂ emission. So it is necessary to remove sulfur from coal before combustion in order to protect the environment. Aiming to produce low-sulfur content char, desulfurization of coal during pyrolysis and copyrolysis with biomass in a free-fall reactor under atmospheric pressure was studied. The main research works and conclusions in this dissertation are as follows:1. Those unstable organic sulfurs in coal can be removed at temperature between 500 and 800℃, more than half of which can be released as organics in tar. The pyrite in coal decomposing at temperature between 500 and 800℃, and the organics in coal can promote removal of the pyritic sulfur. In this process there exists the reaction of transformation from inorganic sulfur into organic sulfur, which can be suppressed effectively at temperature higher than 800℃. At the same time the sulfur fixation reaction aroused by basic minerals in coal happens at temperature between 800 and 900℃. For the high-sulfur content coal containing much pyrite, a longer particle residence time is needed for a better sulfur removal. But for the lower rank coal and for the coal comprising more organic sulfur, a shorter particle residence time is appreciated.2. Considering the very short particle residence time in the free-fall reactor, air was directly used as carrier gas to study the fast pyrolysis and sulfur evolution of coal under oxidizing atmosphere. It shows that the surface area of the char is greatly increased, but the yield of the char only a little changed. Those unstable organic sulfurs in coal can be removed at temperature between 550 and 700℃. And the pyritic sulfur in coal can be removed at temperature lower than 600℃. Also it accompanies the transformation reaction in this process, but its effect can be eliminated at temperature as low as 600℃. It can be seen that the decomposition temperature of pyrite and the release temperature of those unstable organic sulfurs under oxidizing atmosphere are 200℃ lower than those under inert atmosphere. The pyritic sulfur can be effectively removed at a relatively low temperature under oxidizing atmosphere to produce low-sulfur content char. For example, the desulfurization of Yanzhou coal at 700℃ under oxidizing atmosphere can get better results, which are 57.9%, 65.1%, 90.8%, 47.2%, 1.9%, 0.2% and1.7% for the yield of char, the extent of total sulfur removal, the extent of inorganic sulfur removal, the extent of organic sulfur removal, the total sulfur content of char, the inorganic sulfur content of char and the organic sulfur content of char, respectively, than at 900℃ under inertatmosphere. In addition, the pyritic sulfur can be removed within a relatively short particle residence time. Further increasing the reactor temperature and the particle residence time can result in heavy loss of the char without more sulfur being effectively removed. Based on the experiment result, an original two-stage process combining oxidative pretreatment with hydrodesulfurization of the coal is proposed, through which can realize rapid desulfurization of coal to produce low-sulfur content char.3. The existence of the synergetic effect during copyrolysis of coal and biomass in the free-fall reactor is pointed out according to the characters of coal, biomass and the reactor. The experiment of fast pyrolysis shows that the yields of hydrogen-rich gas components (H2+CO) and olefin, such as C2H4, are rapidly increased at temperature higher than 800°C. As well as the difference of blackness between coal and biomass makes their pyrolysis reactions occur almost simultaneously under inert atmosphere in the free-fall reactor. The experiment of copyrolysis indicates that the synergetic effect exists, especially at a higher temperature, a higher blending ratio of biomass to coal and for coals with higher rank. At a low temperature, such as 650QC, biomass can fix sulfur into the char during coal copyrolysis with a large quantity of biomass. But at a high temperature, such as 800°C, biomass can enhance coal desulfurization to produce low-sulfur content char during coal copyrolysis with a little biomass: For bituminous coal, the copyrolysis is in favor ofi sulfur removed from coal to tar. Meanwhile, for lignite, it favors to remove sulfur from coal to gas.4. Nitric acid pretreatment can thoroughly remove the basic minerals and the pyrite from coal, but affects little on the organic sulfur in coal and the yield of organic in char. According to the experimental data from fast pyrolysis of both the raw coal and the HNC>3-pretreated coal, the rate of transformation from inorganic sulfur into organic sulfur can be quantitative calculated during fast pyrolysis of the raw coal.