水泥生产全过程硫循环机制的研究进展

The Sulfur Cycle Mechanism in the Whole Process of Cement Manufacturing:a Review

SO_2是水泥行业产生的主要大气污染物之一。国内外对水泥行业SO_2排放量进行了严格限定。随着劣质原燃材料及替代燃料的大规模使用,更多的硫将被带入水泥生产过程中。这不仅会引起SO_2排放水平的增高,而且会加重硫在水泥窑炉内的循环富集,影响窑炉的正常操作,同时对熟料质量产生影响。水泥生产过程中的硫由原料和燃料带入。其中,部分原料硫在预热器内转变为SO_2并随烟气离开,在生料磨和收尘器内被生料部分吸收,并再次进入预热器,形成了硫在预热器和生料磨、收尘器间的低温循环。未转变为SO_2的原料硫和燃料硫在水泥窑炉高温环境下形成SO_2,并被分解炉内的碱性氧化物吸收,形成硫酸盐进入回转窑,在回转窑烧成区部分硫酸盐发生分解,产生的SO_2再次进入分解炉,由此形成了硫在分解炉-回转窑间的高温循环。围绕水泥生产过程硫的低温循环和高温循环,国内外开展了大量研究。围绕硫的低温循环,首先揭示了水泥生产排放的SO_2主要是由原料中硫铁矿和有机硫的氧化反应引起。硫铁矿的氧化温度约为400℃,反应产物以FeSX、Fe_2O_3为主。根据反应条件的差异,硫铁矿氧化机理包括直接氧化和先分解为硫黄铁矿再氧化。在预热器内CaCO_3和CaO可吸收部分SO_2,其中CaCO_3对SO_2的吸收为直接硫化反应,反应速率受烟气温度、水分含量、CO_2浓度等影响;部分研究者对CaO在预热器内吸收SO_2提出争议,认为CaO会优先与CO_2反应而影响其对SO_2的吸收。生料磨内SO_2的吸收效率被认为随烘干温度升高而降低,随原料水分含量增加而提高,固硫产物以CaSO_3·1/2H_2O为主,其会在生料颗粒表面富集。围绕硫的高温循环,研究结果表明硫酸盐在回转窑内的分解反应受温度、O_2浓度、硫碱比等影响。水泥窑协同处置替代燃料时产生的热解气体会显著降低CaSO_4的分解温度,促进SO_2释放;该现象随着替代燃料粒径的减小、挥发分含量的增加而加剧。熟料中硫的存在形式主要为K_2SO_4、3K_2SO_4·Na_2SO_4、K_2SO_4·2CaSO_4、CaSO_4或替换其他物质存在于硅酸盐矿物中,熟料中硫含量的增加会稳定C_2S而抑制C3S形成。本文综述了水泥生产全过程硫的循环机制,聚焦硫的低温和高温循环。围绕硫的低温循环,阐述了生料含硫形式及其检测方法、硫铁矿氧化及SO_2释放特性、CaCO_3和CaO在预热器环境下对SO_2的吸收机理以及粉磨过程生料的固硫作用;围绕硫的高温循环,阐述了硫在水泥窑炉内的挥发与循环机制、替代燃料对硫释放的影响、硫在熟料中的存在形式及对熟料质量的影响。

As one of the principal air pollutants produced by cement industry,the SO2emission levels have been strictly limi-ted all over the word.However,with the mass adoptions of inferior quality raw materials and alternative fuels in the future,more sulfur will be brought into the cement production process.Hence leading to the increase of the SO2emission level and aggravate the circulation enrichment of sulfur in the cement kiln,which has an impact on the operation of the kiln and the quality of clinker as well.During the cement production process,SO2originated from both raw meal and fuel.Among them,part of the sulfur from raw meal was oxidized to SO2in the preheater system and left with flue gas,then a portion of the SO2was captured by the raw meal in the grinding and de-dusting system and entered the preheater system again,thus,forming a low-temperature sulfur cycle among the preheater,the raw meal mill and the dust collector.The rest of the sulfur from the raw meal and the fuel was transformed to SO2in the kiln and calciner under high temperature,and the most of the SO2was absorbed in the calciner generating the alkali sulfates,afterwards the alkali sulfates would enter the rotary kiln and partially decomposed in the burning zone,the generated SO2would enter the calciner again and a high-temperature sulfur cycle was established in the kiln and calciner.Extensive research on low-temperature cycle and high temperature cycle of sulfurt in the cement production process has been devoted at home and abroad.For the relative low-temperature sulfur cycle,the emission of SO2in the cement production was produced through the oxidation of pyrite and organic sulfur.The oxidation temperature of pyrite begins at about400℃,and the reaction pro-ducts mainly consist of FeS X and Fe2O3.According to the diverse reaction conditions,the oxidation mechanisms of pyrite were classified into the direct oxidation and the reoxidation by thermal decomposition to pyrrhotite first.In the preheater,CaCO3and CaO absorbed a portion of SO2.The SO2absorption by CaCO3belonged to direct sulfuration reaction,whose reaction rate highly depended on the temperature,moisture content and CO2concentration of the flue gas.Some researches deemed that CaO was prior to react with CO2in the preheater circumstance,which would affect the absorption of SO2.The absorption efficiency of SO2in the raw meal mill was considered to decrease with the increasing drying temperature or decreasing moisture content of the raw material particles.The reaction product was dominated by CaSO3·1/2H2O,which would form an enrichment on the surface of the raw meal particles.For the relative high-temperature sulfur cycle,the studies indicated that the temperature,O2concentration,and sulfur/alkali ratio played a pivotal role in the decomposition reaction of sulfate in the rotary kiln.The pyrolysis gas generated when the cement kiln co-disposes the alternative fuel would significantly reduce the decomposition temperature of CaSO4and promote the release of SO2,which would aggravate with the decrease of the size of alternative fuel and the increase of volatile matter content.The sulfur contained in the clinker was mainly in the form of K2SO4、3K2SO4·Na2SO4、K2SO4·2CaSO4、CaSO4,and other substances in the silicate minerals,wherein increasing the sulfur content in the clinker would stabilize the C2S and inhibite the formation of C3S.This paper reviews the cycle mechanisms of sulfur in the whole process of cement production and emphases the low-temperature and high-temperature cycle of sulfur.Revolving round the low-temperature cycle of sulfur,the writer detailedly expounds the sulfur-containing form of raw meal and its detection methods,the oxidation of pyrites and the characteristics of releasing the SO2,the absorption mechanism of CaCO3and CaO in the preheater environment,and the sulfur fixation of the raw meal in the grinding process,respectively.Furthermore,the writer elaborates the volatilization and circulation mechanism of sulfur in the cement kiln,the in-fluence of alternative fuels on sulfur release,the form of sulfur in the clinker and its effects on the quality of clinker on the high-temperature cycle of sulfur.