The preparation materials of SCR de-saltpeter which uses reducing agent ammonia gas are liquid ammonia, ammonia and urea. Since liquid ammonia technology system is simple and the transportation, investment and operation costs are lower, so it has been applied widely in the present project, but the security demand of the system is strict. In line with《the Discrimination Standard for Major Hazardous Source of Hazardous Chemicals》(GB18218—2009), when the reserve of liquid ammonia exceeds 10 tons, then it is regarded as a major hazardous source. Therefore, some special cities (such as Beijing, Guangzhou) demand that the preparation system of reducing agent adopts urea technology of low security demand. The urea technology use pyrogenation method and hydrolyzation method to produce ammonia. The article separately explains the tech-nical principle and technology flow of two technologies.% 选择性催化还原法(简称SCR)脱硝用还原剂氨气的制备原料主要有液氨、氨水及尿素三种。由于液氨工艺系统简单，运输、投资、运行成本较低，在目前的工程中已广泛应用，但存在的缺点是对系统的安全性要求较高。根据《危险化学品重大危险源辨识标准》(GB18218—2009)，液氨储量超过10吨视为重大危险源，因此，某些特殊地区(如北京、广州等地)要求还原剂制备系统采用安全性要求较低的尿素工艺。尿素工艺又分为热解法和水解法制氨，本文分别说明了两种工艺的技术原理、工艺流程。
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