Research advances on the occurrence and release characteristics of chlorine in coal

Chlorine is an important element of harmful trace elements in coal.During the pyrolysis, the combustion and gasification of coal, chlorine would be migrated to flue gas by complex chemical reactions, existed in the forms of various chlorine substances, such as HCl, Cl2 and inorganic salts.It would cause serious pollution to the environment, and endanger human health.At the same time, the volatilization of chlorine would cause slag corrosion problems on the boiler and heated exchange surface, causing safety hazards.It is the premise of clean utilization of high-chlorine coal to understand the determination methods, occurrence forms, release characteristics and control technologies of chlorine in coal.This paper reriewed the relevant research works in this field.Also, the sources and hazards of chlorine in coal, as well as the different methods for determining chlorine content were introduced.The distribution range and mean value of chlorine content around the world were counted, and the chlorine content of typical coals were listed.Then, two main occurrence forms of chlorine in coal are summarized, which are organic forms (including inorganic ions, hydrated ions, inorganic salts and complex minerals) and in-organic forms (including organic exchange ion, organic bond covalent state, organic chloride and organic ion complexes).Meanwhile, three research methods of chlorine occurrence were classified, including correlation analysis method, chemical extraction method and instrument direct analysis method.In addition, this paper described the release characteristics of chlorine during combustion, pyrolysis and gasification of coal.A series of factors which have influence on the release of chlorine were studied, such as chlorine forms, coal characteristics, temperature, heating rate, holding time, pressure and atmosphere.Besides, the changes of chlorine forms and its distribution in char and gaseous during heating were analyzed.At the same time, the main chlorine control technologies and their applicable working conditions were summarized and compared.Lastly, some suggestions were proposed for the investigation regarding the occurrence, release and control of chlorine in coal, which would provide a theoretical basis for the efficient and clean utilization of high-chlorine coal. © 2019, Editorial Office of Journal of China Coal Society. All right reserved.