Study On Environmental Geochemistry Of Bromine In Chinese Coals

Bromine is one of the trace elements in coal. During the combustion of coal, bromine is mainly released in the form of HBr, most of which enter the atmosphere. The bromine in atmosphere plays significant roles in exploration geochemistry and is active in O3 and OxHy free radicals destruction in the troposphere and play an important role in global climate change. Moreover, the bromine can enter the human or animal body through the food chain which may cause bromide poisoning.Based on the progress of study on bromine in coals, this dissertation mainly studies the bromine content and its distribution in Chinese coals, the modes of occurrence of bromine in coals using sequential chemical extraction procedure, the leaching characteristics of bromine in coal, the behavior characteristics and environmental effects during coal combustion.Based on these studies, some conclusions can be draw:1. A method for the determination of bromine in coal using pyrohydrolysis for sample digestion and ion chromatography for the further determination was proposed. The pyrohydrolysis conditions specifically for the release of bromine from coal were optimized:1.0 g coal sample, a temperature of 1050℃,25 min of pyrohydrolysis, 90 mL/min of oxygen flux. The released Br was absorbed by 15 mL receiver solution of 1.8 mM Na2CO3 and 1.7 mM NaHCO3. The limit of detection and quantification of the proposed method were 0.15 mg/kg and 0.49 mg/kg, respectively.2. Bromine contents in Chinese coals are achieved through a designed nationwide samples collecting and consistently determining by pyrohydrolysis and ion chromatography. Compared with other important coal-producing countries, China has lower bromine content, with a concentration scope of 0.12-69.66 mg/kg shown logarithm normal frequency distributions and a geometric mean of 7.04 mg/kg.3. There is great difference for bromine content of Chinese coals in ranks, coal-forming period, coal-forming environment and coal-cumulating areas. Most of coals with moderate and low bromine content in different administrative district in China. When peat developed into lignite through diagenesis, majority of its original bromine was lost because of the release of dissolved organic materials in peat. The sub-bituminous coal has the least bromine content in all rank coals of China, with geometric mean of 2.74 mg/kg. The larger bromine contents in coals, the more influence of marine conditions during their formation had been subjected to.4. In different sulfur forms, the bromine in high-sulfur coals has good relation with pyrite sulfur, poor relation with organic sulfur. While this element in moderate-sulfur coals may be occurred in organic sulfur and nitrogen compound. In the low-sulfur coals, the bromine has no evident relation with different sulfur forms and nitrogen; while it has good relation with ash yield, which indicates most of the bromine in Chinese coal occurs in inorganic bound.5. The experimental results of sequential chemical extraction show that the bromine in coal mainly occurs in the water-soluble, ion exchangeable, carbonate, Fe-Mn oxides and organic fraction. It should be paid more attention for the bromine in coal because its potential leachable rate is 36.62%-86.80% and potential leachable content is 7.092-20.10 mg/kg.6. The results of leaching experiment show that the bromine in coal is mainly released in the early leaching. The released bromine from coal leached with the solution of pH 2 covers 80.65% of the total bromine content in the first leaching 40 h, and 89.42% with that of pH 4 in first leaching 50 h.The stronger the acidity of the leaching solution, the larger the maximum bromine concentration released from coal and the total bromine leaching rate are. Leached by acidic solution, the bromine occurred in carbonate and Fe-Mn oxides which are on the surface of coal particles are released firstly, and then the water-soluble and exchangeable bromine which are inside coal matrix come to be released.7. The anthracite releases the most bromine content at low temperature ashing. Before the bromine release thoroughly, the emission rate of bromine in coal increases with the rise of combustion temperature, retention time and O2 fluid under water vapor passing. The water vapor in flue can improve the emission of bromine in coal obviously. The average increased emission rate of bromine in various coal ranks is 31.9% under water vapor passing.