The Study Of Transformation Regularities Of Volatile Trace Elements (Hg, As, Se)during Coal Combustion

Hazardous trace elements releasing into the environment during coal combustion,especially volatile trace elements which more easily escaping into the atmosphere, cancause critical effects on the environment and human bodies. As a result of theirtremendous harm to the environment and human beings, this thesis is based on thevolatile trace elements (Hg, As, Se) and focused on their contents and occurrence incoal, the transformation mechanism during and after coal combustion. Finally, theeffects of coal upgrading technology on volatilization of trace elements are studiedtentatively in the last part of this thesis.The low concentration (<100ppm) and volatilization of Hg, As, Se leaddifficulties in the quantitative analysis. This thesis applied microwave digestion forpretreatment and inductively coupled plasma mass spectrometry for detection whichset up a prompt and accurate method. This method did not only accelerat thequantitative speed, but also decrease the wastages.Research on the occurrences of trace elements in coal are crucial to the studies oftrace elements in a serious of reactions during coal combustion and predictions oftheir distribution and enrichment. Sequential chemical leaching method was used tostudy the occurrence of three kinds of industrial coal and three kinds of typical lignite.The results showed that the Hg and As were similar in occurrence which mainly insulphide, but Se were equal in sulphide and residue. In the results of lignite, thewater-extractable and sulphide took a large percent in the occurrence which meant ahigh migratory aptitude and may easily escape from combustion.Industrial coal utilization are considered as a main source of Hg, As and Se in theenvironment. The studies on the three elements in industrial conditions are importantto the elements control strategies. The samples were collected from heating stationsand power plants in Tianjin and further discussion of the effects of coal type, boilertechnology and furnace temperature on the transformation were presented in this work.In addition, a mass balance was given and the results showed the three elements had ahigh volatilization of30%-60%in combustion.The fly ash was considered to capture Hg, thereby reducing the amount ofgaseous Hg in emission. This thesis used the fly ash which collected from the powerplants and studied the correlation of trace elements capture with the particle size, pattern, amounts of unburned carbon and the magnetic substances of fly ash. Theresults showed that the contents of Hg in fly ash had a positive correlation with theamounts of unburned carbon and the contents of As had a positive correlation withmagnetic substance.The effects of coal upgrading on the coal characteristic and volatilization of traceelements were tentatively studied at the end of this thesis. Pyrolysis kinetic analysison the upgrading coal was conducted and results showed that the activation energy ofupgrading coal were higher than raw coal which meant the coal characteristic wastruly improved. Furthermore, the combustion experiment of upgrading coal showedthe volatilization of Hg, As, Se were suppressed obviously by upgrading progress.