Experimental Research On Mercury Removal From Coal By Microwave Pyrolysis

China is the world's largest producer and consumer of coal.The technology of thermal removal of mercury from coal before combustion can not only remove mercury from coal,but also save cost,remove moisture and other harmful elements from coal,and reduce mercury emission from source.In this paper,five coal samples,BY5,BY23,FH,SH and CLS,were selected as the main research objects,and the occurrence forms of mercury in coal were studied.Based on the laboratory pyrolysis experiments,the effects of different pyrolysis temperatures,heating rates and microwave power on the migration and transformation of mercury in coal wi th residence time were studied,and the mechanism of mercury removal from coal by microwave pyrolysis was discussed.The mineral components in five coal samples were detected by X-ray energy spectrum,and the occurrence forms of mercury in coal were analyzed by BCR step by step chemical extraction method.The experimental results showed that kaolinite,quartz,kaolinite,quartz,Pyrite and calcite,as common mineral components in coal,are found in almost all coal samples.The main forms of mercury in BY5 and BY23 coal are weak acid extraction and oxidizable state respectively,and the content of mercury in SH and CLS coals is more uniform.The fraction of mercury extracted by weak acid in FH coal was 46.33%,and the content of mercury in other forms decreased in turn.In the microwave pyrolysis experiment system,the coal pyrolysis was carried out in the N₂ atmosphere of 1L/min with the microwave power of 1500W.It is found that the rate of mercury release from coal increases with the increase of heating rate.Microwave power was the main factor that affects the effect of mercury release,and the rate of mer cury release from coal is faster under high power.When microwave power is greater than 1100W,mercury can be completely released from coal in 5min.Due to the non-thermal effect of microwave pyrolysis,isothermal pyrolysis is not suitable for the control of microwave pyrolysis process.The moisture in coal mainly exists on the surface of coal particle or in the capillary hole.The free water will be evaporated rapidly at the beginning of pyrolysis.Its existence has little effect on the mercury release from coal.In order to study the effect of microwave pyrolysis on coal quality,the calorific value and sulfur content of coal samples before and after pyrolysis were measured.It was found that the absolute calorific value of coal samples changed between-14.19%and 9.31%,and the rate of sulfur release was about 20%.This has a good effect on the quality of coal samples.The release form of mercury in pyrolysis process was determined by Ontario Hydro method.The results showed that the contents of CH₄,CO and H₂ in pyrolytic gas increased with the increase of microwave power,and these reductive gases reacted with oxidized mercury to form elemental mercury.In 1500W power pyrolysis flue gas,mercury almost exists in the form of elemental mercury.In order to study the mechanism of mercury release during microwave pyrolysis,the phase changes of XRD in coal samples before and after pyrolysis were analyzed.It was found that the removal of mercury was mainly divided into three aspects:Firstly pyrite and calcite with high dielectric properties are pyrolyzed and the sulfide and carbonate-bound mercury in them are released.On the other hand,due to the different coefficient of thermal expansion of minerals,some organic and residual mercury was released,and most o f the carbonate-bound mercury volatilizes due to the pyrolysis of calcite due to the increase of temperature.