Mercury Distribution And Its Fate During Pyrolysis

Mercury,as a trace toxic heavy metal pollutant,has caused great harm to human beings due to its difficult collection,low temperature escaping,biosphere recycling,bioaccumulation,and hard to degrade.The mercury in the atmosphere is mainly derived from the coal utilization industry.The occurrence of mercury in coal is related to several factors such as coal-forming environment,the coal-forming process,the degree of coal deterioration,and the content and types of minerals.Due to the diversity of coal types and the complexity of the coal molecular structure,there are many occurrences of mercury in coal,which show different release characteristics during coal thermal processing.Recently,many researches are aimed at the emission of mercury during coal combustion and the transformation of mercury in coal-fired flue gas with few studies on the migration of mercury in coal pyrolysis.In this paper,Guizhou high-mercury bituminous coal(GZB)used as raw material is divided into two density grades by floatation test,which are consist of organic-rich density grade Ci(density<1.300 g/cm3)and mineral-rich material density grade C2(density>1.300 g/cm3).The organic solvent progressive extraction technology is also used on C1 to prepare the four organic components which are definited for the CS2/NMP raffinate EG1,CCl4 raffinate EG2,n-heptane raffinate EG3 and n-heptane extract EG4,and the enrichment of mercury in different components have studied.The results indicate that the mercury in the GZB raw coal will be enriched to a certain extent in the minerals.The content of mercury in the four organic group components is ranked in ascending order as EG4<EG3<EG2<EGi indicating that mercury in organic matter of coal is mainly concentrated in organic matter EG1 and EG2,while the content of n-heptane insoluble EG3 and n-heptane soluble components of EG4 is very low.Chemical leaching experiments are used to analyze the mercury species in different samples.The results show that the content of mercury with different forms in the GZB raw coal was in descending order as HCl-Hg>HNO3-Hg>Hg-OM>HgS.The mercury in C1 was mainly composed of Hg-OM,and the mercury in C2 was mainly composed of HCl-Hg,HNO3-Hg.For the two organic components with relatively high mercury content,the mercury in EG1 is mainly consist with Hg-OM with a small amount of HCl-Hg and HNO3-Hg and HgS.In EG2,mercury only consist in organic matter.The release of mercury in the pyrolysis process of different samples was studied by using programmed temperature and constant temperature heat treatment experiments in a small fixed-bed pyrolysis reactor.The results show that main mercury escaping temperature in the GZB raw coal is from 200 to 700?,which are consists of three peaks with peaks at 275?,346?,and 525?,corresponding to Hg-OM,HCl-Hg,and HNO3-Hg,respectively,compared with chemical leaching experiments.Compared with GZB,the area of organic mercury in the mercury release curve of C1 increased significantly,while the area of HCl-Hg and HNO3-Hg decreased significantly,which shows that the occurrence of mercury in C1 is dominated by organic mercury.The area of HCl-Hg in the mercury release curve of C2 is obviously increased,while the area of HNO3-Hg remains basically unchanged accompany with the corresponding release peak of Hg-OM and HgS disappears completely,which shows that the state of mercury in C2 is mainly related to minerals.The Peakfit software is used to fit the peaks of mercury release curves in different samples in order to obtain the specific forms of mercury in different samples.The forms of mercury in GZB are consisted of 7 kinds of mercuic compounds.The domination form of mercury in C1,C2,EG1,EG2 is similar with the result of chemical leaching experiments.When the rate increasing from 5?/min to 20?/min,the mercury release temperature in all the samples moves to the high temperature zone in different degrees at low temperature stage rather than high temperature stage.The results of constant temperature heat treatment experimental show that the main release temperature ranges of mercury in different samples are quite different.The release of mercury in GZB is relatively average at each temperature point within the range of 250-550?.The release temperature range of mercury in C1 is concentrated in 200-250?and 400-550?.The release temperature range of mercury in C2 is concentrated between 250-350? and 450-500?.The release temperature range of mercury in EG1 is concentrated among 150-350?.The release temperature range of mercury in EG2 is concentrated in 350-600?.The kinetics results show that the release of mercury in the pyrolysis process conforms to the Coats-Redfern secondary kinetics model.The kinetics of mercury pyrolysis evolution in GZB raw coal can be divided into five stages,and the activation energy is fifth stage(723-800?)>first stage(200-275?)>fourth stage(527-723?)>second stage(275-340?)>third stage(340-527?).The kinetics of mercury pyrolysis in C1 can be divided into four stages,and the activation energy order is the first stage(200-256?)>fourth stage(422-650?)>second stage(256-312?)>third stage(312-367?).The kinetics of mercury pyrolysis in C2 can be also divided into four stages,which are the first stage(260-350?),the second stage(350-390?),the third stage(390-500?)and the fourth stage(500-600?)with the same activation energy order of C1.The kinetics of mercury pyrolysis in EG1 can be divided into three stages,and the activation energy order is the is the second stage(251-305?)>the third stage(305-350?)>the first stage(200-251?).The kinetics of the mercury pyrolysis evolution in EG2 can also be divided into three stages,and the activation energy order is the the third stage(523-800?)>the second stage(414-523?)>the first stage(300-414 C).With the heating rate increasing,the activation energy and frequency factor increase.