Studies On Release Kinetics And Carbon-based Sorbent Removal Of Trace Elements During Coal Gasification

Toxic trace elements contained in coal will be released during the process of coal gasification. As a typical reducing atmosphere, some toxic trace elements become more volatile during coal gasification than that during coal combustion. It is thus of great importance to clarify the release behaviors of trace elements during coal gasification and to develop emission control methods. There are several important issues needed to be resolved such as the temporal release of toxic trace elements and the adsorption mechanisms of trace elements on carbonaceous surface. Clarifying the temporal evolution processes of trace elements is necessary for further understanding the transformation behaviors of trace elements during coal gasification. Understanding the adsorption mechanisms of trace elements on carbonaceous surface is necessary for the developments of trace elements emissions control technologies.The previous studies on on-line analysis kinetics of semi-volatile trace elements cadmium (Cd), lead (Pb) and zinc (Zn) are mainly focus on solid waste incineration and coal combustion. The release kinetics of Cd, Pb and Zn during coal gasification is still unclear. An on-line analysis system of trace elements was used to study the release dynamics of Cd, Pb and Zn during coal gasification, and compared with the kinetic results of coal pyrolysis and combustion. The results indicate that the temperature and atmosphere have strong influences on concentrations of Cd, Pb and Zn in flue gas. Based on the results of on-line analysis, the kinetic laws for Cd, Pb and Zn release in each condition were determined. The determined kinetic laws can predict accurately the dynamic characteristics of Cd, Pb and Zn release in each condition. The release rates of Cd, Pb and Zn increase with a rise of temperature. In order to predict the transformation behaviors of Cd during coal gasification, pyrolysis and combustion, thermodynamic equilibrium calculations were conducted. Cd presents in Cd(g) dominantly in flue gas during coal gasification and pyrolysis. During coal combustion, the dominant gaseous species of Cd varies with temperature and may react with minerals.By far, there is still lack of knowledge about the kinetic characteristics of arsenic (As) and selenium (Se) release during coal utilization. The temporal release of As and Se from coal during gasification, pyrolysis and combustion in a fluidized bed were measured in situ by an on-line analysis system of trace elements in flue gas. Obvious variations of As and Se concentrations in flue gas were observed during coal gasification, pyrolysis and combustion, indicating strong influences of temperature and atmosphere on As and Se release behaviors. Kinetic laws governing As and Se release during coal gasification, pyrolysis and combustion were determined based on the results of instantaneous As and Se concentrations in flue gas, respectively. The results of thermodynamic calculations indicate that Ca(AsO2)2 is the most probably species during coal gasification and pyrolysis. Ca3(AsO4)2and Ca(AsO2)2 are the possible species resulting from As-Ca interaction during coal combustion. FeSe is the most probably species from Se-Fe interaction during coal pyrolysis and gasification. CaSeO4 is the possible species resulting from reaction of SeO2 with CaO in fly ash during coal combustion.In order to investigate the influence mechanism of H2S on Hg adsorption on carbonaceous surface, cluster models with zigzag edge side and armchair edge side are employed to represent the carbonaceous surface, by applying density functional theory at B3PW91 level of theory. The adsorption of H2S on zigzag edge side and armchair edge side were calculated respectively. The adsorption of H2S on carbonaceous surface belongs to chemical adsorption. H2S molecule is likely to dissociate during the process of adsorption with the formations of C-S or C-SH on carbonaceous surface. Although Hg is not likely to interact with C-S and C-SH formed by H2S adsorption, the presence of S on carbonaceous surface can improve the activity of surface C sites, and then enhances the chemical adsorption of Hg on carbonaceous surface. HgS can adsorb both dissociatively and non-dissociatively on carbonaceous surface. Pyrone and ketone models were further employed to investigate the influences of basic oxygen functional groups on Hg and H2S adsorption on carbonaceous surface. Pyrone and ketone are both active sites for H2S adsorption on carbonaceous surface. The carbonyl oxygen atom in pyrone and ketone can bond with S atom with the formation of C-OS. The adsorption energy of H2S on pyrone is higher than that on ketone because of the present of ether oxygen atom in pyrone. The presence of pyrone can improve the adsorption of Hg on carbonaceous surface, because of the presence of ether oxygen atom. Addition the number of ether oxygen atom in pyrone is able to enhance such improving effect, and make the Hg adsorption belonging to chemical adsorption. The results indicate that carbonaceous materials are able to remove Hg and H2S from flue gas simultaneously. The presence of pyrone can improve the adsorption of Hg and H2S on carbonaceous surface simultaneously.Carbonaceous materials are able to remove As and Se in flue gas of coal gasification, whereas the adsorption mechanisms are still unclear. The adsorption of As and Se species on carbonaceous surface were investigated by density functional theory. The adsorption of As species on carbonaceous surface belongs to chemical adsorption. The stability of gaseous As species adsorptions are in the order of AS2> AS4> ASH3. ASH3 is likely to adsorb dissociatively on carbonaceous surface, whereas the adsorption of AS2 and AS4 on carbonaceous surface are non-dissociative. The adsorption of Se species on carbonaceous surface belongs to chemical adsorption. H2Se is likely to adsorb dissociatively on carbonaceous surface. AsSe can adsorb both dissociatively and non-dissociatively on carbonaceous surface, and the non-dissociative adsorption of AsSe is more stable. PbSe is likely to adsorb non-dissociatively on carbonaceous surface. The structures of As and Se species adsorption are very stable and not likely to desorb. It is can be concluded that carbonaceous materials have the ability to remove H2S, Hg, As and Se in flue gas of coal gasification simultaneously.