Experimental And Mechanism Study On The Correlation Between Mercury And Minerals And Pre-combustion Removal Of Mercury And Sulfur In Coals

Coal is the most popular source of primary energy and the coal-fired power plants are still the main source of the demand for electricity in the world especially in China.With the wide exploitation of coal and the rapid progress of society,clean and efficient utilization of coal has become the most important task during coal utilization.The objective of the thesis is to investigate the methods and mechanisms for Hg/S removal before coal combustion.Several typical high Mercury/Sulfur(Hg/S)containing coals and steam coals in China and Canada were selected in this study.The occurrence of Hg/S in these coals were firstly investigate.Then mild thermal upgrading and Air Dense Medium Fludized Bed(ADMFB) were used for the pre-combustion removal of Hg/S.The thesis revealed the mechanisms and rules for the removal of different forms of Hg/S and also obtained the relative kinetics data by systemic experiments and theoretical analysis. The overall research can provide scientific knowledges for the exploitation and development of pollutants controlling technologies.The occurrence of Hg and S as well as the correlation between those elements and the minerals in different coals were investigated by using variable methods such as float-sink test,sequential leaching,LTA-XRD analysis and XRF.A strong correlation between Hg and an infrequent mineral Chaoite was found in AB coal from Canada.For the other two Chinese coals,Hg was found to be associated with Pyrite and Kaolinite,respectively.The removal and releasing behavior of Hg and S during mild thermal upgrading were studied in the horizontal tube furnace.The results showed that over 90%Hg in coal could be removed by mild thermal upgrading.Mercury in coal can be defined as "volatilizable mercury","molecule bound mercury" and "covalent bond mercury" based on different forms of combination.The study also showed that rapid thermal upgrading and a mild oxidizing atmosphere between 350℃and 400℃can facilitate the release of mercury and pyrite surfur.The kinetics calculation showed that the activation energy of sulfur releasing was decreasd and reaction rate constant k was increased under mild oxidizing atmosphere. It indicated that the mild oxidiazing atmosphere was benifical to the removal of Hg and S for the concern of energy consumption.The formation of more and large sizes of cracks course by rapid thermal upgrading and more complicated pore structure and coarser surface of the char under mild oxidizing atmosphere could be responsible for the increase of Hg,and S removal during thermal upgrading.The thesis indicated that rapid thermal upgrading conducted at 400℃under 4%O₂ oxidizing atmosphere was an optimal working condition for the pre-combustion removal of Hg and S by mild thermal upgrading.Different forms of mercury and sulfur were found to release at different temperatures during thermal upgrading.It was detected that Hg associated with pyrite could be released from 400℃up to 550℃while Hg associated with clay minerals could be released from 250℃up to 350℃.The study also indicated that the unstabilized organic sulfur in coal was the easiest to decomposed at 370℃,and the SO₂ released by the decomposition of pyrite was found at 550℃,the mild oxidizing atmosphere can lower the releasing temperature of sulfide sulfur about 50℃,the most stable thiophene sulfur can only be decomposed when the temperature reached to over 800℃under 10%O₂ atmosphere.The determination of gases released from thermal upgrading indicated that oxygen would selectively break C-S bound than C-C bound under 4%O₂ concentration.With the increasing of oxygen content,more C-C bounds were broken together with C-S bond which will result in the significant loss of heat value of the char.The dry coal cleaning experiments for Hg/Sulfur removal were conducted on ADMFB.Various factors which could influence the efficiencies of the separation of ADMFB were investigated in this study.Under the optimal separation conditions,the fine coal particles with the particle size range from 1 to 3.66mm were separated successfully.The study showed that the characteristics of coal effected theseparation efficiencies distinctly,the best separation for dry coal cleaning could obtain about 75.31% yield of clean coal,45.7%ash rejection,47.2%and 49.3%at the highest Hg and S rejection and also the combustible recovery at about 89.4%.The thesis has systematically studied the occurrence of Hg and S in different coals, and investigated the mechanisms and behaviours of Hg/S removal by using two different methods,and also provided the thermodynamic and kinetic data.The conclusions could be useful for the exploitation of low-cost emission controlling technologies before coal combustion.