The Existing Form Of Arsenic Species In Coal-fired Flue Gas And Its Adsorption By Zeolite

Arsenic is a toxic semi-metal element.During utilization of coal,the emission of arsenic has contributed significantly to the severe pollution for environment and human health.Coal has maintained to be the major primary energy in our country.It has provided energy for power,metallurgy,etc.But combustion of extensive coal has caused the emission of arsenic.At present,there is no mature technology for arsenic pollution control of industrial coal-fired boiler.According to the current research progress and present problems,arsenic pollution control of industrial coal-fired boiler may be divided into three categories.They are pre-combustion arsenic capture,during-combustion arsenic capture,post-combustion arsenic capture,respectively.Though improving rinse effect of coal and optimizing boiler combustion will reduce the emission of arsenic to some extent,yet it is far not enough to meet the increasingly stringent emission requirements.Therefore,the further developing plan of arsenic pollution control of industrial coal-fired boiler is expected,we must take measures to reduce pollution.The technique of adsorption as a simple and viable technique to remove arsenic is becoming more and more promising.Zeolites are widely used as sorbents in the field of adsorption for air pollutant.Their well-defined pore structure and excellent properties make them highly active in a large variety of the adsorption process.Nowadays,the design of sorbent is based on the trial-and-error experiments.There is no doubt that we must take measures to rationally design sorbent with advanced method.In this study,theoretical Grand Canonical Monte Carlo method and Density Functional Theory calculations are applied to predict microscopic behavior of the zeolite and arsenic.The existing forms of arsenic and its chemical structures in different stages of the boiler were calculated by DFT.The different forms of arsenic have differently physical and chemical characteristics.The reasonable controlling technology of arsenic must build on a rational knowledge of the arsenic.Gibbs energy minimum principle was applied to determine the stationary structure of arsenic at four typical temperatures.The temperature is various in different stages of the boiler.The temperature at 1200°C is the typical temperature in the combustion area of the coal-fired boiler.The temperature at800°C is the temperature of flue gas in the horizontal flue.The temperature at 370°C is the temperature of flue gas in the NOx removal reactor.The room temperature wasstudied as well to obtain the stationary structure in air after being emitted from the stack.The results show that trivalent arsenic molecules are thermodynamically stable at high temperatures and pentavalent species are stable at low temperatures.The arsenic species vary with the temperature.At high temperatures,dehydrated compounds are the major species.These compounds will be hydrated and oxidized by O2 when the temperature declines,as implied by the reaction path study.In the area of the NOx remove reactor,trivalent As4O6 is the major species and pentavalent arsenic acid will be produced.Arsenic acid becomes the most thermodynamically stable species at 25°C.Acquiring regularity of absorption for arsenic in zeolite can offer theoretic basis and the practice guide for arsenic pollution control of industrial coal-fired boiler.Based on the discussion above,in the area of the NOx removal reactor,trivalent As4O6 is the major species.In this part,GCMC method and DFT were used to study the adsorption of arsenic in zeolites with various Si/Al ratios and pore sizes.The results show that pores with diameter of 7 ? are optimal for As4O6 adsorption in zeolites.High porosity can also promote the adsorption of As4O6 in a specific amount of zeolite.Al substitutions are beneficial for As4O6 adsorption.Experiments were used to test the adsorption of As4O6 using H-ZSM-5 zeolites with different Si/Al ratios.The comparison between experimental and theoretical results has led to an excellent match.In order to further improve the ability of zeolites for As4O6 adsorption,metal oxide is doped into H-ZSM-5(Si/Al=12.5)zeolite.Test results showed that zeolite doped by Cu demonstrated excellent property for the adsorption of As4O6.The influences of possible compositions in flue gas including SO2,NO,H2 O,CO2 and N2 were also tested using GCMC method.SO2 and NO can cause notable drop of As4O6 adsorption.