The Occurrence Regularity Of AAEM And Sulfur Elements In Particulate Matters Generated From Coal Combustion In A Pressurized Oxy-fuel Fluidized Bed

As an advanced clean coal combustion technology,the pressurized oxy-fuel fluidized bed combustion technology can realize both the high-efficiency of coal combustion and the low-cost capture of CO₂ generated from coal combustion.Existing studies about this technology mainly focused on the coal combustion characteristics and gaseous pollutants under the condition of pressurized oxy-fuel combustion.However,there is no experimental report on the particulate matters generated under the condition of pressurized oxy-fuel coal combustion.Some elements?mainly alkali and alkaline earth metals,and sulfur elements?contained in coal will be easily vaporized during the coal combustion process.These vaporized elements are tend to generate particulate matters in the combustion process,causing atmospheric environmental pollution as well as ash deposition and slagging problems on the heating surface of the boiler.At present,researches on the particulate matter generated from coal combustion were mostly conducted under atmospheric pressure,and the element occurrence characteristics of fine particulate matter under the condition of pressurized oxy-fuel combustion still have not been revealed.Therefore,the key point of this study is to explore the vaporization behavior of AAEM and sulfur elements in coal and their occurrence characteristics in particulate matters under pressurized oxy-fuel combustion.A new pressurized fixed bed combustion reactor was built to conduct an experimental study on the element vaporization behaviors under the pressurized oxy-fuel conditions.The influence of pressure,temperature,atmosphere and residence time on the element vaporization behaviors was analyzed.Based on the ash formation experiment results,the vaporization behavior of AAEM and sulfur element was speculated by ash formation analysis from the data of pressurized oxy-fuel combustion.Moreover,a 10kW_th continuous-feeding pressurized oxy-fuel fluidized bed combustion experiment platform was established.The electrical low-pressure impactor was used to on-line monitoring particulate matter concentrations and collecting fine particulate matter samples by different grades during combustion.The emission characteristics and morphological characteristics of particulate matters generated by the combustion of different types of coal under different pressures and atmospheres were studied.The content and distribution characteristics of alkali and alkali earth metal elements in the particulate matters were analyzed,as well as the content distribution and occurrence characteristics of S element in the particulate matters.The results of the pressurized fixed bed experiment showed that the vaporization rates of Na,Ca and S increased with the increase of combustion reaction pressure in the air atmosphere from 0.1MPa to 0.5MPa,and decreased with the increased pressure in the atmosphere from0.5MPa to 0.7MPa.When the reaction atmosphere was O₂/CO₂ atmosphere?O₂ concerntration is 40%?,with the increased pressure,the vaporization rate of S increased significantly.Under the condition of high pressure,with the increase of combustion reaction temperature,the vaporization rate of Na and S increased significantly.Compared with air atmosphere,O₂/CO₂atmosphere?O₂ concerntration is 21%?had stronger inhibitory effect on the vaporization of Na and K elements,while had less inhibitory effect on the gasification of Ca,Mg and S elements.Under the O₂/CO₂ atmosphere,increasing the oxygen concentration would mainly increase the vaporization rate of Na and K.The experimental results of 10kW_th continuous-feeding pressurized oxy-fuel fluidized bed combustor showed that the increased pressure will reduce the content of carbon black in ultrafine particles.Moreover,the increase of particle volume concentration at high pressure may enhance the agglomeration effect of fine particles,resulting in the increase of the concentration of intermediate mode particles.Two kinds of submicron particles can be distinguished from the morphological characteristics,which were presumed to be carbon black and condensed alkali and alkaline earth metals or other metal suboxide.Higher oxygen concentration may enhance the melting effect and thus form larger and regular shaped residual ash particles.Increasing the pressure will promote the enrichment of Na,K and Mg in submicron particles.Ca was mainly concentrated in supermicron residual ash particles.When the air atmosphere was switched to O₂/CO₂ atmosphere,K enrichment in the submicron particles showed an obvious enrichment,while the enrichment peak of Na elements in the submicron particles shifted to the left,possibly due to the homogeneous nucleation effect of Na caused by a lower combustion temperature.Experiments on the occurrence of S element in fine particulate matter showed that under certain experimental conditions of Xuzhou coal?reaction pressure 0.5MPa and fuel particle size0.001⁰.2mm,or combustion reaction pressure 0.3MPa and fuel particle size 0.08⁰.4mm?,S element was obviously enriched in PM_0.4.The increase of reaction pressure in air atmosphere may lead to the enrichment of S element in submicron particles in the form of sulfate.The larger coal particle size may lead to the enrichment of S in submicron particles in the form of metal sulfide and organic sulfur.Under elevated pressures,air and O₂/CO₂ atmosphere,the element S occurred in the submicron particles in the form of alkali metal sulfate when Zhundong coal was combusted.Pressurized oxy-fuel fluidized bed combustion technology is one of the most promising clean coal combustion technology.In this paper,the occurrence characteristics of AAEM and sulfur in particulate matter generated from coal combustion were revealed through the combustion experiments of the fixed bed and continuous-feeding fluidized bed under pressurized oxy-fuel condition.It provided a basic experimental data of fine particulate matter for the development of pressurized oxy-fuel fluidized bed combustion technology in the future.