Study On The Coal Combustion Behaviors And Heavy Metal Migration Characteristics In A Coupled Combustion Power Plant Of Sewage Sludge And Coal Based On Circulating Flue Gas

Sewage sludge is a by-product of waste water treatment,which contains a large amount of harmful ingredients such as pathogens,hormones and heavy metals.Thermochemical conversion of sewage sludge could not only achieve massive volume reduction of sewage sludge,but also achieve the resource recycling,which is more consistent with the requirement of sustainable development.Under this background,a coupled combustion power plant of sewage sludge and coal based on circulating flue gas was proposed in this paper,and a series of simulation and experimental research were carried out based on this system.Firstly,the process simulation of the coupled combustion power plant of sewage sludge and coal based on circulating flue gas was carried out by using Aspen Plus software.In order to evaluate the feasibility of this system,the thermodynamic performance and economic indexes,i.e.,system efficiency,total capital investiments,internal of return?IRR?,discounted payback period?DPP?were calculated.Moreover,the process simulation of a pulverized coal direct-fired power plant was also carried out for comparison.The results reveal that the system efficiency of the coupled combustion power plant of sewage sludge and coal based on circulating flue gas is lower than that of the pulverized coal direct-fired power plant.However,the economical efficiency of the coupled combustion power plant of sewage sludge and coal based on circulating flue gas is better than that of the pulverized coal direct-fired power plant because of the sludge incineration subsidy.Secondly,the combustion characteristics of bituminous coal under the simulated combustion condition in the boiler of coupled combustion power plant of sewage sludge and coal based on circulating flue gas were investigated by using the single particle combustion system.The effects of O₂,CO₂ and H₂O concentration on the flame structure,ignition delay,burnout time and combustion temperature of bituminous coal were analyzed.The results reveal that bituminous coal ignites homogeneously and the combustion process could be divided in two stages:volatiles combustion and char combustion.Increasing the concentration of O₂ promotes the combustion of bituminous coal,which is reflected in higher brightness of the flame,shorter ignition delay and burnout time and higher combustion temperature.The increment of CO₂ concentration inhibits the combustion of bituminous coal,which would cause lower brightness of the flame,longer ignition delay and burnout time and lower combustion temperature.Increasing H₂O concentration brings negative effects on the combustion of the volatiles,while brings position effects on the combustion of char.Finally,the effects of moisture content on the combustion of sewage sludge were studied by using thermogravimetric analyzer,and the effects of furnace temperature,O₂,CO₂ and H₂O concentration on the heavy metal?Mn,Ni,Pb,Zn,Cu and Cr?migration characteristic of sewage sludge combustion were investigated by using a horizontal tube furnace and a X-ray fluorescence spectrometer.The results indicate that higher moisture content would deteriorate the combustion of sewage sludge,reflects in lower ignition index and comprehensive combustion index.Furthermore,the increment of moisture content would result in higher activation energy.Increasing the furnace temperature would decrease the residual ratio of the investigated heavy metals.With the concentration of O₂ increases,the residual ratio of Mn and Ni firstly increases and then decreases,the residual ratio of Cu and Cr increases and the residual ratio of Zn decreases.On the one hand,the presence of CO₂ would lead to lower combustion temperature and lower porosity of the char,on the other hand,CO₂ would promote heavy metal protoxide would convert to oxide,which is more stable.Therefore,increasing the concentration of CO₂ would lead to higher residual ratio of the investigated heavy metals.On the contrary,increasing the concentration of H₂O would result in lower residual ratio of the investigated heavy metals,which is because H₂O would lead to higher combustion temperature and higher porosity of the char.