Experimental Study On Incineration Characteristics And Pollutants Emission Of Sewage Sludge In Circulating Fluidized Bed

Sewage sludge incineration with the advantages of volume reduction, stabilization, harmlessness and energy recovery, is considered to be the most thorough way among the methods of sludge disposal. During the process of sewage sludge incineration, if the emissions of NOx, SO2, HCl, CO, heavy metals and other secondary pollutants are not effectively controlled, serious environmental pollution will occur. Due to its special fuel properties, incineration and pollutants emission characteristics of sewage sludge are different from those of coal and biomass. Therefore, it is theoretically important to study incineration characteristics and pollutants emission and control of sewage sludge, which is also with application value for the disposal of sewage sludge incineration.In this paper, incineration characteristics and pollutants emission of sewage sludge in circulating fluidized bed (CFB) are investigated. Along with theoretical analysis, a series of experiments in different devices were conducted in order to explore the sewage sludge combustion behaviors, the gaseous emission of sewage sludge incineration in CFB, and the speciation transformation and distribution of heavy metals. Pollutants emission of sewage sludge during the drying and incineration process in the integrated incineration system for wet sewage sludge in CFB is also studied. The detailed contents are as follows.1. The combustion characteristics and kinetics of sewage sludge were studied by thermo-gravimetric analysis. The results show that there are obvious differences between the combustion of sludge and coal or biomass. During sludge combustion, both the stages of volatile volatilization and combustion and the burn-out stage of residue volatile and fixed carbon are the main combustion processes. The ignition temperature of sewage sludge is low, but its combustion rate is also low, leading to poor combustion and burnout performance, which could be remarkably improved by adding coal or biomass.2. The emission characteristics of NOx, SO2and HCl during sewage sludge incineration in CFB were explored. Sewage sludge combustion experiments under different operation conditions were carried out in a15kW CFB experimental system. The results reveal that the concentrations of NO, SO2and HCl increases as the combustion temperature increases, while N2O concentration decreases. The concentrations of N2O and NO reduce with the increase in the secondary air ratio, and the effect is different between sewage sludge with different moisture content. Increasing sewage sludge moisture content leads to substantial increase in the emissions of N2O and HCl, and decrease in NO emission. Compared with coal, higher nitrogen oxides emissions but lower fuel-N conversion ratios are obtained during sewage sludge combustion. Emissions of SO2and HCl can be effectively controlled by limestone injection.3. The effect of phosphorus in sludge on desulfurization of limestone was investigated through experiments and chemical thermodynamic analyses. Chemical thermodynamic analyses show that CaO would react first with phosphoric oxide during sewage sludge incineration, and then the remaining parts may be involved in desulfurization. The same tendency is found by desulfurization experiments. However, during experiments only parts of CaO will react first with phosphoric oxide, and the amount of CaO occupied by unit mole of phosphorus is much lower than analyses results, which mean chemical thermodynamic analyses overestimate the effect of phosphorus. Such effect should be taken into account when the Ca/S ratio is to be decided for sulfur capture.4. The speciation transformation and distribution of heavy metals during sewage sludge incineration were researched by the chemical thermodynamic analyses. The results indicate that Cd, Cu and Pb are transformed to other volatile forms easily during sewage sludge incineration, while Cr, Mn and Zn are not so easy. Cl will promote the volatilizations of Pb, Cd, Cu, Mn and Zn due to the formation of metal chlorides, and S will react with Cd and Pb forming sulfates. Al2O3and SiO2have a good adsorption of Cd and Pb, which will be restrained by Cl and S.5. Pollutants emission of sewage sludge in the drying and incineration process in integrated incineration system for wet sewage sludge in CFB was studied. Experimental results of sewage sludge drying in a fluidized bed demonstrate that the main emitted gases emission in drying process are identified to be NH3, CH4, C3H8, C7H8and C2H6O. Among them, NH3concentration is high, which increases with the increase in incineration temperature. The condensate from drying gases is alkaline, and the concentrations of COD, NH3-N and NH4+are relatively high. The results of incineration test on100t/d integrated incineration system for sewage sludge in CFB show that continuous and stable operation at full load has been achieved with the combustion efficiency of over98.5%in the incinerator and the good drying performance in the drier. The concentrations of CO, NOx, SO2, HCl, Hg, Cd, Pb and dioxin-like pollutants in flue gas are lower than the emission standard limit. The leaching toxicity of heavy metals in fly ash and bottom ash are much lower than the identification standards for hazardous wastes.