Research On The Emission Characteristic And Formation Mechanism Of PCDD/Fs From Cement Kiln During Solid Waste Co-disposal

Co-processing of solid waste in cement kiln is a win-win technology,which not only provides cement kilns with energy and raw materials,but also can dispose solid waste at the same time.However,some persistent organic pollutants(POPs),like polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans(PCDD/Fs)were generated in the co-disposing process simultaneously.Therefore,the key to spreading the technology of co-processing of solid waste in cement kiln is how to control and reduce the generation of PCDD/Fs.This paper will focus on environmental impact of PCDD/Fs,the main region of PCDD/Fs formed,the principal influencing factors and control technology during the co-processing of solid waste in cement kiln.Eventually,the PCDD/Fs and chlorobenzene(CBz)formation mechanism and the mass balance of them will be thoroughly dawned and the PCDD/Fs suppression techonolgy with the appearance of sludge drying pyrolysis gas(SDPG)will also be put forward during the process of co-disposal of solid waste in the cement kilns,which can provide reference for cement companies to carry out the business on co-disposal of solid waste.This paper carried out a series of experimental studies,and made some practical guiding research results and conclusions,including:1)In the paper,we found that the concentrations of dioxins in the flue gas from six typical new dry-process cement kilns below the emission limit of 0.1 ng I-TEQ/Nm3 when solid waste is co-incinerated in the cement kilns.The amount and types of co-processing solid waste have minor effects on the dioxin emission.The emission levels of dioxins decrease with the increasing of the scale of the production line.The results of correlation coefficient analysis showed that the flue gas from cement kilns has minor effects on the concentrations of dioxins in the ambient air and soil,indicating that the dioxins in the flue gas could not largely diffuse and settle into the ambient air and soil.Life cycle assessment results show that the co-processing of solid waste in the cement kilns is conducive to reducing greenhouse gas emissions and does not increase the human toxicity potential.In a word,co-processing of solid waste in the cement kilns is an environmentally friendly technology.2)In the paper,we researched on the distribution of dioxins in the cyclone separator and post-combustion zones.The results reveal that the first stage of the cyclone separators is the main area for dioxin formation and de novo synthesis might be the key formation mechanism.Moreover,the analysis of mass balance shows that the net emissions of PCDD/Fs and CBz are-1268 mg I-TEQ/year and-40 kg/year,indicating that the cement kiln is not a source but a sink process of PCDD/Fs and CBz.Furthermore,the amount of accumulated inorganic chlorine in the kiln decreases from 89 kg/h to 73 kg/h when the bypass system is open.Meanwhile,the concentration of PCDD/Fs emitted from the stack also decreases from 0.037 ± 0.035 ng I-TEQ,/Nm3 to 0.019 ± 0.007 ng I-TEQ/Nm3.The results reveal that the opening of bypass system can destruct the balance of chlorine in the kiln and therefore reduce the amount of dioxins formed.3)In the paper,the influencing factors on the de novo synthesis of PCDD/Fs in the recycled ash and the formation mechanism will be thoroughly explored.The effects of temperature and oxygen content on the desorption characteristics of PCDD/Fs in the fly ash are also studied.The concentration of PCDD/Fs formed in the heated soxhlet fly ash is 157 pg I-TEQ/g,indicating that the recycled fly ash could not significantly increase the emission level of dioxins.Moreover,CuCI2 is proved to be the main catalyst and chlorine source for dioxin formation.Furthermore,the total concentration of dioxins remained in the ash decrease with the increasing of the temperature or the oxygen content.As to the distribution of I-TEQ at the gas and solid phase,the ratio of I-TEQ at the gas phase increases from 18%to 33%as long as the temperature rises from 300? to 400?.On the other hand,the oxygen content has minor effects on the distribution of I-TEQ at the gas and solid phase.4)In this paper,the suppression effects of SDPGs which are produced under different conditions are studied.Moreover,the optimal application condition is given after a lot of tests have been conducted.The main fuctional components and the suppression mechanisms are also investigated to provide a theoretical basis for the synergistic treatment of sludge and supression of dioxin formation.The results reveal that the type,moisture content,dosage and drying pyrolysis temperature of sludge can largely affect the suppression effects of dioxins,while atmosphere for sludge drying has a little influence on that.Eventually,the optimal application condition is suggested to be 300? for sludge drying pyrolysis and the recommended dosage of sludge is given according to the equation of(N+S)/Cl=1.The suppression effiency of PCDD/Fs could be up to 88.9%under the optimal condition.Most importantly,the flue gas at the outlet of Cl stage of cyclone separator can be extracted to the sludge drying equipment and then the SDPG can be returned to the inlet of first stage of cyclone separator to suppress the formation of dioxins in this area.