| Similar to urban domestic waste,the output of rural domestic waste is increasing,and its pollution disposal has been paid more and more attention.For remote villages and towns which are not suitable for the centralized disposal method of"county treatment",pyrolysis gasification technology is a better way to realize decentralized treatment.At present,most small-scale rural waste pyrolysis gasifiers in use have problems such as unstable operation and excessive emission,especially dioxin,a persistent organic pollutant,which will cause many hazards if it cannot meet the emission requirements.Aiming at the generation and emission characteristics of dioxins in the process of pyrolysis and gasification of rural waste,this paper studied the influence of external factors such as gas exhaust mode,air ratio（AR）and temperature through a laboratory scale fixed bed furnace.At the same time,the sampling,investigation and analysis of dioxins in the main parts of a village and town-scale solid wastes gasification system were carried out.The main conclusions are as follows:（1）The pyrolysis and gasification experiment of simulated rural domestic waste was carried out in a fixed bed vertical tubular furnace in the laboratory.The results show that in the range of AR=0～0.35,the total toxic equivalent quantity（TEQ）of dioxins produced by pyrolysis and gasification of unit masss imulated rural waste is positively correlated with the air ratio for the updraft and downdraft mode.In experiments adopting updraft mode,the concentration of dioxin generated by simulated waste at 800℃increased from 0.0187 ng I-TEQ·g-1to 0.0487 ng I-TEQ·g-1.Moreover,In the range of 600℃～800℃,with the increase of temperature,the concentration of dioxins produced by pyrolysis of simulated waste decreases continuously from 1.973 ng I-TEQ·g-1to 1.426 ng I-TEQ·g-1.In general,the dioxin toxicity distribution of tar in the product accounts for the largest proportion,followed by flue gas,and the toxicity proportion of residue is the smallest.In addition,when the updraft mode is adopted,the proportion of dioxin toxicity in the tar（78%-90%）is greater than that in the downdraft mode（40.6%-69.4%）because of its higher tar yield.Therefore,reducing the tar yield in the process of pyrolysis and gasification of rural domestic waste may achieve the effect of dioxin control.（2）The distribution characteristics of dioxin homologues in the products were analyzed by selecting the typical working conditions in the pyrolysis and gasification experiment of updraft fixed bed furnace.The results show that the fingerprint characteristics of dioxins in pyrolysis and gasification reaction products are hardly affected by the air ratio or temperature.The distribution characteristics of dioxin homologues in gas phase and liquid phase are relatively consistent,but the solid phase presents its unique characteristics.In the dioxin mass concentration of flue gas and tar,low chlorinated furans account for the largest proportion.In particular,TCDF and Pe CDF account for 40.98%～63.10%of total dioxins in flue gas.However,high chlorinated dioxins and furans are the main in the residue,the total proportion of Hp CDD,OCDD,Hp CDF and OCDF is 58.42%～69.58%。The common point of the distribution of three-phase product dioxins is that the proportion of PCDFs in total generation is greater than that of PCDDs.Moreover,proportion of2,3,4,7,8-Pe CDF and 2,3,7,8-TCDD in TEQ is prominent.In addition,with the increase of air ratio,the proportion of low chlorinated furans in dioxins gradually increases,while the proportion of high chlorinated furans slowly decreases.（3）According to the sampling investigation of the actual pyrolysis gasifier,it is found that the concentration of dioxin in the slag at the bottom of the gasifier is 0.723μg I-TEQ·kg-1,meeting the landfill standard(<3μg TEQ·kg-1).For the flue gas in the system process,the I-TEQ of dioxin in the flue gas at the outlet of gasification chamber and secondary combustion chamber is low,which are 0.027 ng I-TEQ·Nm-3and 0.167 ng I-TEQ·Nm-3respectively;The concentration before the bag filter is the highest,reaching 10.503 ng I-TEQ·Nm-3;The I-TEQ of dioxin in tail flue gas before and after activated carbon injection were 0.586 ng I-TEQ·Nm-3and 0.029 ng I-TEQ·Nm-3,respectively.The dioxin emission of tail flue gas after activated carbon injection can meet the corresponding standards(<0.1 ng I-TEQ·Nm-3).Besides,the original emission concentration of dioxin in the flue gas during gasification and combustion is low.And low temperature heterogeneous synthesis is the main source of dioxin formation in the flue gas of the new pyrolysis gasifier system.In the future,the flue gas cooling device can be optimized to further inhibit the formation of dioxin.In addition,activated carbon injection has a good adsorption effect on dioxins in flue gas.In terms of the mass concentration distribution of dioxins,high chlorinated PCDD/Fs accounts for a large proportion.Meanwhile,PCDF/PCDD>1,indicating that PCDF is easier to form in the pyrolysis gasifier system.Moreover,2,3,4,7,8-pecdf has the largest contribution to the distribution of I-TEQ.Therefore,the synthetic inhibition of this substance can be explored to reduce the total TEQ emission of dioxin in the future. |
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