Research On Co-gasification Of Municipal Solid Waste In A Three-stage Gasifier

The disposal of municipal solid waste?MSW?has gradually become an important factor restricting the green and sustainable development of our country.As an effective and feasible way to reuse the energy of solid waste,gasification,however,has some problems of low gasification efficiency,poor stability of gasifier and high content of pollutants in syngas.Compared with the gasification process of solid waste alone,the co-gasification process of domestic waste and raw materials with other advantages will also produce synergy,which makes up for the shortcomings of the two raw materials in the separate gasification process,breaks through the restrictions on the selection of gasification reactants,enriches the raw materials and products of co-gasification,and also improves the carbon conversion during gasification process.It should be active to effectively control the production of tar and the emission of secondary pollutants.However,at present,the research on the basic characteristics and synergistic mechanism of co-gasification of MSW is absent.Based on this,a new three-stage co-gasification system of MSW and high alkali coal char or biomass char is developed to solve the problems of high contents of tar,hydrogen chloride and dioxin pollutants,low calorific value and low process efficiency of syngas.The findings of this research is expected to provide a feasible way for further understanding of the basic characteristics and synergistic mechanism of multi-stage collaborative gasification process,and for the treatment of high chlorine content MSW,and has a strong market prospect.In this study,a new three-stage co-gasification system of MSW with high alkali coal char or biomass char was developed.The integrated gasification system is divided into three reactors by using coal char or biomass pyrolysis char with high alkali content as catalyst for tar catalytic cracking,absorbent for removing acid HCl pollutants and reactant for collaborative gasification.The results show that the stable composition of syngas is 41.9 vol.%of hydrogen,29.3 vol.%of carbon monoxide,7.49 vol.%of methane and 3.90 vol.%of carbon dioxide.The calculated low calorific value is 12.2 MJ/Nm³ with the highest yield syngas of 1.57 Nm³/kg,which leads to the best cold gas efficiency of 97.4%and carbon conversion efficiency of 90.3%.With the increase of pyrolysis temperature,the concentration of water vapor increased from 15.7 g/Nm³ to 18.5g/Nm³.The water content in the syngas discharged from the oxidation stage reaches a high value of 18.1 g/Nm³ at the ER of 0.45,while the water content in the syngas discharged at the Er of 0.25is 14.6 g/Nm³.In general,H₂,tar content and low calorific value decreased with the increase of ER.At a higher reduction temperature,the decrease of CO and H₂ content directly results in the decrease of the lower calorific value of syngas.The cold gas efficiency,carbon conversion efficiency and low calorific value of syngas in the co-gasification system all decreased with the increase of reduction temperature,which were as low as 63.4%,66.0%and 10.6 MJ/Nm³ at 1000oC,respectively.The strength ratio of Boudouard reaction and water-gas reaction is less than 1under all experimental conditions,indicating that the water-gas reaction plays a leading role in the reduction stage of three-stage co-gasification system.In addition,with the increase of reduction temperature,the proportion of Boudouard reaction is larger and larger than that of water-gas reaction.As both Boudouard reaction and water-gas reaction are conducive to the yield of syngas at higher temperature,the water-gas reaction is gradually weakened with the increase of reduction temperature.According to the characteristic of high chlorine content in waste,the effect of chlorine on the reactivity and characteristic of char derived from complex MSW components was studied.The effects of inorganic chlorine?NaCl?and organic chlorine?PVC?on the basic physical and chemical properties of char were systematically studied by selecting three typical components of MSW,starch food waste,lignin wood product waste and cellulose waste.In addition,the interaction of different components on the activity and structure of pyrolytic char was also studied.The results show that inorganic chlorine weakens the oxidation reactivity of the pyrolytic char by inhibiting the formation of crystals C?002?and C?440?,and the apparent activation energy of the pyrolytic char sample is smaller than that of the pyrolytic char mixed with PVC.For the lignin char,the addition of PVC will reduce the ratio of I_D/I_G in the Raman spectrum,and the carbon structure tends to be more ordered.The volatilization of alkali metals and nonmetallic elements is relatively strong during pyrolysis process.The interaction between lignin and cellulose improves the order degree of carbon structure,leading to the appearance of crystal carbon peak C?100?in lignin+cellulose char samples.The interaction between starch and lignin results in the increase of the size and thickness of the graphite domain.The interaction between lignin and cellulose makes the plane size of microcrystalline smaller,while has no effect on the thickness of graphite domain.On the other hand,the band area ratio I_D4/I_G values of the mixed char samples are between the two corresponding single char samples,indicating that no interaction occurs in the sp2-sp3 hybrid process.Based on the analysis of the basic characteristics of syngas from the three-stage co-gasification system,the formation characteristics and removal mechanism of two key pollutants,tar and hydrogen chloride,were further analyzed.The results show that the quality of syngas can be greatly improved by using“Zhundong”coal char with high alkali and alkaline earth metal content in the reduction stage,and the concentration of tar and HCl,two key pollutants in the gasification process can be controlled at a lower level,and the lowest concentration of tar and HCl in syngas can be reduced to 9.56 mg/Nm³ and 17.4 mg/Nm³,respectively.The effect of metal chloride slagging on the reactivity of coal char is greater than that of reducing temperature.The tar removal efficiency reached 90.9%at 800oC and 84.6%at 900oC.The ability of metal adsorbent to capture HCl gas decreases with the increase of temperature,and the stability of CaCl₂compound is poor at high temperature.As the higher temperature?over 750oC?promotes the decomposition reaction of CaCl₂,and then promotes the reversible reaction of metal oxide and HCl,thus pushing the reverse reaction to produce more HCl acid pollutant gas.In the reduction stage,it is obvious that the adsorption of CaO on HCl plays a leading role in HCl removal.While the reduction temperature higher than 800oC is needed to produce syngas by catalytic cracking tar at high temperature and Boudouard reaction at the same time.Then,the particles of these two chlorides melt,agglomerate or glue by themselves,resulting in the collapse of the coal char bed materials placed in the reduction zone.Therefore,with the reduction temperature increasing from800oC to 1000oC,the BET specific surface area of coal char after reduction reaction decreased from 127.6 m²/g to 74.9 m²/g.The emission characteristics of PCDD/Fs from three-stage co-gasification of MSW were studied.Two kinds of pyrolytic char with high alkali content and biochar were used as reactants in the reduction stage.The emission characteristics of PCDD/Fs were studied in the three-stage co-gasification system of MSW.The results showed that after the reduction reaction stage,the toxic equivalent value was controlled between 0.068 and 0.085 ng I-TEQ/Nm³ in the three-stage co-gasification unit of MSW due to the degradation reaction of dioxin and anoxic atmosphere at high temperature.The concentration and toxic equivalent of dioxins decreased with the increase of reduction temperature from 800oC to 1000oC,which was attributed to the enhancement of dioxinsd egradation above 350oC.In the reduction stage,low chlorinated PCDDs?TCDD and PeCDD?were more easily converted into high chlorinated PCDDs?HxCDD,HpCDD and OCDD?,while rice husk char was the opposite.In the reduction process,the leading role of chlorophenol precursor reaction on the pyrolysis rice husk char sample is enhanced.The reduction reaction of coal char tends to produce dioxins with high degree of chlorination,while the reduction reaction of rice husk char is more likely to form dioxins with low degree of chlorination.The main gasification reactions,such as water-gas reaction and Boudouard reaction in the reduction section,increase the concentration of the important syngas components dominated by H₂ significantly,which makes the precursor A and precursor B of dioxin reduce respectively,thus greatly reducing the proportion of the key high toxic homologues 2,4,7,8-TCDD and 1,2,3,4,7,8-HxCDD of the two dioxins in all toxic dioxin homologues.In addition,the decrease of HCl has a stronger effect on promoting the reverse reaction of free radicals than the increase of H₂,which leads to the decrease of 1,2,3,7,8-PeCDD,the key highly toxic homologue of dioxin,in the three-stage co-gasification experiment of MSW.