Study On Pyrolysis And Gasification Characteristics Of Rural Waste And Optimization Of Small-scale Pyrolysis,gasification And Combustion System

In our country,the clean and efficient treatment of rural waste is an essential construction of"green,livable villages and towns."The pyrolysis and gasification technology has great potential in reducing contaminant emissions and realizing the nearby disposal of rural waste.However,some drawbacks still need to be resolved to realize the continuous and stable operation of the device and reduce tar production.For this reason,the National Key R&D Program of China financially supported this project（grant number:2018YFD1100600）.Based on its research objectives,this study analyses the effect of mixed waste moisture content on the pyrolysis and gasification process;discusses the pyrolysis and gasification characteristics of mixed waste under the influence of carbon dioxide;and researches the conversion mechanisms of typical tar components in different atmospheres by computation.For the pyrolysis and gasification technology with high requirements of tar content in gasification gas,active metals such as vanadium and molybdenum are adopted to prepare catalysts and research their performance in the degradation of benzofuran and toluene.The study also discusses the optimization of pyrolysis,gasification,and combustion system.In this study,a two-stage continuous feeding apparatus was developed to examine the effect of moisture content of the mixed feedstock on co-gasification.The results show that the pyrolysis residue plays an essential role in the gasification with steam,which can improve the syngas yield,especially the H₂ yield reaches 0.345 Nm³/kg when steam and feedstock mass ratio（S/F）is 0.8.Besides,steam also has an obvious effect on the distribution of tar components.The surplus steam provided abundant H radicals to participate in the hydrogenation reaction.Due to the reduction of dienes and dienophiles,the corresponding cyclization and aromatization reactions are inhibited.Furthermore,the two-stage continuous feeding apparatus is also used to study the effect of CO₂ on the pyrolysis and gasification process.The experiments were carried out when the CO₂participated in the gasification alone or in a mixed atmosphere of CO₂ and steam.It is found that CO₂ is inferior to steam in promoting H₂ formation.But when CO₂ and feedstock mass ratio（C/F）rises from 0.74 to 1.48,the gas yield increases from 60.52%to 77.15%,and the yields of tar and residue decrease correspondingly.As a potential gasifying agent,CO₂ can partially substitute steam in the gasification process.However,when CO₂ coexists with a small amount of steam（S/F=0.4）,introducing excessive CO₂（C/F=1.48）will intensify the competition between steam and CO₂.Furthermore,CO₂ is beneficial to cracking aromatic compounds containing oxygen groups,and especially it can effectively promote a decrease in benzofuran.To further analyze the evolution mechanism of typical tar at the microlevel,the reaction mechanisms of aromatic compounds in different atmospheres are computationally studied using Gaussian.According to the results,H₂O or O/H/OH/CH₃ radicals mainly attack the methyl group on the end-chain of propane to realize decarburization,whereas propylene and propyne are decarburized after radical addition at the double bond or triple bond.Among all these reaction paths,the rate constant of decarburization by H radical reacting with C₃ chain hydrocarbons is the fastest and proportional to unsaturation.When the temperature is more than 800℃,hydrogen consumption in the benzene cracking process is reduced,which is conducive to improving the quality of syngas.Naphthalene is prone to form through the recombination of two cyclopentadienyls.Thus controlling the cyclization of cyclopentadienyls is a critical step in reducing the formation of polycyclic aromatic hydrocarbons.When H and OH participate in cracking phenyl formate,abstracting H atoms on formyl is most likely to stimulate formed intermediate products to further crack the oxygen-containing groups.Besides,H and OH radicals are critical in phenol and benzofuran cracking,respectively.Therefore,when steam and CO₂ are employed as gasification agents,they usually promote tar cracking in the form of O\H\OH radicals.For diverse tar compounds cracking,the performance of these radicals is different.Since steam can provide both H and OH radicals,whereas CO₂ needs to consume H radicals to generate OH radicals in gasification,the gasification effect of steam is better than that of CO₂.Toluene and benzofuran are selected as modelling compounds to study tar catalytic cracking.In this process,the conversions of these two aromatic compounds are used to estimate the performance of the catalyst.Based on the results,vanadium is beneficial for cracking benzofuran and toluene.In particular,the catalytic performance of V/γ-Al₂O₃ is better than that of V/Si O₂ and V/Ca Mg（CO₃）₂.Although the catalytic activity of molybdenum is less than that of vanadium,the two active metals are synergistic.Besides,compared with the impregnation method,the hydrothermal method can further improve the performance of vanadium-based catalyst.However,the hydrothermal method is not suitable for the preparation of molybdenum-based catalysts and can have an inhibitory effect on their performance.The pyrolysis,gasification and combustion integrated device of rural waste（30t/d）was field monitored.According to the actual operation,the device can continuously and stably operate without external heat provided,and the emission flue gas can reach the Chinese national standard（GB18485-2014）.Optimizing the existing process to pyrolysis gasification power generation can effectively improve waste heat utilization.From the simulation results,α_q obviously influences the power generation and energy efficiency of the internal combustion engines.When the waste moisture content is 18.55%,with theα_q rising from 0.20 to 0.35,the power generation of internal combustion engines can increase from 738.91（k W·h/t）,and the energy utilization efficiency of the system also increases to 22.05%.Besides,when theα_q is 0.35,the increase of waste moisture content has a relatively small impact on the change in energy utilization efficiency.Especially when the moisture content increases from 23.55%to 28.55%,the energy efficiency of the system only decreases by 0.38%.More importantly,the optimized system can maintain sustainable operation by relying on the benefits of power generation.