The Experiment And Mechism Study On Tobacco Wastes Pyrolysis And Gasification

As is known to all, exploring novel energy resource and clean conversion technologieshave become an important and timely research way to solve the excessive use of fossilfuels and the concerns over environmental protection. At present, biomass, as the onlyrenewable energy which can be stored and transported has attracted increasing worldwideinterest. Tobacco has been considered as one of the most important agricultural products inChina. However, tobacco wastes accounted for more than30%of the tobacco plantsproduction which counld not be used for cigarwtte production. Thus, how to treat thistremendous amount of wastes by novel and clean technologies, can promote thedevelopment of social economy, improve ecological environment and establish sustainableenergy system. Biomass thermochemical conversion is considered to be one of the mostefficient ways for converting biomass to bio-oil and bio-char especially for fuel gas whichhas high quality. However, high yield of tar in fuel gas prodution inhibits the furtherdevelopment and commercialization of biomass thermochemical conversion. Catalyticpyrolysis and steam gasification are considered as the most potent technique to decrease taryield, thus to improve the quality of fuel gas. In this study, we carried out an in-depthresearch on tobacco wastes catalytic pyrolysis and steam gasification, which were essentialto understand the fundamentals and mechanisms involved in biomass pyrolysis andgasification.This study primarily focused on tobacco wastes as feedstock. For analyzing and labresearch, the tobacco wastes needed to be dried, crashed and sieved in different particlesizes so as to get raw materials. The physicochemical property of the feedstock revealedthat tobacco wastes contained lower fixed carbon and more valatiles comparing to coal andthe harmful elements N and S were less than4%, which were of very low contents infeedstock. Thus, tobacco wastes are environment-friendly bioenergy sources. Meanwhile, the NiO/γ-Al₂O₃catalyst was developed in the study. NiO particles which were prepared byhomogeneous precipitation method were an active componet loaded on commercialγ-Al₂O₃carriers in this work. Based on above work, the supported NiO/γ-Al₂O₃catalystsinvolving γ-Al₂O₃as carrier were further exploited by deposition-precipitation Method.Different analytical methods such as APSP2020, SEM and EDX were used to characterizeand analysis the catalysts. The loaded capacity of active component NiO was more than13wt%. The active component NiO were only concentrated on the surface of the catalystswhich can promote catalytic activity and save cost.Pyrolysis characteristics and kinetics of tobacco wastes were investigated in TGAcoupled with DSC and their kinetics parameters were detemined by using Friedman andDoyle methods, simultaneously, the influence of particle size, heating rate and catalysts onbiomass pyrolysis was analyzed. It was observed that four stage pyrolysis mechanism oftobacco wastes, the first being dehydration, the second at a temperature range oftorrefaction, the third at the pyrolysis temperature range and the fourth stage at thetemperature range of graphitisation. In addition, increasing heating rate and the presence ofcatalysts showed great influence on accelerating thermal degradation.Tobacco wastes gasification experiments were carried out using thermogravimetricanalyzer and gas chromatographic analyzer （TG-GC）. Under the catalytic and non-catalyticcondition, the pyrolysis characteristics, gasification characteristics and kinetics of tobaccowastes were investigated in nitrogen-steam atmosphere. The TGA data indicated the weightloss behavior of the sample in nitrogen-steam mixture was divided into two regions（217³57℃and717⁸05℃）. The volumetric model was used in modeling pyrolysis ofbiomass and the shrinking model was applied for investigating the gasification of pyrolysischars. Both NiO and dolomite could decrease the gasification temperature and enhancewater gas shift reactions. The H2yield （34mol/kg） of tobacco wastes gasification with NiOwas the most. The dolomite had more remarkable effect on improving CO yield （23mol/kg）. For the further information of the characteristics of gas products, tar and charcoal frombiomass pyrolysis, the pyrolysis of tobacco wastes was carried out in fixed-bed reactor withgas chromatograph. The effect of three important reaction parameters such as hearthtemperature, gas/solid residence time and catalysts on the gas product property was focusedon this study. The results showed that higher temperature is favorable for H2and COcontents. With presence of NiO/γ-Al₂O₃catalyst, the yield of fuel gas product reached1.52Nm3/kg with38.6vol.%H2. The calorific value of the fuel gas generated in the process wasabout15MJ/m3, which can be used directly for gas turbine, engine and boiler. Moreover,according to the mass and energy balance evaluation of pyrolysis process of dried tobaccowastes at900℃, the error of mass balance evaluation of dried tobacco wastes is1%. Byenergy evaluation, gas production efficiency, energy recovery and energy consumptionratio of this pyrolysis system were42.16%、89.01%and2.49, respectively.Furthermore, steam gasification of tobacco wastes was investigated by a home designedbubbling fluidized bed system with continuous feeding. The influence of various technicalparameters such as reactor temperature and Steam/Biomass （S/B） on fuel gas yield andcomposition was analyzed experimentally. It was found that H2and CO contents increasedsteadily from50.5%to54.4%and14.3%to18.5%as the temperature increased from700to850℃, while CO₂and CH₄contents exhibited the opposite tendencies. The yield of H2first increased shaply from0.037kg/kg to0.07kg/kg as the S/B increased to0.5, then itincreased slightly to0.095kg/kg as the S/B continuously increased, finally it trend to astable value or decreased. Thus, the optimum quantity of adding vapour is1.32（S/B）.Moreover, a thermodynamics equilibrium model based on equilibrium constant andmaterial balance has been developed to predict the gas composition which has beencompared with experimental results. Calibration of the model with appropriate modelingcoefficients was necessary to achieve close resemblance with the experimental values. Theresults indicated that the trend of changing the gas compositions with temperature and S/Bwas matching with the experimental results and the average RMS error was2.990. To fit the experimental result, the pre-factor were used to calibrate the equilibrium model. It wasobserved that the average RMS value decreases to1.985. So the modified model predictsthe gas composition much closer to the experimental value.