| The high volatile matter and low fixed carbon content of plastics can promote and contribute to gasifying biomass faster and more fully during co-gasification of biomass and plastics.Meanwhile,most plastics do not contain oxygen or have little oxygen content,which can control the oxygen content of products,and obtain high-quality syngas with high hydrogen content and calorific value during co-gasification.However,it is not clear about the effect of synergy between biomass and plastic on the gas production behavior during co-gasification.Meanwhile,biomass,especially crop stalks have a large number of alkali metals,which can influence the gas quality in a catalytic manner,and lead to fouling,slagging,corrosion and other phenomena.In addition,gasification process can be mainly divided into volatilization(pyrolysis)stage and char gasification stage.Given that,co-pyrolysis behavior of biomass and plastics and its influence on the gasification activity of chars were studied firstly.The influence of occurrence modes and content of alkali metals was also investigated.On this basis,the effect of interaction between biomass and plastics,and alkali metals on the gasification characteristics was studied.Meanwhile,the transformation behavior of alkali metals during co-gasification was explored.The following investigations are made in this thesis:Firstly,the effect of interactions between stalk and plastics on the thermal behavior and the evolution of volatile species during co-pyrolysis of stalk and plastics was investigated by thermogravimetric analyzer coupled with fourier transform infrared spectroscopy and mass spectrometry technology(TG-FTIR-MS).The influence of occurrence modes and content of alkali metals was also investigated.Kinetics were investigated by the model-free methods.The results showed that the existence of PE delayed the decomposition of rice straw(RS)and corn stalk(CS)in the first stage,while in the second stage,the existence of RS and CS delayed the degradation of PE.The interaction between RS/CS and PE improved the decomposition rate and reduced the activation energy.The interaction between stalk and PE in the second stage was inhibited by water-soluble and acid-soluble alkali metals/alkali earth metals to some extent.In addition,the effect of the interaction between stalk and PE on the release behavior of volatile matter was not only influenced by PE content in mixed materials,but also related to the content of potassium in raw materials.Secondly,the effect of biomass and plastics mixture on the physicochemical properties of chars obtained from co-pyrolysis was studied.Meanwhile,effects of physicochemical structure of chars,the occurrence modes and content of alkali metals on the gasification reactivity of chars,the speed-limiting step in the process of co-gasification,were explored.In addition,the correlation between physicochemical structure of chars and gasification reactivity was revealed.The results showed that compared with chars obtained from RS pyrolysis alone,the BET specific surface area and pore volume of chars obtained from co-pyrolysis of RS and PE were improved.The specific surface area of chars reached to 17.81m2/g when the content of PE was 60%in the mixture.When the content of PE was higher than 40%,the order degree and gasification reactivity of chars obtained from co-pyrolysis increased with the increase of PE content.The activity of chars obtained from co-pyrolysis of RS and PE in the early stage of gasification was mainly determined by pore structure and order degree of carbonaceous structure,and the activity in the later stage was not only related to these two aspects,but also affected by silicon dioxide content of chars,which can inhibit the gasification reactivity.The existence of water-soluble alkali metals in rice straw can greatly improve the reaction activity of chars,but the promotion effect of ammonium acetate and acid-soluble alkali metals was negligible.When the potassium content in RS was less than 3%,increasing its content could improve the gasification reactivity at low carbon conversion rate.Meanwhile,the gasification reactivity at high carbon conversion rate increased with the increase of potassium content.Based on the above research,co-gasification of rice straw and plastics with steam was carried out in a self-made vertical fixed-bed reactor.Meanwhile,the effects of blending ratio of raw materials,temperature and potassium content in RS on the gas production characteristics were discussed in detail.In addition,the effects of interaction between straw and plastics and alkali metals on the gasification characteristics were revealed.The results showed that the interaction between RS and PE promoted the generation of H2,CO2,CO and CH4.It is found that the appropriate content of PE in the mixture was 40%from the perspective of the effects of interaction between RS and PE on H2 generation and gasification quality(gas calorific value,gasification efficiency and carbon conversion rate).The yields of H2,CO,CH4 and CO2 increased with the increase of temperature during co-gasification of RS and PE,but the variation trend of CO,CH4 and CO2 concentration was opposite.The concentration of H2 at 850? reached to 52.04%.In addition,the gas calorific value decreased with the increase of temperature,from 24.11 MJ/Nm3 at 700? to 13.53 MJ/Nm3 at 850?,while the change trend of gasification efficiency and carbon conversion rate was opposite.From the perspective of gas composition,H2 yield and gasification quality during co-gasifiation,the appropriate content of potassium in RS at 750? and 850? was 6%and 3%,respectively.Finally,the release characteristics of alkali metals and their occurrence modes in ash during co-gasification of biomass and plastics were investigated by combined with thermodynamic equilibrium calculation,mineral composition analysis and microstructureenergy spectrum analysis.The effects of blending ratio of raw materials and temperature on the migration mechanism of alkali metals and other related elements were revealed.The results showed that the release rate and concentration of potassium in gas phase decreased first and then increased with the increase of PE content during co-gasification of RS and PE.The interaction between RS and PE reduced the concentration of potassium and sodium in the produced gas at the content of PE below 70%.When the PE content was less than 60%,the existence of PE had an inhibitory effect on the precipitation of water-soluble potassium,resulting in the decrease of potassium in the gas phase and the increase of water-soluble and acid-insoluble potassium in the solid phase.When the PE content was between 60%and 80%,part of water-soluble and ammonium acetate soluble potassium was precipitated into the gas phase,and part of them reacted with silicon and aluminum to form acid-insoluble potassium.The release rate of gaseous potassium and sodium,and the content of acid-insoluble potassium in solid phase presented an increasing trend with the increase of temperature,while watersoluble potassium in the solid phase had an opposite trend.Moreover,the water-soluble potassium in the solid phase was dominant at 750?,accounting for 84.57%of the total potassium in the solid phase,while the acid-insoluble potassium was dominant at 950?,accounting for 89.17%.The release of potassium during co-gasification of RS and PE at different temperatures mainly came from water-soluble and ammonium acetate soluble potassium in RS. |
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