Study On Dehydration Mechanism And Novel Pyrolysis Method In In-Situ Catalytic Biomass Pyrolysis

Biomass refers to the a type of renewable energy source including animals,plants,microorganisms,and the organic substances excreted and metabolized by these living organisms.It is widely available and inexpensive,and its utilization can also reduce greenhouse gas emissions effectively,so it has become the best alternative to traditional fossil fuels.In-situ catalytic pyrolysis is a technology that selectively promotes the occurrence of specific reactions in pyrolysis by mixing catalysts with biomass feedstock,thereby converting biomass into syngas,bio-oil and bio-char.However,there are still some problems in in-situ catalytic pyrolysis,which limits its wide application in practical industries:Firstly,although existing researches have studied the biomass reaction kinetics,or catalytic effect of catalyst in in-situ catalytic biomass pyrolysis process,the characteristics and catalytic mechanisms of dehydration reaction in the pyrolysis process have still been scarcely studied.Secondly,after pyrolysis,it is difficult to separate and recover the in-situ catalyst from the mixed char products,so the feasibility of in-situ catalytic pyrolysis technology still needs to be further improved.Cattle manure is one of the most typical biomasses.It usually consists of undigested rice husk,corn,straw,etc.,and has the characteristics of traditional agricultural biomass.Besides,the pre-dried cattle manure has uniform particle size and is easy to handle.Therefore,in order to solve the above-mentioned problems in in-situ catalytic pyrolysis,in this paper,we choose cattle manure as the representative biomass,common Ni O/γ-Al₂O₃as representative catalyst,and carried out the following research:Firstly,the characteristics of dehydration reaction and the catalytic dehydration mechanism of catalyst during in-situ catalytic biomass pyrolysis were studied.Secondly,two types of in-situ catalytic pyrolysis methods,i.e.cyclic and monolithic pyrolysis methods were proposed from different angles to overcome the problem of in-situ catalyst separation and recovery difficulties.In addition,the pyrolysis performance of two methods was evaluated based on gas production of biomass pyrolysis.Finally,we come to the following conclusions:（1）In biomass pyrolysis process,dehydration reaction mainly occurs in the temperature range of 250-350℃,and the water produced by the dehydration reaction can account for more than 30 wt.%of the total volatile products.Ni O/γ-Al₂O₃ can promote the dehydration reaction,in which weak acid site ofγ-Al₂O₃ is the main active site for dehydration,and its catalytic process belongs to E2 catalytic mechanism.Besides,since Ni O will preferentially anchor onto the weak acid sites of Ni O/γ-Al₂O₃,thus hindering the contact between active sites and reactants,Ni O instead reduced the catalytic dehydration activity of Ni O/γ-Al₂O₃.（2）A cyclic in-situ catalytic pyrolysis method was proposed.In this method,Ni O/γ-Al₂O₃ was firstly used as initial in-situ catalysts for biomass in-situ catalytic pyrolysis.Then,the mixture of char and initial catalyst remaining after pyrolysis was directly used as a mixed type catalyst for the next round of pyrolysis.After several times of the cyclic pyrolysis,Ni O/γ-Al₂O₃ was separated and recovered from the solid mixture by regeneration.Since the bio-ash is relatively light,the regeneration process can be achieved by calcining and purging the solid mixture.Finally,the regenerated Ni O/γ-Al₂O₃ was used as the initial catalyst in a new cycle of pyrolysis.Through the cyclic pyrolysis method,we can not only effectively reduce the frequency of regeneration and separation of in-situ catalysts,but also realize the resource utilization of biochar.（3）In the cyclic in-situ catalytic pyrolysis method,there is a synergistic catalytic action between the char and Ni O/γ-Al₂O₃.On one hand,during the pyrolysis process,char can in-situ reduce Ni O into Ni with higher catalytic activity.On the other hand,char has large specific surface and pore volume,which can not only provide additional adsorption sites,but also promote mass transfer of macromolecular reactants.Therefore,this method can effectively improve the gas production in biomass pyrolysis,increasing the gas volume and the syngas heating value conversion rate by up to 70%and 82%.After regeneration,the quality and catalytic performance of the initial Ni O/γ-Al₂O₃catalyst will not decrease.Besides,the residual trace amount bio-ash after regeneration is mainly composed of Si O₂,and will not affect the catalyst in the subsequent pyrolysis process.（4）A monolithic in-situ catalytic pyrolysis method was proposed,which uses monolithic catalyst as catalytic bed for in-situ catalytic biomass pyrolysis.Before pyrolysis,the biomass material flows into the overall catalyst from the upper end and contacts with its outer surface to react.Before the pyrolysis,the biomass material flows into the catalyst bed from the upper of the reactor and contacts with outer surface of catalyst for catalysis.After pyrolysis,the generated char flows out from the bottom part of the catalyst bed,thus simplifying the separation process of char and catalyst.Three monolithic catalysts were used for in-situ catalytic pyrolysis,which using Ni O/γ-Al₂O₃as active component,and honeycomb ceramics,metal foam and wire mesh as carrier,respectively.The results showed that the gas yield was respectively increased 40,36and 18%when using corresponding monolithic catalyst,and monolithic catalysts showed better catalytic performance than that of traditional granular catalyst.Besides,the external surface area of the monolithic catalyst is an important factor affecting its catalytic performance,as larger surface can make the active components and reactants contact better.This method shows that monolithic catalysts have great application potential for in-situ catalytic pyrolysis.