Effect Of Torrefaction Deoxygenation And Washing Deash On Pyrolysis Characteristics Of Biomass

Biomass is the only renewable resource that exists in the form of carbonaceous matter,and has the advantages of large resources and zero CO₂ emissions.Biomass pyrolysis can convert biomass raw materials into gas（non-condensable gas）,solid（biomass charcoal,activated carbon）,and liquid（bio-oil,bamboo and wood vinegar,etc.）products with various uses,which has been widely concerned.However,the high oxygen content,high ash content,and low calorific value of biomass raw materials greatly restrict the heat conversion and utilization of biomass.Based on this,this paper uses agricultural and forestry biomass and its three components（cellulose,hemicellulose,and lignin）as experimental raw materials,and uses torrefaction and washing pretreatment as the methods for biomass modification and upgrading.Study on the correlation mechanism between biomass pretreatment and pyrolysis polygeneration.Revealed the three-component torrefaction characteristics of biomass.Discovered the removal effect of aqueous bio-oil on the removal of alkali metals and alkaline earth metals（AAEMs）in biomass.The pyrolysis characteristics and activation energy evolution of biomass before and after pretreatment were analyzed.The regulation effect of washing pretreatment on biomass pyrolysis three-phase products was revealed.A method for the preparation of aromatics by the combined catalytic pyrolysis of straw deoxidizing and deashing was proposed.（1）Research on torrefaction pretreatment of biomass and its componentsDeoxygenation is the core of biomass modification and upgrading,and it is also the key to obtaining high-quality raw materials.In the past,biomass torrefaction research mainly focused on solid product analysis,and it was difficult to fully reveal the mechanism of torrefaction deoxidation process.In this paper,cellulose,hemicellulose,lignin and shaped particles are used as raw materials.From the perspective of the migration of oxygen and carbon elements,the torrefaction deoxygenation mechanism with dehydroxylation and decarboxylation as the core is explored,and it is found that the dehydration reaction is biomass.The core factor of torrefaction deoxidation and quality improvement.From the three-component torrefaction experiment,it was found that after anaerobic torrefaction at 210-300°C,there were 19.76-71.11%,5.85-33.27%,and 16.28-44.89%of the oxygen transfer and desorption of hemicellulose,cellulose and lignin respectively.At all torrefaction temperatures and raw material conditions,CO₂ and CO are the main gaseous product components.The composition of torrefaction liquid products is quite different.The torrefaction liquid products of hemicellulose are mainly acetic acid and furfural,the cellulose is mainly anhydrosugar,and the lignin is mainly phenolic products.The torrefaction experiment of biomass shaped particles found that torrefaction temperature,residence time and raw material types have significant effects on torrefaction performance.Under the same torrefaction conditions,pine sawdust has the highest quality yield and energy yield,followed by rice husk particles,Pine wood shaped particles and rice husk shaped particles;compared with biomass shaped particles,loose raw biomass releases more volatiles during the torrefaction process.（2）Biomass pickling and oil-washing pretreatment researchThe ash and AAEMs in the biomass have a very adverse effect on the reactor.Traditional biomass acid eluted ash（usually dilute hydrochloric acid）is costly and may cause pollution.Aqueous bio-oil is a mixture of water,organic acids and non-acidic organic compounds,with a certain degree of acidity.In this paper,the thermal conversion liquid by-products（aqueous bio-oil,torrefaction liquid,light vinegar liquid,etc.）were used to carry out"oil washing"deashing pretreatment of biomass rice husks,cotton stalks and fir shavings,and the components of the oil washing liquid were studied（acids,phenols,ketones,alcohols,furans,etc.）the differential release mechanism of AAEMs.The results show that acetic acid plays a vital role in the removal of AAEMs.As the p H of the acetic acid solution decreases,the removal rate of AAEMs increases.Non-acidic organic compounds（phenol,guaiacol,ethylene glycol,furfural and hydroxyacetone）can improve the removal rate of K,Na and Mg in biomass.The removal rate of alkali metals in biomass by aqueous bio-oil is more than 90%,and the removal effect of AAEMs is higher than the pretreatment of acid washing with dilute hydrochloric acid solution and acetic acid solution with the same p H value.This also shows that in the pretreatment of aqueous bio-oil washing,there is a synergistic effect between acetic acid and the non-acidic components of bio-oil.（3）The impact of washing pretreatment on biomass pyrolysis polygenerationIn the washing pretreatment process,the organic acids in the aqueous bio-oil are considered to play a key role.However,phenolic substances are also typical components of bio-oils,which also promote the removal of AAEMs.In this paper,the typical phenolic components（phenol and guaiacol）in the bio-oil were used to pre-process the moso bamboo,and the water washing was used as the control group.The results show that phenols play an important role in the removal of AAEMs,which can remove 96.4-96.6%of K,84.6-85.4%of Mg,77.2-81.9%of Na and 66.7-69.1%of Ca.Phenol washing significantly affects the pyrolysis characteristics of bamboo,improves the release of volatile components in bamboo,and makes the pyrolysis peaks of cellulose and hemicellulose separate significantly,and eliminates the corresponding pyrolysis peaks of the extract.Not only that,the phenol washing pretreatment improves the quality of the pyrolysis three-phase product,especially inhibits the formation of acids in the liquid product bio-oil,promotes the formation of dehydrated sugars in the bio-oil.The specific surface area of solid biochar was increased and the volume fraction of methane in gaseous biochar was increased.Pyrolysis polygeneration is not only concerned with a certain product such as bio-oil,but on the quality and application of all products.Phenol washing pretreatment helps to obtain higher-quality biomass pyrolysis polygeneration products.（4）Preparation of aromatics from biomass by deoxidation and washing pretreatment combined with catalytic pyrolysisOn the basis of the above research,the study of biomass selective deoxidation washing pretreatment and shape selective catalytic pyrolysis to prepare aromatics was carried out by combining torrefaction deoxidation and washing pretreatment.Catalytic fast pyrolysis can be used to prepare hydrocarbon rich compounds directionally.The ash and oxygen elements in biomass can be effectively removed by washing and torrefaction pretreatment,so as to further improve the yield of benzene,toluene and xylene（BTX）.Rice straw was pretreated with acid solution,phenol solution and simulated light bio-oil（SBO）,and then torrefaction.The samples were pyrolyzed by Py-GC/MS with HZSM-5 as catalyst.The results showed that the combined pretreatment had the advantages of single pretreatment,and the selectivity of BTX was the highest,which was 64.6%.The acids and phenols in simulated aqueous bio-oil had synergistic effect on the removal of AAEMs in rice straw.These conclusions provide a theoretical basis for further improving the quality of bio-oil and controlling the content of target chemicals（aromatics）.