Effect Of Coal Ash On The Distribution And Transformation Of Biomass Pyrolysis Products

In the process of global carbon neutrality,the use of renewable energy is the inevitable choice to achieve the purpose of energy conservation and emission reduction.Among the renewable energy sources,biomass is the only carbon-based fuel with the most similar chemical properties to coal,so the thermochemical co-utilization of coal and biomass emerged.In most commercial co-utilization cases,biomass is usually added in small proportions,so ash which accounts for more than 10%of coal is non-negligible for the blended biomass.Coal ash comes from incombustible minerals and inorganic substances,which form solid residues coexisting with the glass phase and the mineral phase under the condition of high temperature and rapid cooling.In the process of co-utilization of coal and biomass,these complex ash components affect the thermal conversion behavior and product distribution of biomass,however,little attention was paid to the influence.And in recent years,there have been many new attempts to apply coal ash（CA）to the biomass thermochemical utilization process.For example,coal ash was used as biomass pyrolysis catalysts,catalyst carriers or heat carriers.Therefore,clarifying the influence mechanism of coal ash on biomass not only helps improve the energy utilization efficiency of coal and biomass,but also provides new ideas for the utilization of coal ash.Therefore,starting from pyrolysis,this paper adopted two typical coal fly ash（CA）:type F from bituminous coal（FCA）and type C from lignite（CCA）as the representative of coal ash,and corn straw（CS）as the representatives of biomass.Combined experiments with theoretical calculations,the effects of coal ash on biomass pyrolysis behavior and product distribution with various experimental parameters were studied.And the influence mechanism of key chemical components of coal ash on bio-oil pyrolysis and non-condensable gas generation were explained.The main work and conclusions of this paper are as follows:First of all,the weight loss behavior of co-pyrolysis of CS and CA at a constant heating rate was studied by TG/FTIR device,the kinetic parameters of the co-pyrolysis process were obtained,and the release rules of pyrolysis gas products and functional groups were monitored in real-time.The results showed that both types of CA slightly reduced the proportion of solid phase in the co-pyrolysis products,and reduced the activation energy of different reaction regions.The absorbance of gas phase products showed that CA had a significant effect on the shoulder and peak temperature of CS pyrolysis,and promoted the release of various gas products.Then a two-stage free-falling fixed bed reactor was constructed to provide rapid heating conditions for the pyrolysis reaction,which is closer to the coexistence of coal ash and biomass in industrial utilization.There were two ways of adding CA,one was mixing CA with CS,the other was separating it from CA,the effects of CA on the distribution and properties of gas and solid products during pyrolysis of biomass were investigated.The results showed that in the co-pyrolysis of CA and CS,with a lower pyrolysis temperature,the Si and Al in the CA reacted with the AAEM in CS to form aluminosilicate,and caused the deactivation of AAEM,thus the gas generation was inhibited.As the pyrolysis temperature increased to 700-800℃,more AAEM in CA evaporated in the gas phase and migrated to CS char,Moreover,the promotion effect of various inorganic substances on biomass pyrolysis was enhanced at high temperature,so the gas yield increased.However,with the large CA addition ratio（CS/CA=1:1）,part of the CA adhered to the surface of pyrolysis char,and blocked the pores of char,which was not conducive to the generation and release of gas products.Furthermore,the presence of coal ash also changed the surface crystal phase of solid products,the content of organic functional groups,and ash melting temperature.In addition,with the pyrolysis method of separating CA and CS,the distribution of pyrolysis gas products was changed significantly,but the effect on gas generation of high temperature was weaker than that of co-pyrolysis.Subsequently,to further clarify the effect of CA addition on the conversion mechanism of biomass organic components,a PY-GC/MS device was adopted to explore the composition of CS and CA co-pyrolysis bio-oil at different temperatures.The results showed that the acid sites in CA promoted the deoxidation of bio-oil and slightly reduced the sugars and esters in bio-oil at lower pyrolysis temperatures.However,at high temperature（800°C）,due to the action of acidic sites,AAEM,and basic oxides,CA significantly promoted the decomposition of oxygen-containing components in bio-oil,The maximum reduction in the proportion of misc-oxy in the bio-oil was reduced from 40.09%to 9.93%with CCA addition;Besides,CA promoted the aromatization reaction between small molecular oxygen-containing substances and olefins,which greatly increased the proportion and yield of non-condensable gas,MAHs,and PAHs.Finally,based on density functional theory,Si O₂ and Ca O were selected as the representatives of acid and alkaline substances in coal ash,respectively.And the influence of key chemical components in CA on the bio-oil cracking was explored from the molecular scale.For the bio-oil with a more complex composition,acetone and furan were used as the model.The effects of Ca²⁺,Ca O,Si atom,and two typical defect sites Si O₂on the Gibbs free energy barrier（ΔG）and reaction rate constant（k）of acetone and furan cracking were investigated.The calculated results showed that these inorganic substances changed the bond length and bond Angle of reactants through chemical adsorption,and the dissociation energy of chemical bonds was changed,thus affecting the energy barrier of pyrolysis reaction.The adsorption between Ca and bio-oil is more stable than that between Si and bio-oil.The presence of Ca and Si can reduce the reaction energy barrier of most bio-oil pyrolysis pathways,and also change some reaction pathways.It is proved that alkaline substances,metal oxides,and acid substances in the amorphous phase could catalyze the cracking of bio-oil.This project aimed to obtain favorable conditions for the different types of coal ash to promote the thermochemical conversion of biomass,and provide a theoretical basis for the high-value application of coal ash in the biomass utilization process.