Studies On Demineralization Of Low-lipid Microalgae And Catalytic Pyrolysis Characteristics And Mechanism

Low-lipid microalgae has the characteristics of high carbon fixing efficiency,high growth rate,high productivity per area,good environmental adaptability and does not compete with agriculture for land.Besides,the large-scale cultivation of microalgae can be coupled by technologies of CO₂ emission reduction of power plant and municipal wastewater treatment,which has great potencial of development.Catalytic pyrolysis is one of the thermochemical methods to convert biomass for target products such as aromatic hydrocarbons by adding catalysts（such as zeolite catalysts）.Algal biomass has high ash content.During catalytic pyrolysis process,inorganic metals containing in the biomass,such as Ca、Fe、K and Mg,can deposit on the surface and block the the pores and channels of zeolite catalysts,leading to catalyst poisoning and shortened lifespan.This issue can make difficulties on large-scale industrial application of biomass catalytic pyrolysis.In order to address this problem,demineralization pretreatment before catalytic pyrolysis process has been proposed.Chlorella vulgaris,which belongs to the low-lipid microalgae species,has been taken as the feedstock in our study.The effects of demineralization pretreatment on physiochemical structure of the feedstock,pyrolysis behaviors as well as evolved gas have been deeply studied.By comparing the composition of pyrolysis gas derived from the feedstock before and after pretreatment,the influence to the reaction pathways has been explored.In order to improve the quality of pyrolysis oil,the experiments of catalytic co-pyrolysis of microalgae and hydrogen-rich material have been also conducted.This study provides valuable experimental data for practical technological process design of catalytic pyrolysis of low-lipid microalgae.Firstly,the feedstock was pretreated by different mass concentration of dilute hydrochloric acid and sulphuric acid.The results showed that acid washing could not only remove 84.82%-92.86%of ash,but also more than 98%of alkaline and alkaline earth metal.Meanwhile,part of the hemicellulose has been hydrolyzed during pretreatment,reducing the content of carbohydrate.Different from lignocellulosic biomass,TG analysis showed that the temperatures corresponding to the decomposition peaks for pretreated samples shifted to the lower temperature zone.Comparing with the removal of inorganics,the physiochemical structure changes of microalgae played the dominant role in decomposition of carbohydrate and protein fractions.For evolved gas,it was noteworthy that acid washing suppressed the formation of CO₂ and increased the generation of C-H.Besides,the production of HCN by cracking of dipeptides was promoted clearly.HCl-5%had the lower average activation energy,residual mass（22.97%）and more desired gaseous products（pyrolysis effectiveness index S=0.32）.Secondly,the catalytic pyrolysis behavior and pyrolysis gas derived from Chlorella vulgaris before and after pretreatment with dilute hydrochloric acid at a concentration of 5wt.%were compared.The results of pyrolysis gas chromatography-mass spectrometry（Py-GC/MS）showed that the content of anhydrosugar in the sample HCl-5%pyrolysis gas increased from 6.04%to 7.6%,which was much lower than that of lignocellulosic biomass.This was mainly because the degree of polymerization of carbohydrates in microalgae was low,and Maillard reaction between carbohydrate and protein occurred during pyrolysis,therefore ash had little effect on ring open and bond scission reactions.In addition,the pretreatment of residual trace acid had a certain promoting effect on the dehydration reaction of amino acids as well as the deoxygenation reaction of fatty acids.According to the pyrolysis characteristic parameters,the comprehensive catalytic pyrolysis indexes of the sample HCl-5%at different catalyst-to-feed ratios were smaller than the raw feedstock.When the catalyst material ratio was less than 1/1,the aromatic hydrocarbon content in the sample HCl-5%catalytic pyrolysis gas was slightly higher than the sample RM;as the catalyst material ratio increased to 2/1,the lipid and carbohydrate fractions in the microalgae went through deoxygenation,cracking and aromatization reactions,generating more aromatic hydrocarbons.However,the denitrogenation reaction was difficult to carry out,leading to a lower amount of aromatic hydrocarbons in gaseous product from catalytic pyrolysis of HCl-5%.Thirdly,the ex-situ catalytic pyrolysis of chlorella was carried out using a dual-temperature fixed bed reactor to study the effects of different pyrolysis temperatures,catalytic temperatures and catalyst-to-feed ratios on product distribution and composition,and then the operating parameters were optimized.The results showed that 500oC and 550oC were the optimal pyrolysis and catalytic temperatures,respectively,corresponding to the highest selectivity for aromatic hydrocarbons and non-aromatic hydrocarbons in bio-oil.When the catalyst-to-feed ratio was increased from 0.1/1 to 0.3/1,the O-containing compounds in the bio-oil were significantly reduced,while the denitrogenation reaction was more pronounced at the catalyst-to-feed ratio of 0.5/1.The influence of catalytic temperature on the gas product composition was greater than the pyrolysis temperature.As the catalyst-to-feed ratio increased,the volume concentration of CO increased gradually.The results of in-situ and ex-situ catalytic pyrolysis experiments showed that the two models had the similar bio-oil yields.While,the bio-oil from ex-situ catalytic pyrolysis had higher aromatic hydrocarbon content,low fatty acid content.The gaseous products from ex-situ catalytic mode resulted in higher volume concentrations of syngas（CO,C₂H₄,CH₄ and H₂）and lower volume concentration of CO₂.Finally,different proportions of waste tires were blended into the microalgae for ex-situ catalytic co-pyrolysis.TGA was used to study the co-pyrolysis characteristics of these two materials.It could be seen from the（35）DTG curves that the incorporation of waste tires had the negative effect on the low-temperature pyrolysis section（170³50oC）of microalgae.However,a large amount of free radicals generated during the pyrolysis of microalgae,which promoted depolymerization reaction（350⁴50oC）of waste tire,and the experimental values of the residual mass of the pyrolyzed solid at all blending ratios were lower than the theoretical values.The H/C_eff ratio of the microalgae and the waste tire were 0.31 and 0.92,respectively.As the waste tire increased in the proportion of the mixture,the H/C_eff ratio increased from 0.43 to 0.72,which corresponded to a decreasing yield of the catalyst coke.The waste tires acted as a hydrogen donor in catalytic co-pyrolysis process.When the CV/WT mixing ratios were 4/1 and 2/1,the corresponding synergistic efficiencies were as high as42.88%and 41.82%,respectively.With the increase of the proportion of waste tires,the nitrogen fixing efficiency of co-pyrolysis biochar increased.In addition,the synergistic mechanism of ex-situ catalytic co-pyrolysis of microalgae and waste tires was revealed.