| Rice husk is a kind of agricultural waste with abundant resources and high utilization value.However,the ash content in rice husk is high,and the alkali and alkaline earth metals(hereinafter referred to as AAEM)in the ash will cause serious equipment contamination and slagging during combustion or pyrolysis,which is one of the important factors limiting the industrialized application scale of rice husk.In order to effectively solve the above problems,an environmentally friendly aqueous bio-oil leaching pretreatment method was proposed to remove AAEM from rice husk.Through the study on the removal rate of AAEM and the pyrolysis characteristics of rice husk after removing AAEM,the practical application feasibility of this pretreatment method of rice husk aqueous bio-oil leaching was confirmed.The specific research contents are as follows:Firstly,the effect of hydrothermal liquefaction reaction temperature on the properties of aqueous bio-oil(hereinafter referred to as hydrothermal aqueous)was studied.Rice husk and ultra-pure water was used as raw materials to study the effect of temperature on hydrothermal aqueous properties in a high temperature and high-pressure reactor,and the main compounds were quantitatively detected by gas chromatography-mass spectrometry external standard method.The results showed that with the increase of reaction temperature,the yield of hydrothermal aqueous increased,the content of ketones and phenols increased,while the content of furfural decreased significantly,the acidity increased,and the water content remained unchanged,up to 98 wt.%.Secondly,the removal effect and mechanism of rice husk AAEM by hydrothermal aqueous were studied.Using rice husk as raw material,the effects of three leachate,including hydrothermal aqueous from hydrothermal liquefaction at 250 ℃,aqueous bio-oil from slow pyrolysis of rice husk at 500 ℃ and 1 mol/L hydrochloric acid,on the leaching of rice husk were studied at the same temperature and time.The results showed that for K and Mg ions,the removal rates of hydrothermal aqueous,pyrolysis aqueous and hydrochloric acid were similar(90-97 wt.%);For Na ions,the removal rates of hydrothermal aqueous and pyrolysis aqueous were 58 wt.% and 60 wt.% respectively,lower than 82 wt.% of hydrochloric acid;For Ca ions,the removal rates of hydrothermal aqueous and pyrolysis aqueous were 43 wt.% and 76 wt.%respectively,lower than that of hydrochloric acid(84 wt.%).The organic compounds such as phenol and furfural contained in aqueous bio-oil may improve the removal rate of alkaline earth metals through substitution and chelation reactions.Finally,the effects of hydrothermal aqueous leaching on the pyrolysis process and products of rice husk were investigated.The rice husk after leaching by hydrothermal liquefaction at250 ℃,aqueous bio-oil from slow pyrolysis of rice husk at 500 ℃ and 1 mol/L hydrochloric acid respectively was selected as the research sample.The results showed that compared with the raw rice husk,hydrothermal aqueous leaching made the rice husk pyrolysis more complete,the carbon residue rate decreased from 35 wt.% to 30 wt.%,and the relative content of sugars in the pyrolysis products increased significantly(from 7.14 wt.% to 32.5 wt.%).According to the Coats-Redfern method found that the main pyrolysis stages conformed to the free collision theory,The other two leachate showed similar results.In this paper,it is proved that hydrothermal aqueous can be used as a new leachate to remove rice husk AAEM simply and efficiently,so as to effectively reduce the fouling and slagging tendency of rice husk during combustion or pyrolysis,and improve the quality of its pyrolysis products.Therefore,the concept of integrating biomass hydrothermal liquefaction,bio-oil separation and using aqueous bio-oil to leach AAEM rich raw materials as a circulating bio refinery has broad application prospects and can promote the industrial scale utilization of biomass resources. |
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