| Pyrolysis is a promising technology of utilizing biomass and can converts biomass into solid,liquid and gas products.The liquid product,named as bio-oil,has great potential to be used as a fuel and to extract high value-added chemicals.However,bio-oil has many poor physicochemical properties,such as complex components,high acidity,high oxygen content,high water content and low heating value,etc.,contributing to the difficulties in application of bio-oil,thus the industrial application of biomass pyrolysis has not been realized.Reducing the complexity of the bio-oil components and increasing the selectivity of value-added products are significant to solve this problem.Therefore,from the perspective of optimizing pyrolysis process and regulating pyrolysis reaction process,the stepwise pyrolysis of lignocellulosic biomass was investigated in this study.The stepwise pyrolysis was expected to produce the products step by step,and then reduce the complexity of bio-oil components and concentrate the value-added products.The factors affecting the stepwise pyrolysis were systematically studied,and the characteristics and mechanism of stepwise pyrolysis of lignocellulosic biomass were analyzed.Combined with pretreatment and catalyst,the proper condition and utilization approach of stepwise pyrolysis of lignocellulosic biomass were proposed.1.Study on the influence factors of two-step pyrolysisThe thermogravimetric characteristic of corncob was studied by a thermogravimetric analyzer coupled with FTIR spectrophotometer(TG-FTIR).The results showed that the decomposition process could be divided into moisture evaporation and vitrification process,main decomposition process and carbonization process,and most products with oxygen-containing functional groups were generated before 400?.Pyrolysis-gas chromatography/mass spectrometry(Py-GC/MS)was used to conduct the two-step pyrolysis of corncob,and the effects of pyrolysis temperature and residence time on the two-step pyrolysis were studied.It was found that the first step temperature at 400?-450? was the optimal condition.Compared with one-step pyrolysis,the first step could increase the contents of acids,furans and ketones,and the second step could produce less acids and N-containing compounds but more hydrocarbons.The residence time of the first step could further regulate the selectivity of pyrolysis products.Corncob was washed by water and hydrochloric acid respectively to acquire washed samples with different content of alkaline and alkaline earth metals(AAEMs).The thermogravimetric characteristics of the three samples were studied by TG-FTIR,and the results indicated that the removal of AAEMs increased the decomposition temperature,decreased the yield of bio-char and produced more sugars and furans.The two-step pyrolysis of the three samples were conducted by Py-GC/MS,and the effects of AAEMs on two-step pyrolysis were researched.Compared with one-step pyrolysis,two-step pyrolysis combined with acid washing pretreatment could increase the contents of furans and sugars in the first step,and further increase the content of sugars in the second step.Walnut shell,cotton stalk and corncob with different contents of cellulose,hemicellulose and lignin were used as materials.Acid washing reduced the difference of inorganic component and morphological structure of biomass.The decomposition process of the six samples could be divided into three stages,and acid washing reduced the yield of bio-char of cotton stalk and corncob,but had few effects on the thermogravimetric characteristic of walnut shell because of high lignin content.The effects of components on two-step pyrolysis were studied by using Py-GC/MS.The holocellulose derivatives were mainly generated in the first step for cotton stalk and corncob due to their high content of holocellulose,while walnut shell yielded more phenols in the second step due to its high lignin content.Based on the results above,a new polygeneration scheme of two-step pyrolysis for the efficient utilization of lignocellulosic biomass was proposed.In order to study the two-step pyrolysis characteristics systematically,a small fixed-bed pyrolysis reactor was used to carry out the two-step pyrolysis of rice husk,and the effects of residence time on two-step pyrolysis was analyzed.In the first step,the increase of the residence time of the first step decreased the yield of bio-char product and increased the yields of bio-oil and gas.In the second step,the increase of the residence time of the first step increased the yield of bio-char but decreased the yield of bio-oil and gas.The total bio-oil yield of two-step pyrolysis was lower than that of one-step pyrolysis,while the total yields of bio-char and gas were all higher than those of one-step pyrolysis.Compared with one-step pyrolysis,the bio-oil of the second step had lower acidity.When the residence time of the first step was 3min,the bio-oil of the two steps all had lower water content than that of one-step pyrolysis.Identified with the results of Py-GC/MS,the bio-oil of the first step enriched acids,furans and some phenols with complex side chains,and the bio-oil of the second step concentrated phenols and hydrocatbons.Compared with one-step pyrolysis,the first step could increase the contents of CO and CO2,and the second step could obviously increase the contents of H2 and CH4.2.Research on the two-step catalytic pyrolysis of lignocellulosic biomassTwo-step catalytic pyrolysis of rice husk was conducted by Py-GC/MS,and zinc acetate and HZSM-5 were used as catalysts.The decomposition process of rice husk and rice husk impregnated with zinc acetate could be also divided into three stages.Zinc acetate decreased the decomposition temperature,and increased the yield of bio-char and the maximum mass loss rate.Compared with two-step pyrolysis of rice husk,with the effects of zinc acetate,the first step increased the contents of acids,furans and esters,and decreased the contents of aldehydes,phenols and ketones;in the second step,the contents of sugars,furans and hydrocarbons increased,while the contents of aldehydes,phenols,acids,esters and ketones decreased.Compared with two-step pyrolysis of rice husk,in the first step,HZSM-5 could increased the content of sugars,ketones,hydrocarbons and esters,and inhibited the generation of furans,acids,aldehydes,phenols and N-containing compounds;in the second step,HZSM-5 remarkably increased the content of hydrocarbons,reaching the maximum of 73.23%,and reduced the contents of other kinds of products.The decrease of the complexity of bio-oil components and high selectivity of hydrocarbons could increase the quality of bio-oil.3.Study on stepwise pyrolysis of lignocellulosic biomassStepwise pyrolysis of rice husk was conducted by Py-GC/MS,and the effects of temperature interval and residence time on stepwise pyrolysis were studied.The temperature interval was set as 50?,100? and 150?,and the residence time was set as 5s,10s and 20s.Temperature interval and residence time had significant effects on product distribution,and the product distributions were different under different conditions.On the whole,the larger temperature interval and longer residence time contribute to less pyrolysis steps.Compared with one-step pyrolyusis,stepwise pyrolysis could increase the selectivity of value-added chemicals,but more steps would decrese the yield of products in every step.Therefore,it was appropriate to set the steps of stepwise pyrolysis as two or three. |
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