| With the improvement of people's living standards,the increasing demand for milk products and meat products has promoted the large-scale development of cattle feeding industry.Cattle manure is an unavoidable contaminants during cattle and cow breeding process.If a large amount of cattle manure is not disposed in time,it will not only affect the development of the farm,but also pollute the surrounding environment and pose a threat to people's life and health.Dairy manure contains a large number of organic ingredients,which has the potential to be converted to bio-fuels.The thermochemical method has many advantages,such as high treatment efficiency,short cycle,killing virus and bacteria in the manure during the heat treatment process.Moreover,it can produce high added value fuels such as carbon monoxide,hydrogen,biochar and bio-oil.Hydrogen is a clean fuel and combuste to produce water,which is considered to be one of the most promising future energy to replace fossil fuels.In this study,gasification technology was used to convert cow dung into hydrogen-rich gas,and the gasification mechanism was analyzed.The mechanism of multi-stages gasification and in-situ gasification of cattle manure was studied to achieve an efficient treatment of cattle manure and hydrogen-rich gas production.The main results are as follows:(1)The pyrolysis mechanism of cattle manure was carried out.Based on Van Soest method,cattle manure was treated to obtain the fibers with different compositions.The pyrolysis characteristics of different components of cattle manure were studied by thermogravimetric analysis.The results showed that the neutral detergent soluble extract content was highest in cattle manure,39.35%,followed by hemicellulose and cellulose,and lignin content is low.The pyrolyis rate of acid detergent fiber mainly including cellulose has the largest value among the detergent fibers,14.31%/min,and the smallest pyrolyis rate of strong acid detergent fiber was 1.615%/min.The pyrolysis processes of the different components were intrinsically interacted with each other.The cellulose can change the pyrolysis rate and pyrolysis tempetaure range;hemicellulose inhibited extremely the devolatilization of cellulose,resulting in the decrease of volatile rate of cellulose from 14.31%/min to 7.62%/min.The pyrolysis process of cellulose,hemicellulose,and lignin existed a coupling effect.(2)The thermodynamic model of cattle manure was established.Discrete distributed activation energy model was used to further analyze the coupling effect of these three components.The characteristics of pyrolysis at ultrahigh and ultra low heating rates were predicted and compared,and the characteristic parameters such as the initial temperature,the end temperature,the peak temperature,and the peak rate were deduced.Results showed that the pyrolysis process of cattle manure can be accurately characterized through 27 dominating reactions,and the dominating reactions can be divided into four groups respectively representing four kinds of composition in the cow dung(neutral solutes,washing hemicellulose,cellulose and lignin)of pyrolysis process.The decomposition of each components has no clear boundary,they was interaction and mutual coupling during pyrolysis.As heating rate was chenged,the correlation between the components of the coupling characteristics also change.(3)The key process parameters of in-situ gasification of cattle manure with high moisture for hydrogen-rich gas production were studied.In-situ gasification is a technology that contains evaporation,pyrolysis and steam reforming.The effects of temperature,moisture content,heating rate and feeding temperature on the pyrolysis gas yield,composition,calorific value,hydrogen yield,and carbon convertion rate were investigated in a fixed-bed reactor using single-factor test.The results showed that with the increase of temperature and moisture content,the volume fraction of hydrogen,the yield and calorific value of pyrolysis gas are increased.The effects of heating rate and feeding temperature on the hydrogen volume fraction and gas yield were not significant.In-situ gasification of wet dairy manure showed that cattle manure had the potential to produce hydrogen.The maximum value of hydrogen content and hydrogen yield are 50.57% and 536.64 ml/g,respectively.In in-situ gasification process,the reactions(pyrolysis and gasificaiton)were overlap and complex.And the volatile content producted in in-situ gasification process can inhibite the biochar reactivity and steam gasification reaction.(4)The two-step gasification for hydrogen-rich gas production was studied.During carbonization process,the influence of operating parameters on carbonization reaction mechanism was investigated.The effect of temperature,moisture content,heating rate,feeding amount and reaction time on products distribution was studied by orthogonal test design.The effect of temperature and moisture content on products distribution and biochar physiochemical properties was studied using full-factor experiment design,and the effect of temperature and reaction time on products distribution and biochar physiochemical propertities was studied by one-factor experiment design.The results showed that temperature played a significant role on products distribution and bichar characterisitics.At moisture content of 75%,the biochar yield decreased from 47.30% to 37.70%,as temperature increased from 400 oC to 600 oC.The effect of moisture content on products distribution depended on temperature,and the main effect of moisture content was to change the yields of liquid and gas.The existence of steam in reactor inhibited the second cracking of bio-oil,but promoted the steam reforming reaction of bio-oil.The biochar properties,including elements analysis,proximate analysis,thermal property and crystal structure,was affected by temperature.The effect of moisture content on biochar properties was not significant.The maximum of carbon in biochar was 80.59%,which was obtained at 600 oC carbonization temperature.Therefore,the temperature was very improtant factor for producing biochar during carbonization process.(5)The high temperature gasification of biochar during two-step gasification process was carred out.The mechanisim of biochar gasification and effect of operating factors on gasifcaiton performance were studied.The biochar from different temperatures was used as feedstock to investigate the gasifcation performance.The results showed that the syngas yield and hydrogen yield were increased with increasing temperature.Sutiable steam rate and high heating rate can promote the hydrogen yield;Ash from manure had a positive effect on bio-oil yiled,but had a negative effect on gas yield.The optimal parameters of biochar gasifcation were the temperature of 850 oC and steam flow rate of 1.66g/min at high heating rate mode.And the maximums of hydrogen yield and content were 1105.30ml/g and 59.08%,respectively.The biochar from high temperature was favor to biochar gasifcation for hydrogen production.According to the hydrogen yield,the order of raw material was BC-500 > BC-600 > BC-350 > BC-400 > BC-300 > Cattle manure,and BC-300 to BC-600 represented the biochar derived from carbonization temperature of 300 oC to 600 oC with reaction time of 30 min.The conversion rates of cattle manure and BC-300 were higher than those from BC-500 and BC-600 at same operating factors during gasification process.However,the gas yields from cattle manure and BC-300 were lower than those from BC-500 and BC-600.The results indicated that a large amount of bio-oil was produced during cattle manure and BC-300 gasifcation process.Therefore,biochar derived from high carbonization temperature was sutiable material to produce hydrogen and decrease bio-oil yield during gasification process.(6)The compositions and thermal properties of cattle manure indicated that cattle manure has the potential for thermochemical treatment.The in-situ gasification and multi-stage gasification results showed: a.cattle manure has the potential for hydrogen production by gasification;b.compared to in-situ gasification,the multi-stage gasification technology improved the hydrogen yield and reduced the bio-oil yied;c.the biochar produced from high temperature was the preferred gasification material;d.The hydrogen production from biochar was better than that of cattle manure,and biochar gasification can reduce the bio-oil yield;e.the temperature of 850 oC was the appropriate gasification temperature of biochar;f.the mass of cattle manure biochar was about 40% of cattle manure feedstock,which was benifical to reduce the cost and energy consumption of high temperature gasification equipment.Therefore,the multi-step gasification of cattle manure for hydrogen-rich gas production is a promising and potential technology for treating cattle manure and producing hydrogen-rich gas. |
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