Study on Preparation Technology and Characteristics of Crop Residue Biochar Based on Energy and Fertilizer Utilization

Author:Liu Chao Xia

Supervisor:niu zhi you


Degree Year:2018





Crop residue,as a widely distributed renewable resource with low pollution,is one of the key points in the utilization of all biomass wastes.While pyrolysis carbonization of waste crop residues under anaerobic condition is an effective resource utilization approach.The differences of raw material properties,preparation technology and pyrolysis conditions have great influence on the pyrolysis of crop residues,and the quality characteristics of pyrolysis products of crop residues are also complex.It is of great practical significance for crop residue resource utilization,biochar returning to the field and agricultural sustainable development to carry out the research on pyrolysis technology of crop residue,physicochemical characteristics of biochar and utilization of energy and fertilizer.In this study,rice straw,wheat straw,corn stover,rape stalk and cotton stalk are selected as research objects.The pyrolysis experiments of crop residues are carried out under different carbonization temperatures,holding times and heating rates.The effects on the distribution of pyrolysis products of crop resiudes are studied under different reaction conditions.Then the correlation of solid products,liquid products and gaseous products is discussed thereby revealing the mechanism of decomposition and transformation of crop residues during pyrolysis.The yields and quality characteristics of biochars are also optimized by analyzing the composition,nutrient,pore structure,combustion characteristics and ammonia nitrogen adsorption characteristics.Finally,the energy and fertilizer utilization potentials of different biochar are discussed,which laid a foundation for efficient comprehensive utilization of crop residue biochars.The main work and conclusions are as follows.(1)Combining cellulose,hemicellulose and lignin as three organic model compounds,the pyrolysis characteristics of rice straw,wheat straw,corn stover,rape stalk and cotton stalk under different pyrolysis temperatures,holding times and heating rates are studied.And the pyrolysis kinetic model of crop residues under different reaction conditions is established by using pyrolysis reaction kinetics and mathematical treatment methods.The results show that crop residue pyrolysis consists of four stages,dehydration stage between room temperature and 130℃,transition stage between 130℃and 240℃,main pyrolysis stage between 240℃and 500℃and carbonization stage between 500℃and 900℃.The main weight loss of crop residues occurs in the main pyrolysis stage.The maximum weight loss is from 54.81%to 75.43%and it increases with the increase of pyrolysis temperatures.With the increase of heating rates,the initial temperatures of volatile release and the maximum weight loss rates of crop residues increase,while the comprehensive release characteristic index of volatile decrease gradually.One stage reaction kinetic model is suitable for crop residue pyrolysis,and crop residues have low activation energy.The activation energy and frequency factor of rape stalk is the highest,while that of the corn stover is the lowest.(2)The distribution of pyrolysis products of crop residue is studied under different carbonization temperatures,holding times and heating rates.The composition,carbon and energy conversion of gas and biochars are analyzed.The results show that carbonization temperatures and holding times have significant effects on the composition of crop residue pyrolysis gas and biochars,but the effect of heating rate was not obvious.With the increase of carbonization temperatures,the yields of gas increase,while the biochar yields decrease.The yields of tar increase firstly and then decrease.The yields of tar are the highest between 450℃and 500℃.The yields of biochars,gas and tar tend to stable after 60-90 minutes.With the increase of carbonization temperatures and holding times,the contents of CO2 and CO in pyrolysis gas decrease,while the contents of CnHm,CH4and H2 increase.High carbonization temperatures and holding times can improve the carbon and energy conversion rates of gas.The contents of ash,fixed carbon,C,S and higher heating values of biochars increase with the increase of carbonization temperatures and holding times,while the contents of the volatile matter,H,O,carbon and energy conversion rates decrease.The C and H contents of cotton stalk and rape stalk biochars are the highest and their higher heating values are close to the standard coal.The cotton and rape stalk biochars could be used as solid fuel.HHV and energy conversion rates of the pyrolysis gas of wheat straw and rape stalk are the highest thus they could be used to produce high quality pyrolysis gas.(3)The microstructure and morphological characteristics of crop residue biochars prepared are studied under different conditions.The pH,electrical conductivity(EC)and nutrient composition of crop residue biochars are determined.Finally,the specific surface area,pore size distribution,surface elements and surface functional groups of the biochar are studied.The results show that the pH,EC,P,K,Ca contents of biochars and the specific surface area increase with the increase of carbonization temperature,while the surface N contents,H/C and the number of surface functional groups decrease.More specifically,the pH of biochars ranged from 6.3 to 10.6,and EC ranged from 2.34 ms/cm to 9.12 ms/cm.The contents of N,P and K in crop residue biochars are the highest between 450℃and 500℃.What’s more,the biochars have abundant pore structure,which is honeycomb or straight tubular.The porosity increases with the carbonization temperatures and holding times increasing and the carbonization temperature has a great influence.The K contents of wheat straw biochars are the highest,and the P and Mg contents of cotton stalk biochars are the highest.While the rice straw bicohars and corn stover biochars have higher surface O/C and better surface hydrophilicity.(4)Based on thermogravimetric experiments,the combustion characteristics of rice straw,wheat straw,corn stover,rape stalk and cotton stalk biochars are studied,and the kinetic models are established.The results show that the combustion characteristic curves and combustion characteristic parameters of biochars are different.The ignition temperatures of biochars,the activation energy and the residual contents increase with carbonization temperatures increasing.And the maximum weight loss rates increase firstly and then decrease,while the comprehensive combustion characteristic index of biochars decrease.The ignition temperatures of biochars are between 263℃and 374℃,and the burnout temperatures are between 511℃and 547℃.With the extension of holding times,the ignition temperatures of biochars increase and the comprehensive combustion characteristic index decrease,while the activation energy increase first and then decrease.With the increase of heating rate,the ignition and burnout temperature of biochars,the maximum weight loss rate and comprehensive combustion characteristic index increase,while reaction activation energy decrease.The cotton stalk biochars have the best combustion performance and can be used as solid fuel.(5)Taking the fertizer utilization of crop residue bicohars as the objective,the orthogonal experiments are designed for considering the carbonization temperature,holding time and heating rate.And the comprehensive scoring method is used to optimize the pyrolysis process.The results show that the main order of the quality factors affecting physiochemical properties is carbonization temperature,holding time and heating rate.The optimum combination of the 5 kinds of crop residue carbonization process parameters is that the carbonization temperature is 500℃,the holding time is 30 min,and the heating rate is 10℃/min.The yields of biochars under the optimal conditions are32.38%-38%.Under the optimal conditions,the mass fraction of fixed carbon is more than 45%and the C element is greater than 53%.The mass fraction of the N and K are0.7%-2.5%and 10%-13%,respectively.The H/C and O/C ratios of the biochars are lower than those of crop residues.The ratio of H/C is lower than 0.6 in the corn stover,rape stalk and cotton stalk biochar.The biochar surface has oxygen-containing functional groups and mesoporous structures.The biochars have a good adsorption performance of NH4+-N.Moreover,the adsorption type is heterogeneous adsorption of multi-molecular layers.As a result,crop residue biochar can be used as soil ammonia nitrogen regulator.