Study On Gas Release Characteristics And Char Structural Evolution Duing Pyrolysis And Gasification Of Biomass

With the excessive use of fossil fuels and the concerns over environmental protection, the resource utilization of biomass energy has attracted increasing worldwide interest. Agricultural residues are the main biomass resources available in China. However, at present most of agricultural residues are disposed in open fields, causing environment and public health problems. The research and development of pyrolysis and gasification technologies which can convert agricultural residues to high-quality energy contribute to establish sustainable energy system, promote the development of social economy and improve ecological environment. Based on the research progress and deficiency in biomass pyrolysis and gasification fields, maize stalk, rice straw, cotton straw and rice husk were used in this study as the representatives of Chinese typical agricultural biomass residues and the gas evolution patterns and formation mechanisms and char structural evolution during pyrolysis and gasification were studied in detail, which were essential to understand the fundamentals and mechanisms involved in biomass pyrolysis and gasification.Firstly, the pyrolysis characteristics of typical Chinese agricultural biomass were studied using thermogravimetric and derivative thermogravimetric (TG/DTG) analysis. The effect of heating rate was evaluated in the range of 5-50℃/min providing significant parameters for the fingerprinting of the fuels. Thus, the three-pseudocomponent model was used to simulate the pyrolysis behaviors of the materials studied. Model parameters of pyrolysis were given. The results showed that the thermal decomposition of agricultural biomass mainly occurred between 200 and 500℃. Higher heating rate increased the onset temperature and offset temperature of devolatilization and peak temperature for biomass pyrolysis. The maximum pyrolysis rate increased almost linearly with increasing heating rate. The three-pseudocomponent model with first-order kinetics had capability of predicting the pyrolysis behaviors of lignocellulosic biomass at different heating rates. Activation energy values varied between 98 and 114 kJ/mol for hemicellulose,132-186 kJ/mol for cellulose and 21-26 kJ/mol for lignin.The pyrolysis of agricultural biomasses was studied using a bench scale fixed bed reactor coupled with a Gasboard-3100 gas analyzer and a Gasmet Dx-4000 FTIR multicomponent gas analyzer. The release properties and formation mechnisms of gases produced during biomass pyrolysis were investigated. The influences of pyrolysis temperature, heating rate and residence time on the product yields and gas composition were analyzed in detail. The results showed that the major pyrolysis gases for agricultural residues were similar, including CO2, CO, methane, ethane, ethylene and some organics such as methanol, formaldehyde, formic acid, acetone, etc. The release of CO2 and CO was mainly caused by the cracking and reforming of carbonyl, carboxyl and ether groups. The formation of methane was mainly attributed to the rupture of methoxyl, methylene and aromatic rings. The aliphatic-CH2OH groups in-γposition of the alkyl side chains and aromatic methoxyl groups are the main source of methanol. Moreover, the reforming of free hydroxyl groups and C-0 bonds could also generate methanol. The formation of formaldehyde was probably caused by the Cβ-Cγcleavage in alkyl side chains that have-CH2OH groups or carboxylic acid groups in the-γposition. The products yields and gas composition showed a clear dependence on pyrolysis temperature and heating rate. The results showed that higher temperature led to the increase in gas yield and the decrease in char and liquid yields. As the pyrolysis temperature increased from 600 to 900℃, the CO and CH4 contents in the product gases increased obviously, while the CO2 content exhibited the opposite trend. Lower heating rate favoured the formation of CH4 and CO2, while higher heating rate favoured the formation of CO and C2H4.The structural evolution of biomass chars during pyrolysis was studied. The original samples and chars were characterized by ultimate analysis, FTIR, In-situ DRIFTS, ESEM, helium density measurement and N2 isothermal adsorption/desorption method. The results showed that the chemical composition and physicochemical structure of biomass chars were greatly affected by pyrolysis temperature. At high temperatures, the hydroxyl, aliphatic C-H, olefinic C=C and carbonyl groups in biomass chars were almost lost and the content of ether structure was very low, suggesting that the chars became progressively more aromatic and carbonaceous with increasing temperature. High temperature led to the occurrence of softening, melting and carbon structural ordering. Char structure ordering was responsible for thermal annealing and thus for thermal deactivation. Moreover, the structural shrinkage and pore narrowing occurred during biomass pyrolysis. The change of BET specific surface area during pyrolysis was similar for different agricultural biomass residues. The fractal dimension calculated by fractal Frenkl-Halsey-Hill (FHH) equation could represent pore structure satisfactorily. The fraction dimension was closely related to pyrolysis temperature. The change trends of the fractal dimension and BET specific surface area were similar during slow pyrolysis. However, the change trends of the two showed relatively large difference during fast pyrolysis.Finally, the steam gasification of biomass chars was studied in a bench scale fixed bed reactor. The release properties of gas products were analyzed on-line. Based on chemical equilibrium calculation, the influences of temperature, pressure, H2O/C molar ratio and CaO/C molar ratio on gasification process were studied. Then the structural evolution of biomass chars during gasfication was also investigated by applying ultimate analysis, FESEM, N2 isothermal adsorption/desorption method and XRD. The results showed that the major gases produced during biomass steam gasification were H2, CO, CO2 and CH4. H2 content in the product gases is the highest and it reached more than 50%, while CH4 content is the lowest. Gasification temperature had a notable impact on the release patterns and distribution of gas products. As the gasification temperature rised from 800 to 1000℃, the CO concentration in the produced gases significantly increased, the CO2 concentration decreased, and the CH4 concentration had little change. The change of H2 concentration mainly depended on the gasification reactions between biomass char and steam. The proper temperature, pressure, H2O/C molar ratio and CaO/C molar ratio of gasifier were 850℃, 0.1 MPa,1.5 and 2.0 seperately. The change trends of the fractal dimension and BET specific surface area were similar duing gasification. With the development of gasification, the content of amorphous structure and aliphatic side chains in biomass chars decreased and the crystallite diameter in these solids became bigger, indicating that biomass chars became more ordered with increasing carbon conversion.