Abstract:
China is rich in agricultural and forestry resources, and thus biomass energy has broad prospects for development.Biomass energy utilization technologies are mainly divided into curing molding, thermo-chemical conversion and biochemical conversion.Hydrothermal carbonization (HTC) is one of the thermo-chemical conversion methods.HTC is very suitable to transform wet biomass feedstocks into carbon-rich solid fuels without drying the biomass, commonly referred to as hydrochars, whose physicochemical properties are comparable to that of coal.Sawdust and corn straw, as the representative of woody biomass and herbaceous biomass, respectively, were selected to produce hydrochars accordingly through hydrothermal carbonization under the condition of 230 ℃, 30 min, and gas flow rate of 60 mL/min (the volume ratio of N2 to O
2 was 4∶1).Result indicated that the energy density of hydrochars was greatly improved, and could be used as solid fuel.Combustion characteristics of the hydrochar are different from biomass and biochar when produced at high temperature.Thermogravimetric (TG) analysis was used to investigate the combustion characteristics and kinetics of sawdust hydrochars and corn straw hydrochars.The influences of different heating rates (10, 20 and 40 ℃/min) on hydrochar combustion behavior were analyzed, while raw material was investigated for the comparison at heating rate of 20 ℃/min, too.The combustion characteristic parameters and the kinetic parameters were determined.The results indicated that: 1) The combustion weight loss of biomass and the hydrochar was focused on the burning process of the volatile matter and coke respectively, the ignition temperature and burnout temperature moved to the high temperature zone, and the integrated combustion characteristic index increased when the heating rate increased; 2) Integrated combustion characteristic index of sawdust hydrochars and corn straw hydrochars was increased by about 9 times and 7 times at 40 ℃/min respectively compared to raw biomass, and integrated combustion characteristic index of sawdust hydrochars was much bigger than corn straw hydrochars; 3) At the heating rate of 20 ℃/min, integrated combustion characteristic index of sawdust hydrochars and corn straw hydrochars was decreased a little; 4) The combustion kinetics analysis of hydrochars was performed using the first order reaction kinetics model and the integral method; all correlation coefficients of the fitting curves were greater than 0.9, and most of them were over 0.95 at both high and low heating rate; the activation energy of the volatile combustion phase was greater than that of the fixed carbon combustion stage, which were 43.0-97.7 and 29.6-41.6 kJ/mol respectively.In summary, biomass is suitable for the preparation of solid fuels through hydrothermal carbonization, and the fuel has good combustion characteristics.Combustion characteristics of woody biomass hydrochars are superior to that of herbaceous biomass hydrochars.