Thermogravimetric analysis and kinetic modelling of rice-straw pyrolysis in molten salt of alkali carbonates

The demand for sustainable development has led to a growing interest in biomass energy. Molten salts, such as alkali metallic salts, have been applied in biomass pyrolysis because of their good thermal stability, low vapour pressure, large heat capacity, large thermal conductivity, high solubility, low viscosity, and chemical stability. Using thermogravimetric analysis, this work studied the influence of sodium and potassium carbonates as additives for pyrolysis. Rice straws were mixed at different ratios with two alkali carbonates (Na₂CO₃, K₂CO₃, and their mixture, Na₂CO₃-K₂CO₃). The weight of rice straw decreased at around 200 and 380 ^oC in the presence of carbonates. Temperatures of initial release of volatiles decreased with the increase of mass of salt. The shoulder of the differential thermogravimetric curves appeared earlier under the catalysis by carbonates. The char yield of rice straw with added eutectic molten salt was nearly the minimum compared with that obtained with other salts. Based on this experiment, two reaction models for biomass pyrolysis were proposed. The models can describe the pyrolysis process satisfactorily over the whole temperature range. The kinetic parameters in the different reaction models were calculated by fitting single experiments and by multivariate regression. The results illustrate that the salts can decrease the activation energy and alter the paths of rice straw pyrolysis, and the catalytic activity of eutectic salts is higher than those of the individual salts.