Effect of different torrefaction pretreatment conditions on the grindability of bamboo pellets

Biomass energy is an important part of China's new energy system, and combustion power generation technology is an important way of biomass energy utilization, which contributes to the carbon peaking and carbon neutrality goals. Grindability is an important index to judge the feasibility of biomass utilization, while biomass itself has poor grindability and is not easy to grind fine, which means the pretreatment of the biomass is difficult. Therefore, improving the grindability of biomass is the key to promote the effective utilization of biomass. Torrefaction is a clean and efficient pretreatment method, which can reduce the moisture and oxygen content of biomass, improve the combustion performance, pelletizing performance and transportation performance of biomass, and decompose the fibrous structure, effectively improve the grindability. There are many researches on the biomass torrefaction process, especially in Europe, while the biomass used by European companies is mainly woody biomass, and the research on bamboo biomass is insufficient. Bamboo biomass is widely distributed and abundant in China, hence the torrefaction of bamboo biomass plays an important role in the development of biomass energy in China. In this study, bamboo pellets were selected as raw materials, through proximate analysis, ultimate analysis, laser particle size analysis and other detection means, the effects of torrefaction temperatures and oxygen concentrations on biomass basic physicochemical properties, grindability and particle size distribution were explored. Results showed that torrefaction is helpful to improve the combustion performance of bamboo pellets. With the increasing of torrefaction degree, the mass fraction of C increased continuously and the mass fraction of O decreased significantly, the fixed carbon increased from 12.67% to the highest 50.47%, the calorific value also increased from 21.32 MJ/kg to the highest 29.49 MJ/kg, and the samples are closer to anthracite in the Van Krevelen diagram, the properties of bamboo pellets were obviously improved. The mass yield and energy yield of the sample decrease steadily as the torrefaction temperature and oxygen concentration increase, and the energy yield is always higher than the mass yield, with the highest Energy-mass co-benefit index(EMCI) value at 280 ℃. The main functional groups of the torrefied biomass detected by Fourier transform infrared spectroscopy include O-H, C-Hal, C=O, C-O, C-H, and C-O-C bonds showed a downward trend, with the increase of torrefaction temperature and oxygen concentration. A large amount of CO and CO₂ were released with the decomposition of oxygen-containing functional groups, which effectively reduced the oxygen content of bamboo pellets. The increasing of torrefaction degree also contributes to the improvement of samples’ grindability. With the increase of torrefaction temperature and oxygen concentration, the intensity of torrefaction reaction and oxidation reaction increased continuously, and the consumption and reorganization of biomass components and the generation of a large number of gases led to the continuous destruction of biomass structure, hence the Hardgrove grindability index of the sample increased significantly, the maximum Hardgrove grindability index is 176 in non-oxidative torrefaction and 197 in oxidative torrefaction at 300 ℃. Torrefaction pretreatment can improve the particle size distribution after grinding, and reduce the average particle size of the sample, but can reduce the uniformity of the samples. The best conditions for bamboo pellets torrefaction are obtained by response surface analysis. It was found that the optimal torrefaction temperature of bamboo pellets is about 270 ℃ and the oxygen concentration is between 8% and 12%.