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

Biomass energy is one of the most important parts in of the new energy system. Combustion power generation can be expected to utilize the biomass energy under the carbon peaking and neutrality. Among them, the grindability is one of the most significant indexes to evaluate the feasibility of biomass utilization. While the biomass itself can share the low grindability difficult to grind fine, indicating the low pretreatment of the biomass. Therefore, it is very critical to improve the grindability of biomass for the effective utilization of biomass. Fortunately, the torrefaction can be expected to serve as the clean and efficient pretreatment for the high grindability. The moisture and oxygen content of biomass can also be reduced to improve the combustion, pelletizing, and transportation performance of biomass. The fibrous structure can be decomposed after biomass torrefaction. While the woody biomass is utilized mainly for the torrefaction in Europe. It is still lacking on in the bamboo biomass. Since the bamboo biomass is widely distributed and abundant in China, the torrefaction of bamboo biomass can play an important role in the development of biomass energy. In this study, the bamboo pellets were selected as the raw materials. A series of proximate analysisanalyses, ultimate analysis, laser particle size analysis, and detection were carried out to explore the effects of torrefaction temperatures and oxygen concentrations on the biomass's basic physicochemical properties, grindability, and particle size distribution. Results showed that the torrefaction was greatly contributed to the combustion performance of bamboo pellets. The mass fraction of C increased continuously with the increasing of torrefaction degree, whereas, the mass fraction of O decreased significantly. Specifically, 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. The samples were closer to anthracite in the Van Krevelen diagram, indicating the outstandingly improved properties of bamboo pellets. The mass and energy yield of the sample decreased steadily, with the increase of the torrefaction temperature and oxygen concentration. The energy yield was 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 were detected by Fourier transform infrared spectroscopy. The downward trend was then ranked in the order of the O-H, -CH₂, C=O, C-O, C-H, and C-O-C bonds. A large amount of CO and CO₂ was released with the decomposition of oxygen-containing functional groups. There was the an effective reduction in the oxygen content of bamboo pellets. The increasing of torrefaction degree was also contributed to the grindability of samples. There was the ever-increasing intensity of torrefaction and oxidation reaction, as the torrefaction temperature and oxygen content increased. The continuous destruction of biomass structure was attributed to the consumption and reorganization of biomass components, as well as the generation of a large number of gases. Hence, the maximum Hardgrove grindability index was 176 in the non-oxidative torrefaction and 197 in the oxidative torrefaction at 300 ℃. Torrefaction pretreatment was has improved the distribution of particle size after grinding. While there was the a decrease in the average particle size of the pellets for the high uniformity of the samples. The optimal conditions were obtained for the torrefaction of bamboo pellets using response surface analysis. It was found that the optimal torrefaction temperature of bamboo pellets was about 270 ℃, while the oxygen concentration was about 10%.