Effects of on physicochemical properties of fuel pellets prepared from hydrothermal and hydrothermal oxidation pretreated wood sawdust

Biomass pelletizing treatment can enhance the energy density and decrease the transportation and storage cost of the resulted fuel. In the study, wood sawdust (WS) pretreated by hydrothermal and hydrothermal oxidation process at 170～260 ℃ was used to prepare biomass pellet. The effects of the hydrothermal and hydrothermal oxidation pretreatment on the mechanical property and combustion performance of the wood pellet were investigated using Van Soest method, X-ray diffraction (XRD), thermogravimetric analysis (DTG), industrial analysis and oxygen bomb calorimeter. The results showed that the hemicellulose and amorphous cellulose decomposed rapidly at 170～230 ℃, the crystalline cellulose began to decompose at 260 ℃ and the relative mass fraction of lignin gradually increased with an increase of temperature because of the decomposition of hemicellulose and cellulose after hydrothermal pretreatment. While the hemicellulose was completely decomposed before 230 ℃ and the temperature of crystal cellulose decomposition was advanced to 230 ℃ after hydrothermal oxidation pretreatment. Compared with hydrothermal pretreatment, the relative mass fraction of lignin increased more obviously. The pyrolysis characteristics of wood sawdust pretreated by hydrothermal and hydrothermal oxidation were analyzed. It was found that the samples after hydrothermal pretreatment, the height of hemicellulose pyrolysis peak gradually decreased and completely disappeared at 230 ℃. The pyrolysis peak of cellulose first increased and then decreased with the increase of pretreatment temperature, and the peak of lignin was positively correlated with the increase of pretreatment temperature. Compared with hydrothermal pretreatment, with the increase of hydrothermal oxidation temperature, the pyrolysis peak of hemicellulose disappeared obviously； the pyrolysis peak of cellulose showed a downward trend and the peak was significantly advanced while the pyrolysis peak of lignin continued to rise. The hydrothermal and hydrothermal oxidation pretreatments can greatly enhance the mechanical properties of the resulted biomass pellets compared with that of the unpretreated WS pellets. After hydrothermal pretreatment, the apparent density increased from 1 110.2 to 1 381.6 kg/m³ and the compressive strength increased from 16.2 to 38.9 MPa at 230 ℃, which increased by 24.4% and 140.1%, respectively. The apparent density increased to 1 429.2 kg/m³, and the compressive strength increased to 41.1 MPa at 200 ℃ after hydrothermal oxidation pretreatment, which increased by 28.7% and 153.7%, respectively. Compared with hydrothermal pretreatment, hydrothermal oxidation pretreatment can promote the degradation of the hemicellulose and cellulose contained in WS and enhance its mechanical property, thus, the pretreatment intensity can be alleviated. In the process of hydrothermal and hydrothermal oxidation pretreatment, the crystallinity of samples increased first and then decreased with the increase of pretreatment temperature. After hydrothermal pretreatment, the crystallinity of cellulose increased from 41.4% to 50.7% at 230 ℃. The crystallinity of cellulose reached the maximum value at 200 ℃ after hydrothermal oxidation pretreatment which was higher than the maximum value of hydrothermal pretreatment. When the temperature of hydrothermal oxidation pretreatment rose to 260 ℃, the crystallinity finally decreased to none. The mechanical property of the pellets is found to be positively correlated with the change trend of the cellulose crystallinity and the crystalline cellulose is the key component for the biomass pelletizing process. The hydrothermal and hydrothermal oxidation pretreatments can significantly enhance the heating value, the comprehensive combustion characteristic index and combustion stability index of the resulted WS pellets. Compared with hydrothermal pretreatment, the hydrothermal oxidation pretreatment can promote the carbonization degree of the resulted samples. In sum, the quality and combustion performance of the fuel pellet can be improved by the pretreatment. These results can supply some support for the production of good quality WS pellets.