Effect of hydrothermal and hydrothermal oxidation pretreatment on the physicochemical properties of biofuel pellet

Abstract

The effects of hydrothermal pretreatment with and without H2O2 addition on the properties of the resulted biofuel pellets prepared from cotton stalk (CS) and Chinese fir wood sawdust (WS) were mainly studied in the paper. Based on the changes of three main components (hemicellulose, cellulose and lignin) of biomass during the pretreatment process, the relaxation density, compressive strength, heating values (HVs), and combustion characteristics of the resulted pellets were analyzed to evaluate the feasibility of the utilization as biofuel pellets. The results indicate that the hydrothermal pretreatments with and without H2O2 addition are beneficial to the quality of the resulted biofuel pellets. Particularly, the hydrothermal pretreatment with H2O2 addition, namely the hydrothermal oxidation pretreatment can significantly alleviate the pretreatment severity of the high-strength biofuel pellet preparation. H2O2 addition promotes the decomposition of hemicellulose and cellulose during the pretreatment. Compared with CS, WS is more sensitive to the pretreatment, especially hydrothermal oxidation pretreatment. The crystallinity index and the cellulose content of the biomass sample both increase firstly and then decrease with the increasing pretreatment temperature. In the study, the pretreatment temperature for the biofuel pellets with the best mechanical properties is 230 °C for the hydrothermal process and 200 °C for the hydrothermal oxidation process, respectively. The HVs of the resulted biofuel pellet increases with the increasing pretreatment intensity and the suitable pretreatment can improve the combustion characteristics of the resulted biomass. The mechanical properties of the pretreated biofuel pellets were positively correlated with the crystallinity and the content of the cellulose. HVs of WS pellets is higher than CS pellets, with the better mechanical properties. The cellulose is more favorable for the improvement of the combustion properties of the biomass sample than the hemicellulose and lignin. In addition, HVs and the compressive strength of all the biofuel pellets prepared from the pretreated biomasses meet the Sweden Standard (SS18 7120) and the Ministry of Agriculture of China standard (NY/T1878–2010), respectively.