Optimization of a “coal-like” pelletization technique based on the sustainable biomass fuel of hydrothermal carbonization of wheat straw

The optimization of a “coal-like” pelletization technique based on the sustainable biomass fuel of hydrothermal carbonization of wheat straw was investigated. Wheat straw was selected as the raw material. Single factor experiment was used to study effects of various hydrothermal carbonization and molding conditions on the fuel and pelletization characteristics of hydrochar pellets. Response surface methodology was used to explore the interactions of significant conditions and optimize the wheat straw hydrochar pelletization techniques. Experimental conditions included raw material particle size, hydrothermal carbonization temperature, hydrothermal carbonization time, moisture content, molding pressure, pressure holding time and compression speed. The results showed that effects of hydrothermal carbonization temperature on fuel characteristics of hydrochar pellets was significant (P < 0.05). Hydrothermal carbonization temperature, hydrothermal carbonization time, moisture content and molding pressure had significant effects on water resistance, compressive strength and molding energy consumption of hydrochar pellets (P < 0.05), meanwhile they had interactive effects on water resistance and molding energy consumption. The raw material particle size, pressure holding time and compression speed had no significant effect on the fuel and pelletization characteristics (P > 0.05). The optimized experimental conditions of wheat straw hydrochar pelletization were: hydrothermal carbonization temperature of 231 °C, hydrothermal carbonization time of 25 min, molding pressure of 130 MPa, moisture content of 13%. Higher heating value of the optimum-processed hydrochar pellets was 21.03 MJ/kg, its ash content was 2.11% and comprehensive combustibility index was 9.55 × 10−7%2/(°С3·min2). Using these optimized conditions, it could be produced the hydrochar pellets with similar energy yields and lower ash contents than lignite, as well as a product with better pelletization and fuel characteristics than its parent material pellet.