Influence of Physical and Chemical Properties of Slow Pyrolysis Char and Irrigation Rate on Water Retention of Wood-gasification Residual Chars during Agricultural Use

Japan has declared zero net greenhouse gas emissions by 2050, and substituting fossil fuels with renewable biomass energy is a promising approach to achieve this goal. Among the biomass conversion processes, gasification combined heat and power, which has a high energy conversion efficiency and is expected to revitalize local communities, faces challenges in terms of profitability, one of the main causes of which is the cost of processing char residues. One method to add value to char is to trade credits for carbon sequestration through agricultural land use. However, the relationship between the physical and chemical properties of char and its water retention characteristics has not been fully investigated. In this study, the physical and chemical properties of char were characterized by evaluating its pore properties and electrical conductivity at different temperatures to determine the effects of these properties and irrigation on water retention. The pore properties of the char were measured using the Brunauer–Emmett–Teller (BET) specific surface area, and the electrostatic attraction on the surface of the char was evaluated by measuring the electrical conductivity. The BET specific surface area and electrical conductivity of the char pyrolyzed at 500 °C were 8.55 m2/g and 514 µS/cm, respectively, whereas those of the char pyrolyzed at 800 °C were 375 m2/g and ≥1999 µS/cm, respectively. The BET specific surface area and electrical conductivity increased with increasing char pyrolysis temperature, suggesting increased pore area. When irrigation was 30 g, the water retention capacity increased by approximately 1.4 times with the mixing of char with soil, regardless of the pyrolysis temperature. When the irrigation rate doubled, the water retention capacity increased by an average of 1.5. A robust positive correlation was observed between the water retention capacity and specific surface area, and a positive correlation was observed between the pyrolysis temperature and water retention capacity. These results clarify the factors that contribute to the water retention effect of biochar and show its effectiveness in potential agricultural applications. In addition, CO2 emission reduction of 2.3 t-CO2/t-char can be expected per 1 t of char.