Slow pyrolysis of rice straw: Analysis of products properties, carbon and energy yields

• Slow pyrolysis of rice straw was investigated for temperatures of 300–700 °C.
• Biochar had a mass yield of about 25% from the organic fraction above 500 °C.
• Biochar was the primary product containing 40% of energy and 45% of carbon.
• Bio-oil and light gases had about 60% of energy yield in total.
• Drying of raw material was crucial to efficiently utilize the gases for process heat.

Among many uses of rice straw, application of its biochar from pyrolysis to the soil is receiving greater interest for increased crop productivity and sequestration of CO2. This study investigated slow pyrolysis of rice straw at 300–700 °C to characterize the yields and detailed composition of the biochar, bio-oil and non-condensable gases. Biochar was analyzed for pH, microscopic surface area and pore volume distribution. Although the mass yield for the organic fraction was only about 25% above 500 °C, biochar was the primary product of pyrolysis containing 40% of energy and 45% of carbon from the straw. The utilization of by-products (bio-oil and gases) as energy resources was essential, since the sum of energy yield was about 60%. The gases could be burned to produce the heat for an auto-thermal pyrolysis process, but the heat balance was significantly influenced by the moisture content of the raw material.