Conversion of Agricultural Waste into High-Rank Solid Fuel Through Sequential Wet and Dry Torrefaction Processes

Abstract

The low calorific value and high risk of ash deposition hinder the use of agricultural biomass waste as furnace fuel. However, torrefaction can upgrade agricultural waste into high-rank solid fuel, making it a viable biomass source. This study develops a multi-process conversion method using sequential wet and dry torrefaction to obtain high-rank solid fuel with high calorific value, low slagging and fouling potential, and efficient process energy. The paper focuses on multi-process conversion, revealing its superior product characteristics compared to single-process torrefaction. The wet torrefaction process (180 °C) was followed by dry torrefaction at various temperatures ranging from 200 to 330 °C (200 °C, 250 °C, 300 °C, 330 °C). Rice straw multi-process conversion results show increasing a calorific value from 13.76 to 16.58 MJ/kg (330 °C), reduction in slagging and fouling potential from 0.21 (low potency) and 0.92 (medium potency) to 0.013 and 0.48 (both low potency). For palm oil empty fruit bunches, the calorific value increased from 17.21 to 26.89 MJ/kg (330 °C), with a reduction in slagging and fouling potential from 0.435 (low potency) and 74.84 (high potency) to 0.251 and 28.85 (low-medium potency), indicating low-medium potency. The Energy Return on Energy Invested (EROEI) for the multi-process conversion exceeds 1, utilizing residual energy from other systems. The multi-process conversion has reduced slagging and fouling potential, significantly increased calorific value, and provided a good energy return on energy investment. Therefore, sequential wet and dry torrefaction is recommended to harness the benefits of both processes.