Wet torrefaction of Indonesian agricultural waste biomass: product evaluation and analysis of slagging-fouling potential

Abstract The agricultural waste biomass holds potential as a valuable resource, capable of being converted into high-grade solid fuel for energy production. Despite the ample availability of rice straw and palm oil empty fruit bunch stock, challenges persist with biomass feedstock, such as low calorific values and high slagging-fouling potency. The wet torrefaction process, known as hydrothermal torrefaction, enhances agricultural waste biomass into solid fuel comparable to lignite and sub-bituminous coal. The study investigates the slagging-fouling potential of raw agriculture biomass waste and wet torrefaction-treated variants, evaluating sample characteristics through proximate, ultimate, and calorific value analyses. This study also investigates the potential of the wet torrefaction process as a pre-treatment in multi-process conversion. The moisture content of the wet torrefaction biomass decreased slightly, and the calorific value of the wet torrefaction biomass samples was higher than that of the raw biomass. The result of rice straw wet torrefaction at 240°C was a gross calorific value of 16.29 MJ/kg, equivalent to lignite A, with slagging potential decreased from 0.021 (low) to 0.011 (low), and the fouling potential from 0.921 (medium) to 0.322 (low). The result of palm oil empty fruit bunches, wet torrefaction at 240°C achieved the highest gross calorific value of 20.38 MJ/kg, equivalent to sub-bituminous C coal, with slagging potential reduced from 0.076 (low) to 0.031 (low) and fouling potential reduced from 74.84 (high) to 20.27 (medium). Wet torrefaction shows potential as a pre-treatment for multi-process conversion. Future studies should consider sequential torrefaction (wet and dry methods) that could leverage the advantages of each, reducing slagging and fouling potential while increasing the calorific value.