Effects of water washing and acid leaching on the thermochemical characteristics of canola straw for solid biofuel pellet production

Canola straw is a promising biofuel feedstock; however, its high ash content and alkali metals lead to severe slagging, fouling, and operational challenges during thermochemical conversion. In this study, ultrapure deionized (DI) water and multiple acidic solvents, including acetic, citric, phosphoric, hydrochloric, sulfuric, and nitric acids, were systematically evaluated for washing and leaching to enhance fuel quality. Among the tested treatments, DI water, 1 M acetic acid, 99.7 wt% acetic acid, and 1 M hydrochloric acid significantly improved energy density, carbon content, and combustion behavior. Optimal conditions were identified at liquid-to-solid ratios of 74 and 50 L kg−1 and a leaching duration of 3 h at 60 °C, reducing ash content from 3.2% to 1.1% with DI water and to 0.5% with 1 M acetic acid. DI water effectively removed soluble inorganic species, including Na (73.1%), K (84.8%), P (77.5%), and S (76.4%), while 1 M acetic acid achieved greater than 99% removal of major ash-forming elements. These reductions translated into markedly improved ash behavior, with decreased slagging, fouling, and agglomeration tendencies and enhanced ash stability. The results demonstrate that mild organic acid leaching, particularly with acetic acid, is a highly effective and scalable strategy for upgrading canola straw into a cleaner and more efficient solid biofuel.