Effects of washing pretreatment on the hydrothermal conversion of maize stover under CO₂ atmosphere

Biomass hydrothermal conversion has garnered significant interest among researchers due to its broad adaptability to raw materials, cost-effectiveness, and high conversion efficiency. Utilizing CO₂ as the filling gas for hydrothermal reactions not only contributes to reducing atmospheric carbon emissions but also enhances the efficiency of biomass conversion. Despite these advantages, the mechanisms for hydrothermal conversion of biomass in a CO₂ atmosphere remain poorly understood, particularly the impact of inherent inorganic constituents under such conditions. This study employed maize stover as biomass feedstock, which underwent a pre-treatment process involving rinsing deionized water, hydrochloric acid, and acetic acid. The influence of different solvents on the organic and inorganic compositions of maize stover was investigated by means of elemental analysis, industrial analysis, three component (hemicellulose, cellulose, and lignin) analysis and X-ray Fluorescence Spectrometer (XRF). Subsequent hydrothermal experiments were carried out using the biomass samples before and after the washing pretreatment under a CO₂ atmosphere (3.7 MPa, 270 ℃). The research examined the solubility of CO₂ in the hydrothermal liquid and the characteristics of the hydrothermal products from various feedstocks to determine how changes in the organic/inorganic compositions of maize stover due to the pretreatment affect the CO₂ hydrothermal process. The results revealed that water washing pretreatment effectively removed inorganic elements from maize stover, while it had lessimpact on the organic components. Acetic acid washing pretreatment demonstrated a robust removal capability, achieving an elimination rate exceeding 95% for major inorganic elements in maize stover, such as potassium (K), calcium (Ca), and chlorine (Cl), etc. However, the influence of acetic acid washing on the organic components remained marginal; the removal rates for cellulose, hemicellulose, and lignin were all below 10% following acetic acid treatment. In contrast, hydrochloric acid washing not only efficiently removed inorganic elements but also significantly affected the organic components in maize stover. In hydrothermal experiments, the washing pretreatment increased the solubility of CO₂ in the liquid phase by extracting inherent inorganic matter from maize stover, which significantly enhanced the conversion rates of hemicellulose and cellulose. However, the increased solubility of CO₂ did not significantly promote the hydrolysis of lignin. On the contrary, the absence of inorganic components in the washed samples restricted the cracking of lignin, resulting in a decreased conversion rate during hydrothermal processing. Thus, the washing pretreatment, particularly using acetic acid, not only facilitated the selective hydrolysis of hemicellulose and cellulose but also inhibited the further decomposition of their hydrolysis products, thereby effectively increasing the yields of alcohols and aldehydes. The concentration of aldehydes (e.g. furfural and 5-hydroxymethylfurfural), which are the representative hydrolysis products of cellulose and hemicellulose, in the hydrothermal liquid products demonstrated a notable increase, rising from 14.31% for untreated maize stover to between 25.89% and 39.45% for the washed maize stover. Furthermore, the lignin content in the hydrothermal char significantly augmented following the washing pretreatment, enhancing its potential for subsequent utilization, notably in gasification processes aimed at producing hydrogen-rich syngas. This study holds substantial importance for elucidating the hydrothermal conversion mechanisms under a CO₂ atmosphere and advancing the high-value conversion of biomass through hydrothermal processes.