Role of Moisture Content in Governing the Physical Properties, Flowability, and Fluidization Behavior of Pine Residues, Waste Coal and Organic Fraction Municipal Solid Waste Blends

Abstract

Fossil fuels have been the dominant source of energy and chemicals for over 150 years but have significantly contributed to global warming. Biomass has emerged as a promising renewable alternative because it can be converted into electricity, fuels, and chemicals while being considered carbon neutral, as the CO₂ released during conversion is balanced by that absorbed during plant growth. The United States possesses abundant biomass resources; however, their direct utilization is limited by high moisture content, low bulk density, and low energy density. Blending biomass with waste coal (WC) and the organic fraction of municipal solid waste (OFMSW) offers a promising approach to improve fuel quality while enhancing energy security, reducing greenhouse gas emissions, and diverting waste from landfills. Nevertheless, biomass moisture, often exceeding 50% in freshly harvested materials, adversely affects physical properties, flowability, fluidization behavior, and overall processing. These effects become more complex in blended systems, and comprehensive studies on moisture effects remain limited. This study investigated ten blends of pine residues (PR), WC, and OFMSW with moisture contents ranging from 10–40% (wet basis). Results showed that moisture content and OFMSW proportion were the primary factors governing the physical and flow behavior of the blends. Increasing moisture increased bulk density but reduced particle density and flowability by increasing cohesion, adhesion, angle of internal friction, and angle of wall friction. Machine learning models successfully predicted these properties with coefficients of determination (R²) between 0.83 and 0.96. Thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) were used to characterize thermal behavior and drying energy. PR and OFMSW exhibited greater water affinity because of abundant oxygen-containing functional groups, whereas WC showed lower affinity due to its aromatic carbon structure. Although drying energy of the blends increased slightly with moisture, it remained lower than that of the individual materials, indicating synergistic interactions. Statistical analysis confirmed that drying energy was governed mainly by blend composition and component interactions rather than moisture alone. Moisture content showed the strongest positive correlation with minimum fluidization velocity (Umf), while PR had a weaker positive influence and WC and OFMSW slightly reduced Umf. WC strongly increased bed pressure drop, whereas PR and OFMSW reduced it. A modified Ergun equation incorporating moisture effects significantly improved predictions of pressure drop and Umf, with new coefficients determined for PR and OFMSW. Finally, CFD simulations demonstrated that the Syamlal–O'Brien and Gidaspow drag models provided the closest agreement with experimental pressure-drop data and accurately predicted fluidization behavior for PR and WC, whereas OFMSW exhibited larger prediction errors. Solid volume fraction contours further showed that PR and WC experienced greater bed expansion and solids circulation, while the EMMS model predicted denser bubbling beds.