Optimization of torrefied black liquor and its combustion characteristics with pulverized coal

Background: Black liquor, a byproduct from the pulping process, holds significant potential as a feedstock for biofuels. However, its inherent properties can pose challenges in direct combustion applications. The optimization of torrefaction pretreatment can potentially enhance its fuel properties, making it a more viable energy source. Methods: This study aims to evaluate the combustion behavior of torrefied black liquor within a single pellet burner setup. Performance indicators such as ultimate analysis (EA), proximate analysis (PA), and Poison indices (PI) were employed to gage their efficacy. The Taguchi method was utilized to optimize experimental parameters for these indices. Significant findings: Compared to N₂ flow rate, torrefaction duration, and heating rate, torrefaction temperature had a significant impact on black liquor torrefaction in Taguchi optimization. Higher torrefaction temperatures (300 °C) resulted in lower CO/CO₂ ratios during 800 °C char combustion, dominating overall combustion efficiency. Torrefied black liquor's higher volatile content expedited combustion, while different ratios of torrefied black liquor and coal exhibited linear relationships between mixture proportions and combustion stage durations. Elevated torrefaction temperatures reduced sodium sulfate content in post-combustion ash, effectively preventing boiler fouling.