Co-Firing Characteristics of Biomass with Pulverized Coal in a Drop Tube Furnace: Effects of Co-Firing Ratios and Feeding Methods

Direct cofiring of biomass in coal-fired power plants is a cost-effective strategy for decarbonization. This study systematically investigates the effects of biomass cofiring ratios (0–30%) and feeding methods (cofeeding vs separate feeding) in a 100 kW drop tube furnace, focusing on combustion, emissions, and ash behavior. Increasing the biomass ratio enhanced fuel burnout to a maximum of 98.5% and advanced ignition, but elongated the flame by shifting the flame center downward. A critical trade-off in pollutant control was identified: separate feeding created a pronounced air-staging effect, reducing NOx emissions by an average of approximately 25% compared to cofeeding. Conversely, cofeeding promoted in situ SO2 capture by facilitating alkali-sulfur reactions; at a 30% ratio, cofeeding achieved a sulfur retention rate of 8.49%, which is nearly double that of separate feeding (4.63%), thereby verifying the dominant role of alkali-induced sulfur sequestration. While NOx emissions peaked at a 10% cofiring ratio before declining, biomass addition severely increased slagging risks, evidenced by a decrease in the ash softening temperature (ST) by over 150 °C (from 1494 to 1329 °C). This was attributed to the reaction of alkali metals (K, Na) with aluminosilicates to form low-melting-point minerals like K/Na-feldspar, leading to ash agglomeration. These findings provide crucial guidance for optimizing cofiring operations.