Co-Combustion/Co-Gasification of Organic Sludge and Wood Pellets in Fluidized Bed Reactors

This study investigated the thermochemical reaction characteristics of petrochemical wastewater sludge, wood pellets, and their blends. Owing to the high calorific value and the low sulfur, nitrogen, and ash contents, wood pellets effectively compensate for the low heating value of sludge, thereby enhancing co-combustion and co-gasification performance. Thermogravimetric (TG) analysis of the combustion showed that the addition of wood pellets increased volatile matter release and resulted in two distinct weight loss peaks during the char combustion stage. An increase in the blending ratio (BR) significantly enhanced the combustion characteristics and combustibility indices, exhibiting a positive synergistic effect, particularly at BR30% and BR50%. TG analysis of gasification showed that as the BR increased, two weight loss peaks appeared in a single reaction stage, and both comprehensive gasification and devolatilization indices improved significantly, with the most pronounced synergistic effect observed at BR50%. Kinetic analysis indicated that the addition of wood pellets increased the activation energy in the volatile release region while reducing the activation energy in the fixed carbon (FC) reaction region. In laboratory-scale (10 kW_th) and 100 kW_th-scale fluidized bed combustion and gasification experiments, the reaction temperature and gas emissions of the fluidized bed reached a stable state with increasing BR. Notably, in the 100 kW_th-scale fluidized bed system, the blended fuel demonstrated long-term stable operation at BR85%, effectively reducing CO, NO_x, and SO₂ emissions. In the gasification experiments, the use of olivine as a catalyst enhanced the hydrogen yield in syngas, and the co-combustion of syngas with liquefied petroleum gas (LPG) met the fixed-source emission standards. The results showed that co-combusting or co-gasifying sludge and wood pellets can effectively reduce sludge volume while recovering energy. This study integrated TG analysis with experiments on both lab-scale (10 kW_th) and 100 kW_th fluidized bed systems, emphasizing their complementary roles. Lab-scale tests often require parameter adjustments when scaled up, whereas 100 kW_th-scale trials yield results closer to practical applications. These findings support the feasibility and sustainability of sludge-to-energy technologies.