Experimental study on municipal sludge/coal co-combustion preheated by self-preheating burner: Self-preheating two-stage combustion and NOx emission characteristics

To achieve efficient and low-pollution sludge treatment, this study explored the first-time application of self-preheating combustion technology in sludge/coal co-combustion, and subsequently investigated the influences of sludge blending ratio (BR) and tertiary air types on self-preheating two-stage combustion and NOx emission characteristics in a 30 kW self-preheating combustion system with a novel two-stage down-fired combustor (DFC). This system was well adaptable for fuels, and still ran steadily even if BR was as high as 50 %. During preheating, properly increasing BR improved carbon microcrystalline structure of preheated carbon-based fuel and increased its specific surface area, but its carbon microcrystalline structure was gradually deteriorated at BR>20 %. Fuel properties of preheated coal gas were deteriorated with increasing BR. During combustion, combustion efficiency (η) increased first and then decreased with increasing BR, while NOx emission increased. For tertiary air nozzle located at top of burnout region, the structures of center symmetry and ring hedge had less effect on η and NOx emission, both of which remained at high levels, exceeding 98.5 % and 160 mg/m3. Notably, properly shifting injection position down reduced NOx emission significantly, albeit at the expense of reduction in η. Excessively low injection position had less effect on further NOx emissdddction.