Predictions of NOx and SOx in MILD regime based on thermal conversion of solid sewage sludge surrogates

Abstract

Thermal treatment of sewage sludge using a combination of gasification and combustion of the producer gas is an attractive option to produce sterilized high-notorious bio-char and recover energy from the feedstock. Given the high temperature of the producer gas after gasification, employing the MILD (Moderate or Intense Low Oxygen Dilution) combustion regime in the second stage might benefit the heat balance of the process and result in low emissions. The current work employs multi-component surrogates derived from solid fuel samples of various ultimate elemental compositions and moisture content, a stochastic reactor model for gasification, and a 1D counterflow diffusion flame configuration for combustion with a dedicated, detailed chemical mechanism. Using an improved methodology for emission measurement, we analyzed the combustion emissions of the wet producer gas from gasification, which includes fuel-NO precursors, sulfur components, and tar species up to a size of C11. The analysis confirms that thermal and fuel-NO emissions are reduced in the MILD regime. While high moisture content in the feedstock has been shown to enhance the formation of the pollutant via identified chemical pathways, different representations of fuel samples (ultimate composition) are found to have a significantly more considerable impact on the overall SOx and NOx emissions. The more sulfur is contained in the ultimate composition of the solid surrogate fuel, the more SOX is formed in the combustion stage. This direct relation does not hold for NO. The NO emissions in MILD combustion are the lowest in the performed simulations for the highest amount of fuel-bonded nitrogen. This is caused by the key role of the fuel-NO precursor ammonia (NH3) and its radicals that act in NO formation but also are a driving force in NO reduction. Given this key role of specific fuel-NO precursors, it is further highlighted that there is a significant difference in predicting NO, depending on the complexity of the fuel-NO precursors in the boundary condition of the flame calculation.