Regimes of Nonpremixed Combustion of Hot Low-Calorific-Value Gases Derived from Biomass Gasification

Combustion of the hot, diluted gases derived from biomass gasification was investigated in mixture fraction space. The composition of the fuels corresponds to those measured in industrial gasification plants operated on wood chips and organic residuals. The temperatures of both the fuel and the oxidizer are within the range accessible to gasifiers which naturally produce hot gases. Simulations of counterflow laminar nonpremixed flames show that only the maximum temperatures and temperature profiles of the biomass syngases are comparable to those obtained for methane diluted with nitrogen to the same calorific value. In contrast, the profiles of the heat release rate (HRR) are significantly different, being widely spread across the mixture fraction space when syngas is burning but with a narrow distribution for the methane equivalent. There are also differences in profiles of OH, important from an experimental point of view. From this perspective, methane equivalents cannot be treated as an analog of real biomass-derived gases. Wood-derived gases are generally burned conventionally or, when both reactants are highly preheated, in the high-temperature air combustion (HTAC) regime. Lower calorific-value gases derived from organic residuals are most commonly burned in pilot-assisted regimes, but the favorable regime of moderate and/or intense level of dilution (MILD) can be achieved when the reactants are sufficiently preheated. Moreover, even when the oxidizer is cold, highly diluted and preheated residual-derived gases can be burned within the MILD combustion regime. Wood-derived gases typically do not achieve MILD combustion, even when additional exhaust gas recirculation (EGR) is introduced or when the scalar dissipation rate (SDR) is increased. Although based on the simplified mixture-fraction approach, a survey of the combustion regimes qualitatively agrees with the properties of the combustion processes observed in industrial gasification plants. Gases from wood chips are burned conventionally, whereas less calorific-value gases derived from organic residuals achieve MILD combustion for three different geometrical configurations of the industrial combustion.