Mathematical modeling and experimental study of biomass combustion in a thermal 108 MW grate-fired boiler

Grate boilers are widely used to fire biomass for heat and power production. However, grate-firing systems are often reported to have relatively high unburnout, low efficiency, and high emissions and need to be optimized and modernized. This paper presents the efforts toward a reliable baseline computational fluid dynamics (CFD) model for an industrial biomass-fired grate boiler, which can be used for diagnosis and optimization of the grate boiler as well as the design of new grate boilers. First, on the basis of the design conditions, a thorough sensitivity analysis is done to evaluate the relative importance of different factors in CFD analysis of the grate boiler. In a late stage, a two-day measuring campaign is carried out to measure the gas temperatures and the gas concentrations in the boiler using a fiber optic probe connected to a Fourier transform infrared spectrometer. A baseline model is then defined on the basis of the sensitivity analysis and the measurements. The baseline results show an overall acceptable agreement with the measured data and the site observations, indicating that the baseline model is applicable in the optimization of the boiler and the design of new grate boilers. However, at a few measuring locations larger discrepancies between the baseline results and the measurements are still observed. It is mainly because the boundary conditions used in the baseline model could be different from those in the real boiler. For instance, the noncontinuous biomass feeding and grate movement, the combustion instabilities inside the fuel bed, and the irregular deposit formed on the furnace walls and air nozzles all make it difficult to derive the reliable boundary conditions that the CFD modeling requires. The baseline results and the measured results show the mixing and combustion behavior in the ideal furnace and in the real furnace, respectively. The local discrepancies may quantify the effect of the differences in the boundary conditions used in the baseline model and in the real boiler.