Assessment of experimental 1D and analytical 3D steady approaches of packed bed thermal conversion through the simulation of a 60-kW biomass boiler operating at half and full load

Abstract

Among the different modelling strategies applied for the simulation of packed bed biomass combustion during the last two decades, the present paper detailed describes two models based on two of the most commonly used approaches and tests their performance through the simulation of a 60-kW wood pellet boiler. This contributes to analyze the behavior of both approaches working in different operating conditions and to determine what conditions are favorable for the models application. The first presented model is a 1D experimental method that introduces the products of the biomass thermal conversion through several sections of the bed top surface. The calculations are based on mass and energy balances and experimental determination of the reactive fluxes. The second model is an analytical 3D method that calculates the packed bed thermal conversion inside the CFD domain. This applies more complex calculations with a higher computational cost. For both models, the char oxidation reaction is calculated though four correlations that returns different CO/CO2 ratios. The bed conversion models and the char oxidation correlations are applied to two different tests with the boiler operating at half and full load with different fuels. The results show that both bed models have a similar overall behaviour. The 3D model has a reasonably good behaviour in all cases and is not significantly affected by the different char correlations. Both models give similar results when combustion conditions are favourable (full-load test). The 1D model is highly sensitive to the char oxidation correlations, especially in the half load test. In this case, it has a better behaviour with the correlations that produce lower CO/CO2 ratios. The analysis of the contours in the freeboard shows that, in the 3D bed model, an important part of the combustion process occurs in the bed volume, which is not meshed in the 1D model, and that advance in the combustion compensates the differences in the CO emitted by the bed.