Grate Firing of Biomass: Measurements, Validation and Demonstration: CFD and bed model development

The worldwide concern about global warming because of the emission of CO₂ and other greenhouse gases and the limited availability of fossil fuels has increased the interest in using biomass as a fuel for energy production.

In Denmark, to use biomass (mainly straw) as a fuel for energy production, the development of combustion technology has followed two specific paths: grate firing and co-firing in conventional fossil fuel fired plants. Over the past decade, the research and development work related to biomass grate firing and co-firing has been extensive and the two technologies have now been developed to give viable technical solutions. Particularly, the fundamental knowledge achieved in grate firing makes it a presently preferred technology in Denmark. Grate firing plants have undergone significant development, leading to increased electrical efficiency and greater reliability. However, they are not yet totally problem-free. More efforts are required to further improve and optimize biomass grate-firing technology.

This part of the project focuses on the CFD modelling of two industrial biomass grate-fired furnaces (AVV2 & EV3). The grate fired furnace is an overfeed stoker and can be interpreted as a cross-flow reactor, where biomass is fed in a thick layer perpendicular to the direction of primary air. The bottom of the biomass bed is exposed to preheated inlet air while the top of the bed resides within the furnace. CFD modelling of biomass-fired grate furnaces is inherently difficult due to the complexity of the solid biomass fuel bed on the grate, the turbulent reacting flow in the combustion chamber and the intensive interaction between them. Therefore, development and test of biomass fuel bed models is one of the major concerns in this report, as well as the methodology of modelling grate furnaces. General guidelines for CFD simulation of grate furnaces are also given, and demonstrated through the sensitivity analysis.

The CFD results are finally compared to the data from measurement campaigns, which are one of other parts of the project PSO4792 and the detailed results are documented separately. The comparison shows some discrepancies, indicating that further efforts are needed, e.g., to better evaluate the impact of the combustion instabilities in the fuel bed and the impact of the deposit on both the CFD modelling (particularly the definition of the boundary conditions) and the measurement, and to improve some models.