Grate firing is acknowlegded as one of the oldest firing systems for solid fuels. Nowadays, grate firing systems are state-of-the-art for the thermal utilisation of residential waste and solid biomass on industrial scale. The reasons for that are their low demand on fuel quality, their flexibility and robustness and the relatively low investment and operational costs. Despite that, optimal technological and economic operation is still challenging due to varying fuel properties, increasing costs and legal restrictions. Following the renewable energy policy of the European Union and the growing biomass utilisation, further intensification of grate firing systems for heat and power generation can be expected.
Boosted by the strong evolution of computer technologies, computational fluid dynamics (CFD) has proven to be an effcient and reliable engineering tool for the calculation and optimisation of different firing systems. Here a numerical model representing the relevant physical and chemical subprocesses in conjunction with the thermal utilisation of solid fuels in grate firing systems is developed. Being contrary to most other models, the manifold hydrodynamic and thermal interactions between the fuel bed and the freeboard are accounted for. To keep computing time within a reasonable limit, the fuel bed is implemented based on a multiphase treatment considering two interacting continuous media (known as 'Eulerian' phases).
Sensitivity of the results against crucial model parameters is evaluated and reviewed with respect to their plausibility. Extensive validation of the model results is conducted in comparison with detailed measurements from real grate firing facilities. Independent of the facility's scale, the model proves its ability to reproduce temperatures and gas concentrations in terms of absolute values and profiles.
Using a NEC SX supercomputer platform, computational times of less than 24 hours for a full scale grate firing application can be achieved. Thus, another important prerequisite is fulfilled to enable the usage of the model as an engineering tool for analysing and optimising grate firing systems.