Modeling fixed-bed pellet combustion of fuels with varying ash contents
Paper in proceedings, 2019
Ash transformations occurring in a fuel bed when combusting “difficult” solid biomass fuels risk to initiate significant and potentially catastrophic disturbances that must be avoided and contained for a technique to be commercially viable. The current work is concerned with experimental and numerical investigations of fixed-bed combustion of challenging woody biomass pellets, in terms of combustion behavior and propensity to cause ash-related disturbances in the conversion process.
Experimental investigations of combustion behavior, including ash and slag characterization, were performed for standard wood pellets and for poplar pellets. The experiments were carried out in an insulated 60 L bench-scale stationary-bed reactor. For each experiment, approximately 25 kg of pellets was used, and the bed was ignited from the top while primary air was injected from below. Additional experimental data from forest residue pellet combustion in the same reactor was also available. All experiments resulted in partly sintered ash and no severe slagging (ash melt covering significant portions of uncovered fuel) was observed.
A conceptual model for ash transformations was developed and implemented into a mesh-based biomass fuel particle model. The particle model essentially treats the thermochemical conversion process as a reactive variable-volume 1D transient heat conduction problem, and local particle properties are mass-weighted averages of the four basic constituents (moist wood, dry wood, char and ash). The conceptual model for ash transformations describes the properties of the ash layer at the single-particle level via a model for ash porosity variations, by which the effective mass diffusivity and effective thermal conductivity through the ash are calculated as functions of porosity, which in turn is a function of particle temperature. The particle model with the ash transformation model is implemented into a bed model in ANSYS Fluent where the bed is described as a porous medium.
The performance of the particle model has been validated against single-particle experiments with satisfactory results with regard to temperature and mass loss histories. Front propagation rates, temperature levels and gas phase compositions during fixed-bed combustion are in acceptable agreement at low to medium air-flow rates. Close to the stoichiometric limit, the front propagation is overpredicted, which negatively affects the quality of the prediction for the gas phase composition. The transition from an O2-limited to a kinetically limited regime is captured by the model.
As no catastrophic slagging was observed in the experiments, it was not possible to tune the parameters of the ash transformation model to experimental data. Nevertheless, numerical simulations illustrate how ash transformations occurring during fixed-bed conversion may extinguish the combustion by encapsulating the char and limiting the O2 transport, if the temperatures are high enough to trigger the transformations in the first place. It remains to be seen how these predictions compare to experiments with other difficult biomass fuels.