A comprehensive set of sub-models has been developed addressing the local arrival rates of ash at heat transfer surfaces, the propensity for the ash to stick upon impact and the influence from the deposit on heat transfer properties. The model development was motivated by the severe deposit accumulation rates encountered during straw combustion in grate-fired boilers. The sub-models have been based on information about the combustion and deposition properties of straw gathered from the literature and combined into a single Computational Fluid Dynamics (CFD) based analysis tool which can aid in the design phase of straw-fired boilers. Some of the primary model outputs include improved heat transfer rate predictions and detailed information about local deposit formation rates. This information is essential when boiler availability and efficiency is to be estimated.A stand-alone program has been developed to predict the combustion processes on the grate and the release rate of KCl vapor. These outputs form the boundary conditions for the CFD analysis. The bed model has been validated through comparison with experimental data obtained during batch combustion of straw. It was found that the heat transfer mechanisms have a pronounced influence on the combustion pattern.The combined set of sub-models has been evaluated using the straw-fired boiler at Masnedø CHP plant as a test case. The predicted grate combustion and KCl release patterns are in qualitative agreement with experimental findings and expected trends. Regarding the CFD results, the predicted chemical species concentrations and gas temperatures are in very good agreement with full-scale measurements when the existence of deposits is accounted for. Deposition of KCl vapors has been predicted to dominate in the initial stage of deposit formation, whereas the deposition of silicates is more significant in the later stages. These trends are supported by the structure of deposits collected from the boiler. The accumulation rates of deposits in the later stages are in order of magnitude agreement with values reported for straw combustion in grate-fired boilers. The predicted flyash size distribution is in reasonable correspondence with measurements when the condensation of alkali salts is accounted for.

A comprehensive set of sub-models has been developed addressing the local arrival rates of ash at heat transfer surfaces, the propensity for the ash to stick upon impact and the influence from the deposit on heat transfer properties. The model development was motivated by the severe deposit accumulation rates encountered during straw combustion in grate-fired boilers. The sub-models have been based on information about the combustion and deposition properties of straw gathered from the literature and combined into a single Computational Fluid Dynamics (CFD) based analysis tool which can aid in the design phase of straw-fired boilers. Some of the primary model outputs include improved heat transfer rate predictions and detailed information about local deposit formation rates. This information is essential when boiler availability and efficiency is to be estimated.A stand-alone program has been developed to predict the combustion processes on the grate and the release rate of KCl vapor. These outputs form the boundary conditions for the CFD analysis. The bed model has been validated through comparison with experimental data obtained during batch combustion of straw. It was found that the heat transfer mechanisms have a pronounced influence on the combustion pattern.The combined set of sub-models has been evaluated using the straw-fired boiler at Masnedø CHP plant as a test case. The predicted grate combustion and KCl release patterns are in qualitative agreement with experimental findings and expected trends. Regarding the CFD results, the predicted chemical species concentrations and gas temperatures are in very good agreement with full-scale measurements when the existence of deposits is accounted for. Deposition of KCl vapors has been predicted to dominate in the initial stage of deposit formation, whereas the deposition of silicates is more significant in the later stages. These trends are supported by the structure of deposits collected from the boiler. The accumulation rates of deposits in the later stages are in order of magnitude agreement with values reported for straw combustion in grate-fired boilers. The predicted flyash size distribution is in reasonable correspondence with measurements when the condensation of alkali salts is accounted for.