Reduced order model of a lime kiln for fuel switching

The lime kiln is an essential part of the causticizing loop in the kraft pulp mill. It is also the last operating unit that routinely fires fossil fuels like natural gas and fuel oil. Displacing these fossil fuels with biomass like bark or sawdust is done, as is firing lignin extracted from black liquor. Air-blown gasification and fast pyrolysis are thermal conversion technologies to produce a higher quality biofuel. A reduced-order model has been developed to model both the gas phase and particle phase in a lime kiln. The objective of the model is to study the effect of fuel switching on the performance of the lime kiln and the quality of the solid product. Initial results of the model are in the expected range, but industrial data are needed for validation. The first application of the model will be to a full-scale trial co-firing natural gas with sawdust or kraft lignin.