Deriving guidelines for the design of biomass grate furnaces with CFD analysis ― a new Multifuel-Low-NO_x furnace as example

摘要

In the present work, guidelines for the design of grate furnaces in the typical size range of Austrian district heating plants (0.5 ― 10.0 MW_(th)) were derived from CFD analysis. For this purpose, a case study systematically investigating all important parameters for a successful design of a Low-NO_x furnace was performed. The simulation of solid biomass combustion on the grate and turbulent reactive flow in the combustion chamber was performed via forward coupling. An empirical model was applied for thermal decomposition of the solid fuel. The calculation results were used as boundary profiles for subsequent CFD simulations of the turbulent reactive flow in the furnace. CFD modelling was based on the Realizable k-e Model (turbulence), the Discrete Ordinates Model (radiation) and the Eddy Dissipation Model (turbulent combustion) in combination with a global 3-step reaction mechanism considering the species CH_4, CO, CO_2, H_2, H_2O and O_2. NO_x formation and reduction were not considered explicitly with a CFD postprocessor. The furnace was designed under Low-NO_x aspects, i.e., with air staging technology and a temperature controlled primary combustion zone obtained by flue gas recirculation. As a result of the case study, a Low-NO_x biomass grate furnace for a broad fuel assortment (waste wood, wood chips and bark) was developed and implemented as a pilot-scale plant. Moreover, the modular structured CFD based furnace development allows further combustion chamber geometries to be created depending on the application. Furthermore, guidelines for the design of biomass grate furnaces were derived. In general, a considerable reduction of investment and operating costs is possible by a compact furnace design as well as by reduced air and flue gas fluxes in the furnace. This can be achieved by an appropriate design of the nozzles for air and flue gas injection as well as by adjusting the geometry of the combustion chamber (e.g. barriers) in order to improve the mixing of unburned flue gas and air as well as the utilisation of the furnace volume. A staged secondary air supply leads to a strong reduction of CO emissions, but is linked with increased flue gas temperatures between the nozzles for secondary and tertiary air injection. The higher temperature peaks due to improved mixing conditions have to be considered concerning ash slagging and deposit formation. For dry fuels (e.g. waste wood) additional measures like furnace cooling and enhanced flue gas recirculation are necessary. A scaling of an optimised furnace geometry should be based on approximately constant ratios of the characteristic Reynolds numbers.