CFD-aided modelling / calculation of pellet furnaces
Biomass furnace developments are often, especially for small-scale applications, based on empiric data and connected with long development time and high efforts for test runs (often with several prototypes). CFD simulations are able to reduce both development time and efforts for tests and reduce the required number of prototypes. Moreover, the reliability of the developments can be increased (
CFD).
Despite the complex processes during combustion of solid biomass on a grate and in the turbulent, reactive flow in the combustion chamber BIOS successfully developed and optimised the combustion chambers of several biomass furnaces using CFD simulations. Large (10 to 30 MWth), medium (0.3 to 10 MWth) as well as small-scale (< 300 kWth) furnaces have been developed and optimised, among them several pellet furnaces.
- Iso-surfaces of flue gas velocities [m/s] in a vertical cross-section through the axis of a small-scale pellet furnace (left: basic design; right: optimised design)
The aims of CFD simulations of pellet furnaces can be summarised as follows:
- Achievement of an efficient air staging as a basis for a staged combustion and consequently to reduce NOx emissions.
- To ensure optimised mixing of flue gas with secondary air to achieve a complete burn out of the flue gas at nominal and part load (low CO emissions).
- Improved utilisation of the furnace volume by optimisation of the furnace geometry.
- Reduction of local velocity and temperature peaks to minimise erosion and deposit formation.
- Evaluation of the influences like load condition, fuel moisture content or air staging on the combustion as a basis for optimisation of the control system.
By means of CFD simulations of pellet furnaces the following activities can be done:
- Design and optimisation of furnace and boiler geometry.
- Design and optimisation of boiler cleaning systems.
- Design and optimisation of the secondary air nozzles.
- Efficient reduction of CO and NOx emission in nominal and part load operation.
- Evaluation and optimisation of operating conditions of furnaces and boilers with regard to efficiency, plant availability, part load behaviour and multi-fuel use.
- Reduction of local temperature peaks by furnace cooling or improved operating conditions.
- Simulation and reduction of deposition and material erosion tendencies caused by fly ash.
- Calculation of residence times and flue gas temperatures as a basis for the modelling of formation of fine particulates and deposits.
Pellet furnace development supported by CFD simulations shows clear advantages concerning reduced emissions, higher efficiencies, lower volumes, reduced material wear, increased plant availability, reduced development time and reduced test efforts as well as increased reliability of the developments, which has already been proven by many practical experiences. CFD simulation for the development and optimisation of pellet furnaces has already successfully been introduced to the market, contributes to a better understanding of the combustion processes in the furnace and gains increasing importance as an innovative furnace development tool.
Respective CFD-based developments and optimisations have already been performed for well-known pellet furnace manufacturers, thereunder KWB, Viessmann, Windhager and Guntamatic.