2008 - Numerical simulation of the post combustion zone of biomass firing systems

In the face of the growing demand for energy in our society the utilization of renewable energy sources offers an auspicious method to reduce the net emission of greenhouse active gases. The steam and subsequent electricity generation by combustion of biomass can greatly contribute to a renewable energy supply.

For a maximum contribution to the emission reduction, optimal combustion conditions have to be applied during the thermal utilization. The optimization procedure can be supported in a time- and cost efficient way by the simulation of the process using computational fluid dynamics (CFD).

The proper choice of simulation models is crucial for CFD-simulations. In the present work the submodels for the combustion simulation such as radiation, turbulence and the chemical kinetics system were validated by carrying out simulations for CO/H2/N2-flames and comparison with well-documented experiments.

In this work the gas-phase combustion and flow phenomena in stationary fluidized-bed combustion boilers have been studied in collaboration with Austrian Energy & Environment AG. The main objective of the research was to investigate the conditions in the region of the secondary air injection. This was done by considering several scenarios concerning the position and admission of the nozzles.

Periodic oscillations of the flue gas in the post combustion chamber can be introduced by the conditions in the secondary air injection system. These fluctuations are damped rapidly by choosing a proper admission of the injection nozzles - allowing for steady state and smooth operating conditions in the boiler section.

Figure 1: Height-profile of characteristic parameters in the first pass for various operating conditions. left and centered diagram: area-weighted average of temperature and CO-concentration, respectively; right: standard deviation of vertical velocity.