Methods for NOx emission reduction in BFB combustion:: A CFD study

Fortum and VTT have developed methods for primary NOx reduction in BFB combustion with simultaneous control of CO burnout as well as bed and furnace exit gas temperatures to minimize slagging, fouling and corrosion issues both in lower and upper part of the furnace. Computational fluid dynamics was used as a tool to systemically study features of proper air system design in such a NOx reduction strategy. In this context the CFD code Fluent and VTT's specific sub-models for BFB furnaces were applied to simulate combustion and NOx in a BFB boiler with a capacity of 175 MW fuel. The CFD results show that if special attention has to be paid to temperature control and slagging issues in the near bed region, it's beneficial to choose secondary air elevation differently from the case with the main focus on NOx reduction. Minimum NOx emission is achieved by optimizing secondary air elevation and lower furnace air distribution. The latter is dependent on fuel properties and it can be adjusted with an additional air feed into the near bed zone. Additional air introduction combined with well-designed secondary and tertiary system to provide good mixing ensures efficient heat transfer (low furnace exit gas temperature), complete CO burnout and even temperature distribution in the upper furnace in parallel with favourable NOx performance