New methods for NO_x emission reduction in fluidised bed combustion: CFD modelling results from a stationary fluidised bed boiler

摘要

Primary methods are often a cost-efficient way to reduce NO_x emission in furnaces. They can also be well combined with secondary techniques such as selective non-catalytic (SNCR) or catalytic (SCR) reduction systems. Air staging is perhaps the most commonly used primary method as it is easily applicable for different kinds of combustion processes. However, issues related to burnout, corrosion, slagging, and fouling may arise in air staging cases. These issues should be taken into account in designing a proper air distribution system. This also applies for stationary (bubbling) fluidised bed (SFB) combustion. SFB furnaces are capable of firing a variety of high moisture and high volatile content fuels such as different biomasses, peat, sludge, recovered fuels etc. or typically some kind of mixtures of them. The size range of SFB boilers extends from small-scale units of below 10 MW to larger boilers up to some 300 MW of thermal input. Fortum and VTT have developed new air staging methods for primary NO_x reduction in stationary fluidised bed furnaces with simultaneous control of the above-mentioned issues. Computational fluid dynamics (CFD) was used to systemically study features of a proper air system design. In the following CFD simulation results from a BFBs furnace related to those studies are presented and analysed.