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Overfire air systems

Overfire air (OFA) systems or boosted overfire air systems (BOFA) are a common feature on many of today's fossil-fuelled power stations. Often they were introduced to meet a new level of NOx reduction compliance that could not be achieved through burner modifications alone. In addition, OFA or BOFA provided a low NOx solution that meant the installation of costly selective catalytic reduction (SCR) systems or selective non-catalytic reduction (SNCR) systems to achieve the same result could be avoided.

RJM has considerable experience in installing new OFA and BOFA systems and can deliver incremental NOx reductions of up to 40%, through the implementation of multiple OFA ports, using either forced draft fan pressure air or boosted air delivery systems. RJM's recent experience in this field is based on fine-tuning existing OFA / BOFA systems, in conjunction with either existing low NOx burners or with new RJM Ultra-Low NOx burners. RJM's recent NOx reduction programme at SSE Ferrybridge is a good example of how critical it was to get the BOFA system operating effectively to ensure the plant could a) meet its new low NOx targets as set out by the Large Comubustion Plant Directive and b) run efficiently with correct boiler and burner dynamics. Read full article here.

Conversely, at another plant in Eastern Europe, following an engineering study that showed its existing OFA systems were in fact not well-suited for the boiler and burning configuration, RJM recommended that the OFA system be removed and that a different technology for NOx control be adopted. Once this had been carried out, the plant  reached its emissions targets immediately. 

For new OFA or BOFA installations, RJM will design a system to best suit the project objectives. CFD analysis will ensure a properly designed OFA system prior to installation. RJM's OFA systems are individually designed to suit any make of boiler and combustion system.

RJM's solution will always endeavour to minimise cost but maximise NOx reduction, whilst maintaining zero or limited impact on carbon carryover and boiler efficiency. The final configuration will be dependent on many factors, including OFA jet penetration to ensure burnout of combustibles, the physical space available around the boiler, access to the OFA locations, the fan and windbox air pressures available, constraints on furnace CO concentrations and excess oxygen limitations.

Using our bespoke CFD modelling service together with our highly experienced team of combustion specialists, RJM is able to design, manufacture and install new OFA and BOFA systems or modify and optimise existing systems and has a number of satisfied clients and reference sites where this has been done.


Scale model testing by RJM to measure effectiveness of  windbox and OFA port at AES Kilroot.