1. Background

1.1 The Technologies

Of the combustion generated atmospheric pollutants, NOx has received by far the greatest attention in the last two decades. Although the importance attached to other pollutants is becoming more and more important, NOx is still of central importance.

This Combustion File is intended to overview the main commercially available technologies for NOx control in large fossil fuel utility boilers by modification of the combustion process. Such boilers are typically used in the Power Generation industrial sector.

It relates to a series of Combustion Files, which is presently under development, which will detail each technology individually with the appropriate link in the
Related Combustion Files section at the end of this file.

The overview is predominantly based on pulverised coal technologies but can be applied to other fossil fuels fired in large radiant furnaces.

The technologies to be reviewed for this Industrial Sector are:

• Low NOx Burners
• Flue Gas Recirculation
• Separated Overfire Air/Air Staging
• Gas Reburn
• Coal Reburn

1.2 NOx chemistry

To understand the operation of low NOx technologies, it is essential to appreciate the NOx formation and reduction chemistry. The effects of NOx emissions and the NOx formation reactions needed for the technologies considered in this CF are outlined in
CF66 and CF125.

2. The Technologies

2.1 Low NOx Burners

Low NOx burners (LNB) are a mature, well proven technology for NOx control in both wall fired and tangentially fired furnaces. They are commercially available with full performance guarantees, and there has been a significant uptake of the technology worldwide. The principles of LNB operation and examples of commercially available LNBs for wall fired boilers, are given in
CF126 and
CF127 respectively.

2.2 Flue Gas Recirculation

Flue gas recirculation is an established practise for the control of reheat steam temperature in large utility boiler plant. In this situation it is usual for the recycled flue gas to be supplied through the furnace hopper. By increasing the total mass of flue gas with the addition of a largely inert diluent, whilst maintaining the same thermal input, the bulk furnace radiating temperature is reduced, which can have a significant effect also on NOx emissions.

When utilising [GLOSS]FGR[/GLOSS] for NOx control it is usual to extract the flue gas from downstream of the particulate control equipment. The recycled flue gas can be introduced to various locations in the furnace, and this can have a considerable impact on the achievable NOx reduction.

The use of FGR for any purpose other than reheat control can have a significant impact on the boiler operation. The advantages and limitations of this NOx control technique
may be found in

CF131.

2.3 Separated Overfire Air/Air Staging

Separated overfire air ([GLOSS]SOFA[/GLOSS]), also known as furnace air staging or two stage combustion, is a well established technology for NOx reduction in fossil fuel fired furnaces. The process is equally applicable to both wall and tangentially fired plant – indeed air staging is an inherent part of tangentially fired combustion systems.

In the context of wall firing, SOFA involves the diversion of a proportion of the main combustion air from the burners to separate overfire air ([GLOSS]OFA[/GLOSS]) injectors located at a discrete elevation above the main burners.

Early applications of this technology were undertaken in conjunction with conventional burners, more recently SOFA has been combined with LNB’s. Further details
are in

CF128.

2.4 Gas Reburn

Gas over coal reburning is a comparatively new technology, which combines the principles of air and fuel staging. In this techniques already formed NO reacts with hydrocarbon radicals to form atmospheric nitrogen.

In theory any hydrocarbon fuel could be used as the reburn fuel, but there are a number of benefits in using natural gas (Macphail et al,
1997). Further details are in

CF129.

2.5 Coal Reburn

Coal over coal reburning is a recently demonstrated NOx reduction technology, which integrates the principles of air and fuel staging in the furnace. In this technology pulverised coal is used as an alternative reburn fuel. It can be applied to systems having conventional or low NOx burners, but it is preferable to install LNB’s as this leads to the lowest overall NOx emissions. Further details
are in

CF128

Sources

[1] Michelfelder S & Leikert K. The Stage Mixing Burner and Operating Results of Pulverised Coal Burners for Low NOx Emission. VGB Kraftwerkstechnik 60 (2) 95-103. (1980)
[2] Lam KB, Wilkinson PJ & King JL. Application of Low NOx Axial Swirl Burners to Castle Peak B Power Station. 9th CEPSI Conference – Hong Kong. (1992)
[3] Macphail J, Rhine JM & Hughes M. The Demonstration of Gas Reburning Technology on a 600 MWe Boiler at Longannet, Scotland. (1997)
[4] Macphail J, Rhine JM & Hughes M. Proceedings “4th International Conference on Technologies and Combustion for a Clean Environment”, Scotland. (1997)
[5] Hesselmann G & Chakraborty R. Coal Over Coal Reburn – The Applicaton of Pilot Scale Test Results to Utility Plant Design. Power-Gen Europe 98. (1998).