Fuel staging (burner out of service (boos), fuel biasing, reburning, or three-stage combustion)
One form of fuel staged combustion is the procedure known as burner out of service (boos). The technique involves shutting off the fuel flow from one burner or more to create fuel rich and fuel lean zones achieving some NOx emission control (10%). The technique is not widely used in pulverised coal-fired plants (2 units, 350 MWe).
Another method of fuel staged combustion is fuel biasing. In fuel biasing, combustion is staged by diverting fuel from the upper level burners to the lower ones or from the centre to the side burners. The aim is to create fuel-rich lower or central zone and a fuel-lean upper or side zone in order to achieve complete burnout. The technology lowers flame temperature and improves the balance of the oxygen concentration in the furnace. NOx emissions may be reduced by up to 30% using this technology. This technique is used in 13 pulverised coal-fired units on a total capacity of 2.7 GWe.
Reburning is another form of fuel staged combustion. It is a three-stage (zone) system, which is why sometimes it is referred to as a three-stage combustion process:
- primary combustion zone;
- reburn zone; and
- burnout zone.
In the primary zone, coal is fired through conventional or low NOx burners generally in low excess-air conditions to reduce initial NOx formation. A secondary fuel is injected or blown into the upper section of the furnace. This is a secondary sub-stoichiometric reburn zone without combustion air. Re-circulated flue gas is sometimes used as a carrier for the reburn fuel. Currently natural gas is the most widely used secondary fuel although coal and oil are currently being demonstrated. A primary function is to provide effective mixing of the natural gas with the bulk flue gas in the reburn zone. The secondary fuel breaks down at this stage to produce hydrocarbon fragments, which react with the NOx produced in the primary combustion zone and reduce it to atmospheric nitrogen. In the third, burnout zone, the gases exiting the reburn zone undergo additional combustion with overfire air. This final combustion stage is necessary to consume the carbon monoxide and unburnt hydrocarbons leaving the reburn zone. In general, 10-30% of the total heat input can be used as the secondary fuel. The process creates fuel-rich oxygen-deficient reducing zone, which decomposes the NOx, formed in the primary combustion zone. The technology is capable of achieving relatively high NOx reduction (up to 70%). The cost of the secondary fuel influences the operating costs considerably. Hence reburning costs can be higher than other primary measures for NOx control.
Reburning is a technology applicable to all types of coal-fired boilers. In addition, in some boilers such as cyclones, the typical combustion delaying techniques of reducing excess air to optimise combustion cannot be applied as operation necessarily occurs under excess oxygen conditions to avoid problems such as tube corrosion. Reburning is a suitable and relatively efficient technology which is moderate in cost and can be favoured in this type of boiler.
There are additional advantages in using natural gas with this technology due to the effects of the natural gas burning on other pollutants formed during pulverised coal. Particulate matter, SO2 and CO2 are also reduced in addition to the reduction in NOx emissions combustion. Since natural gas contains no ash, the particulate loading is reduced in direct proportion to the amount of coal displaced. A similar argument leads to the conclusion that SO2 emissions are also reduced, also in direct proportion to the amount of coal displaced. Although CO2 is not normally considered a pollutant it is identified as a greenhouse gas and therefore any reduction in CO2 is also appreciated. The CO2 reduction is due to the greater ratio of hydrogen to carbon in natural gas, compared with coal. A 15-20% of coal substituted by natural gas results in a reduction of 6-9% in CO2 emissions. Factors affecting installation of reburning technology are the availability of natural gas in close proximity and the coal/gas cost differential.
Click on relevant text for details of other primary measures for NOx control:
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