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BOILER EMISSIONS - 1

Combustion equipment such as boilers generate pollutants such as oxides of sulfur (with sulfur containing fuels),unburned hydrocarbons,CO and NOx.The latter two are considered important,particularly in gas fired boilers and gas turbine plants and regulations are in place indifferent parts of the world to limit their emissions.Effect of various variables on their formation is key to understanding some of the measures used to minimize their emissions. In boilers,excess air is an important factor and its impact on CO and NOx is shown below.As the excess air increases,CO reduces,while NOx increases up to a point,above which the combustion temperature is reduced and hence NOx formation reduces.Large amount of excess air increases the boiler size and also lowers its efficiency.Hence excess air has to be within a range.Combustion temperature impacts NOx formation significantly;fuels containing hydrogen,which have a high combustion temperature,for example,generate large amount of NOx. Flue gas recirculation is a commonly used method to lower NOx in addition to methods such as steam or water injection and use of low NOx burners.Table below shows the effect of FGR on combustion temperature.It may be seen that a large reduction in combustion temperature and hence NOx (on the order of 50-70 %)can occur with this technique. Staged combustion,in which air or fuel admission into the burner is staged or admitted downstream of the initial flame front,is also widely used to lower NOx. When large reduction in NOx or CO,on the order of 90 % or more is desired,SCR or catalysts are used.The catalysts speed up the reaction of ammonia with NOx to convert it to nitrogen and water vapor.CO is converted to CO2.
 Package boilers firing natural gas or fuel oils do not use air heaters as heat recovery equipment as it increases the combustion temperature,which can lead to higher NOx. Hence economizers are invariably used as heat recovery equipment. The use of completely watercooled boiler furnace design(click here for a picture of completely watercooled furnace design by ABCO)  minimizes NOx formation by having a cool environment for the flame front,where a large amount of NOx is formed, while refractory lined front boiler walls reradiate energy back to the flame,thus increasing the NOx formation.Refractory lined boilers have other concerns such as maintenance,longer startup periods and leakage of combustion products from the furnace to outside,resulting in casing corrosion. The welded partition wall in a completely water cooled furnace,as compared to skin cased design,also minimizes CO formation by ensuring a leak proof envelope for the flame and ensures that the hot combustion products travel to through the entire furnace length before entering the convection sec tion..Leakage of hot flue gases from the furnace to the convection section,which is likely in a skin cased design, increases CO formation and also lowers the boiler efficiency.

 Effect of FGR on combustion temperature
fuel nat gas nat gas nat gas # 2 fuel oil # 2 fuel oil # 2 fuel oil
Excess air,% 15 15 15 15 15 15
FGR,% 0 15 30 0 15 30
comb temp,F 3227 2892 2619 3354 2994 2713

 

   Techniques used to lower NOx in boilers and HRSGs

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