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Show 6 IV. Overview of NOx Control Technologies NOx control technologies can be divided into two primary types, co~~u~~ modifications and post-combustion controls. As the name implies, combustion modification controls act through changes in the, combustion pr.ocess, and have been very effect i ve' at contro 1 of NOx' emi ss ions f'rom fos's i 1 fue l'S. Post-combust ion controls do not change the combustion process, and can be applied to other than combustion sources. In general, combustion modification controls rely on changing the combustion process to reduce the formation of NOx , while post-combustion controls destroy the NOx that has been formed during the combustion process; this generalization does not hold for reburning, as explained below. In addition, the discussion below does not provide fuel-specific information, but is only a very general overview. For many of the technologies discussed below, the type of fuel can make a significant difference in the appl icabi 1 ity and performance of the particular technology. A. Combustion Modification Controls Combustion modification controls are based on changing the combustion process to reduce the formation of nitrogen oxide (NO) or, in the case of reburning, a combination of reducing formation of NO and destroying ' NO formed in the primary combustion zone. Preventing the formation or achieving the destruction of NO is important, since NO comprises most of the NOx emitted by combustion processes. Combustion modifications can be as simple as reducing the level of excess air or can involve significant redesign of the combustion system. Combustion modifications are designed to achieve two primary objectives: (1) to minimize the peak flame temperature and thereby minimize the reaction of atmospheric nitrogen with atmospheric oxygen to form NO; and (2) to force solid fuels to devolatilize in an oxygen deficient atmosphere to minimize the oxygen available to react with ni trogen found in the fue 1 . In both instances, the net resul tis to reduce the format i on of NO. The primary method used to accomp 1 ish these object i ves is staging, or an incremental mixing of fuel and air, the most ,common form of which is staging of the combustion air (as opposed to staging of the fuel). The simplest forms of combustion modification are low excess air (LEA) operation, burners out of service (BOOS), and biased firing. These modifications , typically do not require any modification to the boiler, but are achieved by altering the air and fuel flows in order to reduce the oxygen available to react with fuel-bound nitrogen. LEA simply reduces the level of excess oxygen available by operating a boiler as near to stoichiometric conditions as possible, with the |
