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Show 1. REBURN I NG 1. 1 P r i nc i pie With a pulverized-coal burning boi ler, almost 100% of NOx is formed with in the primary combustion zone. By blowing natural gas into the top part of the primary combustion zone, el iminating excess oxygen and creating a reducing atmosphere, mathane radical generated in the furnace wi I I react with NOx under the oxygen-starved atmosphere and wi I I convert to molecular nitrogen. If a certain residence time is obtained under high temperature, NOx removal effect wi I I be enhanced. If the top of primary combustion zone can attain a reduction atmosphere of 0.9 air ratio. NOx removal rate wi I I reach 50Y.. This is how the reburning system works with coal-burning boi lers. With MSW-burning furnace with stokers, NOx formation at the primary combustion zone (immediately above the stoker) is less than 50% of the total amount and the balance of more than 50% is considered to be formed after the secondary air inlet. Therefore, natural gas blown into the top of the primary combustion zone, rathe r than dest roy i ng NOx direct I y, lowe rs oxygen I eve I, and by de I ay i ng the secondary air input, wi I I minimize conversion of HCN and NHi to NOx. If a certain residence time can be secured at the reburning zone, HCN and NHi can be converted to N. Also, secondary air can be suppl ied in stages at the upper burnout zone which enables the un-burned gas to be reburnt, thus minimizing the reconstruction of NOx. Due to the above, "Methane DeNOx" would be a better terminology than "Reburning " to describe this process. Figure 1 shows the principles of the deNOx based on this technology. The chemical reactions are listed below. • Rebu rn i ng Zone CnHm+02 -+ Cn'Hm'+CO+H 20 NO+Cn'Hm'-+ Cn"Hm"+N 2 +H 20+CO or NO+Cn'Hm'-+ CN"Hm"+NHi+H 20+CO • Bu rnout Zone Cn"Hm"+02-+ H20+C02 2CO+02 -+ 2C02 NHi+ O2 -+ N2 + H20 NHi+ O2 -+ NO + H20 Radicals o OH H Figure 1 MECHANISM OF deNOx BY REBURNING 2 |
