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Show boilers. Like the previously discussed tests, these are being performed in ABB C-E's Boiler Simulation Facility (BSF). The BSF allows for carefully controlled experiments at a sufficient scale to replicate the design requirements for fuel and air injection to achieve appropriate mixing for the reburn process in a full scale unit . The reburning process is an in-furnace NO control technology which involves diversion of some of the fuel ana combustion air flows from the main burners for injection downstream . ( 7Jn general (see Figure 13 ) , the reburning process involves three zones : 1. Primary Zone : Under overall fuel lean conditions , the major fraction of the fuel is introduced in the main heat release zone. The concentration of NO exiting from this zone will be reduced in the reburn zone. x 2. Reburning Zone : The reburning fuel is injected downstream of the primary zone to create a fuel rich reduction zone . The nitrogen entering this zone comes from four sources : NO , N 2 , N 2 0 leaving the primary zone, and the fuel nitrogen in t~e reburning fuel . According to many investigators, these fuel nitrogen species apparently decompose initially to produce HCN which is then converted to NH 3 , NH 2 , . .. , and N species . These amines can react either with NO or other amines to produce N2 or with the 0 and OH to produce NO . If the conversion to N2 is not complete, some nitrogen rea~tive containing species such as NO , char nitrogen, NH 3 , and HCN would persist to the end of this zone. In order to maximize NO reduction by reburning, it is necessary to minimize the total reactive nitrogen species leaving the reburning zone. 3 . Burnout Zone: In the burnout zone, air is added to produce overall lean conditions in order to oxidize the remaining fuel , but under these conditions any reactive nitrogen is mainly converted to NO . It is vital that O 2 levels in the burnout zone be minimiz~d to prevent significant increases in NO emissions during this final stage of the process . x As can be seen in Figure 13, two discrete combustion stages are created in the furnace where the combustion stoichiometry of each stage is independently controlled. Many researchers have shown that by adjusting the combustion stoichiometry in different stages within the furnace , the emissions level of NO is generally lower than that achieved with other combustion modificati~n techniques . At the time of paper preparation several series of reburning test trials have been completed, all utilizing natural gas as a reburn fuel and firing heavy fuel oil as the main fuel. Analysis of results is not complete , however preliminary results indicate that greater than 50% NO reductions have been achieved . The nominal BSF reburn system design cha~acteristics are shown in Table 1 . 6 |
