NO(sub x) reduction in pressurized fluidized-bed combustion

摘要

Batch combustion experiments were performed in a small bubbling fluidized-bed reactor with the objective of establishing the cause of reduced NO(sub x) emissions from pressurized fluidized bed combustion (PFBC). All variables except for pressure were kept constant in the experiments: fuel batch size, for example, was the same in experiments performed at three pressure levels (0.2, 1 and 2 MPa). Two different types of experiments were conducted: one using air diluted with nitrogen (4.5% O(sub 2)) for the purpose of determining the conversion of fuel N to NO(sub x), and the other with NO-doped diluted air (800 ppM NO, 4.5% O(sub 2)) for the purpose of determining the reduction of bulk-gas NO(sub x) by the burning fuel. A large excess of combustion air was used in all experiments so as to keep the bulk-gas composition relatively unchanged by combustion products. Six different fuels were studied: a bituminous coal, coke prepared from the same coal, three specialty cokes (one of which contained 10 wt % N) and graphite (0%N). The straight-air combustion experiments showed that the conversion of fuel-N to NO(sub x) dropped with increasing pressure (at constant fuel concentration in the bed). The NO-doped combustion experiments showed significantly increased NO(sub x) reduction with increased pressure. This effect was particularly strong between 0.2 and 1 MPa, and was found to continue between 1 and 2 MPa for some of the fuels but too saturate for others. These results imply that an important phenomenon for low PFBC NO(sub x) emission is the reaction of NO(sub x) with reducing N-containing species at the burning fuel surfaces. A simple kinetic model involving parallel paths for the fuel-N is suggested as an interpretation of the results. The NO-doped experiments showed increased N(sub 2)O formation relative to the straight-air experiments implying that N(sub 2)O was a byproduct of the NO(sub x) reduction reaction where N(sub 2) was the main product.