Semi-industrial scale investigations into NOx emissions control using coal-over-coal reburn techniques

Reburning is considered as a technique that would enable the present and future legislation for boiler emissions to be met, as well as potentially providing NOx reduction with reasonable capital expenditure. The technique has not been applied extensively in new or retrofit applications, and this may be due, in part, to the complexity of designing a new boiler or modifying an existing one and a scarcity of research information available to aid in this design. Other factors preventing the implementation of the reburning technique are the perceived problem of carbon burnout and the flame stability over a wide range of boiler operating conditions. Reburning has been demonstrated to be effective in a semi-industrial scale facility, and the results confirm that the mechanism is suitable for retrofit applications. NOx reductions >77% were achieved, relative to a type-II classification flame, when reburning high- and medium-volatile coal in various combinations. Overall, the most promising combinations for reburning were a low- or high-volatile coal as the primary fuel, and a high-volatile, highly reactive coal as the reburn fuel. Typically, reburn fuel fractions >10% were required for effective NOx reduction in combination with a primary zone stoichiometry less than or equal to 0.9.