(146d) Reburning by Mixed Fuels of Waste Materials
A multi-functional, mixed fuel containing natural gas for NO reduction, and lignite ash for reducing the reburning intermediate, HCN, has demonstrated remarkably high efficiency in reburning (AIChE J., 47, 2781 (2001); Fuel, 85, 1781 (2006)). While the price of natural gas has increased and fluctuated significantly in the last five years, there is an incentive to find substitutes for natural gas. Effective substitute for lignite ash is also desirable because of the large quantity required for the target HCN conversion.
Several combinations of hydrocarbon substances and minerals have been chosen as the main reburning fuel and the HCN reducing agent in a reburning apparatus, respectively. This apparatus includes two furnaces in series, one for simulated reburning, and, the other, for combined reburning and burnout stages. For the investigation of reburning intermediates, HCN and NH3, only one furnace is kept at temperatures from 1150 to 1400°C with residence time 0.2 s. When burnout stage is used for the investigation of overall NO reduction efficiency, the second furnace is kept at 1150°C. The stoichiometric ratios of reburning (SR2) and burnout zone (SR3) vary in the ranges 0.8 to 1.0 and 1.1 to 1.3, respectively.
Results indeed show that a wide range of mixed-fuels possess remarkably high overall NO reduction efficiency, up to 85%, from the two-stage experiments at high reburning stoichiometric ratios, 0.95. The nitrogen speciation in both stages is very similar to natural gas. Both components are widely available waste materials. An efficient HCN conversion catalyst is identified. About 60 to 200 metric ton of this catalyst is needed for a 172 MW bituminous coal-fired power plant, depending on the main constituent in the reburning fuel, and it does not cause fouling or slagging problems in the coal-fired boilers. Both components of these substitutes are widely available at low costs. For the fuels showing low overall NO reduction efficiencies, char-nitrogen conversion to NO in the burnout zone is a limiting factor.