After plans to decommission one of its gas-fired power boilers changed, engineers at a refinery found themselves required to meet stringent emissions reductions on the 500,000 lb/hr Riley Turbo boiler. If the company wanted to continue operating the boiler, NOX reductions of approximately 95 percent were required, according to emissions levels set by the local air-quality regulatory body. The required 0.04 lb/MMBTU NOX limit was one that had never been met on this type of boiler without the use of back-end cleanup equipment.
The boiler design was developed as a means to reliably burn petroleum coke, a solid fuel with lower volatility than most pulverized coal, by providing high retention time to ensure complete combustion. The furnaces employ a venturi-shaped furnace with opposed-fired rows of directional flame burners. The burners are angled down toward the bottom of the refractory-lined furnace, which also was designed to fire gas and oil. Above the burners, a row of separate overfire air ports supply additional air to ensure complete burn-out of combustibles. The conditions created in the furnace to ensure adequate burn-out of the petroleum coke also created the high temperatures and long residence times that result in extremely high production of thermal NOX when firing gas.
During the evaluation of NOX control strategies, several different options were presented, including replacement of the existing burners with low-NOX circular registers, flue gas recirculation, and the addition of a flue-gas treatment system utilizing selective catalytic reduction. Two things the plant wanted to avoid in retrofitting the boiler were modifications to the boiler tube walls and the use of selective catalytic reduction.
The boiler burner group of Tulsa-based John Zink Co. designed a burner retrofit kit that would “plug” into the existing burner openings and utilize a combination of NOX control techniques. The burner group determined that all of the refinery's other goals, such as restoring the lost capacity on the unit, returning it to full nameplate rating, and allowing the flexibility of firing a variety of refinery waste gases in the unit, could also be accomplished while still meeting the emission limits.
Using a proprietary emission modeling software, the final ratios of fuel-induced recirculation (FIR), flue gas recirculation (FGR), and steam injection, which were utilized in conjunction with the new burner kits, were optimized to provide the required emissions reduction at the lowest possible operating cost. Once the final design ratios were determined, the custom-engineered retrofit kit was designed to replace the existing burner's fuel-firing components. The new burner kits were designed to fit right into the existing fuel component locations and bolt directly to the existing windbox.
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