Flue gas recirculation (FGR) is a well-known tool for a variety of applications in boilers. These scenarios range from nitrogen oxides (NOX) control for boilers firing fuel oil or natural gas burners to controlling bed temperature for bubbling fluidized bed (BFB) boilers. However, Jansen recently used FGR in a novel manner while supporting a client in the wood products sector.
The client had challenges managing consistent carbon monoxide emissions from one of its boilers. They needed to improve the boiler’s availability while maintaining carbon monoxide (CO) compliance to consume an increasing wood derived fuel (WDF) inventory, provide flexibility with other boilers during maintenance outages, and for power generation. Jansen has considerable experience and the capability to help our clients achieve these goals. But a unique challenge emerged in this case.
During the evaluation of the boiler to determine the specifications of a future overfire air (OFA) system upgrade, the small furnace size and relatively low overall air demand posed this challenge. After allocating air to the combustion air levels (undergrate or UGA, and OFA), fuel distribution, and ash reinjection, the excess air level was significantly higher than required for a stoker fired biomass boiler. The high excess air coupled with the small furnace volume dramatically reduced furnace residence times which diminished the combustion conditions in the furnace.
The computational fluid dynamics (CFD) model of Jansen’s proposed OFA upgrade arrangement predicted CO levels roughly 25% below permitted limits. However, these results considered an ideal fuel and stable operating conditions, which are not always achievable during operations. To provide additional operational margin, Jansen decided to investigate using FGR as the transport medium for the pneumatic WDF distributors and ash reinjection systems. Using the lower oxygen (O2) concentration in the FGR stream at these levels allowed for a further reduction in overall excess air. Our modeling predicted CO emissions to be 65% lower than with a fresh air only Jansen OFA system.
Using FGR is an effective strategy to minimize CO concentrations, especially when the existing boiler design, operation, or steam generating envelope prevents operation at lower excess air levels. This recent modeling of a relatively small boiler furnace using FGR has illustrated the benefit of this strategy in maintaining low CO emissions. These modeling results and learnings will become even more important as lower CO emissions limits continue to be put into effect across the industry.
Author: Joseph M. Klover, P.E. Process Engineer