In part 1 on this topic we presented basic principles of combustion and air system design to show potential benefits of an overfire air (OFA) upgrade. In Part 2 we present data from several of Jansen’s recent OFA upgrades to show measurable reductions in carbon monoxide (CO) emissions.
To review, Jansen’s OFA systems use our patented High-energy Multi-range Combustion Air Nozzle™. These large opening, low pressure drop nozzles are arranged in an interlaced configuration on the boiler sidewalls. This creates air jets that penetrate deep into the furnace and burn out CO and airborne fuel particles. There are typically three to four nozzles per sidewall.
The performance benefits of a Jansen OFA system can be seen in comparing boiler CO emissions before and after an air system upgrade. The figure below depicts CO emissions plotted against furnace volumetric heat input rates for 14 biomass-fired boilers at their pre-upgrade and post-upgrade conditions. Volumetric heat input, which is calculated as the heat input from grate fuel divided by the furnace volume, with the upper nose arch considered as the top of the furnace, is a useful metric in evaluating combustion conditions. A higher volumetric heat input signifies a shorter furnace residence time for combustible gases and airborne fuel particles and therefore usually higher CO emissions.
As the figure illustrates, CO emissions increase with volumetric heat input in both the pre-upgrade and post-upgrade data sets. The improved combustion provided by the Jansen OFA system created an average 33% reduction in CO emissions for the boilers included in the figure, even though heat input rates from grate fuels averaged 8% higher after the upgrade than before the upgrade. In other words, for most of the boilers included in the figure, the air system upgrade led to lower CO emissions even at higher grate fuel firing rates. This is an important point, as many boiler owners today are grappling with the need to maximize steam generation from grate fuels while reducing auxiliary fuel consumption.
Reductions in CO can sometimes be achieved with a boiler’s existing OFA system through optimization tuning. But CO emissions will often remain stubbornly high due to the existing system’s inherent limitations. In other cases, a boiler’s current CO emissions may be acceptable, but upcoming increases in load demand or changes to the fuel supply are expected to require more effective combustion performance. The Jansen OFA system is designed to provide optimal combustion conditions across a wide range of steam loads and fuel quality, making it an ideal choice for boiler owners seeking to get the most out of their equipment. Contact us to learn more about combustion optimization and how our air systems can help you achieve it.
Authors:
Morgan Silverman – Process Engineer
Steve Campbell, P.E. – Senior Process Engineer
Samit Pethe – Manager, Process Technologies