Over Fire Air Upgrades

Combustion Improvement Program

overfire-air-beckMany utility and industrial boilers installed over fire air systems over the years in response to various air emission reduction regulations that ratcheted down emission levels. Power & Industrial has been involved in combustion improvement projects over many years and has seen how the technology and science of lowering NOx emissions has evolved.

Power & Industrial Services has developed an over fire air upgrade formula that can be applied to many existing boilers. There are an entire generation of boilers with burners and overfire air systems that were designed over the years that can benefit from this improvement program. Combustion side NOx reduction is still, by far, the lowest cost NOx reduction and this program is aimed to help our customers remain competitive in this dynamic market.


Power & Industrial Services recently completed a project for a Utility customer that had installed an OFA system, Low NOx Burners and a neural network over the years but still needed more NOx reduction. P&I used the program described below to save this customer the huge expense of installing and operating a SNCR system to get additional reduction needed.

Based on historical data, an additional 10% NOx reduction was needed in order to avoid more costly retrofit options.

Project Goals & Benefits

The project goal was to modernize the OFA system in an effort keep CO emissions within the state guidelines. The result was lower NOx, the ability to operate with lower O2, lower CO and a change in the primary objective of the neural network from minimizing CO to minimizing NOx.

Project Features

Removed twelve OFA ports from service and implemented 4 new larger ports in between the sidewall and outer most burner column. (note: existing OFA system was installed with two elevations of 12 ports each) Reutilized twelve existing ports.

Program Description

Over Fire UpgradesThe engineering team at Power & Industrial is a group of individuals that specialize in design and integration of new combustion equipment along with improving existing systems. Our combustion engineers have designed hundreds of the overfire air systems over the years. A large portion of the designed systems were done utilizing CFD modeling to validate that the design, (a valuable tool unavalible when most systems were initially installed). During a hands on combustion tuning effort in it was noted that certain settings trended toward lower NOx but the system appeared to have limitations as CO would spike. By using CFD modeling, particular OFA port sizing and placement was investigated which resulted in lower NOx in various locations in the furnace. The field data suggested that changing the OFA furnace penetration can reduce the localized NOx formation that takes place with an overfire air system.

This data was used to redesign the OFA system for the customer. The CFD modeling suggested a 10% reduction would be realized. The customer was interested in proceeding further with the project and the hardware was fabricated and delivered prior to a planned outage. New equipment was installed and put into service. Combustion tuning and performance testing showed that system performance was clearly improved and met all goals for the customer.

Redesign of an OFA system is a great way to fine tune combustion on an existing boiler. Reducing NOx and CO are not the only targets. Reducing fouling and slagging, waterwall corrosion concerns and improved efficiency can also be primary targets of a well planned OFA upgrade.

Figure 1: 10% reduction in NOx Emissions (lb/mmBtu)

OFM Yearly Average