The Benefits of Overfire Air and Flue Gas Recirculation Systems

overfire air and flue gas recirculation

When it comes to boilers, efficiency is the name of the game. Lower efficiency leads to worse performance, which impacts the revenue and profit margin of a plant, either directly (for a power plant) or indirectly (for a steel plant, pulp & paper mill, etc.).

Stokers have a lot to do with boiler efficiency. To optimize efficiency, it’s important to modify combustion for a stoker system in a way that best maximizes heat efficiency and capacity while managing emissions to stay on the right side of regulations.

Overfire air and flue gas recirculation (OAFGR) systems can offer significant benefits to a stoker-fired boiler system, ranging from more efficiency to fewer emissions.

Overfire Air and Flue Gas Recirculation Explained

In this configuration, flue gas is taken from downstream of the boiler and combined with combustion air from a draft fan. (The flue gas may also be mixed with overfire air in certain configurations).

What this process does is simple: it replaces the excess air that the stoker usually uses with recirculated flue gas. This is advantageous because the flue gas has a higher heat capacity than simple air, which means it can wick away more heat from the fuel bed, thus dramatically reducing peak temperatures in the unit.

We want peak temperatures to be lower because that reduces clinkering and overheating of the grate. It also has the indirect but noticeable effect of reducing fly ash that is contained in the gas stream leaving the boiler during combustion (because of lower amounts of excess air).

Advantages of OAFGR Systems

One main advantage of an overfire air and flue gas recirculation system is better efficiency for the boiler system.

Higher plant efficiency is accomplished by reducing the amount of excess air in the system (via the process explained below). There is a direct relationship between the amount of excess air and fuel savings, since less fuel is required to feed the stoker and cool the fuel bed. This can result in reductions in excess air by as much as 60% ,and fuel savings as high as 7% or even more. Thus, fuel efficiency can be obtained by implementing an OAFGR system.

Another main advantage is found with fewer emissions, particularly NOx emissions. There is a direct relationship between NOx emissions and the amount of excess air in a boiler system. The higher the amount of excess air, all things considered, the higher the amount of NOx emissions. By reducing excess air, an OAFGR system can reduce emissions at the same time, which keeps a boiler system on the right side of regulations.

A cooler fuel bed also contributes to fewer emissions, thanks to lower fuel bound nitrogen conversion to NOx. Similarly, thanks to how overfire air systems are configured, with the staged air located above the fuel bed, lower emissions are obtained due to controlled stoichiometry of the fuel bed.

Implementing Overfire Air and Flue Gas Recirculation Systems

Installing a OAFGR system for a plant’s stoker-fired boiler system first requires an analysis of the environment, juxtaposed with a survey of the plant’s specs and needs. Then, a team of engineers will design a plan that creates the ideal configuration of the OAFGR system, since there’s no such thing as an out-of-the-box solution that works.

Then, custom equipment will be fabricated and installed by a team of specialists only after being modeled and tested to ensure that it’ll deliver projected results.

ProcessBarron designs, engineers, fabricates, and installs overfire air and flue gas recirculation systems for boilers in a variety of industries and application. Contact the team for more information and a free quote.