Kenneth L. Maloney, Ph.D., Inc.


Flue Gas Recirculation (FGR) is a combustion modification process for stoker-fired boilers that will increase boiler efficiency and steaming capacity, while at the same time reduce particulates, NOx emissions and stack opacity . Emissions of sulfur oxides (SOx) can also be controlled with FGR when limestone is added to the fuel in the form of a coal/limestone pellet.

A System that Pays for Itself

Operators of stoker boilers, particularly those installed several years ago, must contend with changing environmental regulations and increasing fuel costs, in additional to the usual operating problems. Kenneth L. Maloney, PhD, Inc. Flue Gas Recirculation systems provide a unique solution that will solve emission and operational problems while paying for itself with improved boiler efficiency, lowering fuel usage. Simple payback periods of 1 to 2 years are common for most of our installations. The FGR system can be retrofitted to all types and sizes of stoker fired units. The FGR process is applicable to all fuels that are fired on a grate, including coal, wood, and biomass.

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The Concept

Flue gas is diverted from a location downstream of the main boiler bank, returned to and mixed with the combustion air from the forced draft fan. In some cases the flue gas might also be mixed with the overfire air. The recirculated flue gas takes the place of greater amounts of excess air that the stoker would normally use to keep the burning fuel bed cool to avoid clinkering. The FGR system allows the cooling and the combustion requirements of the forced draft air to be "de-coupled." With the use of the FGR system, fuel bed cooling can now be accomplished independent of air requirements for the combustion of the fuel. The FGR system allows the stoker to operate at very low excess excess air levels. Fifteen to twenty percent excess air operation is easily achieved with the FGR system. This reduction in excess air reduces the velocity of the flue gas in the boiler and therefore reduces the amount of flyash that is entrained in the gas and taken out of the boiler.

Frequently Asked Questions About FGR

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Hopper Evacuation of the Mechanical Dust Collector

When the flue gas is taken from the hopper section of the multiclone mechanical dust collector, the pressure drop across the collecting cyclone tubes is increased at all gas flow volumes. This increase in pressure drop increases the "spin" in the tubes and the centrifugal forces on the flyash particulate which results in an increase in dust collection efficiency. The dust collector collects more of the smaller particles: those under 10 microns, which normally escape from the unit. The combination of lower flue gas flow and improved dust collector efficiency result in boiler emission rates on the order of 0.05 pound per million BTU's. In some cases modifications are made to the overfire air system to improve the mixture in the furnace to reduce soot formation and the resulting opacity that it causes. These modifications are aimed at increasing the penetration of the overfire air to reach those portions of the over bed regions that are not well mixed. Flue gas recirculation can be added to the overfire air in certain applications to aid in this mixing objective and to further reduce the excess air in the furnace.

Proven Technology

Kenneth L. Maloney, PhD, Inc. has installed over 25 FGR systems in this country on all types and sizes of stoker fired boilers. The first commercial application of FGR to a stoker boiler in the United States became operational in 1981. Four units at this location have been operating continuously since they were installed. The Kenneth L. Maloney, PhD, Inc. Flue Gas Recirculation system is recognized as acceptable control technology for stoker fired units in Michigan, Pennsylvania, Ohio, Virginia, West Virginia, Wisconsin, and Washington. In those states where FGR has been installed the local EPA's now require that the FGR system be run at all times to meet compliance with the emission codes. In addition to the particulate emission, opacity reductions, and fuel efficiency improvements, the FGR system can also lower flyash and bottom ash carbon content, and reduce NOx and SOx emissions. The stoker owner can buy a cheaper grade of fuel, since the FGR equipped unit will tolerate more fines in the fuel as well as a lower ash fusion point.

The Hardware

The FGR system consists of a series of flues that connect the flue gas recirculation fan and ducts that connect the fan with the gas injection point in the discharge of the forced draft fan. Isolation and control dampers are placed in the flues and ducts to control the flue gas flow and to isolate the system. At any time the FGR system can be isolated and shut down and the boiler will continue to operate in its pre-FGR mode. All flues and ducts are made of heavy gauge steel. The elbows and damper blades are fabricated from abrasion resistant (AR) material. The FGR fan is an armored radial fan with scroll liners and blades of AR material. The fan specifications include a low rotation speed to reduce wear and increase expected life. The flow control damper is positioned by a local drive unit receiving a signal from a controller mounted on the boiler system control panel. The flow control can be operated manually or automatically. In the automatic mode the combustion control signal to the forced draft fan flow controller will be transferred to the FGR flow control damper which will be biased to set the ratio of FGR to fresh combustion air.

Overfire Air

If the unit is deemed to have inadequate overfire air, modifications to the overfire air system will be included in the FGR system installation. New overfire air jets may need to be enlarged. When this is the case the manifolding and overfire air capacities will need to be increased. On larger units there might be the requirement to put flue gas into the overfire air to reduce the amount of fresh air that is entering the furnace above the bed. When FGR is used in the overfire air it acts to dilute the air while maintaining the mass flow and the penetration of the jet to provide the required mixing in the furnace.

Fail Safe

The flow control damper is interlocked with the FGR fan so that the damper cannot be opened until the FGR fan motor is energized and will automatically close if the system fails. The flow logic of the system is such that even if the FGR dampers were shut down, the only thing that would happen is that fresh air would pass through the FGR system into the dust collector and out the stack. There is always plenty of time to close the isolation dampers and prevent the flow of air into the FGR system.

The FGR system can be shut down at any time and returned to its pre-FGR operational configuration. The FGR system will not limit steam generation in any way or cause a forced outage at any time.

Guaranteed Results

Kenneth L. Maloney, PhD, Inc. has had sufficient experience with the FGR system to be in a position to offer performance guarantees for the FGR system in most applications. Kenneth L. Maloney, PhD, Inc. has been successful in meeting the guarantees offered and has always been paid in full. We have a current list of FGR users that can be contacted, visited and will discuss their FGR operating histories with prospective clients.

Opacity & Particulate Reduction Summary Using Flue Gas Recirculation (FGR)

Company Unit Size, PPH Before FGR After FGR
Upjohn Company, Kalamazoo, MI4 - 90,0000.470 (30%)0.133 (15%)
INDSPEC, Petrolia, PA1 - 100,0000.774 (30%)0.208 (15%)
INDSPEC, Petrolia, PA1 - 50,0000.550 (25%)0.300 (17%)
Youngstown Thermal, Youngstown, OH3 - 90,0000.750 (35%)0.128 (7%)
Sonoco Products, Richmond, VA1 - 90,0000.125 (30%)0.091 (12%)
Sonoco Products, Richmond, VA1 - 90,0000.363 (20%)0.166 (10%)
WA State Corrections Center, Shelton, WA1 - 20,0000.242 (28%)
gr/dscf @ 12% CO2
0.052 (12%)

See How FGR Can Reduce Your Emissions and Save You Money

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FGR was featured in Power Magazine

Click here to download the article from the early years of FGR.

FGR was presented at the American Society of Mechanical Engineers (ASME)

Click here to download the article presented at the 1986 Industrial Power Conference.

FGR Brochure

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3D CAD Videos of FGR Installation

FGR Image Gallery

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Copyright 2011
Kenneth L. Maloney, Ph.D., Inc.