CBE CHAVOND-BARRY ENGINEERING CORP.
400 County Route 518 · P.O. Box 205                                       Tel: (609) 466-4900

Blawenburg, New Jersey 08504-0205                                         Fax: (609)466-1231

Flue Gas Recirculation

In

What is Flue Gas Recirculation (FGR)?   FGR is a simple concept wherein you move flue gas from the feed hearth of a multiple hearth incinerator to a hearth below the volatile burning hearth. There are two installations running for several years now; Woonsocket, RI and New Haven, CT. The benefits are listed below.

1.  Reduction in NOx Emissions

Reduction in NOx emissions is an attractive benefit found by using FGR. The design of the incinerator at New Haven, CT specifically needed some form of NOx control. The state permit required a very low emission level. The owner accepted the challenge and set out to design a combustion system that would meet the emission limit without major capital or operating costs. The overall incineration system consists of the 22'-3" 7-hearth multiple hearth incinerator, venturi, tray scrubber, packed scrubber, up flow wet electrostatic precipitator (WESP) and the two chamber Regenerative Thermal Oxidizer (RTO).

The NOx emission limit is only 60.8 ppm dry basis corrected to 7% oxygen (ppmvdc).

 

The final results are pleasingly low, averaging only 2.18 # NOx/dry ton. This is less than 50% of AP-42 which claims sludge incinerators emit about 5.0 #/DT. Experience with other multiple hearth incinerators indicates the emissions are more typically between 6 and 10 #/DT, and in some cases up to 17 #/DT.

There is no official NOx test data available for the Woonsocket, RI facility. However, there is some concentration information available. The data show the dry, corrected NOx concentration ranged between 40 and 60 ppm. This is almost identical to the data from New Haven, which averaged 53.1 ppmvdc.

2.  Reduced Slag Formation and Associated Lost Time.

The New Haven facility began with the FGR installed at start up. We are pleased to say that New Haven has never experienced slag formation! Woonsocket, on the other hand, before installing FGR, had experienced severe slagging resulting in considerable labor cost and lost operation time. A review of the operating records show that prior to FGR, 3.7% down time resulted from the need to remove slag. Monthly, about 10 tons of slag had to be removed from the incinerator. This is particularly troublesome since there is only one incinerator at this facility.

Since the FGR system was installed the lost time for slag removal has dropped steadily. Woonsocket now operates a full year without de-slagging the incinerator.

3.  Increased Incineration Capacity

The experience at Woonsocket shows that before FGR the plant operators processed an average of 382 dry tons/week. After FGR, the rate increased to an average of 419 dry tons/week. This 10% increase is partly due to less slag removal time, partly due to smoother operation and partly due to the incinerator's new ability to maintain the emission rate for THC.

4.    Reduced THC Emissions

Rhode Island enforces the federal THC emission limit of 100 ppmvdc, propane basis. There is presently no carbon monoxide emission limit. As a result, it is not necessary for the Woonsocket facility to operate an afterburner. Without FGR the operators were constantly battling to maintain capacity while not exceeding the THC limit. After the installation, we noted the increase in capacity as well as the ease of maintaining the THC limit.

5.     Increased Stability of Operation

It is difficult to quantify the improvement in the ability of the incinerator to maintain a smooth operation. An attempt has been made to quantify this by selecting two 24 hour periods, one with and the other without FGR. The temperature charts were digitized by taking six data points for each hearth every hour. Each hour was averaged and its standard deviation calculated. Prior to the FGR installation, the most unstable part of the incinerator was hearth 5, with a standard deviation of 101F. Following the FGR installation, the fifth hearth remained the most unstable, but the temperature's standard deviation fell to only 33F.

Added Stability -The operators report that the incinerator is more stable. They need to pay less attention to the incinerator each hour and still maintain control.

Extended Life -Another benefit is anticipated extended refractory and casting life. Maintaining steady temperatures is the best way to preserve the incinerator's refractory and castings.

Higher Incineration Rates -Through the benefit of added stability, it is now possible to operate closer to the limits of the auxiliary equipment. The operators are able to operate closer to the upper limits of the ID fan and scrubber since there is less need to save room for upsets.

6.    Complete Ash Burnout -No "Sludge Balls"

By recirculating the hot gas to the lower hearths we maintain a high enough temperature to complete the burnout. Since the introduction of FGR, there have been no "sludge balls" found in the ash from either installation. It has been noted that the ash is finer and free from lumps and clinkers. Difficulties with the pneumatic ash transfer system at Woonsocket have been eliminated. The fine ash passes easily through the conveying system without clogging.

 

For information on Chavond-Barry Engineering or on the systems described above visit our web site at http:www.Chavond-Barry.com or Contact Chavond-Barry Engineering Corp., Blawenburg, N.J.