FLUE GAS RECIRCULATION IN MULTIPLE HEARTH BIOSOLIDS INCINERATORS

Flue Gas Recirculation (FGR) is a retrofit process for multiple hearth biosolids incinerators, in which flue gas from the feed hearth is recycled to a hearth below the volatile burning hearth. There are three installations in operation for several years now: Hartford, CT, Woonsocket, RI, and New Haven, CT. The benefits include reduction of NOx emissions, increased stability of operation with less temperature fluctuations, reduced formation of slag, increase in capacity due to the ability to operate at lower excess air levels, and less unburned material in the ash.Stack tests during FGR have shown that NOx stack emissions averaged 2.18 lb NOx/dry ton biosolids at New Haven, averaged 1.71 lb NOx/dry ton at Hartford, and averaged 3.3 to 5.78 lb NOx/dry ton at Woonsocket. The average of these three facilities is 2.69 lb/dry ton, which is about half of the AP-42 emission factor (5.0 lb/dry ton) for multiple hearth biosolids incinerators. Concentration of NOx ranged from 53 to 202 ppm dry basis (ppmvd) corrected to 7% oxygen.Use of FGR results in steadier operation. An attempt has been made to quantify this by selecting two 24-hour periods at one of the installations, 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 variable hearth temperature in the incinerator occurred on hearth 5 of 7 hearths, where there was an hourly standard deviation as high as 101°F during a representative period. Following the FGR installation, hearth 5 remained the most variable, but the temperature's standard deviation was reduced to a maximum of 33°F.The benefits of FGR include reduced slag formation and associated lost time. A review of the operating records at Woonsocket 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. The installation now operates for periods of months without de-slagging the incinerator. Because of steadier operation and reduced slag formation, life of the refractory and rabble teeth is extended.Multiple hearth sewage biosolids incinerators with FGR are able to operate with less excess air, with a consequent reduction in oxygen concentration and quantity of flue gas. This makes it easier to meet the THC requirement, in cases where the operation is limited by the available burner capacity and combustion volume in the top hearth of the incinerator. 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 to the incinerator's new ability to maintain the emission rate for THC at a lower oxygen level.The recirculated hot gas to the lower hearths maintains a high enough temperature to complete the burnout of the organic component of the biosolids. Since the introduction of FGR, there have been no “sludge balls” found in the ash from any of the installations. It has been noted that the ash is finer and free from lumps and clinkers. Difficulties with the ash transfer system at Woonsocket have been greatly reduced. The fine ash passes easily through the conveying system without clogging.