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
AFTERBURNER OPTIONS FOR MULTIPLE HEARTH SEWAGE
SLUDGE INCINERATORS
Considering the looming changes to both state and federal regulations limiting the emissions of hydrocarbons and carbon monoxide (CO), [See CBE, Potential Emissions Rule Changes for Sewage Sludge Incinerators], we briefly describe below various afterburner designs proven successful as retrofits to multiple hearth sewage sludge incinerators. In all cases, it is important to have the ability to reach a minimum of 1400F for CO control and allow safety margin in the burner capacity to make up for periods of wet sludge.
Chavond-Barry has successfully designed all types of afterburners
described below; All meet the 100 ppm target for CO, and most operate below 30 ppm.
The owner has a difficult decision to make in balancing capital against operating cost.
The most fuel efficient afterburner configurations are capital intensive. One has no
crystal ball with which to determine the future cost of power and fuel.
1. EXTERNAL CHAMBER
The stand alone external refractory lined chamber is the typical, least innovative afterburner. This design attaches directly to the incinerator gas outlet, and functions to increase gas temperature and retention time. Typically, the venturi and tray scrubber system is located following the external afterburner chamber. Space limitations often make it difficult to add this afterburner to an existing incinerator. Capital cost for this design is usually higher than that of the jumper flue, but is less than the recuperative or regenerative type afterburners. Depending upon the temperatures established by the incinerator's operating permit, fuel cost can be much higher for the external chamber than for the recuperative or regenerative type afterburners.
2. TOP HEARTH
The top hearth afterburner functions exactly as the external chamber. Sludge is fed to the second or lower hearth and the temperature on the top hearth is raised to afterburner levels. This is the least costly installation. However, one must realize that the throughput of the incinerator may suffer due to the loss of one active processing hearth. Most often, the increased fuel required for the afterburner temperature can be used within the incinerator to improve heat transfer and compensate for the lost hearth area. Care must be taken with this design to minimize short circuiting of feed hearth gases around the top hearth causing high CO or hydrocarbon emissions. Fuel requirements are similar to the external chamber.
3. JUMPER FLUE
The jumper flue design has been used on four incinerators with excellent results. Here, as with the top hearth design, the top hearth is used as the afterburner chamber. The feed enters the second hearth. Here, the flue gas is extracted from the side of the feed hearth and injected into the top hearth via the jumper flue and therefore, does not pass through the conventional drop holes.
The jumper flue is external to the multiple hearth incinerator and provides a path for gases to travel between the second, feed hearth and top, afterburning hearth. Burners located at the turn around point of the flue force feed hearth flue gases to pass directly through the burner flames. This results in excellent mixing and emissions control.
The jumper flue is one of the most capital-cost effective options. Furnace configuration and space availability are determining factors for the use of this afterburner design. Depending upon the temperatures established by the incinerator's operating permit, fuel cost can be much higher than for the recuperative or regenerative type afterburners.
Careful design is necessary to minimize by-passing at the center shaft hearth penetration. Adjustment of incinerator temperature profile can be used to compensate for the lost hearth area from using the top hearth as afterburner volume.
4. RHOX PROCESS
The Reheat and Oxidize (RHOX) process is a patented process in which the afterburning function follows the other air pollution control equipment. The particulate and acid gas control equipment is used to cool and condense the majority of the water vapor from the flue gas. The final step is to reduce the unburned components to code level. The cold gas is heated on the way to the afterburner either by recuperative (shell and tube) means or through the use of a Regenerative Thermal Oxidizer (RTO).
RHOX REGENERATIVE SYSTEM
This afterburner design is commonly used for fume incineration. Hot (1500F) afterburned gases are passed down through a bed of ceramic forms. In doing so, the heat content of the gas is transferred to the ceramic mass and the gas is cooled (to about 250F) for discharge to the atmosphere. Cold, dust free gas from the APC equipment is passed up through another bed of ceramic forms which has previously been heated with afterburner gas. Here, the cold gas is preheated to a temperature approaching the afterburner requirement by extracting the heat previously stored in the mass of ceramic forms. A small quantity of fuel is burned in the afterburner to reach the required temperature. The hot and cold gases pass back and forth through two or more beds to achieve preheating and cooling and fuel savings.
These beds are rather large, necessitating a location outside at most incinerator buildings. This is the most fuel efficient of the options, up to about 95%, compared to an external afterburner.
RHOX RECUPERATIVE SYSTEM
The first RHOX installation designed by CBE utilized a shell and tube heat exchanger to preheat the scrubbed gasses on their way to the afterburner. This unit, located in Willow Grove, PA has operated for almost a decade.
The recuperative system requires a separate afterburner with a shell and tube type heat exchanger. Gases leaving the scrubber are pre-heated in the heat exchanger and travel to the afterburner. Gases leaving the afterburner travel through the other side of the heat exchanger, transferring heat to the gases arriving from the scrubber. The area required for this configuration is smaller than that of the regenerative process but still too large to fit within most existing incinerator buildings. This system has a higher capitol cost then the jumper flue or extended chamber designs, but can reduce the afterburner fuel use by up to 70%.
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.