Photo by Mark McLees A "bird's-eye view" of an incinerator including a firefighter to indicate its size. This installation was involved in the fire discussed in the related story on page 66. In the lower half on the right side of the photo is the hydraulic cart/hopper. Once the furnace reaches its operating temperature, the hopper door is shut and the "charge door" is opened by the vertical piston, then the feed ram pushes the trash into the furnace. The secondary chamber is directly above the main chamber.
We all are learning the reason for these barriers. Society with all of its medical advances cannot eliminate infectious diseases, so we must safeguard ourselves via personal protection. We don our EMS gear (sometimes reluctantly) and treat the victims who have called upon us to assist them. When the incident is completed, we drop the disposable bag valve mask and our soiled disposable gloves into the "red bag" and wash up.
But our precautions shouldn't end there. Those "red bags" of infectious waste are still in our communities, and it behooves us to identify the institutions that produce large volumes of infectious or contaminated waste. Some health care institutions dispose of infectious waste on site. The mechanism is basically a high-temperature incinerator. To better understand the various components of these systems, let us look at the process.
The "red bags" from each area or floor in the institution are collected throughout the day by housekeeping staff, then transferred by cart or chute to a collection area. Some incinerators operate 24 hours a day; others burn waste for only a part of the day. In places where the incinerator does not run continuously, the bags are allowed to accumulate until their appointed time of incineration. (This is the least desirable situation, as any type of rubbish accumulation is considered to be a fire hazard.) We need to differentiate between units which operate continuously and those that do not.
The photos accompanying this article depict a "fixed hearth" furnace. It operates in the following manner: Natural gas is used to initially heat the furnace to about 800 to 1,000 degrees Fahrenheit. Then, the waste is fed into the furnace. By feeding a fixed amount of waste per hour (its rated capacity), the furnace becomes self-sustaining without using natural gas. In the case of the unit shown, this feeding of fuel continues for 10 to 12 hours, then the unit is allowed to burn down by stopping the addition of waste and fuel. The burndown takes an additional 10 to 12 hours. Once it has burned down, the furnace is opened and ash is removed. The process then starts all over again.
Other types of incinerators have automatic or continual ash removal and do not have to be shut down except for repairs and inspections. They burn more efficiently than the units that do not burn continuously. A "stepped" hearth furnace and a "rotary" hearth furnace are two examples of the more efficient burners. For now, however, let's keep to the description of the "fixed" hearth unit.
The first step in waste incineration is to weigh the garbage to be burned. For many installations, this is critical, as furnaces are rated by their capacity to burn garbage, measured in pounds per hour. The second step is to place the waste into a cart or hopper system. This hydraulic lift/dump is similar to those found on front-loading garbage trucks. They operate at 300 to 600 psi and offer all the pinch and fire hazards associated with any hydraulic system. The third step entails dumping the waste through the hopper door into a feed tunnel or chamber. Here again, hydraulics are used to power a ram that pushes the trash into the furnace. This occurs once the furnace has reached its operating temperature. For the next 10 hours, waste is pushed down the feed chamber, in this particular unit at a rate of 600 pounds per hour.
Photo by Mark McLees The discharge end of the secondary chamber where it dumps into the boiler.
The "stepped" hearth furnace mentioned earlier typically has three levels, each with its own ash ram. As the waste is burned, the ram pushes it to a lower level where it is agitated and combustion continues. Picture a series of steps, with the final combustion taking place on the bottom step. Again, the point is to accomplish the most efficient combustion with as little ash residue as possible.
The other type of unit mentioned earlier is a "rotary" hearth, similar to a huge drum. Inside, there is no flat surface. The burning waste is slowly agitated by the rotation of the drum. This is the best type for efficiency and creates the least ash residue. However, it is limited in its capacity due to its size and difficulty in making large drums for volume burning.
Let's continue with the description of the process. Directly above the main furnace chamber is a secondary chamber. Just as in the main chamber, natural gas is used to "pre-burn" and heat this chamber. It too becomes self-sustaining with the simple introduction of the gases from the main chamber below it. The secondary chamber operates in a temperature range of 1,800-2,000 degrees. Unlike the main chamber, which typically is double-walled, the secondary chamber is single walled. The outside skin approaches 800 degrees.
The hazard here, besides the obvious burn potential, is the high interior temperature. Never consider directing handlines or any other water into or near these two furnace chambers. If it is necessary to stop the burning process, then you must stop both the introduction of waste and natural gas entering the furnace. Normally, in the secondary chamber, the combustion process continues with the efficient burning of the gases produced in the primary chamber.
Photo by Mark McLees The shortest exhaust stack is the boiler stack. The exhaust stack on the lower roof is the "dump" stack, which only is used in an emergency. The tallest stack is the final exhaust point with products exiting at 90 to 100 degrees.
From this point on in the process, the motivation and direction is for pollution control. From the secondary chamber, the gases vent to a "dump stack" or into a boiler. The "dump stack" vents the hot gases directly to the outside atmosphere and is activated by thermocouples, low water level or flow alarms in the pollution control system. This process occurs only when there is an emergency.
Normally, the high-temperature gases are directed into a boiler where the heat is used to convert water into steam, thereby reducing the temperature of the gases. In larger municipal trash-burning plants, this steam-producing boiler is used to generate electricity. Often, this byproduct (electricity) generates revenue for the municipality. As non-utility generators, these trash-burning plants make money in two ways: by charging to burn trash and by selling electricity back to utility companies.
After the boiler, the normal operating process has the cooled gases entering a metal box filled with filters. Here in the "bag house" cloth filters are used to filter particulates out of the gases. With the gases having been cooled in the boiler process, a draft fan is used to create negative pressure in the "bag house" so that the gases continue to flow through this entire system of ductwork.
Once the gases are drawn through the draft fan, they pass through a series of water sprays. Additional cooling is the main function at this stage. After passing through this "quench section," the gases enter a "scrubber" where, any remaining acids in the gases are neutralized before the gases leave the main stack.
In the installation shown, the "wet" scrubber uses sodium hydroxide (an acid) to neutralize any hydrochloric acid contained in the gases. It operates to attain a neutral pH of 7. The tank itself is fiberglass lined. The acids used in the process are an additional hazard stored in this area. The gases pass through one more dry filter screen before heading up the fiberglass stack at a temperature of 90 to 100 degrees.
Mark McLees is captain of the Syracuse, NY, Fire Department Rescue Company.
