The Refractory and Insulation
The vermiculite based insulation around the side and the back walls is 115 to 200mm thick. This ensures low surface temperatures, higher internal temperatures and lower fuel usage.
The Front face of the incinerator is insulated with calcium silicate that has a very low conductivity to ensure low surface temperatures.
The Feed System
The Hydraulic Feed Ram is designed to handle medical waste with a very broad range of calorific values, shapes and densities. Compact design and limited cross section provide better control over the rate of feed and restrict excessive air ingress around the feed opening. Bulky components, too large to pass through the feed ram can be loaded through a manual loading door. Small to medium animal carcasses sharps containers and buckets can be loaded through the feed ram.
Automatic regulation of the feed rate provides long term control of the rate of combustion. However, modern medical waste is highly volatile. It burns very rapidly. In the short term, a single load of waste can burn so fast as to exceed the design capacity of the incinerator. The combustion rate is controlled by limiting the primary air and the amount of heat on the waste during the initial stages of volatilisation. To facilitate this control the ram operates as follows:
- Up to five boxes or bags of hospital waste or loose waste are loaded onto an inclined stainless steel chute or raised concrete platform. (A roller conveyor can be used in place of the chute if the waste is loaded in rigid boxes. The front box is marked position 1 in the illustration.
- The front box drops over into the inlet chute on top of the ram in position 2. The ram is at rest in the position shown in the illustration.
- When the incinerator is ready for loading (determined on a combined time and temperature function), the ram moves right back. The box in position 2 drops into the feeder ahead of the ram. The next box on the inclined chute then tips into the feeder inlet chute to lie on top the first box.
- The ram moves forward, pushing the first box to position 3. Until this time, the gate is kept closed, preventing any blowback and the ingress of air.
- The ram stops with the box in position 3 while the gate opens
- The ram then pushes the box to position 4 before returning and allowing the gate to close.
- The box in position 4 is shielded from direct heat by a refractory tunnel. Air from the primary chamber is also restricted. The initial rate of combustion is controlled and steady. Volatiles leaving the tunnel are forced to pass through the bed of burning waste. Halogens and fly ash are trapped in the ash.
- As the following box is pushed in, the first box is pushed out onto the hearth, and exposed to the full heat and air supply.
Liquids on the Hearth, in the Feeder and the Fat Pan
The hearth is inclined upwards away from the inlet. Because refractory and firebrick cannot be made impervious to liquids, the under side of the inclined refractory hearth is lined with stainless steel. Fat and liquids, penetrating the refractory, run down the stainless steel sheet into a stainless steel pan, located beneath the refractory at the tunnel outlet. On either side of the tunnel, the pan is exposed to the heat and gases of the chamber. Molten fats and liquids are burnt away in the pan.
The feed ram is slightly inclined towards the incinerator. Liquids spilt during the feed process run down into the tunnel. An inclined stainless steel sheet under the floor of the tunnel ensures that all liquids are conveyed to the fat pan for combustion.
Combustion on the Hearth
A semi-pyramidal shape on the hearth promotes distribution of the waste across the width of the hearth. It also breaks up the solid tube of waste, which would otherwise be formed.
A sharp step at the end of the hearth further assists in breaking and opening up the advancing waste bed. Gentle opening of the bed promotes combustion without entraining excessive fly ash.
A set of cast iron tuyeres, built into the hearth, admits forced draught under fire air underneath the fire bed. The tuyeres are so arranged that the air passages blast downwards preventing molten plastics and other liquids from running into the air holes.
The burnt out ash either falls off the end of hearth onto the riddle flap, which opens periodically to drop the ash into the trolley below.
The trolley is emptied once or twice per shift. The riddle flap is switched off during emptying of the ash trolleys to prevent an inrush of air and entrainment of fly ash.
The Auxiliary Burners
The primary burner is located relatively high in the sidewall of the primary chamber. The flame is angled downwards, but not so far as to fire directly onto the waste. When the rate of combustion of the medical waste is low, a vortex motion of the gases in the chamber carries heat down the opposite wall and under the far side of the fire bed. This promotes rapid primary combustion.
When the rate of combustion is high, volatiles from the waste deflect the burner flame up, away from the fire bed and into the flame port at the entrance to the secondary chamber. Thus whenever the rate of combustion is too high, heat onto the fire bed is reduced and the primary burner provides secondary heat. This action is self-regulating and contributes to automatic control of the combustion rate.
The secondary burner provides heat for combustion of the volatiles after they have left the fire bed. It is located on the sidewall of the primary chamber, directly opposite the flame port. It is not angled downwards. It fires straight across the top, rear of the primary chamber and into the secondary, mixing chamber. This upstream location of the burner provides a longer secondary zone and hence longer secondary retention time. The heat is introduced before the gases pass through the flame port where high velocities and maximum turbulence occur. This vastly improves mixing of the secondary heat with the combustion gases.
The Air Supplies
Secondary air is introduced at the flame port to further improve turbulence and mixing. High gas velocities in the flame port induce secondary air by venturi action. The higher the gas velocities, the more air is induced.
A small percentage of the primary air is introduced through the tuyeres described above. It is temperature controlled.
The balance of the primary air is induced by the draught (negative pressure) in the primary chamber. Most of this air is admitted through the burner quarl and follows a path similar to the primary flame. It is also automatically deflected into the secondary chamber when the combustion rate is high.
Instantaneous, short term, control of the combustion rate is thus achieved by automatic regulation and deflection of the heat and air supplies. The principal is also known as “Controlled air”, “Starved air” or “Pyrolisis”. The feed inlet tunnel also limits the amount of air and heat reaching fresh hospital waste. All of the above effects are instantaneous. No moving parts are involved. The system is rugged and absolutely reliable. Wear and tear is non-existent.