The rotary kiln incinerator

The rotary kiln incinerator consists of a slightly inclined rotating primary combustion chamber.The waste is introduced at its highest end and deashing takes place at its lowest part.Due to its cylindrical form,the heat is equally refracted to the burning waste and the heat transfer and -control is optimal.Because of the (dis)continuous rotation and the inclination of the kiln,bottom ash is continuously and automatically evacuated.Due to the absence of any moving part in the hot zone,the rotary kiln can be operated under very high temperatures.A rotary kiln is the most versatile incinerator available; it can treat a large range of wastes,varying from low to high calorific values and different particle sizes.The rotating primary chamber must be equiped with a static post combustion chamber,for achieving a complete burn out of the flue gases. The plant must be equiped with a flue gas treatment system,dry or wet - or a combination of both -before releasing the flue gases to the atmosphere.

A  rotary kiln incineration plant is typically composed by following items :

01 MAIN COMPRESSED AIR SYSTEM

Compressed air is used to power guillotine doors and hopper lids in a fail safe mode (normally closed). It is also used to atomise liquid waste and sludges into the combustion chambers and for pulsating (for cleaning) the bags in the baghouse (if applicable).

02 HYDRAULIC PACK

Hydraulic pack is driven by an electric motor,has a hydraulic oil tank and the pressurised oil is used to power the skip lift and the feeding ram on the incinerator.

03 SKIP (Waste feeding system)

The skip is hydraulically operated and its operation is controled by the central computer.

04 FEEDER (Waste feeding system)

The waste drops into the hopper by gravity, the ram being retracted. When the hopper door closes (automatically), the guillotine door (fire door) opens and the he ram feeder moves forward to introduce waste into the kiln.When the ram retracts, the guillotine door closes again.

05 ROTARY KILN

There are 2 different types of rotary kilns : the historical co-current - and the modern more elaborated counter-current design.The co-current rotary kiln oxydises the total amount of waste under over-stoechiometric conditions (excess of oxygen),while the counter-current rotary kiln operates partly by under-stoechiometric oxydation (pyrolysis) of the waste.In this case,the pyrolysis gases are used to fuel the post combustion chamber-thus no auxiliary firing is required-while a co-current rotary kiln needs a burner to be fired permanently to maintain the required temperature level in the post combustion chamber.

A counter current rotary kiln allows a precise control of the flue gas stream and therefore its overall dimensions are much more compact than its co-current parent.

Also,the combustion air being introduced at the lowest part of this type of rotary kiln,opposite to the direction of the waste flow,it cools down the ash extraction area and as a consequence the bottom ash is cool and dry by nature and not any slagging occurs.

For more details,please refer to the attachment..

06 MAIN BURNER (natural gas or diesel oil)

The main burner has no other purpose than to preheat the kiln,before feeding any waste.Under normal operating conditions,this burner is not operative.

07 POST COMBUSTION with auxiliary (start) burner

The post combustion chamber(s) is(are) designed to allow a residence time of the flue gases in excess of 2 seconds-at 1100 Degr.C.As the burner will only be in service for preheating the post combustion chamber,before any introduction of waste can take place,it automatically retracts upon reaching the correct operating temperature..

08 SAFETY (emergency) OUTLET

In case of a fatal error (eg. power failure),detected by the control system,the whole plant is automatically and completely shut down (no further feeding will be allowed).The remaining thermal energy is released to the atmoshere by natural draft of a dedicated exhaust system.After the plant returns to its normal operation condition,this safety device will automatically be desactivated.

09 COOLING CHAMBER

The particulates,present in the hot flue gas stream,tend to soften and to become sticky at temperatures above 950 Degr.C.For preventing any slagging at the inlet of any heat exchanger,used to recover and/or to cool down the flue gas,we fit a cooling chamber in which the temperature of the flue gases is reduced to 850 Degr.C,by introduction of a small amount of ambiant air. 

10 ENERGY RECOVERY

Raw flue gases need to be cooled before being treated and being released to the atmosphere.Cooling may be done either by quenching (wet flue gas treatment) or by exchanging the heat to another medium.This medium can be steam or thermal oil or air.If steam is generated,it can be used to drive a turbo genset or to heat other process equipment.Heat exchangers can be installed,either in line or in by pass of the process flow.In case of dry scrubbing with sodiumbicarbonate a bag house is required to filter out the particulates.If hot air is generated,it can be used for materials drying.

Below,we summarise the most frequently used types of energy recovery.

1. Energy recovery: hot air
2. Energy recovery : steam
3. Energy recovery : steam + electricity.

13 DOSING UNIT SODIUM-BI-CARBONATE (dry)

Continuous,proces driven dosing of sodiumbicarbonate for neutralising acid components in the flue gases.The neutralising reaction is completed in a venturi type reactor in which the reaction time is in excess of 2 seconds.

14 DOSING UNIT ACTIVATED CARBON (dry)

Continuous dosing for the absorption of heavy metals and of eventually re-formed dioxins and furans in the heat exchanger.The absorption process is completed in a venturi type reactor (absorption time in excess of 2 seconds)

15 BAG HOUSE (dry scrubbing)

The filtering bags in the baghouse remain permanently coated with a layer of a mixture of sodiumbicarbonate and activated carbon.The neutralising reaction of acids and the absorption of heavy metals and dioxins is further completed during the passage of the flue gases through this layer.At the same time,all particulates are removed from the flue gas stream,before they are evacuated to the atmosphere.This guarantees an overall efficiency of the dry scrubber for HCl abatement of at least 97%. Particulate emission after the fabric filter is less than 10 mg/Nm³

16 ID FAN

One induced draft fan, ebonite coated if a wet scrubber step is added.The rotation speed of the induced draft fan is permanently controled by the system for maintaining a steady negative pressure in the process.

17 CHIMNEY

One chimney for main exhaust.The height can be adapted to local requirements.