2 types of rotary kiln and ash slagging risk

There are two types of rotary kilns, named after the sense of solids- compared to gas flow in the kiln.

Fig. 1: Counter current rotary kiln (TYPE 1)

Fig. 2: Co-current rotary kiln (TYPE 2)

Counter-current rotary kiln

Flue gases flow in the opposite direction of the waste, against the inclination of the kiln. Solids move by the rotary motion and by gravity from the high end to the low end of the kiln.

Incoming combustion air is cold (ambient) and contains max. oxygen level (21%).While flowing to the front of the kiln, the air heats up and gradually loses its oxygen to the combustion of the solids – basically becoming flue gas - so that at the front of the kiln the Exiting gases contain little oxygen (6%) and is hot (1000 deg.C).

Incoming waste (solids) is cold and hits the front zone of the kiln where the atmosphere is low in oxygen and high in temperature.This makes the solids vaporize off all their volatiles in the entry zone which is the entry to the post combustion zone which makes a hot, rich gas in an oxygen-poor atmosphere.

At this point, secondary air is blended in so that the gas phase combustion (post combustion) get going and takes the temperature of the gases further up from 1000 deg. C to 1200 deg. C without the need for support fuel.

Fig 3: Ashes of the counter current rotary kiln

Exiting wastes (ashes) are cooled down by the incoming combustion air and the oxygen –rich atmosphere ensure a perfect burn-out the same time. The low temperature in the ash evacuation zone creates no risk of slagging.

Additional features are that this type of kiln ensures better turbulence and hence the kiln can be kept short and compact. In order to achieve the same residence time for the solids, the rotation is slower the in a co-current kiln which in turn reduces fly-ash carry over.

Co-current rotary kiln

Flue gases flow in the same direction of the waste, with the inclination of the kiln. Solids move by the rotary motion and by gravity from the high end to the low end of the kiln.

Incoming combustion air is cold (ambient) and contains max. oxygen level (21%).While flowing to the back of the kiln, the air heats up and gradually loses its oxygen to the combustion of the solids – basically becoming flue gas - so that at the back of the kiln the Exiting gases contain little oxygen (6%) and is hot (1000 deg.C).

Incoming waste (solids) is cold and hits the front zone of the kiln where the atmosphere is high in oxygen and low in temperature.This makes support fuel necessary to light up the incoming waste. At the back (low end ) of the kiln, the solids do not contain any volatiles any more so that the gases flowing to the post combustion zone have to be heated from 1000 deg. C to 1200 deg. C. by support fuel.

Exiting wastes (ashes) are not cooled down by the incoming combustion air and the oxygen – poor atmosphere does not help to achieve a good burn-out. The high temperature in the ash evacuation zone (1000 deg. C) creates a high risk of slagging and makes ash handling difficult.

Fig 4: Ashes of the co-current rotary kiln (Ash slagging risk)

Additional features are that this type of kiln ensures little turbulence and hence the kiln must be longer. To keep the combustion going, kiln rotation must be faster than in a counter-current kiln, leading to more fly-ash carry over.

Example: The next 2 pictures show a rotary kiln with the same thermal capacity.

	Fig 5. Counter current rotary kiln The kiln is shorter and has a larger diameter. (TYPE 1)
	Fig 6: Co-current rotary kiln The kiln is longer and has a smaller diameter. (TYPE 2)

Manufacturing of counter- and co-current rotary kilns

The main difficulty with the counter-current kiln is the temperature gradient across the fire door which seals off the waste hopper (ambient temp.) from the kiln entry (1000 deg. C).This requires specially engineered designs and a lot of experience to make the fire door reliable and long-lasting. This is the main reason why the counter-current kiln is not more wide-spread.

Conclusion

	TYPE1	TYPE 2
	Counter current rotary kiln	Co-current rotary kiln
Amount of waste in	1000 kg/h	1000 kg/h
Waste inlet temperature	1000 deg.C	200 deg.C
Oxygen % at waste inlet	6%	20%
Waste residence time	>2h	>2h
Ashes
Unburned C in Ash	<0,5%	>2%
Ash slagging risk	No	Yes
Ash Temp.	200 deg.C	1000 deg.C
Mass reduction (%)	>85%	>75%
Post combustion additional support fuel consumption	0 kg/h	>100 kg/h
Fly ashes	750 mg/Nm3	1500 mg/Nm3