Maerz® Parallel Flow Regenerative Shaft Kilns
This kiln type is characterised by two vertical shafts connected to each other by a cross-over channel. This allows for parallel flow heating, i.e. the parallel flow of the feedstock and combustion gases in one shaft and the regenerative preheating of the feedstock by the mixture of combustion gases and cooling air in the second shaft.
The Parallel Flow Principle is ideal for producing highly reactive quicklime and burnt dolomite.
PFR Kilns are used for capacities between 100 and 850 tpd and can be fired with gaseous, liquid and pulverised solid fuels.
Heat consumption figures are in the range of 3600 kJ/kg of lime, (850 kcal/kg or 3.1 MM B.T.U./ton), and are thereby the lowest for all lime kilns.
The grain sizes of the limestone or dolomite to be calcined are usually between 25 and 180mm (1" and 7").
The following illustration shows the principal mode of operation of the PFR Kiln and illustrates the two phases of the gas flow. Two shafts, marke d with 1 and 2, contain the material which is to be burnt. The stone charging system, the reversal flaps for fuel, combustion air and exhaust gas as well as the lime discharge system are not demonstrated in this diagram. Depending on the kiln capacity, the shafts are charged with stone either alternatively or at the same time.
Lime is continuously discharged at the bottom of both shafts. Fuel is only added to one of the two shafts at any one time. In the illustration, the fuel reaches shaft 1, the burning shaft, whilst shaft 2 serves as the non-burning or exhaust gas shaft. Fuel is added by means of a larger number of burner lances, which are installed vertically in the material of the preheating zone. The lower ends of the lance pipes mark the end of the preheating zone and the start of the burning zone. Fuel is added through these lance pipes and is distributed evenly throughout the cross-section of the shaft.
The combustion air is added under pressure at the upper end of the preheating zone above the bed of material. The entire kiln system is under pressure. The combustion air is preheated by the stone in the preheating zone, the regenerator, before it mi xes with the fuel at the end of the lances. The fuel flame is in direct contact with the material to be burnt, while combustion gases and material pass through the burning zone from top to bottom, (parallel flow heating).
The exhaust gases flow from the burning shaft through the crossover channel into the non-burning shaft, where they are passing upwards in counter flow to the material. The exhaust gases transmit their heat to the stone material in the counter flow shaft and thereby make a small contribution to the calcination process.
Each shaft passes burning and counter flow periods of approx. 12-15 minutes. The switching over from the burning to the exhaust gas shaft is called reversal. Throughout the entire burning period, burnt lime is continuously discharged from both shafts, by means of discharge tables, into pressurised hoppers. Cooling air is added continuously to the bottom of both shafts, in order to lower the temperature of the burnt product before it reaches the discharge hopper. During the reversal periods, when the kiln is de-pressurised, the burnt product is discharged from the hoppers below the discharge tables into a common hopper placed underneath, from where it is transported further by means of vibrating feeders and conveyor belts.
The PFR kiln is available in the following type series:
- Rectangular kilns with a direct crossover channel for capacities of 1 00-400 tpd of burnt lime
- Circular kilns with a crossover channel, which is executed as an annular channel, for capacities of 300-850 tpd of burnt lime
- Finelime kilns with a special geometry of the cooling zone and a special charging device for the feed material, for capacities of 300-500 tpd of burnt lime.
A detailed description of the PFR Kiln can be found in the following brochure: