Four Maerz PFR limekilns at the Tata Steel operated Hardendale Quarry at Shap. A PFR kiln (parallel flow regenerative shaft kiln) consists of two vertical shafts and a connecting crossover channel. Both shafts work with each other. While one calcines the product, the other preheats the stone. In the burning shaft the lime is calcined in parallel flow. The hot combustion gases are then transferred through the crossover channel to the non-burning shaft where they preheat the limestone in counter flow in the upper area of the shaft. The flow direction of the gases is reversed at regular intervals. This allows the regenerative preheating of the stone to take place (the stone in the preheating zone of the kiln acts as heat exchanger) and thus for the maximum use of the heat contained in the kiln gases. The principle of calcination in parallel flow is ideal for the production of high-reactive quicklime. A PFR kiln from Maerz processes up to 800 tons of burnt lime per day.