Objective
In the Netherlands pavement bricks are fired most in Hoffman kilns, firing time is appr. 250 hours. Building bricks are most fired in tunnel kilns, firing time is appr. 50-100 hours.
This project is start up to demonstrate that it is possible to produce high quality bricks by rapid firing, due to minimize energy costs and to reduce air pollution. The firing cycle in this case is appr. 35 hours.L%
During the engineering phase it was decided that the kiln width should be fixed at 5 metres, in view of the limited possibilities of controlling the transverse temperature gradient in the kiln.
This meant that the required production had to be based on two identical units next to each other.
A start was made on the construction of the kilns in 1992.
Keller GmbH carried out the construction work. In order to spread the workload of the various construction teams, the second kiln was started 6 to 8 weeks after the first. Special attention was paid to the foundation, the rail heights and transitions of the layout, the quality of the refractory materials, the cars, the instructions for the bricks and the sealing and insulation of the kilns.
The great attention to detail and the unusual dimensions of the kilns required extra care on the part of the project management. The progress of the work was monitored at twice-weekly meetings and corrective action was taken where necessary.
After adrying-out and heating-up period, in August the first kiln was ready to bring a first car to top temperature (approx. 1100 deg. C).
After the loading and unloading machine and the second kiln had been slowly run in and production had reached about 50%, it was decided to take the Hoffman kiln out of operation and dismantle it.
When the tunnel kilns were started up the existing drying units with high air consumption were modified to make their air consumption more suitable for the tunnel kilns.
Work is currently being carried out on optimising the burner groups, control system and extractors in order to achieve the product specifications and increase the speed of the line. The two kilns are operating at almost maximum output. To achieve this the firing, flue gas and control systems have been adjusted.
The firing rate is mainly determined by the largest paving brick sizes.
Because of the weekly format change-overs necessary to supply the market a basic firing rate should be maintained so that all bricks meet the requirements. Practice shows that all bricks are of the same quality throughout the stack.
Each batch is tested by the KIWA testing institute before shipment.
The stacks are stable and the bricks do not stick to each other or to the kiln car. Experience has shown that all preconditions should be met : Clay of a consistent composition, proper clay preparation, shaping, drying and careful kiln car loading.
The conventional method to produce pavement bricks is to make large piles of green bricks in an Hoffman kiln chamber with a lifttruck. While firing until a temperature of appr. 1100 Celcius, the shrinkage of the bricks is appr. 8-10%. About 50-70% of the fired bricks are in the future suitable to sell as pavement bricks, due to the high quality demands. When producing this bricks with a modern conventional tunnel kiln, 25% of the amount has to be sort out, to reach the needed quality.
These production methods cost a lost of energy, i.e. natural gas, and cause severe air pollution. Half of the total fluoride emission in the Netherlands, is caused by the brick industry. This technology makes it possible to implement process integrated measures. The need for end of pipe measures is therefore minimized.
Rapid-firing is known in the ceramics industry for producing tableware, tiles and rooftiles. Not yet in the facade brick industry. The problem until now, was the quality of the low-cost clays, the dimensions of the products and the large production capacities.
PROCESS :
The kiln has high velocity burners in the side walls. These burners are placed under and over the load. By placing them crosswise a intensive circulation of gasses in the kiln is possible. Together with the very low and open setting of piles a very good temperature egalisation is insured.
This kiln don't work with pushes per car, but pushes very short distances. This method gives a very nice temperature profile in the car load.
In the warming up zone, air under high pressure will be injected, and some further high velocity burners will be placed for intensive circulation.
With this system it is possible to reach the limits of the products. The heating up speed in this case can be appr. 100 deg. C/h.
It is known that the emission of fluorides takes place between appr. 700 deg. C and top temperature in the kiln. In this case the warming up speed is much higher than in conventional kilns.
By the extreme good temperature egalisation it is possible to hold very short on top temperature. In this case appr. 4 hours. In a tunnel kiln appr. 35 hours for this product, and appr. 10 hours for building bricks. In a Hoffman kiln appr. 60 hours for this product.
It is possible to rise the temperature higher than normal, because the piles are very low. In this this case, 4000 mm, in conventional tunnel kilns 1000 to 2000 mm. With a higher temperature it is possible to fire the brick faster. The rapid firing is also made possible by the thin brick piles. The temperature in the centre of the brick is much faster reached.
In the cooling zone a rapid cooling is installed. Cold air under high pressure is injected on the sides of the kiln. The same effects are possible as in the heating up zone. Very good temperature egalisation, thin piles and advanced systems make a short cooling zone possible.
After the quartz point an air extractor is mounted to provide a inetnsive cooling in the last part of the kiln.
Programme(s)
Topic(s)
Call for proposal
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DEM - Demonstration contractsCoordinator
6916 KA TOLKAMER
Netherlands