Objective
As the preheating of batch and cullet is not yet very widespread in the glass industry as considerable problems occur during operation of the process, it was therefore the aim of this innovative project to reduce the specific energy consumption by adding preheated batch. The preheating of pure cullet is relatively simple as the cullet can be heated either directly or indirectly, and problems such as conglomeration or transport difficulties do not occur. The preheating of batch, however, causes great problems as the soda tends to hydrate when excess water is present, causing large lumps of batch to form, which hinder the batch transport significantly. Therefore it was the aim and object of this innovation to develop a method for preheating a glass batch, in particular for one mixed with cullet, which avoids the problems mentioned previously, and preheats the raw materials efficiently.
During the cold test run some minor technical problems were detected on the raw material preheater, so that appropriate modifications were made and completed. Due to site conditions it was impossible to install a hopper in front of the preheater so that the preheater could only be fed when a charge was delivered from the batch plant. This resulted in different residence times for the various charges, and, as a consequence, uneven preheating. The initially pursued intention to lead a larger part of the waste gases around the outside as indirect as indirect heating and to let a small proportion flow through the raw material preheater in direct contact with the batch, proved to be impracticable because the pressure difference between the external and internal preheaters meant that the total waste gas volume was sucked through the indirect preheating chamber. Despite these difficulties batch preheat temperatures of about 300 to 350 deg. C were achieved. The batch did not stick or cakeat all, which would have caused batch transport problems.
The batch charging in the doghouse area changed quite suddenly when the material was dried and preheated, and so the batch no longer floated on the glass bath in large piles, but was charged as a relatively thin blanket. This thin layer resulted in much faster heating of the batch as the thermal conductivity of the bacth material normally prevents fast heating of thick piles. The fast heating of the batch, however, resulted in a reduction of the batch cover by about half in the melting end under otherwise similar conditions. This means that the actual speed of melting of the batch increased significantly, so that considerably higher melting rates could be expected in the furnace.
At a later stage some malfunctions occurred which had to be solved but the preheater was kept running during this period. Owing to the fact that the glass problems could apparently not be controlled, the preheater was switched off and the originaloperating conditions were re-established. Therefore it was decided that the batch preheating processes, and especially the effect on the physical decolourisation, should be investigated in laboratory tests.
A certain time was required to carry out the analysis of the glass problems which occured, and the laboratory tests demonstrated clearly that no further problems are to be expected from the chemical and physical decolourisation or from the use of refining agents when the preheating unit is in operation. In the meantime, a second glass melting furnace has been built alongside the existing furnace, and both are supplied by the same batch plant. Consequently the time in operation of the belt conveyor and supply system for the furnaces prevents any intervention at the moment. Therefore an intermediate bunker, urgently required to improve the charging of the preheater, cannot be installed at this point in time.
The innovative concept contains basically a well-known double deck scraper chain conveyor made of adequate high-temperature steel. This scraper chain conveyor has a double casing and is externally completely insulated. The waste gas enters both the double casing and the actual batch carrying zone. The efficiency of the preheater is considerably increased by dividing the waste gas flow, whereby the waste gas velocity can be determined in direct contact with the batch through the choice of the waste gas quantity which flows directly or indirectly through the preheater.
The preheater is installed as bypass to the main batch furnace charger hopper. Therefore the bucket elevator and the main batch belt had to be lifted up so that a distributor could be installed below the belt discharge station. On the one side the discharge station leads the batch to the furnace charger hopper and on the other side into a small bunker located above the metering chute for the preheater. This small bunker simply serves the purpose to seal the batch feeding to the preheater completely to prevent any false air coming in through the partial vacuum of the preheater. The other purpose of this bunker is to guarantee that always enough batch is available with discontinuous charging of the batch so that the preheater works continuously. A vibrating chute feeds the batch to the preheater which draws off the batch to be preheated from the small bunker and feeds it continuously into the preheater. Then the batch is transported horizontally by the scraper chain along the intermediate base plate to the end of the preheater where it falls down to the lower base plate. After the reverse process, the chain collects the batch and leads it into the direction of the furnace charger hopper. Like each scraper chain, the preheater has at its end a rising diagonal surface on which the batch is transported by the scraper chain into the furnace charger hopper. The shafts of the chain cogs and the driveunit are water-cooled and the bearings are placed outside to guarantee a sufficient mechanical stability of the equipment. The waste gas in direct contact with the batch to be preheated, is led into approximately the center of the preheater so that the flow resistance between the waste gas inlet and outlet sleeve is approximately the same in both directions. The gas flows serving for the direct preheating is divided by this. If the gas speed is limited to 1 m/sec to prevent dusting, it follows at a temperature of 600 deg. C that 50% of the available waste gas volume can be lead in direct contact with the batch to be preheated whilst the other part is available externally for indirect heating.
The chain and the metal strips fixed thereto operate as secondary heating zones during the heat transfer so that a very good heat transfer in a relatively small area can be expected. A recirculation was planned to avoid an increase in the waste gas quantities.
Programme(s)
Topic(s)
Call for proposal
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DEM - Demonstration contractsCoordinator
96355 TETTAU
Germany