DE69217503T2 - GAS TREATMENT PLANT - Google Patents

Description

When destroying contaminants by combustion in a gas treatment plant of the type known as a combustion exchanger, electric heaters are often used to maintain the proper operating temperature in the center of the combustion bed.

This is the case in WO-A-8 600 389 (US-A-4 741 690), which is to be considered as the closest prior art. According to its teaching, the contaminants are fed into the combustion exchanger, in which the contaminants themselves burn and/or are processed to decompose themselves. The hot gas leaving the heat exchanger is used to heat the contaminant-laden incoming gas by a countercurrent regenerative heat exchange. However, if the chemical energy developed in the form of heat during the destruction of the contaminants is sufficient, there is no need for the supply of electrical energy. The plant is self-contained in this case, the electrical heating devices only having to be used to start the operation of the plant. The start-up requirement is that the temperature is sufficiently high to ensure that the destruction reaction continues. Once the reaction has started, it generates the heat needed to maintain the temperature level, provided that the chemical energy from the reacting contaminants is sufficient.

If the energy from the contaminant is not sufficient to maintain the operating temperature in the bed on the To maintain the required level, instead of using the electric heaters, it is possible to mix additional fuel into the air to be cleaned to provide the required heat. For example, natural gas or propane can be added to the polluted air stream. Likewise, vaporized solvents are sometimes used. Often, energy prices make it less expensive to add fuel than to resort to heating by electricity.

The dimensions of the electrical heating system in an electrically heated combustion exchanger are generally chosen to ensure that the system can operate at full air flow and to maintain the operating temperature even when the air temporarily contains no energy-producing contaminants. In other words, the system should be able to operate with clean air as well. One consequence of this is that the electrical connections must be dimensioned for considerable energy requirements. However, it is expensive to have to provide considerable energy for the electrical heating devices with consequently large connection energy, which means considerable costs even though the equipment may only be in use very infrequently. The use of equipment designed to supply additional fuel to the air flow to be cleaned can result in cost savings under these circumstances, since it becomes possible to reduce the electrical energy connections. This is a state-of-the-art technology and is used in many cases in conjunction with conventional combustion exchangers.

In a conventionally designed electrically heated combustion exchanger, the electrical heaters are positioned in the hottest zone of the bed. During operation, the temperature in this zone is high (about 1000ºC), with consequent high demands on the corrosion resistance of the materials from which the heaters are made.

This applies whether the heat is supplied by electricity, by the pollutants or by the fuel added to the flow. The problem is identical if other types of heaters are used instead of electric heaters.

By means of the invention defined below, a combustion exchanger can be set up in such a way that any heating devices incorporated therein are spared a long-term stay at high temperatures. An embodiment of the invention is shown in Fig. 1.

The incoming air to be cleaned enters through a duct 1 and is fed to an air direction changing device 2 in which the air is directed alternately upwards to an air distribution space 3 and downwards to an air distribution space 4. From there the air flows through a combustion bed 5 in which the chemical reaction takes place. Via the air distribution space 4, alternating the air distribution space 3, the air then returns to the air direction changing device 2 and the cleaned air finally flows out through a duct 6. For the purpose of maintaining a hot zone in the middle of the bed 5, the direction of the air is changed at regular intervals in accordance with the principle described in the US patent specification referred to above. During normal operating conditions, heat losses are compensated by the heat developed in the reaction process with the contaminants and/or with the fuels mixed with the air for that specific purpose.

Initially, however, a higher temperature is required to initiate the chemical reactions. The necessary starting temperature in the bed is achieved by means of one or more heating devices 7, which are arranged vertically off-center, ie from the middle part of the bed and which are preferably arranged in the upper part of the bed.

When the plant is to be started up from cold conditions, the heaters 7 are activated. They are used to heat a layer in the bed to a starting temperature which need not be as high as the normal operating temperature. It is sufficient if the temperature level is such as to ensure that the mixture of contaminants/fuel and air begins to react during its passage through the bed and that the reaction progresses to a point where more heat is developed within the bed than is extracted from it. The temperature within the bed therefore rises while the plant is in operation. When the correct start-up temperature level has been reached by means of the heaters, the latter are thus stopped and by starting the supply of air flow and the supply of contaminants/fuel the temperature of the hottest layer soon rises to the normal operating temperature.

Since the path from the hot layer to the environment is shorter towards the top of the bed than towards the bottom, the layer loses more heat upwards than downwards. Consequently, the hot layer shows a tendency to move downwards, eventually becoming centered in the bed.

By controlling the air-directing mechanism in such a way that a greater volume of air flows downwards through the bed than upwards, this process can be accelerated and made so rapid that the heating devices occupying stationary positions within the bed are never exposed to the normal working temperature. The simplest way of achieving this is to allow the flow to flow upwards and downwards at intervals of mutually different lengths, but it is equally possible to To obtain interval time of the same length, but instead vary the flow volumes per unit time.

With or without such predetermined control, the end result is an averaging of the hottest layer toward the center of the bed, with the asymmetrically positioned heaters constantly finding themselves in a cooler part of the bed located outside the hottest layer. From now on, the combustion exchanger runs, if desired, with additional fuel added to the flow, without the heaters ever being exposed to the high temperature of the hottest zone.

As a result, the heating devices can be made from cheaper materials and their service life increases, while at the same time they require less maintenance and servicing.

According to the previous exemplary embodiment of the invention, the heaters are positioned asymmetrically within the bed in such a way that they are closer to the upper surface of the bed than to the lower one. This position facilitates maintenance work on the heaters or replacement of the same, since it eliminates the need to remove large amounts of material in order to make the heaters accessible. In principle and within the inventive scope, they can of course also or instead be positioned in the lower part of the bed. Furthermore, a vertical air flow through the bed has been assumed above. Likewise, in the case of a horizontal air flow, the principle of the off-centre position with respect to the heaters can of course be adopted. In the latter case, however, it should be taken into account that the bed arrangement is turned accordingly, with "up and down" being replaced by "left and right".

The most common type of heating device in combustion exchangers is the electric heater. The principle of asymmetrically positioned heaters as defined in the present invention is also applicable to other types of heating devices, such as oil or gas controlled heaters.

Claims (2)

1. Gas treatment plant of the type known as a combustion exchanger, comprising a generative combustion bed in which one or more heating devices are arranged for the purpose of heating the bed and initiating the chemical reaction, characterized in that the heating devices for initiating the chemical reaction are positioned off-centre in the bed, i.e. outside the centre of the bed, and in that the combustion exchanger is provided with air direction changing equipment (2) designed to shift the hottest zone towards the centre of the bed very quickly after start-up by controlling the direction of air flow through the bed up and down either at intervals of mutually different lengths or by varying the flow volumes through the bed per unit time. 2. Combustion exchanger according to claim 1, characterized in that the heating devices (7) are positioned in the upper part of the bed.