Particularly in energy recovery sytems, dust-laden waste gases, for example from production processes, frequently have to be cleaned of entrained particles. At gas temperatures above 400 °C the separation process can be carried out efficiently using the hot-gas filter system with fibre ceramic filter elements described below.
Dry filters, for example bag filters or pocket filters are commonly employed to clean waste gases. However, these filters are not allowed to be used if there is a risk of condensation, because the dust and the filter medium will become dirty and sticky, the bags will be blocked and the gas exit obstructed. A high filtration temperature is essential to avoid falling below the dew point of water vapour or other condensable gas components, such as condensable hydrocarbons like tar. As fabric filter materials normally have a permissable operating temperature from 180 to 230 °C, the gas temperature for filtration is restricted to this value range.
The temperature of the dust-laden waste gases, however, is often between 400 and 650 °C, where energy recovery is preferable. Contamination of the heat exchanger by dusty raw gases not only reduces efficiency, but also leads to easily overlooked maintenance costs due to the shorter cleaning intervals. Experience has shown that self-cleaning heat exchangers fitted with blowers or pellet cleaners for the heat exchanger area tend to be relatively expensive to maintain. All these factors were taken into account in the development of Theisen hot-gas filters, which are designed for raw gas temperatures of approximately 600 to 800 °C.
Fibre ceramic material
Filter elements made of sintered ceramic were originally used, for example, to filter synthetic soot and clean carbide furnace gases. Owing to the relatively high weight of these elements, combined with the need for special clamping devices for installation and the comparatively large pressure loss, tests were carried out with a fibre ceramic filter material. This material exhibited optimum characteristics (refer to the list of filter material features) for the envisaged application, namely dedusting waste gases from fully enclosed electric reduction furnaces. Pyrotex KE85 fibre ceramic filters from BWF Envirotec were already installed in various processes at high temperatures, for example in hazardous waste combustion plants, rotary kilns, roasting lines, calcination plants and pressure-charged fluidized bed furnaces. Theisen employed these high-temperature-resistant ceramic fibre elements with silicate fibres for gas cleaning in a closed reduction furnace used to produce calcium carbide. A hot-gas pilot filter plant, equipped with Pyrotex KE85 filters for instance, serves as a bypass-gas cleaning plant at Donau Chemie in Lan-deck (Austria). The residual dust content of the gas is less than 1 mg/Nm³. The small pressure losses, even with very high filter area loads, facilitated the compact design of the hot-gas filter units.
Integrated filter cleaning
The filter elements are suspended in a cylindrical insulated silo with a conical shaped dust collecting bunker. The raw gas enters the silo centrally at the top. The incoming dust-laden raw gas is turned 180° in the the dust collecting bunker, and by turning the flow the coarse dust is separated from the gas due to inertia and collected in the bunker. The gas, which still contains fine dust, flows inwards through the hanging, candle-shaped filter elements as the fine dust is separated from it. The cleaned gas is then supplied to the conventional clean-gas ring pipe via one of the clean gas chambers which form the top part of the silo. Each clean gas chamber comprises a number of allocated filter elements and can be closed by check valves installed on the clean gas side. Each chamber and its filter elements can thus be isolated from the gas cleaning process by differential pres-sure measurements and/or a timer and cleaned with small quantities of impulse gas. The jet valves of the isolated gas chamber are opened for a fraction of a second to generate these short impulses, which are sufficient to clean both the chamber itself and the filter elements. The dust adhering to the outside of the filter elements drops into the collecting bunker. After a short transition time, the clean-gas check valve opens the cleaned chamber and filter section and the cleaning cycle is applied to the next chamber.
Due to the very small gas volume for cleaning the chambers and the heat-insulating silo shell, the high temperature inside the gas chambers, the filter elements and the dust bunker remains more or less constant. The gas used for the impulses is either compressed clean gas or inert gas (N2 or CO2). Even hydrophobic or pyrophoric dust can be separated and be removed with suitable equipment via the dust collecting bunker. During the dust separation period the high furnace off gas temperature is only slightly reduced, so that hydrocarbons, such as tar are unable to condense on the filter surface. The hot-gas filter unit is heated with a heat transfer medium to prevent condensation during start-up and shut-down.
Hot-gas filter system:
Ceramic fibre filter:
Filter material features
- Very high chemical resistance, even at high temperatures
- Continuous resistant at 850 °C
- Low surface weight (appr. 3500 g/m2)
- Low specific weight (appr. 0,18 kg/m³)
- High porosity of approximately 93 % and therefore low pressure loss
- Dust cannot penetrate more than approximately 1 mm into the filter elements
- Resistant to temperature variations
- Non flammable
- Back-flushing like conventional needled felt filter hoses
- Self-supporting filter elements with no additional support fixtures such as support baskets or rings