GEA Emission Control Technology GEA Scrubber for Exhaust Cleaning

Seeking to provide a flexible unit, capable of mastering manifold industrial off-gas problems, resulted, in 1950, in the development of an adjustable annular gap scrubber which today ranks among the outstanding high-efficiency scrubbers. As far as grain size of solid and liquid dust particles and concentration of gaseous components in a gas mixture are concerned, practically no minimum limit is set for the annular gap scrubber in regard to the separation. In principle, it is possible in all wet-mechanical separation methods to capture coarse and fine dust particles when the forces of inertia are increased by correspondingly higher gas velocities. This however, results in an avoidable high demand of energy in the gas and water medium. The development of the annular gap scrubber took into consideration the formation of district turbulent interface currents by means of a multitude of small turbulence eddies, which due to their extremely intensive mixing motion favored the transfer process. Every wet-mechanical treatment of the disperse system requires a corresponding minimum demand of energy, which can only be approached, if the effects of forces of inertia as well as those of the turbulent transfer processes are utilized simultaneously. Correct coordination of gap width s (h) to gap length 1 (h) is of decisive importance. The main feature of the annular gap scrubber is its axial-symmetric construction. Only co-axial circular cross-sections permit operation with a minimum floor space requirement. This construction exclusively provides uniform water distribution which covers the entire area of the gap and can be attained by means of large, non-clocking spray nozzles aligned symmetrically with the vertical axis.

Radial flow scrubbers are adjustable high-efficiency scrubbers. The raw gas can be fed from the top or from the bottom. The scrubbing liquid is injected through a central nozzle arranged upstream of the scrubbing zone. The scrubbing zone is limited by two rings, one above the other. The gas and the scrubbing liquid enter from above and pass through the gap between these rings in a radial flow, from the center towards the outer casing of the scrubber. The relative velocity between the gas and the scrubbing liquid which is decisive for scrubbing efficiency occurs in the narrowest cross-section of the scrubbing zone. By lifting or lowering one ring, the cross-section of the scrubbing zone can be varied, which allows, even with changing gas volumes, to maintain the gas velocity constant in the gap between the rings and to keep a constant pressure drop across the scrubbing zone. In this way a defined collecting efficiency can be ensured. The differential pressure is the control parameter. In many applications, radial flow scrubbers are combined with an upstream Venturi stage for raw gas saturation and prededusting. Often, a droplet separator is accommodated in the same casing as the scrubber zone in order to prevent any droplets from being entrained to downstream equipment. Advantages: - By adjusting the scrubbing zone, the gas volume can be changed at the ratio 1:10 while maintaining the pressure drop constant. This feature permits optimum adjustment to varying modes of operation. It is especially suitable for batch processes in converters. - The collecting efficiency of the scrubber can be defined as required, irrespective of fluctuating raw gas loads. - Since radial flow scrubbers can be adjusted as required for the respective collecting efficiency the scrubber can be operated at optimum differential pressure which helps to save energy. - The design allows the accommodation of several scrubbing stages in a single vessel; i.e a combination of radial flow scrubber and venturi scrubber or packed column scrubber. - Gas flow can be configured for either top or bottom inlet. Even for critical substances like As, Se or Pb, high collecting efficiencies are achieved. - High inlet temperatures of the raw gas of up to 800 °C can be handled with by combining the appropriate materials with a brick lining. - Lurgi has built more than 100 radial flow scrubbers so far, 40 of which are used in the non-ferrous metallurgical industry.

Empty column scrubbers are used as the first stage of wet gas cleaning for special applications, e.g. for hot and corrosive gases with high dust, arsenic or selenium loads, which occur in some metallurgical processes. In this scrubber type, the gas is quenched to saturation temperature and dedusted at the same time. Long residence times in the scrubbing zone allow arsenic and selenium compounds to be formed and aerosols wetted, so that these small condensed particles can easily be separated. Accretions which may cause problems in other scrubbers in the transition area where the hot gas comes into contact with the scrubbing liquid, is not critical in empty column scrubbers. Empty column scrubbers are made of carbon steel lined with anti-corrosive sheeting which is protected against the high gas inlet temperatures by brick lining. This lining may consist of several layers of different materials, depending on the duty.

Gaseous pollutants like HCl, HF and SO2 are removed by absorbents such as caustic soda (NaOH) solution, limestone (CaCO3) and lime (Ca(OH)2). Caustic Soda Scrubbers When caustic soda solution is used as absorbent, the best scrubber choice is the above-mentioned radial flow scrubber or a packed column scrubber, because there is no danger of buildup as there would be with solid calcium compounds. However, caustic soda should only be used for low raw gas loads and small to mid-size gas volumes, since it is about 10 times more expensive than e.g. limestone. The packed column scrubber shell is made of glass fiber reinforced plastic with a PVC inliner or a chemical barrier. The gas is treated passing through one or two stages of packing. SO2 reacts with the caustic soda to form sodium sulfate. Absorption efficiency depends on the intensity of the contact between gas and scrubbing liquid, which is ensured by structured or piled polypropylene packings. Limestone Scrubbers For cleaning large gas volumes with high contaminant loads, limestone scrubbers have proved to be the best solution. This scrubber is a type of spray tower without internals and is operated in counter-current. The pollutant concentration of the gas to be treated determines the number of nozzle levels accommodated in the scrubber. The limestone scrubber produces gypsum which can be sold at no additional cost to industrial businesses as filling material, as sludge with 50 – 60 % dry substance or dewatered sludge with a residual moisture of approx. 10 %. Absorption technology is increasingly applied in cement plants, where the raw meal is used as absorbent. Nowadays, even small off-gas volumes with high SO2 concentrations such as from metal smelters are cleaned in limestone scrubbers, if there is no downstream sulfuric acid plant. Limestone scrubbers can be made of stainless steel, FRP or rubber-lined or coated carbon steel.

Venturi scrubbers are chiefly used for cooling, saturating and pre-cleaning of gases, e.g. for sub-sequent cleaning in wet-type precipitators. The scrubbing liquid is injected through a central nozzle or in large scrubbers, through several nozzles in the inlet cone preceding the venturi throat. The scrubber may be mounted vertically, inclined or horizontally. For cooling, gas conditioning and pre-dedusting, the scrubber is operated with a pressure drop of only a few mbar. The small pressure drop is obtained by approximately matching the velocity of the injected scrubbing liquid to that of the gas in the venturi throat. During deceleration in the diffusor, the relatively higher mass of the scrubbing liquid causes a further increase in the gas pressure. Depending on the volume of liquid injected, is much energy may be transferred to the gas that not only is there no pressure drop in the scrubber, but the gas pressure is even increased. In contrast, when operating this type of scrubber with a large pressure drop the gas/dust mixture enters the scrubbing zone at a higher velocity that the scrubbing liquid. Thus the liquid droplets are accelerated by the gas stream, causing a pressure drop. As velocity rises, increasing turbulence is simultaneously induced in the scrubber; this brings about intensive mixing of the scrubbing liquid droplets and the raw gas, which are prerequisite for high collection efficiencies. Therefore, the higher the gas velocity in the venturi throat, the larger the pressure drop.

The cyclone scrubber consists of a prewashing zone of square or round cross-section, followed by an acceleration zone and a downstream gas cyclone. This design was developed by Norddeutsche Affinierie and is used for a variety of applications. In 1997 Lurgi acquired a license for the scrubber including all related patents. Cyclone scrubbers are designed for a gas volume of about 5,000 m3/h. For larger volumes, several scrubbers are arranged in parallel. Advantages  Dust collection and gas cooling are accommodated in a single vessel. The longer residence time of the scrubbing liquid in the gas (through several scrubbing stages) provides for intense gas cooling and good mass and heat transfer.  Given the high collection efficiency, especially for arsenic and mercury, it is possible, under certain design conditions, to do without the first stage of wet precipitator stage. The entire vessel is made of polypropylene. This material is anti-corrosive and inhibits accretions even of sticky dust.