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A cement factory in Texas was among the first in the U.S. to install wet flue gas scrubbers in order to reduce the sulfur dioxide emissions. This is a demanding application for pumps because the gypsum slurry that “scrubs” the gas is highly abrasive and corrosive.
Special pump systems with housings and impellers made of mineral cast proved to be an ideal solution for this specific application.
Wet flue gas desulfurization scrubber systems can be vital components in various industries due to strict environmental regulations of those applications. First applications were coal-fired power plants and waste incineration plants. Today, other industries use wet scrubbers in their day-to-day operations as well to scrub flue gas emissions from burners that use coal as a fuel source.
Image 1. Several pumps with housings and impellers made from mineral cast are responsible for the transport of corrosive limestone slurry. (Images courtesy of Deuchting Pumps North America)Image 1. Several pumps with housings and impellers made from mineral cast are responsible for the transport of corrosive limestone slurry. (Images courtesy of Deuchting Pumps North America)
The scrubbers remove toxins from coal- fired flue gas streams in the process.
This is accomplished by spraying a mixture of limestone slurry over incoming flue gas. This creates a chemical reaction that removes or “scrubs” the toxins from the flue gas. Once the limestone slurry reacts with the incoming flue gas, it is converted to a gypsum slurry.
The gypsum is then dewatered and can be sold to end users for use in various industries. Concrete manufacturers are able to use the dewatered gypsum in the concrete production process.
Image 2. Smaller pump in the cement plant’s scrubber systemImage 2. Smaller pump in the cement plant’s scrubber system
The selection of pumps for this application is a crucial task.
Due to its significant corrosive and abrasive characteristics, gypsum slurry is a very difficult medium to handle. An American cement plant with a yearly capacity of about 2 million metric tons has found a good solution for that issue when it installed the first of two scrubber systems in 1998.
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The housings of the eight pumps in the limestone slurry cycle are not made from metal materials, but from a special material that was designed specifically for extremely harsh conditions. This material is a mixture of silicon carbide (SiC) and epoxy resin that is poured into precision molds and heat cured to form the pump components. The material must be machined with diamond tools due to it exhibiting a hardness close to that of diamonds. The material is completely impervious to corrosion and exhibits extreme abrasion resistance given its metallic-free composition and high hardness value.
The scrubber recycle pumps in this plant have operated at a flow rate of 1,310 cubic meters per hour (m3/h), 21.1 meters (m) of head and 630 revolutions per minute (rpm) for nearly 20 years, requiring minimum attention and service. Of course, the mechanical seals were exchanged from time to time. Apart from that, four impellers, one casing cover and some minor components were replaced. An additional 24 pumps are installed in the second scrubber system and other applications in the plant.
Image 3. Horizontal single-stage centrifugal pump in back pullout designImage 3. Horizontal single-stage centrifugal pump in back pullout design
This installation is a true testament to the superior performance of these pumps in the harshest environments. It shows why this type of pump is installed not only in this cement factory, but also in wet flue gas desulfurization systems in multiple power plants and other industrial facilities all across the world.
The pumps are not restricted to flue gas desulfurization. They are also used in processes such as ammonia phosphate and titanium dioxide (TiO2) pigment production, salt solution mining, sugar beet processing and sea water desalination.
Special ceramic nozzles are used to inject the lime suspension into the absorber. These nozzles form many small droplets from the pumped suspension and thus a correspondingly large reaction surface for good mass transfer. The ceramic material permits long service life despite the fact that lime suspension with gypsum content has abrasive properties. In the design we attach great importance to free cross-sections, so that smaller impurities in the suspension cannot set the nozzles. For economical operation, these nozzles can be adapted to the highest efficiency range of the pump. A nozzle can be specified for (almost) every process engineering challenge. In addition to full-cone and hollow-cone nozzles in various spray angles and flow rates, the TwinAbsorb series with patented twist compensation is also available.
The absorption zone consists of several levels of nozzles and a horizontally installed droplet separator system, in order to return the fine droplets carried along in the gas stream to the process. With our high performance droplet separators you can increase the efficiency of your plant.
The solids in the suspension can lead to deposits, e.g. in the droplet separator, in the inlet duct or on pipes, which can lead to problems in operation. Since water is always withdrawn from the circuit through evaporation, water must be fed into the absorber, which can and should be used for cleaning. Lechler tongue nozzles have proven themselves for cleaning the flue gas inlet. Lechler full cone nozzles are usually used for cleaning the droplet separators.
Plastics (e.g. for pipelines) and rubber (e.g. gaskets, rubber linings, etc.) are often used in an absorber whose temperature resistance is lower than the temperature of the uncooled flue gas. Normally, the suspension pumped in a circuit cools the flue gas sufficiently, but if, for example, the feed pump is suspended, the plastics and rubbers could be destroyed. Small special-alloy metal nozzles have proven their worth here, which take over cooling during this time and thus protect the investment of the flue gas desulphurisation plant.
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