Air Pollution Control Measure Technologies for Toxic Exhausts
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Air Pollution Control Measure Technologies for Toxic Exhausts

By Energy CIO Insights | Wednesday, July 31, 2019

Numerous techniques have been designed for complying with the emission standards. The technology can depend on the age of the plant, the type of coal. Various solutions can be applied in combination to achieve emission control targets.

FREMONT, CA: The toxic gases emitted from the exhausts of power plants play a significant role in global warming. In many cases, apart from SOx and NOx, thermal power plants also release a variety of other emissions like particulate matter (PM) and mercury.  With the move toward greener technologies by limiting the emissions, many compliance solutions have been provided. The answers are ESPs, Fabric filters, and mercury removal technologies are being planned for deployment by the thermal power generation industry.

Electrostatic Precipitators:

Electrostatic precipitators (ESPs) are extensively used for controlling particulate emission in coal-based power generation plants. An ESP will electrically charge the ash particles from the flue gas stream to collect and recover the PM. The technology comprises of a series of parallel vertical plates containing electrodes to create an electric field through which flue gas passes and particles are separated and filtered.  A well-designed ESP is characterized by high PM collection efficiency, high dependability, and low flue gas pressure loss, resistance to moisture, temperature changes, and low maintenance. The ESP’s collection efficiency is very high at 99-99.99 percent for a range of 0.01-100 micrometer of particulate matter.

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Nevertheless, it does not function as well for fly ash with high electrical resistivity value. Coal emits high resistivity fly ash when low sulfur fuels are burned. It is difficult to precipitate,  resulting in often limiting the collection efficiency of ESPs. In the same context, the conditioning of fly ash in flue gas is a well-established technique that is applied to restore the performance of ESPs in a coal-based power plant. There are concerns regarding the efficiency and operational performance of existing ESPs relating to PM limits being made much stricter than before. Feasible engineering solutions for ESP augmentation are available for compliance with the emission standards. For instance, increasing the collection area of the flue gas by the addition of pass in parallel and increasing the height of the existing ESPs by a maximum of 15 meters is also a valid option. Another solution is the utilization of a moving electrode electrostatic precipitator (MEEP). A MEEP consists of movable collecting plates and rotating brushes, which allow for a superior collection of highly resistive dust such as coal and sintered ash.

Fabric Filters:

Fabric filters (FFs) have been very effective as particulate collectors in which dust cake is gathered on the surface of bags and is removed by various techniques. The fabric provides a rough surface on which the dust collects. The dust is obtained based on the four mechanisms of inertial collection, interception, Brownian movement, and electrostatic forces. A combination of these mechanisms assists in the formation of the dust cake on the filter, which will maximize the resistance to gas flow. The cleaning of filters is carried out through mechanical shaker baghouse, reverse air or pulse jet baghouse. Baghouse filters are characterized to have a high collection efficiency, 99.9 to 99.99 percent over a broad range of particle sizes, and around 99.7 percent for PM 2.5. Flue gas conditioning is also implemented in fabric filters, enabling lower emissions. The conditioning agents that are commonly used are elemental sulfur, ammonia, and sulfur trioxide. Recently, FFs have experienced facelifts, and several steps have been taken to hike the number of filters and the depth to enlarge the screen without changing the dimensions of the space. New filter materials are being innovated using nanofibre technology and membrane-type fibers as against conventional materials such as glass, cellulose, and synthetic and polymer fibers. By charging the incoming particle from the flue gases, using a corona discharge; the collection efficiency is increased, especially for particles in the sub-micron size range.

Mercury Control:

The primordial goal of mercury control is to oxidize all metallic mercury content to ionic mercury that can filter out from flue gas desulphurization (FGD) systems. The foundation of the process is mainly the oxidation of mercury; the removal is then carried out from downstream equipment before it is emitted via the stack into the atmosphere. The primary technology includes the injection of activated carbon into the filtering mechanisms’ exhaust stream. Mercury can be sifted using the technique of chemical adsorption on powdered activated carbon (PAC). Activated carbon is introduced upstream of an ESP or FF and is cleared out along with fly ash.  The PAC system has a substantial drawback; it will harm ESP performance during the particulate collection. A new entrant to the market, the injection of amended silicates, possess the capacity to negate both the increased SO3 concentration in flue gas and reduce the undesirable effect on ESP performance. However, the long-term viability and economic efficiency of amended silicate have not yet been demonstrated. Another expansively used technology is the halogen injection into coal with calcium bromide or sodium iodide. The process is inexpensive and results in a very high mercury oxidation reaction. The only catch is that, because of the halogen addition to work productively as mercury control technology, retrofitting of an FGD system has to be in place. Oxidized mercury is highly soluble and can be cleared out from FGD systems with ease.

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