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Communication Feature | August 2014

Fabric Filters: The emerging solution for pollution control in power sector

Power development is the key to the economic development. A country´s growth or development yard stick is measured by the Electrical Power it can generate to fuel its industrial growth.

India is power deficit where demand far outweighs supply/ generation. Hence, there is a big gap to cover if the country has to grow as per the Planning Commission forecast. Though, the power sector has been receiving adequate priority ever since the process of planned development began in 1950. Over the years the installed capacity of Power Plants (Utilities) has increased to more than 150000 MW. Although there are different ways to produce electrical power, the largest capacity for power generation still rests with coal based thermal power plant. India and China will heavily depend on this fossil fuel as both countries have their own coal reserves to be exploited and used. The primary objective of the coal burning systems in the process of steam generation is to provide controlled efficient conversion of the chemical energy of the coal into heat energy, which is then transformed to the heat absorbing surfaces of the steam generator. The combustion elements of coal consist of carbon and hydrogen. When combustion is properly completed the exhaust gases will contain carbon dioxide and water vapor. The traces of sulphur present in coal get converted to sulphur dioxide. When carbon burns incompletely it forms carbon monoxide. Nitrogen of air and coal get converted to oxides of nitrogen.

Unfortunately, Coal power plants are the most polluting ones since they emit not only gaseous pollutant (NOx,SOx, Cox) but also suspended particulate matter which is fly ash. A range of pollutants are generated from coal-fired and coal co-fired power generating plants and some are more specific to a particular technology. However, attention has focused mainly on controlling emissions of particulates. Numerous systems have been developed and applied for control of particulates. Some control specifically one type of pollutant; whereas others may integrate several control systems, thus allowing for the control combinations of SO2, NOX and particulates. In most cases, the selection of appropriate emissions control systems will require consideration on a case-by-case basis.

Most of Indian Power plants still use ESP (Electro-static Precipitators) to arrest the polluting fly ash from escaping through stack. The single most important factor influencing the Electrostatic Precipitator (ESP) performance is the resistivity of the dust being caught. The resistivity is only relevant when the dust is caught on the Collecting Electrodes (CEs). Electrostatic Precipitator (ESP) consists of metal plates, which are electrically charged. Dust (fly ash) in the flue gases is attracted on to these plates, so that they do not pass up the stack to pollute the atmosphere. The best average dust emission level that can be achieved with a standard ESP is 75 mg/NM3. But most plants are complying with a norm of 110 mg/NM3. Many of the existing plants that are 15 years old even cannot match these norms because of the rapid deterioration in the coal quality they get form Indian mines. Besides, ESP operation is very much dust sensitive, which means the resistivity of dust should remain constant for a steady operation. This of course is not possible due to different coal varieties (from different mines) the plant have to use.

This uncertainty of complete resistance combined with excess dust coming from new power plants have aggravated the pollution situation, and hence the new regulations are placing the emission levels below 50 mg/NM3 (in the West it is below 15 mg/nm3) and that will soon come into force. Any delay in enforcing these norms will create a situation as in any big city of China today (heavy smog) causing severe health issues. In order to improve the collection efficiency of dust (fly ash) from the flue gases to minimize the emission, an additional Hybrid ESP Fabric Filter system is introduced, especially for thermal power plants. In Hybrid ESP fabric filter, the primary collection of dust (fly ash) will be carried out by ESP and fabric filter will be secondary dust collector. Fabric filters remove dust from a gas stream by passing the gas through a fabric and leaving the dust on the surface of the fabric. The fabric filter is therefore not sensitive to dust resistivity. However due to the ´barrier´ nature of the dust removal, the fabric filter is more sensitive to contamination due to the process. An important consideration for fabric filter is the choice of bag material. Gas Temperature and gas conditions are major factors in this decision.

For Coal Fired Boilers the filter is treating acid gases at 140 to 1500 C. A commonly applied fiber is Polyphenylensulphide (PPS). This has good filtration properties, acid resistance and operational temperatures up to 180 -1900 C. As far as achieving specific emission levels is concerned most fibers used on needle felt filter bags, it can achieve 15 to 20 mg/Nm3. For emissions of 10 mg/Nm3 or below, consideration should be given to fine denier fibres and/or surface coatings and membranes. Filters treating major process gases prefer to be operated under constant process conditions. This is not a problem for ESPs but is an important consideration for a fabric filter. Insulation should be of high quality to maintain heat and filter bags should be kept ´dirty´ so that fibers are protected from Dew Point transitions.

With the current trends to burn alternative fuels and increased environmental pressures, it is easy to see the practicability of fabric filters with the correct filter bag material when emissions of less than 20 mg/Nm3 are being considered and the fabric filter is therefore seen as future proof and likely to meet any future emissions legislation.

The fabric filter has several jobs to perform in a coal-fired boiler application. It needs to maintain pressure drop while meeting all environmental regulations in conjunction with the other pieces of environmental equipment in the system. On top of that, upset conditions in the real world can present challenges to the fabric filter. If the fabric filter can´t handle the upsets, the result could lead to the plant not being able to run at the required load demand or being forced to shut down because of an inability to meet environmental regulations. The other issue is if the upset conditions require continuous cleaning for prolonged periods of time, leading to premature bag failures and shorter bag life than the budget anticipates.

Supreme Nonwoven Industries Pvt. Ltd. is South Asia's largest manufacturer of fabric filters. Supreme provides customization for applications across industries and is a one-stop provider of complete filtration solutions. The range and depth of filtration solutions provided by Supreme are derived by leveraging its long experience spanning technologies, materials, know-how and applications.

With its vast set-up and numerous production lines, Supreme is able to have facilities dedicated to the wide range of fabric filters it manufactures. This capability of Supreme ensures that fibers of each type and grade are manufactured on a line optimally designed for that particular fabric filter type.

The comprehensive range of Supreme's industrial fabric filters are marketed under the Nowofilt brand and are available for both dry and wet filtration. Nowofilt media is used to manufacture a wide range of high quality filter bags. These bags are at the heart of dust collector systems - pulsejet, reverse air and shaker type. To meet requirements of end-user industries and OEM's, the Nowofilt team is equipped with the tools to study each end application and it's working environment. This study is lead by service engineers and supported by a central team of application specialists, development engineers and production staff. The cumulative efforts of Nowofilt team results in only the most appropriate fabric filter being recommended for each end application. The accent is on customer-centricity and, therefore, customization.

We live in an era that increasingly demands improvement in product recovery and reduction in process emissions. Nowofilt from Supreme Nonwoven are engineered to deliver as per these stringent demands of performance, quality and consistency.

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