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Communication Feature | May 2015

Need of Bag Houses in Coal Fired Power Plants

As India is marching towards a high growth economy, there will be a huge requirement for power to accelerate the growth of her manufacturing sector. Most of the power (over 70 per cent) for this will have to come from coal fired boilers as the country is blessed with plenty of coal reserves.

Unfortunately, coal based power plants are by far the most polluting ones since they produce a large amount of residue in the form of ´ash´ (almost 30 per cent avg. by weight of coal burnt) accompanied by other gaseous pollutants (NOx, SOx, COx). The ash that remains air borne due to its fine size constitutes ´fly ash´ and is a major ambient air pollutant. The health hazards of fly ash have been well documented - inhalation of fly ash over a period of time is reported to cause severe lung infection leading to cancer.

Hence, there is a real danger of air pollution in India getting out of control in the coming decade. The Indian government is aware of this and has begun to put into place revised compliance norms for air pollution for industries. From the earlier levels of 110û75 mg/Nm3, the standards have been recently revised to 50 mg/NM3, that is likely to go further down ( < 10 mg/NM3) to be at par with industrialized nations.

Bag Houses
Majority of the Coal Fired Power Stations (old and new) in the country, today, are equipped with ´Electrostatic Precipitator (ESP)´ to filter the large volume of hot, ash laden boiler fuel gas. While this constitutes the main air polluting load in a power plant, there are other areas, such as - Buffer hopper and Silo transfer points, where ´Bag Houses´ are generally installed to take control of the fugitive dust emission. These are the two main ´Air Pollution Control´ (APC) equipments used in Power plants ESP´s (Fig.1) are preferred when handling large gas volumes as their operation costs are lower compared to bag houses for a large volume. But the ESPs´ overall filtration efficiency under actual plant operating conditions is 90 per cent at best. Bag Houses, on the other hand, are 99.9 per cent efficient. The reason for this huge gap is due to their difference in dust separation principle. In ESP, the dust laden gas is first electrically charged by a set of charging electrodes, and then are moved towards another set of oppositely charged collector electrodes. The hot gas leaving the ESP is to most extent dust free. However, the ESP functioning is very sensitive to many factors such as dust quantity, dust chemistry, particle size, current/voltage, moisture, Carbon Monoxide (CO) presence, gas flow distribution, contact time etc. All these influencing factors make the ESP operation very sensitive, resulting in efficiency dropping to 90 per cent and below. Today, many coal fired power plants in the country (mostly state owned) face similar problem with their ESPs. These plants are kept operational due to the energy demand while they breach all pollution norms. It is not uncommon to come across over maintenance of their ID fan impeller, in these plants.

Bag houses of Reverse Pulse Jet type (Fig. 2), in comparison, do not suffer from any of these handicaps, perform with highest filtration efficiency and also highest collection efficiency. The high separation efficiency is largely governed by the filtration phenomenon that plays out in a fabric filter. In a Bag house, the dust laden gas actively tries to permeate through the fine pores of the filter fabric, and in the process, fine and coarse dust particles are captured and trapped, mostly by the many surface fibres in the media, leaving the dust free air to flow through the media pores into the atmosphere. The surface area available for the dust to be captured is enormous in a fabric filter.Today Bag houses offer the most economical solution to meet stringent pollution norms that no other APC system can offer in the market.

ESP-Hybrid Bag Filters
In recent times, a new concept has been introduced to bring ESP and Reverse Pulse Jet bag house operating in tandem, especially where high gas volumes are involved. Instead of a full ESP, only few electric fields are provided to act as a pre-collector. By this way most of the coarse particles will get filtered (over 80 per cent) in the ESP part and leaving only a small dust quantity (less than 10 per cent) of finer size to be get loaded onto the bag filter part. Because of the reduced dust load, the bag house part is expected to perform at low DP levels. The Bag house acts more as a ´Polishing filter´ to separate the very fine dust and contribute to lowering the overall dust emission.

Filter Media Selection
for a Bag House Many different chemical fibre based filter bags have been used to treat the boiler flue gas in US, Australia and South Africa in their power plants. The earliest fibre used was Homopolymer Acrylic (PAN) which was then easily available at an economical cost and able to withstand the acidic and moist nature of the hot flue gas. At the same period, glass fibre based filter bags were also used mostly in US power plants.

But in the last decade, newer and better chemically and mechanically stronger fibres came into market for filter bags - Of these the following two fibres became popular in Coal Fired power plant bag houses.

PolyPhenylene Sulphide (PPS) fibre
Presently this is most accepted fibre used for making filter fabrics for boiler flue gas filtration. Filter bags made of this fibre media, generally operate well in the temperature zone of 140 to 160C and can take surge up to 190C. The filter bags are designed to last a minimum of three years, under properly maintained bag house operating conditions.

The fibre is chemically very resistant, specially to strong acid and moisture. The only drawback is its vulnerability to thermo-oxidation in presence of Free oxygen (less than 6 per cent) and NOx (less than 300 ppm) at high temperature.

Polyimide (P-84) fibre
Another high temperature resistant chemical fibre that can be exposed to temperature levels of 240C. The fibre has a multilobal cross section, thereby able to present more surface area for filtration compared to other fibres. The performance of filter bags made of this fibre have been exceptional and hence are recommended where operating temperatures of the dust laden flue gas is in the range of 190C -200 C.

Because of the high surface area provided by these fibres, P-84 is being used more and more in blends with PPS fibres. By this way, there is improvement in the filtration efficiency (lower dust emission), reduction in Differential Pressure (DP) and bag life. These bags are relatively more expensive than plain PPS bags but in the overall analysis the blended bags prove to be cost effective. Many coal fired power plants are now shifting to use of P84+PPS bags.

Supreme Nonwoven Industries Pvt. Ltd.[SNIPL] - Role in development of new and improved filter media for Power Sector:
The company has been active in developing a variety of filter media for cement, iron and steel, non-ferrous metal industries and power over the past 25 years. The filter media is marketed under the brand name - NOWOFILT. Many of our ´Nowofilt´ qualities are time tested and have become industry standards for their application, due to their improved performance and its suitability to the existing bag house demands.

The company has fully integrated, state of the art manufacturing facilities and is the largest producer of Nonwoven filter media for filter bags in the country. The company has every nonwoven fabric producing technology (using staple fibres), and this gives a clear edge for us to develop and innovate new filter products using these technologies in-house.

For power plants, SNIPL have offered many PPS filter bags to old (ESP Retrofit) and new (ESP-Hybrid) bag houses in the last many years with great success. We offer filter bags using in 100 per cent PPS media, as well as PPS in blends with P-84, PTFE etc. Our Polyester bags and Meta-aramid (Nomex or equivalent) bags are also used in many power plants in Buffer Hopper and Transfer points bag filters.

Conclusion
With the growing population and need for more power for economic growth, there is going to be more power generation in the country. Thermal power using coal will be the first choice, which is bound to put ambient air quality at greater risk. The stricter norms for air pollution that industries have to comply with is possible only by having the most effective filtration of both suspended particulate matter (SPM) and of gaseous pollutants SOx and NOx.ESPs´ will not be able to manage this alone and Bag Houses using filter bags (made of textile fibres) are the only means to achieve these low dust emission levels. Also since bag houses can double as reactors to scrub the gaseous pollutant, they are in away complete APC equipment.

Authored by VS Rajan, Filter Division, Supreme Nonwoven Industries Pvt. Ltd.

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