Power Buildesign: October 2013

Thursday, October 24, 2013

BAG FILTERS

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BAG FILTER TECHNOLOGY

A baghouse or fabric filter is an air pollution control device that removes particulates out of air or gas released from commercial processes or combustion for electricity generation.

Bag Filter/ Houses are often used to control the emission of pollutants in :

Power plants
steel mills
pharmaceutical producers
food manufacturers
chemical producers
other industrial companies

The bag houses for control emission of air pollutants came into widespread use in the late 1970s after the invention of high-temperature fabrics (for use in the filter media) capable of withstanding temperatures over 350°F.

DESIGN ASPECTS:

Following are the scientific procedure is adopted for designing of Bag Filters:

For calculating filtering area selection of filtering velocity is done from reference tables based on material properties.
This filtering velocity needs to be corrected to suit the actual operating conditions.Following four factors are considered.

1.Application Factor : It varies for oily,moist,agglomerating material and depending on continuous/intermittent operation. e.g.0.9 for heavy dust loads and continuous operation.
2.Temperature Factor: It varies as per gas stream temperature. e.g.1.0 for temperature upto 45 deg
3.Particle size factor : It depends on the particle size. e.g.0.8 for below 3 micron.
4.Dust load factor : It varies as per dust loading per volume of gas stream.e.g.1.2 for 10 or less grains/cu.ft.

In addition to the filteration velocity,consideration of "CAN VELOCITY" is critical while sizing bag filter. CAN VELOCITY is the velocity in the flow passages between the filter bags.
With high CAN VELOCITY,the dust particles,which have been cleaned off the bags,get re-entrained in the gas stream. This re-entrained dust gets re-deposited on the bags. This results in higher pressure drop and consequential deterioration in bag filter performance.
The CAN VELOCITY is selected based on the material,bag length. This decides the bag filter housing.

Unlike electrostatic precipitator where performance may vary significantly depending on process and electrical conditions, functioning baghouses typically have a particulate collection efficiency of 99% or better, even when particle size is very small.

Background information

Filtration, using cloth filter as media is one of the most reliable, efficient and
economic methods by which particulate matter can be removed from gaseous
streams. The theory of removal of particulate matter by a bag filter is not
thoroughly known. It is postulated that the initial deposition of particles takes
place through interception and impingement of the particles on the filter bags
because of combined activity due to diffusion, electrostatic attraction and gravity
settling.
A bag filter consists of numerous vertical bags of 120 to 400 mm diameter and 2
to 10 m long. They are suspended with open ends attached to a manifold. The
hopper at the bottom serves as a collector for the dust. The gas entering through
the inlet duct strikes a baffle plate, which causes the larger particles to fall due to
gravity. The carrier gas then flows to the tubes and then outward through the
fabric leaving the particulate matter as a cake on the bag surface.

Efficiency during pre-coat formation is low but increases as the pre-coat (cake) is
formed. Once formed, the pre-coat forms part of the filtering medium that helps in
further removal of the particulate matter. The accumulation of dust increases the
air resistance of the filter media and therefore filter bags have to be periodically
cleaned. They are cleaned by rapping, shaking or vibration or by pulse jet or
reverse jet air flow, causing the filter cake to be loosened and to fall in the
hopper. The normal velocity at which the gas is passed through the bags is 0.4 to
1 m3/minute.
The efficiency of bag filters are affected by the following four main factors:
1. Filter ratios – Filter ratio is defined as the ratio of carrier gas volume to gross
filter area, per minute flow of gas.

2. Filter media – It is important to have filter media that are temperature

resistant, resistant to chemical attack and abrasion resistant.

3. Temperature – Fabric filters do not perform properly if the gas temperature exceeds the upper withstand limit of the fabric material. Generally the upper temperature limit for bag filters is about 290oC. Another temperature related problem occurs when the stream contains a reactive gas like SO2 and SO3 that can form acid if the temperature in the bag filter falls below the dew point.

4. Bleeding – Bleeding is penetration of the fabric by the fine particles and can occur when the weave is too open or if the filter ratio is too high.

Filter Cleaning

Following are the common methods of filter cleaning in a bag filter:

Rapping
Shaking
Reverse air flow (back wash)
Pulse jet

The latest technology of cleaning is high pressure cleaning with pulse jets. In a pulse jet bag filter, periodically a jet of high pressure air is blasted down the inside of the bag which is supported internally by a wire frame. During the cleaning operation, the bag is collapsed on the frame because of the pressure of the gas being cleaned on the outside. When the bag is inflated, the dust cake is loosened and falls into the hopper below. The two important advantages of the method are, there are no moving parts and continuous cleaning is possible.
It is

Bag Filter with showing Duct

not necessary to isolate an entire row or a compartment from service.
In improved pulse jets, the only nozzle for passing compressed air above the bag
is replaced by a venturi on the top of the bag so that all the air pressure is used
to create a pressure wave down the inside of the bag. With this mechanism, even
hygroscopic particles are removed but the disadvantage is due to high
mechanical stress that can rupture the bags.
Pulse jet bag filters were designed to operate at higher air to cloth ratio than
other cleaning styles while handling the same volume of airflow in a small physical shape. Generally requiring less housing, the pulse jet filters relies on filter bags that hang vertically and are firmly held in place by clamps, snapbands or holddowns.
When dust laden gas enters the system and comes in contact with the filters, the dust is collected on the outside surface. To clean the filters, a blast of compressed air is directed into the top opening of the filter. The air is supplied through a blowpipe which feed into venturies (to increase the velocity) located above each filter. The air blast creates a shockwave that causes the fabric to flex down the length of the filter. As the filter flexes, the dust cake fractures and dust falls into the hopper below. The cleaning frequency and cycle for the pulse jet system is critical for maximum efficiency and is set by an adjustable timer to ensure proper cleaning. Pulse jet cleaning requires no moving parts, cleans on demand.

Bag House in Steel Plant

F I LT ER CLEANING USING PULSE JET

A comparison between low ratio bag filters and high ratio bag filters has been
drawn and shown below.

The advantages & disadvantages of high ratio bag filter are as follows:
1. More compact design due to higher filter velocities
2. On-line cleaning of bags, compartment isolation not needed
3. Bags and cage replacement from the clean air side of the unit.
4. Operating temperature limited by synthetic filter media
5. Bag cage required for every bag
6. Limited bag length (upto 8 m)
7. Bag life less than reverse gas type low ratio filter.

The advantages & disadvantages of low ratio bag filter are as follows:
1. Use of woven fiber glass media up to 450o F
2. Bag length up to 36 feet
3. Increased bag life compared to high ratio
4. Larger ground space required
5. Off-line cleaning necessary
6. Increased capital cost

The general arrangement of bag filter based on low ratio design and high ratio
design is shown in figure

Working Principle of Bag Filter:

Filter Media
While selecting the filter media for bag filters, the following characteristics of the
carrier gas needs to be considered.

Carrier gas temperature

Carrier gas composition

Carrier gas flow rate

Size, shape and concentration of dust particles in the carrier gas.

As for the fabric concerned, its abrasion resistance, chemical resistance, tensile
strength and permeability needs to be considered. The physical properties of
some common fabrics are shown below.

Operating temperature 0C is maximum continuous, tensile strength in kg/cm2
The summary of various filter media that are applied in bag filter is given below.

BAG FILTER TECHNOLOGY -WORLD EXPERIENCE

1.Tallawara Power Station

The first medium pressure pulse jet type bag filter was installed on oneunit of 30 MW boiler at Tallawarra Power station in 1982 (by ABB Alstom and named Optipulse filter). The bag filter contained 648 acrylic bags. The pulse jet bag filter uses a medium pressure (200-500 kPa) pulsing system to clean the bags. This optipulse system feature large, fast opeing pulse valves and an air distribution system to the bags of much larger diameter than those normally used on conventional high pressure pulse systems.This is done in order to minimise pressure losses. No venturi is used at the top of the bag so that in combination with the air distribution system, the pulse energy generated by the fast acting valve is not dissipated,allowing effective pulsing of long filter bags. The optipulse features a unique gas distribution system. In earlier pulse jet filters, dust laden gas enters the bag filter via the hopper and risen upwards to the bags. In doing so, the dust cleaned off the bags during the on-line pulsing had to overcome the upward gas flow in order to settle in the hoppers for removal. This upward flow limited the length of bags which could be cleaned. In the optipulse system, gas enters the bag sideways across the bags, so that the gas flow is across and down, actually assisting removed dust to fall to the hopper.

2.Tennyson, Bulimb, Callide A

In Queensland, the Queensland Electricity Commission (QEC) placed order for installing bag filters in twenty eight boiler units at Tennyson, Bulimba and Callide Power stations, which were commissioned in 1983. At the time of placement of order, this was the largest air pollution control contract ever let in Australia. All bag filters were fitted with PTFE filter bags, some of which are still in operation today. The PTFE filter bags can sustain operation for 5 years but the cost was around US 130$ each in 1982. The boilers at Tennyson and Bulimba were of the stroker type, while the
boilers at Callide fired pulverised coal. The bag filters on each 32 MW boiler unit at tennyson were fitted with 1080 filter bags 6 m long. The bag filters on each 16 MW boiler units at Bulimba were fitted with 714 filter bags 6 m long. The bag filters at Tennyson and Bulimba performed satisfactorily. The bag filters at four units of 30 MW boiler units at Callide power station did not perform well and the PTFE filter bags were unable to filter the fine ash particles generated from the power station. The resultant differential pressure was high and the fine ash particles migrated through the filter media. Following investigations, the problem was overcome by extending another 33% filter area to the existing one. Each of the 30 MW boiler unit 21 at callide power station was extended to contain 1440 filter bags 6 m long. The bag filters operated satisfactorily.

3.Munmorah Power Station, Unit 4

In 1986, the ECNSW placed order for optipulse filter for the 350 MW pulverised coal fired unit 4 at Munmorah power station. The fabric filter was to be installed within the existing 4 gas path, 3 field ESP. the workscope included the supply of new goods and personnel elevator, ID
fans, motors and silencers. He bag filter consist of eight separate compartments, each containing 1140 acrylic filter bags 7.2 m long, giving a total of 9120 bags and a filter velocity of 0.02 m/s. The emission level was 50 mg/Nm3 and the DP across the filter bags was 1.65 kPa. Each of the eight compartments can be isolated for servicing. The eight compartments were formed by adding a division wall along the centreline of each of the four ESP gas flows and modifying the inlet ducting system to allow the installation of additional inlet dampers. The entire outlet ducting system was replaced to accommodate new outlet dampers, silencers and ID fans. The plant was commissioned in 1988 and performed satisfactorily. The initial set of bags was replaced after 33,000 service hours, due to gradually rising DP limiting unit load. Less than 100 bags were found to be defective and replaced during the total service hours. There is no bag filter bypass and there was no noticeable effect from boiler start-up or shutdown. Boiler start-up and flame stabilisation at low load is done using light fuel oil having 1% minimum sulphur content. The boiler problems occurred
during the life of bags includes number of incidents of tube leaks and a serious incident of flame out during start-up when 1000 litres of oil reached the bags. At this stage a white plume was visible at the stack top and oil could be seen running down the inside of the bag filter case. The bag filter experienced very high DP and the boiler was shut down. The plant was restarted with oil firing to heat the fabric filter and evaporate the oil, after which the unit was brought back to normal operation. Within three days there was no noticeable effect from the incident.

4.Liddel Power station Units 1 to 4

In 1989 ABB Alstom received an order from ECNSW to retrofit optipulse bag filters within the existing ESP on the four units of 500 MW boilers at Liddell power station. The existing ESP contained 5 gas paths each of 3 fields. The gas flows from primary to secondary air heaters were kept separate in order to avoid high resistivity part of the resistivity Vs temperature curve. The secondary gas stream was running at about 110oC and the primary gas stream at about 170oC. The scope for the project included the following parameters:

Mixing the primary and secondary gas streams before the gas reached

the bag filter Upgrading the ID fans by adding new impellers and
motors

Supply of a compressed air system

Supply of goods and personnel elevator

Supply of all control systems and instrumentation necessary for the

operation of the plant

Modifying the ash removal system.

The bag filter for each boiler unit contained 14664 filter bags 8 m long.The bag filter contains five gas flows. The bag filter was installed during a 10 week outage. The bag filter operated satisfactorily and bags were replaced after 23,000 service hours. The bag replacement was
necessitated by a gradual rise in DP and bag shrinkage. The shrinkage causes the bags to become tight on cages and reduces the effectiveness of the cleaning system.Shrinkage of acrylic filter bags in high ratio filters has become a major concern in recent years. Initially the filter material was fabricated using Dralon T fibre manufactured by Bayer-Germany. Now Bayer has stopped
the manufacture of Dralon T. Therefore, alternative homopolymer acrylic materials are used.

FUME EXTRACTION SYSTEM

BAG HOUSE IN HYUNDAI STEEL

The Bag house used for the fume extraction in Steel Plant have very specific design considering all parameters used in processing of the steel.

Most baghouses use long, cylindrical bags (or tubes) made of woven or felted fabric as a filter medium. (For applications where there is relatively low dust loading and gas temperatures are 250°F or less, pleated, nonwoven cartridges are sometimes used as filtering media instead of bags.).Dust-laden gas or air enters the baghouse through hoppers (large funnel-shaped containers used for storing and dispensing particulate) and is directed into the baghouse compartment. The gas is drawn through the bags, either on the inside or the outside depending on cleaning method, and a layer of dust accumulates on the filter media surface until air can no longer move through it. When sufficient pressure drop (delta P) occurs, the cleaning process begins. Cleaning can take place while the baghouse is online (filtering) or is offline (in isolation). When the compartment is clean, normal filtering resumes.

Baghouses are very efficient particulate collectors because of the dust cake formed on the surface of the bags. The fabric provides a surface on which dust collects through the following four mechanisms:

Inertial collection - Dust particles strike the fibers placed perpendicular to the gas-flow direction instead of changing direction with the gas stream.
Interception - Particles that do not cross the fluid streamlines come in contact with fibers because of the fiber size.
Submicrometre particles are diffused, increasing the probability of contact between the particles and collecting surfaces.
Electrostatic forces - The presence of an electrostatic charge on the particles and the filter can increase dust capture.

A combination of these mechanisms results in formation of the dust cake on the filter, which eventually increases the resistance to gas flow. The filter must be cleaned periodically.