Discussions of boilers often revolve around the type of boiler, its combustion system, water and steam system, or the application it has in a given industry or process. This is very understandable because certainly these aspects do form the core of a boiler’s function. However, another important aspect of boilers is how they handle emissions. Because of this function it is necessary to turn our attention to boiler baghouses and the role they play in the performance of a boiler.
What Is a Baghouse?
A baghouse is a large-scale air pollution control device. It functions by filtering particulates such as dust, ash, soot, and other impurities out of the exhaust from the boiler. As the name implies a baghouse is literally a large enclosure that houses “bags,” which in this case act as filters that remove the pollutants. Industrial baghouses may be entered from time to time by personnel for cleaning, maintenance, and bag replacement.
Why Are Baghouses Needed?
In the earliest years of the industrial era, prior to a greater understanding of the health and environmental risks associated with pollution, boilers typically released their emissions directly into the air. Over the years as greater awareness has developed, emissions have begun to be regulated by organizations such as the EPA (Environmental Protection Agency) in the US and standards have been set on a global scale as exemplified by the ISO 14000 series.
One method of collecting soot and ash from boiler exhaust is electrostatic precipitators, which use an electrostatic charge to remove particulates. However, the performance of electrostatic precipitators can vary depending on processes and electrical conditions. By contrast, baghouses are very reliable, even with very small particle size. They typically have a collection efficiency at or exceeding 99%.
How Do Baghouses Work?
Baghouses are set up above large, funnel-shaped containers called hoppers. The hopper feeds the dust-laden exhaust into the baghouse, where it then passes through long, cylindrical bags that consist of either woven or felted fabric. The fabric of the bag allows air to pass through it, but it captures particles and pollutants, thereby acting as an extremely effective filter. Baghouses typically accomplish their filtration objectives through one of the following four collection mechanisms:
Inertial collection – In inertial collection the dust particles strike the bag fibers at an angle perpendicular to the gas flow. The inertia traps them instead of allowing them to change direction with the gas stream.
Interception – In interception the particles are literally intercepted as they come into contact with the fibers of the bag because the fiber size is too small to allow them to pass through.
Brownian Movement – In Brownian Movement…very tiny, submicrometer particles are diffused. This diffusion increases the probability that contact will occur between the particles and the fiber collection surfaces.
Electrostatic forces – Electrostatic forces also aid in particle filtration. This is because the presence of an electrostatic charge between the particles and the bag fibers increases overall dust capture.
Most baghouses function by using a combination of these collection mechanisms, rather than a single one. Integral to the existence and function of baghouses was the invention and development of high-temperature fabrics in the 1970s, since the bags themselves are comprised of these high-temperature fabrics and can thus resist the high heat that is characteristic of boiler exhaust.
What Baghouse Systems Exist and What Are Their Pros and Cons?
The two most common baghouse systems are Reverse Air Baghouses and Pulse Jet Baghouses. A third type of baghouse called Mechanical Shaker Baghouses also exists, but is less commonly used for boilers because the air to cloth ratio is low and thus they are not space efficient. What follows is a description of Reverse Air Baghouses, Pulse Jet Baghouses, and the pros and cons of each.
Reverse Air Baghouses – Reverse Air Baghouses features bags that are attached to a cell plate on the bottom of the baghouse and which are suspended from a frame at the top of the baghouse. The boiler exhaust flows up from the hopper and through the inside of the bag and the dust thus collects on the inside surface of the bag while the filtered air passes through to the outside. Reverse Air Baghouses get their name because in order to clean them, clean air is injected in the reverse direction. This causes the bags to collapse slightly which in turn dislodges the caked dust and causes it to fall into the hopper below.
- Pros of Reverse Air Baghouses – Reverse Air Baghouses have a high collection efficiency rate. They are also able to tolerate high temperatures very well and they feature a low pressure drop for equivalent collection efficiencies.
- Cons of Reverse Air Baghouses – A few disadvantages of Reverse Air Baghouses include their relatively low air-to-cloth ratio, the need for frequent cleaning, difficulty removing residual dust buildup, and the requirement that filtered air must be used to clean them.
Pulse Jet Baghouses – Pulse Jet Baghouses use bags that supported by a metal cage, which is attached to the cell plate at the top of the baghouse. Exhaust from the boiler passes through the hopper and enters the baghouse from below. Unlike with the Reverse Air Baghouses, with the Pulse Jet Baghouses the dirty air flows from outside the bags to inside the bags, with the metal cage surrounding the bags preventing the bags from collapsing. When it is time to clean the bags a pulse of compressed air is shot through a venturi nozzle mounted at the top of the bag. The air burst travels the length of the bag, causing it to flex, which in turn dislodges the dust buildup and allows it to fall into the hopper below.
- Pros of Pulse Jet Baghouses – Pulse Jet Baghouses also feature a high collection efficiency rate. They also have a relatively high air-to-cloth ratio which makes them more space efficient and thus requires smaller and fewer bags. They can also clean continuously.
- Cons of Pulse Jet Baghouses – On the downside, however, Pulse Jet Baghouses are not as well suited to high temperatures unless special fabrics are employed. They are also not as appropriate for boiler exhaust that has a high moisture and humidity content.
Baghouses play a crucial role in the emission and exhaust system of boilers. Without baghouses emissions levels would be dangerous and would exceed legal levels. This makes it very important that baghouses, bags, and all component parts be cleaned, monitored, and maintained on a regular basis.
