EGEE 101
Energy and the Environment

Baghouse, ESP, or a cyclone?

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black smoke coming out of a smoke stack
smokestack
Credit: AQMD

The emissions from this smokestack are atrocious! Fortunately, we don't see this often in this country. Now, in other countries, that's a different story. Before there was regulation on particulate emissions this is what the stack of a coal power plant would look like. Even when the stack emissions are not dark we are getting pollution in the form of small particles that we need to clean before they reach the atmosphere. Not only is it a health issue, but it is also a waste of coal. Home fireplaces might produce black smoke because of the inefficient method of combustion but many modern fireplaces might have a catalyst to reduce particulate emissions.

What we need is a method of filtering, or pulling the particles out of the hot flue gas before they reach the stack. For coal-fired utilities, there are 3 approaches: a) Electrostatic Precipitators, b) Baghouses for pulverized coal combustion; for a Fluidized bed, c) a cyclone is commonly used.

Watch the following 3:56 minute video about Particulate Matter.

Click here for a transcript of the video about particulate matter.

Dr. Mathews: So when we discuss particulate matter, we are talking about small particles that are smaller than 10 microns, and there's a sub-classification there of small particles smaller than 2.5. So PM 2.5. These very small particles have a number of contributions to environmental challenges.

And so anything that is on the smaller side, less than these 10 microns, can certainly go into the lungs. They can go into the bloodstream. Certainly, the smaller particles have very large surface areas. This is where carcinogenic material can go on as well. And that's why it's the smaller, smaller than 2 and 1/2 microns that are particularly troublesome for environmental issues, for human health, particularly.

Some of these small particles are going to be aerosols. They contribute to climate change. Actually, in a good way. They reflect more of the energy back, sunlight back, and so actually cause the planets. When we have volcanic emissions, you can sometimes see a cooling event on a couple-year period of time, if not longer in some cases, due to all that fine ash being up in the atmosphere.

And so we need to be able to handle these particulates and prevent them to go into the atmosphere. And we can do that by a couple of approaches. So if we'd like to remove them from a gas stream, we can do it through a cyclone. And so with a cyclone, the material comes in and gets rotated. And through that very, very high centrifugal forces, the particles get thrown out to the sides and can be extracted, whereas the clean air comes out.

In some cases, we will have these particles flowing through filters. And so, again, maybe some will go through. Some will stopped and get absorbed. And so your vacuum cleaner, for example-- we can have some filters that will prevent these going through.

We also have examples where we use electrostatic precipitators, for example. And so what we're going to do there is put an electric charge. That electric charge is going to give the particle a charged surface. And if I have a positive plate, then it'll get attracted to the plate, and we can remove the particulates that way.

And so PM 10, PM 2.5 Again, variety of approaches to clean these emissions, depending on what the size is and what the material is. They contribute, certainly, to regional haze. Obviously, some things like sulfur and aerosols make things like smoke worse. And of course, we've got other issues with very small particles going and affecting human health. And so that's PM 10, PM 2.5.

And just to give you an idea of how small that is, a human hair would be ~80 microns. And so we're talking about very small particles that we can't see very well. And because they're small, they can have a long time in the atmosphere. They're not going to fall and be brought down to the ground very easily. They can be blown around.

Credit: JPM
Bag house at Penn State.
The Baghouse at Penn State.
Credit: J. Mathews

This Baghouse used to be at University Park, it gives you some idea of the scale of these systems. It is about a 6 story building, the flue gas passing through before exiting out of the flue. Within the building, there were 1000's of very large bags much like a vacuum cleaner bag (that can stand >110 °C). This building (and the stack) were demolished when we moved away from coal to natural gas for the steam plant. An Office of Physical Plant (OPP) building sits there now.

Baghouse and Cyclone Systems

We have discussed fluidized beds already in the course (Lesson 02). The one shown below has both a baghouse (labeled as a fabric filter) and a cyclone system to return particles back to the fluidized bed if large enough, or if small enough they go through the baghouse (fabric filter) before the other products of combustion exit the stack. The cyclone uses centrifugal forces to separate particles from a fluid.

Watch the following 1 minute video: Baghouse Basics

Click here for a transcript of the Baghouse Basics video.

There are many different sizes and designs, but baghouses all operate in the same basic way. One. The dust-laden or particle laden air or gas stream enters the bag house, travels along the surfaces of multiple fabric tubes and then passes outward or inward through the fabric. Two. The larger particles fall down into a hopper while the smaller particles accumulate on the fabric surfaces. Three. A cleaning mechanism occasionally removes the particles from the fabric tubes and they fall down into the hopper from which they are discharged. Four. the clean air or gas stream exits from the top of the baghouse.

diagram identifiying the baghouse and cyclone systems of a fluidized bed boiler. The process is described in text and video
A baghouse and a cyclone system. 
Credit: DOE

Here is my effort at showing the particle trajectory. The separation due to centrifugal forces occurs because of the mass difference (size and density differences) between air and the particles (char, ash). The smallest particles have the least amount of centrifugal forces and thus are the hardest to separate (larger particles are much easier) to separate from the air. Obviously, there is a cost to adding these components (power-plants are large-scale operations) to reduce the emissions.

Cyclones: air flows into the cyclone, spirals down, particles come out the bottom and the air flows back up and out the top.
An image of the airflow that occurs within the cyclones.
Credit: ARROW Corp.

You can see a much better particle pathway in this video. Watch this 1 minute of the following cyclone animation. You can skip the first 30 seconds of company blurb.

Click here for a transcript of the Hurricane System - High Efficiency Cyclones video.

Hurricanes are customized cyclones to serve different needs and which results from optimization functions such as maximize efficiency, minimize cost, or minimize size. Combining stochastic numerical optimization by changing the eight independent cycling dimensions. With a knowledge of particle agglomeration ACS is able to design the perfect geometry for each case. The colored dots indicate the particulates to be collected and the blue arrow represents the flow of gas that leaves the equipment.

For dusts with tendency to agglomerate such as biomass or coal fly ash among others. Efficiency is maximized thanks to the clustering effects of particles. In fact, the cyclone doesn't see as many small particles but clustered particles with bigger dimensions and thus easier to collect. Agglomeration increases with wide particle size distributions beyond residence times in the cyclone and large inlet particle concentrations. Agglomeration is maximized with the new developed hurricane MK cyclones. Emissions can be under 15% of those of other cyclones being as low as 30 milligrams a normal cubic metre for many industrial processes.

Electrostatic Precipitators

The schematic below shows a boiler with Electrostatic Precipitators that capture the fly ash (ash leaving with the flue) and the small particulates. Also shown is a low-NOx burner to reduce NOx emissions (less smog and what else???)

Diagram of a fluidized bed boiler with the  electrostatic precipitators called out. Described in text.
Electrostatic Precipitators
Credit: LANL

Read

ExplainThatStuff! has a great article, Electrostatic smoke precipitators. Pay particular attention to the section on how it works.