Take a look at this articleabout a proposed power plant here in Western Pennsylvania which will make use of previously discarded “waste coal,” or coal that burns at too low of a heat to be useful in conventional power plants. Pretty much everywhere you go in this area (on backroads mostly) you see this stuff lying in large piles. I think that it is wonderful that they are going to use this stuff and get good energy from it.
Question: Does anybody know anything about these types of plants? Are there other plants like this one in the U.S.? I know that this is the place to go.
. . . I know that this is the place to go for the answer.
Also, what is the pollution like on these plants compared to traditional plants? Does Waste Coal have a higher concentration of sulfur and other “contaminants?”
I could answer every question you would possibly have about this, but I am trying to find more information. The definition of “waste coal” is a very difficult one, and could mean practically anything with regards to energy content, sulfur content, moisture, ash, etc. I will try to find out what I can later tonight, and then I can tell you everything. But the definitions they are using in the linked article are too generic for me to tell anything.
I mean…there are coal plants that burn incredibly poor quality coal, such as Carbon Dos in Mexico, some Texas lignite plants, and a few European plants I know of. But coal is so incredibly variable in qulaity and impact I need to know what it is they are planning on buring here. It is possible that an Eastern “waste coal” is actually better quality than the glorified dirt that Carbon Dos burns, and so forth.
Waste Coal generating plants…interesting topic. Well, now that I can talk more…
Most plants considered for use with waste coal are fluidized bed power plants, specifically circulating fluidized bed plants. These are advanced types of coal plants that can burn a wide variety of troublesome coals (or other fuels), and produce low emissions of sulfur dioxide and very low NOx emissions. Calcium sorbent can be injected into the bed to remove sulfur, and thus eliminate the need for an expensive flue gas desulfurization plant on the back end of the unit - a huge savings in both capital and operations and maintenance costs. They can also be expensive to build and operate, troublesome to operate, and slightly less efficient than a typical pulverized coal power plant. But if the fuel is nearly free…they can work well. An example of this is a plant operated by the Ebensburg power company, Ebensburg, PA. This is a circulating fluidized bed plant that produces about 172 MWt.
Now…what is a “waste coal”? Well, the definition varies, as there is no official ASTM definition for “waste coal”. Thus, a waste coal is what you make of it. Typically, however, waste coal is coal that comes from the boney parts of the mine - portions of the mine which have numerous, small, troublesome rock seams where the coal cannot be easily separated out. Thus, one ends up with a mixture of about 20-50% rocks and ore, and maybe 30-40% coal. Some examples of waste coals are:
Just a few examples, anyways. Pretty poor stuff as coal goes, but still quite a lot of energy. To compare in terms of heat content alone:
Railroad ties range between 5000 and 8000 Btu/lbm
Sawdust ranges from 4000 to 5000 Btu/lbm
Wheat straw ranges from 5000 to 7000 Btu/lbm
So…pretty poor stuff, considering a typical American anthracite will range from 13,500 to 14,500 Btu/lbm heat content, and a typical Illinois Basin coal will be from 11,000 - 12,000 Btu/lbm. Note these coals all have high ash, and high sulfur contents too. Not good for controlling emissions.
I’m still new to the fly ash business. Can you tell me the names of some of the plants in Texas that burn low-quality lignite? Do they all use fluidized beds?
Thanks for the information, Anthracite. As always, much more information than I could have hoped to receive.
After I posted this, I asked a guy here at work about this plant. He hadn’t heard about it, but he did tell me about the plant in Ebensburg (about 30 minutes from here) and he also introduced me to the term “boney coal.” Just up the road from my place, on PA 119, there is a VERY large coal pile which I am infomrmed is Waste Coal which is being trucked to Ebensburg.
Part of the reason this stuff is so abundant is not only the number of mines in the area (mostly closed. There’s actually a town called “Luzerne Mines”) but several power plants. On a clear cool day you can see the cooling tower steam rising from 3 plants. Homer Center plant (which, I understand, has the tallest stack in the U.S. at over 1,250 feet) is about 5 miles as the crow flies from my house and dominates the skyline. I can always tell which way the wind is blowing by looking at the plant.
One more question: In your carefully constructed chart you mention “Pennsylvania Anthracite Coal.” Is it true that Anthracite coal (in PA) is only found in the Eastern part of the state? I’ve always understood that Western PA coal is Bituminous, or do I have it the other way around? Would you have similar information about Western PA Bituminous coal?
Well, it’s found mainly in the Eastern part, or that is to say, mainly in mineable quantities. Now - like I said, the definition of “waste coal” is what you make of it. If you see my chart above - the waste PA anthracite is actually better quality than some of the lignites being burned in Texas and Mexico. Thus, it really is impossible for me to say what this might be. My numbers came from some actual analyses done by Commercial Testing and Engineering for some coal utilization projects I was once involved with.
But…sigh…in general…I guess one might assume that PA Bituminous coal waste would look sorta similar to the anthracite waste, except I would expect a higher amount of volatiles, a little higher hydrogen, and perhaps more moisture. And a lower heating value - maybe 5500-6000 Btu/lbm. But I must stress to you - there is no definition for waste coal. A really finickey Ukrainian anthracite plant, for example, might consider perfectly good Illinois coal at 11,000 Btu to be “waste coal”. It is not really a meaningful term, in other words.
Thank you for the Texas coal information! I think it’s interesting that the places that seem to be burning the crappiest coal are not at all the same ones that make the worst fly ash.
I was wondering about fluidized beds because I had a fly ash salesman call me the other day and bitch at me because my agency doesn’t buy ash from fluidized beds, and being new to all this, I just assumed that fluidized bed = bad coal = bad ash, but now I know that coal quality doesn’t necessarily have anything to do with fly ash quality.
And…in general, fluidized beds have two (or possibly three) problems with their ash:
They can have a high carbon-in-ash content. In fact, fluidized bed boilers rely on recirculating the fly ash to re-burn the carbon contained in it.
They can have a large amount of gypsum and calcium sulfite in their ash if they are using sorbent to remove sulfur.
Since people often co-fire many odd things with coal in fluidized bed boilers (such as tires, petroleum coke, wood waste, black liquor) the ash may contain many strange and mysterious items that render it unusable.
Early on in my solid waste-ish career, I heard that tires are sometimes burned with coal in power plants, but I have not heard much about it lately. How common is that? How well do scrap tires perform? What types of nasties do they leave behind?
Ahh…I know much about tire combustion and TDF (tire-derived fuel) as it is called.
Tires have a decent amount of energy in them - about 14,000 to 15,000 Btu/lbm of tire. This is pretty high up there in heat - similar to a good-quality bituminous coal or anthracite. However, there is one serious problem with tires, and that is how you actually burn them.
You see, most coal furnaces in the world operate on the pulverized coal principle. This is where the coal is fed into pulverizers which grind the coal to a fineness that is typically 70% of all particles being smaller than 200 mesh, or about 74 microns. This is the consistancy of a rough talcum powder. This powdered coal is blown into the burners by primary air fans, or mill exhauster fans, and burned in the furnace. When the coal is ground this finely, it behaves in a way that is somewhat similar to a liquid fuel or gaseous fuel burner.
But tires…tires cannot easily be pulverized like coal can. They typically end up being fed into the furnace in large pieces. Even pea-sized pieces can be too large to combust properly in a pulverized coal furnace. So if you try to feed these pea-sized pieces of tire into a conventional pulverized coal furnace, then they only partially burn, and fall out into the bottom ash hopper. Where they become a hazard and a damn nuisance. Also, you may only get half of the energy out of the tire.
Now, in a stoker type, cyclone type, or fluidized bed furnace, you can feed large particles or chunks of fuel efficiently. In fact, if the fuel is too fine it will cause problems in these types of furnaces. Thus, since the vast majority of all coal power plants in the world are pulverized coal (on a MW-basis), there is limited opportunity for consuming waste tires.
Now, let’s look at what tires are made of:
(from “Standard Practice for Use of Scrap Tire-Derived Fuel”, ASTM Draft No. 1, October 27, 1999)
So…we see that it has a very high level of carbon, but also a very high level of volatile hydrocarbons. Thus, it should burn relatively well, given a proper furnace residence time. It has a pretty low ash, and almost no moisture - this helps boiler efficiency. A low fuel nitrogen content, which should help to lower NOx emissions. Sulfur is high - this is similar to an Illinois coal sulfur content. The trace elements are comperable to those of many coals. So overall, the only real concern from looking at the constituents is the sulfur.
Basically - you need the right furnace to use them, and sulfur is a problem.
Thanks, Anthracite! Scrap tires are a huge problem in our area, and most of them get chopped up and used as daily cover in local landfills. It’s good to know they do have a beneficial use other than playground mulch.
