8 thoughts on “Coal Ash … ick

  1. Also known as fly ash. This went up the chimneys of homes with coal-fired furnaces, which used to be the rule in Appalachia, where I grew up.

    The leftover solid ash on the bottom was usually just dumped or used as landfill. Large facilities would have their bottom ash taken away, I think for making brick.

  2. I happen to know a lot about fly ash. I invented and commercialized a separator to separate unburned carbon from fly ash so it could be recycled and used as a substitute for cement. High carbon fly ash can’t be used in concrete, all you can do is land fill it. My separators have been used to recycle about 8 million tons of fly ash. Each ton of fly ash displaces a ton of cement. To make a ton of cement it takes 2 tons of minerals and releases a ton of CO2 into the atmosphere. If fly ash is recycled into concrete, it reduces CO2 emissions on a ton-per-ton basis.

    Fly ash was not produced by home furnaces. It is only produced in very large industrial sized furnaces. Coal is ground to a find powder (about like flour) then air-conveyed into the furnace and burned as a dispersed powder, sort of like an oil spray, but a powder spray. The coal burns and everything that doesn’t burn, whooshes through the combustion chamber, is quenched in the superheater and air heater. The solid residue is collected and is called fly ash. It is mostly silicate minerals that have been melted and quenched so are now glassy and pozzolanic, that is they form cementitious materials when mixed with calcium hydroxide (hydrated lime).

    The initial innovation of suspension burning of coal, the fly ash was allowed to go up the stack. That greatly reduced the local problem (for the utility) of ash disposal because a lot of it went up and was dispersed over a much larger area. There was some alkali associated with the ash, which helped a tiny amount with the SOx and NOx. There is also iron in the ash, so that may have done some ocean fertilization too. Now, essentially all fly ash is collected. The brown plume that you see coming from a utility stack is NO2, not fly ash. â??Blue plumeâ? is very much fainter and is H2SO4 droplets produced from SO3 when it hits the high relative humidity in an SO2 scrubber. White plumes that disappear are water vapor from the SO2 scrubber or cooling tower.

    A 1000 MW power plant burns about 500 tons of coal per hour and produces about 50 tons of fly ash per hour.

    There is usually unburned carbon along with the mineral. This is coal that simply hasn’t had time to completely burn during the few seconds it is in the furnace. That carbon is somewhat porous and pretty inert. It does have some adsorptive capacity, which is why it is undesirable in concrete because it sequesters the air entraining agents, compounds put in concrete to stabilize tiny bubbles (~100 micron or so) which provide freeze-thaw resistance. Too much carbon and air-entraining is difficult or impossible to make work. The carbon comes from trying to reduce NOx by limiting combustion temperatures and excess air.

    Fly ash is not a highly toxic material. Yes, it does have heavy metals, at ppm levels. Millions of tons of anything can cause a problem wherever it is dumped. The heavy metal content is highly variable. It depends on the coal source, and those are highly variable. You can’t treat all fly ash as if it was all the same.


    The company I work for wants good regulations, regulations that protect the environment and compel companies to do the right thing. Companies doing the right thing have a hard time competing with companies doing wrong things because usually doing the right thing costs more and consumers still want the cheapest price, even if it comes from companies doing bad things.

  3. It’s a little annoying that this video shows nuclear plants (which, obviously, don’t generate any coal ash) while bemoaning the problems of coal ash.

  4. Wow, thanks daedalus2u, that was very informative.

    If I may pick one very small nit, selenium while a relatively large atom, is not a metal.

  5. Steve, I looked through the video again and they all look like coal plants to me. The “smoke” was steam from the cooling towers. They all had stacks which is where the combustion products go.

    I misspoke, The gases do get cooled down when they go though the SO2 scrubber, but they get heated up again so the gases continue to rise as they mix with air (the CO2 in the flue gas gives it a density higher than air at the same temperature). Water vapor is only from cooling towers. Many coal plants have cooling towers too.

    Selenium is classed as as one of the RCRA metals. It is also an essential nutrient.

  6. daedalus2u: Some of the high carbon fly ash is being reburnt as fuel. Proposed Pennsylvania regulations specifically allows for this as a beneficial use of fly ash.

    This Sierra Club video looks to be in response to the Congressional Committee on Energy and Commerce telling EPA
    in a letter that “the proposal to declare this material “hazardous” would cost jobs, raise electricity rates, and deal a death blow to initiatives for reusing ash in building materials and fills.” It simply does not qualify as “toxic” under the definition of the term.

    This issue has more to do with politics than scientific data. Greg, I’m curious, were you swayed by the emotion-generating images? Lots of people are.

  7. S.Hill, I’ve not been swayed by anything. This is one of those things I put up and look for opinions on, and I’m glad to see I’m getting them!

    (I doubt few people would take issue with “ick” to describe anything about coal other than the pretty fossils.)

    I have heard from people who are in the middle of this but not able to comment in public, and I find it quite interesting. When I’ve absorbed the literature people have sent me, I’ll try to sort it out and write something substantive.

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