I thought coal was the result of buried organic matter (plants mostly) which get their oxygen and hydrogen atoms removed (carbonized), but long storage under heat and pressure. So where does the mercury and uraniumcome from? I/m told that wood is a pretty pollution-free fuel-so if wood is a precourser to coal, where does the nasty stuff come from?
It’s All-Natural™ heavy metal and radioactive contamination, so it’s good for you. Not like those nasty substances from that crime-against-god nuclear power.
Seriously, thought, mercury, uranium, thorium, et al. are all present in trace amounts in nature. Most of these things are recognized as health hazards because biological processes tend to concentrate them. When the living things die and get physically and chemically concentrated, you get something more than trace. There’s also some contamination from materials within the earth supplying that heat and pressure.
It varies from deposit to deposit, of course.
No expert but I don’t think so. It may be lower than - say - coal in greenhouse gas emissions but but it is definitely not a good idea for you personal health to breath in too much.
Paging Una Persson.
coal also contains a lot of radioactive Radon that gets released as it burns. This is produced by radioactive decay in the rocks below and surrounding the coal. It absorbs into the coal.
Si
Wood is most definitely not pollution free. Wood burning fireplaces and wood stoves are considered to be the main source of particulate pollution in many areas. Burning wood also results in some nasty chemicals being released, such as sulfur dioxide and formaldehyde. These are not released in huge quantities, but the mercury and uranium released by coal burning are also in small quantities. We’re not talking about clouds of mercury vapor hanging over the air here. The only reason you have a lot of mercury being released into the environment is because you are burning a huge amount of coal. If you burned wood on the same scale, you’d end up with much more pollution (aside from the fact that you’d also probably run out of forest pretty quickly).
I worked in a coal fired power plant for a while, but I’m no expert on coal (I did control systems not chemistry). It’s my understanding that all living things end up with small amounts of mercury and uranium and many other elements in them. That’s the main problem with dealing with mercury is that once it gets inside of you it never comes out. When you die, all of the mercury you’ve ever been exposed to is inside of you. If you get mushed into coal, the mercury and other elements are mostly still there.
Since this is GQ, I’ll stay out of the nuke vs. coal plant debate, which is where the bit about mercury, uranium, etc. being released by coal plants most often comes up.
For figuring out how plant material gets mercury and uranium in it, I refer you to this little-known researcher.
http://www.straightdope.com/columns/070928.html
It absorbs it from the soil, and sometime from the air itself (although researchers are a bit in disagreement on what the proportion is from each source, and whether leaves take it all from dust or if they can trap radon etc. by having it stick to the leaves directly…yes, I did look up some of the references to help Cecil…)
Wood, and leaves do have mercury emissions from their combustion, and I’ve got a few technical papers where the mercury emissions from biomass combustion are measured. In fact, if you search you can find some forestry service papers on how much in the way of mercury emissions are liberated during forest fires (folks were concerned that firefighters, rangers, etc. might get too high of a dose. The answer is, on a per-person scale, they get very little).
And measuring mercury emissions are very hard, and prone to all sorts of arguments over testing methods and measurements. One frequent analogy people use is “imagine the Superdome/Astrodome is full of ping-pong balls, with 30 red ones in the mix. You have 1 minute to find 27/30.” Sort of a fallacious example, because trapping mercury via activated carbon injection is nothing at all like sorting through ping-pong balls… That aside, I’ve seen the EPA tell a power plant that their mercury emissions were X, but when the best independent testing company in the US was called in to do testing, the plant emissions turned out to be 0.1X - or 3.5X. We’re going to be hearing a lot more about this off and on, with the general public never hearing the whole story due to the sound-bite effect.
For examples of how much mercury coal really contains - values range all over the maps, but for Eastern US coals, one expects to see values ranging from 0.04 to 0.2 ppb (parts per billion, dry whole-coal basis), and for low-sulfur Western coals one expects to see values from 0.01-0.08 ppb. Some coals can be under 0.01 ppb, and some above 0.5 ppb. However - and this is a critical detail - mercury in coal, when the coal is burned, is emitted in three primary forms: particulate (trapped in ash particles), elemental (as metal), and oxidized. The particulate and oxidized mercury are relatively easy to trap, and a large amount of them are trapped by existing emissions equipment at power plants. At worst, no more than 70% of the mercury in the coal leaves the stack, and typically the values are well under 50%. If you have a wet scrubber and fabric filter, you might clean up 90% of more of the mercury without even intending to do so.
So while the low-sulfur Western coals have less mercury overall, more of it tends to be harder to trap, which means that for many plants that have switched to those coals to reduce sulfur (and NOx…) emissions, they may be adding activated carbon injection. As a bonus, however, many of my clients are thinking hard about adding other pollution control equipment well ahead of regulatory requirements so they can get the mercury control benefit. One client of mine is moving ahead with a $300M project without the law even telling them to do so.
Uranium content of coal can range from 0.1 to 20 ppb (dry, whole-coal basis), thorium from 0.1 to 10ppb, and lead from 1 to 200 ppb. There really isn’t much study at all about how to trap uranium, thorium, lead etc. at this time, although activated carbon does trap quite a lot of nasty elements.