Coal, like petroleum, is widely understood to have biogenic origins - that is, it is comprised of plants and animals of yore that have matted down and been compressed and/or heated underground until they’ve transformed into the stuff we see today.
So why is coal so laden with mercury? And, if the answer to that question gets made clear here, why is petroleum not similarly laden with mercury?
Both coal and natural petroleum have all kinds of heavy metals and other trace elements.
The reason you might not think so is that petroleum is refined before use. You’re not putting straight crude oil into your gas tank - it’s been separated into each grade of hydrocarbon and most trace minerals have been removed. Lead was particularly prevalent in gas until laws required it to be removed; if anything was going to “settle out” of petroleum naturally, you’d expect lead to.
Coal is just dug up and shoved into a furnace. Because it’s mostly carbon and burns efficiently, the remaining ash concentrates a lot of trace materials - it’s actually fairly radioactive due to traces of things like uranium.
As others have said, there’s small amounts of mercury and what have you in most biological systems. It may be in parts per million, but uts there. If you burn 1 million tons of coal, that means you release a ton of mercury, which is a lot.
Depending on its source, coal can have anywhere from 2 to 25% ash. Ash is primarily inorganic matter that is not combustible. While a geologist could probably offer a better explanation, I imagine that the ash content is a matter of the process, over time, of how the coal was formed and what mineral matter was mixed in.
It’s worth noting that it takes somewhere between 6,000,000,000 to 8,000,000,000 pounds of coal per year to produce 1000 MW of electricity (here’s a reference for amount of coal to produce 100 watts that requires converting to 1,000 MW - http://www.howstuffworks.com/question481.htm.)
Therefore, a trace metal like mercury, that may be present at a concentration of 1 part per million, will have 7,140 pounds per year of it released to the atmoshpere. It is discharged to the atmosphere since mercury would be volatilized in the boiler and carried away in the flue gas. The same would apply to lead which would also be volatilized. Other metals, such as sodium, would remain in solid form as bottom ash or collected as part of the particulate matter removed in the flue gas electrostatic precipitator.
The root cause of the mercury in coal (which is more along the lines of 0.01 to 0.30 ppm, varying widely depending upon the source) is often thought to be a result of the high chloride content when compared to petroleum. There is much dispute over this, however, although it is agreed that chlorides do impact the speciation of coal, which could have an impact upon how much of the total mercury the coal started with can remain in the deposit.
Note that petroleum can contain from almost none to as much as 3 ppm of mercury. The difference here is in preparation - coal around the world typically only goes through the most rudimentary of cleaning and preparation prior to being used. Sometimes you’re lucky if you even get a Bradford Breaker to treat it. In any event, what you burn at a power plant is largely what came out of the ground.
Mercury is not always contained in the ash (which can range from 1% for a coal like Adaro, to more than 50% for lignites and those strange Chinese and Korean anthracites); the organic and elemental mercury species are sometimes well-distributed through the organic matrix of the coal.
In the case of crude oil, however, mercury, vanadium, nickel, and other heavy metals are typically removed or significantly reduced during the refining process, mandating treatment in refinery wastewater systems.
It should be noted that only a fraction of the mercury in coal is emitted to the environment. The EPA ICR program found that passive mercury removal throughout the combustion process and from emissions equipment can remove (roughly) 40% to nearly 90% of the mercury. I’ve personally witnessed mercury tests on power plants with only a fabric filter baghouse which saw 50% passive mercury removal, and I’ve personally witnessed a test at a power plant which had an SCR, scrubber, and baghouse which had 96% passive mercury removal. But the data is all over the map - testing I witnessed at two sister power plants with identical emissions equipment burning the same coal from the same train reclaim showed one had about 45% passive mercury removal, the other 25%. The reasons why were a mystery to us. Honestly, however, I’m not telling anyone anything they shouldn’t already know if they’ve read the EPA ICR reports.
You don’t know the half of it. Bradford took his earnings from the first Bradford Breakers he invented and put them into Western Union stock, then into Westinghouse and then later into an upstart company named “General Electric.” At the time he retired he was worth more than $620,000,000 (about $1e+28 in devalued 2009 currency), owned a private island off the west coast of Africa, and lived in luxury and splendor among his harem of ex-Ottoman Empire sex slaves. He died at age 105, buried in money, women, and erotic-scented oils and unguents. They say he was…happy, but who knows?
I never heard of this guy before. Is there any biographical information available? (I tried looking on Google Books, Google Scholar, the New York Times archives and the local library’s online resources and found very little.) He sounds way too interesting not to have a Wikipedia article on him.
No, I don’t have my units confused. One pound of coal, depending on the coal, will get you somewhere between 5,000 BTU’s (lignite) up to perhaps 22,000 BTU’s (eastern Pa. anthracite).
BYW - A 1,000 MW plant is typical of the boiler/generator units built in the 60’s and 70’s and there are fair number of these in existence today. IIRC, Monroe Generating Station, of what was Detroit Edison at the time, has 4-800 MW coal burning units.
Most power plants burning oil use #6 Crude Oil, which is unrefined, and therefore the metals such as vanadium, etc are not removed particularly so with Venezuelan crude which as the highest levels of vanadium).
I’ve never seen any coal on an as-received basis which could be supplied in quantity at more than 14,500 Btu/lbm. Given that I have about 12,000 coal quality samples in my database and I don’t have anything higher than 15,300 Btu/lbm, I’m thinking you mis-typed 12,000 Btu/lbm as 22,000 Btu/lbm.
DTE Energy Monroe is not the example I would use; I worked at Monroe this last year, in fact, (and have since the early 1990’s) so I have some familiarity with those units.
We need to be very careful about whether we mean “unit” or “plant” as well.
While it is entirely true that #6 has more heavy metals than lighter oils, #6 is not crude oil, it’s residual oil - there is an important difference, and there is an ASTM specification D396 - see Table 1 under that specification, where it shows that #6 has specifications for kinematic viscosity, flash point, and water and sediment (although, in fairness, subsection X22.214.171.124 states that specific distillation percentages are not mandatory for fuel oils #4, #5, and #6. One could take this as implying that distillation is not mandatory for those fuel oils, except that one would be hard-pressed to find oil that met all #4, #5, and/or #6 requirements without distillation. Nor will I preclude that there are some crude oil burning plants out there - there are, but I’ve not seen any on this continent, only when I was working in Caracas in the late 1990’s).
“Residual Fuel Oil: The heavier oils, known as No. 5 and No. 6 fuel oils, that
remain after the distillate fuel oils and lighter hydrocarbons are distilled away
in refinery operations. It conforms to ASTM Specifications D396 and D975 and
Federal Specification VV-F-815C. No. 5, a residual fuel oil of medium viscosity,
is also known as Navy Special and is defined in Military Specification MIL-F-
859E, including Amendment 2 (NATO Symbol F-770). It is used in steampowered
vessels in government service and inshore electric power plants. No. 6
fuel oil includes Bunker C fuel oil and is used for electricity generation, space
heating, vessel bunkering, and various industrial purposes.”
Residual oil may be to a large extent the leftover heavy fractions of crude oil, but that by no means says it’s unrefined. It’s a subtle difference, because crude oil is often treated prior to even entering the primary refinery stream to remove excess water, suspended organic and inorganic matter, etc. The extent to which it is treated varies but it is not crude oil.