Coal power vs. Nuclear: How clean? How safe?

Factual Questions
1

Another thread on nuclear power got me thinking. Since when people say “No Nuclear Power” or “Save the Salmon - Tear Down Dams” the practical effect is for them to say “More Coal Power, please,” I’ve been wondering how they compare. Of course, alternative power sources may render coal AND nuclear obsolete, but for now we’re stuck with them for better or worse.

I’m wondering if anyone (ahem) knows what kind of pollution a coal plant produces?

How dirty is exhaust from a coal plant? Does any of the uranium or thorium (or other radionuclides which naturally occur in coal) end up in the atmosphere for us to breathe? In this thread, Anthracite stated that a noticeable amount of uranium or thorium are generated in the fly ash of a coal plant (which ends up as concrete), I’m curious about other vectors into the environment.

I’d like to know how the pollution from a coal plant compares to the waste from a nuclear plant - barring accidents, at least you can bury the nuclear waste in one place.

With exhaust from a coal plant, we all breath it. Not a problem if it is all H[sub]2[/sub]O and CO[sub]2[/sub], but otherwise…

What other waste does coal produce that is not usable? Fly ash goes into masonry type stuff (bricks, concrete), but what else is there? Is more fly ash produced than is needed? Is there anything that needs to be buried or otherwise disposed of?

2

Considering atmospheric emissions only, you should worry mainly about acid gases: sulphur oxides and nitrogen oxides.

The amount of sulphur oxides produced depends on the amount of sulphur in the coal. For example, if I burn a million tonnes of coal, with a sulphur content of 0.8%, I will produce 35,000 tonnes of SO[sub]2[/sub].

Sulphur oxides can be removed from the flue gases by a number of chemical processes, which convert the harmful gases to valuable products like gypsum (CaSO[sub]4[/sub]) or elemental sulphur and sulphuric acid.

The flue gas desulpurisation process is not 100% efficient (and not every station has FGD), so some SO[sub]2[/sub] still escapes to atmosphere. Some of this gets washed out of the atmosphere as “acid rain” and some combines chemically with other substances in the atmosphere to form secondary pollutants such as ammonium sulphate. These are long-range pollutants with a long residence time in the atmosphere, and a variety of health impacts have been identified by epidemiological studies.

Nitrogen oxides are mainly produced by the chemical combination of nitrogen and oxygen from the atmosphere at high temperatures. The hotter the combustion and the longer the residence time, the more NO[sub]x[/sub] you’ll get. You can reduce the amount produced by designing the combustion process accordingly. Nitrogen present in the fuel also contributes.

It is also possible to remove NO[sub]x[/sub] from the exhaust gases by means of reaction with an ammonia-based reagent. Again, this is not 100% effective.

Nitrogen oxides may be harmful to human health. They also interact with light to form ground-level ozone, which is damaging to plants and human lungs, and attacks rubber, and is also a factor in the formation of photochemical smog.

NO[sub]x[/sub] also contributes to breaking down the ozone layer in the upper atmosphere, allowing the penetration to the earth’s surface of harmful ultra-violet radiation from the sun. N[sub]2[/sub]O is a so-called “greenhouse” gas, implicated in global warming, with a potency factor of around 165 relative to CO[sub]2[/sub]. By combining with water vapour in the atmosphere, NOx tends to form nitric acid, HNO[sub]3[/sub], which falls to earth as acid rain.

In addition, in the presence of ammonia, ammonium nitrate particles will form, which are known to affect human health.

Other than SO[sub]x[/sub] and NO[sub]x[/sub], some particulate emissions will escape the precipitators and be released to atmosphere. These will be of diverse chemical composition reflecting the chemistry of the fuel, including metals such as iron, magnesium, potassium, etc. My guess is that the residence time of these materials in the atmosphere is short and that they are mostly deposited fairly near the plant.

You might also want to consider the global warming effects of CO[sub]2[/sub], which are very uncertain, but may well outweigh all the other impacts.

Overall, is coal worse than nuclear? Probably, yes, very much so. Externalities studies show the impacts of coal generation as being orders of magnitude greater than those of nuclear.

3

What I’m going to do first is post an energy, material, and emissions balance for a very modern US coal power plant running at a high capacity factor.

Note: This is a best case plant. This plant is a very efficient one, which has a flue gas desulfurization (FGD) system to remove SO[sub]2[/sub], a modern electrostatic precipitator to remove particulates, low-NO[sub]x[/sub] burners, and a selective catalytic reduction (SCR) system to remove NO[sub]x[/sub]. So this is a best case plant using the best available control technology. Remember this.

[sub]Disclaimer: I have altered the power output of the plant just slightly, and a few other factors for confidentiality reasons. These will affect the magnitudes, but not the relative magnitudes of the results. I do this all the time for conceptual studies, so I’m confident of the results.[/sub]


Coal Plant Location: Eastern US
Gross MW (what is generated):       465 MW
Net MW (what goes to the customer): 439 MW
Capacity Factor:                    85.0%
Annual Average Net Unit Heat Rate:  9500.4 Btu/kWh (gross)
Unit Type:                          Pulverized coal
Unit Emissions Regulations:         90% SO2 removal
                                    90% NOx removal
                                    0.02 lbm/MBtu particulate
                                    20% Opacity at Stack

And I tried running it with the following representative generalized coals (with summary data only):


                Central       Illinois          Wyoming
              Appalachian    High-Sulfur     PRB Low-Sulfur
Heating Value
Btu/lbm gross:     13,159         11,043              8,802

Moisture, %:         2.98          12.26              23.89

Ash, %:             11.15           9.97               4.46

Sulfur, %:           1.13           2.62               0.30

Nitrogen, %:         1.41           1.18               0.96

I’m going to only look at annual results, and I am going to assume no unit derates occur, and the annual gross capacity factor of 85.0% is achievable.

Here are the results. I will abbreviate the coals “C.App”, “Ill”, and “PRB”.


                           C.App         Ill            PRB
Gross Gen., GWhr/yr:      3477.4      3477.4         3477.4
Net Generation, GWhr/yr:  3276.4      3266.7         3248.6

Inputs:
Coal burned, Mton/yr:       1.18        1.42           1.84
Energy input, MMBtu/yr:    31.13       31.45          32.40
Limestone for FGD,
Kton/yr:                   38.59      107.90          15.73
Ammonia for SCR,
Kton/yr:                   12.29       10.11           7.35
Boiler make-up water,
Mgal/yr:                   124.1       124.1          124.1

Outputs:
Fly ash collected,
Kton/yr:                  105.50      113.57          65.70
Bottom ash collected,
Kton/yr:                   26.38       28.39          16.42
FGD Sludge, Kton/yr:       59.87      167.42          24.41
Mill rejects, Kton/yr:      2.36        2.84           3.68

Gaseous Emissions:
Particulate, ton/yr:       197.4       212.4          123.1
NOx, ton/yr:               462.7       446.0          583.8
SO2, ton/yr:              2660.4      7438.0         1084.6
CO2, Kton/yr:             3187.0      3213.9         4142.2
CO: (not analyzed)
Lead, ton/yr:               1.35        1.05           0.27
Mercury, ton/yr:            0.33        0.23           0.19

Solid Emissions of Radionucleides (in ash):
Thorium, lbm/yr:          1315.7      1278.0          809.6
Uranium, lbm/yr:          1998.0      1610.3          259.0
  (assumes 5 ppm in ash for Thorium all samples)

So, we can look at these on a per-GW*hr (net) basis to see how clean this plant really is:


                           C.App         Ill            PRB
Fly ash, ton/GW*hr:        32.20       32.77          20.22
Bottom ash, ton/GW*hr:      8.05        8.69           5.05
FGD Sludge, ton/GW*hr:     18.27       51.25           7.51
Mill rejects, ton/GW*hr:    0.72        0.87           1.13

Gaseous Emissions:
Particulate, ton/GW*hr:     0.06        0.07          0.038
NOx, ton/GW*hr:             0.14       0.136          0.179
SO2, ton/GW*hr:             0.81        2.28          0.334
CO2, ton/GW*hr:            972.7       983.8         1275.1
Lead, ton/GW*hr:         4.12E-4     3.21E-4         8.3E-5
Mercury, ton/GW*hr:      1.00E-4     7.04E-5         5.8E-5

Solid Emissions of Radionucleides (in ash):
Thorium, lbm/GW*hr:         0.40        0.39           0.25
Uranium, lbm/GW*hr:         0.61        0.49           0.08

Jesus help me if I made some math errors…I know people will jump on me.

4 

1,000,000 tonnes coal * 0.8 % sulfur = 8000 tonnes sulfur

8000 tonnes sulfur * 1.997 tonnes SO2 = 15,976 tonnes SO2
                     ----------------
                      tonne sulfur

Or did I miss something here…?

5

Are we to see the Coal Power Plant Clash of the Titans here now? Start the fanfare… :smiley:

6

Not to get in the middle of this cola plant debate, but the thinking that “No Nukes” and “No Dams” = “More Coal” is not necessarily true. It could also mean “More Natural Gas”. I know anthracite posted numbers about the amount of generation in the US being mostly coal, but the portfolio of generation resources is not uniform across the US. Whereas coal predominates in the Midwest and Northeast, in the Far West (I can get the numbers if necessary), i.e. California, Oregon, and Washington, Coal does not predominate and almost exclusively all new generation being built in California is Natural Gas.

I am not an engineer, so I cannot comment intelligently on the cleanliness of Natural Gas plants relative to other generation resources.

7

Remember also, grasshopper, that a significant amount of the sulfur in the coal can be retained in the bottom and fly ash. This depends on many factors, but has a strong trend as a function of the sodium, calcium, and silica and alumina content of the ash constituents. This sulfur retention can be as high as 73 percent under extreme cases, and for lignites is typically from 10 to 40 percent (percent mass of coal sulfur input retained in the ash and not emitted as SO[sub]2[/sub]). For bituminous coals, this amount of sulfur retention in the ash is commonly assumed to be between 2.5% and 5% of the total.

Sources:

“Some Studies on Stack Emissions from Lignite Fired Power Plants”, G.H. Gronhovd, P.H. Tufte, and S.J. Selle, 1973 Lignite Symposium, May 9-10, Grand Forks, ND, USA

“Estimating Total Sufuric Acid Emissions from Coal-Fired Power Plants”, R. Hardman, R. Stacy, Southern Company Services, E. Dismukes, Southern Research Institute, Sept. 1998, Public Domain Technical Paper supplied by EPRI

8

Another consideration is that the emissions from areas with a high concentration of fossil-fuel power plants combine to cause far greater environmental problems. If concentrations of sulphur oxides, etc. are high enough in soil, they can kill off larger trees. The loss of trees means that soil erodes more quickly, which means even fewer trees, which means less topsoil, until you get barren hillsides.

This has already happened in Central Europe, and I’m sure there are other places at risk as well.

Off-topic plug for small hydrolectrics:

Not everyone who says “Save the Salmon - Tear Down Dams” is advocating the removal of hydroelectric power. I can’t speak for them, of course, but the real problem is the large dams that have a massive environmental impact. “Small Hydrolectrics”, where ‘small’ means likely producing only a few hundred megawatts at the most (at the other extreme, Grand Coulee Dam produces 6000 MW) can be placed in rivers and have much less effect on the environment. Several small stations along a river will produce the same power as a single station, and they can probably deliver it more efficiently. (Total construction cost is likely more than for a large one, though it as at least a well-known technology like coal and so is a currently viable option.)

9

Well, see my first long post for some of these answers.

Yes. Note in my long post, I list several “outputs” from the plant. These are:

Fly ash - light ash which is liberated during the coal combustion process, and is collected in precipitators, fabric filter baghouses, and in flue gas desulfurization (FGD) systems. This light ash can make an excellent concrete, provided that it first has a low unburned carbon content (leftover coal that is not burned properly or completely), and second that it has the proper color, and third that it has the proper calcium and other alkali metal content, and fourth being that if an SCR system is present the level of ammonia entrained in the ash is less than 100 ppm or so.

Bottom ash - heavy ash and slag which rains off of the boiler walls, which is collected in hoppers underneath the boiler. This has the consistancy of gravel/rocks, and has limited use as aggregate and sandblasting media. Often it is landfilled.

Mill rejects - these are oversized pieces of coal, tramp iron, broken pieces of mining machinery, boots, tools, large rocks, etc. that the coal plant pulverizers cannot grind, and thus are “rejected”. They are almost always landfilled, even the large coal pieces, since it is not worth the time to have people pick them out and re-crush them.

FGD Sludge - after the lime or limestone has reached a certain percent solids of CaSO[sub]4[/sub] in the FGD system, these solids must be disposed of so the system keeps the correct solids percent in it’s slurry. This waste of CaSO[sub]4[/sub], CaSO[sub]3[/sub], and some unused limestone is called FGD sludge. It is normally landfilled, although Pure Air corp and others often set up gypsum wallboard plants to produce very high-quality wallboard from the sludge (Bailly plant for NIPSCo comes to mind). However, this is not done that often in the US, due to the very low cost of gypsum.

Used catalyst - SCR systems have used catalyst that must be disposed of. Sometimes this can be sold to reclaimers, but often it is discarded. It is not a significant amount.

Hopefully, the above descriptions will explain some of the line-items in the tables I made.

Well. I will be back later to see if there are other questions.

10

Way cool. I especially dig the per-gigawatt-hour figures, it makes it easy to conceptualize (for me, at least.)

Unfortunately, I don’t fully understand the impact of all outputs. I get acid rain and potential greenhouse effects (though greenhouse is controversial), but I don’t know what all the other products are good/bad for.

Your list:
[ul]
[li]Fly ash[/li]Used for concrete/bricks, I believe. Is all of it used, or is more produced than is demanded? What happens to excess?
[li]Bottom ash[/li]Does this have a commercial use, or is it waste?
[li]FGD Sludge[/li]Sludge doesn’t sound useful. What becomes of this? Is it hazardous (funky hydrocarbon carcinogens, for example?)
[li]Mill rejects[/li]This sounds like input that isn’t really coal (e.g. rocks.) Harmless, I suspect. Sold as gravel/concrete mixings?
[li]Particulate[/li]Dust in the air. People living downwind will have added health problems, just like living in a big city with a diesel bus hub.
[li]NOx[/li][li]SO2[/li][li]CO2[/li](mostly covered by hibernicus)
[li]Lead[/li][li]Mercury[/li]These go up the stack and cause similar problems to particulates, but with the added bonus heavy metal factor. Perhaps I will get motivated and look up how 1 ton Pb per year compares to other pollution sources, such as automobiles in the leaded-gas days.
[/ul]

hibernicus writes:

This probably goes beyond either of your realms of expertise, but I’m wondering how many orders of magnitude? Barring nuclear accidents it is obvious nuclear makes it easier to contain the bad byproducts compared to coal, but what about with accidents? For example, how many years can a coal plant run before it emits enough noxious stuff to make it approximately as bad for the health of the population as, say, Chernobyl?

I don’t have a good feel for this, so it could easily go either way. If Chernobyl is worse than all the coal plants in the world combined burning for a millennium, then nuclear is probably not worth the risk (unless Soviet technology/safety practices were inferior.) However, if it only takes one coal plant 10 years to have the same negative health effects as Chernobyl, then nuclear is the way to go. Anyone have back-of-the-envelope calculations to address this?

11

Doh! And thanks for answering my questions before I ask them. Sorry about the simulpost.

12

Well, since the data is within arms reach I figger I might as well give it. It is not the most up-to-date, but it is certainly illustrative.

According to the WSCC (Western Systems Coordinating Council) 2000 Information Summary, as of January 1, 2000, here is a little bit of the breakdown of the generation resources:

WSCC Total:
Hydro - 38.6%
Steam:Coal - 23.1%
Steam:Gas - 14.8%
Nuclear - 5.8%
Other - 17.7%

NWPP (Northwest incl. Oregon, Washington, Utah, Idaho and large portions of Nevada, Montana, Wyoming and Western Canada):
Hydro - 64.4%
Steam:Coal - 23.8%
Steam:Gas - 3.3%
Nuclear - 1.6%
Other - 6.9%

AZ/NM/SNV (Arizona, New Mexico, Southern Nevada):
Hydro - 19.3%
Steam:Coal - 40.6%
Steam:Gas - 9.3%
Nuclear - 15.4%
Other - 15.4%

CA/MX (California and a teeny bit of Mexico):
Hydro - 24.2%
Steam:Coal - 6.1%
Steam:Gas - 31.4%
Nuclear - 8.2%
Other - 31.1%

I did not include combined cycle plants in these numbers. This is really just meant ot be illustrative.

13

What you missed here was that I started with 2.2 million tonnes per year, then decided to normalise to per 1 million tonnes “for confidentiality reasons”, and forgot to tell my calculator. Damn.

Okay, folks, move along, there’s nothing to see here.

14

This thread proves why we are all here… not only are we seeing a treat (I love it when Anthracite gets going! :smiley: )but we are seeing a rational counterpoint.

I did want to state that I think it is established fact (kind of like the earth is a ball, and goes around the sun) that Sov Union safety and overall design was inferior to ours (the West, and the US specifically).

I have no sites, but if someone really wants to push it I can dig up a truckload…

15

I agree that our design in the West is quite good with respect to safety features at Nuclear Power plants.

If Illinois was a country, and you listed countries with the most power plants, it would be 6th. We’re big on Nuclear Energy here.

If anyone would like more information about nuclear safety, please send me an e-mail. I’m an attorney for the Department of Nuclear Safety and would be happy to provide information.

Of course, if you are very concerned, make use of the Federal Freedom of Information Act and find out what’s going on with your local nuclear power plant. :slight_smile:

e-mail: TibsTanglewood@aol.com

Tibs.

16

Would you be able to give us any information on why Rancho Seco out here in California was shut down before they could even get going?

I’m curious if it will have any bearing on the theories that folks are ignorant.

For those that are uninformed, Rancho Seco was a Nuke Plant our here in Cali that was shut down by popular vote before it could go on-line.

17

I’m in the office tomorrow - I’ll see what I can find out. I can’t give out privleged or confidential info but will tell you anything else I find.

Tibs.

18

In case it hasn’t been mentioned, here a fun fact (I don’t have a cite, unfortunately): A coal burning plant releases much more radioactive material into the environment than a nuclear plant. This is largely because coal contains a small amount of a radioactive isotope of carbon. Nuclear plants have no emissions to speak of.

I heartily support nuclear power. It’s clean, safe (when proper precautions are taken), and (I think) fairly cheap. In fact, I don’t even have the “not in my back yard” sentiment. I really woulndn’t mind it if a nuclear plant were within a few kilometers of my home.

On a side note, I also heartily support hydroelectric power. I just seem to have this silly idea that access to copius amounts of electrical power is well worth the (relatively small) ecological damage that the construction of the dams cause.

19

I’m not doubting you yet, but oddly I have never heard mention of C14 emissions from coal combustion being significant - are you sure that you do not have a link or reference you can point me too? Or else - what do you base this on, or where do you remember seeing it? I have never heard this mentioned in any context before in all my studies - and if it is an issue, I better learn about it.

20

In fact…IIRC, C14 is produced by cosmic rays striking the upper atmosphere, and IIRC if coal was buried and in it’s bed, the C14 contained within it would decay, with no new C14 added. With a half-life of about 5730 years, it seems that after 20 million years or so nearly all of the C14 should have decayed into nitrogen…right? In fact, after 20 million years, the coal should have undergone 3490 half-lives, and only an incredibly small amount of it should be left. Thus, I would hazard that there is only an extremely small amount of C14 in coal.

And Nuke-E’s want to lend me a hand here?