Mercury output for powerplants

[Capt Worley PE]

Does anyone have a good number for the amount in pounds a powerplant generates? Say in terms of a 50MW coal plant puts out 50 tons per year. I'm trying to calculate a trade off, in mercury emissions, for CFLs vs incandescents.

Anybody have any data?

 

[benbo]

Does anyone have a good number for the amount in pounds a powerplant generates? Say in terms of a 50MW coal plant puts out 50 tons per year. I'm trying to calculate a trade off, in mercury emissions, for CFLs vs incandescents.
Anybody have any data?

I don't have a number, but have you checked the EPA website? THey might have some public access permits on there. I think it depends on what kind of plant, because I think plants can put out mercury both into the air and in the discharge of cooling water. So you might want to check NPDES permits as well.

Since you probably just want a ballpark number, this is probably a better question for an environmental engineer rather than me.

 

[mudpuppy]

Quick and dirty calculations:

From the EPA's FAQ on mercury, they say the new law will reduce power-plant emissions by 70% or 33 tons, which back-calculates to about 47 tons of mercury emissions per year--a couple other websites from a Google search confirm this number.

And from the energy information administration, in 1999 about 2 million gigawatt-hours of electricty were produced by coal plants. If my calculations are correct, that would give a small plant like your 50 MW plant, about 20 pounds of mercury emissions per year.

This number is approximately (the same order of magnitude of) what I was expecting (i.e. tens of pounds rather than tons).

 

[Dleg]

It's easy to calculate mercury emissions using the emission factors published in EPA's AP-42 on-line document.

Coal combustion is covered under Chapter 1.1 for bituminous and subbitumunous coal, and Chapter 1.2 for anthracite coal.

Of course, you reallyneed to know some specifics about the power plant, such as what type of combustion system it uses, and what kind of emissions controls are used.

If your 50 MW power plant is burning bituminous or subbituminous coal (anhtracite is pretty limited in use), the mercury output could be calcualted as follows:

Let's assume "controlled" coal combustion. The emission factor for mercury (Hg) would be:

(Ref. Table 1.1-18, AP-42, Sept. 1998)

Bituminous coal: Hg: 8.3E-05 lb/ton (applies per ton of coal feed, as fired)

Note: this factor applies "to boilers utilizing either venturi scrubbers, spray dryer absorbers, or wet limestone

scrubbers with an electrostatic precipitator (ESP) or Fabric Filter (FF). In addition, the factors apply

to boilers using only an ESP, FF, or venturi scrubber."

(I won't calculate it, but I will post the mean emission factor for anthracite coal, which is significantly higher at 1.3E-04 lb/ton, as per table 1.2-7 , AP-42, Oct. 1996)

Now you just need to figure out how many tons per day need to be burned to generate 50 MW. As per the AP-42 chapter 1.1:

As mined, the heating values of typical U.S. bituminous coals range from 10,720 to 14,730 Btu/lb. The heating values of subbituminous coals range from 8,300 to 11,500 Btu/lb on a wet, mineral-matter-free basis, and from 9,420 to 10,130 Btu/lb on an as-mined basis.

Let's just use the mean of the range for as mined bituminous coal - 12,725 Btu/lb, or 2.55E7 Btu/ton.

Converting to kW-hr: 2.55E7 Btu/ton x 2.928E-04 kW-hr/Btu = 7466.4 kW-hr/ton, or 7.45 MW-hr/ton.

To obtain 50 MW of energy from the coal: 50 MW / (7.45 MW-hr/ton) = 6.7 tons per hour.

This is where you would really need to get some specifics from a real 50MW power plant, because obviously there are inefficiencies, and some greater amount of coal would be required to actually generate 50 MW of electricity. I have no idea what those efficiencies might be.

So, just guessing that the boilers and generators are 80% efficient at converting the heat energy, the total coal feed needed would be:

6.7 tph/0.8 = 8.38 tons per hour. So, multiplying by 8760 hours per year, you get: 73,409 tons of coal burned per year. Going back to our emissions factor, we get:

8.3E-05 lb Hg/ton coal x 73,409 tons coal/year =

6.1 pounds per year of mercury, using bituminous coal fired from a plant with emissions controls.

 

[Dleg]

Oh yeah, I suppose for your purposes, you might want to know how much mercury, overall, released from the coal. Because, after all, that mercury goes somewhere, too. Perhaps to the landfill, orwherever it is they dispose of the ash and waste from the emissions controls systems.

Uncontrolled mercury emissions from bituminous coal:

(Ref. AP-42, Sept. 1998, Table 1.1-17)

Hg: 16 lb per 1.0E12 Btu.

So, at 2.55E7 Btu/ton for bituminous coal, mercury is emitted at 4.08E-04 lb/ton, or 30.0 lb/year of mercury for our hypothetical 50 MW plant.

So, we have 6.1 lbs of mercury per emitted to the atmosphere, and 23.9 lbs per year of mercury captured in the emissions controls, being disposed of "somewhere." Regardless of any errors in my assumptions regarding power plant efficiencies, or variations in coal energy content, we do not appear to be talking about values anywhere near "tons."

But please feel free to check my math and my assumptions. I'm not an expert at coal combustion.

 

[Capt Worley PE]

Thanks for the info and links. Just looking at the data, it looks like CFLs put a lot more mercury into the envrinment than the powerplant would generating the extra electricity to run powerplants.

 

[udpolo15]

Thanks for the info and links. Just looking at the data, it looks like CFLs put a lot more mercury into the envrinment than the powerplant would generating the extra electricity to run powerplants.

I think you need to look at exposure. Notwithstanding CFLs breaking in your home, I would say most of the mercury ends up in landfills which limits the exposure. Coal plants on the other hand spread the mercury over a wide area making it available to many receptors. I guess you would also need to consider the bioavailablity of the mercury from each source.

 

[Capt Worley PE]

After doing the calcs, looks like there is 15mg less mercury put into the environment over the life of the bulb. Very rough calcs, though.

 

[C-Dog]

I think you need to look at exposure. Notwithstanding CFLs breaking in your home, I would say most of the mercury ends up in landfills which limits the exposure. Coal plants on the other hand spread the mercury over a wide area making it available to many receptors. I guess you would also need to consider the bioavailablity of the mercury from each source.

you should be recycling the CFLs, so they do not end up in the landfill.

 

[Capt Worley PE]

Recycling isn't convenient. Just tell the grabage guys to handle them carefully.

 

[C-Dog]

Recycling isn't convenient. Just tell the grabage guys to handle them carefully.

I guess it all depends on your local. I do not find any inconvience in recycling them in my neck of the woods.

 

[snickerd3]

we have had no luck with the CFLs...regular bulbs lasted longer. And we are stuck holding onto them for a yr or 2 until the community has a household haz waste collection.

CFLs and thermostats...if the landfill folks just happen to pick that spot in the landfill for complaince sampling they will fail for Mercury and be issued violation notices.

 

[Capt Worley PE]

I hope they don't test where I buried my old Comet.

 

[Guest]

Anybody have any data?

How about a spreadsheet evaluating the exposure due to the a CFL bulb breaking??

It's easy to calculate mercury emissions using the emission factors published in EPA's AP-42 on-line document.
Coal combustion is covered under Chapter 1.1 for bituminous and subbitumunous coal, and Chapter 1.2 for anthracite coal.

Of course, you reallyneed to know some specifics about the power plant, such as what type of combustion system it uses, and what kind of emissions controls are used.

If your 50 MW power plant is burning bituminous or subbituminous coal (anhtracite is pretty limited in use), the mercury output could be calcualted as follows:

Let's assume "controlled" coal combustion. The emission factor for mercury (Hg) would be:

(Ref. Table 1.1-18, AP-42, Sept. 1998)

Bituminous coal: Hg: 8.3E-05 lb/ton (applies per ton of coal feed, as fired)

Note: this factor applies "to boilers utilizing either venturi scrubbers, spray dryer absorbers, or wet limestone

scrubbers with an electrostatic precipitator (ESP) or Fabric Filter (FF). In addition, the factors apply

to boilers using only an ESP, FF, or venturi scrubber."

(I won't calculate it, but I will post the mean emission factor for anthracite coal, which is significantly higher at 1.3E-04 lb/ton, as per table 1.2-7 , AP-42, Oct. 1996)

Now you just need to figure out how many tons per day need to be burned to generate 50 MW. As per the AP-42 chapter 1.1:

Let's just use the mean of the range for as mined bituminous coal - 12,725 Btu/lb, or 2.55E7 Btu/ton.

Converting to kW-hr: 2.55E7 Btu/ton x 2.928E-04 kW-hr/Btu = 7466.4 kW-hr/ton, or 7.45 MW-hr/ton.

To obtain 50 MW of energy from the coal: 50 MW / (7.45 MW-hr/ton) = 6.7 tons per hour.

This is where you would really need to get some specifics from a real 50MW power plant, because obviously there are inefficiencies, and some greater amount of coal would be required to actually generate 50 MW of electricity. I have no idea what those efficiencies might be.

So, just guessing that the boilers and generators are 80% efficient at converting the heat energy, the total coal feed needed would be:

6.7 tph/0.8 = 8.38 tons per hour. So, multiplying by 8760 hours per year, you get: 73,409 tons of coal burned per year. Going back to our emissions factor, we get:

8.3E-05 lb Hg/ton coal x 73,409 tons coal/year =

6.1 pounds per year of mercury, using bituminous coal fired from a plant with emissions controls.

Nice work !!! lusone:

I think you need to look at exposure. Notwithstanding CFLs breaking in your home, I would say most of the mercury ends up in landfills which limits the exposure. Coal plants on the other hand spread the mercury over a wide area making it available to many receptors. I guess you would also need to consider the bioavailablity of the mercury from each source.

Check out my spreadsheet above!

Recycling isn't convenient. Just tell the grabage guys to handle them carefully.

News article about the EASE of handling CFLs ... DEP Encourages Use and Recycling of Compact Fluorescent Lamps

we have had no luck with the CFLs...regular bulbs lasted longer. And we are stuck holding onto them for a yr or 2 until the community has a household haz waste collection.

The Florida Dept of Environmental Protection is currently negotiating an agreement with Walmart to implement a CFL collection and recycling program. From what I understand thier is A LOT of interest and will most likely be pushed as part of the governor's overall energy conservation/sustainability campaign.

JR