Monday, April 30, 2007

Mercury in compact fluorescents

Ah drat. A slashdot article appeared about some poor lady in Maine broke a compact fluorescent lamp and freaked out about the small amount of mercury in it. A questionable journalist wrote an article about the freakout.

I had a draft article about the issue: Compact fluorescents contain mercury. How does this amount compare to the amount of mercury released by using a standard incandescent lamp? So enjoy my sketchy calculations below.

Lamp Amount (mg.)
Pre 1988 T-12:~45
Post 1988 T-12: ~11.6
Typical T8: ~4 to 5
Low Mercury T8: ~3
CFL: ~4 to 5

Type Mercury content mg/unit*
Fluorescent lamp: 10
Straight fluorescent with diffusion barrier coating: 6
Compact fluorescent lamp: 4 - 23 W: 5
26 - 55 W: 10

26 watt CF: 8 hours a day, per year uses 75.92 kWh. At roughly 10 cents/kwh (new IL prices), $7.59 a year.
100 watt incandescent 8 hours a day, per year: 292 kWh. $29.20.

every 1000kWh produces 619lbs CO2 for ComEd power, which was 27% coal. (12 months prior to march 2004) Excelon environmental release
(Although recently it's been very highly nuclear, up to 92% in fall 2006).
2/3 of mercury in coal escapes to air

0.17ppm average Hg content in US Coal
Indiana coal varied from 0.31ppm to 0.02ppm

# The thermal energy content of coal is 6,150 kWh/ton. Even though coal-fired power generators are very efficient, only about 40% of the thermal energy in coal converts to electricity. So the electricity generated per ton of coal is (0.4 ton)(6,150 kWh) = 2,460 kWh/ton.
(ISA: Maximize energy buck with efficient bang)

For our 100W incandescent, it burns 292kWh per year, of which 27% is coal, which means 64lbs (29kg) of coal is burned in IL. 0.17ppm of that is 4.93mg Hg, which 66% escapes: 3.29mg Hg.

Our compact fluorescent will produce 26% of that much mercury, or 0.86mg Hg. At 8000 hours lifetime, that's roughly just short of three years of 8hrs/day. So we'd release 2.37mg of Hg for the lifetime of the bulb, while the incandescent would release 9mg over that same time period. Add the existing 4 to 5mg per CFL, and the CF still is releasing less mercury into the environment.

This calculation gets worse and worse the more coal your power is made from. Illinois has the highest production of nuclear power of any state, so using compact fluorescents reduce the release of mercury into the environment, if you ignore renewables like hydroelectric. The nation as a whole uses 50% coal as its power source.

1 comment:

Jennifer said...
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