Monday 7 December 2009
Tuesday 1 December 2009
electricity footprints
About those fuels...
I was trying to calculate my carbon footprint with various calculators on the web. One of the main differences between the calculations is due to how electricity use is counted. In the UK, where most electricity is produced by coal burning power plants, we produce 0.537 kg CO2 for every kWh of electricity. In California, PG&E estimates 0.238 CO2 / kWh. That's more than a factor of 2! Why the big difference?
You don't have to go very far to find a lot of statements like 'natural gas produces less CO2 than coal'. This bothered me because it seemed like both were carbon compounds from more or less the same source (plants and animals of the carboniferous, mostly), and combining them with oxygen shouldn't be that different. But it is.
It turns out that the main difference is due to breaking carbon-hydrogen (C-H) bonds and forming hydrogen-oxygen (H-O) bonds.
When these fuels burn, the carbon and hydrogen in them combine with oxygen to form CO2 and water.
fuel + O2 ---> CO2 + H2O
The different grades of coal have different ratios of carbon to other atoms, but commercial coal used in power plants (bituminous coal) is at least 70% carbon. The rest is mostly water. Wikipedia lists sub-bituminous coal as 6% or less hydrogen. Moreover, if this hydrogen is already tied up in water, it isn't available to be burned.
In contrast, methane, the main component of gas delivered to my water heater, has a formula CH4. Each mole contains 12 g of carbon and 4 g of hydrogen, so it is 25% hydrogen by weight. Also, that hydrogen is all bonded to carbon, so it is available for burning.
The result is that a power plant burning coal produces something like 1.2 kg CO2 per kWh of electricity, while a power plant burning natural gas only produces 0.7 kg CO2 for the same energy output.
Now add in a few wind farms, solar projects, and a hydroelectric dam and the differences between the UK and California electricity footprints make a bit more sense.
Too bad my computer doesn't run on natural gas.
I was trying to calculate my carbon footprint with various calculators on the web. One of the main differences between the calculations is due to how electricity use is counted. In the UK, where most electricity is produced by coal burning power plants, we produce 0.537 kg CO2 for every kWh of electricity. In California, PG&E estimates 0.238 CO2 / kWh. That's more than a factor of 2! Why the big difference?
You don't have to go very far to find a lot of statements like 'natural gas produces less CO2 than coal'. This bothered me because it seemed like both were carbon compounds from more or less the same source (plants and animals of the carboniferous, mostly), and combining them with oxygen shouldn't be that different. But it is.
It turns out that the main difference is due to breaking carbon-hydrogen (C-H) bonds and forming hydrogen-oxygen (H-O) bonds.
When these fuels burn, the carbon and hydrogen in them combine with oxygen to form CO2 and water.
fuel + O2 ---> CO2 + H2O
The different grades of coal have different ratios of carbon to other atoms, but commercial coal used in power plants (bituminous coal) is at least 70% carbon. The rest is mostly water. Wikipedia lists sub-bituminous coal as 6% or less hydrogen. Moreover, if this hydrogen is already tied up in water, it isn't available to be burned.
In contrast, methane, the main component of gas delivered to my water heater, has a formula CH4. Each mole contains 12 g of carbon and 4 g of hydrogen, so it is 25% hydrogen by weight. Also, that hydrogen is all bonded to carbon, so it is available for burning.
The result is that a power plant burning coal produces something like 1.2 kg CO2 per kWh of electricity, while a power plant burning natural gas only produces 0.7 kg CO2 for the same energy output.
Now add in a few wind farms, solar projects, and a hydroelectric dam and the differences between the UK and California electricity footprints make a bit more sense.
Too bad my computer doesn't run on natural gas.
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