Demonstrating the CO2 Weight of a Therm of Natural Gas

We use weights (bottles filled with specific weights of dyed water to represent one gas therm's actual weight of, carbon in the fuel, oxygen for combustion, CO2 from shrinkage (pipeline consumption) and Equivalent CO2e from leakage of methane.

We tie the bottles together with string to indicate you can't get one thing without getting all the things.

e.g. you can't burn the carbon without using the oxygen and you can't get the methane to your home without incurring the shrinkage and leakage.

A therm is 100,000 British Thermal Units of energy. It is about the same as the energy in 3/4 gallon of gasoline. It is about $1.20 worth of natural gas at retail prices in California in 2016. A therm weighs 4.25 pounds but at atmospheric pressures a therm of natural gas has a volume of 100 cubic feet (5' by 5' by 4' ).

A therm is enough energy to heat 100,000 lb of water by one degree F. e.g. heat about 12,000 gallons by one degree F or heat 120 gallons by 100 Degrees F. A therm is about what a family of 3 uses in their water heater for 2 or 3 days. It's enough to heat water for 74 minutes of showering with a 2 gallon per minute shower-head.

Natural Gas is fossil methane released from underground and compressed into pipelines and recompressed a couple of times along long pipeline crossing states or whole countries on its way from the gas field to your home.

Every couple hundred miles along the way, some gas is piped out into combustion turbines that burn it and use its mechanical energy to re-compress the rest of the gas in the pipeline to send it further on its way. This is called "shrinkage" and results in some CO2 emissions from the combusted gas in the turbines. At the same compressor station location, some leakage of fugitive methane from the compressor seals around the compressor shaft. The fugitive emissions of raw methane are called "leakage".

It shows the calculations that find a Therm of natural gas (Methane or CH4)

A therm has 3.2 pounds of Carbon and 1.06 pounds of Hydrogen.

The 3.2 lbs of Carbon (C) is oxidized by 8.5 pounds of atmospheric oxygen (O2) to create 11.7 pounds of CO2.

The 5% shrinkage (Gas consumption to pressurize the remaining gas Amounts to .6 pounds of CO2 emissions.

The really big problem is leakage. Each 1% amount of leakage with a 100 year Global Warming Potential of 84 times its weight in CO2e amounts to an added 3.5 pounds of CO2e emissions. Since natural gas drilling, compression, storage and distribution systems are now being found to be pretty leaky, (>4% leakage) that amounts to more than half of the total CO2e emissions of a therm.

3.2 pounds of Carbon (We dyed it yellow) see bottle to the right

8.5 pounds of O2 (We dyed it blue in 2 two-liter plastic bottles.)

1-2 pounds upstream emissions (we dyed it brownish) it included a little leakage weight. See bottle to the left.

Four 3.5 pound bottles with each representing 1 % methane leakage. This way we could discuss leakage reduction and remove one or two bottles at 1% leakage reduction, each. See clear bottles in the cloth bag.

The all in weight for a therm with 4% leakage amounted to a very provocative 28 pounds of CO2e.

The weight bottles are all tied together. We have the "student" first lift the 3.2 pounds of carbon, then lift higher to pick up the 8.5 pounds of O2 making 11.7 pounds of tailpipe emissions. We then hand them the upstream shrinkage of 2 pounds. So far, so good.

We next ask them to lift higher to lift the bag with 4% leakage (4 times 3.5 pounds of CO2e = 14 pounds from leakage) for a total COe2 weight of 27 pounds CO2e per therm.

The little "e" in CO2e means it's an equivalent Global Warming Potential (GWP) weight for a mix of CO2 and some methane. Methane has an 84 GWP but it has a lower molecular weight of 16 (1 C @12 and 4 H @ 1 =16) compared to CO2's molecular wt of 44 (1 C @ 12 + 2 O @ 16 each)

When we combine the two factors, (84GWP * 16/44 = 30.5) we get 30.5 times the warming when we release methane rather than combusting it,

So 11.7 * 1%* 30.5 = 3.57 pounds per therm for each 1% of leakage.

With 4 % leakage, most of the equivalent CO2e is coming from leakage not combustion. (14 pounds CO2e for leakage and 11.7 lbs for combustion.)

The Students really get the weight of the issue. I think they will remember this for a long time.