Sunday, January 25, 2015

A 2.7% methane leak rate represents ~10% of Massachusetts' entire greenhouse gas emissions inventory

Can natural gas pipeline leaks really amount to a full 10% of the total state greenhouse gas emissions (including every sector of human activity in the state - transportation, electricity, agriculture, residential/commercial buildings, industrial processes, and waste)?  

This would be an enormous contribution to climate change forcing from a single activity.  

That's what results from our recent PNAS paper would indicate, if our regional results from eastern MA and environs were to be scaled to the Commonwealth.

Here's how I arrive at this remarkable estimate: 

In our PNAS study we estimated a 2.7% (+/- 0.6%) leak rate from natural gas (primarily methane, or CH4).  

Assuming that percentage is leaked at the same percentage across the state of Massachusetts (not a bad assumption; old cast iron and bare steel pipes are distributed across the state), and given that MA used about 420 billion cubic feet of natural gas annually in 2012-13, a leak rate of 2.7% X 420 billion cubic feet equals 11.3 billion cubic feet leaked gas.

Let's convert to cubic meters: 11.3 x 10^9 ft^3 x 0.028 m^3/ft^3 = 3.20 x 10^8 m^3 CH4

or, 3.20 x 10^11 liters of CH4

assume (accepting a bit of temperature and pressure error) 22.4 liters per mole for an ideal gas at standard temperature and pressure.

3.20 x 10^11 liters x 1 mol/22.4 liters = 1.43 x 10^10 moles CH4

use 1 mole CH4 = 16 grams

1.43 x 10^10 moles CH4 x 16 grams/mol = 2.3 x 10^11 grams CH4

assuming CH4 warming potential is 34X that of CO2 over 100 years (Table 8.7 here) 

= 7.8 x 10^12 g eCO2, or 7.8 Million Metric Tonnes equivalent CO2 per year in Massachusetts in lost natural gas.

This is about 10% of the total state greenhouse gas emissions inventory, using the most recently available state estimates (2011, to stand in for our study's 2012-13 time frame). The +/- 0.6% confidence limits around the mean of 7.8 million metric tonnes give a range of 6.1 to 9.5 million metric tonnes, or about 8% to 13% of the state's entire GHG emissions inventory for 2012-13.

Note this is not even the amount of natural gas consumed (which, when combusted releases CO2); it's just the 2.7% of that total that leaked.

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