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.

1 comment:

1. Using some updates, its approx 11% :

Total gas consumption statewide 2017 449,463,000,000 cubic feet
leak rate 2.70%
leaked gas 12,135,501,000 cubic feet
leaked gas 343,638,555 cubic meters
leaked gas 343,638,554,717 cubic liters
moles of gas 15,341,006,907 moles CH4
in grams 245,456,110,512
CH4 GWP 34
eCO2 grams 8,345,507,757,408
lost natural gas eCO2 million metric tonnes 8.35
2011 MA GHG MMTCO2e (AR4) 78.6
lost natural gas as % of MA GHG 11%