The carbon footprint of burning natural gas

In another article, the amount of water vapour produced in burning natural gas was calculated. The same technique can be used to obtain the amount of carbon dioxide produced, and some of the text of that article has been copied here.

When natural gas, consisting mostly of methane is burned completely in air, carbon dioxide, water vapour and heat energy are produced. The chemical reaction is:

CH4  +  2O2  >>>> CO2  +  2H2O + energy

methane   +    oxygen         carbon dioxide + water

(In words: one unit of methane, consisting of one carbon atom and four hydrogen atoms, combines with 2 units of oxygen, an oxygen molecule consisting of two oxygen atoms, to produce one unit of carbon dioxide plus two units of water vapour. The chemical reaction gives out energy as heat.)

It’s quite straightforward to work out the relative masses involved, from the relative masses of the atoms of the elements:

Element

Symbol

Relative weight

Hydrogen

H

1

Carbon

C

12

Oxygen

O

16

So we can put the relative weights under the reaction equation:

 CH4  +  2O2  >>>> CO2  +  2H2O + energy

16    +   64              44    +     36

The masses of methane and carbon dioxide are in the proportion 16 to 44, or 4 to 11. So, burning 4kg of methane results in 11kg of carbon dioxide.

The density of methane is around 0.66kg per cubic metre, so 1m3 of methane produces 0.66 x 11/4 = 1.8kg CO2.

The calorific value of natural gas has a range of values, but for present purposes, we’ll take it as 38.7 MJ/m3 (MJ = megajoule = 1 million joule). In other words, burning 1m3 of natural gas releases 38.7 MJ of heat energy.

Gas consumption is now shown on our energy bills in units of kilowatt.hours (kWh).

1kWh = 3.6MJ

To obtain 1kWh, we need to burn 3.6/38.7 = 0.093m3, which produces 0.093 x 1.8 = 0.17kg (170 gram) of CO2 . However, this figure is based on recovering 100% of all the heat obtainable, by cooling the flue gases and condensing all the water vapour – in other words on the gross calorific

 

value. If we assume all the water vapour is lost from the flue, we only obtain the net calorific value, which is 1.11 times lower. If we use this figure, then to obtain 1kWh of useful heat involves producing 0.17 x 1.11 = 0.19kg or 190 gram, not far from the figure given in the references below:

 0.2kg per kWh or 200 gram per kWh.

A household averaging 48kWh per day from burning gas, produces around 10kg CO2 per day, or in the region of 3500 kg or 3.5 tonnes per year.

Footnote: it’s very rewarding to arrive at useful number from considerations of basic science, but we ought to pause to remember the vast amount of effort that went into giving us the figures and ideas that we can so easily look up.

 

References:

http://www.volker-quaschning.de/datserv/CO2-spez/index_e.php

http://www.engineeringtoolbox.com/co2-emission-fuels-d_1085.html

http://en.wikipedia.org/wiki/Carbon_dioxide