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Discussion > CO2 and carbon intensity of fuels

Martin A and others interested in the CO2 released by various fuels -

Some will remember that as part of a comment in the “Unthreaded” area, a week or so back, I stated that wood releases 40 times the CO2 of natural gas (methane) for a unit of energy. Martin A took me to task over this “40 times” figure and produced some convincing calculations based on the chemistry of the various fuels to show it was not true.

As I stated at the time, it is not my area of knowledge, and therefore I use data from various people who have more insight into the world of energy and engineering. The 40 Times figure came from an excellent piece by Matt Ridley -

Matt references the work on the carbon/hydrogen ratio of various fuels by Jesse Ausubel of Rockefeller University which leads to the result that the ratio of Carbon to Hydrogen is

10 times for wood – being based on the fact that wood is mostly cellulose which is carbohydrate, with oxygen atoms bonded to hydrogen, then many of the hydrogen atoms do not get oxidised exothermically when burned because they are already oxidised. Wood isn't pure carbon as some free hydrogen is in the lignin component of the wood.

The ratio in other fuels is easier to see from their chemical make-up and leads to a

1 times for coal, 0.5 for oil, and 0.25 for natural gas.

So the carbon intensity of wood is 40 times that of natural gas. Therefore gas has the highest energy density of these fuels. Of course we are ignoring nuclear energy here.

It is when we move to assessing the amount of CO2 released per unit of energy produced that matters become less clear.

I emailed Matt Ridley (as one commenter suggested) and he kindly confirmed his source and his understanding of the science as stated above. He also copied Prof. Ausubel in on the reply and he provided this link to the “wood” part of this paper.

I also contacted Prof. Colin McInnes since he has also referenced the work of Ausubel when talking about energy density and the dangers of policies that seek to move to sources of lower energy density such as wind and biomass.

Among the many thought provoking articles on his blog is

He confirmed that converting from energy density to CO2 released per unit of energy produced is not straightforward and that he had not found a clear practical comparison. He also mentioned that the combustion efficiency of the fuel would come into it, and that gas was particularly efficient in this regard when used in modern turbines.

Amongst Matt Ridley's follow-up thoughts were that

1. Efficiency of combustion is an issue, as you say (in response to my quoting Prof.McInnes)

2. The heat released by oxidising a hydrogen atom is not the same as the heat released by oxidising a carbon atom. But there is no straightforward formula when they are in compounds.

So where does this leave us?

It is often quoted that coal releases twice as much CO2 as gas. This is one reason that many believe that we could lower our CO2 emissions and our costs by embracing shale gas and converting/building more power stations to run on gas. Some papers have quoted CO2 release gas/coal of a third, some just say less than a half.
The carbon intensity of coal is from Ausubel's work 4 times that of gas, so this would suggest a quarter, but who knows?

I would like to thank Matt Ridley, Colin McInnes and Jesse Ausubel for their input and kind replies to a simple meteorologist trying to grasp the realities of energy. I hope that in summarising I have not misrepresented their views.

Mar 6, 2012 at 8:43 PM | Registered Commenterretireddave

Hi Dave,

May I ask what is the information you actually require - or its purpose? I'd have thought for most purposes, the mass of CO2 released by unit of usable energy* released would be what was needed.

*eg not used to evaporate water contained in the fuel.

I think that "coal" can vary greatly, from anthracite with very high carbon percentage (>90%) to coals containing lots of tar and releasing lots of gas when heated. Complicating things further is the variable proportion of water and ash in different types of coal. So I imagine that any general figure is likely to be rather imprecise.

Mar 7, 2012 at 3:10 PM | Registered CommenterMartin A

Hi Martin,

The figures quoted above are effectively carbon to hydrogen ratios and obviously, taken at face value, wood has a lot more carbon to produce CO2 when burnt.

BUT simply the question is how does this relate to the amount of CO2 emitted per unit of energy for each fuel? Matt Ridley's original article translated one to the other and an engineer of Colin McInnes eminence says he hasn't seen a straightforward comparison. Obviously wood produces a fair bit more CO2, but how much more in reality? - agreed that some wood and coal will vary as you say. Perhaps oil and gas less so

Which is where we started with the 40 times statement originally you may remember.

Those who favour biomass for fuel always assumed it to be carbon neutral, but this is not true for trees which have a carbon debt in centuries as Matt Ridley points out.

The only thing for certain seems to be that, short of nuclear power, gas will release the least CO2.

We all know nuclear fission produces large amounts of energy but I was surprised by the figure on Colin McInnes's website, that one pound of uranium produces 16,000 times the electricity of a pound of coal.

Mar 7, 2012 at 3:55 PM | Registered Commenterretireddave

Might I suggest Robert Laughlin's excellent 'Powering the Future' for use in this context?

Mar 7, 2012 at 4:00 PM | Unregistered CommenterShub

Mar 7, 2012 at 3:55 PM retireddave

"Obviously wood produces a fair bit more CO2, but how much more in reality? "

As Matt Ridley said in his recent article (and not too far from the figures I calculated) wood produces about twice the CO2 per unit energy produced, relative to gas. I think that, to a reasonably close approximation, gas is equivalent to oil, while wood is not far different from coal.

To be fair to wood burners, a lot of wood that is burned for heat was still CO2 in the atmosphere well into your and my lifetime, rather than being underground since pre-dinosaur days.

Mar 7, 2012 at 10:05 PM | Unregistered CommenterMartin A

The 40x figure does seem a bit implausible, but then domestic woodburners do consume a fair bit of fuel, especially if it’s not as dry as it should be. I throw a few lumps of coal on, if that happens...

The pelleted stuff currently being lionised by the greens (and catching fire in storage at Tilbury power station) has already used up a fair bit of energy in the drying process, so that’s more CO2!

Mar 8, 2012 at 11:58 AM | Unregistered CommenterJames P

Martin A -

Now you have lost me (not a difficult feat, given that I lose a million brain cells a day at my age - or is that another thing I read in an article that isn't true??).

You say - "As Matt Ridley said in his recent article (and not too far from the figures I calculated) wood produces about twice the CO2 per unit energy produced, relative to gas."

Which article is this of Matt's? In the one I referenced of his from December 2011 he says "Burn wood and you make 40 times more carbon dioxide for each unit of energy than if you burn gas. It’s the worst thing you can do in carbon terms."

Has he changed his mind?

Mar 8, 2012 at 12:21 PM | Registered Commenterretireddave


Thanks for the hint. I will purchase the book - most reviews seems to feel it is a realistic look at energy for the future - except for a few who want to believe in fairy stories.

Can you say whether he gives any figures for CO2 released per fuel as we are discussing?

Mar 8, 2012 at 12:22 PM | Registered Commenterretireddave

The short answer is: no. Laughlin does not get into CO2 caculations. But he does a lot of inter-energy source comparisons, which might be useful, or stimulating

For eg:

Because electricity pipelines have limited capacity, we can profitably compare them with oil and gas pipelines. The amount of power transmitted by one of the three wires of a big line is roughly the power consumed by a jumbo jet at full throttle on its takeoff roll. This fact inadvertently answers a question that has perplexed all air travelers since the dawn of aviation, which is how such a massive machine could possibly fly. The answer is that it requires the power of a small city.


The power delivered by the entire transmission line is equivalent to three times this flow, or ninety times the pumping rate of gasoline into a car. A typical natural gas pipeline, which has a diameter of about one meter, delivers about ten times this power. A major oil pipeline, which also has a diameter of about one meter, delivers about one hundred times this power.

and, the question of energy density, indirectly

Thus, how much energy a fuel can pack away in a small space, something central to its function, is fixed by quantum mechanics, not serendipity. It isn’t an accident that carbon-based fuels carry the amount of energy they do. It’s more or less the maximum energy density we can ever get with any substance without resorting to millions of atmospheres of pressure. Not only is it impossible to increase gasoline’s energy content using processing tricks, it’s also impossible to do it by substituting other atoms for carbon in the fuel.

Laughlin, Robert B. (2011-09-27). Powering the Future: How We Will (Eventually) Solve the Energy Crisis and Fuel the Civilization of Tomorrow. Perseus Books Group.

Mar 8, 2012 at 6:08 PM | Unregistered CommenterShub

Has he changed his mind?
Mar 8, 2012 at 12:21 PM | retireddave

In a very recent article to which BH drew attention, Ridley said "gas produces less than half the CO2 of wood" (or words to that effect).

Don't have it at my fingertips but I'll try to find it next week.

Mar 10, 2012 at 9:17 AM | Unregistered CommenterMartin A

Martin - but that is gas to coal, not gas to wood. Going back to the Ausubel et al paper. Wood has 10 times the Carbon to Hydrogen ratio of coal. Coal has the much higher energy density, which is why it fuelled the industrial revolution and wood couldn't have done.

The Ausubel ratios being - wood (10), coal (1), oil (0.5), and methane (0.25)

The C:H ratio is 4 times for coal against gas (and would suggest a quarter CO2 for gas against coal, which is less than half - as you said before, coal is variable) - it is 40 times for wood against gas which gets us back to where we were.

Mar 12, 2012 at 5:20 PM | Unregistered Commenterretireddave

Dave - gas to coal or gas to wood, I don't think there is much difference. In burning coal or wood, you are burning mostly carbon, so the ratio of CO2 mass produced per unit energy released is about the same.

I assume that the quantity that meets your needs best is CO2 mass produced per unit energy released. Ratios such as the ratio of carbon to hydrogen, or the ratio of carbon to total material are not relevant to anything, I would have thought.

I found a website The Engineering ToolBox that tabulates the mass of CO2 (in kg) per kWh of energy released for different fuels.

Here are their figures, sorted by kgCO2/kWh

Coal (bituminous/anthracite)................0.37

Light Oil..............................................0.26
Crude Oil.............................................0.26
Liquid Petroleum Gas..............................0.24
Natural Gas............................................0.23

It seems that fuels fall in two categories:

(a) Fuels such as wood and coal where you are burning mostly carbon. These release around 0.4kg of CO2 per kWh of energy released.

(b) Hydrocarbons - where you are burning both hydrogen and carbon. These release around 0.25kg of CO2 per kWh of energy release.

There is a ratio of 1.5 between the first group and the second and it does not make much difference which one you select from each group.

I hope this clarifies and finally gives you what you are after. I also hope it agrees with the figures I posted earlier - I could not track down my posting to check.

Mar 13, 2012 at 1:21 PM | Registered CommenterMartin A

Martin - Thanks very much for that. I am sure your posts on the subject have been consistent throughout.

I totally agree with your second paragraph and that is what I have been trying to get at.

Thanks very much for the link to the engineering toolbox - it could prove useful in many other areas as well.

Thank you very much for your interest, time and effort.


Mar 14, 2012 at 11:54 AM | Registered Commenterretireddave

Martin A -

I copied a link to this thread to Matt Ridley and Colin McInnes for their information, also copying in Jesse Ausubel. I thought you might be interested in a reply from Jesse Ausubel with some very interesting comments and a couple of fascinating links to the work of his team.


You seem to continue conflating two issues.

One is the proper H:C ratio for calculating the secular evolution of the energy system. This is what my work is about, and the 40:1 ratio is sound. And, watching the news these days, I find the imminent dominance of natural gas for the next few decades fits the forecast. H and C compete for market niche as horses and cars, or CDs and MP3s. This is a fundamental insight from the 1980s, of which my colleagues and I are proud.

The second is the emissions from a given energy system. The emissions, as many have remarked, are finally the result of many factors including efficiency of engines, temperature and completeness of combustion, tightness of distribution system, detailed quality of fuel, etc. For a given power plant (or stove), one can actually measure the gas emissions and relate them to the input. I am sure many people have done this. It is harder to measure emissions from an entire “system” from mining to chimney and transmission wires or heat, but people do this too. The numbers are interesting, and show engineers where to improve practice. Basically parts of systems (such as a stove) or systems become more efficient over time, to some ceiling. The Franklin stove for example, saved a lot of American forest.

Have a look at figures 4 & 7 at:

If you visit, you will see that most of our work is about biodiversity and protection of habitat, both terrestrial and marine:

Fortunately, the reversal of the destruction of Nature is beginning. We need to spare lots more land and sea for Nature.

I think most of your correspondents have the picture. ""

Mar 14, 2012 at 1:40 PM | Registered Commenterretireddave

"One is the proper H:C ratio for calculating the secular evolution of the energy system."

Not too sure just what that means but presumably it's useful to Mr Ausubel in predicting future trends.


"Mr. Ausubel was a main organizer of the first UN World Climate Conference (Geneva, 1979), which substantially elevated the global warming issue on scientific and political agendas."

Mar 17, 2012 at 9:13 AM | Registered CommenterMartin A

Martin A - Thanks, yes I was a little unsure of exactly what he meant by that sentence, but a good look through is work is required I think to see where he is coming from.

At a simple level his remarks (I feel) underline the fact that wood and coal continually need transporting to their use, but gas once piped does not to the same extent and releases its energy more efficiently as well.

He is obviously a person who cares deeply about the environment (so do I) and his work speaks for itself, being based on science and data not dogma. The fact that Matt Ridley sent me a short message agreeing with Jesse's remarks is good enough for me.

I don't believe that truth is compromised, wherever it comes from.

Mar 17, 2012 at 2:09 PM | Unregistered Commenterretireddave