From the AGU Journal Highlights:
Replacing coal with natural gas would reduce warming
A debate has raged in the past couple of years as to whether natural gas is better or worse overall than coal and oil from a global warming perspective. The back-and-forth findings have been due to the timelines taken into consideration, the details of natural gas extraction, and the electricity-generating efficiency of various fuels. An analysis by Cathles, which focuses exclusively on potential warming and ignores secondary considerations, such as economic, political, or other environmental concerns, finds that natural gas is better for electricity generation than coal and oil under all realistic circumstances.
To come to this conclusion, the author considered three different future fuel consumption scenarios: (1) a business-as-usual case, which sees energy generation capacity continue at its current pace with its current energy mix until the middle of the century, at which point the implementation of low-carbon energy sources dominates and fossil fuel–derived energy production declines; (2) a gas substitution scenario, where natural gas replaces all coal power production and any new oil-powered facilities, with the same midcentury shift; and (3) a low-carbon scenario, where all electricity generation is immediately and aggressively switched to non–fossil fuel sources such as solar, wind, and nuclear.
The author finds that the gas substitution scenario would realize 40 percent of the reduction in global warming that could be achieved with a full switch to low-carbon fuel sources. The benefit for mitigating warming revolves around the fact that to produce an equivalent amount of electricity burning natural gas would release less carbon dioxide than burning oil or coal. Though atmospheric methane traps more outgoing radiation than carbon dioxide does, at reasonable leakage rates its atmospheric concentration is much lower and what is released decomposes much more quickly. The author suggests that over timescales relevant to large-scale warming—decades to centuries—the effect of any methane released during natural gas extraction would be inconsequential.
Source:
Geochemistry, Geophysics, Geosystems,doi:10.1029/2012GC004032, 2012
Title:
“Assessing the greenhouse impact of natural gas”
Authors:
- L. M. Cathles
- Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA.
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Key Points
- Natural gas substitution achieves 40% warming reduction of low carbon fuels
- Duration of substitution does not affect 40% benefit
- Full benefit at gas leakage 1% of production or less
Abstract
The global warming impact of substituting natural gas for coal and oil is currently in debate. We address this question here by comparing the reduction of greenhouse warming that would result from substituting gas for coal and some oil to the reduction which could be achieved by instead substituting zero carbon energy sources. We show that substitution of natural gas reduces global warming by 40% of that which could be attained by the substitution of zero carbon energy sources. At methane leakage rates that are ∼1% of production, which is similar to today’s probable leakage rate of ∼1.5% of production, the 40% benefit is realized as gas substitution occurs. For short transitions the leakage rate must be more than 10 to 15% of production for gas substitution not to reduce warming, and for longer transitions the leakage must be much greater. But even if the leakage was so high that the substitution was not of immediate benefit, the 40%-of-zero-carbon benefit would be realized shortly after methane emissions ceased because methane is removed quickly from the atmosphere whereas CO2 is not. The benefits of substitution are unaffected by heat exchange to the ocean. CO2 emissions are the key to anthropogenic climate change, and substituting gas reduces them by 40% of that possible by conversion to zero carbon energy sources. Gas substitution also reduces the rate at which zero carbon energy sources must eventually be introduced.
“Well DInostratus it looks like you know it all so no need for any further explaination”
If nothing else, I need a spelling lesson. “vocalization”? I can’t believe I let that mistake pass.
George E Smith @ur momisugly 11.59am
George I guess energy wise, hydrogen gives you more bang but obviously that can also be a problem – much easier to transport and handle the carbon variety.
On the western vs eastern coals, I’m on a different continent (Dinostratus might say a different planet 😉 ) but my understanding is that the important parameter for the US clean air conditions is the actually amount of SO2/Btu produced from the power plant, so although the eastern coals are higher in energy they are much higher in total sulphur compared to the sub-bituminous coals from the Powder River Basin which have very low sulphur.
Another problem with using true anthracite as a fuel is that it is more difficult to ignite compared to say high volatile bituminous coal which is probably considered the ideal. Anthracite also generally demands a higher price as it is less common and has other specialised uses, – too good for a dirty old power plant.
One thing about the Chinese – they are the largest coal producer in the world, 3200Mt in 2010 (USA 932 Mt) and don’t actually import a huge tonnage of coal, about 177 Mt in 2010 which was less than Japan.
Dinostratus says:
July 19, 2012 at 8:20 pm
If nothing else, I need a spelling lesson. “vocalization”? I can’t believe I let that mistake pass.
Heck, I jest figgered them them bytes and pieces of coal were screaming in pain as you shot a cruel laser beam at their little bitty innocent particulars …..
Dinostratus
Might be time for you to cite the quacker principle (oh I hope I’ve spelt that right)
My suggestion of the Wikipedia page was for George’s benefit, I didn’t quote anything beyond the figures.
Yes you quote LHV sometimes known as net calorific value (NCV) and HHV, also known as gross calorific value (GCV) correctly, however coal laboratory analysis reports HHV, LHV is calculated using the coal moisture and hydrogen contents. As I mentioned calorific value I should not have used Mj/kg as that is for specific energy, kcal/kg for calorific value is that ok.
Obviously you have a processing problem regarding what is reported as ash. Coal does not contain ash. Ash is the residue remaining after the combustion of coal under specified conditions (ASTM D-3174; ISO 1171; AS 1038.3) i.e. the ash value reported from coal analysis is the result of the combustion process.
Well ghee (correct spelling)
I never suspected such a simple and easily verifiable statement to get such static.
Easy really
http://pubs.awma.org/gsearch/journal/1990/6/40_06_861.pdf
The lowest CO2 emissions are calculated for the high-volatile bituminous coals (198 to 211 lbs CO2/MMBtu) and the highest for lignites and subbituminous coals (209 to 224lbs CCVMMBtu).
On average, the lower rank coals produce 5 percent more CO2 upon combustion than the high volatile bituminous coals, on the basis of gross calorific value. This difference increases to 9 percent on the basis of estimated net calorific value.
or
http://www.epa.gov/nsr/ghgdocs/electricgeneration.pdf
In addition to the lower CO2 emissions rate per unit of heat input (lbs CO2/MMBtu), due to the inherent moisture in subbituminous and lignite coals, all else being equal a bituminous coal-fired boiler is more efficient than a corresponding boiler burning subbituminous or lignite coal. Therefore, switching from a low to a high-rank coal will tend to lower GHG emissions from the utility stack.
Well it’s the end of the week for me so I’m off for a beer (better run the spell check first)
CCCcccOOOoooHHHhhh. Confusion thrice confounded.
1. There is no such thing as a carbon molecule. Just atoms.
2. While Carbon’s atomic number is 7, Carbon’s atomic mass ranges from 12-14, because of the neutrons. Hydrogen, unless you’re talking about the rather scarce deuterium, has none. So carbon is about double the “7X” density figure you cite. Sub-factor: carbon links up in solids (coal) but hydrogen is always a gas unless you do some real serious chillin’.
3. In chemical terms, it takes 2 hydrogens to reduce one oxygen atom, but two oxygen atoms to oxidize one carbon atom. So that’s a 4X advantage to carbon.
The question of “higher heat” is a complex of all those factors, and whether you are talking mass, moles, or volume.
Correction to above: If you are treating the H2 as a unit, your numbers are closer. But in combustion chambers, molecules break down, and the atoms go their separate ways.
Hydrogen’s volume really dominates it’s deployment. Even liquified, it is hugely less dense than its combustion partners. Check out this schematic for the proposed Skylon: most of its sleek volume is for liquid hydrogen. There’re tiny chambers for liquid oxygen for use in the rocket mode of its flight. http://www.gizmag.com/sabre-engine/23304/pictures#10
typo: its deployment. (Drop-kicked by my own hobby horse! :-/p )
As for the energy calcs of various coals, etc., what is driving the takeover of shale gas is energy per $. Gas plants and fuel are currently and for the projected future insanely cheaper. Responsible utilities couldn’t and can’t justify any other choice financially.
Brian, can’t really argue with this other than to say that the volatility of short term gas prices and an historical reluctance of gas companies to commit to long term contacts at realistic prices has gone against gas. Has shale gas changed that ? Not sure but as gas prices drop due to supply then exploration and development will also decline, when will the bounce come? What is pretty certain is that it pushes the so called take-over by renewables further out. Interesting times.
“…the lower rank coals produce 5 percent more CO2 upon combustion than the high volatile bituminous coals, on the basis of gross calorific value.”
That’s all I was trying to say. It makes comparing CO2 emissions from natural gas and CO2 emissions from just about any coal easy.
Brian H says:
July 20, 2012 at 3:00 am
George E. Smith; says:
July 19, 2012 at 11:59 am
1. There is no such thing as a carbon molecule. Just atoms.
2. While Carbon’s atomic number is 7, Carbon’s atomic mass ranges from 12-14, because of the neutrons.
No, Z of C is 6.
phlogiston;
Indeed. I knew/know that. But apparently my phat phingers didn’t. Duh. Doh. Dum. Sorry!