What are the biggest environmental issues with using methane, $\ce{CH4}$, (which is found as the major energy source in natural gas, shale gas, biogas, and synthetic natural gas), as a combustible fuel?
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2 Answers
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Methane has many lovely properties. It's a very dense source of energy. It is plentiful, it seemed to be cheap to extract (until we worked out the real cost), and similarly seemed to be cheap to transport long distances. It can be stored for many months in vast quantities.
But.
It's a greenhouse gas - a powerful one at that. On a 100-year horizon, it has about 20 times the global warming potential of $\ce{CO2}$. That needn't be a problem, if natural-gas extraction had no leakages, and distribution systems had no leakages. But they do have leakages, which means the supply chain worsens global warming.
Furthermore, when methane is burnt, $\ce{CO2}$ is released into the atmosphere, worsening global warming even further.
We've learnt to handle methane's other main problems: that it's colourless and odourless, so can just accumulate unnoticed waiting to explode, if you let it; and that it's also dangerous if handled carelessly in any other way.
It also generates $NO_x$ when burnt in air, and they are local as well as global pollutants. It is possible to have flue-scrubbers to remove the $NO_x$.
That doesn't mean that that methane is permanently bad. It can be created in a carbon-neutral fashion (from biological sources, or synthesised from atmospheric $\ce{CO2}$), and if leakages are minimised, offers a potential useful form of energy storage. (note that in these forms, it would be an energy & storage vector, not a net energy source, so its economic value would be very different - but that's economics, not chemistry)
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2$\begingroup$ Interesting, you brought up some really great points that were not immediately obvious. I'll have to look into ways that leakage can be minimised. $\endgroup$ Commented Dec 6, 2012 at 21:42
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$\begingroup$ Some interesting points, but many are also true of any other hydrocarbon fuel. The issues that differentiate methane are mostly about the difficulty of transport and storage (much has to be liquefied to move it across large oceans). $\endgroup$ Commented Dec 7, 2012 at 21:28
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From what point of view? Environmental? Industrial? Practical?
In Europe (I live in Italy) we use methane a lot, both for automotive and for house heating/cooking.
From the environmental point ov view, $\ce{CH4}$ is one of the best choices (together with LPG). When burnt (both in thermodynamics engines and in free-flames) it releases $\ce{H2O}$ and $\ce{CO2}$ only, even when burn at low temperatures and pressures (gasoline would produce a lot of pollutants in this case).
From an industrial point of view it is even better. There is a large abundance of $\ce{CH4}$ underground. We import a huge amount of it from Russia and north-Africa at a quite low price. Even better: $\ce{CH4}$ is easy to produce from other sources. For example, here in my area we use $\ce{CH4}$ (and LPG) produced (as a by-product) from petrolchemical industry and oil refineries.
From a practical point of view, it is relatively easy to manage. In my house (as in 99% of European houses) we use $\ce{CH4}$ both for heating and for cooking. We just have a high/low pressure pipeline connected to the national gas network. Nothing actually more complicated that the usual drinkable water supply pipeline.
I drove $\ce{CH4}$- and LPG-powered automobiles for years in my life. $\ce{CH4}$ forces you to have heavy, high-pressure bottles in you car (usually in the hood) but still can give a car that reach 90% of the usual performance and costs you something like 50% of a normal gasoline-powered car.
LPG is even better. My wife has a LPG car in this period. The LPG bottle is relatively light (50% of the high-pressure bottle used for $\ce{CH4}$) and cheap (30% less than a $\ce{CH4}$ system). Her car reach 90% of the gasoline-powered equivalent car and she spend something like 60% of a corresponding gasoline-powered car. It is even competitive with a corresponding diesel car.
The version of $\ce{CH4}$ (and LPG) that you can buy for automotive and in-house uses in Europe is always added with a specific odorizer that makes very easy to spot any leakage. Our gas sensors are aimed at spotting this specific odorizer so using $\ce{CH4}$ and LPG is actually quite safe.
Of course, $\ce{CH4}$ is still a fuel gas. It still requires high-pressure bottle and high-pressure pipeline so it still is a dangerous product and still have to be handle with care. For this reason, for example, you cannot refill your car by yourself and you have to wait for the service station staff.
In the same way, every $\ce{CH4}$ pipeline used for in-house cooking or heating must be verified and certified once per year by a certified technician.
In EU and IT, there are plans to produce $\ce{CH4}$ and LPG from many new, sometime weird, sources (organic waste, biomasses, etc.) so we expect to see an even larger diffusion of $\ce{CH4}$ in the future.
Please note that many of those sources are totally renewable so there isn't any new $\ce{CO2}$ added to earth atmosphere. There is just a circulation of existing $\ce{CO2}$.
[title](URL). ANyway, I fixed it for you :) $\endgroup$