Lifecycle: Coal versus Natural Gas

UPDATE: See the next post about lifecycle analysis

It is very clear that combustion of natural gas in power plants produces substantially less — about half — the carbon dioxide (CO2) that is produced by a coal fired power plant for the same amount of electricity produced. A question has been raised, however, about whether the natural gas is still significantly better than coal if you take into account the entire life cycle of emissions: exploration, extraction of the fuel from the ground, transportation to the power plant, leakage of green house gases (GHGs), etc. Lifecycle analysis is not always straightforward and there can be controversy over weighting factors used and uncertainties. For example, methane (a potent GHG) is released in considerable quantities into the atmosphere when coal is mined (methane is the major source of explosions and fires in coal mines — indeed, methane which is odorless and tasteless, is one of the major reasons for canaries in coal mines). But different mines have different accumulations of methane and exact quantities are difficult to measure.

I am not an expert in lifecycle analysis, but on digging through the science and technology literature, I did find a recent article comparing the the GHG emissions of coal and natural gas. According to Jaramillo et al. (2007) [full reference at the end of this post], natural gas remains the clear winner over coal in terms of GHG emissions, even when taking the full lifecycle into account (figure below -- click to see a larger version). With current technologies, even liquefying natural gas to transport it from a distant area (say, the Middle East) results is less overall emissions, though the advantage is less than using local, domestic sources. 

lifecycle.014

In some of the groups that oppose shale gas drilling, the comments from Lovelock (2006) about how just over 2% natural gas loss during production and transportation makes natural gas worse than burning coal have received some attention. The Environmental Protection Agency (EPA) estimates that just over 1% of methane is lost during natural gas production in the United States during production, transmission, and storage, in contrast to Lovelock's cited figure of 2-4%; the difference may be due to different practices in different countries (Lovelock is located in England). I need to do more research into Lovelock's approach, but it appears that he did not do the full lifecycle analysis and is comparing just coal combustion to combustion, transport, and production of natural gas. For example, the Environmental Protection Agency (EPA) estimates that, 57.6 million metric tons of CO2 equivalent methane were released to the atmosphere in 2007 by coal mining. Because of better practices, methane emissions from both coal mining and natural gas production are declining. Jaramillo et al. take these factors into account in their calculations.

Jaramillo et al. (2007) also considered the lifecycle emissions of coal versus natural gas for future, highly efficient power plants that capture and sequester CO2. The result (figure below) shows that natural gas still has fewer overall emissions across its lifecycle than coal, although the advantage of natural gas is reduced somewhat; with carbon capture and sequestration (CCS), natural gas power plants would emit about 75% of the GHGs that are emitted by coal fired power plants. With CCS, however, liquefied natural gas from distant areas would have moderately more GHG emissions than coal. 

lifecycle.015

Note that we do not yet know whether we will ever have large scale CCS; none of the power plants in the Finger Lakes area are equipped for CCS, and CCS would probably raise the cost of electricity considerably (either directly or indirectly via a carbon tax or cap and trade system). Note, too, that Jaramillo et al. (2007) do not specify whether they modeled traditional natural gas or shale gas (i.e., like the gas in the Marcellus). I suspect that they modeled traditional natural gas. Even so, with current technologies, it is likely that shale gas would still come out as superior to coal; it is unlikely that shale gas could have more emissions that liquefying and transporting gas from distant areas.

The bottom line appears to be that, taking into account the total lifecycle, natural gas produced locally is much superior to coal in terms of GHG emissions per unit of energy generated. Use of advanced technologies, including CCS, can make coal a better performer than liquefied natural gas transported from distant areas, but domestic natural gas still wins. Synthetic gas from coal is always the worst performer with either current or advanced technologies. It would be nice to see a shale gas analysis, but my gut reaction is that at least for current technologies, shale gas would be superior to coal, taking into account the entire lifecycle.

One final comment: to realize CCS, we will have to drill wells, hydrofracture and inject the CO2 into the ground locally, or build regional pipelines to transport CO2 to distant collection points. These processes are similar to those that some shale gas opponents object to. It will also cost a significant amount extra.

References

Jaramillo, P., Griffin, W. M., and Matthews, H. S., 2007, Comparative life-cycle air emissions of coal, domestic natural gas, LNG, and SNG for electricity generation: Environmental science & technology, v. 41, no. 17, p. 6290-6296, doi: 10.1021/es063031o.

Lovelock, J., 2006, The Revenge of Gaia: Basic Books, 208 pp.

R. W. Allmendinger © 2006-2020