Is Natural Gas Better Than Coal?

This question has a rather obvious answer, right? But of course, it depends on what question we're really asking!

Coal is associated with our early industrial past- and with devastating pollution. One event, the "killer fog" in London England in 1952, sent over 100,000 people to hospital and killed over 4,000 - a catalyst for clean air regulations and a massive kick-start for the environmental movement worldwide.

(image credit: dailymail.co.uk)

Coal however isn't just something from our past. It's a part of our present, still, in a massive way. Images of Chinese cities, and their sickly orange skies, make this quite obvious.

(image credit: citymetric.com)

Natural gas, in comparison, is seen as a "clean" fuel- or at least a cleaner fuel. And in toxic pollution terms, it absolutely is. Natural gas burns cleanly, with vastly less particulate, heavy metals such as mercury, and sulphur emissions than coal, leaving no ash to dispose. Natural gas still generates toxic NOx when burned in air. And if your natural gas comes from a well, it is a fossil- just like coal- and it emits fossil CO2 into the atmosphere when you burn it. CO2 is a greenhouse gas, which leads to climactic forcing as its concentration increases in our atmosphere.

It also seems rather obvious that natural gas is a cleaner fuel in terms of CO2 emissions. Taking electricity production as an example, natural gas has two things going for it.

1) Natural gas produces less CO2 per joule or BTU of heat produced from its combustion, because every atom of carbon comes along with two molecules (four atoms) of hydrogen. Hydrogen burns in air to make water- also a greenhouse gas, but one which is in rapid physical equilibrium with water in the oceans, soils and biosphere. (No need to worry- the steam from your shower isn't worsening the greenhouse effect!)

2) Natural gas power plants are, or at least can be, significantly more efficient than the average coal-fired power plant. Whereas a typical coal plant may convert only 30% of its chemical potential energy to electricity, a combined cycle gas turbine (CCGT) plant may convert up to 60% of the lower heating value (LHV) of the gas fed to it to electricity.

The Energy Information Administration of the USA (EIA), provides data about the "heating rates" and CO2 emissions for each fuel type used in electrical power generation. The data is averaged across all the power plants in the USA, so it includes old and new plants.

For coal in 2017, it took 10,465 BTU/kWh, and coal direct uncontrolled combustion emissions were 93.3 kg CO2 per million BTU. That's 976 g of direct CO2 emissions per kWh of electricity produced from coal in the USA in 2017.

In contrast, for natural gas in 2017, the figures were 7812 BTU/kWh and 53.07 kg direct CO2 per million BTU, which is 414 g CO2/kWh. Less than half as much CO2 intensity. Fantastic! Our job is done here, right? We have found the clear winner! Natural gas is much cleaner AND much lower in CO2 emissions!

(Feeling rather good about Ontario's incredible 40 g CO2/kWh grid at the moment, too- we burned our last coal for power generation back in 2013)

(video credit: CTV News)

D'oh! We forgot something, or more properly, EIA didn't include something, and it's rather important...

Natural gas is primarily methane- and methane is an even more powerful greenhouse gas than CO2. And here's where it gets a bit more complicated: methane is also much shorter-lived in the atmosphere than CO2. Whereas CO2 hangs around for over 100 years, methane is converted to CO2 in the upper atmosphere in about 12 years. So when we compare methane to CO2 in terms of its GHG potential, we have to use a factor which integrates methane's effects over a time horizon as it is gradually destroyed by the atmosphere. The typical time horizon is about 100 years, and the factor people commonly (still) use is that methane is 25x as bad as CO2 per unit mass. The IPCC however changed their factor in 2013, though it seems not too many noticed: they now say that methane is 33x as bad as CO2 on the 100 year time horizon, and 86x as bad on the 20 year time horizon.

The other thing we forgot is that mining coal and producing and distributing natural gas both result in methane emissions to the atmosphere. And where methane is concerned, a little bit matters quite a lot.

Coal mining methane emissions are usually ignored, because mines have to be vented for the safety of coal miners. An excellent publication by NREL in 1999 did an exhaustive study of emissions from coal power production, from mine to smokestack, and came up with an estimate of average methane emissions of about 0.91 g of methane per kWh of electricity generated from coal. Relative to 976 g of CO2/kWh, that doesn't sound like much- but if you were to use the 86x factor on the 20 year time horizon, that adds 8% to coal's total emissions, bumping it up to about 1054 g CO2 equivalent (CO2e) per kWh.

For natural gas, the issue is venting and leakage.

Howarth, Energy Sci and Eng 2014 (linked above) did a fairly careful evaluation of methane emissions during all phases of conventional and shale/tight sand (gas produced by hydraulic fracturing aka "fracking") production and distribution. They estimate that 1.4% of conventional and 3.3% of "frack" gas is lost "upstream", i.e. during well construction, development etc., and a further 2.5% is leaked "downstream" (from natural gas distribution equipment and pipelines). Accounting for the ratio of conventional and "fracked" gas in 2012 (40% vs 60%), that works out to an estimate of 5.04% of the gas produced by the natural gas industry being lost along the way.

That is a positively incredible amount of fossil methane being leaked...

Howarth's estimates are similar to those of several other authors- and of course are disputed entirely by the natural gas industry itself. Who would throw away 5% of their valuable product knowingly?

While I can't knowledgeably comment on the "upstream" figures, the downstream estimate of 2.5% is definitely way too high for power plants. Howarth's figure is based on an average for the gas industry, including antiquated cast iron gas distribution systems in east coast US cities. Power plant gas losses are going to be quite a bit lower than 2.5%, and that's half the total leakage right there.

But let's use the 5% leakage rate from Howarth and do some comparisons. 7812 BTU per kWh works out to about 177 g of natural gas (LHV) per kWh. 5% of that is 8.85 g of methane per kWh: 292 g CO2e/kWh for the 100 yr and a whopping 761 g CO2e/kWh for the 20 year time horizon- more CO2e than the entire amount generated by burning the gas. The total for natural gas power on the 20 yr time horizon is 1175 g CO2e/kWh- higher than coal at 1054 g CO2e/kWh!

Is natural gas, as Howarth concludes in the title of his paper, "a bridge to nowhere"? Is it making things better, or worse?

My take on this is simple: in order for Howarth to come to his headline-grabbing conclusion, he had to use the 20 year time horizon for the comparison, ignore the methane emissions from coal production (which admittedly is easy to do), and then look at ONLY the GHG emissions of power production, forgetting about the toxic emissions. While surely this 5% leakage claim needs to be investigated further, and if it's even close to that, it needs immediate action- even with this figure, natural gas is providing a tremendous toxic emissions reduction in return for a GHG emissions situation that is, within the accuracy of the estimates, roughly the same as that provided by coal.

Our best course of action is to pursue energy efficiency, and reduce the amount of energy of all kinds that we waste, because all energy generation technologies have environmental impacts. We also need a fossil carbon tax that applies the appropriate factors to all emissions including fossil methane and CO2, so we stop treating our atmosphere like a free and limitless public sewer and encouraging the combustion of fossils as a result. Doing that will force the natural gas industry to get very, very serious about leakage, very quickly. Significant and sustained fossil carbon taxes will hasten the transition to renewable electricity generation (wind and solar plus storage) and possibly nuclear generation too, which is already underway. But is natural gas- both renewable natural gas from anaerobic digestion and fossil natural gas from hydraulic fracturing, part of that transition? Absolutely. Coal needs to stay in the ground- that is the cheapest and most effective way to permanently sequester fossil carbon that exists. Coal needs to be the first fossil fuel out.

(A reminder: the opinions expressed in this article are entirely my own. Anything I've said which can be demonstrated, with references, to be incorrect, I will gratefully correct. And my employer, Zeton Inc., has no opinion on these matters- we design and build pilot plants for the whole chemical process industry, love doing it, and stick to our knitting!)