Education About Gas

The end of coal, the rise of natural gas

 By Luca Longo
About gas

Coal or natural gas to produce electricity? Not all the energy sources are the same. Luca Longo explains why the most sustainable choice, to protect our planet and fight climate change is to use natural gas…

(Click on to enlarge the infographics below)

Every year we consume more than 24 PWh of electricity: 24,000,000,000,000,000 Wh/year, in other words, the energy that would be used by a million billion 24W energy-saving bulbs kept on for an hour. To be clear, it is as if every one of the seven and a half billion human beings on the planet kept 15 24 W light bulbs on all day and night for the whole year.

Two-thirds of this enormous quantity of electricity that we consume comes from fossil fuels, 10.7% from nuclear power, and 23.9% from renewable sources. Even though world averages don’t provide a very useful picture, ranging from France, which gets 75% from its nuclear power plants, to China, which uses coal for 78%, and Norway, which derives 98% of its electricity from hydro.

But let’s look in more detail at the three sources used to generate electricity:

  • Among renewable sources the lion’s share is currently taken up by hydroelectric turbines (16.6%). Followed by wind (3.7%) and biofuels (2.0%, mostly first generation). At the bottom of the list there is solar photovoltaic (1.2%), while thermal solar (CSP), geothermal, tides and waves combined make up for the remaining 0.4% of the 23.9% total share of renewables. We still need to invest in research because if the most technologically advanced sources (photovoltaic, biofuel, tides, wind, …) have to replace of the less efficient ones we have been using for hundreds of years.
  • It is expected that there will be a downsizing of nuclear energy. This is mainly for political reasons, rather than serious technical issues related to plant safety or environmental protection (it is, in fact, an entirely fossil source, but is among those available it has one of the lowest levels of environmental impact).
  • In order to meet the growing demand for electricity, today and in the immediate future, we will have to continue to rely mainly on fossil fuels. Currently, coal, gas and oil account for 40.8%, 21.6% and 4.3% respectively of electricity generation. And, returning to the example above, four in ten electric light bulbs operate on coal, two on gas, a half on oil, one uses atomic fission, two on water, while the last half bulb is powered by a mix of all the remaining renewable sources.

Substantially 66,7% of electric energy global production (equal to 15,9 PWh) is made from fossil fuels (coal, gas, oil). These energy sources, in soild, liquid or gaseous forms, are burnt and turned into heat. This evaporates water and creates high pressure steam that drives turbines which, in turn, are connected to a generator that generates electricity. In modern installations, this transformation of the chemical bond energy stored in the fuel into thermal energy, and then into mechanical energy and finally into electricity, has an overall efficiency of about 40%. This means that 60% (or more) of the energy is dispersed in the environment, mainly as heat. The most virtuous plants try to reuse part of this heat for preheating incoming streams or for the heating of surrounding facilities or homes. While coal and fuel oil plants have a yield of around 40%, and technological development has enabled combined cycle gas systems to reach yields of up to 55%.

Weighted average efficiency of thermoelectric plants: Coal (35%), Oil (40%), Gas (47%). Data from Ecofys

Another important consideration is that fossil fuels are not pure substances but contain variable amounts of other elements and compounds that turn into particulates, fumes or gas when burned. The same combustion process can produce toxic compounds. Even at the extraction phase, coal is a particularly dirty fuel compared to others. The pollution produced by prospecting, drilling and oil and gas extraction impacts much less on the environment than the open or underground mines needed to extract coal. Moreover, the health of mine workers is much more at risk, both at depth and in the vicinity of mines, than that of oil and gas well technicians.

In addition, all fossil fuels burn by oxidising carbon and producing carbon dioxide. While the various forms of coal produce from 350 to over 400 grams of CO2 per kWh, fuel oil produces emissions of between 240 and 260 g/kWh and natural gas stops at just 200 g/kWh. In fact, CO2 is the main cause of the greenhouse effect and global warming. The main advantage in the use of gas rather than coal for the production of electricity is precisely this: while the chemical energy contained in coal is in the carbon-carbon chemical bond, in gas it is stored in the carbon-hydrogen bond. To release the energy trapped in these fuels millions of years ago, both the carbon – which produces CO2 – and the hydrogen – which produces H2O water vapour – must be completely oxidised. This is why, for the same amount of energy produced, the complete combustion of natural gas produces about half of the CO2 produced by burning coal. Consequently, replacing coal-fired thermoelectric power plants with gas could save up to half of the carbon dioxide emitted into the atmosphere, with obvious benefits in terms of global warming.

It is not easy to compare the average cost of electricity from different sources. In addition to the variable costs of raw materials, the costs of building and managing more or less advanced, more or less efficient and more or less polluting plants has also to be considered. A Fraunhofer ISE study shows that in Germany the cost of one MWh of electricity ranges from €63 to €80 using coal to €75-98 for combined cycle gas systems. In the United States, the EIA estimates a cost of $95/MWh for coal and $75/MWh for gas. But if carbon capture and storage systems are installed, the costs rise to $144 and $100 $/MWh respectively. Despite lower costs, it is clear that the less attention paid to purifying fuels and breaking down fumes – the more polluting plants will be are.

This is why less developed countries that are less aware of the risks of global warming tend to prefer coal-fired plants built without such scruples. The consequences have been obvious, for example, to the Chinese government after another consecutive Beijing winter was invaded by a level of smog that made it impossible to see more than a few metres and made the airports unusable. The Chinese Centre for Prevention and Control on PM10 published, in the British Medical Journal, an estimate of 1.2 million deaths attributable to particulate matter in just one year.

In conclusion, coal is a dirty fuel. It is dirty when it is extracted, dirty when it burns and dirty when you have to treat the dust it produces. Its only value is that it costs less. So, what can we do? Since it is not possible to replace it directly with green energy sources, we need, on the one hand, to invest more in the research and development of renewable sources, and, on the other hand, use gas as a “bridge” towards a low carbon future. But, how? For example, stepping up the progressive replacement of coal and gas fired power plants would be a decisive step forward in the decarbonisation process and environmental protection requirements set out in the Paris climate agreement that has already been ratified by 195 countries.

Moreover, in the future, it will be possible to use modern gas-fire power stations in conjunction with renewable energy to offset seasonal and daily fluctuations in the energy demand with changes in the production of energy from renewable sources.

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about the author
Luca Longo