Three-eighths of a coal power station

Some notable milestones to pass on the way to 100% renewable energy are one-quarter, one-half, and three-quarters renewable electricity generation.

The average CO2 emissions per kilowatt-hour for all electricity generated at each of these milestones might be 660 grams, 440 grams and 220 grams respectively.

But they could be much less.

We'll look at the halfway milestone to see why this is so:

At this milestone, one-half of all electricity is delivered from renewable energy sources with no fossil-fuel CO2 emissions - solar PV and solar thermal, wind farms, hydroelectric including pumped hydroelectric storage, and battery storage.

The other half of electricity is delivered from fossil fuel power generators. These power plants are only dispatched at times when total demand exceeds the total capacity of all the available renewable energy sources.

These fossil fuel power plants may have average CO2 emissions per kilowatt-hour of electricity of 880 grams.

Average CO2 emissions and efficiency of a coal-fired power plant

In this case the average CO2 emissions per kilowatt-hour for all electricity generated at the halfway milestone will be 440 grams: (Zero for the half from renewable energy sources plus 880 grams for the half from fossil fuel power plants) divided by two.

It isn't necessary for the CO2 emissions from the electricity generated by fossil fuels to be nearly this high. They can be reduced to three-eighths of 880 grams per kilowatt-hour of electricity.

A way of doing this allows the use of power plants that are far more efficient than coal-fired power plants, are far cheaper to build, and are able to start more quickly in response to increases in demand.

A further advantage is that they use only three-eighths of the coal to generate each kilowatt-hour of electricity so the cost of mining and transporting coal for electricity generation is cut to just three-eighths of the cost with the less efficient, more expensive coal-fired power plants.

This way of supplying electricity at the halfway milestone reduces the average CO2 emissions for all electricity generated to just 165 grams: (Zero for the half from renewable energy sources plus 330 grams for the half from fossil fuel power plants) divided by two.

Average CO2 emissions and efficiency of a combined cycle power plant

The reduced quantity of coal for fuel for the combined cycle power plants can converted to methane by a reaction with hydrogen. The hydrogen can be produced by electrolysis using excess renewable energy generated whenever total demand is less than the output of renewable energy sources.

A coal-fired power plant that is emitting 880 grams of CO2 per kilowatt-hour burns coal containing 240 grams of carbon for one kilowatt-hour of electricity. Coal containing just 90 grams of carbon (three-eighths of 240 grams) is all that's needed for a combined cycle power plant to generate a kilowatt-hour of electricity.

Coal may be converted directly to methane by reacting it with hydrogen:

Hydrogen - A Key to the Economics of Pipeline Gas from Coal, C. L. Tsaros, Institute of Gas Technology, Chicago, Illinois

The objective in manufacturing supplemental pipeline gas is to produce high- heating-value gas that is completely interchangeable with natural gas - essentially methane.

The basic problem in making methane from coal is to raise the H2/C ratio. A typical bituminous coal may contain 75% carbon and 5% hydrogen, a H2/C mole ratio of 0.4:1; the same ratio for methane is 2:1. To achieve this ratio it is necessary to either add hydrogen or reject carbon. The most efficient way is to add hydrogen. The hydrogen in the coal can supply about 25-30% of the required hydrogen, but the bulk must come by the decomposition of water, the only economical source of the huge quantities needed for supplemental gas.
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In the second, or direct, method, methane is formed directly by the destructive hydrogenation of coal by the reaction:

C + 2H2 → CH4

There is a steadily growing list of commercially available systems to produce hydrogen using excess renewable energy:

Clean and Low-cost Hydrogen for Industry

The Sunfire steam electrolysis system, based on solid oxide cell (SOC) technology, promises lower onsite hydrogen production costs compared to legacy technologies. The ability to supply steam directly to the electrolysis module is unique and maximises efficiency.