LONDON (Reuters) - U.S. President Donald Trump has promised to end “the war on coal” waged by the previous administration and help put coal miners back to work.

In practice, coal production and employment have been victims of the shale gas revolution rather than government regulation.

Coal-fired power generation has been steadily losing market share to natural gas since 1988, according to data from the U.S. Energy Information Administration.

Coal accounted for 57 percent of electricity generation in 1988 but that share fell to 53 percent in 1998 and 50 percent in 2008 (tmsnrt.rs/2o3nRSL).

Gas has been the main beneficiary with its share of generation climbing from 9 percent in 1988 and 13 percent in 1998 to 20 percent in 2008.

For much of that period, coal’s relative decline was cushioned by growing demand for electricity which provided an expanding market for all fuels.

Coal consumption by power producers peaked at 1.05 billion short tons in 2007, up from 758 million tons in 1988 (tmsnrt.rs/2nE69Sv).

But since then the coal industry has been hit a perfect storm of stagnating electricity demand, a sharp fall in the price of gas, and a record warm winter in 2015/16.

Coal has continued to lose out to gas in the generation system but since 2008 the decline has been absolute as well as relative terms because demand for electricity is no longer growing.

Gas prices have fallen sharply thanks to the shale revolution, encouraging power producers to build more gas-fired power plants and run them for more hours each year.

And the unusually mild winter of 2015/16 resulted in an enormous build up of coal stockpiles at power plants and a sharp reduction in new shipments from the mines (tmsnrt.rs/2nDS2fR).

The resulting wave of bankruptcies, mine closures, and job losses and layoffs crashed over the coal industry during 2015/16.

But mine closures and job losses were the result of market forces rather than job-killing government regulations introduced by the Obama administration.

MORE COAL CLOSURES

Most mining companies and coal analysts are cautiously optimistic about the outlook for production and employment in 2017/18.

Excess coal stocks at the country's power plants have been worked down and a modest rise in gas prices should encourage power producers to run gas-fired plants for fewer hours in 2017/18 (tmsnrt.rs/2o32b9q).

But the medium-term outlook for the coal industry remains poor, with more coal-fired power plants scheduled to close over the next five years to be replaced by a combination of gas and renewables.

Gas-fired generating capacity is scheduled to expand by another 8 percent over 2017/18 according to data from the U.S. Energy Information Administration.

At the same time, more coal plants are set to close, which will cut potential demand further even if gas prices increase modestly.

The problem is that most coal-fired power plants are now growing very old and nearing the end of their service life.

The capacity-weighted average U.S. power plant started generating in 1987, according to an analysis of EIA data (tmsnrt.rs/2o3hwqh).

But the average for all power plants conceals significant differences between coal and non-coal power generating units.

The capacity-weighted average coal-fired power plant started generating in 1976 and has now been producing power for 41 years (tmsnrt.rs/2p0ZYsB).

In contrast, the average non-coal power plant started generating in 2000 and has been producing electricity for less than 17 years (tmsnrt.rs/2nE3wA5).

COAL FLEET AGEING

Most coal plants were originally planned to operate for a minimum of 25 years without significant modification and their service life can be prolonged to 40 or even 50 years.

But as plants age they are increasingly likely to suffer mechanical breakdowns and need expensive components replacing (“Life extension of coal-fired power plants”, IEA, 2005).

The main problems are associated with the steam generator and the turbines, which suffer from creep, fatigue, corrosion and erosion.

Repeated heating and cooling of the system accelerates the ageing process and shortens the service life even further.

Most coal plants were designed to operate as baseload, with a limited number of cold and warm starts to reduce the thermal stress on the components.

But many are now forced to operate on a two-shift system, operating only during peak hours and then coming off load when electricity demand drops.

Two-shifting involves more starts, and much more thermal stress on the components, speeding up the ageing process.

Many of the coal-fired power plants scheduled for closure in the next five years started generating in the 1950s and 1960s and are basically wearing out.

Some of them are small-scale and relatively inefficient, burn low-quality coal, or emit lots of mercury and other air pollutants.

In most cases, there is no economic case for replacing worn out boilers and turbines or fitting new pollution-control equipment.

Power plant operators mostly plan capacity expansions and refits on a 20-year horizon or longer and few see any benefit from upgrading or adding coal-fired units.

No new coal-fired power plants are currently planned, in contrast to many additional gas-fired and renewable power units, according to notices filed with the EIA.

Even if the Trump administration tries to throw a lifeline to coal-fired power plants, its four or eight-year time-span is too short to have much impact on capacity planning.

As the existing coal fleet continues to age, more power plants are likely to be retired because they are too expensive to maintain.

The Trump administration can end the “war on coal” but it cannot halt the march of time and fatigue which is gradually culling the coal fleet.

The amount of coal-fired generation capacity will continue to shrink. Any revival in coal consumption depends on a rise in gas prices to encourage power producers to use the remainder for more hours each year.