(Repeats with no change to text. John Kemp is a Reuters market analyst. The views expressed are his own)

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By John Kemp

LONDON, Nov 14 (Reuters) - President Barack Obama’s administration has been blamed for the closure of a record number of coal-fired power plants across and heavy job losses in the mining industry.

Since the current administration began in January 2009, more than 400 coal-fired power units have closed across the United States and around 33,000 coal-mining jobs have disappeared.

Coal now accounts for only a third of electricity generated in the United States down from almost half when the president took office.

The administration’s opponents criticise it for waging a “war on coal” to support cleaner forms of energy including wind, solar and natural gas.

The administration's supporters credit it with forcing the closure of power plants that were a major source of air pollution as well as greenhouse gases (tmsnrt.rs/2eT5xUI).

But the reality is most of the coal-fired units that have closed since the president took office were very old and inefficient and would likely have closed anyway.

OLD AND WORN OUT

The average coal-fired power unit closed during the Obama administration started generating electricity in 1960 and almost all of them began generating before 1971.

So most of the retired units were already over 50 years old and had been producing power much longer than the typical generating unit (tmsnrt.rs/2eT8oNf).

The average retired unit had a capacity of just 80-100 megawatts, much smaller than the 500-1,000 megawatts now considered the minimum efficient size for a coal plant.

Aging power plants require much more maintenance to keep them running safely which means they are typically available to generate for far fewer hours each year than more modern ones.

Most components in a coal-fired power plant will show wear and tear as a result of prolonged operation and eventually need replacing (“Coal-fired electric power plant life extension”, Martin Marietta, 1986).

Power plant components are subjected to high pressures and temperatures, repeated cycles of heating and cooling, constant exposure to steam and corrosive impurities including sulphur.

The result is a range of damage including creep, fatigue, erosion and corrosion.

Boiler tubes and drums, main steam lines, turbine blades and forgings, scrubbers and generator winding supports are among the expensive items that need replacing.

Power companies must make a commercial decision whether to incur large capital costs to extend the life of existing coal plants or replace them with other sources of generation.

MORE FLEXIBLE WITH GAS

In practice, it has been cheaper to replace coal-fired power plants with combined-cycle gas turbines which are quicker and cheaper to build, easier to run, and offer more generation flexibility.

CCGTs can ramp their production up and down much faster than coal-fired units making them much more suitable for load-following and two-shifting operations (running during the day while switching off at night).

CCGTs are therefore much more attractive for generators needing flexibility to operate in deregulated wholesale power markets.

Most new thermal power plants built in the United States since the early 1990s have therefore been fuelled by gas and employ a CCGT design.

Cheaper gas prices thanks to the shale revolution have entrenched the advantage of gas-fired power generation even further.

Coal units might have survived if demand for electricity had continued growing, in which case it might have made sense to keep them running while building gas-fired power stations to meet incremental demand (tmsnrt.rs/2fqZiLy).

But growth in electricity demand has been slowing for decades. Consumption has been essentially flat since 2007, according to an analysis of data from the U.S. Energy Information Administration (tmsnrt.rs/2fhgq6D).

In a stagnant electricity market, coal-fired power generation has been pitted head to head with natural gas, and lost.

CONTINUED DECLINE LIKELY

Most utilities and independent power producers have opted not to extend the life of coal fired units when the time for the next major refit has arrived.

The Obama administration’s encouragement of more solar and wind generation, both by utilities and by households, has worsened the predicament for coal-fired generators.

Stricter pollution and emissions regulations for new power coal-fired power plants (finalised) and existing ones (currently being challenged in court) have added to the compliance burden for coal-fired plants.

But most of those power plants would have retired in any case because it was no longer commercially viable to keep them running.

Coal retirements look set to continue in the medium term even if the forthcoming Trump administration ends the “war on coal”.

The average age of coal-fired units still in operation is 39 years (the capacity-weighted average unit first operated in 1977).

By contrast, the average age of combined-cycle gas units is just 13 years (the median plant began generating in 2003).

Very few new coal-fired power units have been constructed in recent years and it is hard to see that changing unless gas prices rise significantly or the demand for electricity starts growing faster.

So the coal fleet will continue shrinking as existing plants reach the end of their lives and are replaced by other forms of generation.