On Mon, 20 Jan 2020 12:12:25 +0000, Chris Hogg wrote:

The company I used to work for had their own coal-fired power station
up until about 1970, to supplement the grid or to use when the grid
went down. The man who managed the coal stocks told me that the stock
decreased naturally every year, by about 10% IIRC (which seems a lot,
I may have mis-remembered), simply due to aerial oxidation of the
stockpile. In the extreme, they can suffer spontaneous combustion.
http://tinyurl.com/wcv5jay

Indeed it does degrade, I can't recall by how much and preventing
degradation and ultimately fire in the heap was always top prority.

Most of the sites using coal pulverisation rather than chain grate
boilers always had three separate heaps, with only one at a time
having coal taken from it, one being full and the other being built
with new deliveries. They normally entered the winter period with
maximum coal stocks with the lowest level as they entered the summer
main overhaul periods (except when a miners strike was predicted and
then all three heaps were the size of a mountain)

There was a strict rotation of usage and regular compression of the
heaps by driving huge dozers/scrapers over them to keep the airspace
to a minimum.

Coal could also come straight off the trains and into the boiler
hoppers such as overnight and in bad weather when driving up coal
heaps is a bit unsafe.

The net weight of coal delivery is determined by weighbridge and was
the basis of payment rather than the actual energy value, but with
predicted coal sources and washing out of stone etc before delivery
the quality would be relatively consistent.

Coal was randomly sampled every day off the conveyors on the way to
the boiler hoppers and the labs on site did a calorific value test
that formed the basis of the thermal efficiency figures that
ultimately determined the order of despatch, at least within the
specific regions, with National Control determining the energy
contribution from each region and the transfer across the region
boundaries.

All this is from a very rusty memory because it was many decades ago.
Power stations, at least then, were mucky places, now they are clean
but just as dangerous