For coal to be used for making steam it has to swell and cake somewhat
during combustion - if a grate is used in the boiler. If it does not do
this, holes are left in the bed and incoming air takes the line of least
resistance and passes through leading to inefficient use of air, poor
steaming and hence poor fuel efficiency. If the coal swells too much then
it chokes the bed - such coals are good for making coke. The NCB ranked
coal types on the basis of their caking characteristics and other attributes
such as volatiles content are related in the ranking. Steam coals come
somewhere in the middle of the range. Anthracite does not swell and would
not be suitable for routine use as fuel for a boiler with a grate.
Coal containing a high proportion of smalls will tend to smoke for the
reasons you have stated and these arise because of poor screening, rough
handling and fragile coal.
Coals with a low ash content should also be avoided as a layer of ash is
said to protect the grate and give it a longer life; coals with low-melting
ashes choke the grate.
Industrial boiler operators usually changed their coal source with the
greatest reluctance and it has always surprised me how ships were able to
take on coal at ports throughout the world and keep going.
----- Original Message -----
From: "Ken J Smith" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, March 16, 2011 2:02 AM
Subject: Anthracite v. Steam Coal
> Dear All,
> From the early C20th, Welsh smokeless steam coals (as given in the
> 'A' List for, eg, 1912) were used by the Royal Navy's top fighting ships
> because, inter alia, they gave a tactical advantage; if your fleet made
> smoke than an enemy's, you were likely to become aware of his presence
> before he became aware of yours. Further, most of the smoke from burning
> coal comes from its volatiles. (The rest is from the fly ejected during
> charging.) But most anthracites had lower volatiles than most smokeless
> steam coals (&, being harder, made less dust & hence less fly), so ... why
> didn't the Royal Navy use anthracite?
> Kindest regards,
> Ken (Smith).