All about Coal

There must have been some quality that Welsh coal had compared with say Newcastle area coal. Why else would Jellicoe's Grand Fleet at Scapa Flow in the Orkneys require Welsh coal? Jellicoe specials were run from Pontypool Road to Grangemouth on the Firth of Forth for transhipment, round the clock, during the 1914-1918 war. [Hauled by Churchward's 2-8-0s as far as Chester or Warrington, via Hereford and Shrewsbury]

 

The evidence at Minehead is that Welsh tends to burn at a higher temperature which can cause problems with fire bars. The answer on the home fleet is to have bars cast with a significant chrome content. They are significantly more expensive but have a far longer life with a lower propensity for clinker to stick to them.

I'm not in Mr Aberdare's league when it comes to expertise however it appears that Welsh (Anthracite) does not have the highest heat content. Whether this means that low volatile bituminous burns at a lower temperature but for longer, I'm not sure. The table below is quite interesting.

Coal Grade Heating Value (Btu/lb) (kJ/kg)
Anthracite 12910 30080
Semi-Anthracite 13770 32084
Low-volatile bituminous 14340 33412
Medium-volatile bituminous 13840 32247
High-volatile bituminous A 13090 30499
High-volatile bituminous B 12130 28262
High-volatile bituminous C 10750 25047
Sub-bituminous B 9150 21319
Sub-bituminous C 8940 20830
Lignite 6900 16077

edited to say "oh bugger". The format of the table was fine when I wrote it.

 

Worth remembering that "heat" and "temperature" are not the same thing. A small glowing ember is very hot (i.e. has a high temperature) but contains far less heat (energy) than a glass of lukewarm water (*). So the fact that a type of coal on that list has a high energy content does not necessarily mean it will burn at a high temperature, relative to a different coal that may burn hotter but with less energy output per unit mass.

(*) If you don't believe me, drop a small ember in a glass of cold water - it will transfer all the heat it has until it and the water equilibrate, at which point the water has hardly warmed up at all, but contains all the heat that was initially in the ember).

Tom

 

This discussion has encouraged me to leaf through various BR Performance and Efficiency tests, specifically Nos. 3, 4, 5, 6, 7 , 10 and 20.
The following coal calorific values in B.ThU/lb may be of interest, figures are as received and dried.
Blackwell 11800 & 12350
Lilleshall 12660 & 13640
Blidworth 12930 & 13890
Bedwas 13970 & 14220
S.Kirkby 13994 & 14334

The Brittania 7MT 4-6-2 with exhaust steam injector (Test No.5) was tested on both Blidworth and Bedwas. The extrapolated maximum evaporation was 22490 and 24190 lb/hr of steam.

With regard to other parts of this discussion a Duchess or a Merchant Navy for example clearly have a capacity to 'boil more water' than a King. They have not only larger grates but corresponding larger evaporative areas. Maximum recorded power outputs not surprisingly support this.

Within reason any locomotive will benefit from a higher calorific fuel (coal) whether ex GWR or not.

For interest
King grate area 34.3sq.ft total evaporation area 2514 ratio total evap:grate = 73
Merchant Navy 48.5 sq.ft. 3116 sq.ft = 64
Duchess 50sq.ft 3629sq.ft = 72 (the last two built post 1948 = 78.

Indicates how much smaller the King is. Interestingly maximum outputs suggest the MN boiler can sustain an evaporation rate equal if not marginally greater than a Duchess (thermic syphons ?)

What would be very special might be a MN boiler mated with the Duke of Gloucester 'bottom'. Highest evaporation combined with highest front end efficiency.

Michael Rowe

 

These figures substantiate my statement that there is little difference between a good Welsh coal and a good hard coal from most other areas of the UK. Some additional comment:
Anthracite is not Welsh steam coal. I doubt that any loco would steam well on anthracite as it would go out if there was no reasonable draught.
The Welsh coal people rave about is a low volatile bituminous coal.
It is little use comparing the dry C.V. as the coal in the tender is not of that quality. It is also worth noting that the coal actually has to boil the water in itself so a coal with a high moisture content is going to have less heat available for boiling the water in the boiler.
C.V. is not the be all and end all of coal choice. Of probably more importance are the ash content and the ash fusion temperature.

 

Report No 10 on the Merchant Navy states on p12 under the heading "Comments (Without Syphons)" "The removal of the thermic syphons has virtually no effect on the maximum output of the boiler...because the limit is set by the amount of fuel that the draught can burn and not by the ability of the heating surfaces to transmit heat ...The removal of the syphons and the change of brick arch resulted in an increase in the inlet steam temperature of some 40deg to 60deg F." This is a precis of a couple of sentences and whole of this section needs to be read (for others, the report (and others) is available at http://users.fini.net/~bersano/english-anglais/BR-tests/ ).

 

Perhaps the fact that it makes less smoke if fired properly may have been a factor? Mind you, once it had been loaded at the mine, discharged to colliers in northern Scotland and then loaded into the ship’s bunkers it must have consisted largely of dust!

 

That quality was its low volatile content. In the days before radar you wanted to stay as undetected as possible. If you had black smoke billowing from your several funnels you would be spotted well before your actual ship came into view. 'Make smoke' is only a tactical advantage to use once you have been spotted.

 

Continuing the lesson in coal, it is inevitably a complex subject that will bore all but the most interested. However, it is probably worth putting down a few basic facts. The first thing to realise is there is no international standard and various people/organisations have attempted this in the past and even today. The NCB attempted to classify all UK coals back in the 50's and this resulted in a rather complex system, starting at 101 for Anthracite and going through to the 900's, ranking coal in descending order of volatile content. Other schemes have used carbon as the prime factor. In the training that I give on combustion I try to keep it simple and this is probably the best thing to do on here. The following are slides from my death by Powerpoint presentation, which I hope will give some understanding to the subject.
Firstly, what exactly is coal? It is a mix of many elements, most of which may be of interest to Chemists but are of no concern to us using it on a steam locomotive.

Coal can broadly be classified as Anthracite, Bituminous and Lignite. There is no hard and fast cut off to split them up but Anthracite is generally reckoned to have a minimum carbon content above 92%. Some systems quote a semi-anthracite between 86-92%, others refer to this as a bituminous coal. Anthracite is of no use in a locomotive firebox as it is difficult to ignite and requires a forced draught for combustion. Its high combustion temperature is often ruinous of firebars. Bituminous coal essentially covers all coals down to the brown coal/lignite classification. It is this coal that we use in our locomotives and covers all coals from the majority of the UK.

Bituminous coals have various properties and can be broken down further. Prime amongst these is the caking ability. Caking coals swell up, soften and then solidify when heated. Caking coals can be further sub-divided (confusingly) into coking and non-coking coals. The former are those that form a stable matter when heated and are therefore suitable for further refinement into coke. Caking coals that collapse during combustion are not suitable for this. Historically, bituminous coals that can be turned into coke are referred to as coking coals. All other bituminous coals are generally lumped together as steam coals. This term has nothing to do with its suitability for use in a loco boiler but the fact that its prime use would be for steam raising in factories/ships/etc.

Coal can also be classified as hard or soft. Generally, the older the coal the harder it is and these are at greater depths but faults and other geological features can throw up anomalies. Anthracites are the hardest coal and lignites the softest in geological terms. However, we tend to use the terms hard and soft differently, referring to the coal's friability. Thus Welsh, Somerset and Kent coals are regarded as soft whereas Midlands, Yorkshire and Scottish coals are generally said to be hard.


There are many other descriptions associated with coals. For, example, some of the Welsh coals were marketed as dry steam coals because of there low inherent moisture content. The term house coal is more a reference to the suitability of the coal for use in an open grate and is usually a washed product of suitable lump size, just as we now tend to get for use in our locomotives; it is not a reflection on its suitability for this purpose, or otherwise.

Back to that first slide and what is in the coal. Carbon and hydrogen both contribute substantially to heat production, the burning of hydrogen especially contributing to the heat output. One of the reasons why higher volatile coals have a similar C.V. to the Anthracite is because of the greater amount of hydrogen present. Sulphur adds marginally to the combustion equation but is not kind to copper fireboxes so is to be avoided. Chlorine is another unwanted, leading to corrosion and the formation of birds nests. This slide shows well shows the effect of chlorine on a boiler (not on a loco) burning a high chlorine content fuel.


The constituency of the ash is also important but difficult to quantify. Ash is a eutectic mix, meaning that the fusion temperature will vary as the mix varies. A good ash (is there such a thing?) may not melt until 1400degC but a poor ash can be molten at 800degC. What is little known is that the ash will melt at a lower temperature in a reducing atmosphere. i.e., one starved of oxygen. That can happen when you shut the dampers.



That's probably enough for now. Posted on the West Somerset thread for continuity but perhaps the mods should see fit to move the whole relative correspondence elsewhere.

 

For water tube power station boilers a significant ssh content stops the tubes overheating. As to friability , not all midlands coals were hard. I did my face training at Florence (Stoke on Trent). We were in the Winghay seam. You could pick up a shovelfull and throw it st the pansa conveyor where it would shatter into dust. The pansa or face conveyor was a metal trough with chains down both sides with bars between then to pull the coal along. Winghay was almost exclusivly a power sation fuel

 

One of the many things about coal was that it was impossible to draw lines and say that coal from this colliery/area would have these characteristics. Even the same seam could have different characteristics in different parts of the colliery.
On a point of pedance, armoured face conveyors (AFC's ) were referred to as a Panzer and not Pansa. Invented in Germany where the word panzer means armour.

 

I am an engineer - you dont expect me to spell as well

 

Am I right in assuming that Winghay was used in power stations due to its friability and hence ease at which it could be pulverised in a ball mill for fluidisation.

 

More likely they could not ger rid of it in any other market than power stations. Ball mills are pretty effective amd could manage Staffs Hardmine much of which went to power statioms

 

Just spotted this, Dave, My grandad, Noah Stubbs was foreman platelayer at Florence colliery, having previously been underground at Hanley Deep Pit and Florence, but put to work on the surface after being entombed twice. My first footplate ride was on "Florence No. 2," which, I believe is at Foxfoeld now.

 

At the risk of thread drift I finished up as ventilation engineer at the combined Florence/Hem Heath i.e. Trentham Colliery in 1993. Never a dull moment. As a relevant point though, in the latter days of the coal industry many pits may have mined a number of different seams but it all went through the same washery and produced products of different sizes but a mixture of varying seam proportions. In my last couple of years at Trentham we were working, from the top, Great Row, Winghay, Rowhurst, Yard Ragman, Hams & Bowling Alley. It all came via various bunkers & up the surface drift belt at Hem Heath straight to the washery.

 

In NCB/British Coal days, coal was coal, as far as the marketing department was concerned. Unless you were a big purchaser, you couldn't specify what coal you got. Although marketing would endeavour to supply a suitable fuel, leading costs usually meant it came from the nearest pithead unless you were prepared to travel and collect it yourself from the landsale.
Thread drift is always OK if we're mining coal!