Steam & Excursion > What is 'Green Coal' and a 'Clinker?'

I was reading the latest Fall 2016 issue of Classic Trains, and an article on page 50, it mentions 'Green Coal' and 'Clinker'. What are they?

​The paragraph:
​"After the engine had been serviced and set out here, the fire had cocked over, and the roundhouse boys had been keeping her alive by throwing green coal on top. After three days, she had a huge clinker with tons of green coal on top"

​Thanks for any explanation, from a too late-era railroader :-)

"Green coal" is fresh/new coal thrown into/onto a spot that is already NOT burning in the firebed.

A clinker is caused when coal melts together instead of burning. The clinker thus cause a cemented-over area on the grates where absolutely no air can pass through. An "old head trick" sometimes used to break-up a large clinker, was to squirt the clinker with cold water from the Fireman's deck hose. The quick blast of cold water will generally cause the clinker to crack into smaller pieces, and thus can be raked out of the firebox through the firedoor.

Some information..... Relative to stoker-fired, modern era steam locos...

Green coal sometimes means a purposely built, low combustion-rate mound of coal , typically built at the rear corners of the firebox and the area around the stoker fitting table or stoker 'pot''.  It can be wet coal and it's purpose is to eliminate massive amounts of cold air from entering the firebox at it rearward section.  Because of the vacuum drafting of the fire, caused by the Venturi-jet up the smokestack generated by the geyser-like sream exhausted at the end of each piston stroke, the wind tends to be the strongest up the rear of the firebox.  That mass of cold air is very deleterious to the firebox sheets, and ruins thecattemps to increase firebox temperatures.

Since the coal burns brightest and quickest under the strongest wind, the rear corners, and that area under the firing table, burn down quickly, -----and when burnt out, become cold, & the light remaining ash which then gets sucked up the stack, leaves the grates bare---- leading to large areas of the coal grate in virtually bare conditions ---allowing torrents of cold air into,the furnace. Such action, if not prevented, is very hard on the '"door sheet's staybolts",  as well as the fireman.   So, a careful fireman builds a substantial rear 'bank' of green coal,  at the rear of the grates.
Over time, this green coal solidifies into a glowing, carbon-mass of coke.
Not adding materially to the boiling effort; but,  by maintaining an over-all higher firebox temperature.

The wind up the the rear firebox sheet tends to rapidly consume the areas of the bank right next to the firebox sheets, and those areas must occasionally be re-built by temporariy cutting back on the stoker jet force ( but leaving enough steam jet flow to prevent clogging of the stoker jets with bits of green coal)  and speeding-up,the rotational speed of the stoker feed-screw --- maybe rolling in a ton, or two,  of green coal to fill-in the bare corners.  Sometimes it's best to do this several times, over 4or 5 minutes.

The simple concept, often taught, is to build the firebed to mimic  the shape of a regular coal shovel, scoop, 
You want the rear corners to be built up, rounded steeply in the back, like the rear corners of the scoop, a mound of coal around the stoker firing table --- similar to the area of the scoop where the handle attaches to the shovel, ---- and the rest of the grate area nice & thin, and flat....

That thin and flat area is the bright area of the fire where the greatest heat is produced,,and the greatest rate of fuel burning occurs.
That area is sometimes called "the reduction area", referring to the oxygen-reduction of the carbon, and volatiles in the burning coal.  As soft coal burns, it swells, expands, releases 'volatiles' ,  and must be converted to gas before it will ignite.  
That whole process is called oxygen-reduction

( That expansion property, of going from a solid to a gas,  is why you don't want unburned coal getting into the stoker jet ports....such small, cold pieces of coal will try to burn, but they only swell in the confined space of the jet orifice, the cold metal surrounding the jets prevents the swollen coal from igniting, and now the jet(s) have become plugged, solid.  [the use of the pointy 'jet hook', typically results in the jamming of the unburned coal further into the jet orifice, NOT A GOOD THING],

That area of the thin, flat fire typically covers 80% of the grate.  A good stoker fire burns very thin, only a couple of inches, which burns white hot,----- depending on the load behind the tender, ------ and extinguishes ( for lack of fresh fuel)  in 'under a minute',
if unattended to, and constantly fed at a matching rate of fuel consumption ------to however-hard the engine is being worked.

On heavy grades, just starting a heavy train, and during photo run-bys, ---- having the heavy 'bank' at the back of the grate makes the forward part of the fire really hot, for maximum steam production.   Also, in those periods of inattention by a distracted fireman
( non-gender specific) the reduction area of the fire goes out very quickly---- setting up an impending crisis for the the crew.

In such cases, a breaking-up that big bank of glowing, 'coked' coal in the rear bank, and pushing large chunks of glowing fire liberally over the cold fire bed, and then immediately stoker-feeding a thin layer of green coal over the chunks of the burning 'bank' will almost instantly re-ignite the entire grate, and if fed modestly, the fire will soon be fully recovered.  HOWEVER, that large 'bank' that had been in the rear of the great MUST immediately be re-built--- order to give all,that green coal enough time to form a solid mass of red hot coals.  That can take a long time.  So your  'safety reserve' of available fire is vital to a successful trip.

Most of the time, the large, glowing heat bank serves to help maintain more even firebox temperatures.  Wildly fluctuating firebox temps cause constant expansion/ contraction ( lateral movements) of the firebox sheets, and can cause staybolt threads to tear as a result of metal fatigue.   The furnace sheets are subjected to the greatest bad effects of thermal cycling, of all the boiler sheets.

The other sheets of the boiler, not part of the furnace, proper, --- are submerged under ( stable temperature)  water, or are subject to steam temps that are the same as the boiling water temperatures,  and the rapid temperature cycling does not affect these sheets as badly as the furnace sheets.

[ Note:  Oil burning locos are subject to the most deleterious temperature swings in the firebox due to the fire temperature  being capable of rapid, severe temperature changes caused by careless firing practices.  Because the amount of heat in the fire can so easily by be changed simply by widening the oil firing-valve, the firebox can go from very cold to white-hot, with a flame tha fills the furnace void with 3,000 degree fire.  Coal burners rely on the quality of the firebed for their highest temperatures, so the thermal cycling times of coal burners are lower, and firebox temperatures ten to be more even in their severity of change]


CLINKERS....

Clinkers can be confused with the big banks of unburned, green coal.
Clinkers are the result of melting the silica-like particles remaining in the cold ashes, as distinguished from green, unburned coal.
These masses of unburned coal, are best dealt with by 'starving' the area of the big mass of adding more coal, on top.
If stoker fed, simply reduce the stoker jet pressure to the affected area, in about 10 to 15 miles, the green coal bank will burn down, nice & thin.....

Some poor quality coals form clinkers simply as a result of the combustion process.
BUT, self clinkering coal is a really rare occurrence.

IMPORTANT....
Virtually all clinkers are caused by improper use of, and very aggressive work with the 'clinker-hook'.
A clinker hook is a long steel, firing tool,with two tines at the end like a two tinned fork.  The tines stick down,,at right angles to the long shaft.

Improper use of the clinker hook typically results in forming a thick bed of ashes building up on the grates.  The best bed is a thin bed, less than 2-inches thick, that form an insulating layer on the cast iron grate segments -- to keep them from melting.
If the firebed begins to get much thicker than that, the air flow through the bed slows down combustion temperature lowers, the fireman panics, adds more green coal, the green coal lowers the firebox temperature, and soon the firebed build up with partly burned coal, a deep bed of ashes and a stalled engine.

So, now the fireman, too late, grabs for the clinker hook and proceeds to drag the hook back and forth through the deep ashes and tries in vain to raised the combustion temperature.... Soon it appears that his tactic appears to be workin, the steam pressure builds back up, the water leve can be brought up, and the trip might be re-started.

However, the firebed is is no condition to produce very much heat, as the engineer vainly tries to beat his steed harder, in order to  get the draft rate higher.....

Remember that word 'silica', from above.  That is the same silica as glass is made of. NOW, the attempts to raise the fire's temperature are too feeble, and the temperature only melts the collections of silica into puddles of molten glass.

Typically theses puddles of molten glass, the size of small bed pillows, which may congeal together &  block any oxygen from getting to the fuel, the clinker begins to form as the lowering temperature glass solidifies.  It is common for this mass of melting glass to grow swiftly ----- covering virtually all of the grate area with a solid, gooey mass of solidifying glass clinker.  It's become a mix of glass, ash and unburned coal...the result is a stalled train and a couple of hours worth of hard work to remedy the clinkered fire.....

The remedy of a deep buildup of the ash bed, is to properly shake the grates, while stopped.
Flush and dump the ash pan, and begin rebuilding the layered firebed as originally intended....

NEVER SIMPLY SHAKE THE GRATES TO GET THE HOT FIRE UNDER THE GRATE BARS...THE RED-HOT HEAT WILL BURN THE GRATES OUT OF THE ENGINE, THE FIRE WILL FALL INTO THE ASHPAN, AND THE ENGINE MUST BE TOWED TO WHERE IT CAN BE REMOVED FROM THE TRAIN.  AFTER SHAKING THE GRATES, ALWAYS ENSURE THAT NO ASHES ARE BUILT UP,  BLOCKING COLD AIR FROM COOLING THE BOTTOM OF GRATE BARS... AND BE SURE TO CLOSE THE ASHPAN DUMP-DOOR, AND LOCK IN THE CLOSED POSITION.....

SO, now we'll stand back and read the common practices and suggestuions, used to prevent clinkering, and ways to manage clinkers, in locos,  as they show up in the firebox....

Gentlemen, and Esteemed Ladies, the 'floor is yours...'

W.



 



Edited 5 time(s). Last edit at 09/29/16 01:50 by wcamp1472.

A real simple definition of "clinker" is "melted ash".  As a coal fire burns the ash works its way down toward the grates as new coal is applied from the top.  As the ash cools is eventually either falls through the grate or just ends up on the grate as cold ash.

If a fireman has trouble with the fire and develops humps or "banks" in the fire, or if the fireman fires too heavy and the firebed becomes too thick, the tendency is take the fire rake and churn the whole mess up to get some air moving through the fire again.  This mixes the cold ash with the the burning live fire.  The ash proceeds to melt and the resulting goo-ey mess sticks together and forms a natural ashphalt-type substance that effectively blocks all air from getting to the fire from below.

Then your're screwed.

Some coal is more likely to cause clinkers than others.  One of the things one looks for in purchasing coal is the "fusion temperature" of the ash, which is the melting point of the ash.  The higher the fusion temperature, the less likely the coal will be to clinker.  Coal with high ash content and low fusion temperature is a recipe for disaster.

The best practice is if you need to put the rake to the fire, do it gently, and only move the coal on the surface of the fire.  Don't dig deep and stir the coal ashes up into the live fire.