Simple Combustion

Very simply, to have fire there are three main requirements.

1. Something to burn (e.g. coal)
2. Something to allow the coal to burn. i.e. oxygen Oxygen comprises about 21% of the earth’s atmosphere, and is the only bit that enables combustion.
3. Something to get the coal and oxygen to join together i.e. an initial heat source Having got the stuff to burn, the heat has to be got to the water.

Having got the stuff to burn, the heat has to be got to the water. There are only three ways.

1. Radiation – usually noted by the visible light produced by the fire (how a camp fire works)
2. Conduction – getting the heat through the metal, after radiation and convection have done their work
3. Convection – the turbulent motion of the products of combustion moving hot gases to the firebox metal, and a similar operation in the water

When burning any fuel, there is a wonderfully technical phrase called the “stoichiometric mixture”.
Simply, this means that you have exactly as much oxygen and fuel as is needed to achieve 100% combustion of the fuel. You will never achieve this magic state. The closest that you are likely to get ( and be able to observe) is with a very light grey smoke at the funnel, when using a coal that normally makes smoke.
No smoke either means you are using a smokeless fuel, or are allowing “excess air” into the system.

There are many grades of coal, so this is not a definitive description.

1. Brown such as Victorian coal from the Yallourn region
   1. Generally this coal has a large amount of moisture in it, probably non-burnable mineral matter, and some burnable material
   2. This coal can have between 40-78% water as mined, with 65-70% carbon, and very little ash
2. Bituminous black coal
   1. This can have up to 40-50% volatiles. This is usually hydro-carbons and might make the coal seem oily. It could have over15% ash, and the remainder solid carbon
3. Anthracite
   1. This is a black coal, that may have considerable ash, but will otherwise be almost pure solid carbon. i.e. no oily stuff. It generally will not make smoke
   2.  The wide firebox (Wooton) was developed specifically to burn anthracite, as it needed a lager firebed to generate the same amount of available heat as an “oily” coal. It has less radiant energy.

In all cases, the coal has to get hot enough to burn. Basically, this means that the carbon gets to a state where some of it on the surface will combine with some of the oxygen to form carbon monoxide (CO). With additional oxygen, often provided by secondary air through the fire hole door (or other holes/pipes if a gas producer firebox), this carbon monoxide will then burn with a reddish flame to form carbon dioxide.
The chemistry is much more complex when there is the “oily stuff” present. Here there is a process not unlike that in an oil refinery where oil is heated up to separate different types (lube oil, diesel petrol, etc). The difference is that we have oxygen there waiting to combine with the boiled off oily stuff. With insufficient oxygen, or too much oily stuff, combustion will be a bit like the visible part of a candle flame – nice and yellow and sooty. The stuff does not combust until it is in the vapour form.

The size of the fuel, the depth of the firebed, and the frequency of firing all have effects on the success of steaming, and the stuff coming out the funnel.
Big lumps of coal burn slowly, simply because there is a high weight to surface area ratio. In full size locos, big lumps are usually reserved for the heel in a wide firebox, as the fireman does not want this to disappear quickly.
Small lumps will burn more quickly because they have a higher surface area to weight ratio. Using the standard railway practice of “light an often” means that these smaller lumps can burn quickly (therefore helping to make steam). By not covering the firebed all over at each firing this allows sufficient oxygen to let it burn off most of the smoke causing particles.
You might like to consider the similarity of this to the lighting up process where small combustibles (kindling) is used to get bigger stuff alight and then hot enough to get the coal to burn.
In full size, when it used to be “necessary“to make smoke for the rail fans, one method was to stack many shovels of coal onto the heel while backing up. Once the train began moving to the photographers, all this partially heated coal was shoved forward with a fire iron onto the very hot front of the firebed. Result, instant cooking of the coal with the result of a lot of unburnt volatiles suddenly being generated, and grossly in excess of the available oxygen. Instant clag!!

Moisture in coal is undesirable, firstly, because it doesn’t burn, and secondly, because you have to boil it while getting the coal to burn. As this water requires exactly the same amount of heat per kg to get it to boil as the water in the boiler, it is clearly not desirable as it just wastes any heat you get from the coal.

By using smallish coal lumps, you can minimise the amount of air into the system. This is very desirable, as along with the oxygen in the atmosphere, there is a soup of other gases (mainly nitrogen) that do not assist in burning, and like the water in wet coal, have to be heated up, thus using up some of the energy that you would rather have boiling water in the boiler.

If you have thick smoke coming out the funnel, you have at least two undesirable situations,

1. All that black stuff is unburnt, and therefore wasted, fuel
2. Some of that smoke will be plain carbon particles and might put a cinder in your eye
3. If the coal is bituminous, then the black will contain boiled off hydrocarbons (the oily stuff), and when cooled down by contact with the fire tubes, some will condense and begin to clog up the tubes. This can occur when you have a very yellow firebox flame that is still visible as it gets into the tubes. Think of the tip of a candle flame against a spoon, and you will get the idea.

Contrary to what most people think, the material your boiler is made of is not the main hindrance to getting heat from the fire into the boiler.
It is actually four other things;

1. A thin stationary air layer up against the metal of the firebox
2. Dirt on the fireboxmetal
3. A still layer of water on the wet side of the firebox
4. scale on the wet side of the firebox metal.

Combined, these four things can significantly exceed the insulation afforded by the metal (whether it be steel, or copper).

By increasing turbulence in the air on the fire side, and water on the other, the stationary layers can be overcome to a degree. But the other two can only be overcome by good boiler maintenance, and good firing techniques.

Clinker
Clinker is simply ash that has got so hot that it has melted.
The obvious solutions to this are, low ash fuels, or a cooler fire.
For those who have heard of the “gas producer” fireboxes of the late South African steam era, the use of these was primarily to stop the high ash content from melting.
In models, a nice uniform (small lumps) and deep firebed with no major holes can be an asset in doing the same. Holes in a fire allow a lot more oxygen available to the coal surrounding the hole.
This acts a bit like a local air blower, and results in a very hot region (white hot even in a wood fired boiler) that can achieve the ash melting point. Not all ash is the same, so some fuels will have a greater propensity for melting than others. However, in general a cooler fire is less likely to clinker.
Having said this not all boilers are the same, and some will work very well with a very thin fire. The CR NM class, as an example seem to love a fire so thin that the grate can sometimes be seen.
That raises a whole other subject of the overall combustion path through the boiler, from ash pan to funnel top. We are not going there in this discourse!