Black smoke from steam engines

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animal12

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Me & a bud were talking about the awesome engines you folks here build & we're both wondering what the real black smoke is that you sometimes see from steam engines ? Looks like when a guy hammers on his truck diesel engine .
Thanks
animal
 
The engine exhaust creates a vacuum in the smoke box [front of the boiler] by channeling the exhaust thru a venturi. When the throttle is opened, the exhaust and vacuum increase and so does the draft and burn rate in the fire box. As you say when the engineer hammers the throttle. Pretty slick, semi self governing system. I think the later, oil fired engines responded very rapidly to the increase in fuel demand and the towering columns of black smoke resulted. I often wonder what the EPA would think.

Other opinions welcome. I had no clear how this worked until I made a locomotive.
 
British Railways footplate crew were issued with a copy of the Handbook for Railway Steam Locomotive Enginemen. There are 13 pages on coal, combusion, and firing methods. Briefly, black smoke occurs when there is insufficient air to burn the volatile matter coming off the coal, and a clear exhaust when there is too much air, which cools the fire. The ideal exhaust, from the point of view of efficient combustion, is a light grey. This is largely a matter of good firing technique, but these days it also depends on what coal you can get. When fresh coal is added to the fire, the firedoor is ideally left partly open to admit 'top air' to burn the volatiles. At night this all becomes rather academic. The fireman probably cannot see the colour of the smoke he is making, and the driver may well slam the firedoor shut so that the glow does not prevent him from seeing signals (an oil lamp behind coloured glass).
 
The engine exhaust creates a vacuum in the smoke box [front of the boiler] by channeling the exhaust thru a venturi. When the throttle is opened, the exhaust and vacuum increase and so does the draft and burn rate in the fire box. As you say when the engineer hammers the throttle. Pretty slick, semi self governing system. I think the later, oil fired engines responded very rapidly to the increase in fuel demand and the towering columns of black smoke resulted. I often wonder what the EPA would think.

Other opinions welcome. I had no clear how this worked until I made a locomotive.
I had several cab rides on the locomotives running at the North Freedom Rail museum in Wisconsin, and the train crews were VERY sensitive to what their stacks were putting out. They told me their procedures, and they seem to work because a grey or clear exhaust was what I saw nearly always when I visited and when I rode. They were doing their utmost to be good neighbors in the areas their tracks passed through. (Their locomotives were all oil fired, which I think is easier to control than a coal fire.) The fireman controlled the flame, and also the oil pressure and rate of flow, and they seemed pretty good at it. They were not weekend duffers, playing with trains, but real enginemen who knew their equipment and all of the idiosyncrasies of each locomotive.
 
According to at least one study of locomotive steam engine efficiency that I read, a lot of the black in the black smoke is wasted (unburned) coal that is sucked through the system when the throttle is suddenly opened. This was reported in a book "A Study of the Locomotive Boiler" by Lawford Fry, around 1924. The book is available online as a free download.
 
When I teach newbies how to coal fire on marine boilers, I explain that black smoke derives from unburned carbon. As carbon is a combustible that could heat water in the boiler, its loss is a loss of heating potential. We endeavour to see firemen (stokers) add additional top air via a partially open firedoor, or by a slightly clear grate just inside the firedoor deadplate. On our Yarrow watertube boiler, we can use the corner of the rake to pierce half a dozen air holes in the firebed to get some extra top ait into the furnace to help consume the smoke.
 
When I teach newbies how to coal fire on marine boilers, I explain that black smoke derives from unburned carbon. As carbon is a combustible that could heat water in the boiler, its loss is a loss of heating potential. We endeavour to see firemen (stokers) add additional top air via a partially open firedoor, or by a slightly clear grate just inside the firedoor deadplate. On our Yarrow watertube boiler, we can use the corner of the rake to pierce half a dozen air holes in the firebed to get some extra top ait into the furnace to help consume the smoke.

Hmmmmmmmm - - - - very interesting!

Thinking - - - wondering - - - if one were firing using biomass (wood chips) how could one effect that " corner of the rake to piercing half a dozen air holes in the firebed" by some 'mechanical' means?

Any ideas?
Would pushing air from under the firebed suffice?
(Want as complete a combustion as possible as well as clean combustion - - - - how to burn the gasses coming off the top of the bed is another question - - - argh - - - so many questions and so little time and even less knowledge (on my part!!!).)

TIA for any ideas!!!
 
There is bottom/primary air fed from the ashpit and up through the grate, and then top/secondary air enters via the holes in the firedoor. After a pitch, we keep the firedoor open a bit to allow extra top air to enter to help burn the volatiles driven from the fresh pitch. We might also clear a small section of the grate just inside the deadplate to achieve the same result. You can clearly see the reduction - The engineer may ask the firemen to crack open the firedoor a bit more to minimise smoke after a pitch. Fireman will shut the firedoor when volatiles are driven off.
 
Usually happens when the load on my 1/3 scale traction engine is removed also but there it allows the fire to reduce dropping steam generation a tad, better than popping the relief valve.
 
Hi AJ. With no experience of wood burning, except the rare firing of log stoves when in holiday homes.... but I can imagine the problem...
Wood - naturally - varies widely, as does its derivative, coal. Considering it is simply a lump of mixed hydrocarbons... all of mixed complexity, almost every chip is different from the next.
But my simple understanding of hydrocarbon combustion is as below:
The heat of the existing fire - and firebox - in 2 forms (conduction of adjacent hot or burning gases and radiant heat from further away, but which can penetrate some depth into the lump of wood) simply causes the lighter hydrocarbons to flash-off quickly, and without adequate air and turbulence will just form a rich combustion mixture. This makes CO and unburned hydrocarbons. Just like in a petrol engine that is rich. The CO is fuel that needs oxygen from the air, and a temperature over 350C so that it burns. Otherwise it is clear gas that is wasted fuel (half of the carbon is "unburnt"). The hydrocarbons, due to heat and the random way they are formed from having most of the hydrogen stripped off, then recombine into heavier hydrocarbons that need a much higher temperature and oxygen from air to burn. This is the black smoke that is a lot of unburnt fuel. Yes there is also free carbon in that mix of unburnt hydocarbons, but also other HC compounds like waxes, heavier oils, etc. It is the problem of the "oil refiner" to manage the re-engineering of mixed hydrocarbons (crude oils, etc.) into their various fractions that are created by "cracking the crude", I.E. separated, at different temperatures and pressures, then re-combined as appropriate to make other things like plastics, oils, etc.. The wood or coal fire in your boiler is just like uncontrolled cracking! But inside the fire, there are zones where all the hydocarbons molecules have been stripped away and pure carbon is left. This charcoal burns really well as it is in the air-rich lower levels of the fire, where air is drawn-in through the bottom bars. The temperature is over 800 C and glowing red as the carbon burns with air to CO and CO2. It also gives off lots of radiant heat that heats fuel above and walls of the firebox (thus making steam), as well as hot exhaust gas.
So a closed throttle reducing the draught, stops a lot of air being drawn through a fire, that is at conditions for combustion for the wider throttle combustion air draught. Thus a rich mixture is formed and causes the black smoke. Closing the throttle progressively allows the fire temperature profile to adjust and less black smoke is formed.
When new fuel is added, it is rapidly heated from the fire beneath and lots of light hydrocarbons are emitted, thus making the gas above the fire very rich (also with depleted oxygen from air passing through the fire) necessitating opening the fire door to allow extra top air to burn these hydrocarbons. But cooling of exhaust gases from the fire beneath, by both the cold new fuel and extra cold top air, can prevent complete combustion of these hydrocarbons, so making it difficult to avoid black smoke, especially with very "wet" or oily fuels. Hence drier "steam" coal is best to avoid this, or harder woods, of less oily varieties. Making holes in the fire allows extra air in, that is preheated by the fire, which helps to keep the above-fire temperature up, and helps this combustion to happen before the flue-tubes. Thus reducing black smoke.
If your boiler using wood pellets is a commercial device, I would expect it to have temperature sensors and even O2 sensors to feed an automatic controller to avoid over-fuelling and black smoke. But if a simple burner, then you have to imagine the temperatures and control fuelling and air factors yourself.
Have fun!
Hope this is of some use,
K2
 
Last edited:
Hi AJ. With no experience of wood burning, except the rare firing of log stoves when in holiday homes.... but I can imagine the problem...
Wood - naturally - varies widely, as does its derivative, coal. Considering it is simply a lump of mixed hydrocarbons... all of mixed complexity, almost every chip is different from the next.

snip
If your boiler using wood pellets is a commercial device, I would expect it to have temperature sensors and even O2 sensors to feed an automatic controller to avoid over-fuelling and black smoke. But if a simple burner, then you have to imagine the temperatures and control fuelling and air factors yourself.
Have fun!
Hope this is of some use,
K2

Grew up where our primary heat was a wood stove.
Using similar is somehow not so appealing - - - except I'm trying to re-think the stove.
The idea is to fire wood chips - - - like landscaping ones not the teeny weeny stuff that is sold for 'home' use.
I get the sensors stuff - - - what I'm trying to figure out is how to feed (that is not simple - - -needing to use the under fire air to get good burn) and how to do a clean burn of the gasses produced in the combustion.
I'm a thinking of something like a high temperature material heavy mesh for the grate (to let ash fall through yet not impede the under fire air).
Where I'm striking out is how to force the burn of the gasses produced.
Ideas - - - suggestions?

TIA
 
My engineering interest area is primarily other related topics. I had no idea firing a boiler was so complicated. Very interesting. Thanks.
Henry
 
Hi Henry. As an engineer (infernal combustion engines, power station gear and cars for 50 odd years...) I became fascinated by the boiler side of steam engines when I realised how fuel became a thread of power that we feed into an engine to convert it to motion, or whatever. Just like an electric cable in fact. So a boiler is an engine in itself. - A converter of the form of energy flow. As a hot source allows the heat to run down to a cold source, we tap into it and take off a proportion that we can use in another engine. I.E. Hot gases from combustion (or whatever) IN, Exhaust (cooler) OUT and we have tapped off the steam flow full of energy for use as we choose.
And as good engineers, we can work out how much we need, and work backwards to determine how much fuel we need to consume for our steam needs. Then we can design the boiler (and burner) properly. Plus feed pumps, etc. to maintain the working conditions. In fact on a railway steam locomotive, it takes 85% of the mass to make the steam, and 10% to convert the steam to power at the wheels. And 5% to stop it and keep the rain off the driver!
Cheers.
K2
 
AJ: re: Post #11.
Consider this.
  1. A relatively modern large grate design for locomotives and industry uses a chain grate... where the coal etc. is fed onto the chain conveyor, which transports the cold stuff at the required speed into the boiler combustion zone, where it burns and the ashes drop through a grate beneath the conveyor (where the air enters).
  2. A Cornish boiler, Scotch boiler or other (in industry) has a screw feed for the coal (or whatever) from a bunker into the combustion tube. The coal is forced along the pipe by the screw, at the chosen and variable rate, where it meets an air feed at the end with grate for exiting ashes. The fire stays at the grate where it drops ashes and receives air.
These similar ideas are in many books so I am sure you can decide if your wood chips are suited to a screw feed or chain grate feed so you can make one and see? The main design feature is where you locate the air supply, and grate to feed it through. On the cold side there isn't any free air so the fuel entering doesn't burn. Then when it meets the air, the combustion is sustained at the required rate by adjust the fuel feed rate and air feed in synchrony.
chain grate horizontal boiler – Industrial Boiler Catalogue
Grate Limit and Boiler Efficiency | Advanced Steam Traction (advanced-steam.org)
Steam boiler chain grate - Search (bing.com)
https://www.swynnertonparishcouncil.org.uk/1645616473/household-pellet
1647279516267.png

STEAM BOILER SCREW UNDER FEED FIXED GRATE (SOLID BIOMASS) - LKBOILER


Any use?
K2
 
I was always told to add coal to just one side of the fire alternating sides each time. The hot fire on the side seems to burn off the smoke from the fresh side fairly well. I think letting in cool top air and putting a hole in the fire are not very efficient things to do and are hard on the boiler.
 
Thanks folks , looks like I have some reading to do
thanks
animal
 
The engine exhaust creates a vacuum in the smoke box [front of the boiler] by channeling the exhaust thru a venturi. When the throttle is opened, the exhaust and vacuum increase and so does the draft and burn rate in the fire box. As you say when the engineer hammers the throttle. Pretty slick, semi self governing system. I think the later, oil fired engines responded very rapidly to the increase in fuel demand and the towering columns of black smoke resulted. I often wonder what the EPA would think.

Other opinions welcome. I had no clear how this worked until I made a locomotive.
There is an additional effect of more practical value. It makes the stack work as if it was taller. Without that boost in draft the smoke will spill out across the top of the engine right into the engineers cab and they are not likely to survive that for very long. That venturi is designed to act as a thermal compressor where the energy of the steam is designed to draw in the low pressure smoke and expel it out the stack with increased velocity.

And the EPA would indeed write a law. Its the only legislation that writes laws by act of promulgation without the involvement of congress. Every one should have the distinct pleasure of working with them. Believe me it is not fun.
 
As you say HMEL. The variation of blast through the venturi (chimney) naturally regulates the forcing of the draught - the rate of flow of air into the fire. I guess this can change within 1 beat of the engine? Modern infernal combustion engines with computers controlling the fuel injection "per beat" can react quickly to engine load/demand. But a coal fire takes minutes to change! So shutting the regulator rapidly on a full fire goes to a very rick mixture and black smoke, or whacking the regulator open from a light setting to full can suck a light fire up the flue tubes!
K2
 
"Little and often" is the rule I was taught for fuelling solid fuel fires..... Also, learn the road, anticipate changes needed for the fire, and fuel to a plan in advance of changing load. Do not "react to changing loads". That is too late to get it right!
K2
 
AJ: re: Post #11.
Consider this.
  1. A relatively modern large grate design for locomotives and industry uses a chain grate... where the coal etc. is fed onto the chain conveyor, which transports the cold stuff at the required speed into the boiler combustion zone, where it burns and the ashes drop through a grate beneath the conveyor (where the air enters).
  2. A Cornish boiler, Scotch boiler or other (in industry) has a screw feed for the coal (or whatever) from a bunker into the combustion tube. The coal is forced along the pipe by the screw, at the chosen and variable rate, where it meets an air feed at the end with grate for exiting ashes. The fire stays at the grate where it drops ashes and receives air.
These similar ideas are in many books so I am sure you can decide if your wood chips are suited to a screw feed or chain grate feed so you can make one and see? The main design feature is where you locate the air supply, and grate to feed it through. On the cold side there isn't any free air so the fuel entering doesn't burn. Then when it meets the air, the combustion is sustained at the required rate by adjust the fuel feed rate and air feed in synchrony.
chain grate horizontal boiler – Industrial Boiler Catalogue
Grate Limit and Boiler Efficiency | Advanced Steam Traction (advanced-steam.org)
Steam boiler chain grate - Search (bing.com)
https://www.swynnertonparishcouncil.org.uk/1645616473/household-pellet
View attachment 135061
STEAM BOILER SCREW UNDER FEED FIXED GRATE (SOLID BIOMASS) - LKBOILER


Any use?
K2

Hmmmmmmmm - - - - a moving chain grate on a 75 to maybe 100 kW boiler is going to be a right royal pita to not only design.
I would bet that its would also be an even great nuisance to maintain!!!

I was thinking something like a heavy wire mesh with openings of at least 1.5 if not 2cm square (1/2 to 3/4") and some what of pushing the air in under the grate. Very much still working out ideas!!!!!
 

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