Steam & Excursion > How fire works

BentnoseWillie wrote:

> Snip <
>
> Just trying to get my noodle around the dynamics of
> combustion, I remain:
> B-Dubya

OK. Here\'s the crash course in bituminous fireing.

A good steam coal will consist of the followin 5 components:

1) 50% coke (by weight), also called fixed carbon. This is the stuff that burns by glowing on the grate, with little or no flame. It contains 45% of the available energy.

2) 35% volatiles, also called hydrocarbons. This is cooked out of the coal in the firebox and burns as a gas in the combustion space of the firebox. It contains the remaining 55% of the available energy.

3) 9% ash. Made up of rock, iron, dirt, and anything else that won\'t burn. Does not contribute to the heat of fire, in fact takes heat out and drops it into the ashpan. Fireman must control ash, as when it builds up on top of firebed it will melt and form clinker.

4) 3% sulfur.

5) 3% tar. Sulfur and tar burn, but it takes as much energy to burn it as it gives back in heat, so is not of concern to fireman while on the road. You don\'t want to leave it in the ashpan or on the grate between trips, however, as it produces acid which eats steel.

Working on the road, the volatiles are cooked out of the coal within 15 seconds, and are burned in the combustion space. The coke, however, takes about 3 minutes. Thus, the fireman is actually controlling two fires in the firebox - a slow burning coke fire, and a rapid burning volatiles fire. By proper firing methods, he controls the thickness of the firebed to regulate the amount of air (oxygen) entering the firebox thru the grate. A thicker fire allows less air.

The temperature in the firebox is around 2200 to 2500 degrees F when working hard. (Steel melts at around 2000 degrees, which is why you want to keep the firebox surrounded with water at a temperature of around 350 to 400 degrees.)

As a piece of coal is thrown into the firebox, here\'s what happens to it.

Almost immediately, - no, let\'s say immediately - the hydrocarbons begin to distil out of the coal. It only has to reach 700 degrees for this to happen, and when the firedoor is open one can see the bits of coal being stoked to burst into flame even before they hit the grate. At 1750 degrees, the hydrocarbon molecules break down into individual atoms of carbon and hydrogen.

At 750 degrees the carbon molecules break down into individual carbon atoms.

At 730 degrees, an individual carbon atom will combine with an molecule of oxygen and yield carbon dioxide plus 14,500 btu\'s of heat for each pound of carbon. So, you can see that the carbon will scarf up all the oxygen available as soon as the carbon molecules and hydrocarbon molecules break down. You also see that it is possible, in fact easy, to maintain a fire at a temperature between 750 degrees and 1750 degrees in which only 14% of the available energy will be put to use, as the hydrocarbons will go up the stack unburned!

Coninueing this discussion assuming a temperature above 1750 degrees, hydrogen will combine with oxygen at a temperature of 1200 degrees. A pound of hydrogen combining with oxygen will yield 62,000 btu\'s of heat! And, hydrogen has a much stronger affinity for oxygen than does oxygen, so it will actually strip away oxygen from the CO2 molecules created by the coke fire on the grate (at a cost of 10,100 btu\'s per pound), converting the CO2 into carbon monoxide. If there is not enough remaining oxygen at this point to permit the CO2 to burn again to regain that 10,100 btu\'s per pound, this energy is lost. Also, you will then not have sufficient oxygen to burn the carbon atoms released from the hydrocarbon molecules, and this heat will be lost and you will get black smoke.

Put coal in too slow, and you\'re depending too much of the coke fire alone, and not getting enough heat out of the engine. Put in coal too fast (a more common error), and the volatiles will scarf up the oxygen and 2/3 of the heat of the coke fire is lost. Too thick a fire, and you don\'t have enough oxygen. To thin a fire, and there\'s not enough heat give off to heat the air available (remember air is 90% uncombustable nitrogen), and the temperature drops below critical values.

Got that? The test will be next week <g>.

That\'s a great explanation of the science behind the fire. Grate stuff!

Seriously, the science and art of railroading constitute the real reason why some of us are here and information like this is exactly what we want to learn about.

Very well stated MTM, the only thing missing is that you base your calculation on the fact that all coal is the same fixed content ratio. Coal from different regions and even from mine to mine locally can change the percetages to each other and need to be compensated for when acually firing. Another intangeble is the amount of moisture content in the coal.

Percent of ash in coal can range anywhere from 4.25% on the low end to as high as 16.5 % ( truely considered burnable dirt). Of the theorectical 9% you use in your calculation only 4% of that should actually reach the ashpan the rest should be drawn off the fire as the coke breaks down during burning and through out the stack. Also a reason why some engines had extended smokeboxes to accomodate the extra fly ash build up that would accumulate there. Suprisingly some of the highest ash content coal is from PA, Maryland region and out west. And the lowest ash highest BTU coal is from southern Ilinois and Indiana.

Also to add, ash if left mixed in with the coke will be heated by the volitiles to the point where it actually fuses together and melts causing "glazing stituation or clinkers"

LLW 8673

Nice explanation ... one thing to add .... 2000F is the rolling temperature of most steels, the melting temperature is up around 2800F. I am attaching a little chart to show how the tensile strength of steel decreases as the temperature increases. As you can see your boiler is getting pretty "soft" if you let the temperature get over 1000F. If you are a blacksmith, 2000F is around "orange" color, 1800F is "red" and the hump you see the dotted line making at 500F is the "blue brittle" range ... the color a RR wheel turns if you let the brake shoe drag.

---

MTM,

Thanks very much for the detailed explanation! I have one correction and one question. First, nitrogen makes up something less than 80% of air, not 90%. I haven\'t looked it up, but I have a vague memory that it\'s 78% nitrogen, 21% oxygen and 1% everything else. The question: You said that water around the firebox needs to be between 350-400 degrees. What pressure is the water under so that it is still water at those temperatures?

Thanks!

RobR

RobR wrote:

> MTM,
>
> Thanks very much for the detailed explanation! I have one
> correction and one question. First, nitrogen makes up
> something less than 80% of air, not 90%. I haven\'t looked it
> up, but I have a vague memory that it\'s 78% nitrogen, 21%
> oxygen and 1% everything else. The question: You said that
> water around the firebox needs to be between 350-400 degrees.
> What pressure is the water under so that it is still water at
> those temperatures?
>
> Thanks!
>
> RobR
>
Yeah, I noticed a couple typos in there. 80% is the figure I meant to have used.

IIRC, at a pressure of 170 psi the water temperature is 365 degrees. Most locomotives operate in the 175 to 200 psi or so range, though 250 or even higher was not uncommon in later years. The important thing is that water can cool the sheets and keep them from failing, whereas steam will not.

Someone else pointed out that the melting point of steel is around 2800 degrees. That\'s true, to get liquid steel. As also mentioned in the same post, it starts to soften at a much lower temperature. As far as the fireman is concerned, he can start damaging the sheets at a much lower temperature, but reach failure at around 2000 degrees.

I also said hydrogen has a much higher affinity for oxygen than does oxygen... DUhhhh! That should be "than does carbon".

I have a detailed chart from the late thirties that lists alomost every mine in the US and even different Veins of coal within the same mines giving the ratios you describe,BTU output, total BTU\'s per pound, ETC.. it also gives the optimum pounds of air needed to combust the the different grades to extract the most BTU\'s out of each perticular grade of coal. You wouldn\'t happen to know the calculation to get a pound of air , or how many cubic feet a pound of air is off hand ?

The list show that the range is anywhere from 18 lbs of air to 48 lbs of air needed to burn 1 pound of coal. Most being in the 28 to 36 lbs. range needed.

LLW 8673

Not offhand. (I\'ve never had to shovel air.)

I\'ll try to remember to check those figures for ya tonite.

Somehow I think I just found out more than I want to know about coal fire. All I know is when I put it in my engine and it gets drafting hard the steam pressure goes up. If the engine stops smoking I\'m either running out of coal or it has caked over on the grates shutting off the air. So either add some more coal or bust up the cake.

If you want to work less hard, use less coal, make less smoke, spend less time and money maintaining your engine, get more power out of the engine, make the engine last longer, and have more fun doing it, then pay heed to what has been written here.
:-)

LLW8673 wrote:
> You wouldn\'t happen to know the calculation to get a pound of
> air , or how many cubic feet a pound of air is off hand ?
>
> The list show that the range is anywhere from 18 lbs of
> air to 48 lbs of air needed to burn 1 pound of coal. Most being
> in the 28 to 36 lbs. range needed.
>
> LLW 8673
>
> [%sig%]

More then you ever wanted to know:
http://www.usatoday.com/weather/resources/basics/density-calculations.htm
In addition, stick with Pocahontas coal and you\'ll never go wrong.

152 cu ft, 11.6 lbs, of air is required for compete combustion of 1 pound of carbon.

230 cu ft of air is required for complete combustion of 1 pound of hydrocarbons.

whew,After that calculation GE, I\'m going to go to Tomstp\'s theory of firing.

Black to much coal
Clear Not enough coal
Light gray haze , Just right

Actually one of my lessons in oil firing was just that exactly, except exchange coal for oil.

In the words of one of the great literary icons of modern times.

" Eeeghhh, Like , I hate numbers. Cause there\'s like too many of \'em. : Beavis

LLW 8673

Using the information given it shuld take 156.5 cubic feet of air to oxidize one pound of coal, 76 cubic feet for the carbons, 80.5 for the hydrocarbons. Did I fgure it right?

grande473 wrote:

> Using the information given it shuld take 156.5 cubic feet of
> air to oxidize one pound of coal, 76 cubic feet for the
> carbons, 80.5 for the hydrocarbons. Did I fgure it right?
>


You get A+

LLW8673 wrote:

> whew,After that calculation GE, I\'m going to go to
> Tomstp\'s theory of firing.

LOL!


> Black to much coal
> Clear Not enough coal
> Light gray haze , Just right
>
> Actually one of my lessons in oil firing was just that
> exactly, except exchange coal for oil.

Those rules are good advice, except to keep in mind that too much coal can also give a clear stack. That\'s called "plugging" the fire.

LLW8673 wrote:

> whew,After that calculation GE, I\'m going to go to
> Tomstp\'s theory of firing.
>
> Black to much coal
> Clear Not enough coal
> Light gray haze , Just right
>
> Actually one of my lessons in oil firing was just that
> exactly, except exchange coal for oil.

And the "old timers" could literally do it in their sleep! Leeetle bit more complicated then kicking her into notch 8 and watching the stack smoke.

ge13031 ...I learned you can\'t shovel coal lefthanded in a NKP700!

ge13031 wrote:

> ge13031 ...I learned you can\'t shovel coal lefthanded in a
> NKP700!

Why would you want to?

And, even if you did want to, why couldn\'t you?

Well, I certainly never meant to spark quite this much information transfer!

However, I\'m glad it happened. Now I won\'t ask any more stupid questions about combustion!

Moving on to new stupid questions, I remain:
B-Dubya

I can\'t shovel coal righthanded in a NKP 700 either. Is that a trick question Did you mean it is physically impossible to do it, or just not right now! That goes along with " You can\'t put to much water in a steamlocomotive" :.)

MTM , been on the recieving end of that situation. My first thought is what the^%#%**%^ do I do! Then you figure it out.

LLW 8673