Air Requirements for Coal Combustion

Investigating a strong draft blowing toward my hand-fired insert stove, I wondered how much of that draft was going to support combustion (versus, for example, the amount going to the convection blower fans, which obviously play no major or direct role in combustion).



Assume you are firing your stove somewhat above an idle—for example, you burn 24 pounds of coal in 24 hours, or one pound per hour. What volume of air is necessary, per minute, to support this combustion of one pound of coal in one hour? (Assume you are at sea level, and air temp is 59 degrees F)




I found a link ( http://renaud.ca/wordpress/?p=49 ) that went through several chemical calculations regarding combustion of coal which shed light on the issue of air flow I'd wondered about. In the example, the author uses an approximation of 70% as the percentage of carbon in coal. So, he's using bituminous coal for his estimate. Anthracite is closer to 90% carbon. He assumes—which is fair, based on other research I found—that all the carbon is converted to CO2 or CO (carbon dioxide or carbon monoxide). Thus, in a pound (454 grams) of bituminous, 317.5 g is carbon; and a pound of anthracite is 408.6 grams carbon. In his calculation with bituminous—which you can go through at the link—625 liters of carbon dioxide (at sea level barometric pressure and at about 59 degrees F) is produced by burning 1 pound of bituminous. This translates to 804 liters of carbon dioxide from burning 1 pound of anthracite. Since for every molecule (or mole, or any other amount) of CO2 produced, one molecule of O2 (oxygen) must combine with one atom of C (carbon), 804 liters of O2 are required for this combustion of one pound of anthracite. (I re-did the calculation using a molar mass of 32 for O2, as well) Since air is 21% oxygen, this means that 3829 liters of air are necessary for the combustion of one pound of anthracite. This converts to 1010 gallons of air, or about 16.8 gallons per minute. It seems hard to believe this much air is required, but I believe the calculations are correct.

Figuring out how to get this much air to the stove without having it sucked in through gaps and creating drafts, especially in already-cool rooms far removed from the stove, is a critical problem.

You aren't really burning a full pound of coal per minute, that's 60 pounds per hour... most stoves either stoker of hand feed run about one pound to two pounds per hour at lower settings.. 24-50# per day..

Even my AA260 cranking away in -10* weather is only burning between 5 and 8 # per hour [ 120-180# per day]. And that fan is really pulling air through the fire at those burn rates.. !!

Most outside air requirements for small to large stoves and boilers can be supplied by a 3" to a 6" duct, as long as the duct isn't too long..frictional losses start to effect the flow rates if the duct is too long.

Greg L.

I just ran into problems with combustion air trying to figure out why my daughter's hand fired wouldn't get hot......Her house is so tight that there wasn't enough available fresh air to fire up her stove! (now she opens the bulkhead a bit until they can run a dedicated air line to the base of the stove)

You aren't really burning a full pound of coal per minute, that's 60 pounds per hour

??? Not sure where your getting this "full pound of coal per minute" thing. ???

most stoves either stoker of hand feed run about one pound to two pounds per hour at lower settings.. 24-50# per day

yup, that (plus ease of calculation) is exactly why I figured it out with 24 pounds in 24 hours, or one pound per hour

The point of the post was to quantify the amount of air required for even a slow-burning fire. Devil505's experience with his daughter's stove is quite consistent with the idea that LARGE amounts of air are needed. I was trying to identify a reasonably accurate number regarding this issue.

Give your stove its own outside air supply and let it use as much as it wants and just sit back, enjoy the heat.

Hi Ashcat,, look at the time in the morning that I posted my 'answer' .. I was half awake.. sorry for the misinterpretation.

The problem with coming up with a quantity of air is what I was [trying] to get at.. at an idle,, I'd say you could supply the fire in a Harman Mark I with a large garden hose,, but with any larger fire, I can't mentally or on paper do the math.. or convert from CFM's to GPH or whatever is needed..

It's a tough question to answer,,but there is no question that an outside air source is a good idea..

Greg L..

Now, back to cooking.

Gambler--
I've been thinking of ways of testing that, short of putting any holes in the floor at this point. I have a window on either side of the insert, about 4 feet away. I was thinking of setting up a Rube Goldberg contraption involving a 3-4 inch high box, the width of the window, and running a hose/flexible duct from the box (which by now has been placed in my window and the window closed down to the top of the box), from the box to an outlet box roughly the size of the ashpan door, where it would discharge outside air to serve as the supply for combustion.

Would a vacuum cleaner hose (approx 1.5-2 inch diameter) work, or would resistance to flow be too high in that narrow a tube? (I suspect it would be adequate, at least for testing purposes)
Also, how do you modulate the airflow to the stove, given wind variability? Is there such a thing as a barometric damper for an intake air supply?

By the way, I tried slightly opening one of the windows overnight, and awoke to a cold first floor and a wife who had already closed the window. It seemed to make no difference to the upstairs temps overnight.

can't mentally or on paper do the math.. or convert from CFM's to GPH or whatever is needed..

OK, 16.8 gallons per minute converts to about 2.25 cubic feet of air per minute to support combustion of one pound of coal in one hour (US gallon = 0.133680556 cubic feet)
Burning 72 lbs in 24 hours--not unusual among members with hand-fireds, but certainly a hotter fire--requires 50.4 gallons of air per minute, or if you prefer, 6.75 cu ft of air per minute.

Ashcat wrote:

can't mentally or on paper do the math.. or convert from CFM's to GPH or whatever is needed..

OK, 16.8 gallons per minute converts to about 2.25 cubic feet of air per minute to support combustion of one pound of coal in one hour (US gallon = 0.133680556 cubic feet)
Burning 72 lbs in 24 hours--not unusual among members with hand-fireds, but certainly a hotter fire--requires 50.4 gallons of air per minute, or if you prefer, 6.75 cu ft of air per minute.

Are you having problems burning due to draft? Have you checked the draw with a monometer? As you are solving it, the air @ 21% is only for stoichiometry . Draft is variably constant (or constantly variable?!?), regardless of the combustion. More air is needed other than for combustion. Fire or no fire, it will increase and decrease due to many variables; temperature difference, barometric pressure etc. I had an experience like Devil's only with an oil burner installed in a garage I built. I had the draft gauge hooked up and was struggling with the efficiency of the burner and little draft. Dad walked out of the open door and the draft spiked -.05! Door closed, next to nothing again. Solved that one easily enough with the window trick.

If you don't have a supply air shortage, supplying outside air to any burner will decrease the efficiency but probably not noticeably. The cold air will carry away BTUs much like wet fuel will. It takes BTUs away from warming the house & up the chimney. It might only be measurable with instrumentation commonly used to assess burner efficiency. Then again, you might end up with a cold floor! Part of the calculation is the differance between the pre-combustion air temperature and the final flue temperature.

Are you having problems burning due to draft?

No problems with my draft. Burns fine--in fact my problem is slowing down the draft. Except for starting new coal, my ashpan vents are never greater than 10% open, with MPD fully "closed". Adjusting from 0 to 10% allows adequate temp control, of 350* to 500* stove temps. I don't own a manometer.

If you don't have a supply air shortage, supplying outside air to any burner will decrease the efficiency but probably not noticeably.

Maybe efficiency would be less from using cooler combustion air, but the house may be warmer for any given amount of coal burned, because that combustion air won't be sucked into the house thru gaps, etc. This, I believe, is Gambler's point, which I agree with. 1010 gallons (135 cu ft) of air are required for each pound of antrhacite burned, and this air rushing into the house would, I believe, far exceed the loss of efficiency you mention that results from using colder combustion air.

VigIIPeaBurner wrote:[

If you don't have a supply air shortage, supplying outside air to any burner will decrease the efficiency but probably not noticeably. The cold air will carry away BTUs much like wet fuel will. It takes BTUs away from warming the house & up the chimney. It might only be measurable with instrumentation commonly used to assess burner efficiency. Then again, you might end up with a cold floor! Part of the calculation is the differance between the pre-combustion air temperature and the final flue temperature.

I can't agree with this.. adding 20* air to a fire that is going to be burning at 2200* instead of burning 70* room air will have essentiall zero effect on the fire or the stove's efficiency...

However, supplying an outside air source directly to the combustion air inlet in the stove will GREATLY improve the efficiency of heating the house.. The reason being that the air going up the chimney HAS to come from somewhere.. without a dedicated outside air source, the fresh air that is burned in the fire and going up the chimney comes from the whole house,, and this air is replaced by cold outside air infiltrating into the house through gaps in and around windows, door seals, floor/wall junctions and floor joist/sill plate/foundation gaps and openings.. All of this infiltration of cold outside air is what in most homes we call drafts..

My first house was built very inexpensively,, the windows and doors were as cheap as the builder could buy.. I had terrible drafts, cold air infiltration. Really bad drafts everytime the gas boiler was firing,, So I made a dedicated outside air source for the boiler from 4" dryer vent.. it worked great,, the rooms distant from the house thermostat were much warmer, with greatly reduced drafts and infiltration.

Greg L

A very interesting discussion. I would like to try to rig up something to try using outside air, but my draft/thermostat setup would make it tricky.

I have been thinking about ducting in outside air to feed the baro. damper instead of pulling out heated room air. my chimney pulls a strong draft. With much wind it will open completely when set at .05 and the air is just rushing up the chimney through the damper. I know cold air reduces chimney draft , but I have a draft in the chimney even with a light breeze and no fire. If I added baro loss to combustion air loss from the house it is a wonder I can heat the place at all. My old house is insulated but has a cellar only under a small addition. Getting enough air infiltration has not been a problem from the standpoint of coal stove needs. Keeping the place warm on a windy winter day can be a problem though.

VigIIPeaBurner wrote:If you don't have a supply air shortage, supplying outside air to any burner will decrease the efficiency but probably not noticeably. The cold air will carry away BTUs much like wet fuel will. It takes BTUs away from warming the house & up the chimney. It might only be measurable with instrumentation commonly used to assess burner efficiency.

I believe that because the colder the air the more dense it is. And this dense high oxygen air will offset any loss due to warming up the air in the combustion process. Like was said you would probably need instrumentation to be able to see any of this. So in the real world it is a mute point. But what you will notice is the lack of a drafty room because the flue isn't sucking up warm house air.

LsFarm wrote:

VigIIPeaBurner wrote:[

If you don't have a supply air shortage, supplying outside air to any burner will decrease the efficiency but probably not noticeably. The cold air will carry away BTUs much like wet fuel will. It takes BTUs away from warming the house & up the chimney. It might only be measurable with instrumentation commonly used to assess burner efficiency. Then again, you might end up with a cold floor! Part of the calculation is the differance between the pre-combustion air temperature and the final flue temperature.

I can't agree with this.. adding 20* air to a fire that is going to be burning at 2200* instead of burning 70* room air will have essentiall zero effect on the fire or the stove's efficiency...
...>8..
Greg L

Greg, we're both correct. I should have worded it more carefully. The efficiency I was referring to was the total combustion of the carbon. Not how efficiently we're heating the space. The temperature of the air feeding the fire is subtracted from the flue temperature is used as a divisor in the calculation, as I recall - it's been a long time. It would be a small contribution but you can see if it's 450 flue - 73 room temp and using outside temps of 20 will yield a larger divisor. I don't recall how substantially it contributed to the final calculation. The temperature of the flame/fire was not in the calculation. (see below for more...)

I had just the opposite of drafty in the bank barn I built. I had to crack the window just to get it to burn! I never did add a cold air combustion feed to it because when I built the top floor, it was solar oriented and heated the top to workable temps most of the time. I did beekeeping on the side and all the carpentry was in the winter. If you have loose construction you will definitely benifit, from more than just comfort inex, from an air supply line.

gambler wrote:I believe that because the colder the air the more dense it is. And this dense high oxygen air will offset any loss due to warming up the air in the combustion process. Like was said you would probably need instrumentation to be able to see any of this. So in the real world it is a mute point. But what you will notice is the lack of a drafty room because the flue isn't sucking up warm house air.

Yes, I was referring to using instruments to assess combustion efficiency. I posted the whole process on another thread but a small recap is due since I've created some confusion by only partially stating the process. My dad had been a oil burner tech and had the instrument kit. To assess combustion efficiency; room, over fire and flue temps were measured, CO2 was measured and smoke opacity was measured. As I recall, the variables were put into a calculator and the efficiency was the result. I know - we're talking coal here

The combustion efficiency tools used by service technicians measure the CO2 and O2 in the flue. Then knowing the fuel type a slide rule calculator gives the combustion efficiency. The old Bacharach brand slide rules had scales for coal. The modern electronic version has the slide rule equivalent programed in the tool.

The calculation method assumes a known chemical formula for the fuel. Then measuring the two gases produced by the combustion you can work backwards and calculate combustion efficiency. Since even Anthracite coal composition and hence it's chemical formula varies it's only an approximate efficiency. Good for relative adjustments, not good for absolute measurements. That's why the instruction manuals call them "indicators" not "instruments".