Air Requirements for Coal Combustion

Thanks Yanche. I'm sure it was an older Bacharach, I remember the slide rule. The CO2/O2 adsorbent would certainly need replacing. It sounds as though there is some experience behind your comments on applying the measurements to a coal appliance. Have you worked with them on a coal fire and is it really worth the effort of going through the steps?

After just buying a Leisure Line Lil' Heater, I have given some thought to using a pre-existing dryer vent to draw fresh outside air in for combustion.

The steps needed to construct an air-supply duct is very simple and should provide enough air for the 45 cfm stoker blower.

1). Remove the existing dryer vent from the rim joist and nail a small wood frame over the top of a 6" square sheet of metal window screen to keep pests from entering the house where the dryer vent was located.

2). Screw a toilet bowl flange over the hole cut into the rim joist and then run a 4" pvc pipe inside a basement ceiling joist and hang using joist hangers. Next, terminate the pvc pipe just above the rear of the coal stove. Use screws to hold the loose pvc pipe and elbows together without the need for pvc glue.

3). Dry fit a 90 degree elbow to the pipe and run another section of pipe vertically until at a right angle with the combustion fan cage, but 2' foot distant from the stove. Connect a second 90 degree elbow and then run 4" black pipe horizontally until contact is made with the combustion fan's open squirrel cage.

Now for the questions!

A). Should I install a barometric damper into the pvc pipe to prevent a loss of fresh air should the vent ever become clogged or blocked for any reason?

B). Will a 4" pvc pipe, with two changes in direction, provide enough fresh air to the combustion blower? I assume yes since the 4" vent provided enough diameter to vent the clothes dryer. The typical plastic dryer exhaust wheel is 3" inches deep by 8" inches in diameter with about 20 curved fins but not sure about the spec's on the blower motor, looks fairly small.

Carbon monoxide is produced from the partial oxidation of carbon-containing compounds; it forms when there is not enough oxygen to produce carbon dioxide (CO2), such as when operating a stove or an internal combustion engine in an enclosed space. In the presence of oxygen, carbon monoxide burns with a blue flame, producing carbon dioxide.

Based on the above information, it's easy to conclude that by providing your coal stove/furnace/boiler with fresh outside air via a supply duct directly to the combustion blower, the fire is less likely to produce carbon monoxide.

Making sure you have enough combustion air obviously is important, but I'm not sure about jumping from the quote to the conclusion about ducting in combustion air, as opposed to, say, keeping a window open a little. I thought about doing something similar once, but the stoker pro I asked didn't recommend it. In addition to the air pressure and blockage issues, the stoker would be subjected to wide swings in the characteristics of the combustion air, including temperature and moisture content, that would have unknown impacts on its performance. I think the most effective CO protection comes from ensuring appropriate draft and ventilation, and having functioning detectors. I wouldn't rely much on the idea that perfecting the fire can prevent CO formation in any type of way that people should rely on.

Mike

I agree with Mike on this. Outside air and adequate amounts of air for combustion are two separate issues. The last thing I want to feed my stoker is damp air from outside. I trimmed the bottom of the basement door a little to allow more make-up air, and called it good. I admit that my house probably has sufficient make-up air on its own , but I would rather pull it through the basement than the living room windows.

It's not the amount of air that produces Carbon Monoxide, but; the temperature of the air as it comes into contact with the fuel. Perfect combustion results in 3 products: Carbon Dioxide, Water and Inert Matter (ash). For the hydrocarbons in any fuel to completely combust, the temperature must be up around 1800 degress and above. Otherwise you get carbon monoxide, which means half of the theoretical heat value in the hydrocarbons is lost.
It is impossible to achieve these temperatures and level of combustion in a naturally aspirated appliance. So even if the air to fuel ratio is perfect, the goal is still unreachable simply because you can't get those high temperatures without a forced draft of some kind. Also at those temperatures your stove wouldn't last very long. The loss is much greater with Bituminous Coal as it is mostly hydrocarbons. The loss is minimum with Anthracite as it has a much lower hydrocarbon content. It is mostly fixed carbon which burns at a much lower temperature, around 750 degrees or so. It's well within the capacity of a normal appliance to reach optimal temperatures to burn fixed carbon. It's nothing to lose sleep over. You still get plenty of efficiency with coal. Theory is just that. It is a level of perfection that we know exists, but: we'll never quite get there.

I'll take a one shot ballpark stab at this one:

1. There is roughly 21% oxygen in air by volume
2. Air weighs approximately 0.0807 lbs. per cubic foot (per a random website I pulled up)
3. By weight, 23% of a cubic foot of air is oxygen (per a random website I pulled up)
4. 23% of 0.0807 lbs. = 0.0186 lbs. of oxygen in a cubic foot of air
5. 1 cubic foot = 7.4805 gallons
6. 1 gallon of air (at standard temperatures and pressures) therefore has 0.002487 lbs. of oxygen.
7. The molecular weight of carbon is 12
8. The molecular weight of O2 is 32
9. The percentage of carbon in 1 pound of anthracite is 86%
10. The weight of carbon in 1 pound of anthracite is therefore 0.86 lbs.

32/12 * 0.86 lbs. of carbon = 2.2933 lbs. of oxygen needed to perfectly burn 1 lb of anthracite

2.2933 lbs. oxygen / 0.002487 lbs oxygen/gallon of air = 922 gallons of air

It takes 922 gallons of air to burn one pound of anthracite. Somewhat less if some percentage of CO is considered

If you are burning 1 lb of anthracite per hour, then it takes 922/60 = 15.4 gallons of air per minute

Someone please check my figures.

DING ! DING ! DING ! DING!!!!!!
...I think we have a winner !!!!! larry

...1lb of anthracite require a little over 12 lbs of AIR for complete (theoretically) combustion
...which is about 154 CUBIC FEET
..over 60 minutes ( 1 hr) is about 2.5 cubic feet per minute (c.f.m.)

of course this is under perfect conditions..
make up air will vary alot depending on stove type ,coal quality , temperature,humidity, altitude , alignment of the stars , moon phase and shoe size....

....way to much math in this post- love it !!

I found this but have no idea what this is in English measurement but here it is.

http://www.brighthubengineering.com/power-plants/ ... -required/

The theoretical air required to burn the coal is

= 1.64 / 23.2% = 7.1 kg of Air for 1 kg of Coal.

EDIT: Online conversion says 7.1kg = 15.6lbs and 1kg = 2.2lbs OR pretty much what Larry said.

I don't believe anyone touched the comment Grizzly had about supplying outside air to his barometric damper. Just from reading posts from others, this is a bad idea. I don't quite remember the reasoning (something about directing outside air to the damper messes with the pressure differences between inside and out that the damper needs to work properly). But the amount of inside air you lose through the damper is insignificant.

Post by Dallas - Fresh Air Supply for Barometric Damper?

especially if its windy outdoors...can defeat the draft controlling function of the baro altogether

Found some info on this topic. See attached. Look at Table 32 located on page 4 of the pdf

vfw3439 wrote:Found some info on this topic. See attached. Look at Table 32.

Sorry you are way over my understanding And I like tech stuff, sorry Help us understand the pdf.

Sorry I found the meat of your post.
CALCULATED THEORETICAL AMOUNT OF AIR REQUIRED PER POUND OF VARIOUS FUELS
Fuel
Weight of Constituents in One Pound Dry Fuel Air Required per Pound of Fuel Pounds Carbon Per Cent Hydrogen Per Cent Oxygen Per Cent Coke 94.0 … … 10.8 Anthracite Coal 91.5 3.5 2.6 11.7 Bituminous Coal 87.0 5.0 4.0 11.6 Lignite 70.0 5.0 20.0 8.9 Wood 50.0 6.0 43.5 6.0 Oil 85.0 3.0 1.0 14.3

So according to this table, one pound of Anthracite Coal needs 11.7 pounds of air for combustion..

1 cubic foot of air at standard temperature and pressure assuming average composition weighs approximately roughly 0.08 lbs. according to two sources I found..

11.7 pounds/.08 pounds per cubic ft = 146 cubic ft per one pound of coal right?

So to keep it simple lets say I burn 48 pounds in a day which is 2 pounds per hour, using 292 cubic feet of air volume per hour.

My basement is roughly 15ft x 15ft x 7ft cube = 1575 cubic feet..

SO if this is all correct then my furnace uses one full volume of the basement in combustion air every 5 and half hours..

And that doesn't count what the baro is sucking up the chimney which I would think is grossly more than combustion.