Coal Size and Combustion Air Settings.. What Do They Prove?

Coaledsweat....

Are you saying that for a given primary airflow opening size that pea will result in higher stove surface temps than nut or stove?

It's my thought that pea can ultimately result in higher surface temps, albeit at a higher draft and primary setting than larger sizes, simply because more (weight) coal is burning in any given size pot with smaller coal.

I have another theory....and it could be completely wrong....but here it goes. Say lightnings measurements are correct, and nut and peas sized coal have the same pressure differential. Coaledsweat mentioned that the pea packs tighter and has less surface area than nut, however smaller sized coal has more surface area per lb, (that's why we use rice in stokers).

I have no data to support this, but I bet if you took temp measurements at different depths in the firepot you would find that the larger the coal the more consistent the temp across the bottom, middle and top of the coal bed. With smaller coal, the bottom of the bed wold be hot (at least as hot as the larger sized coal) but because of the increase surface area of the smaller coal, it is using more of the O2, and thus starves the upper layers rendering them cooler. so even with a consistent draft measurement, smaller coal may burn more O2 faster at the bottom and not allow the top to access as much, making the pot cooler as whole...

please let me know if this makes any sense at all

coldinmaine wrote:please let me know if this makes any sense at all

It intuitively makes sense to me, but intuition alone is often a bad way to do science.

Coldinmaine, I understand what you are saying and it deserves consideration.

Scalabro, I think you aren't quite understanding what I am trying to determine. If small coal and large coal have the same amount of combustion air going thru them, would the smaller coal burn slower-cooler or would it burn at the same rate as the larger coal?

It seems we have a variety of views. Maybe it's time to do some experiments and careful observations to see what's really happening..

scalabro wrote:Coaledsweat....

Are you saying that for a given primary airflow opening size that pea will result in higher stove surface temps than nut or stove?

No, the larger chunk of coal will burn faster, thus hotter than the smaller size. For the same weight of coal, the larger size has more surface exposed to air. Smaller sizes pack more densely, they have a higher contact percentage ratio with the surrounding coal. Get a big glass jar and fill it with stove coal and shine a flashlight through it. Then do the same with pea. With stove, most of the surface of a piece of it is exposed to air. With pea, more of its surface is exposed to more pea coal.

Lightning wrote:
Scalabro, I think you aren't quite understanding what I am trying to determine. If small coal and large coal have the same amount of combustion air going thru them, would the smaller coal burn slower-cooler or would it burn at the same rate as the larger coal?

In my mind this would result in colder stove temps simply because there is more coal in the fire pot when using pea, so you need more draft, ie more primary air.

coaledsweat wrote:

scalabro wrote:Coaledsweat....

Are you saying that for a given primary airflow opening size that pea will result in higher stove surface temps than nut or stove?

No, the larger chunk of coal will burn faster, thus hotter than the smaller size. For the same weight of coal, the larger size has more surface exposed to air. Smaller sizes pack more densely, they have a higher contact percentage ratio with the surrounding coal. Get a big glass jar and fill it with stove coal and shine a flashlight through it. Then do the same with pea. With stove, most of the surface of a piece of it is exposed to air. With pea, more of its surface is exposed to more pea coal.

In my mind stove burns "faster" because air flows through the bed easier at any given draft, resulting in more airflow throught the bed. BUT, pea can result in ultimately higher stove temps (with greater draft) because you have the ability to put more coal in the stove.

scalabro wrote: In my mind stove burns "faster" because air flows through the bed easier at any given draft, resulting in more airflow throught the bed. BUT, pea can result in ultimately higher stove temps (with greater draft) because you have the ability to put more coal in the stove.

I don't believe that is so. Just because you get more coal into it doesn't mean you can burn more in a given period of time.

coaledsweat wrote:

scalabro wrote: In my mind stove burns "faster" because air flows through the bed easier at any given draft, resulting in more airflow throught the bed. BUT, pea can result in ultimately higher stove temps (with greater draft) because you have the ability to put more coal in the stove.

I don't believe that is so. Just because you get more coal into it doesn't mean you can burn more in a given period of time.

What has more BTU potential 5 lbs of coal or 10 lbs of the same coal?

coaledsweat wrote: I don't believe that is so. Just because you get more coal into it doesn't mean you can burn more in a given period of time.

Sure it does, pea just requires more draft.

What has more BTU potential 5 lbs of coal or 10 lbs of the same coal?

Thank you everyone for contributing your thoughts.
For the sake of simplicity lets isolate the variables...

Keep everything the same except the coal size.
Same amount of coal mass.
Same amount of primary combustion air going the fuel bed.

Would smaller coal burn cooler-slower or at the same rate as the bigger coal?

There are some good points brought up by all of you...

franco b wrote:With the same air intake setting the pea will run cooler which proves there is a pressure differential even if you can't measure it. If air were getting through just as easily as with nut I think the temperature would be the same assuming equal quantities.

coaledsweat wrote:The pea settles down and packs tighter, the nut coal has a lot more surface area exposed to the air therefore it burns faster, ie: hotter because more of it is burning at one time.

coldinmaine wrote:I bet if you took temp measurements at different depths in the firepot you would find that the larger the coal the more consistent the temp across the bottom, middle and top of the coal bed. With smaller coal, the bottom of the bed wold be hot (at least as hot as the larger sized coal) but because of the increase surface area of the smaller coal, it is using more of the O2, and thus starves the upper layers rendering them cooler. so even with a consistent draft measurement, smaller coal may burn more O2 faster at the bottom and not allow the top to access as much, making the pot cooler as whole...

scalabro wrote:Because the smaller pea size restricts airflow through the coal bed. It has the same effect of closing the MPD and/or closing primary air.
No?

This is definitely a challenging issue. Cause if it were wood, we know wood will burn hotter faster in smaller pieces than bigger wood given the same combustion air (I think ) because ultimately more surface area is exposed to heat. Sooo, why doesn't that work for coal? But I'd rather not make this a debate on comparing wood and coal, I just wanted to throw that out there to see if you guys think some of the same principals apply.

At this point I believe it has something to do with the surface area available for the oxygen to react with the coal. My thinking is that more surface area equals more of the available oxygen gets used which means a hotter-faster burn rate. Although the smaller coal has more surface area, I'll offer this which is a combination of suggestions.. Suppose that since the smaller coal packs more tightly together, there are pockets in the fuel bed that are cut off from receiving a supply of oxygen. So even though the larger coal has less total surface area than the smaller coal, the larger coal has more surface area that can receive an oxygen supply allowing for the hotter-faster burn..

scalabro wrote:

coaledsweat wrote: I don't believe that is so. Just because you get more coal into it doesn't mean you can burn more in a given period of time.

Sure it does, pea just requires more draft.

What has more BTU potential 5 lbs of coal or 10 lbs of the same coal?

It has more potential heat because it is more coal. That doesn't mean you can burn it faster in the real world. People use stove when it is very cold out and service the appliance more often. They turn to pea when it is warmer and service less frequently. Why do you suppose they do that?

While technically you are correct, the amount of air you would have to blow through pea to get it to burn hotter you would no longer have a residential heating device as most of the heat from it would be going up the chimney and into the environment.

coaledsweat wrote: It has more potential heat because it is more coal. That doesn't mean you can burn it faster in the real world. People use stove when it is very cold out and service the appliance more often. They turn to pea when it is warmer and service less frequently. Why do you suppose they do that?

While technically you are correct, the amount of air you would have to blow through pea to get it to burn hotter you would no longer have a residential heating device as most of the heat from it would be going up the chimney and into the environment.

That has totally NOT been my experience with the three different stoves on the same chimney.

I use stove coal in my Crawford because of its "airflow restrictive" design and the grate size. In other words, it was specifically designed for that size coal. Pea would burn in it just as hot as stove but I would need much more draft and a smaller grate.

In my Harman MKII, running pea would result in stove temps just as hot as nut and stove size.

I would NEVER run pea in my MKII in warm weather due to lower drafting conditions especially if I had a weak chimney. For the exact reasons you state. Less draft would equal larger coal.

In fact some of the highest heat outputs delivered by my MKII were using pea in the dead of winter.

Scalabro, do you have a manometer installed on any/all of your coal appliances?

They all burn hot, especially if you leave the ash door open lol the reason I change from pea coal to nut coal then to nut/stove mix as it gets colder is that it takes less time to ramp up when tending.