"Over" Air and Efficiency of Stove

In general, with a hard coal burner.... How does the over air effect the getting heat out efficiency?

On my Hitzer 82FA, it seems to be more efficient with lower amounts of the over air.

Anyone got the engineering knowledge on this?

Many anthracite stoves don't even have overfire air and work efficiently.

When I load my large stove I have preheated air pipes inside and when I open them it roars while burning up all the gasses but it doesnt burn them for real long. If open when its not burning gasses it cools my stove down, slowing down the under fire air. So it helps some at first and thats it, at least for me. Mines kind of a clone of your Hitzer only a smaller size. Im betting you will gain some efficiency but Im not sure its enough to worry about.

I'll take a stab at it. My HITZER's didn't have over fire air. They burnt just fine. The gasses travel around the hopper, and out. Now, for the D.S. Circulators, as well as the Harman MARK series; they pocket gasses. They have over fire air.The over fire air dilutes the gasses, in hopes of a lesser BANG!, should it happen. At the same time, over fire air washes the glass, helping to keep it cleaner, and cooler.

When first experimenting with the D.S., I plugged all the secondary air holes. I had an explosion, and it was a BIG one.

So, my conclusion of over fire air is; it aids in burning / diluting excess gas build-up. After that, it's not needed. Not with anthracite anyways.

For bituminous coal the over fire air vents is necessary. I'm adding air vents to load door cause I burn bituminous. But I sampled Anthracite a while back it went very well without above fire air.

This post by William Sherrick a year or so ago. It has the best explanation of what happens in a coal fire that I have ever come across.

wsherrick @ VERY Stupid Question for Those With Baseburners

If it doesn't go right to the post, it's the 14th post down on page 6.

Paul

warminmn wrote:Many anthracite stoves don't even have overfire air and work efficiently.

When I load my large stove I have preheated air pipes inside and when I open them it roars while burning up all the gasses but it doesnt burn them for real long. If open when its not burning gasses it cools my stove down, slowing down the under fire air. So it helps some at first and thats it, at least for me. Mines kind of a clone of your Hitzer only a smaller size. Im betting you will gain some efficiency but Im not sure its enough to worry about.

Yes Warminmn, I have to agree with you. Not being a chemist, I did read what William Sherrick had to say, which kind of explains what's happening. As you say, it's not for long. Which is probably the reason D.S. made the new anthramax with adjustable over fire air. After the gasses are burnt off, over fire air can be closed.

Sunny Boy wrote:This post by William Sherrick a year or so ago. It has the best explanation of what happens in a coal fire that I have ever come across.

wsherrick @ VERY Stupid Question for Those With Baseburners

If it doesn't go right to the post, it's the 14th post down on page 6.

Paul

Read the link and the chemistry did not make sense to me. I am quite sure you need a hydrocarbons to make fire. I know CO2 is used to put out fires and CO takes O2 out of the air so how they combust escapes me. CO is from incomplete combustion and CO2 from complete combustion. If I missed something please point it out, not trying to start a rake fight just don't follow it.

ddahlgren wrote:

Sunny Boy wrote:This post by William Sherrick a year or so ago. It has the best explanation of what happens in a coal fire that I have ever come across.

wsherrick @ VERY Stupid Question for Those With Baseburners

If it doesn't go right to the post, it's the 14th post down on page 6.

Paul

Read the link and the chemistry did not make sense to me. I am quite sure you need a hydrocarbons to make fire. I know CO2 is used to put out fires and CO takes O2 out of the air so how they combust escapes me. CO is from incomplete combustion and CO2 from complete combustion. If I missed something please point it out, not trying to start a rake fight just don't follow it.

The description is about what happens over the fire after the hydrocarbons have separated into their base elements (carbon and hydrogen) by the fuel being exposed to heat and air. When CO is converted to CO2 in the combustion process you release a whole lot more heat than if the fuel is just converted to CO.
Somewhere I wrote a long post describing the whole thing. If I feel up to it I might redo this post and include the entire combustion process from cold fuel to ashes.

ddahlgren wrote:

Sunny Boy wrote:This post by William Sherrick a year or so ago. It has the best explanation of what happens in a coal fire that I have ever come across.
wsherrick @ VERY Stupid Question for Those With Baseburners

Read the link and the chemistry did not make sense to me.

I read the link and it IS a great explanation but I don't think it is true. I agree the chemistry doesn't make sense -- CO2 is pretty stable and it's hard to break up the molecule. A more likely explanation is that there's not enough oxygen coming from under the fire, so CO (incomplete combustion) is formed, rises above the coal bed, and secondary air further burns the CO to make CO2, providing more heat than if the CO just went up the stack. So yes, the secondary air is valuable.

Also there are volatiles (the hydrocarbon stuff) within the coal which are driven off by heat and the secondary air allows those volatiles to burn. Oxygen from under the fire has largely been used up already, lower down in the coal bed, so without secondary air there would be no oxygen to combine with the hydrocarbon volatiles above the coal. Once the coal bed is hot the volatiles are largely gone, which is why secondary air helps the most early in the burn cycle (after new coal has been added).

That's my theory, I may be full of poop but until a chemistry professor weighs in I'll stick to it.

If you have an approximate date I am happy to do a search sort of date and name based. I would think there are a host of other reactions going on making sulfuric acid nitric acid or at least oxides of nitrogen. How much no doubt linked to the percentage of sulfur in the coal. I had thought the biggest problem the EPA had with coal was ash and acid rain. I used to know a lot more chemistry than I do now so a use it or lose it for me. Shooting from the hip I would think that secondary spent most of it's time burning volatiles and thinking locked in methane or similar.

That's what I get for coming back on here.
This information came out of two very authoritative books on coal combustion in relation to boilers and steam.
I guess they didn't know crap about the subject. They just made it all up.

wsherrick wrote:

ddahlgren wrote: Read the link and the chemistry did not make sense to me. I am quite sure you need a hydrocarbons to make fire. I know CO2 is used to put out fires and CO takes O2 out of the air so how they combust escapes me. CO is from incomplete combustion and CO2 from complete combustion. If I missed something please point it out, not trying to start a rake fight just don't follow it.

The description is about what happens over the fire after the hydrocarbons have separated into their base elements (carbon and hydrogen) by the fuel being exposed to heat and air. When CO is converted to CO2 in the combustion process you release a whole lot more heat than if the fuel is just converted to CO.
Somewhere I wrote a long post describing the whole thing. If I feel up to it I might redo this post and include the entire combustion process from cold fuel to ashes.[/quote]

I for one, would appreciate it if you did, William.

Paul

wsherrick wrote:That's what I get for coming back on here.
This information came out of two very authoritative books on coal combustion in relation to boilers and steam.
I guess they didn't know crap about the subject. They just made it all up.

Please relax, your input is appreciated.

My next question: What ratio of under/over air gives maximum efficiency? Just taking a stab... 40-1 ?!?

Who has the math?

You still the man in my book William!!

wsherrick wrote:That's what I get for coming back on here.
This information came out of two very authoritative books on coal combustion in relation to boilers and steam.
I guess they didn't know crap about the subject. They just made it all up.

wsherrick wrote:That's what I get for coming back on here.
This information came out of two very authoritative books on coal combustion in relation to boilers and steam.
I guess they didn't know crap about the subject. They just made it all up.

No offense meant. So the books said passing very-hot CO2 over carbon causes the CO2 to dissociate, or whatever the term is? CO2 + C >> 2CO. I'm surprised but my chemistry knowledge is limited.

This Wikipedia article supports what you said:
A major industrial source of CO is producer gas, a mixture containing mostly carbon monoxide and nitrogen, formed by combustion of carbon in air at high temperature when there is an excess of carbon. In an oven, air is passed through a bed of coke. The initially produced CO2 equilibrates with the remaining hot carbon to give CO. The reaction of CO2 with carbon to give CO is described as the Boudouard reaction.[67] Above 800 °C, CO is the predominant product:

CO2 + C → 2 CO (ΔH = 170 kJ/mol)