Which brings me to a ? for the thermodynamic pyrotechnicians among us

[ISeeDeadBTUs]

[quote author="slowzuki" date="1233785178"]Strange how a glowing mass that fills the whole stove doesn't throw much heat.[/quote]

A bed of coals the size we can end up with in a hydronic, if placed in a wood stove, would drive everyone out of the house or have us open the windows. But without FLAME in these 'Gassers' you just won't get the quality heat production.

What's the technical X-Plain-nation for dat??

 

[Ugly]

That's a good question and I was thinking about that as I design my prototype for testing some other wood gas ideas. The following is my personal reasoning, I could be totally wrong, but I don't care and welcome any further education on the matter.

Here's what I assumed and why; The volatile gas being released from charcoal is less dense than the gas from chunk wood. There's no moisture in charcoal and the heavier gasses have already been driven off from charcoal. Charcoal gasification basically (mainly) releases H and CO where chunk wood gassification releases a whole pile of other volatiles (such as methyl gasses) . In a chunk wood boiler, your secondary burn takes place right there below the fire and the gasses don't need to be cooled and condensed as they are in a typical Imbert style gassifier where the gas is scrubbed of tars and condensed by cooling to increase the btu/cubic m . They are already dense enough the way they are straight from chunk wood and most boilers with gassification are designed around this fact.

So if all you have left is charcoal with it's lighter gasses and the design of the boiler is such that there's no way to cool and condense the gas, you're not going to get much of a reaction in the secondary. The gas is too light by that point.

I was thinking if I could just turn off (block off) the secondary entirely and use the primary (full of charcoal) with a standard boiler jacket design (or watertubes) - with just the charcoal left I'd get more heat at that point. Or I could suck the gas off the primary, condense it using heat exhange and reintroduce the condensed back into the secondary chamber as a seperate process.

In a nutshell, with no way to condense the light gas produced by charcoal, the secondary chamber designs don't lend themselves to being very functional for the gassification of charcoal.

Ugly

 

[jebatty]

Ug... makes some sense regarding CO and H, which combust at about 1200F and 1400F respectively. On the first question though, I do not end up with many coals in my Tarm, they burn quite well to ash, albeit at a lower stack temp than during the first 1/2 of the burn. Putting these two together leads me to suggest that it is the design of the gasser which is at issue. Either the gasification tunnel and/or the draft air supply is not well tuned to the fuel being combusted (wood coals). Since these gases combust at these high temps, either the combustion is not taking place and the gases are escaping and/or there is insufficient O2 to allow the combustion to take place.

I think a second issue may be the density of the wood being burned. I burn almost only pine and aspen, although this year so far it has been mostly birch, all of which are less dense than oak, hickory, hard maple, etc. These less dense woods tend to burn to ash with less coals than the more dense woods. If I had a load of oak, I would try it and see if the result is the same as with what I normally burn. I will have some oak next heating season, but the oak was too green to attempt to burn it this season.

 

[pybyr]

Strange how a glowing mass that fills the whole stove doesn't throw much heat.

A bed of coals the size we can end up with in a hydronic, if placed in a wood stove, would drive everyone out of the house or have us open the windows. But without FLAME in these 'Gassers' you just won't get the quality heat production.

What's the technical X-Plain-nation for dat??

My Econoburn can and does still kick out a _lot_ of heat when there's big bed of coals in the upper firebox, even though they've burned their way back away from the nozzle and there's no gasification going on.

As I understand it, the Econoburn has a full water jacket, and does not rely solely on the firetubes, so that's what I attribute this phenomenon to with my set-up

 

[Nofossil]

First of all, there's a really good sanity check available. There have been very careful instrumented tests that show a good gasifier can extract 90% of the available chemical energy in the fuel and deliver it as temperature gain in the water passing through it. Of the remaining 10%, virtually all of it is heat lost in water vapor and heat of the gases going up the flue. There is virtually zero unburned fuel being lost up the chimney, and virtually no other energy loss of any other kind, They do lose around 1.5% through the doors, side panels, and boiler-attached plumbing.

If the coals aren't generating as much heat as coals do in other situations, it must be that the wood has given up that heat earlier. It isn't being lost.

I have noticed that charcoal left behind after a fire in my EKO is extremely light. I suspect that it has had a good deal more of the flammable components driven out of it than the charcoal generated in a conventional fire.

If that's the case, it could be that conventional charcoal, with a dense core of unpyrolized wood, has a great deal more chemical energy left to release per unit volume.

 

[Hydronics]

I believe it's simply a function of the available energy in the fuel. The coals have significantly less combustibles than the solid wood it once was (the fuel is partially used up). This is most notable in a gasser because after all their main design intent is to utilize what would go up as smoke in natural combustion.
The wood stove extracts considerably less energy than the gasser from solid wood, this is why there is less difference in heat output between flames & coals.
Consider the basic camp fire, you can hold your hand over red coals longer than you can over active flames.

 

[DaveBP]

Solid chunk fuels are just not consistent in heat output. Big 1st half and then the long slow finish. Sound like wine label talk? Pellet burners are probably better, but it's because of how they load incrementally, not how the pellets burn. I think this aspect of wood boilers is the main source of scorn one hears from oil and gas burner guys sometimes (usually around a few beers ).
That pile of coals would seem hotter in a wood stove, sure. They'd feel even hotter in your lap, but if you could measure the flow rate through the boiler and the delta T you'd see those coals are producing heat at the rate you feed them air and I'd bet more heat than if it were in a woodstove because more of the heat would go into the water and not up the stovepipe. I betcha...

 

[Ugly]

(snip)
I have noticed that charcoal left behind after a fire in my EKO is extremely light. I suspect that it has had a good deal more of the flammable components driven out of it than the charcoal generated in a conventional fire.

If that's the case, it could be that conventional charcoal, with a dense core of unpyrolized wood, has a great deal more chemical energy left to release per unit volume.

Agreed. The little gas that is being released is low density from light charcoal. While the total energy yield from the entire burn might be highly efficient, the gassification of that light charcoal isn't going to produce high heat since it just doesn't have the energy to give up. Since there's no way in a wood boiler to increase the density of the gas I'd argue that you're giving up much of the energy left in this charcoal right in the primary chamber and just sort of pushing the hot air down to the secondary that was designed for a much richer fuel mix. It's still a super clean burning mix since it's mostly H and CO and therefore a robust secondary burn isn't all that essential at that point in time from a cleanliness/burn efficiency aspect.

I just wonder how efficent it is to push what is basically hot air around the secondary burn path. Not necessarily in the sense of consumption of wood, but the overall ablative impact and the need to push (or pull) that air with a strong blower all around the secondary path and basically introduce more cold than is really needed into the unit at that point to get what residual heat there actually is from said lightweight charcoal.

Anyhow... happy burning

 

[jebatty]

It's often during this last part of the burn that I hear the roar of a gasification burn with no visible flame. Hydrogen and CO both burn colorless, I believe.

 

[Fred61]

It has always been my understanding that those red glowing embers are about 2000 degrees. This could be why there is no change in appearance at the low end of the nozzle.

 

[Duetech]

ISeeDeadBTUs,
Let's not forget the insulative quality of the water jacket which is missing from the wood stove and the the intensity of infrared heat that a standard wood stove puts out. Some wood stoves at full bore put out nearly 400*f at the surface and the stove stays warmer (for a while) than the surface of the gasser gets. (Which make me wonder, if we could introduce wood at the bottom of the coals in an old wood burner, if we could turn an old wood stove into an updraft gasser?)
While my gasser is running full tilt one of my cats likes to sleep right under the secondary door.
The infra red radiation of an old potbellied stove can be felt across a room where with a gasser you pretty well have to cross the room to feel the heat.

 

[ISeeDeadBTUs]

Ok, I should have mentioned . . .

1) No brawlin'
2)Term yer answers in such a way as your typical dumbass - such as myself - can understand ya

 

[ISeeDeadBTUs]

ISeeDeadBTUs,
Let's not forget the insulative quality of the water jacket .

What the hell is a water jacket? Is that like a full-metal jacket?

My GW has 8 gals of water in it . . . the T-Pex for delivery to the house DOUBLES the volumetric capacity of the heat exchange medium.

 

[cguida]

(snip)
I just wonder how efficent it is to push what is basically hot air around the secondary burn path. Not necessarily in the sense of consumption of wood, but the overall ablative impact and the need to push (or pull) that air with a strong blower all around the secondary path and basically introduce more cold than is really needed into the unit at that point to get what residual heat there actually is from said lightweight charcoal.

Hello Ugly,

It looks like Igor Kuznetsov, the Russian stove designer, has spent his whole career dealing with that question -- the ablative effect of excess gasses ("ballast gas") he calls it -- not directly involved in the combustion process, but being dragged through any forced draft system soaking up heat and costing efficiency. If you are into stove/furnace/boiler design, yhou'll find lots of interesting ideas to chew on at his website

http://stove.ru/index.php?lng=1

Lately he has put up a power point presentation that explains his 'free gas movement' designs (as opposed to 'forced gas movement'). There isn't much new under the sun in the wood-burning world. But I've never seen or heard of anything like this -- something for sure to cogitate on and investigate. Below are 3 sketches from his website which hopefully convey some idea of what he is talking about.

 

[Dave T]

All being said, does my gasser burn more efficient with a full load and the ability to break it down to coals before the second burn??
Or is it more efficient to tend the boiler and feed it fresh wood more often?? Is the difference if storage is avalible LOAD IT, and if
burning without storage feed it a little at at time?? I am a seasonal roofer so I have the ability to tend often..Thanks, Dave

 

[Ugly]

All being said, does my gasser burn more efficient with a full load and the ability to break it down to coals before the second burn??
Or is it more efficient to tend the boiler and feed it fresh wood more often?? Is the difference if storage is avalible LOAD IT, and if
burning without storage feed it a little at at time?? I am a seasonal roofer so I have the ability to tend often..Thanks, Dave

I think the words efficent and output are getting crossed up in your first question. I'd suggest you get more BTU's earlier in the burning cycle with a lot more wood gas density coming out with fresh wood than at the light charcoal stage.

However as nofossil pointed out, the overall cycle when measured against the potential stored BTU in the wood is "efficient".

I personally believe the majority of these units are designed to take best advantage of the pre charcoal stage, which makes sense as most of the energy is available then and the need to burn without smoking is one of the best selling points.

Weigh your wood and feed it as consistently as possible using both methods during similiar outdoor temps and find out what works best. Keep notes over a period of time and find out what works in your install. I think this type of gassification as a process is better suited to small hot burns with as little idling as possible. Manufacturers likely agree or they'd have built units with massive primary chambers to reduce loading frequency. Be nice to load once a week