secondary burn questions on gasification boilers

[welderboyjk]

Just wondering for those of you with gasification boilers, how long does it take for the "secondary" burn chamber to come up to temp and start burning.
Secondly I see that most of the gasification boilers are downdraft design. Does anyone make an updraft design.
Thanks.

 

[jebatty]

Solid gasification within 5-10 minutes. What's your question as to downdraft or updraft?

 

[timberr]

I usually have good gasification after 10-15 min, sometimes it takes +30 min. if the wood has too much moisture. I have a downdrafter.

 

[WRVERMONT]

Time to good gasification depends on a few things and boiler make,size. Larger boilers can take a little longer. Placement and amount of kindling is the greatest factor to fast gasification. Takes me about 10-20 minutes if I pay attention to building a kindling fire. Some boilers seem to reach gasification quicker, I have been able to reach gasification quicker with the Econoburn and smaller EKO's. I think the "tall" pathway and nozzle may make for more immediate gasification with the Econoburn.
It takes some people a lot longer, usually not starting with kindling splits small enough. The temperature of the wood is also a factor. One other factor would be how warm is the water in the boiler when you start, if you have thermal storage or not? Small factors, but not inconsequential. I'll stop before I get carried away!

 

[WoodNotOil]

Until there is a good bed of coals and the boiler water temp is above a certain point (140*? can't recall of the top of my head). If there are coals left from the last firing and boiler temp is up enough it can be immediate. Following a good fire start procedure the 5-20 minutes people mention is pretty typical. If the MC of the wood is too high, you may not get any gasification though. This is one of the stumbling blocks new users sometimes run into when learning how to operate gassers.

 

[stee6043]

For me it depends almost entirely on how cool the boiler gets before I restart the next fire. I don't typically get good solid gasification until the boiler get's to 160+/-. So a starting boiler temp of 120 get's running a lot faster for me than starting it at 65 degrees. With thermal storage, depending on the whether, I see both situations quite regularly.

 

[welderboyjk]

I was curious because I built an OWB for a friend of mine and I tried to copy my lopi endeavor stove but with a powered fan draft. It is presently hooked up to a hx in his furnace and a cast iron radiator in the basement with a box fan on it that seems to be doing a decent job of heating his house by itself. I've got the cycle seet between 140-160 because he's got younger kids and I didn't want anyone to get scorched on the radiator.
The outer jacket is a 500 gal propane tank (ugly but cheap) and minus the insert it should hold about 300 gal of water. Upon initial firing is smokes like a SOB but after 20-25 minutes you can hear a whoosh out the flue pipe and shortly after that there is a significant reduction in smoke. I just wish there was a way to get there faster. I've got about a 4 inch ring of exposed steel around the top of the firebox. Maybe putting more brick in here would help. Maybe turning up my cycling stat a little.
Downdraft I was meaning like pulling the fire downward as opposed to a traditional woodstove.
Thanks.

 

[WoodNotOil]

Where is your draft fan located and how does the eshaust exit the unit? If gasification is occuring, what is that 2000* flame hitting? Did you line it with refractory or is it hitting bare steel? I am interested in understanding your design. I love DIY projects...

 

[jebatty]

Downdraft I was meaning like pulling the fire downward as opposed to a traditional woodstove.

I think most, if not all, gasification wood boilers are downdraft. The Garn would be similar to an updraft, but I'm not sure it would be correctly classified as a gasification boiler, although it appears to be very efficient. Garn users can add their input on this.

 

[welderboyjk]

Here are a couple of pics of the fireboxes;
http://s469.photobucket.com/albums/rr56/welderboyjk/boiler/
I kind of drew lines where the firebricks lay in the bottom. They were meant to slide into a 500 gal tank so they are about two feet square by four deep.
One is up and running the other(mine) got the bottom cut out and 10 inches added to the depth.
The channel running around the outer topside are the air channels with the bungs for the "air tubes". 2" thick by 12 by 24(trimmed) lay on top of the channel. There are holes in the front channel pointing down and there is a pattern of holes in the rear channel expecially where the hole is trimmed in the top baffle. On top there is a little "doghouse" 18"(two brick) long by about 3 1/4" tall. There is a steel baffle running front to back in the upper chamber. Flue exits on the RH side top. The little square drawn on the front plate is where the draft fan plenum runs in.
when we first tried to run it we had to leave the door open for a while to get a good fire going and then it would go by itself. Restarts were tough so I added a couple of 1/2" pipe drop tubes in the front to get some air to the bottom of the fire. Restarts great now. But I think I may have to throttle the draft fan down a little bit. The air holes are all about 5/32 and with the drop tubes it gets plenty of air.
It is easy to tell when the smoke lights off because you can hear the fan change tone and there is a whoosh sound. It blew loose creosote out of the flue once. If you open the door there are some nice flames by the air tubes and by the firebox exit near the back channel air holes, but they look like there is too much air. They don't dance nicely.
It may not be perfect gasification but with the 10" added to the bottom of the other box I may have enough room to do a downdraft setup on mine.

 

[ken999]

For you folks that take 10-20-30 minutes for gasification, how much smoke do you boilers produce while coming up to good gasification temps?

 

[TCaldwell]

Dectra calls it a '' secondary reaction chamber'', not as independant as a downdrafter design secondary burn chamber, however will reach temps in excess of 2000f. Basically and combustor design that will present unburnt fuel and wood gas at a temp of 1200f,to a insulated chamber with oxygen will create a clean burn. with dry wood a garn will be hot enough to reburn under 10 minutes.

 

[sgschwend]

FYI
I don't know what the practical flame temperature of wood is (I wish I did), it is quite a bit less than 2000F:
Here is one reference: http://www.doctorfire.com/flametmp.html

When one consults combustion textbooks for the topic of 'flame temperature,' what one normally finds are tabulations of the adiabatic flame temperature. 'Adiabatic' means without losing heat. Thus, these temperatures would be achieved in a (fictional) combustion system where there were no losses. Even though real-world combustion systems are not adiabatic, the reason why such tabulations are convenient is because these temperatures can be computed from fundamental thermochemical considerations: a fire experiment is not necessary. For methane burning in air, the adiabatic flame temperature is 1949°C, while for propane it is 1977°C, for example. The value for wood is nearly identical to that for propane. The adiabatic flame temperatures for most common organic substances burned in air are, in fact, nearly indistinguishable. These temperatures are vastly higher than what any thermocouple inserted into a building fire will register!

 

[jebatty]

Perhaps the 'adiabatic' flame temperature is the near instantaneous heat energy released at the moment of the chemical (burn, flame) reaction occurring, but this cools almost as fast by dissipating the heat into the surrounding atmospheric gases, which in turn is transferred to the refractory, hx tubes, and everywhere else. It would be interesting to know the temperature of the surface of the refractory tunnel in a gasifier, and similarly for the surface of the secondary reaction chamber of the Garn.

I don't the reliability of this Info, but fwiw:

The simple facts of temperatures:

* 1535ºC (2795ºF) - melting point of iron
* ~1510ºC (2750ºF) - melting point of typical structural steel
* ~825ºC (1517ºF) - maximum temperature of hydrocarbon fires burning in the atmosphere without pressurization or pre-heating (premixed fuel and air - blue flame)

Diffuse flames burn far cooler.
Oxygen-starved diffuse flames are cooler yet.

The 1517F number may give a clue as to the fire temperature in the refractory by adding to that the preheated fuel/air mixture coming from the firebox. That is an oxygen controlled (starved ?) flame, as is the flame in the refractory. The published numbers of 1800-2000F logically would seem to be within reason, but again, that flame cools very quickly.

 

[Rick Stanley]

Dectra calls it a '' secondary reaction chamber'', not as independant as a downdrafter design secondary burn chamber, however will reach temps in excess of 2000f. Basically and combustor design that will present unburnt fuel and wood gas at a temp of 1200f,to a insulated chamber with oxygen will create a clean burn. with dry wood a garn will be hot enough to reburn under 10 minutes.

Tom, how can you tell if it's re-burning or gasifying? Well, I mean me. How can I tell? Knowing you, you've installed thermocouples everywhere, right? Anyway, I can't really say I see an event or a stage of burning that is "secondary" burning. At least nothing like the Tarm and Eko guys describe. I have nice dry wood that routinely raises the tank temp 20-30 degrees, depending on the demand at the time, on a single burn. So I know it's a hot sumbitch, but secondary burn? gasification? Who knows. Seems like it's working right..................

 

[stee6043]

For you folks that take 10-20-30 minutes for gasification, how much smoke do you boilers produce while coming up to good gasification temps?

They smoke a bit. I guess it's hard to describe "how much" smoke, though. On a standard weather day my smokiest 10/15 minutes will produce a smoke column that disspates in say....maybe 20' or less from the output of my stack? But this is with the bypass open. I run my boiler on startup with the bypass open until I get 400 degrees (external) on my flue. Once I have 400 I shut the bypass and turn on the fan. With the fan running, even before I achieve "good" gassification, my smoke is less than half what it is with the bypass open. Another 30 minutes or so of that and the smoke is completely gone...

 

[muncybob]

My chimney does not seem to smoke as much as my neighbors chimneys even at a "cold" start. They have indoor wood stoves of some sort. The smoking won't normally last any longer than 10 min or so and will be less than 5 if there was a good bed of coals before it lit up. It always amazes me how little I see coming out after it gets into gassing.

 

[Nofossil]

I think it's VERY safe to say that gasifiers are not best modeled as a diffusion flame. Diffusion flames are those in which mixing of fresh air happens only by diffusion, as in a candle flame. Anything that provides mechanical mixing of the fuel and fresh air will result in higher combustion temperatures.

One measure of combustion temperature is to observer whether "ash fusion" has been reached. That's the temperature at which the ash starts to melt and fuse together. Ash fusion temperatures vary by species, but a reasonable range is about 2100 °F to 2500 °F. In my EKO, I definitely reach ash fusion temperatures in the bottom of the combustion area in the lower chamber. I've attempted to measure the flame itself, but ended up ruining an inconel jacketed type K thermocouple that was rated for 2400 °F.

The temperature drops very rapidly when you get out of the flame zone. I now measure combustion temperature a few inches aways from the combustion chamber, near the water jacket. Temperatures there rarely exceed 1400 °F.

I'll try to get my hands on a set of potter's cones that are designed to melt at different temperatures in order to get a better idea of actual combustion zone temperatures.

 

[welderboyjk]

Well I don't think i'm too far off on my time then. I am glad that my reburn time isn't too far out of line with the high dollar units, but I will be looking for ways to fine tune the system for sure. Next year it should be getting hooked up to a 1500 gal fiberglass tank so the burn times will be able to be longer which should decrease the amount of "startups". The only bad thing is the mfgr. of the tanks said the max temp that the tanks should see is 150. I may have to use mine for summer solar storage and look for something else for the boiler.
Thanks for the replies.

 

[WoodNotOil]

Well I don't think i'm too far off on my time then. I am glad that my reburn time isn't too far out of line with the high dollar units, but I will be looking for ways to fine tune the system for sure. Next year it should be getting hooked up to a 1500 gal fiberglass tank so the burn times will be able to be longer which should decrease the amount of "startups". The only bad thing is the mfgr. of the tanks said the max temp that the tanks should see is 150. I may have to use mine for summer solar storage and look for something else for the boiler.
Thanks for the replies.

You could line the tank with poyiso insulation and put in a liner that can withstand a hotter temp. That should reduce the temp the fiberglass is exposed to and allow you to get a hotter tank. Just a thought... you would loose a little volume, but end up with more btus stored...

 

[Fred61]

I think it's VERY safe to say that gasifiers are not best modeled as a diffusion flame. Diffusion flames are those in which mixing of fresh air happens only by diffusion, as in a candle flame. Anything that provides mechanical mixing of the fuel and fresh air will result in higher combustion temperatures.

One measure of combustion temperature is to observer whether "ash fusion" has been reached. That's the temperature at which the ash starts to melt and fuse together. Ash fusion temperatures vary by species, but a reasonable range is about 2100 °F to 2500 °F. In my EKO, I definitely reach ash fusion temperatures in the bottom of the combustion area in the lower chamber. I've attempted to measure the flame itself, but ended up ruining an inconel jacketed type K thermocouple that was rated for 2400 °F.

The temperature drops very rapidly when you get out of the flame zone. I now measure combustion temperature a few inches aways from the combustion chamber, near the water jacket. Temperatures there rarely exceed 1400 °F.

I'll try to get my hands on a set of potter's cones that are designed to melt at different temperatures in order to get a better idea of actual combustion zone temperatures.

I'm pulling ash "cookies" out of my U blocks after a burn. Is that because I'm approaching ash fusion or is it ash that is packed like a snow drift? They aren't clinkers (fused silica).

 

[Nofossil]

I'm pulling ash "cookies" out of my U blocks after a burn. Is that because I'm approaching ash fusion or is it ash that is packed like a snow drift? They aren't clinkers (fused silica).

I had a boiler engineer visiting and he described the 'cookie' effect as evidence that I'm reaching the ash fusion point. Full melting takes quite a bit more heat, I think. Bottom line is that secondary combustion temps in a gasifier are considerably higher than an ordinary wood flame. It's literally a blast furnace, with forced introduction of secondary air.

 

[ohbie1]

I think it's VERY safe to say that gasifiers are not best modeled as a diffusion flame. Diffusion flames are those in which mixing of fresh air happens only by diffusion, as in a candle flame. Anything that provides mechanical mixing of the fuel and fresh air will result in higher combustion temperatures.

One measure of combustion temperature is to observer whether "ash fusion" has been reached. That's the temperature at which the ash starts to melt and fuse together. Ash fusion temperatures vary by species, but a reasonable range is about 2100 °F to 2500 °F. In my EKO, I definitely reach ash fusion temperatures in the bottom of the combustion area in the lower chamber. I've attempted to measure the flame itself, but ended up ruining an inconel jacketed type K thermocouple that was rated for 2400 °F.

The temperature drops very rapidly when you get out of the flame zone. I now measure combustion temperature a few inches aways from the combustion chamber, near the water jacket. Temperatures there rarely exceed 1400 °F.

I'll try to get my hands on a set of potter's cones that are designed to melt at different temperatures in order to get a better idea of actual combustion zone temperatures.

I embeded a 2400 deg. K couple in 1/2" black pipe by filling it with Hi-Temp mortar, just left the very tip exposed. I put it in the lower chamber and measured a high of 2200 °F on several occasions. Most of the time it burned around 1700-1900, but most of my wood is 30%, not optimum. I was trying to see what pri and sec settings produced the hottest temp. The couple lasted about 3 wks. or so, but it was fun while it lasted.

 

[dogwood]

Ash fusion? I have some eighteen year old locust splits with 0% moisture content, burning in my Tarm should produce nuclear fusion.

Mike

 

[TCaldwell]

Rick, I have a type k with a ceramic shield, omega engineering centered at the very back of the secondary reaction chamber where it meets the flue tube. the hottest i have seen was 2340 with a flue temp on the last pass thermometer of 520, on average 1500-1800degf . I have noticed these deli sized cookies , when i clean out the firebox basically in the vecinity of where the primary air blasts the the load of wood towards the end of the fire, the refractory in the firebox glows in this area also. With a standard issue garn air delivery system it will not direct enough air to do this.