Learning - control burn rate

[maurice]

Since I am handy like Red Green but maybe more cautious than his show... lol...I'm learning outdoor wood boiler (OWB?) or outdoor wood gasifier boiler (OGB?) to see if this is something I might make instead of buying. (Prices are a bit above my budget.)

I grew up with wood heat (old farmhouse, not insulation, wake up seeing my breath in the bedroom) but have gone away from wood. I now see such wasteful clearing of trees and wood in the farm area I live in and the nearby community, add to that high propane prices. So a bit of backgound; I'm not an old nut.

As I understand from my past, the burn rate can be controled with how much primary air enters. So on an OWB, assuming tight leak free contruction, it this accurate?

Or on an OGB, the gasifier chamber has primary air, but the "burn" rate is controlled by the opening to the secondary burn chamber where added air is introduced (if I understand correctly.) Can the burn rate be controlled by the size of the opening to the second chamber?

 

[Nofossil]

Building your own gasifier is not a trivial exercise. There are complexities in the physical dimensions and dimension ratios as well as in the controls. Primary and secondary flow rates need to be matched to each other and to the cross sectional area of the nozzle. There are books and resources out there that address the physics of wood gasification as well as some of the construction and materials issues. The 'resources' sticky at the top of the boiler room has a lot of good links in it. Suffice to say that they're more complicated than they look.

To address your specific question, gasifiers are throttled by the amount of primary air that's introduced into the gas generation chamber (the top chamber on a downdraft unit). The amount of secondary air needs to provide enough oxygen to assure complete combustion of the wood gas that was generated. Most gasifiers maintain a set ratio between primary and secondary air, with a secondary air target of about 1.6 times the stoichiometric volume. More sophisticated models use lambda sensors to close the loop and maintain ideal secondary air flows.

The peak output of a gasifier is determined by the cross sectional area of the nozzle (assuming that the blowers can achieve sufficient air flow to reach ideal velocities). They can be throttled down by reducing primary and secondary air, but there's a limit to how far down you can throttle them. As airflow is reduced the velocity of wood gas through the coal bed and the nozzle drops below the ideal values. Read up on (broken link removed to http://www.woodgas.com/Superficial%20Velocity.pdf) if you're really interested in how all of this works.

A gasifier that's idling is not a gasifier. A gasifier that's running 'in the zone' burns about 40% less wood than the best conventional boilers, and produces virtually no smoke, odor, or creosote. It's a great goal, but there's some hard science hidden in the dimensions and the controls.

Best of luck. My brother built a condensing gasifier based on the EKO 25 dimensions, and it works very well.

 

[maurice]

Nofossil, thanks for the info! Yes, I understand that building a gasifier is not a trivail experience. I have read that sticky and followed the links. I am looking for deeper info. Your reply has more info than I found. Unfortuanately, I can't get your Super Velocity link to work.

As I wrote in my first question, I am not looking to steal trade secrets from patent holders.

I do understand that a gasifier that's idling is not a gasifier. Your advice about the 40% less wood, low smoke, odor, and creosote would be motivation to try for a gasifier. And I thank you agian for your helpful info!

If I were to build a OWB (non-gasifier - am I getting the acronyms correct?) I would try to get a hot, efficient burn and to capture the heat so I would not have to throttle the fire. I was very interested in the VTHR burner- http://www.sredmond.com/vthr_index.htm , he generated much excitement, but he could not make it foolproof (my words) so he stopped. I had hoped to find a website carrying on the research, but not finding any.

If I were to build something, I am thinking of ways to make changes easily. Maybe I can't make a gasifier (OGB?), so if I tried and failed, I'd plan to revert the build to an OWB

Well, I may be talking too much and boring y'all, if anyone is interested, I'll mention some ideas I've had.

 

[Nofossil]

Nofossil, thanks for the info! Yes, I understand that building a gasifier is not a trivail experience. I have read that sticky and followed the links. I am looking for deeper info. Your reply has more info than I found. Unfortuanately, I can't get your Super Velocity link to work.

Sorry about the link - it's a Windows thing. Some idiot in Redmond decided to let people put spaces in file names. Bad idea - breaks a lot of stuff. You'll have to go here and then figure out where the link is to the SV article.

Go for it. It can be done. Just don't want anyone investing a lot of time and energy without realizing what's involved. Post your progress - I expect a lot of folks would be interested.