"owb with gasification"????

Jeremy Q said:

Ok here goes, first post so bear with me.Im looking at building an owb and would like to incorporate a gasification design into it. Ive been doing research and think i have an idea in mind(if i can put my thoughts in words). Now you guyz can shoot the holes in it so i can redesign before building.
What i have in mind is a 250gal owb with 500 gal storage. If i understand gasification correct, its the superheating of secondary air to ignite the unburned gases from the fire? So what im thinkn is if you have a main fire in the owb being feed on a primary air system you can "catch" the flue gases and burn them with a secondary air system. This would allow you to have a hotter clean burning fire. Ive messed around with sum updraft "soup" can gasifers with alittle success. So im basing my ideas off what i've played with and read. There's a lot of good ideas here from what ive read. DONT WANT TO BUY A OWB or GASIFER. More just a project I want to build and play with n the heat would just be a bonus. Maybe im all washed up and out of my mind!?!? Sorry for the lengthy first post. Hope you guyz can guide me on my endeavor.

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So.... Another intrepid experimenter. Welcome to the club. I don't want to discourage you at all, but be prepared for the magnitude of the engineering task that's involved. Gasifiers are a great example of something that looks really simple but is actually quite complex. Many of the designs out there are 'evolutionary' - that is, they are the result of many years of tweaking and improvements based on studying how the previous designs worked. Almost all are downdraft in some form, because the coal bed acts as a cracking tower - it breaks down the complex hydrocarbons in an oxygen-starved but extremely hot environment. This creates wood gas which is easy to burn and doesn't require a long dwell time.

There are many variables which need to be right in order to get good performance. Here's an incomplete list:

1) Free oxygen to fuel gas ratio - should be about 1.6 x stoichiometric
2) Primary air flow rate / nozzle cross section ratio
3) Primary / secondary air flow ratios
4) Secondary combustion dwell time
5) Control algorithms - air flow vs. outlet temp vs. fire phase
6) Primary combustion chamber airflow patterns

If you decide to embark on this journey, I'd strongly suggest making a copy that's as exact as possible of a working design as a starting point. There are changes that are 'safe' - increased heat exchanger surface area, for instance. Other changes may or may not produce unexpected results. For instance, increasing the volume of the primary chamber without changing anything else. Might be fine, might cause massive problems.

Be prepared for the need to make changes and adjustments.

Good luck.....

Jeremy,
I've been contemplating the same but plan to use my updraft boiler as the heat exchanger and connect the discharge of the gasifier to it.

From a hands-on standpoint the goal is "simply" to achieve complete complete combustion (the gasification) and transfer as much energy as possible to the water (heat exchanger). Reaching this goal is not simple, however certainly possible. In an ideal world your flue temps. would equal your boiler water discharge (complete transfer of energy), this is economically and practically unfeasable.

As Nofossil advised, you would benefit from studying quality gasifiers (Tarm, eko, Econoburn). The wood gun is a more difficult design to build in your garage and the Garn is a horse of a different color, both are very high quality designs.

I would base the design on these proven units. Companies with large resources have done the research and development already. Try to keep it as flexible/tuneable as possible: adjustable secondary air, ability to remove the nozzle if refractory fails, access to the exchanger for cleaning and inspection. You can adjust primary/overall airflow by partially blocking the inlet of the blower.

If you decide on a larger firebox, make it taller rather than wider so that less of the wood is exposed to the intense heat of the refractory. Cast iron may be the ideal material for the heat exchanger from a durability standpoint. You can control the circulator with one aquastat and the blower with another. If you don't have storage you can maintain combustion with a timer or adequate draft may address this by pulling air through the blower inlet.

If one does their homework, has determination, fabrication and troubleshooting skills, the goal can be achieved. Good luck.

Jeremy Q said:

This is kinda what I've been thinkin, with a secondary air and control going into the upper chamber. Like i said KINDA. My idea is slightly different and larger to increase times between loadings. Check it out and imagin a secondary air system n tell me if im out of my mind(my wife does all the t

http://www.woodheat.org/technology/outbobpen2.htm

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I had looked at that when I had my CB and was going to try it but it would void out the warranty. I looked around allot but couldn't find out if this person had ever updated his report after using it awhile. I don't think you'll get allot of secondary burn without figuring out how to add just the right amount of air to the top chamber, but it would burn a little cleaner because your fire would be insulated from the water.

Jeremy, I was wondering if you have found anymore information on building your own outdoor gasification? I was planning on building my own outdoor boiler this summer, but would like to try to incorporate gasification into it somehow. I have been looking at some of the different models made, and can't figure out which way to go.

I've examined most of the common gassifiers on the market and come to the conclusion they are fairly efficient at what they do: turning chunk wood into gas and then into heat. Not that they couldn't be better since the entire airflow and burn pathway is designed around a best compromise situation and making the boilers much more complex would require a lot more sophisticated controls and materials. I mean my dream furnace would sample the gas coming off my primary and automatically adjust my nozzle size, airflow pathway and volumes and be able to reconfigure (resize and reshape) the secondary combustion chamber on the fly. Short of involving a NASA engineer, I don't think I'm going to realize that particular dream. That's why it's not a goal.

The lack of standardized testing in the industry however leaves one scratching his or her respective ass and wondering what's good and what's not so good. In terms of actual burn efficiency, there's so little raw data concerning exactly what went into the furnace because wood itself is so variable in composition, size, moisture content etc... and then I look at the BTU ratings and wonder if they just came up with those figures based on an efficiency calculation or did they actually measure the BTU/hr using a heat dump with calculated loss over the measurement period factored in.

In other words, lots of questions and little data. Some of the best data and statistics I've ever seen surrounding this topic come from nofossil. I'd still like more and I'm going off to gather my own by simple prototyping.

If your goal is heat and you want tried and true, I agree with the previous statement that cloning an existing unit is the best way to start. I wish there more contruction resources from an engineering standpoint, but there aren't so we'll have to create some.

Regards,
Ugly