Is there a connection between split size and burn time in a gasification boiler?

[cguida]

Is there a connection between split size and burn time in a gasification boiler? For example, if you had larger wood (didn't split it so fine), would you get a longer (and more efficient) burn time? Or wouild you just get more creosote?

What is the preferred diameter? Is there such a thing as going overboard, and splitting stuff up too fine? Is the preferred diameter pretty much the same for all boilers, regardless of log length?

Even if it burns up quicker, finer split wood will dry out faster... right?

How fine do you split your wood?

 

[Nofossil]

Bunch of really good questions. Here's my take on it. Keep in mind that I'm making this all up.

For any specific gasification boiler, there's an optimum airflow that's dictated by the nozzle cross-section and maybe the size of the secondary burn chamber. Given that airflow, you need a certain volume of wood gas to mix in and provide a good air/fuel ratio.

The rate of wood gas generation depends on several things including the amount of surface area, the type of wood, the moisture content, and perhaps the phase of the moon.

Wood that's split too small and is really dry and is really fast-burning (white pine, for instance) will generate too much wood gas and give you a mixture that's too rich.

Wood that's split too large and has higher moisture content and burns more slowly generates too little wood gas and results in a mixture that's too lean. You can even lose secondary combustion.

A boiler with a lambda sensor can adjust primary and secondary airflow to compensate, but us peasants have to play with our wood instead. I find that an average split size of around 3" to 4" is about right, and a mix of different sizes and species is good. If I'm burning mostly poplar and pine, I'll aim for mostly larger pieces.

 

[pybyr]

I'm a newbie compared to NoFo (my gasifier "went live" 1-15-09), but here is what I have found so far-

with my medium quantity of well dried hardwood (sugar maple and cherry) that I started the season with, my Econoburn 150 seemed pretty flexible with sizes of wood

With some of the other wood that I have been burning since mid-February, and that was cut in summer 08 but not split or gotten out of the woods until Fall 08 (not ideal, but factors beyond my control got in the way), and even though these are generally pretty fast-drying species (white ash, sugar maple, red pine) the size of the wood makes a MUCH bigger difference.

With that less than ideal wood, limbwood and things that are split so that no single dimension exceeds about 4-5 inches seem to work best.

The gasifier's taste in wood seems nearly the opposite of my old wood/ hot air furnace. With the old unit, once a fire had been successfully started, it seemed to do best with the biggest chunks of wood that I could handle- the really big stuff maintained the steadiest, longest, and most efficient burn. My Econoburn seems to yield more heat, and a better/ more efficient/ longer lasting gasification with small to medium wood.

At least based on my learning curve so far, gasifiers really sing when you quickly attain and consistently maintain a bed of coals in the bottom of the firebox-- and small to medium wood does a good job of getting there quickly, and continuing to collapse into that pile of coals, whereas bigger pieces of wood end up simply sitting there as smoldering logs.

Also, FWIW, the "net heat" that I get out of a big armload of small logs burned well in my Econoburn dwarfs the net heat that I got out of a wheelbarrow full of bigger logs in my old wood/ air unit. So it's not just about burn time, it's about overall efficiency, which is where storage comes in (to serve as the flywheel so that "burn time" becomes less of a focus).

 

[pybyr]

P.S.- an interesting thought about gasifiers/ wood size

in the US, we are used to thinking of split cordwood as fuel-- trees are allowed to grow big, then significant-sized areas are cut down simultaneously, cut up, split, and trucked moderate distances; then the cycle starts over, and especially unless the just-cut area is thinned as it goes back through succession into mature woodland (and it's seemingly the exception that anyone comes back to manage it that well), you get a lot of crowded growth that will take quite a while to develop into a healthy stand of what folks consider "firewood sized" trees.

Given the gasifiers' ability to thrive on wood that is smaller than US firewood norms, potential seems to abound to learn more about "coppicing" as practiced in Europe, which seems (from what I know so far, which is not a lot) to yield more biomass, more quickly, with less impacts, while doing good things for habitat and biodiversity.

 

[Piker]

It's kind of a catch 22. The more wood you put in the burn chamber of a gasser that is going to idle, the less efficient it's likely going to be. But if you need long burn times, 5" or 6" splits stacked neatly and tightly will probably get you the best burn times, at least in an Econoburn. Larger wood tends to stack up leaving great gaps of airspace between pieces that could otherwise be filled with wood... and if you put too big a piece of wood in, you're likely to get some fashion of bridging occur. On the other end of the spectrum, mass quantities of very small pieces of wood could induce an overheat, especially if there is no barometric damper installed.

Again, I really think storage is the way to go. Specifically pressurized. I have been strongly recommending to my customers that they at least install a provision for storage in their piping, because someday they will probably want it. Storage has just a fantastic way of doing away with the whole "burn time" issue. can't wait to get my own piped up.

cheers

 

[cguida]

Thanks very much Mr Fossil and Pybyr

When I get around to the softwood, I'll split it alittle coarser than the hardwood I'm working up now. Given that the wood will be only a year old and not two, and applying the Pasta Standard, I figure I better go for Angel Hair instead of Thin Spaghetti.

Has anyone ever seen a situation where the wood-gas/air mixture was so rich it didn't ignite, or failed to completely combust? Hard to imagine such a thing, but I guess it is theoretically possible.

Has anyone tried intalling a lambda sensor just to see what kind of readings it produced, and what that indicated about the state of the fire in the stove? Seems like that would be a first step toward retrofitted lambda control.

About European stoves and European wood -- somebody correct me if I'm wrong, but it must be that most of Europe just doesn't have big wood like we do. If they did have big wood, they'd probably be exporting Outside Wood Boilers. But as it stands, they are under more pressure than we are to get the most "milage" out of their forests.

 

[tom in maine]

This is a fun question that we all work through with our own assortment of hardware and fuel.

It seems that the bottom line on stick wood burning is that it is an issue of carburetion. We need to properly mix air with wood.
The wood is a big hunk of fuel that does not mix too well with air, until we gasify it by whatever scheme works best.
We also have to remove the heat so it can do useful work.

A lot of these variables can be handled by a lambda controlled device or user interface. Are we all not lambda controllers, after all!

I have been playing with a vertical fed wood boiler for my shop that burns sticks that stand on end. This is what Dick Hill did
back in the 70's. It is interesting to see how his basic concept has evolved into the current state of design that are currently being built in Europe
and in this country by Econoburn and Garn.

Anyway, every time I fire up the unit, it is interesting to sort out the wet from the dry and the small sticks from the big ones.
They all do burn eventually. And we are the carburetor controls!

 

[pybyr]

About European stoves and European wood -- somebody correct me if I'm wrong, but it must be that most of Europe just doesn't have big wood like we do. If they did have big wood, they'd probably be exporting Outside Wood Boilers. But as it stands, they are under more pressure than we are to get the most "milage" out of their forests.

If, as they say, necessity is the mother of invention, then scarcity is the taskmaster of efficiency.

That said, why burn large quantities of something at low efficiency/ high emissions when you could burn smaller amounts with high efficiency/ low emissions- unless you've just been stuck in "that's the way we've always done it"

 

[pybyr]

This is a fun question that we all work through with our own assortment of hardware and fuel.

It seems that the bottom line on stick wood burning is that it is an issue of carburetion. We need to properly mix air with wood.
The wood is a big hunk of fuel that does not mix too well with air, until we gasify it by whatever scheme works best.
We also have to remove the heat so it can do useful work.

A lot of these variables can be handled by a lambda controlled device or user interface. Are we all not lambda controllers, after all!

I have been playing with a vertical fed wood boiler for my shop that burns sticks that stand on end. This is what Dick Hill did
back in the 70's. It is interesting to see how his basic concept has evolved into the current state of design that are currently being built in Europe
and in this country by Econoburn and Garn.

Anyway, every time I fire up the unit, it is interesting to sort out the wet from the dry and the small sticks from the big ones.
They all do burn eventually. And we are the carburetor controls!

As someone who spent a lot of time with and had a good knack for carburetors when they were still around, that's a fascinating observation.

 

[gorbull]

What about the length of the cordwood, does that have any bearing on the efficiency of the burn? Just wondering if it is worthwhile to invest in a bigger unit that can take 30-34" pieces?

Thanks.

 

[Huskurdu]

What about the length of the cordwood, does that have any bearing on the efficiency of the burn? Just wondering if it is worthwhile to invest in a bigger unit that can take 30-34" pieces?

Thanks.

Econoburn makes a 1,000,000 btu boiler, it handles 46" logs, but where does cost override fewer loadings?

np

ps... if you get one of those million btu units can I come over and hug it?

np

 

[jebatty]

Like most who have burned for two or more seasons, I have settled on 3-5" splits + 2-5" round wood (branches, generally), well dried. Let the gasser burn full bore, about 4 hours of good burn with some coals left to turn to dust, and capture the full output with zero idling to storage. And if more heat needed, reload when the first load has burned down pretty close to low coals.

The coppice concept has lots of merit. If we are growing wood fiber to burn in a gasser, then it makes no sense at all to grow big trees, except to use the branches as fuel and sell the round wood for lumber, chipping for strand board, pulp, etc. It does make lots of sense, though, to grow willow, maybe hybrid poplar, alder, young aspen and other quick growing brushy type material that produces 2-4" stems, or to use unmerchantable thinnings from a timber stand improvement. A well managed gasser "forest" probably can be rotated on a 5 year cycle or less, and very little land is needed to produce a continual fuel supply.

Although we have plenty of land and trees, we do not cut trees for the gasser. The gasser gets fed with split slab wood, branches, and split round wood that is of such low quality it can't be used for anything else. The trees are cut for lumber or we sell a stand to a logger, and then scavenge the leftover slash for gasser fuel.

 

[SolarAndWood]

It does make lots of sense, though, to grow willow, maybe hybrid poplar, alder, young aspen and other quick growing brushy type material that produces 2-4" stems,

We live a 1/4 mile from SUNY ESF's experimental willow fields. Maybe I should offer them the opportunity to use my new boiler instead of scrounging this year...

 

[cguida]

No Kidding!

I have to go out and tend sheep at the moment, but when I get back I'll try to find the video of the SUNY coppice demo, with the giant mechanized catipillar eating its way through the stands...

 

[SolarAndWood]

The last time they harvested, they used what looked like an older smaller combine with a custom head on it. The field went down fast.

 

[Fred61]

I've found over the years that un-split wood dries real slow. When I have wood that is too small to split, I usually run the tip of my chain saw the length of the piece causing a gap in the bark. It's quick and easy to do. After the wood has been stacked and dried for a period, I'll usually find that the piece has split directly under the cut.

 

[Nofossil]

Is there a connection between split size and burn time in a gasification boiler? For example, if you had larger wood (didn't split it so fine), would you get a longer (and more efficient) burn time? Or wouild you just get more creosote?

It's probably worth mentioning that in most cases, with the same primary/secondary/fan settings, longer burn times are LESS efficient in a gasifier. This isn't entirely true in all cases, but if you use large green pieces of wood you'll get longer burn times - not better efficiency. By the same token, if you aren't carrying the heat away as fast as it's being produced, the boiler will go into idle. Same result: longer burn times AND lower efficiency.

As far as I can determine, the only way you get longer burn times and higher efficiency is if your initial settings involved too much primary air so that you're generating too much wood gas. That can result in incomplete secondary combustion and higher flow velocity through the heat exchangers, both of which can reduce efficiency. If you have that problem, changing the adjustments could result in longer burn times and higher efficiency.

 

[Como]

We went to see some National Trust properties Wife wanted to see th houses, then I found out about what they were doing with biomass:

http://www.nationaltrust.org.uk/main/w-global/w-news/w-news-further_news/w-news-enviro-renovate.htm

So the wife was taking photo's of the houses and I was taking photo's of their installs. Most of the properties have large areas of woodland that have to be managed so its a win win. They were chipping everthing, which makes sense for them.

 

[Trex83]

No Kidding!

I have to go out and tend sheep at the moment, but when I get back I'll try to find the video of the SUNY coppice demo, with the giant mechanized catipillar eating its way through the stands...

Hi guys,
Late post, I know...
Does someone has the link to the video or the contact for that SUNY campus. I am writting a proposal to study practical considerations of burning purposely grown biomass in downdraft gasifier. There are plots in QC that I will look into also.
Thanks
Trex83

 

[Como]

This does not directly answer your question, but I remember some years back coming across a site for an Irish company that offered a complete package of equipment and fuel and sold the energy. I remember they had contracts with local farmers to provide the biomass from cultivated woods.

 

[Reggie Dunlap]

My Tarm seems to like smaller splits roughly 19" long. My experience is that finely split wood dries a lot quicker than big chunks due to size and less bark to hold the moisture in. Smaller splits are also easier to handle and stack better than big chunks.

As for burn time I usually don't worry about it too much. With storage i don't really care how long my boiler runs, as long as those BTU's are heating my storage tank. The only time I care about burn time is when it's 20 below or colder and I need to keep the boiler going almost full time to keep up with heat demand.

 

[Eric Johnson]

I cut whole trees, so I have a nice mix of wood of different sizes and split configurations. I generally try to use smaller pieces when starting a fire, graduating to larger chunks once a nice bed of coals develops later in the burn. On cold nights or when I want a longer burn, I put in big, unsplit rounds if I have them dry, and turn down the blower speed. I'm sure you can dope it out scientifically, but I've found that experience with your unit and the kind of wood you typically burn is a pretty reliable guide.

 

[TCaldwell]

As discussed above, split size and mc are variables that impact the performance of each boiler differently. The physical boiler combustor and airflow design coupled with control algorithm if any really determine what size split and mc is efficient for a given boiler. Lambda or o2 is really only feedback to a controller, it is up to the controller algorithm to calculate a output for fan or damper control. The setpoint you choose will also determine how long and how hot the fire will burn, a lower sp=shorter and hotter and vice versa. Really many obvious tactiles, sound, smell, flue temp ect. will tell as much as expensive controls, trust me on this!

 

[TCaldwell]

the general answer for a longer burn time would be larger dry split hardwood, smaller splits even at a higher mc will not last as long or burn as hot. Dry smaller splits will burn fast/hot however are more of a challenge for boiler to effiecently control, and begins to puff.

 

[heaterman]

Very good discussion that really relates to how best to use the concept of gasification. As anyone can read in the above posts there are 2 things that stand out. The first being that best practice is to burn the wood hot and fast with no idling. Just load it and let it rip. This of course points directly to the next item of business and that is what to do with excess heat generated which of course means storage. Get your mind and your equipment wrapped around those two things and practically any gasser with decent wood will very efficient.

The analogy of the carb is very accurate. Wood gas has to thought of the same way as fuel in your vehicle. If there is more fuel available than there is air to burn it you get a dirty, over rich fire with lots of wasted fuel (smoke and particulates) going out the stack. If you lean if the "mixture" and provide too much air for the fuel (wood gas) being generated, you can lose gasification and decrease efficiency at that end of the combustion spectrum also.