Wood Burn Rate

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Snail

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Jul 8, 2011
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New Zealand
Does anyone know of a source of information regarding the variables that control the rate at which wood fuel will burn and how to calculate that burn rate if those variables are known? Also, what controls the minimum possible burn rate achievable by limiting the primary air to just enable continuing gasification, i.e. the file is just self-sustaining?

I have a free source of wood that is unfortunately all mixed sizes, from normal splits to almost twigs. I would like to get a handle on how much faster the smaller bits could be burning and if this can damage the combustion chamber if I overload it with such small stuff.

Aside from that, the information would just be interesting to know!
 
Off hand, there is a ratio for lbs of air required per lb of wood at stiochiometeric or complete combustion. Then that based on your boiler or stove combustor will also require primary/ secondary division. I wish I could be more helpful but the right google search should work
 
Tuning a wood boiler for a specific burn rate would be impossible due to all the variations in the fuel. Even if all pieces of wood were the same size when loading, shortly after commencement of the burn everything changes from size to moisture content. Not like oil which stays constant and is metered. The one thing that is metered in your boiler is the air mixture so those twigs are only going to burn as fast as the availability of oxygen allows.

Go ahead and feed your boiler what you have but I think that over time you're going to decide that it's not worth your while to bend over to pick up pieces that start approaching twig size.
 
I don't know if I can adjust my burn rate? But I can set my fan speed to start at a % of 100 % plus or minus of 100%. With it starting lets say -2 setting the fan will modulate lower than if I started at 100%.
 
I believe most gasification boilers will burn wood at a constant rate of X lbs per hour. According to the manual for our old Jetstream that rate is 25 lbs per hour.
The most important factor related to output is the moisture content of the wood. Burn green or wet wood as opposed to seasoned or dry wood and your boiler hourly output can be cut by 50%!
 
Most gasification boilers are not going to be very happy trying to burn a load of twigs! In our boiler the burning of a load of twigs would get very exciting. The burning of a load of twigs would generate more smoke than the oxygen available to burn the smoke, momentarily suffocating the fire and reigniting with a explosion when enough oxygen was again available to support a fire. This condition is known as puffing and in some cases the explosions can be come quite violent!
 
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Hobbyheater, do you know of a link where the puffing phenomenon is explained? I would have thought that with a gasifier, you would have enough controls to prevent that being a problem? Doesn't the smoke burning happen in the secondary chamber? And wouldn't reducing the amount of wood per charge control the problem completely, albeit with the downside of having to make repeated reloads?

Fred61, I have to pick up the "twigs" anyway to keep the place tidy and our local rubbish disposal charges a frightening amount to dispose of them if I don't burn them. We are retired and have more time than money, so I just look at it as good exercise.

I can manage the heat levels OK with an attended wood stove in the lounge( If I am careful that is, I warped one stove over-firing it with small wood). However, I also want to have an unattended furnace in the basement, hence my questions.

The basement heater will heat a conditioned crawl space. This would solve a cold floor problem with our exposed wooden floors and just take the chill off the whole house in the evening, supplemented by spot heat in living areas. This system seems to me to be about the most affordable approach, given a mild climate but an air- leaky, poorly insulated light timber construction house.
 
Snail said:
Hobbyheater, do you know of a link where the puffing phenomenon is explained? I would have thought that with a gasifier, you would have enough controls to prevent that being a problem? Doesn't the smoke burning happen in the secondary chamber? And wouldn't reducing the amount of wood per charge control the problem completely, albeit with the downside of having to make repeated reloads?
.
Click to expand...

The Term (Puffing) from what I have seen only exists on this forum. The Jetstream does not use secondary combustion. It was designed to burn clean at high temperatures with no need for secondary combustion. The burn chamber and entire refractory base weighs in the neighborhood of 1,400 lbs. So picture this large mass of refractory at 2000 F and introduce a large amount of small dry fuel which will generate more smoke than can be burnt with the available oxygen, in turn momentarily extinguishing the fire and when enough oxygen is again available to the burn chamber, there is a gas explosion. The gas is the smoke and Gasification is a simple term for the burning of this smoke.
The recommended fuel moisture content for the Jetstream is between 17 and 20 percent fuel moisture content. The fuel moisture content of the firewood which I burn is below 7% so to prevent puffing and still maintain a clean burn, I do reduce the fuel charge which does result in more frequent loading of the burn chamber as you have suggested.
 
I have seen the term "whuffing" used to describe what is called puffing on this forum. Hobbyheater, I gain the impression from your explanation that a firebox with a large thermal mass will make the problem more likely, is that so? Elsewhere the problem seems to be linked to rapidly closing down the air supply on a very fast-burning fire. In both cases the problem arises from a lot of heated wood surface with insufficient air supply to support combustion of the resulting copious gasification?

With the Jetstream you have a stick feed system, which I am considering. So the "fuel charge" at any one time is not the total charge in the stove but only the amount in the combustion zone, it's a semi-continuous feed system, unlike other furnaces which are basically batch processors. I have always wondered if this gives a long period of fairly steady state combustion, unlike most systems where conditions change throughout the burn, from gas burning to charcoal burning.

Would using a taller feed tube and longer sticks solve the problem of shorter burn times with your system?

How much do you need to cut down the cross-section of the fuel feed when you get thinner sticks, in order to keep the same burn rate and air requirement? E.g. would feeding 6 sticks of 1.5 inch diameter timber, deliver the same heat and have the same air demand as 3 sticks of 3 inch? That would be only half the cross-section area feeding through but the same circumference. Or have you found some other rule best to apply?

7% moisture? That's low! You must have very low humidity levels in your area.
 
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Snail said:
.
Click to expand...

We live on the coast high humidity.
On the fuel moisture - the wood is split and air dried under cover for 2 years then 2 1/2 cords is moved into the boiler room which can obtain temperatures of 100 F or better after a 6 -7 hour batch burn. After several months in the boiler room, the wood's moisture content will not read on the moisture meter. The boiler room is separated from the house by a concrete fire wall and door so no problem with excess moisture in the house.

According to the owners manual, a full charge of hardwood will yield a 4 hour burn. But only soft wood is available in this area.

A taller loading tube would make a big difference.

I split our wood so that three pieces in the fire box would be ideal, but when the wood gets really dry, I only burn 2 at a time. The BTU output remains the same but the feeding interval is more frequent.

On the puffing. Our home was rented out for a long period of time and our last set of tenant attempted to burn old cedar roofing shakes for fuel. The result was the loading door was blown or shattered off the loading chamber and the refractory base required major repairs.
Several years ago I was looking into purchasing a newer boiler. Most of the manufacturers and knowledgeable dealers that I talked with said the super dried wood would represent a problem for most of the units that I inquired about. Two years ago the problem of a new boiler was solved. I found a brand new Jetstream boiler; never used and still its original shipping pallets. I'm currently using unit #528 with 30+ years of service and have unit #175 for a spare.

Recommended wood sizing.I base this sizing on a 12 " round .
pg 14.jpg

Jetstream drawing.jpg
 
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Thanks Hobbyheater. Can you give the approximate dimensions of the sticks please, what are the biggest and smallest cross-sections you use? And how many of each size would you put in?
 
The section of the Jetstreams loading chamber the steel section that passes through the heat exchanger is 12 inches in diameter . So as I'm splitting a 12 inch round I want 3 to 4 splits from that round , so when splitting larger rounds its just a case of eyeball to split the right size , have never used a tape measure .

Ideal wood length for the Jetstream is 32 " , the splitter will do 26" , but our pickup can only pack 3 rows of 22" length .

These rounds are 22" in length .
IMGP3574.JPG
Another shot of split size .
IMGP3575.JPG
Maybe the saws on the splits will give you an idea of the size .
IMGP3584.JPG
 
Thanks Hobbyheater. Your nom-de-plume seems a bit misleading though. The photos show a pretty impressively professional setup, way beyond a mere hobby!
 
I grew up the youngest of seven, my mother passing before I had any memory of her and by the time I had started school, all my older siblings had left home. My Dad was the Steam Engineer and Millwright at this 1880s style sawmill with a waste wood- sawdust fired steam boiler. I just could not spend enough time with Dad in that Boiler Room and would have loved to have been a steam engineer but the new boilers just lacked the charm.
Picture taken in 1958 the year the boiler was condemned!
Broughton Lumber And Trading Company,Telegraph Cove, BC
Telegraph Cove (7).jpg
 
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Lovely photo! I must go and visit our local steam museum again next "live steam" day. A few years ago, we rented an old farm house in the back of Taihape. The farm had a small sawmill of about the same vintage as your one was. It still works and is in occasional use. The original power was a kerosene engine rather than steam though. That still worked too, although the day to day power is now an agricultural tractor of 1950's vintage, mounted on blocks.

That 100 year old homestead was cold. 500m altitude makes a difference when the house is as heat leaky as that. Mind you, in comparison with an original homesteaders cottage/shack that is still standing nearby, it would have been the height of luxury. Those settlers were tough!
 
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hobbyheater: I believe most gasification boilers will burn wood at a constant rate of X lbs per hour.
Click to expand...
Don't you suppose this is related to the rated btuh output or burn rate of the boiler? A boiler rated at 50kw or 170,000 btuh would burn at a rate of about 28 lbs/hr.

According to the manual for our old Jetstream that rate is 25 lbs per hour.
Click to expand...
That would compute to about 150,000 btuh.

These computations are based on the assumptions of 20% MC and 400F stack temperature. Also assumed, I would think, is ordinary splits of wood. For the Tarm, about 6" on the high end, for example. my usual splits are about 3-6", some larger, but most splits are not uniform in size. Most 6" rounds I will split once, one dimension remaining about 6" and of course the other about 3". A 12" round normally would yield about 4 splits. I also will burn some branch rounds down to less than 3", but not often less than 2".
 
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jebatty said:
Don't you suppose this is related to the rated btuh output or burn rate of the boiler? A boiler rated at 50kw or 170,000 btuh would burn at a rate of about 28 lbs/hr.

That would compute to about 150,000 btuh.

These computations are based on the assumptions of 20% MC and 400F stack temperature. Also assumed, I would think, is ordinary splits of wood. For the Tarm, about 6" on the high end, for example. my usual splits are about 3-6", some larger, but most splits are not uniform in size. Most 6" rounds I will split once, one dimension remaining about 6" and of course the other about 3". A 12" round normally would yield about 4 splits. I also will burn some branch rounds down to less than 3", but not often less than 2".
Click to expand...


To be honest I've always just excepted the numbers as written .
I've often thought that the Jetstream gains an advantage in heat exchange because the heat exchanger sites totally on top of the refractory base (1,400 lbs)much like a kettle on top of the stove .That area of the heat exchanger that contacts the top of the refractory base is about the same as the surface area of the fire tubes.
In the summer time when I get a 6-7 hour burn with little draw from storage (1,000 imp. gal. ) the top of storage will peak at 195 f and the bottom 165 -170 F the circulator will be left running for another 3 hours after the fire is out and the bottom temperature will climb to 180-185 F with no little or no drop or rise in the top temperature.
The first hour of the burn cycle yields a tank rise of 3-4 F , hour 2 to 3 about 7-10 F after hour 3 I have seen as much as 15F rise .
Many things here that I do not understand .
I burn Douglas Fir or Hemlock with a moisture content of 7% or less.
 

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After the fire has been out for a couple of hours!
Shake bake chicken & spuds to be roasted!
100_0684.JPG
Real campfire flavor !100_0686.JPG
Next Rolls!
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Done100_0783.JPG

We get the maximum benefit from those stored BTU'S. This firewood thing can be a lot of FUN !;)
 
Sounds like you are one of the longest user off a Jetstream boiler,they were ahead of their time.Looks like we are the only users here.The inexperance and price scared me off in 1979 from buying one when I was building a new 2000 sq.ft.home.I have mine now 19 years and think they are great.Too bad knowlege and experance could be easier to come by.Oh,oh my wood supply looks like it's going to run out in this cold winter.Will be prepared next season.
 
A.MacLeod said:
Sounds like you are one of the longest user off a Jetstream boiler,they were ahead of their time.Looks like we are the only users here.The inexperance and price scared me off in 1979 from buying one when I was building a new 2000 sq.ft.home.I have mine now 19 years and think they are great.Too bad knowlege and experance could be easier to come by.Oh,oh my wood supply looks like it's going to run out in this cold winter.Will be prepared next season.
Click to expand...

There is another member in New Brunswick Slowzuki he joined Hearth in Feb 1 2007. I believe he has two units and possible a third . I Know of
one on Lasqueti Island and another in Prince George BC and two in Yukon. These owners are all very individualistic in there approach to keeping there boilers alive .
On wood storage I only have enough room to stay ahead three years.
 
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