Some on this site have said the new gasifiers can burn close to 50% less wood than the standord OWB. Does most everyone beleive this to be true? Also, does the wood really need to be seasoned hard wood? Thanks.
Do the new gasifiers really burn 1/2 the wood
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WoodNotOil
Minister of Fire
I have a tarm and my father has an OWB. Our experience is that I burn about a 1/3 of the amount of wood he does, but his house is larger than mine too. So 50% sounds right on to me. Hardwood has more btu's. Any dry wood would theoretically work, but you would burn more softwood to get the same required btus.
Nofossil
Moderator Emeritus
I think there's no doubt. OWBs have been tested to yield efficiencies around 25%. Gasifiers run around 85% to 90%, both as tested by independent labs. In both cases, this is efficiency of the boiler itself. OWBs can lose more in the underground piping, and any heat lost from the boiler itself heats the outdoors.
OWBs are designed to operate at or near idle much of the time, and are typically installed that way. A traditional (non-gasification) boiler can attain much higher efficiency if it burns well seasoned wood and is run flat out - with no idling.
Here's a baseline for you: My brother ran a traditional indoor wood boiler as efficiently as possible for several years. He burned only well seasoned hardwood, and kept careful records. He weighed the wood that he burned, and correlated it do degree-days. He always burned flat out, heating a large radiant slab and a small amount of storage.
When that boiler finally gave out, he replaced it with a gasifier. His wood consumption dropped by 40% - to about 3 cords per year, here in Vermont. The boiler is his only heat source.
His house is much better insulated than most, but he has very detailed records of wood consumption that give me very high confidence that a gasifier will use substantially less wood than even the most optimally operated conventional boiler. OWBs are not typically in that last group, so I would expect the difference to be even more dramatic.
In my case, I don't have any history of wood consumption before my gasifier. However, I'm heating a 3500 square foot house including a large hot tub and 100% of my domestic hot water from mid-October through the end of April with just over four cords of less than ideal firewood.
The wood does not need to be hardwood, but it does need to be at least somewhat seasoned, and drier is better. Moisture interferes with the secondary combustion by cooling it too much.
OWBs are designed to operate at or near idle much of the time, and are typically installed that way. A traditional (non-gasification) boiler can attain much higher efficiency if it burns well seasoned wood and is run flat out - with no idling.
Here's a baseline for you: My brother ran a traditional indoor wood boiler as efficiently as possible for several years. He burned only well seasoned hardwood, and kept careful records. He weighed the wood that he burned, and correlated it do degree-days. He always burned flat out, heating a large radiant slab and a small amount of storage.
When that boiler finally gave out, he replaced it with a gasifier. His wood consumption dropped by 40% - to about 3 cords per year, here in Vermont. The boiler is his only heat source.
His house is much better insulated than most, but he has very detailed records of wood consumption that give me very high confidence that a gasifier will use substantially less wood than even the most optimally operated conventional boiler. OWBs are not typically in that last group, so I would expect the difference to be even more dramatic.
In my case, I don't have any history of wood consumption before my gasifier. However, I'm heating a 3500 square foot house including a large hot tub and 100% of my domestic hot water from mid-October through the end of April with just over four cords of less than ideal firewood.
The wood does not need to be hardwood, but it does need to be at least somewhat seasoned, and drier is better. Moisture interferes with the secondary combustion by cooling it too much.
muncybob
Minister of Fire
On the subject of dry firewood...I noticed a newspaper ad yesterday for kiln fried firewood. I would imagine it really doesn't matter how the wood is dried(kiln, naturally seasoned/dried, etc) as long as the mpisture content is low? I'm sure kiln is not optimally cost effective but it may be a choice for me for the first heating season.
webbie
Seasoned Moderator
I would agree that 1/2 is a conservative figure. The new efficient boilers use almost 1/2 that of an INDOOR older generation boiler - and OWB are quite a bit less efficient than indoor standard wood boilers. Consider the size of the firebox alone - a small OWB has a box about 4-5x the size of an efficient indoor boiler....and while an OWB may have a longer burn time, that still means vastly more wood used....or, on the other end it means you are smoldering your wood and getting very low efficiency.
The new EPA OWB will be MUCH more efficient than the older ones and they are hitting the market now.
The new EPA OWB will be MUCH more efficient than the older ones and they are hitting the market now.
Eric Johnson
Mod Emeritus
Kiln dried firewood tends to be very pricey. Typically, it's sold in small quantities to upscale customers, i.e., ski resorts, etc. But no matter how you dry it, dry wood is dry wood. The kiln just cuts down on the drying time involved and produces a cleaner-looking product, free of bugs.
heaterman
Minister of Fire
2theHunt said:
Case and point.
We installed a Garn for a customer last year who had previously been heating his old 3,000 sq ft farmhouse and DHW with an OWB. A Hardy to be specific but the brand does not matter. Every independent test I have knowledge of shows <45% efficiency........but I digress, back to the story. During the project this customer ADDED 4,800 sq ft of well insulated, radiant floor heated space, with 16' ceilings no less and he reported that his wood consumption was about 2 full cords LESS for the winter than usual. He doubled his heating load and used about 15% less wood. Go figure.
The people who don't believe the reduction in wood use are ignoring the facts or have something else to sell. Simple as that.
Dry wood makes a huge difference in ANY AND ALL wood burning stoves, boilers, furnaces or whatever. The wood burning industry in general and thousands of customers have been done a major disservice by the OWB industry claiming that you can burn green wood.
A manufacturers statement claiming the you can burn green wood is nearly akin to someone telling you that you can pick up a hammer and smash your other hand with it. Yes both can be done but why would you do either? It's beyond belief to me that anyone would not realize that burning green wood wastes half of it's heat content by using it to evaporate moisture rather than heating your space.
Green wood can be burned. However it is the amount of heat to evaporate the water that reduces the efficiency. Burning green would is for the emergency or the lazy!
Eric Johnson
Mod Emeritus
It would be hard to find a better return on an investment (and a guaranteed one at that) than buying green firewood and letting it dry for a year.
muncybob
Minister of Fire
Eric...no comment on "kiln fried" wood?? I just realized my mistype...kinda funny
I bet fried wood should burn faster! Yes, it's pricey @ $25/chord(face I would imagine) but it's an option unless I can find enough standing dead trees this fall for my first wood heating season.
I bet fried wood should burn faster! Yes, it's pricey @ $25/chord(face I would imagine) but it's an option unless I can find enough standing dead trees this fall for my first wood heating season.When I bought my GW, the sales guy was careful to say that it would burn higher MC wood, but he didn't recommend burning GREEN WOOD. On idle, the GW will 'bake' the moisture out of wood as long as it is up off the coal bed. But if it's cold out, ya definitly want avoid the wetter stuff. I have never measured the MC of any of my wood. I try to get only oak that has been cut down at least a few years ago.
But this kiln-fried wood is intriguing. . . sounds like the deep-fried Twinkies from a few years ago . . . .
But this kiln-fried wood is intriguing. . . sounds like the deep-fried Twinkies from a few years ago . . . .
it is simple math
Vaproization of water reqires 540 calories per gram of water. How many grams of water is in a wet log?
Since vaprization is a cooling process, that heat is truly lost.
Vaproization of water reqires 540 calories per gram of water. How many grams of water is in a wet log?
Since vaprization is a cooling process, that heat is truly lost.
webbie
Seasoned Moderator
Ah, but the Greenwood "bakes" it off instead of burning it off.....
I think a more accurate way of saying this might be that "some boilers will tolerate slightly wetter wood better".......meaning you can hit your hand with the hammer, but the hammer has a flat head as opposed to a "waffled" head (like a framing hammer).
I think a more accurate way of saying this might be that "some boilers will tolerate slightly wetter wood better".......meaning you can hit your hand with the hammer, but the hammer has a flat head as opposed to a "waffled" head (like a framing hammer).
Nofossil
Moderator Emeritus
I think it's fair to point out that while any boiler will be less efficient burning green wood, gasifiers will be MUCH less efficient. If the wood is too green it will extinguish the secondary combustion, making gasifier into a non-gasifier.
I suspect that the gasifier would be no less efficient than any other boiler burning green wood. However, if the wood is dry enough it can be much more efficient than ordinary boilers.
Green wood might drop an ordinary boiler from 50% to 35%. It would likely drop the gasifier from 85% to 35%.
My experience is that you don't want wood any wetter than about 30% to 35% MC, and you need some wood at 20% to get the secondary combustion started. Well seasoned firewood is around 20% - a bit lower if it's stored in a dry or covered location.
I suspect that the gasifier would be no less efficient than any other boiler burning green wood. However, if the wood is dry enough it can be much more efficient than ordinary boilers.
Green wood might drop an ordinary boiler from 50% to 35%. It would likely drop the gasifier from 85% to 35%.
My experience is that you don't want wood any wetter than about 30% to 35% MC, and you need some wood at 20% to get the secondary combustion started. Well seasoned firewood is around 20% - a bit lower if it's stored in a dry or covered location.
webbie
Seasoned Moderator
With the Tarm, I found that 25% or less was about the breaking point. I didn't try mixing to solve this.....one year I set up at the Trade Show.....and they supply the wood there (at the outdoor burn area). The wood tested at over 25%, and so I have to do some splitting and get a decent bed of coals before the boiler would act correctly.
muncybob
Minister of Fire
I understand why gasification boilers would use less wood for the same amount of heat as non gassers. My questions is if a gasser goes out to the point of needing to start a new fire due to prolonged period between loads how long (on the avgerage because I know there are some varialbes) would I expect it to take before a new fire is to the point of gasification again?
Nofossil
Moderator Emeritus
kooP said:
Nope - 970 BTU/lb to vaporize, 144 BTU/lb to thaw. Boiling is the really big one.
If a log weighs 20 pounds and is at 50% moisture, then it will require 9700 BTU to boil (or bake) the water before combustion can take place, The remaining 10 pounds of wood has about 86,000 BTU of heating value. Subtracting the 9700 btu, that means you could get at most 76,300 BTU. The energy loss due to boiling the water in this case is 11% of the available total.
In any wood burning appliance, the water vapor will also dilute the air surrounding the combustion zone, reducing the available oxygen and leading to more incomplete combustion. 10 pounds of steam is more than 250 cubic feet. This is almost equal to the amount of air required to burn the remaining wood, so oxygen levels would be reduced by almost 50%.
nofossil- Besides being a Pyro Extraordinaire, what kind of engineer are you. I really enjoy your posts.?
Adios Pantalones
Minister of Fire
On kiln-dried wood- it's very expensive- prohibitively so, IMO, for someone using wood as a big chunk of their heating in cold climate. It does reduce bug issues and dries the wood before any mold etc can start. If used in a short time, it is probably drier than air dried wod, but will; eventually equilibrate to ambient moisture.
This ws important to me in making bows- the lowest (and optimum) moisture content of any wood should be 6% to remain flexible. This was acheived at 30% RH air, regardless of temperature.
This ws important to me in making bows- the lowest (and optimum) moisture content of any wood should be 6% to remain flexible. This was acheived at 30% RH air, regardless of temperature.
Nofossil
Moderator Emeritus
chuck172 said:
By education, I'm a mechanical engineer. I went over to the dark side in 1980 and started doing computer hardware and software. I've also done systems engineering in aerospace. I now work as a consultant on business processes. I do embedded system design in a vain attempt to stay somewhere near current. I'm happy if my ramblings are helpful or at least amusing.
Nofossil
Moderator Emeritus
Nofossil
Moderator Emeritus
kooP said:
I've gotten much worse things than that backwards. I'm still struggling with understanding what really happens in the primary and secondary combustion zones. A while back, Tmonter posted some information on a very comprehensive analysis of combustion and system efficiencies. I've taken that and I'm working on applying it to residential level gasifiers.
In my case, between 55% and 60% of the theoretical heat energy from the wood that I burn is delivered in a measurable way to my heat loads. About 12% is vaporization and other 'up the stack' losses.
About 30% of my heat energy is unaccounted for. Some of it is leaking into the house directly from the boiler and plumbing, but I suspect there's more to be found.
Some days I feel pretty bad that I'm misplacing that much heat. On the other hand, I seem to be burning less wood than most for the size heat load that I have, so I must be doing something right.
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