Kiln dried Firewood - The future ?

[Logsnstuff]

Compared to what MC most sellers sell air dried at then a properly kilned product is undoubtedly superior.

 

There are sellers with air dried at around 10% at present, I am one, but this takes a long time to get there. I currently have imported kilned at well below that, over the winter though this will draw moisture and get somewhat damper. The other benefit of kilning is that it kills boring insects, with treated timber in houses these days it should not be a problem but woodworm in house timbers does not do them any good.

 

A

 

and you have the American study which says there is no advantage in a domestic stove burning below 15%mc timber because it can't cope with the increased gasses and they end up going up the chimney and polluting the air.

Page 20 http://www.epa.gov/burnwise/workshop2011/WoodCombustion-Curkeet.pdf

 

I tested a log at a stove shop it was kiln dried guy was claiming 2%mc and he stuck the meter on the end with little penetration to get his 2% when I put mine in the side I got 12%.

[openspaceman]

and you have the American study which says there is no advantage in a domestic stove burning below 15%mc timber because it can't cope with the increased gasses and they end up going up the chimney and polluting the air.

Page 20 http://www.epa.gov/burnwise/workshop2011/WoodCombustion-Curkeet.pdf

 

 

I haven't visited your link but I think it concurs with what Tom Reed found, essentially the enthalpy of vaporising that bit of water keeps pyrolysis in check, without it pyrolysis proceeds like a chain reaction and the offgas comes off in a peak which it is difficult to supply enough secondary air to.

[Alycidon]

Logs,

 

I was not aware of the study but that does surprise me somewhat. Generally the drier the better as less energy is wasted evaporating moisture so more energy available for conversion into heat. Seems I may need to do more research.

 

Openspaceman: The more modern stoves are now fitted with a terciery air system, this is an additional air supply coming into the rear of the stove above the fire through a row of holes usually about a third to half way down the rear firebrick, this is uncontrolled and fully open all the while. This would help burn excess gases as it doubles the air being admitted through the secondary air supply.

 

You still have the issue re boring insects, while not a problem in the winter they do emerge in the spring.

 

We live and learn.

 

A

[ash_smith123]

I can understand using one if your yard isn't big enough to hold stock, the problem I have is the people saying it's superior to air dried logs of the same mc or as I mentioned before those who say air seasoned to 30% whilst kiln dried is sub 20% and totally misleading the public.

 

 

How is that misleading the public? We are telling them exactly what we are selling. It's then upto the customer if they want to buy seasoned or kiln dried from me at av 30% or under 20% or look for another supplier?! Its up to you to market your product at the moisture content you are selling.

[openspaceman]

Logs,

 

 

Openspaceman: The more modern stoves are now fitted with a terciery air system, this is an additional air supply coming into the rear of the stove above the fire through a row of holes usually about a third to half way down the rear firebrick, this is uncontrolled and fully open all the while. This would help burn excess gases as it doubles the air being admitted through the secondary air supply.

 

First I'll make the point that this phenomena is not something that you'll notice in Britain, the equilibrium moisture content outside here is probably 16% and indoors in my home it seems to be 10% so there is always enough water to prevent this happening during a normal burn.

 

It is a problem with batch loading without some sort of feedback loop like a lambda sensor can provide. Something can, and does, happen with a stove left to smoulder, the solid lump of log effectively carbonises with no flame present the offgas fills the stove but is below its autoignition temperature, the temperature rises to the point where nearly all the oxygen is used up but the temperature is higher than the autoignition point of the offgas, then someone opens

s the door,,,

 

This actually took the life of one of the drivers who collected wood from us in the early 80s when he opened the door and inhaled the burning offgas. A similar deflagration took the roof off a school boiler room

 

The other thing is that the effect on heat efficiency of burning logs is not as significant as people expect, I posted figures in another thread to show the difference between 20% and 300% (Cornishwoodburner queried this in a PM so I rechecked my figures and I had made a mistake but it was in minor and in favour of the drier wood).

 

 

What I said then was that excess air was likely to be more significant in terms of extracting heat from the stove and it is this that limits efficiency in batch loaded stoves ( it's less of a problem with a woodchip stoker because the air and fuel ratios are fairly well maintained as constant. So most log burners run with sufficient excess air to provide enough chances of an offgas molecule meeting an oxygen molecule inside the combustion chamber. This means at other times, e.g. just prior to throwing a log on and just after there is far too much air passing through the system, all this air has to be heated up and as it is not necessary for combustion it passes through having a free ride but is ejected at flue gas temperature. If you consider a kg of dry wood only needs about 6kg of air to burn but you have to share all the heat produced by the ,now, mass-flow of 7kg with an additional 12 kg of excess air then you'll appreciate the waste.

 

Also this is where the moisture in the fuel becomes more significant than the energy cost of its evaporation, in order to burn wood the primary air has to burn char, to give heat which drives the pyrolysis and gasification, wet wood needs more primary air.

 

Now given that you need to meet the requirements of Time for the fuel to burn, high Temperature for the reaction to take place and Turbulence to mix the gases and make sure a fuel particle meets sufficient oxygen molecules you will see that increasing mass-flow, by the need for more air, actually reduces Time the air is in the burner, it lowers the temperature, because moisture has to be evaporated at 100C before anything else can happen plus all the excess air has to be heated up and although turbulence may even increase, especially with forced draught, any steam in the combustion chamber works to reduce the chance of offgas meeting an oxygen molecule.

 

 

 

You still have the issue re boring insects, while not a problem in the winter they do emerge in the spring.

 

 

This was very much one of the selling points of our high speed dryer which was designed to produce kindling in boxes for sale in a supermarket chain. You cannot have creepy crawlies emerging from the boxes when opened.

[R5log]

First I'll make the point that this phenomena is not something that you'll notice in Britain, the equilibrium moisture content outside here is probably 16% and indoors in my home it seems to be 10% so there is always enough water to prevent this happening during a normal burn.

 

 

 

It is a problem with batch loading without some sort of feedback loop like a lambda sensor can provide. Something can, and does, happen with a stove left to smoulder, the solid lump of log effectively carbonises with no flame present the offgas fills the stove but is below its autoignition temperature, the temperature rises to the point where nearly all the oxygen is used up but the temperature is higher than the autoignition point of the offgas, then someone opens

 

s the door,,,

 

 

 

This actually took the life of one of the drivers who collected wood from us in the early 80s when he opened the door and inhaled the burning offgas. A similar deflagration took the roof off a school boiler room

 

 

 

The other thing is that the effect on heat efficiency of burning logs is not as significant as people expect, I posted figures in another thread to show the difference between 20% and 300% (Cornishwoodburner queried this in a PM so I rechecked my figures and I had made a mistake but it was in minor and in favour of the drier wood).

 

 

 

 

 

What I said then was that excess air was likely to be more significant in terms of extracting heat from the stove and it is this that limits efficiency in batch loaded stoves ( it's less of a problem with a woodchip stoker because the air and fuel ratios are fairly well maintained as constant. So most log burners run with sufficient excess air to provide enough chances of an offgas molecule meeting an oxygen molecule inside the combustion chamber. This means at other times, e.g. just prior to throwing a log on and just after there is far too much air passing through the system, all this air has to be heated up and as it is not necessary for combustion it passes through having a free ride but is ejected at flue gas temperature. If you consider a kg of dry wood only needs about 6kg of air to burn but you have to share all the heat produced by the ,now, mass-flow of 7kg with an additional 12 kg of excess air then you'll appreciate the waste.

 

 

 

Also this is where the moisture in the fuel becomes more significant than the energy cost of its evaporation, in order to burn wood the primary air has to burn char, to give heat which drives the pyrolysis and gasification, wet wood needs more primary air.

 

 

 

Now given that you need to meet the requirements of Time for the fuel to burn, high Temperature for the reaction to take place and Turbulence to mix the gases and make sure a fuel particle meets sufficient oxygen molecules you will see that increasing mass-flow, by the need for more air, actually reduces Time the air is in the burner, it lowers the temperature, because moisture has to be evaporated at 100C before anything else can happen plus all the excess air has to be heated up and although turbulence may even increase, especially with forced draught, any steam in the combustion chamber works to reduce the chance of offgas meeting an oxygen molecule.

 

 

 

 

 

 

 

 

 

This was very much one of the selling points of our high speed dryer which was designed to produce kindling in boxes for sale in a supermarket chain. You cannot have creepy crawlies emerging from the boxes when opened.

 

 

Do you sale logs?

[richy_B]

This is quite a thread and I maybe repeating someone else but for me kiln drying offers the advantage of space. I operate in London where I do not have the benefit of a lot of area to have wood air drying. The kiln allows me to have 36 cubes of split wood waiting to go in, 12 cubes drying for a week which then generally sell out in the following days. I appreciate this doesn't result in a a huge turnover of product but without the kiln I'd have logs to dispose of instead of something to sell. There is also the key eco advantage of keeping a small loop, often less than 5 miles from tree to kiln to customers.

 

The future of firewood? I don't think so. Regrettably I think the future of firewood in the UK will be imported material from eastern Europe/Baltics.

[cornish wood burner]

How is that misleading the public? We are telling them exactly what we are selling. It's then upto the customer if they want to buy seasoned or kiln dried from me at av 30% or under 20% or look for another supplier?! Its up to you to market your product at the moisture content you are selling.

 

I think you have missed log's point. He is happy to market his wood but the problem comes with generalisations that slander the competitions product. His air dried may or may not be drier than your kiln dried, but would you be happy if your competitors advert said "air seasoned is better for the environment and also has a lower MC than kiln dried."

[Logsnstuff]

I think you have missed log's point. He is happy to market his wood but the problem comes with generalisations that slander the competitions product. His air dried may or may not be drier than your kiln dried, but would you be happy if your competitors advert said "air seasoned is better for the environment and also has a lower MC than kiln dried."

 

that's exactly my point.

[Logsnstuff]

one of the reasons I never pursued the kindlinginabox idea was the creepy crawley factor that's the only reason I would consider a kiln in the future. it has left me with a 1,000 boxes sitting taking up space for the last 5 years.

 

This was very much one of the selling points of our high speed dryer which was designed to produce kindling in boxes for sale in a supermarket chain. You cannot have creepy crawlies emerging from the boxes when opened.