Apparently logs can be too dry

[daveatdave]

I thought I was: thumbup:

supposed to be a smiley on the end

[openspaceman]

I've read that too dry wood creates smoke before, but the reason given was weak. The reason is, very dry wood burns quickly so users often restrict the air to the stove too much in an attempt to slow the burn. So the faulty logic, very dry wood = slumbering=smoke therefore dry wood = smoke. What tosh. Like much of that article I fear.

 

My theory is a bit complicated to explain but if you consider the "smoke" created by the heated log is in fact gases and vapours from the pyrolysis which then has an inadequate supply of air OR becomes too cold to burn out completely then that forms the products of incomplete combustion which are smoke. Smoke is an aerosol of soot and condensate droplets so if there is a lot it appears white (as does milk) or if less it looks blue because of the way the droplets/particulates split the light (which presumably why the sky is blue)

 

I can attempt to explain my theory how a little moisture might modify the burn to avoid the sudden evolution of this offgas should you wish.

[neiln]

Go ahead.

[Alycidon]

I think a lot of the results of thesa science experiments/tests are skewed to suit the persons way of thinking.

 

Commen sense tells me that the dryer the wood the more efficient the log burner is all secondary gas are burned off.

 

Agreed, ask any stove manufacturer, all want a maximum moisture level of 16%, Morso is 15%. The drier the wood the more heat is generated as less energy is used to evaporate the water remaining in the log. These also burn far cleaner and put less soot and tars into a chimney or flue. In practise softwood can go a bit wetter ( up to about 20%) and get similar results.

 

All stoves sold in the UK have to be CE marked and emissions compliant. The last lot of emissions regs was put into place in 2014, there is a further tightening planned from 1 Jan 2022 when all solid fuel space heaters must conform to Commission Regulation (EU) 2015/1185 requirements for efficiency and emissions. This will be known as Ecodesign.

 

Stoves that are 2022 ready are already in the market, some five years early, all are dedicated wood burners ( solid fuel is to dirty to be allowed) these stoves emissions are 90% less than an open fire and 84% less than a stove design of ten years ago. Ecodesign stoves are measurably better for the environment, but at present they carry a price premium over an older and dirtier design.

 

The Aarrow Farringdon is one such stove.

 

https://publications.europa.eu/en/publication-detail/-/publication/c6ccf626-2f6d-11e5-9f85-01aa75ed71a1/language-en

 

This regulation will not mean that older designs of stove cannot still be used and sold, but new designs coming forward will need to be Ecodesign compliant. It then comes down to people like me to sell the clean credentials to prospective purchasers. Morso have not done any development work on multifuel stoves since 2013, every new model is dedicated wood burner.

 

A

[dig-dug-dan]

All this compliance of stove manufacturers is all well and good, but the biggest problem is people buying cheap logs, often unseasoned, or wet, mas they see a bargain, and then continue to burn it.

year after year, the same woman rings me up to buy logs, tellz me I am too expensive, and buys a load of crap cheap.

the nieghbours told me she said it was wet and didnt burn very well, yet she still buys it because its cheap.

there were rumours of some kind of legislation to stop it in the form of licencing, but who will enforce it, and what stops the consumer buying from an unlicenced source?

[Alycidon]

All this compliance of stove manufacturers is all well and good, but the biggest problem is people buying cheap logs, often unseasoned, or wet, mas they see a bargain, and then continue to burn it.

year after year, the same woman rings me up to buy logs, tellz me I am too expensive, and buys a load of crap cheap.

the nieghbours told me she said it was wet and didnt burn very well, yet she still buys it because its cheap.

there were rumours of some kind of legislation to stop it in the form of licencing, but who will enforce it, and what stops the consumer buying from an unlicenced source?

 

Yep and buying cheap stoves, some with fake CE markings and then doing self installs.

 

I cannot ever see any form of legislation coming forward on firewood quality, the Hetas scheme is a non runner in my book as it allows moisture contents of up to 25%. I have spoken to Helen about that and getting it reduced but to no avail,

 

A

[ash_smith123]

All this compliance of stove manufacturers is all well and good, but the biggest problem is people buying cheap logs, often unseasoned, or wet, mas they see a bargain, and then continue to burn it.

year after year, the same woman rings me up to buy logs, tellz me I am too expensive, and buys a load of crap cheap.

the nieghbours told me she said it was wet and didnt burn very well, yet she still buys it because its cheap.

there were rumours of some kind of legislation to stop it in the form of licencing, but who will enforce it, and what stops the consumer buying from an unlicenced source?

 

 

They could go down the route of something like the BSL but Like you say it will be almost impossible to enforce. There will be a "black market" for firewood!

[Woodworks]

Yep and buying cheap stoves, some with fake CE markings and then doing self installs.

 

I cannot ever see any form of legislation coming forward on firewood quality, the Hetas scheme is a non runner in my book as it allows moisture contents of up to 25%. I have spoken to Helen about that and getting it reduced but to no avail,

 

A

 

Self instals or just bad instals?

 

25% is about as low as you can get in a typical Devon winter or do you think we need to start kiln drying? As you say though I cant see legislation over fuel coming any time soon as it would be almost impossible to enforce.

[openspaceman]

Go ahead.

 

Firstly consider the other fuels we burn, with petrol we meter in just about the right amount of fuel for the air present in an engine, with diesel we inject a measured amount of fuel into excess air. With a gas flame we use the gas passing through a nozzle to entrain just the right amount of air to burn cleanly. Similarly with a pellet burner we trickle a few pellets in at a time that on average the conditions in the burn pot remain the same and air supplied to burn the pellets but wood is a bit different from the other fuels as it doesn't burn directly but in two stages (actually diesel in an engine burns in two stages also).

 

The primary stage is when wood is heated to pyrolysis temperature and evolves gases and vapours. The heat from this comes from burning charge that has already been formed and this char ignites at a relatively low temperature, about 250C. The pyrolysis products and products from burning the char then rise as gases and vapours where they burn in the secondary flame. Conventional fires aim for the air to the primary combustion to control the power by restricting the air supply to the bottom of the combustion, the evolved offgas then entraining sufficient secondary air to burn these products completely. Unless air is well mixed into the gases from the primary combustion the oxygen must combine with the gases at the flame surface, if the dwell time in the flame is insufficient or there is not enough air Products of Incomplete Combustion (sooty particles) form.

 

Something like 70% of the heat from wood comes from burning in the flame and 30% from burning out the char which is why it is important to maintain a flame and not let logs smoulder.

 

In the log stove we have a major difference from when fuel is metered in in that a whole batch of fuel is in the firebox at once. So not only are the outsides of the log burning but also the interferon of the log is heating up. As I said pyrolysis of the log is initiated by the is heat at about 270C and is mildly exothermic, which means it produces a little of its own heat without needing further input from the fire below. During pyrolysis all the gases are released from the log but if the fire is turned down low there may not be enough secondary air to burn them, hence soot is formed. So a very dry log can heat up and evolve a lot more gas than the fire can provide air for.

 

However to evapourate water needs 2.3MJ /kg which is hundreds of times more heat than is necessary to raise dry wood to its pyrolysis temperature. So the small amount of moisture in the wood slows down the heating up of the interior and the wood progressively burns from the outside inward, releasing offgas in a more regular manner

 

You can demonstrate the concept by taking a green log and splitting it in half, dry one half in the oven. Then put them both in a fire, you will see the oven dry log quickly evolves gases which ignite, shielding the log, then when the pyrolysis is complete the charcoal then turns grey as it burns and ash is exposed. The wet log steams for a while and then the outside chars, this char is burn as it is exposed and the log burns away to ash from the outside till nothing is left.

[richardwale]

Self instals or just bad instals?

 

25% is about as low as you can get in a typical Devon winter or do you think we need to start kiln drying? As you say though I cant see legislation over fuel coming any time soon as it would be almost impossible to enforce.

 

And 25% firewood burns absolutely fine. Agree that in the depths of cold damp winters forget anything less than 20%. The huge log piles outside every Scandinavian home are they 16%, nope. They might be once inside and warmed by the stove but do the stove sellers tell the totally unknowledgeable customers this?

I've been burning firewood a long time and anything 25 or less is perfectly acceptable to sell and burn.