Apparently logs can be too dry

[Logsnstuff]

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

 

clearview have <25% on their website.

 

 

think it was November when I posted this up from the FC renewables or whatever his title was:

 

I am certainly aware that most combustion equipment prefers the fuel not to be bone dry. In the case of wood chip boilers they will be commissioned for a specific range of moisture content, and log stoves too are designed to operate optimally at a specific range of moisture content, typically 15-25% (wet basis) as this is the value that dried wood will tend to stabilize at if stored outdoors, under cover in the British climate, even if it’s initially kiln dried lower. Lower MC woodfuel can be burned perfectly well, and very efficiently, as is the case with wood pellets that are typically <10% MC, however that’s because the boiler/stove is designed and optimized for this range. I haven’t seen the specific EPA report you mention, but it is likely that a stove designed for 15-25% will not work as well at lower values. I’d be interested to see their specific concerns. In the case of woodchip boilers, if the fuel is too dry it can lead to higher levels of NOx emissions as a result of the combustion temperature being too high

 

I am certainly aware that most combustion equipment prefers the fuel not to be bone dry. In the case of wood chip boilers they will be commissioned for a specific range of moisture content, and log stoves too are designed to operate optimally at a specific range of moisture content, typically 15-25% (wet basis) as this is the value that dried wood will tend to stabilize at if stored outdoors, under cover in the British climate, even if it’s initially kiln dried lower. Lower MC woodfuel can be burned perfectly well, and very efficiently, as is the case with wood pellets that are typically <10% MC, however that’s because the boiler/stove is designed and optimized for this range. I haven’t seen the specific EPA report you mention, but it is likely that a stove designed for 15-25% will not work as well at lower values. I’d be interested to see their specific concerns. In the case of woodchip boilers, if the fuel is too dry it can lead to higher levels of NOx emissions as a result of the combustion temperature being too high

[neiln]

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.

 

That makes sense to me, a good explanation of something I'd been wondering. However, if the user is content to have a fast and hot burn, and opens the air sufficiently, it should burn clean.

[Billhook]

We used to supply dry seasoned wood to an old fella some years ago who insisted on sprinkling the load with water to make it last longer.

We used to laugh about it but it seems he was light years ahead of his time!

 

On the subject of harmful fumes, my parents and their parents and their parents all sat beside roaring open log fires for most of their long lives and not one died before they were 87, many well into their 90s.

Has anyone known of a person dying from lung problems or lung cancer by using wood to fuel their fires?

[neiln]

I've been reading stuff on hearth. Com and arborists it's so picked up a few things the Us do. I guess having alot of rural communities they have a lot of wood burners, and their EPA regs seem tight, and are due to get tighter. They have a lot of catalytic stoves, where the cat does the secondary burn. I believe they specify <16% MC for wood. I suspect that is largely to ensure the cat 'lights off' in hot smoke even when the primary air is down low and the primary burn is just a smolder, but it does show the technology trend and the need for dry wood for it to work.

[Alycidon]

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.

 

25% is just not good enough IMHO yet according to Hetas its fine !!.

Logs at 15% would generate about 40% more heat, have a look here:

 

http://www.forestry.gov.uk/pdf/eng-woodfuel-woodasfuelguide.pdf/$FILE/eng-woodfuel-woodasfuelguide.pdf

 

If you cant naturally get the internal MC down that far then KD is the way to go, either yourself maybe with RHI or buying in ready KD logs from the Baltic. Under an open dutch barn in vented bags I can get air dried logs down to about 10% external in September having processed in April. Cord is usually a couple of years felled. I am in the midlands so have no sea fogs which I assume is an issue for you. Bear in mind that wood is hydroscopic so will draw moisture in from the atmosphere once out of the kiln.

 

I have a customer driving 40 miles every week to buy 10 packs of my kiln dried Birch in small nets as he has no storage space for a large crate, in an Audi A5 !!. Another does a longer run in a Porsch 4x4.

 

A

[Billhook]

No serious harm in breathing wood smoke | ScienceNordic

[Alycidon]

I've been reading stuff on hearth. Com and arborists it's so picked up a few things the Us do. I guess having alot of rural communities they have a lot of wood burners, and their EPA regs seem tight, and are due to get tighter. They have a lot of catalytic stoves, where the cat does the secondary burn. I believe they specify <16% MC for wood. I suspect that is largely to ensure the cat 'lights off' in hot smoke even when the primary air is down low and the primary burn is just a smolder, but it does show the technology trend and the need for dry wood for it to work.

 

Stoves with secondary burns are fitted with terciery air systems, this puts a third layer of air into the firebox to ensure that there is always enough oxygen available to ensure a clean burn. Terciery air systems on most current model stoves are not controllable normally and are fully open all the time.

 

terciery is the green arrows here,

 

What is a Primary Flow, Secondary Flow and a Tertiary Flow? - Green Homes Online

 

Arada Farringdon and some new model Charnwoods MAY have controllable terciery, they have all their air controls on one lever.

 

A

[neiln]

Thinking about how 'better/cleaner' burning could be encouraged or enforced, while making sale of better performing stoves is possible and will help in time, I can't see how wood MC can be enforced nor burning practice, and those later 2 have a big affect. Ok stoves can and are made to prevent no less than a minimum air supply already if Defra approved but that only tackles slumbering. Perhaps it should be a compulsory part of Part J to fit a stove flue thermometer of some kind? The visual guide to good burning would help, but not cure bad practice.

[openspaceman]

That makes sense to me, a good explanation of something I'd been wondering. However, if the user is content to have a fast and hot burn, and opens the air sufficiently, it should burn clean.

 

I agree but there are a couple of points, even with a fast hot burn the rate of evolution of pyrolysis offgas will not be steady with bone dry wood.

 

You cannot keep bone dry wood that dry, even pellets start disintegrating in a damp garage

 

If you supply enough secondary air for the worst case you end up sending too much heat up the flue most of the time, this is insignificant on the domestic scale but not so on a commercial scale , which is why they use wide band oxygen sensors in the flue in order to supply just the right amount of air.

 

NOX was mentioned in a recent reply, perceived wisdom is that NOX forms above about 1500C at ambient pressure, the flame temperature of wood is only around 1600C (or else you could smelt iron with wood rather than charcoal) and by the time you add above 50% excess air to ensure a clean burn you will be below that.

 

I'm all for burning dry wood and 20% mc is good and easily attainable in this climate under cover but as long as you can get up to temperature is the real condition for clean burning and a stove with an insulated fire chamber will manage this where a metal box won't. My Jotul is essentially a metal box.

[neiln]

Stoves with secondary burns are fitted with terciery air systems, this puts a third layer of air into the firebox to ensure that there is always enough oxygen available to ensure a clean burn. Terciery air systems on most current model stoves are not controllable normally and are fully open all the time.

 

terciery is the green arrows here,

 

What is a Primary Flow, Secondary Flow and a Tertiary Flow? - Green Homes Online

 

Arada Farringdon and some new model Charnwoods MAY have controllable terciery, they have all their air controls on one lever.

 

A

 

Yes I know, my stovax has primary, airwash and then what it calls secondary, the heated air that enters the top of the fire box and should ignite the smoke. US stoves meeting current EPA regs can be tube/air feed secondary but a good proportion use a catalytic matrix. Just like the cat in a car exhaust, it triggers the burn of the polutants at a low temp. I think the big advantage those stoves have is very long burn times, as the primary air can be turned right down, giving a slumber, yet the cat ensures a clean burn.