drying wood

[markus]

That's quite a high starting point for ash, it's notoriously difficult to give a general rule for a species moisture content and it can vary between parts of the tree.

 

Anyway to get where you want from 38% to 23% from an initial tonne you need to lose about 200 litres of water.

 

There's still not enough details but a couple of points, the recirculating air affects how saturated it becomes. Theoretically you maximise use of fan power when the air leaving the wood is saturated. At saturation air holds different amounts of water according to its temperature and it isn't linear. Without going into detail if you draw a graph of absolute humidity verses temperature at saturation you will see there is a point around 25C where the moisture carrying capacity starts kicking off. Below this and you have to pay to circulate a lot of air for no great drying effect.

 

A chap posting here on another forum gave some useful figures which illustrate this:

http://arbtalk.co.uk/forum/milling-forum/38987-kilning-theory-discussion.html#post610301

 

And this sort of chart contains a lot of useful information if you take the time to figure it out.

 

 

http://upload.wikimedia.org/wikipedia/commons/0/05/PsychrometricChart-SeaLevel-SI.jpg

 

 

From this you can see that it your container is uninsulated in a british winter the ~ 20 degree difference is losing you a lot of heat.

 

This has a knock on affect on your dehumidifier, which is really just a heat pump that recovers latent heat from the circulating vapour and dumps it back into the container but the container loses most of this recycled heat because it isn't insulated...

 

Aloso then consider the COP of the heat pump and compare the cost of electricity to drive it with heat from other sources.

 

thx catweazel

are you advising not to try to hard to get below 25%mc as the cost to get to 23% does not do it justice?

[openspaceman]

thx catweazel

are you advising not to try to hard to get below 25%mc as the cost to get to 23% does not do it justice?

 

 

Not at all, though I would be interested in an experiment on the equilibrium moisture content of a 23% mc wwb log in a shed on a rainy cold february.

 

You haven't really given enough details of what cost benefit you are aiming for. Usually the only benefit of drying is to meet a standard or charge a premium. If the latter then you have to cost the operation to see what the premium is.

 

In general firewood is a luxury good and the price paid has little relation to to its competitiveness with other available fuels. Having a product that is easy to light, has a lively smokeless flame and doesn't pollute may justify the cost and secure the premium. On the other hand a large industrial consumer will stomach the ~10% flue losses of burning green whole tree woodchip because the double handling alone doesn't justify a drying stage and he's only paying £25/tonne delivered.

 

Having said that early in one of the NFFO rounds one biomass burning power station ran a big grain dryer with and extra oil burner to get the wood dry enough to cofire with coal because the subsidy was so high.

[openspaceman]

The FITs for solar pv and wind push up the price of electricity which hits people on low incomes hardest. If all the FITs were paid from central taxation then the bulk of the subsidy burden would fall on the wealthy, which seems to me to be fairer.

 

OK this is getting too far off topic, I'm happy to take this to a more suitable forum or e-mail but I will comment:

 

Yes it does increase the price of electricity and yes it was a huge incentive which did not take account of the changing costs of solar PV but the cost of failing to have enough installed PV would have been of a similar magnitude. Not having sustainable energy is rapidly becoming a non option especially since pan european public opinion has changed post Fukishima.

 

There is no way the wealthy are willing to have a fairer tax system which is why we are seeing the demise of the NHS. We are governed by wealthy people who believe in the market economy and no limits to growth. Economies depend on success not also rans or lame ducks because capitalism mimics darwinism. Society tries to keep it within bounds.

 

Wealthy people also avoid tax better than most.

 

 

It's absolute madness

 

I've snipped the rest of your post which contains some misconceptions but is too far off topic for here.

 

Have you read Mckay's book "sustainable Energy Without Hot air" it's a no cost download , some very heavy going but well worth trying to understand, I admit much of the maths was beyond me, especially the stuff about flight.

[Stereo]

I take your points Spruce and largely agree but I think laying into Nick personally is a little harsh.

[nick channer]

£4000 seems like a lot to charge, how big are the houses?

 

A couple of guys up here looking at doing a similar thing, mini hydro plants seem popular as well.

 

whys it harsh... there paying more than that now for heating oil.....the houses are let except the one my dad lives in.... the money from them is whats paying for the boiler.. were not doing it for the fun of it were doing it to make... and savr money..

[markus]

Not at all, though I would be interested in an experiment on the equilibrium moisture content of a 23% mc wwb log in a shed on a rainy cold february.

 

You haven't really given enough details of what cost benefit you are aiming for. Usually the only benefit of drying is to meet a standard or charge a premium. If the latter then you have to cost the operation to see what the premium is.

 

In general firewood is a luxury good and the price paid has little relation to to its competitiveness with other available fuels. Having a product that is easy to light, has a lively smokeless flame and doesn't pollute may justify the cost and secure the premium. On the other hand a large industrial consumer will stomach the ~10% flue losses of burning green whole tree woodchip because the double handling alone doesn't justify a drying stage and he's only paying £25/tonne delivered.

 

 

 

Having said that early in one of the NFFO rounds one biomass burning power station ran a big grain dryer with and extra oil burner to get the wood dry enough to cofire with coal because the subsidy was so high.

 

you seem to know what you are talking about catweazel ,the other option i have considered is as my container is next to my unit to pull dry air into my container from the unit through a ducted fan and extract it out again with an extraction fan making sure i am only pulling in dry air form inside my unit,letting the sun heat the container and as was said earlier painting it black,the other note to mention is whether to insulate container or not because if i insulate the container it is preventing natural heat penetrating the container from the sun, by the way i work for an insulation company and can insulate for free.Your thoughts appreciated.

[Dean Lofthouse]

I would insulate, it,s ok warming the unit during the day with free heat from the sun, the problem is then in the evening when the exterior temp is low and inside relatively warmer causing the moisture to condense heavily on the inside.

 

If you insulate well very little heat will be needed to raise the internal temperature, i would have thought you wouldnt need to renew the air in the container, just remove the moisture from the air and drain to the outside

[openspaceman]

the other option i have considered is as my container is next to my unit to pull dry air into my container from the unit through a ducted fan and extract it out again with an extraction fan making sure i am only pulling in dry air form inside my unit,letting the sun heat the container and as was said earlier painting it black,the other note to mention is whether to insulate container or not because if i insulate the container it is preventing natural heat penetrating the container from the sun, by the way i work for an insulation company and can insulate for free.Your thoughts appreciated.

 

You are changing tack too quickly for me. The dehumidifier may not be a bad option and any extra heat will benefit but as I said currently you are wasting a lot of energy through the container walls. I'd go for insulation and keep it dry. Solar passive is great but should be a separate box on the side and configured not to lose heat at night. For the small extra electricity cost active solar with using a black cloth absorber is likely worthwhile, probably better suited to cold dry climates than UK.

 

Unlike drying planks you don't have problems with checks and splits but you tend to have greater cross sections and there in is the limiting factor. The 19 gram piece of ash I took and hung in this study this morning has already lost 4 grams because it has a large surface area to lose moisture from and a small distance for the moisture to migrate through. As a log gets bigger this ratio gets worse and drying slows down. The only thing that speeds it up is temperature. So you have a complicate equation to work out, firstly to supply enough energy to evaporate the total amount of water you need to remove per hour, then the necessary airflow to carry this water away, and the power to drive it and then the amount of log surface area you present to the airflow. The more logs you have to blow through the higher the back pressure and more electricity used.

 

This tends to mean that fan electricty is the highest cost as low grade heat is cheap, which is why the Border Biofuels chip dryer ran air at about 40C, it was designed to use waste engine heat.

 

I've got a feeling its not worth blowing air less than ~25C and you get much more useful work at higher temperatures, so insulation is a must.

 

The dehumidifier has the great advantage that the latent heat of evaporation isn't lost to the outside air as is with a through flow dryer but I'd like to know the inlet temperature and outlet temperature plus water condensed to see how efficient it was.

[Spruce]

I take your points Spruce and largely agree but I think laying into Nick personally is a little harsh.

 

Yes, and I apologise to Nick. I'm immensely frustrated by the absurdities of the carbon reduction subsidy system and I clicked 'post' before thinking instead of after.

[Spruce]

yes but spruce next time you buy fish and chips or a ham sandwich and get change from a tenner remember if it wasn't for subsidies food would be alot dearer and that tenner wouldnt get you squat. its not for the benefit of the man selling its for the man buying!!

 

The subsidies for renewables etc are for the benefit of the man selling, not the man buying. These subsidies make electricity and taxation more expensive for you and I, not cheaper.