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Following the recent announcement by Defra to ban the sale of wet wood and coal in February 2021, there has been a flurry of activity and discussions over various social media channels and responses to media articles. We always like a good healthy debate!

So, I thought it would be useful to answer some of the main questions over a series of blogs.

Several people have asked me the question – How much energy is needed to kiln dry logs? A perfectly good question when there is an ever-increasing focus on energy costs and the environmental balance between using one energy source over another and which uses the least energy or is the most sustainable for the future - always a difficult balance. And of course, there are always valid arguments either way.

Put quite simply, should we be using any energy to dry firewood? In the ideal world, I would say no, when it can dry naturally, over a period of time of course – up to 2-3 years. However, as we know, we just don’t’ live in the ideal world, so what are the issues?

If you own a wood burning stove (a standard 5kw stove) and burn most evenings in the winter months, say October to April and also at weekends, then you are likely to need about 3-4m3 (loose fill) of kiln dried logs. Just over 2 of our 1.6m3 bulk bags will probably be enough to see you through one winter and for convenience you can have two deliveries to minimise storage space.

If you decide to cut, split and dry your own firewood, not only can it be a lot of work to produce, but also handle, you will need enough storage space for a max of 3 years to rotate your stock. You will therefore need space for 9-12m3 of logs. I don’t know of many people who would have this sort of capacity, let alone desire to manage the logistics.

Solar panelsSo, to the question, how much energy do we use? Surprisingly little is the answer. Firstly, we cut the timber into log lengths of generally 23-25cm and then split into diameters of 5-15cm. Yes, this uses some electricity to run the processors. We then air dry for a few weeks before placing in the kilns which are 40ft shipping containers which get their heat from large boilers which run on our wood waste and wood chip. We use some more electricity here to run the fans on the kilns, but we have solar panels to produce some, but not all of our requirement. We put 18 tonnes of wood into each kiln and take 12 tonnes out. This brings the moisture content down from about 50-60% down to well below 20% in approx. 50 hours! That’s 6 tonne of water we drive off as steam.

Having got the logs very dry, its important to know that the difference in heat output between the logs going into the kiln, and those coming out is about 3kWh/kg. Wet logs will produce about 1 kWh/kg, that’s the equivalent of a single bar electric fire whereas the kiln dried logs produce approx. 4.5kWh/kg. You will therefore need significantly less logs for the same heat output and in terms of delivering dry wood to a customer, this is far more cost effective than delivering wet wood, which has little energy value at the time of delivery.


  • The question "So, to the question, how much energy do we use? " is still not answered and you deleted my previous post.

    For your example above, I say 4000kwh to vaporise the water alone, plus the energy for the fans and heatlosss through the walls (how much insulation? from the photo very little) at a high temp difference of c. 60 degrees C for 50 hrs

    Also green wood at 55% Moisture Content has a Calorific Value of 2 kWh/kg, twice what you claim. Ref › documents › 1958Wood as Fuel – Forest Research

    Air drying is much kinder to the environment than kiln drying.

    Iain on
  • “…So, to the question, how much energy do we use? Surprisingly little is the answer. "

    Can you put a number on that?

    Burning in the stove the reduced output of the wet logs is due to having to evaporate the excess water, so ball park it takes 3kWh to kiln dry a kg of wet wood from 55% to 20%

    Iain on
  • So dealing only with energy inputs/outputs: another piece of ‘information’ (because I do not know if it has been reviewed by independent experts and approved) I found on the internet: "The “caloric content” (energy content combustion) of firewood depends mainly on how dry it is. “Green” wood is about 10 MJ/kg ( megajoule per kilogram), air-seasoned wood about 16 MJ/kg, while kiln dried wood is about 19 to 20 MJ/kg "
    What ‘caloric content’ does your boiler use per 12 tonnes end product?

    Joanna O'Brien on
  • You say that kiln dried logs give off a lot more heat and we agree at that point only. Whilst they burn a lot hotter they also use about 3 times more logs than the unseasoned logs and taking that into the equation the unseasoned logs are not only cheaper to buy they work out cheaper to burn as they do not use the same amount of logs.

    Mick Waddington on
  • I wonder 4 things when reading your report here.

    1. Where are you figures derived from, in terms of kWh/Kg.
    2. Since when did logs need more than a year of air drying to become seasoned? Even oak and denser woods such as hawthorn only need one year if they’re stacked and stored well.
    3. How do you filter the 1/3 mass which is driven off the logs in your kilns, as this is considered to be a toxic output, and part of the reason for implementing this intended ban/restriction.
    4. How much power/gas/kWh does your kiln use per occasion for drying your 18 tonnes down to 12 tonnes?
    5. What kinds of wood are you refering to, and do you use a single source for consistency?

    Kind regards

    Yemach Shemo.

    Yemach Shemo on

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