Tuesday 10 January 2017

Season of mists

the wood fire handbook: the complete guide to the perfect fire [sic, in the gorgeous design of this mini coffee-table book they’ve forgotten where the capital letters are in the font tray] was part of my Christmas loot and also part of my 12 days of Christmas reading binge when the internet went down. It’s fierce didactic; Vincent Thurkettle is quite school-marmy in his style of transmitting a life-time’s accumulated knowledge about the properties and handling of fire-wood.

One of the key issues in successful wood-burning is seasoning the wood. It is one of the reasons why wood is split and each species and each micro-climate will have its special requirements for being seasoned effectively. Ireland is going to take longer than Corsica; pine Pinus sylvestris is going to take longer than oak Quercus petrea. Seasoning is really about drying out the wood until the moisture content is lowered sufficiently for the stuff to combust. As Thurkettle says, ash Fraxinus excelsior is a peculiar boon for the woodpile because it will burn green or unseasoned but it will burn a damn sight better, and hotter, if its moisture content is 20% rather than 50%.

When I first came to The Institute, I had great fun (and learned a lot) teaching Environmental Chemistry. It was mainly about the chemistry of water: so very important as it’s the soup that sustains all life. One of the many peculiarities of water, compared to other hydrides of the first line of the periodic table methane CH4, ammonia NH3, OH2, hydrofluoric acid HF is its fabulously high freezing and boiling points. It’s the hydrogen bonds innit? It has a really high specific heat – which means that it retains heat really well and is thus the optimal, not merely the cheapest and most available, stuff to have in radiators. It also has huge latent heat. This is usually measured in Joules. For each gram [that’s, by definition, a cc = cu.cm] requires 4.2J to increase its temperature by 1oC. To get from freezing to boiling point water therefore requires 420J/g.

And this has to do with the price of milk seasoning wood? Well if the wood weighs 1 kg and is 20% water and the ambient temperature outside [winter] is 0oC then it’s going to require 200g x 100oC x 4.2 = 84kJ to raise the temperature of the water in the log so that it can start its final [boiling] shedding of water so that the carbon in the wood can start to burn and produce heat and a carbon footprint. If you bring the log inside next to the fire [20oC] only 80% of this energy will be required in your fire box. If the wood is green, if it burns at all, it will require an *additional* 300g x 100oC x 4.2 = 126kJ.
But this is all in the ha’penny place because the final stage of conversion into steam involves a ‘phase transition’ from liquid to vapour. The hydrogen bonds of water makes it very sticky and for a molecule to wrench itself from the cosy bosom of its liquid state into the independent, zingier, life of a molecule of water vapour requires a fabulous 2260J/g. To get from 99oC to 100oC takes 4.2 J/g but the final lift off requires 500 times the energy! For your 1kg green log this is 500g x 2260J = 1.13mJ [+ 126kJ + 84kJ to get the log up to 100]. This latent heat is one of the reasons why a steam burn is so serious. The vapour molecules are still fizzy with the extra energy for lift-off. Thus, while you can with practice get an egg out of boiling water with your fingers, you want to stay well away from the spout of the kettle.

This is why it is a super good idea to reduce the moisture content of your logs. You want the existing fire to be heating the room and your family rather than merely getting the next log up to escape speed.

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