Sustainable Carbon Cycling

21 11 2011

Comments regarding “pollution” from our AGA has prompted me to post again.  I was introduced to sustainable energy by my friend Bruce, mentioned elsewhere on this blog, who actually helped me move the AGA from where we first became acquainted.  He burns wood exclusively for all his cooking, and most of his hot water, boosted with solar.  I say boosted, because Mt Glorious, where he lives, is less than ideal for solar, even though the entire house runs on nothing else but solar as a stand alone/off grid system.  Bruce is the most amazing person I have ever met, and he absolutely, without a doubt, changed my life…. and I can never thank him enough for it.

Bruce and his wife Erika plus three children live on six acres, high in the hinterland of Brisbane.  He planted a forest of sort on this land, just for energy need.  He is adamant that done properly, wood burning is efficient, and sustainable.  It is because of Bruce we have a combustion stove.

Wood burning has received a sometimes well earned bad reputation for pollution, not because of the practice, but due to the uncaring or perhaps ignorant attitude of many people who practice wood burning.  There is no doubt that poor combustion leads to high emissions of smoke and inappropriate firewood harvesting is contributing to ecological damage.

The combustion processes taking place when burning wood are quite different to other solid, liquid or gaseous fuels.  An external heat source is required to start the process of drying and thermal decomposition of wood.  If you attempt to burn green wood, most of the energy you seek from your heater will be wasted to just boil off any moisture in the wood.  The fire will be too cold, and it will smoke.  At temperatures above about 250°C, exothermic reactions (i.e. giving off heat) commence and the decomposition process can become  self-sustaining.  I have seen the top plate on our firebox glowing dull red, which, according to research I have done on cast iron, means this top plate had reached ~550°C, and obviously at such temperatures, complete combustion occurs. This is where the AGA’s insulated design really shines, keeping the heat inside the stove where it is needed.  It also means less firewood needs to be burned to achieve the desired result.

Burning unseasoned wood is a really wasteful thing to do.  You cannot reach the full potential of the device you’re burning it in, and it causes unnecessary pollution.  Patience is of the essence, because Bruce tells me it takes two years out of the weather for wood to dry sufficiently to become firewood.  When I first fired up the AGA, I was going to excitedly post a photo of the smoke coming out the flu (it does smoke for the first five minutes or so on firing up), but by the time I had found the camera and run outside to do so, the smoke had already disappeared.  There was no point even taking a photo, the smoke was almost invisible, and the flu looked like the stove was not even on!

The process of thermal decomposition causes chemical changes in the complex organic molecules that constitute lignin, cellulose and hemicellulose. Organic gases (a fuel in their own right at the right temperature) are released, leaving a carbon-rich solid residue. Under controlled conditions it is possible to produce a solid residue of relatively pure carbon. The carbon (charcoal) burns as a result of surface reactions (with little gas release) leaving a residue of ash. All this ash ends up on our food garden to enrich the soil.

The energy content of wood does not vary much from one species to another; hardwoods typically release 19 MJ/kg (oven dry wood basis) and softwoods 20 MJ/kg, provided combustion is complete.  Pine works just fine, but because it is far less dense than hardwood, you need to burn more of it, in volume at least.  When in continuous mode, the AGA burns 12 kg of wood a day, for an estimated 240MJ/day, or roughly 10MJ/hour or 2.8kW continuous power.  I can actually foresee that our stove could become a community energy source, with neighbours bringing food to cook as the firewood is shared out….

This energy heats our water, half the house, and accounts for all our cooking.  All on an armful of firewood.  The AGA is also unlikely to be used much in Summer, unless it rains for several days in a row.

The big difference between doing this with wood over fossil fuels is that you can grow wood, but you can’t grow fossil fuels, and furthermore, as you grow the wood, all the CO2 emitted is 100% absorbed by the new growth in the trees that will be future fuel.  This is the Carbon Cycle.  Wood is a truly renewable resource.

The biggest problem I see with a solution such as this, is that it is only achievable for a few people.  The Earth does not have the resources for everybody to start burning firewood, there are simply way too many people to accommodate now, we as a species are well and truly already in overshoot.  I’ve even been accused of being elitist for doing these things that only a few others will be able to duplicate.  I don’t know how to answer to this.  I think I’ve been lucky to have met Bruce who opened my mind to new and different ways of seeing things, I’ve been smart enough to work out the future wasn’t going to be as rosy as the media makes out, and I worked on a plan to survive the looming collapse.  This blog is here to assist people who think like Bruce and I do, and are prepared to take the plunge to change the way they live.

Take it or leave it…….




2 responses

13 06 2012
John Cornel

Thank you for the all interesting information.
Regarding the Aga running on wood instead of coal/coke, what conversions did you do on the stove? Do you feed the wood through the normal small feed hole in the hot cooking area? It appears to be to small for efficient usage with wood.
I am trying to convert a standard solid fuel Aga to a wood burner but am not sure what to do?
Please help.

13 06 2012

Hi John…….

No, you do not feed the firewood through the small hole, you need a conversion kit which, in part, totally replaces the hot plates above the firebox. kits, I believe, are available here The new hot plate has a hole about 7 inches/180mm in diameter to allow feeding big pieces of wood down the beast’s throat!

The conversion also involves replacing the original 4 inch flue with a 5 inch one (wood generates more gases than fossil fuels), and the griddle at the bottom of the firebox which may or may not be there in a gas fired AGA (I’ve never seen one).

Good luck….


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