Tuesday, February 09, 2010

Keep the home fires burning


Freshly emancipated from my job in the city, I have reverted into suburban Victorian mode.

The 3 hours not spent battling with commuter trains are now either spent asleep or doing something useful.

This year we have had the addition of a wood burning stove and backbolier in our living room. This helps keep the core of the house cosy and offsets the amount of gas that we use. However, it must be stated that keeping a woodstove stocked with wood can become a distraction to other activities, more a labour of love.

Whilst man has been using wood to keep warm for millennia, there is a certain amount of skill needed to use it efficiently. Open fires are notoriously inefficient, and even some stoves have much to be desired. In the 21st Century, new wood based fuels are becoming available, and some of the mechanical handling associated with wood fuels can be automated. For the moment, I'm happy and contented to lie on the heath rug and gaze into the embers. Here's a few notes on how I keep this one burning.

Whilst you can just go out an buy a load of logs from numerous local log merchants, the quality is often dubious and you will need somewhere dry to keep them whilst they continue to dry out. Logs are often supplied quite chunky - meaning that further splitting is needed to make them more manageable for the woodstove -- despite its 18" log-length.

Another source of wood is from waste constructional timber. There are many loft conversions going on in this area, and I happened upon a load of roof timbers which were being skipped in the next road - so I offered to liberate them for nothing. These are generally older than 40 years - and so are not contaminated with pressure treatment or wood preservative.

Having cut the longer lengths into 6' pieces, so that they would fit in the back of the van, I hauled a van load back around the corner. These timbers are a nominal 5"x2" but when they are sawn the size is actually about 47 x122mm. They are probably Douglas Fir, and are very dry - so split easily with a hand axe. They are however full of roofing nails and felt tacks - so have to be handled a little carefully with leather faced riggers gloves.

After getting them home, they need to be chop-sawn into a suitable size for the stove. My chop saw has an end stop at 250mm - so this determined the average sawn length. Each 6' timber was sawn into roughly 7 pieces, carefully checking the path of the blade for nails and tacks before committing to the cut.

Once cut, the 10" lengths are split with a hand axe into two, generally unequal parts. These "halves" will burn rapidly and release their heat quickly. For a slower burn, whole pieces are used.

Eight of these 6 foot timbers will provide enough wood for two 12 hour burns. If we are both home, and the weather is cold, we light the stove in the early afternoon and keep it fired until past midnight.

The process of cutting up and splitting the roof timbers takes on average 30 minutes including the time to get out the chop saw, carry the timbers to the cutting area and tidy up afterwards.

The box on the left contains 30 of the 10" pieces and weighs 22kg. This gives the wood an average density of 511g/litre, and I have seen these pieces vary in mass from 500 to 800g.

The calorific value of woodfuel is approximately 4kWh/kg so the box represents about 88kWh of energy. Unfortunately, a basic cast iron woodstove is not a particularly efficient converter of wood into heat - so roughly half of this energy will reach the occupants in the room, the rest will be lost up the chimney. This is however about twice as good as an open fire.

Wood can vary in moisture content, depending on how it has been stored, and different species have different calorific values and burn rates. Better stoves do exist, but what is needed is a change in the technology in order to get the most out of wood. This is where wood gasification comes in.

Wood gasification is the conversion of woody biomass into a mixture of flammable gases, which can be burnt at optimum efficiency in a modified stove or burner. Woodgas primarily consists of carbon monoxide CO and hydrogen H2, with small amounts of CO2, methane and half of the mixture consists of the inert nitrogen from the atmosphere. Woodgas varies in calorific value from between one eighth to one sixth that of natural gas. Wood gas can be used as an engine fuel for riunning vehicles, generators and other machinery. In principle, it should be possible to run a whole household heated and powered from woodgas. Well that's the plan.

Next week I travel to Berkeley, California to learn all about wood gasification and how to build wood gasifiers. There is a project planned to convert a Lister diesel engine to run on wood gas using a spark ignition conversion - so that's what I'm hoping to do, learn how it's done and bring the technology home.

4 comments:

Dale Asberry said...

Are you going to All Power Labs?

Ken Boak said...

Hi Dale,

Yes, I'm off to see Jim Mason and the gang at the Shipyard.

I've already got a GEK here, now I'm off to find out all I can from the experts.

mikethebee said...

You raise a good point in that older construction timber is okay while newer waste will be contaminated with chemicals. I noted when looking to buy a multi-fuel burner that the manufacturer advised against using such wood at all, indicating the the acids would rot the metal.

Good luck with the trip, CA is always a fun place to go anyway.

Anonymous said...

This reminded me of the Philips Woodstove project where the electronics giant tried to design a more efficient cooking stove to conserve wood and reduce pollution for developing countries. See http://www.research.philips.com/technologies/projects/woodstove.html