Gasification - Some Woods Better Than Others?

Five or so years ago there was a post-apocalyptic social experiment quasi-reality show called The Colony on the Discovery Channel. In one of the episodes they successfully ran a petroleum gas powered electrical generator on wood gas, and I was blown away. That was my first exposure to wood gasifiers. Last year on the History Channel show Mountain Men, Eustace Conway and Preston Roberts successfully converted an old pickup truck to run on woodgas using a gasifier constructed pretty much of junk that was lying around the farm, and again I was fascinated.

I realize that most people would be most interested in just converting scrap wood to gas, but if someone wanted to devote a section of woodlot to wood gas production, do some woods perform better than others? Or would fast growing trees be a more important consideration?

John

I have several young copses planted to black locust, willow and poplar. I plan on using the poles produced for fence posts and fire wood. Do these species make good wood stock for a wood gasifier? My farm also has tens of thousands of young cedar trees - how does cedar do in a gasifier - the oil in cedar wood is famous for being "different".

Does using properly cut up small diameter branches with the bark on make any difference as compared to chopped up larger pieces of wood with no bark? I guess I am wondering if the chemicals in the bark produce a different end product.

I really like the idea of being able to independently make "natural" gas from wood. I had no idea that this process was so widely used during WW II when supplies were scarce. They well may be scarce again and in difficult times I would like to be able to use vital pieces of machinery to keep the homestead going and even help out my neighbors.

Thank you for organizing this information so clearly.

John & Mary great questions. When considering wood species you first want to think about ash content. A low ash content is needed because inside the gasifier the temperatures can exceed 2000 degrees F and the ash can fuse together if there is alot of it, making clinker stones. Species to avoid are Cotton wood. A little ash acts as an insulator though and is a good thing.

The next consideration is density. The more dense it is the more energy is packed in there and the more run time you will get. This is why chunks are preferred over chips because by the 5 gallon bucket load the chunks weight about 50% more. Then we get into the topic of hardwoods vs. soft woods. I like softwoods (Doug Fir) for small gasifiers because the wood breaks down easier. I like hard woods (Oak) for bigger engines because they last longer.

The next consideration is tree growth time and/or availability of free wood on your property or nearby by the truckload. I would try to grow the most marketable wood for sale, rather than the best gasifier fuel since so many woods work fine. 90% of the tree should be used for lumber in my opinion. Sawmill waste is the honey pot for a wood gasser.

I am a Pacific Northwest guy so the woods I typically run are douglas fir, oak, pine, alder and some exotic woods when I can get scraps from a cabinet maker. On a day to day basis I use old oak pallets or fallen fir limbs. Pine boards and free construction debris is another good one. I use some cedar too but it tends to be oilier and coats the feed hopper inside walls, so I mix it in at 25% to keep that in check. Cedar is really best suited for kindling and the fireplace, but it will work.

Bark does seem to have some natural fire defenses built into it, but a small amount around the edges of branches has never been a problem for me. I wouldn't try to use just big chunks of bark though. Gasifiers run best with wood because they make great charcoal and it's the charcoal's surface that helps the chemistry inside make the fuel.

I'm not sure about willow or black locust as a feedstock, but poplar is supposed to be a great one and fast growing.

The two critical things to consider are:
Moisture content of the wood. Start with 20% or less moisture. With experience you can go wetter. Peaked cold frames are a good way to dry wood out in the sun.

Wood chunk or chip size. Wood sizes are best determined by the rate of gas production needed. A wood gasifier that passes more gas can gobble up larger pieces of wood. An ideal small chunk would be like 2 of your thumbs together and be most ideally suited for small engines. An ideal large chunk would be about the size of a small fist and good to power your V-8 truck down the road.

I didn't get to watch those two shows mentioned, but I think it's awesome people are waking up to this forgotten technology.