Sunday, May 25, 2008

Wood Gas - The Future Biomass-Derived Fuel

Let's assume, that you own a farm. You produce grain and have a lot of straw. You'd like to make your energy on site from the biomass you have available. You might plant your fields with rapeseed or canola and produce biodiesel. But you'd need to get methanol and lye first, since both of them can't be made there! And you can use your biodiesel only in compression ignition (diesel) engines. And what if you have an old genset powered by spark-ignition (gasoline / petrol) engine? I think I might have a solution for you...

You'll make use of a technology that's more than 200 years old. It's greatest development took place during World War II in 1940s in Europe. At that time most of petroleum-derived fuels were used by military and there was no fuel left for civilians. So they made their own fuel, the wood gas from wood or charcoal. In some countries 95% of civilian transport (trucks, buses, even fishermen's boats) were fueled with this gas.

What is this wood gas? From chemical point of view it's a mixture of combustible carbon monoxide (CO), hydrogen (H2) and methane (CH4), dilluted with some water vapor, carbon dioxide (CO2) and nitrogen (N2). This gas can be easily used for almost every internal combustion engine. Well... except for the most modern ones, they'd require to have their electronics to be modified.

How wood gas is made? When you heat wood it is pyrolysed, i.e. all the tars vaporize. When you add some oxygen, the carbon bound within the wood cells is burnt to carbon dioxide. The vapors burn too. When those burnt gasses pass through a layer of very hot char, they are reduced to carbon monoxide and hydrogen. A small amount of methane is also produced. When the gas is cooled and cleaned of any tars and particles, it can be used in internal combustion engine, boiler or any other device!

What do you need to produce your own wood gas? You'd need a device that's called gasifier. It consists of fuel hopper, gasifying chamber and a set of filters. It's very easy to build, the people during WWII made it out of old water heaters and stuff like that.

Is wood gas safe? Well, if you don't inhale it, you'll be OK. Carbon monoxide is very toxic and poisonous, so caution is required when using wood gas. From the engine point of view, if it's clean, it's safe. The tars might condense inside the engine, for example on valves, which might cause the engine to fail. The particles would probably destroy the contact surfaces on pistons and cylinders. If the gas is too hot the engine will not work (hot gas is diluted and has small energy content per unit volume), but it will not harm it.

Any drawbacks? One that's worth mentioning. Petrol engines will suffer 1/3 power loss -- the wood gas does have small heating value. Diesel engines will not suffer such power loss, since they work on stratified mixtures and because of that you can introduce more wood gas to recover the power loss. You'd need to leave some small amount of diesel to be injected to the cylinders to ignite the wood gas.

Interested? One liter of petrol / gasoline can be replaced with 2.5 - 4 kg (5.5 - 9 lbs) of wood. How cool is that!

What is the wood gas?

Wood gas (holzgas in german) is the product of wood gasification. It consists of combustible gases such as carbon monoxide, hydrogen, methane and inert gases - nitrogen, carbon dioxide and water vapor. The composition of the wood gas depends on many factors, e.g. temperature inside the gasifier, moisture content of fuel gasified etc.

Gasification of solid fuels was invented over 200 years ago. In the beginning of 19th century town gas (manufactured from coal) was used to fuel kitchen stoves, industry and street lights. In general any combustible gas manufactured by gasification of solid fuel is described as manufactured gas or producer gas.

Gasification is the process in which the solid fuel is being subject to many termochemical reactions, at the end of those reactions you have combustible gas. This process takes place in a device called gasifier or wood gas generator.

Wood gas can be used for fueling internal combustion engines, both with spark ignition (gasoline fueled) and compression ignition (diesel fueled). The latter case needs some extra explanation.

Wood gas was very popular until the industry started using natural gas as energy source. Prior to development of natural gas supply with pipelines almost all fuel gas was manufactured from coal. Producer gas became popular during World War II, when civil customers couldn’t purchase liquid petroleum derived fuels because of the large military demand. At that time the wood gas renaissance took place.

Volumetric composition of wood gas may be as following [1]:

  • carbon monoxide CO - 19%
  • hydrogen H2 - 18%
  • methane CH4 - 1,25%
  • carbon dioxide CO2 - 12%
  • water vapor H2O - 2,50%
  • nitrogen N2 - rest.

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