Wood Gas

Burning wood in an open fire produces light, some heat, and lots and lots of smoke — as you know if you’ve ever forgotten to open the fireplace flue. The process isn’t terribly efficient, and it obviously doesn’t do wonders for our air quality. But when you burn that same wood in a very hot, oxygen-restricted environment, you break down the compounds of the wood into clean-burning combustible gases, along with ash or charcoal. Those gases — wood gas — can be used to heat a cook stove or even power a car, and their emissions consist of carbon dioxide and water vapor. There’s no smoke.

Gas produced from wood and other carbon-based materials (primarily coal) has been used since the beginning of the industrial revolution. It’s had many names — synthesis gas, syngas, producer gas, gengas, town gas — and has been used variously to provide heat, light, and transportation fuel. During World War II, over one million European cars and trucks traveled with onboard gasifiers that provided fuel for their modified gasoline engines. In this chapter, we’ll look specifically at making and using wood gas.


Wood Doesn’t Burn:

Strik e a match and look at the flame. Notice that the flame is not actually in contact with the match but rather surrounds it. Likewise, wood in a fire does not actually burn; it is being heated to the point where combustible gases are released. When the hot gases combine with oxygen in the air, they oxidize (combine chemically with oxygen) and burn. It is the liberated gases that are burning while the wood becomes carbonized, turning to charcoal, and eventually the charcoal burns to ashes. Charcoal is wood that has been reduced mostly to carbon. Ashes are minerals that are left over after all the carbon has been burned off the charcoal.

The blue part of the flame is burning hydrogen gas (oxidizing to form water vapor); the orange flame is carbon monoxide oxidizing to produce carbon dioxide (CO2); and burning soot and tars produce a yellowish flame. With the addition of more oxygen to support further combustion, the carbon is reduced to ash. The ashes cannot be further oxidized. If you choked off the air from hot wood coals and stopped the burning, you would be left with charcoal suitable for use in a charcoal barbecue grill. When making charcoal, combustion is controlled so that no additional oxygen will further oxidize the carbon coals.

How Wood Gas Generators Work:

Gasification works by heating the biomass fuel — wood or almost any other carbonaceous material — to nearly 500°F, releasing flammable gases from the fuel without immediately burning them. The combustible gases produced are hydrogen (H2) and carbon monoxide (CO). When burned, they create only carbon dioxide and water vapor (H2O). The gases can either be burned within the unit (as with a gasification cook stove) or pulled off for external use as a gaseous fuel. A wood gas generator consists of a chamber for holding and heating the fuel to create flammable gases in a controlled environment, a system for cooling and filtering the gases, and a system for distributing the gases to where they will be burned. Each of these pieces must be carefully designed to create an integrated system for managing the thermochemical decomposition of biomass.