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The Chemistry of Gasification

flame match

Since the earliest recorded history we have known fire. It has kept us warm, cooked our food, and eventually we were able to control it enough to get it to perform work for us. Despite our long history with fire, few people know, or care, what actually happens when you strike that match and light your campfire.

As you strike the match on a rough surface, friction heats up the chemical patch on the end, causing it to combust. The heat of this combustion rapidly heats the wooden match stick. As the wood is heated it turns to char, releasing a flammable mixture of gases. Assuming there is enough heat and oxygen present, this gas will then combust, producing the sustained flame that we see coming from the match. The char produced then combusts, producing more heat which in turn produces more char, and gas from the remaining wood in the match stick. This process is noticeable if you look closely at a burning match. You will see that once lit the flame isn’t actually coming from the wood its self, but it seems to materialize in the air just above it. When this burning match is placed in the tinder of your campfire the heat produced by the burning gases heats up the tinder, reducing it to char, and causing it to release its flammable gases. As you can see, this process continues to apply to the entire campfire.

Wood fires smoke for two reasons. One occurs when the wood gas that’s being produced doesn’t receive enough oxygen to fully combust before it cools, and becomes diluted to a point that it will no longer ignite. In this case the “smoke” that’s seen actually contains the flammable gases (CO and H2), along with ash from the combusting char, and various nitrogen compounds that make up air. The other reason a fire smokes is because there is too much oxygen present as the gas is being produced. To understand this one must understand the actual chemical reactions taking place in the ideal production of wood gas: Gasification.

Wood gas is produced when wood is heated to high temperatures without enough oxygen to actually combust. Instead it is reduced down chemically to char, which is essentially carbon in a process called pyrolysis. Pyrolysis is the decomposition of organic molecules into simpler molecules that occurs in a high heat environment with a lack of oxygen. Next, provided the temperature is high enough (at least 700 degrees C), and no oxygen is present, a series of reactions occur between the carbon and water vapor, carbon monoxide and char. This process, called reduction, produces hydrogen gas, carbon monoxide. Hydrogen and carbon monoxide are the flammable gases in wood gas. The resulting charcoal and carbon then combusts with oxygen gas to provide heat, water vapor and carbon dioxide, all essential reactants for the drying, pyrolysis, and reduction processes. The balanced chemical reactions for each of these processes are listed below.

Combustion:

  1. CxHx(s) + O2(g) -> CO2(g) + H20(g) + Heat
  2. C(s) + O2(g) -> CO2(g) + Heat
  3. C(s) + ½ O2 -> CO(g) + Heat

Reduction:

  1. Heat + C(s) + H2O(g) -> H2(g) + CO(g)
  2. Heat + C(s) + CO2(g) -> 2CO(g)

Now, thinking back to the smoking camp fire. The process and reactions detailed in the above paragraph is what happens in gasification which in a way is “ideal burning”. When optimized it results in essentially only CO2 and heat emissions. However this only occurs in a controlled environment when the right amount of each reactant is introduced at the right time. This is not the case in a camp fire. As I mentioned earlier if too much oxygen is present during the gas production stages the fire will smoke. The extra oxygen cools the reaction, and means that the reduction reactions will run with an excess of oxygen and lack of heat. This essentially eliminates the critical reduction process, and simply combusts the rest of the material. Without reduction you have no gasification. The reason gasification can be so efficient is because it breaks “burning” into two stages, allowing them two be optimized separately.

 

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Copyright Ethan Dreissigacker 2008