When charcoal is heated to very high temperatures, the carbon content of the charcoal becomes very reactive, and exhibits a very strong affinity for oxygen, enabling it to reduce (the opposite of oxidize; in this context, to ‘reduce’ is to reverse oxidation) oxidized substances such as carbon dioxide and water vapor. As the combustion products from the combustion stage are percolated through the hot charcoal, these reduction reactions convert carbon dioxide and water vapor into carbon monoxide and hydrogen, while consuming carbon from the charcoal to do so. This process produces roughly half of the combustible molecules in syngas.
In the process of reduction, the charcoal chips are perforated at the molecular scale as carbon atoms are individually removed from the surface of the charcoal. This makes the resulting charcoal quasi-activated, which is a desirable quality for filtration, and may potentially have benefits for the charcoal when used as biochar, due to the apparent beneficial influence of increased porosity on ammonia, methane, and N2O emissions abatement.
Char-Ash as a byproduct
In the course of gasification, wood chips turn into charcoal chips, and these charcoal chips give up their carbon content during the reduction process to produce carbon monoxide. This loss of carbon causes the charcoal chips to shrink. Eventually, the shrunken charcoal chips pack too densely to permit the rate of gas percolation needed to feed the engine drawing the gas from the gasifier. In order to restore the rate of gas production, the reactor purges the shrunken chips of charcoal, which are then pushed out of the reactor as char-ash. This byproduct of gasification is one of the two material outputs of gasification; the other output is the syngas.
Gas production and charcoal production
In the process of reduction, the charcoal chips are perforated at the molecular scale as carbon atoms are individually removed from the surface of the charcoal. This makes the resulting charcoal quasi-activated, which is a desirable quality for filtration, and may potentially have benefits for the charcoal when used as biochar.
Depending on what product is desired (gas or charcoal) the gasifier may be operated to produce more of the desired product by regulating how long the charcoal is exposed to reduction reactions before being purged