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All About Wood Gasification and Secondary Combustion

The term wood gasification may come across as a bit cryptic or feel like it requires a graduate degree in chemical engineering to truly understand. However, for making the right wood boiler purchasing decision, there are a few things you need to know.

Definition of Gasification

According to Wikipedia, "Gasification is a process that converts carbonaceous materials, such as coal, petroleum, or biomass (such as wood), into the simpler elements carbon monoxide and hydrogen by reacting the raw material at high temperatures."

In a temperature range of 395 ° F to 535 ° F (i.e. a regular fire), the majority of the gases are released from the wood in combination with the smoke. Those gases contain about 50% to 80% of the heat content of the wood. This process results in a gas mixture that is often referred to as synthesis gas or syngas or in the case of a wood fuel – woodgas.

If we stop the reaction at this stage, the woodgas may be used as a fuel for a number of purposes. The gas may even be cooled and cleaned to remove tars and particulates and used as fuel in a variety of other applications, including engines!

How Wood Gasification and Complete Combustion is Achieved in Wood Gasification Boilers

If you are looking to purchase a wood boiler, the bottom line is that wood gasification boilers that provide complete combustion are better than traditional water-jacketed wood boilers. Some key points to take away from this article include:

  1. Continuous burn: Dual combustion chambers. The wood gasification models imported from Europe employ this technique. These units are designed to operate properly when they burn a load of wood in one continuous burn and transfer the resulting heat to a water storage container (usually 400-1000 gallons or more) where it is stored until the heat is needed. In these systems, the gases flow down through the fire into a secondary chamber where firebrick (or a ceramic material) creates the superheated environment necessary to complete the efficient combustion process. Keeping this secondary chamber at high temperatures is key to the performance of the overall system, hence the need for one continuous burn so that this chamber does not cool and lower the boiler efficiency.
  2. On-demand burn: Single combustion chamber. The Greenwood wood boiler uses a single burn chamber to foster wood gasification. During normal operation, a patented thermal mass maintains the firebox at the high temperatures required for complete combustion. The CrossFire Combustion System regulates air flow and the amount of heat being delivered to the home. This enables the system to operate as an on-demand system, thereby removing the need for the water storage tank outlined above. This design not only simplifies the operation and maintenance of the unit, but also enables a greater variability of fuel composition (e.g. whole log wood and wood moisture content). Click here for a video demonstration of updraft wood gasification.


In gasification wood boilers the wood gases don't just go up and out the chimney, as is the case with standard wood boilers. Instead, the reaction is continued and the resulting woodgas is superheated and mixed with air resulting in complete combustion. The heat is then transferred to a boiler for efficient distribution. An additional benefit of the gasification process is that the complete combustion leaves little or no ash or creosote.

There are several common schools of thought for applying wood gasification and secondary combustion principles in the design of a residential or small-scale wood boiler, here are two of the most common approaches:

  1. In general, gasification is very efficient in extracting energy from different types of organic materials, including wood.
  2. Extremely high combustion efficiency is obtained by gasification, thereby creating minimal emissions.
  3. With respect to wood boilers, wood gasification means:
    - Less wood is required,
    - Significantly lower emissions/smoke, and
    - Less ash.

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