In a wood gasification boiler, 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 emitted wood gas 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.
There are several common thought processes for applying wood gasification and secondary combustion principles in the design of a wood boiler, here are two of the most common approaches:
Continuous burn: Dual combustion chambers. Many of 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 275-1000 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.
On-demand burn: Single combustion chamber. The boiler uses a single burn chamber to foster wood gasification. During normal operation, the firebox is maintained at the high temperatures required for complete combustion. The gasification 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.
So, if you are researching a purchase of 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:
In general, gasification is very efficient in extracting energy from different types of organic materials, including wood.
Extremely high combustion efficiency is obtained by gasification, thereby creating minimal emissions.
With respect to wood boilers, wood gasification means:
Less wood is required,
Significantly lower emissions/smoke, and