I’m interested in all aspects of wood pellets, from how wood pellets are made to how they are used. When it comes to wood pellet combustion there is the conventional pellet stove and boiler route. However, there is also a growing trend to use wood pellets in alternative combustion systems, in this case, rocket mass heaters. If your not sure what a rocket mass heater is I’ll talk more about that below. If you already understand the basic principles of rocket mass heaters, watch the video below on how rocket stoves can be designed to work more efficiently on wood pellets.
Automated vs Gravity Wood Pellet Stoves
Wood pellet heating comes in two forms. You have the highly automated pellet stoves and boilers such as the ÖkoFEN units and you also have none powered wood pellet stoves. Both interest me and are important in their own way, but for different reasons. Highly automated conventional pellet stoves and boilers serve as a transition product, to get people away from conventional oil and gas stoves and boilers.
However, improving the design of none powered/gravity stoves is really more important. In most of the world where solid fuel is burnt, there is either no electrical supply or its highly irregular. Wood is typically burnt on highly inefficient open fires or basic stoves. This not only wastes most of the wood it creates significant amounts of smoke pollution which kills hundreds of thousands of people every year.
The Benefits of Rocket Stoves
Rocket stoves and in the case of this post rocket mass heaters use only a fraction of the wood of a conventional stove. In some cases, fuel consumption can be reduced by as much as 80-90% using a rocket stove design. Therefore significant issues around deforestations for stove fuel can be reduced. Its cheaper for the consumer and considerably better for their health.
Rocket stove designs which can also use wood pellets can further help to resolve the issues stated above. Continued improvements in designs could even potentially burn biomass pellets from agricultural residues (grasses/straws). However, there are significant challenges around the increased ash content and clinker formations compared to wood pellets.
How does a Rocket Stove work?
A rocket stove is designed to run on small diameter fuel, typically small chunks/chips of wood. Wood pellets are not only small they have a high energy density. The rocket stove design is about making sure the draft of the fire chamber is as good as possible without using power-assisted fans. That is what makes rocket stoves are so interesting.
It’s amazing that rocket stoves can achieve such a high efficiency with no electrical fan forcing air into the fire. Rocket stove designs work by ensuring that air predominantly enters either through the fuel or from underneath the combustion zone. This essentially gasifies the wood and its the gas and smoke from the wood which is actually being burnt. I’ll talk more about gasification in later posts.
But what is a Rocket Mass Heater?
The rocket stove principles have improved combustion efficiency significantly over a conventional wood stove. However, a rocket mass heater can improve combustion efficiency even more. Essentially a rocket mass heater is integrated into the design of the property. The flue pipe is extended in length through the property. Then a large heat absorbing mass (typically of clay) encases the flue pipe. This mass extracts the heat and will releases it gradually over time.
Avoiding the Dangers of Creosote
As you can see from the video above about what a rocket mass heater is, one of its impressive features is how it avoids the dangers of creosote. Creosote forms from burning wood with a high moisture content or poor combustion efficiency.
As stated in the video the previous trade-off has been to insulate the chimney to avoid creosote formations, but this also wastes a lot of energy. With a rocket mass heater stove design it is the combustion chamber which is insulated. This creates a much hotter combustion zone which burns off any potential volatiles which would form creosote.
Why Rocket Mass Heaters and Wood Pellets go together
With a properly designed rocket mass heater, you can take advantage of one of the unique features of wood pellets, how they flow. As wood pellets have such a high density they flow well through hoppers, therefore it’s possible to create a none powered stove which can run for many hours with a full hopper of wood pellets.
In the first video above, you first see the design of the grate which will hold the pellets but will also let the ash through the grate so combustion is continuous. Next, you see the various feed shoot designs. The later designs improve the spread of fuel over the grate. This, in turn, will also provide a better flow of wood pellets from the hopper.
It is interesting to see how much of a difference installing and removing the restrictor plate makes. The restrictor plate provides better air direction and also encourages more air to come in below the grate. It’s also important to note that higher temperatures were recorded on the heat exchanger than with any wood previously used. This is due to the high density of the pellets and their very low moisture content.
Conclusions on using Wood Pellets with a Rocket Mass Heater
I do really like the idea of a fully designed rocket mass heater which can run on wood pellets via a gravity hopper. It would mean that a highly efficient and low maintenance off grid wood pellet heating system is possible. When I get the chance I would like to have a go at building my own rocket mass heater which runs on wood pellets. I think it would be a really rewarding project. If you would to learn more about making, burning and using wood pellets, please visit my home page for other posts.