The great and upsetting thing about YouTube videos is that they don’t go away! A lot of this old video now seems incomplete:

Our understanding of how to come closer to complete combustion has changed and includes details missing before. The following describes a hopefully less wrong set of design principles. David Evitt is currently deep diving into clean combustion and who knows what details will be added in the next few years?

Nine years ago, we thought that close to complete combustion could be achieved by forcing all gases and smoke into flame for a sufficiently long amount of time.

  1. Today we add that the air/fuel ratio in the combustion zone needs to be air rich.
  2. We add that the flame/gas/smoke/air mixture needs to be thoroughly mixed. 
  3. We add that the temperature in the combustion zone needs to be above 850°C to allow short residence times to become effective.
  4. We add that at a minimum of 850°C the residence time needs to be more than 0.2 seconds. Longer is better.
  5. Wet wood, un-preheated air jets, and mass tend to reduce temperatures to below 850°C.
  6. Optimized heat transfer efficiency helps to reduce harmful emissions since less fuel accomplishes tasks.

The U.S. Department of Energy (DOE) Bioenergy Technologies Office (BETO), in collaboration with Brookhaven National Laboratory (BNL), Lawrence Berkeley National Laboratory (LBNL), and the Alliance for Green Heat, today announced the winning teams for the Wood Heater Design Challenge (WHDC). 

Aprovecho Research Center, from Cottage Grove, Oregon, came in second place and won $25,000 with a novel burn pot, airflow configuration, and sensor package for pellet heaters. Davidon Industries from Warwick, Rhode Island was awarded first place for their mechanically automated, combustion-air control technology for cordwood heaters. Kleiss Engineering from Cloverdale, Indiana, won the third-place prize with a smart wood stove heater.

“Embracing innovation allows us to challenge existing norms, push boundaries, and discover new solutions that can reshape the entire industry,” said Dr. Valerie Sarisky-Reed, Director of BETO. “Wood stove research is part of DOE’s overall strategy to develop affordable bioenergy technologies and convert our nation’s renewable resources into fuels, power, products and in this case, more efficient wood stoves for homeowners.”

Aprovecho, Davidon, and Kleiss were selected from nine teams competing at the Wood Heater Technology Slam in September 2022. Teams pitched new wood stove ideas to retailers, the public, and experts, who assessed which stoves were the most innovative, efficient, and offered the greatest market potential. The three finalist teams moved forward to the testing phase of the competition, which was held this past spring at BNL in Upton, New York.

Read the full press release on the DOE Website. Learn more about the BETO-funded Wood Heater Design Challenge.

Watching a Rocket stove or a pellet stove (as above), it becomes obvious that metering the fuel is a primary factor in achieving close to complete combustion. When too much fuel is introduced into the combustion chamber, the emissions of smoke increase almost immediately.

For the clean burning of biomass, the controlled metering of fuel seems to be as necessary as it is in the engine of an automobile. The rate of reactions (how fast the solid biomass is being converted into wood gas) is then matched with the corresponding amounts of Time, Temperature, and Turbulence required to minimize CO and PM2.5.

ARC has added Metering to Time, Temperature, and Turbulence while unsuccessfully searching the thesaurus for a synonym that starts with the letter T. Maybe someone can succeed where we have failed?

Dr. Larry Winiarski wrote the ten rocket design principles

We have been having a lot of fun doing a modern literature search: Surfing YouTube. YouTube is often years ahead of the slower, but probably more accurate, information in peer reviewed journal articles. I suppose that many people are looking at both.

A shared misunderstanding seems to be that making the combustion zone hotter cleans up combustion. Yes, it is great to keep the temperatures around 900°C, which shortens the residence time needed to burn up the wood gas. However, just raising the temperature misses other necessary components that also move stoves closer to complete combustion. They are:

1. FUEL/AIR RATIOS: Fuel and air are needed for complete combustion.

2. MOISTURE: Biomass has to be relatively dry to burn.

3. MIXING: Turbulence needs to completely mix the fire, air and wood gas.

4. RESIDENCE TIME: Less time is required at higher temperatures to burn up the wood gas.

5. TEMPERATURE: Good – higher temperatures decrease the residence time. Bad –higher temperatures increase the rate of reactions possibly producing more wood gas than can be cleanly combusted.

6. METERING: As Dr. Winiarski wrote in his Rocket Design Principles: “Only make the amount of wood gas that can be combusted.”