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Wood Stove Challenge

The Puget Sound is home to more than 100,000 woodstoves, many of which are older and emit a lot more pollution than newer, certified stoves. These devices are the largest source wintertime air pollution in the Puget Sound. Replacing older stoves can be expensive. There is limited financial assistance available in some areas with the highest pollution levels, but there just isn’t enough for the rest of the region.


 The Woodstove Retrofit Open Challenge


Fine particulate matter (PM) pollution is a widespread wintertime problem in the Pacific Northwest. Over the years, many members of the public and agency staff have wondered if some type of pollution reduction device could be added to older wood stoves. In 2013, the Puget Sound Clean Air Agency, working with Washington State Dept of Ecology, obtained a grant to search for and test devices that can reduce PM and PAH (polycyclic aromatic hydrocarbons) emissions.

In late 2014, we ran an open competition on the crowdsourcing site, Innocentives, to find new and emerging retrofit technologies. We received 32 proposed solutions which we narrowed down to four semi-finalists. The four semi-finalists were then tested in an EPA accredited lab in Portland, OR. The lab tests looked at the emissions from two old, uncertified stoves first without the retrofits. Then, each of the retrofits was installed and run through a series of tests.  All four of our semi-finalists showed significant reductions in PM, with three devices beating our Challenge goal of a 75% reduction!  Congratulations to all of our semi-finalists and especially our three Finalists! Our three semi-finalists also showed similar reductions in PAHs!

Each of the three Finalists has a somewhat different technical design and so has different strengths and weakness. Two of the devices have small heaters that help insure steady pollution reduction performance, while the third uses the latest catalytic technology and doesn’t need a heater. The two active devices reduced fine particle emissions by about 90%, and the passive device reduced fine particles by about 80%. In the end, our panel of judges was split on how to the rank the devices and settled on a three-way tie. Initial cost estimates are in the range of $500-$1000, but this could change depending on the production quantity and other factors. Below is a figure of the average particle pollution reduction for each retrofit for both stoves.

So what does the future hold for these retrofits? A real-world pilot study in a small town or community would be a great next step.  But, there are still a number of issues and challenges to be addressed, including finding a funding source. All of our finalists are eager to move forward and would be able to begin production in early 2016. We are working on pulling together all of the pieces for a pilot study, but there are still a lot of unknowns.  If a pilot study goes well, the air quality community would then work to find the best way to take advantage of a new air quality protection tool.

A Final report on the Challenge is being prepared and should be released in a few weeks. Once the report is available, a link will be added here and in our document library.

Below is further information and links provided by our semi-finalists.





ClearStak Pollution Control Device and Intelligent Biofuel Controller

ClearStak’s Pollution Control Device (PCD) is composed of a short section of stack, a catalytic converter, a thermocouple, a catalyst heater, and an optional oxygen or lambda sensor. The catalyst provides a reactive surface that all emissions must come in contact with before exiting a stack, effectively reducing the release of carbon monoxide (CO), volatile organic carbons (VOCs), and particulate matter (PM). Many catalysts rely on the heat of the fire to maintain an effective temperature, and for this reason they are often ineffective during start up. BioMass Controls' Intelligent Biofuel Controller (IBC) was developed to pre-heat the catalyst to prevent it from falling below its activation temperature. By ensuring this temperature is maintained before combustion is initiated, the IBC further prevents smoke during start-up and catalyst clogging. Furthermore, optional oxygen sensors can be utilized to maintain high combustion efficiency through the optimization of air-to-fuel ratios.

ClearStak LLC is a privately held environmental engineering and consulting company located in Putnam, Connecticut whose mission is to invent and manufacture technologies that improve efficiency and reduce visible emissions from the combustion of densified biomass, providing renewable energy solutions that are safe for the ClearStak LLC is a privately held environmental engineering and consulting company located in Putnam, Connecticut whose mission is to invent and manufacture technologies that improve efficiency and reduce visible emissions from the combustion of densified biomass, providing renewable energy solutions that are safe for the environment.

Manufacturing of the PCD and IBC will be achieved along with partners Johnson Controls Inc. of Grand Rapids, Michigan and Applied Ceramics Inc. of Atlanta, Georgia.

More information can be found by visiting and Products can be purchased through Biomass Controls, LLC at (860) 928-0712.  Additional information can also be found on the following YouTube videos:

Grace Fire

The Grace Fire StoveCAT

The Grace Fire LLC StoveCAT uses patented catalyst coated ceramic medium from Clear Skies Unlimited (CSU) that can survive the harsh operating environment of a wood stove while achieving the desired pollution reduction.  CSU catalyst coated medium can not only withstand flame impingement, but unlike the first generation of catalyst support structures, has material characteristics that not only gives it increased performance but gives it a dramatically longer life. 

In addition, the device has several additional critical innovative characteristics:

  1. An array of catalyst coated medium allows exhaust gases to safely continue flowing even if the catalyst were to be totally blocked.

  2. The device provides supplemental ambient air to the bottom of the catalytic array to aid in more complete conversion of the pollutants into harmless compounds. 

  3. The supplemental air is preheated by flowing downward around the outer surface of the catalytic array to maintain the higher temperatures required for efficient catalysis. 

  4. A VcV or “Kiwi Valve” from Flamekeeper automatically maintains a properly balanced flow of supplemental air throughout the various phases of the burn cycle.

The result is a retrofit device that exceeds all of the preferred level requirements of the Puget Sound Clean Air Agency Request for Proposals

  1. Does not increase the hazard for the user, the house or the general environment outside the house.

  2. Reduces fine particle by greater than 75% in real-world conditions.

  3. Costs less than $600 for a resident to purchase and install.

  4. Is robust enough that annual (or longer) maintenance can be performed by unskilled residents, or can be performed by an appropriate maintenance person for less than $100.

  5. Has an expected lifetime, assuming adequate maintenance, of 10 years or more.

  6. Doesn’t require intervention, adjustment, or monitoring by the user.

    Additional information can be found at the following sites and YouTube videos:

    The following photos of the device (placed on the stove next to the normal flue for comparison purposes only), demonstrate the eye-appealing, sleek profile of the device, displaying the aesthetics demanded by the consumer for acceptance in their household.





An energy efficient afterburner retrofit for wood burning stoves

This technology is the brain child of Dennis Grahn. The afterburner is extremely efficient in removing noxious hydrocarbon emissions from the exhaust stream of any combustion source. The noxious emissions are thermally combusted in a secondary hot spot maintained above the spontaneous combustion temperatures of the hydrocarbon materials contained in the exhaust stream.

Drs. Grahn and Heller, Biologists at Stanford University, study mammalian physiology. The energy efficiency of the afterburner is based on principles of mammalian thermoregulatory physiology - insulation and cn principles of mammalian thermoregulatory physiology - insulation and countercurrent heat n principles of mammalian thermoregulatory physiology - insulation and countercurrent heat exchange to optimize energy utilization. For the wood stove application, the afterburner consumes less energy than a common household appliance. To date, the development of the technology has been a science project.

Kevin Haley is a principal of Clearstreme. He is a mechanical engineer with a MBA. Kevin has over 30 years of experience in industry facilitating the transition of prototype designs into functional equipment. Sam Shawe is the founder of Outback manufacturing - a well-established 15 year old precision manufacturing company. Sam has the technical capabilities to design and execute the manufacturing process.

The mission is to transition the technology from a science project platform into production-quality commercially viable merchandise. Critical design considerations will be optimizing function and vastly improving aesthetics.

The Grahn Team:

Dennis A  Grahn  | Stanford University | 650-725-8723 |

H. Craig Heller | Stanford University | 650- 723-1509 |

 Kevin Haley | Clearstreme, LLC | San Jose, CA | 408-833-8188|

 Sam Shawe | Outback Mfg. | Bend, OR | 541-330-1046|


MF Fire

MF Fire Afterburner

Designed by fire scientists, MF Fire’s Afterburner wood stove retrofit dramatically lowers emissions by reducing start-up times and using secondary and catalytic combustion to eliminate pollutants in a wood stove’s exhaust. The start-up phase, the most polluting phase of wood burning, can contribute up to 70% of a wood stove’s total particulate emissions. By speeding and cleaning the start-up phase, emissions can be reduced by nearly 50%. Secondary and catalytic combustion further reduce emissions, scrubbing the remaining particulates so your wood stove emits nothing but carbon dioxide and water.

The Afterburner is as simple as plug and go, connecting directly to your existing wood stove’s exhaust, replacing the bottom two feet of the stack, and plugging into any wall outlet. To start burning, you flip a switch, light the wood, and shut the door. Inspired by technology used in the Mulciber Stove, the Afterburner is another exciting design from MF Fire.

Visit the MF Fire website or find them on Facebook and Twitter.