Last week Dean and Travis went to the Hearth, Patio & Barbeque Association Expo in New Orleans to get updated on the latest heating stove technologies and to meet and talk to stove companies. Three days was a short time but we made many friends and learned a lot!
The CENTRAL BOILER outdoor log and pellet furnaces stood out as one of the newest innovations. It can be imagined that in a sustainable biomass future, heating big buildings (even high-rise apartments?) with logs or pellets might be as useful as freestanding stoves heating rooms. The same (bigger/smaller) clean burning combustion systems deliver renewable, home-grown heat to occupants in bigger/smaller buildings. Maybe pellets could be poured down the same delivery chutes that are used for heating oil in New York City buildings?
Admiring the automated house-sized furnaces on display at the Expo made the possibilities seem so much better defined.
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.
https://aprovecho.org/wp-content/uploads/2014/08/Aprovecho-Favicon.png120120Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2023-09-07 18:31:512023-09-08 16:46:24Aprovecho Announced as a Winner of the Wood Heater Design Challenge
Since 1976, ARC has been investigating how to improve heat transfer and combustion efficiency in Low Middle Income Countries’ wood burning cook stoves. Emissions of Particulate Matter have been shown to kill millions of people annually. PM concentrations are frighteningly high in homes without chimneys but emissions into outdoor air are an increasing health/climate concern. Incomplete combustion in cooking and heating stoves is an obvious problem especially when compared to the very clean combustion in more mature technologies like automobiles.
The EPA biomass heating standard allowing two grams per hour of PM to pollute the environment is very lenient. National standards in Europe also allow biomass stoves to endanger health/climate. Cook stoves are forced to burn much more cleanly by stricter WHO standards and ISO benchmarks.
The Guardian’s Environment Editor Damian Carrington reported in 2021, “Despite their severe impacts on air pollution and human health, domestic heating emissions are under-regulated in the EU, especially when compared to other sources such as traffic. Neither the EU EcoDesign requirements nor the more ambitious Nordic ecolabel succeed to keep particle emissions from new stoves within acceptable levels. In 2022 a new EcoDesign stove will be allowed to emit 60 times as much particulate matter as an old truck from 2006, and 750 times as much as a newer truck from 2014.”
https://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.png00Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2023-04-28 16:40:222023-04-28 16:40:24Clean Burning Also Necessary for Biomass Home Heating
Turbulence is very important for close to complete combustion. Swirl, Squish, and Tumble are used to create turbulence in internal combustion engines.
Kirk Harris discovered that a static fan shape with overlapping 70 degree blades creates lots of fast moving swirl at the approximately one to two meter per second velocities found in TLUDs.
1. Swirl:
The rotational motion of air within the cylinder is called Swirl. Swirl enhances mixing and makes the flue air mixtures homogeneous. Swirl is the main mechanism to spread the flame within the combustion zone.
2.Squish:
The radial inward movement of air is called Squish. Squish can be defined as an inward flow of air towards the combustion recess.
3.Tumble:
Squish generates secondary motion about the circumferential axis near the outer edges. This motion is called ‘tumble’. To achieve this either the fuel is directed towards air or air is directed towards the fuel.
When the goals for biomass cook stove interventions were raised to include protecting health, it was obvious that adding a chimney or cooking outdoors continued to be the historically proven solutions. USA heating stoves create more smoke than cook stoves but the smoke is transported outdoors in the chimney and diluted by clean air to meet EPA outdoor air standards for PM2.5. Cooking outdoors, especially in a bit of wind, directly dilutes the PM2.5.
When the outdoor air is cleaner, the emissions from the stove can be higher. When the outdoor air is dirtier, the emissions need to be cleaner. Simple! Aprovecho published a model that estimates emissions based on the quality of the outdoor air. See: https://aprovecho.org/portfolio-item/project-planning/
ISO Tier Mapping for CO and PM2.5 per MJdelivered for the natural (blue triangle) and forced draft (orange dot) cases. Note the log scale on both axes.
As seen on the upper right side of the graph above, stick burning stoves (even in the lab) emit very high levels of PM2.5. That can be OK when used with a functional chimney or outdoors in rural locations with limited numbers of cooks per hectare. But in many more crowded situations the emission rates need to be much lower to protect health.
Adding forced draft mixing to many types of stoves, including the open fire, can be very effective in reducing the emission rates of PM2.5. The Jet-Flame shoots primary-air-only jets into the bottom of the fire and this simple technique reduces emissions of PM2.5 and CO, while reducing fuel use and time to boil. We hope that technologies like the Jet-Flame can assist stove projects to protect health especially when combined with chimneys and/or outdoor cooking.
Kirk Harris has been investigating TLUDs for decades and, as far as I know, his natural draft TLUD burning pellets achieved the lowest natural draft recorded score for PM2.5: 0.7mg/minute at high power (Lawrence Berkeley National Laboratory). This video shows Kirk in China at Shengzhou Stove Manufacturer where Mr. Shen built a copy of his stove to start the process of possibly manufacturing it.
The fascinating aspect in the video is how fast the flame is swirling, keeping the flame below the level of the pot and increasing dwell time.
Adding a fan shaped static mixer between the hole in the concentrator ring and the bottom of the pot has become commonplace in various TLUDS since Kirk invented the technique. We recently added a fan shaped static mixer in a natural draft TLUD to get rid of creosote. The tars were burned up in the hot, swirling flame.
Keeping the flame below the cold surface of the pot is always helpful and can be achieved with the Jet-Flame and in both natural draft and forced draft TLUDs.
To last long enough for commercial/carbon success, the combustion chamber has to be made with cast refractory ceramic. Making the static mixer and combustion chamber from cast refractory ceramic dramatically increases longevity. The Oorja stove in our lab has lasted for about 20 years!
I imagine Dr. Tom Reed smiling in heaven as the stove community moves closer to optimization with:
Electrostatic Precipitation: Smoke particles are negatively charged and attracted to positively charged metal plates that are automatically cleaned.
Both automobiles and the biomass industry rely on improving combustion efficiency and post combustion reduction of PM2.5 to achieve “clean burning.” It’s really hard to rely on the combustion chamber to burn up enough of the harmful smoke to protect health, especially in large scale applications. Of course, all efforts should be made to be as efficient as possible. The goal is to burn up everything! In cookstoves, with limited space and a pressing need to be affordable, the problem becomes more acute.
Biomass heating stoves are larger and can cost a lot more than cookstoves. Industrial technologies are even less constrained. For decades, home heaters have tried catalysts to reduce emissions. Factories have used a wider array of technologies including filtration, catalysts, and electrostatic precipitation. Chapter 8 in “Clean Burning Biomass Cookstoves, 2nd edition, 2021” includes explanations of these technologies.
Generally, filtration can work very well to capture dust and smoke with reported efficiencies of up to 99% (Frisky, et al., 2001). Catalytic converters are placed into the hot exhaust path where temperatures are hot enough (above 426°C). They work well with CO (30% to 95%) but not so well to remove PM2.5 (30% to 40%) (Hukkanen, et al., 2012). The Swiss electrostatic precipitator (ESP) called the OekoTube has been measured to reduce PM2.5 by 80.2% to 97.7% (Brunner, et al., 2018). However, as in industrial uses, routine cleaning is necessary to remove creosote and other coatings that interfere with proper function. Unlike filters and catalytic converters, the low wattage ESP does not reduce the draft in the stove, which could be potentially advantageous.
ARC has been experimenting with post combustion of PM2.5 since 2017 as a result of the EPA SBIR funded work to create a clean burning biomass heating stove. We believe that if ESP is to be useful, automatic self-cleaning must be included, as in some industrial products. The hope is to invent super clean combustion but it’s great that post combustion approaches already exist. On the other hand, forced draft mixing, which is relied upon for combustion efficiency in industry, is largely missing in both cookstoves and residential biomass heaters. Perhaps its addition will be sufficient to reach the goals of protecting health and carbon neutral fuel use with renewably harvested biomass?
https://aprovecho.org/wp-content/uploads/2022/02/ESP-illustration.png14962082Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2022-02-16 18:31:212022-02-16 18:53:06Post Combustion Reduction of PM2.5
The Gates funded Global Health Labs and ARC/SSM invented the Jet-Flame
Global Health Labs, Shengzhou Stove Manufacturer and Aprovecho teams show off the Jet-Flame
Shengzhou Stove Manufacturer manufactures the Jet-Flame (jet- flame.com) that is being field tested in over 30 locations. Our lab helped to create this accessory that is designed to reduce emissions while increasing thermal efficiency and reducing time to boil. 30 pre-heated primary air jets shoot up into the fire resulting in increased molecular mixing and elevated temperatures. Smoke is reduced by about 90% compared to an open fire.
A forced draft stove can be very clean burning, but start up may create a lot of PM2.5. This is because the cold combustion chamber can allow a higher percentage of the smoke and Carbon Monoxide to escape unburnt. David Evitt, COO of ASAT (the for-profit arm of ARC), invented a method for lighting the Jet-Flame that can be a lot cleaner.
Wet 30 grams of left over charcoal with 10 grams of alcohol.
Place the small pile of charcoal on top of the holes in the Jet-Flame.
Light the charcoal.
Turn on the Jet-Flame.
Push the tips of the sticks of wood against the pile of burning charcoal.
Keep on pushing the sticks into the fire as the tips are consumed.
Here’s a video showing how we light fires in the lab:
https://aprovecho.org/wp-content/uploads/2021/11/11.3-no-smoke.jpg309580Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2021-11-02 21:34:322021-11-03 15:47:24Video: Lighting a Fire With The Jet-Flame – No Smoke
The Jet-Flame in the CQC high mass brick Rocket stove
I ask for help when moving the CQC stove. We built it on a piece of plywood and two folks can, with care, move it around the lab but it is heavy. The sand/clay/cement bricks are dense at 1.4 grams per cubic centimeter after being baked many times in the stove. Dr. Winiarski advised that, when possible, Rocket stoves should float in water at less than 1 gram per cubic centimeter.
For a long time, people have added sawdust and other lightweight materials into earthen mixtures to try to lighten up stoves. I ended up at Shengzhou Stove Manufacturer (SSM) in China because for hundreds of years ceramicists had manufactured (and sold in Africa since 1407) durable earthen stoves that weighed around 0.7 grams per cubic centimeter. Their amazing clay floats when dug out of the ground! It is full of diatomaceous earth. The Shens own a 100 year supply of clay in two mines next to the factory.
Why go to
all of this trouble to lighten stoves?
The heat from the fire is diverted into the mass of the stove body and less heat is available to cook food. It is harder to start a hot, intense fire in a high mass combustion chamber. In a natural draft stove, this can be disadvantageous. The open fire has other problems but, out of the wind, the hot gases from the flames directly contact the pot and it’s common for open fires to have higher thermal efficiencies compared to high mass stoves, including Rocket stoves. Lightening the bricks helps to address this difficulty. Heat is still diverted into the stove body, but less. Well insulated, mostly metal, Rocket stoves successfully avoid most of these losses.
Indigenous cooks, experts at using fire, often use grasses and twigs to start a hot, fast fire in a high mass stove. You need to pour the BTUs into the stove to quickly prepare food. Speed to cook is almost always the first priority when talking to cooks around the world. When the SSM Jet-Flame is added to the high mass stove, the mini blast furnace immediately starts a hot, over 1,000°C fire that delivers relatively hot gases into the channel gap around the pot created by the pot skirt. (The CQC skirt creates a 5mm channel gap that is 7cm high.)
The Jet-Flame creates a surprising result
The thermal efficiency in the first CQC/Jet-Flame test (see below) was 33%. The 5 liters of water boiled in 12.5 minutes. After the first 12.5 minutes of heating, the over 1,000°C fire started to heat up the mass and the water boiled more quickly in 10.2 minutes at 38% thermal efficiency. Three more short, but intense, heating phases resulted in the thermal efficiency incrementally rising to 41%, 42%, and 45%. The progressively hotter gases scraping against the sides and bottom of the pot in the small channel gap were more and more successful at transferring heat through the metal walls of the pot into the water.
When thermal efficiencies are in the 40% to 45% range, the performance of the high mass stove is similar to low mass, insulated Rocket stoves. This similarity was completely unexpected at ARC.
Results of five tests of the CQC Stove with Jet-Flame.
https://aprovecho.org/wp-content/uploads/2021/03/3.24.21-cqc-jf.png648864Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2021-03-24 16:13:222021-03-24 16:13:25Looking at High Mass and Insulation
ARC is investigating how to optimize the performance of the SSM Jet-Flame in the CQC earthen brick stove. Forty six thirty-minute ISO 19867 Water Heating Tests were completed under the LEMS hood at seven fan speeds. Two 4 cm x 4 cm douglas fir sticks were burned side by side. Five liters of water in a seven liter pot were heated, and the CQC pot skirt was used in all tests.
Results
Tier 4 ISO Voluntary Performance Targets:
Thermal Efficiency 40% to 49%
CO
<4.4g/MJd
PM2.5 <62mg/MJd
Time to boil: The time to boil decreased with an increase in fan speed.
Thermal efficiency: The thermal efficiency stayed
close to 35% in most cases and was higher at 3 and 8 volts (around 40%).
Firepower: The firepower rose to 6.8kW at 8
volts, starting at 2.6 kW at 2 volts.
Emissions of Carbon monoxide: Generally emissions decreased with
increasing fan speed.
Emissions
of PM2.5: 7 and 8 volts scored the best, at half of the result of 5
volts.
Combustion chamber temperatures: The mid combustion chamber temperatures rose with increases in fan speed from 382C to 730C.
Excess
air: Lambda fell as voltage increased from
4.1 to 1.9.
We recommend that the project do enough field testing to determine what settings are preferable to local cooks, remembering that higher voltages consume more power. In this way, the Jet-Flame/CQC stove can be tailored to regional cooking, keeping in mind the power output and use patterns of the CQC photovoltaic solar system.
Here’s what the flame looks like when varying the
voltage:
https://aprovecho.org/wp-content/uploads/2021/02/2.24.21-stove-test.jpg438577Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2021-02-24 17:03:032021-02-24 17:49:34Varying Fan Speed in the SSM Jet-Flame/CQC Stove
