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.
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.
The radial inward movement of air is called Squish. Squish can be defined as an inward flow of air towards the combustion recess.
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: http://aprovecho.org/portfolio-item/project-planning/
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.
The Journal “Energy for Sustainable Development” has just published Aprovecho’s most recent research paper, “Retrofitting stoves with forced jets of primary air improves speed, emissions, and efficiency: Evidence from six types of biomass cook stoves.” It was authored by Samuel Bentson, David Evitt, Dean Still, Dr. Daniel Lieberman and Dr. Nordica MacCarty (Energy for Sustainable Development 71 (2022) 104–117)
Incorporating jets of forced air into biomass cook stove combustion has been shown to potentially decrease harmful emissions, leading to a variety of designs in recent years. However, forced draft stoves have shown mixed success in terms of real world performance, usability, and durability. The Shengzhou Stove Manufacturer Jet-Flame forced draft retrofit accessory was developed by the Gates funded Global Health Labs and ARC, to implement forced jets of primary air at a low cost into a wide range of types of cook stoves using a small 1.5-W fan housed in a low-cost cast iron body to be inserted beneath the fuel bed of a biomass cooking fire.
This research sought to quantify the potential efficiency and emissions performance impacts of the Jet-Flame when installed in six different types of biomass cook stoves (three open or shielded fires and three rocket stoves) versus the natural draft performance of each. The effect of the operating fan voltage was also measured. A series of tests following a modified ISO 19867-1:2018 protocol were performed in the laboratory using the Aprovecho Laboratory Emissions Measurement System (LEMS) equipped with additional oxygen and temperature sensors.
Results for each stove, carefully tended with a single layer of sticks, showed that the global average PM2.5 reduction with the Jet-Flame was 89% relative to the natural draft cases, with larger relative improvements seen in the most rudimentary stoves. CO was reduced by a global average of 74%, reaching Tier 4 or 5 for all stoves. Thermal efficiency was also improved by 34% when calculated without taking into account the energy content of the remaining char (or 21% with char), illustrating the value of burning char to provide cooking energy rather than leaving it unburned in the combustion chamber as is common in many natural draft stoves. Time to boil was also reduced by 8%.
In addition, adjusting the voltage of the jet-flame assisted in modulating firepower, possibly improving the usability of the stove.
http://aprovecho.org/wp-content/uploads/2020/08/jet-flame-in-cqv.png520352Kim Stillhttp://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2022-10-14 16:31:422022-10-14 16:32:49Jet-Flame Research Results From Six Types of Biomass Cookstoves
The Rocket stove requires forced draft to create sufficient mixing in the flame to achieve Tier 4 for PM2.5. In addition, when sticks are pushed too fast into the Rocket, too much wood gas enters the combustion zone and a lot of smoke is emitted. Metering of the fuel is necessary to keep the Rocket stove even moderately clean burning. Unfortunately, that is unlikely to happen in real life.
On the other hand, the TLUD supplies a constant amount of wood gas into the combustion zone. A five inch in diameter reaction vessel produces around 4kW of energy (5 liters of water boils in about 20 minutes using a pot skirt with a 6mm gap).
The emissions of CO and PM2.5 can be very low in a natural draft TLUD when:
The primary air controls the rate of reactions (how fast the wood pellets are turned into wood gas).
The jets of pre-heated secondary air provide sufficient mixing of wood gas, flame, and air by completely covering the top of the fuel bed.
The burner section allows sufficient Temperature, Time, and Turbulence to complete the combustion process.
Nothing is perfect. The TLUD, whether forced draft or natural draft, has trouble achieving enough Turn Down Ratio to simmer food efficiently. It’s also generally necessary to burn pellets to lower emissions to Tier 4 levels. At the same time, we love to cook on TLUDs and it is great not having to constantly adjust the fuel!
As Dr. Tom Reed said, “Now you’re cooking with the real natural gas.”
Before starting to develop cleaner burning cook stoves
in the 2013-2015 DOE project, ARC researchers completed a survey of best
performing existing stoves. Improving combustion efficiency to protect health
(and climate) has continued as various cook stove organizations have worked
tirelessly to meet the WHO 2015 PM2.5 Intermediate
Emission Rate Target of 1.75mg/minute, calculated to protect health in homes using
biomass to cook.
With a 6mm channel gap pot skirt, many stoves scored close to 50% thermal efficiency.
TLUDs were not able to achieve enough Turn Down Ratio (TDR) to simmer water.
Burning wood does not emit much CO so meeting the WHO CO Target (.35g/min) was easy.
Forced draft TLUDs scored between 2mg/min PM2.5 to around 5mg/min.
Stoves with chimneys met the aspirational WHO PM2.5 Emission Target of 0.23mg/min since the smoke was transported outside.
SupaMoto Forced Draft TLUD Bests WHO Goals
The new SupaMoto stove from Emerging Cooking Solutions with combustion technology from partner company Zemission has made great progress! For information contact Mattias Ohlson at: firstname.lastname@example.org. As seen below, in Water Boiling Tests at ARC, the SupaMoto Forced Draft TLUD achieved:
51% to 56% thermal efficiency (without pot skirt)
0.1g/min to 0.6g/min for CO
0.19mg/min (simmer) to 1.11mg/min (high power) for PM2.5
temperature corrected time to boil the 5L of water was fast, about 18 minutes
As in the FD TLUD Mimi-Moto stove, turn down in the Supa-Moto is achieved by inserting an accessory into the combustion chamber. The Supa-Moto Turn Down Ratio (TDR) varied between 1.91 to 2.11. When a lid is used on a pot, a TDR of around 3 saves more fuel when a lower firepower is needed to simmer food to completion. Reducing the forced air jets in a TLUD does not create sufficient TDR.
It is so gratifying to witness progress! I never
thought that we would see a biomass stove come so close to meeting the
aspirational PM2.5 WHO Emission Rate Target.
To test a stove that easily meets the Intermediate PM2.5 Target is
amazing. Mattias and the Zemission team have moved TLUD technology forward and
it is a very valuable achievement!
Thank you for your work! Learning how to cleanly
combust biomass has important ramifications in all parts of the world now that
climate change reinforces the importance of renewable biomass as a health and
climate friendly energy source.
How can smoke, extremely dangerous for health and climate change, be ignored in carbon credit equations? Carbon dioxide and methane are counted but not smoke. Carbon dioxide is reduced when heat transfer is improved resulting in less wood being burned. Wood doesn’t make appreciable amounts of methane.
Because smoke is not counted to earn carbon credits, smoky stoves with good heat transfer efficiency make as much money as clean burning stoves even though the Black Carbon in smoke is something like 680 times worse than CO2 by weight for warming. Because smoke is not included in climate credit math, adding clean burning to biomass cook stoves usually has to be as inexpensive as possible.
We know that adding high pressure mixing to Rocket stoves dramatically reduces smoke. As of 2022, forced draft is required to achieve adequate amounts of mixing. Mixing requires high pressures that (so far) cannot be made with natural draft. We know how to improve the Rocket but are in the process of completing the transformation to clean burning.
http://aprovecho.org/wp-content/uploads/2022/09/44527589921_65c870e76f_c.jpg370799Kim Stillhttp://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2022-09-16 16:19:192022-09-16 16:24:57A Recent History of the Rocket Stove: 2022
The investigation of how to reduce emissions and fuel use in biomass stoves continued with support from an EPA SBIR award. Two products were manufactured by our Chinese partner SSM, a heating/cooking stove and the Jet-Flame, a $12 insert that has made stoves 67% cleaner burning in field tests. https://www.jet-flame.com/
The Gates funded Global Health Labs (Dr. Daniel Lieberman) also worked with ARC/ASAT and BURN (Peter Scott) to improve the Rocket stove. BURN and ARC/ASAT added fan driven mixing to the Rocket stove.
Learning how to optimize the use of high pressure jets of air at high, medium, and low power required hundreds of experiments. Different pressures are needed as firepower is adjusted. The size of the fuel also affects emission rates. Experiments under the LEMS hood determine the location of jets, pressure, and volume of air for varying applications.
http://aprovecho.org/wp-content/uploads/2016/03/Sidefeed-1.jpg276187Kim Stillhttp://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2022-09-02 16:19:492022-09-02 16:45:36A Recent History of the Rocket Stove: 2016-2021
In 2011, Dr. Samuel Baldwin at the Department of Energy (who wrote the Bible on cook stoves in 1987) organized a two-day 100 person conference to identify how cook stoves could be improved and manufactured. Key recommendations were:
At least 90% emissions reduction and 50% fuel savings are appropriate initial targets for biomass cook stoves.
Multiple stove designs will be needed to accommodate a variety of cooking practices, fuels, and levels of affordability.
Technical R&D should guide and be guided by field research, health, social science, and implementation programs. At every stage, laboratory and fieldwork should be integrated into an iterative cycle of feedback and improvement.
The cost and performance tradeoffs associated with the use of processed versus unprocessed fuels should be explored. While processed fuels can improve stove emissions and efficiency, the processing adds additional costs and these fuels may require a fuel distribution system.
From 2013-2015, ARC received a grant from DOE and spent three years establishing a baseline of stoves in use and then improved five types of stove prototypes with the iterative development process using the LEMS emission hood. The lab testing showed how combustion and heat transfer could be improved in those five types of stoves with the hope that field testing would evolve useful products that use less fuel and make less smoke. A book was written: Clean Burning Biomass Cookstoves, (2015) available on the publications page. The book was updated in 2021.
http://aprovecho.org/wp-content/uploads/2016/03/Sidefeed-1.jpg276187Kim Stillhttp://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2022-08-25 14:44:172022-08-25 15:13:36A Recent History of the Rocket Stove: 2011-2015
In 2009, The New Yorker published an article about the Rocket stove entitled Hearth Surgery: The quest for a stove that can save the world. One year later, USAID funded field tests in Africa showed that the insulated Rocket stove was not cleaner burning than the open fire. The Rocket with skirt saved 40% of the fuel to cook and emissions were only reduced by that amount.
Not a Planet Saver, yet!
The insulated Rocket combustion chamber raised temperatures but as Dr. Winiarski realized at the time, flame, air, and gases were not adequately mixed to achieve sufficient combustion efficiency. Larry knew that the Rocket was smoky but it was simple to make and with a pot skirt saved fuel. He wanted to provide folks with a stove that was helpful and he realized that it wasn’t perfect.
Larry’s idea went viral worldwide and continues to be a favorite on the internet and in many low- and middle-income countries. Millions of Rocket stoves are manufactured and sold yearly by factories large and small.
Going viral is great but can have a downside especially when the initial products are not technically mature. It’s normal for first generation products to be improved as time goes by. The process of development continues in 2022.
http://aprovecho.org/wp-content/uploads/2016/03/Sidefeed-1.jpg276187Kim Stillhttp://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2022-08-19 13:25:292022-08-19 14:02:59A Recent History of the Rocket Stove: 2009
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: