If wood gas passes into flame, it can ignite and less un-combusted smoke escapes. Which of the following patterns has the greatest potential for clean burning? Maybe “the devil is in the details”?
- The pattern that Dr. Winiarski often tries is downdraft/down feed. The wood is burned at the bottom of a vertical stick that can fall down as it is consumed. Air is pulled down alongside the sticks and into the fire. The charcoal falls below the sticks and in front of the flame path as flame is pulled horizontally into an insulated space by the draft in the Rocket short chimney.
- Side feed/side draft is how most people feed a fire. The sticks are pushed into the fire as they burn. In this pattern, the fire creates charcoal that lies underneath the burning sticks of wood and helps to keep the fire going. When the tips of the sticks burn combustion is fairly clean. The sticks and fire are directly under the short chimney in the Rocket stove and the flame is pulled up towards the pot.
- Top Lighting a batch of fuel. Sticks, for example, are loaded vertically, packed fairly tightly and hold each other up in a crucible. The entire top of the fuel bed in lit and is on fire. The fire slowly travels down into the crucible. The wood gas rises and enters the fire from below.
If you wanted to minimize escaping smoke would you light the fire on the bottom or side or top of the sticks?
If the wood sticks are lit on the sides or on the bottoms the wood gas must be forced by the stove to join into the fire. Lighting the entire top of the batch of fuel has the natural advantage that all wood gas passes into flame. Masonry heating stoves have often used this top burning technique to clean up combustion. In each case the same principle applies: 1.) All the wood gas must go into the hot flames and 2.) Be well mixed there with air 3.) For a long enough time for complete combustion to occur.
Diagram of Top Lit UpDraft Stove (TLUD)
Happy Holidays Clean Combustion Techniques
1.) When wood is burned a lot of smoke is produced but the made charcoal at the tip of the wooden stick contributes heat but does not make much smoke. Rocket Stove: Increase the time that the charcoal at the tip is burning. TLUD: A layer of hot charcoal covers the fresh wood.
2.) If the stove begins smoking the rate of reaction (solid turning into gas) is probably too fast. Too much wood gas is being produced and un-combusted fuel is escaping. Rocket Stove: Pull the sticks back until just the tips are burning. TLUD: Reduce the primary air.
3.) Mixing the smoke, gases, flame, and air can reduce emissions dramatically. Rocket Stove and TLUD: Cut up the laminar flames with natural draft mixing devices. When using secondary air increase the draft until fast moving jets completely cover the top of the burning fuel.
4.) Create a space filled with fire that forces the smoke, gases, flame, and air to mix more completely. Rocket and TLUD: Orifices successfully increase mixing.
5.) Increase the dwell time to improve combustion efficiency. Rocket and TLUD: Do not make the mixing chamber above the fire too short. With sufficient draft install fixed fan blades to induce swirl that makes at least two revolutions.
6.) Secondary air more effectively enters flame when the pressure difference is assisting the mixing process. Rocket and TLUD: In natural draft stoves the pressure is lower in fast moving flame. Add higher pressure secondary air downstream of the mixing device.
HEPA home furnace filter reduces PM2.5 emissions
There are a number of methods to reduce personal exposure to household air pollution associated with using biomass fuel for the daily cooking and heating taking place in nearly 40% of global households. These most commonly include 1.) Increasing ventilation rates, 2.) Installing a chimney and 3.) The use of cleaner fuels and cook stoves. A recent ARC paper available (free for a limited time) at:
investigates two less-commonly considered methods: 1) Reducing exposure through filtration and capture of PM2.5 and 2) Avoiding making emissions by using made charcoal and retained heat for cooking.
The smoke is pulled through the filter and less smoke exits the room
When cook stoves are operated inside an enclosure from which smoke is pulled through an inexpensive HEPA-type furnace filter before exiting to the outside, the personal exposure levels, room concentrations, and external pollution are reduced. To test this method, an enclosure was built from which a box fan pulled the air and PM2.5 through four different furnace filters. The rate of PM2.5 production (mg/min) exiting the filter was monitored with gravimetric measurement under a LEMS emissions hood during the high and low power phases of the Water Boiling Test 4.2.3 conducted on a biomass rocket stove with forced draft.
The average of seven baseline emissions tests with no filter was 7.5 mg/min of PM2.5. The average of seven tests using the highest quality furnace filter (3M 2200) was reduced to 1.5 mg/min and the difference was significant at 95% confidence. The use of retained heat to simmer dramatically reduced emissions of PM2.5 by burning the boil-phase-made-charcoal and using retained heat in the stove while 5 liters of covered water were simmered for 35 minutes.
It was great to attend the Global Alliance Forum a week ago in Delhi, meeting and learning from so many colleagues. Dr. Omar Masera presented a paper that summarized a survey of fugitive emissions from plancha type stoves in Mexico. The World Health Organization Indoor Air Guidelines figure that 25% of the total emissions going up a chimney end up inside the house because stoves and chimneys are leaking that much into the room air.
It’s starting to rain here in Oregon and on my daily drive to work smoke is pouring out of chimneys again while the indoor air stays clean as the draft in the chimney puts the emissions outdoors. These chimneys are well built and are maintained so almost no smoke pollutes the home. The chimney is located several feet above the peak of the roof to encourage the pollution to drift away from the windows and doors. Without chimneys the high levels of smoke emitted from these heating stoves would make living indoors very uncomfortable.
The five plancha cooking stoves in the study were also not very clean burning (Tier 1 for PM 2.5 for high and low power) but results from 54 tests showed that fugitive emissions into the room were only 1% of the totals resulting in Tier 4 for Indoor Emissions of PM 2.5 and CO (Medina, et al., Development Engineering Volume 2, 2017, pages 20-28). Why have chimneys, the historical, relatively inexpensive, and most practical technology to protect health, not become a most popular intervention? At ARC we try to combine clean combustion and superior heat transfer efficiency with a chimney so that the stove brings a synergy of improvements to the consumer.
Dr. Tom Reed: The Father of Clean Burning?
Well, Tom is certainly one of a small group including Paal Wendelbo and Ron Larson who started making “Tier 4” stoves in the 1980’s. I think of Tom when I light his wonderful forced draft TLUD camp stove which I do to demonstrate a simple ‘no smoke’ stove. Tom’s webpage says,” In 1972 Tom Reed became concerned about the energy and fuel futures of the U.S. and began working on alternate fuels on the side while working at MIT in the field of solid state research. He was the first person to use alcohol blends during this period and when he wrote “Methanol – A Clean Fuel for the 21st century”, for Science magazine, it changed his career. In this article he said that for the short term methanol would be made from natural gas, but in the long term biomass could supply our needs forever.
Looking out of the ARC office windows, the rural Oregon road that curves past the campus is lined with 100 foot tall trees and the forest, even after decades of logging, is immense. Learning how to cleanly combust wood seems important here in the US as it does in other countries. Sustainably harvested energy is only amazing if it does no harm when burned and complete combustion opens a carbon neutral alternative that may be necessary in all sorts of applications.
How close is the complete combustion of biomass? Pellet stoves, industrial burners, and forced draft in general gets pretty close. Scrubbing the remaining emissions gets closer. In the cook stove world progress has been faster than I imagined as back yard R&D coupled with university analysis, supported by the DOE and EPA, inches science closer to the zero emission goal. As I envision the steps needed to complete the understanding leading to complete combustion, I’m thinking that reaching the goal is almost inevitable, with thanks to true believers like Dr. Tom Reed who helped to start the ball rolling.
How to Install a Catalyst in an Existing Heating Stove
When starting a cold stove the catalyst is disengaged from the gas stream by pulling on a steel rod. A green light will indicate when the stove gases are above 600F. The catalyst can then be engaged by pushing in the rod.
In one experiment in the ARC lab the installed catalyst reduced the emissions of PM 2.5 in a simple steel box stove made in Mongolia to about 1g/hour which meets the 2020 EPA Heating Stove Standard.
The following is a summary of a January/February 1983 article from Mother Earth News that describes design principles for installing a catalytic converter in a wood burning stove:
- Ceramic or metal catalytic converters are coated with platinum and/or palladium and/or rhodium.
- A catalytic converter can reduce the ignition temperature of carbon monoxide and hydrocarbons from upward of 1300°F to the 500-700°F range.
- Once the smoke passing through the catalyst reaches that threshold, it will oxidize as long as the combustibles in it are well mixed with a sufficient supply of air.
- After it is “lit,” the converter will produce enough heat to maintain ignition, even though the temperature of the incoming “wood gas” may drop slightly below 500°F.
- The basic concept can be applied to just about any roughly cubic metal (steel or iron) wood burner.
- Since the converter must reach a temperature of at least 500°F before it “lights off,” a location close to the fire will insure that the unit starts working as soon as possible and continues to do so throughout the burn.
- It is strongly suggested that a baffle arrangement is incorporated, diverting flame, residues, and ash, since eliminating the baffle could result in drastically shortened catalyst life.
- There must be adequate oxygen at the catalyst, however, it was determined that specific provisions for secondary air weren’t needed. Under all burn conditions, there was always an adequate air supply remaining at the converters.
- Hotter air (both primary and secondary) means better performance. Well warmed primary air encourages efficient primary combustion, and hot secondary air is vital to maintaining ignition temperature at the catalyst.
- A well sealed, strong bypass valve is mandatory. Even though the cell structure of the catalyst is relatively open (in comparison with that of the automotive variety), the unit does restrict natural draft to some extent, particularly when the catalyst isn’t lit. For that reason, there must be a valve that allows you to shunt smoke around the catalyst while a new fire gets going and whenever the stove’s doors are opened.
- As we’ve already mentioned, the single most important precaution is to always bypass the converters when the doors are opened. Furthermore, that same valve will have to be open while you’re getting a fire going.
- And should the catalyst go out for some reason, we’ve found the thermometer to be the only easy way to tell.
- After the charge of wood burns down — but not out — you can open the bypass, add more fuel, and then close the bypass immediately.
- Materials such as coal, wood pellets, paper with colored ink, tires, plastics, and treated or painted lumber, may “poison” the catalyst and render it ineffective.
Aprovecho Research Center
PO Box 1175
Cottage Grove, OR 97424, USA