Discussing Stove Design with Dr. Thatcher
Dr. Tracy Thatcher of the Civil Engineering department is an expert on air quality and has an understanding of the combustion process and how it applies to our design. After speaking with Dr. Thatcher, it became evident that a complete stove redesign was necessary in order to optimize the rice hull combustion process.
First, as with any combustible organic matter, the rice hulls need to get hot enough to gassify. Yellowed or browned rice husks is a sign of an insufficient temperature because the hulls did not burn completely. See the following graph of volatile matter removed versus temperature from the paper posted above, titled “Thermal Characteristics of Rice Hulls”:
As you can see, the rice hulls must reach 400-500C to separate volatile matter from the hulls (known as “outgassing”). The key to starting the rice hull stove and achieving a successful burn is to use proper kindling. We tried to light the rice hulls with an acetylene torch but that only charred the top layer of rice hulls and the remaining hulls did not sustain a burn.
According to Dr. Thatcher, a quality stove would allow the user to regulate the air flow and fuel feed. Sometimes, you need more air flow initially to begin a burn, but then the air needs to be choked slightly to prevent too much of a burn. She liked the idea of a stovetop and agreed that an exhaust system is critical for an indoor stove that burns a silica-rich biofuel such as rice hulls.
The fuel-to-air mixture is important, and one can test the quality of burn with a CO/CO2 meter. If you have too much CO, it means there is not enough air getting into the stove.
We plan to take this feedback from Dr. Thatcher and put together a useful portfolio of information for next year’s group so that they are not bogged down trying to achieve a successful burn. Rather, they can hopefully start with a better understanding of the science and mechanisms behind combustion that we did not understand until recently.
Literature Review: Mayon Turbo Stove Group – Winter 2011The purpose of this literature review is to collect information related to our problem statement and acknowledge ways that people have already attempted to solve the various problems.
Benefits of the Turbo Stove:
- Slows deforestation by running on agricultural waste
- rice hulls work well as fuel and are easily abundant as rice is the #1 food crop in the world
- Fast boiling
- Convenient to use
- Cheap ($15-$20) to buy
- Low fuel consumption/cheap and abundant fuel
Future Plans/Goals for the Stove:
- Figure out what exactly is successful about the Turbo Stove so that it may be manufactured and used in mud stoves
Science Behind Rice Hull Combustion
To burn rice hulls effectively, the turbo stove must be constructed with several design considerations, including:
- Air flow into the combustion area
- Prevention of excess air
- Complete combustion of fuel
- Air circulation and mixing
- Steady yet gradual replacement of ash with new rice hulls
- Insulation of the fuel bed
Many studies have been done to characterize the pyrolysis, combustion, and gasification of rice hulls. Some of these published papers are provided below.
Thermal Characteristics of Rice Hulls.pdf
Pyrolysis of Rice Husk in a Fixed Bed Reactor.pdf
Preliminary Investigation on Combustion Characteristics of Rice Husk in FBC.pdf
OVERVIEW OF COMBUSTION AND GASIFICATION OF RICE HUSK IN FLUIDIZED BED REACTORS.pdf
Combustion and Emission Characteristics of Rice husk in a Rectangular Fluidized Bed Combustor.pdf
Most of these studies analyze rice hull combustion by analyzing the composition of the post-burn residue.
- Exhaust System: http://en.wikipedia.org/wiki/Exhaust_pipe
Piping used to guide exhaust gases away from a controlled combustion site. This Wikipedia page defines what an exhaust system is, what design criteria is needed, and explains other terminology related to exhaust pipes. Everything on this page is applicable for the most part. However, the systems that are focused on are relevant to cars and motor vehicles. Even though the focus is not on stove exhaust, we can still use the page for ideas, basic design, and terminologies.
- What Pipes Should be Used to Exhaust a Woodburing Stove? http://answers.yahoo.com/question/index?qid=20081120125455AAMGye5
This website clearly explains the best system for creating an exhaust system for a wood burning stove. We can interpret this into rice hulls as well as they are very similar. It gives different suggestions of ways to install an exhaust pipe as well as different variations of systems.
- Pellet Stoves: http://en.wikipedia.org/wiki/Pellet_stoves
Although this is not the type of stove we are designing, the pellet stove already has an exhaust system installed. We can look at the system in use to determine what is relevant to our Turbo Stove and how it can be used in other countries for cheap and with easy accessibility and installation. This website includes benefits from the stove, principles of operation, and backgrounds of other stoves. All of which useful in achieving our goal, having the background of other stoves will also give us other options and references.
Rice Hull Combustion
- Stoves for Rice Husk and Other Fine Residues: http://practicalaction.org/stoves-for-rice-husk-and-other-fine-residues
Any stove that is burning fine residues (e.g. sawdust, rice hulls) requires a precise fuel-to-air ratio in order to burn effectively. Consequences of incomplete combustion include toxic smoke, large amounts of waste, and a smaller proportion of useful energy extracted from the fuel. Three ways to manipulate the fuel-to-air ratio in a stove design are:
– Size of air inlet valves
– Fuel feed rate into the combustion chamber
– Removal of ash from the burn area
Signs of incomplete combustion include smoldering and smoke events. Smoldering is flameless combustion resulting from insufficient air flow, and this reaction occurs at the surface of the solid rather than flaming which occurs in the gas phase (http://en.wikipedia.org/wiki/Smoulder)
Existing Mayon Turbo Stoves:
- Features of the MTS: http://www.reap-canada.com/bio_and_climate_3_3_1.htm
- High efficiency and clean combustion
- Affordable ($15-20 USD) to buy
- Fast boiling
- Convenient (to introduce new fuel, the user simply taps the stove every 7-10 minutes)
- Low fuel consumption (approx. 1.5 rice hull sacks per family per week)
- Compatible with a variety of fuels, including rice hulls, corn cobs, peanut shells, cocoa shells, crushed coconut shells, and sawdust
- Structure of the MTS: http://apptechcalpoly2010des.wetpaint.com/page/Biofuel+Stove
- This particular prototype is designed for indoor use in the Congo where outdoor cooking is impractical due to high winds:
Alternative Turbo Stove Designs
- Lo Trau Stove of Vietnam
Apart from the Mayon turbo stove, there are a number of alternative turbo stove designs that could offer design solutions to the engineering problems that our group faces. One such variation is the Lo trau stove of Vietnam that gained popularity in the region in the 1950s. This stove can burn a variety of fuels including wood chips, sawdust, small branches, and rice hulls. Additionally, the Lo Trau stove has multiple designs including a fixed and portable version.
- Lacai Cooking Stove for Developing Nations
Another model of the turbo stove is the Lacai Cooking Stove for Developing nations. These stoves can be purchased in batches of 500 or more for assembly and sale in developing countries. The stoves are built with stainless steel construction and utilize a fan in the base to move air through the unit. Although they seem to be well built and probably burn well, the cost of manufacturing these stoves would be much higher than other turbo stove designs.
Build a Mayon Turbo Stove to be used for indoor cooking and modify it by adding an exhaust system, providing insulation to the body, and making the removal of burned fuel more efficient so that it can be an inexpensive and appropriate way for people in the Congo to cook indoors.
Helpful Links:Projects that have already been done:
- Mayon Turbo Stove
- Mayon Turbo Stove – Full Performance Report (PDF)
- Lo Trau Stove
- Turbo Wood-Gas Stove
- Biomass Cook Stove – Wikipedia
- Microsoft Word – residue_stoves.doc.pdf
- Rice Husk Stove Handbook
- Practical Action – Stoves for rice husks and other residues
For Design and general FYI: