Soot

A group of women came to my Spanish school earlier this week as part of the next group that will receive free or near-free stoves from the school.  Another student and I gave a talk to this group on “Las estufas y los beneficios para la salud.”  As the engineer, I explained about soot from stoves, and the other student, a medical student, explained about the health effects:  “el humo de las estufas tradicionales contiene miles de sustancias que con similares [a cigarros] y que son muy peligrosos para el cuerpo.  La peores incluyen la ceniza y hollín que se forman a causa de combustion en el fuego.  Estos cambian el calor de una olla nueva a negro.  Y la ceniza y el hollín pueden mezclar con el aire y entran nuestros cuerpos... (insert medical stuff here)... Las estufas de la escuela son mas eficientes de las estufas tradicionales.  El proceso de combustión in nuestras estufas es mas completo y las partículas de hollín cambian a gases mas limpias.  Entonces el aire es mas limpio tambien.” 

Here are some initial results of the soot measurement in the other stove project that I’ve been volunteering with.  This was from my first time lighting the stove, so this may not be the best gauge of how well the stove is performing.  It’s obvious though that even with this simple diagnostic we can tell that there’s soot forming from the flame.  Now to figure out how to reduce the amount formed. 

Before/antes (top/arriba) and after/despues (bottom/abajo)

Planning experiments

Understanding the theory of combustion and heat transfer in a stove can only help so much with the design.  To understand the performance of the current rocket stove design more, experiments need to be performed (experimentos son necesario para entender como es la functión de la estufa de cohete).  Unfortunately, we don’t have the funds for laser absorption diagnostics in Xela.  So what tools can we use for experimentation?  (No tenemos herrmientas por absorbción de lasers en Xela.  ¿Entonces, que herrmientas pedemos usar medir cosas en experimentos?) 

The company I’m working with has a few tools for experimentation.  I have two thermocouples, one with a limit near the boiling temperature of water, and one with a limit of roughly 773 Kelvin.  I can also use an IR temperature sensor to measure surface temperatures on non-reflective surfaces.  Furthermore, I have access to a scale to measure weight.  What would really be useful would be if I could monitor the air flow velocity, and that could give me a sense of the air-fuel ratio.  Additionally, a method to monitor soot would be helpful to determine how complete the combustion process is.  We started up the stove and played with the tools we had, and also several other methods of monitoring things.  The temperature measurements seem fine.  I explored a poor man’s soot diagnostic by putting a piece of clean pumice stone in the gas flow chamber (la idea gracias a Adela) – after a few minutes, the pumice stone will begin to darken from the soot, and using this monitoring technique I may be able to determine whether more or less soot is formed in different stove configurations.  To make more quantitative soot measurements, a particle counting device can be purchased for about $200, though this won’t happen in the time that I’m here.  We explored the air flow velocity by lighting another starter stick of wood and placing it near the air inlet of the stove, and by watching the flame enter the air inlet we could tell that there was substantial airflow.  I doubt this would yield a very good quantitative measurement of air flow velocity, but maybe by measuring the angle of the flame entering the air inlet I might be able to determine whether changing the flow geometry inside the stove has a significant effect on the air flow velocity. 

My planned experiments include time-to-boil tests.  In these tests, I will make lots of temperature measurements and ad hoc soot and velocity measurements.  And maybe a tortilla cooking test will occur as well. (Voy a medir la temperatura en muchos lugares y voy a tratar de medir hollín y velocidad de aire.  Haré un experimento de cocinar tortillas también).

Results to come.  Experimental suggestions welcome.

Rocket stoves

What is a rocket stove and how does it function?  (¿Que es y cómo función una estufa de cohete?)  The basic design of a rocket stove was introduced by Dr. Larry Winiarski.  A rocket stove consists of a fuel magazine for wood or other combustible fuels, supported above the entrained air into the flame by a grate or other supporting structure.  A rocket stove also has an internal chimney where the heated flue gases can further react. 

Rocket stoves are one of the primary types of fuel-efficient wood burning stoves in current programs that build and distribute cheap wood burning stoves to developing countries.   Why is a rocket stove more efficient than a normal stove?  (¿Porque es la estufa de cohete mas eficiente de una estufa normal?)

When wood is heated in a basic burning process, organic compounds in the wood gasify and combust with the surrounding air.  This behaves similar to a diffusion flame where the oxygen required for combustion diffuses into the flame from the outside.  If there is not enough oxygen to react with the gasified compounds, then unburned gases will result and appear as soot.  This leads to a loss in flame temperature, and thus lower energy conversion efficiency in the combustion process.  The rocket stove addresses these issues in several ways.  First, because the air enters the combustion process from under the flame, more mixing of the air and the fuel occurs before the flame (primero, la aire entra la proceso de combustion abajo de el fuego, la aire y los combustibles mezclan mas antes de el fuego).  Thus, the flame more closely resembles a premixed flame, which has the potential for a higher combustion temperature and less unburned gases than a diffusion flame if the air/fuel ratio is optimized.  Second, even if unburned gases remain, they can further react in the internal chimney in the rocket stove before heating the pot (segundo, si hay gases que no quemar en el fuego, los gases pueden quemar en la chimenea adentro antes de cambia calor con la olla).  Therefore, a properly designed rocket stove can be very efficient at converting energy to heat with clean combustion (entonces, una estufa de cohete con una deseño inteligente puede cambiar energía y calor muy eficiente y tiene combustion mas limpio).

To improve the cooking ability of the stove, the heat transfer efficiency must also be examined (necesitamos examinar la teoría de cambio de calor para mejorar cómo buen function la estufa).  Both convection and radiation heat transfer are important for a pot directly above the fire (Hay does tipos de cambio de calor que es muy importante por una olla arriba del fuego).  For two or three pot stoves, with a plancha/cooking surface over the internal chimney, the rear burners receive less radiation, and thus convective heat transfer is the predominant energy transfer mechanism (por estufas con dos o thres ollas como una plancha arriba de la chiminea adentro, las ollas segundaria no reciben calor para radiación y entonces conveción es mas importante).  To improve the convective heat transfer coefficient, the flue gas channels under the rear pots can be reduced, or turbulence can be induced, both strategies which have the potential to restrict the airflow and cause there to not be enough air and lead to soot formation.  These are the tradeoffs for stove design (estes es los intercambios para deseño de estufas).  Most of the literature on heat transfer in cook stoves are on single pot stoves, so many improvements are likely possible for plancha-topped multipot burners.

Types of stoves in Xela

The stove in my host family’s kitchen is a normal box stove where the combustion chamber is filled with wood and air enters through a small opening in the door (la estufa in la cocina de mi familia en Guatemala es un pollo y las leñas llenan la caja de combustión y la aire entra la caja por un hoyo en la puerta).  I’ve eaten lots of delicious rice, beans, eggs, tortillas, tamalitos and meat cooked by this stove (comí muchos arroz y frijoles y huevos y tortillas y tamalitos y carnes deliciosos que mi madre cocinó con esta estufa).  This stove looks like it’s made mostly of concrete and bricks, similar to the stoves my Spanish school installed for many families, and therefore, cannot be moved.  

In addition to these permanently fixed stoves, various different types of portable stoves can be purchased in Xela.  I saw a few different types at several hardware and appliance stores, including a portable enclosed box type stove (se llama un pollo) and portable gas stoves.  The wood stove shown below is about $125 (la estufa de leña es un mil quetzals) and the portable gas stove is about $200 and comes with one free cylinder of propane (la estufa de propano es un mil seiscientos quetzales y incluye un cilindro gratis). 

The portable gas stove costs no more than 10% more than the efficient rocket stove (la estufa de propano cuesta no mas de dies por ciento mas de la estufa se llama rocket).  This may beg the question why would Guatemalan women buy the wood-burning rocket stove over a gas stove.  I’m not sure what the cost of propane is in Guatemala, but I’ve been told that gas cooking is considered a more expensive affair, therefore, it’s likely that an efficient wood-burning stove that costs the same as the gas stove will be much cheaper in the long run.  Furthermore, I’ve also been told that Guatemalans prefer their beans to be cooked over a wood fire, and that beans cooked over a gas flame or beans cooked in a pressure cooker “do not taste as good.”  Therefore, many Guatemalan women who own gas stoves also have a wood burning stove in their kitchen if they can afford it.

Stove building

My teacher, several students, and I went to a small community about 8 km from Xela to finish the last step in completing the stoves that the school provided them (mi maestro y cuatro estudiantes y yo fuimos a comunidad peqeño mas o menos ocho kilometers lejos de Xela y nosotros terminados paso cuatro y final para construir las estufas de la escuela).  To install the door, we marked where the holes to attach the doors needed to be, then we used a hammer and chisel to make two holes in the bricks, and then we put the door in place and filled the holes with a mixture of cement and sand (marcamos donde necesitamos los hoyos para la puerta y entonces usandos un madrillo y cincel para tajar dos hoyos y entonces ponimos la puerta en sitio y llenandos los hoyos con una mezcla de cemento y arena). 

It was neat to see the completed stoves, which each had a combustion chamber much smaller than I originally envisioned when I saw the stove design.  One of the stoves we worked on was being used, and it was clear that there the small space for the combustion chamber was filled mostly with burning wood, and therefore there was not a lot of excess space with gas to be heated, and this part of the design is probably what makes the stove fairly efficient.

There weren’t really any streets or addresses in this rural community so we spent a lot of time looking for the houses that had the stoves from the school.  It would be interesting to learn more about how these types of communities get food, water, electricity, etc.

Intro to stove building

There are primarily two different types of cook stoves that burn solid biomass in developing countries.  One is a normal cook stove where the firewood is completely contained inside the stove and all of it can burn, and air can enter the combustion chamber through various locations, including openings in the stove top and through the fire door if it is open.  The other type of cook stove is called a “rocket stove” where the firewood is inserted into an opening in the combustion chamber, and only the tip of the firewood burns and the air required for combustion enters the chamber from primarily below the burning firewood for best efficiency.  These are the two types of stove for the two stove projects I will be helping with in Xela.  Both are more efficient that the traditional cooking method of creating a “three-stone stove” (using three stones to support a pot over an open fire), and in addition to the improvements in efficiency, these two types of cook stoves can direct the smoke resulting from the fire to outside of the house, improving the health of the Guatemalan women and children who typically spend lots of time in the kitchen.

The cook stove with only the firebox is very easy to build (la estufa con solo la caja de fuego es muy facil para construir).  It costs only about $100 and can be build in a four mornings in four steps by student volunteers (la estufa cuesta solo mas o menos ochenta quetzales y es posible construir en cuatro mañanas en cuatro pasos con estudiantes voluntarios).  The materials and tools that are required for the stove are also very simple (Las materiales y las herrmientas es necesario para construer la estufa son muy simple).  The materials include cement blocks, bricks, white sand, cement mixture, mud, and sugar for the base and the firebox (las materiales necesario es blocks de cemento y ladrillos u sacos de arena blanca y sacos de barro molido, y tapas de panela).  Also needed is the stove top, the fire door and the chimney (la plancha y la puerta y la chimenea tambien es necesario).  The first step is to build the base which has three layers of bricks and cement blocks (el primero paso es contruir la base; hay tres hiladas de ladrillos y blocks de cemento en la base).  The second step is to lay out another three layers of bricks to build the firebox with holes for the door and the exhaust (en el segundo paso los estudiantes volantarios ponen tres hilados de ladrillos con dos hoyo, uno para la puerta y uno para la humo sala).  The third step involves building a ramp inside the firebox for the hot gasses to heat up the burner in the rear of the stove (el tercero paso es construer una rampa adentro la caja de fuego para la gases calientes ir a la humo sala).  The third step also involves installing the chimney and the cover to keep out rain (el tercero paso tambien necesita la instalación de la chiminea y la sombrero de la chininea).  In the fourth step, the firebox door is put in place (en el paso cuatro los estudiantes voluntarios poner la puerta).  In each step, the spaces between the bricks are filled with a mixture of mud, and after the each step, it is necessary to wait for the cement and mud mixtures to dry and this is why the stove cannot be finished in a single day (en cada paso necesario llenar las espacios con una mezcla de barro y despues de terminar cado  paso, es necesario esperar, y es la razón los estudiantes nececitan quatro dias para construer la estufa). 

I’m still learning about the “rocket stove.”  From what I currently know, in a rocket stove only the tip of the firewood is in the combustion chamber, allowing for more complete and cleaner burning which leads to the potential of a higher efficiency stove.  Furthermore, if the air enters below the coals, the combustion process is also expected to be better than in the enclosed firebox stove (si la aire entrar la caja de fuego abajo de la leña la proceso de combustion es mas eficiante de la estufa normal).  Commercial stoves with the rocket design can be 50 to 100% more expensive, but the trade off for the extra cost in the stove can be recovered by the savings in firewood costs.  In many engineering projects tradeoffs exist, and different stove designs may be better for different families (en muchos proyectos de ingenieria hay intercambios y deseños diferentes para las estufas pueden mas bien para familias diferentes).

Energy in Guatemala

I am in Quetaltenango, Guatemala (Xela) for the next two weeks learning Spanish and volunteering with two different cook stove projects.  My Spanish teacher is also an engineer (mi maestro también es un ingeniero) and finished his PhD in agricultural engineering (doctorado de ingeniera agricultura).  From him, I learned a lot about the different types of energy resources in Guatemala, most of them are from renewable resources (más energías en Guatemala es renovable).  

Most electricity comes from hydropower (más electricidad es de hydroelectrica), but some solar, wind, geyser, diesel, and natural gas are also used.  Solar and wind power (electricidad de sol y viento) are primarily found in smaller residential neighborhoods.  There is a local project nearby Xela for a electricity from geysers (Hay una proyecto circa de Xela por electricidad de geisers).  Electricity from propane and methane (electricidad de propane y methano) is considered very expensive (es muy caro).  Some diesel is also used for electricity generation, but the diesel is not biodisel (pero no es biodiesel).

We also talked about what supplied energy to cars in Guatemala.  I was disappointed to find out that pretty much all cars are run on petroleum fuel, a non-renewable fuel (todos los automóviles usan petróleo, un combustible no renovable).  There is no methanol or ethanol biofuel (no hay methano o ethano biocombustibles).  There is some use of biodiesel in Guatemala (I think it’s used in cars), and the biodiesel in Guatemala is made from a plant or vegetable oil (pinón o aceite vegetal).  Other technologies are clearly too expensive for Guatemalans for powering their automobiles, such as electric cars and hydrogen fuel cells (automóviles electrico y celdas de combustíon de hidrogeno).

My teacher also taught me a lot about different agricultural things in Guatemala (agricultura organica, vermicultura, plantes medicinales, y lentrinas secas).  He even suggested a farm (una finca) where I could visit to see all of these projects, though I don’t know if I’ll have time.  The biggest problem in Guatemala are water, trash, and CO2 (hay problemas de agua, basura, y CO2), and these are additional areas where more research and/or development could be helpful.

Lastly, I learned about cook stoves in Guatemala.  My teacher designed the improved cook stove (mi maestro deseña esta estufa mejorada) that the Spanish school builds for select local Guatemalan families.  The intelligent design is very simple (el deseño intelegente es muy facil): easy to build, easy to understand, and functional, and compared to an open fire, the improved stove uses 55% less firewood (la estufa mejorada usa 55% menos leña).  The stoves consist of a base, a firebox (una casa de fuego), and cook top (una plancha).  The main disadvantage of these stoves is that the metal cook top is very expensive (una desventaje es la plancha de metal es muy caro).

After my morning Spanish lesson, I went to visit a company in Xela that builds and sells improved cook stoves (visito una compania en Xela construyen y venden estufas mejoradas).  In contrast to the stoves built at my Spanish school which are donated at low to no cost to select Guatemalan families, this company operates with a business model where all their stoves are sold at cost, but all families can buy them.  The stoves built by this company are designed to be much more efficient than both a three-stone fire and a normal enclosed cook stove.  I will be volunteering with this company for the next two weeks, working on questions including the design of the entrance for the firewood, the optimal geometry for combustion chamber and internal chimney, and additional heat transfer issues.

I started to do a literature search on cook stove research, and successful designs are (a) backed up by sound principles of heat transfer, (b) targeted to a particular region, (c) require no substantial behavioral modification from users, and (d) provided with follow-up support [Ahuja 1990].  Unfortunately, most of the research in the literature seems to be for cook stoves in rural Africa and India, where single pot stoves are sufficient.  In Guatemala (and other Latin American countries I presume) where tortillas need a plancha to be cooked on and multi-pot stoves are common, the stove designs need to differ from the successful stoves distributed in Africa and India.  One challenge is to figure out how to most efficiently get heat to the rear burners (since the main flame is under the primary burner only), which do not exist in the single-pot stove designs.  Other challenges include lowering the cost, making the stove more easily transportable and deliverable, and designing the stove such that it can burn all day if desired (apparently this is desired by Guatemalan women).