Rice Hull Pulverizer and Insulating Brick Making

Problem Statement:
One half of the world cooks on open fires. Smoke from open fires leads to many problems, such as constant eye irritation, respiratory diseases, and their houses burning down. Rocket stoves are an excellent way to combat these issues because they use less wood and are more efficient than typical three stone cook fires. To make a brick for a rocket stove, organic material is combined with clay and, when fired, the organic material burns out to leave a clay brick that is lighter and more efficient than normal. AidAfrica has developed a six brick rocket stove, as shown in the picture below, made of local clay and sawdust to make cooking safer and more efficient for the rural people of Uganda and the environment (see AidAfrica).

Original Rocket Stove.png
Six brick rocket stove developed by AidAfrica

With a device like one in which we propose to build, rice hulls will finally be able to be used as organic material in the building of a rocket stove like one in which AidAfrica distributes. Sawdust has gotten more expensive to buy and transport to build six brick rocket stoves for the poor rural villages in Uganda. Currently, rice shells in Uganda are unused, and left to rot or burn in the fields.

Saw dust is currently used in the bricks because it can be burned off when fired. The whole goal of these bricks is to insulate the fire so it is more efficient, and, by having tiny air bubbles left behind from the saw dust, the bricks are more effective at creating a hotter fire that cooks faster. Plain rice hulls are too large and put the integrity of the bricks in peril, so they need to be made smaller.

Solution: Use broken down rice hulls as an organic material for the creation of a rocket stove.

Our group (from left to right): Mrs. Keller, Mr. Keller, Scout, Daly, Beth, Ryan
Our group (from left to right): Mrs. Keller, Mr. Keller, Scout, Daly, Beth, Ryan

Group Names:
Ryan Hodgens – 3rd year business entrepreneurship major. Interests include traveling, meeting new people, and learning.

Beth Hotchkiss- 3rd year Civil Engineering major. Interests in international development, systems thinking, and media.

Daly Sombat – 4th year Mechanical Engineering major. Interest in innovations/designs to better the lives of others while also learning from them. Hobbies include cooking and sharing it with others, playing lacrosse, reading and napping.

Scout Vernon

Mr. and Mrs. Keller – Co-founders of AidAfrica

Within our group, we have split into 2 groups to maximize our time efficiently since half of us are in the Monday night lab and the other two are in the Tuesday morning lab. Daly and Scout are working on building a prototype to break down rice hulls (to make it similar to sawdust). Beth and Ryan are working toward making the insulating bricks with the rice hulls.

Sketches of Mechanism:
design 1.jpgdesign 3.jpg

Mechanism Building:
We have tried different methods to see various ways to break down the rice hulls. Some methods were pounding it with a hammer, grinding it with a rock to concrete, and using an electric coffee grinder (see pictures below). We found that pounding it with a hammer and grinding it with rock to concrete didn’t really break down the rice hulls into smaller pieces; instead it flattened it, changed shape and or had minimal break down of the rice hulls. But the electric coffee grinder worked really well and broke the rice hulls down to a similar texture as sawdust. But due to the minimal availability of electricity in Uganda, we decided it will be hand-powered or human-powered.

We observed when we tried folding and ripping the rice hulls apart by hand, it was reluctant to break. But when we pulled them, it broke more easily than folding and ripping. We also noticed that something sharp would be ideal in breaking the rice hulls down due to the success from grinding it using an electric coffee grinder.

So to get inspiration and see what materials were available to us, we looked in the storage unit at the back of the lab to get some ideas. We found an aluminum honeycomb mesh and a sheet of metal with holes; the holes on the metal sheet were big enough where the rice hulls could fall through but not unless shaken. Before we attached anything to our metal mesh mechanism design (as shown in Figure 7), we tested to see what would break down the rice hulls the best. Though we assumed and observed that metal against metal might not be the best method, we tried it out anyways to see if it would break down the rice hulls. We tried aluminum honeycomb against aluminum honeycomb, sheet metal against sheet metal and sheet metal against aluminum honeycomb. We found that the sheet metal against aluminum honeycomb broke down the rice hull more than the others. So we decided to use the aluminum honeycomb and the sheet metal.

When building the mechanism, we ran into a few problems. One example was when we were going to use a flat metal bracket to connect the two wood pieces but then we realized that the walls might be too thin and the nail might interfere with the grinding/spinning motion. Due to that, we just glued on two wood pieces on two of the sides, which is not as appealing. Our prototype is shown in the pictures below.

After the construction of the mechanism was done, we tested our rice hull pulverizer. After using our design to break down the rice hulls, we found many flaws with it. Firstly, it took a long time to even get half a sandwich bag full of broken down rice hulls (maybe an hour grinding), so very inefficient. It couldn’t handle too much rice hulls at a time, and some of the rice hulls would get stuck on the cracks of the edges.

Though our mechanism wasn’t as what we hoped for, we learned a lot from taking analysis of the rice hull, designing and building. We looked into using a bike model design to break down the rice hulls, which EWB (Engineering Without Borders) had a prototype but unfortunately didn’t have it working by the end of the quarter. However, using a bike might be limited to only men being able to use the bike model design because women in Uganda usually are dressed in skirts/dresses. But we hope to try the bike design that EWB has developed to see if it would break down the rice hulls once they have it working.

Final Thoughts on Possible Solutions for Upscaling:
Peter Keller originally had in mind for our project to design a mechanism to break down rice hulls by the truck loads. But due to limited time and resources, we decided to go the small scale route. But we did think about some designs that can possibly work with high bulk rice hull intake. We really want to be able to use oxen into our design because that’s an available resource in Uganda (see sketch below for using oxen for our design). Also, the bike model design sounds like a possible solution but one of the drawbacks would be limited to only men. With the more resources and time to experiment and build, we can come up with a mechanism using local resources in Uganda that can be used by men and women.


Brick Making:
We are currently in the process of testing the durability, strength, and the insulating properties of bricks with different amounts of sawdust per set amount of clay to find the best ratio. At the moment, we have created ten different clay brick/pucks with a different and measured amount of sawdust in each. The next step is to wait for the bricks to dry and, in turn, fire them in a kiln. The most insulative bricks are, we hypothesize, the ones with the least amount of clay per cubic inch: at this level, the bricks will have more air bubbles in the clay. Because the air pockets take a long time to transfer heat, these bricks will be able to heat slowly thereby allowing most of the heat to be directed into the pot for cooking food.

To create the initial bricks, different amounts of sawdust were mixed with Hawaiian Red Clay and formed into tiny pucks for testing. After finding the sawdust brick that best fits the characteristics that we want, we planned to apply the ratio to pulverized rice hulls and see if it the two organic materials are similar. However, due to time, we instead made rice hull bricks in conjunction with sawdust bricks.

Using the method described above, we have made several saw dust bricks and (almost) fired bricks containing various amounts of whole rice hulls and ground rice hulls. Unfortunately the kiln is malfunctioning and there is not firing for the remainder of the quarter. We were unable to fire our ground rice hulls bricks. This was a bit of a disappointment because we planned on applying the knowledge from this sample to our finished product: a half-scale rocket stove.


Pictured above:
(top left) Various amounts of saw dust were tested to make insulated bricks similar to the already existing versions in Uganda.
(top right) One member of the team labels the test bricks so the group will know what proportion of saw dust the brick contains.
(bottom left) Team members mix clay and organic material: the two main components of insulating bricks
(bottom right) Team members use a mini mold to form bricks for the mini rocket stove.

During the beginning of the quarter, the team measured the insulating properties of the bricks by conducting a simple float test. For this test, our team placed the fired bricks into a plastic bag, sealed the bag, and placed the bag in water to see if the pucks would float. The team rationalized that the most insulating bricks would have a lot of air pockets and low density. We hoped our bricks would be able to float on water like the ones produced by AidAfrica. Unfortunately after a month of testing different ratios, none our our bricks were able to achieve a density less than water.

Therefore, our team attempted a different method to test the insulating properties of bricks: the stove test. For this test we observed the time it took for one of our brick pucks to evaporate 1/4 of a teaspoon of water on a hot stove. The most insulating bricks should, theoretically, take the most amount of time for heat to travel through the material to evaporate the water. Having low conductivity/ high insulating properties is good for rocket stoves because the heat is directed toward the pot instead of the stove itself.

The last tests that our team did was a break test. For this experiment, we dropped the small brick pucks from shoulder height onto a flat surface and observed the outcomes.

Testing clay puck brick in the pan on the stove
Testing clay puck brick in the pan on the stove

Pictured above is the stove test
Float Test-
We hoped our bricks would be able to float on water like the ones produced by AidAfrica. Unfortunately after a month of testing different ratios, none our our bricks were able to achieve a density less than water. Our team had to reevaluate our goals and determine a new test.

Stove Test-
The team successfully tested 5 of the sawdust bricks and 2 of the rice hull bricks before our pan malfunctioned. The rice hull bricks out performed the sawdust bricks with times of 3-5 minutes vs 1-3 minutes respectively. Furthermore, the bricks with more organic material also tended to have longer times. These long times to evaporate water suggest that the bricks are insulating and will be better suited for cookstoves. Unfortunately, the team believes more testing will be needed due to the small sample size.

Break Test-
Each puck broke in a unique way. Unfortunately, we are not sure if the breakage is due to the material or the angle the puck hit the ground at. This test will need to be more standardized for any conclusions to be drawn. Pictured below are some of the breakages. We could copy the method used by Cal Poly students in Guatemala.

Problems Encountered:
A couple weeks into our project, we had questions for Peter Keller about how they would fire the bricks (what kiln is being used in Uganda and what the temperature ranges) and wondered how the process of making bricks went. He replied back to us answering our questions but also sent this video on making test disk (see video Making Test Disk). This really confused our group and definitely discouraged us because our problem seemed to have been solved and done. At this point, we weren’t sure if we should continue or find a new project. We decided to just continue and email Peter Keller on this, and he got back to us in a day and let us know that the video was a lie and they’ve never done rice hull bricks. So, we had our project and our passion/spirit back.

When testing how efficiently our bricks could evaporate water, we ran into a few problems with inadequate heating devices. We burnt both a cookie pan and a cooking pan with the high temperatures of the stove. Bummer. “Why is the pan turning white?”

The result of heating the clay puck brick in a pan on the stove on high heat
The result of heating the clay puck brick in a pan on the stove on high heat
The result of heating the clay puck brick on a cookie pan on the stove on high heat
The result of heating the clay puck brick on a cookie pan on the stove on high heat

A common theme toward the end of the quarter was a broken kiln and a near-halt in our project. Below is a picture of our ground rice hull bricks after hanging out in the same spot for two full weeks – unfired. We were unable to test our pictured bricks for the main characteristics that we were looking for: durability, strength, and low heat conductivity. As a result, we had to make an educated guess based on past data and information when designing the ratios for our final rocket stove.

Sample Pucks.png
The unfired rice hull clay puck bricks left on the shelf

Rocket Stove:
Our final goal for this project is to create a half-scale rocket stove. Because we were unable to fully test the properties of our hockey puck test bricks, we were forced to make educated guesses on ratios pertaining to our final rocket stove. And because we were unable to measure directly the most efficient organic material to be used, we made a six-brick rocket stove with three different kinds of material. Two bricks made with whole rice hulls, two bricks made with sawdust, and two bricks made with ground straw. Again, unfortunately we were unable to fire our finished product due to a broken kiln. We are planning on firing when the kiln starts working again because we would love to see the results of our rocket stove.
Mini Rocket Stove.pngIMG_3718.JPG