The Maputo Ceramic Stove A novel approach to the manufacture of small charcoal stoves for low income households

Transforming the low income cooking sector in a major city requires replacing the technologies and techniques in common use. A typical stove used at a market in Maputo. There is little difference between this and an open fire The Maputo Ceramic Stove can be placed on the floor, its own stand, or on top of an existing metal frame.

But clay stoves crack. Why? Internal stress from differential thermal expansion. What can be done? Heating a clay component on the inside causes it to expand on the inside only, resulting in cracks. Slotting the clay stove body can reduce fracturing. This one opens up 5mm when a fire is lit inside.

Finding a clay mix that is less likely to crack means finding a mix that has a much lower thermal expansion coefficient, which translates into less internal stress at any given temperature. A PK11 plate was made to simulate a worst-case scenario grate, trying to crack it by applying heat to the centre. Many different mixes of local clays were tried until one called PK11 was found to be the best of all.

Heating the centre while keeping the edges cool showed that PK11 does not expand enough to create high stress at moderate temperatures. The close-up is a picture of a single hole near the centre at yellow heat.

Transformation of a mix of clays into a low thermal expansion material The graph on the right shows the physical size of a sample being heated to 1200 degrees C and cooled again. The red line starts at the top left. As the material is heated, it expands, shown by the red line rising as the temperature reaches about 800 C. The top arrow shows the point at which components in the mix start to melt which is the cause of the reduction in size of a clay product when it is fired. As the temperature increases, more and more components melt and the sample shrinks each time one does C is reached and the overall shrinkage is about 12%. As the sample cools the red line moves to the left. Note that the physical size of the sample decreases very little as it cools from 700 to 0 degrees. This change in the expansion rate is permanent. Melting starts Region of interest is C Note small change in size Expansion

Initially it was expected that it would be necessary to separately control the primary and secondary air. On the left, air flowing through the grate can be separately limited, reducing the fire power without increasing the CO emissions, a major problem with the Jiko Stove. Primary air enters through the holes in the grate and secondary air moves around the top lip. This 2-part design proved unnecessarily difficult to manufacture. In this later model the grate sits on supports and the air is directed around the top of the grate sending the secondary air towards the centre in a conical flow.

The stove body on the left is well-fired so the clays have transformed into a low thermal expansion material. On the right is an under-fired stove (made from the same mix) that will crack almost as soon as a fire is lit in it as the clay is ‘ordinary’.

Increasing the production speed means using modest mechanisation. The machine is called a Jigger. The white larger piece is a mould holder and inside it is a mould made from a type of Plaster of Paris. About 100 moulds are used twice each per day. The Jigger forms the stove body by pressing the clay mix against the inside of a rotating mould After a few minutes the mould absorbs some of the water and the stove body drops out.

The air holes and final trimming are done by hand. The grate supports are added then it is dried slowly for a few days. The stove bodies are stacked on top of each other. They are strong enough to be stacked 3-high. The dark ones are freshly made.

Fired stoves and grates coming out of the oven. They are fired to 1170 degrees. The shelves and supports are also made from a low thermal expansion material. The final product can be glazed or branded with a logo or educational message.

Consumer acceptance Months of use and abuse have cracked this MCS, however it has not broken and performs well for many hours per day. Almost every user reports significant fuel savings

Some of the cracked Maputo Ceramic Stoves were given to market cooks to see if they would eventually break into pieces. Some have been in use for 6 months. The fired PK11 is very strong, not like flower pot material and most stoves continue to stand up well.

Testing Some tests are done with ‘real pots’ and incorrectly made parts to see how it affects performance (not much). A typical test setup showing combustion analysis and recording equipment.

Surveys are being conducted is a number of locations selling at $4.45 retail

In general there is a favourable reception for the most important reasons: fast lighting, fast boiling, good turn down ratio, fuel saving, adequate power for pots under 10 litres.

Problems still requiring attention Grates still crack, though if properly fired, they tend not to break right through to the opposite side. This stove broke into two parts months ago and has been tied together with a piece of wire. It continues in daily use. We continue work on the clay mix and firing to further reduce this type of problem.

Typical profile of the failure rate of the stove components – Report by Peter Coughlin Sept 2007

Ease of Use and Practicality of the Stove’s Design The vast majority of people report that they had no problem at all in learning how to use the ceramic stove and no one had serious problems. Small problems were mentioned by 36% of the respondents in learning to reduce the volume of charcoal that they had always been using. Other minor problems was mentioned by a few people. In all cases, the respondents reported that, with practice, they readily overcome their difficulties. – Report by Peter Coughlin Sept 2007

Customer support Training materials have been produced to support the household management portion of the project. This is only one example. At this time the focus is on market development and matching production to sales with innovative new equipment. Oct 2007 END