Productive use of coffee husks - How to make briquettes from coffee husk in Karagwe, Tanzania
Sammanfattning: In the Tanzanian district of Karagwe the NGO KADERES is working with agricultural development. In relation to this they produce fair trade coffee obtained from local farmers. Coffee husks are created as a waste product from the plant and are currently not used for any purpose. Instead they are left outside the factory. The propose of this project is to find a way to create briquettes from these husks. On a technical level, there is two ways in which to produce briquettes; the carbonize-first and the densify-first method. The carbonize-first method, is a technically more simplistic method and is the one most commonly used in East Africa. In this method, the coffee husks are first carbonized into charcoal using a charcoal kiln. The charcoal is then grinded and mixed with hot water and a chemical binder, usually a form of starch like cassava flour. The starch in this process binds the charcoal particles together, meaning that the binding of the briquettes is created chemically. The mixture is then pressed through an extruder that increase the density of the mixture, creating briquettes. The mixture is then dried until it reaches a sufficient moisture content. The densify-first method is more high-tech. In this method the coffee husks are densified through a physical process. The lignin in the coffee husk welds them together, when enough pressure and temperature is applied. This is done using a briquetting press followed by a cooling belt with a preheating step preceding these two steps. There are two main types of briquettes presses, namely piston presses and screw presses. Both with pros and cons where piston presses are efficient if the bulk density is high. For densification to be optimal, the coffee husks need the right physical and chemical properties. Otherwise additional steps before densification are required. If the particle size is too big the husks needs to be crushed, if the moisture content is to high they need to be drayed and if unwanted material is in the biomass then sieving is needed. Both the densify-first and the carbonize-first methods have their positive and negative sides. The carbonize-first method is adaptable for low to mid-range production within a price range suitable for Tanzania. The process can moreover use other biomasses then coffee husks under the same process. The efficiency is however low compared to briquettes made using the densify-first method. This method on the other hand, while creating better briquettes, is also more expensive in regards to the initial investment and requires equipment not found in the region. As such this study proceeds from this theoretical background to a methodologic section that aims to study which method is the best and what unit operations that are needed for that method. The properties of the husk that where studied was their size, their bulk density and their moisture content. The particle size was measured in relation to height, width and length. The result indicated that the husks are bigger then the optimal size for the densify-first method and that crushing equipment is needed. The bulk density was measured using a one litre container. This was done both by shaking the husks to decrease the amount of air in the container and by not shaking the husk. The result indicated then when the husks where shaken the bulk density was high enough for a piston press to be used. The moisture content was measured using a microwave. The husks where put into a container and their initial weight was measured. The husk where dried in a microwave oven for two minutes and additionally 30 seconds at the time until no more loss of weight was recorded. There was then assumed that this means that there was no water left in the husk. The result from the experiment indicated that the husks were dry enough for briquetting whiteout the need for drying. This however, requires that the husks are kept dry and not exposed to rain. A simple cost analysis based of the two methods was also done. This using data from other plants in East Africa as sources of reference. This analysis showed that the carbonize-first method had a lower cost of initial investment, but a higher cost of production then the densify-first method due to the need for cassava flour and water in the process. The densify-first method requires advanced machinery that has to be imported, but there is no major cost of production. Using a local sales price for briquettes the analysis showed that there is profit to be made from both methods. A possible problem that could arise is the lack of a market for the briquettes, some plants is the region have been force to close down due to lack of sales. As this is a technical report and not an economical one, it’s of key importance that a proper economic analysis is made before plant construction start. This report can only conclude what is technically possible and needed, not if its economically viable. With this information obtained the report concludes that a densify-first method is the better process. As a method it creates better briquettes for a lower cost of production then one made using the densify-first method. The initial cost is more expensive, but this will be worth it in the long run. The report further recommends that this method uses a crusher, a cooling belt and a preheater along with the briquetting press. A screw press is preferred over a piston press, due to several reasons. There are however presses made explicitly for coffee husk, so they are most likely the best.
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