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| Fig. 1: The evolution of coffee from berries to ground.(Source: Wikimedia Commons Berries, Beans, Ground) |
Ethiopia is the worlds 5th biggest coffee exporter. Annually it produces up to 495,000 metric tons (tonnes) of coffee. Ethiopia consumes roughly half of the coffee it produces. Coffee accounts for more than 30% of Ethiopia's GDP. Throughout the lifetime of coffee beans, starting out as coffee berries and ending as grinded coffee, a significant portion by mass of the coffee berries ends up not being used. The leftover coffee is mainly generated at two stages of coffee processing; during the coffee conversion process from berries to beans and after the extraction process of the c spend coffee grounds. The conversion ratio from coffee berries to clean green bean is around only 18%. [1] Additionally, only 18-22% of the coffee is extracted when hot water is poured over the coffee ground. [2] This leaves almost 96% of the original coffee unused. There is currently lots of research going on to use coffee waste to generate energy. From drying and burning the leftover coffee waste to drying the coffee leftovers and extracting biofuel from them, a range of alternatives are being explored to generate energy.
This paper will focus on how much energy could be generated annually if Ethiopia pursued the biofuel route of coffee waste energy generation. The biofuel route. There are multiple ways that oil can be extracted from coffee grounds. We have processes such as Soxhlet extraction, Supercritical fluid extraction, ultrasound extraction and microwave extraction. [3] The analysis in this paper is going to method agnostic as we will average the amount of biodiesel that could be extracted from the coffee waste amongst the different methods. One paper found that biodiesel extracted would amount to up to 15% of the weight of the dried waste. [4] We will calculate the amount of energy that can be generated by the remnants of the coffee extraction process and coffee conversion process and evaluate to see if there is potential to meet Ethiopia's unmet energy needs.
When calculating the amount of energy that could be generated, I will be assuming that the amount of energy generated from the berries is going to be the same as the energy generated from the grounded coffee and this will be 15% of the whichever coffee mass we are looking at. Of the 4.95 × 105 tonnes Ethiopia is expected to produce in 2022/2023, it is projected to export 2.81 × 105 tonnes. So since the berry-to-bean conversion all happens in Ethiopia, all the waste generated would be in Ethiopia. For the coffee ground stage, as 2.81 × 105 tonnes of the coffee gets exported to other countries, only 2.14 × 105 tonnes will remain in Ethiopia. We will only be accounting for the leftover ground coffee from Ethiopia when we are looking at coffee extraction.
| Leftovers from berry-to-bean conversion | = | 5.00 × 105 tonnes × 0.18 |
| = | 9.00 × 104 tonnes | |
| Biodiesel generated from Coffee conversion | = | 9.00 × 104 tonnes × 0.2 |
| = | 1.80 × 104 tonnes | |
| Leftovers from beans-to-grind extraction | = | 2.14 × 105 tonnes × 0.2 |
| = | 4.29 × 104 tonnes | |
| Biodiesel generated from extraction | = | 4.29 × 104 tonnes × 0.2 |
| = | 8.58 × 103 tonnes |
Up to 36 GJ energy can be extracted from a ton of Biodiesel. Taking into account of the amount of energy input into biodiesel production plants the energy extracted goes down to 4 GJ per 1 ton of biodiesel, which corresponds to 1111.1 kWh of energy. This would mean the 1.80 × 104 tonnes of biodiesel generated from conversion will equate to 1.7 × 107 kWh.
The total annual energy consumption of Ethiopia in 2019 (including biomass firewood) was 4.9 × 1011 kWh. [5] The above calculations would indicate that the amount of energy generated from coffee waste would be able to meet 0.0035% of the yearly power consumption of Ethiopia. Further research needs to be done on the assumption that the amount of biodiesel that could be generated from coffee berries waste is 20 percent of the mass of the berry waste. Additionally, the use of organic solvents such as cyclohexane during the leaching of oil presents some environmental risks, such as being dangerous to aquatic life so more research should be done to investigate alternative solvents.
The final energy calculations from above are not taking into consideration the energy that could be generated from the leftovers of the ground coffee extraction. They were not taken into account because of the complexity of the logistics of collecting the leftovers. Had they been accounted for the total amount of energy would have risen by 50% without accounting for energy that would be consumed collecting the coffee ground leftover.
© Temesgen Worku. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants manpermission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] T. Espinosa-Solares, "Raw Coffee Processing Yield Affected More By Cultivar Than By Harvest Date," J. Agric. Univ. P.R. 89, 169 (2005).
[2] X. Wang and L.-T. Lim, "Modeling Study of Coffee Extraction at Different Temperature and Grind Size Conditions to Better Understand the Cold and Hot Brewing Process," J. Food Process Eng. 44, e13748 (2021).
[3] N. Kondamuri, S. K.Mohapatra, and M. Misra, "Spent Coffee Grounds as a Versatile Source of Green Energy," J. Agric. Food Chem. 56,11757 (2008).
[4] J. Y. Zhu and X. S. Zhuang, "Conceptual Net Energy Output For Biofuel Production From Lignocellulosic Biomass Through Biorefining," Prog. Energy and Combust. Sci. 38, 583 (2012).
[5] "Africa Energy Outlook 2019," International Energy Agency, 2019.
