Injera baking is one of the most energy intensive cooking activities. In response to this, improved stove like Mirt has been introduced. However, we still need to cut trees. An innovative and sustainable alternative to this is the use of biogas. However, less is known about its application in Injera baking. Individuals have come with various designs of the burner system and bio-Mitad (see figure 1).
Fig.1 Biogas burner and biogas Mitad [1, 2]
The burner designs on the left and center are developed by innovative individuals but the report [1] claims they have low performance as the gas doesn’t burn as required. This however could also be due to low gas pressure or high pressure drop. The biogas Mitad on the right is developed by Selam Vocational training center [2].Similarly, the design is unable to attain uniform heat distribution and therefore bake good Injera.[5] Overall, there is a need for formal research despite the seemingly trivial causes of the problems.
Burner system design begins with question like how much heat is required to cook Injera. This determines the gas flow rate and hence the geometry and size of the burner system. Burnham, [3] has shown that 1 KW cooking power is required for baking Injera which is 20KW fire power in the test Mitad. (5% efficient) This value is the average result from 5 bakers and baking of 10 Injera/batch. The author shows that the results for cooking power agree with previous similar works but difference exists in the stove efficiency.
From energy efficiency point of view baking on clay that may be up to 60cm diameter is not good. The study [3] reported a thermal conductivity of 0.41W/m. K for Mitad design in Eretria. Low thermal conductivity is an inherent problem in Mitad leading to more biogas use. The specific energy consumption of classic Mitad is reported to as 10MJ/kg and 17MJ/kg Injera [3].
This is a challenge in small scale digesters because of the competition from cookers and light bulb (rural areas). Assuming 0.3kg/Injera and 55% biogas stove efficiency, 45lit biogas/Injera or 900lit/batch (20 Injera/batch) is required. Assuming a farmer with four cows (20-25kg dung/day), this gas consumption takes major portion of the daily gas use. (0.8-1.0cu.m/day[8]) Although, the baking is not daily, cooking can not be done simultaneously.
Some research issues on previously developed bio Mitad have been suggested by report [8].
They can be generalized as,
1. Heat transfer and distribution over the plate,
2. Heat loss from bio Mitad
3. Improved burner for better heat distribution,
4. Optimization of Mitad dimensions
The issues 1 and 4 share a central theme on thermal performance aspects Mitad. Research issue 2 focuses on heat loss which could be thought of at the design stage of the stove. Issue 3 focuses on burner design aspects.
In conclusion, three research areas could be identified; Mitad plate, stove and burner system. Optimization of thermal performance of Mitad is logically the first step if residential application of the Mitad is considered. In larger scale (8cu.m and above) and institutional plants, this can be skipped since the gas shortage is not as significant. Secondly, design and development of the burner system is essential. Finally, the stove is optimized for heat loss.
References
[1] Bilhat L., 2009, National survey on current status of institutional biogas systems installed in Ethiopia, GTZ-German technical cooperation, Ethiopia
[2] Web page, http://www.selamethiopia.org/biomitad_big.htm as accessed on 18, August, 2010
[3] Burnham H., 2008, breeding a better stove: the use of computational fluid dynamics and genetic algorithms to optimize a wood burning stove for Eritrea, PhD thesis, University of Nottingham
[4] David F., 1996, Biogas stove design, a short course, Kingdome bioenergy Ltd also available online http://www.kingdombio.com/BiogasBurner1.pdf
[5] Alemayehu Tadesse customer service head, Selam technical and vocational training center, xxx@gmail.com, August 21, 2010, Reply to Biogas Mitad. Email to: Moges Ashagrie xxx@yahoo.com
[6] Chandra A., Tiwari G.,Srivastavay K., Yadav P., 1990, “Performance evaluation of biogas burners”, Energy conservation and management, Vol.32 No 4, pp.2
[7] H.R.N Jones, 2005,” The application of combustion principles to domestic gas burner design”, Taylor and Francis e-library, UK
[8] Getachew E., et al, 2006, report on the feasibility study of a national program for domestic biogas in Ethiopia, SNV, Ethiopia