NEXT-GENERATION FUEL CELL: FROM WASTE-TO-ELECTRICITY

The food waste treatment-based anaerobic digestion has been proven to play a primary role in the electricity industry with high potential economic benefits, which could reduce electricity prices in comparison with other renewable energy resources such as wind and solar power.

 --Ngoc Bao Dung Thi, TDTU, Viet Nam

Food and Agricultural Organization of the United Nations mentioned that over 1.3 billion tons of food waste (FW) represents a third of the global food production that is churned out annually [1a, 2]. Both the developed and developing nations have 15-63% of their municipal waste streams to be from FW [3]. In the same way, the world has doubled its total energy consumption over the last two decades, especially due to the population surges in many countries, which was noted by International Energy Agency (IEA) [4]. In this field some of the scientists aims to evaluate the electricity generation from the anaerobic fermentation (AnF) of FW and spent animal beddings (SAB) which can be used as a fuel cell for the conversion of the produced fermentation products.

Figure 1. Showing waste to energy generation plant [1b].  

The batch treatment was done by butyric fermentation with acetic acid as the major product based upon the suggestion of Liu et al. [5] when a very high rate of acetic acid conversion occurred during the fermentation liquid from the FW treatment which was used in a fuel cell. In their study, the fermentation liquids from FW and SAB were generated from mean open-circuit voltages of 0.64 and 0.53 V and P_mean of 1.6 and 1.3 mW/cm², respectively. 

In reality, FW fuel has a higher power density than the value (1.2 mW/cm²) obtained by Liu et al.[5] but lower than the 3.4 mW/cm² were reported by Xu et al.[6] when glucose was used as fuel. Interestingly, SAB fuels were also performed better than the earlier report for FW [7]. The reason for these observations could be directly due to the relatively high organic load of the two fermentation liquids in thier study i.e. 23.4 and 20.9 g/L for FW and SAB respectively [7]. However, these are still far lower than the 180 g/L glucose concentration used in the study by  Xu et al. [6] which yielded a better result. In the use of fermentation liquid for fuel cell therefore, a higher concentration of the liquid is required to enhance the performance efficiency.

The efficiency of a fuel cell in treating fermentation liquid is the composition and oxidation pattern of totally organic carbon (TOC). In thier work, when the fuel cell was operated in continuous mode, there was a rapid degradation of TOC. The TOC values at the end of 12 h operation were 4.3 and 4.5 g/L from the initial concentrations of 6.5 and 6.1 g/L for FW and animal beddings (AB). After this, degradation was slower and picked up again at about 25 h of the experiment. At the end of 48 h, the final values of TOC in the fuel cells treating FW and spent animal beddings (SAB) were 0.3 and 0.5 g/L respectively (Figure 2). This meant that at the end of the experiments, 95% and 92% of the initial TOC in FW and SAB fermentation liquids have been utilized. It was also noticed that the bulk of the TOC in the liquids was drawn from the volatile fatty acids (VFAs) which is evident from the high diversity and concentrations of the VFAs in this work. This phenomenon can also impact greatly on the degradation or conversion of the VFA for the efficiency of the cells in both FW and SAB experiments.

Figure 2. This figure shows the pattern of total organic carbon (TOC) conversion for (A) food wastes and (B) spent animal beddings during the experiments [1].

Figure 3. This shows the schematic diagram of the experimental set up showing both the first and second phases of the experiment [1].

A fuel cell (Figure 3) can efficiently be used for energy generation from food wastes and spent animal beddings. Anaerobic fermentation of the waste streams was first carried out and the products were very rich in nutrients and volatile fatty acids which were subsequently and efficiently converted to electricity.

The initial extraction of lipids gave the fermentation liquids a better chance of performing as no clogging or inhibition was observed up to 97% VFAs conversion was achieved. In the same manner, there was high mineralization of carbon to CO₂ while most nitrogen was degraded to ammonia and subsequently to N₂.

The experiment has also shown efficiency in the generation of additional electricity from the effluents in a second treatment with the addition of an external carbon source. Even though the food waste has been treated using many technologies in the past, the result of this work showed one of the very best treatment efficiencies achieved from food wastes. Also, spent animal bedding is a profound energy resource as it compared favorably well with food waste in this study.

Therefore, the use of fuel cells for the treatment of highly volatile waste streams such as different food wastes and others is hereby solicited as this treatment shows higher efficiency than anaerobic digestion within a very shorter period.

References

1. a) S. O. Dahunsi, Scientific Reports, 10, 10735 (2020). b) N. B. Dung Thi, C-Y. Lin, G. Kumar, American journal of transportation and logistics, 26, 197-202 (2016).
   
2. FAO, Food and Agriculture Organization of the United Nations. Summary Report of Food Wastage Footprint: Impacts on Natural Resources (FAO, Rome, 2013). ISBN: 978-92-5-107752-8.
3. M. Kim, et al. Water Res. 96, 208–216 (2016).
4. IEA, International Energy Agency. Key World Energy Statistics 6 (IEA, Paris, 2014).
5. Y. Liu, K. Feng, H. Li, Appl. Energy 233–234, 395–402 (2019).
6. F. Xu, H. Li, Y. Liu, Q. Jing,  Sci. Rep.-UK 7, 5142 (2017).
7. C. Liu, H. Li, Y. Zhang, C. Liu,  Biores. Technol. 219, 252–260 (2016). 

Blog Written By

Dr. S. THIRUMURUGAN

National College, Thiruchirappalli

Tamilnadu, India

Blog Editors

Dr. A. S. Ganeshraja

Dr. K. Rajkumar

Dr. S. Chandrasekar

Blog Reviewers

Dr. Y. Sasikumar

Dr. K. Vaithinathan