Anaerobic digestion of cocoa waste within a circular economy context

Ondertitel:Anaerobe vergisting van cacaoafval in een circulaire economie context

The role of anaerobic digestion in the present bioeconomy concept exceeds the boundaries of on-site electricity and heat production, as it can serve as a process for renewable energy recovery and the production of bio-based products. To apply this concept, three agricultural feedstocks were evaluated for potential biogas production: cocoa waste, pumpkin and animal manure. First, the optimization of cocoa waste anaerobic digestion was evaluated employing batch and fed-batch reactors in high (dry AD) and low (wet AD) total solids content of the feedstock. Dry AD performed significantly better than wet AD. A case study included a theoretical energy potential calculation for a full-scale plant with data obtained experimentally. AD from cocoa waste could supply up to 82% of the electricity demand of a rural region of Ecuador where cocoa is grown (20,000 inhabitants). It was confirmed that the use of synthetic nutrients and cow manure improved biogas and methane yields from cocoa waste, significantly. Interestingly, the treatment of synthetic nutrient addition and co-digestion with sterile cow manure had no significant differences. However, the best treatment was co-digestion with raw cow manure in terms of stability and methane yields in the long-term operation. A possible explanation behind the long-term stabilization of co-digestion with raw cow manure might be the high presence of Metanosaetaceae in the cow manure feedstock, which remained stable throughout the experiment. After AD optimization, a cyclic valorization of agricultural residues with different processes was performed. AD was integrated with slow pyrolysis to evaluate the energy and mass balances towards new products. The integrated process of co-digestion (cocoa waste + cow manure), followed by the slow pyrolysis of the digestate at 500 °C recovers most of the intrinsic energy of the waste into the gas phase in the form of biogas and non-condensable gases (up to 48%). In synthesis, these integrated processes recover more energy than each of processes separated, being equivalent to 61% mass conversion from the feedstock to fuel (liquid and gas). Finally, we demonstrated that electrochemical biogas upgrading is a successful method to separate CO2 from biogas. The process achieved almost perfect CO2 removal efficiency. Electrochemical biogas separation produces ‘customized’ gas mixtures. The final cathode and anode off-gas blend varied with the current applied, indicating that it is possible to have specific gas blends according to our necessities by changing operational parameters of the electrochemical upgrading unit. Animal feed, in the form of microbial protein, were generated from real stream biogas, cathode and anode off-gases. Cathode off-gas achieved the highest biomass concentration and the highest protein content in the microbial biomass. In conclusion, co-digestion proved to be a powerful technology in dealing with agricultural residues and brings stability and buffer capacity in the overall anaerobic digestion process in the long term. Slow pyrolysis has a strong potential for dealing with agricultural waste. Digestate, biogas, biochar, bio-oil and even syngas can supply agricultural areas that have a high demand for fuel and soil amendments. The best-case scenario is the integration of co-digestion and slow pyrolysis. This alternative has the highest energy efficiency in terms of gross energy recovery and in terms of net energy return. Finally, new products can be obtained from agricultural waste, such as microbial protein using anaerobic digestion as central technology.

ISBN:9789463572286

Jaar van publicatie:2019

Toegankelijkheid:Open