(written by: Willy Yanto Wijaya)
There are many ways to obtain energy from biomass. In general, we can classify into three types: thermal conversion, chemical conversion, and bio-chemical conversion. Thermal conversion methods include combustion, pyrolysis, and gasification. The principle of this method is utilizing heat as the dominant mechanism to convert the biomass into other chemical forms, where combustion specializes on the heat extraction via exothermic reactions, while the latter two processes specialize mainly on the syngas (CO, H2) extraction. Biomass can also be processed chemically via chemical conversion using catalysts to become other types of fuels which are more convenient to be used in applications. Another method of biomass conversion, which is our particular interest in this writing, is bio-chemical conversion. Bio-chemical conversion makes use of the enzyme of bacteria or other micro-organisms to break down biomass into biogas or liquid bio-fuels and other types of chemical species.
There are several kinds of bio-chemical conversion, two of which are biogas fermentation and microbial fuel cell. This writing aims at describing these two methods, explaining how we can obtain energy from these two methods, as well as pros and cons of these two methods.
I. Biogas Fermentation
Biogas fermentation is a series of processes where microorganisms break down biodegradable materials (usually in the absence of oxygen), thus it is often attributed as anaerobic digestion. The break-down of these biodegradable materials (such as biomass) will produce simpler molecules, where some of these products are in the form of bio-gas. The detailed mechanism of how biogas fermentation occurs depends on the microorganisms involved, as well as types of feedstock (biomass) used and operating conditions (temperature, pH, etc). In general, we can describe the mechanism into four key biological and chemical stages: hydrolysis, acidogenesis, acetogenesis, methanogenesis, which is described in Fig. 1. (click to enlarge figure)
Fig.1. Biological and chemical stages of biogas fermentation processes. The exact chemical species produced at each stage depend considerably upon the kinds of microorganisms used as well as processing conditions.
As shown in Fig. 1, polymeric form of organics such as carbohydrate, protein, lipid, cellulose are degraded into their constitutive monomers by many kinds of microorganisms. The monomers are then converted mainly into organic acids (such as butyric acid, propionic acid) by the action of acid producing microorganisms. Next step is the conversion of these organic acids into acetic acids as well as hydrogen. Finally, methane can be recovered by the methane-producing bacteria. Since some of the hydrogen produced cannot be converted, the final product of biogas usually contains methane (CH4) and hydrogen (H2). These methane and H2 (which are the main constituents of biogas), after desulphurization and deodorization, can later be used in the combustion system or fed to the fuel cell system to generate electricity. This is how we obtain energy from the biogas fermentation method.
II. Microbial Fuel Cell
Microbial fuel cell is a device that converts chemical energy into electrical energy by the catalytic reaction of microorganisms. Figure 2 shows the schematic of microbial fuel cell. In the microbial fuel cell, biomass is oxidized at the anode part producing CO2, proton (H+) and electron. Electrons will go through external circuit to the cathode part producing current, and protons go through the exchange membrane. At the cathode part, oxygen reduction reaction occurs just like in the typical chemical fuel cell. The role of microorganisms in microbial fuel cell is very important. Some microorganisms cannot transfer the electron to the anode, and therefore mediators such as thionine, methyl viologen, etc have to be used to facilitate the electron transfer. However, recent development has shown that there exist electrochemically active bacteria (such as: Shewanella putrefaciens, Aeromonas hydrophila) which can transfer electron directly to the electrode without mediators.
Fig.2. Schematic of microbial fuel cell.
From Fig. 2, we can see that the microorganisms in microbial fuel cell convert the biomass substrate directly into electrical energy, while in the biogas fermentation method (Fig. 1), biomass substrate is degraded into eventually the mixture of CH4 and H2 (chemical fuels). Therefore, appropriate kinds of microorganisms used in microbial fuel cell are very crucial since they determine the actual mechanism of the oxidation reactions as well as mechanism of electron transfer in the anode part of the fuel cell, which will influence how effectively we can obtain electrical energy from this system.
III. Merits and demerits of both methods
Several merits and demerits of biogas fermentation are as follow:
–> The biogas production process is a well-established technology, especially since the improvements done by UASB (Upflow Anaerobic Sludge Blanket) configuration.
–> Biogas fermentation can handle many kinds of heterogeneous wastes, specifically it is good for treating biomass substrates with high COD (chemical oxygen demand).
–> Since output of biogas fermentation is in the form of gas fuels (low volumetric energy density), it is difficult to store. Besides, the biogas produced usually still contains H2S where the removal will add to the cost.
–> At temperature below 20°C, the fermentative process will not function well.
Several merits and demerits of microbial fuel cells are as follow:
–> Since its nature of direct conversion to electrical energy, theoretically, microbial fuel cell can deliver higher kWh for each amount of organic matters consumed (estimated at 3 kWh/ kg organic matter (dry weight)).
–> Microbial fuel cells have potential application for treating low concentration COD substrate at low temperature (10-20°C)
–> The actual performance of microbial fuel cell however is still far below its theoretical expectations. There are still many issues to solve, particularly the limiting factors, such as: the activity of bio-catalysts (which has not been well understood), the mechanism of bacterial electron transfer, and loss due to internal resistance.
–> So far, microbial fuel cell does not work well with the high COD biomass substrates.
–> Since the output energy is in the form of electrical energy (flowing electrons), the stability of the bacterial oxidation reactions is an important issue. Besides, electricity storage system such as battery is needed, specifically when there is no load in the external circuit.