Using Brewery Effluent in Fuel Cell Power Generation

Introduction

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The use of fuel cells to generate power is quickly growing in popularity. Along with this increasing attention goes increasing research and applications. New uses for fuel cells as well as new variations on the basic current generating process are now being developed. One of the more interesting recent variations on fuel cell power generation is the use of brewery effluent to power a microbial fuel cell. Breweries across the world are now taking advantage of this unique opportunity to recycle their wastewater while producing significant portions of electricity to power their beer manufacturing establishments.

What is a Microbial Fuel Cell and How Does It Work?

Basic Theory and Design

A microbial fuel cell is a distinct type of fuel cell that utilizes the oxidization of a material by bacteria. It consists of an anode and a cathode physically separated by a cation exchange membrane (CEM), but connected through a resistance circuit. The CEM allows the flow of protons from the anode to the cathode, while still physically separating the two. The oxidization process produces both protons and electrons. The electrons are then transferred from the oxidized material to the anode and flow through the resistor to the cathode, producing a current. The protons are transferred from the reaction site through the CEM to the cathode chamber. Once the protons reach the cathode chamber, they ideally react with oxygen to form pure water. As long as there is a constant source of electrons from oxidization, a constant current is produced, and an MFC has been built.

There are many different MFC designs. The different configurations and materials all have an effect on the power output of an MFC. When choosing MFC component materials, electrical and chemical properties must be considered. Conductivity and chemical stability are very important for anodes. The most typical anode material is carbon, usually in the form of graphite plates or rods. The cathode material needs to be an excellent electron acceptor, such as oxygen. The CEM needs to separate the anode and cathode, but still be permeable to protons.

Despite properly setting up an MFC with anodes and cathodes, current and electrons will not be produced unless the oxidation reaction is thermodynamically favorable. This is evaluated in terms of electromotive force (EMF). For a standard microbial fuel cell, the theoretical EMF is calculated by the following equation:

(1)
\begin{align} E_{emf}=\frac{-\Delta(G_{r})}{nF} \end{align}

where:

(2)
\begin{align} \Delta(G_{r})=\Delta(G_{ro})+RTln(\Pi) \end{align}

In these equations, $\Delta(G_{r})$ is the change in the Gibbs Free Energy of the system, n is the number of electrons produced per reaction mole, F is Faraday's constant, and $\Pi$ is the activities of the products divided by the activities of the reactants. For a thermodynamically favorable equation, the $E_{emf}$ must be compared to the $E_{emfo}$ at standard temperature and pressure conditions. This means the following equation must be greater than zero for the oxidization reaction to occur.

(3)
\begin{align} E_{emf}=E_{emfo} - \frac{RT}{nF}ln(\Pi) \end{align}

The actual voltage produced in a MFC is much less than the theoretical voltage from equation 1 due to losses from many places. These losses include:

• Activation losses: Losses due to the activation energy required for the oxidization reaction
• Bacterial Metabolic losses: Losses due to the generation of metabolic energy for sustaining the reaction
• Ohmic losses: Losses from the resistance to electron flow

Above image: Model of a basic microbial fuel cell [4].

Evaluation of MFC Performance

Four common ways to evaluate MFC performance are:

• Power Output
• Power Density
• Coulombic Efficiency
• Energy Efficiency

Power Output: The power output of an MFC is simply the product of the voltage and the current. However, it is much more difficult to measure current than resistance in an MFC, so combining the above principal with Ohm's Law ($V=IR$) the power output of an MFC can be calculated as:

(4)
\begin{align} P=\frac{(E_{cell})^2}{R} \end{align}

Power Density: The power density of an MFC is the power output per unit volume. The power density is a suitable means to evaluate performance because it of normalizes the power output for use in comparison to other power generating devices. It is calculated by simply dividing the power output from equation 4 by the volume.

Coulombic Efficiency: Coulombic efficiency is the ratio of the actual number of Coulombs transferred to the anode from the oxidizing material to the maximum possible number of Coulombs transferred.

Energy Efficieny: Energy efficiency is the ratio of power produced by the cell to the heat of combustion of the reacting material. It is calculated by dividing the power output by the heat of combustion value times the number of moles of substrate [5].

How is Brewery Effluent Used in MFCs?

Using brewery effluent to power MFCs can not only help generate power, but is also effective as a method of wastewater treatment. In China, brewery effluent makes up 1.5% to 2% of the country's total wastewater. Current biological methods of treating brewery effluent are effective but energy intensive. Using MFCs to treat this wastewater will not only yield cost savings, but will help power the brewery, reducing harmful emissions such as CO2.

Brewery wastewater is effective as fuel for microbial fuel cells because it contains sugar, starch, and protein. It is derived from a consumable good, and contains a lack of ihibitory substances like ammonia [6]. The effluent goes through anaerobic digestion, yielding methane, which powers the fuel cell. This process is shown in Equation 5 [7].

(5)
$$CH_{4} + 2O_{2} -> CO_{2} + 2H_{2}O$$

Several factors can affect the performance of a brewery effluent powered MFC. When the temperature of the effluent is lowered, the MFC has less voltage output, a lower power density, and a lower Coulombic efficiency (CE). In research conducted by Feng, Wang, Logan, and Lee, the power density decreased by 17% when the temperature was dropped from 30 oC to 20oC. This is a result of a reduced amount of reactions occurring at the cathode. However, this temperature dependence is much less severe than that of current anaerobic wastewater treatment processes.

The addition of a buffer to the wastewater can improve MFC operation. When the buffer PBS is added, the power density increases, the conductivity of the solution increases, the CE increases, the ohmic resistance decreases, the proton flow between the electrodes increases, and the PH near these electrodes in neutralized. In addition, the removal of CODs (chemical oxygen demands) is increased (CODs must be minimalized in wastewater treatment).

The power output is also in fact dependent on the initial concentration of these CODs. Power output is higher for higher initial concentrations of CODs, and can be calculated using Equation 6, where Pmax is the maximum power output at a fixed internal resistance of 100/omega in mW/m2, S is the concentration of the wastewater in mg COD/L, and K is the half-saturation constant in mg COD/L.

(6)
\begin{align} P = P_{max} \frac{S}{K_{s} + S} \end{align}

While the power output of an MFC using brewery effluent is less than that of an MFC using domestic wastewater, it still provides a viable economical and environmentally friendly solution to dealing with wastewater processing [6].

A 6-liter beer MFC [8]

Applications

The Sierra Nevada Brewery in Chico, California contains a 1-MW fuel cell power plant. The plant contains four DFC300 fuel cells from FuelCell Energy, Inc. The biofuel obtained from the effluent is mixed with natural gas and fed into the cells. This mixture of fuel yields 100% of Sierra Nevada Brewery's power. When not mixed with natural gas, the fuel cells can produce up to 400 kW of energy from brewery effluent alone.

Not only are the fuel cells used to produce power, but they are used in the brewing process as well. Steam is collected from them at 650oF and 125psi. This steam is then used for heating the brewery and to boil the beer in the brewing process. In this way, 1.5 million Btus of waste heat harvested each year.

The implementation of the fuel cells is beneficial both environmentally and financially. They are categorized as Ultra-Clean under California State Law. They allow the local power company to operate with less demand and thus more efficiently [7]. Installing the fuel cells in 2005 and the digesters in 2006 cost about $7 million. However, the brewery received$2.4 million from Pacific Gas & Electric Co. under the California Public Utility Commission (CPUC) Self-Generation Program. It also received $1 million form the US Department of Defense Climate Change Fuel Cell Program. The cells allow$400,000 per year in fuel cost savings. With all these financial factors combined, the project will be paid off in about 6 years [9].

Other Applications

Other breweries applying MFC technology to their wastewater are Kirin, Asahi, and Sapporo [10]. Fosters in Australia is also implementing this technology with the assistance of research at the University of Queensland. The university received a \$150,000 state government grant to implement the MFC at Fosters. Upon completion, it is to be a 660-gallon fuel cell that produces 2 kW of power [11].

Pilot MFC at Fosters Brewery [12]

How YOU Can Make Your Own MFC!

The following procedure is quoted directly from www.microbialfuelcell.org:

Building a Two-Chamber Microbial Fuel Cell (after a tutorial presented by The Logan Group)

This webpage aims to help someone to build a microbial fuel cell (MFC) using relatively inexpensive and readily available materials. The method is based on the microbial fuel cells built by Abbie Groff, a student at Conestoga Valley High School in Lancaster, PA. The research she performed with her MFCs helped her win the Grand Champion Award at the 2005 Lancaster County Science Fair. More information about her research can be found on her website.

This website is intended to be a rough guide to constructing a MFC, not an exact step-by-step procedure.

Materials

Unless otherwise noted, all materials should be available at local stores.

Two heavy duty plastic bottles with sealable lids
Short section of plastic pipe (polyethylene or PVC) for salt bridge
Means to connect pipe to bottles (plastic flanges, end caps with holes drilled)
Agar1
Salt (NaCl, KCl, KNO3, etc)
Carbon cloth2
Bacteria3
Food for the bacteria4
Fish tank air pump with plastic tubing
Sealing materials (epoxy)
Resistors
Copper wire (plastic coated)
Wires with alligator clips
Multimeter for electrical measurements

Construction Procedure

1. Collect materials

2. Connect end caps of flanges to bottles

• Epoxy end caps or flanges to sides of plastic bottles.
• After epoxy has hardened, drill or cut holes through plastic bottles to allow for contact between liquid and the salt bridge.

3. Assemble Salt Bridge

• Dissolve agar into boiling water (at concentration of 100g/L).
• Add salt to the agar/water mixture while the mixture is still hot.
• Seal one end of plastic pipe.
• Pour agar/salt mixture into plastic pipe while it is still warm and before it begins to thicken.
• Allow the agar/salt mixture to cool and solidify.

4. Assemble electrodes

• Connect copper wire to piece of carbon cloth.
• Use epoxy to fasten the wire to the carbon cloth and to help protect from corrosion.
• Test electrodes with multimeter - there should be a small amount of resistance between a point on the carbon cloth and the end of the wire opposite the cloth.
• For anode, pass wire through a hole in the bottle lid and seal with epoxy. Cathode chamber does not necessarily need a lid.

5. Assemble MFC

• Connect salt bridge between the two plastic bottles and use epoxy to seal.

One of Abbie Groff's MFCs (from www.geocities.com/abigail_groff).

1. Add inoculum (wastewater, anaerobic benthic sediments) to anode chamber

2. Add conductive solution (saltwater) to cathode chamber

3. Insert anode (connected to lid) into anode bottle. Add cathode to cathode bottle. Begin bubbling air in cathode bottle with fish pump.

4. Connect external circuit through a resistor, and start measuring voltage.

Important Hints for Operating your MFC

1. Oxygen must be kept out of the anode chamber

2. For long-term operation, electrodes should be constructed in a way that limits corrosion of copper wire due to contact with liquids

3. Power can be significantly increased by using a catalyst (typically platinum) on the cathode. Note: Platinum is expensive.

Material Notes

1 Agar should be available in most high school science labs. If not, it can be purchased from several sources online.

2 Carbon Cloth can be purchased online from www.etek-inc.com. The carbon cloth necessary for the electrodes is standard carbon cloth without wet proofing.

3 Bacteria for a MFC can be obtained from several sources. A sample of wastewater from a local wastewater treatment plant would contain the proper microorganisms. Some locations at the plant may be better than others for obtaining the proper organisms. Animal wastewater from a farm would also work. Anaerobic benthic sediments in a creek or lake would also be likely to contain the proper organisms.

4 Most likely, wastewater or anaerobic sediments will initially contain enough organic matter to serve as food for the bacteria, but this will eventually run out. A food source (substrate) such as glucose or acetate (vinegar) can then be used to maintain the MFC.

References:
[1] http://www.dallasnews.com/sharedcontent/dws/img/04-07/0412beer2.jpg
[2] http://www.greenjobs.com/Public/images/fuel-cell.jpg
[4] http://www.microbialfuelcell.org/images/MFC.jpg
[5] Logan, Bruce, Bert Hamelers, Rene Rozendal, Uwe Schroder, Jurg Keller, Stefano Freguia, Peter Aelterman, Willy Verstraete, and Korneel Rabaey. "Microbial Fuel Cells: Methodology and Technology."Environmental Science and Technology. 2006. Microbial Fuel Cells. 19 November, 2008. http://www.microbialfuelcell.org/
[6] Feng, Yujie, Xin Wang, Bruce E. Logan, and He Lee. "Brewery Wastewater Treatment Using Air-Cathode Microbial Fuel Cells."
Applied Microboilogy and Biotechnology. 78 (2008): 873-880. EBSCO Host. University of Pittsburgh Lib. Pittsburgh, PA.
19 November, 2008. http://www.ebscohost.com
[7] Skok, Andy. "Pristine Power, Premium Beer." Sustainable Facility. LexisNexis. University of Pittsburgh Lib. Pittsburgh, PA.
19 November, 2008. http://www.lexisnexis.com
[8] http://www.microbialfuelcell.org/Presentations/First%20MFC%20symposium/MFCconfPennState08-compressed.pdf
[9] Lipman, Tim. "Sierra Nevada Brewery 1 MW Fuel Cell CHP System." LexisNexis. University of Pittsburgh Lib. Pittsburgh, PA.
19 November, 2008. http://www.lexisnexis.com
[10] http://www.fuelcells.org/basics/apps.html
[11] http://www.msnbc.msn.com/id/18440305/
[12] http://www.microbialfuelcell.org/Presentations/First%20MFC%20symposium/JK%20presentation%20MFC%20Pilot%20v3.pdf

Page Created by AnnMarie Rowland and Sarah Dotson

page revision: 40, last edited: 21 Nov 2008 19:31