Molten Carbonate Fuel Cell Combined Heat, Hydrogen and Power System: Feedstock A

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Highlights

Biogas can be produced from wastewater， organic， agricultural and industrial waste.

Biogas produced from these feedstocks is a potential source of renewable energy.

Methane present in biogas can be used to fuel a molten carbonate fuel cell （MCFC）.

The MCFC can be used for a combined heat， hydrogen and power （CHHP） system.

The CHHP system reduces fossil fuel usage and greenhouse gas emissions.

Abstract

Biogas is an untapped potential in regards to an alternative energy source. This immediately available resource will allow countries to reduce their greenhouse gas emissions， energy consumption， and reliance on fossil fuels. This energy source is created by anaerobic digestion of feedstock. Sources for feedstock include organic and inorganic waste， agricultural waste， animal by-products， and industrial waste. All of these sources of biogas are a renewable energy source. Specifically a fuel cell can utilize the methane present in biogas using integrated heat， power， and hydrogen systems. A study was performed concerning energy flow and resource availability to ascertain the type and source of feedstock to run a fuel cell system unceasingly while maintaining maximum capacity. After completion of this study and an estimation of locally available fuel， the FuelCell Energy 1500 unit （a molten carbonate fuel cell） was chosen to be used on campus. This particular fuel cell will provide electric power， thermal energy to heat the anaerobic digester， hydrogen for transportation， auxiliary power to the campus， and myriad possibilities for more applications. In conclusion， from the resource assessment study， a FuelCell Energy DFC1500TM unit was selected for which the local resources can provide 91% of the fuel requirements. Key words： Molten carbonate； Tri-generation； Feedstock； Hydrogen； Fuel cell

INTRODUCTION

Biogas is a potentially enormous source of renewable energy. It is produced by the anaerobic digestion of wastewater， organic and inorganic waste， agricultural waste， industrial waste， and lastly animal by-products. Biogas can be treated to produce Hydrogen， Power and Heat （CHHP） by utilizing a molten carbonate fuel cell. This paper will examine the development of a CHHP system at the Missouri University of Science and Technology （Missouri S&T） campus located in Rolla， Missouri， USA. The CHHP system is capable of producing enough power for the campus so that air pollution will decrease； in turn， making the community healthier（Hamad， et al.， 2013； Agll， et al.， 2013； Yu， et al.， 2013）. The electric power purchased by campus will consequently reduce. An additional benefit of the CHHP system is the higher efficiency at which it operates compared to other distribution plants of similar dimensions. The hydrogen produced can be a power source for diverse purposes on the university campus. These can include but are not limited to personal transportation， reserve power supplies， portable power， and mobility/utility applications. Within the vicinity of the Missouri S&T campus are a variety of feedstock that can be utilized for consumption to produce biogas were ascertained. A study on energy flow and resource availability was executed to pinpoint the type and source of feedstock necessitated to continuously run the CHHP at maximum capacity to produce electricity， heat recovery， and hydrogen （Pecha， et al.， 2013； Braun， 2010； Ghezel-Ayagh， McInerney， Venkataraman， Farooque， & Sanderson， 2011）.

3.4 Gas Treatment System and Fuel Storage

The gas treatment system uses the biogas from the anaerobic digestion system as its input feed. The gas treatment system is comprised of the PSA unit that helps in deriving pure form of methane （Hamad， et al.， 2013； Agll， et al.， 2013； Krishna， 2012； Adhikari & Fernando， 2005； Locher， Meyer， & Steinmetz， 2012）. The design has a total of four adsorbers to ensure a continuous stream of high quality methane. While carbon dioxide （CO2）， hydrogen sulfide （H2S） and other impurities in one set of tanks are desorbing， biogas will be fed to the second set of tanks for adsorption. The product from this gas treatment system is pipe line quality natural gas which is fed into the fuel cell.

Even though the DFC？ fuel cell units can handle 60% methane and 40% carbon dioxide without affecting its efficiency， the design included the PSA unit for the following reasons：

a. The DFC？ fuel cell units cannot accept H2S， water（H2O）， and other impurities in its input fuel. Therefore， biogas treatment is necessary before feeding it into the fuel cell under all conditions.

b. Inlet fuel pressure to the fuel cell should be between 2-2.4 bar. If the fuel contains 40% carbon dioxide， it will impact the sizing of the equipment downstream the fuel cell. For example， the design will require a higher capacity heat exchanger， water gas shift reactor， and hydrogen purification or separation system. The DFC1500TM requires 307 m3/h of natural gas at 37 MJ/m3. If biogas is utilized， the fuel cell system will require 477 m3/h of biogas as fuel to operate. This will increase the size of the equipment downstream the fuel cell by 55% and will increase its capital cost which is not desirable.

c. The biogas output from the digester can vary due to disruption in the feedstock availability or other unforeseeable reasons. In this case， the system will have to use natural gas purchased from utility company to provide any unmet fuel demand by the fuel cell. It was estimated that the systems downstream the fuel cell will run at 78.5% of its normal capacity if the fuel quality changes from 100% biogas to 50% biogas and 50% natural gas.

d. The product gas from the PSA unit is expected to have an average heat content of 37 MJ/m3 which is roughly equal to the average heat content of natural gas consumed in Missouri （38 MJ/m3） through 20072010. Hence， the fuel cell unit will receive a consistent fuel throughout its operation.

An energy analysis that determined the net of fossil fuel savings， and the savings in green house gases， has been performed in detail. The same can be found in Agll et al （2013）.

CONCLUSION

This paper provides the feedstock analysis and design of combined heat， power， and hydrogen systems to be used at a university campus. An energy flow and resource availability study was performed to identify the type and source of locally available feedstock， required to continuously run the fuel cell system at peak capacity. It was found that the anticipated methane production after biogas treatment is 260 m3/h with a heat content of 37 MJ/ m3. Following the resource assessment study， a FuelCell Energy DFC1500TM unit was selected for which the local resources can provide 91% of the fuel requirements. The CHHP system provides electricity to power the university campus， thermal energy for heating the AD， and hydrogen for transportation， back-up power and other needs.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the Hydrogen Education Foundation for their support of the annual Hydrogen Student Design Contest which challenges university students to design hydrogen energy applications for realworld use.

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