Energy Fuels 2010, 24, 4743–4747 Published on Web 02/19/2010
: DOI:10.1021/ef901260k
Power and Hydrogen Co-generation from Biogas† Samir Bensaid, Nunzio Russo, and Debora Fino* Department of Materials Science and Chemical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy Received November 2, 2009. Revised Manuscript Received February 4, 2010
The present work investigates the implementation of molten carbonate fuel cells (MCFCs) for power generation from landfill biogas, coupled to the on-site production of hydrogen for vehicle-fueling purposes. The system modeling of the plant has been performed in steady-state conditions, with the aim of assessing the overall power efficiency. Because MCFCs are highly exothermic and their working temperatures (650 °C) are compatible to steam reforming, the syngas is produced directly inside the vessel containing the fuel cell stack, with the reaction being thermally self-sustained. Moreover, the hightemperature flue gases from the MCFC are expanded in a turbine, thus increasing the total power generated. Hydrogen is produced through a pressure swing adsorption system, whose feed can be from either the MCFC anode outlet or a split of the reformate before the anode inlet. The overall net power efficiency of the system is similar for both configurations, being 56 and 55%, respectively. However, other parameters apart from efficiency must be taken into account for the proper selection of the most suitable configuration, such as raw material consumption and system flexibility. The two options are evaluated and discussed on the basis of these considerations.
modern engines. This, however, is much less than the value aimed at with modern fuel cells. The most suitable ones for biogas exploitation are solid oxide fuel cells (SOFCs), for power generation in the 5-50 kW1,2 range, and MCFCs, for systems from 100 kW up to multi-megawatt solutions.3,4 In a particular application,5 an alkaline fuel cell (AFC)-based system has had the aim of exploiting biogas in territories that are far from grid connections. MCFCs are suitable for biogas valorization because of their high efficiency (>50%), their temperatures around 650 °C (which lead to higher Nernst potential than SOFCs and can thermally sustain fuel reforming), their flexibility because they can operate with H2/CO/CO2 mixtures, and their reduction cost potential.3 Moreover, the high temperature of the gases exiting from the fuel cell can be further expanded in a gas turbine to generate additional power and be thermally integrated with the preheating section of the system. As far as hydrogen purification and production are concerned, pressure swing adsorption (PSA) has been selected.6
1. Introduction Landfills will remain the main waste-disposal means for Turin until 2010. The Turin landfill at “Basse di Stura” is the largest in Europe. It will ensure biogas production for at least another 20 years. The Piedmont waste management policy makers favor anaerobic digestion as a treatment option for both waste sludge from sewage treatment and the increasing amount of recovered organic civil waste. Furthermore, the Piedmont Regional framework is very oriented toward clean transport, in both the public sector (the GTT public transportation fleet has a multitude of natural gas-fueled buses) and the private one (FIAT has decided on methane cars as a market target in the short term, and Centro Ricerche FIAT has already developed several generations of H2-fueled car prototypes). Piedmont is also the region where the first Italian hydrogen bus was developed. In this context, the paper describes a biogas-fueled power generation system based on molten carbonate fuel cells (MCFCs) with higher efficiency than 50%, coupled with a methane and hydrogen production and distribution station for road transport. The main objective of this study was to assess the feasibility of a power unit (250 kW) based on MCFCs, integrated with a biogas fuel processor for decentralized methane and hydrogen production, especially tailored for a fueling station that is capable of supplying about 20-100 vehicles per day. This should correspond to a minimum production of 750 N m3/day or about 0.4 mol/s pure hydrogen (99.99%). Current biogas energy use is almost exclusively accomplished in internal combustion engines that enable up to a 40% energy conversion efficiency but only with the most
2. System Description 2.1. Pretreatment. Biogas may be both a landfill gas or an anaerobic digestion gas produced from industrial wastewater treatment, stabilization sewage sludge, or the recycling of biowaste, agricultural waste, or manure as organic fertilizers. Dependent upon its source, biogas can contain CH4 (48-75%), CO2 (25 - 47%), N2 (0-20%), O2 (0-5%), H2S (100-2000 ppm), and mercaptanes (0-100 ppm), together with other substances (1) Staniforth, G.; Kendall, K. J. Power Sources 2000, 86, 401–403. (2) Van herle, J.; Membrez, Y.; Bucheli, O. J. Power Sources 2004, 127, 300–312. (3) Bischoff, M. J. Power Sources 2006, 154, 461–466. (4) Bove, R.; Lunghi, P. J. Power Sources 2005, 145, 588–593. (5) Gair, S.; Cruden, A.; McDonald, J.; Hegarty, T.; Chesshire, M. J. Power Sources 2006, 154, 472–478. (6) Ruthven, D.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; John Wiley and Sons: New York, 1994.
† This paper has been designated for the Bioenergy and Green Engineering special section. *To whom correspondence should be addressed. Telephone: þ39011-090-4710. Fax: þ39-011-090-4699. E-mail:
[email protected].
r 2010 American Chemical Society
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Energy Fuels 2010, 24, 4743–4747
: DOI:10.1021/ef901260k
Bensaid et al.
Figure 1. Plant process flow diagram.
of the stack through the blower, together with the air from compressor 1. The outlet stream from the cathode sides goes directly into the pressurized vessel and surrounds all of the equipment installed there: the MCFC stack, the catalytic burner, the steam reformer, and the blower. As a result, the gaseous atmosphere inside the pressurized vessel is partly fed to the catalytic burner or sucked by the blower and then recycled (representing about 80% of the total cathode outlet flow rate). The remaining part of the cathodic atmosphere goes directly outside the pressurized vessel, to the APS system. The air process system was conceived to expand the flue gases from the FCS in a turbine, to produce additional power and drive compressor 1. Part of the compressed air is conveyed to the MCFCs, while the remaining part is mixed with the cathode outlet flow and processed in the burner to increase the temperature of the gases that expand in the turbine. As can be observed in Figure 1, part of the FCS outlet gases bypass the turbine; this ensures that an acceptable balance of the APS flow rates is maintained, with a maximum mass ratio of the flow rates to the turbine and compressor 1 that does not exceed 1.2. The fresh air from compressor 1 is regulated to ensure the best oxygen/fuel ratio in the fuel burner, despite the variation in the bypass flow rate. The heat exchanger called “recuperator” in Figure 1 preheats the air, therefore, exploits the sensible heat from the turbine flue gases, which are also used to generate steam for reforming and finally purged. The PSA system has a set of equipment that is used to treat the stream that flows from the MCFC anode; gas coolers reduce the stream temperature from 650 to 200 °C, so that it undergoes a
in traces (
