A STUDY CONCERNING THE POSSIBILITIES FOR USING FUEL CELLS SYSTEMS FOR MARITIME PROPULSION 1

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1 A STUDY CONCERNING THE POSSIBILITIES FOR USING FUEL CELLS SYSTEMS FOR MARITIME PROPULSION 1 CLAUDIU GEORGESCU, EDEN MAMUT Department of Engineering, The Ovidius University of Constanţa, Constanţa, , Romania Received December 21, 2004 The reduction of emissions on modern vessels is an important design issue. Using fuel cells as a power source solves this problem but raises other concerns. These concerns are about designing the ship in order to accommodate the reforming systems, hydrogen storage and distribution systems and the fuel cell stacks. In this paper are discussed the influences that fuel cells systems would have on the ship s architecture. Letting these problems aside, using fuel cells for maritime propulsion has the advantage of being cleaner and more efficient than other propulsion systems. 1. INTRODUCTION Reduction of exhaust emissions, improvement of thermal efficiency, lowering the noise and vibration levels are the main objectives of designing and exploitation moderns naval systems. 2. THEORY THE FUEL CELL STRUCTURE Generally, a fuel cell is a floored structure and consists of a multi-cell stack of individual fuel cells. Each cell have an anode and cathode, separated by a porous ceramic layer filled where it is dispesed a specific electrolyte. The reforming catalyst is placed both in the anode gas flow field and special individual reforming units dispesed inside fuel cells. The direct current produced is proportional to the extent of the electrochemical reaction in accordance with Faraday s law. 1 Paper presented at the 5 th International Balkan Workshop on Applied Physics, 5 7 July 2004, Constanţa, Romania. Rom. Journ. Phys., Vol. 51, Nos. 1 2, P , Bucharest, 2006

2 50 Claudiu Georgescu, Eden Mamut 2 To apply these technologies in the maritime propulsion, 2 types of fuel cell were tested: Molten Carbonate Fuel Cell (MCFC); Solid Oxide Fuel Cell (SOFC). FuelCell Energy, Inc. (FCE) is developing a 625 kw fuel cell power plant for marine applications based on its Direct Carbonate Fuel Cell (DFC) technology. The power plant is designed for operation on Mil-F-16884J naval distillate fuel designated as NATO F-76. This fuel is characterized as a 385 C max. end boiling point diesel fuel with up to 1% sulfur by weight. Thanks to sulfur content, for the fuel processing, in the marine power plant design, it is included a high pressure hydrodesulfurizing, as a first stage producing desulfurized liquid fuel. The second stage employs an adiabatic prereformer, which reacts the desulfurized distillate with steam producing a methane rich fuel gas. The converted fuel is expanded through a turbo generator, reheated and directed to the anodes in the DFC stacks. The methane is then converted to hydrogen in the DFC stacks which in turn is electrochemically converted to water, thereby producing DC power. The water for the steam is recovered from fuel cell exhaust SULFUR REMOVAL It is important to underline that the sulfur level in the fuel which supplies the fuel cells must be below 0.1 ppm. Although worldwide standards are constant lowering sulfur levels in transportation fuels, however it will be necessary to have sulfur removal steps in fuel cell power plants. Deep desulfurization can be achieved by hydrodesulfurization (HDS). Regenerable sulfur solution retrieved downstream of the HDS is being tested to minimize maintenance, size and weight of the equipment PREREFORMING Adiabatic prereforming of desulfurized NATO F-76 is being used to convert the naval distillate to a methane-rich gas useable at DFC. As shown by the reactions below, the endothermic reforming reaction is driven by the exothermic shift and methanation reactions, thereby eliminating the need to provide heat reactor. 1. Reforming (Endothermic): m n m + + ( + ) CH nho nco n H Shift (Exothermic): CO + H2O CO2 + H2 3. Methanation (Exothermic): CO + 3H2 CH4 + H2O

3 3 Fuel cells systems for maritime propulsion POWER PLANT PROCESS DESCRIPTION The nominal power plant design rating is 625 kw. A simplified process flow diagram is shown in Figure 1. Two stacks of direct carbonate fuel cells in series provide VDC to the power conditioning system which converts the DC to 60 Hz AC power at 450 volts. In the first stage of fuel processing, the NATO F-76 fuel is mixed with recycled hydrogen and small amount of make-up hydrogen. The mix stream is hydrodesulfurized at elevated pressure. The stream then passes into a bed with ZnO where the H 2 S reacts with ZnO forming ZnS. The high tide leaving catalyser is cooled, condensed and the H 2 is recirculated. The ZnS produced is regenerated to ZnO in situ using depleted air from the fuel cells. In the second stage of fuel processing, the desulfurized fuel is mixed with steam, heated and then passes in an adiabatic prereformer which produces a methane-rich fuel high tide. Fuel processing and fuel cell are thermally integrated for high efficiency. The processed fuel is eased in a turbo expender generator producing about 50 kw of a.c. power. The methane-rich fuel is reheated and pumped to the anodes in the DFC cells producing DC power. Water produced in the cells is recovered by condensation and vaporizated using waste heat from the fuel cells. Fig. 1 Ship service fuel cell power plant simplified process flow diagram. 3. EXPERIMENTAL THE MODELING OF FUEL CELLS SYSTEM A dynamic computer simulation model of the proposed U.S. Coast Guard Molten Carbonate Fuel Cell (MCFC) power plant was developed by John J.

4 52 Claudiu Georgescu, Eden Mamut 4 McMullen Associates, Inc. (JJMA) in cooperation with FuelCell Energy, Inc., the fuel cell developer. The model was designed to simulate fuel cell power plant operation and electrical distribution system integration, and was developed using a variety of software tools including: SIMSMART TM dynamic simulation software, MATLAB TM analytical software and the Visual Basic programming language. In order to simplify modeling of the complex MCFC power plant, the system was represented by a block diagram, shown in Figure 2. This diagram decomposes the fuel cell system into the following major operational areas: Fuel Cell direct current (dc) Block Power Conditioning System (PCS) Control System (CS) Motor Control Center (MCC) Load Demand 1) The Fuel Cell dc Block is the key component responsible for generation of power. It may further be broken down into two major subsystems: the fuel cell itself and the Balance-of-Plant (BOP). The BOP is responsible for providing suitable gases to the fuel cell s anode and cathode, and for receiving and processing anode/cathode output streams. Major components of the BOP include: a fuel processing system; a water recovery system; the heat recovery units. 2) The PCS is primarly responsible for the conversion of direct current produced by the plant to alternating current (ac). The PCS mainly consists of inverters, ac switch gear, dc no-load disconnect switches, isolation transformers and an internal control system. The inverter is the key component of the PCS and is responsible for producing 600 Vac, 3 phase 60 Hz ac power from the dc source efficiently and with minimal harmonics. 3) The CS also performs automatic load control functions. The CS functions as the master system controller and coordinates power plant subsystems and decision tasks. All the dc block process control and data acquisition tasks are also performed by the CS, as well as regulatory control and operational state sequencing. 4) Auxiliary power is provided by the PCS through a Motor Control Center (MCC), which controls component on-off switching, variable speed requirements and transformer operation.

5 5 Fuel cells systems for maritime propulsion 53 Fig. 2 MCFC Power Plant Power Control Diagram POWER / CONTROL STRATEGY During all operating modes, the CS will provide the coordination and control required for fuel feeding, high tide flow, process temperatures and pressures and electric power load generation. The power which can be provided by the fuel cell is directly proportional to the concentration and high tide of reactants (H 2, CO, CO) within the fuel cell stacks which are continuously monitored and controlled by CS. On a vessel, the load according to various equipment will be applied at ship s main bus, being covered by the power provided by PCS. As the dc power demand from the fuel cell increases, the dc current will increase while the voltage decreases. The interdependency of voltage and current is governed by fuel cell type and design. The direct current is a primary control variable in the power plant operation. The control system for the DFC is designed to provide two major process system management functions: Adjusting the fuel feed at fuel cell. Control of oxidant gas (passing through the cathodes) in order to maintain the fuel cell stacks within temperature regimes required by the functioning conditions. The BOP dynamic response to load change is very important in determining power plant transient behavior. Therefore, as a first step, is necessary a response analysis of BOP equipments.

6 54 Claudiu Georgescu, Eden Mamut THE SYSTEM INFLUENCE CONCERNING SHIP S ARCHITECTURE The USCGC Vindicator, a TAGOS-1 Class ship, was selected as the test platform for this study. The ship has an electric integrated propulsion power system powered by four, 600 kw, Caterpillar diesel generators and two fixed pitch propellers, each powered by an 800 hp Direct Current (DC) motor. Fig. 3 USCGC Vindicator. A conceptual arrangement of the Vindicator s machinery space and associated auxiliary system interfaces was developed. A comparison of MC fuel cell dimensions with those of the existing diesel generators needed modifications to the current machinery room arrangement. In particular, removal of void bulkheads on both sides of the space will be required in order to provide access to the four fuel cell modules. The auxiliary service systems, such as seawater, lubrication oils, fresh water, fuel and compressed air are also affected, although to a relatively minor degree. While the MC fuel cells had somewhat greater total system weight than the diesel generator sets they replaced, the ship s performance in terms of stability and sea keeping were evaluated and are expected to remain acceptable. Limited maneuvering simulations, including forward acceleration and reversing were performed. These simulations indicated that the application of power produced by fuel cells is expected to cause insignificant changes in the ship s maneuvering performance. While some additional volume and footprint area is needed to accommodate the current MC fuel cell design relative to equal power diesel generator sets that they replace, there is sufficient room in the existing machinery spaces, with minimal impact on interfacing auxiliary systems. 4. RESULTS ADVANTAGES AND DISADVANTAGES The advantages of fuel cell system consist of: increase thermal efficiency, the reduction of exhaust emissions, the diminuation noise and vibration levels on ship board. The equipment in the power plant is arranged as shown in Figure 3.

7 7 Fuel cells systems for maritime propulsion 55 Fig kw power plant equipment arrangement. Fuel processing layers are located with maintenance access for periodic catalyst replacement at 1-2 year intervals. The LHV efficiency of the ship service fuel cell (SSFC) power plant is compared to a high-speed diesel generator set and a gas turbine generator set in Figure 4. The high efficiency of the SSFC over its entire power range has significant advantages in marine applications where the ship service load varies widely during anchoring in port and cruising at sea. The high efficiency achieved is due to the high efficiency of the fuel processor in converting naval distillate to fuel gas and the fuel cell s high efficiency in converting fuel gas to power. Fuel processing units are located with access for maintenance. The disadvantages are: cost very raise, the work with high temperatures, specific weight bigger, costs of exploitation and maintenance raises. Fig. 5 Efficiency of different power systems.

8 56 Claudiu Georgescu, Eden Mamut 8 5. CONCLUSION Processing of naval distillate NATO F-76 can be achieved efficiently for the internally reforming carbonate fuel cell, thereby making this fuel compatible with the DFC TM for marine fuel cell power applications. A fuel cell power plant for marine applications based on FuelCell Energy s DFC TM technology promises significant fuel saving advantages over power generation alternatives due to its high efficiency. Dynamic simulation modeling of the MCFC continues as the fuel cell module evolves. Early simulation results have contributed to current fuel cell design efforts, which will ultimately transform this developmental technology into a viable power source for future marine applications. REFERENCES 1. E. Mamut, E. Budevski, Hierarchical Processes in Fuel Cells, 1st Int. Seminar on Thermofluid Design, Gabrovo, Bulgaria, (2004). 2. G. Steinfeld, R. Sanderson, H. Ghezel-Ayagh, S. Abens, Mark C. Cervi, Distillate Fuel Processing for Marine Fuel Cell Applications, AICHE Spring 2000 Meeting in Atlanta, March 5 9, (2000). 3. Erkko Fontell, SOFC Development in Wärtsilä Corporation, WFCP, (2003). 4. S. Allen, E. Ashey, D. Gore, J. Woerner, M. Cervi, Marine Application of Fuel Cells, Naval Engineers Journal, January, (1998). 5.