Technical Realizations

Power Efficiency Guide

Ultimate Guide to Power Efficiency

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12.3.1 Cell Stack

A single fuel cell, as depicted in Figure 12.1, is capable of delivering a voltage of about 0.7 to 0.9 V under operating conditions. According to today's standards, a typical current density of an MCFC is at about 150 mA/cm2, so a cell of 1 m2 size delivers approximately 1 kW electric power. To obtain systems with higher power, several cells are combined in a cell stack (Figure 12.2). From the chemical engineering point of view, these are parallel reactors; from the electric point of view, this is a series of current sources.

Channel structures between the cells distribute the gases across the electrode area and simultaneously collect the produced charges at the electrodes and transfer them to the neighboring cell. Because they connect the anode of a cell to the cathode of the next one, they are referred to as bipolar plates. Cell stacks can contain up to several hundred cells.

One practical issue with the stacking of fuel cells is the sealing. Due to temperature gradients in the system, and because of different expansion coefficients of the materials, the layered structure of the stack has to be pressed with a pressure of several bar.

The plates at either end of the stack are oftentimes massive structures. This is not only because they have to take up the pressure the stack is under, but also because here the electric current is collected in one point. While in bipolar plates the electric current flows mainly directly through the plate in a direction perpendicular to the cell plane, the current in the end plate has to flow along the plane. To minimize resistance, a high cross-sectional area is required; thus the end plates are comparably thick.

12.3.2 Peripheral Devices

In addition to the fuel cell itself, a fuel cell system requires several additional peripheral process units. These are mainly: a desulfurization unit for the feed gas, an evaporator for the steam necessary for the reforming process, an upstream prereformer, a combustion unit between the anode exhaust and the cathode inlet, and eventually one or more heat exchangers to recover the thermal energy of the exhaust stream.

Anode Cathode

Anode Cathode

FIGURE 12.2 Fuel cell stack, bipolar plates, and end plates.

Especially for fuel gases from fermentation processes, sulfur plays an important role. For a MCFC — as for any other fuel cell — sulfur is a strong catalyst poison and has to be removed from the feed gas. Depending on the fuel gas in the respective application, this unit has to fulfill different requirements. Processed natural gas contains a well-defined, constant portion of sulfuric components. In this case, active coal filters are applied, but this choice is not optimal due to the high replacement frequency and its costs. As an alternative, an adsorption bed with zinc oxide under cyclic operation is being discussed. It transforms the sulfur-containing components to hydrosulfide and then adsorbs it during the first cycle period. Once the catalyst bed is loaded to a certain extent, the feed gas is switched to a second identical adsorption bed in parallel, and air is fed into the first one. With this, the sulfur is oxidized and removed from the bed as sulfur dioxide, leaving a cleaned adsorption bed behind.

Gases containing larger portions of sulfur require additional measures. Biogas, for example, often contains significant amounts of sulfur with frequently changing concentrations and varying molecular composition. Here, biological anaerobic processes have been proposed, which produce elementary sulfur. The advantage of this concept is that the sulfur is not emitted as sulfur oxide, but it is available in a pure and less toxic form. On the other hand, these processes cannot clean the gas sufficiently for application in MCFC. Consequently, a combination of biological and adsorptive desulfurization methods seems advisable here [5-7].

In addition to hydrocarbons or carbon monoxide, steam is an important component in the feed gas. The amount of water in the feed gas is usually described by the so-called steam-to-carbon ratio, S/C. In most applications, this ratio is between 2 to 3. Mixtures with significantly less steam tend to reversibly deposit carbon on the surfaces of the pipes and electrodes. This leads to the blocking of the anode electrode and the reforming catalyst in the fuel cell and quickly decreases the performance of the system [8-12]. On the other hand, too high a steam dosage only dilutes the feed gas and leads to decreased system efficiency, especially because of the energy costs of the water evaporation. Prior to usage in the MCFC, the water has to be cleaned. Reverse osmosis is a preferred principle here to remove most of the undesired ions from the water, which could possibly act aggressively toward the piping, the catalysts, and the electrolyte at the high temperature inside the MCFC. Furthermore, an evaporator is obviously required in state-of-the-art MCFC systems.

While in principle the reforming process can take place exclusively inside the anode channel — as depicted in Figure 12.1 — a part of it usually is relocated to an external, separate reactor outside the cell. This concept, which is commonly used with low-temperature fuel cells, is known as external reforming (ER) or prereforming (PR). Another concept is the indirect internal reforming (IIR), in which the reforming takes place inside a stack or is thermally directly attached to it, but not within the cells themselves. Consequently, the concept of performing the reforming process inside the anode channel is called direct internal reforming (DIR). All three concepts are shown schematically in Figure 12.3 [13-21].

The advantage of the ER is that the design of the reactor and its operating parameters are largely independent of the fuel cell itself, giving several degrees of freedom with respect to geometry, heat management, fuel flexibility, and control. On

External Reforming

ER: External Reforming

IIR: Indirect Internal Reforming

DIR: Direct Internal Reforming

FIGURE 12.3 Reforming concepts: external reforming (ER), indirect internal reforming (IIR), and direct internal reforming (DIR).

ER: External Reforming

IIR: Indirect Internal Reforming

DIR: Direct Internal Reforming

FIGURE 12.3 Reforming concepts: external reforming (ER), indirect internal reforming (IIR), and direct internal reforming (DIR).

the other hand, as already mentioned, the reforming process requires heat and high temperature, which in the case of ER have to be provided from the outside of the reactor. This can either be done by using the heat of the exhaust gas, accepting a limit process temperature equal to that of the cells exhaust gas, or by direct oxidation of a portion of the fuel with air inside or outside of the reforming reactor. The choice of direct oxidation not only contradicts the fuel cell principle of turning reaction enthalpy into electric energy, but it also tends to lower the overall electric efficiency of the system. Furthermore, the ER is subject to the chemical equilibrium of the reforming process, which at an operating temperature of 400°C is at about 50% conversion, and for 800°C approaches full conversion, depending on the feed gas composition and pressure. The application of the ER is still advisable in MCFC systems for two reasons. First, it increases the fuel flexibility. If the feed gas contains short-chained hydrocarbons like ethane or propane, these have to be converted to methane before being reformed to hydrogen. This cracking of carbon-carbon bonds is significantly slower than the steam reforming process and thus requires larger amounts of catalysts. These can be provided by a larger external reforming reactor. Thus the external reformer makes it possible to adapt the MCFC system to the specific fuel of each individual application without the need to manipulate the design parameters of the fuel cell itself. Secondly, the external reformer serves as a kind of warning system in applications with strongly alternating sulfur content. In the event of a sulfur breakthrough, the external reformer is the first unit to be poisoned by sulfur. This can easily be detected by a set of thermocouples that show the position of the reaction zone moving through the catalyst bed. In this case, a shutdown or the fix of the desulfurization unit can save the fuel cell from harm. Even in the worst case, that is, if the external reforming unit is irreversibly damaged, it is cheaper to replace than an irreversibly damaged fuel cell stack. Thus, application of the ER concept is advisable, especially in systems with an unreliable desulfurization unit or with strongly alternating sulfur content.

The IIR concept is based on the idea to provide the required heat for the reforming process by direct coupling to the heat-releasing electrochemical reactions in the fuel cells. This can be realized either by a separate reactor inside the hot housing of the cell stack or by inserting a reforming reactor between several fuel cells. This promotes the heat exchange toward the reforming process, so it takes place at about cell temperature.

The highest degree of process integration is the DIR concept, in which the reforming process is located directly inside the anode channel. This not only leads to an intense energetic exchange between heat-releasing electrochemical reactions and the endothermic reforming process, but it also includes the mass coupling of the production of hydrogen and carbon monoxide and their consumption at the anode electrode. This direct consumption of the reforming products accelerates the reforming process and shifts its chemical equilibrium toward an extent of conversion to almost 100%, even at lower temperature of about 600°C.

As already mentioned in the technical introduction, the anode exhaust and the cathode inlet are coupled via a combustion device. In the MCFC, a catalytic unit is preferred because it allows oxidization of diluted gas/air mixture over the complete cell stack without a pilot light. In addition, it helps to avoid extreme temperatures and thus suppresses the formation of nitrogen oxides, NO,. In fact, nitrogen oxide concentration in the exhaust gas is below the detection limit.

12.3.3 Actual System Design

MCFCs are developed by different companies around the world. Although their products are working on the same basic principle, they differ in system design, power class, and intended applications. In the following, the technical solutions of the most prominent developers are discussed.

12.3.3.1 MTU CFC Solutions

In Germany, the MTU CFC Solutions company has developed the so-called Hotmodule MCFC. MTU and Fuel Cell Energy are partners to a technology and supply exchange contract. Development started in 1990, and the functionality of the concept was first publicly demonstrated in 1997 at a prototype plant in Dorsten, Germany [22, 23]. This was followed by a series of about 25 field-test plants, which where installed in various applications in Germany, Europe, and other parts of the world. Exemplary applications are power supply in telecommunications, combined heat and power supply in hospitals and a university, and combined steam and power supply at a tire manufacturer. Currently, preparations for a series production of the Hotmodule are ongoing.

The Hotmodule is a complete MCFC system with a nominal power of 250 kW. The stack consists of 343 cells, in which the anode and cathode gas channels are arranged in a cross-flow design. The stack is in horizontal position so that the cells are standing upright, with the anode channels running from the bottom to the top of the stack and the cathode channels going from one side to the other. The stack is located in a cylindrical vessel in which all the hot compartments of the system are located — thus the name Hotmodule (Figure 12.4).

As shown in the flow scheme (Figure 12.5) the Hotmodule includes all three reforming concepts. The IIR is realized by inserting a flat reforming reactor after each package of eight fuel cells. Before the external reformer, the feed gas is heated up and mixed with steam in a combined heat exchanger and humidifier.

The Hotmodule has all the general advantages of the MCFC concept and combines them with a simple, efficient design. The cylindrical housing of the rectangular

Mcfc Kawagoe

FIGURE 12.4 Cross-section of the Hotmodule.

FIGURE 12.4 Cross-section of the Hotmodule.

Exhaust

Reverse

Evaporator

Water

osmosis

Desulfuri-sation

Fuel

Desulfuri-sation

External Reforming

DIR Anode

Cathode

Combustion Chamber

FIGURE 12.5 Flow scheme of the Hotmodule. The dotted line indicates the cylindrical vessel inside which the hot compartments are located.

cell stack automatically segments the surrounding space into four compartments. One of them contains the gas manifold, which distributes the prereformed feed gas to the anode channels, and another one contains the combustion chamber. The other two volumes connect the combustion chamber with the cathode inlet, thereby splitting the cathode exhaust gas into the cathode recycle stream and the exhaust gas stream. The fact that the cell stack is completely surrounded by hot gases reduces the mechanical stress due to thermal gradients. As the stack is mounted on rails, it can be replaced by simply opening the vessel front cover, pulling the old stack out and moving the new one in. A complete replacement procedure takes about 3 to 4 days. This is mostly due to the long time required to heat up the new stack and cool down the old one.

12.3.3.2 Ansaldo Fuel Cells

In Trieste, Italy, Ansaldo Fuel Cells has successfully demonstrated the feasibility of a 100-kW MCFC system and is working on its "2TW" model, a 500-kW system including four stacks [24-26]. In contrast to the Hotmodule by MTU, the 2TW system is operated at an elevated pressure of several (3 to 5) bar. The compressor for the feed gas is coupled with a turbine in the exhaust gas stream, and the energy balance of these two is positive, meaning that the 2TW provides electrical power not only from the cell stack, but also from the combined turbine/compressor.

Another remarkable characteristic trait of the 2TW is the focus on a single reforming unit (Figure 12.6). This unit is combined with the combustion chamber to provide the energy required by the endothermic reforming process and is located inside the containment vessel of the hot system parts. The second advantage of this combination is that high operating temperatures and consequently high degrees of conversion can be obtained in the reformer. Although this reforming unit is located in the hot cell housing, it must be considered as an ER because its temperature is dominated by the combustion temperature instead of the cell temperature. The air is inserted at the cathode inlet, so it is heated up by the hot combustion exhaust gas, and a high oxygen concentration is obtained at the cathode electrode. In the Hotmodule, the cool air is fed into the combustion chamber, which reduces the temperatures inside the chamber, but in the 2TW system, a high temperature in the combustion and consequently in the reforming unit is favorable.

The MCFC systems by Ansaldo are also realized within a cylindrical vessel, which is advantageous for pressurized operating conditions.

12.3.3.3 Ishikawajima-Harima Heavy Industries

In Japan, MCFC development is conducted by a consortium of companies led by Ishikawajima-Harima Heavy Industries (IHI) [27, 28]. The MCFC system they are focusing on is a 300-kW stack with features similar to those of the 2TW by Ansaldo. It is operated under pressurized conditions and only contains a single ER unit combined with the combustion chamber. An important addition in this system is the water-recovery system, which by condensation of the steam in the exhaust gas provides the water required for the reforming process, so under regular operating

FIGURE 12.6 Flow scheme of the MCFC system by Ansaldo.

conditions, no external water supply and cleanup is required. A first plant has been demonstrated at the Kawagoe test site, and plans also include the development and demonstration of a plant of the size of several MW.

12.3.3.4 Fuel Cell Energy

Fuel Cell Energy (FCE), located in Danbury, CT, is one of the earliest suppliers of MCFC components and complete systems. They offer a cargo-container-sized 250-kW system as well as 1- and 2-MW plants based on MCFC. Just like the Hotmodule of the MTU, FCE's fuel cells feature both internal reforming concepts (IIR and DIR), using the external reformer solely to crack longer-chained hydrocarbons. In fact, early MTU systems were based on FCE cell design. A large number of demonstration plants inside the United States in different areas of application prove the technical feasibility of the FCE MCFC systems [29-31].

12.3.4 Actual Applications

MCFC systems have proven their potential in a large number of trial plants in quite different applications. They demonstrate the full functionality of the fuel cell and its superior electric system efficiency in comparison with classical units of the same size. The efficiency reaches up to 48%, depending on the system size and the individual application.

One typical application is the implementation of an MCFC system in a power plant delivering electricity and heat to a larger building complex, as is the case at the university hospital in Magdeburg, Germany. The specific Hotmodule system in Magdeburg is fed with natural gas, and it is combined with a tube-bundle heat exchanger utilizing the exhaust gas for the heat generators and cold chillers of the plant (Figure 12.7). While the system has an overall electric efficiency of 47%, its combined electric and thermal efficiency is at about 70%.

A similar MCFC system operated with natural gas is installed at a technical center of a telecommunications provider in Munich, Germany. The highly complex electronic devices in telecommunications require direct current, which is produced by the fuel cell, so AC/DC converters can be omitted and their energetic losses can be avoided. Secondly, the MCFC serves as an on-site uninterruptible power supply, which helps to avoid costly power failures in critical electronic systems. Finally, the heat produced by the Hotmodule is used to operate absorption chillers for the air-conditioning of the offices and technical rooms of the facility. In this application, the full range of the potential products of the MCFC is unfolded [32].

The application of the Hotmodule with biogas has already been demonstrated on a smaller scale, and specifications for the feed gas have been determined [33]. The sulfur content must be lowered to 20 ppm before the gas is convenient for the Hotmodule. It seems as if ammonia does not spoil the cell performance; indeed, it is electrochemically converted to nitrogen, water, and electric energy. No nitrogen oxide can be detected in the exhaust gas whatsoever. Only siloxane is a major issue, as it is a catalyst poison and has to be removed from the feed gas. In 2005, the installation of a Hotmodule at a wastewater treatment plant near Ahlen,

Mcfc Kawagoe
FIGURE 12.7 The Hotmodule MCFC system made by MTU in the IPF power plant at the University Hospital in Magdeburg, Germany.

Germany, is planned. It will mainly be fed with the sewage gases from the fermentation process and will supply the plant with electricity and heat for the offices and operations buildings.

One of the main fields of future applications of MCFC is seen in the use of secondary gases, i.e., waste gases from biological or industrial processes. Currently, only a small part of these gases are used for electric power supply; another part is burned to provide heat; and the rest is simply burned without any beneficial use. Here, the MCFC could exploit additional attractive applications due to its potential in combined heat and power supply, even with lean gases.

Other intended applications include the use of gasified coal as a fuel for MCFC, which is currently favored by IHI in Japan. While this is not a renewable technique in itself, it offers a highly efficient alternative to classical coal power plants and helps to preserve limited resources.

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