Ind. Eng. Chem. Res. 2005, 44, 1535-1541
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Integrated Fuel Cell Processor for a 5-kW Proton-Exchange Membrane Fuel Cell G. M. Ratnamala, Nilesh Shah, Viral Mehta, and P. V. Rao Process Development Division, National Chemical Laboratory (NCL), Pune 411008, India
Sukumar Devotta* National Environmental Engineering Research Institute (NEERI), Nagpur 440020, India
A fuel processor is a combination of a few reactors to generate hydrogen required for a fuel cell. The study deals with the production of hydrogen suitable for a 5-kW proton-exchange membrane fuel cell (PEMFC) for household applications using liquefied petroleum gas (LPG) as the fuel. The aim is to energy integrate the five reactors in a fuel processor, i.e., desulfurizer, steam reformer, high-temperature shift reactor, low-temperature shift reactor, preferential oxidation reactor, and various heat exchangers. Heat-exchanger network synthesis analysis has been carried out for the entire process to make the process efficient. The results obtained from the studies show that the steam reforming with LPG gives a higher concentration of hydrogen in the product of about 74%. The fuel cell efficiency is around 34%, and the thermal efficiency including lean gas is about 93%. The model developed can serve as the basis for the development of an integrated PEMFC decentralized power pack for household applications. Introduction Fuel cells (FCs) have received increased attention in recent years for power generation owing to their potential higher thermal efficiency and lower CO2 emissions per unit of power produced. FCs are intrinsically much more energy efficient and could achieve 70-80% system efficiency.1 A FC is an electrochemical device where a suitable fuel reacts with oxygen to generate electricity and produce combustion byproducts. Different types of FCs are in various stages of development. Among these, the proton-exchange membrane FC (PEMFC) is useful for small-scale applications such as household power packs and automobiles. PEMFCs operate at relatively low temperature (70-90 °C), have a high power density, and can vary their output quickly to meet the power demand. The electrolyte used is an ion-exchange polymer membrane, which is a very good proton conductor. The fuel used is a H2-rich gas with a small trace of CO, which may be a poison for the membrane. H2 is not freely available. So, it has to be generated from H2-rich fuels. It is generally generated by the chemical conversion of fuels such as hydrocarbons and alcohols. A FC system can be grouped into three parts: FC processor, FC stack, and power conditioner. In the FC processor, a fuel-like liquefied petroleum gas (LPG) is processed to produce H2-rich gas through several steps including reforming, water gas shift reactions, and preferential oxidation.2 Then the H2 and air are fed to the FC stack for generation of a direct electric current. The direct electric current produced by the FC is converted to an alternating electric current by the power conditioner. PEMFCs for stationary household applications should be compact. If H2 production is done by steam reforming, * To whom correspondence should be addressed. E-mail:
[email protected].
which is an endothermic reaction, external heating has to be supplied by combustion or electrical heating. If combustion is used, then the integrated FC system should include a combustion chamber. Such integrated systems can be evaluated in terms of the energy and size. Energy integration or process integration can be applied to enhance its thermal efficiency. By improvement of the energy integration of the system and by optimization of operating conditions, the efficiency can be increased from 35% to 49% for an optimized design.2 In the present paper, the following methodologies were used: (1) A spreadsheet model was developed for mass and energy balance for the whole system. (2) The energy requirements were computed for defined hot and cold streams in the process. (3) A heat-exchanger network design by pinch technology is applied for the integrated system design to achieve an optimum design by minimizing the number of heat exchangers and utility load. Fuel Selection for the FC and Fuel Processor H2, being difficult to store and transport, may be conveniently produced at the site from easily transportable and stored fuels.3 Numerous researchers have shown that FCs utilizing fuel-containing H2 would produce near zero emissions. Fuels for the fuel processor may be any hydrocarbon fuel or any organic fuel. For the present work, LPG has been chosen. LPG has a reasonably high H2 content and is also available readily in almost the entire Indian subcontinent. LPG may also be the most suitable fuel for stationary household applications.4,5 Fuel Processing Fuel processing is defined as conversion of any hydrocarbon or organic fuels to a fuel gas reformate suitable for FC anode reaction. It comprises the reduc-
10.1021/ie0498450 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/04/2005
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Figure 1. Major components of the FC processor.
tion of harmful contents such as sulfur, the conversions of raw fuel to fuel gas reformate, and final processing to make it suitable for the FC. It is carried out in an integrated unit called the fuel processor to produce the suitable fuel needed for the FC. A typical PEMFC processor is shown in Figure 1. The major components of the fuel processor are as follows: (a) desulfurizer; (b) reformer; (c) water gas shift reactors; (d) preferential oxidation reactor. Desulfurizer (DS). The first step in processing is to reduce the sulfur content to the ppm level from the raw fuel because it acts as a poison for the catalysts used in the reactors and FC membrane. Sulfur may be present in the form of mercaptans in hydrocarbon fuels such as LPG. Hydrodesulfurization is usually achieved by decomposing the mercaptans present in the fuel to H2S and organic compounds. It employs a ZnO- and NiSbased catalyst. Decomposition is carried out by NiS in the catalyst, and ZnO absorbs the H2S formed. The temperature is generally around 300-350 °C. Reformer. The conversion of fuel to H2 in the presence of catalyst at high temperature is called reforming. Reforming can be of three types: steam reforming, autothermal reforming, and partial oxidation. Of the three reforming processes, the steam reforming process yields the highest H2 concentration in the product.6 Steam Reformer (SR). In the reformer, the fuel is heated, vaporized, and mixed with steam. The reaction starts, in the presence of nickel-based catalysts, in the temperature range 400-750 °C, to produce H2, CO, CO2, CH4, and H2O. Because the reaction is endothermic, it requires some additional heat. Partial Oxidation Reactor (POX). In this reactor, fuel reacts with oxygen, in the temperature range 700900 °C, to generate H2 and CO. The catalysts used are usually rhodium, platinum, and cobalt supported on oxide substrates. The reaction is highly exothermic and requires external cooling. Autothermal Reformer (ATR). In the ATR, steam reforming and partial oxidation reactions are coupled. The heat generated by the POX is utilized by the endothermic SR reaction. The ATR has a lower H2 concentration in the product compared to SR. Thermodynamic analysis of the natural gas fuel processor under simultaneous partial oxidation and steam reforming shows that an optimal H2 yield of 36.3-36.6% can be achieved.7 For the present work, SR is considered because it offers a higher H2 concentration and it is more energy effiecint. Water Gas Shift Reactors. CO produced from reforming reactions must be brought down to the ppm level because it gets adsorbed on the noble catalyst of the PEMFC and poisons it. The CO level is reduced by water gas shift reactors, where CO reacts with steam
to give H2 and CO2. This step also leads to additional H2 generation. Usually, two water gas shift reactors [a high-temperature shift (HTS) reactor and a low-temperature shift (LTS) reactor] are used in series to reduce the CO content and increase the H2 concentration. Preferential Oxidation Reactor (PROX). PROX is the preferred method, compared to other methods such as membrane separation, to bring down the CO level to a few ppm because of its lower cost, lower H2 losses, and lower energy consumption. In this process, steam from the LTS reactor and a small amount of air is fed to the PROX reactor, where selective catalysts, like Pt or Pd supported on a base oxide, oxidize CO without oxidizing a large amount of H2. Once the fuel is processed, it is introduced to the FC stack to generate electricity. Fuel Processor Efficiency and FC Efficiency Fuel processor or thermal efficiency defines the energy efficiency of a fuel processor.1 The thermal efficiency of a fuel processor ηTE may be defined as
ηTE ) LHVH2/LHVFuel where LHVH2 is a lower heating value of H2 fed to the FC stack (kW) and LHVFuel is a lower heating value of fuel used (kW). The FC efficiency is defined as the ratio of the dc power output of the FC stack to the LHV of fuel.
ηFC )
dc power ouput LHVFuel (reforming + burning)
where LHVFuel (reforming + burning) is a lower heating value of fuel used in the SR reaction and combustion. The FC stack efficiency8 is defined as
ηFS )
dc power ouput LHV of the anode feed gas
where LHV of the anode feed gas is H2 used in the FC. Optimum Temperature In the present study, three reactions are assumed to occur simultaneously in the SR with a steam-to-carbon (S:C) ratio of 3. To determine the concentration of each species a function of the temperature at the end of the reactor, the method of Lagrange’s undetermined multipliers is used. For any reaction, assuming an ideal gas law and constant pressure, the equation is10
∆Gfi° + ln Yi +
∑k (λkaik) ) 0
(1)
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Figure 2. Variation in the concentration of species in SR with the equilibrium temperature.
Figure 3. Variation in the concentration of species in the HTS with the equilibrium temperature.
By the use of Polymath, a set of nonlinear equations is solved simultaneously and concentrations of each species are obtained for the SR, HTS reactor, and LTS reactor for various reaction temperatures. The water gas shift reaction also takes place along with a reforming reaction. The CO produced from the reforming reaction reacts with steam to give CO2 and H2, by which the CO content is reduced and H2 is enriched. Because the present study deals with PEMFCs, H2 should contain CO of less than 5 ppm. CO in PEMFCs acts as a poison not only for the catalyst but also for the membrane. Therefore, the temperatures are selected such that the maximum concentration of H2 and the minimum concentration of CO are obtained. All thermodynamic data
for various reactions have been obtained from the literature.11 The results are presented in Figures 2-4. From the graphs, the following can be concluded: (i) In the SR, the maximum H2 and minimum CO concentration occurs in the temperature range 9501000 K. Therefore, for further calculation for the SR, the reaction temperature is assumed to be 973 K. (ii) The temperature range for HTS reactors and for the specified catalyst was selected from the literature. The maximum concentration of H2 and the minimum concentration of CO were obtained at around 550 K. However, an HTS reactor was running successfully at 623 K, without any coke formation, at our laboratory. Therefore, for the present work 623 K is considered.
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Figure 4. Variation in the concentration of species in the LTS with the equilibrium temperature.
Figure 5. Flow diagram for the fuel processor with SR for LPG.
(iii) The variation of the concentration with the temperature is almost a flat line for the LTS. On the basis of the experiments in our laboratory, the temperature considered is 453 K, where this temperature gave optimum results. Therefore, this temperature is used in the LTS reactor for further calculations. Process Description The flow diagram for steam reforming of LPG is shown in Figure 5. The reactions that occur in the reactors are listed in Table 1. LPG is heated to 350 °C and is fed to a DS unit. The sulfur content of the LPG is assumed to be 30 ppm. The catalyst considered for the DS unit is ZnO modified with molybdenum. The S impurities are brought down to
