Article pubs.acs.org/EF
Fuel Processing of Diesel and Kerosene for Auxiliary Power Unit Applications Joachim Pasel,*,† Remzi Can Samsun,† Ralf Peters,† and Detlef Stolten†,‡ †
Electrochemical Process Engineering (IEK-3), Institute of Energy and Climate Research (IEK), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany ‡ IEK-3 Fuel Cells (FZ Jülich), Faculty of Mechanical Engineering, Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen University, 52072 Aachen, Germany ABSTRACT: Apart from necessary balance-of-plant components, such as pumps, blowers, sensors, and heat exchangers, the fuel processing unit of a high-temperature polymer electrolyte fuel cell (HT-PEFC) system based on autothermal reforming contains three main components: the autothermal reformer, the water-gas shift reactor, and the catalytic burner. In Jülich, several generations of these catalytic reactors have been designed, constructed, and manufactured, with a wide range of thermal powers between 13 and 140 kW. Characteristic common features of the respective reactor generations are described as well as their specific structural features. The experimental part of this paper concentrates on investigations with different generations of reactors for autothermal reforming using different diesel and kerosene fuels, which were produced either via the gas-to-liquid or bio-to-liquid process or in a conventional manner from crude oil. They mainly differed from each other with respect to their boiling ranges and mass fractions of aromatics. It was proven in the scope of the current work that autothermal reforming of synthetic kerosene and diesel fuel can be performed for 6000 h of time on stream at a fuel conversion of 99.99 and 99.67%, respectively. The O2/C molar ratio of the educts was found to be an important reaction parameter strongly affecting the monolith temperatures inside the autothermal reformer and the chemical composition of the product gas. Furthermore, fuel characteristics had a significant impact on reactor performance.
1. INTRODUCTION The strategic approach of work in the fuel processing and systems group at IEK-3 is governed by the aim of developing a high-temperature polymer electrolyte fuel cell (HT-PEFC) system based on the autothermal reforming of middle distillates, such as diesel and kerosene. The envisaged power class is 5−10 kWe. This class is particularly interesting for using HT-PEFC systems as auxiliary power units (APUs) for onboard power supply in aircraft and trucks. A process flowchart was developed for this purpose and is described in detail by Samsun et al.1 The main components of the system besides the HT-PEFC stack are the autothermal reformer (ATR) (Scheme 1), the water-gas shift (WGS) reactor (Scheme 2), and the
Scheme 3. Reaction Equations for Catalytic Combustion
and 2). The exothermic partial oxidation and the endothermic steam reforming reaction of the hydrocarbons (HCs) contained in diesel fuel and kerosene proceed consecutively on the same monolith. In parallel, the WGS reaction and the methanation reaction also take place in an ATR to the thermodynamic equilibrium of the reactor temperature. The WGS reactor reduces the concentration of carbon monoxide in the hydrogen-rich reformate to a level of 1 vol % (cf. eq 5). Otherwise, the catalyst of the downstream fuel cell would be poisoned by adsorbed molecules of carbon monoxide, thus significantly reducing the cell voltage. The anode off-gas of the HT-PEFC stack is converted in the CAB according to the reactions shown in Scheme 3. The anode off-gas contains considerable amounts of carbon monoxide, which was not oxidized in the WGS reactor, as well as unconverted hydrogen and methane. Methane was formed according to eq 4 during the reforming process. Such gas concentrations (approximately
Scheme 1. Reaction Equations for Autothermal Reforming
catalytic burner (CAB) (Scheme 3). These three reactors constitute the fuel processing unit. The ATR converts liquid fuels, such as diesel fuel or kerosene, together with air and steam into a hydrogen-rich gas in a catalytic process (cf. eqs 1
Special Issue: Accelerating Fossil Energy Technology Development through Integrated Computation and Experiment
Scheme 2. Reaction Equation for the WGS Reaction
Received: December 3, 2012 Revised: February 7, 2013 Published: February 11, 2013 © 2013 American Chemical Society
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injection and atomization, (3) fuel evaporation using the heat of the superheated steam, which prevents possible fuel ignition because of the lack of an oxidant, and (4) addition of air to the mixture after the fuel evaporation zone. In the literature,5−15 Rh, Pt, Ni, and RhPt are the preferred catalytically active species for autothermal reforming of liquid HCs. These species were deposited on a number of different washcoats, e.g., CeO2, ZrO2, Al2O3, TiO2, SiO2, La2O3, MgO, and Y2O3. The various papers describe different routes for the preparation of these catalytic systems as well as their catalytic activity and characterization by a number of analytical methods, such as temperature-programmed desorption, oxidation and reduction, pulse chemisorption, or transmission electron microscopy. Parmar et al.16 describe in their paper kinetic studies of the autothermal reforming of the diesel surrogate tetradecane over a Pt/Al2O3 catalyst. They found that a Langmuir−Hinshelwood−Hougen−Watson model, in which HC is adsorbed on the catalyst surface as alkyl intermediates by scission of the C−H bond, gave physically meaningful results. The Korea Advanced Institute of Science and Technology (KAIST) published a number of papers about autothermal reforming of liquid HCs, gasoline, and diesel fuel for fuel cells in recent years.17−20 They investigated different catalytic systems, varied the operating conditions (temperature, O2/C molar ratio, and H2O/C molar ratio), and applied a diesel ultrasonic injector to obtain a homogeneous mixture of the reactants. In Table 1, ATR 8 is the base case with a thermal power of 13 kW, a power density of 2.8 kW/L, and a specific power of 2.2 kW/kg. For reactors ATR 9.1 and ATR 9.2, these values were increased. ATR 9.2 has a thermal power of 28 kW, a power density of 3.5 kW/L, and a specific power of 3.3 kW/kg. The mixing chamber of ATR 9.2 was made of sheet metal instead of ceramics as in earlier ATR generations. This makes it possible to manufacture the mixing chamber more easily. Additionally, the reaction chamber of ATR 9.2, which is exposed to temperatures between 800 and 1000 °C, was also made of metal sheet instead of turned tubes as used before. This will make it possible to reduce manufacturing costs if large numbers of this reformer are produced. ATR 10 in Table 1 is an exception with respect to its thermal power of 140 kW. ATR 10 was specifically designed and manufactured for aircraft applications, where air conditioning, ice and rain protection, cabin systems, engine starting, landing gears, flight controls, etc. consume large amounts of electricity to be potentially supplied by fuel cell systems. A special characteristic of ATR 10 is that some of its parts (torospherical head and deflecting annulus) were manufactured by means of precision casting, for which minimum wall thicknesses of 2−3 mm are necessary inside the reformer. Precision casting is much more economical than metal cutting, which was applied for all other reformer generations in which the wall thicknesses of the pressureloaded parts were in the range of only 1 mm. The maximum value of 3.6 kW/L for the power density was reached in the case of ATR 13. The design and construction of this reactor was particularly optimized with respect to its power density because ATR 13 will be assembled in an allocated and limited space at the bottom of a conventional truck. The characteristic values of reformers ATR AH1 and ATR AH2 in Table 1 are comparable to those of reformers ATR 9.1 and ATR 9.2. However, ATR AH1 and ATR AH2 are industrially manufactured by a medium-sized German company, thus optimizing production costs and material input, while all other
32.0 vol % H2O, 1.4 vol % CO, 7.6 vol % H2, 20.2 vol % CO2, 0.3 vol % CH4, and 38.1 vol % N2) cannot be released into the environment. Recently, target values for APUs based on fuel cell systems in the range of 1−10 kWe were published by the United States Department of Energy (U.S. DOE). They are valid for systems operating on standard ultra-low-sulfur diesel fuel and can be used for truck applications.2 The value for electrical efficiency was defined as 30% for 2013. The final target of 40% must be reached in 2020. A power density of 30 We/L was defined for 2013, while that for 2020 was set to 40 We/L. Peters and Westenberger evaluated a number of scientific publications in the area of fuel-cell-based APUs for aircraft.3 The goal for electrical efficiency was defined as 40%. This is identical to the above-mentioned target for trucks. The power density target of 750 We/L is much higher than that for trucks.
2. REACTOR GENERATIONS FOR FUEL PROCESSING In Jülich, several reactor generations for autothermal reforming have been designed, constructed, manufactured, and experimentally tested in recent years. These generations cover a wide range of thermal powers from 13 to 140 kW. The thermal power is calculated by multiplying the lower heating value of hydrogen with the number of moles of hydrogen produced via fuel processing. The reformer efficiency refers the thermal power to the lower heating value of the fuel and its molar flow. For example, ATR 9.2 from Table 1 delivers a molar stream of Table 1. Reactor Generations for Autothermal Reforming of Liquid Fuels (Diesel and Kerosene) reactor generation ATR ATR ATR ATR ATR ATR ATR
8 9.1 9.2 10 13 AH1 AH2
thermal power (kW)
power density (kW/L)
specific power (kW/kg)
13 18 28 140 13 18 28
2.8 3.3 3.5 3.3 3.6 3.3 3.3
2.2 2.3 3.3 3.2 2.6 2.3 3.3
hydrogen with a thermal power of 28 kW. Assuming a hydrogen use of 83% in the fuel cell and an efficiency of the HT-PEFC itself of 43.0% at a selected operation point, an electric power of the system (with ATR 9.2) of 10 kW (gross power) can be achieved. For each reactor generation in Table 1, the volume of the catalytically coated monolith was calculated to obtain 100% reformer load at a gas hourly space velocity (GHSV) of approximately 30 000 h−1, an O2/C molar ratio of 0.47, and a H2O/C molar ratio of 1.9. Table 1 shows the different reactor generations for autothermal reforming together with their thermal power, power density, and specific power. The fundamental design of ATR 8 and its experimental operation during startup and steady state is explained in detail by Pasel et al.4 It should be noted here that the general structure of ATR 8 was maintained for all reactor generations in Table 1. It is a common characteristic of all reformer generations from Table 1 that they are equipped with an integrated heat exchanger to produce superheated steam to be used as the educt for the reforming reaction (cf. eq 1). Further common features of all reformer generations are (1) use of a RhPt/Al2O3−CeO2 catalyst from Umicore AG and Co. KG, (2) use of a standard pressure-swirl nozzle for fuel 4387
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4. RESULTS AND DISCUSSION A statistical experimental design was derived to characterize the ATR 9.1 described in Table 1. It comprises 11 single experiments, in which the O2/C molar ratio was varied between 0.43 and 0.47, the H2O/C molar ratio was varied between 1.7 and 1.9, and the GTL kerosene mass flow rate was varied between 1215 and 2025 g/h. Three experiments at the center point of the design completed this series of investigations to exclude a decline of the catalytic activity and to verify the reproducibility of the results. The ranges for the variables were selected on the basis of preliminary experiments.23 It was shown in the scope of that work that, when applying these molar ratios, the deposition of carbonaceous substances on the catalyst as well as high temperatures in the monolith, leading to sintering of the catalytically active species, can be avoided. Samsun and Peters24 and Box et al.25 give a very detailed description of the methodology of statistical experimental design in their papers. For the present contribution, the influence of these reaction parameters on characteristic variables was investigated, such as the temperatures in the monolith, the concentrations of reaction products in the reformate, and the efficiency of the reforming process. The temperature of the steam upon entering the reactor was 460 °C. Some of these dependences are presented and described in the following as examples. Figure 1 shows the impact of the O2/C molar ratio and the H2O/C molar ratio on the temperature in the monolith of the
ATR generations are produced by the workshop at Jülich. This transfer of production technology from a research institute to a medium-sized company constitutes an important step toward commercialization of the reformer technology developed at Jülich. The following discussion concentrates on experiments with different reactor generations for autothermal reforming, applying different kinds of diesel fuel and kerosene.
3. EXPERIMENTAL SECTION Seven different fuels were used for the autothermal reforming experiments. They are described in Table 2 with respect to some
Table 2. Different Kinds of Diesel and Kerosene Applied for Autothermal Reforming aromatics (wt %) /S fraction (ppm)
average molecular formula
185
