On-board reforming of biodiesel and bioethanol for high temperature PEM fuel cells: Comparison of autothermal reforming and steam reforming
Research highlights
▶ ATR-HTPEM system for on-board reforming is competitive with SR-HTPEM system in terms of efficiency. ▶ Max. SR fuel processing efficiency: 75.6% for biodiesel, 76.3% for bioethanol. ▶ Max. ATR fuel processing efficiency: 74.1% for biodiesel, 75.1% for bioethanol. ▶ Max. system efficiency for SR-HTPEM systems: 30% for biodiesel and 31% for bioethanol. ▶ Max. system efficiency for ATR-HTPEM systems: 26% for both biodiesel and bioethanol.
Introduction
Today there is great interest in developing fuel cell systems for the transportation sector. Fuel cells are considered as a promising, environmental-friendly option for powering future cars and auxiliary power units (APU) for all kinds of vehicles. Polymer electrolyte membrane (PEM) fuel cells are considered as the most promising option for transportation applications because of their high power density which is an order of magnitude higher than for any other type of fuel cell [1]. The operating temperatures are typically between 70 and 90 °C. In order to avoid poisoning of the platin electrode the CO concentration in the feed gas has to be reduced below 20 ppm [2].
This paper focuses on hydrogen production from biofuels for high temperature PEM (HTPEM) fuel cells. HTPEM fuel cells are operated at slightly higher inlet temperatures of 120–180 °C [3]. Li et al. reported that a HTPEM based on polybenzimidazoles, a high-temperature polymer which has been firstly synthesized by Carl Shipp Marvel in the 1960s, can tolerate up to 1 Vol.-% CO and 10 ppm SO2 in the fuel stream, allowing for simplification of the fuel processing system [4]. Thus it is possible to reach the CO requirements for a HTPEM by using only a water gas shift stage.
As there is no existing infrastructure for hydrogen available, the feed hydrogen for high temperature PEM fuel cells has to be supplied by on-board reforming of existing transportation fuels such as gasoline, diesel and biofuels [2]. Especially liquid biofuels have recently attracted increasing attention as alternative sources for the transportation sector [5]. Demirbas comes to the conclusion that bioethanol and biodiesel are the two liquid transportation fuels with the highest potential to replace gasoline and diesel fuel in the future [6]. Currently, bioethanol which is derived mainly by fermentation of sugar cane and starch is by far the most widespread non-fossil alternative fuel in the world. World production of bioethanol increased from 24.5 million metric tons in the year 2004 to 60 million metric tons in 2009 [7]. This is about 4% of the worldwide gasoline consumption. World production of biodiesel, a synthetic diesel-like fuel produced by transesterification of vegetable oils, increased from 2 million metric tons in the year 2004 to 15 million metric tons in 2009. This accounts for approximately 0.2% of diesel consumed for transport [6].
In a first step the liquid biofuels have to be converted into a hydrogen-rich gas by the means of reforming. Steam reforming (SR) is the most widely practiced commercial process for hydrogen and synthesis gas production [8]. It is well known, that SR has the highest hydrogen efficiency amongst the available reforming options [9], [10], [11]. One of the early applications of steam reforming was the catalytic synthesis of ammonia from hydrogen and atmospheric nitrogen, a process which was developed by Fritz Haber and Carl Bosch in 1909.
If H2O is replaced by CO2 (CO2 reforming, CR) a synthesis gas with a lower H2:CO ratio is obtained. By combining SR and CR a synthesis gas ideal for conventional methanol plants can be produced.
Synthesis gas can also be produced by partial oxidation of hydrocarbons with oxygen (POX). POX is divided into thermal partial oxidation (TPOX) at temperatures higher than 1200 °C being used for sulphur-containing heavy hydrocarbon fuels and catalytic partial oxidation (CPOX) for low-sulphur feedstock taking place at temperatures of 900–1000 °C.
Autothermal reforming (ATR) is a mixture of SR and POX using steam and oxygen to produce a hydrogen rich synthesis gas. In order to obtain pure hydrogen from the reformate gas of either SR, CR, POX or ATR (with subsequent water gas shift) further gas purification steps are necessary, e.g., preferential methanation, preferential oxidation and pressure swing adsorption [8], [12].
Ersoz et al. [13] compared SR, ATR and POX for a combined reformer PEM fuel cell system using natural gas, gasoline and diesel as hydrocarbon sources. They come to the conclusion that steam reforming and autothermal reforming appear as the most competitive options in terms of fuel processor efficiency for PEM fuel cells.
Giunta et al. [14] analyzed hydrogen production from steam reforming for PEM fuel cells using bioethanol as raw material. They conclude that hydrogen production using ethanol as raw material is a very attractive alternative to those technologies based on fossil fuels.
Ersoz et al. [15] studied the performance of autothermal reforming for two different hydrocarbon mixtures. The results indicate very similar behaviour for both of the investigated fuels. The maximum fuel processing efficiency is reported at T = 700 °C.
Benito et al. [16] performed extensive thermodynamic analysis of a 1 kW bioethanol steam reforming processor for PEMFC operation. A processor efficiency of 73.7% for S/C = 3.2 was achieved taking advantage of the heat released in the exothermic stages. By using the energy content of the unconverted hydrogen of the exhaust anode gas stream an energy efficiency of the processor-fuel cell system of 30% was achieved.
The objective of this article is to evaluate different reforming concepts for on-board reforming of biodiesel and bioethanol for use with high temperature PEM fuel cells in the transportation sector, e.g., APU applications for cars, trucks and ships. Conversion of biodiesel and bioethanol into a hydrogen-rich gas is analyzed with the simulation tool Aspen Plus. Two different reforming options (SR and ATR) are compared in terms of hydrogen efficiency, fuel processing efficiency, dynamic behaviour, product gas composition, system complexity and sulphur resistance. An extensive parameter study is carried out in order to find optimum operating conditions for a 5 kW biofuel processor suitable for HTPEM applications. As on-board reforming of liquid biofuels requires compact, low-volume, low-weight reformers the approach within this work was to keep the system as simple as possible, using only waste heat streams from within the system to preheat air, water and fuel. An elaborated thermal heat integration system is set up for both reforming concepts to achieve high preheating temperatures and thus a high overall system efficiency.
Section snippets
Bioethanol and biodiesel
In this work bioethanol is represented by pure ethanol C2H5OH. Analysis of biodiesel was performed with a commercial GC-FID system. Based on the analysis methyl-oleate (C19H36O2) was chosen as a reference substance for biodiesel. Methyl-oleate is derived by transesterification of triolein, the triglyceride of oleic acid, which is the dominating fatty acid of rapeseed oil. In Table 1 the chemical and physical properties of biodiesel are compared with those of the model substance methyl-oleate (C
Hydrogen efficiency without heat integration
For the steam reforming concept without considering heat integration (Fig. 1) hydrogen efficiency and effective hydrogen efficiency were calculated in the temperature range 550–850 °C. The influence of preheating H2O-REF and AIR-B on hydrogen efficiency was investigated. Analogue simulations have been conducted for the autothermal reforming concept (Fig. 3). Hydrogen efficiency was calculated for different preheating temperatures of and TAIR-REF with air ratios ranging
Conclusions
Hydrogen production from bioethanol and biodiesel for use with HTPEM fuel cells has been studied within this paper. Extensive simulation work including a variation of reforming temperature, air ratio and steam to carbon ratio has been carried out for two different reformer concepts; one based on steam reforming (SR) and the other based on autothermal reforming (ATR).
Simulation results show that preheating of feed water and feed air has a positive effect on hydrogen efficiency. By applying high
Glossary
- APU
- auxiliary power unit
- ATR
- autothermal reforming
- CR
- CO2 reforming
- FID
- flame ionization detector
- GC
- gas chromatography
- HTPEM
- high temperature polymer electrolyte membrane fuel cell
- LHV
- lower heating value
- POX
- partial oxidation
- PEM
- polymer electrolyte membrane fuel cell
- S/C
- steam to carbon ratio
- SR
- steam reforming
- WGS
- water gas shift
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