Andrew Dicks

Intrinsic reaction kinetics of methane steam reforming on a nickel/zirconia anode

Abstract For the purposes of optimising important system parameters in direct internally reforming (DIR) solid oxide fuel cell (SOFC) systems, a detailed knowledge of the methane steam reforming rate on the anode is needed. In order to shed light on the present poorly understood kinetics, a study of the methane steam reforming rate given by a typical thin electrolyte-supported nickel/zirconia SOFC anode has been carried out using a tubular plug flow differential reactor. These tests were essentially gradientless. The reaction rate was studied as a function of temperature (700–1000°C) and the partial pressure of methane (2–40 kPa), hydrogen (10–70 kPa) and steam (10–70 kPa). The total pressure was nominally 1 atm. The reaction was first order in methane with a weak positive effect of hydrogen, and a stronger negative effect of steam. The kinetics were complicated by the fact that reaction orders in hydrogen and steam were either temperature dependent and/or depended on the partial pressures of other components in the gas mixture. Furthermore, Arrhenius-type plots gave gradients which were dependent on the steam partial pressure. It is clear from this study that the reaction cannot be represented as simply as is generally attempted in the literature. An improved rate equation has been derived.

Modification of coal as a fuel for the direct carbon fuel cell

As a promising high-temperature fuel cell, the direct carbon fuel cell (DCFC) has a much higher efficiency and a lower emission as compared with conventional coal-fired power plants. To develop an increased understanding of the relationship between the microstructure, surface chemistry, and electrochemical performance of coal as a fuel for the DCFC, a coal sample from Central Queensland has been subjected to various pretreatments, including acid washing, air oxidation, and pyrolysis. It has been found that an acid treatment of the coal enhanced its electrochemical reactivity due to an increase in oxygen-containing surface functional groups. By contrast, heat treatment of the coal results in a sharp decrease in the electrochemical reactivity in the DCFC due to a decrease in the oxygen-containing surface functional groups, particularly CO(2)-yielding surface groups. A higher surface area of coal may also be helpful, but much less important than surface chemistry.

A study of SOFC-PEM hybrid systems

Abstract The benefits of a system combining high- and low-temperature fuel cell types have been assessed using computer predictions. A high-temperature solid oxide fuel cell (SOFC) may be used to produce electricity and carry out fuel reforming simultaneously. The exhaust stream from an SOFC can be processed by shift reactors and supplied to a low-temperature polymer electrolyte membrane (PEM) cell. The overall efficiency predicted for the hybrid system is shown to be significantly better than a Reformer–PEM system or an SOFC-only system. Approximate capital and running cost estimates also show significant benefits compared to the other two systems.

Assessment of commercial prospects of molten carbonate fuel cells

Abstract The commercial prospects of molten carbonate fuel cells have been evaluated. Market applications, and the commercial criteria that the MCFC will need to satisfy for these applications, were identified through interviews with leading MCFC developers. Strengths, weaknesses, opportunities and threats (SWOT) analyses were carried out to critically evaluate the prospects for commercialisation. There are many competing technologies, but it is anticipated that MCFCs can make significant penetration into markets where their attributes, such as quality of power, low emissions and availability, give them a leading position in comparison with, for example, engine and turbine-based power generation systems. Analysis suggests that choosing the size for MCFC plant is more important than the target market sector/niche. Opportunities will exist in many market sectors, though the commercial market would be easier to penetrate initially. Developers are optimistic about the commercial prospects for the MCFC. Most believe that early commercial MCFC plants may start to appear in the first decade of the next century, the earliest date suggested for initial market entry being 2002.

PEM fuel cells - Applications

The proton-exchange membrane fuel cell (PEMFC), emerged in the early 1990s, is a device that can produce clean power by the direct electrochemical oxidation of hydrogen. This chapter outlines the main features of the PEMFC and its unique operating characteristics. It charts progress in the development of PEMFC technology, focusing on materials of construction and applications. Still subject to considerable R&D, many demonstrations have been set up and are in progress for applications in both stationary power (small portable, backup systems, and power generation) and in transport (road, rail, marine, and aviation).

Low energy plasma treatment of a proton exchange membrane used for low temperature fuel cells

A low energy (∼30 V) plasma treatment of Nafion, a commercial proton exchange membrane used for low temperature fuel cells, is performed in a helicon radiofrequency (13.56 MHz) plasma system. For argon densities in the 10 9 –10 10 cm −3 range, the water contact angle (hydrophobicity) of the membrane surface linearly decreases with an increase in the plasma energy dose, which is maintained below 5.1 J cm −2 , and which results from the combination of an ion energy dose (up to 3.8 J cm −2 ) and a photon (mostly UV) energy dose (up to 1.3 J cm −2 ). The decrease in water contact angle is essentially a result of the energy brought to the surface by ion bombardment. The measured effect of the energy brought to the surface by UV light is found to be negligible.

FUEL CELLS – MOLTEN CARBONATE FUEL CELLS | Overview

The molten carbonate fuel cell (MCFC) emerged during the twentieth century as one of the key fuel cell types. It uses an electrolyte of alkali metal carbonates, operates typically at 650 °C, and is best suited to hydrocarbon fuels such as natural gas, coal gas, or biogas. The high operating temperature enables such fuels to be fed directly to the MCFC stacks, leading to conversion efficiencies greater than 50%. Molten carbonate fuel cell systems are ideally suited to applications that need continuous base load power. The first commercial systems, at the 300 kW scale, are therefore being used in applications such as hospitals and hotels.

Methane electro-oxidation on a Y0.20Ti0.18Zr0.62O1.90 anode in a high temperature solid oxide fuel cell

A high temperature solid oxide fuel cell has been operated in low humidity (3 % H2O) methane using Y0.20Ti0.18Zr0.62O1.90 (YTZ) as the anode. The mechanism of methane electro-oxidation was investigated using ac and dc techniques at different anodic overpotentials and methane concentrations in the temperature range 788 – 932 °C. It was found that YTZ did not support methane cracking and that its electrocatalytic activity was stable over a long period of operation. Anode performance was significantly enhanced under positive polarization. Although the system showed good stability under low humidity methane conditions, the electrochemical performance was inferior to that observed for conventional anodes, albeit under high humidity methane or hydrogen fuel conditions. The overall area specific polarization resistance decreased from 167.88 Ω cm2 to 10.14 Ω cm2 between open and short (Ecell = 0 V) circuit. Altering the fuel to steam ratio showed that the steam reforming of methane was the main source of power generation at low methane concentrations. Direct methane oxidation was too slow to be discerned under these conditions, but could co-exist with steam reforming at higher methane concentrations.

Providing and Processing Fuel

Hydrogen, the preferred fuel for fuel cells, can be obtained from many sources. Fossil fuels such as oil, natural gases and coal, as well as bio-fuels can all be chemically converted to hydrogen. The basic chemistry of the various steps in the conversion is well known. However, each type of fuel cell has different fuelling requirements and therefore the design of fuel processors depends not only on the availability and form of fuel but also the application. For stationary power plants natural gas is an ideal fuel. It is best converted to hydrogen as close to the fuel cell as possible. In the case of the MCFC and SOFC this ensures high efficiency by using heat that would otherwise be lost from the stack. Recent advances in micro-channel catalytic reactor design may also lead to higher efficiencies and more compact stationary and portable systems. For transportation applications, hydrogen appears to be the preferred fuel in the long term. In the near term, methanol is a good fuel to use in vehicles, since it can be converted relatively easily on-board to hydrogen. Hydrogen can be generated by electrolysing water, and in combination with a fuel cell, this offers a means of storing energy from intermittent renewable power sources. In the future, hydrogen may be generated by direct solar electro-photolysis, or by biological methods. As such technologies advance, the transportation and storage of hydrogen stands out as perhaps the major barrier to the realisation of commercial fuel cell systems.Copyright

How do we fuel fuel cells

While there are a number of different fuel cell technologies, they all need to run on a fuel of some kind. Usually this will require some degree of pre-treatment. This article discusses the various fuelling options, and the processes available to make them usable in fuel cells.