Isothermal oxidation

Ferritic stainless steels depend on chromium for high temperature corrosion resistance. A Cr202 scale may form on an alloy above 600°C when the chromium content is ca 13 wt % (36,37). This scale has excellent protective properties and occurs iu the form of a very thin layer containing up to 2 wt % iron. At chromium contents above 19 wt % the metal loss owiag to oxidation at 950°C is quite small. Such alloys also are quite resistant to attack by water vapor at 600°C (38). Isothermal oxidation resistance for some ferritic stainless steels has been reported after 10,000 h at 815°C (39). Grades 410 and 430, with 11.5—13.5 wt % Cr and 14—18 wt % Cr, respectively, behaved significandy better than type 409 which has a chromium content of 11 wt %. 

FIGURE 4.6 Transmission electron microscopy (TEM) bright field image of the scale grown on a Crofer 22 APU coupon during an isothermal oxidation at 800°C in air after 300 h. 

The laboratory-to-laboratory reproducibility of the isothermal oxidative stability procedure for measuring the oxidative stability of polyolefin insulation used in telecommunications applications has been improved from approximately 45% to approximately 85%. The improved protocol is described in some detail. 12 refs. 

Decomposition of N,0 on a catalyst and isothermal oxidation of CO on platinum catalyst in circulating reactor Hugo (37) X ... 

Isothermal oxidation of CO on Pt catalyst in a recirculating reactor Hugo and Jakubith (38) X X... 

Figure 31. Weight-loss of unidirectional carbon/carbon composites by isothermal oxidation in air, as affected by Zn2 207 inhibitor or by SiC coating (32,49) The composites were fabricated with 50 vol.-% high-modulus Modmor I fibers, coal-tar pitch as matrix precursor, four densification cycles, and final heat treatment to 1400°C.

Fig. 12.4. Effect of RE additions on isothermal oxidation and spalling behaviour [29].

This introduction is rather long, since it includes that matter which is common to all of the following chapters in the book, some of which are new and unique to the book. A first-time lead is provided into the revolutionary new technology of isothermal oxidation, detailed in the thermodynamic appendix (Appendix A). The reaction in a fuel cell is isothermal, charge exchange, oxidation. Combustion does not occur, and its theory does not relate to fuel cells, nor does its main parameter the calorific value or combustion enthalpy. 

The author s position is roughly that implied in Burns remark above. The fuel cell community is young and interdisciplinary. Retired fuel cell technologists are still rarities, so that distillations of long-term extensive experience are hard to come by. Moreover, such experience will not have been against the new background fully developed in this book for the first time. The new fuel cell isothermal oxidation theory is remarked upon in the foreword, and was partly introduced by Barclay... 

It is a noteworthy worldwide failure that isothermal oxidation is not the industry-recognised modus operandi of the fuel cell, although it is without douht that it should be, and must be in the future, in the author s view. 

Isothermal chemistry in fuel cells. Barclay (2002) wrote a paper which is seminal to this book, and may be downloaded from the author s listed web site. The text and calculations of this paper are reiterated, and paraphrased, extensively in this introduction. Its equations are used in Appendix A. The paper, via an equilibrium diagram, draws attention to isothermal oxidation. The single equilibrium diagram brings out the fact that a fuel cell and an electrolyser which are the thermodynamic inverse of each other need, relative to existing devices, additional components (concentration cells and semi-permeable membranes), so as to operate at reversible equilibrium, and avoid irreversible diffusion as a gas transport mechanism. The equilibrium fuel cell then turns out to be much more efficient than a normal fuel cell. It has a greatly increased Nernst potential difference. In addition the basis of calculation of efficiency obviously cannot be the calorific value of the... 

Gerry Agnew of Rolls-Royce predicts in his foreword to this book that grudging admission of this efficiency difficulty may take an extended period. The author is impatient to improve on that prediction, and presented his work (Barclay, 2002) at an IMechE conference on 30 November 2005 in pursuit of that objective. The conference chairman accepted that the calorific value was no longer a valid performance criterion. In a discussion, the author presented an extensive calculation of the power yield of isothermal oxidation, relative to the yield of combustion allied to a heat cycle. 

This book versus alternative textbooks. Four book titles follow, which do not follow the line of Barclay (2002). In any discussion of isothermal oxidation at the fuel cell electrolyte-electrode interface the four example books suffer from a lack of thermodynamic rigour. 

New perspectives arising from isothermal oxidation. The next chapter of this book describes the greatly altered perspective of the fuel cell industry, when Grove s ideas are updated. The second chapter describes the detail of Regenesys, or ESS-RGN. This system has changed hands, as noted above, and information is available from http //www.vrbpower.com/. (The initials VRB stand for Vanadium Redox Battery, a low-power alternative to Regenesys.) The new 2005 VRB Power Systems shorthand is ESS-VRB for 2.5 to 10 MW and ESS-RGN for 10 to 100 MW. In Chapter 2 the reader will be acquainted with ESS-RGN, one of the two VRB fuel cell systems (incompressible liquid based) which can be termed complete . The redox battery uses small pumps as circulators. 

In Weibel etal. (2005a, b) Harvard University has announced that it is developing a new, incompressible reactants, liquid-based fuel cell for the isothermal oxidation of a coal/water/sulphuric acid slurry, and producing compressible carbon dioxide. Eurther detail is given at the end of Appendix A. 

Power from the sun. From the foregoing, it becomes clear that human utilisation of the power output of the sun, by combustion (non-isothermal oxidation) of photosynthesised fuel to feed heat engines, is extremely inefficient, relative to muscle power. Relatively efficient isothermal oxidation in complete fuel cells needs to be made practical and economic, and needs to be seen as the equal and opposite analogue of isothermal photosynthesis, as described above. 

The nascent ability of some SOFC versions to oxidise methane directly (Perry Murray etah, 1999 Park etah, 1999 Gorte etal., 2000) using appropriate catalysts represents a major challenge to the rival MCFC, which cannot emulate the new technology. The direct isothermal oxidation of methane as in Figure A.5 means that the new system has no need for a hydrogen mine , although the need remains as an essential for vehicle fuel cells. 

In a qualification of the above quotation, this book points out that the causes of things in the fuel cell industry literature are written without a logical thermodynamic basis. The author introduces such a basis, namely reversible isothermal oxidation. 

A fusion between these direct hydrocarbon proposals and IT/SOFCs such as that of the Mitsubishi Materials Corp. (Section 4.5), or that of Ceres Power Ltd (Section 4.6), could make a hugely competitive, simplified and cheaper future system, with further major development potential to take account of the points in Appendix A of this book, that is fuel cells allied to concentration cell circulators, namely complete fuel cells. Such a development will await judgement of this book, and departure into the history of the application of combustion theory to fuel cells using isothermal oxidation. 

Like the competing IT/SOFC, the MCFC is an incomplete system which has not considered isothermal oxidation with its circulator problem, as defined in this book. The system is also immature as it stands, since the mobility of its electrolyte is a newly recognised fact, for which the author has suggested surface tension gradient as the mechanism. Moreover, a major increase in power density is under consideration, which would interact with electrolyte mobility via higher temperature gradient. 

Complete fuel cells are engineered for isothermal oxidation by the addition of perm-selective membranes and isothermal concentration cells. They would, if developed, generate much higher Nernst potential difference than an existing incomplete fuel cell. That would give a chance for fuel cells to draw level, a chance the industry does not seem to understand. 

A last-minute incursion on the scene has been the announcement by Harvard University of its initial attempt to develop a low-temperature coal fuel cell (Weibel etal., 2005a, b). The language of the two papers produced is that of isothermal oxidation. There is resemblance to the... 

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