Andrew E. Lutz

Thermodynamic analysis of hydrogen production by steam reforming

Abstract This paper presents thermodynamic analysis of hydrogen production by steam reforming. The analysis treats the chemistry at two levels: a global species balance assuming complete reaction and solution of the equilibrium composition at the specified reformer temperature. The global reaction allows for an energy balance that leads to analytical expressions for the thermal efficiency. We use this to determine the maximum efficiency, and to distinguish between various definitions of efficiency. To obtain a more realistic estimate of the efficiency, the chemical equilibrium solution is combined with a system energy balance, which compares the energy required to vaporize and heat the fuel–steam mixture to the reformer temperature with the heat available from combusting the residual fuels in the reformate stream. The equilibrium solutions are compared to experimental measurements of the species and thermal efficiency of reforming diesel fuel, obtained with prototype compact steam reformers. The observed efficiency is significantly lower than the equilibrium prediction, indicating that both incomplete reaction and heat transfer losses reduce the performance.

Thermodynamic comparison of fuel cells to the Carnot cycle

Abstract This paper compares the theoretical maximum efficiency of a fuel cell to the efficiency of a Carnot cycle driven by the same net reaction. The comparison dispels the misconception that an ideal fuel cell is potentially more efficient than an ideal heat engine. The paper presents expressions for the thermal efficiencies of an ideal fuel cell and a Carnot heat engine. To show that the maximum efficiency is the same, the analysis of the Carnot cycle is modified to consider an engine driven by a combustion reaction. The derivation invokes the approximations that the enthalpy and entropy changes for the reaction are independent of temperature; these approximations are justified by the hydrogen-oxidation reaction. The analysis extends that presented by Appleby and Foulkes (Fuel Cell Handbook, Van Nostrand Reinhold, New York 1989) by showing that with proper accounting for heat addition, the maximum efficiency of a fuel cell is 100%—not larger—for reactions with a positive entropy change. In addition, this paper explains the difference between the combustion temperature, at which an idealized Carnot cycle would operate, and the adiabatic flame temperature.

Dynamic effects of autoignition centers for hydrogen and C1,2-hydrocarbon fuels

A chemical kinetics model is used to compute the dynamics of a local autoignition center in a combustible mixture. Such centers arise from inhomogeneities in the bulk mixture and ignite earlier than the surrounding mixture. The model of an exothermic center considers a small, homogeneous mass of reacting mixture that is surrounded by an inert mixture. The model combines energy and species equations for the reacting mixture with a gas dynamic constraint for the expansion of the exploding center. The induction time, the time of chemical energy release, and the exothermic energy and power of the centers are evaluated for a variety of stoichiometric fuel-air mixtures. Fuels include hydrogen, methane, acetylene, ethylene, and ethane. An important dynamic effect of the center is the compression wave it produces in the surrounding mixture. The computations show that compression ratios of 1.2 to 1.8 are produced by a center in these fuels at high pressure.

A Turbulent Jet Chemical Reaction Model: NOx Production in Jet Flames

The Two-Stage Lagrangian turbulent jet reaction model is described, and results presented for NOx emissions from methane, CO-H2, and hydrogen jet flames for a variety of conditions. The model includes complete chemical reaction mechanisms and treats the effects of buoyancy and radiation, the influences of which are shown to be significant. Non-equilibrium chemistry influences are also examined. The extensive comparisons made between the model and experimental results for the three fuels suggest that the model incorporates the essential processes that control NOx production in fuel jet flames.

The response of palladium metal-insulator-semiconductor devices to hydrogen-oxygen mixtures: comparisons between kinetic models and experiment

Abstract The operation of hydrogen-sensitive metal-insulator-semiconductor (MIS) devices in the presence of oxygen is described using a detailed model of the surface and interface kinetics. The solution methods developed here build on existing models by considering adsorbed oxygenated species in the interaction between atomic hydrogen at the metal-semiconductor interface and the external surface. The net effect of the adsorbed oxygenated species is to increase the amount of interfacial hydrogen predicted to exist within the structure at equilibrium. These theoretical predictions are compared to computed results from a previously existing model; furthermore, both mechanistic models are analyzed in light of new and previously published experimental response trends for MIS devices. Although the two models considered in this work are each found to be useful in understanding some aspects of the response, elementary reaction mechanisms appear to be inadequate for prediction of response curves. The results of these comparisons suggest that the kinetics for operation of MIS sensors in hydrogen–oxygen mixtures are quite complex, and may be strongly morphology-dependent.

Scaleable stagnation‐flow reactors for uniform materials deposition: Application to combustion synthesis of diamond

We describe two inherently scaleable geometries for chemical‐vapor‐deposition and heat‐transfer processes that are based on stagnation flows. The ‘‘coflow’’ and ‘‘trumpet‐bell’’ designs result in radially uniform fluxes to surfaces and they optimize the use of reagents. Using a trumpet‐bell burner, we have grown uniform films of diamond from a substrate‐stabilized flat flame of C2H2/H2/O2.

Thermodynamic analysis of fermentation and anaerobic growth of baker’s yeast for ethanol production

Thermodynamic concepts have been used in the past to predict microbial growth yield. This may be the key consideration in many industrial biotechnology applications. It is not the case, however, in the context of ethanol fuel production. In this paper, we examine the thermodynamics of fermentation and concomitant growth of bakers yeast in continuous culture experiments under anaerobic, glucose-limited conditions, with emphasis on the yield and efficiency of bio-ethanol production. We find that anaerobic metabolism of yeast is very efficient; the process retains more than 90% of the maximum work that could be extracted from the growth medium supplied to the chemostat reactor. Yeast cells and other metabolic by-products are also formed, which reduces the glucose-to-ethanol conversion efficiency to less than 75%. Varying the specific ATP consumption rate, which is the fundamental parameter in this paper for modeling the energy demands of cell growth, shows the usual trade-off between ethanol production and biomass yield. The minimum ATP consumption rate required for synthesizing cell materials leads to biomass yield and Gibbs energy dissipation limits that are much more severe than those imposed by mass balance and thermodynamic equilibrium constraints.

Analysis of a Distributed Grid-Connected Fuel Cell During Fault Conditions

The effect of a short circuit fault and a voltage sag fault on a distributed grid-connected solid oxide fuel cell (SOFC) is investigated in this paper. The fuel cell is modeled in Simulink and the performance is verified against experimental load testing data. Grid faults are simulated, and conclusions are drawn on the fuel cell response and the effects of the fault condition on internal fuel cell parameters.

Rich flammability limits in CH3OH/CO/diluent mixtures

ABSTRACT This investigation is an attempt to predict theoretically, using existing flame modeling capabilities, rich flammability limits for gas mixtures—parameters that are of immense interest to the chemical industry. The system chosen for study is methanol/carbon monoxide/diluent mixtures, where the diluent is either nitrogen or carbon dioxide at pressures of 1,11, and 21 atm, respectively. The critical oxygen concentration needed to sustain a flame, for several mixtures and pressures, was determined experimentally in a spherical vessel with a central ignition source. Burning velocities of 1-D, planar, freely propagating, premixed flames were calculated to determine the minimum oxygen concentration required for these flames to propagate. This minimum O2 concentration was found to be consistently larger than that observed in experiments; however, the effects of pressure and diluent composition agreed well with experimental measurements. In order to understand better all the phenomena involved, a transie...

A model for detailed chemical kinetics in turbulent nonpremixed jet flames

This paper describes a model that predicts detailed chemical kinetic behavior in turbulent nonpremixed jet flames. The model uses two perfectly stirred reactors to represent regions of uniformly mixed fluid and strained-flames in the jet. Entrainment into these reactors and exchange of fluid between them is governed by relatively simple jet mixing rules that come from dimensional analysis and experimental observations. With the mixing described by these rules, the model can include extensive chemical kinetics at very low computational cost. 29 refs.