Fuel utilization coefficient

For the following tests, the fuel utilization coefficient is set at the constant value of 0.90, the stoichiometric ratio ranges between 6 and 1.8, for a stack power from 700 W to 12 kW, With regard to the stack humidification, the inlet air is saturated at room temperature (290-300 K) to minimize the amount of energy required for the best membrane hydration and reduce the electric consumption of the humidifier and the purge frequency [4]. As a consequence, the stack temperature is controlled at about 315 K, to avoid any dehydration of the membranes. 

In addition, manufacturers boiler operational manuals provide hardcopy data ratings for heat transfer coefficients, local heat flux, fuel utilization, furnace heat release rates, maximum continuous rating... 

Example 16.4 The process in Figure 16.2 is to have its hot utility supplied by a furnace. The theoretical flame temperature for combustion is 1800°C and the acid dew point for the flue gas is 160°C. Ambient temperature is 10°C. Assume A7 = 10°C for process-to-process heat transfer but A7 = 30°C for flue gas to process heat transfer. A high value for A 7 mln for flue gas to process heat transfer has been chosen because of poor heat-transfer coefficients in the convection bank of the furnace. Calculate the fuel required, stack loss and furnace efficiency. 

The coefficients in these equations were correlated from performance data for cells (45) operating at 120 psia (8.2 atm), 405°F (207°C) (16) with fuel and oxidant utilizations of 85% and 70%, respectively, an air fed cathode, and an anode inlet composition of 75% H2, and 0.5% CO. Similarly, at atmospheric conditions, the magnitude of this loss can be approximated by... 

Introduction of room-temperature ionic liquids (RTIL) as electrochemical media promises to enhance the utility of fuel-cell-type sensors (Buzzeo et al., 2004). These highly versatile solvents have nearly ideal properties for the realization of fuelcell-type amperometric sensors. Their electrochemical window extends up to 5 V and they have near-zero vapor pressure. There are typically two cations used in RTIL V-dialkyl immidazolium and A-alkyl pyridinium cations. Their properties are controlled mostly by the anion (Table 7.4). The lower diffusion coefficient and lower solubility for some species is offset by the possibility of operation at higher temperatures. 

The diffusion of molecular species has also been studied in concentrated solution environments (25,26). Yeo and McBreen measured the diffusion coefficients of H2 and Cl2 in 1200 EW Nafion membranes immersed in HC1 solutions, and that of Br2 in HC1 and HBr solutions as a function of electrolyte concentration and temperature (25). In concentrated HC1 solutions the order of diffusion coefficients is H2>Cl2>Br, as expected from molecular size. Activation energies of diffusion for H2 and Cl2 in 4.1 M HC1 were found to be 21.6 and 23.3 kJ mol 1 respectively over the 25°-50°C temperature interval. These values are very similar to those for water diffusion in the same membrane in dilute solution, as seen in Table III. The authors utilize these results to estimate a coulombic loss of about 2% in a hydrogen-chlorine fuel cell, due mainly to chlorine migration through the membrane. 

It is worth noting that the remarkable effect described for the carbon support porosity on the metal utilization factor and hence on the specific electrocat-alytic activity in methanol electrooxidation was only observed when the catalysts were incorporated in ME As and measured in a single cell. The measurements performed for thin catalytic layers in a conventional electrochemical cell with liquid electrolyte provided similar specific catalytic activities for Pt-Ru/C samples with similar metal dispersions but different BET surface areas of carbon supports [223]. The conclusions drawn from measurements performed in liquid electrolytes are thus not always directly transferable to PEM fuel cells, where catalytic particles are in contact with a solid electrolyte. Discrepancies between the measurements performed with liquid and solid electrolytes may arise from (1) different utilization factors (higher utilization factors are usually expected in the former case), (2) different solubilities and diffusion coefficients, and (3) different electrode structures. Thus, to access the influence of carbon support porosity... 

BOG conditions and about +3 x 10" / C for EOC conditions over the normal operating temperature range. In the calculation of the total reactor isothermal temperature coefficient of reactivity, the fuel and moderator temperatures up to about 1700 C (3092 F) have been varied isothermally. The inner and outer reflector temperatures on which the reflector contributions to the temperature coefficient calculations are based, are assumed to be in equilibrium with the respective fuel temperatures as discussed later. Table 4.2-12 lists the assumed temperature conditions used to determine the temperature coefficients of reactivity that have been plotted as a function of the active core temperature in Figures 4.2-6 to 4.2-8. A nine neutron group radial diffusion calculational model with cross sections based on the temperatures indicated in Table 4.2-12, was utilized to determine the temperature coefficients of reactivity. 

The problem of energy price volatility can be resolved by dividing the utility price into separate terms, each of them being a product of a certain coefficient and the price parameter [34]. The first term reflects the conventional inflation rate while the second one represents the contribution from the fuel price ... 

Where A(s) is the catalyst precursor and B(s) is the fuel, the two solids react to form the catalyst, Qs) and the D(g) or the gases evolved. Ng is the stoichiometric coefficient of the gaseous products. Reaction (1) is assumed to take place in a one dimension finite geometry. The model utilizes following assumptions ... 

Because of the complexity of combustion kinetics, coupling kinetics and hydrodynamics into a single comprehensive model is not generally pursued. Instead, many successful hydrodynamic studies vary operational parameters and study the effect on combustion performance parameters. Moe et al. [22] characterized combustion performance with seven parameters (1) heat transfer, (2) combustion efficiency, (3) bottom ash/total ash, (4) bed grain size, (5) limestone utilization, sulfur capture, and Ca/S (6) CO emissions, and (7) NO and NjO emissions. Eight operational variables they listed that impact one or more of the performance parameters were (1) bed temperature—affects carbon burnout, emissions, sorbent utilization, and heat absorption (2) primary/secondary air split—impacts NO emissions, temperature distribution, and pressure drop (3) excess air—changes thermal efficiency, emissions, and carbon burnout (4) solids circulation rate—controls load, heat absorption pattern, heat transfer coefficient, and pressure drop (S) fuel size—determines carbon burnout, bed vs. fly ash split, and pressure drop (6) limestone size—determines Ca/S ratio required and bed vs. fly ash split (7) Ca/S ratio—impacts sulfur capture, limestone utilization, waste/disposal volumes, particulate generation, and emissions and, (8) load—effects heat absorption, emission, carbon burnout, thermal efficiency, and temperature distribution. 

Core reactivity is controlled by means of chemical poison dissolved in the coolant, burnable poison rods and control rod assemblies. Soluble boron and burnable poison rods are utilized for shutdown and fuel bumup reactivity control. Control rod assemblies (37 clusters) are used for power regulation and hot shutdown. The core consists of 3 regions with enrichments of 2.4%, 2,67 % and 3. 0%, It has a negative temperature coefficient of reactivity. The core has a fuel cycle of 12 to 16 months with a discharge bumup of 30,000 MWd/tU. 

Burnable poison (Gd203) is used to partly compensate the fuel bum up reactivity, and soluble boron is utilized for reactor shutdown only. This results in a negative temperature coefficient of reactivity over the complete core life. 

To illustrate this technique, a PWR fuel storage pool calculation was performed to determine the Ke-eigenvalue as a function of assembly spacing. A transport calculation was performed for the base-case separation spacing of 12 cm to furnish data to determine the transport diffusion coefficient. This transport diffusion coefficient was utilized in subsequent dlffusioin calculations in which the... 

At even higher temperatures in the range of 800-900 °C, ionic conduction can also be observed in solid oxides. Certain doped metal oxides become conductors hence, they can be utilized as electrolytes in the solid oxide fuel cell (SOFC). The most common oxide used is yttria-stabilized cubic zirconia (Zr02), called YSZ, with a doping level of 8% Y2O3. This material is a ceramic the thermal expansion coefficient of its thin layers has to be matched with the other fuel cell components. 

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