Air feed stoichiometry

The results show that, at temperatures below 60 °C and an air feed stoichiometry below three, the cathode exhaust is fully saturated (nearly fully saturated at 60 °C) with water vapor and the exhaust remains saturated after passing through a condenser at a lower temperature. In order to maintain water balance, all of the liquid water and part of the water vapor in the cathode exhaust have to be recovered and returned to the anode side before the cathode exhaust is released to the atmosphere. Because of the low efficiency of a condenser operated with a small temperature gradient between the stack and the environment, a DMFC stack for portable power applications is preferably operated at a low air feed stoichiometry in order to maximize the efficiency of the balance of plant and thus the energy conversion efficiency for the complete DMFC power system. Thermal balance under given operating conditions was calculated here based on the demonstrated stack performance, mass balance and the amount of waste heat to be rejected. 

From 200 to 1150 h, the air feed stoichiometry was reduced to 2.2. The stack current during the entire life test period is plotted in Figure 2.16. These test results shows that a stable DMFC stack performance can be achieved over an extended period with a low air feed stoichiometry with our current stack design. 

Figure 2.4 Amount of water vapor in the air cathode exhaust at cathode exit point of an operating DMFC stack as a function of stack operating temperature and air feed actual stoichiometry.

Hence the maximum air feed actual stoichiometry is a function of water vapor partial pressure corresponding to the air exhaust release temperature (pw) and the total pressure on the air cathode exhaust (ptotai)- Figure 2.8 shows the maximum air feed actual stoichiometry calculated from Equation 2.5, using water vapor partial pressure from the CRC Handbook of Chemistry and Physics [D.R. Lide (ed.), 72nd edn, 1991-92], as a function of air cathode exhaust release temperature. 

Figure 2.8 Maximum allowed air feed actual stoichiometry for a DMFC power system as a function of air cathode exhaust release temperature.

It shows that, for a condenser outlet temperature of 40 °C, which can be reasonably achieved at commonly encountered environmental temperatures, in order to maintain the water balance for a DMFC power system operated with ambient air feed, the air feed actual stoichiometry is limited to 2.7 at Los Alamos altitude (7200 feet above sea level) and to 3.6 at sea level. 

From the foregoing discussion, it is clear that, in a DMFC, the air cathode has to be operated under rather challenging conditions, that is, with a low air feed rate at nearly full water saturation. This type of operating conditions can easUy lead to cathode flooding and thus poor and unstable air cathode performance. To secure better air cathode performance, we have made great efforts to improve the ell cathode structure and cathode flow field design to facilitate uniform air distribution and easy water removal. The performance of our 30-cell DMFC stacks operated with dry air feed at low stoichiometry is reported in the following section. 

Figure 2.12 Voltages of the individual cells in a 30-cell DMFC stack at 14 Voperated at 60 °C with a 0.5 M methanol solution feed at 125 ml min at the anode and with 0.76atm dry air feed at 7.35 SLPM at the cathode. At a stack current of 6.1 A, the corresponding methanol and air feed actual stoichiometries are 2.8 and 2.0, respectively.

The acid-base features of the catalysts were studied by the reaction of isopropanol conversion to propene (acidic feature) and acetone (basic feature) under N2 in the feed and the redox features by the reaction under air in the feed. It was observed at 230°C (table 4 from ref 40) that the pyrovanadate sample was much more basic than the other two pure phases and that excess MgO with respect to crystallized phase stoichiometry induced even more basic character (table 4). 

Figure 5.9. Water partial pressures and / as a function of position in the gas channel. The feed is countercurrent with dry hydrogen and air with the conditions T = 80°C, i = 0.6 A/cm, psi—pc= 1.5 bar, hydrogen and air stoichiometries of 4 and 2, respectively. The partial pressures are given for the anode and cathode gas channels (aGC and cGC) and GDL / membrane interfaces (aM and cM). The water vapor pressure at this temperature is about 0.2 bar. (The figure is reproduced from Ref. [71] with permission of The Electrochemical Society, Inc.)...

Docter et al. [61] developed an ATR for gasoline with an electrical power equivalent of 10 kW . It was composed of a mixing zone with fuel, air and water injection, and a metallic monolith of 0.51 volume coated with catalyst. The monolith was heated by electricity at the inlet section and operated at a very high 0/C ratio of 1, which is the stoichiometry of partial oxidation. Steam was added to the feed at S/C ratio of 1.5. These operating conditions resulted a low hydrogen content of about 27 vol.%, which was determined for the reformate. The reactor could be turned down by a ratio of 1 10 within 2 s while operating temperatures decreased from 800° C to about 660° C. The efficiency of the reactor was still in the range of 80% at more than 2 kW power output. 

In real stacks, air stoichiometry is large since air plays the role of a cooling medium. The linear temperature shape can be distorted by fuel exhaustion along the channel. If hydrogen utilization is large, the domain close to the outlet of the respective channel suffers from fuel starvation the local current there is lowered and we may expect a flattening of the temperature shape in the middle of the channel (here we discuss co-flow feeding). 

The autothermal reformer was operated at temperatures between 600 and 800 °C [618]. According to the stoichiometry and thermodynamics, the highest efficiency of the reactor was achieved at the lowest 0/C ratio of 0.75 (here expressed as an air to fuel ratio of 0.25) as shown in Figure 9.43. This was almost independent of the S/C ratio, which was varied from 1.0 to 2.5, and was also independent of the inlet temperature of the feed, which had been changed in parallel. 

We have already discussed under practical stoichiometry how the air requirements can be estimated based on the fuel composition (ultimate analysis). The primary and secondary air requirements for combustion of pulverized coal or coke are best estimated by mass and heat balance at the mill. In Appendix 6A we show a calculation taken from Musto (1997) for the primary and secondary air required for coal pulverizer with 4.5 metric ton per hour (10,0(X)lb/hr) coal feed rate at initial moisture of 15 percent which is required to be ground and dried to 2 percent with a 200 HP mill. In order to estimate the actual primary and secondary air, one has to make some estimation of the evaporation rate, the amount of gas entering the coal mill, and the bleed air required so that the quantity of air that should be vented from the hood off-take can be properly estimated. It shows that for a take-off gas temperature of 315°C (600° F) and vent gas temperature of 76°C (170°F) and allowing ambient air infiltration of 10 percent at 15°C (60°F) the primary air will be about 22 percent of stoichiometric air and 21 percent of total air. The remaining air (about 79 percent) will be the secondary air. With this information we can size a burner using a burner pipe diameter based on a Craya-Curtet parameter of choice bearing in mind the conditions that ensure the desired jet recirculation patterns described in Chapter 3. 

The first term is an abstract representation of the dynamic feed-forward controller while the remaining terms are for PI control. The air mass flow rate is the output flow rate from the compressor. This controller provides air stoichiometry recovery that is twice as fast as the static feed-forward controller when I is changed in step increments. However, this controller suffers from a low bandwidth so it will not be able to reject rapid changes in I, the system disturbances. 

Fig. 18 Fuel cell performance for the p-PBI membranes from the sol-gel process. Polarization curves of fuel cells under H2/air (squares) and H2/O2 (circles)) without any feed gas humidification. The membrane PA doping level was approximately 32 mol PA/PRU. The catalyst loading in both electrodes was l.Omgcm" Pt, and the cell was operated at 160 °C at constant stoichiometry of 1.2 stoic and 2.5 stoic at the anode and the cathode, respectively...

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