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Chapter 3. Sustainable Hydrocarbon Fuels by Recycling CO2 with Renewable/Nuclear Energy 74 interconnects. Although the cells can be run at more than –1.5 A/cm2 at the thermoneutral voltage based on this area-specific resistanceviii, an initial current density of –0.5 A/cm2 is assumed to minimize cell degradation, which can occur at high current densities [37, 38] (and see Chapter 4). Therefore the remaining heat needed is supplied by electrical heating very close to the cells; summing the internal Joule heating and this additional electrical heating, a voltage corresponding to the thermoneutral voltage plus 8% (to accommodate heat exchange losses) is supplied in total. Cell performance has been observed to degrade at a low rate of 0.003 mV/h for co-electrolysis operated at a fixed –0.25 A/cm2 [37, 38] (and see Chapter 4) – we assume this same degradation rate can be achieved when operated at the higher current density of –0.5 A/cm2 assumed above. The cell stack is assumed to operate with a high CO2 + H2O conversion (reactant utilization), which should give a composition sufficiently rich in CO + H2 for Fischer Tropsch synthesis. The ability to operate efficiently at >90% conversion has been demonstrated [101], but no long-term durability tests have been reported. Overall, the assumptions are closely based on today’s reported solid oxide cell technology with reasonable minor improvements. Figure 3-6 shows a simplified process diagram and Table 3-2 presents an energy balance estimate based on the assumptions. From the energy balance, electrolysis is clearly the major energy-consuming step and therefore electricity is expected to dominate the operating cost. Fischer-Tropsch synthesis (FTS) is exothermic, putting out waste heat and steam, both of which are not well utilized in existing fossil-to-liquids synthetic fuel processes [230]. However, in this process, the low-grade heat is enough to preheat the CO2 and H2O (to make steam) to 250 °C for the electrolyzer. This lowers the thermoneutral voltage to 1.35 V and increases the net system efficiency. The voltage applied to the cell and resistive heater is slightly higher to compensate for the assumed heat exchanger efficiency of 93%: 1.35 V / 93% = 1.45 V. The higher heating value of –CH2– is 647 kJ/mol. Assuming the process is entirely driven by electricity and the waste heat is utilized as described, the total electrical energy input would be 928 kJ/mol (Table 3-2) and the net electricity-to-fuel efficiency would be about 70%. This is approximately double the efficiency estimated for a low temperature electrochemical process which does not include CO2 capture nor liquid hydrocarbon fuel synthesis (it yields a gaseous mixture of CH4, H2, C2H4 and CO) [121]. Heat losses are almost entirely due to the excess heat resulting from Fischer-Tropsch synthesis. Less exothermic fuel synthesis would improve the net efficiency. As described in section 3.2.3, hydrocarbon or methanol synthesis from CO2 and H2, instead of CO and H2, is less exothermic. If this type of synthesis matures and proves to be efficient, it may therefore be viii Assuming a reactant stream composed of >90% reactants with H2O/CO2 = 2 to yield the proper syngas ratio for Fischer-Tropsch and the remaining 1-10% as recycled syngas, the OCV is 0.7-0.8 V. Since the reactants are preheated by the exothermic Fischer-Tropsch reaction, the thermoneutral voltage in this case is between the steam electrolysis thermoneutral voltage (1.29 V) and that of CO2 electrolysis (1.47 V). See references [37] and [38] for a polarization curve that illustrates this.PDF Image | Electrolysis of CO2 and H2O
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