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MASS AND ENERGY BALANCE A full description of the C2CNT Energy and Mass Balance is given in sections 3-7 of the C2CNT 2016 paper “Thermodynamic assessment of CO2 to carbon nanofiber transformation for carbon sequestration in a combined cycle gas or a coal power plant” (attached) with the pertinent features that all carbon dioxide is consumed and yields a net of only two products (carbon nanotubes an pure oxygen), that only a small fraction of the electricity is consumed in the electrolysis and that useful heat is liberated in the dissolution of carbon dioxide in carbonate. In addition, an abbreviated mass and energy balance here is derived from a comparison to that known of a comparable, mature industry: aluminum production. The C2CNT process bears many similarities to aluminum smelting. Both processes consist of molten electrolysis, and do not use noble or exotic materials. Aluminum smelting produces aluminum metal from alumina (using bauxite, sodium hydroxide and electricity), while C2CNT produces carbon nanotubes from carbon dioxide (using carbon dioxide and electricity). Aluminum smelting operates at 960°C in a molten cryolite electrolyte. The C2CNT process operates under somewhat milder conditions at 770°C in a less exotic, molten carbonate electrolyte. Both processes operate at high rate (hundreds of mA per cm2) and low polarization. In both cases the electrolysis chamber consists of common metals, common insulators (such as kiln or “firebricks”), and control equipment. Electrolysis in the Aluminum smelting process is driven at approximately 4 volts using 3 electrons per aluminum. The C2CNT electrolysis is driven at approximately 1 volt using 4 electrons per carbon dioxide. C2CNT and aluminum smelters have approximately equivalent output (tonnage) rates. Aluminum costs ~$1,880 per metric ton, of which ~32% of the cost is electricity (Djukanovic, 2012). Today’s newer, more efficient Al plants require 12 MWh per ton, whereas older Al plant require 15 MWh per ton. 13 MWh is measured and calculated from a 94% efficient 3 electron per Al electrolysis at 4.1V (Naixiang, 2014). $600 for 12 MWh = $0.05 per kWh. This Al electricity cost varies from lowest (in the Middle East) at $300, to mid range $650 (US), to highest (China with high energy tariffs) of $1,020/ton. Al smelter cost structure (modified from Djukanovic, 2012). Production costs per metric ton (tonne) of Al, based on market costs consist of: Consumable Expenses (32% electricity & 52%; reactants = 84%), Electricity: 32%, Labor: 8%, and Capital Expenses (amortized cost of electrolyzers, processing equipment, and miscellaneous overhead). Note that the energy to drive the aluminum production originates from two sources (electricity and energy released from the consumed carbon anode). Table 1. Comparison of aluminum and C2CNT production costs. Process Aluminum C2CNT $US Cost (% of total) alumina carbon 733(39%) $244(13%) CO2 - $0 $0 electricity $602(32%) electricity $360 labor $150 (8%) labor $150 capital $150 (8%) capital $150 total $1880(100%) total $660 As compared in Table 1, and unlike Al smelting, the C2CNT process uses a no-cost oxide as the reactant (carbon dioxide, rather than bauxite). Both are straightforward, high current density electrochemical (molten electrolytic reduction of oxide) processes. The C2CNT process operates under somewhat milder conditions at 770°C in a less exotic, molten carbonate electrolyte at similar rates of output, and to a first order of approximation, both processes will be assumed to have the same labor and capital costs. Whereas, Al production requires ~13 MWh per ton of aluminum, C2CNT production requires less energy (7 MWh) per ton of carbon nanotubes. This energy is calculated here from the C2CNT 1 volt electrolysis consuming 4 electron per carbon dioxide splitting efficiency. The observed electrolysis voltage varies from 0.8V to up to 2V, decreasing with higher concentrations of added lithium oxide, and increasing with current density and with mixed molten carbonate electrolytes (Ren, 2015; Ren, 2017). Using the formula weight to convert mass carbon dioxide to moles, and Faradays constant at 1V yields 2.4 MWh per ton CO2, which decreases to 2.0 MWh per ton CO2 (which is 7.2 MWh per CNT) by the 20% energy recovered through driving the turbine more efficiently with pure oxygen (looped in from the C2CNT electrolysis), rather than regular air, combustion (Lau, 2016). This yields an electrical cost of $360 per ton CNT, and as summarized in Table 1, a total cost of $660 per ton of CNT. The electrical cost falls per ton CNT based on less expensive wind electric, equivalent to (x12.01/44.01) $50 per ton of CO2 (Licht, 2017). Higher production rates will increase this cost, while the imposition of a carbon tax or carbon credits will lower this cost. 8PDF Image | Xprize
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