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State-of-the-Art Components - Conversion of Fuels Generic Fuel Conversion: Considering the spectrum of fuel conversion from steam reforming to partial oxidation should convey a basic understanding of the reforming processes. An elegant, general equation published by the ANL describes fuel conversion throughout the spectrum. Autothermal reforming falls within this spectrum so that the equation encompasses processes of interest to fuel cells. The equation does not apply to complete combustion, but that conversion process is not relevant to fuel cells (20, 22, 23). The general, idealized equation is: CnHmOp +x(O2 +3.76N2)+(2n–2x–p)H2O=nCO2 +(2n–2x–p+m/2)H2 +3.76xN2 (8-4) where x is the molar ratio of oxygen-to-fuel. This ratio is very important because it determines: • The minimum amount of water that is required to completely convert the carbon in the fuel to carbon dioxide (2n – 2x – p). Excess water is used in practice to ensure the conversion, resulting in water in the reformate (right side of the equation). Typically, one or two moles of water for every mole of oxygen are used. • The maximum hydrogen yield (2n – 2x – p +m/2) • The maximum concentration (percentage) of hydrogen in the reformate {[2n – 2x – p +m/2]/[n + (2n – 2x – p +m/2) + 3.76x] all times 100} • The heat of reaction {∆Hr = n(∆Hf,CO2 )– (2n – 2x – p)∆Hf,H2O - ∆Hf,fuel}. Decreasing the oxygen-to-fuel ratio, x, results in increasing demand for water (water-to-fuel ratio), with commensurate increases in the yield and concentration of hydrogen in the reformate gas. When x = 0, the equation reduces to the strongly endothermic steam reforming reaction. The reaction becomes less endothermic with increasing oxygen. It becomes thermoneutral44 at x = x0 (0.44 for methane). Above this point, the reaction becomes increasingly exothermic. At x = 1 with methane, the pure POX reaction, the feed contains sufficient oxygen to convert all of the carbon in the fuel to CO2. No water needs to be added. The equation is a mix of the steam reforming reaction and the POX reaction at values of x between 0 and n. Beyond x = [n – (p/2)] = n (when p = 0), where water is a product, the heat of reaction is determined by the phase of the product water. At still higher values, the excess oxygen oxidizes the hydrogen to produce water. Finally, at stoichiometric combustion, all carbon and hydrogen are converted to carbon dioxide and water. Here, x = Xc = [n – (p/2) + (m/4)]. The value of x reduces to 2 with CH4 as the fuel. Equation 8-4 depicts a total reaction where the fuel input is converted to carbon dioxide. Actually, the initial reforming step is carried out at elevated temperatures, where a mixture of carbon monoxide and carbon dioxide is formed. In the subsequent reformate conversion step, the carbon monoxide is converted via the water-gas shift to carbon dioxide: CO+H2O↔H2 +CO2 (8-5) 44. The thermoneutral point (of oxygen-to-carbon ratio) is where the enthalpy of the reaction is zero, (∆Hf,298 = 0). 8-15PDF Image | Fuel Cell Handbook (Seventh Edition)
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