ABSTRACT
Methanol is arguably the easiest organic molecule to chemically convert to a hydrogen-rich synthesis gas stream. This process, ge-nerically called reforming, requires an oxidant-usually either air or water, although hydrogen peroxide is also an option. Regardless of the choice of oxidant, the desired reaction pathway is oxidation of carbon to carbon dioxide with the liberation of hydrogen. If water is the oxidant, the process is called steam reforming. If air is the oxi-dant, the process is called partial oxidation. And if both water and air are oxidants, the process is called autothermal reforming.For hydrogen fuel cell systems methanol has long been recog-nized as an attractive potential source of hydrogen. Indeed, meth-anol has been described as an ideal hydrogen storage medium, or hydrogen carrier, when one considers that methanol is made by reacting hydrogen with carbon monoxide and carbon dioxide, and then subsequently decomposed to the reactants. 3.1 Methanol Steam ReformingAt modest temperatures of 200°C to 400°C methanol readily reacts with steam in the presence of a catalyst to form a mixture of carbon oxides and hydrogen. Formation of excess by-product methane can
be a problem at higher temperatures. Under ideal conditions the fa-vored reaction stoichiometry is shown as equation 3.1. CH3OH(g) + H2O(g) → CO2 + 3H2 ΔH°298 = +49.3 kJ/mol (3.1) ΔG°298 = –3.5 kJ/molThis ideal reaction stoichiometry illustrates one of the major ad-vantages of steam reforming: ⅓ of the product hydrogen is derived from water. For this reason, and since water and methanol are mis-cible in all proportions, methanol-steam reforming is the most com-mon approach to extracting hydrogen from methanol.If the reactants are in the liquid phase, the heat of vaporization of methanol (ΔHv = 37.6 kJ/mol) and of water (ΔHv = 44.0 kJ/mol) increases the magnitude of the reaction enthalpy. CH3OH(l) + H2O(l) → CO2 + 3H2 ΔH°298 = +131.4 kJ/mol (3.2) ΔG°298 = +9.3 kJ/molThe most widely accepted mechanism for methanol-steam re-forming is the sequence of the two reactions shown below: CH3OH → CO + 2H2 (3.3) CO + H2O ↔ CO2 + H2 (3.4)Equation 3.3 is the thermal decomposition of methanol, and is considered to be practically non-reversible under typical reforming conditions. On the other hand, equation 3.4, the water-gas shift re-action (WGS), is reversible and likely to be at least partially under equilibrium control. Thus, carbon monoxide is normally a significant by-product. Methane may also form in substantial concentrations at high temperature and low space velocity. A more realistic reaction equation is CH3OH + H2O → xCO2 + yCO + zCH4 + mH2 + nH2O (3.5)In equation 3.5 it is assumed that all methanol reacts with a stoichiometric amount of water and is consumed, and that the
formation of methane results in production of by-product water ac-cording to equations 3.6 and 3.7. CO + 3H2 → CH4 + H2O (3.6) CO2 + 4H2 → CH4 + 2H2O (3.7)Because formation of methane represents a loss of 3 or 4 molar equivalents of hydrogen, it is usually not desirable to operate under conditions in which more than a fraction of a percent of methane forms. Thus, for practical purposes formation of methane can usu-ally be ignored. If we assume that by-product formation of methane is effectively suppressed, a more realistic steam-reforming reaction stoichiom-etry is shown in equation 3.8. CH3OH + H2O → 0.33CO2 + 0.67CO + 2.33H2 + 0.67H2O (3.8)The relative ratio of CO and CO2 reflected in equation 3.8 is typical for a platinum catalyst operating at 350°C to 400°C. The stoichi-
ometry reflects complete conversion of methanol and only partial reaction of water. In fact, the conversion of water to desired prod-ucts is largely governed by the water-gas shift reaction kinetics and equilibrium. Thus, the relative concentration of CO in the product reformate stream is expected to increase with increase in operat-ing temperature assuming adequate reaction rate for the shift re-action. If the combination of catalytic activity for the shift reaction, reaction temperature, and space velocity is such that the overall rate of CO reaction with water is slow under the reaction condi-tions, then the concentration of CO may increase as temperature is decreased. An evaluation of CO concentration plotted against space velocity at constant temperature will indicate if reaction kinetics are limiting (if CO concentration increases with increasing space velocity at constant operating temperature, then the WGS reaction rate is limiting and true equilibrium control is not governing the CO concentration).Referring to equation 3.8, it is clear that the concentration of hy-drogen in the reformate product stream is relatively high at 58.25% by volume. If the product stream is dried, the concentration of
hydrogen is even greater, 69.97% by volume. This high concentration of hydrogen is a benefit when considering pressure-driven hydrogen purification methods for purifying hydrogen from the refor-mate stream; for example, using a membrane process. 3.1.1 System Design and Energy BalanceGiven that methanol and water are completely miscible and that steam reforming is accomplished at temperatures less than 450°C using a variety of commercially available catalysts, the typical system design is relatively simple. An example is the process flow dia-gram shown in Fig. 3.1. This system design assumes that the refor-mate stream from the steam reformer subsequently passes through a temperature-controlled single-stage shift reactor to increase the hydrogen yield. A two-stage shift reactor could easily be substituted for the single-stage reactor at an increase in cost. But if the steam re-former is operated at a relatively high temperature, e.g., greater than about 300°C, the added cost might be worthwhile. The orifice in the feed line just prior to the vaporizer/reformer yields a constant flow rate to the reformer, and is optional.Since shift reactors alone cannot achieve suitably low CO concen-tration for use of the product reformate stream in low-temperature PEM fuel cells, the diagram indicates that the product stream is to be sent to a high-temperature PEM fuel cell (implying a CO concentration ≤3% by volume). The anode purge gas stream comprises the fuel gas stream. The purge ratio should be chosen to satisfy both
acceptable kinetics at the anode side of the fuel cell and sufficient heating value to meet or exceed the enthalpy requirements of the steam reformer. A different hydrogen purification means is required to produce hydrogen from methanol-steam reforming that meets or exceeds purity requirements for low-temperate PEM fuel cells. Hydrogen
purification is addressed in more detail in Chapter 4. However, as a comparison to Fig. 3.1 a process flow diagram for a methanol-steam reformer using a membrane-based purifier is shown in Fig. 3.2. Very high purity hydrogen can be achieved from reformate in a single stage, thus allowing integration of the reformer with low-tempera-ture PEM fuel cells.
