The thermodynamic calculations are done with a Gibbs free energy minimization model13 using the predictive Soave-Redlich-Kwong Equation of state to calculate the required fugacity coefficients.14 In the thermodynamic calculations the non — gasified part of the feedstock remains as solid carbon. Biomass is taken as C6H10O4 in these calculations. As mentioned before, a thermodynamic analysis gives good insight in the possible product yields, because most reforming catalysts are actually designed to obtain chemical equilibrium.

Figure 20.2 shows the carbon decomposition boundaries for several steam over carbon rations (S/C = 1, 2, 3) against the background of the phase diagram of water. Operating points located above the carbon boundary lines give thermodynamic coke, while points below do not. Obviously, the absence of thermodynamic coke does not give much information about kinetic coke. On the other hand, if thermodynamics predicts coke, there is bound to be coke in practice. In

Figure 20.2, the operating regimes for steam reforming and reforming in hot compressed water are also depicted.

From thermodynamic point of view, steam reforming of biomass can be done without coke formation already for S/C = 1 at temperatures above ~700°C. Reforming below 700°C, thus including pre-reforming toward methane, is certainly free of thermodynamic coke for S/C > 2. Reforming in hot compressed water will produce thermodynamic coke for concentrated feedstock solutions of 50 wt% organics or more (S/C = 1, ~ 57 wt% organics). Above 450°C feeds of up to 40 wt% organics (S/C = 2, ~40 wt% organics) can be handled. For the whole hot compressed region it holds that feeds below 30 wt% (~ S/C = 3), organics do not produce thermodynamic coke. Dry reforming of biomass (reaction equation [20.2]) always produces thermodynamic coke. To avoid coke formation, dry reforming should be combined with steam reforming.

The carbon distribution of the product gas and the hydrogen yield are depicted in Figure 20.3 for relevant conditions for steam reforming and reforming in hot compressed water. The carbon distribution is given as fraction of the total carbon content of the gas and the hydrogen yield is given as fraction of the maximal amount of hydrogen that can be produced according to:

C6H10O4 + 8H2O ^ 6CO2 + 13H2 [20.8]

The data for reforming in hot compressed water are given for 250 bar, temperatures between 250°C and 700°C and 10 wt% and 20 wt% organics. More concentrated feeds turned out to be very susceptible to coking in practice; more diluted feeds suffer from a too low energetic efficiency. Steam reforming is evaluated between 500°C and 1000°C, 1 bar and 30 bar for S/C = 1 to 12.

For reforming in hot compressed water it can be seen that thermodynamics dictate a CH4/CO2-rich gas below 400°C while gas mixtures containing CH4, CO2, and H2 are obtained at higher temperatures. H2/CO2 gas can be only achieved thermodynamically at high temperature (>600°C) and for unrealistic low reactant concentrations (<2 wt%). There are some attempts reported6,15 to decrease catalytically the methane formation rate via C-O bond cleavage and hydrogenation by poisoning while maintaining the high rates of C-C bond cleavage and shift for hydrogen production. Gas produced by reforming in hot compressed water typically has a (very) low CO content because of the high water concentration in combination water-gas-shift activity.

At 30 bar, steam pre-reforming (~500°C) creates according to thermodynamics CH4 and CO2, while at 1 bar already quite some hydrogen is produced. Complete methane conversion is obtained at moderate S/C (2-3) for 1 bar at 700°C and for 30 bar 900°C is required. The H2/CO and CO/CO2 ratio can be easily manipulated with the steam over carbon ratio. For typical CH4 steam reforming conditions (S/C = 3, 30 bar) the gas yields are also presented in Figure 20.3. The differences between CH4 and biomass can be explained by the fact that biomass contains ‘internal’ water in its molecular structure: C6H10O4 = C6H2(H2O)4.

Temperature [°С] Steam over carbon [-]

20.3 Carbon distribution and hydrogen yield of the product gas for relevant steam reforming and reforming in hot compressed water conditions as predicted by thermodynamics. Biomass = C6H10O4. For 30 bar and S/C = 3 also the lines for methane steam reforming are given (dotted lines).