As mentioned earlier in this chapter, hydrogen production can be maximized per fed ethanol through pure steam reforming. However, the highly endothermic feature of this reaction limits its widely industrial application for hydrogen production. In order to lessen its heavy dependence on external energy supply, part of ethanol is sacrificed to provide required energy for steam reforming through the introduction of oxygen, which is named as oxidative steam reforming (Reaction 5). Depending on the value of 8, the enthalpy change of

Reaction (5) will become less positive, indicating less energy requirement from surroundings. The reaction will finally become autothermal at the point where little or no energy is needed from external sources (e. g., if 8=0.6, AHr,298K =4.4 kJ/mol).

C2H5OH(l) + 8 O2 + (3-2 8) H2O(l) (6-2 8) H2 + 2 CO2 (5)

Although the products from the desired reactions are only CO2 and H2, in reality, depending on the reaction conditions and catalysts used, the product distribution can be governed by a very complex reaction network. Possible reactions involved can be as follows.

CHsCH2OH CH4+CO+H2 (ethanol decomposition) (6)

CHsCH2OH CH3CHO+H2 (ethanol dehydrogenation) (7)

CHsCH2OH C2H4+H2O (ethanol dehydration) (8)

CH3CH2OH+H2O 2 CO+4H2 (ethanol incomplete reforming) (9)

2 CH3CH2OH (C2H5) 2O+H2O (ethanol dehydrative coupling) (10)

CH3CH2OH+H2O CH3COOH+2 H2 (acetic acid formation) (11)

CH3CHO CH4+CO (acetaldehyde decomposition) (12)

2CH3CHO CH3COCH3+CO+H2 (acetone formation) (13)

CO+3 H2 CH4+H2O (methanation) (14)

C2H4 coke (polymerization) (15)

CH4+2 H2O CO2+4 H2 (methane steam reforming)1 (16)

CH4 C+2 H2 (methane cracking) (17)

CO+H2O ^ CO2+H2 (water-gas shift) (18)

2 CO CO2+C (Boudouard reaction) (19)

There are many side reactions that might take place during ethanol steam reforming, complicating the product distribution. To get the highest possible H2 yield for industrial applications, it is essential to investigate the effects of temperature, reactants ratio, pressure, space velocity as well the catalytic parameters. A thermodynamic analysis was performed using the software HSC® Chemistry 5.1. All possible products, including solid carbon were included among the possible species that could exist in the equilibrium state. In the thermodynamic analysis, the following definitions are used.

H2 Yield % = moles of H2 produced x 100 6 x (moles of ethanol fed)

„ , mol of a certain product _ „„

Selectivity % = x 100

mol of total products

„ T moles of ethanol converted _ „„

EtOH Conv. % = x 100

moles of ethanol fed

The thermodynamic analysis in Fig.2 shows ethanol conversion, yield and selectivity of main products starting from a reactant composition similar to a bio-ethanol stream from biomass fermentation (ethanol-to-water ratio of 1:10). Ethanol conversion is not thermodynamically limited at any temperature. The methanation reaction, which is exothermic, is thermodynamically favored at lower temperatures (below 400 oC). At higher temperatures (above 500 oC) the reverse of this reaction, i. e., steam reforming of methane to CO2 and H2 becomes favorable. This would suggest that, if operated in a thermodynamically controlled regime, in order to minimize CH4 concentration in the product stream, the reaction temperature should be kept as high as possible. However, as shown in Fig.2, once the temperature is increased above 550 oC, the reverse-water-gas shift reaction takes off, i. e., CO formation becomes significant and hydrogen yield decreases. At this ethanol-to-water ratio, there is no solid carbon at the equilibrium state.

Fig. 2. Product distribution from ethanol steam reforming at thermodynamic equilibrium with EtOH:Water=1:10 (molar), CEtOH=2.8%, and atmospheric pressure

Fig.3 shows the effect of ethanol-to-water molar ratio on H2 yield. Lower molar ratios of ethanol-to-water can increase the hydrogen yield, since both water gas shift reaction and CH4 reforming reactions would shift to the left with increased water concentration. In Fig.3, solid carbon selectivities for the lowest water concentrations are also included. At high ethanol-to-water ratios, solid carbon deposition becomes thermodynamically favorable, especially at lower temperatures.

The effect of dilution with an inert gas on the equilibrium H2 yield is shown in Fig.4. The addition of inert gas increases the equilibrium hydrogen yield at low temperatures and has no effect at high temperatures. At low temperatures, the dominant reaction is the methanation/ methane steam reforming. Diluting the system favors the methane steam reforming, and hence we see a difference at low temperatures. At high temperatures, the main reaction is the reverse water gas shift reaction, which is not affected by dilution, since there is no change in the number of moles with the extent of this reaction. Increased pressure has a negative influence on hydrogen yield at lower temperatures and no effect at higher temperatures (Fig.5).

Fig. 3. Effect of EtOH-to-water molar ratio on equilibrium H2 yield and C selectivity at (no dilution)

Fig. 4. Effect of dilution on equilibrium hydrogen yield (Dilution ratio used: Inert:EtOH:H2O = 25:1:10)

Although it is important to be aware of the thermodynamic limitations, these analyses do not provide any information about the product distribution that would be obtained under kinetically controlled regimes. However, the study is still meaningful for guiding the choice of the desirable reaction parameters such that reaction is always controlled by kinetics under thermodynamically favorable conditions.

Due to its simplicity, flexibility, maturity, and high hydrogen yield, thermal bioethanol steam reforming has been extensively studied and a variety of technical improvements and researches directions have been proposed and implemented over the past several decades. The discussions of the following sections will focus on this technique.

Fig. 5. Effect of pressure on equilibrium hydrogen yield (EtOH:Water=1:10 (molar ratio), no dilution)