Bioethanol reforming for H2 production. A comparison with hydrocarbon reforming

Nicolas Bion, Florence Epron and Daniel Duprez

DOI: 10.1039/9781847559630-00001

Hydrogen is essentially produced by steam reforming (SR) of hydrocarbon fractions (natural gas, naphtha, ...) on an industrial scale. Replacing fossil fuels by biofuels for H2 production has attracted much attention with an increased interest for bioethanol steam reforming. Kinetics and mechanisms of hydrocarbon-SR and alcohol-SR present some similarities but also some very important differences due to alcohol reactivity much more complex than that of hydrocarbons. The scope of this report is to compare the two processes in terms of reaction mechanisms. Attention will also be paid to the case of crude bioethanol.

1. Introduction

Whereas hydrogen is the most abundant element of the Universe, it is relatively rare on Earth (0.9 atom % in the outer shell of our planet). Virtually, it does not exist as dihydrogen: it is associated with oxygen in water, with carbon in fossil hydrocarbons, both with oxygen and carbon in bioresources (carbohydrates, cellulosic and lignocellulosic matter, lignin, ...) and more rarely with other elements. Water is by far the main source of hydrogen on Earth (Table 1).

The stock of hydrogen available in fresh waters (lakes and rivers) is then of 1.3 × 1013 tons while the total ressources in hydrogen in oceans ans seas amount to 1.5 × 1017 tons. Comparatively, the ressources in hydrogen available in fossil fuels are modest (Table 2).

Assuming a mean H/C atomic ratio of 1.66 in crude oil, of 3.8 in natural gas and of 0.8 in coal, the stock of hydrogen in fossil fuels would not exceed 111 × 109 T (23 in crude oil, 30 in natural gas and 58 GT in coal reserves), i.e. two orders of magnitude less than the amount of hydrogen contained in fresh waters. Unfortunately, water is a stable molecule needing a high energy input to recover hydrogen as H2. This energy may be provided by (i) a chemical source by oxidizing the carbon of an hydrocarbon into CO and CO2 (steam reforming); (ii) by electricity (water electrolysis) or (iii) by photons (water splitting). Steam reforming is by far the main process for H2 production and only this way of hydrogen production will be examined in this Chapter. On an environmental point of view, steam reforming is not a green process since all (or almost all) the carbon of the hydrocarbons is transformed into carbon dioxide. To avoid this drawback, fossil fuels may be replaced by biofuels. Carbon dioxide is still produced but it may be recycled to new biomolecules by photosynthesis. The annual production of biomass in the World would be comprised between 150 and 420 × 109 metric tons. The mean hydrogen content in biomass being comprised between 5 and 7 wt-%, the stock of hydrogen in this renewable matter would be close to 11 × 109 T/year. In other words, ten years of biomass production would be sufficient to recover all the hydrogen content of fossil fuels. However, a great part of this biomass is composed of wood, difficult to transform into valuable products. For that reason, only products derived from cellulosic and hemicellulosic biomass have been considered for hydrogen production. Since ten years, intensive researches have been devoted to the steam reforming of bioethanol which is a fuel well-adapted to the production of hydrogen. This Chapter deals for a great part with this process, with a special attention paid to the use of crude bioethanol. In a first part, however, the steam reforming of hydrocarbons (aromatics and alkanes) will be reviewed as the model of many mechanistic investigations. This will allow one to compare the steam reforming of hydrocarbons with the steam reforming of ethanol, an alcohol leading to more complex kinetic schemes.

2. The steam reforming of hydrocarbons

2.1 Thermodynamics

For methane, four main reactions can occur:

1. The steam reforming reaction leading to CO and H2

CH4 + H2O -> CO + 3H2 ΔH0298 = +206 kJ mol-1 (1)

2. The steam reforming reaction leading to CO2 and H2

CH4 + 2H2O -> CO2 + 4H2 ΔH0298 = +165 kJ mol-1 (2)

3. The water gas shift reaction (WGS)

CO + H2O -> CO2 + H2 ΔH0298 = -41 kJ mol-1 (3)

4. The coking reaction

CH4 -> C + 2H2 ΔH0298 = +75 kJ mol-1 (4)

For higher hydrocarbons, a similar set of reactions may be written; for instance the reactions of n-heptane and of toluene leading to CO and H2 become:

C7H16 + 7H2O -> 7CO + 15H2 ΔH0298 = +1107 kJ mol-1 (5)

C7H8 + 7H2O -> 7CO + 11H2 ΔH0298 = +869 kJ mol-1 (6)

Except for the WGS reaction, all the reactions involved in the hydrocarbon steam reforming are strongly endothermic with an increase of the number of molecules. They are thus favored at high temperatures and low pressures. The temperature effect is illustrated in Fig. 1 which shows the change with T of the gas composition at equilibrium in the methane steam reforming (initial conditions H2O/CH4=1, P=1 bar). Calculations were carried out by minimizing the sum of the Gibbs free energies of formation of all the compounds (reactants and products) while keeping constant the number of moles of each element (here C, H and O). Details of the procedure are given in Perry's Handbook. Thermodynamic data (molar Gibbs free energy of each compound) are taken from Stull et al. Maximal H2 production is observed around 700 °C. Above 700 °C, the H2 mol% does no longer increase because of the preferential formation of CO at high temperature. This is coherent with the WGS equilibrium: reaction 3 being exothermic, CO2 is favored at low temperature while the reverse reaction (RWGS) yielding CO is favored at high temperature. At 900 °C, total conversion of methane can be achieved yielding quasi-exclusively a syngas with the composition given in equation 1 (75% H2 + 25% CO).

The equilibrium gas compositions in methane, n-heptane and toluene steam reforming at 700 °C are compared in Table 3. The initial state is a steam/ hydrocarbon mixture with a molar ratio corresponding to the stoichiometry of equations 1, 5, 6 written with H2 and CO as products of steam reforming.

Methane and steam conversions are very high but not total even at 700 °C. The vol.% of hydrogen expected in dry gases amounts to 70%. The formation of methane and CO is favored in the steam reforming of C7 compounds at 700 °C, which tends to decrease the hydrogen content in dry...