Page 220 - Advances in bioenergy (2016)

REFORMATION OF BIOETHANOL

The use of bioethanol as renewable hydrogen carrier is attractive because of its wide
availability, storage, and handling safety, and because it can be produced from several biomass
sources. Reformation of ethanol by steam [Eq. (1), Table 9.1] offers significant advantages
over competing processes. Steam reforming (SR) of ethanol produces high yields of hydrogen
at elevated temperatures. Recently, alternative processes operating at lower temperatures are
being investigated.

High Temperature Ethanol SR

SR of ethanol is a strongly endothermic reaction and, thus, it exhibits maximum performance
with respect to hydrogen yield at high temperatures, typically >700°C. Depending on the
catalyst used and operating conditions, several reactions may run in series and in parallel,
producing acetaldehyde, hydrogen, methane, carbon oxides, and ethylene, as per Eqs (2)–(8) in
Table 9.1. Equation (2) can occur either directly, or via intermediate formation of acetaldehyde
as the sum of Eqs (3) and (9).

The production of hydrogen via SR of ethanol at high temperatures has been studied over a
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variety of metal catalysts, including Co, Rh, 16,17 Pd, 16,17 and Ni. Catalytic performance tests
obtained over Pd, Rh, Ni, and Co supported on MgO showed that Rh exhibits the best
performance in terms of ethanol conversion and stability, whereas Ni exhibits the highest
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hydrogen selectivity (>95%). For low-loaded Al O supported catalysts, Rh is significantly
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more active and selective toward hydrogen formation compared to Ru, Pt, and Pd, which show
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a similar behavior. Catalytic activity and selectivity is strongly affected by the nature of the
support. For 10 wt% Ni-based catalysts ethanol conversion reaches almost 100% at 650°C for
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all catalysts investigated. However, selectivity toward H follows the order:
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Ni/ZnO ≈ Ni/La O > Ni/MgO > Ni/γ-Al O . Similarly, it has been found that Ni catalyst
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exhibits higher activity and selectivity toward hydrogen production and, most important, long
term stability when supported on La O compared with Al O , YSZ, and MgO. 18
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Typical results obtained over several catalytic materials are presented in Figure 9.1, in which
conversion of ethanol and selectivity toward hydrogen are plotted as functions of reaction
temperature. It is observed that the homogeneous conversion of ethanol becomes significant at
temperatures above 750°C and reaches 80% at 850°C (Figure 9.1a). However, selectivity
toward hydrogen is very low under these conditions and increases from ca. 35 to 55% upon
increasing reaction temperature from 650 to 850°C (Figure 9.1b). The Ni/(La O /Al O )
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catalyst exhibits superior catalytic performance with X EtOH reaching 100% at temperatures
higher than 800°C and selectivity toward H exceeding 95% at 840°C. The activity of 0.5%
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Rh/Al O catalyst is comparable with that of Ni/(La O /Al O ). However, selectivity toward
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hydrogen is poor over the rhodium catalyst, especially at temperatures below 750°C. The 20%
Co/Al O catalyst is almost inactive under the experimental conditions employed. The
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