ReviewA review on reforming bio-ethanol for hydrogen production
Introduction
Energy is an indispensable element in our everyday lives. However, most of the energy we use nowadays comes from fossil fuels—a non-renewable energy source. Furthermore, our dependence on fossil fuels as energy sources has caused serious environmental problems, i.e. air pollutants and greenhouse gas emissions, and natural resource depletion. The need for renewable alternatives is becoming ever more urgent. Solar, wind, and biomass are promising renewable resources but are generally site-specific, intermittent, and thus, not stable. Hydrogen has been identified as an ideal energy carrier to support sustainable energy development [1], [2], [3], [4]. Hydrogen can be used in a fuel cell to generate electricity with high efficiency. It is extremely clean as the only by-product is water. In order to support sustainable hydrogen economy, it is crucial to produce hydrogen cleanly and renewably.
At present steam reforming of hydrocarbons, i.e. natural gas, is the most commonly used and generally the most economically competitive method for hydrogen production [1], [2], [3]. Natural gas is a kind of fossil fuel, and its usage fails to provide a solution to deal with the huge amount of carbon dioxide emissions during the reforming processes. In addition, the use of fossil fuels for secondary energy production is non-sustainable. As a result, there is a growing interest in the search for effective alternatives to produce renewable hydrogen cleanly and safely. Among the various feedstocks, ethanol is very attractive because of its relatively high hydrogen content, availability, non-toxicity, and storage and handling safety. More importantly, ethanol can be produced renewably by fermentation of biomass sources, such as energy plants, agroindustrial wastes, forestry residue materials, and organic fraction of municipal solid waste. The ethanol produced in this way is called bio-ethanol, which is a mixture of ethanol and water with molar ratio 1:13 (about 12 wt% ethanol) [5], [6], [7]. As biomass takes in carbon dioxide from the atmosphere for its growth, reforming of biomass-derived ethanol does not contribute to global warming.
Ethanol reforming processes for hydrogen production can be generally classified into two groups: (1) steam reforming and (2) autothermal reforming. Compared with autothermal reforming, steam reforming of bio-ethanol has received more attention due to its relatively higher conversion efficiency. This paper aims to review the technological developments in bio-ethanol steam reforming for hydrogen production, with an emphasis on catalyst development. Studies on autothermal reforming and short contact time reactor development are also discussed. In the end, the prospect of bio-ethanol derived hydrogen in fuel cells is discussed.
Section snippets
Bio-ethanol production
Bio-ethanol is produced by fermentation of biomass materials. When oxygen is insufficient for normal cellular respiration, anaerobic respiration takes place by yeasts, converting glucose into ethanol and carbon dioxideSugar cane, switchgrass, potatoes, corns, and other starch-rich materials can be effectively converted to ethanol by fermentation. This method is mature and easy to control. However, the cost of ethanol production is rather high mainly due to the
General aspects
The reaction pathways and thermodynamics of ethanol steam reforming have been studied extensively recently [9], [10], [11], [12], [13]. The possible reaction pathways of ethanol steam reforming are summarized in Table 1. It can be seen that hydrogen production varies significantly with different reaction pathways. In order to maximize hydrogen production, it is crucial to ensure sufficient supply of steam and to minimize ethanol dehydration and decomposition.
Catalysts for bio-ethanol steam reforming
In the above-mentioned reversible
Autothermal reforming of bio-ethanol
Steam reforming is an endothermic process in the absence of oxygen gas and requires energy input to initiate reactions. Alternatively, hydrogen can be obtained by partial oxidation of ethanol at a temperature of about 773 K according to the following reactionHowever, hydrogen selectivity of ethanol partial oxidation is generally low. In order to enhance hydrogen production, autothermal reforming can be applied. Autothermal reforming, also called oxidative steam reforming, is
Prospects of bio-ethanol and bio-ethanol-derived hydrogen in fuel cells
As the products of biomass conversion are mainly hydrogen-rich gases, they have been used in conventional internal combustion engines or gas turbine to provide power or heat. However, it would be more effective to use fuel cells to convert hydrogen energy into electricity efficiently, cleanly, and silently.
Fuel cells are electrochemical devices that continuously generate electricity and heat by electrochemical reaction between fuel (hydrogen) and oxidant (oxygen or air). As combustion is not
Conclusion
Bio-ethanol reforming processes, including bio-ethanol steam reforming and autothermal reforming, are reviewed in this paper. Focus is given to catalyst development for hydrogen production by bio-ethanol steam reforming.
From comprehensive literature surveys and analyses, it can be seen that Rh and Ni exhibited the best performance in terms of bio-ethanol conversion and hydrogen selectivity. In order to maximize hydrogen production, proper preparation of the catalyst and suitable supports are
Acknowledgment
The work described in this paper was supported by a grant from the Research Grants Council of the Hong Kong Special Administration Region (HKU7150/05E).
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