The technical advantage of ethanol steam reforming over direct ethanol combustion for power generation is the improvement of thermal efficiency through hydrogen production exclusively used for fuel cell. In addition to stationary electricity generation, fuel cell is also designed for powering portable devices such as automobile. It is unsafe to travel around with compressed hydrogen tank on board. Therefore, there is a necessity for on-board steam reformer development where liquid ethanol rather than compressed hydrogen gas is fed into the storage tank. In order to get better mileage per gallon ethanol fed, the very important requirement of on-board steam reformer development is its light weight, which generates great demands on size reduction of on-board reformer. To fulfill the miniaturization and compactness requirements, various types of micro-structured reactors have been developed in recent years, which is typically composed of stacks of channeled blocks. Each micro-channel coated with active catalyst acts as the steam reformer for hydrogen production. Partial ethanol is combusted in the other side of the channel to supply heat required for reforming. Such design provides many technical advantages including rapid mass and heat transport due to large surface area to volume ratios, lower pressure drop, good structural and thermal stability, and precise control of reaction conditions leading to higher hydrogen yield [159, 160]. The main challenges faced by this technique before it becomes final commercialization are system integration, reactor fabrication process, and catalyst regeneration or replacement.

Combinatorial method originally developed for drug discovery has been introduced into the catalyst discovery field in the last decade to accelerate the catalyst screening process. By using this high-throughput approach, large and diverse libraries of inorganic materials can be prepared, processed, and tested simultaneously under the same reaction conditions for quickly obtaining potential candidates with desirable catalytic performance, which is beneficial for significant reduction of time and money spent on catalyst development [161, 162]. However, the relatively complicated algorithms for testing matrix determination, expensive testing instrument, and representability of the screening results should be better handled before it can be widely accepted as a standard catalyst development strategy.

The influence of external field (e. g., electric and magnetic field) on catalytic performance during BESR could be another interesting area to study. Because any chemical reaction involves electron transfer and rearrangement facilitated by the addition of catalyst, the application of external field which can exert impact on electron movement is expected to have influence on catalytic reactivity. Such effect has been recently evidenced by L. Yuan, et al. that hydrogen yield and selectivity were significantly enhanced when an AC current passed through Ni/Al2O3 catalyst [163].

According to LeChatelier’s Principle, referring to Reaction (1), continuous removal CO2 from product stream can shift the reaction equilibrium toward products side, leading to the improvement of hydrogen production. Based on literature review, there are mainly two methods for CO2 in-situ removal: addition of CO2 sorbent and CO2 selective membrane. The CO2 sorbent used for this purpose has to be regenerated at temperature higher than reaction temperature for reuse. For doing so, the high temperature CO2 sorbent has to be circulated between reactor and regenerator [164]. The CO2 sorbent is usually regenerated under the hot air environment and has good resistance to high temperature and attrition. According to literature reporting, CaO and lithium silicate are among the most commonly used CO2 sorbents for hydrogen production. For CO2 selective membrane, CO2 is either rejected by the membrane and stays in the retentate side, or diffuses through the membrane and swept out as permeate. In order to in-situ remove CO2 or perform hydrogen purification within the reformer, various types of membrane reactors have been developed in recent years to obtain hydrogen rich gas stream. Moreover, catalytic membrane reactor has also been invented to perform water-gas shift (WGS) and separation simultaneously through applying certain catalyst onto the membrane surface, among which Pd — impregnated membrane is the most reported one for getting purfied hydrogen product [165, 166]. Nevertheless, many technical problems including cost reduction, selectivity and permeation efficiency improvement, and rigidity enhancement have to be solved before it becomes economically attractive.

The high cost of ethanol feedstock for steam reforming mainly comes from the downstream distillation and purification steps of the crude ethanol obtained from fermentation. If the crude ethanol can be directly used as the feedstock for hydrogen production from BESR, the large amount of energy wasted during distillation for water and other impurities removal can be eliminated, leading to the significant cost reduction of ethanol feedstock and in turn hydrogen produced from BESR. In addition, other oxygenated hydrocarbons contained in the fermentation broth can also be steam reformed to generate extra 7% hydrogen if crude ethanol is employed compared to steam reforming of pure ethanol. Although this approach sounds promising for final commercialization of BESR technique, the challenge still remains at the catalyst’s tolerance to the impurities present in the crude ethanol solution. According to related publications, several researchers have conducted such study to evaluate the impact of impurities on catalytic performance toward hydrogen production. A. Akande and his coworkers investigated the influence of crude ethanol simulated through adding small amount of lactic acid, glycerol, and maltose to ethanol aqueous solution on the catalytic performance of Ni/ Al2O3 [128, 167]. Initial catalyst deactivation was observed followed by stable run within 12 hours test. Similar study has also been performed by our group over Co/CeO2. ~90% hydrogen yield is achieved and well maintained within 100 hours run. A more systematic research has been recently implemented by A. Valant, et al. over Rh/MgAl2O4 [168]. More oxygenated hydrocarbons including esters, aldehydes, amine, acetic acid, methanol, and linear or branched alchols have been tested for its influences on catalytic performance of BESR. Catalyst deactivation is observed for certain impurity additions. Through catalyst modification, much better stability has been achieved using Rh — Ni/Y-Al2O3.

Although high pressure operation will result in inhibition of hydrogen production, as predicted thermodynamically referring to Section 2.7, it is still worth investigating, because high pressure operation will significantly lower down the hydrogen compression cost for storage and transporation. In order to compensate the hydrogen production loss, hydrogen selective membrane reactor has been recently proposed in combination with high pressure operation by Argonne National Laboratory [169]. By doing so, the formed hydrogen can be continuously removed leading to the thermodynamic equilibrium shift toward hydrogen production.

2. Acknowledgment

We gratefully acknowledge funding from the U. S. Department of Energy through grant DE- FG36-05GO15033. The Ohio Supercomputer Center (OSC) is also acknowledged for generous computational support of this research.

[1] News on: Sci Focus Direct on Catalysts, 2005

[2] Genencor, relations, 21 October 2004, avaible from: http:/genencor. com/cms/connect/ genencor/media_relations/news/archive/2004/gen_211004_en. htm

[3] Genencor, products, 14 January 2010, avaible from: http:// www. genencor. com/ wps/ wcm/ connect/ genencor/ genencor/ products and services/ business development/ biorefineries/ products/ accellerase product line en. htm

[4] Novozyme, brochure, 29 January 2010, Viable from: http:// www. bioenergy. novozymes. com/ files/ documents/ Final%20Cellic%20Product%20Brochure_ 29Jan2010.pdf