Biodiesel, as a renewable energy, has got fast development and large-scale production, resulting in the significant overcapacity of its by-product glycerol. Crude glycerin can be obtained by treating biodiesel through a simple process. Besides glycerol, there are other impurities in crude glycerol. In order to apply it to food, cosmetics, medicine and other industries, crude glycerin must be refined. But at present, the refining process of crude glycerol is rather complicated, with high cost and low economic feasibility. Therefore, it is necessary to develop the application space of crude glycerol and improve the added value of crude glycerol, such as using crude glycerol to make hydrogen.
Hydrogen is a kind of clean and efficient secondary energy. With the continuous expansion of hydrogen application and the increasingly prominent world’s energy and environmental problems, biological hydrogen production technology has received extensive attention. Among them, hydrogen production from crude glycerol is also an important comprehensive utilization of the by-products of biodiesel, which has attracted more and more attention. The main processes of making hydrogen from glycerol include steam reforming, partial oxidation, autothermic reforming, water phase reforming and supercritical water reforming. And steam reforming technology is widely used in chemical industry.
ADHIKARI et al. prepared hydrogen by steam reforming process and catalyzed the hyper endothermic reaction between glycerol and water to produce hydrogen. They’ve investigated the catalytic reforming properties of Ni/ MgO, Ni/TiO2 and Ni/CeO2, finding that the activity of hydrogen production is the most high for Ni/MgO, the yield of hydrogen can reach 56.5%, when the reforming temperature is 650 ℃. SLINN et al. investigated the feasibility of steam reforming of glycerol, a by-product of biodiesel, to produce hydrogen. They used Pt-al2o3 as the catalyst, and found that the higher reaction temperature is, the higher gas phase yield will be. And the highest yield is close to 100%, the selectivity is 70%. Under the optimum conditions for hydrogen production by steam reforming of glycerol, the carbon deposition of glycerol, a by-product of biodiesel, was slightly higher than that of pure glycerol. However their catalyst activity is similar.
BYRD et al. adopt supercritical water reforming process, used glycerin, the by-product of biodiesel, as raw material to produce hydrogen, with Au/Al2O3 catalyst. The reaction was carried out in a tubular fixed bed reactor, reaction temperature is 700 ~ 800 ℃, and the glycerin concentration (mass fraction) in the feed is 40%. The highest yield was close to the theoretical yield, with 7 moles of hydrogen for every 1 mole of glycerol. As the excessive impurities in crude glycerol will affect the activity and service life of the catalyst, the purity of glycerol is required during the process. Therefore, in order to increase the efficiency of making hydrogen from crude glycerol and reduce the production costs, it is also necessary to develop catalysts with strong environmental adaptability, corrosion resistance to impurities and high activity.
Glycerol can also be converted to hydrogen in the form of microbial catalytic conversion. GUILLAUME et al. utilizes photosynthetic bacteria Rhodopseudomonas palustris to convert crude glycerol to hydrogen. The yield is high during this process, producing 6 mol of hydrogen per 1 mol of glycerol (75% of theory, theoretically 8 mol of hydrogen per 1 mol of glycerol). It was also found that impurities in crude glycerol, a by-product of biodiesel, had no inhibitory or toxic effects during the whole fermentation process.
According to the above analysis, hydrogen can be prepared from crude glycerol, the by-product of biodiesel, through various chemical catalysis technology or microbial transformation technology, with high hydrogen production efficiency. This process has the advantages of renewable raw materials, clean and pollution-free, and is one of the efficient ways of hydrogen production, with great development space.
