The rapid increase in global biodiesel production is resulting in a worldwide surplus of glycerol, which is generated as a by-product of transesterification. Once considered a valuable co-product, crude glycerol is rapidly becoming a waste product with an attached disposal cost.
A growing body of research is focusing on developing new glycerol platform chemistry and product families to take advantage of a substance that is increasingly abundant and cheap. At the 232nd National Meeting of the ACS in San Francisco, researchers described various approaches to utilizing glycerol (C3H8O3) as a feedstock for different fuel outcomes: a low-temperature catalytic approach to using glycerol as a source for fuel and chemicals; the steam reformation of glycerol to produce hydrogen; and glycerol as a feedstock for microbial hydrogen production.
|Various glycerol pathways. Click to enlarge. Source: Soares, Simonetti and Dumesic.|
Low-temperature catalytic processing. Dante Simonetti from the University of Wisconsin described work that puts glycerol through a two-step process involving low-temperature catalytic conversion to a syngas (H2 and CO) and subsequent Fischer-Tropsch or methanol synthesis.
The group found that gas mixtures of H2 and CO can be produced at high rates and selectivities from glycerol over platinum-based bi-metallic catalysts at temperatures (e.g., 500 K to 620 K) that are significantly lower compared to conventional gasification of biomass.
The two-step process can also serve as an energy-efficient alternative to processes used to convert starch-based materials to fuel-grade ethanol, because glycerol can be produced in high concentration (e.g., 30 wt%) by fermentation of glucose. Accordingly, this process opens new pathways to more effectively utilize renewable biomass resources to provide liquid fuels and chemical intermediates.
The University of Wisconsin Group, led by Prof. James Dumesic, have also developed a low-temperature aqueous phase reforming process that can use glycerol as a feedstock. Dumesic is one of the co-founders of Virent. (Earlier post.)
Steam reforming of gylcerol to produce hydrogen. Several papers tackled the issue of hydrogen generation via the steam reforming of glycerol, with the focus being the discovery of the optimum catalyst and process.
A team from Spain presented experimental results indicating the catalysts they tested are all able to convert the glycerol completely with values very close to the theoretical results predicted by thermodynamic equilibrium.
The experiments were carried out in a fixed-bed catalytic reactor at 773 K and 873 K with nickel catalysts supported on g-alumina and modified by different contents of MgO, ZrO2, CeO2 or La2O3. The feed composition was increased from 1 to 10% of glycerol in water which is a similar content to that obtained in the first phase glycerol separation from biodiesel.
The team found that the addition of promoters significantly improves hydrogen selectivity and avoids the formation of undesirable by-products if compared with non-promoted catalysts. The best performer was a promoted catalyst with 5 wt.%.
Although the focus of a paper from Mississippi State was the steam reforming of sugar, the researchers found that the process was problematic, due to caramalization resulting from the process temperature. The team is in parallel investigating glycerol in its experimental process, which apparently works fine, although no results were presented.
A separate paper from the Mississippi State team described a thermodynamic analysis of the steam reforming of glycerol to produce hydrogen.
The group analyzed the steam reforming process of glycerol over the following variable ranges: pressure 1 atm, temperature 600-1000 K and water-to-glycerol feed ratio 1:1-9:1. The study revealed that the best conditions for producing hydrogen is at a temperature >900 K and a molar ratio of water to glycerol of 9:1. These conditions minimize methane production and inhibit carbon formation.
Microbial hydrogen production. A team from Brookhaven National Laboratory is investigating the processes under which Thermatoga neapolitana, an anaerobic, thermophilic bacterium, efficiently processes glucose feedstock—in this case, glycerol—to produce hydrogen.
One surprising finding was that T. neapolitana produced hydrogen most efficiently in a moderately low-oxygen—but not oxygen-free—environment. Previously, hydrogen production by bacteria has only been reported under anaerobic conditions.
The ability to operate with some oxygen in the production lines would make this process more economically feasible. The team is further studying the mechanistic aspects of the hydrogen production system, and is beginning to work on scaling up the process to a larger 14-liter reactor.
“Glycerol as a Source for Fuels and Chemicals by Low-Temperature Catalytic Processing”; Ricardo R. Soares, Dante A. Simonetti, James A. Dumesic; Angewandte Chemie International Edition Volume 45, Issue 24, Date: June 12, 2006, Pages: 3982-3985