Glycerol girds for new role
Any list of alternative fuels promising to have a major impact is bound to include biodiesel and hydrogen. Production of biodiesel certainly is increasing but so too are concerns over what to do with byproduct glycerol (see www.ChemicalProcessing.com/articles/2007/099.html and www.ChemicalProcessing.com/industrynews/2007/020.html). Meanwhile, the potential of hydrogen, either for use in fuel cells or for direct combustion, is blunted to some extent because more than half of the worlds supply comes from natural gas. Now, however, researchers in Britain have come up with a way to tackle both issues using a sorption-enhanced steam reforming process to convert glycerol into hydrogen. It should permit production of a high purity hydrogen stream that doesnt need downstream purification steps, hopes Valerie Dupont, a senior lecturer in the Energy & Resources Research Institute at the University of Leeds, Leeds, U.K.
Our process is a clean, renewable alternative to conventional methods. It produces something of high value from a low-grade byproduct for which there are few economical upgrading mechanisms, says Dupont.
The route couples the thermal decomposition of glycerol (C3H8O3) with the water gas shift reaction. The glycerol decomposition is favored by low pressures, making the reaction intrinsically safe, notes Dupont. The thermodynamics of the combined reactions are such that at around 630°C, with a 3:1 steam:carbon ratio, there is negligible CH4 byproduct by methanation, little carbon formation and a glycerol conversion of nearly 100%, she adds. This corresponds to a peak yield of about 1.5 mol H2 per mol of 2H in the glycerol feed. Using a catalyst provided by Johnson Matthey, Royston, U.K., which is collaborating on the project, will enable the reaction to take place at about 550°C.
The adsorbent particles provide two important benefits. They capture the CO2 in situ as soon as it is formed, driving the kinetics to the production of more H2. In addition, the adsorption is exothermic, reducing process energy demands for the endothermic steam reforming.
The process consists of a continuous fluidized-bed reactor/adsorber packed with catalyst and two desorbers, one of which is online at any time (Figure 1). The steam/glycerol feed stream receives regenerated hydrotalcite adsorbent particles discharged from the desorber before passing into the reactor. There, the C3H8O3 and H2O react to yield H2, CO2 and CO.
Gas leaving the reactor contains adsorbent and so is sent through a gas-solid separator; recovered adsorbent particles return to the desorber. Once all fresh adsorbent in that unit is used, operation switches to the second desorber while the first unit is regenerated. The CO2 is released in concentrated form, easing sequestration if required.
The concentration of CO in product gas is negligible. Water is removed and recycled to produce more steam, leaving >90 vol%-pure H2. For use in proton exchange membrane fuel cells, the H2 will require purification, notes Dupont.
Laboratory-scale trials will begin in February. Dupont and her coworkers plan to conduct a thorough economic evaluation, including catalyst and adsorbent life and waste disposal costs, by early 2009. If the economics prove favorable, the process could reach commercialization in less than 10 years, she estimates.
The project, which has received a £270,000 (about $550,000) grant from the U.K.s Engineering and Physical Sciences Research Council and started in October, will run through March 2009 and involves two industrial collaborators. Biodiesel producer D1-Oils, Middlesbrough, U.K., is supplying crude glycerol from its plant, and Johnson Matthey is providing steam reforming expertise as well as catalyst. However, neither firm will have exclusive rights to technology developed.