Stefan Rabe

Synthetic natural gas from biomass by catalytic conversion in supercritical water

Biomass can be effectively converted to synthetic natural gas (Bio-SNG) in water near or above its critical point (374 °C, 22.1 MPa). If an active and selective catalyst is used, no tars or char are formed. The onset of the gasification reaction was visualized in sealed quartz capillaries as high pressure batch reactors, by using an optical microscope. By pressure differential analysis of batch experiments, the onset temperature was found around 250 °C, which is much lower than conventional atmospheric gasification processes operating at 800–900 °C. The temporal evolution of the gaseous products in the batch experiments was consistent with a sequential gasification–methanation mechanism, where methane is formed from CO2 and H2. A Ru on carbon catalyst exhibiting excellent long-term stability was tested at 400–500 °C and space velocities up to 33 gHC gcat.−1 h−1 in a continuous test rig at 30 MPa.

Towards Understanding the Catalytic Reforming of Biomass in Supercritical Water

Biomass conversion to transportation fuels (such as biodiesel, Fischer–Tropsch diesel, ethanol, dimethyl ether, methanol, biomethane, and hydrogen) has been the subject of many studies. 2] Biogenic synthetic natural gas (Bio-SNG) is particularly interesting as it is an attractive alternative that can be produced with a high efficiency from almost any kind of biomass. Furthermore, the combustion of Bio-SNG produces less atmospheric pollutants compared to liquid and solid fuels, and Bio-SNG can be distributed using the existing natural gas grid. Biomass with a high water content (“wet biomass”) usually poses a great challenge to thermochemical processes. The water in the biomass needs to be removed to a residual content of 10–15 wt % before thermal processing. Water removal therefore requires a lot of energy for wet biomass with an initial water content greater than 80 wt %. Processing biomass in hot pressurized water was found to have many advantages over gas-phase thermochemical processes such as pyrolysis and gasification by steam and/or air. Evaporation of the water in the biomass is avoided when working above the critical pressure of pure water (that is, at p> 22.1 MPa). Nearand supercritical water is a green solvent that may replace organic solvents for a number of organic syntheses. We have shown that waste biomass can be catalytically converted to Bio-SNG in supercritical water. The process has a high efficiency and low environmental impact. 7] A catalyst with ruthenium supported on granular carbon showed good gasification efficiency and was found to be stable for at least 220 hours on stream with a clean feed. Ruthenium catalysts also showed good performance for the production of hydrogen from ethanol in supercritical water at higher temperatures. Ethanol can be regarded as a simple model compound for the supercritical water gasification (SCWG) of wet biomass, since it contains both carbon–carbon and carbon–oxygen bonds. The catalytic reforming of ethanol can be formally described as shown in Equations (1)–(3): C2H5OHþH2O! CH4 þ CO2 þ 2 H2 ð1Þ

Catalytic partial oxidation of methane to synthesis gas over a ruthenium catalyst: the role of the oxidation state

The catalytic partial oxidation of methane to synthesis gas over ruthenium catalysts was investigated by thermogravimetry coupled with infrared spectroscopy (TGA-FTIR) and in situ X-ray absorption spectroscopy (XAS). It was found that the oxidation state of the catalyst influences the product formation. On oxidized ruthenium sites, carbon dioxide was formed. The reduced catalyst yielded carbon monoxide as a product. The influence of the temperature was also investigated. At temperatures below the ignition point of the reaction, the catalyst was in an oxidized state. At temperatures above the ignition point, the catalyst was reduced. This was also confirmed by the in situ XAS spectroscopy. The results indicate that both a direct reaction mechanism as well as a combustion-reforming mechanism can occur. The importance of knowing the oxidation state of the surface is discussed and a method to determine it under reaction conditions is presented.

X-ray Absorption Fine Structure Study of the Effect of Protonation on Disorder and Multiple Scattering in Phosphate Solutions and Solids

Phosphorus K-edge X-ray absorption fine structure (XAFS) was explored as a means to distinguish between aqueous and solid phosphates and to detect changes in phosphate protonation state. Data were collected for H(3)PO(4), KH(2)PO(4), K(2)HPO(4) and K(3)PO(4) solids and solutions and for the more complex phosphates, hydroxylapatite (HAP) and struvite (MAP). The X-ray absorption near-edge structure (XANES) spectra for solid samples are distinguishable from those of solutions by a shoulder at approximately 4.5 eV above the edge, caused by scattering from cation sites. For phosphate species, the intensity of the white line peak increased for solid and decreased for aqueous samples, respectively, with phosphate deprotonation. This was assigned to increasing charge delocalization in solid samples, and the effect of solvating water molecules on charge for aqueous samples. In the extended X-ray absorption fine structure (EXAFS), backscattering from first-shell O atoms dominated the chi(k) spectra. Multiple scattering (MS) via a four-legged P-O(1)-P-O(1)-P collinear path was localized in the lower k region at approximately 3.5 A(-1) and contributed significantly to the beat pattern of the first oscillation. For EXAFS analysis, increasing Debye-Waller factors suggest more disorder in the P-O shell with addition of protons to the crystal structure due to the lengthening effects of P-OH bonds. This disorder produces splitting in the hybridized P 3p-O 2p band in the density of states. For aqueous samples, however, increased protonation reduced the structural disorder within this shell. This was linked to a change from kosmotropic to chaotropic behavior of the phosphate species, with reduced effects of H bonding on structural distortion. The intensity of MS is correlated to the degree of disorder in the P-O shell, with more ordered structures exhibiting enhanced MS. The observed trends in the XAFS data can be used to distinguish between phosphate species in both solid and aqueous samples. This is applicable to many chemical, geochemical and biological systems, and may be an important tool for determining the behavior of phosphate during the hydrothermal gasification of biomass.