For high temperatures (>500°C), alkalis have been proposed as catalysts.18 Alkalis promote the water gas shift and methanation reactions leading to more hydrogen or methane production and a carbon monoxide lean gas. The studies on whether or not alkalis enhance the extent of gasification are contradictory.64,65 Recovery of alkalis from the process may be a problem, because alkalis hardly dissolve in supercritical water. Antal et al.66 reported that leading the effluent of their empty tube reactor over a fixed bed of activated carbon derived from coconut increased the extent of gasification from 0.7 to 1.0. Kersten et al.65 used the Ru/TiO2 of PNNL and found complete gasification of glucose (1-17 wt% solutions) at 600°C and approximately 60 seconds residence time. The produced gas was at chemical equilibrium. The reaction is much faster at 600°C compared to 350°C, which is beneficial for the size of the reactor. However, no information is yet available concerning the stability of catalysts in the high temperature range supercritical water.

Reported problems with respect to the catalysts are poisoning through trace components such as sulfur, magnesium, calcium and the growth of the active metal crystals during operation (sintering). A general problem of the near and super critical region is that it enhances leaching of the catalytic active phases and degeneration of the support. Furthermore, if coke is formed on the surface of the catalysts, the high H2O concentration helps in keeping it clean via gasification. In accordance with that it was found that coke formation on the catalyst surface is a minor problem.67

20.2 Conclusions

Biomass can be converted via reforming into synthesis gas, H2/CO2 gas, and CH4/CO2 gas. The technology is in the R&D stage with some pilot work ongoing.

Applying natural catalytic materials (dolomite, olivine) in biomass gasifiers can lower the tar and the higher hydrocarbon content of the gas, thus reducing the load on downstream tar removal and reforming units. Engineered catalysts (primarily Nickel based) for inside gasifiers seem to be a dead end as there are too strong cooking, attrition, and poisoning issues. Tars and hydrocarbons can be removed downstream of the gasifier in relatively standard fixed bed type reformers. It is however essential that the feed gas of the reformer is cleaned from e. g. S, Cl and tertiary tars. Another great challenge will be dealing with the impurities in synthesis gas made from biomass for upgrading in secondary conversions (FT, alcohols, etc.). Downstream upgrading of bio-based fuel gas is technologically feasible (e. g. see the Sasol process for coal gas), but an expensive alternative (e. g. cleaning and pressurization).

Bio-liquids such as pyrolysis oil and its fractions and aqueous waste streams from other (biological) biomass conversions are considered as interesting feedstocks for reforming. These liquids are easy and cheap to pressurize and contain less contaminants than raw biomass. Besides these technical advantages, bio-liquids support a logistic scheme in which the primary conversion can be performed near the source of the biomass feedstock (e. g., remote, rural areas), with large-scale production of the finished bio-fuels in refineries near the market. Reforming of bio-liquids can become an important element of bio-refineries for hydrogen production. For quick introduction and growth of large amounts of bio-fuels, it is essential to integrate and to partner with existing industries and markets. In case of reforming, this can be done by co-feeding natural gas and naphtha reformers with bio-liquids. Reforming of bio-liquids can be done in the gas/vapor phase, the liquid phase or in the supercritical phase. All technologies have potential, but there are still challenges ahead. Optimal process and reactor configurations still have to be developed. Important issues here are handling of coke formation, mineral deposition, catalyst make up, heat addition, and biomass feeding systems. For reforming in hot compressed water feeding of biomass slurries is a real challenge while for the vapor phase system controlled atomization still requires R&D. Dedicated catalyst systems will be mandatory in biomass reforming. Extensive prior knowledge and experience with coal, oil, and natural gas can be used to modify, adapt, or design efficient catalysts. Most importantly, integration of catalyst, reactor, and process design and engineering in an early stage is needed.