20.4.1 Biomass feedstock for reforming

Solid, liquid, and gaseous biomass can be used for reforming. Solid biomass first has to be gasified or evaporated, liquid biomass can either be first evaporated or processed in the liquid/supercritical phase. Gaseous biomass is handled as such.

Solid biomass (waste) is available from already existing industries (like agriculture, wood production, and first-generation biofuels). It is available in its bulk volume or has been densified using for instance pelletization allowing easier handling. The biomass can then be fed to a gasifier using a screw feeder or a hopper system. To convert solid biomass to a liquid has several advantages over using the solid biomass directly.

• A liquid is produced from bulky solid biomass, which is usually difficult to handle. By eliminating void volume which is inevitably present with solid biomass, the energy volumetric density is significantly increased. This makes trans-shipment and transport, especially over longer distances, much more effective.

• It can be stored in tanks. It is often more stable against biological decomposition and cannot ignite at ambient temperature.

• Liquids are easier to process especially when pressurized conversions are envisaged.

With liquefaction, a solution is being given for effectively utilizing biomass: to bridge the large gap of biomass supply and demand, and to do it in a sustainable way. Biomass is available decentralized (where it is being grown) but processing needs to be done centralized to benefit from the economy of scale. Biomass can be liquefied where the biomass is available and then be transported over long distances (road, water) to central processing units of similar scales as the current petrochemical industry. Besides technical and logistic advantages, this conversion chain will also give incentives for economical development and job creation especially in rural areas.

Fast pyrolysis (see Chapter 14) is a liquefaction technology which seems to be very attractive for handling relative dry biomass streams on a worldwide scale. Fast pyrolysis technology produces pyrolysis oil (or bio-oil) via rapid heating of biomass to approximately 500°C in absence of oxygen. In this way, the biomass is thermally decomposed and produces gases, vapors, and char. The vapors are condensed yielding the pyrolysis oil with a yield up to 70 wt%. The gases can be combusted to supply heat for the process and the char can be used as a fuel or it can directly be recycled back to the land since most minerals and metals are concentrated into it. Pyrolysis oil as such can be reformed directly. Additionally, more water-rich fractions of pyrolysis oil are co-produced within various bio-refinery concepts (see Section 20.4.2) which allows ‘milder’ reforming than the full oil.

Hydrothermal liquefaction can be used to produce oil from wet biomass streams which is the subject of Chapter 18. Very large quantities of low organic water streams are available (e. g., from municipal, fermentation, digestion waste and in the future from algae) which essentially could be used for reforming at elevated pressures. However, the organic concentration must not be too low (> 10 wt%) for the energetic efficiency of the process. Also bio-gas (CH4 rich) from anaerobic digestion (Chapter 12) can be used as reformer feed which, after a gas cleanup, is essentially a mixture of CH4/CO2.

Trace components in biomass such as sulfur and chlorine are a serious issue in both reforming and downstream catalytic conversions. Sulfur removal is manageable using commercial technologies, such as adsorption (e. g., ZnO, Ni, etc.) and hydrodesulfurization (HDS). HDS (e. g., Albemarle’s NEBULA, BASF, Haldor Topsoe) can bring S levels down to single-digit ppm, but is expensive. High chlorine levels pose a greater challenge. The best option for chlorine, ammonia, and metal contaminants is to use dedicated sorption processes for each contaminant. To summarize, various general and specialized cleanup solutions need to be developed and used, depending on the contaminants in gas and the downstream catalysts.