Liquid fuels contribute a significant share to the energy supply of modern societies, primarily in the transportation sector and for heating purposes. They will remain important even after heat demands of buildings have decreased significantly and electrical drive systems for passenger cars have become important. The main reason for this is the high energy density which is of major importance in mobile applications. Today’s predominant raw material petroleum will be complemented and replaced for reasons of price increase, supply security, and fossil CO2 emissions (Schaub and Edzang, 2011). One option with high CO2 reduction potentials is the complete use of lignocellulosic biomass through cleavage of the biopolymers into a mixture of carbon monoxide and hydrogen, called syngas. Subsequently, this gas mixture can be used as a basis for the generation of various platform chemicals and/or synthetic fuels (see diagram below).
Syngas conversion technologies (Woolcock and Brown, 2013)
For the generation of syngas from biomass there exist in principle two options:
- Direct gasification of the solid biomass in special designed gasifiers to a raw gas with subsequent gas cleaning steps and pressurization for the synthesis reaction;
- Thermal pre-treatment to produce energy dense intermediates e.g. by torrefaction or fast pyrolysis of biomass to obtain a liquid (bio-oil), char and gas.
The mixing of bio-oil with char and gasification of the slurry in a pressurized entrained flow reactor is being demonstrated at a pilot scale of 2-5 MWth at KIT, Karlsruhe, Germany. Straw is used as lignocellulosic feedstock as it is available in large amounts at fairly low prices. Bio-slurry preparation appears beneficial as it has a higher heating value between 18 to 25 GJ/m3 which corresponds to 1/2 up to 2/3 of the volumetric energy density of heating oil (HHV 36 GJ/m3). In addition, the liquid-like bio-slurry is easy to feed into the gasifier at pressures adjusted to the subsequently following synthesis process.
The conceptual process overview is shown below. Many decentralized fast pyrolysis plants prepare the bio-slurry which is transported to a centralized gasification site of reasonable, industrial scale.
Latest developments of the Karlsruhe bioliq® process can be seen at http://www.bioliq.de/english/.
bioliq® process overview (Dahmen et al., 2012)
Dahmen, N., E. Dinjus, T.Kolb, U. Arnold, H. Leibold and R. Stahl (2012). “State of the Art of the bioliq® Process for Synthetic Biofuels Production.” Environmental Progress & Sustainable Energy 31(2): 176-181.
Schaub, G. and R. Edzang (2011). “Synthetic Fuels from Natural Gas and Biomass – Status and Perspectives.” Chemie Ingenieur Technik 83(11): 1912-1924.
Woolcock, P.J. and R.C. Brown (2013). “A review of cleaning technologies for biomass-derived syngas.” Biomass and Bioenergy 52(0): 54-84.