SYNTHESIS GAS UTILIZATION FOR TRANSPORTATION FUEL PRODUCTION - Biomass as a Sustainable Energy Source for the Future - page 534

water as a coolant, thus generating steam. This is called a multitubular packed bed reac-
tor. As catalyst development progresses and catalysts become more active, the heat pro-
duction per unit volume also increases. A possible solution is to replace the randomly
packedbeds by structured packings that force convective transport in the radial direction,
giving better heat transfer (Pangarkar et al., 2008). Such packing can be coated and/or
filled with catalysts. The investment costs of such systems — which are not yet used

in practice will be larger, while the benefit is that the better heat transfer enables the

use of tubes with larger diameters, so less tubes are needed per vessel. The latter brings
down the investment; so there is a trade-off between tube size and degree of structuringof
the packing. Currently, commercial multitubular reactors are operated at low conver-
sions per reactor, typically considerably lower than 50%. By recycling and/or putting
reactors in series, a much higher overall conversion is reached for the complete process.
Currently, these two reactor types are both operated at large scale (>30,000 bar-
rels

—

day −1 ): slurry bubble columns by Sasol and multitubular packed bed by Shell.
However, in both cases, the syngas is not produced from biomass but from natural
gas. The plants are located very close to natural gas fields, enabling economies of
scale. Transportation of liquid fuel is much cheaper than that of natural gas. It is also
possible to base relatively large plants on a feedstock of biomass, but this will require a
lot of transportation of biomass. According to Hamelinck et al. (2004), the optimum
scale for a BTL production plant is about 2000 MW th ; for comparison, the Sasol plant
in Qatar produces about 4000 MW th . However, if cheap biomass is locally available,
smaller, distributed plants may be an option too, especially since these do not require
shipping of raw biomass over long distances. A number of smaller companies are
developing small-scale FTS reactor technology, e.g., Rentech, Syntroleum, and Vel-
ocys. Some of these companies rely on scale-down versions of the large-scale tech-
nology, whereas others propose radically different concepts. For example, Velocys
commercializes a millichannel reactor: a reactor with many parallel channels of 1
or a few mm (so an order of magnitude smaller than for a conventional multitubular
reactor), filled with particles of around 250 m diameter (Deshmukh et al., 2011).

Because the ratio of the tube diameter to the particle diameter is relatively small,
the pressure drop stays limited, even with these small particles: wall effects lead to
higher bed porosity. Because of the small channel diameter, a good heat transfer
can be ensured, and a highly active catalyst can be applied. Velocys claims to achieve
a production of 12 barrels day −1

μ

kg −1 reactor mass, 1.5 4 times higher than the com-

mercialized large-scale technologies (Deshmukh et al., 2011).
Summarizing, the production of liquid transportation fuels from biomass via
gasification and FTS has the advantages that it can work with a wide variety of
second-generation feedstocks, that it uses mature technology, and that the product is
easily integrated in the existing fuel distribution system. It requires, however, large
investment costs, and the efficiency is not veryhigh, aswill be discussed inSection17.5.
There is an issue with respect to a potential downscaling of FTS technology for regional
distributed applications. The purpose of such downscaling is the minimization of
the costs of the entire logistic production chain from biomass to energy product(s).
One could consider producing the raw hydrocarbon mixture locally in smaller units,
while refining could take place in larger refineries.

−