Page 143 - Advances in bioenergy (2016)

transporting problematic. To take advantage of the efficiencies of scale, biomass conversion
plants are designed to be fed by biomass feedstock collected from a surrounding radius inside
5
of 100 miles. Even at the scale of some hundreds of tons per day of processing, the scale of
fuel production is dwarfed by typical fossil energy refineries. Fast pyrolysis provides a means
to take advantage of this distributed nature of biomass feedstock and have smaller scale
pyrolysis plants feeding collectively to a larger central upgrading plant to produce finished

fuels. In this scheme, the bio-oil becomes the transported form of the biomass. Bio-oil is more
energy dense allowing for efficient transportation. The liquid handling properties also add to
the advantage over solid biomass transportation. However, bio-oil properties, such as its
acidic nature and its thermal instability, must be addressed in any transport system.

Reactor Configurations

Reactor engineering for fast pyrolysis must account for the properties of biomass and the
process requirements. Biomass is a solid. It can be reduced in particle size in either the wet
(green) stage or the dry stage. As biomass is not a good conductor of heat, rapid heating
requires that it be in a small particle size. Furthermore, a good heat transfer mechanism needs
to be incorporated to accomplish the fast heat-up required. The most efficient means, with
scaleable designs, have been fluidized beds using a solid particulate heat transfer medium
mixed with the biomass particles. Fast heat-up of the biomass particles (<2 s) is facilitated by
drying the biomass prior to the pyrolysis step. The introduction of the large heat load required

by the water in green biomass (30–50 wt% moisture) has been difficult to accomplish for short
residence times at reactor temperatures of 450–550°C, well above the boiling point of water.
As a result, fast pyrolysis typically requires a dry biomass with a moisture content less than 10
wt%. The heat input requirement to pyrolyze biomass (and to some degree, dry the biomass)
can be met by combustion of the solid and gas by-products.

Fluid Bed Systems

In the bubbling fluid bed option, the gaseous by-product is burned and the hot flue gas is
injected to fluidize the bed. Char can be recovered as a by-product. In the circulating bed
option, solid char is recovered by cyclones with carried-over heat transfer bed solids. The
char is combusted to reheat the solids, which are returned to the bed that is fluidized by the flue
gases. Transport reactors have also been used that do not include fluidized bed solids but
incorporate radiation heating and partial heating by conduction from the reactor wall.

Ablative Systems

In ablative systems, the heat input comes not from small particles but by continuous contact
with solid surfaces. A similar short residence time on the hot surface is accomplished by the
nearly immediate volatilization of the bio-oil products from the hot surface. The surfaces are
typically metal to maximize the heat transfer rate. The force to contact the biomass to the hot
surface can come from centrifugal forces or mechanical pressure application, depending on the
engineered design. There are even designs that combine ablative surface pyrolysis with
fluidized bed operation.