Environmental Engineering Reference
In-Depth Information
Consequently, cofiring wood chips or agricultural residues in a coal-fired power
plant necessitates installation of additional machinery, leading to additional capital
costs. Furthermore, for biomass, the energy required for grinding is several times
higher, leading to high operating costs. The most common practice, circumventing
most grinding problems, is the use of wood pellets, which are essentially compacted
sawdust. However, wood pellets require closed handling and storage facilities and, at
high cofiring percentages, may reduce the rating (power output) of the power plant
considerably. Torrefied biomass pellets or briquettes potentially do not require
closed handling and storage nor special grinding equipment. Furthermore, due to
their larger heating value, there will be no or only minor derating at high cofiring
percentages.
The favorable properties in terms of grindability and energy density are even more
important in dry feed oxygen-blown entrained flow gasification (especially with
respect to pneumatic feeding), which is seen as a key technology in the future produc-
tion of liquid biofuels and biochemicals. There also may be opportunities for torrefied
biomass as a fuel in smaller-scale pellet boilers and stoves. Finally, torrefaction can
potentially enable the utilization of a wide range of agricultural residues for the pro-
duction of electricity, heat, fuels and chemicals from biomass.
12.6 THE FUTURE OF TORREFACTION
The initial main driver for torrefaction has been the increasing demand for solid bio-
fuels in Western Europe emanating from legislation and incentives in favor of cofiring
biomass in large power utilities. As the availability of clean woody biomass in this
region is limited, North America serves as the principal source of imports. Conse-
quently, several developers in Europe and North America are aiming to commercial-
ize this technology, with several projects having made it to the pilot/demo phase. The
continuing interest in torrefaction and its development relies on the sustained presence
of these drivers, in addition to a further exploration of alternative raw biomass supply
chains for which torrefaction seems to be a strong enabler.
More recently, interest in torrefied biomass as a renewable fuel has grown in other
parts of the world, e.g., in Brazil, South Africa, and in several Asian countries includ-
ing Malaysia, Indonesia, Japan, and South Korea. There is also a growing interest in
the use of torrefied biomass for applications such as entrained flow gasification (for
biofuel production) and smaller scale heat and power production.
Even as a lot is known about the chemistry of torrefaction reactions, process
scale-up, optimization, and control will benefit from a more thorough fundamental
understanding. Furthermore, in view of economics and sustainability, it is also
imperative that the developed technologies focus on employing more efficient heat
integration strategies.
Handling, storage, grinding, feeding, and conversion (e.g., combustion, gasifica-
tion) of torrefied biomass need to be explored extensively in large-scale test cam-
paigns to optimize product quality and inspire end user confidence. This requires
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