THERMOCHEMICAL CONVERSION: AN INTRODUCTION TO FAST PYROLYSIS - Biomass as a Sustainable Energy Source for the Future - page 361

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11.1 INTRODUCTION

Fast pyrolysis is a thermochemical process to convert biomass into bio-oil that can
be further upgraded or refined for the production of heat, electricity, (transportation)
fuels, and chemicals. Lignocellulosic streams such as sawdust, forest thinning, straw,
and empty fruit bunches are targeted feedstocks. At temperatures between 200 and
550 C in the absence of oxygen, biomass particles decompose into char, liquids
(removed from the solid as vapors or as aerosols), and gases by a process known as
pyrolysis. The liquid product, termed bio-oil or pyrolysis oil, is captured downstream
of the reactor in condensers. When the pyrolysis is conducted at temperatures between
450 and 550 C, combined with a high heating rate of the biomass particles and rapid
quenching of the produced vapors, maximal liquid production is achieved. This process
is termed fast pyrolysis. Typically, small particles (millimeters) are used as feedstock, to
achieve high heating rates. Processes conducted at lower temperatures (200 350 C),

often using larger particles (centimeters), resulting in high char yields are typically
known as slow pyrolysis or carbonization. Pyrolysis processes are typically operated
at atmospheric pressure or below. In addition to its practical importance for the
production of bio-oil, pyrolysis is also the initial chemical step in all biomass
gasification and combustion processes.
Advocated advantages of bio-oil produced via fast pyrolysis over the bulky
inhomogeneous biomass from which it originates are (i) the increased volumetric
energy density (

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5×); (ii) the ease of storage and transportation, which facilitates
the decoupling of the refining location from biomass production; (iii) better
processability; and (iv) the possibility to return minerals to the soil via the carbo-
naceous by-product. At the time of writing, several fast pyrolysis demonstration
plants were in operation or being built, aiming at maturing the technology,
maximizing oil production, and producing sufficient oil for application tests.
Research is focusing on understanding the underlying processes at all relevant
scales, ranging from the chemistry of the cell wall compounds to optimization
of production plants, in order to produce better quality oils for targeted uses.
For wet biomass (>ca. 60% moisture), a process called hydrothermal liquefaction
has been studied and operated at pilot scale. In this process, which is operated at high
pressure (typically 100 300 bar), water is used as a solvent and reaction medium.

Also, other solvents, such as ethanol, are considered. In this case, the process is called
solvolysis. Fast pyrolysis, hydrothermal liquefaction, and solvolysis are all liquefac-
tion processes: processes to convert biomass into a liquid. This chapter deals with the
fast pyrolysis process, while the other liquefaction processes are briefly touched upon.
Fast pyrolysis is described at the level of chemistry and processes. Bio-oil proper-
ties are reported together with proposed applications. This chapter is intended as a first

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