Page 144 - Advances in bioenergy (2016)

Moderate Heating Rate Systems

Other engineered designs attempt to accomplish fast pyrolysis with perceived simpler designs.
These designs, often using auger transport systems, typically accomplish pyrolysis more akin to
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slow pyrolysis used in charcoal production. In these systems, the liquid yield of bio-oil is less
than recovered in true fast pyrolysis, while the gas and char yields are higher. Also, the bio-oil
product composition will include a lower level of residual oxygen, suggesting secondary
reactions, and a higher yield of water product, which often causes the bio-oil to phase
separate. An improved laboratory-scale auger system has been recently reported with a high

gas sweep, which can achieve results more similar to the fluid bed systems. 7

Alternatives: Catalytic Pyrolysis, Hydropyrolysis, Hydrothermal Processing

There are other technology options under development, which attempt to produce a more
deoxygenated product bio-oil, which will require less upgrading to hydrocarbon fuels
products. These other options include catalytic pyrolysis, hydropyrolysis, and hydrothermal
liquefaction. Catalytic pyrolysis involves the addition of catalysts, typically similar to the acid
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catalyst used in petroleum cracking. The point of catalyst addition remains an issue of
development with some attempting direct incorporation of the catalyst into the fluidized
pyrolysis bed and others working on vapor phase catalysis of the pyrolysis product following
char separation. Hydropyrolysis also involves the addition of catalyst but in addition high-
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pressure hydrogen is injected into the reactor system. Hydrothermal liquefaction is an entirely
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different processing concept, which operates at higher pressure and lower temperature for
longer residence time. The reactor typically incorporates a liquid water-processing phase,
although some concepts substitute a solvent other than, or in addition to, water.

Bio-Oil Product Properties

Fast pyrolysis has been developed to maximize the liquid product yield from biomass.
Examination of the liquid phase suggests that it is a single phase, which includes a wide range
of oxygenated functional types with a substantial level, typically less than 30 wt%, of water. 11
The water is both dissolved in the liquid organic phase and is also found as microdroplets

coating the small pieces of char that escape the cyclones and are collected in the condensate. 12

Bio-oil from fast pyrolysis has an assigned CAS number of 1207435-39-9. The product is
described as liquid condensate recovered by thermal treatment of lignocellulosic biomass at
short hot vapor residence time (typically less than about 5 seconds) typically at between 450
and 600°C at near atmospheric pressure or below, in the absence of oxygen, using small
(typically less than 5 mm) dry (typically less than 10% water) biomass particles.

A number of engineered systems have been used to effect high heat transfer into the biomass
particle and quick quenching of the vapor product, usually after the removal of solid by-
product “char,” to recover a single-phase liquid product.

Bio-oil is a complex mixture of, for the most part, oxygenated hydrocarbon fragments derived
from the biopolymer structures. It typically contains 15–30% water. Common organic