review and discussion
Fast pyrolysis uses much faster heating rates than ancient pyrolysis.
Advanced processes are fastidiously controlled to present high liquid yields.
Pyrolysis liquids are formed by rapidly and simultaneously
depolymerizing and fragmenting cellulose, hemicellulose, and lignin with a
rapid increase in temperature. Bio-oils contain many reactive species, which
contribute to unusual attributes.
Fast pyrolysis bio-oils are supposed to replace fuel oils in many
stationary applications including boilers and furnaces. However, these bio-oils
are completely different from petroleum fuels and other bio-oils in the market,
like biodiesels, as regards both their physical properties and chemical
composition. When the unusual properties of these bio-oils are carefully taken
into account, their combustion with-out a pilot flame or support fuel is
possible on an industrial scale. Even blending of these oils with alcohols in
order to improve combustion is not necessarily required.
In the latter industrial scale bio-oil combustion tests, bio-oil has
been found to be technically acceptable for replacing heavy fuel oil in district
properties of fast pyrolysis bio-oils
Bio-oils are composed of differently sized molecules derived primarily
from the depolymerisation and fragmentation reactions of three key biomass
building blocks: cellulose, hemicellulose, and lignin, as previously discussed.
Perfect reproduction of all the bio-oil processing conditions can produce
varying bio-oils, because the feed composition (biomass or wood) influences the
nature of the final product.
Even though bio-oils are typically considered to be homogenous
single-phase liquids, there are a number of reasons why two or more phases
might be gone through during product recovery, handling or storage. If ignored,
this phenomenon may cause serious problems in combustion applications.
i. Phase-separation due to chemical composition
The amount and type of neutral
extractives in the wood feedstock causes the separation out of a distinct top
layer from highly polar bio-oils. Forest residues, in particular, yield a
liquid with a 5–20 wt. % top phase that is low in polarity. The amount of top
phase depends on the feedstock distribution, as well as on the process and
product accumulation conditions. Examine in contrast with the bottom phase, the
top phase is inferior in water, oxygen, and density and superior in heating
value and solids content. Extractives can appear as dissolved in bio-oil, as
oily droplets, or as crystals in the bio-oil 5.
ii. Phase separation due to high water content
Bio-oils can be considered as
micro emulsions of water and water-soluble organic compounds with
water-insoluble, mostly oligomer, lignin-derived material. The ratio of these
fractions depends on the feedstock, process conditions, and production and
storage conditions. The water-insoluble fraction, mainly lignin-derived
oligomers, usually accounts for about 20–25 wt. % of the liquid (wet basis),
while the water concentration typically ranges from 20 to 30 wt. %. Two-phase
product with a larger aqueous phase and viscous oily phase may be produced if
high-moist (> 10 wt. %) feedstock is used. Alkaline metals, especially
potassium, catalyse pyrolysis reaction, producing more water.
The solubility of bio-oils in organic solvents is affected by the
degree of polarity. Good solvents for highly polar bio-oils are low molecular
weight alcohols, such as methanol, ethanol and iso-propanol. These solvents
dissolve basically all the bio-oil, excluding solids (char) and some
extractives. Acetone is also a good solvent for wood bio-oils but may cause
reactions yielding to sedimentation with straw pyrolysis bio-oils. An increment
in the pH of the bio-oils can, in principle, be carried out by adding basic
organic solvents, such as amines or alkali hydroxides.
composition of bio-oil is significantly different from that of petroleum fuels.
It consists of different compounds derived from decomposition reactions of
cellulose, hemicellulose, and lignin. The chemical composition of bio-oil
varies depending on the type of biomass feedstock and the operating parameters.
Generally speaking, bio-oil is a mixture of water and complex oxygen-rich organic
compounds, including almost all such kinds of organic compounds, that is,
alcohols, organic acids, ethers, esters, aldehydes, ketones, phenols, etc.
Normally, the component distribution of bio-oil may be measured by GC-MS
bio-oil derived from lignocellulose is a dark-brown, viscous, yet free-flowing
liquid with a pungent odor. Crude bio-oil has an oxygen content of 30-50 wt. %,
resulting in instability and a low heating value.
bio-oil is a homogeneous liquid containing a certain amount of solid particles.
After long-term storage, it may separate into two layers and heavy components
may be deposited at the bottom.
solvent fractionation at a moderate temperature, fast pyrolysis bio-oil can be
divided into water-soluble (WS) and water-insoluble (WIS) fractions. These
fractions are not pure, but their main compound types determine their properties.
of Bio-oil Preparation
Fast pyrolysis is the leading method for producing liquid biofuels. The
advantages include low production costs, high thermal efficiency, low fossil
fuel input, and CO2 neutrality. Pyrolysis to a liquid offers the
possibility of decoupling (time, place, and scale), easy handling of the
liquids, and more-consistent quality, compared to any solid biomass. A liquid
intermediate is produced for a variety of applications by fast pyrolysis.
A fast pyrolysis process includes
drying the feed to a water content of typically < 10% (although up to 15% can be acceptable), to reduce the water content in the product oil. The feed must be ground to give sufficiently small particles to ensure rapid heat transfer and reaction. After pyrolysis, the solids (char) must be separated and the vapours and aerosol are quenched. The liquid bio-oil then is collected. There are two major upgrading methods used to produce fuel grade product from bio-oil namely catalytic cracking and hydrodeoxygenation. A catalytic cracking of bio-oil leads to the oxygen removal in the presence of selective catalysts for example zeolites and mesoporous molecular sieves.