Pyrolysis involves heating materials in
a reducing atmosphere (with a limited oxygen supply), sometimes
with steam. It was used in the UK for many years to produce coke
and "town gas" from coal, before the development of
the North Sea gas fields. It still has applications in the production
of activated carbon for waste treatment.
In fast pyrolysis of biomass, the material
being treated is heated rapidly for only a few seconds, and this
breaks it down into a large number of relatively small molecules.
The resulting mixture is then cooled rapidly to prevent further
reaction, giving a dark, oily liquid, known as bio oil or pyrolysis
oil. Other products include char, mostly carbon solids, and a
mixture of gases. Both of these can be used as fuels on site.
Content of Bio Oil
This depends on the biomass source used, but typically includes
chemicals with functional groups such as carbonyl, carboxyl, hydroxyl
and methoxyl. These are all oxygen-containing groups, whereas
the oil extracted from the ground is almost wholly hydrocabon
Uses of Bio Oil
Given its instability, it is not possible to distil bio oil, but
it may be used directly as an alternative to diesel and other
fuel oils. The heating value of bio oil is about 50% that of conventional
petroleum-derived fuel oil, requiring adjustments to the rate
of feed of the fuel.
As it cannot be distilled, other separation methods must be used,
including making use of the fact that part of the oil is water
miscible, part is not. Although the range of substances present
is wide, several tend to predominate, and these are potential
candidates for chemical stock.
- Bio oil can also be used as a chemical
resource without separation:
- Reaction with ammonia and amines results
in a cocktail of non-toxic compounds suitable for use as a slow-release
- Reaction of the carbonyl and carboxyl
groups with ethanol (which
itself can be produced from renewable resources)
gives a mixture that is more stable than raw bio oil.
As a fuel, this has a higher thermal output, and a significant
proportion can now be distilled to obtain many other useful
- Control of conditions can result in
oil that is particularly high in compounds of phenol, and this
can be used directly to make
phenolic resins for wood products
like plywood and MDF.
here to find out more about the product routes from Bio Oil
Bio oil is in fact potentially quite reactive,
and must be maintained at no more than 25°C. Even then, it
is unstable and will change in composition over time. It is also
a mixture of hydrophilic and hydrophobic compounds, which means
it will tolerate the addition of extra water (neat bio oil contains
water), but is immiscible with most organic solvents.