Bio Oil from Fast Pyrolysis

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 in content.

Uses of Bio Oil

Fuel 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.

Chemical Feedstock
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 fertiliser.
  • 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 products.
  • 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.

Click 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.

diagram: functional groups in bio oil

  back to top