Biomass gasification is a process that converts solid fuel into combustible gas, but this technological process also has a ‘dark side’ in the form of tar. This represents one of the biggest obstacles to the wider practical use of wood gas. In this article, we will explain exactly what tars are and how to tackle them during the gasification process.

What are tars?

From a technical point of view, tars are a mixture of hydrocarbons which exist in a liquid state at normal temperature and pressure. In raw gas, their concentration usually ranges from 5 to 75 g/m³. Their presence in the gas depends mainly on the type of gasification plant, the operating temperature, the pressure and the composition of the fuel used.

Why do tar deposits form?

Tar formation is a natural and ever-present by-product of biomass gasification. The main source is the volatile substances released during the pyrolysis phase. The process of their formation and transformation can be categorised according to temperature:

1. Primary tars: These are formed at lower temperatures (450–500 °C) directly from the volatile components of biomass. They are chemically unstable.

2. Secondary tars: These are formed by the transformation of primary tars and are characterised by the presence of volatile phenols.

3. Tertiary tars: As the temperature and the gas residence time in the reactor are further increased, their final conversion takes place, with benzene as the main component

A key factor influencing the amount of tar produced is the moisture content of the fuel. Experiments confirm that when damp wood chips (e.g. 25 per cent moisture content) are used, significantly more tar is produced during the process than when pre-dried fuel is used.

How do carbon deposits affect the engine and technology?

Tars are an undesirable component in wood gas, as they cause serious technical problems during the subsequent use of the gas. The greatest risk they pose is condensation. Tars in the gas begin to condense even at temperatures below 300 °C.

If such gas enters an engine or turbine without being adequately purified, the condensed tar can clog the equipment, cause mechanical parts to seize up and, ultimately, lead to malfunctions and plant shutdowns.

How can tar build-up be prevented?

Although there is currently no cleaning method that is 100 per cent effective and, at the same time, easy to apply in practice, there are several ways to minimise their quantity:

1. Use of large generators: By using high-capacity gasification generators, it is possible to effectively reduce tar formation.

2. Fuel moisture control: One of the most effective measures is the use of dry wood chips. Reducing the moisture content of the fuel before it enters the reactor has been shown to reduce the production of tar in the gas.

3. Gasification temperature: A higher temperature in the reactor promotes better transformation and decomposition of tar-like substances.

4. Filtration: The exhaust gas must pass through special fabric filters, which trap impurities and condensed tars.

Choosing the right type of gasification generator and ensuring the fuel is properly prepared are key to producing high-quality wood gas with a minimal impurity content.