The tar problem
The presence of tars in the product gas is a big problem in the commercial utilisation of biomass product gas as source of sustainable energy. Tar is formed in the gasifier and comprises a wide spectrum of organic compounds, generally consisting of several aromatic rings. Simplified tars can be distinguished in heavy tars and light tars:
- Heavy tars condense out as the gas temperature drops and causes major fouling, efficiency loss and unscheduled plant stops (see pictures on the right). The tar dew point is a critical factor.
- Light tars like phenol or naphthalene have limited influence on the tar dew point, but are not less problematic. Light tars like phenol chemically pollute the bleed water of downstream condensers and aqueous scrubbers. Naphthalene is important as it is known to crystallise at the inlet of gas engines causing a high service demand (see picture on the right).
Tar defined
A well accepted definition states that tars are all organic compounds with a molecular weight higher than benzene. A better and more detailed tar description is given by the classification of tars. The type of tar components can be determined by the tar guideline. The tar guideline is a standardised tar measurement and analysis method.
The tar dew point, a critical parameter
The lowest temperature in the process is determined by downstream equipment and the application of the product gas. As typical tar dew points are between 150 °C and 350 °C, and the lowest process temperature is typically 30 °C (compressed gas is even more critical), massive tar condensation and tar problems are inevitable. It is important to realize that the actual tar concentration is not the most important parameter. It is the tar dew point which defines the point at which tars start to be problematic. One of the most important goals for the OLGA technology is to lower the tar dew point to a level at which problems can be excluded.
OLGA targets:
- Tar dew point < 20°C
- Total tar concentration efficiency 99 percent
- Optimised phenol removal, < 2,5 mg/Nm3 downstream
- Optimised naphthalene removal efficiency, 99 percent
ECN have developped a quick calculation method for tar dew points based upon the tar composition. This tool is published at the Thersites website.
Contact person: Jan-Willem Könemann, +31-10-5991114
