In the OLGA unit, tars must be removed from the gas to specifications of:

1. a tar dewpoint <10°C; 
2. less than 10 mg/mn3 phenol, and 
3. less than 30 mg/mn3 naphthalene in the gas.

OLGA inlet temperature of the cooled and de-dusted product gas is 375°C, i.e. a 25°C temperature decrease due to heat losses is assumed over cyclone and the piping to the OLGA inlet. Total pressure drop over OLGA is less than 50 mbar.

The tar removal in OLGA comprises three stages. In the first stage, ‘Collector’ the heavy tars are condensed from the gas by contacting the gas with cool oil scrubbing liquid. Most of the remaining dust is also removed with the scrubbing liquid. The remaining fine dust and entrained oil aerosols are removed in the second stage, i.e. the ‘Demister’. The combined scrubbing oil streams are cooled, passed through a separator, and returned to the first scrubber. In the separator the dust is removed as filter cake and the liquid heavy tars are continuously separated from the oil and recycled as “feedstock” to the combustor of the MILENA, resulting in complete destruction of the tars.

In the third stage, the ‘Absorber’, the light tars are removed with oil. The Absorber is operated above the water dew point to avoid mixing of tar and water. The product gas after OLGA is “tar-free”, which means that downstream the OLGA tar-related problems are avoided. The Absorber scrubbing oil is regenerated in the ‘Stripper’, which is operated with air. The Stripper is equipped with a condenser to minimise oil losses. The stripper air is returned to combustor section of the gasifier where the tars are reused/destructed. The regenerated Stripper oil is cooled in a cross heat exchanger by heating the tar-loaded oil from the Absorber.

With the OLGA process tars can be removed from up to 40 g/mn3 to a level in which the tar dewpoint (i.e. the temperature at which tar condensation may occur) is below  5°C. Furthermore, the OLGA has also been demonstrated in lab-scale operation for cleaning of gasification gas for Fischer-Tropsch synthesis [1]. The OLGA process was developed at ECN and is commercialised by the Dutch company Dahlman Industrial Group. OLGA has been demonstrated in a 700 hour duration test downstream the ECN CFB gasifier from January to March 2006. Mid 2006), a 4 MWth pilot OLGA was commissioned, which is part of a semi-commercial pilot CHP plant in France.

References:

1. H. Boerrigter, H.P. Calis, D.J. Slort, H. Bodenstaff, A.J. Kaandorp, H. den Uil and L.P.L.M. Rabou: Gas cleaning for integrated biomass gasification (BG) and Fischer-Tropsch (FT) systems. ECN, ECN-CX--04-004, 2004, The Netherlands (confidential).