Gas cleaning
Initially, the cracking of tar using metallic catalysts was studied, but by the end of 1997 this was abandoned because the long-term clean-up result was far from satisfactory. The next attempts were based on scrubbing the gas using water, and this was soon considered more efficient and operationally stable.
However, the remaining problems now appeared to be the intimate mixing of tar compounds with the scrubbing water (creating problems for the subsequent separation of tar and water) and also a large quantity of tar/water aerosols (microscopic droplets) carried from the gas clean-up system. The scrubbing system was replaced by a gas-cooler (cooled by district heating water) and it was verified that the remaining aerosols could be caught by a wet Electrostatic Precipitator (ESP).
Preliminary verification was also obtained in a parallel project that the tar-contaminated water from the gas cleaning process could be cleaned in a two-step process before discharge to the municipal sewerage system:
- Removal of the heavy tar compounds in a state-of-the-art oil separator
- Subsequent removal of the water-soluble tars (light tars and acids with molecular weights below 200) in a system using a micro-filter followed by a two-stage reverse osmosis process.
The product gas was therefore cleaned to a level acceptable for use in gas engines without adverse environmental effects, and it was decided to proceed with electrification of the Harboøre plant.
It became apparent during autumn 2000 that the water clean-up system would never be able to meet the requirements for plant effluent discharge, and the system was abandoned.
Our R & D Centre subsequently invented and demonstrated a small-scale system for highly efficient water clean-up.
