Dahlman OLGA

OLGA vs other systems

Tars from biomass product gasses can be removed with a thermal tar cracker, a catalytic tar cracker or a scrubbing process.

 

Thermal tar cracking

A thermal tar cracker heats up the product gas to a temperature of 1200°C. At this temperature the tars are removed almost completely leading to a very low tar concentration <100 mg/m_n³ and tar dew point <10°C. The disadvantage of the application of a thermal cracker is the reduction in efficiency. To increase the temperature of the product gas a part of the product gas is combusted with oxygen. Consequently, the system efficiency (biomass to electricity) is reduced as well as the calorific value of the product gas. The reduction in calorific value makes the application of the product gas from a direct air blown gasifier in a gas engine difficult.

 

Catalytic tar cracking

A catalytic tar cracker does not heat up the product gas and thus eliminates the disadvantages of a thermal cracker. In theory the tar removal efficiency can be complete. However, soot formation and deactivation of the catalyst are serious problems to deal with, resulting in limitations in the process. At the moment, the tar concentration at the inlet of the cracker should remain below 2 g/m_n³ and the presence of alkali metals and sulphur should be controlled. Several projects have shown that a catalytic tar cracker can be a vulnerable part of the system. Bad tar removal by e.g. catalyst deactivation directly leads to heavy tar problems downstream. In principle tar removal is less efficient than a thermal cracker but can be good enough for the application of the product gas in a gas engine.

 

Tar removal by aqueous scrubbers

Aqueous tar removal systems cool down the product gas and remove the tars by condensation. In most aqueous systems dust and tars are collected simultaneously. The product gas is cooled down and aerosols of dust, tars  and water are collected with a wet ESP downstream. Some systems use a dry hot gas filter (HGF) upstream for dust removal instead of a wet ESP. The tar dew point downstream an aqueous system is similar to or higher than the operating temperature of the system. Therefore, the total tar content downstream an aqueous system can exceed 1 g/m_n³. To avoid tar condensation and fouling of piping the gas should not cool down. In the aqueous scrubber system a tar/water problem is created. Mixing (heavy) tars with water will lead to operational difficulties in the scrubber and huge maintenance costs. The most important disadvantage is formed by waste water handling especially because of tars like phenol which seriously poison the waste water. Waste water handling is often so expensive that the plants economical feasibility is at stake.

 

Tar removal by OLGA

Based on the problems with aqueous scrubbers ECN and Dahlman developed the oil based tar removal system OLGA. In OLGA the tars are removed by condensation and by absorption.
The temperature remains above the water dew point to avoid mixing of dust and tar with water.
Due to the absorption step in OLGA the tar dew point is decreased far below the operating temperature of OLGA, typically below 10°C. The total tar concentration is reduced to 200 mg/m_n³. Tars downstream OLGA are composed of light compounds like Xylene and Indene. These compounds do not cause fouling problems in the downstream equipment. Phenols are almost completely removed in OLGA to avoid the production of poisoned condense water and expensive waste water cleaning.

 

Comparison between  OLGA & aqueous scrubbers

ECN operated and tested two aqueous systems and one oil based system, OLGA, downstream the 100 kg/h (500 kWt) air blown circulating fluidized bed gasifier. The gasifier produces a product gas with a tar load of 10 to 20 g/m_n³ on dry basis, divided in 12% heavy tars, 84% light tars, and 4% hetero-cyclic tars.

The figure below compares the tar removal efficiency of the three tested gas cleaning systems. In the aqueous scrubber the gas was not on specification for a gas engine. With the addition of a wet-ESP the heavy tars were almost completely removed and the tar dew point decreased to 60°C. The product gas could be applied in a gas engine, but the system suffered from wastewater problems. The OLGA removed the tars almost completely. The tar dew point was reduced well below a temperature of 10°C. The water condensate did not contain phenols and the gas could be applied in a gas engine.

Conclusion is that the performance of the OLGA system is better than aqueous systems. The catalytic tar cracker competes with OLGA. However, deactivation of the catalyst is still a drawback of these systems and reduces the flexibility of the application.