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the ECN tar dew point site

Validation of the tar dew point model

The in-house tar dew point model has been validated downstream the fluidised bed gasifier (WOB) at ECN. The model has been validated by:

  1. using a tar condensor
  2. using an on-line tar dew point analyser (TDA)

The air blown gasifier has a capacity of 1 kg biomass/hour and was operated at atmospheric pressure and a temperature of 850°C.

As a result, the in-house tar dew point model has an accuracy of ± 3°C in the temperature range of 20°C to 170°C. The heavy tar compounds dominate the tar dew point, which means that even at low concentrations these compounds start to condense at relatively high temperature. Therefore, when heavy tars are present in the gas, but not included in the measurement, a large error is potentially created and the calculated dew point will be an underestimate.

Model validation using a tar dew point analyser (TDA)
The process flow diagram for the model validation with the TDA is given in Figure 1. Raw product gas was drawn through a filter and tar condensor and subsequently fed to the TDA. The TDA is an analyser developed by ECN and Michell Instruments that continuously measures the tar condensation temperature in biomass product gas, using a sensor cell with an optical surface. For more information please download the report ECN-C-05-009, Tar dew point analyser. At the inlet of the TDA the tar composition was measured with the SPA method. Subsequently, the tar dew point was calculated from the tar composition and compared with the tar dew point measured with the TDA.

Process flow diagram
Figure 1: Process flow diagram for validation of tar dew point model with tar dew point analyser, TDA.

Figure 2 compares the calculated tar dew point with the measured tar dew point. The measured tar dew points are similar to the calculated values. In the temperature range of 25°C to 150°C the difference is at most 2°C. At a dew point of approximately 170°C the measured tar dew point was 7°C to 14°C lower than the calculated tar dew point.

Process flow diagram
Figure 2: Comparison of the calculated tar dew point with the measured tar dew point. On the Y=X line the calculated tar dew points equal the measured tar dew points.

Model validation with the tar condensor
The process flow diagram for the model validation with the tar condensor is given in Figure 3. Raw product gas was drawn through a filter and fed to the tar condensor for the saturation of the product gas with tar. Downstream the condensor the tar composition was measured with the SPA method. The tar dew point was compared with the temperature of the tar condensor.

Process flow diagram
Figure 3: Process flow diagram for validation of tar dew point model with the tar condensor

Figure 4 compares the calculated tar dew point (Y-axis) with the temperature of the tar condensor (X-axis). The solid squares in Figure 4 represents the calculated tar dew point. The open rhombus and the Y=X line is the situation when the tar dew point mimics the tar condensor temperature, which can be considered as the ideal line. The difference between the calculated tar dew point and the ideal Y=X line in Figure 4 is in most cases lower than 3°C.

Process flow diagram
Figure 4: Comparison of the calculated tar dew point with the temperature of the tar condensor. On the Y=X line the calculated tar dew point equals the temperature of the tar condensor.

The difference in the tar dew point and the temperature of the condensor can be caused by a sum of errors: