Thersites: website for tar dew point calculations
Introduction
The purpose of “Thersites” is to give insight in the phenomenon of the tar dew point and to allow calculation of tar dew points of producer gases from its composition. Tar condensation occurs when the gas
phase gets saturated with a specific tar component. Higher temperature allows for higher saturation
pressures of the tar components, hence the vapour pressure of the tar component in gas the phase can
be higher. This allows for a relation between the tar concentration and tar dew point. Using the two models
on the website (www.thersites.nl) gives users insight in the tar dew point and the relation between
concentration of the different tar components and the effect on the tar dew point.
Tar is commonly described as (aromatic) hydrocarbons that have a larger molecular weight than
benzene. Based on the different types of gasifiers
or gasification conditions a large spread in tar
components can be obtained. The tar dew point
is the temperature at which the real total partial
pressure of tar equals the saturation pressure of
tar. Based on the Antoine constants (A,B and C)
and temperature, the vapour pressure of a
specific tar component can be calculated.
( )
log Pi * = A −
B
T +C
The tar dew point is determined by molecular
mass and by the concentration of that molecule
in the gas phase. Combining information on the
concentration of the tar component in the gas
phase with Raoult’s law it is possible to calculate
the tar dew point.
Classification system
In the spectrum of different tar components, many
different types exist. The range is basically from
hetero atom containing, polar tar components to
poly aromatic non soluble tar components. To
structure all these tar components, a classification
scheme was designed in which all these different
tar components would fit. With the Guideline
Sampling method1 all these different tars can be
quantified. The Solid Phase Adsorption (SPA)
method is only applicable to class 2 till class 5
type tar components. It is an adsorption
technique for ‘light’ tars, with coronene being the
heaviest tar. In the following table this scheme is
being depicted.
Class 1 GC undetectable tars. This class
includes the heaviest tars that
condense at high temperature even at
very low concentrations.
Class 2 Heterocyclic components (like phenol,
pyridine, cresol). These are components that generally exhibit high water
solubility, due to their polarity.
Class 3 Aromatic components. Light hydrocarbons that are not important in condensation and water solubility issues.
Class 4 Light polyaromatic hydrocarbons (2-3
rings PAH’s). These components
condense at relatively high concentrations and intermediate temperatures.
Class 5 Heavy polyaromatic hydrocarbons
(4-5 rings PAH’s). These components
condense at relatively high temperature at low concentrations.
To show the relevance of a classification scheme
the following graph depicts the effect of
concentration on the tar dew point for the class
2-5 type tars.
II
III
IV
V
300
250
Temperature (0C)
Tar dew point
200
150
100
50
0
-50
-100
-150
0.1
1
10
100
1000
10000
concentration (m g/nm3)
b-08-020
1
ECN publication, ECN-C--02-090
ECN Biomass, Coal & Environmental Research
T. +31 224 56 4504
Contact: Berend Vreugdenhil
P.O. Box 1, 1755 ZG Petten, The Netherlands
F. +31 224 56 8487
[email protected]
www.ecn.nl
Simple model
Six key tar compounds in the simplified model
have been selected based on the tar classification system and molecular size of tar
compounds. Toluene and naphthalene are
normally present in biomass producer gas at high
concentration and have a relatively low dew point
at high vapour pressure. Pyrene and Benzo(ghi)
perylene are present at low concentration but
have a relatively high dew point at low vapour
pressure. Phenol is an oxygenated tar compound
and is therefore different in element composition.
The second method is by using the tar dew point
analyzer (TDA) for measuring the tar dew point
online. The off-line tar measurement is done with
SPA and analyzed to obtain the concentrations of
the different tar components.
Flare
Tar condensor
On-line
Tar dew point Analyzer
Filter
1 kg/h Biomass
Off-line
Tar measurement
Tar dew point
Model
BFB Gasifier
Tar dew point
1 mn 3/h Air
=?
Tar dew point
Complete model
Validating the model
The model has been validated using two
methods. The first one is by using a tar condenser, which is set at a specific temperature.
This will be the condensation temperature of the
tars in the gas phase. Information on tar concentration is obtained with the SPA method.
Analyses of the SPA samples produce tar concentrations of the different components and this
is used for the calculation of the tar dew point.
Comparison show deviations of less then 3°C.
Flare
Off-line
Tar measurement
Tar condensor
Tar dew point
Model
Filter
1 kg/h Biomass
3
1 mn /h Air
BFB Gasifier
Temperature
condensor
=?
Tar dew point
The results depicted in the following graph show
the accuracy between the temperature set by the
tar condenser and the calculated tar dew point.
The results of the model are very good.
180
Calculated tar dew point [°C]
The complete model consists of 37 different tar
components that are typical for a producer gas
as can be expected from biomass gasification.
These components cover the entire range from
class 2 till class 5 tars.
The website has included an option to save a
calculation to a ‘csv’ file, which can be imported
into excel. From -20°C to 180°C the model can
predict quite accurate what the tar dew point will
be for a given gas phase concentration. Within
the range of 20°C to 170°C the model has an
accuracy of ±3°C as validated using the tar dew
point analyzer (TDA) and controlled experiments.
160
140
120
100
80
80
100
120
140
160
180
Temperature of tar condenser [°C]
Y = X line
Dew point [°C]
Further information
Publications
www.ecn.nl/publications
Tar dew point calculations
www.thersites.nl
ECNECN Biomass, Coal & Environmental Research
T. +31 224 56 4504
Contact: Berend Vreugdenhil
P.O. Box 1, 1755 ZG Petten, The Netherlands
F. +31 224 56 8487
[email protected]
www.ecn.nl