Teren uit pyrolyse en vergassing van biomassa en reststromen : definities, vorming, eigenschappen, bemonstering en analyse
report
Tars from the pyrolysis or gasification of biomass can cause a problem when pyrolysis or
gasification gases are used in gas engines or gas turbines. Therefore, knowledge on tars is
indispensable when integral pyrolysis or gasification installations are to be developed.
This report summarises knowledge on tars. A literature study has been performed towards the
definition, formation, properties, sampling and analysis of biomass tars. Knowledge on coal tars
is included in this report whenever this knowledge is applicable to biomass tars.
Definition of tars
A general definition of tars is “all hydrocarbons that are liquid at standard temperature and
pressure”. Recently, in an EU/IEA/DOE meeting a number of experts has come to a more
specific definition of tars from biomass:
Tar = all organic contaminants with a molecular weight larger than benzene
Formation of tars
The formation of tars is inherent to the thermal decomposition of large, organic and solid fuel
particles. During the thermal conversion of biomass, initially primary tars are formed
(compounds such as levoglucosan and coniferyl alcohol), which are fragments of the original
biomass. The primary tars decompose into secondary tars (compounds such as phenols en
cresols). At high temperatures and/or long residence times, these secondary tars are converted
into tertiary tars. Tertiary tars are subdivided into condensed tertiary tars (aromatic
hydrocarbons or polynuclear aromatic hydrocarbons (PAH’s) such as benzene, naphthalene,
pyrene) and alkylated tertiary tars (methyl-, ethyl- ....derivates of condensed tertiary tars).
With respect to pyrolysis processes, more tar is formed in fast and flash pyrolysis processes than
in conventional (slow) pyrolysis processes. The flash processes yield primary and secondary
tars, the fast and conventional pyrolysis processes yield secondary tars. In gasification processes
the type of process determines to a large extent how much tar is formed. Biomass gasification in
downdraft and cross-flow fixed-bed gasifiers results in low tar concentrations of 0,01 to 10
g/mn. Biomass gasification in bubbling and circulating fluidised bed gasifiers results in the
formation of intermediate tar concentrations of 1-50 g/mn, whereas updraft fixed bed gasifiers
yield high tar concentrations of 10-200 g/mn Chemical and physical properties of tars
Tars differ in chemical composition (C, H, N and O concentrations), which results in different
O/C and H/C ratios. Primary tars (fast pyrolysis and entrained flow gasification) have the
highest O/C and H/C ratios of 0.2-1.0 and 1.2-2.2, respectively. Tars from fixed-bed gasifiers
ECN-C--99-102 3
have lower values of 0.2-0.3 and 0.6-1.5, respectively. Fluidised bed gasifier tars have the
lowest O/C and H/C ratios of 0.05-0.1 and 0.6-1.1, respectively.
Differences between the varying types of tars can also be found in values for the combustion
enthalpy, which varies from 20 - 40 MJ/kg, in values for the viscosity (range 10-27000 cps),
density (range 1.13-1.27 kg/dm3
) and acidity (range pH of 2-7).
Tars are harmful for human health; they are irritating for the skin and can cause skin and lung
cancer after exposure to high concentrations for a long period of time.
Sampling and analysis
Several institutes have developed methods for the sampling and analysis of tars. The sampling
methods are based on condensation, absorption and adsorption and are briefly described in this
report. Analysis can be performed on-line, which is however hardly ever done, and off-line.
Two on-line analysis methods for tars that are under development are based on mass
spectrometry and on a flame ionisation detector. Off-line analysis methods are most commonly
used, such as (i) evaporation or distillation of the solvent followed by gravimetric detection; (ii)
gas chromatographic detection; (iii) a ‘Total Organic Carbon (TOC) Analyser’; or (iv) ‘Size
Exclusion Chromatography’.
Recently, two standard methods (Protocols) have been proposed by a group of EU/IEA/DOE experts in order to define a reference for tar measurement, in particular for the measurement of
tars on which important (financial) decisions are based. One protocol has been developed for
small biomass gasifiers that allow sampling at a low temperature (i.e. fixed bed gasifier systems
where sampling can be performed after the scrubber and upstream of the gas engine). The other
protocol has been developed for large-scale gasifiers (i.e. fluidised bed gasifiers where sampling
is performed at high temperature and possibly at a high pressure). Both protocols are based on
absorption of tars in a solvent (dichloromethane), followed by gravimetric and/or gas
chromatographic analysis of the tars.
gasification gases are used in gas engines or gas turbines. Therefore, knowledge on tars is
indispensable when integral pyrolysis or gasification installations are to be developed.
This report summarises knowledge on tars. A literature study has been performed towards the
definition, formation, properties, sampling and analysis of biomass tars. Knowledge on coal tars
is included in this report whenever this knowledge is applicable to biomass tars.
Definition of tars
A general definition of tars is “all hydrocarbons that are liquid at standard temperature and
pressure”. Recently, in an EU/IEA/DOE meeting a number of experts has come to a more
specific definition of tars from biomass:
Tar = all organic contaminants with a molecular weight larger than benzene
Formation of tars
The formation of tars is inherent to the thermal decomposition of large, organic and solid fuel
particles. During the thermal conversion of biomass, initially primary tars are formed
(compounds such as levoglucosan and coniferyl alcohol), which are fragments of the original
biomass. The primary tars decompose into secondary tars (compounds such as phenols en
cresols). At high temperatures and/or long residence times, these secondary tars are converted
into tertiary tars. Tertiary tars are subdivided into condensed tertiary tars (aromatic
hydrocarbons or polynuclear aromatic hydrocarbons (PAH’s) such as benzene, naphthalene,
pyrene) and alkylated tertiary tars (methyl-, ethyl- ....derivates of condensed tertiary tars).
With respect to pyrolysis processes, more tar is formed in fast and flash pyrolysis processes than
in conventional (slow) pyrolysis processes. The flash processes yield primary and secondary
tars, the fast and conventional pyrolysis processes yield secondary tars. In gasification processes
the type of process determines to a large extent how much tar is formed. Biomass gasification in
downdraft and cross-flow fixed-bed gasifiers results in low tar concentrations of 0,01 to 10
g/mn. Biomass gasification in bubbling and circulating fluidised bed gasifiers results in the
formation of intermediate tar concentrations of 1-50 g/mn, whereas updraft fixed bed gasifiers
yield high tar concentrations of 10-200 g/mn Chemical and physical properties of tars
Tars differ in chemical composition (C, H, N and O concentrations), which results in different
O/C and H/C ratios. Primary tars (fast pyrolysis and entrained flow gasification) have the
highest O/C and H/C ratios of 0.2-1.0 and 1.2-2.2, respectively. Tars from fixed-bed gasifiers
ECN-C--99-102 3
have lower values of 0.2-0.3 and 0.6-1.5, respectively. Fluidised bed gasifier tars have the
lowest O/C and H/C ratios of 0.05-0.1 and 0.6-1.1, respectively.
Differences between the varying types of tars can also be found in values for the combustion
enthalpy, which varies from 20 - 40 MJ/kg, in values for the viscosity (range 10-27000 cps),
density (range 1.13-1.27 kg/dm3
) and acidity (range pH of 2-7).
Tars are harmful for human health; they are irritating for the skin and can cause skin and lung
cancer after exposure to high concentrations for a long period of time.
Sampling and analysis
Several institutes have developed methods for the sampling and analysis of tars. The sampling
methods are based on condensation, absorption and adsorption and are briefly described in this
report. Analysis can be performed on-line, which is however hardly ever done, and off-line.
Two on-line analysis methods for tars that are under development are based on mass
spectrometry and on a flame ionisation detector. Off-line analysis methods are most commonly
used, such as (i) evaporation or distillation of the solvent followed by gravimetric detection; (ii)
gas chromatographic detection; (iii) a ‘Total Organic Carbon (TOC) Analyser’; or (iv) ‘Size
Exclusion Chromatography’.
Recently, two standard methods (Protocols) have been proposed by a group of EU/IEA/DOE experts in order to define a reference for tar measurement, in particular for the measurement of
tars on which important (financial) decisions are based. One protocol has been developed for
small biomass gasifiers that allow sampling at a low temperature (i.e. fixed bed gasifier systems
where sampling can be performed after the scrubber and upstream of the gas engine). The other
protocol has been developed for large-scale gasifiers (i.e. fluidised bed gasifiers where sampling
is performed at high temperature and possibly at a high pressure). Both protocols are based on
absorption of tars in a solvent (dichloromethane), followed by gravimetric and/or gas
chromatographic analysis of the tars.
TNO Identifier
856333
Publisher
ECN
Collation
67 p.
Place of publication
Petten
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