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Characterisation of Tar produced in the Gasification of Biomass with in situ Catalytic Reforming
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INTRODUCTION
Biomass gasification as a source of hydrogen and other valuable gases provides a
convenient energy vector for a wide range of applications. It consists of the
thermo-chemical conversion of biomass using air or oxygen or steam, or
combinations of these gases as the gasification agent and results in a fuel gas rich
in hydrogen and carbon monoxide, with a significant content of methane; carbon
dioxide, steam and nitrogen are also present in the product gas in addition to
organic (tar) and inorganic (H2S, HCl, NH3, alkali metals) impurities, and
particulates.
The organic impurities are collectively known as “tar”, a generic term for
all the organic compounds present in the gasification product gas excluding the
gaseous hydrocarbons (C1 to C6). In fact, tar is a complex mixture of condensable
hydrocarbons, which includes single ring to 5-ring aromatic compounds along
with other oxygen-containing and complex polycyclic aromatic hydrocarbons
(Maniatis at al., 1998; Li at al., 2009). Tar deposition can block gas coolers, filter
elements and engine suction channels and also interferes with the catalyst
performance. In all cases, the presence of tar means lower gas yields (Bangala et
al., 1997). The yield of these undesirable contaminants can be reduced by careful
control of the operating conditions (temperature, biomass heating rate, etc.),
appropriate reactor design and a suitable gas conditioning system (Van Paasen et
al., 2004).
In order to drastically reduce particulate and tar contents however, gas
cleaning systems are required. This operation is normally carried out by filtration
and scrubbing of the product gas, but this results in a decrease in gas temperature
to close to ambient, with a resulting loss of efficiency for subsequent drying and
heating applications. This factor penalizes the overall economic performance for
“green” biomass-based electricity generation. On top of that, the increased
complexity in the plant scheme brought about by such add-on gas treatment units
can contribute significantly to the overall investment and operating costs. Such
negative effects become more significant for small-to-medium scale gasification
units. Process simplification and intensification can therefore play an important
role in biomass-based green energy production. (Rapagnà et al., 2009).
Catalytic filters represent a promising technology in the hot gas cleaning
field. A suitable tar reforming catalytic filter element has been developed by Pall
Filtersystems GmbH (Nacken et al., 2007; Nacken et al., 2009). This involves a
modification to the conventional ceramic hot gas filter candle to provide a porous
inner tube in which a catalyst particle layer can be deposited (Nacken et al., 2007).
Such an arrangement provides for high flexibility in potential applications.
Tar formation is highly dependent on the reaction conditions − in
particular temperature, residence time and catalyst type. The problems that it
Rapagnà et al.: Characterisation of Tar produced in the Gasification of Biomass 1
Published by Berkeley Electronic Press, 2010
gives rise to appear to be more to do with its properties and composition than with
its quantity (Li at al., 2009), so that an analytical method for its rapid screening is
clearly desirable. Its characterisation is a key factor in the evaluation of gasifier
performance.
With regard to tar levels observed in biomass gasification applications,
data presented in the literature show a bewildering array of values; this is mainly
due to the different definitions of tar adopted by the various authors as well as the
different sampling methods and the diverse treatments of the condensed organics
before analysis (Milne et al., 1998). This has led to the proposal for a “Tar
Protocol” for sampling and analysis methods of tar fractions (Maniatis et al.,
1998; CEN CEN/TS 15439, European Committee for Standardization, 2006).
There are however some problems with the proposed standard, in particular with
the adoption of GC/MS as the major analytical tool: the higher molecular weight
hydrocarbons in the tar tend not to elute from GC columns; conventional
chromatographic methods are time consuming, expensive and often require a
preliminary preparative step (Patra et al., 2001); and accurate measures of the
individual components of the tar are not required. All this suggests that GC/MS is
perhaps not the most appropriate technique to consider for the purpose.
Because of the strong fluorescence emission of aromatic compounds,
fluorescence measurements and in particular synchronous fluorimetry would
appear to be techniques worth considering, especially as they may be used to
analyze mixtures without the need for pre-treatment. A number of studies on
qualitative and quantitative analysis of polycyclic aromatic hydrocarbons in
different media using these techniques have been reported in the literature:
Schwarz et al., 1976; Vo-Dihn, 1978; Lázaro et al., 2000; Patra et al., 2001;
Sharma et al., 2007.
In conventional fluorescence applications an emission spectrum is
obtained by scanning the emission wavelengths at a fixed excitation level.
Otherwise, a synchronous fluorescence scan (SFS) consists of a record of the
spectrum where both monochromators are scanning, keeping a constant
appropriate (delta lambda, Δλ) between excitation and emission wavelengths.
That results in a narrowing of spectral bands, simplification of emission spectra
and contraction of spectral range (Patra et al., 2002).
The work presented here concerns the incorporation of a catalytic filter in
a biomass gasification plant. Such an arrangement represents a promising
alternative technology for biomass gasification (Heidenreich et al., 2008).
Experimental tests of biomass steam gasification were carried out in order to
compare the effect on fuel gas quality of a conventional ceramic candle filter with
that of the innovative catalytic one − both manufactured by Pall Filtersystems
GmbH. For each test, the gas product composition was measured along with the
tar quantity − by Total Organic Carbon analysis (TOC). Preliminary results are
2 International Journal of Chemical Reactor Engineering Vol. 8 [2010], Article A30
also presented on the characterization of the tar in accord with technical
specification CEN/TS 15439 and HPLC-UV and fluorescence spectroscopy
analyses.
MATERIALS AND METHODS
Experimental apparatus and gasification operating conditions
a sketch of the biomass gasification plant used for the
experimental tests. This is mainly composed of a bubbling fluidized bed gasifier
with an internal diameter of 0.10 m, externally heated by means of a 6 kW electric
furnace. In order to realize in-situ tar reforming and particulate abatement, a
catalytic ceramic filter candle manufactured and provided by Pall-Schumacher
GmbH (Nacken et al., 2009) is placed in the freeboard.