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Title:
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Conversion of biomass, prediction and solution methods for ash agglomeration and related problems: contract JOR3-95-0079, Final report 1 March 1996 to 1 March 1999
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Author(s):
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Published by:
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Publication date:
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ECN
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1-11-1999
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ECN report number:
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Document type:
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ECN-C--99-090
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ECN publication
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Number of pages:
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Full text:
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89
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Download PDF
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Abstract:
When biomass is used as fuel for thermal conversion plants, minerals fromthe fuel can be responsible for major problems. Generally, these problems are
associated with the existence and development of low melting compounds or
eutectics, which form sticky layers. In a fluidised bed, this can result in
bed-agglomeration and defluidisation. This causes local high temperature,
which often accelerates the process. It ultimately can lead to a completely
sintered bed content with a glassy phase gluing the bed particles together
and shut-down of the plant. The main objective of the title project is to
develop a methodology to predict ash/bed agglomeration and sintering
problems, to indicate related problems and, furthermore, to identify solution
methods to make different types of biomass streams more viable for energy
production. Within the present study, selected fuels are subjected to
different existing methods together with some new ones, in order to determine
the agglomeration temperature. The selected fuels are verge grass, Danish
wheat straw (both stored dry and partly leached due to rainfall), sewage
sludge, cacao shells and willow as a reference. The methods used within the
study are chemical analysis of fuel and ashes, determination of standard ash
melting temperatures, compression strength measurements of the ash, DTA/TG
analysis of the ash, SEM and ESEM (high temperature environmental scanning
microscopy), two different lab-scale bubbling fluidised bed combustion
facilities, a lab-scale bubbling fluidised bed gasifier and a circulating
fluidised bed gasifier. The lab-scale facilities have been used to test
potential measures to reduce the problem of agglomeration. These measures are
the use of additives (kaolin, magnesite, dolomite, gibbsite and sewage
sludge) and non-quartz bed materials (alumina and mullite). The work
performed within the project has lead to the following results. Chemical
analysis of the fuel can give a first indication of whether there might be an
agglomeration problem during thermal conversion. In general a high K-content
means an increased risk for agglomeration. However, the K-content alone is
not a good indicator. Also chlorine proved to be very important. From the
methods used in the project, laboratory fluid bed agglomeration experiments
seem to give the most reliable information about conditions and temperatures
where agglomeration takes place. Contrary to methods like DTA, compression
strength and ash melting temperatures, all processes that. might be relevant
for agglomeration actually can occur during fluid bed experiments: fuel-bed
material interactions, volatilization and condensation, shear forces,
temperature homogeneity and accumulation. Standard tests have been developed
where process temperature is gradually increasing until agglomeration. These
tests have been applied in the project at three different laboratories: ECN
(Netherlands), ETC (Sweden) and VTT (Finland). They have proved to be
accurate and reproducible. It has been shown that the addition of kaolin,
magnesite, dolomite and sewage sludge significantly reduce the risk of
agglomeration. The agglomeration temperatures increased with at least 60C in
these cases. During combustion experiments, measured particle temperatures
appeared to be up to 100C higher than the bed temperature. This might have a
large influence on agglomeration. Because gasification, contrary to
combustion, is a process where peak temperatures are lower or even absent,
one might expect that agglomeration during gasification will occur at higher
temperatures than during combustion. This however can not be concluded from
the experiments, illustrating the importance of other factors like the design
of fluid beds (the gas distribution, the type of nozzles, etc.). It can be
concluded that not only the type of fuel and other chemical 'input' is
determining the agglomeration temperature, also other factors like gas
distribution, size of bed material, type of nozzles, cyclone efficiency in
CFBs, etc. can have an important role. This means that results from lab-scale
facilities can be interpreted in a relative way (comparing fuels and evaluate
possible solution) but should always be used with great care when trying to
draw conclusions for full-scale plants. Nevertheless, standardised lab-scale
bubbling fluidized bed experiments, as developed and used in this project,
seem to be the most reliable tools for the prediction of agglomeration. 119
refs.
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