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Title:
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Verkennende evaluatie kwaliteitsbeïnvloeding poederkoolvliegas : bijstoken van biomassa in een poederkoolcentrale of bijmenging van biomassa-assen met poederkoolvliegas
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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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1-8-2000
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ECN report number:
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Document type:
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ECN-C--00-058
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ECN publication
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Number of pages:
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Full text:
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88
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Download PDF
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Abstract:
In this literature survey the consequences of co-firing of biomass andmixing of biomass ash with coal fly ash on the coal fly ash quality is
evaluated. Biomass ash considered in this context is produced by
gasification, pyrolysis or combustion in a fluidized bed. The irregular shape
of biomass ash obtained from gasification, pyrolysis or combustion has a
negative influence on the water demand in concrete applications of the coal
fly ash resulting from mixing biomass ash and coal fly ash. In case of
co-firing, high concentrations of elements capable of lowering the ash
melting point (e.g. Ca and Mg) may lead to more ash agglomeration. This leads
to a less favourable particle size distribution of the coal fly ash, which
has a negative impact on the water demand in cement bound applications.
Gasification, pyrolysis and combustion may lead to significant unburnt carbon
levels (>10%). The unburnt carbon generally absorbs water and thus has a
negative influence on the water demand in cement-bound applications. The
contribution of biomass ash to the composition of coal fly ash will not be
significantly different, whether the biomass is co-fired or whether the
biomass ash is mixed off-line with coal fly ash. The limit values for Cl, SO4
and soluble salts can form a limitation for the use of coal fly ash
containing biomass for cement-bound applications. As side effects of biomass
co-firing, the level of constituents such as Na, K, Ca and Mg may lead to
slagging and fouling of the boiler. In addition, a higher emission of flue
gas contaminants As, Hg, F, Cl and Br may be anticipated in case more
contaminated biomass streams are applied. This may also lead to a higher
contamination level of gypsum produced from flue gas cleaning residues.
Relatively clean biomass streams (clean wood, cacao shells, etc.) will hardly
lead to critical levels of elements from a leaching point of view. More
contaminated streams, such as sewage sludge, used and preserved wood, petcoke
and RDF (refuse derived fuels), will most likely lead to increased leaching.
This will be more prominent for oxyanions than for metals. In the evaluation
of the application of coal fly ash in cement production or in partial cement
replacement, it is important to assess the materials behaviour in recycling
stages in unbound form besides its leaching behaviour of the intact product
in its service life. This aspect has not been addressed before. If a material
performs poorly from an environmental point of view in its recycling stages,
one should be more critical in allowing (too) high levels of co-firing or too
high mixing ratios of biomass. In general, the oxyanions will be more
critical than most metals. The variability in several types of biomass is
rather high. This holds limitations for plant operation and availability.
Premixing of biomass during size reduction leads to more consistent input and
thus to more constant ash quality. Co-firing may lead to increased Cr-VI
levels in the fly ash due to oxidation of Cr, which is more prominently
present in flue gas upon biomass co-firing than in case of regular coal
firing. Elevated Cr-VI levels are more leachable. The following
recommendations have been made: measurement of leaching behaviour of coal fly
ash from co-firing of different biomass streams with a special emphasis on
Cr-VI leachability; measurement of flue gas quality relative to pure coal
combustion during co-firing of contaminated biomass streams; verification of
durability of cement-based products containing coal fly ash with ash from
biomass; evaluation of the leaching behaviour of recycling products from the
primary uses of biomass ash or fly ash. 35 refs.
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