DISPOSAL AND UTILISATION OF ASH RESIDUES FROM PRESSURISED FLUIDISED BED COMBUSTION. Final report

Directorate-General For Energy

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• Publié par: DIRECTORATE-GENERAL-FOR-ENERGY
• Nombre de lectures: 53
• Langue: English
• Poids de l'ouvrage: 3 Mo

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Commission of the European Communities
technical coal research
DISPOSAL AND UTILISATION
OF ASH RESIDUES
FROM PRESSURISED FLUIDISED
BED COMBUSTION
Report
EUR 13050 EN
Blow-up from microfiche original Commission of the European Communities
technical coal research
DISPOSAL AND UTILISATION
OF ASH RESIDUES
FROM PRESSURISED FLUIDISED
BED COMBUSTION
BRITISH COAL CORPORATION
Coal Research Establishment
Stoke Orchard
UK-Cheltenham, Glos. GL52 4RZ
Contract No 7220-EA/817
FINAL REPORT
Directorate-General Energy
1991 EUR 13050 EN Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Telecommunications, Information Industries and Innovation
L-2920 LUXEMBOURG
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting on behalf
of then is responsible for the use which might be made of the following
information
Catalogue number: CD-NA-13050-EN-C
© ECSC — EEC - EAEC, Brussels - Luxembourg, 1991 III
SYNOPSIS
Pressurised fluidised bed combustion (PFBC) is being developed as a possible
alternative to the existing pulverised fuel (PF) generation system as it can
operate at higher efficiency and with control of sulphur dioxide and
nitrogen oxides emissions. An environmentally acceptable means of ash
disposal is an essential requirement of any coal-fired power generation
scheme and as part of the overall PFBC development programme, a study has
been carried out to assess the environmental effects of disposing of PFBC
ash, to .consider suitable ash disposal schemes and to investigate its
utilisation potential in commercial outlets.
The PFBC ashes examined during the study were obtained from combustion
ι
trials undertaken on the 80 MWth PFBC facility at the Grimethorpe PFBC
Establishment and a 2 MWth PFBC unit at the Coal Research Establishment.
The major differences in chemical composition between PFBC ash and ash
from conventional power stations, PFA, are the higher levels of calcium (and
magnesium, if dolomite is used as S0„ sorbent) and sulphate within PFBC ash.
As the combustion temperature for PFBC (typically 850CC) is relatively low,
little, if any, fusion occurs and the ash is more crystalline than PFA, with
the principal phases being calcium sulphate anhydrite, calcium carbonate,
calcium oxide and magnesium oxide. Calcium-bearing silicates (plagioclase
and diopside) were also detected in the ash by X-ray diffraction.
In contrast to residues from atmospheric FBC systems, a significant
proportion of the residual limestone sorbent remains as calcium carbonate
within PFBC ash and the calcium oxide (free lime) content is generally
lower. Ash from the Grimethorpe plant which, of the ashes examined, was
considered to be more representative of material produced on a commercial
plant, had a free lime content <1Z.
The most abundant trace elements in PFBC ash were usually barium and
manganese and the least abundant were cadmium, mercury and selenium. The
concentrations of trace elements were generally in the same range as for PFA
and were present at levels at which they would be considered to pose little
potential health hazard with respect to ingestion or inhalation of the ash. IV
The results of a cytotoxicity assay, a rat lung inflammation assay and
the Ames plate incorporation assay confirmed that the fibrogenic and
mutagenic/carcinogenic potential associated with inhaling the ash should be
low and, at this stage, there should be no basis for classifying PFBC ash
other than as a 'nuisance dust'.
The chemical characteristics of PFBC ash are such that it should not be
deemed a hazardous or special waste under current UK legislation.
PFBC fines are coarser than PFA and the bed ash is somewhat finer than
the other offtake from a PF plant, furnace bottom ash. The optimum moisture
content values for compaction were similar to those of PFA but the maximum
dry densities were higher, which indicates that the disposal volume
requirement should be less for PFBC ash. The shear strength parameters
(cohesion, 'c' and angle of shearing resistance'φ') also compared favourably
with those of PFA and ash mounds constructed from PFBC ash should be
intrinsically more stable at current design slopes and heights.
Simulated conditioning tests indicated that no flash setting or
pelletising occurred when conditioning PFBC fines with 15% water at ash
temperatures up to 200°C. Therefore conventional PFA conditioning plant
should be suitable as a means of suppressing air-borne dust. The generation
of heat during conditioning, as a result of hydration of the ash, should
also not be a problem
PFBC ash self-hardens when wetted and this was mainly attributed to the
hydration of the calcium sulphate anhydrite to gypsum and, to a lesser
extent, to the hydration of calcium silicates. However, significant
self-hardening, in the short term, only occurred when the ash was well
compacted and this should not pose particular difficulties in handling
conditioned ash or in pumping ash slurries.
PFBC ash could be disposed of by the wet (ash slurry to lagoons) and
dry (landfilling of conditioned ash) routes already established for PFA,
though the latter route is likely to be the preferred option. In the wet
disposal route, because of the potential alkalinity of PFBC ash, the pH of
lagoon effluent would need to be monitored to ensure the discharge consent
limit (typically, pH not greater than 9) was being met. Though lagoon
effluent from typical PFBC ash (free lime content <1Z) may not require
treatment, schemes for reducing the pH of highly alkaline effluent were
still investigated. Three schemes for controlling the high pH arising from
lime dissolved in the effluent were identified: treatment with sulphuric
acid, mixing with large volumes of cooling water from the station or river water (reaction of calcium bicarbonate in the water with dissolved lime) and
forced aeration (reaction of C0„ with dissolved lime). All three schemes
were considered feasible and the most appropriate option, from a technical
and economic point of view, is likely to be dependent on site-specific
factors.
The leaching of soluble components from the ash is generally considered
the most important aspect when assessing the environmental effects of ash
disposal and, in this study, the leaching behaviour of PFBC ash was examined
by laboratory shake and column leaching tests and field lysimeter
experiments. Data from these experiments, particularly those under field
conditions, indicated that the generation of leachate of high pH from
typical PFBC ash (free lime <1Z) may be somewhat less of a potential problem
than with PFA. The predominant dissolved component in the leachate was
generally calcium sulphate and this maintains a concentration at its
saturation value as leaching continues in the long term. The more soluble
calcium, potassium and sodium components, such as chloride, were leached out
more rapidly and this initial flush increased the total dissolved solids
content of the initial leachate, but only to a level similar to that in
corresponding PFA leachate.
The overall trace element content of PFBC ash leachate was low, with
typically only molybdenum (Mo) being present in any significant
concentration (>1 mg/i,). In general the leachability of trace elements in
PFBC ash was lower than in PFA.
The self-hardening property of PFBC ash results in rapid reductions in
permeability and values of 10 ' cm/sec were typically achieved. This
reduction in permeability would minimise the rate of leachate generation
from a PFBC ash deposit and thereby limit the pollutant loading effect of
the leachate.
Overall it is considered that leachate from PFBC ash should generally
not have a significant adverse environmental impact in most disposal
situations, and it should pose no more of an environmental problem than that
associated with PFA.
In the disposal of ash from power stations, steps are usually taken to
restore the surface of the disposal site to agricultural or amenity use and
conditions requiring the inclusion of a full landscape scheme are routinely
added to planning consents for ash disposal. Plant growth experiments were
undertaken in a greenhouse and in the field to investigate factors
influencing vegetation establishment and growth on PFBC ash and how this
affects landscape development. Fertiliser availability problems and VI
phytotoxic effects from trace species are unlikely to be encountered with
PFBC ash. The constraints to plant growth were high pH, salinity, poor
water retention capability and, in particular, its self-hardening
properties. Despite these poor growth conditions, with the use of topsoil
or amendments, adequate surface vegetative cover can be established and
successful restoration and landscaping of PFBC ash disposal sites should be
achievable.
A preliminary examination was made of the disposal of PFBC ash with
colliery spoil. The incorporation of ash with spoil improved the
engineering properties (compressive and shear strength) of the material
which could introduce an additional safety factor in rela