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Potentials for utilisation of Pfbc ash
Environmental Aspects of Constnrcfion wifh Waste Materials JJJ.M. Goumans, H A . van der Sloot and Th.G. Aalbers (Editors) G.1994 Elsevier Science B.V. AN rights reserved.
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Potentials for utilisation of PFBC ash J. Rogbeck and P. Elander Swedish Geotechnical Institute, S-581 93 Linkoping, Sweden Abstract Pressurised Fluidised Bed Combustion (PFBC) is a coal combustion technique commercialised during the last ten years. Development of PFBC has been carried out for more than 20 years and has resulted in a combustion technique with low emissions to the atmosphere, producing ashes with favourable properties concerning utilisation. Two types of ash are produced, fly ash and spent bed material. For utilisation, a mix of the two ashes and water has been found to be of the greatest interest. In this paper, laboratory investigations and field tests carried out to clarify significant properties of PFBC ash mixtures in different utilisation objects are presented. It is concluded that the ash has excellent potentials for utilisation regarding mechanical properties as well as environmental effects. The paper is based on research kindly supported by ABB Carbon AB, EFO Coal & Oil AB, Sweden and Stockholm Energy AB.
1. THE PFBC TECHNIQUE Pressurised Fluidised Bed Combustion (PFBC) is a coal combustion technique which has been commercially developed by ABB Carbon AB. The development of PFBC has been carried out for more than 20 years and has resulted in a combustion technique with low emissions to the atmosphere and solid residues with favourable properties concerning utilisation. The PFBC technology uses a combined cycle, involving generation of electricity by a gas turbine and a steam turbine. This ensures high efficiency, giving something like 15 % lower fuel consumption than with conventional technologies. Combustion takes place in the fluidised bed at elevated pressure, 5-12 bar depending on load. The combustion temperature is about 820O to 880° C. This means that the nitrogen oxide emissions can be kept down, since thermal NO, is only formed at higher temperatu-res. Prior to combustion, the coal is crushed to a maximum size of about 5 mm and is then mixed with a sorbent (limestone or dolomite). During combustion, the sorbent will capture released sulphur to form calcium sulphate. Residues from a PFBC plant are created in three streams; as granular bed material, as fly ash captured by the cyclones and as filter catch from the final back end filter. The ratio between spent bed material and fly ash varies, but is normally between 50/50 and 30/70, with fly ash dominating. The amounts of ash from the back end filter are very small, less than 1 to 2 %, and are normally added to the fly ash from the cyclones. Regarding the grain size distribution, the fly ash can be characterised as sandy silt and the bed material as coarse sand.
590
2. MECHANICAL PROPERTIES The results from laboratory investigations and field tests presented in this paper are mainly based on a comprehensive study carried out in 1988 on ashes from the pilot plant at ABB Carbon in Finsping (I). The results have then been complemented with recent tests on ashes from the commercial Vartan PFBC plant in Stockholm. The bed material in the tests is mainly based on limestone as sorbent, althoug dolomite ashes have also been investigated. At an early stage in the investigations it was found that, even if spent bed material and fly ash can be used separately, a mix of them is of the greatest interest. If these ashes are mixed with water and vibro compacted, a concrete-like material with high bearing capacity and compressive strength is obtained. The results presented below refer accordingly to mixes of the ashes. Although the mixing ratio between bed material and fly ash has been varied in the investigations, the most common ratio has been 50/50 or 30/70 with fly ash dominating. In the tests, bed material and fly ash have almost always been mixed in a dry state before adding water. The amount of water varies, depending on whether the intention is to obtain a wetted mixture or a slurry. The samples are then vibro compacted. Curing normally takes place under high humidity (RH about 90 %) at a temperature near +20° C. Some test series have been cured under other conditions, such as under water or at a high (+50° C) or low ( - 1 8 O C) temperature. Curing times up to one year have been investigated. An important factor when estimating the potential for utilisation of fine grained combustion residues is their strength properties. For ashes, the strength is usually determined (at least in Sweden) as the unconfined compressive strength. This is normally assessed from samples with varying curing times so that a time strength relation can be achieved. The growth of compressive strength concerning PFBC ash mixes is relatively fast and values of over 5 MPa are achieved within a few days. The final strength is normally between 20 and 45 MPa, even if values over 50 MPa have been measured. Figure 1 presents examples of the development of strength of ash mixtures. In this case, has the ratio been 30 weight-% bed material and 70 weight-% fly ash (Polish coal, limestone as sorbent). Other results have shown that when dolomite is used as sorbent, the compression strength may decrease (2). Maximum measured values so far for dolomite based bed material vary between 15 and 25 MPa. However, the test series in this case are too small to be statistically significant.
Strength properties 100
Compression strength (MPa)
10
1
"I'
1
10
100
1000
Curing time (Days)
Figure 1. Examples of the development of strength of PFBC ash mixtures (70 %fly ash and 30 % bed material). Polish coal, limestone as sorbent.
591
To clarify the long term durability of the compression strength, one test series based on limestone as sorbent was cured for 28 days and then placed in pressurised cell permeameters. Water under high pressure (170 Wa) was forced through the samples for up to 8 months. The unconfined compression strength was then determined. The results indicated no decrease in strength compared with samples cured in the normal way. However, it should be observed that the high strength properties are very dependent on curing the mixture can be cured above freezing temperature until the hardening process is completed or at least has progressed for a certain time. In some laboratory and even full scale tests, the ash mixture has been exposed to temperatures below 0 C from the beginning or only after a few days of curing, which has resulted in almost no increase in strength. Results from freeze-thaw tests also show that the ash mixtures are sensitive if the curing time is too short before freezing. As a result of the concrete-like material obtained, the permeability of the ash mixtures is very low. Test results show that the permeability after 28 days curing is less than 1 0 1 0 d s . Se Figure 2.
Permeability 1OE-09
Permeability (m/s)
1OE-10
1OE-1 1 .___
I OE-12
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95100
Time (days) -50/50 bed ashhyclone ash *30/70 bed ash/cyclone ash --Swedish clay
Figure 2.
Permeability ofPFBC ash mixtures (Polish coal, limestone as sorbent, curing time 28 days).
In addition to the above mentioned parameters, a large number of laboratory tests have been carried out, for example to clarify the properties of synthetic aggregates produced from PFBC residues. These tests included compacting characteristics, degradation of aggregates, Swedish flakiness index and Swedish impact value. In general, the results have shown that after curing and crushing, PFBC ash mixtures are well suited for use as synthetic aggregates in fills and road building. 3. LEACHING CHARACTERISTICS
Like coal ash, PFBC ash mostly consists of a mixture of amorphous glass and crystalline phases and some unburnt material. The main components of the ash are compounds of silicon (Si), calcium (Ca), aluminium (Al) and iron (Fe). The desulphurisation process in a PFBC results in capture of sulphur as (CaSO,), while calcium surplus is obtained as calcium carbonate (CaCO,). The chemical composition of the ashes may vary to a certain extent, depending on the type of coal burned and the type of sorbent used (limestone or dolomite). If dolomite is used as
592
sorbent, the ash will also contain a certain amount of magnesium. In the 1988 study, a 50/50 mix of fly ash and bed material was analysed. The content of different trace elements in the mixture was generally less than 200 mgkg and for most of the elements less than 30 mgkg. In general, the environmental consequences of utilisation of residues are associated mainly with the release of contaminants, in particular trace elements, with percolating water. The possible release of contaminants from PFBC ashes were evaluated by means of leaching tests according to the Swedish ENA-method. This test is a serial batch test with four of leachings at the L/Sratio 4 (accumulated L/S-ratio 16) and with 24 h agitation by horizontal oscillation. Leaching at the L/S-ratio 1 is also determined. To achieve this build-up in L/S ratio concentration is used in the same procedure as above, but the material is renewed instead of the leachant. The particle size of materials used in ENA-tests should be <20 mm and the sample mass should be 125 g. Demineralized water adjusted to pH 4 by means of sulphuric acid is used as leachant. Centrifugation and 0.45 m filtration are used for leachate separation. The leachates in question were analysed for a considerable numbers of elements by means of ICP-ES. AAS were used for analysis of some leachates in order to achieve lower detection limits (for lead, mercury and cadmium). Leaching tests on PFBC ash were performed with 70/30 mixtures of fly ash and bed material (samples No 1-3) and 50/50. For the 70/30 mixture, tests were performed both with untreated samples and with samples that were cured and thereafter crushed to a gravel-like material (synthetcic agglomerates). For the other mixtures, tests were performed only on synthetic agglomerates. The ash showed a high buffering capacity and the pH was high in all the leachings, between 10 and 13, with the lower value in the leachates with higher L/S-ratios. In Figure 3, the accumulated leaching of Cr, Mo and Pb is given as a function of the L/S-ratio as determined in the successive leachings L/S 4 to L/S 16, and compared to results from identical leaching tests with ordinary pulverised fly ash, references (3) and (4), and a normal Swedish moraine (4). It should be mentioned that the L/S 12 leachate was not analysed in the tests, but the leaching in this step was estimated as the mean of the L/S 8 and the L/S 16 leachings. From the results, it appears that leaching from the synthetic agglomerates was in the same range as leaching from the untreated sample (No. 1). Neither were any significant divergences observed between samples with different mixing ratios. Possibly, the leaching from sample No. 3, which originated from combustion with ammonia in order to reduce the emissions of nitrogen oxides, was somewhat higher than from other samples. However, it is difficult to draw any firm conclusions as many elements in the leachates were below the detection limits. It was concluded that the leaching of chromium and molybdenum was higher than leaching of other trace elements from PFBC ash. From Figure 3, it can be seen that leaching of chromium was higher than from a normal moraine, but leaching of Cr as well as of Mo was lower than from the pulverised fly ash. Leaching of lead was in the same range as from PFA, but considerably lower than from the moraine. The reason for the high leaching of Pb from the moraine is not clear, but has been noticed for several samples from different places in southern Sweden (4). Leaching of other elements from the PFBC ash was low or very low. In Sweden, leachate levels from the L/S 1 leaching test have often been used for calculating leaching from various end-products in landfills and thereby estimation of the environmental consequences of disposal and utilisation. In Figure 4, maximum levels of some elements in leachate from PFBC are compared to results from identical tests with other materials. Data for pulverised coal fly ash (PFA) and coal bottom ash (BA) are taken from references (3), (4) and (5), and data for moraine from (4). Also normal background levels in unaffected fresh water, as suggested by the Swedish National Environmental Protection Agency (6) to be used in environmental impact assessments, and drinking water criteria in Sweden are shown in the figure. The elements shown are considered to be the most critical for PFBC ash from an environmental
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Cr ---
PFBC 1
--O-
PFBC 2
-*-
PFBC 3
--
PFBC 4
--
PFBC 5
1
10
100
PFA
-O-
LIS
Mo
Moraine
--
PFBC 1
/
PFBC 2
+
--c
PFBC 3
--
PFBC 4 PFBC 5
1
10
100
-*-
PFA
L/S
Pb
-
11
PFBC 1 PFBC 2
--
---
PFBC 3 PFA
1
10
100
Moraine
LIS
Figure 3. Accumulated leaching of chromium, molybdenum and lead as afirnction of the USratio for the PFBC ashes, a fly ash fiom pulverised coal combustion (PFA) and a normal Swedish moraine.
594
point of view, taking into account the leachate levels compared to background levels. For the elements shown, the leachate levels from the investigated PFBC ashes are in the same range as that measured in leachates from pulverised fly ashes. For other elements, the leachate levels have been generally low and in many cases not detected at the detection limits used (established for most elements by the ICP-ES technique). AS
i
.
PFBC
PFA
BA
Moraine
Cd
-I
PFBC
PFA
BA
Moraine
BA
Moraine
Cr
Moraine
- -
_-
-2
1000
I
0,l
Pb
- BACKGROUND LEVEL
-DRINKING WATER CRITERIA
-I
PFBC
PFA
Figure 4. Leachate levelsJEomPFBC ashes, other coal ashes and moraine, compared to background levels and drinking water criteria..
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4. POTENTIALS FOR UTILISATION
The residues from PFBC, in particular hardened mixtures of spent bed material and fly ash, have proved to be one of the important competitive edges of this valuable technology. Mechanical properties such as high strength, high bearing capacity and low permeability combined with a low environmental assessment, make PFBC residues well suited for a range of utilisation purposes. Some examples of commercial exploitation are; till material road construction material stabilising agent 4.1. Fill material
Due to their mechanical properties, residues from PFBC are well suited for a range of fills. By adding water to a mixture of spent bed material and fly ash and then vibro compacting the material, a monolithic fill with high strength can be obtained. Half-scale tests have demonstrated that this is possible even directly into water, as would be necessary for extending a harbour area for example. In the test, a mixture of spent bed material and fly ash was mixed in a dry state and poured into a basin filled with fresh water. The mixture was then vibro compacted under water. After one week of curing, standard penetration test (SPT) was performed on the fill. The results showed a coefficient of elasticity modulus of more than 300 MPa. Investigations are in progress both in Sweden (the Viirtan plant in Stockholm) and in Japan (the Wakamatsu plant) to clarify the possibilities for using PFBC ashes for land reclamation. Another way of using PFBC ash mixtures as fill material is to first produce synthetic aggregates from the material. This can be done by casting large slabs which are cured a certain time, after which the slabs are crushed in a conventional rock crushing plant. It is also possible to produce synthetic aggregates by manufacturing paving stones. This has already been performed on an industrial scale test. The brick-sized pieces obtained can be used either directly as fill material, or as synthetic gravel after crushing. One of the advantages is that production can take place during the firing season (normally the winter months), after which the bricks can easily be stored (cured) until the summer season when demand is greatest.
Casting of PFBC ash.
Brick production.
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4.2. Road construction material In the same way as mentioned above, synthetic aggregates produced from PFBC ash mixtures can be used as road construction material. Various full scale tests have already been performed. In one case, a mixture of spent bed material and fly ash was cast into a homogeneous slab which, after curing, was crushed to a coarse gravel. This was then used for an embankment and sub-base for an industrial road. The road was built in 1989 and the results so far are very good.
Test road in Linkoping, Sweden, uses synthetic gravel madefrom PFBC ashes. In a similar full-scale test, synthetic aggregates were used as road-base material on an industrial site at ABB in Finsphg. Above the road-base, a mixture of fly ash and conventional crushed bedrock was used as a sealing layer. The sealing layer was allowed to cure for about six weeks. All of the goods transports in the area were then made to cross the test area. Also in this case, the results proved very good even after a couple of years and no damage could be found on the surface. 4.3. Stabilising agent Due to their self-binding properties, PFBC residues, especially fly ash, are well suited as a stabilising agent. A project financed by the Swedish National Road Administration (7) showed that fly ash from PFBC can be of interest as a stabilising agent in the construction of embankments on soft andor fine grained soils, e.g. clay or silt. In the project, different stabilising agents were mixed with natural silt. The results showed that when adding 10 weight-% of pure PFBC fly ash to silt, the shear strength increased from 30 kPa to about 140 Ha. An additive of 15 weight-% fly ash resulted in a more than ten times higher shear strength (450 H a ) than for the natural silt. Consequently, the results indicated that using PFBC fly ash as a stabilising agent in embankments either increases the quality of the road or allows the thickness of the sub-base andor roadbase layers to be reduced. Similar tests have been performed to demonstrate the use of PFBC fly ash as a stabilising agent in mining with back filling. Due to the demands for a fast hardening time, the fly ash in this investigations was mixed with small amounts of cement. The results showed that PFBC fly ash increased the shear strength, Figure 5. This means that the conventional addition of cement can be reduced, resulting in lower costs for the back filling. Another interesting result from the laboratory investigations was that an additive of PFBC fly ash give a more plastic shear failure than conventional cement admixtures. This allows the fill to accommodate rock movements better before the mixture fails.
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Figure 5. Increase in shear strength as a function of time for mining backfill using PFBC jly ash as stabilising agent. A similar use of PFBC fly ash as stabilising agent is in lime columns. Fly ash can then be used as an additive to lime or cement, which will reduce the costs. However, before this is done on a commercial basis, complementary laboratory tests, especially concerning long-term durability, are required. Since PFBC residues can be used to produce a material which has a compressive strength comparable to that of concrete, it should be possible to use them as an additive in the manufacture of cement and concrete. One limiting factor when using ordinary coal fly ashes in the manufacture of concrete is the amount of unbumt carbon. Comprehensive measurements of fly ashes generated in Stockholm VWan indicate that during normal operation the unbumt carbon content is less than 3 %, which is well below the requirement for Sweden. The possibility of using PFBC residues in this way, although variable according to the feedstocks, would seem to be very high.
4.4 Environmental aspects If the ash is to be used in aggregates such as gravel, results from the leaching tests mentioned above may be used for environmental impact assessments. From the leaching tests, it can be concluded that the leaching of trace elements from PFBC ash is generally low. However, the results in relation to background levels indicate that leaching of chrome possibly could be problematical and should be observed. This can be illustrated by an environmental impact assessment performed for a planned utilisation object in Sweden, a fill for a reclamation harbour in Stockholm (8). The projected fill is small, about 10 000 m3. For conservative reasons, the leaching was calculated assuming a relatively high permeability, such as for silt, even if the hardening could lead to a much lower permeability. Calculations of the leaching from the fill, based on the leachate levels from the L/S 1 tests, showed that the leaching of chrome should be in the same range as the wet deposition on the surface of the close recipient (7 square kilometres), and that 520% of the discharge with urban run-off in the neighbourhood led directly to the recipient.
598 For the same fill, also a more probable scenario was calculated, on the assumption of a hardened fill of low permeability. For estimation of the leaching, tank tests were performed with ash from the VSirtan PFBC plant in order to determine leaching by diffusion from a monofill. An environmental assessment based on these assumptions showed a considerably lower leaching of chrome, about 5 % of the wet deposition on the close recipient and less than 1 % of the urban run-off in the area. The lower calculated leaching of chrome was probably related mainly to different leaching properties of the tested ash sample, compared to the ash samples investigated in the 1988 study, and not only to the different test procedures. A simple batch test on the same sample, performed with L/S ratio 2, resulted consequently in a chrome level considerably lower than that from L/S 1 tests as well as from L/S 4 tests in the 1988 study. The relatively high leaching of chrome calculated for the first case is a consequence of the high permeability of an unhardened fill in combination with changing water levels, causing frequent pore water exchanges in the fill. This effect would be even more pronounced if the fill were constructed with synthetic agglomerates, because of the higher permeability of such a fill. Leaching of a permeable fill constructed on land would be considerably lower, since the amount of water flowing through the fill decreases, and the release of chrome would consequently be less important in relation to other sources. Recent leaching tests indicate also that leaching of chrome may not be of the same importance for all ashes, but can vary. Varying leaching properties may, for instance, depend on the type of coal burned and the combustion conditions. Finally, it should be emphasised that leachate levels measured in laboratory leaching tests should be used with caution when predicting leachate levels and environmental impact from real fills. For instance, Figure 4 shows that also leaching tests on conventional filling materials, such as normal moraine, result in leachate levels in the same range or higher than the tested PFBC ashes, disregarding chrome. Taking this into account, the summarised test results indicate that PFBC ash in most utilisation objects causes only a limited impact on the environment. 6. REFERENCES
1 Rogbeck, J. (1988). Utilisation of residues from PFBC. Swedish Geotechnical Institute Dnr. 1-227/88. In Swedish. 2 Rogbeck, J. (1991). Investigation of strength of PFBC ash from VSirtaverken. Swedish Geotechnical Systems AB, No 9107. In Swedish. 3 Hartlkn, J. Elander, P. Kullberg, S. Lundgren, T. & RosCn, B. (1986). Residues from semi-dry flue gas desulphurisation. REFORSK FoU nr 10. In Swedish. 4 Nilsson, C. (1987). Residues from fludised bed combustion - properties in disposal and utilisation. Stiftelsen for v h e t e k n i s k forskning, No 276. 5 The Swedish Coal Health Environment Project (1983). The Swedish State Power Board. 6 Swedish National Environmental Protection Agency (1990). Basis for forming estimates for lakes and watercourses. SNV AllmSinna Rid 90:4. In Swedish. 7 Elander, P. (1991). Soil stabilisation for road construction. Laboratory study on the effects of some stabilising agents. Swedish Geotechnical Institute, Dnr 1 -366/89. In Swedish. 8 Elander, P & Rogbeck J. (1992). Filling with ash in Stockholm harbour. Swedish Geotechnical Systems AB, No 9107. In Swedish.
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Potentials for utilisation of PFBC ash J. Rogbeck and P. Elander Swedish Geotechnical Institute, S-581 93 Linkoping, Sweden Abstract Pressurised Fluidised Bed Combustion (PFBC) is a coal combustion technique commercialised during the last ten years. Development of PFBC has been carried out for more than 20 years and has resulted in a combustion technique with low emissions to the atmosphere, producing ashes with favourable properties concerning utilisation. Two types of ash are produced, fly ash and spent bed material. For utilisation, a mix of the two ashes and water has been found to be of the greatest interest. In this paper, laboratory investigations and field tests carried out to clarify significant properties of PFBC ash mixtures in different utilisation objects are presented. It is concluded that the ash has excellent potentials for utilisation regarding mechanical properties as well as environmental effects. The paper is based on research kindly supported by ABB Carbon AB, EFO Coal & Oil AB, Sweden and Stockholm Energy AB.
1. THE PFBC TECHNIQUE Pressurised Fluidised Bed Combustion (PFBC) is a coal combustion technique which has been commercially developed by ABB Carbon AB. The development of PFBC has been carried out for more than 20 years and has resulted in a combustion technique with low emissions to the atmosphere and solid residues with favourable properties concerning utilisation. The PFBC technology uses a combined cycle, involving generation of electricity by a gas turbine and a steam turbine. This ensures high efficiency, giving something like 15 % lower fuel consumption than with conventional technologies. Combustion takes place in the fluidised bed at elevated pressure, 5-12 bar depending on load. The combustion temperature is about 820O to 880° C. This means that the nitrogen oxide emissions can be kept down, since thermal NO, is only formed at higher temperatu-res. Prior to combustion, the coal is crushed to a maximum size of about 5 mm and is then mixed with a sorbent (limestone or dolomite). During combustion, the sorbent will capture released sulphur to form calcium sulphate. Residues from a PFBC plant are created in three streams; as granular bed material, as fly ash captured by the cyclones and as filter catch from the final back end filter. The ratio between spent bed material and fly ash varies, but is normally between 50/50 and 30/70, with fly ash dominating. The amounts of ash from the back end filter are very small, less than 1 to 2 %, and are normally added to the fly ash from the cyclones. Regarding the grain size distribution, the fly ash can be characterised as sandy silt and the bed material as coarse sand.
590
2. MECHANICAL PROPERTIES The results from laboratory investigations and field tests presented in this paper are mainly based on a comprehensive study carried out in 1988 on ashes from the pilot plant at ABB Carbon in Finsping (I). The results have then been complemented with recent tests on ashes from the commercial Vartan PFBC plant in Stockholm. The bed material in the tests is mainly based on limestone as sorbent, althoug dolomite ashes have also been investigated. At an early stage in the investigations it was found that, even if spent bed material and fly ash can be used separately, a mix of them is of the greatest interest. If these ashes are mixed with water and vibro compacted, a concrete-like material with high bearing capacity and compressive strength is obtained. The results presented below refer accordingly to mixes of the ashes. Although the mixing ratio between bed material and fly ash has been varied in the investigations, the most common ratio has been 50/50 or 30/70 with fly ash dominating. In the tests, bed material and fly ash have almost always been mixed in a dry state before adding water. The amount of water varies, depending on whether the intention is to obtain a wetted mixture or a slurry. The samples are then vibro compacted. Curing normally takes place under high humidity (RH about 90 %) at a temperature near +20° C. Some test series have been cured under other conditions, such as under water or at a high (+50° C) or low ( - 1 8 O C) temperature. Curing times up to one year have been investigated. An important factor when estimating the potential for utilisation of fine grained combustion residues is their strength properties. For ashes, the strength is usually determined (at least in Sweden) as the unconfined compressive strength. This is normally assessed from samples with varying curing times so that a time strength relation can be achieved. The growth of compressive strength concerning PFBC ash mixes is relatively fast and values of over 5 MPa are achieved within a few days. The final strength is normally between 20 and 45 MPa, even if values over 50 MPa have been measured. Figure 1 presents examples of the development of strength of ash mixtures. In this case, has the ratio been 30 weight-% bed material and 70 weight-% fly ash (Polish coal, limestone as sorbent). Other results have shown that when dolomite is used as sorbent, the compression strength may decrease (2). Maximum measured values so far for dolomite based bed material vary between 15 and 25 MPa. However, the test series in this case are too small to be statistically significant.
Strength properties 100
Compression strength (MPa)
10
1
"I'
1
10
100
1000
Curing time (Days)
Figure 1. Examples of the development of strength of PFBC ash mixtures (70 %fly ash and 30 % bed material). Polish coal, limestone as sorbent.
591
To clarify the long term durability of the compression strength, one test series based on limestone as sorbent was cured for 28 days and then placed in pressurised cell permeameters. Water under high pressure (170 Wa) was forced through the samples for up to 8 months. The unconfined compression strength was then determined. The results indicated no decrease in strength compared with samples cured in the normal way. However, it should be observed that the high strength properties are very dependent on curing the mixture can be cured above freezing temperature until the hardening process is completed or at least has progressed for a certain time. In some laboratory and even full scale tests, the ash mixture has been exposed to temperatures below 0 C from the beginning or only after a few days of curing, which has resulted in almost no increase in strength. Results from freeze-thaw tests also show that the ash mixtures are sensitive if the curing time is too short before freezing. As a result of the concrete-like material obtained, the permeability of the ash mixtures is very low. Test results show that the permeability after 28 days curing is less than 1 0 1 0 d s . Se Figure 2.
Permeability 1OE-09
Permeability (m/s)
1OE-10
1OE-1 1 .___
I OE-12
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95100
Time (days) -50/50 bed ashhyclone ash *30/70 bed ash/cyclone ash --Swedish clay
Figure 2.
Permeability ofPFBC ash mixtures (Polish coal, limestone as sorbent, curing time 28 days).
In addition to the above mentioned parameters, a large number of laboratory tests have been carried out, for example to clarify the properties of synthetic aggregates produced from PFBC residues. These tests included compacting characteristics, degradation of aggregates, Swedish flakiness index and Swedish impact value. In general, the results have shown that after curing and crushing, PFBC ash mixtures are well suited for use as synthetic aggregates in fills and road building. 3. LEACHING CHARACTERISTICS
Like coal ash, PFBC ash mostly consists of a mixture of amorphous glass and crystalline phases and some unburnt material. The main components of the ash are compounds of silicon (Si), calcium (Ca), aluminium (Al) and iron (Fe). The desulphurisation process in a PFBC results in capture of sulphur as (CaSO,), while calcium surplus is obtained as calcium carbonate (CaCO,). The chemical composition of the ashes may vary to a certain extent, depending on the type of coal burned and the type of sorbent used (limestone or dolomite). If dolomite is used as
592
sorbent, the ash will also contain a certain amount of magnesium. In the 1988 study, a 50/50 mix of fly ash and bed material was analysed. The content of different trace elements in the mixture was generally less than 200 mgkg and for most of the elements less than 30 mgkg. In general, the environmental consequences of utilisation of residues are associated mainly with the release of contaminants, in particular trace elements, with percolating water. The possible release of contaminants from PFBC ashes were evaluated by means of leaching tests according to the Swedish ENA-method. This test is a serial batch test with four of leachings at the L/Sratio 4 (accumulated L/S-ratio 16) and with 24 h agitation by horizontal oscillation. Leaching at the L/S-ratio 1 is also determined. To achieve this build-up in L/S ratio concentration is used in the same procedure as above, but the material is renewed instead of the leachant. The particle size of materials used in ENA-tests should be <20 mm and the sample mass should be 125 g. Demineralized water adjusted to pH 4 by means of sulphuric acid is used as leachant. Centrifugation and 0.45 m filtration are used for leachate separation. The leachates in question were analysed for a considerable numbers of elements by means of ICP-ES. AAS were used for analysis of some leachates in order to achieve lower detection limits (for lead, mercury and cadmium). Leaching tests on PFBC ash were performed with 70/30 mixtures of fly ash and bed material (samples No 1-3) and 50/50. For the 70/30 mixture, tests were performed both with untreated samples and with samples that were cured and thereafter crushed to a gravel-like material (synthetcic agglomerates). For the other mixtures, tests were performed only on synthetic agglomerates. The ash showed a high buffering capacity and the pH was high in all the leachings, between 10 and 13, with the lower value in the leachates with higher L/S-ratios. In Figure 3, the accumulated leaching of Cr, Mo and Pb is given as a function of the L/S-ratio as determined in the successive leachings L/S 4 to L/S 16, and compared to results from identical leaching tests with ordinary pulverised fly ash, references (3) and (4), and a normal Swedish moraine (4). It should be mentioned that the L/S 12 leachate was not analysed in the tests, but the leaching in this step was estimated as the mean of the L/S 8 and the L/S 16 leachings. From the results, it appears that leaching from the synthetic agglomerates was in the same range as leaching from the untreated sample (No. 1). Neither were any significant divergences observed between samples with different mixing ratios. Possibly, the leaching from sample No. 3, which originated from combustion with ammonia in order to reduce the emissions of nitrogen oxides, was somewhat higher than from other samples. However, it is difficult to draw any firm conclusions as many elements in the leachates were below the detection limits. It was concluded that the leaching of chromium and molybdenum was higher than leaching of other trace elements from PFBC ash. From Figure 3, it can be seen that leaching of chromium was higher than from a normal moraine, but leaching of Cr as well as of Mo was lower than from the pulverised fly ash. Leaching of lead was in the same range as from PFA, but considerably lower than from the moraine. The reason for the high leaching of Pb from the moraine is not clear, but has been noticed for several samples from different places in southern Sweden (4). Leaching of other elements from the PFBC ash was low or very low. In Sweden, leachate levels from the L/S 1 leaching test have often been used for calculating leaching from various end-products in landfills and thereby estimation of the environmental consequences of disposal and utilisation. In Figure 4, maximum levels of some elements in leachate from PFBC are compared to results from identical tests with other materials. Data for pulverised coal fly ash (PFA) and coal bottom ash (BA) are taken from references (3), (4) and (5), and data for moraine from (4). Also normal background levels in unaffected fresh water, as suggested by the Swedish National Environmental Protection Agency (6) to be used in environmental impact assessments, and drinking water criteria in Sweden are shown in the figure. The elements shown are considered to be the most critical for PFBC ash from an environmental
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point of view, taking into account the leachate levels compared to background levels. For the elements shown, the leachate levels from the investigated PFBC ashes are in the same range as that measured in leachates from pulverised fly ashes. For other elements, the leachate levels have been generally low and in many cases not detected at the detection limits used (established for most elements by the ICP-ES technique). AS
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4. POTENTIALS FOR UTILISATION
The residues from PFBC, in particular hardened mixtures of spent bed material and fly ash, have proved to be one of the important competitive edges of this valuable technology. Mechanical properties such as high strength, high bearing capacity and low permeability combined with a low environmental assessment, make PFBC residues well suited for a range of utilisation purposes. Some examples of commercial exploitation are; till material road construction material stabilising agent 4.1. Fill material
Due to their mechanical properties, residues from PFBC are well suited for a range of fills. By adding water to a mixture of spent bed material and fly ash and then vibro compacting the material, a monolithic fill with high strength can be obtained. Half-scale tests have demonstrated that this is possible even directly into water, as would be necessary for extending a harbour area for example. In the test, a mixture of spent bed material and fly ash was mixed in a dry state and poured into a basin filled with fresh water. The mixture was then vibro compacted under water. After one week of curing, standard penetration test (SPT) was performed on the fill. The results showed a coefficient of elasticity modulus of more than 300 MPa. Investigations are in progress both in Sweden (the Viirtan plant in Stockholm) and in Japan (the Wakamatsu plant) to clarify the possibilities for using PFBC ashes for land reclamation. Another way of using PFBC ash mixtures as fill material is to first produce synthetic aggregates from the material. This can be done by casting large slabs which are cured a certain time, after which the slabs are crushed in a conventional rock crushing plant. It is also possible to produce synthetic aggregates by manufacturing paving stones. This has already been performed on an industrial scale test. The brick-sized pieces obtained can be used either directly as fill material, or as synthetic gravel after crushing. One of the advantages is that production can take place during the firing season (normally the winter months), after which the bricks can easily be stored (cured) until the summer season when demand is greatest.
Casting of PFBC ash.
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4.2. Road construction material In the same way as mentioned above, synthetic aggregates produced from PFBC ash mixtures can be used as road construction material. Various full scale tests have already been performed. In one case, a mixture of spent bed material and fly ash was cast into a homogeneous slab which, after curing, was crushed to a coarse gravel. This was then used for an embankment and sub-base for an industrial road. The road was built in 1989 and the results so far are very good.
Test road in Linkoping, Sweden, uses synthetic gravel madefrom PFBC ashes. In a similar full-scale test, synthetic aggregates were used as road-base material on an industrial site at ABB in Finsphg. Above the road-base, a mixture of fly ash and conventional crushed bedrock was used as a sealing layer. The sealing layer was allowed to cure for about six weeks. All of the goods transports in the area were then made to cross the test area. Also in this case, the results proved very good even after a couple of years and no damage could be found on the surface. 4.3. Stabilising agent Due to their self-binding properties, PFBC residues, especially fly ash, are well suited as a stabilising agent. A project financed by the Swedish National Road Administration (7) showed that fly ash from PFBC can be of interest as a stabilising agent in the construction of embankments on soft andor fine grained soils, e.g. clay or silt. In the project, different stabilising agents were mixed with natural silt. The results showed that when adding 10 weight-% of pure PFBC fly ash to silt, the shear strength increased from 30 kPa to about 140 Ha. An additive of 15 weight-% fly ash resulted in a more than ten times higher shear strength (450 H a ) than for the natural silt. Consequently, the results indicated that using PFBC fly ash as a stabilising agent in embankments either increases the quality of the road or allows the thickness of the sub-base andor roadbase layers to be reduced. Similar tests have been performed to demonstrate the use of PFBC fly ash as a stabilising agent in mining with back filling. Due to the demands for a fast hardening time, the fly ash in this investigations was mixed with small amounts of cement. The results showed that PFBC fly ash increased the shear strength, Figure 5. This means that the conventional addition of cement can be reduced, resulting in lower costs for the back filling. Another interesting result from the laboratory investigations was that an additive of PFBC fly ash give a more plastic shear failure than conventional cement admixtures. This allows the fill to accommodate rock movements better before the mixture fails.
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Figure 5. Increase in shear strength as a function of time for mining backfill using PFBC jly ash as stabilising agent. A similar use of PFBC fly ash as stabilising agent is in lime columns. Fly ash can then be used as an additive to lime or cement, which will reduce the costs. However, before this is done on a commercial basis, complementary laboratory tests, especially concerning long-term durability, are required. Since PFBC residues can be used to produce a material which has a compressive strength comparable to that of concrete, it should be possible to use them as an additive in the manufacture of cement and concrete. One limiting factor when using ordinary coal fly ashes in the manufacture of concrete is the amount of unbumt carbon. Comprehensive measurements of fly ashes generated in Stockholm VWan indicate that during normal operation the unbumt carbon content is less than 3 %, which is well below the requirement for Sweden. The possibility of using PFBC residues in this way, although variable according to the feedstocks, would seem to be very high.
4.4 Environmental aspects If the ash is to be used in aggregates such as gravel, results from the leaching tests mentioned above may be used for environmental impact assessments. From the leaching tests, it can be concluded that the leaching of trace elements from PFBC ash is generally low. However, the results in relation to background levels indicate that leaching of chrome possibly could be problematical and should be observed. This can be illustrated by an environmental impact assessment performed for a planned utilisation object in Sweden, a fill for a reclamation harbour in Stockholm (8). The projected fill is small, about 10 000 m3. For conservative reasons, the leaching was calculated assuming a relatively high permeability, such as for silt, even if the hardening could lead to a much lower permeability. Calculations of the leaching from the fill, based on the leachate levels from the L/S 1 tests, showed that the leaching of chrome should be in the same range as the wet deposition on the surface of the close recipient (7 square kilometres), and that 520% of the discharge with urban run-off in the neighbourhood led directly to the recipient.
598 For the same fill, also a more probable scenario was calculated, on the assumption of a hardened fill of low permeability. For estimation of the leaching, tank tests were performed with ash from the VSirtan PFBC plant in order to determine leaching by diffusion from a monofill. An environmental assessment based on these assumptions showed a considerably lower leaching of chrome, about 5 % of the wet deposition on the close recipient and less than 1 % of the urban run-off in the area. The lower calculated leaching of chrome was probably related mainly to different leaching properties of the tested ash sample, compared to the ash samples investigated in the 1988 study, and not only to the different test procedures. A simple batch test on the same sample, performed with L/S ratio 2, resulted consequently in a chrome level considerably lower than that from L/S 1 tests as well as from L/S 4 tests in the 1988 study. The relatively high leaching of chrome calculated for the first case is a consequence of the high permeability of an unhardened fill in combination with changing water levels, causing frequent pore water exchanges in the fill. This effect would be even more pronounced if the fill were constructed with synthetic agglomerates, because of the higher permeability of such a fill. Leaching of a permeable fill constructed on land would be considerably lower, since the amount of water flowing through the fill decreases, and the release of chrome would consequently be less important in relation to other sources. Recent leaching tests indicate also that leaching of chrome may not be of the same importance for all ashes, but can vary. Varying leaching properties may, for instance, depend on the type of coal burned and the combustion conditions. Finally, it should be emphasised that leachate levels measured in laboratory leaching tests should be used with caution when predicting leachate levels and environmental impact from real fills. For instance, Figure 4 shows that also leaching tests on conventional filling materials, such as normal moraine, result in leachate levels in the same range or higher than the tested PFBC ashes, disregarding chrome. Taking this into account, the summarised test results indicate that PFBC ash in most utilisation objects causes only a limited impact on the environment. 6. REFERENCES
1 Rogbeck, J. (1988). Utilisation of residues from PFBC. Swedish Geotechnical Institute Dnr. 1-227/88. In Swedish. 2 Rogbeck, J. (1991). Investigation of strength of PFBC ash from VSirtaverken. Swedish Geotechnical Systems AB, No 9107. In Swedish. 3 Hartlkn, J. Elander, P. Kullberg, S. Lundgren, T. & RosCn, B. (1986). Residues from semi-dry flue gas desulphurisation. REFORSK FoU nr 10. In Swedish. 4 Nilsson, C. (1987). Residues from fludised bed combustion - properties in disposal and utilisation. Stiftelsen for v h e t e k n i s k forskning, No 276. 5 The Swedish Coal Health Environment Project (1983). The Swedish State Power Board. 6 Swedish National Environmental Protection Agency (1990). Basis for forming estimates for lakes and watercourses. SNV AllmSinna Rid 90:4. In Swedish. 7 Elander, P. (1991). Soil stabilisation for road construction. Laboratory study on the effects of some stabilising agents. Swedish Geotechnical Institute, Dnr 1 -366/89. In Swedish. 8 Elander, P & Rogbeck J. (1992). Filling with ash in Stockholm harbour. Swedish Geotechnical Systems AB, No 9107. In Swedish.