Utilization of coal ash from fluidized-bed combustion boilers as road base material

Utilization of coal ash from fluidized-bed combustion boilers as road base material

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Resources, Conservationand Recycling 14 (1995) 69-77


conservation and recycling

Utilization of coal ash from fluidized'bed combustion boilers as road base material T. Takada a,*, I. Hashimoto a, K. Tsutsumi a, y . Shibata ", S. Yamamuro a, T. Kamada b, K. Inoue c, K. Tsuzura d, K. Yoshida e "Research and Development Office, Kawasaki Heavy Industries Ltd., Akashi-shi 673, Japan b Center for Coal Utilization, Okubo 2-3-4, Shinjuku-ku, Tokyo 169, Japan c Nippon Hodo I/o. Ltd., Kyobashi 1-19-11, Chuo-ku, Tokyo 104, Japan d Naruto Salt Mfg. Ltd., Muya-cho, Naruto-shi 772, Japan c Department of Chemical Engineering, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

Accepted 17 January 1994


A process has been developed to utilize the large quantities of ash discharged from coal-fired fluidized-bed combustion boilers as road base material. Since the 1980s, the fluidized-bed boiler has been the subject of much study because it can burn fuels such as anthracite that cannot be burnt in pulverized coal-fired boilers and other conventional boilers, and because it eliminates the need for a flue gas desulfurizing facility. However, the disposal of fluidized-bed combustion ash is increasingly difficult, and therefore costlier, because of tighter legislation on waste management. Meanwhile, the excavation of road base material is also subject to stricter regulations due to fears of environmental impact. This has caused a shortage of material for road construction in some areas. It would be useful if ash from fluidized-bed combustion could be processed into road base material. There would be an enormous demand for such a product. A simple and low-cost road base material production process has, therefore, been developed where the ash is mixed with water, molded, steamed, aged and crushed into small pieces. This paper discusses the conditions required to yield optimum products and the paving test results. Keywords: Coal ash; Fluidized combustion;Road construction

1. I n t r o d u c t i o n

More than 550 million tonnes o f coal ash were produced in 1992 ( I E A Coal Information), more than 10 million t of it by fluidized-bed combustion ( F B C ) boilers. In Japan, the total * Correspondingauthor. Elsevier Science B.V. SSD10921-3449 (94) 00008-S


T. Takada et al. /Resources, Conservation and Recycling 14 (1995) 69-77

production of coal ash in 1992 was about 6 million t (Coal Note) of which 0.5 million t came from FBC boilers. This huge volume of coal ash poses problems because it is increasingly difficult to find suitable disposal sites and because it is necessary to take the environmental impact, such as coastal pollution due to land reclamation, into consideration. There is a pressing need to develop technologies to make effective use of this waste. Various processes have been suggested: two possibilities are its use as cement material and light-weight aggregate. These applications alone, however, cannot possibly bring about a major reduction in the amount of waste, yet many countries currently face difficulties in disposing of the ash. The ash from coal-fired FBC boilers contains calcined limestone employed for desulfurization and calcium sulfate, a byproduct of that process. Therefore, when the ash is mixed with water then steam-processed to accelerate strength development, hydration takes place, turning the coal ash into a hard solid substance. Noting this characteristic, we started research on a process to convert this coal ash into road base material, and have already completed a pilot plant in Naruto City, Tokushima Prefecture to produce such material with a capacity of 70 t/day. This report introduces the results of the studies conducted so far and the performance of a test road using crushed solid produced by the plant.

2. Properties of coal-fired FBC ash

Ash from coal-fired FBC boilers which use limestone as sorbent can be divided into three types based on the particle size: bed material offtake ash with a median diameter of 1 mm removed from the bottom of the boiler, multicyclone discharge ash having a median diameter of 100 p~m,and bag filter ash having a median diameter of 10/xm. The general characteristics of the coal ash can be described briefly: (a) the ash consists of both coal ash and spent sorbent from desulfurization such as CaO and CaSO4; (b) the ash is highly porous and active because the FBC temperature is low, around 800-900°C; (c) because of the above, the coal ash is hydration-responsive (see [ 1 ] ). In the present study, two kinds of imported coal A and B were employed. Table 1 shows the chemical composition of the mixtures of bed material ash, multicyclone discharge ash and bag filter ash. Table 1 Chemicalcompositionof fluidized-bedcombustionash Type of coal

Chemicalcomposition Char

Coal A 100% Coal A 20% + coal B 80% Coal A 40% + coal B 60%

Coal ash








30.0 13.0

24.4 33.2

16.2 19.1

6.9 16.0

11.3 9.05

1.0 1.45

10.2 8.20








T. Takada et al. / Resources, Conservation and Recyclb~g 14 (1995) 69-77


3. S o l i d i f i c a t i o n test Table 2 summarizes the solidification characteristics o f the coal ash shown in T a b l e 1. Coal ash which contains calcined limestone and calcium sulfate can be turned into a very hard substance by m i x i n g it with water, leaving it to pre-cure at r o o m temperature (the Table 2 Solidification characteristics of fluidized-bed combustion ash Type of coal

Coal A 100% Coal A 20% + coal B 80% CoalA40% + coal B 60%

Coal ash

Solidified ash

Blaine's specific activity" surface.area (cm2/g) (ml/100g)

bulk density (g/cm 3)

compressive strength (kg/cm 2)

9540 9950

152 383

1.51 1.67

73 225





'q~heactivity was obtained by keeping 100 g of test specimens at 40°C, dripping I N HCI into each specimen, and by measuring the HCI consumption after 10 min when the pH of the sl~ecimen was 8.2.

Fig. I. Micrograph of the solid.


1". Takada et al. / Resources, Conservation and Recycling 14 (1995) 69-77

temperature rises to 20--400C during the pro-curing period because the reaction is exothermic) for about 6 h, then exposing it to a steam environment at 60°C for 12 h for curing. This is because the chemical reaction shown below produces ettringite which consists of needle-shaped crystals that entangle with each other as shown in Fig. 1: AI2Oa + ( 3CaO + 3CaSO4) + 32H20 --*3CaO. A12Oa•3CASO4.32H20 Coal ash

Spent sorbent



A test was conducted to study the effects of various factors upon compressive strength by employing three types of ash at the typical mixing ratio ( 10% bed material offtake ash, 30% multicyclone discharge ash and 60% bag filter ash).

~ 250

- 400 300




200 .~ A: Activity O: Compressive strength







lOO ~

20 0

CaO in coal ash [%] Fig. 2. Effects of CaO content in coal ash on compressive strength.

250 200





1.65 1


o~ I00 50


1.50 ~ A: O: Compressive strength








Char in coal ash [%] Fig, 3. Effects of char in coal ash on compressive strength.

T. Takada et al. / Resources, Conservation and Recycling 14 (1995) 69-77


250 200 150 .t,.l



50 1.50

1 1.55



Bulk density [g/cm3] Fig. 4. Effectsof bulkdensityof the solidon compressivestrength.

CaO content in ash Fig. 2 shows the relationship between the amount of CaO, ash activity and compressive strength of the solid. It is found that to attain a compressive strength of 150 kg/cm 2, which is a prerequisite for a road base material, it is necessary to have at least 10% of CaO in the coal ash. Less CaO produces too little ettringite. On the other hand, excessive CaO, 20% or more, hinders the interlocking of ettringite. Char in coal ash The relationship between char in the ash, the bulk density and compressive strength of the solid is shown in Fig. 3. The more char in the ash, the less the compressive strength of the solid. This is because the more char there is, the lower the bulk density of the solid, preventing the ettringite crystals from interlocking. It is necessary, therefore, to limit the char in the ash to less than 20%. Bulk density of the solid substance Fig. 4 shows the relationship between the compressive strength and the bulk density of the solid. It can be seen that it is necessary to keep the bulk density above 1.65 g/cm 3 to obtain a compressive strength over 150 kg/cm 2. Standing time after mixing The relationship between the pre-curing time and the compressive strength of the solid substance is shown in Fig. 5. If the pre-curing time when the mixture is left at room temperature after mixing is too short, many cracks are generated during the curing period, preventing full compressive strength from being reached. Preventing trace elements from leaching from combustion ash In some cases, a minute amount of harmful heavy metals is contained in the combustion ash. It has been found that the leaching of heavy metals from the solid form is well below the J-IS reference level for landfill, and even below detectable levels. This is because these metals are chemically locked inside the ettringite generated during the hydration reaction.


T. Takada et aL / Resources, Conservation and Recycling 14 (1995) 69-77 O: Pre-curing temp. 20°C A:

. 200 I 1801160

i 140 r~


I 1
















Pre-curing time [h] Fig. 5. Effectsof pre-curingtimeon compressivestrength. 4. Mass production


The following procedure was tested to clarify the solidification characteristics in the mass production process. Ash mixture Bed material offtake ash: 3 kg Multicyclone discharge ash: 9 kg Bag filter ash: 18 kg 12-13.5 kg of water is poured in over a period of about 1 rain at a water/ash ratio of 0.40--0.45:1. Mixing Capacity: 75 1 High-speed mixer (350 rpm) Mixing time: 3 rain Molding Concrete block machine Mold size: 300 X 300 X 100 (h) mm Production time: 20 s per cycle Molding pressure: 1-2 kg/cm 2 Vibration time: 4 s Pre-euring 20"C for 6 h Curing 60"C for 12 h Product 300 X 300 X 100 (h) mm block Fig. 6 shows a view of the blocks obtained in the above procedure. The blocks have a bulk density of 1.65 or greater and a compressive strength above 150 kg/cm 2. These tests confirm that a mass production process is possible.

T. Takada et al. / Resources, Consera,ation and Recyclhzg 14 (1995) 69-77


Fig. 6. External view of the product. 5. T e s t i n g as r o a d b a s e m a t e r i a l Physical properties

Crushed stone up to 37.5 mm diameter was obtained from the blocks for use as road base material. The product met all the requirements of the Ministry of Construction for road base material and had the following physical properties in comparison with the reference crushed stone M-30 (grain size below 37.5 mm): (a) the maximum density in the compaction test is less than one half that of M-30 (dry basis); (b) it has great hydraulicity; (c) it absorbs much water-up to 45% by weight. Test road construction

A test road was constructed, 7.5 m wide and 56 m long divided into two sections, one 25 m long using M-30 standard crushed stone and the other 31 m long using the crashed stone produced from combustion ash. Both sections had a surface layer 5 cm thick on a road base 25 cm thick. This test road had a simplified construction to allow test results to be obtained in a short period: actual road paving standards in Japan require a total thickness of 41 cm. The test period was from December 1985 to December 1988. As the crushed stone from the combustion ash tends to break and become finer, its compaction strength develops well


M-30 crushed stone

o= 4



],11 ~ ~ _

0 I I 01 3

- °








O~C~SS I:1" ~ a sh9


Time [months] Fig. 7. Change in deflectionwith time.




T. Takada et al. /Resources, Conservation and Recycling 14 (1995) 69--77








o N ~

M-30 crushed stone

/ .....






20 10


Processed ash









6 12 Time [months] Fig. 8. Changein 2.5-ramsievingweightpercentagewithtime. and the finished surface is smoother. Visual observation and comments from the construction workers indicated that the two road base materials had equal workability. 48





I l I











M-30 crushed stone /i!


24 20

/ //



12 8



01 3





Processed ash


/ w 24

Time [months] Fig. 9. Changein the crack ratiowithtime.

o_ I 36

T. Takada et al. / Resources, Conservation and Recycling 14 (1995) 69-77


Fig. 7 shows the change in deflection with time. The crushed stone from the combustion ash had a drastically reduced road deflection, which may be attributed to its high hydraulieity. Samples taken from the road were loosened and sieved. The sieving results are shown in Fig. 8, and the change in cracking against time is shown in Fig. 9. Localized cracks on the test road in the area of the crushed stone made from combustion ash were noted during the first month; however, very few new cracks developed after 6 months. Meanwhile, on the test road where the M-30 was used, cracks started to appear after 3 months and continued to develop until the end of the test period, with ultimately about 5-times more cracks than in the crushed stone from the combustion ash. These test results lead us to believe that crushed stone made from combustion ash is a useful substitute for the standard M-30 crushed stone.

6. Development of an actual plant As a result of these tests, the Center for Coal Utilization, Naruto Salt Mfg., Nippon Hodo Co., and Kawasaki Heavy Industries started joint construction of a plant in October 1992, subsidized by the Japanese Government. The plant was scheduled for completion in September 1993; ash will be supplied from a coal-fired FBC boiler owned by Naruto Salt Mfg. and the plant will be capable of fully automatic and continuous maintenance-free operation. The main specifications of the plant under construction are a production capacity of 70 t/day (at the condition of dry ash surface), producing a crashed stone equivalent to M-30. The total market in Japan alone for such products is some 1000 billion t/year.

7. Conclusions The results of the fundamental tests started in 1982 have now been put into practice. As demand for energy sources increases, the demand for coal, especially in developing countries, is expected to grow and the production of coal combustion ash will also continue to increase. It is hoped that this process will help utilize the ash that is otherwise regarded simply as waste and eventually contribute to betterment of the global environment.

Acknowledgements This work, in particular the leaching test, was partly supported by research funds from the Electric Power Development Company.

References [ 1] Yasuda, M. and Yamada, K., 1992. A review of ash utilization technology. J. Energy Inst. Japan, 71 (8): 743.