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Fluidized-bed combustion of oil shale retorting solid waste.

Background

The most important local solid fuel in the Republic of Estonia is oil shale (OS). It is used mainly as a fuel in electricity production, but also processed thermally to produce crude oil. A minor part is processed using solid heat-carrier technology [1], the main part in vertical retorts.

Semicoke (SC) is a harmful waste of the last process. Its annual production in different plants altogether is almost 1 million tonnes. The overall amount of SC in dumps is about 100 million tonnes. SC is characterized by a considerable carbon content (10-12%), a remaining energetic potential of 2.5-4 MJ/kg, and a high content of mineral matter (60-70%). The sulphur content of SC is 2.2-2.7%, and it is present in different forms, where sulphides (among them watersoluble ones) as well as organic and pyritic sulphur compounds prevail.

Due to the high content of different harmful compounds [2], the leachates of SC dumps in the amount of about 6,000-7,000 [m.sup.3] per day, containing 300-500 mg/L phenols, including relatively stable 5-methylresorcinol and p-cresol, sulphide ions, polyaromatic hydrocarbons and other toxic compounds [3], are one of the greatest sources of contamination of surroundings and water in the region close to the Finnish Gulf and cause a high load on the wastewater purification system. So, SC is a large-scale waste with a high remaining energetic value, its use for landfilling is environmentally dangerous and due to high and growing taxes (about 20% per year) economically burdensome.

The most effective solution of these problems would be development of the retorting process in a way allowing to achieve complete usage of the OS organic part in the main process. Another promising way to utilize the remaining organic part of SC could be its combustion allowing to convert the waste material into an energy source and eliminating the emission of toxic pollutants. The task of the present work was to analyze the prospects of utilizing the energetic potential of SC by combustion.

Utilization of SC as a Fuel

Taking into account the actual location of fuel consumers in the North-Eastern part of Estonia (Ida-Virumaa county) the following possibilities were under discussion:

* pulverized firing of SC in the existing boilers at AS Narva Elektrijaamad (Narva Power Plants Ltd.)

* using SC as additional fuel in cement clinker production at Nordic Cement Plant

* combustion of SC in specially designed boilers with circulating fluidized bed (CFB) at the place of its formation

Combustion of SC in the Existing Pulverized-Firing Boilers

Because of the same origin of SC and OS, there should be no specific cocombustion problems, but problems related to fuel and ash handling as well as to exhaust gas devices could rise due to higher capacities. The problems of pulverized firing (PF) of SC mixed with oil shale were discussed in paper [4], where for SC the calorific values [Q.sup.d.sub.b] = 1.2 and 4.4 MJ/kg were used as the basis for calculations.

It was shown that PF of SC in power plants is technically feasible, but it deteriorates the exploitation data of boilers. It leads to an increase in the fuel load, increasing the total amount of waste products even more (Fig. 1). Fuel transport and handling problems would rise, especially in wintertime. Economical efficiency is disputable while transportation of SC from oil plants to power plants is expensive. Expenses to fuel handling as well as to ash removal and flue gas extraction will also grow, respectively. The efficiency of boiler goes down and to keep heat production at the targeted level it is necessary to increase fuel load, which, in turn, increases the amount of waste products. Furthermore, this approach does not solve all the problems of the oil producer either. In conclusion, this utilization scheme is inexpedient and can be recommended only as an exception.

SC as an Alternative Fuel in Cement Production

Industrial-scale tests carried out at Kunda Nordic Cement Plant in 2001-2002 showed that adding SC to fuel in a certain proportion is possible without secondary negative effects on the process and production quality. However, for stabilizing fuel calorific value on a fixed level it is necessary to increase the percentage of expensive coke in the fuel mixture. It leads, taking into account also additional expenses on the transport of SC from the oil plant in Kohtla-Jarve to Kunda and some additional fuel handling expenses, to the increase in fuel cost. If this question finds an answer, it will be possible to utilize, according to the estimations of cement producers, 100-150 thousand tonnes of SC annually. Consequently, by these means the problem of SC utilization could not be solved totally.

Test Combustion in Fluidized-Bed Device

Earlier Data and Experience

The idea to utilize and defuse SC by combustion in fluidized bed (FB) is not quite new. Laboratory-scale investigations have been made by Uuesoo [5] and Kuusik et al. [6-9], and pilot-scale experiments on OS : SC mixtures with the ratios from 100 : 0 to 25 : 75 performed under supervision of Martins [10]. Analysis and generalization of the existing data made by Opik and Jefimov resulted in the recommendation to combust SC at Kohtla-Jarve and Kivioli plants using the CFB technique [11]. Arro et al. [4] have compared and analyzed different possibilities of SC combustion. Combustion of mixtures with low oil shale content has not been investigated. The objective of this work started in 2002 was gaining and updating the experience in burning pure SC and its mixture with small additions of OS in fluidized bed.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Experimental

The initial samples were supplied by AS Viru Keemia Grupp (VKG--Viru Chemistry Group Ltd.) at the town of Kohtla-Jarve (Table 1). During the last test combustion in 2002, the mass ratio of the fuel mixtures OS : SC was varied from 20 : 80 to 0 : 100 (LHV 4.66-6.06 MJ/kg--Table 2). The layout of the 50-[kW.sub.th] test rig used is shown in Fig. 2.

Fuel was supplied from the hopper 2 to the furnace 1. The primary air was preheated in the air preheater 8 and directed to the furnace through the furnace bottom grate 3. The bottom ash was removed from the furnace by an ash screw. The screw feeder of fuel and ash screw operated synchronously, and it was possible to change smoothly the turns of the ash screw and thus to adjust the height of fluidized bed in the furnace. The fly ash precipitated from flue gases in the cyclone 4 was partially recycled to the furnace together with secondary air by an ejector-type air transporting system. The amount of circulating ash depended on the secondary airflow. Flue gases from the furnace and fly ash were cooled down in two cooling towers 5 (by water) and 6 (by secondary air). Fly ash precipitated from flue gases partially in the cooling towers 5 and 6; the final cleaning of gases was carried out with a bag filter.

The test device was started by preheated primary air ([t.sub.p.air] = 380-400[degrees]C) directed through the OS and ash mixture on the grate. After ignition of OS and complete warm-up of the test plant by burning pure OS, the fuel mixture was fed into the furnace. The actual test started when the temperature of fluidized bed had settled down at the given level, the OS ash in fluidized bed was replaced by the ash of the respective fuel mixture, and the ashes accumulated in cooling towers during firing-up were removed from the unit.

The experiments for studying the S[O.sub.2]-binding efficiency of the samples were carried out using Q-derivatograph (MOM, Hungary) under isothermal conditions in the air--S[O.sub.2] flow at 700[degrees]C during 30 min. A multi-plate platinum crucible was used, the weight of samples was 100 mg, thickness of the sample layer 0.2-0.3 mm, the rate of gas flow 270 mL per min and S[O.sub.2] partial pressure in the air--S[O.sub.2] mixture 190 mm Hg [12].

Results of the Tests

The experimental data of the principal tests are given in Tables 2 and 3. The time curves of temperatures fixed during test 1 are given in Fig. 3. It can be seen that, for example, during the test at the average temperature in fluidized bed 830-850[degrees]C a good temperature distribution was achieved--temperature in the freeboard along its height differed from the that of fluidized bed by about [+ or -] 40 K.

[FIGURE 3 OMITTED]

The following conclusions can be drawn:

* A stable combustion regime in fluidized bed was obtained at temperatures of about 800-850[degrees]C (see Fig. 3). During the test the percentage of primary air was 60-65% of the total air amount introduced into the furnace, and the velocity of gases in the freeboard was 3.1-3.4 m/s. The heat load of the furnace cross-section was 1.47-1.86 [MW.sub.th]*[m.sup.-2]*[h.sup.-1].

* 15-24% of the ash was bottom ash, 76-85% fly-ash (cyclone ash, ash from cooling towers and the bag filter). 91-96% of the total ash formed was captured in the test plant.

* The burn-up factor of fuel carbon at burning SC and OS mixtures was 95-98% in the bottom ash, 91-96% in the cyclone ash and 89-93% in the bag filter ash. This shows that the higher residence-time of fuel particles in the furnace is a prerequisite for their complete burning.

* The test burning confirmed the essential impact of combustion temperatures on the decomposition rate of carbonate compounds in fuel ash--at the fluidized bed temperature 855[degrees]C it was 0.84 (Ca[O.sub.free] = = 21.1%) and at 787[degrees]C 0.44 (Ca[O.sub.free] = 14.6%), respectively.

* The content of sulphide sulphur in the ashes removed from the test plant was low, only 0-0.05%. The ashes of cyclone and bag filter did not contain any sulphide sulphur when burning pure SC. Complete oxidation of sulphide sulphur requires longer residence time of ash particles in the furnace and appropriate conditions (O2 availability and temperature).

* Determination of the reactivity of ashes towards S[O.sub.2] confirm their high S[O.sub.2]-binding ability, which is a good prerequisite for elimination of S[O.sub.2] emissions as compared to the data of burning pure OS (Fig. 4).

* Pure SC of low moisture content [W.sup.r] < 10% can be burnt in fluidized bed in a stable mode. At [W.sup.r] > 10% an addition of OS in the amount of 10-20% is needed. In the last case the ash amount formed is about 2.7-4 times higher as compared to the burning of OS to get the same heat capacity. Therefore, introduction of more efficient fuel feeding and ash removal systems in FB boilers is required.

* The ashes formed were tested in order to estimate their S[O.sub.2]-binding ability. These experiments showed that the S[O.sub.2]-binding rate of the bottom ash formed at SC combustion is comparable with that of the bottom ash formed at OS combustion under CFB conditions (see Fig 4). It means that nearly full binding of S[O.sub.2] that occurs at OS combustion tests in CFB device can be foreseen also at SC combustion.

[FIGURE 4 OMITTED]

Utilization of Shale Oil Production Waste in CFB Boiler

The results of earlier test firings show that the CFBC technology is suitable for combustion of Estonian OS [13]. The atmospheric CFBC technology may be recommended also for utilization of SC, which is by composition of the same origin as OS. In addition, this technology is suitable regarding fuel quality. Whereas the composition of ash and conditions in fluidized-bed boiler do not differ from those of OS test firing, it could be presumed that similar binding of S[O.sub.2] will occur. These considerations were confirmed by the results of the SC combustion tests described above.

For designing new CFB systems for utilizing OS retorting wastes with residual energetic potential (fine fractions of OS, SC, retorting gas), the heat amounts forming at combustion must be known. Calculations to estimate the amount of heat available for production needs or for generating electrical power in the possible process of utilizing SC were carried out using the processing data of VKG. The calorific value (LHV) of OS and of its fines (~9% from total OS) taken for calculations was [Q.sup.r.sub.i] = 11.05 MJ/kg. The wet discharge of SC is used in existing retorts, so in the calculations [W.sup.r] = 25% was taken for the basis. The amount of SC per tonne OS is 600 kg, and the calorific value of SC is [Q.sup.r.sub.i] = 4.12 MJ/kg (on dry basis). The amount of wet SC per tonne OS is 800 kg, and recalculated calorific value according to the above moisture content is [Q.sup.r.sub.i] = 2.47 MJ/kg. The amount of generator gas per tonne oil shale in the calculations was 450 [m.sup.3], the calorific value of generator gas 3.9 MJ/[m.sup.3]. The calculations made for the capacity 700,000 t/y (80 t/h) of wet SC and 525,000 t/y (59 t/h) of dry SC see in more detail in [4].

The results of calculations show that the heat output of a new boiler-utilizer using wet semicoke and operating at efficiency 80% will be 105 [MW.sub.th]. The heat output of the boiler will be 114 [MW.sub.th] if it operates at the same efficiency and uses dry semicoke. The increase is 9%, approximately. Physical heat of hot (500[degrees]C) semicoke adds 7 [MW.sub.th] to the boiler's heat output, and the total heat output will be 121 [MW.sub.th] (Table 4).

Conclusions

1. It has been shown that SC could be fired in pulverized form in the mixture with OS in existing PF boilers, but it deteriorates their exploitation data. This utilization scheme is inexpedient and can be recommended as an exception.

2. Utilization of SC in cement production by adding it to fuel in a certain proportion is possible without secondary negative effects on the process and production quality. However, the amounts that could be utilized are moderate, and by these means the problem of SC utilization cannot be solved totally.

3. The first test combustion of pure SC and its mixtures with small additions of OS was performed on the 50-[kW.sub.th] test rig with CFB. The main technical parameters of the burning process were elaborated, mass balances and chemical composition of ash as well as its S[O.sub.2]-binding ability were determined and calculated. It was shown that pure SC with low moisture content [W.sup.r] < 10% can be burnt in fluidized bed in a stable mode. At [W.sup.r] > 10% an addition of OS in the amount of 10-20% is needed.

4. The amount of heat that could be produced by combusting energy-containing waste of OS retorting process was calculated. The heat output of a new boiler-utilizer using wet SC and operating at the efficiency of 80% will be 105 [MW.sub.th]. Heat output of the boiler will be 114 [MW.sub.th] if it operates at the same efficiency and uses dry SC (increase 9%). Utilisation of the physical heat of hot (500[degrees]C) SC adds 7 [MW.sub.th] to the boiler's heat output and increases the total heat output up to 121 [MW.sub.th].

5. On the basis of the results of test combustion of OS and its mixtures with SC the atmospheric CFBC technology can be recommended as the most convenient one regarding both fuel quality and environmental safety. It is the most promising solution for utilizing OS processing waste at the place of its formation. To enhance the efficiency of utilization it is reasonable to modify the SC discharge from wet to dry. Using physical heat of SC is recommendable; for that the distance between boiler-utilizer and OS retort must be minimized.

Acknowledgements

The study was partly funded by Joint USA--Estonia Oil Shale Research and Utilization Program.

Received April 22, 2004

REFERENCES

[1.] Golubev, N. Solid heat-carrier technology for oil shale retorting // Symposium on Oil Shale, 18-21 November 2002, Tallinn : Abstracts. P. 45-47.

[2.] Jefimov, V., Doilov, S., Pulemyotov, I. Development of ecologically acceptable technology for processing large-particle kukersite in vertical retorts // Oil Shale 1997. Vol. 14, No. 1. P. 77-83.

[3.] Trapido, M., Munter, R., Kallas, J. Oil shale ash dumps waste water as polycyclic aromatic hydrocarbons and phenols pollution source. Short communications of Estonian 6th Conf. in Ecology "Problems of Contemporary Ecology". Tartu, 24-26 April 1994. P. 203-205 [in Estonian].

[4.] Arro, H., Prikk, A., Pihu, T., Opik, I. Utilization of semicoke from Estonian oil industry // Oil Shale. 2002. Vol. 19, No. 2. P. 117-125.

[5.] Uuesoo, R. Combustion of oil shale residue // Proc. Acad. Sci. Estonian SSR. Tech. and Phys.-Math. 1956. Vol. 1. P. 69-81 [in Russian].

[6.] Kuusik, R., Veiderma, M. Combustion of oil shale semicoke at the fluidized bed conditions // Goryuchie Slantsy / EstNIINTI. 1977. No. 9. P. 16-19 [in Russian].

[7.] Kuusik, R., Kaljuvee, T., Trikkel, A. Utilization of oil shale semicoke and its ash : Report of Tallinn Technical University. 027F.--Tallinn, 2001 [in Estonian].

[8.] Kaljuvee, T., Kuusik, R., Trikkel, A. S[O.sub.2]-binding into the solid phase at thermooxydation of blends based on Estonian oil shale semicoke // J. Therm. Anal. Cal. 2003. Vol. 72. P. 393-404.

[9.] Trikkel, A., Kaljuvee, T., Kuusik, R. S[O.sub.2]-binding at the combustion of oil shale in the mixture with semicoke // The Baltic Sea and its Environment: ESTO-96 Twin Symp. Tallinn, Aug. 9, 1996 / Estonian Academy Publishers, Tallinn, 1997.

[10.] Martins, A., Pesur, A., Kallaste, E., Siirde, S. Co-combustion of oil shale semicoke and oil shale mixtures in fluidized bed : ESF Grant No. 2210 Report, Tallinn, 1997. P. 4.

[11.] Opik, I., Yefimov, V. An analysis of the RAS "Kiviter" energy balances and development plans // Oil Shale. 1995. Vol. 3, No. 3. P. 247-258.

[12.] Kaljuvee, T., Trikkel, A., Kuusik, R. Decarbonization of natural lime-containing materials and reactivity of calcined products towards S[O.sub.2] and C[O.sub.2] // J. Therm. Anal. Cal. 2001. Vol. 64. P. 1229-1240.

[13.] Prikk, A., Arro, H. Circulating fluidized-bed technology--the technology exact for Estonian oil shale // Oil Shale. 1997. Vol. 14, No. 3 Special. P. 209-214.

Presented by J. Kann

R. KUUSIK (a), * (2), A. MARTINS (b), T. PIHU (c), A. PESUR (b), T. KALJUVEE (a), A. PRIKK (c), A. TRIKKEL (d), H. ARRO (c)

(a) Department of Chemical Engineering, (b) Estonian Energy Research Institute, (c) Department of Thermal Engineering, (d) Department of Chemistry

Tallinn University of Technology 5 Ehitajate Rd., 19086 Tallinn, Estonia

* (1) Presented at Symposium on Oil Shale in Tallinn, Estonia, November 18-21, 2002.

* (2) Corresponding author: e-mail rkuusik@edu.ttu.ee
Table 1. Characterization of Oil Shale and Semicoke Samples
Used in Tests

Item OS SCI SCII

Moisture content [W.sup.r], % 6.10 8.10 9.90
Ash [A.sup.d], % 49.40 68.90 68.70
Carbonate [(C[O.sub.2]).sup.d], % 16.30 16.90 17.89
[C.sup.d], % 32.74 17.02 16.32
[H.sup.d], % 4.33 1.14 1.05
[S.sub.total.sup.d], % 2.00 1.70 1.80
Calorific value in the bomb, MJ/kg 13.21 4.92 4.22
[S.sup.d] in the bomb residue, % 1.14 0.96 0.99
Non-disintegrated carbonates in the 4.04 10.00
bomb residue, %

Table 2. Combustion Test Data

 SCI/OS = SCII/OS = SCI/OS =
Description 80/20 (%) 90/10 (%) 100/0 (%)

 without with without with with
 bag bag bag bag bag
 filter filter filter filter filter

Fuel mixture:

LHV Qir, MJ/kg 6.06 4.83 4.66
Moisture Wr, % 11.33 7.42 6.15
Ash content 65.0 66.77 68.9
 Ad, %
Flow rate B, 16.24 16.42 17.7 16.8 16.88
 kg/h
Pressure in air 112 105 143 146 142
 box,
 mm[H.sub.2]O
Draft, Pa 46.5 38.3 55 42 16

Primary air:

 [m.sup.3]/h 26.63 25.67 22.95 23 23
 % 63.5 59.63 64.56 64.19 64

Secondary air:

 [m.sup.3]/h 15.31 17.38 12.6 12.83 12.94
 % 36.5 40.37 35.44 35.81 36
Gas velocity 3.4 3.1 3.14
 in
 freeboard,
 m/s
Fluidised bed 857 855 826 787 793
 temperature
 (EPR-09M),
 [degrees]C

Ash balance,
 %:

 Bottom ash 21.69 15.19 21.7 24.16 20
 Cyclone ash 53.61 53.8 56.6 60.4 56.36
 Cooling 12.05 10.76 10.38 6.71 7.27
 chamber I
 Cooling 8.43 9.49 7.55 4.03 7.27
 chamber II
 Bag filter 4.22 10.76 3.77 4.7 9.1

 Total
 separated
 ash:

 kg 16.6 15.8 10.6 14.9 16.5
 % 100 100 100 100 100
 kg/h 7.49 8.62 10.43 9.93 10.31

Separation rate, 80 91.07 95.24 95.64 94.5
%

Average 0.76 0.62 0.74
decomposition
rate of
carbonates

Table 3. Characterization of SC and OS Mixtures, Pure SC
and Ashes Taken from Different Points of the Test Rig, %

Description Moisture Mineral Heating Ca
 [W.sup.r] C[O.sub.2] loss [O.sub.total]

SCI/OS =
80/20 (%)

Fuel 11.33 15.02 36.74 22.06
Ash:

bottom 2.56 3.70 40.86
cyclone 4.04 4.99 33.60
bag filter 0.44 3.65 5.33 29.29

SCII/OS =
90/10 (%)

Fuel 7.42 16.92 34.37 25.26
Ash:

bottom 10.59 10.82 42.14
cyclone 6.41 6.69 33.13
bag filter 0.33 3.10 5.14 27.12

SCI/OS =
100/0 (%)

Fuel 6.15 15.75 31.56 25.31
Ash:

bottom 6.76 7.08 38.66
cyclone 3.80 5.30 32.74
bag filter 0.18 3.22 4.49 27.95

Description Ca DCR [Cl.sup.-] [C.sub.org]
 [O.sub.free]

SCI/OS =
80/20 (%)

Fuel 0.24 21.74
Ash:

bottom 21.26 83.55 0.17 1.14
cyclone 13.59 74.43 0.95
bag filter 4.43 76.96 1.68

SCII/OS =
90/10 (%)

Fuel 17.45
Ash:

bottom 14.87 44.15 0.23
cyclone 12.27 64.66 0.28
bag filter 3.34 82.62 2.40

SCI/OS =
100/0 (%)

Fuel 15.81
Ash:

bottom 14.93 60.10 0.32
cyclone 12.96 77.14 1.50
bag filter 4.19 80.48 1.27

Description C burn-out [S.sub.total] [S.sub.sulphide] Insoluble
 rate residue

SCI/OS =
80/20 (%)

Fuel 2.00 0.36 27.02
Ash:

bottom 94.94 4.00 0.04 23.11
cyclone 95.85 2.58 0.03 31.10
bag filter 92.75 2.86 0.03 32.79

SCII/OS =
90/10 (%)

Fuel 2.05 0.4 26.65
Ash:

bottom 98.83 3.50 0.10 18.46
cyclone 98.51 2.29 0.05 32.67
bag filter 88.91 2.62 0.02 41.02

SCI/OS =
100/0 (%)

Fuel 2.10 0.45 27.68
Ash:

bottom 98.10 2.90 0.03 22.44
cyclone 91.00 1.96 0 26.23
bag filter 92.35 2.76 0 33.68

Table 4. Initial Data and Calculation Results for the
Boiler-Utilizer

 Wet semicoke Dry semicoke
 [W.sup.r] = 25% [W.sup.r] = 0%

Types and amounts of inserted fuels

Semicoke:

 tonnes per year 700,000 525,000
 tonnes per hour 80 59

Oil shale

 tonnes per 1 tonne of semicoke 0.1125 0.150
 tonnes per hour 9.0 8.8

Retort gas

 [m.sup.3] per 1 tonne of semicoke 562.5 750.0
 [m.sup.3] per hour 44,944 44,175

Heat input by fuel mixture components

Semicoke:

 LHV [Q.sup.r.sub.i], MJ/kg 2.47 4.12
 Heat input, MJ/h 197,353 242,668
 Oil shale:
LHV, MJ/kg 11.05 11.05
Heat input, MJ/h 99,450 97,240
LHV of retort gas and gas plant 3.9 3.9
gasoline, MJ/[m.sup.3]
Heat input by retort gas, MJ/h 175,282 172,283
Sum of heat inserted, MJ/h 472,085 512,191
Thermal capacity of boiler by fuel 131 142
inserted, M[W.sub.th]

Thermal capacity of boiler-utilizer:

at 80% efficiency, M[W.sub.th] 105 114
with physical heat of hot semicoke -- 121 *
added, M[W.sub.th]

* Semicoke temperature is 500 [degrees]C.
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Author:Kuusik, R.; Martins, A.; Pihu, T.; Pesur, A.; Kaljuvee, T.; Prikk, A.; Trikkel, A.; Arro, H.
Publication:Oil Shale
Date:Sep 1, 2004
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