Behavior of sulphur compounds at combustion of oil shale semicoke.

Introduction

In Estonia, total annual consumption of oil shale (OS) was 12-13 million tons during last years, from which 90 % were used in the power industry and 10 % in shale oil production [1]. Approximately one million tons of semicoke (SC), a by-product of shale oil production, has been formed and stored in open-air dumps. At present its total amount exceeds 100 million tons.

SC storage causes serious contamination of surroundings, especially of water bodies in the region close to the Gulf of Finland, by different environmentally harmful compounds like sulphides, phenols, PAH, etc. [2-4], and is accompanied by high and increasing environmental taxes [1]. The energetic potential of SC formed at OS thermal processing is not utilized--the gross heat value of SC formed can be as high as 3.8 MJ/kg, its organic matter content reaches 9 %, and the total content of sulphur occurring in various forms exceeds 2 % [4].

In [5], an attempt was made to study the distribution of total sulphur as well as of its different forms between solid, liquid and gaseous phases at OS semicoking. It was found that the total sulphur content of SC formed was in the range of 0.5-3.3 % depending on the origin of the OS sample. High sulphide sulphur content of SC can be explained by [H.sub.2]S formation in reactions between sulphur and hydrogen liberated at destruction of pyrite and OS organic part during semicoking. A part of [H.sub.2]S can react with oxides of different metals, decomposition products of carbonates present in the OS mineral part, yielding the sulphides of these metals.

In [6], it was determined that the content of different sulphur forms in the solid residues of OS processing using the solid heat carrier depends on the temperature and air excess during SC combustion in the technological furnace. For example, the sulphide sulphur content varied from 0.52 to 2.78 % (total sulphur 1.36-3.08 %). To eliminate sulphide sulphur formation, the air excess within [alpha] = 1.3 is needed [7].

Our earlier studies concerning the S[O.sub.2] emission during thermooxidation of OS and its mixtures with different coal samples confirmed that a part of S[O.sub.2] formed was bound in the solid phase [8, 9]. Preliminary investigation of thermal treatment of SC samples showed that a great part of sulphur present in SC was also bound in solid phase [10].

By calculating the parameters of pulverized firing (PF) of SC [11] and taking into account the results of OS test combustion under circulating fluidized-bed (CFB) conditions [12, 13], it was concluded that CFB combustion should be most suitable for SC. The same conclusion was reported in papers [8, 9] basing on the results of thermooxidation studies. Taking into account the real situation in the oil shale processing industry, where a notable amount of fine-grade oil shale is formed at OS preparation for thermal processing, the SC and fine-grade OS blends could be considered a fuel to be used [14].

Since the data characterizing the changes in different sulphur forms during SC combustion are lacking, the aim of the present research was to study the behavior of sulphur compounds during thermooxidation of SC and OS samples, and their different mass ratio mixtures. In addition, the fuel mixtures modified with SC ash additives were studied.

Experimental

Materials

Two SC samples, two OS samples and their mixtures of different mass ratios were studied. Different forms of sulphur (total, sulphide, sulphate, pyrite) were determined as described in [15].

The organic sulphur content was calculated as

[S.sub.organic] = [S.sub.total] - ([S.sub.pyrite] + [S.sub.sulphide] + [S.sub.sulphate])

and the content of organic matter in dry samples as

[100 - [A.sup.d] - [(C[O.sub.2]).sup.d.sub.M] (%)

where [A.sup.d] is the content of ash, % (dry basis);

[(C[O.sub.2]).sup.d.sub.M] is the content of mineral carbon dioxide, % (dry basis).

The SC samples (SC I and SC II) obtained from AS Kiviter (Viru KeemiaGrupp AS) at various times were quite similar in the content of organic matter (13.1 and 12.0 %), ash (68.9 and 73.9 %) and mineral carbon dioxide (18.1 and 14,1 %), as well as in the content of different sulphur forms ([S.sub.pyrite] 0.60 and 0.51 %, [S.sub.sulphide] 0.60 % both, and [S.sub.organic] 0.74 and 0.87 %, respectively). Only their sulphate sulphur content (and, consequently, the total sulphur content) was different (0.44 and 0.80 %, respectively) (Table).

The OS samples from the Aidu deposit (OS I and OS II) differ much from each other in the content of organic matter (29.8 and 63.1 %), mineral carbon dioxide (19.8 and 5.8 %), and total sulphur (1.63 and 1.22 %), as well as in the content of different sulphur forms ([S.sub.pyrite] 1.20 and 0.47 %, [S.sub.sulphate] 0.10 and 0.04 %, and [S.sub.organic] 0.33 and 0.71 %, respectively). The gross calorific value for SC I and SC II was 4.12 and 3.70 MJ/kg and for OS I and OS II 10.24 and 22.43 MJ/kg, respectively. Hence, the OS II could be qualified as nearly pure OS organic matter and it is not a typical material used in industry.

In the laboratory-scale bubbling fluidized-bed (LBFB) kiln experiments, two SC II fractional classes were used: 1-2 and 2-3 mm. They differed slightly (about 5-8 %) from each other as well as from the mean sample in the gross calorific value (3700-3940 kJ/kg), and in the content of total carbon (13.6-14.4 %), sulphur (2.7-3.0 %), hydrogen (0.74-0.88 %) and nitrogen (0.04-0.05 %). Differences in the specific surface area were in the range of 2.5-3.2 [m.sup.2]/g. In the SC II 2-3-mm fraction, the total CaO content was 25.05 %, that of free CaO 0.83 % and insoluble residue 28.8 %. Total sulphur, and pyritic, sulphide, organic and sulphate sulphur content was 2.75, 0.50, 0.61, 0.86 and 0.78 %, respectively.

Methods

Experiments in the Thermogravimetric Equipment

Different methods were used for estimation of transformations of S compounds taking place at thermooxidation (burning) of the fuels studied. S[O.sub.2] emission was estimated experimentally by using thermogravimetric equipment (Q-Derivatograph, MOM, Labsys [TM], Setaram) and dynamic heating (5 K per min up to 900[degrees]C) in the air stream, with absorption of evolved gases in water and titration of the solution at pH = 4.0 [16, 17]. That enabled simultaneous fixation of TG, DTG, DTA, TGT and DTGT curves. Multiplate Pt crucibles were used; each sample mass was 275-325 mg (or 10-15 mg when using Labsys [TM]). Total sulphur content was determined in the solid residue. The initial samples as well as the solid residues (ashes) were subjected to X-ray diffraction and BET specific surface area analyses.

For modeling the CFBC process, the influence of SC ash addition was studied varying ash share in the fuel-ash mixtures within Ca[O.sup.free]/S mole ratio from 1.0 to 1.7. The ash used was produced combusting the SC II 2-3-mm fraction at 850[degrees]C in the LBFB kiln. Total CaO and free CaO content of the ash was 42.8 and 25.4 %, respectively.

Experiments in LBFB Kiln

A series of experiments was carried out in a LBFB kiln (height 0.65 m and diameter 50 mm) to obtain ash samples for determining their residual activity towards S[O.sub.2] and to establish the main technological parameters (temperature, burning time, fuel, gas and air flows, etc.). The combustion temperature was varied from 800 to 950[degrees]C. During the experiments (duration 20 min) ash samples were taken every five minutes. Chemical and mineralogical analyses of the ash samples obtained were carried out.

Results and Discussion

Experiments in the Thermogravimetric Equipment

Thermooxidation of SC, OS and their mixtures started at 250-280[degrees]C and continued up to 540-560[degrees]C. Two intensive exoeffects accompanied by mass losses with maximums in the DTA curve at 320-340 and 440-480[degrees]C and one endoeffect with a minimum in the DTA curve at 760-780[degrees]C were observed. The first exoeffect corresponds to thermooxidation of volatile compounds, the second one to thermooxidation of the heavier part of organic matter and pyrite; the endoeffect corresponds to the decomposition of carbonates in the SC and OS mineral part (Fig. 1,a-c).

The exoeffects of SC samples were much less intensive than those of OS samples because the main part of volatile compounds had evolved during OS thermal processing. The second exoeffect maximum in the DTA curve of SC samples had shifted by 20-40[degrees]C and that of SC-OS mixtures by 5-20[degrees]C towards lower temperature compared with OS samples.

The total mass loss of SC samples was 13-18 % up to 540[degrees]C and 25-33 % up to 850[degrees]C instead of 30.5 and 51.7 % in the case of OS I. During thermooxidation of OS II, which contains twice as much of organic matter as OS I, the corresponding numbers were 57.7 and 64.4 %.

In the case of OS samples and OS-SC mixtures, S[O.sub.2] emission from the samples studied started at 220-300[degrees]C and proceeded in two steps--at 340-380[degrees]C and from 440-460[degrees]C (in some cases a third peak was observed in the DTGT curve with a maximum at 500[degrees]C) up to 580-620[degrees]C. S[O.sub.2] evolved in the temperature range from 220-250 to 400[degrees]C originates from the organic matter volatile part, at 400-620[degrees]C from the organic matter heavier part and pyrite. S[O.sub.2] emission from SC started at 260-280[degrees]C and proceeded practically at a constant rate up to 640[degrees]C.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The amount of sulphur evolved as S[O.sub.2] was 9.1 and 4.2 % of the sample total sulphur content in the case of SC I and SC II, and 57.6 and 48.8 % in the case of OS I and OS II, respectively. The amount of sulphur evolved from the fuel mixtures of different constituents and various SC-OS mass ratios varied from 9.6 to 37.5 % of the total sulphur content (Fig. 2).

The influence of the SC-OS mass ratio in the fuel mixtures can be well observed in the DTGT curves (Fig. 3). It is noticeable that the amount of S[O.sub.2] emitted during thermooxidation was about 10-20 % less than calculated considering these mixtures to be simply mechanical ones.

During fuel mixture thermooxidation, the influence of SC ash additions on the sulphur binding in the solid phase is considerable. S[O.sub.2] emission from the mixtures with SC ash additives started at 220-300[degrees]C as well, but stopped at temperatures 40-60[degrees]C lower than in the case of the mixtures with no ash addition. The amount of sulphur emitted decreased depending on the fuel mixture composition, and Ca[O.sup.free]/S mole ratio varied from 9.6-37.5 to 2-15 % (Fig. 4). It indicates that ash present in the fuel mixture intensively binds S[O.sub.2] even in the stationary layer, and its more intensive effect in fluidized-bed conditions must be evident.

Experiments in LBFB Kiln

The results of experiments carried out in the LBFB kiln in the temperature range of 800-950[degrees]C confirmed that under fluidized-bed conditions SC burned intensively--within 5-10 min, and the formation of ash with maximum content of free calcium and magnesium oxides occurred at 850[degrees]C. At lower temperatures the organic carbon burnout was not complete, at higher temperatures the reaction between free Ca and Mg oxides formed during decomposition of carbonates and silica-containing part of SC forming silicates is possible.

Essential changes in the distribution of different sulphur forms can be followed at SC combustion. In twenty minutes the sulphate sulphur share increased from 0.80 % (28.8 % rel.) in SC up to 2.77-2.96 % (93.3 % rel.) in ashes, that of pyritic sulphur decreased from 0.51 % (18.3 % rel.) in SC to 0.20 % (6.2-6.7 % rel.) in ashes. The organic sulphur and sulphide sulphur content decreased from 0.87 (31.2 % rel.) and 0.60 % (21.8 % rel.) in SC to 0.02-0.03 % (0.6-0.9 % rel.) in ash formed at 850[degrees]C, disappearing in the ash formed at 950[degrees]C (see the Table).

[FIGURE 5 OMITTED]

The distribution of different sulphur forms in OS I, SC II and SC II ashes formed in the LBFB kiln at 850 or 950[degrees]C is fairly demonstrated by Fig. 5. In OS pyritic sulphur prevailed, in SC all sulphur forms were present within 20-30 % (rel.), in the SC ashes sulphate sulphur content remained 92-93 % and that of pyritic sulphur 7 % (rel.).

Changes in the Sample Mineralogical Composition during Thermooxidation

According to the X-ray analysis results, in OS and SC samples the main phases were calcite CaC[O.sub.3], quartz Si[O.sub.2], dolomite CaMg[(C[O.sub.3]).sub.2] and pyrite Fe[S.sub.2]. The presence of muscovite K[Al.sub.2][Si.sub.3][O.sub.10][(OH).sub.2] and microcline Kal[Si.sub.3][O.sub.8] was also determined.

In SC samples anhydrite CaS[O.sub.4] and portlandite Ca[(OH).sub.2] were present. Their forming was caused by the formation of some amount of free CaO during OS semicoking in the neutral gas atmosphere followed by CaO reaction with sulphur compounds formed during OS thermal processing, as well as with water at SC emersion from convertor.

SC mineral composition changes notably during thermooxidation. The occurrence of characteristic peaks of different phases on diffractograms and their intensity depend on thermal treatment conditions. Thus, in the samples thermooxidated up to 600-650[degrees]C dolomite and periclase MgO, calcite and lime CaO were simultaneously detrmined (Fig. 6). Comparison of the intensity of the respective peaks confirms that most of dolomite is decomposed, but calcite decomposition is only starting. In the samples heated up to 800-900[degrees]C, lime is present as the main phase instead of calcite.

[FIGURE 6 OMITTED]

In the thermooxidation products two main phases are quartz and anhydrite. On the diffractograms characteristic peaks of anhydrite are more intensive in the case of using ash additives--on the one hand, due to the anhydrite presence in the ash samples, and on the other hand, due to the binding of additional S[O.sub.2] by the ash.

In some thermally treated samples muscovite was not determined which means that at higher temperatures it could decompose completely. In the thermooxidated samples pyrite is not present, instead of it a new phase--hematite [Fe.sub.2][O.sub.3] was found. In some samples thermally treated up to 600-650[degrees]C, calcium sulphide CaS and double sulphate Ca[Mg.sub.3][(S[O.sub.4]).sub.4] were recorded as traces.

Considering the results of thermooxidation experiments with SC, OS and their mixtures the following transformations of S compounds would occur (as a rule these transformations started at temperatures below 600-650[degrees]C, but achieved their maximum rate at different temperatures):

Formation of S[O.sub.2]:

[S.sub.sulphide] + [O.sub.2] [right arrow] S[O.sub.2] (1)

[S.sub.organic] + [O.sub.2] [right arrow] S[O.sub.2] (2)

Fe[S.sub.2] [right arrow] FeS + [S.sub.pyritic] (3)

4FeS + 7[O.sub.2] [right arrow] 2[Fe.sub.2][O.sub.3] + 4S[O.sub.2] (4)

[S.sub.pyritic] + [O.sub.2] [right arrow] S[O.sub.2] (5)

Decomposition of carbonates:

CaMg[(C[O.sub.3]).sub.2] [right arrow] CaO + MgO + 2C[O.sub.2] (6)

CaC[O.sub.3] [right arrow] CaO + C[O.sub.2] (7)

Binding of S[O.sub.2]:

2CaO + 2S[O.sub.2] + [O.sub.2] [right arrow] 2CaS[O.sub.4] (8)

CaS and CaMg[O.sub.3][(S[O.sub.4]).sub.4] traces in the solid phase at temperatures [less than or equal to] 600-650[degrees]C could be explained by the occurrence of reactions

CaO + S[O.sub.2] [right arrow] CaS[O.sub.3] (9)

4CaS[O.sub.3] [right arrow] CaS + 3CaS[O.sub.4] (10)

CaO + 3MgO + 4S[O.sub.2] + 2[O.sub.2] [right arrow] CaMg[O.sub.3][(S[O.sub.4]).sub.4] (11)

At temperatures >650[degrees]C oxidation of CaS and decomposition of CaMg[O.sub.3][(S[O.sub.4]).sub.4] take place:

CaS + 2[O.sub.2] [right arrow] CaS[O.sub.4] (12)

CaMg[O.sub.3][(S[O.sub.4]).sub.4] [right arrow] CaS[O.sub.4] + 3MgO + 3S[O.sub.3] (13)

As S[O.sub.3] also forms at a low rate under the conditions used, the following reaction takes place after formation of CaO:

3CaO + 3S[O.sub.3] [right arrow] 3CaS[O.sub.4] (14)

Conclusions

Investigation of SC thermooxidation in the thermogravimetric equipment as well as in the LBFB kiln confirmed that during combustion under fluidized-bed conditions various and intensive interactions between S compounds and SC mineral part took place. At moderate temperatures (800-900[degrees]C) and moderate oxygen content of the gaseous phase these interactions led to effective binding of S compounds in the solid phase, mainly with formation of CaS[O.sub.4], and to the almost complete elimination of S[O.sub.2] emission. The results obtained refer to the advantages of fluidized-bed technique, first of all the circulating fluidized-bed technique for utilization of semicoke.

* Acknowledgements

The financial support of Estonian Scientific Foundation and (G4262) is highly appreciated.

Presented by J. Kann Received October 9, 2002

REFERENCES

[1.] Lahtvee, V. Energy and environment // Estonian Energy 1999. Tallinn, 2000. P. 52-59.

[2.] Molder, L., Elenurm, A., Tamvelius, H. Sulphur compounds in a hydraulic ash-disposal system // Proc. Estonian Acad. Sci. Chem. 1995. Vol. 44, No. 2/3. P. 207-211.

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

[4.] Yefimov, V. Oil shale processing in Estonia and Russia // Ibid. 2000. Vol. 17, No. 4. P. 367-385.

[5.] Gubergrits, M., Elenurm, A. Raspredelenie i transformatsiya raznovidnostej sery pri polukoksovanii slantsa-kukersita (Distribution and transformation of sulfur different forms at semicoking kukersite oil shale) // Goryuchie slantsy (Oil Shale) / EstNIINTI. 1981. Vol. 23, No. 4. P. 11-19 [in Russian].

[6.] Elenurm, A., Rohtla, I., et al. Sulphur compounds in the solid residues of oil shale processing by solid heat carrier // Oil Shale. 1988. Vol. 5, No. 3. P. 285-296.

[7.] Kuusik, R., Veiderma, M. Combustion of oil shale semicoke at the fluidized-bed conditions // Goryuchie slantsy (Oil Shale) / EstNIINTI. 1977. No. 9. P. 16-19 [in Russian].

[8.] Kaljuvee, T., Kuusik, R., Veiderma., M. Emission of sulphur dioxide by thermooxidation of Estonian oil shale and coal // Proc. Estonian Acad. Sci. Engng. 1998. Vol. 4, No. 3. P.199-208.

[9.] Kaljuvee, T., Kuusik, R. Emission of sulphur dioxide during thermal treatment of fossil fuels // J. Therm. Anal. Cal. 1999. Vol. 56. P. 1243-1251.

[10.] Kuusik, R., Kaljuvee, T., Trikkel, A. Mitigation of negative environmental impact of semicoke // 1st Intern. Congr. on Petroleum Contaminated Soils, Sediments and Water. London, UK, Aug. 14-17, 2001 : Abstract and Directory. London, 2001. P. 27.

[11.] Arro, H., Prikk, A., Pihu, T., Opik, I. Utilization of semi-coke from Estonian oil industry // Oil Shale. 2002. Vol. 19, No. 2. P 117-126.

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

[13.] Arro, H., Prikk, A., Kasemetsa, J. Circulating fluidized-bed technology--test combustion of Estonian oil shale // Ibid. P. 215-217.

[14.] Opik, I., Yefimov, V. An analysis of the RAS Kiviter energy balances and development plans // Ibid. 1995. Vol. 12, No 3. P. 247-257.

[15.] Solid fuels. Sulphur Content. Determination of total sulphur and its bonding forms. EVS 664 : 1995 / The Board of Sertificate of the Republic of Estonia. 1955. P. 13 [in Estonian].

[16.] Paulik, J., Paulik, F., Arnold, M. Simultaneous TG, DTG, DTA and EGA technique for the determination of carbonate, sulphate, pyrite and organic material in minerals, soils and rocks. 1. Principles of the method // J. Therm Anal. 1982. Vol. 25. P. 327-340.

[17.] Paulik, F., Paulik, J., Arnold, M. Simultaneous TG, DTG, DTA and EGA technique for the determination of carbonate, sulphate, pyrite and organic material in minerals, soils and rocks. 2. Operation of the thermo-gas-titrimetric device and examination procedure // Ibid. 1984. Vol. 29. P. 327-340.

T. KALJUVEE (*1), R. KUUSIK (*1) A. TRIKKEL (*2), N. MALJUKOVA (*1)

(*1) Department of Chemical Engineering,

(*2) Department of Chemistry, Tallinn Technical University 5 Ehitajate St., Tallinn 19086, Estonia

Main Characteristics of the Samples

Sample                              Content, % (dry basis)

                           Organic           Ash         [(C[O.sub.2])
                           matter                          .sub.M]

Semicoke I (SC I)           13.1             68.8            18.1
Semicoke II (SCII)          12.0             73.9            14.1
Oil shale II (OS II)        63.1             31.2             5.8
Oil shale I (OS I)          29.8             50.4            19.8
SC II ([??] 2-3 mm) burnt in BFBC *:
  at 850[degrees]C:
     5 min                  <0.5            >99               0.57
    20 min                  <0.5            >99               0.60
  at 950[degrees]C:
     5 min                  <0.5            >99               0.39
    20 min                   0              >99.5             0.30

Sample                  Content, % (dry basis)

                        [S.sub.total]    [S.sub.sulph]   [S.sub.pyr]

Semicoke I (SC I)           2.38              0.44            0.60
Semicoke II (SCII)          2.78              0.80            0.51
Oil shale II (OS II)        1.22              0.04            0.47
Oil shale I (OS I)          1.63              0.10            1.20
SC II ([??] 2-3 mm) burnt in BFBC *:
  at 850[degrees]C:
     5 min                  2.98              2.69            0.13
    20 min                  3.21              2.96            0.20
  at 950[degrees]C:
     5 min                  3.05              2.83            0.14
    20 min                  2.97              2.77            0.20

Sample                           Content, % (dry basis)

                       [S.sub.sulphide]    [S.sub.org]          C

Semicoke I (SC I)           0.6               0.74           17.9
Semicoke II (SCII)          0.6               0.87           14.4
Oil shale II (OS II)        0                 0.71           48.5
Oil shale I (OS I)          0                 0.33           28.3
SC II ([??] 2-3 mm) burnt in BFBC *:
  at 850[degrees]C:
     5 min                  0.11              0.05            0.49
    20 min                  0.02              0.03            0.4
  at 950[degrees]C:
     5 min                  0.08              0.45           <0.01
    20 min                  0                 0               0.36

Sample                           Content, % (dry basis)

                              N               H               Cl

Semicoke I (SC I)           0.52              1.21            0.21
Semicoke II (SCII)          0.05              0.79            0.21
Oil shale II (OS II)        0.09              5.96            0.12
Oil shale I (OS I)          0.53              3.00            0.10
SC II ([??] 2-3 mm) burnt in BFBC *:
  at 850[degrees]C:
     5 min                 <0.01              0.37            0.10
    20 min                 <0.01              0.43            0.12
  at 950[degrees]C:
     5 min                  0.34             22.90            2.42
    20 min                 <0.01              0.41            0.07

Sample                                   Specific        Gross
                                         surface         calorific
                            Free         area,           value,
                             CaO         [m.sup.2]/g     MJ/kg

Semicoke I (SC I)             -               3.21            4.12
Semicoke II (SCII)            -               3.12            3.70
Oil shale II (OS II)          -               3.26           22.43
Oil shale I (OS I)            -               8.26           10.24
SC II ([??] 2-3 mm) burnt in BFBC *:
  at 850[degrees]C:
     5 min                 20.20              4.02             -
    20 min                 25.37              3.82             -
  at 950[degrees]C:
     5 min                    -
    20 min                 23.65              2.40             -

* Laboratory bubbling fluidized-bed kiln.