Sulfur dioxide release in fluidized bed combustion of Jorban oil shale

Fuel Processing Technology, 6 (1982) 245--254

245

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

S U L F U R DIOXIDE RELEASE IN FLUIDIZED BED COMBUSTION OF JORBAN OIL SHALE

R.A. HADDADIN

Chemistry Department, University of Jordan, Amman (Jordan) (Received June 2nd, 1981; accepted in revised form January 18th, 1982).

ABSTRACT

Due to high concentrations of sulfur and carbonates in Jordan oil shale, it was anticipated that the deposits would be a suitable burning fuel in an atmospheric fluidized bed system. The SO2 capture by calcium oxide and calcined dolomite in spent shale prompted this experimental program to verify spent shale reactivity and SO~ retention. Concentrations of SO~ in effluents were analyzed vs. time in a fluidized bed material o f silica sand, pure limestone rock (99.6% carbonates) and spent shale. The SO 2 release was studied in batch tests for each bed material. The effect o f particle size in spent shale FBC was tested. Some model sulfur containing compounds were impregnated on limestone carrier solids and combusted to give time-release data. A sample of pyrite was charged to the bed of limestone sand in order to study its sulfur release and compare the data. A co m p o n en t balance was attempted to trace sulfur in the various bed materials.

INTRODUCTION

The growing demand for electrical p o w e r and continually rising prices of crude oil will continue to draw on solid fossil fuels as a source of energy. At the same time, the need to control and reduce environmental pollution will dictate the manner by which these fuels will contribute to our future energy supplementation. For Jordan shale (E1-Lajjun area), which is rich in S and N compounds, the solution towards its utilization lies in its potential conversion to clean non-polluting fuel. It is looked upon as a source of syncrude when upgraded for a refinery feedstock or fuel for direct combustion to generate electricity. Fluidized bed technology as applied to combustion of fossil fuels is still on the experimental scale. In the U.S., emphasis is placed on fluidised bed combustion (FBC) potential to burn low grade fuels cleanly and efficiently [ 1--3 ]. Although a n u m b e r of performance studies on fluidized beds have been conducted b y chemical engineers on coal burning [3--5], oil shale combustion in fluidised beds (FB) has n o t gained much attention because of its attrition, flaking and elutriation properties. Recently, a few tests have been attempted to burn shale in experimental systems n o t exceeding 60 cm in

0378-3820/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

246 diameter [6,7]. When tests in MERC (U.S. Energy Dept.) were performed on U.S. shale, the equipment was plagued with dust. However, bed temperatures were reported to have reached 1600°F [ 7]. Lignite, another high ash fuel, when burned in FB showed desirable combustion characteristics with bed temperatures o f up to 1093°C (2000°F) [8]. The immediate objective of these batch tests is to determine SO2 release when shale is combusted in a bed of fluidized spent shale as a prospective sorbent. SO2 release data were checked against a limestone bed as well as SiO2 (silica sand) in order to justify and verify S retention data. In spite of the severity of conditions in handling the sampling, sulfur tracing was pursued and checked whenever possible to supplement SO2 release data. EXPERIMENTAL

Equipment Figure 1 shows a schematic diagram of the experimental arrangement. Figure 2 shows the solids sampling and charge injection probe. The bed was a 316 stainless steel cylinder (20 mm thick, 15 cm in diameter × 75 cm long) which was insulated with 7 cm thick Watlow Electric insulating wool material with a surface temperature n o t exceeding 175°C. The distributor plate

1. 2. 3. 4. 5, 6. 7.

FEEDHOPPER I / 2 HP MOTOR& DRIVE 3 HP BLOWER 8UTAGAZCYLINDER F B COMBUSTOR SOLIDFEEDPORT FREE80ARD ZONE

8. 9. 10. 11. 12. 13. 14. 15.

SAMPLECHARGEWINDOW PLENUM SOLIDCOLLECTIONVESSEL CYCLONE OUSTBAG GAS STREAMFILTER INFRARED ANALYZER(L & N) SUCTIONPUMP AND DISCHARGE

Fig. 1. Flow diagram and equipment.

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1. HANDLESTO CONTROL SPRING DISTANCE 2. SPRINGTO RELEASECOVER FROMPROBE 3, OUTSIDEFLEXIBLECONDUfT (I" O.D.) 4. INSIDECONDUIT('/2" O.D.) 5. CENTERGUIDES 6. SAMPLECUP 7. SAMPLERETAINERCOVER

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60 mm

i ",,J-~6 O mm

=',I

Fig. 2. In-bed solids injection probe.

was 1Ainch (6.4 ram) thick with 80 holes, 2 mm in diameter each. The holes (orifices) were distributed on equilateral triangular pitch with a 250 mesh s.s. screen welded to the plenum space. In order to prevent damage to the screen at operating temperatures, a layer (5 cm thick) of 6--8 mesh silica sand was used to isolate the screen from the h o t bed. The effluents of the bed passed through a cyclone and the entrained dust was collected in a large canvas filter. A double filtered gas stream was withdrawn to analyze for SO2 using a L & N IR analyzer. The vessel was equipped with temperature probes protruding 5 cm horizontally inside the bed. A n o t h e r was located in the freeboard. Pressure taps to measure Ap across the distributor and the bed were used to verify bed fluctuation and stability. A water cooling coil was incorporated inside the bed in order to operate at temperatures below 830°C. The solids injection probe is shown in Fig. 2. The device was operated successfully to inject about 100 gm of shale inside the FB and above the distributor.

The fluidized bed operation A preliminary test was run under typical conditions as shown in Table 1. The superficial velocity at m i n i m u m fluidization conditions was obtained from the quadratic equation in terms of Umf [9],

248 1 . 7 5 (dp Umfpg) 2 + 150 ( 1 - e m f ) (dp U m f P g ) =dp 3 Pg (Ps-Pg) g p2 --2E3 Cse3mf P ~s mf P

which reduces to give the simplified form Urn f =

(dPsdp2)2nsops-pg p

When

(e3mf._._~)9 1-emf

Re < 20 f o r small particles, in general Umf can be e x p r e s s e d as:

Um f = dp 2 (P s - P g) 9 1650 U T h e b e d is c h a r a c t e r i z e d as stable w h e n (Us-Umf)/0.35 (g dp) 1/2 < 0.2, w h e r e Us is t h e superficial v e l o c i t y . H o w e v e r the u s e o f t h e d e n s e p h a s e over t h e d i s t r i b u t o r p l a t e (5 c m t h i c k o f 6 - 8 m e s h ) m a k e s this r a t i o m u c h higher, as n o t e d f r o m T a b l e 1. Silica s a n d was u s e d as t h e b e d m a t e r i a l . T h e b e d was h e a t e d b y igniting t h e f l u i d i z a t i o n air t o w i t h i n + 10°C f r o m t h e i n t e n d e d TABLE 1 Operating parameters, stable fluidization Bed Material

Silica

Static bed height (cm) Fluidized bed height (cm) Particle size (bed material) Particle size of sample, (unless otherwise stated) M i n i m u m fluidization vel. (cm/sec) Superficial vel. (cm/sec) Total press, drop, (mm H20) % excess air (vol)

Limestone

Spent shale

20 25 30/50

20 25 30/50

20 26

30/50

30/50

30/50

14 60 360 15--20

11 55 360 15--20

10 52 360 15--20

30/50

TABLE 2 Degree of Calcination (40 minutes residence time at indicated T) limestone & spent shale, 30/50 mesh T (°C)

Limestone*

Spent shale**

600 800 830 850 900

5 35 61 85 96

7 38 43 95 94

*Analysis by weight loss in TGA apparatus **Tests in FB system --electric heating

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temperature. When preparing the limestone bed t w o semi-cylindrical electric heaters (4400 Watts) clamped around the fluidization zone were utilized to bring the bed to calcination temperatures. The degree of calcination vs. temperature is shown in Table 2. To prepare the spent shale as a FB material, the vessel was half charged with shale and heated electrically at 5°C/min up to 500°C at which time the fluidized bed material began to volatilize and ignite simultaneously. After the danger of a sudden eruption of volatile matter was eliminated, additional fresh shale was fed continuously to bring the expanded bed height (overall) to 25 cm. This practice eliminated the elutriation of bed material due to the sudden combustion of organic matter. Samples of spent shale were withdrawn to analyze for carbonate decomposition as reported in Table 2. RESULTS AND DISCUSSION

Time -- SO~ release data were obtained at various temperatures, ranging from a b o u t 800 to 900°C {Figs. 3--7). Such temperatures were sustained by burning premixed butagas with air at a b o u t 15% excess air. Once the bed was stable within the expected temperature range a charge of 100 gm of shale was injected and released above the distributor. The sample size was chosen so that no significant temperature spikes were detected at the time of sample unloading. The temperature only fluctuated + 10°C from the test temperature during the injection. This was

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Fig. 3. Time--SO~ release in silica sand bed (30--50 mesh).

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900°C, 21% RELEASE 865°C, 16% RELEASE 830°C, 12% RELEASE 810oc, 6% RELEASE 650°C.

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Fig. 4. T i m e - - S O 2 release in l i m e s t o n e b e d m a t e r i a l ( 3 0 - - 5 0 m e s h ) a t various t e m p e r a tures.

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Fig. 5. Particle size e f f e c t u p o n SO 2 release in c a l c i n e d lime bed.

within experimental uncertainties. The gas stream sample was withdrawn continously from the instant of discharge to the end of the test. It is interesting to note here t h a t when fresh shale was injected into the silica sand bed, the charge quickly erupted into flame and extended into the freeboard and cyclone region. This was n o t observed when the bed material was limestone or spent shale.

251

One test was performed to determine the temperature of combusting shale fines in a fluidized bed. The bed was heated with a butagas--air mixture (20% excess air) up to 750°C. The gas was shut off and the shale was fed by t w o screw feeders at a rate of 120 gm/min for 200 minutes, sustaining an operating temperature of 830 + 30°C. Although the attrition and elutriation A

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15

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Fig. 7. Time--SO 2 release in spent shale be(] material (30--50 mesh).

2.0

2.5

252

problems were severe the bed height had to be maintained constant by withdrawing spent shale. The tests were conducted for three bed materials (Figs. 3--5) to show the spent shale capacity to absorb SO2 as compared with limestone (calcined) and to show the absence of sorbent in an inert bed of silica sand. Tracing sulfur in its various product forms showed the capacity of spent shale as a suitable material to scrub combustion products. Even though the data on the S balance seemed scattered due to the severity of conditions and the experimental limitations, the results showed that calcined shale can absorb SO2 and retain it favorably. Table 3 shows the sulfur balance in the gas stream calculated from time-release peak areas, the amount of sulfur retained in the FB as unburned effluents and fines. In the latter, organic sulfur was the dominating component. Losses as reported appear to be significant since they were calculated b y difference. With regards to spent shale suitability, temperatures between 850 to 890°C give best S retention conditions. TABLE 3 S u l f u r b a l a n c e a p p e a r i n g as SO~ in gas, in filter bag a n d c a p t u r e d as sulfates Bed Material

Silica s a n d Limestone

S p e n t shale

T

% Sulfur

(°C)

released

elutriated*

captured

loss* *

870 865 810 650 890 855 660

31 16 6 5 10 7 18

42 13 21 38 22 17 29

-63 50 11 40 48 17

27 8 23 46 28 28 36

* C o m p o s e d o f 6 0 - - 8 0 ( b y w e i g h t ) organic m a t t e r sulfur, m o s t likely partially c o m b u s t e d shale **calculated by difference

The runs at low temperature were affected by the cooling coil where elutriant formation increased. This was probably due to the disruption of the fluidization conditions u p o n sample probe injection in the center of the coil. The elutriated matter was mainly partially c o m b u s t e d shale fines and was analyzed for sulfur and organic matter content by burning the fines in air at 550°C. The effluents of the tests were checked continously using the IR scanner for SO2 concentration. The inorganic S in solids filterate was analyzed for, by decomposition in a high temperature oven with N2 as the carrier gas to the scanner. When using pyrite, (Fig. 6), a slower rate of SO2 release was noted. Only a b o u t 6% of the S was accounted for as a pollutant, with the remainder (78%) as sulfated lime with no S detected in the filter bag. When using model

253

c o m p o u n d s containing sulfur, such as sulfone, (mp. 192°C) and 3,7 -dimethylbenzo (b) thiphene (bp 122°C), the evolution of the impregnated hydrocarbons by rapid heating in an air atmosphere allowed oxidation and release of SO2 (Fig. 6). However, n o t all sulfur fed with the sample was traceable in the gas. Only a b o u t 23% of the S as SO2 was detected. A b o u t 30% of the S was also found absorbed as sulfacted limestone. A detailed tracing of S in the various forms of products was n o t a t t e m p t e d in this work and therefore can n o t be elaborated upon. The experiments performed t o investigate the suitability of spent shale, which is mostly carbonates, to scrub the FB atmosphere of SO2 were encouraging. The release data was somewhat compatible with that from using pure limestone material. The gaseous effluents contained from 10 to 20% of SO2, (calculated from the peak area of the curves). The total SO2 was released for a duration of a b o u t 2 minutes and observed for over t w o minutes, or until the instrument dial showed no reading above that calibrated for background. In some tests the gas monitoring ran for over 10 minutes. Feeding large particles of shale to the FB, as shown in Fig. 7, reduced the abrupt evolution of organic sulfur and permitted better particle--oxygen reactions at considerably slower rates, which allowed enhanced SO2 retention and sulfation. When using smaller particles, the opposite was clearly observed from the data where considerable sulfur oxidation probably t o o k place in the freeboard. The temperature in this zone was recorded in the range 600--700°C. CONCLUSIONS

1. The batch tests aimed at evaluating the time--SO2 release data could n o t be simulated to conditions of continous feeding of shale. The sample immediately volatilized and c o m b u s t e d partially in the free-board zone, especially when silica sand was the bed material. 2. Some SO2 capture was evident in the case of limestone and spent shale. Due to elutriation of shale fines, the retention of SO2 in this case was n o t inclusive and needs to be further expanded using larger particle sizes. 3. In the case of pyrites, release was much slower; the SO2 capture was significant and S balance was more traceable in the bed giving 78% S charged as sulfate. 4. For model c o m p o u n d s , volatilization was still a problem, even though SO2 capture was evident as compared to SO2 released. The volume of the gaseous effleunts was large and condensation recovery of these sulfonated hyrocarbons was n o t effective. 5. Larger particles o f shale burned much more efficiently in the bed and gave a better sulfation reaction than when using for example, 30--50 mesh. However, this size was chosen in order to reduce the time of sample withdrawal into the analyzer, which could increase contamination and lead to drift in the measurements.

254

6. Even though the Ca/S mole ratio in the case of spent shale FB material is large and well in excess of 10 to 1 for these batch experiments, SO2 capture seems promising and a continous process should be attempted to test various Ca/S ratio effectiveness. ACKNOWLEDGEMENT

While a visiting professor (1978--79), at West Virginia University, Chem. Eng. Dept., the author received considerable training and consultation in fluidized bed technology from Professor C.Y. Wen (Chairman), for which he is indebted.

particle diameter (cm) acceleration of gravity (980 cm/sec 2) A P b pressure drop across the fluid bed (cm H20) A P d pressure drop across the distributor (cm H:O) particle Reynold number (dimensionless) Re Umf minimum fluidization velocity (cm/sec) Us superficial velocity (cm/sec) e m f void fraction at minimum fluidization conditions viscosity of air--gas mixture (gm/cm sec) P density of the air and gas (gm/cm 3) Pg density of solids (gm/cm 3) Ps sphericity of particles (dimensionless) ~s dp

g

REFERENCES 1 Wen, C.Y., 1976. Fluidized bed combustion -- state of the art, Chem. Eng. Dept., W. Va. Univ., Morgantoun, W. Va. 2 Ulerich, N.H., O'Neill, E.P. and Keairns, D.L., Aug. 1977. The Influence of Limestone Calcination on The Utilization of the Sulfur-Sorbent in Atmospheric Pressure FluidBed Combustors, EPRI--FP 426 (Project 720-1). 3 Malts, P.C. and Rees, D.P., 1979. In: L.D. Smoot and D.T. Pratt, (Eds.) Mechanism and Kinetics of Pollution Formation During Reaction of Pulverized Coal, Pal. Coal Comb. & Gasification, Plenum Press, N.Y. 4 Orning, A.A. and McCann, C.R., 1968. A study of fluidized combustion of coal, presented at annual meeting, AIME, New York, N.Y. 5 Hoke, R.C. and Ruth, L.A., April 1979. Control of emissions from the pressurized fluidized bed combustion of coal, International Fluidized Bed Combustion Symposium, Boston. 6 Langsjoen, P.L. and Flectcher, E.A., April 1976. Observations on the combustion of oil shale in a fluidized bed, The Combustion Inst. meeting, S.L.C., Utah. 7 Grimm, U., personal communication, METC, Morgantown, W.V. 1979. 8 Mai, J.S., Grimm, U. and Halow, J.S., April 1978. Fluidized bed combustion test of low quality fuel, U.S. Dept. of Energy, MERC, Morgantown, W.V. 9 Kunii, D. and Levenspiel, O., 1969. Fluidization Engineering, John Wiley and Sons, N.Y., p. 73.