Entrained flow gasification of coal

Entrained flow gasification of coal

Entrained 1. Evaluation measurements Nicholas Chemical (Received flow gasification of mixing Fi. Soelberg and reaction *, L. Douglas Engineerin...

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Entrained 1. Evaluation measurements Nicholas Chemical (Received

flow

gasification

of mixing

Fi. Soelberg

and reaction

*, L. Douglas

Engineering Department, 1 March 1984; revised

of coal

Smoot

Brigham Young 23 July 7984)

and

processes

Paul

University,

0.

from

local

Hedman

Provo,

UT 84602,

USA

Local mixing and reaction processes were studied within a laboratory-scale, entrained coal gasifier at atmospheric pressure, using a Utah high-volatile, low-sulphur bituminous coal at a design flow rate of 24.5 kg h-r. The coal-oxygensteam feed mass ratio was 1.00:0.91:0.27. A waterquenched sample probe was used to collect radial gas and char samples at seven different axial positions in the 124 cm long reactor for the measurement of gasification products and residual char composition. The observed carbon conversion was 79+ 3%. Coal hydrogen and oxygen were converted more rapidly and more completely than carbon. Devolatilization, which occurred very rapidly near the inlet, led to most of this carbon conversion; heterogeneous char reactions with CO, and steam apparently accounted for the balance. Oxygen was consumed through reaction with volatiles very quickly in the upper gasifier region. These data were used to

evaluate mixing and reaction characteristics within the reactor. Agreement ofmeasurements from a generalized two-dimensional entrained ‘coal gasification model was good. (Keywords: coal; gasifKation;

mixing and reaction processes)

Entrained flow gasification of coal has been considered by many to be a viable source of fuel gas and chemical feedstocks. Entrained coal gasification has several advantages over other gasification processes, including high throughput for a given reactor volume and simple mechanical design and flexibility in coal type used, regardless of caking and swelling tendencies. Several entrained flow gasification studies have been conducted over the last 30-40 years. Von Fredersdorff and Elliott’ reviewed gasification processes up to 1963, discussing reactor design, thermodynamics and kinetics. Bissett’ reviewed entrained coal gasification work up to 1978, emphasising research by Sebastian3 at the US Bureau of Mines on an experimental gasilier. McIntosh and Coates4 operated an entrained gasifier up to 1.07 MPa to evaluate high-pressure reactor performance. Skinner et al.’ and Lewis et aL6 made optimization studies on the same gasifier as used for the present work, measuring mixing and reaction rates and maximizing carbon conversion. Several variables studied included oxygen percentage in primary and secondary streams, secondary temperature, input stoichiometry and secondary swirl number. Problems encountered in these studies included uncertainty in coal feed rate and carbon measurements. Several hardware and conversion procedural modifications were made, to improve measurement accuracy and reliability. For the present study, local gas and char samples were collected at selected radial and axial positions within the gasifier to 1. determine mixing and reaction characteristics, 2. evaluate carbon conversion for validation of previous work by Skinner et a1.5 and Lewis et aL6, and * Present

address:

EER Corp., Irvine, CA 92714, USA

001~2361/85/06077&06-o6$3.00 0 1985 Butterworth & Co. (Publishers)

776

with predictions

Ltd.

FUEL, 1985, Vol 64, June

3. compare the experimental results with predictions from a generalized coal gasification model developed by Smith et al.’ and Fletchers.

EXPERIMENTAL Facility

The reactor (Figure I) consisted of four flanged 30.5 cm sections and one flanged half-section of 15.2 cm. Two of the sections had ports through which the reaction process could be observed. Another section was equipped with ports for sample probe insertion. Castable alumina ceramic insulation lined the interior of each section, the reactor top and bottom and the observation ports. The nominal inside diameter was 20 cm and the overall length was 124 cm. Test procedure

Eleven tests were conducted at atmospheric pressure to obtain gas and char samples from selected radial positions. A Utah high-volatile bituminous coal was used, pulverized to 70% < 74 urn. The gross calorific value of the coal was 29.4 MJ kg-’ and the mass mean particle diameter was 41.5 urn. Soelberg’ reports the measured size distribution. The proximate analysis was 45 wt% volatiles, 44 wt% fixed carbon, 8.5 wt% ash and 2.5 wt% moisture. The ultimate analysis (db) was 72 wt% C, 5.5 wt% H, 1.4 wt% N, 0.6 wt% S and 12 wt% 0. The design coal feed rate was 24.5 kg h - ‘. The coalloxygensteam mass ratio was 1.0:0.9 1: 0.27. The primary stream of premixed coal, oxygen and argon was preheated to 366 K, and the secondary stream of steam and helium was preheated to 430 K. The argon (also used to entrain the coal) and helium trace gases were supplied

Entrained ,+Secondary

Primary Stream

Stream -

Hydrogen Preheat

Igniter Methane

Top Plate

flow gasification

of coal. 7: Al.

Glass bottles retained the probe liquid and char. The char was filtered, dried and analyzed for C, H, 0, N, S and ash content with an elemental analyser and a sulphur analyser. Local mass balances were conducted to determine elemental conversion using independent argon and ash balance methods. Inert argon was used as a tie element to find the fraction of a coal element released to the gas: C, = [(M [email protected],, Y)/ (M w,,

Liner

Ceramic Quartz

I IF!15.73 cm

Viewport

J

cm

Quarl

Flange

Water -

Reactor Figure 1

Diagram

of high-pressure

entrained

flow gasifier

at 3.31 and 0.076 kg h- ‘, respectively. Nitrogen gas, for sight window purging, was supplied at 0.81 kg h-‘. Samples were taken from within the gasifier with a stainless steel water-quenched probe. At each axial position, this probe was traversed radially across the reactor to collect samples at selected radial positions from the centerline to the wall. Attempts to adjust the probe pressure to sample isokinetically were unsuccessful, because of turbulent fluctuations, sampling time constraints and the frequent plugging of probe pressure taps by char and water. To obtain suficient quantities of gas and char for analysis within the sample time of 3-5 min at each position, the probe velocity was often at least twice that required for isokinetic sampling. The nonisokinetic sampling method used probably caused some bias in the particulate samples toward the smaller particles. However, isokinetic sample removal from turbulent media is itself an uncertain procedure. Gas compositions should be more reliable than the particulate data, profiles of which are not reported here owing to this uncertainty.

Evaluation of results Gas samples were cooled, separated from the liquid and char and filtered. An on-line, wet test meter measured the probe gas flow rate and an on-line chemiluminescent analyser measured the NO concentration. Dry gas samples were collected in inert commercial polymer sample bags for subsequent chromatographic analysis.

Y,,w,km,)l I 00%

(1)

where M B$ and M WA,= the molecular weights of elements i and argon, m,l, and mk = the inlet mass flow rates of argon and dry coal, K and Y,, = the mole fractions of elements i and argon (wet or dry basis) in the local gas sample and w:= the weight fraction of i (dry basis) in the raw coal. Carbon and sulphur conversion values were evaluated by this method, since coal was the only source of these species. Equation (1) requires complete primarysecondary stream mixing and the assumption that the fuel gas in a sample comes from the char in the sample. This method is most reliable near the reactor exit but is invalid near the reactor inlet, where neither gas nor particle mixing was complete. The other method for determining local elemental conversion used a forced ash balance to evaluate the amount of a coal species left in the char, relative to the amount in the coal: c, = [ 1 - +V;:“/(w;“w;,]

Quench

R. Soelberg et al.

100%

(2)

where wp = the weight fraction (dry basis) of ash in the raw coal, wyh= the weight fraction (dry basis) of element i in the local char sample, wp = the weight fraction (dry basis) of ash in the local char sample and w: = the weight fraction (dry basis) of element i in the raw coal. In the ash method, ash is assumed to be inert. Toward the exit of the gasifier, ash loss due to devolatilization, wall slagging and dissolution in probe water may be as much as 40%‘jv9. Thus, near the reactor exit, Equation (2) may be inaccurate. RESULTS

Data accuracy The feed stream flow rate accuracies (standard deviations) were: coal f 1.2%, oxygen -t4.5%, steam _+6.8%, argon + 1.5%, helium 15.3% and nitrogen (window purge) f3.1’~. The gas species concentrations were accurate to within +_1.7% for CO, CO,, H,: CH,, 02, Ar and He, and + 3.3% for N,. The accuracies of the coal and char ultimate analyses were: carbon +1.2%, hydrogen &-3.0%, nitrogen + 1.5%, oxygen and sulphur +4.5% and ash f 14%. The propagated error of the individual experimental errors in data reduction calculations was determined using differential error analysis”. Equations (1) and (2) for elemental conversion were differentiated with respect to each error contributor. Summation of all the contributing errors gave the total propagated error. Where the argon method was valid in the lower half of the reactor, overall carbon conversion was accurate to f 3%. In the upper half of the reactor, where the ash method was probably more accurate, local carbon conversion was accurate to * 14%. Themass balance closure over the reactor from the inlet to the sampled axial station indicated the overall

FUEL, 1985, Vol 64, June

777

Entrained flow gasification

of coal. 1: N. R. Soelberg

et al. Over the same distance, the levels of the fuel-rich species CO and H, rose from zero to 44 and 20x, respectively.

45 0 Line (Total Coal Conversion,

daf)

I 0

25

50

% Total Coal Conversion

75

100

Scanning electron micrographs Scanning electron micrographs (Figure 4) of representative coal and char samples from within the gasifier showed that extremely rapid reaction was initiated very quickly in the forward region of the reactor (by the 28 cm axial sample position) with general particle blowhole formation, fragmentation and lacy ash development. No distinct swelling was noted. Only a few particles developed into hollow cenospheres, when they presumably failed to form blowholes to release gas from the particle interior. The cenospheres were never larger than the largest coal feed particles. Perhaps they were formed from some of the smaller coal particles, or from particle fragments. The particle heating rates were estimated to be in the region of 4 x lo5 K s-l. Solomon et al.” suggested that at such high heating rates, little swelling is expected, even for typically swelling coals.

(daf)

Figure 2 Centreline carbon, hydrogen and oxygen conversion compared with total conversion. 0, Carbon; 0, hydrogen; A, oxygen

40

+=/=!6~SEz~ ‘0 ‘2 0

for a test. The carbon and total mass balances ranged between - 12% and + 18%. The hydrogen, oxygen and sulphur balance accuracies were generally within *20x. The greatest sources of these errors may be inaccuracies in solid species concentrations in the char due to losses from ash devolatilization and errors from non-isokinetic sampling and in determining the steam concentration in the sampled gases. Since a water-quench probe was used, the steam concentration could not be measured directly. measurement

0

accuracy

20-

n/ ;/”

O co

I

I

0

I

Elemental conversion The gasifier outlet carbon conversion assessed by the argon method was 79 f 3%. Lewis et ~1.~ reported 89% carbon conversion using the same gasifier and coal type and similar test conditions, but with a different coal feed system. Mass balances for the Lewis results indicated a possible error in the coal feed rate, which was corrected for in subsequent coal feed system modifications. When corrected for this error, the Lewis tests also show z 79% carbon conversion. Local conversion values of carbon, hydrogen and oxygen relative to total coal conversion are shown in Figure 2. Hydrogen conversion was most rapid and complete, followed by oxygen and carbon. This is the same sequence as reported by Skinne?. The ash method for evaluating elemental conversion was used in Figure 2, since there are large errors in hydrogen and oxygen conversion based on the argon method, caused by inaccurate determination of the steam concentration in the sampled gases. Cubic polynomials fitted to the data points had correlation coefficients R* =0.997 for carbon, 0.933 for hydrogen and 0.935 for oxygen.

Gas species profiles Figure 3 shows radial profiles for CO, CO,, H2 and O,, all on the dry basis. CO, and 0, were high near the reactor inlet, 0, was rapidly depleted to zero and the CO, concentration decreased toward the exit from 45 to 27%.

778

FUEL, 1985, Vol 64, June

02

0 1

3 Radial

5

7

Distance from Centerline

9 (cm)

Figure 3 Measured radial profiles of CO, CO,, H, and 0,. Axial probe location (cm): 0, 13; 0,20; A, 28; 0,34; V, 51; X, 112

I

Entrained

DISCUSSION Mixing

and reaction

zones

Three regions of mixing and reaction were evident within the reactor. Most evident was a region of intense, rapid reaction extending out radially and axially from the inlet to z 35 cm downstream. In this region, rapid particle heating and devolatilization took place as gas-phase oxidation of the devolatilized products supplied the heat.

flow

gasification

of coal.

1: N. R. Soelberg

et al.

The estimated time (from model computations) for complete devolatilization and oxygen consumption was z 11 ms. The mixture changed quickly from oxygen-rich to fuel-rich; as the oxygen was depleted to zero, CO, decreased from 45% to 30x, CO increased to 40% and H, increased to 20%. The maximum inside wall temperatures were z 1400 K. The second region was a zone of recirculation in the upper corner of the reactor. Gas and particle samples in this region indicated reverse flows and low char flux. Those particles in the recircuIation zone were smaller and more completely consumed, as indicated by elemental conversion and scanning electron micrographs. A third region, in the lower two-thirds of the reactor, was a region ofgas and particle mixing and heterogeneous reaction. The Reactor inner wall temperatures decreased from a maximum of 1400 K near the inlet to ~950 K at the outlet. Radial profiles of gas concentration rapidly became uniform, indicating essentially complete mixing by =c35 cm downstream. Only slight overall change in gas composition was observed beyond that. An increase in carbon conversion from ~70 to 79% showed that slow heterogeneous char reactions with CO,, H,O or both had occurred. In addition, the water gas shift reaction (CO + H,O = CO, + HZ) adjusted the gas composition. The CH, level increased only from 0.3 to l.O%, which is normal for high-temperature, atmospheric pressure gasifiers. At temperatures between 1200 and 1800 K, the rate of the char-H, reactions is one to two orders of magnitude slower than the rates of char reactions with CO, or H,O, which are similar over that temperature range. Calculations show that the gas phase is very near equilibrium at the outlet. This conclusion is consistent with the findings of others for high-temperature gasifiers’.‘2. Considering the decrease in CO, level from 28 to 23x, the decrease in H,O Ieve from 40 to 35x, the increase in H, level from 20 to 25% and the constancy of the CO level, char-CO, or char-steam reactions may proceed to some degree, with a buffering action on the CO level by the water gas shift reaction. Comparison

with other experimental

gusijers

Table 1 compares

Figure 4 Selected scanning electron micrographs of raw coal and char samples obtained within the gasitier: a, raw coal ([email protected]; <74pm): h, char from 28cm axial, 4cm radial position

Table I

Comparison

of test results with those of McIntosh

_____ -___ Coal feed rate (kg h-r) Coal type Coal:O,:H,O mass ratio Coal throughput (kg mm3 hh’ kPa-‘) Carbon conversion (y’, dafb) (CO + H, + CH,) yield (kg kg- ’ coal) Dry gas composition (argon-free) (molX) co CO, HZ CH, N, etc. Dry gas caloritic value (MJ mol-‘) Cold gas efficiency (“/;;) Reactor heat losses (%) ” From

an overall

heat balance,

and Coate?

results from this study (test 25) with those of McIntosh and Coates4 and Sebastian3. The reactor configuration, construction and size of the gasifiers used by these workers were similar to those of the

and Sebastian3

Test 25

McIntosh-Coates

24.5 Utah bituminous

20.5 Utah bituminous

1:0.91:0.27

1:0.92:0.3

23.6 79 0.84

2.6 91 1.04

45.9 25.1 25.9 1.0 2.1 202 42.7 11

using 298 K, 101 kPa and liquid water as the standard

47.3 16.7 33 0.2 2.8 248 64.0 17

(No. 251F)

Sebastian

(No. 69)

23.7 Sewickley bituminous 1:0.77:0.15 16.8 72 0.96 48.5 13.9 33.2 4.4 232 59.1 14

state

FUEL, 1985, Vol 64, June

779

Entrained

flow

gasification

of coal.

1: N. R. Soelberg

et al. Interpretation

0

0

02

0

0.2

04

0.6

04

06

Normabed

Figure S centrations

0

Comparison (mol%, db)

posItIon

axial

of measured

(a) and

08

10

08

10

(x/L)

predicted

(b) CO con-

a

FO.6 ,04 P r 0.2 8 aO 0

02

0

04

0.6

08

10

08

10

02 0 0

02

0.4 Normallzeci

Figure 6 centrations

Comparison (mol%, db)

06 axial

of measured

positlon

(x/L)

(a) and predicted

(b) CO2 con-

present gasilier. However, neither obtained samples from inside the gasifier. The McIntosh-Coates gasifier used two stages, with an i.d. of 7.6 cm and a combined length of 165cm. This gasifier was operated at 1.01 MPa. The gasifier used in Sebastian’s study was 15.2cm i.d. and 213 cm long and was operated at 115 kPa. The coal feed rate, coal type, input stoichiometry and exit gas composition in all three studies were similar. Although the observed exit carbon conversion is bracketed by the values from the other studies and the percentage heat loss is lower, the cold gas efficiency is lower in the present study. This may be due in part to the higher coal throughput in this study, resulting in a lower residence time. The use of trace gases, nitrogen purge gas and lower preheat temperatures and the presence of the sample probe also contribute to this lower efficiency. In addition, the McIntosh-Coates gasifier incorporated recycle of the outlet gas to preheat steam and oxygen, giving energy savings of ~4% of the coal heat content.

780

FUEL,

1985,

Vol64,

June

using predictions

Fletcher* conducted preditions for these tests using a generalized model (PCGC-2, pulverized coal gasification and combustion, 2-dimensional) developed by Smith et model describes particlea1.‘s8. This two-dimensional laden, turbulent diffusion flames. Gas-phase reactions were assumed to be micromixing-limited and not kinetically limited, approaching partial thermodynamic equilibrium. Partial equilibrium allows for the presence of carbon in the form of the fuel-rich gas species CO and CH, instead of solid carbon. Without this assumption, the predicted carbon conversion was low. Two predictions were made, one assuming no reactor heat loss and the other assuming z 40% heat loss. With no heat loss, the predicted outlet carbon conversion was 92x, which was 13 higher than the experimental value of 79%. Assuming 40% heat loss, an outlet carbon conversion of 75% was predicted, 4 lower than observed. An estimated heat loss of only z 11% was determined from an energy balance over the entering and exit streams. The more realistic heat loss value was not considered in the model computations. A higher carbon conversion resulting from a lower heat loss was also observed experimentally in tests previous to the mapping test series, when reactor section 1 was externally cooled. Cooling of the reactor section resulted in a 200 K wall temperature decrease and a 10% decrease in carbon conversion. Recent gasifier studies have incorporated external insulation to reduce heat loss and a lengthened reactor to increase the residence timei3. These changes have also yielded a significant increase in carbon conversion. The model predicted the same three zones of reaction and mixing as found experimentally. Gas species concentration were maps constructed by double interpolation between radial and axial profiles. Experimental and predicted maps (the latter shown for the higher heat loss level considered) for CO, CO,, and H, are compared in Figures 5-7. Good qualitative agreement was found in the respective trends, peaks and concentration levels for those species. The model predicted that coal devolatilization was complete within the first

E

02

02

04

0.4 Normalued

Figure 7 centrations

Comparison (mol%, db)

of measured

axial

06

0.8

1.o

06

0.8

10

posltlon (x/L)

(a) and

predicted

(b) H,

con-

Entrained

one-third of the reactor and accounted for 88% of the predicted carbon conversion. Gas mixing and heterogeneous reactions took place in the lower two-thirds of the reactor. CharCO, and char-H,0 reactions occurred, but still accounted for only 12% of the total predicted outlet carbon conversion. The average particle heating rate was ~4 x lo5 K s-l, requiring ~6 ms to reach a temperature of 2300 K. The estimated time for complete devolatilization was = 11 ms. Such rapid devolatilization had been observed at high temperatures and high heating rates by by Kobayashi et coal and lignite. The qualitative all4 for bituminous agreement between observations and calculations in Figures 5-7 suggests that these reaction times are approximately correct.

flow gasification

of coal. 1: N. R. Soelberg

et al.

REFERENCES 1

2 3 4

5

6

7

8 9

ACKNOWLEDGEMENTS

10

This work was supported principally by the US Department of Energy, Morgantown Energy Technology Center, Morgantown, WV. Dr Holmes Webb was the USDOE Project Officer. Financial support from the Research Division and the College of Engineering and Technology of Brigham Young University is also acknowledged. Contributors to this work include Drs Angus Blackham, Thomas Fletcher and Philip Smith and Mr John Highsmith. The assistance of Ronald Anderson, Blaine Brown, Joel Glassett, Brent Kelly and Jeffrey Lindsay in testing and analysis is recognized.

11

12 13

14

Von Fredersdorff, C. G. and Elliott, M. A. in ‘Chemistry of Coal Utilization’, Supplementary Volume (Ed. H. H. Lowry). Wiley. New York, 1963 Bissett, L. A. US Dept of Energy, Morgantown Energy Research Center, MERC/RI-7812, 1978 Sebastian, J. J. S. Ind. Eng. Chem. 1952,44, 1175 McIntosh, M. J. and Coates. R. L. Final Report on US Dept of Energy Contract No. EX-76-C-01-1548, Eyring Research Institute, Provo, UT, 1979 Skinner, F. D., Smoot, L. D. and Hedman, P. 0. American Society of Mechanical Engineers, Conference Paper No. 80WA/HT-30, Chicago, November 1980 Lewis, G. H., Smoot, L. D. and Hedman, P. 0. Paper to Combustion Institute, Western States Section Meeting, Salt Lake City, UT, 1982 Smith, P. J., Fletcher,T. H. and Smoot, L. D. in ‘18th Symposium (International) on Combustion’, Combustion Institute. Pittsburgh, 1981 Fletcher, T. H. PhD Dissertation. Department of Chemical Engineering, Brigham Young University, Provo, UT, 1983 Soelberg, N. R. MS Thesis, Department of Chemical Engineering, Brigham Young University, Provo, UT. 1983 Holman, J. P. ‘Experimental Methods For Engineers’, 3rd Edition. McGraw-Hill. New York. 1978 Solomon, P. R., Hamblin, D. G. and Best, P. P. Quarterly Report for June 4, 1982 to August 31, 1982, Contract No. DE-ACZl81FE05122, Advanced Fuel Research, Inc., East Hartford, CT 1982 Pilcher, J. M., Walter, P. L., Jr and Wright, C. C. Ind. Eng. Chm. 1955,47, 1742 Hedman, P. O., Smoot, L. D., Fletcher, T. H., Smith, P. J. and Blackham, A. U. Interim Report for US Dept of Energy Contract No. DE-AC21-81MC16518, Brigham Young University. Provo, UT, 1984 Kobayashi, H., Howard, J. B. and Sarotim, A. F. in ‘16th Symposium (International) on Combustion’, Combustion Institute, Pittsburgh, 1977

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