-1

UNDERGROUND COAL GASIFICATION AT

HANNA, WYOMING

G. G. CAMPBELL, C. F. BRANDENBURG, 'R. M. BOYD, AND T. E. STERNER

LARAMIE ENERGY RESEARCH CENTER LARAMIE, WYOMING

PRESENTED AT THERMAL POWER CONFERENCE, WASHINGTON STATE UNIVERSITY, PULLMAN, WA

OCTOBER 16, 1975

ABSTRACT

UNDERGROUND COAL GASIFICATION AT HANNA, WYOMING

G. G. Campbell, C. F. Brandenburg, R. M. Boyd and T. E. Sterner

Laramie Energy Research Center Laramie, Wyoming

A second underground coal gasification experiment is being conducted at the Laramie Energy Research Center's field site near Hanna, Wyoming. This paper preSents the results from the first phase of the Hanna II experiment in which coal seam permeability was evaluated, pneumatic linking and linking via reverse combustion were studied, and in which a sustained gasification process was maintained between the two linked vertical wells. Air injection alone did not increase permeability to levels which .JOuld allow sustained gasification. Reverse combustion linking was determined to be a local, relatively low temperature process which advanced at a rate of 5 ft/day and produced a high permeability carbonized path. Subsequent gasification was maintained over a 38 day period. Overall, injection of 1.9 }~ scf/day of air yielded 2.7 MM scf/day of gas with an average heating value of 152 Btu/scf. Material balance calculations indicate that 1700 tons of coal in place were utilized.

INTRODUCTION

Underground coal gasification (UCG) has been the subject of investiga­tions for more than 100 years (1). The process has been shmm to be technically feasible, but has been economically unattractive in competition with petroleum resources. The recent shortages of petroleum fuels have rekindled interest in research on efficient in situ gasification of coals in the United States. Russian activities in underground gasification of coal have been the most sustained (1). Electricity has been generated from low-Btu gas at several large scale plants in the Soviet Union from sub­bituminous coals and lignites since 1960. Recently, diminished Soviet utilization of UCG for energy production is believed due to the discovery of adequate local petroleum fuel sources. In England, UCG was studied extensively and field experiments produced processes ,vhich were techni­cally feasible and nearing commercial utilization but were abandoned in

1959 because they were not sufficiently economical at that time (2). Similarly, investigations were c6nducted in the United states by the Bureau of ~lines from 1946 to 1959 (3-6) and by Gulf Research in 1968 (7). These studies were also discontinued for lack of immediate need and economic incentive.

The Bureau of Mines started a reassessment of UCG in 1971 (1). This study showed that a renewed field effort was appropriate. A field site near Hanna, Wyoming, was selected and the Hanna I experiment was conducted be­tween March 1973 and March 1974 (8-10). Tne Hanna I experiment showed that western subbituminous coals were suited to underground gasification because they are generally thicker, more permeable and more reactive. Tne Hanna I experiment also showed that a low Btu gas, 125 Btu/ft 3; could be produced for an extended period.

A second field experiment, Hanna II, was designed to answer questions generated by the. first experiment and to include an expanded instrumentation effort. The instrumentation was provided by the Sandia Laboratories and included thermal, acoustic and resistivity measurements.

Hanna II is now being conducted in three phases: Phase 1, an experi­ment to examine pneumatic and combustion linking of wells to conduct a limited gasification test, and to evaluate instrumentation; Phase 2, two two-well burns to gasify coal berween an injection and production well in preparation for the third phase, a line-drive burn to gasify coal along a 60 foot combustion front.

OPERATION

A basic problem in underground coal gasification is the formation of an underground reaction vessel before actual gasification of the coal. This is necessary because the permeability of the virgin coal is too low to allow gasification by either a forward or a reverse combustion process. Various approaches to underground gaSification differ mainly in the technique used to create the necessary permeability. These techniques include mining, directional drilling, fracturing, electrolinking and combustion (1). The Hanna experiments use the combustion oriented lL~ked vertical well technique. In this concept, the reaction vessel is prepared by generating a permeable path through the coal seam between adjacent vertical wells by reverse combustion.

FIELDING DESCRIPTION

The Hanna II experiment is being conducted on a site near Hanna, Wyoming, at a location approximately 700 ft west of the site of the Hanna I experiment. The location and geology of the formation have been described previously (8,9,13). The ceiling of the thirty-foot-thick coal seam lies at a depth of 275 it compared to a depth of 400 ft in the Hanna I experiment.

The surface layout of the eight wells for Phase I of the Hanna II experiment is shown in Fig. 1. Wells I, 2, 3, and 4 were cased injection

and production wells. The original experiment plan called for Wells 2, 3, and 4 to be on a 60 ft radius from Hell 1; however, the original Well 3 was plugged and abandoned due to a parted casing and cement bonding proble1!lS. Wells AA, BB, CC, and DD were 'instrumentat,ion wells containing thermocouples, geophones, resistivity probes, and a gas sampling canister.

Wells 1, 2, and 4 were drilled two-thirds of the way through the coal seam and 6 in diameter casings were ,set at this level and cemented with high temperature cement containing 35% silica flour. After cementing, the hole was reentered and drilled out to the bottom of the coal, seam., The second Well 3 was drilled out to a level 15 ft below the bottom of the coal seam and the 61n casing set at this level. After cementing with high temperature cement, the casing in the bottom third ,of the coal, "as' perforated with 4 perforations per ,foot. This completion method, with access to only the bottom third of the coal seam, was intended to confine the linkage pathway in the lower portion of the seam.

The air injection and gas production facilities used for the experiment are shown schematically in Fig. 2. All static and differential pressure, and temperature measurements were made with electronic transducers. The signals from these sensors were recorded with a system built around a mini­computer. Computer software was developed so that both injection and production flow measurements were calculated at five minute intervals from which;,hourly average flow parameters were obtained.

TEMPERATURE INSTRUMENTATION

Phase 1 was planned as a preliminary experiment (11). For this reason, the temperature sensing array, shown in Fig. 3, was sufficient to provide only limited spatial resolution in the vertical direction and to observe typical thermal histories at isolated points. Such a sparse array does not allow clear definition of the velocity and direction of reaction zone propa­gation, the horizontal location of linking paths, nor the lateral extent of the reaction zone at any horizon.

EXPERIMENT~ RESULTS

,Phase 1 of the Hanna II experiment was conducted in the following sequence: (1) air injection tests to determine the permeability and directional flow characteristics of the coal seam and to attempt to create a pneumatic link, (2) ignition and formation of a link by reverse combustion, and (3) sustained gasification in a forward burn mode.

DIRECTIONAL PERHEABILITY AND PNEU¥.ATIC LINK TESTS

Since the flow of air through the natural fractures in virgin coal is an important parameter during linkage, the Hanna II experiment was designed to evaluate not only the directional permeability under seam conditions but also to evaluate permeability enhancement via continued air injection through the seam without combustion (pneumatic linking). Based upon directional permeability measurements and visual analysiS of an oriented core taken from

WeIll, Well 2 was located from WeIll along the major fracture direction of" the coal, Well 3 was located along the minor fracture direction, and Well 4 o was located 100 from the major fracture direction. This is shown in Fig. 1.

The measurement of the directional flow of air from WeIll to Wells 2, 3, and 4 was conducted April 7-16, 1975 (Julian days 97 'to 106; reference January 1, 1975). The injection pressure and injection rate during this period are shown in Fig. 4. The injection pressure was maintained below the overburden pressure to prevent pneumatic fracturing. The average air pro­duction rates of Wells 2, 3, and 4 during this period were 2,30, and 9 scfm, respectively, clearly indicating preferred flow in the direction towards Well 3. This coincides with the updip and minor fracture direction of the coal seam--Fig. 1 (13). This directional flow portion of the experiment was prematurely halted when Well CC showed signs of leakage. Air injection was switched to Well 3 in order to reduce the pressure at Well CC.

Air injection was initiated at Well 3 on April 17 (107), and was continued for 31 days--Fig. 4. Injection pressure was increased progressive­ly over a five day period, then held steady at approximately 225 psig during this experiment. At constant pressure, the injection rate increased only slightly. However, the air recovery rate was approximately 40 to 50 percent. Two wells are considered to be linked when injection capacity is at least 1000 scfm of air at pressures of less than 50 psig.

IGNITION AND REVERSE COMBUSTION LINKAGE

The slightly enhanced permeability observed during the pneumatic linking phase was judged inadequate to provide sufficient flows for gasification. Such permeabilities and resultant flows can be obtained by reverse combustion as demonstrated in the Hanna I experiment (10).

The coal was ignited in the lower 10 ft of 1-1ell 1 with air injection into Well 1 and Well 3 open for production--Fig. 5b. A 24 Kw electric heater was used to achieve ignition. Injection and production flows during this ignition phase are shown in Fig. 6. The temperature of the heater shell was maintained between 800° and 10000F from initiation of heating on May 21 (141) until Hay 25 (145). Hithin 12 hours after initiation of heating, an abrupt increase in C02 content of the gas produced from Well 3 indicated that combustion had been initiated. As indicated in Fig. 6, injection and production rates both decreased during this period even though the air injection pressure increased to 270 psig. This decreasing flow behavior was typical of prior attempts to effect forward combustion in a coal seam without preliminary permeability enhancement.

In the second step of combustion linking, flow is reversed to achieve the reverse combustion mode--F·ig. 5c. The flow of the oxidizing air through the fissure structure toward the ignite,d region progressively reacts with the coal at the exposed faces near the burning region, thus enlarging the flow cross-section along local paths.

The downhole heater was removed from Well 1 and air injection into Well 3 was begun to initiate formation of a link by reverse combustion. The flow parameters of the system during this period are shown in Fig. 7. The injection pressure during this period was main.tained between 240 and 270 psig. The initial air injection rate was about 50 scfm; this rate gradually increased to almost 90 scfm by June.3 (154). The gas production during this period increased gradually until, on June 1(152), the produced'gas rate exceeded that of air injection.

A nitrogen balance ratio (nitrogen in the produced gas versus the nitrogen injected) during ,this period ranged from 40 to 80%. Some nitrogen loss to the formation was expected due to the high injection pressure. The oxygen content.of the. produced gas ranged from 1 to 2%; indicating that a small .amount of inj ected air bypassed thereac tion zone. The overall gas composition of produced gas during this period was like that of Hanna I and is shown in Fig. 8.

The'completion of linkage is a sudden and dramatic occurrence. It is evidenced .by an abrupt increase in inject,ion and production rate and decrease in injection.pressure as was observed in this work and as reported previously (12). An hourly plot of inj ection pressure and flow rate is shown in Fig. 9, and, shows. a drop from 220 to 141 psig ,at 0900 on June 4 (155). The 500 cfm plateau in the flow rate resulted from the limited flm. capacity of the single air compressor on line at the time of linking. The addition of another compressor resulted in the increase in pressure and flow rate at 1400 hours. The injection pressure continued to decrease over an additional period, but linkage v,as virtually complete after 15 hours. As shown in Fig. 8, a distinct change in the product gas composition also occurs upon linkage: CO, H2 and CHI.! all increase and the concentration of CO exceeds, tha t of ;C02'

This linking process utilized 33 Mscf of air injected pe.r lineal foot of linkage, the same quantity utilized during the Hanna I experiments. This is less than the Russian experience for which values of 80-240 Hcf/ft have been reported ,(14). Haterialba1ance calculations (10, 15) during the 1inkipg.process indicate that a total of 15.1 tons of coal in, place were affected.

GAS IFICATION

Upon linking, the increased flows were sufficient to sustain gasifica­tion. The same injection and production wells ,.ere used, and the differences from the preceding step were only in the flow parameters and changes in the coal seam produced by combustion linking. The reaction zone proceeded from Well 3 toward \.Je1l 1 in a forward burn mode with the reaction air flowing both along the reaction surface and through the reaction: zone into the unreacted c.oal. The geometry of the burn front' was dictated by the permeable link and .was affected by the relative flow resistance of the linking path and the less permeable but larger area of the unreacted coal.

The gasification phase of the experiment was continued for 38 days from the time of linking. During this period, the air injection rate usually

exceeded 1000 scfm (1.4 MMscf/day) and gas" production rates were generally greater than 1.4 times the air injection rates. The average daily flow conditions during that period are shown in Fig. 10. This figure shows that the injection pressure gradually decreased during the course of this experi­ment. Occasional variations in the injection rate reflected deliberate or unavoidable operational changes. Significantly, none of the variations of flow observed at the surface resulted "from subsurface conditions peculiar to the in situ oblique forward bu"rn" reactor system, but.were the result of operational changes in air compressor equipment.

Measured and calculated values for the gas produced are both shm-m in "Fig. 10. There are inherent difficulties in accurately measuring the flow rate of a gas containing unknown and variable quantities of water and tar. It is felt that the flow rate calculated from a nitrogen balance and assuming zero leakage, as was observed in Hanna I (9), gives a more realistic value of the gas production rate.

The daily average compositions of the produced gases are shown in Fig. 11. The daily average gross heating value and product temperature are given in Fig. 12. The overall average gas composition for the gasification phase is found in. Table 1. This gas composition is similar in most respects to air­blown producer gas (16). Table II presents a summary of the average operational parameters during the gasification phase.

The quantity of affected in place coal was calculated by a material balance method used previously by Elder (15) and was also used for evalu­ation of the Hanna I experiments (10). Daily material balance results for the gasification phase are presented in Fig. 13. The analysis assumes that the in situ coal reaction occurs in two steps: Carbonization followed by gasification. The method. is based on the stoichiometric difference between carbonization and gasification products. Only the carbon, hydrogen, and oxygen are considered in this calculation and a steady state reaction is assumed. The calculated amount of "coal-carboni zed-only" includes that portion of coal from which only the carbonization products have been removed prior to complete gasification. Although variations of production rate were experienced, the calculated material balance is considered to offer a good approximation of the amount of coal utilized. From this analysis, the total amount of coal affected during gasification \Vas 1000 tons (moisture and ash­free) which corresponds to about 1700 tons of in situ coal. Of· the 1000 tons, total, 500 tons w-erecarboriized, and 500 tons ","lere gasified completely.

CONCLUSIONS

This first phase of the Harina II experiment utilizing the linked vertical well technique has again demonstrated the technical feasibility of lli"i.der ground coal gasification and has been helpful in developing a description of the basic process parameters involved.

Evidence from this experiment suggests that the dip direction, as well as the fracture direction, play an important role in determL,ing the preferred directional flow of air injected into the Hanna I coal seam.

An attempt to create a link between adjacent wells sufficient to sustain gasification simply by air injection was not feasible. The ability to effect a linkage by reverse combustion in a reproducible and dependable manner has been confirmed.

During the 38 day gasification period, injection of 1.9 MMscf/day of air yielded 2.7 MMsef/day of produced gas with a heating value of 152 Btu/scL Material balance calculations indicate that 1700 tons of coal in place were utilized. Two.features distinguished the gas production in this experiment: the heating value of product gas was both relatively high, 152 Btu/scf, and relatively uniform over the life of the experiment. Both factors favorably affect economic feasibility~

ACKNOWLEDGMENT

The author thanks all personnel of the Laramie Energy Research Center and Sandia Laboratories associated with various aspects of this field experiment. Special acknowledgment is given to the Rocky Mountain Energy Company, a subsidiary of the Union Pacific Railroad, for the use of their land as the test site at Hanna, Wyoming.

RE:E'ERENCES

.1. A. D. Little, Inc. itA Current Appraisal of Underground Coal Gasifica­tion." Report No. C-73671, Dec. 1971, 121 pp.

2. Sir Alexander Gibb and Partners, The Underground Gasification of Coal, Sir Isaac Pitman and Sons, Ltd~ ,London (1964).

3. Dowd, J.J., J.L, Elder, J.P. Cohen. Gasification of Coal, Gorgas, AL." 62 pp.

·"Experiment in Underground BuMines RI 4164, Aug. 1947,

4. Elder, J.L., M.H. Fies, H.G. Graham, R.C. Montgomery, L.D. Schmidt, and E.T. Wilkins. "The Second Underground Gasification Experiment at Gorgas, AL." BuMinesRI 4808, Oct. 1951, 72 pp.

5. Capp, J.P., J.L. Elder, C.D. Pears, R.W. Lowe, K.D. Plants and M.H. Fies, "Underground Gasification of Coal--Hydraulic Fracturing as Method of Preparing a Coalbed." BuMines RI 5666, 1960, 50 pp.

6. Capp, J.P., and K.D. Plants, "Underground Gasification of Coal with Oxygen-Enriched Air." BuMines RI 6042, 1962, 14 pp.

7. Raimondi, P., P.L. Terwilliger, and L.A. Wilson, Jr., "A Field Test of Underground Combustion of Coal, " SPE4.J13, presented at the Eastern Regional Meeting of the SPE of AIME, Pittsburgh, Penn., Nov. 7-9, 1973, 11 pp.

8. Campbell, G.G., C.F. Brandenburg, and R.M. Boyd, "Preliminary Evaluation of Underground .Coa1 Gasification at Hanna, Wyoming," BuHines TPR 82, Oct. 1974, 14 pp.

9. Schrider, L.A., J.\.[. Jennings, C.F. Brandenburg, and D.D. Fischer, "An Underground Coal Gasification Experiment, Hanna, Hyoming," SPE 4993, presented at the SPE Fall Meeting of AIME, Houston, TX, Oct. 6-9, 1974, 25 pp.

10. Brandenburg, C.F., D.D. Fischer, G.G; Campbell, R.M. Boyd, and J.K. Eastlack, "The Underground Gasification of a Subbituminous Coal," presented at the Div. of Fuel Chem., Am. Chem. Soc. National Spring Heeting, Philadelphia, PA, Apr. 1975, 11 pp.

11. Brandenburg, C.F., R.P. Reed, R.M. Boyd, D.A. Northrop, and J.W; Jennings, "Interpretation of Chemical and Physical Measure­ments From an In Situ Coal Gasification Experiment," SPE 5654, presented at the SPE Fall Meeting of AIME, Dallas, TX, Sept 28-Oct. 1, 1975, 28 pp.

12. Kreinin, E.. and M. Revva, "Underground Gasification of Coal" Kemerovskoe Knizhnoe Izdatel'stvo, 1966, 85 pp.

13. Schrider, L.A., C.F. Brandenburg, D.D. Fischer, R.M. Boyd and G.G.- Campbell, "The Outlook for Underground Coal Gasification," presented at the 1975 Lignite Symposium, Grand Forks, ND, May 1975.

14. Lavrov, N.V., G.O. Nuginov, D.K. Semenenko, "Results of Studies of Underground Gasification of Brown Coal," Khimiya Tverdogo Top1iva 1 (1968) 113-120.

15. Elder, J.L., M.H. Fies, H.G. Graham, J.P. Capp, and E. Sarapuu, "Field-Scale Experiments in- Underground Gasification of Coal at Gorgas, Alabama; Use of Electrolinking-Carbonization as a Means of Site Preparation," BuMines RI 5367, October 1957, 101 pp.

16. Fischer, D.D., and L.A. Schrider, "Comparison of Results from Underground Coal Gasification and From a Stirred Bed Producer," presented at the National AIChE Heeting, Houston, TX, Har. 16-20, 1975, 34 pp.

"

TABLE I

Overall Average Composition of Product Gas (volume percent)

Hz 17.3 Nz 51.0

CO 14.7 Ar. 0.6

COz 12.4 H~S 0.1

CHIf 3.3 CZ-CIf 0.6

TABLE II

Average Operational Parameters

Heating Value of Gas (Gross) 152 Btu/sci

Daily Gas Production 2.7 MM scf

Daily Air Injection 1.9 MM scf

Daily Btu Production 420 MM Btu

Ratio: Heat/Air Volume 223 Btu/sci

N60W

FIGURE l~

TRUE NORTH

1 MN

o 10 20 30 40 feet

H I I I I S20W

WELL PATTERN; PHASE OF THE HANNA II EXPERIMENT

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LINE - GATE VALVE -1,1- ORIFICE

FIGURE 2, - SCHEMATIC OF SURFACE PIPING

WELL 3 WELL 1 DO SURFACE CC

GROUTED INSTRUMENT

WELLS

275' OVER-BURDEN

3

4

5 6 LEVEL 7

8 30' COAL SEAM

9

52.5 45 30 15 0 , , I I ,

FEET FROM WELL I

FIGURE 3, - CROSS SECTION OF WELLS 1, CC, DD, AND 3 SHOWING WELL COMPLETIONS AND THERMOCOUPLE LOCATIONS

DIRECTIONAL PERMEABILITY TEST

INJECTION INTO WELL 250

200 0> (I)

a.

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JULIAN DAY, 1975

FIGURE 4. - DIRECTIONAL PERMEABILITY AND PNEUMATIC LINK TESTS

.,

(A.) Virgin Coal (low permeability)

HIGH PRESSURE GAS AIR INJECTION PRODUCTION

(C.) Combustion Linking Front Proceeds to Source of Air

P

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-ION .l1'4 u t:.l-TION I I~

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(B.) Ignition of Coal

LOW PRESSURE AIR INJECTION

GAS PRODUCTION

--tl-I ~I--

(D.) Linkage complete when combustion zone reaches injection well (system ready for gasification)

FIGURE 5. - SEQUENCE OF EVENTS IN LINKAGE BY REVERSE COMBUSTION -.'

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I I 70 ! -l

E ! I If-

I u (/)

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-----~40 ~ PRODUCTION RATE

AT WELL 3

141 142

JULIAN 144

1975

FIGURE 6. - FLOW PARAMETERS DURING 'IGNITION

o 145

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W I­« 0:: 1000

Z o -I-o :::> Q o a:: a.. Q

Z « Z o

INJECTION PRESSURE

PRODUCTION RATE

280

240

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w a:: :::>

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120

W a:: a.. Z o I­o W ..., Z H I­

o W ..., Z H

0E-"3 T··~·;·=-~ I -=- Jo 145 150 155

JULIAN DAY, 1975

FIGURE 7, - FLOW PARAMETERS DURING LINKAGE VIA REVERSE COMBUSTION

I­Z UJ 60 u a:: UJ a.

50 UJ :E. ;:, c; 40

~ I z .

ri ~ 20t __ L C02

___ ---- --

u

8 10 ________________________ L_~~ _____________ ---------·-------------

°1~5:::;2:-----;-1.;-53:;-------;1.;-5-=4----~-=-----1:-!:5:-:6-------l157

JULIAN DAY,

W I­<t a::

FIGURE 8. - PRODUCED GAS COMPOSITION DURING LINKING AND EARLY GASIFICATION

TIME. hours J500r-~lr2rrrl~6~TT~~~~4~~8~~1~2~~16~~20;,~~~~rr~~~12300

FLOW RATE too; :::J fJ) fJ) w a:: 0...

Z o

z o H I­U w 500 --,

100 ~ U W --, Z

H PRESSURE

FLOW RATE

OL-__________ ~~--------------------~--------~O 156

JULIAN DAY. 1975

FIGURE 9. - FLOW PARAMETERS AS LINKAGE WAS ATTAINED

Z H

>-0

"'0 ....... -0 C/l

~ ~ .

w I-« 0::

Z 02 H I-U ::> 0 0 a:: 0...

0 Z « z 0 H I-U W J Z H

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MEASURED FLOW

CALCULATED FLOW ,',

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, ,

~~ ... ~

I ~,

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~/

, . '

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t[ 60 •

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50 ::> (f) (f)

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30 Z 0

20 H I-U

10 w J Z OLI ____ -L ________ ~L_ ________ ~ ________ _L ________ ~ __ . ________ L_ ________ _L ________ ~ __ ~ 0 H

152 155 160 165 170 175 180 185 190 192

JULIAN DAY. 1975

FIGURE 10. - AVERAGE FLOW PARAMETERS DURING GASIFICATION TEST c,

I , I

I

.-.60 .­Z W u 50 a:: w n..

w 40 ~ :::> ...J o > 30 '-'

z o ~_ H20/""-.- '-­« --

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~ ~ ............. ..

u Z o U OIL ______ L-____ -L ______ ~ ____ ~~ ____ ~~--~~ ____ ~~

155 160 165 170 175 180 185 190 192

JULIAN DAY, 1975

fIGURE 11. - AVERAGE COMPOSITION Of THE GASES PRODUCED DURING GASIfICATION TEST

,.

(!) Z H I­<{ W I

I I I 1 I I , ,

I I , , ,

I I I

/ ,_/

I I I I

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HEATING VALUE W lY :::> I-

300 <{ lY W a.. :E

200 w I-

01 I I I I I I I I 10 152 155 160 165 170 175 180 185 190 192

JULIAN DAY, 1975

FIGURE 12. - AVERAGE HEATING VALUE AND TEMPERATURE OF THE GASES PRODUCED DURING GASIFICATION TEST

'-,

i ,

I I I

I I !

....J <i o u

o COAL CARBONIZED ONLY

E223 COAL COMPLETELY GASIFIED

01 ~~1r1r/r1rLr/r/r/r/r/r/r/r/r/fLr/r/r/r/r/i

155 160 165 170 175 JULIAN DAY, 1975

FIGURE 13. - MATERIAL BALANCE CALCULATIONS, DURING GASIFICATION TEST