Kinetics of Coal Gasification - Design of Atmospheric Pressure

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT

Kinetics of Coal Gasification DESIGN OF ATMOSPHERIC PRESSURE GASIFIERS HOWARD R. BATCHELDER' AND ROBERT M. BUSCHE2

U. S. Bureau

of Mines, Louisiana, Mo.

P

RECEDING poi tions of this study have presented the basis for and development of a method of calculating the kinetic changes occurring during atmospheric pressure, dilute-phase suspension gasification. This method has been applied systematically to the solution of a number of cases over a range of operating conditions. This paper deals with the assembled results of those calculations. The authors intended to compare the calculated results with those actually obtained from a number of experimental units now in operation. It was hoped that this comparison would substantiate the calculated results or furnish indication of revisions in the method of computation that would lead to results con&tent with those obtained in the pilot plants. A substantial start had been made on this comparison, using data then available from three experimental units. Indications were that the calculated results were within 5% of the experimental data. However, it was found that any precise comparison would involve extensive recalculation of the existing data and new experimental data to cover the desired ranges. Because of the time required to accumulatc and correlate the necessary data, and because of termination of the work of the U. S. Bureau of Mines a t Louisiana, Mo., this comparison has not yet been completed satisfactorily. Therefore, the results of the calculations are presented a t this time with the understanding that they must be used with caution until they have been validated experimentally. Many of the cost elements in the production of synthesis gas from coal can be estimated satisfactorily from material balance equations and assumed carbon conversions. This does not, howeuer, provide information as to the relation between gasifier volume and feed rate or the effect of operating variables on this Ielation. The capital charges on the gasifier itself may be a substantial part of the total cost of the gas produced, and residence time must be one of the factors considered in making a choice of operating conditions. The three most important process functions in gasification are the percentage carbon gasified, the synthesis gas yield per unit weight of carbon fed, and the oxygen requirements per unit of synthesis gas yield. These are plotted as functions of oxygencarbon ratio and residence time for three particular steam-carbon ratios in Figures 1, 2, and 3. The calculations were carried out for an inlet process steam temperature of 1000" F., a coal temperature of 60' F., and a 100% oxygen temperature of 60' F. Standard gas volume was calculated at 60' F. and atmospheric preseure. The gasifier heat loss was estimated to be 500 B t.u. per pound of coal gasified or 667 B.t.u. per pound of carbon converted. The heat loss may be converted to the units of B.t.u. per pound of carbon fed by multiplying by the carbon conversion. This conversion takes into account variations in heat loss with variations in temperature level and residence time. Conversion is, of course, fostered by increases in the latter two quantities.

* 2

Present address, Battelle Memorial Institute, Columbus, Ohio Present address, E. I. du P o n t de Nemours & Co , Inc , Chaileston,

TV. Va.

December 1954

The calculations were based on the Rock Springs, Wyo., coal analysis given in Table I. If the charts are used for other coals, adjustments must be made in the results to compensate for the difference in coal analysis. Carbon conversion is probably the least sensitive to changes in coal composition. (At the present time, the validity of this assumption as applied to coals of widely different rank is not established. There is good reason to believe, however, that for bituminous coals the correction can be made as indicated xvithout serious error.) Consequently, for use with other coals, the synthesis gas yield and oxygen requirement may be estimated from the carbon conversion by use of the formula

lvhei e

J!f

=

x

= = = = =

C H 0 Y S

standard cubic feet of carbon monoxide plus hydrogen per pound of carbon fed fraction carbon converted fraction carbon in dry coal fraction hydrogen in dry coal fraction oxygen in dry coal standard cubic feet of oxygen per pound of coal fed = fraction sulfur in dry coal

Methane formation is slight for atmospheric pressure gasification at high temperature and has been neglected in the above equation. However, where appreciable amounts are present, the conversion term in this expression should be adjusted accordingly.

Table 1.

Analysis of Rock Springs, Wyo., Coal

Dry basis Constituent, ? '& Hydrogen Carbon Nitrogen Oxygen Sulfur Ash Heating value, B.t.u /Ib. Moisture, Surface average particle diameter, microns

4.98 75.02 1.62 11.14 1.08 6.16 13,180 5 0 40

Supplementary graphs are presented as parts of Figures 1, 2, and 3, which relate residence time for varying oxygen-carbon ratios to coal throughput, expressed as pounds carbon per (hour) (cubic foot of gasifier volume). This relationship was obtained by integration, along the time axis, of flows calculated by the method outlined in earlier papers (1). From Figure 4 the enthalpy and temperature of the gas leavingthe generator may be obtained a3 functions of residence time and oxygen- and steam-carbon ratios. The temperature is that a t which the effluent gases would enter a waste-heat boiler, and the 500" F. base for enthalpies is assumed to be the temperature a t which the gases would leave the boiler. Thc cnthalpies shown represent the heat available for steam generation, and appropriate steam credits can be assigned.

INDUSTRIAL AND ENGINEERING CHEMISTRY

2501


IO0

60

20

c:

2

. 0

d

IO

: z 0

m

a:

5.0

d

I.

m

_I

0

> 2.0 2

1.0

II

IC

( 3

STD CU F T O X Y G E N

Figure 1.

2502

Process Functians

/ LE3 CARBON

for Steam-Carbon Ratio of 0.8


Vol. 46, No. 12

December 1954


2503

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Flame-Temperature Curve Is Useful in Refractory Selection

Oq-gen requirement = .-;-13 O( 1000)

T h e variation of flame temperature with time for various inlet ratios may be obtained from Figure 5 . These temperatures Ivere calculated by assuming complete lateral mixing and no longitudinal mixing along the flow axis of the reaction system. They, therefore, represent the maximum temperatures achieved under idealized conditions. Extreme turbulence would cause flattening of the peak presented, although it was pointed out in a previous paper t h a t temperat'ures of t'he magnit'udes sho1v-n have h c e i ~ found to exist through studies made of high temperature erosion of refractory ( I ) . The curves may be used to estimate the most severe temperatures t,o n hich the refractory may be exposed lor various operating conditions. A s is expected, peak flame temperature decreases with increase in steam-carbon ratio and decrease in oxygen-carbon ratio. The time required to reach the peak temperature also iricreascs n ith an iiim ease in either oxygen or steam ratio. Typical Problems Illustrate Use of Graphs

The following are illust,rations of the t,j-pe problems that ma!be solved viith the aid of the graphical correlations. Example 1. Calculation of Residence Time. Rock Springs coal containing 75qc carbon is fed to a generat,or with an effect,ive volume of 300 cubic feet a t a rate of 2000 pounds per hour. Process steam is also fed a t a rate of 1800 pounds per hour, and 95% oxygen at, a rate of 20,050 standard cubic feet per hour. What is the residence time of the gas in the generator?

(hour) (cubic foot) Oxygen-carbon ratio =

20 050 (0.95) 2000 (0.75)

feet per pound carbon 1800 Steam-carbon ratio = 2ooo (o,'75)

=

=

13.0 st.andurd cubic

1.2 pounds per pound

carbon From the supplementary curve of Figure 2, the residence time for the above conditions is 1.8 seconds. Example 2. Calculation of Yield. For the conditions of Example 1, what carbon conversion, synthesis gas yield, coal requirement, and oxygen requirement are obtained? From Figure 2 at,a residence time of 1.8 seconds and a n osygencarbon rat,io of 13.0 standard cubic feet per pound: Carbon conversion = 90% Synthesis gas yield = 40.6 standard cubic feet carbon monoxide plus hydrogen per pound carbon Oxygen requirement = 320 st,andard cubic feet oxygen per 1000 st,andard cubic feet carbon monoxide plus hydrogen 1000 Coal requirement = -___-_ = 32.8 pounds coal per (40.6)(0.7502) 1000 standard cubic feet carbon monoxide plus hydrogen Example 3. Use of Different Coal. coal of the follon-ing a n a l p i s (in weight per ccnt) is used at the caarbon throughput and ratios of Example 1. What yields are to be rspected? 80 0 5.3 7.0 0.8 1 .o 5 9

Carbon Hydrogen Oxvgen suifur Nitrogen Ash

If i t is assumed that variation in coal composition hae a negligible effect on conversion, conversion is again 90%. Synthesis gas yield is calculated from Equatiuri 1

= 41.17 standard cubic feet carbon nionoxide plus hydro-

gen per pound carbon

2504

=

316 standard cubic fcet

41 1 ,

oxygen per 1000 standard cubic feet carbon monoxide plus fly.drogen 1000 Coal requirement, = = 30.3 pounds caoal per (41.17)(0.800) 1000 standard cubic feet carbon monoxide plus hydrogen Example 4. Determination of Operating Conditions for Minimum Raw Materials Requirement. A% gasifier haying essentially- a I-second residence time operates a t a steam-carbon ratio of 1.2 pounds per pound of carbon. TT-hat oxygen-carbon ratio should be used for ( 1 ) minimum o x q p i requirement or (2) minimum coal requirement? llininium osygen requirement at I-second residence timc rniLTbe obtained by sight from the curves a s 333 standard cubic feet oxygen per 1000 standard cubic feet carbon monoxide plus hydrogen a t a n oxygen-carbon ratio of 11.5. Minimum coal requirement is obtained at a niaximuni s~-rithesis gas yield of 39.5 standard cubic feet carbon monoxide p l u ~ hydrogen per pound carbon by operating a t an oxygen rat,io of 14.5. This example illustrates that the minimum oxygen requirement and coal requirement are obtained a t different oxygen ratios. The calculation of optimum oxygen-carbon ratio for minimum total cost of oxygen plus coal depends on the unit cha,nges for the raxv materials and is illustrated in the follon-ing example. Example 5. Determination of Minimum Cost of Coal plus Oxygen. For t h e gasifier given in Exaniple 4, what ouygencarbon ratio d l result in the minimum total cost of oxygen plus coal if oxygen is available a t $5.00 per ton of 100% oxygen and coal, containing 75% carbon, is available a t 34.00 per ton? Select any oxygen-carbon ratio and obtain from Figure 2 the corresponding synthesis gas yields and oxygen requirement at a residence time of 1.0 second. At 12 standard cubic feet oxygen per pound carbon the synthesis gas yield i j 35.7 standard cubic feet carbon monoxide plus hydrogen per pound carbon and oxygen requirement is 334 standard cubic feet oxygen per 1000 standard cubic feet carbon monoxide plus hydrogen. (300)(1000) cents per 1000 standard Coal cost = ( 3 5 , T ) ( 0 , 7 5 ) ~ 2 0= 0~~ cubic feet carbon monoxide plus h?,drogen

Oxygen cost

=

( ~ ~ ~ & 7! cents ! ~ per~ 1000 ~ staritln1d

-___

=

cubic feet carbm monoxide plus hydrogen The calculation is repeated for a number of oxygen-carbon ratios. A graph may then he made of cost V ~ I S U Sosygen ratio, a n d the optimum ratio taken at the minimum point on the C U I V C . Example 6. Estimation of Enthalpy of Product Gas Available for Waste Heat Steam Generation. X generator having :I residence time of 1.5 seconds is operated a t an oxygen-c:arbo11 ratio of 12.0 and a steam-carbon ratio of 0.8. K h a t is the ternperature of the product gas, and JThat heat is available for waste heat steam generation if the temperature of t,hc gas leaving the boiler is 500' F.? K h a t credit may be made for waste heat steam for unit credit of 25 cents per million B.t.u.:' From Figure 4 a t the conditions given. the gas exit tc.mpc'rature F. Enthalpy 01 the gas r6,lative to 500" F. = 2620 B.t u. prr pound carbon fed From Figiirc 1, the synthebis gas j icld is found to be 30 14 standard cubic feet, carbon monoxide plue hydrogen per pound carbo 11. = 2480O

Steam credits

=

2.262'25)(3000) = 1.0 cents pcr 1000 standat.ti 106(39.9)

cubic feet carbon monoxide plus hydrogen

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

Vol. 46, No. 12


December 1954


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Vol. 46, No. 12

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT constant throughput would alter the shape of the individual curves somem-hat but would not change the location or value of the optimum conditions. It must be remembered that the costs shown cover only the elements mentioned, and then only apply a t the unit prices assumed for this calculation. A t a feed rate of 2 pounds per (hour) (cubic foot), the most economical operation is a t fairly high carbon conversions, which can be achieved with relatively low oxygen-carbon ratios. The cost a t optimum conditions is high, because the capital charges against the gasifier more than offset the low materials requirements. As the throughput is increased to 5 pounds per (hour) (cubic foot), a marked reduction in gasifier capital charges and only a small increase in coal and oxygen requirements result in a considerable drop in total cost. The optimum is still a t high carbon conversion. Further increase in feed rate to 7 pounds per (hour) (cubic foot) effects a further reduction in total cost, but the optimum is a t slightly lower carbon conversion, as the oxygen requirement begins to increase rather sharply.

Figure

5. Variation of Flame Temperature with Time

Example 7. Choice of Operating Conditions for Minimum I n the design of a gasifier, what throughput and operating conditions correspond to minimum synthesis gas cost if coal is available a t 84.00 per ton; 100% oxygen at $5.00 per t o n ; steam superheated to 1000" F. a t 40 cents per 1000 pounds; and waste heat credit a t 25 cents per 1,000,000 B.t.u.? Capital charges are assumed to be 11 cents per day per cubic foot of gasifier volume. No credit is taken for unconverted carbon and no charges are made against its renloval from the gas. Cost.

Each of the preceding examples is utilized in the calculation. The following steps are involved: A number of throughput rates are arbitrarily chosen. F o r each of these, and for a particular steam-carbon ratio, the synthesis gas yield and oxygen requirement corresponding to a number of oxygen-carbon ratios are obtained by the procedure given in Examples 1 and 2. For a coal other than that used in calculating the charts, the correction procedure given in Example 3 is used. Costs of coal and oxygen are calculated from the material requirements in a manner similar to that given in Example 5. Cost of process steam is obtained in a similar manner from the steam-carbon ratio, the corresponding synthesis gas yield, and the unit charge. These costs are added to those of coal and oxygen. Credit for waste heat boiler operation is obtained by the calculation procedure given in Example 6. Capital investment cost is obtained by dividing the unit capital charge by the throughput and by the synthesis gas yield. The algebraic sum of individual costs and credits may then be plotted and the conditions for minimum cost obtained by inspection. .4 calculation summary for one throughput is given in Table IT.

A summary plot of such a series of calculations made by using the charges assumed in this example appears in Figure 6. Calculation on the basis of constant residence time rather than December 1954

14

5 0

~

I 70

00 90 100 PER CENT CARBON CONVERSION

Figure 6. Minimum Cost Calculations (Example 7)

Table II. Cost Study Illustrating Selection of Operating Conditions for Minimum Cost (Example 7) For f 2 / J 7 G = 5 0 Ib carbon/(hour) (cu ft. gasifier volume) Oxygen, std. cu. i t . / Ih. carbon 10 12 14 16 Residence time, sec. 2 25 2 00 I 75 1.10 Carbon conversion, % 68,O 84.5 94.5 97.0 (CO Ha) std. cu. ft./lb. carbon 33.1 38.7 41.5 39.0

+

Materials per 1000 Std. Cu. Ft. (CO 34.5 Coal, lb. 40.4 Oxygen. std. cu. it. 302 310 31.0 36.2 Steam, lb. 74.1 Wasteheat, 1000B.t.u. 72.6 Cost Cents/1000 Std. Cu. F t . (CO 2.37 Capital investment 2.77 8.08 6.90 Coal 6.54 Oxygen 6.37 Steam 1.45 1.24 Subtotal 18.67 1 x 5 Waste heat credit 1.85

h-et total

+ Hz) 32.2 337 28.9 80.5

34.2 410 30.8 100.0

+ Hz)

L e . - _ _2 . 0 1-

2.35 6.84 8.65 1.23 19.07 2.50

15.20

16 57

16 86


2.21 6.44 7.11 1 16 1 m

14.91

2507

ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT ht a feed rate of 10 pounds per (hour) (cubic foot,),the decrease in capital charges just about balances the increase in materials requirements, b u t the optimum is a t a carbon conremion of about 88%. Further increases in feed rate have relativelj- little effect on the capital charges, and the increased roal and oz)-gen required raise the total cost. In this case the nio::t cxonomical

Time required for the flame t o reach the wall:

Oj

=

6/150

=

0.04second

From Figure 5 a t a n oxygen-carbon ratio of 12.0 and a stcamcarbon ratio of 1.2, the flame temperature after 0.04 second ie 3490" F. This temperature is obviously the highest temperature obtained under the conditions given. Turbulence and espansion of t h e jet vi11 result in a lower actual temperature. The valuc predicted by t h e graph consequently contains a fa,otor of safety for selection of the proper conditions to prolong reflacw r y life. Heat Loss R e d u c e s Gasifier Yield

01

03

1.0

30

10

REACTION TIME ( S E C )

Figure 7. Effect of Heat LOSS on Carbon Conversion operation \I-oulci be a t just over 807, c a r b ~ nmnversio~i. If this series was continued, the total cost a t optimum conditions rrould rise steadily with increased throughput, and the mininium costs would be reached a t progressively lolver carbon conversions. Obriously, changes in the unit price of any elements oi total cost will alter the relationships s h o r n , both as to optimum throughput and opt,imum carbon converjion a t a given throughput. It would, however, t,alce a large increase in niaterials costs to justify operation at very low throughputs. The UT of the mat,rix chart,s facilitat,es the construction of the cost, curves, so t,hat a wide variety of circumstances can be investigated in a relatively short time. Example 8. Estimation of Flame Temperature. A single burner is diametrically mounted in a generat,or. K h a t t'eniperatures u-ill be encountered at the point of impingement, of the flame xyith the refractory wall 6 feet away if the velocity of the jet leaving the nozzle is 150 feet per second and is remain constant?

2508

An equivalent heat loss of 500 B.t.u. per pound of coal wa,+ used in this study in order t o compare the calculat~ed results with existing pilot, plant data. Full scale generators mill have a lower heat, loss per pound of coal because the gasifier area per much less than in t,he rniall units. A fened out a t different heat losses to csaminc this cffect on gasification yields. These results arc shorx-n in Figure 7. At a constant oxygen ratio of 12.0 standard cubic iect per pound of carbon and a steam ratio of 1.2 pounds per pound of carbon, decrease in heat loss irom 1000 t,o 500 B.t.u. per pound of coal results in n 0% increase in carbon conversion a t corresponding residence time. If a 500 B.t.u. heat loss is compensated by a thermally equivalent, increase in steam temperatures from 1000" t,o 2000" F., n 6y0 increase in conversion results. Thip latter example is thermally equivalent to a zero heat loss and indicates the variation in conversion to be expected from varying heat loeses. Conclusion

T h e results of a large number of theoretical calculations have been assembled in the forin of charts from which the operating conditions for coal gasification can hc chosen, and the approsimate cost of the elements variable IT-ith operating conditions ran be est,imated. Substantiation of these calculated results by (Loinparison with experimental data has not been completed, and the charts should be used n-ith reservations until t'his is done.

Literature Cited ( I ) Batchelder, H. E S G . CHCILI.,

R.,Busche, R.A I . , and Armstrong, W. 45, 1850 (1953).

E'., I r u .

RLCBIVED for review Octoher 20, 1953. ACCEPTEDSeptember 22, 1954. Presented before t h e Dirieion of Gas and Fuel Chemistry a t the 124th M e e t i n g Of the .k>lERICAX C H E \ i I C A L S O C I E T S , Chicago, 111.


Vol. 46, No. 12

Kinetics of Coal Gasification - Design of Atmospheric Pressure

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