Gasification of subbituminous coal and lignite in externally heated retorts V. F. PARRY, E. 0.WAGNER, A. W. KOTH, AND J. B. GOODMAN Bureau of Mines, Golden, Colo.
DURING the war the Bureau of Mines conducted experimental work on two pilot plants for gasifying low-rank fuels to produce industrial gases for beneficiation of low grade iron ores. The experimental data resulting from operation of the externally heated alloy retorts for 3500 hours are summarized in graphs and tables, and theories of gasification and heat transfer are discussed. The theory of the annular retort and mechanism of gasification therein are presented. Carbon monoxide and hydrogen can be made a t rateis of 65 to 70 cubic feet per hour per square foot of heated surface i n retorts heated to 1900" F. The performance of various processes for production of carbon monoxide and hydrogen is compared on the basis of cubic feet per hour per square foot of fuel bed area. Internally heated processes make 1000 to 8600 cubic feet of carbon monoxide and hydrogen per square foot, while the externally heated processes make 1000 to 2300 cubic feet. Various grades of water gas having hydrogen-carbon monoxide ratios ranging from 1.9 to 10 were made in the pilot plants, but lowest-cost gases and highest efficiency and capacity are attained in making gases of 2.0 to 2.5 ratio. The performance of three externally heated processes, the Didier-
Bubiag, the Freiberger, and the annular retort, is reported with operating data. The Didier-Bubiag process, operated in Germany, supplied about 7qo of the synthesis gas converted to liquid fuel by the Fischer-Tropsch process. This retort employs refractory chambers and each unit makes about 17,500 cubic feet of gas per hour a t an efficiency of about 78oJ,. Heat is transferred through the walls a t a rate of 2060 B.t.u. per hour per square foot. The annular alloy retort a t Grand Forks has about the same capacity and has a present indicated efficiency of 71%, but the rate of heat transfer is about 6000 B.t.u. per hour per square foot. Calculations show t h a t the efficiency of externally heated continuous retorts equipped with heat-recovery devices should be about 80%. In externally heated retorts the cost of alloy is about one cent per 1000 cubic feet of carbon monoxide and hydrogen, assuming t h a t a n alloy plate 0.25 inch thick will have a n average life of 10,000 hours a t 1900' F. When carbon monoxide and hydrogen are made a t a rate of 70 cubic feet per hour per square foot in the externally heated retort, the equivalent cost of oxygen to make the same gas in an internally heated process would be 3 to 5 cents per 1000 cubic feet.
I
cates greater capacity when operating on 35- t o 40-mm. top sizes. The capacity of the Lurgi producer is therefore about one half t h a t of the modern water-gas machine; moreover, i t requires oxygen, which is relatively expensive. The Lurgi generator would not be used for production of carbon monoxide and hydrogen because of the synthesis of methane at elevated pressures. Experimental results at Bohlen have proved t h a t 210 cubic feet of oxygen were required to make 1000 cubic feet of carbon monoxide and hydrogen. The Winkler producer, operating under low pressure, had the greatest capacity and gasified a wide variety of coal dusts at rates up to 8600 cubic feet of carbon monoxide and hydrogen per hour per square foot, but i t consumed about 317 cubic feet of oxygen per 1000 cubic feet of carbon monoxide and hydrogen produced (11). The other low-pressure processes, with the exception of the Leuna slagging producer, have less capacity than the watergas process but also consume less oxygen than the Winkler.
N THE production of carbon monoxide and hydrogen, the
gas required for industrial hydrogen or for synthetic liquid fuels, three basic techniques are employed for gasification of solid fuels: (1) the intermittent water-gas process; (2) gasification of coal or coke continuously in a fixed or suspended bed by blasting with mixtures of steam and oxygen; and (3) gasification of mixtures of coal and steam by heat transmitted through walls. Table I compares the performance of several producers employing the above techniques. -I IS The modern intermittent water-gas process theoretically can make carbon monoxide and hydrogen at a rate of 3600 cubic feet per hour per square foot of fuel-bed area. This is about the maximum t h a t can be attained with a 4-minute cycle consuming 29,000 cubic feet of air per cycle at a rate of 320 cubic feet per minute per square foot, as indicated from the experimental work by Dashiell. The present performance of the water-gas generators is 2000 t o 2400 cubic feet of carbon monoxide and hydrogen per hour per square foot. This process supplied most of the gases for synthetic products in the United States and for about two thirds of the Fischer-Tropsch plants in Germany because of its reliability and the quality of gas produced. The high-capacity machine requires special-quality coke passed over a 2-inch screen and freed from fines. The efficiency is about 65%. Gasification of coal in fixed beds or in the fluidized state, using oxygen and steam, has recently received much attention.
I n considering the above techniques for gasifying solid fuel, it is shown t h a t relatively expensive raw materials are required. The cost of screened coke from coking coal and the relatively high cost of oxygen for internally heated processes might compete unfavorably with gas producers employing external heating at low capacity while treating low-rank fuels that require little preparation. An externally heated retort requires no oxygen; it is continuous, and low-value fuels are gasified. By recuperation of heat, over-all efficiencies of 80% can be attained. The Didier-Bubiag process, which was employed extensively in Germany for making synthesis gas from brown coal and broken briquets, supplied about 7% of the gas for making synthetic fuel in the Fischer-Tropsch plants (7). This externally heated process makes carbon monoxide and hydrogen a t a rate of about 1150 cu-
The Lurgi pressure producer at Bohlen made 1000 to 1340 cubic feet of carbon monoxide and hydrogen per hour per square foot of fuel-bed area when gasifying graded briquets and dried brown coal of 10-mm. top size (1,8,9,11,12, 21). Later experience indi627
*
1.N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
628
Steam Heat (1850 . + ----. +--=
Natural subbituminous coal
3.15 Hz
+ 4.78 C + 1.09
0 2
4.06 HzO
O
F.)
216,800 B.t.u.
3,120 Cubic feet of mixed water gas 320 B.t.u. per cubic foot
4.5 H2
d
Synthesis gas 2555 cubic feet
333,200 B.t.u. (net) ( = 107 B.t.u. per cubic foot of gas made)
Steam Heat (1850" F.) +++ =
Natural lignite
+ 3.66 C + 1.48
+ 12.6 lb. of residue
+ 2.25 CO + 1.08 COz + 0.31 CH, + 0.09 C2H4 + 0.96 C + 1.78 HzO
L
3.64 Hz
Vol. 40, No. 4
7
0 2
1.86 HzO
-
d
-
186,800 B.t.u. 2390 Cubic feet of mixed water gas 3.44 Hz
318 B.t.u. per cubic foot
+ 9.6 lb. of residue
+ 1.72 CO + 0.85 CO? + 0.23 CH4 + 0.065CzH4 + 0.73 C + 1.36 H20
7 -
Synthesis Gas 1950 cubic feet Figure 1.
250,900 B.t.u. (net) ( = 106 B.t.u. per cubic foot of gas made)
Equations for gasification of subbituminous coal and lignite in an externally heated annular retort, based on 100 pounds of ash-free coal
For simplicity in writing the equations, the nitrogen and hulfur in the coal and residue, and the hydrogen and oxygen in the residue are omitted. The sensible heat in products leaving the retort a t 750° F., amounting t o 58,000 B.t.u. for subbituminous coal and 44,600 B.t.u. for lignite, is not included in the written equations. .4 slight correction for illuminants in the gas, which have a n average composition corresponding t o Cg.s&.a, is included in the equations mhich consider the illuminants as CzHi, The datum line is 60' F.
bic feet per hour per square foot of fuel-bed area, and the ovens employed are about 35 feet high, 13 feet deep, and 13inches wide (10, 19). The Freiberger process (Reiche-Zeche), which was not developed to a commercial scale, makes carbon monoxide and hydrogen at a rate of 2300 cubic feet per hour per square foot of retort cross section when passing brown coal through 6-inch alloy tubes. heated externally to about 1800" F. (17, 19). In the small Bureau of Mines pilot plant at Golden, carbon monoxide and hydrogen are made at a rate of 2300 cubic feet per hour per square foot of retort cross section when gasifying coal in an alloy tube 14.25 inches in inside diameter. I n the large plant a t Grand Forks, N. Dak., carbon monoxide and hydrogen are made at a rate of 1100 cubic feet per hour per square foot when gasifying natural lignite of 0.25 by 1.5-inch size in a 48-inch retort. Even though the capacity of a single externally heated retort is low, the capacity per square foot of cross-sectional area compares favorably with the internally heated processes. In 1943 the Coal Division of the Bureau of Mines started development work on processes t o make reducing gases (carbon monoxide and hydrogen) with the object of making sponge iron from low grade iron ores in connection with investigations on ram materials resources for steel production. The initial object of these investigations was t o make hydrogen from lignite. A small
OF GASIFICATION PROCESSES MAKING TABLE I. PERFORMANCE CARBON MONOXIDE AND HYDROGEN
Oxygen Capacity Used, CO H;, Cu. Ft./ Cu. Ft.," Cu. Ft. of Hour/Sq. Ft.a CO H 2
+
Process
+
Literature Cited
Internally heated processes Intermittent water gas on sized coke 2400 None (6, 12) Winkler low-pressure producer, carbonized brown coal 8630 317-f (11) Thyssen-Galocsy low-pressure producer 1800 325 (11) Lurgi high-pressure producer using brown coal briquets moo 210 (11,18) Koppers low-pressure producer 270 (11) Leuna slagging producer, on sized coke 4000 275 ( 1 1 , 18) Externally heated procemes Didier-Bubiag, 13-inch wide, brown coal briquets 1150 Xone (19) Freiberger, 6-inch tubes, brown coal 2300 None (19) Annular retort, Golden, Colo., 16inch diam., natural subbituminous 2300 None (16) coal 0.5 X 1.0 inch Annul& retort, Grand Forks, X. Dak., 48-inch diam., using natural 1100 h'one (16) lignite Hz is sum of pure carbon monBased on upper area of fuel bed. CO oxide and pure hydrogen in product gas.
+
pilot plant with a capacity of 75,000 cubic feet per day was built a t Golden, Colo., and a large plant with a capacity of 400,000 cubic feet per day was erected and tested a t Grand Forks, N. Dak. These plants have operated a total of about 3500 hours under various conditions to develop the technology of gasification of low-rank coals in externally heated retorts. TWOreports describe the results of the investigation but give no analysis of the experimental data (14, 1 5 ) . The report summarizes the essential findings of the investigation and reviews the technology of gasification of noncoking coals in externally heated retorts. Water-Gas Reactions on Subbituminous Coal and Lignite
The problem was attacked by considering the gasification of subbituminous coal and lignite as chemical reactions for which equations can be written. Figure 1 shows approximate equations for gasification of low-rank fuels when 80% of the carbon is converted. These are derived by considering the process of gasification in an externally heated retort as a series of steps t h a t can be differentiated to fit the mechanism of gasification. The probable mechanism of gasification is discussed later in this report. Products of low temperature distillation from any coal can be calculated, and their probable analyses are known from previous experimental work ( 2 , 6, 13, 16). These products, plus additional steam, then become the reactants which move in parallel flow through the upper gasification section of the annular retort. The combined hydrogen derived from the tar oils and hydrocarbons is determined by experiment; and when this is known, the water-gas reactions for any hydrogen and carbon monoxide composition of final gas can be calculated if the ultimate analyses of the original coal and the residue are known. The method of doing this is discussed briefly in this report but a more detailed explanation will be presented in a following paper. The first step is to determine the proximate and ultimate analysis of the coal. The next is to assume or determine the percentage of carbon converted to gas which fixes the carbon in the residue. The ultimate analysis of the residue is then determined or estimated from average analysis of similar residues. Generally speaking, the average analysis of ash-free residues from subbituminous coal and lignite is about as follows, depending upon the degree and temperature of gasification: carbon, 92 to 94y0; hydrogen, 2%; oxygen, 4 t o 5%; heating value is about 14,000 B.t.u. per pound. The elements in the residue are then subtracted from those in the coal. The next step is to determine or calculate the total
I N D U S T R I A L A N D EN G I N E E R I N G C H E M I S T R Y
April 1948
629
the data of Clement (6). If carbon monoxide and hydrogen are extracted from the mixed water gas, the combined hydrogen in the form of methane and illuminants added t o the net heat in the residue furnish 333,200 net B.t.u., which is about sufficient t o carry out the reaction a t a n efficiency of 65%. The net heat available is 107 B.t.u. per cubic foot of gas made. The equation for gasification of natural lignite tells the same story but with different figures. These basic equations may be worked out for any fuel and are useful in guiding experimental work. The fundamental water-gas reactions between carbon and water t o make carbon monoxide, carbon dioxide, and hydrogen, and the net heat of reaction above 60" F. are shown graphically in Figure 2. The ratio of water to carbon entering the reaction is designated as 2 and is the abscissa of this chart. This is a useful figure when these reactions are considered, because the analysis of the product gases can be readily calculated when Z is known. The following relationships apply: moles of hydrogen formed per mole of carbon converted 2 - 2 = moles of carbon monoxide formed per mole of carbon converted 2 - 1 = moles of carbon dioxide formed per mole of carbon converted
2
=
The net heat of reaction, with products at 60' F., can be calculated from the following equation: Net heat of reaction, per mole of carbon 17,740 (2 1) converted = 56,410
-
In making a water gas having a hydrogen-carbon monoxide ratio of 2.0 from pure carbon and steam in a n externally heated retort, Figure 2 shows that the value of 2 equals 1.333, which can be read from the chart or calculated from the equation. The composition of the water gas will therefore be:
MOL RATIO H 2 0 / C OR H2/CO+C0,=Z
Figure 2.
-
Water-gas reactions
Moles 1.333 0.666 0.333
Hn
co water formed by evaporation and decomposition, followed by a determination or calculation of the total combined hydrogen resulting from distillation and cracking of volatile matter. This can be determined by correlation of experimental data or by direct testing in a n assay retort. The oxygen in the coal not included in the total water or residue is converted t o carbon dioxide, which is in turn converted to carbon monoxide, depending upon the apparent equilibrium. The available hydrogen is that remaining after accounting for the combined hydrogen, the hydrogen in the water, and the hydrogen in the residue. The available hydrogen appears as a gas. By proper stoichiometric hand!ing of the above quantities, the pure carbon remaining to be converted t o water gas is calculated, and the apparent equilibrium attained upon gasification depends upon the final ratio of hydrogen t o carbon monoxide. The water-gas reactions, the additional steam required, and heats of reaction are then calculated and the elemental and heat balance is established. The equation for gasification of any coal to a mixed water gas of any hydrogen-carbon monoxide ratio can be calculated in about 1.5 hours using forms prepared for the calculation. Figure 1 is an approximate statement of the gasification of natural lignite and subbituminous coal; some elements are omitted for simplicity in arranging the equation. Table I1 gives a more accurate statement of the gasification of 100 pounds of ash-free natural subbituminous coal. The equations of Figure 1 show that 100 pounds of ash-free natural subbituminous coal, consisting of 3.15 HS f 4.78 C 1.09 02 moles when mixed with 4.06 moles of steam and then heated t o the range of 1850 a F. while supplying 216,800 E3.t.u. t o the system, will combine to form 3120 cubic feet of mixed water gas of the molal composition stated. The apparent equilibrium temperature of these reactions is about 1450" F., calculated from
+
co2
% 57.2 28.6 14.2
The net heat of reaction will be 56,410-17,740 X 0.333 = 50,503 E3.t.u. per- mole of carbon converted (products at 60OF.). It is thus possible to calculate readily the important facts on heat of reaction, rate of heat transfer, composition of gases, and mass velocity of reactants by considering these fundamental water-gas reactions in company with the analysis of the coal and residue. The design of pilot plants and the experimental work on various fuels were worked out by considering these relationships, keeping in mind the need for high capacity and efficiency.
TABLE 11. ELEMENTAL BALANCE
In
. I
Coal, moles Steam, moles Heat through retort wall Total, molee out Hydrogen, moles Carbon monoxide, moles Carbon dioxide, moles Methane moles IlluminaAts, Cn.sHr.6, moles Nitrogen, moles Sulfur (H2S), moles Excess steam, moles Residue 12.6 Ib. Sensibli heat in products 750° F. Unacobuntedfor, moles Total
C
4.774
... ... 4.774
..
.
Hz
09
7.215 3 118 0.031 0.024
216,800 1,135,300
4.496
...
1.079 0.311
0:622
1.079 ,..
0.173
0.173
...
0:955
60' F.
911,400 7,100
... . . . . . . ... .. --1.124
...
S
3.152 1.087 0.031 0.024 4.063 2.031 ...
2,248
...
Nz
Net Heat, B.t.u.,
...
... ...
467,470
..
... ...
......
..
...
0 ;026 0 : o i o : : : .., 0 ;oio 1.783 0.892 0.131 0.021 0.005 0:004
. ..
.. . ..
273,610 107,360 48,610
......
2,550 10,400 176,500
. 47,600 0:OOS -0:oiO 0:002 . . . 1,200 4.774 7.215 3.118 0.031 0,024 1,135,300
630
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. 40, No. 4
2200
2 z
$ I
4.0
2000
3.0
1600 100
1800
1400
90
1200
eo
1000 70
t
t: 5
60 50
V
HC /, O
R A T I O IN GAS
Figure 4. Products of reaction per mole of carbon converted
The water-gas reactions practically cease a t 1300" F., even with highly reactive fuels; therefore, the width of the coal layer heated should be such that the point farthest from the sourcc of heat is 1300' to 1400" F. (about 730" C.) when water gas is being produced a t the maximum rate.
For purposes of illustration, consider two vertical tubes having 6- and 24-inch diameters heated t o 1800 ' F., through which coal and steam are moving. S e a r the inner surface of the tubes the temperature of the mixture will approach 1800 F. The temperature of the system 1 inch from the heated wall will be considerably lower (about 200" F.) because of the endothermic reactions, and the temperature 2 inches from the heated wall will probably be 350" F. lower than the wall temperature. Near the center of the tubes, an equilibrium temperature of about 1200 O F. should be approached, because a t this temperature the endothermic reactions are practically nonexistent. In this example, the active zone for the water-gas reactions extends 2 to 2.5 inches from the heated wall where the temperature level is 1300 O to 1800 F. Therefore, if heat is being transferred a t a constant rate, the capacity of each tube is proportional to the surface area and not to the cross-sectional area. In the example considered the 24-inch tube will have four times the heat-transfer capacity of the 6-inch tube. The interior of the large tube within 3 inches of the wall is useless with respect to capacity and is probably detrimental because heat is dissipated by decreasing turbulence in the reaction zone. Therefore, it is proper t o pass the fuel down through a narrow annular space to achieve high capacity and efficiency. The optimum width of the annulus must be determined by experiment. I n a very narrow annulus, the reactions proceed rapidly, and the velocity may be high enough to sweep the solid fuel out of the system. The optimum width of the annulus is t h a t which will transmit heat a t a maximum rate while retaining the solid fuel within the reaction zone. O
Figure 3.
H 2 :CO RATIO Gasification of natural lignite 80% carhnn Converted
Figure 3 shows the range of analyses and yields of mixed water gas that can be made from natural lignite when 807, of the carbon in the original lignite is converted t o gas. This chart incorporates the carbon dioxide, available hydrogen, and combined hydrogen derived from the lignite and indicates the theoretical yields when different grades of water gas are made. Such charts can be calculated for any coal and are valuable guides in conducting experimental work. The heating value of the gas is calculated to the base of 60" F. and 30 inches of mercury saturated, which is 1.73y0 less than on the dry basis. All other references to heating value of gases given in the report are given on the dry basis, which is a more useful base. Heat Transfer and Movement of Materials in Externally Heated Retorts
When heat is moved through a diaphragm to effect the watergas reactions, several factors must be considered to obtain the maximum rate. When carbonizing coking coal in externally heated retorts, the rate of movement of heat through the coal is inversely proportional to the square of the distance the heat travels (3). )Then heat is transferred to noncoking coal, through which steam and gases travel under turbulent conditions, the rate of movement of heat is indicated by experiment t o be inversely proportional to the distance the heat travels. Therefore, in an externally heated system for making water gas, the coal layer should be as thin as possible t o achieve the maximum rate of heat transfer.
It has been observed in experiments a t Golden that the lowrank fuels shrink as moisture is removed and the coal is carbonized. Lignite shrinks about 0.90 cc. for each gram of moisture removed, and subbituminous coal shrinks about 0.75 cc. per gram of moisture. liberated. When carbonized t o 1450' F., subbituminous coal shrinks about 4570 and lignite about 60% (6). These natural shrinkage properties invite the coal t o move through narrow reaction zones. Once the coal is in the heated zone it will move readily through a narrow annular space with parallel malls. If solid fuel and steani could be moved through annular reaction zones 1 inch wide, the temperature of the heated walk mould not have to exceed 1650' F. (900" C.) t o achieve rapid heat transfer while water gases of low hydrogen-carbon monoxide ratio are made. As the width of the reaction zone increases, the temperature of the heated wall must increase t o maintain the same capacity while the same grade of gas is made. Figure 4 shows the materials that must be handled when a mole of carbon in either subbituminous coal or lignite is con-
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1948
OF:
2,600 2,400
2,200 2,000 1,800
1,600 1,400 1.200
631
As shown in this diagram, if the flue temperature is 2600 F., the temperature drop across the brickwork is 730" F. while heat is being transferred at a rate of 2050 B.t.u. per hour per square foot. The heat thus transferred can be used a t a temperature level of 1600" to 1700"F. to make 28 cubic feet of water gas per hour per square foot. This is about the limit for silica walls. The dia ram shows silicon carbide walls of the same wid& as the silica brickwork to attain gas-tightness. However, thinner gas-tight silicon carbide walls, that would be desirable, can probably be fabricated. As this diagram shows, the 5-inch silicon carbide wall can transmit heat at a rate of 13,000 B.t.u. per hour per s uare foot with a temperature drop of 600' F. a n 3 a heated surface temperature of 2600" F. The heat thus transferred will'make 175 cubic feet of water gas per hour in the annular retort at a temperature level of 1600 O to 1800" F. O
Figure 5. Heat transfer and maximum rates of gasification in externally heated retorts employing alloy, silicon carbide, and silica brick walls
w,20 K , 183 B.t.u., 7300 T I 1RFdl
?: iSZ0
Ts,1400 Gas. 100
5.0
110 13 000
2:600 2,000
1,300 175
Width, inohes Conduotivit B.t.u./hour/Y,q. ft.
OF.
,
OF.
OF.
Cu. ft./hour/sq. ft.
verted to gas. The volume and weight of materials given are about the same for all fuels gasified because the e n d products of hydrogen, carbon monoxide, and steam dictate the equilibrium. As the hydrogen-carbon monoxide ratio in the water gas varies from 2 t o 9, the excess steam increases abouts even fold, and the pounds of gas plus steam per mole of carbon converted increase from 40 to 100. Therefore, it is evident that higher capacities and greater efficiency can be attained when gases of low hydrogen-carbon monoxide ratio are made. This is important in the design and operation of the annular retort. Because of the natural balance of hydrogen and carbon in lomrank fuels, it is easier t o make water gases having hydrogencarbon monoxide ratios of about 2.5 with lignite and 2.2 with subbituminous coal. Operating difficulties increase when the hydrogen-carbon monoxide ratio exceeds 3.0 because of the large quantity of steam required to maintain the equilibrium. If hydrogen is the desired product it is better t o conduct the "shift" reaction by a separate operation. Therefore, when considering these general principles of external heating and movement of reactants, one is inevitably led to the desirability of thin reaction zones t o achieve high capacity and efficiency and to maintain the heated walls at a safe maximum temperature. I t is also indicated that the lowest-cost industrial hydrogen or synthesis gas will be obtained when gases of low hydrogen-carbon monoxide ratio are made.
The maximum practical rate of heat transfer across metallic walls t o effect water-gas reactions 2.0 is indicated by experiment and calculation to be 13.7 7300 B.t.u. per hour per square foot, which will 2,050 2,600 produce about 100 cubic feet of gas, as indicated 1,860 1,440 in Figure 5. Greater rates of heat transfer could 28 be attained a t the expense of higher metal' temperatures. A combination retort emdovinp - " -silicon carbide walls in the hottest section and heat-resisting alloy steel walls in the cooler section might be designed for maximum capacity. Preliminary calculations on such a combination retort 20 feet long and 6 feet in diameter indicate a gas-making capacity of about 50,000 cubic feet per hour.
Natural sub.bituminous coal 100 Ib. 860,400 net B,t.u.
w Furnace temperature
Flow'of Heat through Walls of Externally Heated Retorts
The rate of transfer of heat through various materials suitable for construction of retort settings can be calculated from available data. The average conductivity of silica brick a t 2225' F. is 13.7 B.t.u. per hour per square fodt per inch of thickness per F. temperature djfference. The conductivity of silicon carbide increases from about 105 at 1200' F. to 109 a t 2200' F., and the conductivity of Type 446 heat-resisting alloy is about 183 a t 1900" F., while the conductivity of the nickel-bearing heat-resisting alloys such as 25 Cr-20 Ni (Type 310) is about 147 a t 1900"F. The maximum practical rate of heat transfer through the
above materials is illustrated in Figure 5. Walls of silica brick about 5 inches thick are now used for externally heated coke ovens and continuous vertical retorts.
Figure 6. Mechanism of gasification of natural subbituminous coal in externally heated annular retort
632
INDUSTRIAL AND ENGINEERING CHEMISTRY Probable Mechanism of Gasification of Low-Rank Fuels in Annular Retorts
Figures 6 and 7 give diagrammatic pictures of the annular retort when natural lignite and natural subbituminous coal are gasified to make a "synthesis-type" water gas. By increasing the concentration of steam and by adjusting temperature to lower levels, "high-hydrogen-type" water gases are maae in the upper annulus. Reactive fuel size t o 0.25 by 1.5 inches is introduced in the top of the upper annulus. Steam is added, if necessary, to control the
concentration of water vapor with respect t o reactants. When the coal is heated to about 212' F., free moisture evaporates and travels downward with the steam and dry coal. As the coal is heated from 750" to 900" F. (399 to 482 C.), i t decomposes into primary products of distillation: combincd water, tar, light oil, and gas, plus a char residue. In Figures 6 and 7, the weight of products of distillation from 100 pounds of the fuel considered is indicated in the top box, to the right of center. The mixture of steam, carbon, hydrocarbon gajes, and tar vapors moves downward into the higher-temperature gasification zone where i t undergoes further decomposition and where water gases are formed. This is the gasification zone of the upper annulus. The products of gasification, made up of fixed gases from primary distillation, water gases from the reactions between steam and carbon, and hydrocarbons from the coal, leave the annulus at the center offtake. All tar vapors are decomposed if the retort is operated a t high temperatures. The combined hydrogen in the products of gasification, consisting of methane, traces of ethylene, and illuminants having the average composition C,.BHS.~, slightly exceed the combined hydrogen in products of low temperature distillation, indicating some formation of methane. The unconsumed char, now substantially free of hydrocarbons, passes into the lower annulus, where it comes in contact with reacting gases and steam passing countercurrent to the char. Steam is introduced at the top, as shown in the figures, and preheated O
O
Vol. 40, No. 4
before entering the lower annulus. The amount of steam admitted is regulated to give the desired final gas analysis. Watergas reactions proceed as heat is taken up by the reactants. Fuel is moved through the system a t a rate which allows about 80% of the carbon to be converted. This i: a brief account of what occurs along the vertical axis of the retort. The rate or velocity of the reactions and the apparent equilibrium attained depend upon the average temperature in the reaction zone and the rate a t which heat can be transferred through the wall of the outer retort. This, in turn, depends upon the temperature outside the tube and the velocity of gases traveling within the annulus. The latter is controlled by the width of the annulus and to some extent by the volume of steam used. The relationships that exist across the horizontal axis of tho retort are complex, because the source of heat is from only one side. The reactions proceed rapidly near the surface of the heated side, and the temperature drops very rapidly mith distance from the source of heat. Table I11 indicates the magnitude of this drop as the width of annulus chnngcs. O F TEMPERATURE DROP TABLE 111. MACKITUDE
V i d t h of Reaction
Zone, Inches
Temperature of Heated Wall, O F.
Temperature of Inner Wall, O F.
Average Temperature, F.
Tempeiature Dikferenoe, 0
F.
Because heat reaching the inner wall is transferred principally by convection and t o a lesser degree by condudion and radiation, and the endothermic reactions proceed a t faster rates a t high temperatures, the temperature distribution across the reaction zone probably is a curve. LikeTvise, the apparent gaseous equilibrium across the annulus, and the rate of reaction, change rapidly because of the change in temperature. As the reactants pass through this system, the equilibrium of the water gases formed will tend to approach the equilibrium dictated by the temperature environment. If the gases travel in turbulent flow, the distribution of temperature and the composition of the gases across the reaction zone will change less rapidly because of the more rapid lateral movement associated with turbulent flow. It is evident that thin reacticm zones nre best. Description of Pilot Plants
Figure 7. Meohanism of gasification of natural lignite bituminous coal in externally heated annular retort
Complete descriptions of the small pilot plant a t Golden, Colo., and the larger plant a t Grand Forks, N. Dak., have been given in previous publications which record the various changes made in the plants during the course of development (14, 16). Figure 8 shows the arrangement of the small pilbt plant during 1947 and gives the details of constructional features. I n this unit, the width of the annular reaction zone is now 1.75inches and the alloy employed for the outer retort is Type H I casting with a '/&-inch wall. As shown in Figiire 8, the system is heated by adding combustible gas to a preheated mixture of products of combustion and air. The products of combustion and air are preheated t o about 1300" F. when entering the gas ports and travel at high velocity around the reaction tube. KO localized hot spots develop with this method of heating because the air is diluted with about 3 parts of products of combustion and restricted to 3 t o 59" more than is rcquired for complete combustion of the gas. The combustion takes place slowly around the tube, and a good deal of heat is transferred directly by radiation from the gas. The heating gases swirl around the tube a t an angle of about 30' with the horizontal and travel at a velocity of 12 to 15 feet per second.
April 1948
.IN D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
633
Slack Gar
Net Btu used/cu ff.d gas made =
15A115B I I5C 1561 141 I124
Average furnace temperature (Points 1,2.38 4) 15A-1835 158-1610
OF.
15C-1595
Figure 8. Heat and material balances aud operating data during run 15
The large pilot plant a t Grand Forks is shown in Figure 9, which gives the design and typical operating data since March 1946. This plant has five times the capacity of the small unit, as its heating surface is 200 square feet compared with 40 in the small plant. The structural features of the large plant are essentially the same as of the Golden plant, but there is some improve: ment in design and materials. Both plants have the same characteristics, and operating results in the small unit can be duplicated in the larger plant.
Operating Data and Results The small plant has operated about 1500 hours under various conditions to o%tain data on different fuels and to settle points of design. A typical run starts on Monday and lasts until Friday night, during which time about four variables are studied. In making a test, the plant is brought into balance from 12 noon t o 4 P.M. and is then run without changes for 20 hours. The rate of movement of materials, temperatures, and analytical data are obtained continuously during the balanced period. Figure 10
INDUSTRIAL AND ENGINEER ING CHEMISTRY.
634
f.C.
Ash
02
8
S
1'
= *
30.9
= =
44.6 0.7
Vol. 40, No. 4
5.3
Blu/cu.fi
299
.=
C Z b
=
0.4
111.
=
0.3
cu. 11 pf pas made
Figure 9.
U. S. Bureau of Mines pilot plant for gasification of lignite
Arrangement and design of retort from March 1946 to January 1948.
shows certain operating data obtained during a typical balance period on the small plant. Fifteen tests have been made on the small Plant, Covering the study of about 50 variables and changes in operating conditions. The data for each test have been reported previously (I,$), but t h e significant results of several runs are given in Table 11'. ThP terms used in this table are defined as follows:
Operating data for run 4-13
Percentage gasified indicates the percentage of carbon in the original coal which is converted t o gas, including the carbon in the methane and illuminants. The steam used per mole of carbon gasified represents the total process steam plus t h a t derived from the drying and destructive distillation of the coal. The steam Converted, z, represents the steam decomposed during gasification of the pure carbon to form the mater gas. This is a calculated figure and does not consider the hydrogen or carbon dioxide derived from the coal by decomposition. The average furnace
April 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLEIV.
GASIFICATION OF COALIN SMALLPILOT PLANT, GOLDEN,COLO. 5-B Colo. subbituminous
5-c Steamdried lignite
8-B North Dakota lignite
9-D Wyoming subbituminous
1.44 76.6 1540
1.24 94.4 1605
1.45 89.8 1690
1.68 74.7 1730
2.95 73.4 1800
3.25 64.2 1750
2.33 82.4 1745
Steam Lb. /hr./sq. f t . b Upper annulus Lower annulus Used per mole of carbon gasified Converted = Zd
4.0 8.0 7.0 1.75
3.8 8.2 6.25 1.65
1.9 2.2 2.93 1.49
2.6 4.4 3.65 1.47
2.3 4.6 1.96 1.29
2.8 3.8 2.81 1.36
2.7 2.7 2.07 1.32
Net heat used B.t.u./cu. ft. of gas made M B.t.u./hr./sq. ft. b
190 8.19
180 9.40
202 8.03
180 9.29
166 10.52
141 10.10
141 8.71
124 9.08
Gas made, 60" F., 30 inches Hg, dry Hz:CO ratio Iz Cu. ft./hr./sq. ft., total b Cu. ft./lb. of coal, total CO HZ CU. ft./hr./sq. f t . b CO Hz: cu. ft./hr./sq. ft,.e
7.18 47.5 34.2 33.8 1280
4.73 51.1 42.7 37.5 1430
2.0 80.1 27.1 67.5 2240
2.11 62.1
50.6 1610
2.01 70.4 30.3 58.9 1870
2.06 60.6 25.5 49.6 1890
2.64 72.0 28.4
Retort arrangement Width of annulus, inches Ratio of length of top t o bottom annuli
3 2.45
3 2.45
2 2.79
2 2.11
2 2.11
1.75 2.00
1.75 2.00
Test No.@ Kind of coal Coal rate, lb./hr./sq. ft. b Percentage gasifiedc Average furnace temperature, '
635
F.
-
+ +
Q
b 0
d 6
12-c North
13-C
15-0
15-B
Colorado Subbituminous
13 11.2
3 2.45
3 2.45
12-Inch retort used for tests 5 to 9 and 16-inch retort used thereafter. Based on external area of retort heated surface = 40 sq. ft. Percentage of carbon in coal converted to gas.
10-B Colorado subhituminous
19.1
2 38 70.7 1610
1.8 3.0 2.16 1.33
2.54 74.1 1696
'
2.7 4.6 2.73 1.43
56.6
2040
Per mole of carbon gasified. 2 = Hz/(CO f COi) in water gas formed. Based on cross section of retort.
TABLE. V. GASIFICATIONOF COALIN LARGE PILOTPLANT, GRANDFORKS, N. DAK. temperature is the average value of temperature points 1, 2, 3, and 4 in Figures 8 and 9. The gas made per square foot of retort surface area is calculated on the basis of 40 square feet in the small plant and 200 square feet in the large plant. Table V gives comparable operating data on the large plant, which has operated about 2200 hours. The analyses of typical gases made from various coals in both plants are given in Table VI. The yields and heating value of gases obtained in both plants when making various grades of water gas from several ranks of coal are shown in Figures 11and 12. The lines on these graphs were previously calculated for 80% gasification of subbituminous coal and lignite, respectively, and many of the points are for gasification of less than 80%. Because of slight discrepancies in published data on the heat capacity and heats of combustion and reaction of gases, all thermal data in this investigation are referred t o Bureau of Standards data (80). For convenience in correlating these with other information, the ' basic data in Table VI1 have been compiled. A heat and material balance &I run 4-H on the large plant is given in Table VIII. Other heat balances were made but
Test No.
Preliminary A
Kind of coal Coal rate, lb./hr./sq. ft. b Percentage gasified C Average furnaoe temperature, F.
Lignite 3.00 48.7
1-B
2-E
3-B
4-H
4-0 Lignite Lignite Lignite Lignite Lignite char 3 46 3.64 3.30 3.07 1.67 47.0 74.9 77.4 70.3 46.6
5-B
6-D
X5
Lignite Lignite Lignite 1.62 1.90 3.71 71.6 58.6 80.0
1825
670
1790
1785
..
..
1510
1535
1750
3.5 2.9
3.0 9.7
2.4 4.0
2.5 4.6
3.7 2.1
1.6 1.5
3.0 3.7
2.5 3.5
2.6 3.4
4.86 1.56
5.76 1.65
2.20 1.42
2.61 1.48
2.59 1.49
2.08 1.42
5.67 1.76
5.08 1.67
1.91 1.34
Net heat used: B.t.u./cu. ft. of gas made M B.t.u./hr./sq. ft. b
144 6.47
130 6.64
127 9.84
123 9.23
118 8.26
132 5.39
128 5.00
139 4.98
110 9.35
Gas made, 60° F., 30 inches Hg dry H2:CO ratio = R Cu.ft./hr./sq. ft totalb . cU. ft./lh. of ooai: total CO iHz, cu.ft./hr./sq.ft.b CO Hz, cu. ft./hr./sq. f t . 6
3.47 45.3 15.1 32.4 516
4.58 49.4 14.3 35.0 558
2.48 78.3 20.9 60.7 1070
2.71 73.6 22.3 57.6 915
3.06 68.9 22.4 54.2 862
2.53 40.2 25.8 32.9 525
7.17 38.3 23.7 26.9 428
5.01 35.2 18.5 25.4 405
2.0 84.0 22.8 68.6 1090
Retort arrangement Width of annulus inches Ratio of length oi top to bottom annuli
4
4
3
3
3
3
3
3
2.25
2.02
4.03
2.26
2.11
1.86
1.86
1.86
1.86
1.78
Steam Lb./hr./sq. ft. b Upper annulus Lower annulus Used per mole of carbon gasified Converted = Zd
+
Q
b c
d e
.
Desired oonditions for 2000-hour test. Based on external area of retort heated surface, 200 sq. ft. Percentage of carbon in coal converted t o gas. Per mole of aarbon gasified. 2 = Hz/(CO COz) in water gas formed. Based on cross section of retort = 12.55 sq. ft.
+
TABLE VI.
TYPICAL GASESMADEFROM COALSIN EXTERNALLY HEATED ANNULAR RETORT
Run No. Plant Coal gasified Percentage converted Av. temp., F. Heat used, B.t.u./cu. ft. of gas made Gas made M cu. ft./ton Hz: CO ratio Gas analysis,
coz co
9%
'
Hz
Ill. CH4 CnHs
Nn
B.t.u./cu. f t . B.t.u./cu. ft., COz-free
10-A 12-B 14-A 15-C Golden Golden Golden Golden Subbit. Subbit. Bit. Subbit. 78.5 64.3 86.1 71.8 1805 1750 1775 1595
4-H 4-N 7-C G.F. G.F. Golden Lig. Lig. S. D. Lig. 74.8 71.3 53 1800f lSOO+ 1540
5-B
3-A G.F. Lig. 58 8 1345
G.F. Lig. 71.5 1510
7-A Golden Subbit. 66 1500
138
131
204
124
118
118
169
201
126
178
59.6 1.92
56.4 2.16
77.6 2.36
57.7 2.64
45.7 3.06
45.6 3.12
50.6 4.1
38.2 5.4
44.3 7.18
58.9 10.7
56.4 29.3 0.2 3.3
10.8
13.3 56.6 26.2 0.2 3.2
13.4 59.1 25.1 0.2
17.4 57.0 21.6 0.4 2.8
18.0
22.5 59.8 14.6 0.1 2.5
015 302 349
26.3 59.7 10.7 0.3 2.6 1.0 0.4 274 366
24.8 61.6 8.6 0.2 3.2 0.6 1.0 266 354
25.9 64.0 6.0
ii3 551
19.1 58.7 18.8 0.2 2.5 0.2 0.5 285 352
..
1.8
0.1 0.3 292 338
018 288 349
59.7 19.1 0.3 2.0 0.4 , 0.5 294 358
6:5 265 343
0.6 2.8
0.7 0.6 272 367
636
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. 40, No. 4
a
A.m.
C 0 2 in gas made
Specific g r ~ t yo f gu made Figure 10. 4
P.N.
330 320
310
3 00 290 280
270 260
I
2
3
4
5
6
7
8
9
IO
Operating data for run 15-B
Feb. 11. 1947, t o noon, Feb. 12, 1947
/I
H2:C0 Ratio in water gas Figurell. Yield and heating value of gas made from subbituminous coal i n Golden pilot plant Curves show theoretical data when coal is 80% gasified
are not reported in this paper. The potential heat in the gas varies from44 t o 68% of the heat in the materials entering, but the total heat in gas plus that in char residues was 83 to 85% of the total heat in materials used. Radiation and sensible heat losses in the present Grand Forks plant are therefore approximately 15%. The distribution of net heat used for gasification of subbituminous coal and lignite when various grades of Rater gas are made is shown in Figure 13. This distribution was calculated from several heat balances and reveals an important characteristic difference in the heat necessary for the various coals. Approximately 1070 less heat is required to gasify subbituminous coal because of the difference in moisture content. Part of the exothermic heat of decomposition of subbituminous coal is used in converting the water-gas reactions; but no heat is derived from this source from lignite, even though the exothermic heat of reaction is slightly greater. If the lignite were dried, advantage from the exothermic hcat of decomposition 1%-ouldbe appreciable. Figure 13 shows that approximately 56,000 B.t.u. must pass through the retort wall for each mole of subbituminous carbon gasified, while about 63,000 B.t.u. must be transmitted for lignite. Therefore, the gross heat necessary in the heating chamber is proportionallg greater. As shown in the graph, about 93,000 B.t.u. per mole of carbon gasified must be supplied when gasifying subbituminous coal and about 105,000 B.t.u. are required for lignite. Intcrmediate grades of low-rank coals, such as subbituminous C ratilr, will fall between these limits. The graph is useful for estimating the advantage of drying. For example, in the lolver section of the column the single-line
TABLE VII. HEATSOF COMBUSTION AND HEAT CAPACITY OF DRYGASESPER POUND MOLE Constituent H2 co co:
637
I N D U S T R I A L A N D E N G I N E ERING C H E M I S T R Y
April 1948
Heat of Combustion, Net B t u er Mole 60° F.: iOl?iohes H i 103 975 121:714
CH4
784'000 345:194
c
1'6i3;ij7
Ill. (Ci.8Hr.a)
.....
Nt HnO
Heat Content,
Natural Subbituminous
Coal
Natural Lignite
Molal Volume, 60°,30 Inches Hg 378.3 378.2 376.1 378.0 377.8 376.6 378.6
B.t.u. per Mole, 60° t o 750° F. 4 820 4'900 7:210 14 000 7'550 4'900 5:800
I
....
...
TABLE VIII. HEAT AND MATERIALBALANCEON R U N 4-H, GRANDFORKS PILOT PLANT= Temp.,
F.
In
60 420 84 a4 77
Natural lignite as oharged Process steam, sensible heat Heating gas, potential heat Heating gas, sensible heat Air to retort Total
-
B.t.u., Thou- B.t.u. Moles Pounds sands %
1D:43 16.45 *3
. . . . . .
614 '3937b 69.6 1.0 350 58 29.2 257 1652 4 0.1
& 7 0.1 2365
5658
100.0
%
out COZ
18.0 19.4 59.3 0.3 2.2 0.4 0.4
co Ha
Ill., Cz.aHr.s CHI
CsHa Na
730 730 730. 730 730 730 730
6.31 6.80 20.78 0.10 0.77 0.14 0.14
... ...
..*
.
.
I
... ... ... ...
...
.
I
.
...
... ... ... -
- m
184C 3.2 Subtotal 1oo.o ... 3659 64.7 730 ... Make gas, potential heat Undecomposed steam, sensible 91 I.6 730 Is.ls 291 heat 69.5 ... 860. 3934 Subtotal 18.8 ... 104 1061 Char and dust, potential heat 17 0.3 ... Char and dust, sensible heat ... i4oi 225 4.0 Stack &sea, sensible heat 421 7.4 . . . . . . ... Radiadon from system ... COX lost Unsocounted for ,.. ... ... 2365 5658 100.0 ... Total a Net heat basis, hourly operation. Gas volumes are dry a t 60' F. and 30 inches Hg. b Potential net heat in coal is equal t o gross heat minus 18,350times mole fraction of total hydrogen. C Sensible heat in make gas.
... ... ...
-
-
-
-
-... Figure 13. Distribution of net heat used when subbituminous coal and lignite are gasified in externally heated annular retort
Commercial Processes for Gasification of Coal i a Externally Heated Retorts ANNULAR RETORT PROCESS
2
*
3
H,:CO
4
5
6
7
Ratio in water gas
Figure 12.. Yield and heating value of gas made from lignite i n Grand Forks pilot plant Curves show theoretical data when lignite ia 80% gasified
The two pilot plants for developing and testing the annular retort system of gasification have supplied information for the design of the next stage. Figure 14 shows an arrangement of a 48inch retort which incorporates the factors of design developed in the previous tests. A retort of this design, or one of 60- or 72-inch diameter, can be considered a single commercial unit, and its probable performance on lignite has been estimated in column X of Table V. The 48-inch unit, as shown in Figure 14,should produce 16,000 cubic feet of 300-B.t.u. gas per hour from 740 pounds of natural lignite at an over-all efficiency of 71% in an integrated plant. By adding waste heat recovery the over-all efficiency can be increased to about 80%. The development plan of the bureau was t o test this unit during 1948 to determine the probable life of the alloys when operating a t a maximum temperature of 1850' to 1900" F. These contemplated tests will probably be delayed by restrictions in appropriations. Final answers on cost of making gas and the life of suitable alloy materials cannot be answered until tests totaling several thousand hours of operation have been completed. Table I X shows probable heat and material balances on this
638
INDUSTRIAL AND ENGINEERING CHEMISTRY Coal
\
\Hopper/ \i7
\
:
L
Heat-Resisting Alloys. Final sclection of suitable alloys for the annulai retort illustrated in Figure 14 has not been made, but favorable considwation has been given to the following alloys :
/'
J
Increment
d I b I I-
L_----
"
Vol. 40, No. 4
Charger
