INDUSTRIAL A N D ENGINEERING C;%IEMISTRY
782
These residues were analyzed according to the methods for proximate coal analysis as given by Mahin,* and their heating values on the Parr calorimeter according to instructions accompanying the instrument. The results are tabulated in Table IV. The volatile matter, sulfur, and B. t. u. decrease with increasing still temperatures, while the fixed carbon and ash increase. The heating values of these residues are fairly high, the coke obtained above 540” C. having a heating value slightly above 14,000B. t. u. Heating just above 360” C. produces a residue from this,coal with a volatile matter value of but 8.95 per cent. Above 540” C., a t which quantities of gases are produced, a coal residue with less than 1 per cent volatile matter is obtained. I n this manner a product is formed that can be used in the household without fear of crumbling to a dust or producing dangerous back fires and excessive smoking and sooting, thereby increasing the safety and efficiency in the use of the fuel. 8
“Quantitative Analysis,” 2nd ed., 1919, p. 308.
Vol. 16, No. 8
CONCLUSIONS 1-The excessive smokiness of Farmville, N. C., coal is due to the liberation of complex paraffin hydrocarbons a t relatively low temperatures. 2-The characteristic disintegration of this coal on exposure to the air is due to the absorbed gases and the loose combination of the heavy hydrocarbons in the coal. 3-A positive decomposition point occurs a t 540” C. in addition to the secondary change occurring above 700’ C., as found by other investigators. 4-No marked decomposition point occurs below 540” C., the coal yielding complex hydrocarbons and other by-products without a well-defined decomposition point. 5-This high volatile and smoky coal can be converted into a safer, more efficient, and cleaner fuel for domestic consumption by low-temperature distillation. 6-Low-temperature distillation of this coal produces a commercial gas.
Producer Gas’ Apparent Equilibrium between Its Constituents and Influence of Depth of Fuel Bed By R. T.Haslam MASSACHUSETTS INSTITUTEOP TECHNOLOGY, CAMBRIDGE, MASS.
The constituents of producer gas come to an apparent equilibrium value dependent on the thickness of the fuel bed alone and independent of gas velocity (rate of firing). ratio of pounds of coal to pounds of steam. or temperature of the exit gases. A hypothesis explaining this phenomenon is gioen, based on the reaction HzO CO = COZ Hz being catalyzed by hot surfaces which the gases can reach only by digusion. The relationship between the apparent equilibrium oa of fuel bed may be expressed as
+
+
The amount of water in producer gas may be calculated from the
+
HE importance of the reversible reaction HzO GO = COZ Hz to indicate the direction in which producer gas composition is changing has been recognized by all investigators in this field, but one of the unsettled questions is the time necessary to complete the change. Pu’euman2 considers that the time of contact between the gas and fuel is far shorter than that required for equilibrium, which according to his findings depends not only on time of contact and temperature, but also on the nature of the boundary surface between the solid and gaseous phases. He further noted in all his investigations (seventeen in number) that the producer gas came to an apparent equilibrium corresponding ~ ,the ~ other hand, to a temperature around 600” C. H U S S Oon basing his deduction on the work of LeChatelier, assumes in his mathematical treatment of this subject that the gas is in equilibrium with the coke a t the temperature of the gas. Haber4 states that in a free flame above 1500” C. the equi-
T
+
1 Presented in part under the title, “Calculation of the Amount of Water in Producer Gas from the Orsat Analysis,” before the Section of Gas and Fuel Chemistry at the 66th Meeting of the American Chemical Society, Milwaukee, Wis., September 10 t o 14, 1923. 2 Stahl u. Eisen, 83, 394 (1913). a Rev. ind. mindvale, 1922, 373. 4 “Thermodynamics of Technical Gas Reactions,” 1908.
usual Orsat analysis to within 10 per cent by the equation
where IHzO] equals the oolumes of moisture in the producer gas per 100 volumes of dry gas, (COz), (Hz), and (CO) equal the percentages of these gases as determined by the Orsat analysis, and L equals the depth of fuel bed in feet. The use of this equation aooids the necessity for the direct determination of water i n producer gas, a relatioely dificult analysis to carry out in the plant. Its use also brings out the important point that greater dilution of producer gas is brought about by the presence of the undecomposed water than by the carbon dioxide.
librium value for this reaction shifts rapidly with temperature. Below this temperature numerous writers have pointed out the great effect of catalytic surfaces. While going over the recent investigations of Clements5 it was noticed that the producer gas from each series of experiments, in which the entering air was humidified to varying degrees, came to an apparent equilibrium constant independent of the degree of humidification, but that the constant differed for each series. The only variable differentiating the two series of experiments was the depth of fuel bed, 3.5 feet in the first series and 5 feet in the second. Therefore, a search of the literature was made to bring out the effect of depth of fuel bed on the apparent equilibrium between the constituents of producer gas, and two other satisfactory investigations were found-those of Bone and Wheeler6 and recent work of Hunt, Johnson, and Willi~.~ Data from all these sources are tabulated in Table I. Engineering, 116, 8 J . Iron Steel Inst. (London), 1928 (advanced proof); 597 (1923). 4 J . Iron Steel Inst. (London),76, 126 (1907). 7 “The Determination of Optimum Operating Conditions for a Commercial Morgan Gas Producer;’ M. I. T. Thesis, 1923.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
August, 1924
783
TABLE I
Run
Pounds Steam er Pound goal
Pounds Coal Fired er Hour per gquare Steam De- Temperature Foot Grate of Gas at composed Area Per cent Outlet, O C.a
-DRY COz
?-foot fuel beds 1 2
3 4 5
0.455 0.544 0.81 1.12 1.57
29.2 25.8 24.5 25.4 23.4
87.4 80.0 61.4 52.0 42.0
0.28 0.38 0.51 0.64 0.855
21.6 20.6 22.1 18.1 17.2
59.3 57.5 56.2 55.6 50.6
677 674 665 634 59 1
45 63 55 68 69 63 70
775 775 775 777 789 648 666
48.7 47.2 45.8 43.8 40.4
805 746 717 696 657
5.25 6.95 9.15 11.65 13.25
GAS COMPOSITION, P E R CENT-co Ha CHI
Nz
Volumes Total Moisture per 100 Dry Gasb
Equilibrium Apparent Constant K’C
27.3 25.4 21.7 18.35 16.05
16.6 18.3 19.65 21.80 22.65
3.25 3.40 3.40 3.35 3.50
47.50 46.90 46.10 44.83 44.55
6.87 7.27 12.16 18.76 28.16
0.47 0.68 0.68 0.75 0.67
24.7 23.2 21.3 19.8 19.0
11.5 11.6 12.3 13.1 13.9
4.8 5.1 5.2 5.1 5.1
53.1 03.2 53.2 52.7 52.5
5.6 7.0 9.4 11.7 17.0
0.48 0.49 0.49 0.53 0.41
9.8 11.36 10.1 12.3 12.5 10.9 12.0
2.5 3.0 3.1 2.8 3.4 3.6 3.5
59.34 56.71 57.1 55.0 33.3 55.1 53.8
8.5 8.1 7.9 8.9 7.6 6.2 7.7
0.44 0.45 0.37 0.42 0.43 0.38 0.35
9.5 10.8 11.3 13.0 14.1
4.6 4.5 4.3 4.0 4.1
55.1 54.4 54.5 53.9 62.5
7.4 9.3 12.0 16.3 24.0
0.31 0.35 0.32 0.35 0.35
5i-fOOl fuel bed
1 2 3 4 5
-
5.-9 6.9 8.0 9.3 9.6
4.5-foot fuel
1 2 3 4 5
0.33 0.41 0.54 0.72 1.00
20.3 18.1 17.8 17.8 17.8
bedl
-
.
7.6 19.83 7.2 20.76 6.4 22.3 6.7 22.2 5.9 23.6 5.3 24.6 5.5 24.2 3.5-fool fuel beds 6.0 24.8 7.05 23.3 22.4 7.5 20.3 8.8 10.9 18.4
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-
a Not given in Bone and Wheeler’s data. b Includes the free and combined moisture given off by the coal. Moisture calculated in the case of Bone and Wheeler but reported directly by the others, (Cod (Hz) c K‘ = (co, (HzO) for the reaction Hz0 CO = COz H 2 , where the values (COz), (Hd, (CO), and (HzO) equal the volumes of COz, Hz,CO, and Hz0, respectively, per 100 volumes of dry gas found in the producer gas, whether or not the producer gas actually reached equilibrium.
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+
RESULTS Table 11 is a summary of the apparent equilibrium constant for each depth of fuel bed, the various values having an average deviation Of between and lo per cent. In Fig* the average apparent equilibrium constant is plotted against depth of fuel bed, and it is to be noted that the value K‘
(Hz) ( C O ) (HzO)
= (COz)
which bears a linear relationship to the depth of fuel bed, may be expressed by the equation K’ = 0.096 L, where L is the depth of the fuel bed in feet. Table I11 gives the proximate and ultimate analyses of the coals used in each test. TABLE11-APPARENT EQUILIBRIUM CONSTANT Apparent Depth of Equilibrium Average Fuel Bed Const,ant Deviation INVESTIGATOR Feet K Per cent Bone and Wheeler 7 0.66 10 Clements 5 0.48 5.9 Hunt, Johnson, and Willis 4.75 0.405 9.0 Clements 3.5 0.34 5.0
Fuel Bed Runs Feet 7 5 4.25 3.5
TABLE111-COAL ANALYSIS Proximate (as fired) Fixed Volatile Carbon Matter Moisture Per cent Per cent Per cent 50: 34 52.10 56.64
------_ 78.41 80.30 73.6 80.4
5.51 5.35 4.68 5.25
10.03 8.54 6.02 8.38
1.39 0.98 1.49 1.16
TABLEI V Pounds Coal Fired per Hour per Square Foot
Run
Apparent Equilibrium Consfant
K
+
Again the true equilibrium constant for the reaction HzO CO = COZ Hz goes down with an increase in temperature, but the apparent equilibrium constant of the producer gas is
+
Ash Per cent 3 to 5.3
3 to 4 4.0 1.51 4.0
Ultimate ( d r y basis) Percentage-----Hz 0 2 Nz
C
7 5 4.25 3.5
3k:b6 35.4 34.5
steam per pound of coal. T o illustrate how K’ seems t o. be ._ independent of these variables, consider the following: I n Table IV, by rearranging Some of the data from the investigation of Hunt, Johnson, and willis it is Seen that Kr is constant within the experimental error, although the rate of firing is increased 350 per cent. This would indicate that the time of contact between the gas and upper layers of fuel is not the controlling factor.
3.9 12.5 3.89
S
Ash
0.83 0.78 1.49 0.81
3.83 4.06 12.7 4.0
DISCUSSION OF RESULTS Inspection of the results given in Table I1 and Fig. 1 shows that the apparent equilibrium constant, K’, is dependent only on the thickness of the fuel bed, being independent of gas velocity (which is proportional to the rate of firing or pounds of coal per square foot of grate area per hour), the temperature of the gas leaving the producer, or the pounds of
FSG.1
independent of the temperature of the exit gas, as may be seen from the data from Clements’ 3.5-foot fuel bed runs in Table V. The apparent equilibrium constant corresponds to a temperature much higher than the reading of the pyrometer placed in the exit gas.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
784 TABLE V Run
Temperature of Exit Gas
c.
True Equilibrium Value Correspond- Apparent Equiing to These librium Constant Temperatures K' 2.24 0.35 1.81 0.35 1.63 0.32 1.44 0.35 1.12 0.31
That the apparent equilibrium constant, K',of the producer gas is not dependent on the ratio of steam to coal, evenwitha 300 per cent change in this ratio, is shown by the data of Bone and Wheeler in Table VI. TABLE VI Apparent Equilibrium Constant
Pounds of Steam per Pound of Coal 0.28 0.38 0.51 0.64 0.855
Run 1 2 3 4 5
K'
0.48 0.49 0.49 0.53 0.41
The reason for the relationship between the apparent equilibrium value for the constituents of producer gas and the depth of fuel bed is at present a matter of conjecture. However, the following hypothesis (the basis of this hypothesis is due to W. K. Lewis) correlates these outstanding facts: (1) The apparent equilibrium constant is proportional to the depth of fuel bed. (2) It is independent of time of contact between the producer gas and upper layers of fuel. ( 3 ) It is independent of the temperature of the gas leaving the fuel bed.
Vol. 16, No. 8
Considering the second effect, it is to be noted that the cooling of the ascending gases is due to the loss of heat to the solid fuel by conduction through the same film through which the gaseous molecules reacting according to the equation H 2 0 CO = GOz Hz must diffuse, and also to the loss of heat which is absorbed by the reduction of carbon dioxide to monoxide. Because of this rapid loss of heat the composition of the gas changes with an insufficient speed to reach anywhere near to the equilibrium value corresponding to the rapidly changing gas temperature. Fig. 2 illustrates
+
+
the change in the actual value of ('02) (H2) (in this paper
(eo) WaO)
called the apparent equilibrium constant) vs. depth of fuel bed. On the same plot, for comparison, is the larger, true equilibrium value of
(Hz) which increases a t a greater rate with
(CO) 030)
depth of fuel bed, since cooling, which controls the true equilibrium value, is not only by loss of sensible heat to the coke, but is also due to heat absorption from the reduction of carbon dioxide to monoxide. Thus the apparent equilibrium constant is controlled only by depth of fuel bed, and the composition of the gases is so far from true equilibrium that gas temperature has no effect on the apparent equilibrium. This hypothesis is insufficient in that it does not satisfactorily account for the failure of the gas to come to an equilibrium value dependent on some temperature. If the true gas temperature were known this difficulty might be overcome.
The main chemical reactions, the combustion of carbon to carbon dioxide, and the subsequent major reduction of carbon dioxide and water by the incandescent coke, occur in the bottom of the fuel bed. I n the upper layers two things take placefirst, the interaction between the gaseous constituents according to the equation HzO CO = GO2 Hz, and, second, the cooling of the hot gases.
+
+
TABLE VII-RANGE COVERED BY THESE INVESTIGATIONS VARIABLES Minimum Maximum 3.5 7 Depth of fuel bed in feet 8.9 29.2 Pounds coal per square foot per hour 0.28 1.57 Pounds steam per pound of coal 5.25 13.25 Per cent COz 12.9 27.3 Per cent CO 22.65 8.6 Per cent Hz Per cent Ha0 decomposed 40.4 87 Volumes of total water per 100 of dry gas 5.6 28.07 Exit gas temperature 591" C.a 805' C.= a Not including runs on 7-foot fuel bed.
Considering the first effect, Haber and other investigators have shown that this reaction proceeds a t an extremely slow rate below 1500" C. unless catalyzed by hot surfaces. Before the molecules of gas can get to the hot surfaces they must diffuse through the stationary gas film that surrounds each solid particle, I n all probability the rate of the catalyzed reaction HzO GO = COz H2 is greater than the rate of diffusion of the molecules through the gas film, and therefore the rate a t which the reaction proceeds depends wholly on the rate of diffusion of the gaseous molecules. Now it has been shown by numerous investigations in many fields that the rate of diffusion is independent of gas velocity. For example, while the thickness of the film is about halved by doubling the gas velocity, the time in which diffusion can take place is also halved, so the number of molecules that diffuse through a gived area of stationary gas film remains constant. The rate a t which the reaction H 2 0 GO = C02 Hzproceeds is therefore independent of gas velocity (time of contact, or pounds of coal fired per square foot per hour) and is dependent only on the area of surface through which diffusion takes place, which, in turn, is proportional to the depth of fuel bed.
+
+
+
+
omn
OF FUEL BED
-
FIG.2
While the data are derived from the investigations covering a wide range of operating conditions, as shown in Table TTII, still it should be noted (Table 111) that in all investigations a similar grade of fuel was used. Outside of the further light which these results throw on theory of the gas producer, their main value consists in affording a ready means of calculating from the Orsat analysis the percentage of water vapor present in producer gas. Since the equation connecting K ' with depth of fuel bed is K ' = 0.096 L, then by substituting (coz)(H2) for K'and trans-
(eo)(HzO)
posing, we get ( ( 2 0 2 ) (Hz) [HzO1= (0.096 L) ( C O )
where
[HzO] = the volumes of total moisture (undecomposed steam plus the free and combined moisture in the coal) per 100 volumes of dry producer gas (COz), (Hz), and (CO) = the per cents of COa, Ha, and CO as determined in an Orsat analysis L = the depth of fuel bed in feet
I n general, this equation appears to be accurate to within 10 per cent.
