January, 1927
INDUSTRIAL AND ENGINEERING CHEMISTRY
__
119
Reactions in the Fuel Bed of a Gas Producer' 11-Effect of Depth of Fuel Bed and Rate of Firing By R. T. Haslam, R. F. Mackie, and F. H. Reed DEPARTMENT OF CHEMICALEXGIXEERING,MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAXBRIDGE, MASS.
By means of a n experimental gas producer burning anthracite coal a t industrial rates, a study has been made of the effect of two primary variables, depth of.fuel bed, and rate of firing on the reactions taking place in the fuel bed. Throughout this series the steam admitted with the blast was kept constant a t 0.4 pound of steam per pound of coal. Samples were drawn simultaneously from different points in the bed through water-cooled sampling tubes, precautions being taken to prevent channeling of the bed. Temperatures a t different points in the bed were also measured. Runs were made with fuel beds of 1.5-, 3.0-, and 4.5-foot depth and with firing rates of 10,40, and 70 pounds of coal per square foot of grate area per hour. I t was found t h a t the heating value of the gas produced, cold gas efficiency, and per cent steam decom-
posed increased both with increased depth of fuel bed and rate of firing. The main effect of increasing depth of fuel bed and rate of firing is to increase the temperature of the primary reduction zone (i. e., where steam is decomposed by carbon). This increase in temperature increases the proportion of CO and HP in the gas and cuts down the proportion of COS and undecomposed H20. The effect of the increased time of contact in the runs having low gas velocity (low rate of firing) is secondary to the low temperatures resulting. The thickness of the primary reduction zone and the thickness of the oxidation zone (C O2 = COS)is not affected by increasing the total thickness of fuel bed or the rate of firing. Steam passes through the oxidation zone undecomposed.
+
. . . . . . . . .... .. HE work here presented is a continuation of the gas
T
producer studies begun by Haslam, Entwistle, and Gladding.2 The previous article described apparatus and experimental technic. The various reactions taking place in different zones of the producer and the functions and extent of the zones were described. It is the purpose of the present work to report the effect of two primary variables in gas-producer operation on the performance of the producer in gasifying coal. These were depth of fuel bed and rate of firing (pounds of coal per square foot of grate area per hour), The other primary variable, amount of steam admitted with the air blast (pounds of steam per pound of coal), was kept constant. I t s effect will be reported in the next article oi this ~ e r i e s . ~ A few minor changes in apparatus and experimental procedure tended to simplify technic and contribute to the ease of maintaining steady conditions. The sampling holes on the producer were fitted with 1-inch standard pipe nipples secured to the wall of the producer by sturdy clamps. A hole was drilled in a pipe cap, into which the water-cooled sampling tubes were soldered. This arrangement made a gas-proof joint between the sample tube and 1,he producer which proved much simpler to handle than the luted joint formerly used. The sand seal on the top was replaced by a water seal. The coal feed pipe was fitted with a water seal. When charging, the cover of the pipe was removed. A clapper valve on the bottom of the charging pipe was held shut by a chain connecting the cover and the valve. These water seals prevented gas leakage from the top of the producer. To the sample-hole nipples could be screwed a quartz window set into a drilled pipe cap. Through these an optical pyrometer could be sighted for the purpose of determining coke surface temperatures at difl'erent points in the bed. The gas samples were collected in 300-cc. gas collection tubes fitted with glass stopcocks, using ZnSOc solution (30 per cent ZnSOd 1per cent H2S04jas a siphoning liquid. I n these tubes the gas samples were kept out of contact with liquid, thus minimizing any tendency to a change
+
1 2
8
Received August 17, 1926. THIS JOURNAL, 17, 586 (1925). See Haslam, Ward, and Mackie, page 141 of this issue.
in composition as a result of absorption. I n all these runs a carefully sized anthracite pea coal (Table I) was used as a fuel. The use of this coal and the method of operation eliminated the distillation zone of the producer. Before samples were drawn the bed was brought to a dull, even red on top. Analysis of Coal a s Received Per cent Moisture 2.35 Volatile combustible matter 7.70 Fixed carbon 76.55 Ash 13.40
T a b l e I-Proximate
Total 100.00 Total carbon (as received) 78.84 Heating value, B. t. u. per pound dry coal 12,690
Results The results of this work are summarized in Table 11. The plots in Figures 1 to 9 show how gas composition and temperature vary with distance from the grate bars with three different rates of firing (10, 40, and 70 pounds of coal per square foot per hour) and three depths of fuel bed (1.5, 3.0, and 4.5 feet). Figures 10 to 18 show the changes in the stoichiometric ratios (total carbon : N2; fraction water decomposed; and C0:COQ ratio) with distance from the grate bars for different depths and rates. T a b l e 11-Summary
of R e s u l t s ~~~
DEPTH l.5feet
OF
FUELBED
3.0feet
Rate o j f i r i n g , 10 lbs. per sq. j t . per hour Actual depth, ft. 1.73 2.84 Actual rate, Ibs. per sq. f t . per hr. 8.00 8.50 B. t. u. per cu. f t . gross, 30 in. Hg, 60' F., HzO satd. 74.2 89.3 Cu. ft. gas per lb. coal, 30 in. Hg, 60' F., Hz0 satd. 90.3 87.5 Cold gas efficiency, per cent 54.3 63.4 Cu. ft. air per Ib. coal, 30 in. Hg, 60' F., Hz0 satd. 74.3 69.0 Lbs. steam per lb. coal 0.480 0.450 K', apparent equilibrium constant = (Cod (Hz) 0.80 0.95 (CO) (Hz0) Depth primary reduction zone, in. 7.5 12.0 1660 Mean temperature, P. R. Z . , F. 1600 k', reaction rate constant 0.228 0.675 Per cent steam decomposed 67.0 55.2
4.5feet 4.13 9.72 102.0 80.7 66.8 61.1 0.405 1.2 9.0
1800
0.811 77.0
INDUSTRIAL AND ENGINEERING CHEMISTRY
120
Table I1 (Concluded) DEPTHOF FUELBED 1.5 feet
Rate of firing, 40 lbs. per sq. ft. per Actual depth 1.46 34.4 Actual rate 92.6 B. t. u. of gas 85.5 Cu. f t . gas per Ib. coal Efficiency, per cent 64.1 Cu. f t . air per lb. coal 66.7 0.443 Lbs. steam per Ib. coal K' (equilibrium) 0.89 P. R. 2. depth 11.2 P. R. 2. temperature 1700 k' (reaction rate) 2.08 67.9 HzO decomposed, per cent Rate of firing, 7 0 Ibs. per sq. ft. p e r Actual depth 1.79 Actual rate .. 75.2 118.4 B. t. u. of gas Cu. ft. gas per lb. coal 74.8 Per cent efficiency 71.9 Cu. f t . air per lb. coal 54.6 Lbs. steam per Ib. coal 0.348 0.89 K' (equilibrium) P. R. 2. depth 6.8 P. R. 2. temperature 2100 k' (reaction rate) 11.9 HzO decomposed, per cent 84.4 ~~
3.0 feet
Vol. 19, No. 1
tillation zone kept the hydrocarbon content of the final gas down to a negligible quantity, or zero.
4.5 feet
Table 111-Heating
hour
Value of Final Gas
(B.t. u. per cu. ft. gross, 30 in. H g , 60' F., H20 saturated)
3.21 4.12 40.6 40.2 108.4 114.5 76.0 79.2 68.2 73.5 56.8 57.4 0.366 0.366 1.11 21.0 6.5 13.0 2200 2300 4.34 2.06 80.4 98.5 hour 2.82 3.82 73.0 71.6 118.2 115.1 76.9 77.4 73.8 72.2 55.2 56.5 0.359 0.365 7.6 2.3 8.0 7.0 2400 2400 9.9 4.7 96.9 90.0
DEPTHOF FUELBSD
RATEOF FIRINO Lbs. coal/sq. ft./hr.
1 . 5 feet
3 . 0 feet
4 . 5 feet
10 40 70
74.2 92.6 118.4
89.3 108.4 118.2
102.0 114.5 115.1
rate of firing of primary reduction zone (P. R. Z . ) (HzO)o concentration of HzO at bottom of E'. R. 2. or initial concentration (HzO), = concentration of Hz0 a t top of P. R. 2.
0 ' X (HzO), depth
Reaction rate constant = k' = log
-
Discussion of Results
The experimental difficulties of drawing continuous samples from different points in a fuel bed over long/periods of time are very great. Likewise, it is difficult to maintain a bed in equilibrium condition over a long period. For this reason comparatively short-time samples had t o be used. This equilibrium was particularly difficult to maintain in the runs a t high rate (70 pounds of coal per square foot per hour) and deep bed (4.5 feet). Working with a bed so close to the top of the producer it became difficult to get a sufficient thickness of red coal before the bottom part of the bed was burned out. This difficulty manifested itself only in run 10; in the others equilibrium conditions were found comparatively easy to reach and maintain. r
IS
12
II
i
i
1.
'
Figure 2-Run 7 8.5 Ibs. coal per sq. ft. per hr.; 2.84-it. fuel bed; 0.45 Ib. steam per lb. coal
The heating value increases as shown above, owing to the greater amounts of Hz and CO in the final gas. The percentage of these constituents is increased because of the larger amounts of steam capable of being decomposed in runs having the greater firing rates and deeper fuel beds. Table IV shows the change in steam decomposition. Table IV-Percentage
I7
16
l
'
'
DEPTHOF FUELBED
RATEOF FIRINQ Lbs. coal/sq. ft./hr. 1 . 5 feet
SAMPLE HOLL N-
1s
of Entering S t e a m Decomposed
I
10 40
70
3.Ofeet
4.5feet
55.2 67.9 84.4
The increased steam decomposition is due to higher temperatures in the primary reduction zone-i. e., where carbon decomposes steam. The previous articles of this series showed that steam is decomposed in two principal zones in the bed. AUPLL "OLL
*.
c Figure 1-Run 2 8.0 lbs. coal per sq. ft. per hr.; 1.73-ft. fuel bed; 0.480 Ib. steam per Ib. coal
That the heating value of the gas generally increases with increasing depth of fuel bed and increasing firing rate is shown in Table 111. The low heating values are due to the use of anthracite coal. Moreover, the bed was first brought to a dull red on top to eliminate the distillation zone which would be present. Thezuse of anthracite coal and the elimination of the dis-
,*C*L,
molt
GRAOL
8.113
Figure 3-Run 8 9 72 Ibs coal per sq it per hr ; 4 13-ft fuel bed, 0 405 Ib. steam per lb. coal
I N D U S T R I A L A N D ENGI.VE'E:RI.YG CHEEMISTRY
January, 1927
1-The primary reduction zone, where the decomposition of steam by carbon took place: C HsO = CO Hz-70,900 B. t. u. per Ib. mol C 2H20 = CO? 2H~-71,600 B. t. u. per lb. mol 2-The secondary reduction zone which served t o transfer heat from the hot gas t o the colder fuel, and in which a small amount of additional steam x a s decomposed by the interaction of carbon monoxide: CO HzO = COt Ht-700 B. t. u. per lb. mol
++
+ +
+
121
where HzO is the concentration ol water vapor in mols of water per mol of wet gas at any time. 8 . Integrating between limits of (H2O)o and (HzO)~, and 0 and 0 we get
+
The.time of contact, 8,is proportional to the depth of the primary reduction zoiie and inversely proportional t o the rate of firing, so t h a t rate of firing k' = log- (HzOh (H20)1 depth of primary reduction zone where (H2O)a is the concentration of steam in the air blast at the bottom of the primary reduction zone and (HzO),,the concentration at the top.
'
?
>
Fieure +Run I
I*U*L/
,I
4
,E
-0.5
1,
w
b
I
19
,
1
33.4 HJS. coal per sq. f t . per hr.; 1.48-ft. fuel bed; 0 443 Ib. steam per Ib. coal
The limits of these zones are found by reference t o the plots of gas composition and gas ratios. The bottom of the primary reduction zone (P. R. Z.) is found a t the point where the products of decomposition of steam (viz., CO H,) first appear in the gas. The top of the primary reduction zone is found where the total C:N2ratio becomes practically constant. The secondary reduction zone reaches in this case from the top of the primary reduction zone t o the top of the fuel bed (usually to the bottom of the distillation zone). An attempt to get a quantitative measure of the rate of steam decomposition was made by calculating an apparent rate of reaction constant. Haslam, Hitchcock, and Rudow4
+
I"
Figure 6-Run 5 40.2:Ibs. coal pcr a q . ft. per hr.; 4.12-ft. fuel bed; 0.366 Ib. steam per Ib. coal
The reaction rate constant, calculated in this manner, illcreases with the rate of firing as shown in Table V. l'ahle \--Reaction RATEO F FinIPic I h s . coal/sri. ft./lir.
1.3 feet
0.675 %08 11.9
10 40
70
Rate Constant, k' DEPTH OF
3 . 0 feet 0 . z ~ 4.34 9.9
FUELBED 4 . 5 feet 0.811 2.0G 4.7
0.571 2.83 8.83
I'he agreement between the constants for a given rate at different depths is not good, but the increase in these 5 M N MU
Figure +Run
Mean
*-
4
38.3 lbs. coal per sq. ft. per h r . ; 3.2-ft. fuel bed; 0.1(78Ib. steam
per Ib. coal
found that the priniary reactions by which steam is decoriiposed, viz: C and
+ H20
C f 2HzO
+ H~-T0,900 = CO2 + 2&-71,60U =
CO
R . t.
11. per
lb. tiiol
B. t. u . p2r lb. tnul.
are both first order reactions, which means that the rate of decomposition of water is proportional to its concentratioii at m y given t'ime. Mathemnticnlly expressed, this is : "I%IVJOUHNAI.,
10, 115 (1923).
Figure 7-Run 3 -- 2 lb3. coal per sq. f t . per hr.; 1 79-ft. fuel bed; I J
0.3-18 Ib. s t e n t n per Ib. coal
wnstants with iucreasiiig rat,e is apparent arid considerable. A fifteenfold increase iu the coilstant results froin :I seven-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
122
fold increase in firing rate. It is well known that reaction rate increases as a power function of the temperature. This fact partially explains the increase shown here. The variation in the average temperature of the primary reduction zone is shown in Table VI. Table VI-Average
Table VIII-Cold
VOl. 19, KO. 1 Gas Efficiency, Per cent DEPTH OF FUELBED
RATEOF FIRING Lbs.coal/sq. ft./hr. 1 . 5 feet
3 . 0 feet
54.3 64.1 71.9
10 40 70
4 . 5 feet
64.3 68.2 73.8
66.8 73.5 72.2
Temperature (" F.) in Primary Reduction Zone DEPTHOF FUELBED
RATEOF FIRING Lbs. coal/sq. ft./hr.
1 . 5 feet 1660 1700 2100
10 40 70
3 . 0 feet
4 . 5 feet
1600
1800 2300 2400
2200 2400
The increase shown above is due mainly to the increase in the heating value of the gas (Table 111). The cubic feet of gas per pound of coal decreases slightly with increased rate of firing. This follows from less air (containing diluent 1
I,
j2
1.
p
,*
5,"PLL 17
I(0.L
c
w
IS
I
1
10
,
That the depth of the primary reduction zone has little, if anything] to do with the reaction rate constant is shown by the constancy of these depths when tabulated against firing rate and fuel-bed depth (Table VII). Table VII-Thickness
DEPTHO F FEELBED
RATEOF FIRING Lbs. coal/sq. ft./hr.
10 40 70
of Primary Reduction Zone, i n Inches
1 . 5 feet '
7.5 11.2 6.8
3 . 0 feet 12.0 6.5 8.0
4 . 5 feet
..
9.0 13.0 7.0
The depth of the primary reduction zone is independent of either the rate a t which the fuel is being burned or the total over-all depth of the fuel bed. The variation in depth-
Figure 9-Run 10 T l . 6 Ibs. coal per sq. ft. per hr.; 3.82-ft. fuel bed; 0.365 lb. steam per Ib. coal
nitrogen) being used per pound of coal a t higher rates. The apparent equilibrium constant is considerably in error in some cases as a result of the error in calculating small concentrations of water. The concentration of water vapor was obtained by calculation of the H, and O2appearance in the gas (the latter as CO and C02, of course) rather than from direct analysis] because of the experimental difficulty. -4 tabulation of these apparent equilibrium constants ( K ' ) is shown in Table IS. Table IX-Apparent
RATEOF FIRING Lbs. coal/sq. ft./hr. 1 . 5 feet
i. e., from 6.8 to 13.0 inches-is apparently due to sampling and channeling. The increase in steam decomposition and also the rate of reaction constant that results from the use of a deep fuel bed is due to the fact that the fuel comes into the primary reduction zone hotter with deeper beds, thus raising the temperature of this zone. The increase in the total depth of fuel bed increases the steam deconiposition by increasing the temperature a t which the steam decomposition reactions take place, rather than by increasing reaction time. It will be noticed that the teniperature of the primary reduction zone increases 200" to 600" F. by an increase in the total depth of fuel bed from 1.5 to4.5 feet. Increasing the rate of firing from 10 to 70 pounds of coal per square foot per hour increases the temperature of this zone 500" to 800" F. Cold gas efficiency] which is the product of heatingvalue of the final gas (B. t. u. per cubic foot, gross] 30 inches Hg, 60" F., HO saturated) and cubic feet of gas per pound of coal divided by the heating value of the coal times 100, increases with firing rate and fuel bed depth (Table V I I I ) .
0.95
10 40 70
Figure &Run 9 7 3 Ibs. coal per sq. ft. per hr.; 2.82-ft. fuel bed; 0.369 lb. steam per Ib. coal
NE
'3 Ihq.
0.89 0.89
Equilibrium Constant
DEPTHO F FUELBED 3 . 0 feet
0.80 1.11 7.8
4 . 5 feet 1.2 21.0 2.3
0 0
i"
'
z'
Figure 10-Run 2 coal per sq. ft. per h r . ; 1.73-ft. fuel bed; 0.48 111. stentn ucr Ib. c o d
January, 1927
IA'DUSTRIAL A N D ENGINEERI,VG CHEMISTRY
123
Figure 12-Run 8 9 . i 2 Ibs. coal per sq. f t . per hr.; 4.13-ft. fuel bed; 0.406 Ib. steam per Ib. coal
4
I
-
'
Figure 14-Run 4 38.3 Ibs. coal per sq. ft. per hr.; 3 2-ft. fuel bed; 0 378 Ib. steam per Ib. coal Figure 13-Run 1 34.1 Ibs. coal per sq. f t . per h r . ; 1.46-ft. fuel bed; 0.443 Ib. steam per Ib. coal
5AWD.E
1
Figure 15-Run 5 40.2 Ibs. coal per sq. ft. per hr.; 4.12-ft. fuel bed; 0.366 Ib. steam per Ib. coal
I n the run made a t the 40-pound rate and 4.5-foot depth, a very small amount of water was left in the final gas so that the error in its measurement was very large; hence, the accuracy of this calculated equilibrium constant is small. The runs a t 1.5-foot depth had the most water in the final gas and therefore the greatest accuracy in calculating the equilibrium constants. The constants from these runs do not vary greatly with a sevenfold change in rate of firing. This bears out the conclusions from previous work5 which showed that this constant was independent of rate of firing. 5
Haslam, THISJ O U R N A L , 16, 782 (1924).
8
3
.
5
6
to-c
N=
I
FiLure 16-Run 3 75.2 Ibs. coal per sq. f t . per hr.; 1.79-ft. fuel bed; 0.348 Ib. steam per Ib. coal I
Table X-Thickness
of Oxidation Zone, in I n c h e s
RATEOF FIRING Lbs. coal/sq. ft./hr. 1 . 5 feet 10 40
io
a
2.5
3.0
2 5
DEPTH OF
FUELBED
3.Ofeet
4 . 5 feet
1.0 3 0
15.P 3.0
4 0
z n
This is in error because trouble with one sampling hole caused chan-
neling.
The carbon in the fuel burns with the oxygen of the air in the oxidation zone. This zone supplies all the heat in the producer. The thickness of this zone is found to be
124
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 19, No. 1
Figure 18-Run 10 hr.; 3.82-ft. fuel bed; 0.365 Ib. steam per lb. coal
71.6 Ibs. coal per sq. f t . per 73
Ibs. coal per
Figure 17-Run 9 sq. ft. per hr.; 2.82-ft. fuel bed; 0.359 per Ib coal
Ib. steam
Table XII-Percentage
of Inert8 i n Final Gas (Co1, Hz0. Ns.
RATEO F FIRING
unaffected within the limits of experimental error by changing rate of firing or depth of fuel bed, as shown in Table X. The union of oxygen gas and carbon is a diffusion reactioni. e., i t is independent of the time of contact but is greatly affected by the velocity of the oxygen past the surface of the carbon face. Next to the carbon surface is a stationary film of gas through which the oxygen must diffuse up to the carbon surface. If the velocity of the gas past the surface is doubled, the thickness of the film is about halved (and, therefore, the rate of diffusion is doubled), but the time of contact is also halved, so the net effect is zero. This independence of the disappearance of oxygen through the reaction : C
+
0 2
= COZ
+ 174,600 B. t . u . per
lb. mol
from changes in gas velocity shows why the thickness of the oxidation zone remains practically constant over the sevenfold change in air rate used in this work. Table XI shows that the percentage of combustible matter in the gas increases with the firing rate and fuel-bed depth. Table XI-Percentage
of Total Combustible Matter i n Final Gas (Hp, C O , CHI) DEPTHOF FUELBED
RATEO F FIRING ft./hr.
1 . 5 feet
3.Ofeet
4.5feet
10 40 70
21.9 28.1 36.8
26.7 31.4 37.1
31.4 35.6 35.6
Lbs. coal/sq.
This result also follows from less air per pound of coal (thus bringing in less diluent nitrogen) a t high rates of firing and depth of fuel bed. This is shown in Table XII, where the percentage of undecomposed H20, CO,, and Nz is tabulated against firing rate and fuel-bed depth.
Lbs. coal/sq. 10 40 70
ft./hr.
01)
DEPTHOF FUELBED 1 . 5 feet
3.0 feet
4 . 5 feet
78.1 71.9 63.2
73.3
68.6 64.4
68.6 62.9
64 4
Conclusions
1-Cold gas efficiency is increased by increasing the rate of firing at constant depth and constant blast saturation. (Pounds of steam per pound of coal.) 2-Cold gas efficiency is increased by increasing depth of fuel bed a t constant rate of firing and constant blast saturation. (Pounds of steam per pound of coal.) 3-Percentage steam decomposition and higher heating value of gas (B. t. u. per cubic foot, gross, 30 inches Hg, 60' F., H2O saturated) also increase with increasing depth of fuel bed and rate of firing a t constant blast saturation, 4-The increased steam decomposition is due to increased temperature in the primary reduction zone. The effect of time of contact of water vapor with carbon is secondary to the temperature effect. &-The thickness of the primary reduction zone is independent of the depth of fuel bed and rate of firing. 6-The thickness of the oxidation zone is independent of the depth of fuel bed and rate of firing. 7-The main effect of increasing rate of firing and depth of fuel bed is to increase the temperature of the primary reduction zone. 8-Steam admitted with blast passes through the oxidation zone-i. e., to the point where oxygen in the gas has practically disappeared-without appreciable decomposition. 9-The percentage of combustible matter (hydrogen and carbon monoxide) in the final gas increases with increasing depth of fuel bed and increasing rate of firing.
Hydrogen Replaces Exhaustion of Oil in Airship Fuel Use of hydrogen in fuel for airships has recently been the subject of experiment in Great Britain, according to a report by Trade Commissioner H. s.Fox, London. A series of exPeriments on a solid-injection fuel engine, for the purpose of ascertaining the effects of the admission of small quantities of hydrogen during the suction stroke, were conducted by the engineering laboratories of Manchester University. Three series of trials were run with hydrogen, each with a different load. As the supply of fuel is consumed by an airship in flight, a corresponding amount of hydrogen must be released-a fact developed during the discussion-and the experiments in question were undertaken t o determine whether this waste hydrogen might be utilized to replace a portion of the oil fuel supplied t o
the engines. The maximum amount of hydrogen used in the experiments was slightly more than 3 per cent by volume of the corresponding air supply a t the lightest load, to approximately 14 per cent by weight of the oil-fuel supply. Three corresponding series of trials were also made using coal Of gas used gas in place Of hydrogen. The was 5 per cent of the air supply, corresponding at the lightest Oil used. load to about 24 times the weight Of It was stated t h a t these quantities of coal gas or hydrogen could be satisfactorily employed in the type of engine considered, and t h a t the engine appeared to run more smoothly when the gas was being used.
