ILITD USTRIAL A N D ENGINEERING CHEMISTRY
Apri!, 1923
355
Producer Gas from Powdered Coal'" By R. T. Haslam and Louis Harris MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGS, MASS
I
N g e n e r a t i n g elec-
The literature describes several attempts to ma& a satisfactory nary experiment^^,^ were trical O r m ~ h a n i c a l producer gas from powdered coal, but the process has not been exmade with the results noted P0Wer f r 0 C0a1 b e 10 w . Efforts w e r e tensioely developed. The following article is a report of some exthrough the use of steam, periments which were carried out to find the reason for this lac,+. made to determine the most an over-all efficiency of of development, and, if possible, to devise means for overcoming any favorable conditions for the about 18.5 per cent is the difficulties which stood in the way. partial combustion (to carmaximum obtained to date. bon monoxide) of powdered This efficiency has been coal. reached only by the extraordinary efforts exerted during PARTIAL COMBUSTION OF COAL the last two decades in the development of efficient steamThe combustion of coal with an amount of air insufficient boiler and turbine practice. compared with efforts in the development of efficient power through steam, relatively for complete combustion to COZ may be divided into three little attention has been paid to the production of power Stages-namely, (1) distillation of volatile matter, ( 2 ) cornthrough the gas engine, although the theoretical operating bination of residual carbon with oxygen to form COZ, and efficiency of the gas engine at reasonable compression ratios (3) reduction of the COz to CO by the interaction of the COz carbon. is considerably higher than the theoretical efficiency of a with the The rate at which the first two reactions take place above condensing steam turbine, even a t such pressures as 300 lbs. per sq, in, ~h~ lack of development in the production of 1000° C. is so rapid, compared to the third reaction, that the power through producer gas may be attributed to the dig- rate a t which coal and air unite to form CO depends entirely reacts with the coz. culty of generating a satisfactory gas, the chief disadvantages On how fast the EFFECTOF TEMPERATURE in the production of producer gas from bituminous coal, the only COmm~cialcoal to-daY, being (1) dirty gas due to Rhead and Wheelerg found that the equilibrium ratio of tar, (2) nonuniform composition of gas, and (3) lack of CO : COz in the equation COz C = 2CO increased rapidly flexibility. The last two disadvantages are caused by the with temperature, so that a t 8000 C. the ratio of : coZ thick fuel bed required by the gas producer. was 6.3 : 1, while a t 1100" C. the ratio was about 500 : 1. The rate at which the reaction came to equilibrium also inPOWDERED COALFOR GASPRODUCERS creased rapidly with increase in temperature. The following A scheme mentioned a number of times in the literature, table, in which kl represents the kl in equation which seems capable of eliminating these three disadvantages, d is the burning of powdered coal with an insufficiency of air - (CO) = bi (COz) kz (CO)', for complete combustion to carbon dioxide. Any tar formed at would be decomposed by the high temperature, leaving a shows this quantitatively,1° since above 900" C. the reaction dry dust (ash) the only material to be removed. The ease C COZ = 2CO is practically irreversible. of regulation in the feeding of coal and air would insure a TABLE I constant gas composition, while the small amount of fuel in Temp. kl (Corrected for C. 100-Mesh Particles) the "producer" hearth a t any one time would insure flexi900 0.000046 bility in the handling of varying loads. 1000 0.00046
+
-
+
Several attempts t o carry out such a process have been made. Lattaa mentions a producer in operation which yielded a gas containing 23 t o 27.5 per cent carbon monoxide. In Germany,4 a producer using powdered anthracite-coal refuse was used t o generate a gas having a composition of 17.6 t o 24.2 per cent carbon monoxide. In the Marconnet6 producer, a vertical-shaft type, a mixture of powdered coal and air is fed in at the bottom. Satisfactory results were obtained with powdered coals having an ash from to per cent, Bourcou(i6 mentions several other types of powdered-coal-gas producers. H e proposed a vertical type of producer in which the presence of powdered coal in the resulting gas would be minimized. and a lone time of contact of Dowdered coal with gas effected by causing t h e flow of powdered-coal-gas stream to-be alternately up and down through three vertical compartments in t h e producer.
In view of the remarks made in the literature, it is surprising that such an apparently promising field has not been developed more extensively. In order to obtain some idea as to the cause for the lack of development in this field, prelimi1 Presented before the Section of Gas and Fuel Chemistry a t the 64th Meeting ob the American Chemical Society, Pittsburgh, P a , September 4 to 8, 1922. 2 Contribution No. 34 from the Department of Chemical Engineering,
M . I. T. 8
4
6 0
"American Producer Gas Practice." Glaser, Annalenffzlr Gewerbe und Bauwesen, 1909, 111. Gdnre cieril, 51 (1907), 22. Chem. M e t . Eng., 24 (1921), 600.
1100 1200 1300
0.00268 0,0032 0.0269
The necessity of maintaining a high temperature is therefore apparent. Bourcoud6 considers that the reaction velacity data are not applicable in the case of powdered coal. I l e believes that diffusion-the removal of the carbon monoxide, formed from the surface of the coal-is the deciding factor which determines the rate of reaction. However, that this reaction is not controlled by diffusion may be deduced from the work of ~ ~ ~ovits,i and ~L4ugustine.~~ i ~ ~ ~ ~ EFFECTOF PREHEATING AIR Under ordinary circumstances carbon and oxygen burn to COz directly, and any CO formed comes from the subsequent reduction of COS by carbon. Above 1000" C. there can be no appreciable amount of COS present in contact with carbon after equilibrium has been attained. Carbon monoxide may be formed by burning the powdered coal with cold air and then reducing the COz to CO by excess carbon; but conditions for the immediate formation of CO are more favorable 7 8
Harris andsanders, M.S., Chemical Engineering Thesis, M. I. T., 1921. Canfield and Didisheim, M.S , Chemical Engineering Thesis, M. I T.,
1922. 9 10
11
J . Chem. SOG.(London), 1921. Buy. Mines, Bull. 7 (1911). Ibid , Tech. Paper 137.
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Vol. 15, No, 4
INDUSTRIAL A N D ENGINEERING CHEMISTRY
356
Water Coded Feed
I
.
S/l/i-alubes for dplL/a/Pyromefer Measure~en fs
J
nJ
1
\
Side Elevafion of furnace SKETCH OF FURNACE AND P R E N R A T E R
NOTE:Cross-hatched portion shows section of combustion chamber bricked in i o increase vzlocity of gases.
if the carbon is burned with air a t a temperature already above the point where COZ can exist in appreciable quantities in contact with carbon. Therefore, it would seem desirable to preheat the air for combustion to 1000" C. or over, in order to reduce the time of contact or size of furnace required. Furthermore, the preheating of the air to this temperature would actually lower the temperature of the flame if carbon only burns to the equilibrium ratio of CO : GOz corresponding to the temperature of the entering air. Thus, the reaction would be hastened and the destructive action of the flame on fire brick minimized by preheating the air to about 1000" C. TIMEREQUIRED FOR REDUCTION Calculation of the size of furnace required to reduce a given amount of COZto GO by powdered coal shows the great effect reaction temperature has on the size of combustion chamber required. The general rate of reduction .equation d
di. (CO)
=
ki (COz) - kz (CO)'
(1)
may be simplified by considering the speed of reversal negligible at the temperature in the furnace
Integrating, 2.3
log [ m - ( I
=
where t
1
= time
+m)
+C x ]
in seconds
x = the fraction of CO a t the time t m = the fraction of COZ a t the start of reduction k1 = the velocity constant for the reaction C COz = 2 C 0 , as given in Table I
+
C = the constant of integration.
Using Equation 3, a iurnace as shown in the accompanying drawing was built large enough to burn and reduce 12 lbs. of coal per hour. The sectional area of the furnace was designed i o give a gas velocity above the critical range-thus
affording turbulent or a churning motion t o the powderedcoal-gas mixture. The furnace was built sufficiently long to permit a study of the progressive reactions in the furnace.
COALFEEDINQ Two distinct types of coal feeders were used. I n the first the coal fell into an air stream, and after traveling about 8 in. was introduced into the furnace. However, this method of coal feeding permitted less than one-half of the air to be preheated. In the second type of feeder the coal fell directly into the combustion zone, being picked up by all the preheated air. This method of coal feeding had the disadvantage that a considerable portion of the coal settled out in the combustion zone, owing to the low velocity there. A contraction in the cross section of the combustion zone as illustrated in the drawing, helped prevent the settling of coal and gave a gas richer in carbon monoxide. The advantage in better mixing of the first type of feeder over the second is counteracted by the beneficial effects of higher temperature of entering air in the case of the second type of feeder. PREHEATISG AIR For the suns in which only part of the enterinq air mas preheated, the preheating was carried out by passing the air through a double row of l/rin. pipes in series, laid alongside the flue on each side of the furnace. Where all the air was preheated before entering the furnace, it was passed through 1-in. pipes, as above, and then through an externally heated silica tube filled with crushed fire brick. The air passing through the first preheater was heated to 540" to 776" C., depending on the conditions of the run. In the second preheater, all the air was raised to approximately 475" C. Calculations on runs made with former types of ,preheater indicate that approximately 12 per cent of the total heat of the chemical reactions (C O2 = CO1, C l / 2 0 2 = CO) was used to preheat the air. This factor causep a lowering in the temDerature of the Dowdered-cod-gas mixture. and necessitates the -use of R larger furnace. TI& preheating of the air is
+
+
I N D UXTRIAL A N D ENGINEERIXG CHEMISTRY
April, 1923
best carried out by the use of a regenerative checkerwork system after the completion of the reduction reaction. TABLE I1 TEMPERATURES
Run 8 c (Final) 5 (Middle)
7(First) (Middle) (Final)
Final Sample Gas Analysis Point COa 0 2 CO " C . 12.3 2 . 0 1 0 . 4 895 10.2 0.6 9 . 4 865 10.4 4.1 12.5 1 . 2 , , , ,
Run 8c 5(Middle) :(First) , (Middle)
.
,
,
6.5 8.3 ,
..
900
Entering Point OC. 495 775
690
Mols Air per Hr. 0.670 0.577
Lbs. Coal Burned per Hr. 1.96 1.84
0.485
1.16
TABLE I11 C v . FT. OF COMBUSTION SPACE PER LB. OF COALPER H R . Calculated Observed 0.7 1.2 1.0 1.0 0.8 0.9 1.5 0.9 Average.
Air Preheated All Less than one-half Less than one-half
Ratio of Observed to Calculated 1.72 1.0 1.12 1.67 . . 1.38
. ... .. . . .
RESULTS Table I1 illustrates the type of results obtained with two different coals. I n Run 7 the temperature fell from 1300" C. a t the combustion chamber to 900" C. at the point where the final sample was taken. These runs, as well as the others, showed conclusively that a gas richer in CO was obtained the higher the temperature of the entering air. However, it is to be noted that in the case where all the air was preheated, an air temperature of only 495" C. was obtained, whereas 1000" C. was desired. A comparison (Table 111) is given between aalculated and observed reduction rates by integrating Equation 2 between the limits zero per cent CO and the per cent
357
CO formed in the run, upon introducing the proper fraction of COn initially formed in the run. The difference beween the calculated and observed capacities may be due to the coal not being a t 900' C. (the temperature used for calculation) throughout the run or by incomplete mixing of the coal and flue gas. Although the design of the furnace was changed several times, great difficulty was encountered in maintaining a uniform mixture of coal and gas. Coal dust had a tendency to settle out and the gas stream became stratified. I n a number of runs incompletely consumed coal was carried out of the furnace by the gas stream. CONCLUSIONS 1-It is much more difficult to generate producer gas from powdered coal than the literature would lead one to suspect. However, the problem does not seem hopeless and in view of the possibilities of obtaining cheap power additional work should be carried out. 2-The difficulties seem to be the production of a sufficiently high temperature in the reacting zone, and the maintenance of the necessary intimacy of mixing between the powdered coal and reacting gas. 3-With each increase in the temperature of the incoming air, better results were obtained. It seems desirable to preheat the air to about 1000" C. 4-The maximum amount of CO obtained in any reliable run was 10.4 per cent, although for short periods 12 to 13 per cent was reached. &-For the condition of the experiments the observed size of combustion space required for reduction of COZ to CO by the powdered coal was 1.38 times that calculated by Equation 3.
Note on the Use of Phosphorus in Gas Analysis' By August Holmes 47 N. 1 6 T ~ST.,
Pyrogallic acid, widely used for the determination of oxygen in gas, has several disadvantages. It will absorb carbon dioxide if present; if not properly prepared it is liable to give off carbon monoxide; it exhausts its absorbing power for oxygen so gradually that it is difficult to determine when absorption is complete. It has the advantage, however, of being rapid, accurate when freshly prepared, and uninfluenced by traces of illuminants. It has been the writer's experience that phosphorus is a better reagent for oxygen because, although a little slow, it is never exhausted as long as there is any present. By observing a few points it is quite rapid, and complete oxygen absorption is easily determined by the disappearance of the white fumes. T o remove the last traces the sample should be replaced in the measuring pipet, mixed, returned to the phosphorus pipet, and allowed to stand until the disappearance of the white fumes. After the gas has passed through the bromine water and absorbed the illurninants, the oxygen is easily absorbed by the phosphorus. This is also true for flue gases from boilers and burners, but not for the exhaust from a gas engine, nor the residue from the explosion in a gas analysis. Frequently, the oxygen in these cases is not easily acted upon by the phosphorus, which leads to the conclusion that there is no excess oxygen present. By passing such gases through bromine 1 Received
February 16, 1923.
EASTORANGE,N. J.
water, although there will be no change in reading after absorbing the bromine in the caustic potash, it will be found that the oxygen has been activated and is easily absorbed. It is not necessary, however, to pass the entire sample through the bromine water but merely to inoculate it with bromine by passing 2 to 3 cc. into the bromine pipet and then directly to the phosphorus pipet. A rapid absorption of the oxygen takes place at once. This bromine inoculation has been found advantageous for sluggish oxygen absorption. It is not necessary to pass the gas, after this treatment, through the caustic, since no change in reading takes place. If the room is cold, inoculation assists the absorption. I n case the phosphorus has become contaminated by the illuminants or, more likely, by the cuprous chloride, the black spots may be removed by displacing the water on the phosphorus with concentrated nitric acid or, if the spots are very bad, with concentrated nitric acid to which a little concentrated hydrochloric acid has been added. The acid should, of course, be removed with clean water. The phosphorus will be light in appearance, and very active for some time. The time required for a complete analysis of illuminating gas for carbon dioxide, illuminants, oxygen, carbon monoxide, explosions for hydrogen and paraffins, and absorption of the excess oxygen is about 20 min. with phosphorus, which is not very long.
