EFFECT OF VARIABLES JOHN J. S. SEBASTIAN U. S. BUREAU OF MINES, MORGANTOWN, W. VA.
ROCESSES for the low cost production of synthesis-gas from various types and grades of powdered coals by entrainment in oxygen and superheated steam are being developed in small and large pilot plants a t the Morgantown, W. Va., Station of the Bureau of Mines, U. S. Department of the Interior, under a cooperative agreement with West Virginia University. The initial work of development and operating results obtained as progressive improvements advanced the project toward the solution of many hitherto unknown problems have been d e scribed from time to time in the literature (1,3,8-11). From the start it was realized that, since there are enormous differences in chemical and petrographic composition as well as chemical and physical properties of the many varieties, ranks, and grades of coals in the United States, there would be strong necessity for a practical laboratory scale method to test various fuels for their utility in synthesis-gas production. Accordingly, a laboratory scale method was developed for the continuous production of synthesis-gas (essentially CO Hz) by entrainment of powdered coals in oxygen and superheated steam. The laboratory scale pilot plant, which gasifies various types of fuels a t rates u p to 55 pounds per hour, has been in operation for about 4 years (IO) and has furnished design data for building considerably larger pilot plants. Principally, however, the information obtained in testing various types and grades of coals served as a guide t o operating conditions for other, larger scale gasifiers at the Morgantown Station. The suitability of the coal for synthesisgas production on commercial basis was thus determined. It soon became evident t h a t all coals do not gasify alike in a generator of given design but differ vastly in their behavior, and coals t h a t appear to give the worst results are frequently available in given localities at the lowest price. It was desirable, therefore, that an investigation of the effects of process variables should determine under what conditions a given coal can be gasified with maximum efficiency, lowest oxygen and steam requirement, and, hence, lowest cost. I n the work described, an attempt was made to approach this objective by analyzing the independent and dependent process variables and to determine the effects of the former on the latter. The respective significance of each is discussed.
P
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EQUIPMENT AND PROCEDURE
~
Figure 1 shows t h a t the apparatus consists of ( a ) pneumatic feeding system, ( b ) gasifier, ( c ) residual dust-removal system, and ( d ) auxiliaries (condensers, exhauster meter, storage tank, vent pipe, and instrument-control system). Pictures of t h e l a b o r e tory scale pilot plant are shown in Figures 2 and 3, representing the upper and lower floors, with the gasifier, next to t h e control board on the upper floor, extending t o t h e floor below. Details of construction of various parts of t h e unit, including t h e gasifier,
have been given in two Bureau of Mines' reports, one of which has already been ublished (IO). Pulverized coa]Pis fed continuously, at rates up t o 55 pounds per hour, int.0 the top of the gasifier from the continuous feeder column (10 inches in diameter by 8 feet in height), in which the coal is fluidized with air. A more or less intermittently operating batch-feeder column is used to charge the coal into t h e continuous feeder vessel, from which it is transported pneumatically through a J/,,-inch diameter alloy-steel feed pipe into the gasifier. By virtue of a pressure difference of 3 t o 4 pounds per s uare inch, t h e coal flows as a dense stream at a uniform rate into ?he oxygen mixing chamber, 1 pound of air carrying 150 t o 200 pounds of coal. When thoroughly mixed with oxygen t h e coal is injected in the gasifier head at a velocity of 110 t o 130 feet per second and mixes with the superheated steam injected at an adjacent point. Before each run the silicon carbide-lined gasifier tube (7 feet long by 6 inches internal diameter) is preheated by air-fed natural gas flames until the upper half of the tube reaches 2100 O t o 2500 F. The temperature of preheat depends on the type of coal gasified and other operating conditions; i t should approximate the temperature gradient of the silicon carbide lining developing, from top t o bottom, as a result of equilibrium after several hours of continued gasification. The run is then begun by shutting off the burners and simultaneously starting t o feed coal and inject oxygen and steam. The amount of air conveying the coal into the oxygen mixing chamber is negligible. I n the gasification chamber, where pressure is maintained by control instruments a t 1 t o 2 pounds per square inch, the coal particles entrained in the turbulent atmosphere of oxy en and steam gasify, and t h e residual dust is carried downwar! in entrainment in the product gases. Agglomerated pieces of slag with coarse residue particles drop into t h e collector drum below the gasifier. The amount of residue here recovered plus that removed by the subsequent knockout chamber comprise 40 to 60% of the total residue. Slightly less than half of the total residue is removed in two water scrubbers, and about 0.5% of t h e total is recovered in two intermittently operating, porous metal filters. Droplets of water carried by t h e gas stream from t h e scrubbers are removed in t h e baffle chamber, and excess water vapor in the condensers. The gas is then moved by an exhauster through a dry meter and either vented or collected in a storage holder of 5000-cubic foot capacity. Owing t o the intentional use of excess carbon ( I O ) with respect to oxygen and steam injected, the carbon in the coal was not expected to gasify completely. An extremely light and fluffy carbonaceous residue, comprising 15 to 50% of the carbon from the coal, was recovered. The residue yield can be reduced considerably by decreasing the excess in carbon input; this means higher oxygen and steam input ratios, and results in a poorer quality synthesis-gas of high carbon dioxide content. Obviously such procedure would not be economical. As pointed out later, however, in a gasifier of much larger size, designed to provide higher mass-transfer rates with the rea'ctants preheated t o higher temperatures, a much more complete gasification should be achieved without the use of excessive quantities of oxygen and steam. 1175
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
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INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1952
An electron micrograph of the collected residual dust, shown in Figure 4, clearly reveals a distinct resemblance to carbon black, T h e carbon particles, in chains or clusters, are characteristic t o both, the difference in this instance being the presence of many dark particles of ash, with a few isolated black spheres, probably cenospheres of fly ash. Elutriation tests indicated that approximately 50% of the residual dust is less than 7.5 microns in size. T h e residue, containing 22 to 37% ash, depending on the conditions of gasification, can be recycled and gasified in admixture with freah coal (IO). It can also be gasified alone and fprther concentrated in ash content or used in powdered-fuel burners for generation of power. Industrial utilization of this product to replace or supplement carbon black in some uses is yet to be investigated. ACCURACY AND REPRODUCIBILITY O F OPERATING RESULTS
Approximately 80 runs have been made covering several stages of development marked by many improvements and modifications in the equipment and changes in the original procedure. Some of the runs were continued for 10 to 24 hours, but most were terminated after 3 to 5 hours when the run had accomplished its purpose. The latest runs, especially those following the first 60, proceeded smoothly, under controlled conditions, in all inst&nces. These could have been continued but were terminated voluntarily. Reasonably close duplication of the operating results of several runs, carried out under conditions controlled to be as nearly identical as possible, showed good reproducibility or precision, as seen by a typical example in Table I. Not only the composition of the gas made, exit-gas temperature, and gas output, but also the coal to gas, oxygen to gas, and steam to gas ratios, gas yields, percentage of carbon gasified, heat losses, and thermal efficiencies check closely. However, the accuracy-that is, deviation from the true and correct results obtainable under a given set of conditions-is unknown because of an obvious lack of established standards. Nevertheless, from studies in progress a t the Morgantown Station of the Bureau of Mines, it appears that it will be possible to develop equations, by statistical correlation of operating results available from the large scale pilot plants, t o permit the prediction of the completion of gasification, oxygen requirement, and other essential operating results obtainable by gasifying a coal of a given composition. HEAT EFFECTS AND REACTION RATES
I n addition to various side reactions, five fundamental reactions appear to take place principally in the gasification of solid fuels with oxygen and steam. At the gasification temperature of 2240" F. (150O0 K.), assuming that reactants and reaction products are mamentarily at this same temperature, water is in the gaseous state, and carbon is in the form of graphite, the heats evolved (+) or absorbed ( -), in B.t.u. per pound-mole (IO, l a ) , are: c '/*Oa = co +49,581 (1) c 0 2 = coz +170,199 (2) c02 = 2 c o -71,035 (3) C HzO = CO Hz -58,077 (4 1 GO H2O = GO2 HB +12,958 (5)
+ + c+ + +
+ +
In gasifying coal entrained in steam and oxygen a t temperatures from 2100" t o 2400" F., the oxygen reacts with carbon in the coal to form carbon monoxide and carbon dioxide; the ratio of these two depends essentially on gasification conditions, including the temperature of the generator chamber, the ratio of oxygen injected to coal charged, and the ratio of steam to coal. The exothermic combustion reactions furnish the heat required for the endothermic reaction of the steam with the remaining carbon t o form hydrogen and carbon monoxide, the essential constituents of synthesis-gas. Let i t be assumed that the oxygen used is just sufficient to burn part of the carbon with which it reacts t o carbon monoxide
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TABLEI. REPRODUCIBILITY OF OPERATING RESULTS Operating oonditions Type of coal gasified Size of coal % through 200-mesh Duration o! run hours R a t e of coal fee6 lb./hr.a Coal throughput ;ate, lb./cu. f t . space/ h* a
O&en
hr. b
input (100% 011, std. cu. ft./
Steaminput, lb./hr.c Oxygen: coal ratio, Ib./lb. dry, ash-free coal Steam: coal ratio, lb./lb. dry, ash-free coalc Steam temD.. _ . F. Operating results Output of synthesis gas, std. cu. ft./hr. Anal sis of synthesis-gas, % C& HP nn %tal HC H2:CO ratio Temp. of gas rtt outlet e F . d D r y residue output lb:/hr. Total ash in residud % Coa1:gas ratio lb./lt)00 cu. f t . CO HP OXY en:gas rktio, CU. ft./1000 CU. ft. C 8 Hn Steam:gas ratio, lb./1000 cu. f t . CO+Hzc Carbon easified. lb./100 lb. carbon in
+
Run 71 Subbituminous 89.0 4.57 54.3
R u n 72 Subbituminous 89.0 4.57 54.3
41.8
41.8
368.5 7.735
386.4 7.616
0.64
0.68
0.16 709
0.16 760
1406
1407
310.2 6.51
324.2 6.39
84.3
84.7
+
72.8
72.1
24.66
24.72
016 918 spacelhr. 728 713 Heat losses. B.t.u./lb. raw coale On as-charged (moist) basis. b All gas volumes refer to 60' F., 30 inch- of Hg, dry. Steam in'ected; steam from moisture i n c o d not included. d Measured by high velocity thermocouple. e Radiation and unaccounted for losses.
(without forming any carbon dioxide) and that the heat developed by Reaction 1 is absorbed entirely by Reaction 4,consuming the stoichiometric amounts of steam and carbon. Then, if there are no heat losses and if Reactions 1 and 4 proceed to completion a t 2240" F., the theoretical oxygen requirement will be 185 cubic feet per 1000 cubic feet of carbon monoxide plus hydrogen. Actual gasification yields are less, of course, because of heat losses and incomplete reactions. In the early tests with low coal throughputs, the heat losses were so excessive in the internally heated gasifier of the size used (6 inches inside diameter and 82 inches long) that about three times the theoretical oxygen input was required per 1000 cubic feet of synthesis gas produced in order to maintain the gasifier temperature above 2000" F. Furthermore, it was necessary to use a very low steam input, otherwise the gasifier temperature would have dropped and the carbon dioxide content of the make gas would have gone up to 20 to 30%. By lowering the steam input and increasing the coal throughput, it waa possible t o make a low carbon dioxide (6 to 7%) synthesis gas without an excessively high oxygen input, using less than twice the theoretical minimum of 185 cubic feet per 1000 cubic feet of carbon monoxide plus hydrogen. For a high synthesis-gas yield per unit weight of coal charged, it is essential that the heat losses from the gasifier chamber due to radiation, convection, and conduction be held to a minimum. Expressed as heat units lost per unit volume of gasifier chamber or per unit volume of synthesis-gas produced, small scale gasifiers compare unfavorably with commercial scale generators. The heat losses from the latter are lower, as their surface area exposed per unit volume of generator chamber is considerably less. As a result, large scale generators require much less or no insulation. For the same reason, if small gasifiers are run much below their maximum capacity, the heat losses per unit volume of synthesis gas become very high. However, elimination of heat losses alone Fs not sufficient for the effective gasification of coal, the objective being to increase the percentage of gasification of carbon in the coal charged, al-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Figure 2. Apparatus Used for Testing Fuels for Synthesis-Gas Production Upper floor shows pneumatic feeding system, upper part of gasifier, and control board
though complete elimination of a carbonaceous residue might be impractical and uneconomical. To increase the proportion of carbon gasified, it is necessary t o bring the gasification reactions closer to equilibrium within the available time of contact between the reactants. It is debatable whether the rate of surface reaction or mass transfer controls the over-all reaction rate a t 2240" F. or at other temperatures prevailing in the reaction chamber. There appears to be a consensus of opinion that the mass-transfer rate is the controlling factor in the carbon-oxygen reaction a t all temperatures above the ignition point of the coal. But investigators ( 6 )have claimed some evidence to indicate that below 2100°F. the surface reaction rate may control the carbon-steam reaction under certain conditions, although in general the mass-transfer rate is held t o be the chief controlling factor of steam reduction reactions. Thus, to improve the gasification, it is highly essential to increase the rate of diffusion of the reactants through the film of reaction products which cover the particles of fuel. Although a high degree of turbulence in the generator chamber and thorough mixing of the reactants are undoubtedly helpful, the rate of diffusion can be augmented by increasing also the relative velocities of the reactants. An effective way of accomplishing this is tangential injection of the reactants. Owing to great differences in the specific gravities of oxygen-steam mixture and solid-fuel particles, the centrifugal force considerably increases the velocities of the solid particles with respect t o the speed of motion of gaseous molecules. (Attempts to accomplish this in the small scale gasifier failed owing to the rapid oxidation and destruction of the silicon carbide lining opposite the point of injection.)
-4 novel method of providing for increased relative velocities by producing pulsations or vibrations of given frequency in the generator chamber is under investigation at the hIorganton n Station of the Bureau of Mines. If the surface reaction controls the over-all rate of carbon-steam reaction belon- a given limiting temperature (2100 F.) and under certain conditions, as has been claimed recently ( 6 ) ,the chemical reactivity of the fuel should be increased for greater completion of gasification. Since the reaction velocity is a function of the temperature, reduction of heat losses, achieved by incieased coal throughput rates as well as by preheating the reactants, should raise the temperature of gasification and give better results with greater thermal efficiency. Furthermore, so-called reactive coals such as high-volatile coals of high oxygen content should give higher yields of synthesis gas and increased efficiency. The fact that higher gasifier temperatures gave generally better results and more reactive loa. rank coals of younger geologic age gasified more efficiently with higher gas yields also seems to indicate that the rate of surface reaction is an important factor in the over-all control of gasification rates. Irrespective of the factors that control the rates of oxidation of carbon and reduction of steam, it is essential that sufficient time of contact be provided between the solid and gaseous reactants. For any given tube diameter and length there is a maximum velocity (or minimum residence time) for the entrained reactants, which should not be exceeded. A4shigher coal-throughput and, consequently, increased gas-output rates decrease the contact time, i t is t o be expected that the tube diameter and length determine the maximum econon~icalthroughput of the coal. O
INDUSTRIAL AND ENGINEERING CHEMISTRY
M a y 195%
The effect of increased viscosity of reactants and reaction products, at higher temperatures, on the rates of surface reaction a n d diffusion has not been evaluated. Nevertheless, the importance of a kinetic study of the effect of this variable is recognized. Also, the effect of pressure on the thickness of the gaseous film surrounding each coal particle appears to merit investigation.
TABLE 11. EFFECT OF TYPE AND COMPOSITION OF COAL GASIFIED
Operating conditions Size of cpftl, % ’ through 200-mesh Composition of coal as charged. % ,” Moisture Ash Volatile matter Hydrogen Carbon Oxygen Sulfur Duration of run hours Rate of coal feed Ib./hr.d Coal throughput ;ate, lb./cu. ft. space/ I
PROCESS VARLABLES
Independent process variables (or operating variables) are . defined as operating conditions, independently chosen and varied at will but maintained as constant as possible during a given run. Dependent process variables, on the other hand, are defined as operating results obtained as functions of the operating conditions chosen. Although there may be other ways of classifying both groups of variables, a convenient method of classifying them from a practical operating standpoint is as follows:
I. Independent process variables (operating variables) 1. 2. 3. 4. 5. 6.
Shape and dimensions of gasifier (design) Method of feeding reactants Composition and physical properties of the coal charged Particle size (screen analysis) Coal throughput rate Injection temperature of reactants 7. Oxygen t o coal input ratio 8. Steam t o coal input ratio 11. Dependent process variables (operating results) 1. Per cent of coal (or carbon in coal) gasified 2. Com osition of gas produced 3. Y i e d o f CO Hz per unit weight of coal 4. Synthesis-gas output rate 5. Exit-gas temperature 6. Type, quantity, and size of residue 7 . Oxygen requirement per unit volume of GO H, 8. Heat losses from gasifier 9. Thermal efficiency
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hr. d
COP
H2
H9:CO ratio Temp. of gas at outlet O F.A Dry residue output Ib:/hr. Total ash in residud % Coa1:gas ratio, lb./
