INDUSTRIAL A N D ENGINEERING CHEMISTRY
586
Vol. 17. No. 6
Reactions in the Fuel Bed of a Gas Producer‘ Ry R. T. Haslam, F. E. Entwistle, and W. E. Gladding M A S S A C H U S E T T S I N S T I T U T E OF TECHNOLOGY, CAMBRIDGE.
A
N EXACT knowledge of the composition of the gases as they travel up through the fuel bed of a gas producer ’ is of value in determining the mechanism of producer gas formation. A number of investigation^^.^^^^^ along this line, however, have yielded inconclusive results, for one or more of the following reasons:
MASS.
pieces of pipe being used as plugs. A hopper having a damper and cover was used for coal feeding. (b) Air Supply. A 10-horsepower turbo-blower delivering air up to 10 ounces per square inch pressure was used to force the air through the fuel bed. The air was preheated and humidified bv steam led into the air line through a series of nozzles. The-amount of air reaching the producer was (1) Channeling in the fuel bed caused the composition of the controlled by a by-pass and measured by means of a sharpgas even at the same level to vary widely. (2) The distance between sampling points was so great that edged orifice (5.08 cm. or 2 inch diameter) placed in the air insufficient data were obtained in some of the more important line just before the air entered the furnace. The air temperazones. ture and humidity were de(3) No samples were taken termined by wet- and dryin the oxidation zone of the bulb thermometers placed producer. Gas samples were withdrawn from t h e fuel b.ed of a between the orifice and the (4)Gas samples were taken gas producer, precautions being taken t o prevent chanA baffle in tpe producer. t h r o u g h uncooled quartz neling of t h e gases and to stop t h e reactions once ashpit distributed the air tubes and reaction between t h e gases entered t h e sampling tubes. the constituents of the gas blast over the grate surface Analysis of these samples shows: could t h e r e f o r e c o n t i n u e and thus decreased any edge (a) The oxygen is first completely consumed in a very after withdrawal from the leakage. fuel bed. thin zone with the formation of carbon dioxide. (c) Gas Sampling. Water(b) No water is decomposed and only a small amount cooled c o p p e r sampling The present investigation, of carbon monoxide is formed until all the oxygen is tubes (gas space 3.18 mm. by guarding against these consumed. or 0.13 inch diameter) were sources of error, is an at(c) Steam is reduced by t h e solid carbon fuel only in designed to give high gas tempt to gain a further int h e lower portion of t h e reducing zone immediately velocities and rapid cooling. sight into p r o d u c e r g a s above t h e oxidation zone. The portion of the fuel bed These sampling tubes were f o r m a t i o n a n d thereby is termed t h e “primary reduction zone.’’’ Carbon diinserted in holes in the proclear up certain disputed oxide is also largely reduced within this zone. ducer walls spaced 7.62 cm. points. (d) Above t h e primary reduction zone the only water (3 inches) apart between decomposed is by the reaction 3.81 and 50.80 cm. (1.5 and Apparatus CO € 1 2 0 = CO1 Hz 20.5 inches) above the grate Simultaneously some carbon dioxide is reduced by and then every 22.86 cm. (9 The apparatus consisted carbon. This portion of t h e fuel bed is termed t h e inches) up to 138.5 em. (56.5 of ( a ) an experimental up“secondary reduction zone,” and its main function inches) above the grate. draft producer, ( b ) a device seems t o be t h e exchange of heat from t h e hot uprising T h e h o l e s were evenly for supplying to the progases to t h e cooler downward moving fuel. spaced radially, the samples ducer under p r e s s u r e a being taken from the cenknown quantity of air and tral &xis of the fuel bed. steam, and (e) means of withdrawing gas samples from adjacent points in the pro- Aspirator bottles connected with I-liter common bottles were used for withdrawing gas samples from the producer ducer fuel bed. (a) Producer. The experimental producer had a 43.2-cm. fuel bed. A standard Williams and a standard Burrell ap(17-inch) diameter grate with four grate bars located about paratus were used for gas analysis. 46 cm. (1.5 feet) above the foundation, thus providing Experimental Procedure. an ashpit and windbox. The circular producer walls 1.8 Before starting a run layers of ashes, excelsior, and wood meters (6 feet) high were constructed of hard-burned fire brick, bonded with a high-temperature plastic cement. 9 were put into the producer and 45.36 kg. (100 pounds) of coal sheet-metal casing with a 7.6-em. (3-inch) clearance space added. The producer top was placed in the sandseal, the was built around the walls. The space between the metal stack connected on and all joints sealed with asbestos furnace casing and the producer walls was packed with Sil-o-cel cement. The excelsior was then ignited, a door bearing the blast pipe placed in the opening of the ashpit, and a blast of powder. The producer top was sealed with tightly packed sand and air admitted. When the disappearance of heavy smoke was equipped with a 15-cm. (6-inch) welded stack connection showed the wood to be all burned, steam was turned on to and a 15-em. (6-inch) hole for feeding. Two 1.9-cm. prevent clinkering and coal added until the fuel bed was built (0.75-inch) poke holes were bored and threaded, two capped up to the desired thickness. The bed was poked frequently during the process to prevent channeling. When a good 1 Presented under the title “Reactions in the Fuel Bed ’* before the fire had been made, the blower was shut down and the sampSection of Gas and Fuel Chemistry a t the 68th Meetmg of the American September 8 to 13, 1924. Chemical Society, Ithaca, N Y., ling tubes inserted, leaks around them being prevented by 2 Wendt, Forsch. Ingeneure, 31, 57 (1906). luting with asbestos cement. After ashes were removed, a Neumann, Slahl u. Etsen, 33, 394 (1913). the ashpit door (bearing the blast pipe) was fixed and ce4 Bone and Wheeler, J . Iron Steel Inst. (London),76, 126 (1907). mented in place. The temperature and amount of air were 6 Terres and Schierenbeck, Gas u. Wasserfach, 67, 326 (1924). I
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INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1925
then adjusted and the fire was allowed to burn under these conditions, coal being added to maintain a constant depth of fuel bed. During the period just before sampling, readings of the depth of fuel bed, manometer, and wet- and dry-bulb thermometers were taken a t 10-minute intervals to insure constant conditions. The time for sampling was determined by the dull red appearance of the top of the fuel bed. Samples from all the sampling tubes were withdrawn simultaneously, collected over saturated brine solutions, and analyzed as soon as possible in both the Burrell and Williams apparatus. I n order to avoid corriplicating the problem by having a distillation zone in the producer, and greatly to decrease troubles due to channeling in the fuel-bed, a carefully sized anthracite pea coal was used, which had the following analysis : Proximate Analysis as Fired Per cent 1.07 Moisture 6.66 Volatile matter 76.55 Fixed carbon 15.72 Ash 13,200 R . t. u. per lb. Heating value
Results
The results obtained in Run 5 , which are typical of results from several runs, are giren in Table I and Figure 1. Table I Dry-bulb temperature of air entering, 50.5’ C. Firing rate, 77 4 pounds per square foot per hour Pounds steam per pound coal, 0.407 DISTANCE Sam- ABOVE GR.4TE -GAS ANALYSIS, P E R CENTple Inches Cm. CO COz Hz CH4 Oz 0 0 20.9 0 0 1 1.9 4.8 20.9 0 0 0 2 5 . 1 13.0 0 0.2 0 0 3 8 . 1 . 2 0 . 6 3.7 18.6 11.4 28.9 18.2 10.6 1.3 0 0 14.1 35.8 23.4 7.8 3.2 0 0 6 16.6 4 2 . 1 . . . ... 7 2 0 . 1 5 1 . 0 24:O 7 . 8 7 . 5 0 . 1 0 8 29.1 73.9 26.2 5.2 8.2 0.1 0 9 38.1 96.7 26.7 5 . 2 8 . 6 0 . 5 0 11 56.1142.5 24.8 6.1 9.3 0.7 0
d
Mols w h r per 100 mols dry producer
Nz 79.1 79.1 77.5 69.9 65.6
15.6 15.6 15.3 12.4 9.7
60.3 59.0 59.1
4.2 6.7 6.4 5.6
. . . . . . 6 0. .. 6.
gas
by carbon occur simultaneously. Thus, Dyrssen? states: “the heat required to decompose a molecule of C02 or H2O is supplied by a nearby molecule of 0 2 oxidizing C to COZor CO.” Similarly, the following statement appears in the catalog of a well-known producer manufacturer: “At this point (where air and steam come in contact with the hot coal) three distinct gases are liberated-( 1) carbon dioxide (COZ) formed by the union of carbon and oxygen, ( 2 ) hydrogen (Hz)liberated from the steam, etc.” Figure 1 shows clearly, however, that hydrogen does not appear in the gas until all the oxygen has been consumed, and that water decomposition starts further up the fuel bed than the carbon monoxide formation. The hydrogen content of the gas increases rapidly a t first (through a zone whose depth is about 30 cm. (1 foot) after the point of oxygen disappearance), and then. increases slowly to the top of the fuel bed. Reactions in Producer Gas Formation
The actual combustion reactions by which the carbon dioxide, carbon monoxide, and hydrogen are formed may be determined from the changing composition of the gases. In passing through the first 7 to 10 cm. (3 or 4 inches) above the ashes the oxygen is all consumed and carbon dioxide only is formed. In this section of the fuel bed, the combustion or oxidation zone, the only reaction is
c + 0 2 = coz
(1)
It is in this zone that all the heat of the fuel is liberated. As the hot gases pass out of the combustion zone, carbon dioxide is reduced to carbon monoxide and hydrogen is formed by the decomposition of steam. This layer of the fuel bed is therefore called the reduction zone, and in it four reactions take place:
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Discussion of Results
With Reactions 2 and 3 there may be an increase in the proportion of steam decomposed, Hz/(HzO Hz), only by a corresponding increase in the amount of carbon gasified,
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I t 4
The table and the figure both show that the entering airsteam mixture is unchanged in composition as it passes through the ashes, which merely serve to preheat the mixture. At the end of the ash zone (about 12 cm. or 5 inches) above the grate in Run 5 the mixture comes into contact with incandescent fuel and combustion begins. The rapid disappearance of oxygen indicated in Figure 1 closely parallels the disappearance of oxygen within the fuel bed of a hand-fired furnace, as observed by Kreisinger, Ovitz, and Augustine.6 Figure 1 shows that the oxygen is entirely consumed as the gases traverse the first 7 to 10 cm. (3 or 4 inches) of fuel above the ashes and that the carbon dioxide content of the gas rises with corresponding rapidity. After this rapid initial increase the carbon dioxide is a t a maximum a t about 18 cm. (7 inches) above the grate, and from this point to the top of the fuel bed gradually decreases in amount. Carbon monoxide formation does not take place appreciably until all the oxygen has disappeared and the carbon dioxide content has nearly reached its maximum. The monoxide is rapidly formed in the next 30.48 cm. (12 inches) of fuel bed after the oxygen has disappeared, and from then on shows a slight increase until the gases pass into the gas space. As shown by both Table I and Figure 1, the water is not decomposed until all the oxygen is consumed. This contradicts a view frequently expressed in the literature that the union of carbon and oxygen and the decomposition of steam U.S. Bur. Mines, Tech. Paper 137 (1916).
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