Topochernistry of Fuel Beds J. R. ARTHUR, D. H. BANGHAM, AND H. G. CRONE British Coal Utilisation Research Association, Leatherhead, Surrey, England
T h e work was undertaken to obtain a closer understanding of the chemical reactions in fuel beds in active combustion, and in particular to ascertain the manner in which the oxygen is consumed. Experiments with actively burning fuel beds showed the extreme localization of the gasification zone and also disclosed an apparent anomalous increase in total energy of the gases as they-traversed the bed. The latter is probably due to combustion within the gas sampling probes and the importance of detailed design is emphasized. Experiments with chemical inhibitors fully confirm the role assigned to gas phase reactions (the burning of carbon monoxide). Experiments with single carbon tubes in the presence and absence of inhibitors show up clearly the entirely different pattern of carbon gasification with distance through the tube. The high carbon monoxide contents of gases sampled in the presence of the inhibitors are compared with the concentrations obtained during the last stages of burning of underfeed fuel beds. Experiments demonstrating the movement of gases into the interior of burning particles are described and discussed in the light of the “internal burning” of solid fuels. The work provides a more complete understanding than was previously available of the reactions in which oxygen is consumed in fuel beds, and brings out clearly the importance of the burning of carbon monoxide within a fuel bed. The work has been applied in the design of the Downjet furnace, in which no metal grates are used and which when burning coke can produce very nearly the theoretical yield of carbon dioxide with no excess of oxygen.
. )
*
takes place most rapidly in the hottest portion of the bed (10)i.e., in the combustion zone itself and its near neighborhood. It becomes increasingly unimportant on receding from this zone. Much of the experimental work later described was undertaken to decide between these two working hypotheses. THEORETICAL CONSIDERATIONS
The quantity of carbon in a fuel bed in active combustion a t any given time far exceeds the stoichiometric equivalent of the oxygen present. This fuel presents, however, varying degrees of accessibility to the oxygen molecules, a portion being exposed on the particle surfaces swept by the main stream of oncoming gas, and a larger portion being buried deep within the particles themselves. Between these two extremes there will be an intermediate degree of accessibility. A simple model of a fuel bed of sized particles would be a channel between walls of. carbon through which air is made to flow under turbulent conditions. The matter is simplified still further by considering a straight-sided channel since several of the experiments actually relate t o such a system (Figure 1). In an actual fuel bed the surface-volume ratio of the voids between the particles clearly plays an important part in deciding the relative probabilities of those reactions which can take place only a t the fuel surfaces and those which take place in the gas phase. In a random packing of closely graded fuel particles the voids occupy about 35% of the bed volume. Since the boundary surfaces of these voids are identical with the boundary surfaces
A
T
HIS paper attempts to summarize some of the results of experiments carried out in the laboratories of t.he British
I
It
Coal Utilisation Research Aseociation over a period of 10 years. While some of this work already has been reported (6),the cxigencies of the war and of its aftermath have hindered the presentation of any description of the work as a whole. Experiments carried out in 1939 to 1940 ( 4 ) in connection with attempts to design a small high-speed producer for road transport threw serious doubt on the validity of the classical “twozone” theory of producer bed reactions, and gave results more in harmony with the findings of Grodzovskif and Chukhanov (IO). Thus, in particular, the fuel consumption was confined almost completely to the first few centimeters of the bed; and the temperature distribution in the bed could not be accounted for in terms of Le Chatelier’s roncept of distinct “oxidizing” and “reducing” zones. In these experiments the fuel anthracite or reactive coke (size ”8 X l/g inch) was fed by gravity across the path of the air blast. However, it was an open question as to whether a substantial amount of carbon monoxide was formed as a primary product of combustion (some of it survived passage through the voids in the bed, notwithstanding the presence of unconsumed oxygen), or whether the results could be sufficiently well explained merely by the overlapping of Le Chatelier’s oxidizing and reducing zones. The latter modification of Le Chatelirr’s theory is in any event rendered necessary by the high temperature coefficient of the velocity of the reaction CO2 $- C = 2CO. This reaction,
f
3; C
6
Zr
C
D
Figure 1. Model of Fuel Bed with StraightSided Channel of the particles themselves, the surface-volume ratio for the voids is about twice that for the particles. Referring to Figure 1, if A B and CD represent fuel surfaces bounding a cylindrical channel of radius r, each distance r traversed by the air stream on its passage through the channel may be considered roughly equivalent t o one particle diameter. In general, if z is the length of path down the channel to some point where a gas sample is taken, the equivalent path length in a bed of sized fuel may be taken as approximately x / r . It is convenient to use the function W / W o ,defined as the ratio of the weight of carbon W gasified by passage of a given quantity of air after a path length z t o the theoretical weight, Wo,which would be gasified in the event of the complete conversion of the oxygen contained in this air into carbon monoxide. This function may be designated as the relative carbon saturation (or gasification ratio); its value is readily calculated from the gas analysis bv the relation [COI [COZl WIW0 = [CO] 2[COZl 2 0 1
+
+ +
where the terms in brackets represent percentages of the sppropriate gases. 5 25
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
526
W / W o takes the values 0, 0.5, and 1.0, respectively, for 1111changed air, theoretical combustion gas, and producer gas which contains no carbon dioxide. An important property of W / W ois that it is unaffected by reactions occurring in the gas phase, in particular the reactmionCO 0.502--+ CO?.
+
-AIR
BLAST
ATER-COOLED
Figure 2.
General View of Fuel Bed
The experimental results which refer to actual fuel beds and those which refer to experiments carried out with carbon tubes of different length are discussed below-. EXPERIMEVTS WITH FUEL BEDS
The semitechnical scale apparatus used in these experiment6 has been described by Nichols (18). The general arrangement is shown in Figure 2. The fuel-closely graded anthracite or inch being a size commonly used-was reactive coke a/8 X fed in transversely to the direction of the air stream. I n so far as the air stream encountered fresh fuel particles on entering the bed the conditions in this e-rperiment correspond in some degree to those described by h’icholls ( 1 1 )as those of “underfeed” combustion. Strictly speaking, however, the conditions of this experiment were neither those of underfeed nor of overfeed combustion, as the movement of particles was greatest in the plane normal to the air stream. An advantage of this apparatus was that it permitted long periods of steady running during which no appreciable change occurred in the temperature of the brickwork and insulation. The results of the very large number of experiments carried out with this apparatus are summarized in the following statements:
VOl. 43, No. 2
The heat balance anomalies probably arose becauso the gas sampled contained Pome form of chemical energy which became degraded t o heat on passage through the sampling tubes-for example, the copper surfaces of these tubes, though water-cooled, might be effective catalysts of the CO 0.502-+CO2 reaction. Conclusions based on the measured values of the function W / W O (such as statement 2 above) would in no way be invalidated by the errors thus introduced into the gas analyses. Confirmation of the role assigned t o gas phase reactions waE obtained by carrying out the follow~iiiglaboratory-scale experiment suggested by the work of Dufraissc and Horclois (9).
+
A bed of coke particles coiitained in a tube of transparent quartz was ignited and maintained in active combustion by delivery of air at a controlled rate. At a specified time about 5% by volume of the vapor of carbon tetrachloride was then introduced with the air stream, all other conditions being kept constant (8). The bright glow from the bed immediately disappeared, but was slomly restored on cutting off the supply of the tetrachloride. As it mas improbable that resuscitation of the bed occurred aiter the observed time intervals if the carbon tetrachloride (a known inhibitor of the combustion of carbon monoxide and other gas-phase combustion reactions) had completely suppressed the reactions taking place a t the carbon surface, it was decided t o use it (and similar reagents) as a means for distinguishing between the primary and the secondary procems of combustion. EXPERIMENTS WITH SINGLE CHANNELS BETWEEN CARBON BLOCKS
The use of a single channel as an esperiniental fuel bed has several practical advantages. The path length from the point of oxygen entry may be more exactly defined, visual observation is easier, and the location of probes for the withdrawal of gas may be ascertained with greater certainty in relation to the solid-gas interfaces.
0.0
1 I I I I 1.0 2.0 3.0 4.0 5 0 CARBON TUBE LENGTH, cms
’
Figure 3. Cumulative Carbon Consumption Upper czcrve. Air stream with inhibitor (1% h> vol. of POC18) Lower curwe. Air stream with no inhibitor
1. The curve of .W/W, plotted against the path len.gth expressed in particle diameters ( Z / T ) was to a first approximation independent of particle size and (within certain limits) of the flow rate (14). 2. While small proportions of oxygen occasionally survived a t ath lengths equivalent to about three particle diameters, W/%Oapproached or surpassed the value 0.5 (corresponding to combustion gas) after a path length of about a single diameter (14). 3. As the gas passed further through the bed the sum of its sensible heat and its chemical energy-as calculated from the analysis of gas samples withdrawn through probe sample tubesshowed an apparent and anomalous increase (6).
Such a channel has been used to attempt to investigate void rcactions by the technique of gas sampling, the side walls being made up of a series of graphite blocks between which a fast (turbulent) stream of air was blown. The graphite blocks were externally heated and maintained a t a temperature in the range of 850” to 900’ C. The results of these experiments were as follows :
The above experiments proved time-consuming and were somewhat unsatisfactory in so far as the shorter path lengths could not be investigated with sufficient accuracy for good reproducibility. It was also suspected that the gas samples withdrawn might not be representative. Xot only does the sampling tube disturb the packing of the bed, but therc is a high probability that the sample will be withdrawn from near the center of an unrepresentatively large void.
The samples, even when taken from the neighborhood of the carbon surface-the tip of the probe being in the region of the blue flame near the point of air entry-were characterized by very small carbon monoxide-carbon dioside ratios, rarely exceeding 1/20 on a volume basis. By redesigning the tips of the sampling probes 80 as to effect the most drastic chilling of the gases a t their point of entry, slirrhtlv higher carbon monoxide contents (occasionally up to 3%) +ere f k n d .
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1951
While providing no substantial positive evidence of extensivc gas-phase burning of carbon monoxide, these experiments always gave a cluc to the explanation of the energy-balance anomaly already mentioned. A different state of affairs was revealed, however, when an inhibitor of the reaction CO $. O.502+CO2 was included in the air stream fed to the carbon channel ( 1 ) . The blue glow was extinguished and there was a rapid fall in temperature. The carbon monoxide content of the sampled gases was greatly increased.
-1
zok
\ 0
0
1
INHIBITED COMBUSTION
1.0
3.0 4.0 5.0 6.0 CARBON TUBE LENGTH, cms.
2.0
7.0
8.0
Figure 4. Oxygen Remaining in Gases after Passage through Carbon Tubes in Presence and Absence of rnhibi tor
.
Arthur and Bowring (3) give a systematic study of the effects of various additives upon the composition of the exhaust gases from a burning carbon tube under approximately steady temperature conditions. These provide evidence that such halide inhibitors as phosphorus oxychloride and carbon tetrachloride exercise their function either by reducing the concentration of hydrogen atoms or by having this effect on some species giving rise to hydrogen atoms (or t o both of these actions). All the gaseous additives investigated and proved effective as inhibitorschlorine gas, hydrochloric acid, phosphorus oxychloride, and phosphorus trichloride-were such as would give rise to chlorine atoms on thermal decomposition. The extent to which some of the above reagents lead to a retardation of the solid-gas reactions is being &died. EXPERIMENTS WITH CARBON TUBES
k
T o illustrate the effect of one of these inhibitors of gas-phase combustion the results of a series of experiments carried out with artificial “graphite” tubing of low hydrogen content (CO.l%) and low ash content (
