The production of nitric oxide during the combustion of methane and air in a fluidized bed

The production of nitric oxide during the combustion of methane and air in a fluidized bed

The Production of Nitric Oxide during the Combustion of Methane and Air in a Fluidized Bed M. PILAWSKA,† C. J. BUTLER,‡ A. N. HAYHURST,* and D. R. CHA...

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The Production of Nitric Oxide during the Combustion of Methane and Air in a Fluidized Bed M. PILAWSKA,† C. J. BUTLER,‡ A. N. HAYHURST,* and D. R. CHADEESINGH

Department of Chemical Engineering, Cambridge University, Pembroke Street, Cambridge CB2 3RA, England Fuel-lean mixtures of CH4 and air were burned when fluidizing electrically heated beds of silica sand particles maintained at a temperature between 1000 K and 1300 K. The amount of sand in the bed was varied, as also were U/Umf and the size of the sand. Measurements were made of the concentrations of NOx (i.e., {[NO] ⫹ [NO2]}, CO2, and CO. At the highest temperatures (⬃1300 K) [NOx] in the off-gases was found to be as little as 10 p.p.m. and was independent of the amount of sand fluidized. Likewise with the sand at the lowest temperature of ⬃1000 K, [NOx] was also low (2– 6 p.p.m.), but was proportional to the depth of the sand in the bed. At intermediate temperatures of ⬃1123–1250 K, [NOx] was higher and reached a maximum of ⬃22 p.p.m. in the gases leaving the bed. How these gases burn depends on the temperature of the bed and also on whether the bubbles ascending through the sand are slow or fast (i.e., small or large). The ignition criteria are different for slow and fast bubbles and are formulated here. At the lower temperatures below ⬃1123 K the behavior was as follows. Quite surprisingly, at 1 ⬍ U/Umf ⬍ 3, combustion of these O2-rich mixtures does not occur completely to CO2 in a shallow bed, with the sand ⬃20 mm deep, when roughly speaking [CO2] ⬀ U/Umf. Measurements of axial temperature and also of both [CO] and [CO2] in these systems indicated that at temperatures below ⬃1123 K, no combustion occurred in the bed either between the sand particles or in rising bubbles. Instead, bubbles ignited just after leaving the fluidized sand. It was found that larger bubbles were hotter after combusting this way and retained their identity for longer times than small bubbles. The indications are that below ⬃1170 K, NO is partly produced via N2O, as well as by the “prompt mechanism.” By contrast, above ⬃1250 K combustion occurred in smaller, “slow” bubbles, close to the distributor. The “prompt” mechanism is the principal source of NOx above 1170 K, because at these higher temperatures there is insufficient time for NO to be produced via N2O as the intermediate. © 2001 by The Combustion Institute

NOMENCLATURE A Ab Ci D D crit D min E g H

pre-exponential factor for the chainbranching reaction cross-sectional area of the fluidized bed concentration of species i mean diameter of a bubble in a fluidized bed critical diameter of a bubble with the relative velocity, Ub ⫽ Umf/⑀mf minimum diameter for a “slow” bubble to explode activation energy of the chain-branching reaction acceleration due to gravity total depth of fluidized sand

* Corresponding author. E-mail: allan_hayhurst@cheng. cam.ac.uk † Currently at the Institute of Thermal Engineering, and Air Protection, Cracow University of Technology, ul. Warszawska 24, 31-155 Krakow, Poland. ‡ Currently at the Health and Safety Laboratory, Harpur Hill, Buxton SK17 9JN, England. COMBUSTION AND FLAME 127:2181–2193 (2001) © 2001 by The Combustion Institute Published by Elsevier Science Inc.

h k [M] R T U Ub U mf

⑀b ⑀mf

height (above the distributor) up a fluidized bed pseudo-first-order rate constant for production of NO concentration of all molecules, M universal gas constant absolute temperature superficial velocity of gas through a fluidized bed the velocity with which a bubble rises up a fluidized bed relative to the surrounding particles the minimum value of U, i.e., for incipient fluidization the voidage in a fluidized bed because of the bubbles the voidage in the particulate phase

INTRODUCTION The combustion of mixtures of a hydrocarbon fuel and air in a fluidized bed of, e.g., sand presents many unusual features, mainly deriving 0010-2180/01/$–see front matter PII 0010-2180(01)00319-4

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from the sand inhibiting combustion. The mechanism of inhibition depends on the removal and recombination of free radicals (e.g., very diffusive, free atoms of hydrogen) on the sand’s large surface area [1]. Alternatively, this inhibition has been attributed entirely [2] to thermal effects without any participation from radicals; such disagreement really ought to be resolved. This paper explores the extent to which NOx (i.e., NO and NO2) is produced, when a bed of sand is fluidized at a well-defined temperature by a mixture of CH4 and air. In such a situation, the production of NO might be expected to be by one of four mechanisms, each of which is based on N2 being attacked chemically by a species in the pool [3, 4] of free radicals created during combustion. Firstly, NO can be produced by a chain reaction in Zel’dovich’s [5, 6] thermal mechanism: O ⫹ N2 3 NO ⫹ N N ⫹ O2 3 NO ⫹ O

(I)

whose rate-determining step (I) has the large activation energy of 319 kJ/mol [7]. Next, Fenimore’s “prompt” mechanism [8, 9] appears to depend on a reaction such as: CH ⫹ N2 3 HCN ⫹ N

(II)

with HCN oxidizing to NO and with free N atoms reacting with almost any species containing oxygen to give NO. Thirdly, NO can be generated [7, 10] in fuel-rich flames via N2H formed endothermically as an intermediate in: N 2 ⫹ H ⫹ M ⫽ N 2H ⫹ M N2H ⫹ O 3 NO ⫹ NH

(III)

Here M is any molecule; NH can react with either H or OH radicals to give free atoms of N and thereby again produce NO. Finally, well below 2000 K the fairly similar reaction involving the exothermic formation of N2O [11] in: N 2 ⫹ O ⫹ M ⫽ N 2O ⫹ M

and thereby “fix” atmospheric N2 and yield NO. Again, such a scheme is favored by low temperatures to produce N2O, as well as by oxygen-rich conditions as here. Reaction (IV) has an activation energy of 97 kJ/mol [12]. Of course, in all these schemes any observed NO2 most probably originates from NO. As well as being important in its own right, studying the production of NO is likely to reveal details of how CH4 and air burn in a fluidized bed; this is important for the utilization of waste gases and the combustion of volatile materials from coal, biomass, and waste solids in such beds. Of all the aliphatic hydrocarbons, CH4 oxidizes in the absence of sand with extreme properties: it has the highest spontaneous ignition temperature (905 K) [13], the lowest burning velocity [14], and the longest ignition delay time [14]. In this sense, methane should not be regarded as a typical hydrocarbon. The fluidized beds in this study were at a well-defined temperature between 1003 and 1303 K and at atmospheric pressure. Under normal circumstances (i.e., in the absence of large amounts of sand) chain branching would be by: H ⫹ O2 3 OH ⫹ O

(V)

above 1100 K [14], but is mainly by: HO2 ⫹ RH 3 H2O2 ⫹ R H2O2 ⫹ M 3 2OH

(VI)

at 1 bar and the lower temperatures of 900-1100 K [14]. It is thus possible that the burning of CH4 and air in these fluidized beds is affected by such mechanistic changes, the largest of which is that heterogeneous chemical reactions might be expected to play an unusually important role for the gas passing interstitially through the fluidized sand (the particulate phase), but this is less true for the gas passing through the bed as bubbles and constituting the bubble phase, according to the two-phase theory [15] of fluidization.

can be followed by: N2O ⫹ O 32NO N2O ⫹ H 3 NH ⫹ NO N2O ⫹ CO 3 NO ⫹ NCO

EXPERIMENTAL METHODS (IV)

The bed has been described before [16]. The sand was supported on a porous plate of sin-

NO IN FLUIDIZED BED

2183 TABLE 1

Values of Umf, (U/Umf) for a cold feed rate of 1 L/s, Dcrit and ␶ig Temp./ °C 730 730 830 830 930 930

Temp./K

Size sand mm

Umf m/s

(U/Umf)

Dcrit mm

␶ig/s

1003 1003 1103 1103 1203 1203

0.25–0.355 0.50–0.60 0.25–0.355 0.50–0.60 0.25–0.355 0.50–0.60

0.032 0.13 0.030 0.12 0.028 0.10

9.1 2.2 10.6 2.7 12.3 3.4

1.2 20 1.1 17 0.94 12

32 32 3.0 3.0 0.5 0.5

tered quartz and held in a vertical, cylindrical, quartz tube (inner diameter 122 mm). Below this distributor plate was a plenum chamber to which a mixture of CH4 and air was fed after being metered by separate rotameters. Quartz sand, previously sieved to two size ranges (0.250 – 0.355 mm and 0.50 – 0.60 mm) was fluidized. The mass of sand in the bed was varied from 0.300 to 1.600 kg; this resulted in the unfluidized depth of the sand being ⬃ 85 mm for 1.6 kg of the larger sand present. However, the height was less (⬃70 mm) for 1.6 kg of the finer sand, presumably due to a lower voidage for a wider range of particle sizes. This meant that with only 0.300 kg of sand in the bed, the stagnant depths were as little as ⬃16 and 13 mm for the coarser and finer sands, respectively. Of course, when fluidized, the depth of the sand increased [15] as the superficial velocity, U, was made to exceed its value, Umf, for incipient fluidization of the sand. No experiments were performed without sand in the bed. Around the quartz tube were six vertical electrical heating rods and around them was material for thermal insulation. A thermocouple was kept in the sand and formed part of the control system to hold the sand at a pre-set target temperature. A window enabled the vertical bed to be viewed from the side, in addition to its upper surface being seen via a mirror. The fluidizing gas normally had a total flow rate of 1.00 ⫻ 10⫺3 m3/s, as measured at room temperature and pressure. The ratio [air]/[CH4] was always 12.85 on a molar basis, corresponding to there being 35% excess air. With such a constant flow rate of gas mixture to a bed, which varied in temperature and had different sizes of sand, etc., the result was that U/Umf also varied. Further details are given below.

The gas some 200 mm above the fluidized sand was analyzed by continuously withdrawing a small fraction of it through a quartz tube (inner diameter 5 mm), then drying it in a tube containing fresh silica gel and passing the sample on to infrared analyzers to measure [CO] and [CO2]. It was not expected that much CO would be present, because the gases were very oxygen rich (see above). The gas from the CO2 analyzer was passed [17] to a chemiluminescent (Thermo Electron: Model 10; Hopkinton, Mass.) analyzer to measure [NOx]. Table 1 lists Umf for both sizes of sand and three temperatures of the bed; Umf was calculated from Wen and Yu’s correlation [18]; the resulting values were confirmed experimentally. Clearly from Table 1, Umf depends on (in fact, on the square of) the size of the sand. In addition, Table 1 gives values of (U/Umf) for the usual flow rate of 1 L/s of fluidizing gas as measured at room temperature; (U/Umf) is large (⬃10) with the smaller sand particles, suggesting that back mixing of fluidizing gas [19] might occur. In fact, at 1003 K the value of U ⫽ 0.29 m/s compares with an expected value of the burning velocity of ⬇3 m/s for gas pre-heated to that temperature [14]. This means that any flame in the freeboard would move downwards on to the upper surface of the fluidized sand, where the burning velocity in all probability falls due to radicals recombining on the sand. The result is that the flame doesn’t propagate downwards through the bed. Also listed in Table 1 are values of the ignition delay times [14] for stoichiometric mixtures of CH4 and air at the temperatures concerned; these values, of course, refer to the absence of hot tiny sand particles, but nonetheless could refer to a large bubble. It is striking that at 1003 K, ␶ig exceeds

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M. PILAWSKA ET AL. TABLE 2 Values of the mean effective diameter (in mm) of the bubbles at various heights, h, above the distributor for 1 L/s (at room temperature and pressure) of CH4 and air fed to the bed h/mm

Sand size/ mm

T/K

U/Umf

10

30

100

0.25–0.355 0.25–0.355 0.25–0.355 0.50–0.60 0.50–0.60 0.50–0.60

1003 1103 1203 1003 1103 1203

9.1 10.6 12.3 2.2 2.7 3.4

5.0 5.2 5.4 4.1 4.5 4.9

12.1 12.6 13.1 9.8 11.0 11.7

31.6 32.9 34.2 25.8 28.7 30.6

30 s. In practice, gas leaving a bed at 1003 K will ignite, despite ␶ig being so long, because disengaging bubbles encounter combustion products in the splash zone immediately above the fluidized sand. The two-phase theory of fluidization [15] assumes that for a bed of cross-sectional area, Ab, there is a volumetric flow-rate of fluidizing gas, AbUmf, percolating through the sand; the rest of the gas passes through the bed as bubbles with a volumetric flow rate equal to (U ⫺ Umf) Ab. The effective diameter of the bubbles varies with h, the height above the distributor, according to: D⫽

0.54 共U ⫺ U mf兲 0.4h 0.8 g 0.2

(1)

because of bubbles coalescing [20]. Here g is the acceleration due to gravity. This formula refers to mean values of D, there being, of course, a spread of values about the mean. In principle, there are two types of bubble: “fast” and “slow” [15]. If (Umf/⑀mfUb) ⬎⬎ 1, where ⑀mf is the voidage in the particulate phase and Ub is the rise velocity of a bubble relative to the surrounding sand, the bubble is said to be “slow.” In fact, Ub ⫽ 0.711 (gD)1/2 [15]. Otherwise, if (Umf/⑀mfUb) ⬍⬍ 1, the bubble is said to be “fast.” Inside a slow bubble [15] the gas moves upwards from the surrounding sand, through the slower moving bubble and then into the surrounding particulate phase, not to return again to the bubble (or void). However, a fast bubble has gas circulating within it and only a small fraction of that gas leaves the bubble to enter the sand around such a large bubble [15]. Table 1 also

lists values of Dcrit, the critical effective diameter of a bubble, obtained by equating the ratio (Umf/⑀mfUb) to unity, with ⑀mf ⫽ 0.41 for sand [21]. It will be noted that for the smaller sand particles, Dcrit is small (⬃1 mm) and much less than for the larger sand. Some information on the approximate sizes of bubbles is presented in Table 2, where are listed for both sizes of sand and three temperatures, values of D in mm at various heights above the distributor. Values at other heights can be obtained by scaling according to h0.8 from Eq. (1). As an example, if 0.400 kg of the larger sand is in the bed at 1123 K, the stagnant depth of the sand is ⬃21 mm, but this rises to ⬃35 mm, when (U/Umf) ⫽ 4. Table 2 does not reveal this actual dependence on (U/Umf). A comparison of Tables 1 and 2 demonstrates that when the smaller sand particles were used, the bubbles soon have a diameter D ⬎⬎ Dcrit and so are likely to be “fast.” By contrast, the larger sand particles (diameter 0.50 – 0.60 mm) usually result in the bubbles being too small to be “fast,” except for deeper beds, with h ⬎ 0.1 m.

RESULTS AND DISCUSSION Dependence of [NOx] on Depth of Sand and on Temperature Figures 1 and 2 are different plots of the same observations, made when the bed was not in a steady state, but heating up slowly from room temperature to a pre-set target temperature of ⬃1323 K. In every case, the smaller sand parti-

NO IN FLUIDIZED BED

Fig. 1. Plots of [NOx] in the off-gases vs. the mass of finer sand (0.250 – 0.355 mm) in the fluidized bed. The plots are for different temperatures in the sand (in °C) while the bed was heating up to 1050°C (1323 K), with a constant flow rate (1 L/s at room temperature and pressure) of air and CH4.

Fig. 2. The same data in Fig. 1 replotted against the bed’s temperature for particular masses of sand (as shown) in the bed.

2185 cles were used and the total flow rate of methane and air was 1.0 L/s, when measured at room temperature and pressure, with 35% excess air throughout. Such a large flow of fluidizing gas ensured that the finer sand was always fluidized, even when the electrical heaters were not switched on and also when there was no combustion. Figure 2 gives plots of [NOx] in the off-gases vs. temperature for a particular mass of the finer sand in the bed, while the bed heated up from 1000 to 1300 K. Surprisingly, a maximum is observed for [NOx]. The visual observations were that at 1003 K (730°C, when U/Umf ⫽ 9.1) the methane burned fairly quietly above the bed. In fact, at 1003 K the concentration of NOx increased linearly with the mass of sand in the bed, as shown in Fig. 1. Herein lies an initial problem: if the CH4 burns only above the sand when the bed is at 1003 K, the yield of NOx would be expected to be independent of the depth of the sand, assuming that NO is a by-product of combustion. However, Fig. 1 indicates that at 1003 K, [NOx] in the gas leaving the bed is strictly proportional to the mass of sand, so the straight line for 1003 K passes through the origin of Fig. 1. Quite possibly, the majority of the NO is produced either in the particulate phase or in rising bubbles. At this relatively low temperature of 1003 K there is no evidence for combustion occurring in rising bubbles, but the bubbles do ignite when they disengage from the bed. There is no significant disappearance of CH4 in the bed [22], but there is a hint [22] of small quantities of C2H4 and CH2O, probably produced in the particulate phase. In this case, the small quantities (⬍6 p.p.m.) of NOx detected at 1003 K could also be produced in the particulate phase, while there is minimal oxidation of CH4. This picture of [NOx] being proportional to the quantity (or depth) of sand in the bed holds at 1053, 1103, and 1153 K in Fig. 1. This is interesting because it is clear from looking at and listening to the bed that when it has warmed up to ⬃1093 K, bubbles of gas ignite noisily near the top of the bed. On doing so, they emit an audible and in fact loud “popping” sound. At even higher temperatures, smaller bubbles appear to ignite lower in the bed (i.e., nearer the distributor) and do so more quietly than at ⬃1093 K. Eventually the bed is relatively silent

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at or above ⬃1173 K. So far there is no evidence that the linear plots in Fig. 1 at 1003–1153 K are entirely attributable to combustion in rising bubbles, because no such thing occurs at 1003 and 1053 K. It is thus possible that the linear plots in Fig. 1 derive from chemical reactions occurring in the particulate phase, possibly catalyzed by the sand. This idea will now be investigated. It is easiest to suppose that NOx is generated by some pseudo-first-order chemical reaction. The resulting concentrations of NOx are low (⬍24 ppm at 1153 K when U/Umf ⫽ 11.1) The simplest explanation of the linear plots in Fig. 1 at 1003, 1053, 1103, and 1153 K is that the bed is behaving as a pseudo-homogeneous reactor [23] with plug flow of reactant gas between the sand particles. Consequently, for the simple firstorder reaction A 3 B: CB/CAo ⫽ 1 ⫺ exp(⫺kH/Umf)

(2)

Here CB is the concentration of B after a residence time t, CAo is the initial concentration of A, k is the first-order rate constant (defined with respect to unit volume), and H is the total depth of the fluidized sand, through which the superficial velocity of fluidizing gas is Umf. Equation (1) simplifies to: CB ⫽ CAokH/Umf

(3)

when (kH/Umf) ⬍⬍ 1. The mass of sand in the bed is clearly proportional to H, which depends also on U. It is readily seen that Eq. (3) is consistent with the linearity of the four plots in Fig. 1 for 1003–1153 K, because the mass of sand in the bed is: W ⫽ HAb(1 ⫺ ⑀b)␳(1 ⫺ ⑀mf)

(4)

where ⑀b is the voidage of the bed due to the rising bubbles, ⑀mf is the voidage in the particulate phase and ␳ is the density of a sand particle. Taking Eqs. (3) and (4) with k ⫽ Aexp(⫺E/RT) gives: ln共CNO/H) ⫽ constant ⫺ E/RT

Fig. 3. A plot of the logarithm of the slopes of the four straight lines in Fig. 1 against the reciprocal of the bed’s temperature.

(5)

Equation (5) is checked in Fig. 3 by plotting the slopes of the four straight lines in Fig. 1 against (1/T). A tolerably straight line can be fitted to Fig. 3; its slope indicates an activation energy of 89 ⫾ 15 kJ/mol. This is much lower than that

(319 kJ/mol) for NO being produced thermally with Reaction (I) as the rate-determining step. The activation energy for Reaction (II) is ⬃92 kJ/mol [24] and so appears to be a possibility, although a dependence of [CH] on temperature would increase the activation energy above 92 kJ/mol and so make this scheme less likely. Otherwise, the sequence [7, 10] via N2H in Reaction (III) has an activation energy, which is probably the sum of that (70 kJ/mol) for producing radicals in (V) and the endothermicity (⬃23 kJ/mol) for Reaction (III). Finally, the route to NO via N2O has an activation energy of 97 kJ/mol associated with step (IV). These considerations appear to rule out the production of NO via Zel’dovich’s thermal mechanism. At this stage “prompt” NO and that from N2O cannot be discarded; however, NO from N2H is most unlikely for the fuel-lean conditions [7], as here. These questions will be returned to again after examining more results. Dependence of [NOx] on U/Umf One difficulty with Fig. 1 was that U, Umf, and also U/Umf were all changing with the temperature of the bed. Thus, the nature of the fluidization was being altered when the bed’s temperature was changed. The dependence of [NOx] (in the off-gas) on (U/Umf) is investigated in Fig.

NO IN FLUIDIZED BED

Fig. 4. Plots of [NOx] in the off-gases vs. U/Umf for 400 g of sand (both sizes were used, as shown) fluidized in the steady state at either 800 or 1000°C.

4 by varying the total flow rate of gas to beds with two different sizes of sand and with the bed at either 1073 or 1273 K. In each case there was 0.400 kg of sand in the bed, giving the unfluidized depth of sand to be ⬃21 or ⬃18 mm, which is not very deep, for the larger and smaller particles, respectively. The result is that most of the gas bypasses the sand, because of rising up the bed as bubbles, with minimal interchange, in such a shallow bed, between the gas in the particulate and bubble phases. Examining first the results in Fig. 4 for the lower temperature of 1073 K (800°C), one has to bear in mind that the bed is then relatively noisy, because of bubbles igniting when they break through the upper surface of the fluidized sand. Above, it was conjectured that at 1073 K, NO is entirely produced in the particulate phase, e.g., on the surface of sand particles. Had that been the case, one would have expected [NOx] above the bed to fall when (U/Umf) was increased above unity, because NOx is then diluted by additional fluidizing gas from the bubbles. The indications in Fig. 4 are that at 1073 K, [NOx] increases linearly with (U/Umf) in the range 1 ⬍ U/Umf ⬍ 4. In fact, the intercept is such that [NOx] 3 0 as U/Umf 3 0. That [NOx] increases with U/Umf is important evidence for rejecting the idea that NO at 1073 K is only produced heterogeneously on the sand in the particulate phase. In Fig. 4 at 1073 K there could be a maximum in [NOx] at U/Umf ⬃4 with

2187 a subsequent slight fall to a steady, constant value of ⬃7 ⫾ 2 p.p.m for 4 ⬍ U/Umf ⬍ 9. These measurements of [NOx] in Fig. 4 each have an uncertainty of ⫾2 p.p.m. This rise and subsequent constancy of [NOx] at U/Umf ⬎ 4 suggests that at 1073 K, NO is produced not just in rising bubbles and not solely in the particulate phase. The problem now is to reconcile these observations with those in Fig. 1, which demonstrates that for U/Umf ⬃10 at 1003–1153 K, [NOx] is proportional to the mass of sand in the bed. An alternative hypothesis might be as follows. When a bed of sand at ⬃1073 K is fluidized by a mixture of methane and air, the gas in excess of that required to fluidize the sand forms bubbles. The ignition of a “slow” bubble has been considered in very simple terms [1] and roughly speaking before such a bubble can ignite it must have a diameter larger than D min ⫽

冉 冊

9U mf E exp 2 A关M兴 RT

(6)

Otherwise, the convection of radicals from a bubble is faster than chain-branching in e.g., (V) or (VI). Equation (6) is for chain branching being controlled by the general chemical reaction R ⫹ M 3 2R, where R is a radical and M is a molecule like O2 in Reaction (V) or RH in Reaction (VI). This branching step has a second-order rate constant equal to Aexp(⫺E/RT). The important point here is that a slow (or tiny) bubble of methane and air explodes when it has grown to a threshold size, which in turn is characterized by Umf and the temperature of the bed. Thus, an increase in temperature causes Dmin to diminish, as also does a decrease in the size of the sand, with the consequent fall in Umf. On the other hand, the ignition criterion for a bubble which has somehow become sufficiently large to be classified as “fast” (i.e., its diameter is much larger than Dcrit) is that it will ignite if its subsequent residence time in the bed exceeds the ignition delay of its contents; some values of ignition delay times were presented in Table 1. Of course, at a particular height up a fluidized bed (e.g., the top) there is a spectrum of sizes for the bubbles, with their mean falling lower in the bed according to Eq. (1). This means that at 1003–1153 K in Fig. 1, when more and more of the finer sand was present, the likelihood of all

2188 the bubbles being “fast” was increased, because bubbles have more chance to grow by coalescence in a deeper bed. They will not ignite as fast bubbles at the four lower temperatures in Table 1, because ␶ig is much longer (see Table 1) than the rise time of these bubbles, which is typically less than 0.2 s. All this is illustrated by the temperature of 1003 K in Fig. 1, where U/Umf ⫽ 9.1, so only 1/9.1 ⫽ 11% of the entering gas passes through the particulate phase; the other 89% passes through as bubbles. Here Table 1 indicates that with the finer sand present, if a bubble is to have the properties of a fast bubble, it must grow to an effective diameter much greater than 1.2 mm, say to 5 mm, for which they must rise to a height h ⬃10 mm above the distributor, as shown in Table 2. In fact, the unfluidized depth of the sand in Fig. 1 (size: 0.25– 0.355 mm) varied from ⬃13 mm with 300 g in the bed to ⬃30 mm with 700 g. This means that at 1003 K the bubbles always become “fast,” even with the minimum of 300 g of sand present. The consequence is that, because ␶ig is so large, these fast bubbles can only ignite on leaving the bed of sand. However, there is a spread of bubble sizes and it is possible that only the very largest bubbles ignite, whilst they disengage from the bed. The disengagement of bubbles is complex; sand particles are usually carried in the wake of a bubble and are eventually ejected into the freeboard [23], so there is a non-zero concentration of sand particles just above the bed in the splash zone. In this case, it is maybe not surprising that only the faster, i.e., larger, bubbles are free of the inhibiting effects of ejected sand particles. Looking at the plot in Fig. 1 for 1003 K and a large U/Umf of 9.1 (see Table 1), it is clear that [NOx], like the increase in depth of the sand when fluidized, is proportional to the mass of sand. In all probability this could derive from there being (at the upper surface of the sand) more bubbles above some critical size for ignition, when more sand is present in the bed. Here then is the basis for an explanation of Figs. 1 and 2. Temperature Profiles Some further light is cast on this problem by Fig. 5, which plots the axial temperature against height up one of these beds held at 973 K. These

M. PILAWSKA ET AL.

Fig. 5. The axial temperature (measured with a shielded thermocouple) at different heights above the distributor, for sand (size 180 –212 ␮m) fluidized with a fuel-rich mixture ([air]/[CH4] ⫽ 6.0 on a molar basis) for three different U/Umf, as shown. The approximate locations of the upper surface of the bed are as shown for the three U/Umf.

measurements were made with a thermocouple [22], inside a thin-walled, stainless-steel tube (inner diameter 23.5 mm) to shield the thermocouple from radiation emitted by the electrical heaters. In the bed, the gases are seen in Fig. 5 to be at the same temperature as the sand, but they become hotter on entering the splash zone. At even greater heights above the bed’s upper surface, the gases eventually cool. Figure 5 reveals that this rise to a maximum temperature in the freeboard is bigger with a larger U/Umf, i.e., with larger bubbles leaving the bed. Figure 5 is for sand (0.180 – 0.212 mm) fluidized at 973 K by fuel-rich mixtures of methane and air ([air]/[CH4] ⫽ 6.0) with three different values of U/Umf. The rises in temperature above the sand are 50, 125, and 168 K for U/Umf ⫽ 1.5, 2.5, and 3.5, respectively. Presumably, this maximum temperature affects the extent to which NO is produced. Because of this, Fig. 3 is probably spurious, because the temperatures quoted there are too low, being those of the bed, rather than of the splash zone above it. Even so, Fig. 5 confirms the above supposition that with a bed

NO IN FLUIDIZED BED at or below 1150 K, all the combustion occurs above the sand. The sizes of the bubbles leaving the sand are important in at least two respects: larger bubbles are seen from Fig. 5 to become hotter after igniting; also, they probably retain their identity in the freeboard for longer times than smaller bubbles do. Both the higher temperature and the longer time for NO to be produced inevitably increase the final yield of NOx. It is interesting in Fig. 4 that at the lower temperature it looks as if [NOx] 3 0, as U/Umf 3 0. This suggests that all the gas (i.e., that from both the particulate and bubble phases) is contributing to give NOx in the freeboard. Finally, it is important to notice (not shown in Fig. 5) that with the bed at the higher temperature of 1173 K, the temperature measured above the top of the fluidized sand fell monotonically with increasing height. This is in accord with the majority of the heat then being released near the distributor. Figure 5 thus confirms all the previous notions on where combustion occurs. In particular, there is evidence that with relatively large U/Umf and a bed below 1150 K, combustion occurs in the larger bubbles above the bed, so giving hotter “bubbles” of products, which retain their identity for longer times. Production of CO2 Before proceeding further with Fig. 4, these ideas are checked in Fig. 6, which is a plot of [CO2] in the off-gases vs. U/Umf. The conditions are identical to those in Fig. 4, so both Figs. 4 and 6 refer to shallow beds of only 0.400 kg of sand at either 1073 or 1273 K and with two sizes of sand particles. Also plotted in Fig. 6 is the level [CO2] would reach for complete combustion of all the carbon to CO2. Of course, the measurements in Fig. 6 have errors (actually ⬃10 –20%), but the general shapes of Figs. 4 and 6 are similar. Quite strikingly [CO2] falls when (U/Umf) is progressively reduced below ⬃4. In this case the indications are that at the lower temperature of 1073 K, combustion to CO2 is incomplete for (U/Umf) ⬍ 4. There is, however, an important difference between Figs. 4 and 6: in Fig. 6 the production of CO2 is almost complete whenever (U/Umf) ⬎ 4, but although the production of NO in Fig. 4 reaches an asymptote when (U/Umf) ⬎ 4, this asymptotic

2189

Fig. 6. Plots of [CO2] in the off-gases against U/Umf for 400 g of either size of sand fluidized in the steady state at 800 or 1000°C.

concentration of NOx is proportional to the depth of the sand, as shown by Fig. 1 for temperatures of 1153 K or less. This again is as if larger bubbles are not only slightly hotter, but also retain their identity for longer times on entering the freeboard, so that the production of NO can, e.g., proceed for longer times, as well as at higher temperatures. This would argue against all the NO being produced by the “prompt” mechanism, which would not be affected by the contents of a bubble being able to react for longer times in the freeboard. However, the alternative scheme, whereby NO is produced from N2O, would give more NO in a bubble surviving for a longer time, before being disrupted by sand particles above the bed or by mixing with the surrounding gas. At this stage, the production of NO from both the prompt mechanism and N2O in the larger, faster bubbles after they leave the bed appears to be a possible explanation for the observations at the lower temperatures of 1003–1153 K. Finally, it is worth noting that at the lower temperatures in Fig. 6, [CO2] in all probability tends to zero as U/Umf 3 0. This confirms that CO2 and NO are produced in the gas above the bed and not within it on the surface of the sand particles. That complete oxidation to CO2 is not achieved

2190 for U/Umf ⬍ 3 with the bed at the lower temperature of 1073 K is in accord with only the larger bubbles igniting on leaving the bed. This is confirmed by including more sand in a bed at the lower temperature 1073 K; the result is that [CO2] is increased for U/Umf ⬍ 3. The results in Fig. 6 for the higher temperature of 1273 K are discussed below. It has been noted before [25] that, when U/Umf ⬎ 6, there can be some recycling of the off-gases back to the bed. This occurs by sand particles ascending up the center of the bed in the wakes of bubbles and thereby forcing the fluidized particles to descend, most probably near the walls containing the bed. This circulation or “gulf-streaming” of the sand can result in gas (otherwise leaving the bed) being entrained by sand particles returning downwards into the bed in order to descend further toward the distributor. Gulf streaming starts [19] at U/Umf ⫽ {1 ⫹ Vb/⑀mfVw,}, where Vb/Vw is the ratio of the volume of a bubble to that of its wake. It should be asked whether the observations in Figs. 1 and 2 could be attributable in any way to “back-mixing” of gas driven by such “gulf-streaming.” In such a situation the fluidized bed tends to have the gases mixed well and their residence time in the bed is proportional to the mass of sand present. The audible evidence is that at up to ⬃1150 K, bubbles ignite on leaving the bed, suggesting that little is occurring by way of chemical reactions in the bed. This point is returned to again below. Behavior at Higher Temperatures Returning now to Fig. 2 to consider the highest temperature of 1303 K (when U/Umf is ⬃14) it is seen that a low [NOx] of ⬃10 p.p.m. is always measured, irrespective of the depth of the sand. However, Fig. 4 shows that at the slightly lower temperature of 1273 K, [NOx] falls to zero, when U/Umf is reduced from ⬃4 to unity, rather than zero. In fact, Fig. 4, like Fig. 2, shows that [NOx] is, totally fortuitously, pretty well the same at the two temperatures of 1073 and 1273 K. The explanation of why Fig. 2 indicates that at the highest temperature of 1303 K [NOx] has the same, but relatively low value (⬃10 p.p.m.), irrespective of the quantity of sand, is probably the reason why combustion is then much quieter

M. PILAWSKA ET AL. than at a lower temperature of ⬃1100 K, when large bubbles ignite noisily on bursting from the fluidized sand. At the highest temperatures (⬎1200 K), [NOx] in Fig. 2 falls as the temperature in the bed increases. All this suggests that on increasing the bed’s temperature above ⬃1173 K more combustion occurs close to the distributor, until at ⬃1300 K all the burning occurs in gas in excess of that required for fluidization (i.e., in bubbles close to the distributor). Figure 2 indicates that combustion is rapid in these first bubbles to be formed. In fact, such bubbles will probably be sufficiently tiny to be “small” and will ignite because their initial size exceeds Dmin in Eq. (6). Such a scenario is in accord with [NOx] at the highest temperature in Fig. 2 being independent of the mass of sand fluidized; likewise, [NOx] 3 0 in Fig. 4 as U/Umf 3 1, as expected from there being negligible mixing of the gas in the bubble and particulate phases in such shallow beds as used here. Likewise, at the higher temperature in Fig. 6, there is a suggestion that [CO2] 3 0 as U/Umf 3 1, in line with there being no burning in the particulate phase. If this were true, it would mean that the gas initially entering the particulate phase does not even burn in the freeboard above the bed. The errors in Fig. 6 are such that it is possible that [CO2] 3 0 when U/Umf 3 0 for all conditions. Free Radical Mechanisms It is not clear whether an increase in temperature causes a change of chain-branching from Reaction (VI) to (V) and this is a contributory reason for the ignition of bubbles moving from above the sand (at ⬃1150 K or less) to the very bottom of the fluidized bed at 1250 K or above. That apart, it is noteworthy in Fig. 6 that at U/Umf ⬃2 more CO2 is produced with the larger than smaller sand at 1273 K; in fact, Eq. (6) predicts that Dmin is smaller for the smaller sand particles. Presumably the dominant factor (for whether or not a bubble ignites) is the actual size of the bubbles, this being larger for the bigger sand particles, according to Eq. (1). One must ask if NO originates from the N2O mechanism at all the temperatures used in this study. One major reason for [NOx] in Fig. 2 being so small at the highest temperature is that

NO IN FLUIDIZED BED with combustion occurring so quickly in the very first bubbles to be formed, there is little time for reactions like (IV) to proceed before they are interrupted by one bubble coalescing with another. This suggests that with so little time available, NO is mainly produced by the prompt mechanism above ⬃1170 K. In this case, it would be expected to make a contribution at the lower temperatures of this study, where [NOx] is larger than expected solely from prompt NO, because of the additional contribution via N2O. In broad outline then, the observations so far can be explained by combustion being confined at the lower temperatures (⬍1150 K) to the larger, faster-moving bubbles leaving the sand and persisting beyond the splash-zone into the freeboard. In addition, at or above 1250 K (see Fig. 2) the gas entering the bed starts to ignite in the first bubbles created just above the distributor. Here also lies the explanation of the maximum in [NOx] revealed in Fig. 2. At the lower temperatures, when combustion is mainly in the larger bubbles leaving the fluidized sand, the amount of NO from the N2O mechanism increases with the temperature in the bed. However, in gradually hotter beds, combustion occurs increasingly lower in the bed. In fact, above ⬃930°C combustion occurs increasingly at the distributor, where only the prompt mechanism operates and the yield of NOx becomes insensitive to temperature. Of course, at intermediate temperatures of ⬃850 to 950°C, some bubbles will be large enough to ignite in the middle of the bed for the conditions of Fig. 2; on exploding, such a bubble is sufficiently disrupted for its contents to be mixed with sand, so the N2O mechanism is halted. Finally, it is worth noting that there is little evidence for chemical reactions occurring on or between the sand particles in the particulate phase; this confirms that sand inhibits combustion by removing radicals, because no variations of temperature were found in a bed, apart from in those outer regions adjacent to the cooler confining walls. Measurements within the Fluidized Bed These matters are explored further in Fig. 7, which plots [NOx], [CO], and [CO2] as measured at points up the axes of two beds. Each of them contained 1.200 kg of the larger sand

2191

Fig. 7. Measured [NOx], [CO] and [CO2] along the axes of two beds at 1123 and 1223 K. In each case, 1.200 kg of the larger sand was fluidized with a low U/Umf of 2.0: The approximate position of the upper surface of the fluidized sand is shown.

(0.50 – 0.60 mm) and had U/Umf ⫽ 2.0 and the same oxygen-rich mixture of CH4 and air. The only difference was the temperature in the fluidized sand, which was either 1123 or 1223 K. The unfluidized depth of the sand was ⬃64 mm, which rose to ⬃90 mm when fluidized with U/Umf ⫽ 2.0. Figure 7 shows that at the lower temperature, [CO], [CO2], and [NOx] are all low near the distributor, but grow to maxima near the top of the bed, which of course moves up and down quite markedly and vigorously. In this case at 1123 K, evidently most of the burning and production of NOx is in the splash zone. The plot of [CO] for the lower temperature displays a maximum closer to the distributor than the maxima for [CO2] and [NOx]. Presumably, this indicates that CO is an intermediate which is oxidized to CO2 relatively slowly in one of these beds. Such a statement is based on the reasonable assumption that with U/Umf ⫽ 2.0, as in Fig. 7, the bed behaves more as if the bubbles passed through the sand in plug flow.

2192 This picture contrasts with the plots of [CO2] and [NOx] in Fig. 7 for the higher temperature of 1223 K, which show maxima near the distributor, followed by considerable drops for measurements at progressively greater heights above the distributor. As for CO, it is striking that it was never detected in or above a bed hotter than 1150 K, i.e., when combustion occurred close to the distributor. In such cases it is evident that combustion of these O2-rich mixtures goes completely to CO2 immediately the gases enter the bed. Quantitative analysis of the profiles of [CO2] and [NOx] at the higher temperature in Fig. 7 is difficult because of the considerable difference in sampling conditions, e.g., between the probe in the sand and well above the sand. Sampling problems probably account for the [CO2] profile at 950°C, which is otherwise hard to explain. Nevertheless, Fig. 7 confirms the previous conclusion that at higher temperatures all the chemical reactions occur near the distributor, but at lower temperatures they are confined to the larger bubbles “bursting,” as they disengage from the fluidized sand. This picture leaves many details vague, particularly the precise nature of what occurs near the distributor at high temperatures and in the splash zone at the lower temperatures. The conclusions are not at variance with previous work, although little has been done so far on the generation of these relatively tiny amounts of NOx in these systems. However, it has been noted before [26] that with a bed at 1153 K [NOx] peaks in mixtures close to stoichiometric; this reinforces the importance of temperature. The conclusion of this previous study [26] was that “how the NO is formed . . . . must remain an open question,” although the thermal mechanism was firmly discounted. That the final [NOx] is surprisingly low (see Fig. 2) at the highest temperatures has been noted before [26]. Otherwise, the combustion of methane appears to proceed along broadly similar lines to that of, e.g., propane [25]; thus, at the higher temperatures, the combustion of propane occurs close to the distributor, but at the lower temperatures it is in bubbles in the splash zone. However, there is a difference between the highest temperatures at which a gas mixture can burn above the sand: for CH4 and air this seems to be ⬃1150 K, but for C3H8 and air this critical

M. PILAWSKA ET AL. temperature can be as low as 1030 K [25]. Finally, even though relatively large values of U/Umf were used in this study, no effects of back mixing were found. CONCLUSIONS When fuel-lean mixtures of CH4 and air are used to fluidize a bed of quartz sand particles, two modes of behavior are found, depending primarily on the temperature of the sand. With the bed below ⬃1123 K (850°C) the situation is as follows: combustion mainly occurs in bubbles disengaging from the top of the bed. If the bed is also shallow (⬃20 mm deep) at these lower temperatures, combustion to CO2 is incomplete when U/Umf ⬍ 4, but is complete when U/Umf ⬎ 4. This is interpreted as only bubbles above a certain critical size ignite on leaving the fluidized sand. The production of NOx (i.e., NO and NO2) is remarkably sensitive (in fact proportional) to the depth of the sand at these lower temperatures. Also [NOx] is proportional to U/Umf for U/Umf ⬍ 4, but otherwise is constant. Carbon monoxide is detected in the splash zone of these beds below 1123 K. The temperature of the gas just above the bed exceeds the temperature of the bed; this is more pronounced with larger U/Umf, i.e., when the bubbles leaving the fluidized bed are larger. The indications are that NO is produced by both the prompt mechanism and that with N2O as an intermediate. The contribution from the latter varies with temperature. When discussing the ignition of a bubble in a fluidized bed, it is important to distinguish between slow and fast bubbles. Here the size of the sand particles, among other parameters, determines whether bubbles are slow or fast. The criteria for when slow and fast bubbles ignite are different. By contrast when the bed is hotter than ⬃1250 K, [NOx] is independent of the depth of the sand. Also for U/Umf ⬍ 4, the conversion of CH4 to CO2 in the off-gases is slightly less than 100%; for U/Umf ⬎ 4, the yield of CO2 is 100%. Likewise, [NOx] is proportional to (U ⫺ Umf)/ Umf for U/Umf ⬍ 4, but for U/Umf ⬎ 4, [NOx] is constant. Carbon monoxide is hard to detect in these hotter beds. The explanation appears to be that at these higher temperatures the first

NO IN FLUIDIZED BED bubbles of CH4 and air are slow and ignite soon after being formed, just above the distributor, because Dmin from Eq. (6) is small at higher temperatures. However, even at 1300 K there is no evidence for CH4 and air burning interstitially in between the sand particles. This inhibition of combustion is due to radicals recombining on the large area of the sand particles, rather than any thermal effects [2]. NO is produced in relatively small amounts at these highest temperatures, because only the “prompt” mechanism operates. At intermediate temperatures (1123–1250 K) a hybrid of the above two limiting cases holds. Thus in hotter beds, slow bubbles can ignite low in the bed, but appreciably above the distributor. At the lower end of this intermediate range of temperatures, rising bubbles can ignite below the bed’s upper surface; moreover, they tend to do so as fast bubbles in deep beds of tiny sand particles, but as slow bubbles in shallow beds of large particles. The amount of NO produced reaches a maximum at these intermediate temperatures. M.P. thanks Queens’ College, Cambridge, and the Cambridge Colleges’ Hospitality Scheme for East European Scholars for their generous financial and other support.

2193 7. 8.

9. 10. 11. 12.

13.

14.

15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

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Received 22 January 2001; revised 5 August 2001; accepted 22 August 2001