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An experimental study of non-premixed combustion in a turbulent fluidized-bed reactor
Fuel Processing Technology 88 (2007) 847 – 858 www.elsevier.com/locate/fuproc
An experimental study of non-premixed combustion in a turbulent fluidized-bed reactor R. Sotudeh-Gharebagh a,⁎,1 , J. Chaouki b , P. Sauriol b a
b
Process Design and Simulation Research Centre, School of Chemical Engineering, University of Tehran, PO Box 11365-4563, Tehran, Iran Chemical Engineering Department, École Polytechnique de Montreal, P.O. Box 6079, Stn. “Centre-Ville”, Montreal, Quebec, Canada H3C 3A7 Received 26 October 2006; received in revised form 31 March 2007; accepted 11 April 2007
Abstract Although the theory of fluidized bed technology is fairly well understood, non-premixed reactions in such beds have recently garnered considerable attention, particularly with respect to combustion applications, which still present a number of practical engineering and operational problems at start-up, during operation and at shutdown. While studies of such processes in fluidized beds remain essential to validating theoretical models, young researchers often perceive them as dirty and tedious experiments due to the complex operational concerns involved. Nevertheless, the planning and implementation of experiments, the analysis of the full array of interactions between system components and bed materials, internal surfaces and measurement devices, and interpretation of results to diagnose abnormal operating conditions present operators with a range of difficult intellectual challenges. Some of the important experimental issues involved in the operation of non-premixed reactions in fluidized beds are presented and discussed. In particular, widening the range of fluidized bed system operations to include direct fuel injection can expand the range of reactant compositions beyond those normally allowed by safety constraints. Therefore, the aim of this study is to understand the behaviour of non-premixed natural gas combustion in turbulent fluidized bed using various spargers under high temperature conditions. The results of this study can also provide insights into how volatiles burn inside fluidized bed reactors. © 2007 Elsevier B.V. All rights reserved. Keywords: Operation; Non-premixed combustion; Sparger; Fluidized bed; Highly exothermic reactions
1. Introduction Fluidized bed reactors are particularly attractive for highly exothermic reactions because of their superior heat management ability, proper mixing and excellent temperature control. Despite such obvious advantages, these systems have not been widely exploited by industry. Combustion of gaseous mixtures, mostly methane–air, has been successfully implemented in fluidized bed reactors [1–18]. Detailed reviews of such studies on premixed fuel combustion in bubbling and volatile combustion in circulating fluidized bed reactors have been presented by [19,18,12].
⁎ Corresponding author. Fax: +98 21 6646 1024. E-mail address: [email protected] (R. Sotudeh-Gharebagh). 1 Sabbatical leave (2006-7) Chemical Engineering Department, Qatar University, Doha 2713, Qatar. 0378-3820/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2007.04.009
In bubbling fluidized bed studies, feed composition was maintained well below the lower explosion threshold. However, such studies have been conducted in small diameter fluidized beds wherein localized data for model verifications cannot be reliably obtained. Two critical transition temperatures exist for combustion, and vary according to the type of fluidized beds (e.g. [19]). Below the lower critical temperature (T1), combustion only occurs above the bed surface, while between (T1) and the higher critical temperature (T2), combustion begins to move into the bed; combustion takes place entirely within the bed when its temperature exceeds T2. However, whether or not combustion occurs within the particulate phase during the transition period as well as after the combustion moves into the bed is still an open question [19]. Use of gaseous mixtures in bubbling fluidized beds under premixed conditions has very limited industrial applications since high throughput is always desired for industrial processes. Such high throughputs can be achieved through moderately
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high superficial velocities (turbulent fluidization) combined with the direct injection into the reactor of higher concentrations of feed within the usual range of flammability. This would result in significant cost savings, due to the greater per-pass conversion and yield [20,21]. Furthermore, given that natural gas availability has increased while emission standards have become more stringent, turbulent fluidized-bed combustion technology has seen an increase in natural gas utilization for a wide range of applications. These include co-firing, gas re-burning, foundry sand reconditioning, incineration of high-moisture sludge, cleaning of metallic parts and direct combustion for heating. Thus, a systematic understanding of the non-premixed gasphase combustion in turbulent fluidized beds is more important that ever before. In a study of liquefied petroleum gas (LPG) combustion in a rectangular pilot bubbling fluidized bed, mixing of LPG and air is found to be difficult because the LPG's high calorific value led to incomplete combustion [22]. Ross et al. [23] reported combustion of coal volatiles, as simulated by propane, and its interaction with char gasification reactions in a bubbling fluidized-bed at 750–950 °C. Foka et al. [24] investigated the catalytic premixed combustion of natural gas (with a poor inlet mixture of 4% methane) in a turbulent fluidized bed at moderately low temperatures (400–600 °C). The full success of this type of bed was strongly dependent upon the nature of the catalyst used, its effectiveness and resistance to attrition as well as corresponding costs. Unfortunately, combustion catalysts are usually expensive and subject to attrition. In order to make turbulent fluidized bed reactors applicable for residential, industrial and waste-to-energy applications and to decrease this cost substantially, catalysts should be replaced by inert materials. Despite the large impact of using inert particles as bed materials in industrial turbulent combustors, limited experimental data are available in the literature regarding the combustion mode and regime of fluidization. Sotudeh et al. [25] reported the feasibility of non-premixed natural gas combustion at moderately high temperatures (b 900 °C) in a turbulent fluidized bed of inert particles with an in-bed CO profile. They used a single gas sparger configuration in injecting natural gas into the bed. Since the rate of combustion of gaseous fuel under non-premixed conditions is limited by the rate of mixing of fuel and oxygen and the chemistry of reaction, these should be thoroughly understood in any experimental and modelling efforts. Beside the moderate superficial velocities occurring under turbulent conditions, the lower region of circulating fluidized bed combustors used in coal combustion, where the volatile matters are largely burnt, operates under turbulent fluidized bed conditions [26,27]. In modelling efforts, combustion of volatile matter is either ignored, assumed to occur close to the distributor, or assumed to occur throughout the emulsion phase [6,28]. However, the contribution of the bubble phase to volatile combustion process is still more poorly understood and requires experimental verification. This issue is of interest for two reasons: (i) with solid fuels, volatiles account for an appreciable proportion of the combustibles [5,16], and (ii) fluidized bed combustion may burn gas streams carrying organic vapours [29] or solid fuels of low calorific value.
In order to make turbulent fluidized bed reactors applicable for highly exothermic non-premixed reactions, Good Experimental Practice (GEP) is needed in order to address key issues. The non-premixed reactions in fluidized beds are partially controlled by the rate of mixing with oxygen at a given temperature. If the mixing is slow, which is the case in bubbling fluidized beds, the reactant bubbles (methane bubbles) may pass un-burnt through the bed and burn above the bed. This decreases the amount of heat released within the bed, leading to higher freeboard temperatures than are desirable. Therefore, the main objective of the present study was to address the key operational issues and mixing behaviour of non-premixed combustion using various sparger configurations in fluidized reactors operating in a temperature range of 400–1000 °C. This would be required for studies of the reaction parameters and system hydrodynamics. Therefore, an extensive experimental
Fig. 1. Experimental unit (a) and the gas injection port (b).
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program is conducted in this study which may lead to a more fundamental understanding of non-premixed combustion. 2. Experimental The pilot fluidized bed reactor used in this study consists of a 200 mm ID and 2 m tall refractory-lined reactor (Figs. 1a and 2). The reactor is divided into three zones: the inlet zone (wind-box and distributor), a fluidized bed, and a freeboard. An external natural gas burner, supplied with 20 kW of power, was located at the bottom of the bed, providing the partial heat required for the reactor to achieve the desired temperature. Fluid Catalytic Cracking catalyst (FCC) (mean diameter of 70 μm, ρ = 1.45 gr/m3) and sand particles (mean diameter of 543 μm, ρ = 2.65 gr/m3) were used during the experiments. Several ports were provided along the axial position and around the circumference of the reactor. Providing great flexibility for detailed in-bed measurements, these ports were used for pressure measurements, sampling and natural gas injection into the reactor. Natural gas (NG) was supplied to the reactor through a secondary injection port (Fig. 1.b) with spargers of different geometries (Figs. 3 and 4). Type-K thermocouples were also placed along the reactor center-line to monitor
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temperature profiles. An absolute pressure transducer was used to monitor the level of particles in the reactor by continuous recording of pressure fluctuations. The bed inventories for FCC and sand particles were 15 and 20 kg, respectively. Normally, each experimental run began with cold and hot hydrodynamic tests; i.e. measurement of minimum fluidization and transition velocities. Minimum fluidization velocities of 0.23 m/s and 0.003 m/s for sand and FCC particles, were measured, respectively, under cold conditions. Due to issues of temperature stability and operational problems, it was not possible to measure the minimum fluidization velocity under high temperature conditions. When bed temperature reached the desired level, NG was injected through the secondary injection port and combustion took place inside the bed. The detailed procedure involved in reactor heating has been reported elsewhere [25]. Local bed structure was characterized by measuring the local concentration and temperature profiles under various operating conditions (Fig. 5a). Gas sampling probes, their tips protected by a glass wool filter (Fig. 5b), were placed along the reactor central axis, and connected to the gas chromatograph where the samples were withdrawn with a variable pressure vacuum system. A number of spargers with different tip geometries were also tested to study their effects on mixing (Fig. 6). The distributor plate or spargers were designed in such a way as to overcome the problem of poor gas mixing in fluidized beds. The pilot plant used
Fig. 2. Detailed schematic of a Pilot Plant Turbulent Fluidized Bed Reactor.
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Fig. 3. Spargers used in experimental studies. was very flexible and could be quickly modified to operate under a wide variety of operating conditions. The particular design chosen for this study, (i) facilitated the injection of air and NG at different locations in the bed, (ii) provided operating temperatures up to 1000 °C, and (iii) allowed a turn down or complete shut down of gas and air flow without problems arising from backfilling of the distributors with particles. The experiments studies were repeated several times in order to check the reproducibility of all observations and measurements.
3. Results and discussion 3.1. Reactor operational issues The typical duration of individual experimental runs are presented in Table 1. Various problems including attrition and
Fig. 4. Detailed scheme for spargers C and D (Spargers A and B have the same tip as sparger C).
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Fig. 5. Gas sampling scheme (a) and Probe tip (b).
Fig. 6. Sampling probes, gas spargers (a) and distributor (b).
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erosion, and improper operation may occur during an experimental run, such that special attention is required to overcome these issues and ensure proper operation. Occurring in many locations and through a number of mechanisms, the phenomenon of particle attrition in fluidized bed reactors is an important one, since it alters the reactor solid inventory. For the fluidized bed reactor used in this study, two instances of difficulty in maintaining control over the bed level occurred. In the first case, starting preheating experiments at minimum fluidization based on cold conditions, led to a sharp dynamic decrease in bed level due to thermal break-up of bed materials. In the second case, attrition was attributed to the purging system. During the preheating period of non-premixed experiments, air had to be used to keep the natural gas injection sparger clean. However, the introduction of air, even at a low rate caused a highly turbulent jetting area at the injection site, rendering bed inventory control difficult. This suggested that if bed materials were subject to attrition, jetting conditions should be prevented inside the bed by placing shrouds around grid holes so as to reduce the velocity at the gas–solid interface to below 30 m/s. For these cases, continuous solid injection and bed inventory control solved the problem. In all experiments, the bed level was closely controlled. The typical bed pressure signal was 16 ± 5 mA indicating a constant bed inventory. Given that attrition and abrasion may damage sampling probes and injection systems, probes should be carefully inspected (Table 1) and the fact that gas jets can be serious sources of erosion in fluidized beds should be taken into account. Under the experimental conditions, the erosion caused by jet impingement as a result of the creation of a highly turbulent area around the sparger required that the probe be changed frequently. Improper shutdown and operation of the bed may damage the sampling probes and distributor (Fig. 6). In order to avoid undesirable damage at high temperatures, a gradual cooling of the bed (Table 1) is mandatory before a full shutdown. 3.2. Experimental results and discussion For all experiments, conversion was 100% at the top of the bed. For a given temperature, the rate of gaseous fuel combustion in fluidized beds is normally assumed to be limited by the rate of mixing of fuel and oxygen [28,30]. However, our experimental results show that the amount of over-bed burning
increases rapidly as the bed temperature drops below a given critical value for the premixed combustion. This is consistent with the work reported in other studies [5,14,31,32].. As temperature is increased, the 100% conversion point moves inside the bed as a result of free radical formation dominating the inhibition process caused by the emulsion phase [5,6]. Since the combustion is inhibited by the removal of either radicals or heat, the combustion within the emulsion phase begins to occur at temperatures well above those required for bubble burning inside of the bed [6]. For non-premixed combustion to occur successfully in a fluidized bed reactor, both mixing and temperature requirements must be met. Three combustion regimes were identified: (i) at low bed temperatures (b 700–750 °C), the fuel likely did not burn within the bed; (ii) at moderate bed temperatures (about 825 °C), the fuel likely only burned in the bubbles; and (iii) at high bed temperatures (N 900 °C), combustion occurred within the bubbles and in the particulate phase of the bed. These regimes are consistent with those reported in other similar studies [6,12,19]. As mentioned before, sand particles within the emulsion phase would decelerate the reaction process considerably. In order to simulate the contribution made by these particles, separate experimental and modelling studies were conducted in a fixed bed reactor. Details of the study have been reported by Sotudeh and Chaouki [33]. In Fig. 7, the experimental data obtained in a fixed bed reactor of sand particles was compared to predictions generated by the plug flow model developed with the Gas Research Institute's (GRI) homogeneous methane combustion mechanism [34]. As seen in the figure, the model overpredicted the experimental data. The difference between model and the experimental data is attributed to the deceleration effects caused by the presence of sand particles. The figure also shows that sand particles may lower conversion at lower temperatures (related to low experimental conversion). However, at the higher temperatures (N 900 °C) (related to high experimental conversion), sand inhibitory effects seem negligible, as is also reflected in other studies [5,11,18,35]. Given its wide availability and lesser cost compared to other bed materials (alumina and combustion catalysts), sand would therefore be an appropriate choice of bed material when developing a novel reactor for the wider applications envisaged in the industry. Axial temperature profiles in the bed under various bubbling and turbulent fluidization conditions are shown in Fig. 8. It is
Table 1 A typical experiment duration Action
Activities/measurement
Time
Preparation Preheating Stable fluidization Data acquisition
Bed inspection, device preparation and installation, loading Bring the bed from ambient temperature to 800 °C To ensure the stable operation at the temperature desired Gas analysis (5 probes, 9 measurements/probe, 20 mpm a) Pressure fluctuation (during the gas analysis) Temperature profile (3 probes, 12 measurements/probe, 3 mpm) Bring the bed from the reaction temperature to 400 °C (to prevent damaging the distributor) Analyze the data and generate the corresponding graphs
6h 14 h 2h 14 h – – 4h – 40 h
Cooling down Data analysis Total time required for each experiment a
Minutes per measurement.
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Fig. 7. Parity conversion plot for the emulsion phase at the temperature ranged 650–1000 °C (XCH40 = 2%, η = 107, τ = 3 and dp = 543 μm).
worthy to mention that the experimental transition velocities from bubbling to turbulent were measured experimentally as 0.7 m/s and 1.5 m/s for FCC and sand particles, respectively. Radial temperature measurements showed the temperature fluctuations in the fluidized bed to be small under all operating conditions. The standard deviation of axial temperature profiles under bubbling condition varied from 5 to 21 °C, while the deviation under turbulent conditions varied from 3 to 12 °C. This means that the bed can easily be operated under isothermal conditions, because of the intense and rapid solids mixing under turbulent conditions. The temperature measurements reported in this study are consistent with those reported by [22], where the maximum temperature fluctuation was reported to be less than 5 °C. The wind-box temperature remained near 200 °C throughout, preventing any damage to the distributor and allowing the safe operation of the reactor (Fig. 8).
Fig. 9. Methane concentration profile around the sparger for FCC at 820 °C, U = 0.8 m/s and Uj = 56 m/s.
A plot of methane concentration at the injection point is presented in Fig. 9. This figure shows that upon introducing methane into the bed through the sparger, a reducing atmosphere is generated around the sparger, where methane is consumed within a short distance of the sparger. The methane coalesces with the air bubble to form a less rich methane bubble. The repeatability (standard deviation) of methane measurements are presented in Table 2. As seen in this figure, the methane concentration at 23 mm from the sparger tip was considerably greater than that at − 2 and 74 mm. The high concentration of methane around the sparger wall can be attributed to the fact that bubbles, as formed at the sparger tip, end up forming a train of bubbles around the sparger wall. Under these conditions, the mixing is limited to the sparger wall itself. As soon as the bubbles reach the sparger level, due to the intense gas–solid mixing which occurs under the turbulent fluidization regime, the tracer gas is very well purged to the emulsion phase. Radial symmetry was exhibited under turbulent fluidized bed conditions due to the intensive mixing of solids as shown in Fig. 10. It might be very difficult and tedious to establish such radial symmetry with bed-mixing in bubbling fluidized bed conditions because of vigorous bubble passage through the bed and their heterogeneous behaviour. The conditions for this figure are the same as those reported for Fig. 9. A radial concentration profile of CO at different axial positions around the sparger is shown in Fig. 11. The figure shows a
Table 2 Standard deviation for methane measurements Methane concentration (% mole)
Fig. 8. Temperature profile of the pilot fluidized bed reactor under turbulent bed conditions (U = 1.5 m/s) and bubbling fluidized bed condition (U = 0.7 m/s) for sand particles.
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0.3321 ± 0.0935 0.6611 ± 0.1521 1.7000 ± 0.3439 2.4600 ± 0.1878 3.5300 ± 0.3365
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Fig. 10. Symmetrical methane concentration profile around the sparger for FCC at 820 °C, U = 0.8 m/s and Uj = 56 m/s.
Fig. 12. Oxygen concentration profile around the sparger for FCC at 820 °C, U = 0.8 m/s and Uj = 56 m/s.
high CO concentration at 23 mm from the sparger tip, where CO bubbles are protected by the sparger wall. Thus, due to inhibition phenomena, CO-rich bubbles were formed. For low bed temperatures, an observation of the CO profile in the reactor leads to the conclusion that, due to the quenching effect of the emulsion phase, CO and volatile materials may burn in the freeboard or inside bubbles. Unlike other fluidized bed applications where coalescence of the bubbles is an undesirable feature, in non-premixed air-fuel combustion mode, this promotes effective mixing of unlike bubbles to form CO bubbles, which are then easily ignited if the bed temperature is sufficiently high. Furthermore, at the injection point, the distributor bubbles are generally small and consequently mixing of gas and air would effectively occur at the vicinity of the gas distributor.
The oxygen profile around the sparger is shown in Fig. 12. The figure shows that a reducing zone is formed around the sparger similar to that in the dense bed of the circulating fluidized bed combustors. As the bubble moves upward in the vicinity of the sparger wall, the oxygen shortage becomes critical. The figure also confirms the experimental findings reported in Figs. 9–11. Upon injecting a high velocity jet into the fluidized bed reactor, the creation of high turbulent area results in all the oxygen found in the immediate vicinity of jet being rapidly consumed in forming CO bubbles. It was initially assumed that methane injection through the sparger would lead to a hot spot at the vicinity of the sparger, but Fig. 12 shows this assumption not to be valid. In order to confirm isothermicity around the sparger, the temperature profile around the sparger was measured as shown in Fig. 13, and was seen to vary little around the injection point. What little
Fig. 11. CO concentration profile around the sparger for FCC at 820 °C, U = 0.8 m/s and Uj = 56 m/s.
Fig. 13. Temperature profile around the sparger.
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temperature variation occurs around the sparger is related to the rise in gas temperature. It seems when a single step reaction is chosen, a hot spot may form around the sparger if the heat of reaction exceeds the amount transferred to the surrounding medium by different heat transfer mechanisms. However, the hypothesis should be verified experimentally. This could have large practical applications, particularly with respect to hazardous waste destruction. Besides the previously mentioned temperature requirements, safe operation of the fluidized bed reactor in non-premixed mode requires a proper sparger or injection system. The hydrodynamics at the injection point and the geometry of the sparger would play a very important role in promoting intense gas–solid mixing. Four different sparger tips were used to study the injection point hydrodynamics and the effect of sparger geometry thereon. Plots of the mixing behaviour under different sparger designs at 400 °C (Fig. 14), show, as expected, that sparger geometry significantly affects mixing, the controlling step in non-premixed combustion. Some spargers tip designs allow the spreading of bubbles, with a concomitant enhancement of mixing. Four mixing zones could be identified around the sparger. At the injection point, level with the axial position of − 2 mm, the tracer concentration is low showing the bubbles tendency to move up. Depending on the tip of the sparger used, some bubbles are spread downward, providing partial mixing. At 23 mm from the sparger tips, bubbles are seemingly moving close to the sparger walls and well protected. This is particularly evident with the sparger B which shows high methane concentrations, whereas for sparger D, bubbles exiting the sparger spread downwards into the fluidizing medium, resulting in better mixing. Normally in this zone, the wall protects the bubble and it is only when it passes the level of the sparger base that the mixing is improved (zone 3, Fig. 14). In zone 4, the bubbles from all spargers reach the splash zone and no geometry effects would be identified here.
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3.3. Good experimental procedure (GEP) Based on the comprehensive experimental program undertaken, operation of non-premixed highly exothermic reactions (HER) in fluidized bed reactors under GEP requires the following parameters to receive particular attention: 3.3.1. Proper unit preparation The experimental unit should be carefully prepared and inspected before start-up by cleaning all probes, checking all measurement devices and their calibration. 3.3.2. Cold experiments In order to study the limitations associated with the fluidizability of the particles and the bed ability to withstand the abrasion associated with solid fluid movement, the fluidization experiments should be conducted in a cold unit. 3.3.3. Probe and sparger maintenance The measurement probes and spargers should be flushed with bottled oxygen-free nitrogen N2 upon loading the bed materials in order to keep them solid-free. The experiments are started by heating up the reactor contents. Usually a porous sintered steel plug fitted in the tip of the probe permitted the collections of gas samples, free of sand and dust. In this study, glass wool is placed at the tip allowing the proper dust-free sampling. 3.3.4. Leak development This would be mostly likely cause of accidents. A very careful and serious inspection should be performed prior to and during each experiment. 3.3.5. Start-up A systematic methodology should be followed in the operation of combustion reactions in order to obtain the temperature level desired [19,25]. 3.3.6. Particle heating: two operation modes exist for particle heat up 3.3.6.1. flame-based particle heating. This would be done by the gradual addition of sand though a well-designed solid injection system [19]. In this case, the empty chamber was preheated to a temperature above 600 °C by the ignition of a NG–air mixture inside the bed. Sand was slowly added to the unit while maintaining the temperature well above the light-off limit, until the operating temperature of 800 °C was reached. By then the superficial velocities were adjusted. By increasing the sand particles and adjusting gas superficial velocity, a point would be reached where the flame disappeared.
Fig. 14. Mean methane concentration around the spargers at 400 °C and Uj = 36 m/s.
3.3.6.2. non-flame based particle heating. Particles are loaded into the bed and combustion would be forced inside the bed through under-bed (wind-box) and over bed combustion. As used in this study, this technique would be more efficient in heating up the bed contents. Direct gas injection at different
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levels would be needed in order to start up the reactor. In/over bed gas firing is used in order to raise the bed temperature to either an intermediate level or to an elevated level depending on the operating conditions. 3.3.7. Bed level measurement A catalytic reaction in a fluidized bed may “run away” if the fines content or porosity of the particles increases due to a change in the source of raw material. Therefore, the bed level should be closely monitored and controlled. Attrition may alter bed inventory, and this may, in turn, create a number of problems at high superficial velocities if the cyclone performance is inadequate to capture the particles and recycle them back to the bed. Sometimes a two-stage cyclone may be required. This is also a serious problem for in-bed particle heat up, due to the superficial gas velocity increase during temperature rise-up. 3.3.8. Choice of operating conditions and stable fluidization Highly exothermic reactions are quite rapid. The proper gas– gas–solid efficiency makes the process continuous and robust, thus greatly reducing the potential hazard from this extremely hazardous reaction. Some difficulties may initially occur in maintaining the fluidization for any length of time due to attrition, but the addition of bed materials could eliminate the problem. 3.3.9. Gas accumulation within the reactor To reduce the risk associated with the exothermic reactions in a fluidized bed reactor, the natural gas is fed in gradually so as to control the heat generated by the chemical reaction. In practice, the added component may not be immediately consumed and would partly accumulate and get mixed into the bed, depending on the hydrodynamic conditions. The amount accumulated would be a direct measure of the hazard potential. From the point of view of safety, an accurate selection of operation and design parameters is required to ensure a minimum accumulation. If the bed is carefully operated, a smooth and stable temperature profile is obtained in the reactor. This is easy when handling the single-step reactions. 3.3.10. Multiple reactions The undesirable runaway reactions often encountered in practice (i.e. methane reaction) are generally caused by multiple reactions. The rate of these reactions would be negligible at initial conditions but may become significant at higher temperatures. In fluidized bed reactors, the control of hydrodynamic conditions would be a very effective means of controlling the reactions at different steps of the process, thus leading to an inherently safe operation. 3.3.11. Runaway reactions In order to prevent a runaway, the reaction zone should be located inside the bed. Situations could occur under which the reaction zone is either at the entrance or at the exit of the reactor, leading to serious operation issues. In case of the two-step reactions considered in this study, complications
arise and one has to discern between the heat production rates of the different reactions. Since the methane combustion scheme is rather complex, the interaction of parameters in a multiple reaction system makes the development of an unambiguous criterion impossible. However, with a proper temperature measurement around the sparger, this issue would be easily addressed. To guard against accumulation of a flammable mixture in the wind-box, gas detectors and alarms should be fitted to the reactor and the surrounding media. 3.3.12. Burning back As the distributor temperature approaches the bed temperature, the reaction may spread back into the wind-box. Under these conditions, the fuel is immediately shut off in order not to destroy the plate and the gas is then injected through the spargers located within the bed. The highly exothermic reaction is safe provided that it is located within the bed, as burn-back cannot occur in this mode. If the degradation and attrition of the sand during the test is significant, a close control of the bed inventory would be needed to attain stable fluidization and to prevent burn-back conditions. 3.3.13. Temperature and reaction control The temperature of the reactor at different axial and radial positions, in the feed unit and surrounding the reactor are measured by thermocouples. When the reactor temperature exceeds a certain unacceptable level, the gas injection flow is lowered to reduce the temperature. During the experiments, gas samples are taken automatically for later analysis. Successful long term pilot operation requires careful mapping of the operating procedure, careful control over reactions and rapid corrective action as needed. 3.3.14. Safe operation The process can be regarded as invariably safe when no runaway reactions can occur at any temperature, under any given operating conditions. This can be achieved when the reaction zone is located inside of the bed. The location of the reaction zone is very critical and sensitive to small changes in the bed temperature. By adopting GEP, a few experiments would be sufficient to establish conditions for large-scale reactor operations to be invariably safe. To this end, one has to avoid accumulation of the reactant in the reaction zone and outside of the bed and wind-box. In methane combustion, CO accumulation would necessarily occur and proper actions would be required to prevent its accumulation. 3.3.15. Shutdown Conducting high temperature reactions in fluidized beds is often associated with problems, such as a tendency for the bed to agglomerate and collapse during the shutdown of the reactor as well as resultant damage to the distributor. Therefore, it is necessary to blow the bed and keep it fluidized by lowering the gas superficial velocity, which will in turn provide sufficient time for bed materials to cool down.
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3.3.16. Environmental concerns Highly exothermic reactions in fluidized bed reactors are characterized by a gaseous exit stream carrying vapours and dusty materials. The gases as well as the dusty materials are often hazardous to health or outright poisonous. In addition to enhanced efficiency cyclone and dust collection systems, precautionary systems are needed in order to protect the operators and operation environments. Particulates smaller than 10 μm in diameter are often picked up by relatively gentle air currents, do not settle quickly, and may cause damage to eyes and lungs. This is a particular issue if the materials are toxic, corrosive, radioactive or biologically active. 4. Conclusions A non-premixed reaction mode was implemented using fluidized bed technology using various spargers. These experiments served to establish a strategy for the operation of highly exothermic reactions in fluidized bed reactors. This study shows that methane, when injected into the bed, is initially converted to CO as an intermediate component, with the full reaction of CO to CO2 occurring inside of the bed. Three nonpremixed combustion regimes, similar to those occurring under premixed combustion were observed: (i) at low bed temperatures (b700–750 °C), the fuel likely does not burn within the bed; (ii) at moderate bed temperatures (about 825 °C), the fuel likely only burns in the bubbles; and (iii) at high bed temperatures (N 900 °C), combustion occurs within the bubbles and in the particulate phase of the bed. Non-premixed combustion was shown to allow the expansion of the safe range of operation and improve reactor performance and that an increase in the number of injection points would generally be helpful. At the injection point, 4 mixing zones were identified, ranging from poor to completing mixing, depending on the sparger type employed. The outcome of this study may also have implications for the combustion of gaseous fuels and volatile matters in the lower region of fluidized bed coal combustors. Nomenclature D Reactor diameter (m) dp Mean particle size (μm) FCH40 Initial methane molar flux (mol/s) T Temperature (°C) U Superficial gas velocity (m/s) Uj Sparger jetting velocity (m/s) XCH40 Initial methane mole fraction (−) z Height along the bed (m) τ Mean residence time (s) η Contact time index (W/FCH40) (g s/mol) Acknowledgments The University of Tehran, Iran, is gratefully acknowledged for granting sabbatical leave to R. Sotudeh. The comments and fruitful discussions with Dr. Navid Mostoufi from the University of Tehran are highly appreciated. The authors are
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also grateful for the critical and helpful comments of the anonymous referees. References [1] R.V. Rameshwar, V.F. Stepanchuk, Combustion of gaseous fuels in fluidized beds, Indian J. Technol. 5 (1967) 245–252. [2] W.E. Cole, R.H. Essenhigh, Fluid bed combustion of natural gas, Second conference on natural gas research and technology, Vol. 4 (1972) 1–28. [3] P.V. Sadilov, A.P. Baskakov, Temperature fluctuations at the surface of a fluidized bed with gas combustion occurring there in, Int. Chem. Eng. 13 (3) (1973) 449–456. [4] I. Yanata, K.E. Makhorin, A.M. Glukhomanyuk, Investigation and modelling of the combustion of natural gas in a fluidized bed of inert heat carrier, Int. Chem. Eng. 15 (1) (1975) 68–73. [5] J.S. Dennis, A.N. Hayhurst, I.G. Mackley, The ignition and combustion of propane/air mixtures in a fluidized bed, nineteenth symposium on combustion, The Combustion Institute, Pittsburgh, 1982, 1982, pp. 1205–1212. [6] E. Turnbull, J.F. Davidson, Fluidized combustion of char and volatiles from coal, AIChE 30 (1984) 881–889. [7] D.R. Van Der Vaart, The chemistry of pre-mixed hydrocarbon air combustion in a fluidized bed, Combust. Flame 71 (1988) 35–41. [8] D.R. Van der Vaart, Freeboard ignition of pre-mixed hydrocarbon fuels in a fluidized bed, Fuel 767 (1988) 1003–1008. [9] D.R. Van der Vaart, Mathematical modeling of methane combustion in a fluidized bed, Ind. Eng. Chem. Res. 31 (1992) 999. [10] J.F. Stubington, S.W. Chan, S.J. Clough, A model for volatiles release into a bubbling fluidized bed combustor, AIChE 36 (1) (1990) 75–85. [11] R.P. Hesketh, J.F. Davidson, Combustion of methane and propane in an incipiently fluidized bed, Combust. Flame 85 (1991) 449. [12] R.A. Srinivasan, S. Sriramulu, S. Kulasekaran, P.K. Agarwal, Mathematical modeling of fluidized bed combustion-2: combustion of gases, Fuel 77 (1998) 1033–1049. [13] P. Pre, M. Hemati, B. Marchand, Study on natural gas combustion in fluidized beds: modelling and experimental validation, Chem. Eng. Sci. 53 (1998) 2871–2883. [14] S. Dounit, M. Hemati, D. Steinmetz, Natural gas combustion in fluidised bed reactors between 600 and 850 -C: Experimental study and modelling of the freeboard, Powder Technol. 120 (2001) 49. [15] E.M. Bulewicz, W. Zukowski, S. Kandefer, M. Pilawska, Flame flashes when bubbles explode during the combustion of gaseous mixtures in a bubbling fluidized bed, Combust. Flame 132 (2003) 319–327. [16] W. Zukowski, A simple model for explosive combustion of premixed natural gas with air in a bubbling fluidized bed of inert sand, Combust. Flame 134 (2003) 399–409. [17] L. Ribeiro, C. Pinho, Generic behaviour of propane combustion in fluidized beds, Chem. Eng. Res. Des. 82 (A12) (2004) 1597–1603. [18] W. Wu, P.A. Dellenback, P.K. Agarwal, H.W. Haynes, Combustion of isolated bubbles in an elevated-temperature fluidized bed, Combust. Flame 140 (2005) 204–221. [19] J. Friedman, H. Li, Natural gas combustion in a fluidized bed heat-treating furnace, Combust. Sci. and Technol. 177 (2005) 2211–2241. [20] D. Kunii, O. Levenspiel, Fluidization Engineering, 2nd eds, ButterworthHeinemann, Boston, 1991. [21] T.M. Leib, P.L. Mills, J.J. Lerou, Fast response distributed parameter fluidized bed reactor model for propylene partial oxidation using feedforward neural network methods, Chem. Eng. Sci. 51 (1996) 2189–2198. [22] L. Wang, P. Wu, Y. Zhang, J. Yang, L. Tong, Temperature distribution and control in liquefied petroleum gas fluidized bed, J. Univ. Sci. Technol. Beijing 11 (2004) 202–206. [23] D.P. Ross, H.M. Yan, D.K. Zhang, An experimental study of propane combustion in a fluidized-bed gasifier, Combust. Flame 124 (2001) 156–164. [24] M. Foka, J. Chaouki, C. Guy, D. Klvana, Natural gas combustion in a catalytic turbulent fluidized bed, Chem. Eng. Sci. 49 (24A) (1994) 4269–4276.
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[25] R. Sotudeh-Gharebagh, R. Legros, J. Chaouki, High temperature combustion of natural gas in a turbulent fluidized bed, Chem. Eng. Sci. 54 (1999) 2029–2037. [26] A. Chehbouni, J. Chaouki, C. Guy, D. Klvana, Characterization of the flow transition between bubbling and turbulent fluidization, Ind Eng. Chem. Res. 33 (1994) 1889–1898. [27] F. Berruti, J. Chaouki, L. Godfroy, T.S. Pugsley, G.S. Patience, The hydrodynamics of circulating fluidized beds: a review, Can. J. Chem. Eng. 73 (1995) 579–594. [28] D. Park, O. Levenspiel, T.J. Fitzgerald, A Comparison of the plume model with currently used models for atmospheric fluidized bed combustion, Chem. Eng. Sci. 35 (1980) 295–397. [29] J. Baron, E.M. Bulewicz, W.Z. Ukowski, S. Kandefer, M. Pilawska, Combustion of hydrocarbon fuels in a bubbling fluidized bed, Combust. Flame 128 (2002) 410–421.
[30] J.F. Stubington, J.F. Davidson, Homogeneous combustion in fluidized beds, AIChE 27 (1) (1981) 59–68. [31] Turnbull E., 1983. The fluidized combustion of char and volatile from coal, Ph.D. Dissertation, Univ. of Cambridge, UK. [32] A.P. Baskakov, K.E. Makhorin, Combustion of natural gas in fluidized beds, Inst. Fuel Symp. Ser., vol. C3, Fluidised Combustion, London, 1975, pp. 1–9. [33] R. Sotudeh-Gharebagh, J. Chaouki, Heterogeneous and homogeneous combustion of natural gas over the inert particles, Can. J. Chem. Eng. 81 (2003) 1182–1191. [34] GRI, Annual report, Gas Research Institute, 1995. [35] Van der Vaart D.R., 1985. The combustion of gas in a fluidized bed. PhD Dissertation, University of Cambridge, UK.
An experimental study of non-premixed combustion in a turbulent fluidized-bed reactor R. Sotudeh-Gharebagh a,⁎,1 , J. Chaouki b , P. Sauriol b a
b
Process Design and Simulation Research Centre, School of Chemical Engineering, University of Tehran, PO Box 11365-4563, Tehran, Iran Chemical Engineering Department, École Polytechnique de Montreal, P.O. Box 6079, Stn. “Centre-Ville”, Montreal, Quebec, Canada H3C 3A7 Received 26 October 2006; received in revised form 31 March 2007; accepted 11 April 2007
Abstract Although the theory of fluidized bed technology is fairly well understood, non-premixed reactions in such beds have recently garnered considerable attention, particularly with respect to combustion applications, which still present a number of practical engineering and operational problems at start-up, during operation and at shutdown. While studies of such processes in fluidized beds remain essential to validating theoretical models, young researchers often perceive them as dirty and tedious experiments due to the complex operational concerns involved. Nevertheless, the planning and implementation of experiments, the analysis of the full array of interactions between system components and bed materials, internal surfaces and measurement devices, and interpretation of results to diagnose abnormal operating conditions present operators with a range of difficult intellectual challenges. Some of the important experimental issues involved in the operation of non-premixed reactions in fluidized beds are presented and discussed. In particular, widening the range of fluidized bed system operations to include direct fuel injection can expand the range of reactant compositions beyond those normally allowed by safety constraints. Therefore, the aim of this study is to understand the behaviour of non-premixed natural gas combustion in turbulent fluidized bed using various spargers under high temperature conditions. The results of this study can also provide insights into how volatiles burn inside fluidized bed reactors. © 2007 Elsevier B.V. All rights reserved. Keywords: Operation; Non-premixed combustion; Sparger; Fluidized bed; Highly exothermic reactions
1. Introduction Fluidized bed reactors are particularly attractive for highly exothermic reactions because of their superior heat management ability, proper mixing and excellent temperature control. Despite such obvious advantages, these systems have not been widely exploited by industry. Combustion of gaseous mixtures, mostly methane–air, has been successfully implemented in fluidized bed reactors [1–18]. Detailed reviews of such studies on premixed fuel combustion in bubbling and volatile combustion in circulating fluidized bed reactors have been presented by [19,18,12].
⁎ Corresponding author. Fax: +98 21 6646 1024. E-mail address: [email protected] (R. Sotudeh-Gharebagh). 1 Sabbatical leave (2006-7) Chemical Engineering Department, Qatar University, Doha 2713, Qatar. 0378-3820/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2007.04.009
In bubbling fluidized bed studies, feed composition was maintained well below the lower explosion threshold. However, such studies have been conducted in small diameter fluidized beds wherein localized data for model verifications cannot be reliably obtained. Two critical transition temperatures exist for combustion, and vary according to the type of fluidized beds (e.g. [19]). Below the lower critical temperature (T1), combustion only occurs above the bed surface, while between (T1) and the higher critical temperature (T2), combustion begins to move into the bed; combustion takes place entirely within the bed when its temperature exceeds T2. However, whether or not combustion occurs within the particulate phase during the transition period as well as after the combustion moves into the bed is still an open question [19]. Use of gaseous mixtures in bubbling fluidized beds under premixed conditions has very limited industrial applications since high throughput is always desired for industrial processes. Such high throughputs can be achieved through moderately
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high superficial velocities (turbulent fluidization) combined with the direct injection into the reactor of higher concentrations of feed within the usual range of flammability. This would result in significant cost savings, due to the greater per-pass conversion and yield [20,21]. Furthermore, given that natural gas availability has increased while emission standards have become more stringent, turbulent fluidized-bed combustion technology has seen an increase in natural gas utilization for a wide range of applications. These include co-firing, gas re-burning, foundry sand reconditioning, incineration of high-moisture sludge, cleaning of metallic parts and direct combustion for heating. Thus, a systematic understanding of the non-premixed gasphase combustion in turbulent fluidized beds is more important that ever before. In a study of liquefied petroleum gas (LPG) combustion in a rectangular pilot bubbling fluidized bed, mixing of LPG and air is found to be difficult because the LPG's high calorific value led to incomplete combustion [22]. Ross et al. [23] reported combustion of coal volatiles, as simulated by propane, and its interaction with char gasification reactions in a bubbling fluidized-bed at 750–950 °C. Foka et al. [24] investigated the catalytic premixed combustion of natural gas (with a poor inlet mixture of 4% methane) in a turbulent fluidized bed at moderately low temperatures (400–600 °C). The full success of this type of bed was strongly dependent upon the nature of the catalyst used, its effectiveness and resistance to attrition as well as corresponding costs. Unfortunately, combustion catalysts are usually expensive and subject to attrition. In order to make turbulent fluidized bed reactors applicable for residential, industrial and waste-to-energy applications and to decrease this cost substantially, catalysts should be replaced by inert materials. Despite the large impact of using inert particles as bed materials in industrial turbulent combustors, limited experimental data are available in the literature regarding the combustion mode and regime of fluidization. Sotudeh et al. [25] reported the feasibility of non-premixed natural gas combustion at moderately high temperatures (b 900 °C) in a turbulent fluidized bed of inert particles with an in-bed CO profile. They used a single gas sparger configuration in injecting natural gas into the bed. Since the rate of combustion of gaseous fuel under non-premixed conditions is limited by the rate of mixing of fuel and oxygen and the chemistry of reaction, these should be thoroughly understood in any experimental and modelling efforts. Beside the moderate superficial velocities occurring under turbulent conditions, the lower region of circulating fluidized bed combustors used in coal combustion, where the volatile matters are largely burnt, operates under turbulent fluidized bed conditions [26,27]. In modelling efforts, combustion of volatile matter is either ignored, assumed to occur close to the distributor, or assumed to occur throughout the emulsion phase [6,28]. However, the contribution of the bubble phase to volatile combustion process is still more poorly understood and requires experimental verification. This issue is of interest for two reasons: (i) with solid fuels, volatiles account for an appreciable proportion of the combustibles [5,16], and (ii) fluidized bed combustion may burn gas streams carrying organic vapours [29] or solid fuels of low calorific value.
In order to make turbulent fluidized bed reactors applicable for highly exothermic non-premixed reactions, Good Experimental Practice (GEP) is needed in order to address key issues. The non-premixed reactions in fluidized beds are partially controlled by the rate of mixing with oxygen at a given temperature. If the mixing is slow, which is the case in bubbling fluidized beds, the reactant bubbles (methane bubbles) may pass un-burnt through the bed and burn above the bed. This decreases the amount of heat released within the bed, leading to higher freeboard temperatures than are desirable. Therefore, the main objective of the present study was to address the key operational issues and mixing behaviour of non-premixed combustion using various sparger configurations in fluidized reactors operating in a temperature range of 400–1000 °C. This would be required for studies of the reaction parameters and system hydrodynamics. Therefore, an extensive experimental
Fig. 1. Experimental unit (a) and the gas injection port (b).
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program is conducted in this study which may lead to a more fundamental understanding of non-premixed combustion. 2. Experimental The pilot fluidized bed reactor used in this study consists of a 200 mm ID and 2 m tall refractory-lined reactor (Figs. 1a and 2). The reactor is divided into three zones: the inlet zone (wind-box and distributor), a fluidized bed, and a freeboard. An external natural gas burner, supplied with 20 kW of power, was located at the bottom of the bed, providing the partial heat required for the reactor to achieve the desired temperature. Fluid Catalytic Cracking catalyst (FCC) (mean diameter of 70 μm, ρ = 1.45 gr/m3) and sand particles (mean diameter of 543 μm, ρ = 2.65 gr/m3) were used during the experiments. Several ports were provided along the axial position and around the circumference of the reactor. Providing great flexibility for detailed in-bed measurements, these ports were used for pressure measurements, sampling and natural gas injection into the reactor. Natural gas (NG) was supplied to the reactor through a secondary injection port (Fig. 1.b) with spargers of different geometries (Figs. 3 and 4). Type-K thermocouples were also placed along the reactor center-line to monitor
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temperature profiles. An absolute pressure transducer was used to monitor the level of particles in the reactor by continuous recording of pressure fluctuations. The bed inventories for FCC and sand particles were 15 and 20 kg, respectively. Normally, each experimental run began with cold and hot hydrodynamic tests; i.e. measurement of minimum fluidization and transition velocities. Minimum fluidization velocities of 0.23 m/s and 0.003 m/s for sand and FCC particles, were measured, respectively, under cold conditions. Due to issues of temperature stability and operational problems, it was not possible to measure the minimum fluidization velocity under high temperature conditions. When bed temperature reached the desired level, NG was injected through the secondary injection port and combustion took place inside the bed. The detailed procedure involved in reactor heating has been reported elsewhere [25]. Local bed structure was characterized by measuring the local concentration and temperature profiles under various operating conditions (Fig. 5a). Gas sampling probes, their tips protected by a glass wool filter (Fig. 5b), were placed along the reactor central axis, and connected to the gas chromatograph where the samples were withdrawn with a variable pressure vacuum system. A number of spargers with different tip geometries were also tested to study their effects on mixing (Fig. 6). The distributor plate or spargers were designed in such a way as to overcome the problem of poor gas mixing in fluidized beds. The pilot plant used
Fig. 2. Detailed schematic of a Pilot Plant Turbulent Fluidized Bed Reactor.
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Fig. 3. Spargers used in experimental studies. was very flexible and could be quickly modified to operate under a wide variety of operating conditions. The particular design chosen for this study, (i) facilitated the injection of air and NG at different locations in the bed, (ii) provided operating temperatures up to 1000 °C, and (iii) allowed a turn down or complete shut down of gas and air flow without problems arising from backfilling of the distributors with particles. The experiments studies were repeated several times in order to check the reproducibility of all observations and measurements.
3. Results and discussion 3.1. Reactor operational issues The typical duration of individual experimental runs are presented in Table 1. Various problems including attrition and
Fig. 4. Detailed scheme for spargers C and D (Spargers A and B have the same tip as sparger C).
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Fig. 5. Gas sampling scheme (a) and Probe tip (b).
Fig. 6. Sampling probes, gas spargers (a) and distributor (b).
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erosion, and improper operation may occur during an experimental run, such that special attention is required to overcome these issues and ensure proper operation. Occurring in many locations and through a number of mechanisms, the phenomenon of particle attrition in fluidized bed reactors is an important one, since it alters the reactor solid inventory. For the fluidized bed reactor used in this study, two instances of difficulty in maintaining control over the bed level occurred. In the first case, starting preheating experiments at minimum fluidization based on cold conditions, led to a sharp dynamic decrease in bed level due to thermal break-up of bed materials. In the second case, attrition was attributed to the purging system. During the preheating period of non-premixed experiments, air had to be used to keep the natural gas injection sparger clean. However, the introduction of air, even at a low rate caused a highly turbulent jetting area at the injection site, rendering bed inventory control difficult. This suggested that if bed materials were subject to attrition, jetting conditions should be prevented inside the bed by placing shrouds around grid holes so as to reduce the velocity at the gas–solid interface to below 30 m/s. For these cases, continuous solid injection and bed inventory control solved the problem. In all experiments, the bed level was closely controlled. The typical bed pressure signal was 16 ± 5 mA indicating a constant bed inventory. Given that attrition and abrasion may damage sampling probes and injection systems, probes should be carefully inspected (Table 1) and the fact that gas jets can be serious sources of erosion in fluidized beds should be taken into account. Under the experimental conditions, the erosion caused by jet impingement as a result of the creation of a highly turbulent area around the sparger required that the probe be changed frequently. Improper shutdown and operation of the bed may damage the sampling probes and distributor (Fig. 6). In order to avoid undesirable damage at high temperatures, a gradual cooling of the bed (Table 1) is mandatory before a full shutdown. 3.2. Experimental results and discussion For all experiments, conversion was 100% at the top of the bed. For a given temperature, the rate of gaseous fuel combustion in fluidized beds is normally assumed to be limited by the rate of mixing of fuel and oxygen [28,30]. However, our experimental results show that the amount of over-bed burning
increases rapidly as the bed temperature drops below a given critical value for the premixed combustion. This is consistent with the work reported in other studies [5,14,31,32].. As temperature is increased, the 100% conversion point moves inside the bed as a result of free radical formation dominating the inhibition process caused by the emulsion phase [5,6]. Since the combustion is inhibited by the removal of either radicals or heat, the combustion within the emulsion phase begins to occur at temperatures well above those required for bubble burning inside of the bed [6]. For non-premixed combustion to occur successfully in a fluidized bed reactor, both mixing and temperature requirements must be met. Three combustion regimes were identified: (i) at low bed temperatures (b 700–750 °C), the fuel likely did not burn within the bed; (ii) at moderate bed temperatures (about 825 °C), the fuel likely only burned in the bubbles; and (iii) at high bed temperatures (N 900 °C), combustion occurred within the bubbles and in the particulate phase of the bed. These regimes are consistent with those reported in other similar studies [6,12,19]. As mentioned before, sand particles within the emulsion phase would decelerate the reaction process considerably. In order to simulate the contribution made by these particles, separate experimental and modelling studies were conducted in a fixed bed reactor. Details of the study have been reported by Sotudeh and Chaouki [33]. In Fig. 7, the experimental data obtained in a fixed bed reactor of sand particles was compared to predictions generated by the plug flow model developed with the Gas Research Institute's (GRI) homogeneous methane combustion mechanism [34]. As seen in the figure, the model overpredicted the experimental data. The difference between model and the experimental data is attributed to the deceleration effects caused by the presence of sand particles. The figure also shows that sand particles may lower conversion at lower temperatures (related to low experimental conversion). However, at the higher temperatures (N 900 °C) (related to high experimental conversion), sand inhibitory effects seem negligible, as is also reflected in other studies [5,11,18,35]. Given its wide availability and lesser cost compared to other bed materials (alumina and combustion catalysts), sand would therefore be an appropriate choice of bed material when developing a novel reactor for the wider applications envisaged in the industry. Axial temperature profiles in the bed under various bubbling and turbulent fluidization conditions are shown in Fig. 8. It is
Table 1 A typical experiment duration Action
Activities/measurement
Time
Preparation Preheating Stable fluidization Data acquisition
Bed inspection, device preparation and installation, loading Bring the bed from ambient temperature to 800 °C To ensure the stable operation at the temperature desired Gas analysis (5 probes, 9 measurements/probe, 20 mpm a) Pressure fluctuation (during the gas analysis) Temperature profile (3 probes, 12 measurements/probe, 3 mpm) Bring the bed from the reaction temperature to 400 °C (to prevent damaging the distributor) Analyze the data and generate the corresponding graphs
6h 14 h 2h 14 h – – 4h – 40 h
Cooling down Data analysis Total time required for each experiment a
Minutes per measurement.
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Fig. 7. Parity conversion plot for the emulsion phase at the temperature ranged 650–1000 °C (XCH40 = 2%, η = 107, τ = 3 and dp = 543 μm).
worthy to mention that the experimental transition velocities from bubbling to turbulent were measured experimentally as 0.7 m/s and 1.5 m/s for FCC and sand particles, respectively. Radial temperature measurements showed the temperature fluctuations in the fluidized bed to be small under all operating conditions. The standard deviation of axial temperature profiles under bubbling condition varied from 5 to 21 °C, while the deviation under turbulent conditions varied from 3 to 12 °C. This means that the bed can easily be operated under isothermal conditions, because of the intense and rapid solids mixing under turbulent conditions. The temperature measurements reported in this study are consistent with those reported by [22], where the maximum temperature fluctuation was reported to be less than 5 °C. The wind-box temperature remained near 200 °C throughout, preventing any damage to the distributor and allowing the safe operation of the reactor (Fig. 8).
Fig. 9. Methane concentration profile around the sparger for FCC at 820 °C, U = 0.8 m/s and Uj = 56 m/s.
A plot of methane concentration at the injection point is presented in Fig. 9. This figure shows that upon introducing methane into the bed through the sparger, a reducing atmosphere is generated around the sparger, where methane is consumed within a short distance of the sparger. The methane coalesces with the air bubble to form a less rich methane bubble. The repeatability (standard deviation) of methane measurements are presented in Table 2. As seen in this figure, the methane concentration at 23 mm from the sparger tip was considerably greater than that at − 2 and 74 mm. The high concentration of methane around the sparger wall can be attributed to the fact that bubbles, as formed at the sparger tip, end up forming a train of bubbles around the sparger wall. Under these conditions, the mixing is limited to the sparger wall itself. As soon as the bubbles reach the sparger level, due to the intense gas–solid mixing which occurs under the turbulent fluidization regime, the tracer gas is very well purged to the emulsion phase. Radial symmetry was exhibited under turbulent fluidized bed conditions due to the intensive mixing of solids as shown in Fig. 10. It might be very difficult and tedious to establish such radial symmetry with bed-mixing in bubbling fluidized bed conditions because of vigorous bubble passage through the bed and their heterogeneous behaviour. The conditions for this figure are the same as those reported for Fig. 9. A radial concentration profile of CO at different axial positions around the sparger is shown in Fig. 11. The figure shows a
Table 2 Standard deviation for methane measurements Methane concentration (% mole)
Fig. 8. Temperature profile of the pilot fluidized bed reactor under turbulent bed conditions (U = 1.5 m/s) and bubbling fluidized bed condition (U = 0.7 m/s) for sand particles.
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0.3321 ± 0.0935 0.6611 ± 0.1521 1.7000 ± 0.3439 2.4600 ± 0.1878 3.5300 ± 0.3365
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Fig. 10. Symmetrical methane concentration profile around the sparger for FCC at 820 °C, U = 0.8 m/s and Uj = 56 m/s.
Fig. 12. Oxygen concentration profile around the sparger for FCC at 820 °C, U = 0.8 m/s and Uj = 56 m/s.
high CO concentration at 23 mm from the sparger tip, where CO bubbles are protected by the sparger wall. Thus, due to inhibition phenomena, CO-rich bubbles were formed. For low bed temperatures, an observation of the CO profile in the reactor leads to the conclusion that, due to the quenching effect of the emulsion phase, CO and volatile materials may burn in the freeboard or inside bubbles. Unlike other fluidized bed applications where coalescence of the bubbles is an undesirable feature, in non-premixed air-fuel combustion mode, this promotes effective mixing of unlike bubbles to form CO bubbles, which are then easily ignited if the bed temperature is sufficiently high. Furthermore, at the injection point, the distributor bubbles are generally small and consequently mixing of gas and air would effectively occur at the vicinity of the gas distributor.
The oxygen profile around the sparger is shown in Fig. 12. The figure shows that a reducing zone is formed around the sparger similar to that in the dense bed of the circulating fluidized bed combustors. As the bubble moves upward in the vicinity of the sparger wall, the oxygen shortage becomes critical. The figure also confirms the experimental findings reported in Figs. 9–11. Upon injecting a high velocity jet into the fluidized bed reactor, the creation of high turbulent area results in all the oxygen found in the immediate vicinity of jet being rapidly consumed in forming CO bubbles. It was initially assumed that methane injection through the sparger would lead to a hot spot at the vicinity of the sparger, but Fig. 12 shows this assumption not to be valid. In order to confirm isothermicity around the sparger, the temperature profile around the sparger was measured as shown in Fig. 13, and was seen to vary little around the injection point. What little
Fig. 11. CO concentration profile around the sparger for FCC at 820 °C, U = 0.8 m/s and Uj = 56 m/s.
Fig. 13. Temperature profile around the sparger.
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temperature variation occurs around the sparger is related to the rise in gas temperature. It seems when a single step reaction is chosen, a hot spot may form around the sparger if the heat of reaction exceeds the amount transferred to the surrounding medium by different heat transfer mechanisms. However, the hypothesis should be verified experimentally. This could have large practical applications, particularly with respect to hazardous waste destruction. Besides the previously mentioned temperature requirements, safe operation of the fluidized bed reactor in non-premixed mode requires a proper sparger or injection system. The hydrodynamics at the injection point and the geometry of the sparger would play a very important role in promoting intense gas–solid mixing. Four different sparger tips were used to study the injection point hydrodynamics and the effect of sparger geometry thereon. Plots of the mixing behaviour under different sparger designs at 400 °C (Fig. 14), show, as expected, that sparger geometry significantly affects mixing, the controlling step in non-premixed combustion. Some spargers tip designs allow the spreading of bubbles, with a concomitant enhancement of mixing. Four mixing zones could be identified around the sparger. At the injection point, level with the axial position of − 2 mm, the tracer concentration is low showing the bubbles tendency to move up. Depending on the tip of the sparger used, some bubbles are spread downward, providing partial mixing. At 23 mm from the sparger tips, bubbles are seemingly moving close to the sparger walls and well protected. This is particularly evident with the sparger B which shows high methane concentrations, whereas for sparger D, bubbles exiting the sparger spread downwards into the fluidizing medium, resulting in better mixing. Normally in this zone, the wall protects the bubble and it is only when it passes the level of the sparger base that the mixing is improved (zone 3, Fig. 14). In zone 4, the bubbles from all spargers reach the splash zone and no geometry effects would be identified here.
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3.3. Good experimental procedure (GEP) Based on the comprehensive experimental program undertaken, operation of non-premixed highly exothermic reactions (HER) in fluidized bed reactors under GEP requires the following parameters to receive particular attention: 3.3.1. Proper unit preparation The experimental unit should be carefully prepared and inspected before start-up by cleaning all probes, checking all measurement devices and their calibration. 3.3.2. Cold experiments In order to study the limitations associated with the fluidizability of the particles and the bed ability to withstand the abrasion associated with solid fluid movement, the fluidization experiments should be conducted in a cold unit. 3.3.3. Probe and sparger maintenance The measurement probes and spargers should be flushed with bottled oxygen-free nitrogen N2 upon loading the bed materials in order to keep them solid-free. The experiments are started by heating up the reactor contents. Usually a porous sintered steel plug fitted in the tip of the probe permitted the collections of gas samples, free of sand and dust. In this study, glass wool is placed at the tip allowing the proper dust-free sampling. 3.3.4. Leak development This would be mostly likely cause of accidents. A very careful and serious inspection should be performed prior to and during each experiment. 3.3.5. Start-up A systematic methodology should be followed in the operation of combustion reactions in order to obtain the temperature level desired [19,25]. 3.3.6. Particle heating: two operation modes exist for particle heat up 3.3.6.1. flame-based particle heating. This would be done by the gradual addition of sand though a well-designed solid injection system [19]. In this case, the empty chamber was preheated to a temperature above 600 °C by the ignition of a NG–air mixture inside the bed. Sand was slowly added to the unit while maintaining the temperature well above the light-off limit, until the operating temperature of 800 °C was reached. By then the superficial velocities were adjusted. By increasing the sand particles and adjusting gas superficial velocity, a point would be reached where the flame disappeared.
Fig. 14. Mean methane concentration around the spargers at 400 °C and Uj = 36 m/s.
3.3.6.2. non-flame based particle heating. Particles are loaded into the bed and combustion would be forced inside the bed through under-bed (wind-box) and over bed combustion. As used in this study, this technique would be more efficient in heating up the bed contents. Direct gas injection at different
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levels would be needed in order to start up the reactor. In/over bed gas firing is used in order to raise the bed temperature to either an intermediate level or to an elevated level depending on the operating conditions. 3.3.7. Bed level measurement A catalytic reaction in a fluidized bed may “run away” if the fines content or porosity of the particles increases due to a change in the source of raw material. Therefore, the bed level should be closely monitored and controlled. Attrition may alter bed inventory, and this may, in turn, create a number of problems at high superficial velocities if the cyclone performance is inadequate to capture the particles and recycle them back to the bed. Sometimes a two-stage cyclone may be required. This is also a serious problem for in-bed particle heat up, due to the superficial gas velocity increase during temperature rise-up. 3.3.8. Choice of operating conditions and stable fluidization Highly exothermic reactions are quite rapid. The proper gas– gas–solid efficiency makes the process continuous and robust, thus greatly reducing the potential hazard from this extremely hazardous reaction. Some difficulties may initially occur in maintaining the fluidization for any length of time due to attrition, but the addition of bed materials could eliminate the problem. 3.3.9. Gas accumulation within the reactor To reduce the risk associated with the exothermic reactions in a fluidized bed reactor, the natural gas is fed in gradually so as to control the heat generated by the chemical reaction. In practice, the added component may not be immediately consumed and would partly accumulate and get mixed into the bed, depending on the hydrodynamic conditions. The amount accumulated would be a direct measure of the hazard potential. From the point of view of safety, an accurate selection of operation and design parameters is required to ensure a minimum accumulation. If the bed is carefully operated, a smooth and stable temperature profile is obtained in the reactor. This is easy when handling the single-step reactions. 3.3.10. Multiple reactions The undesirable runaway reactions often encountered in practice (i.e. methane reaction) are generally caused by multiple reactions. The rate of these reactions would be negligible at initial conditions but may become significant at higher temperatures. In fluidized bed reactors, the control of hydrodynamic conditions would be a very effective means of controlling the reactions at different steps of the process, thus leading to an inherently safe operation. 3.3.11. Runaway reactions In order to prevent a runaway, the reaction zone should be located inside the bed. Situations could occur under which the reaction zone is either at the entrance or at the exit of the reactor, leading to serious operation issues. In case of the two-step reactions considered in this study, complications
arise and one has to discern between the heat production rates of the different reactions. Since the methane combustion scheme is rather complex, the interaction of parameters in a multiple reaction system makes the development of an unambiguous criterion impossible. However, with a proper temperature measurement around the sparger, this issue would be easily addressed. To guard against accumulation of a flammable mixture in the wind-box, gas detectors and alarms should be fitted to the reactor and the surrounding media. 3.3.12. Burning back As the distributor temperature approaches the bed temperature, the reaction may spread back into the wind-box. Under these conditions, the fuel is immediately shut off in order not to destroy the plate and the gas is then injected through the spargers located within the bed. The highly exothermic reaction is safe provided that it is located within the bed, as burn-back cannot occur in this mode. If the degradation and attrition of the sand during the test is significant, a close control of the bed inventory would be needed to attain stable fluidization and to prevent burn-back conditions. 3.3.13. Temperature and reaction control The temperature of the reactor at different axial and radial positions, in the feed unit and surrounding the reactor are measured by thermocouples. When the reactor temperature exceeds a certain unacceptable level, the gas injection flow is lowered to reduce the temperature. During the experiments, gas samples are taken automatically for later analysis. Successful long term pilot operation requires careful mapping of the operating procedure, careful control over reactions and rapid corrective action as needed. 3.3.14. Safe operation The process can be regarded as invariably safe when no runaway reactions can occur at any temperature, under any given operating conditions. This can be achieved when the reaction zone is located inside of the bed. The location of the reaction zone is very critical and sensitive to small changes in the bed temperature. By adopting GEP, a few experiments would be sufficient to establish conditions for large-scale reactor operations to be invariably safe. To this end, one has to avoid accumulation of the reactant in the reaction zone and outside of the bed and wind-box. In methane combustion, CO accumulation would necessarily occur and proper actions would be required to prevent its accumulation. 3.3.15. Shutdown Conducting high temperature reactions in fluidized beds is often associated with problems, such as a tendency for the bed to agglomerate and collapse during the shutdown of the reactor as well as resultant damage to the distributor. Therefore, it is necessary to blow the bed and keep it fluidized by lowering the gas superficial velocity, which will in turn provide sufficient time for bed materials to cool down.
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3.3.16. Environmental concerns Highly exothermic reactions in fluidized bed reactors are characterized by a gaseous exit stream carrying vapours and dusty materials. The gases as well as the dusty materials are often hazardous to health or outright poisonous. In addition to enhanced efficiency cyclone and dust collection systems, precautionary systems are needed in order to protect the operators and operation environments. Particulates smaller than 10 μm in diameter are often picked up by relatively gentle air currents, do not settle quickly, and may cause damage to eyes and lungs. This is a particular issue if the materials are toxic, corrosive, radioactive or biologically active. 4. Conclusions A non-premixed reaction mode was implemented using fluidized bed technology using various spargers. These experiments served to establish a strategy for the operation of highly exothermic reactions in fluidized bed reactors. This study shows that methane, when injected into the bed, is initially converted to CO as an intermediate component, with the full reaction of CO to CO2 occurring inside of the bed. Three nonpremixed combustion regimes, similar to those occurring under premixed combustion were observed: (i) at low bed temperatures (b700–750 °C), the fuel likely does not burn within the bed; (ii) at moderate bed temperatures (about 825 °C), the fuel likely only burns in the bubbles; and (iii) at high bed temperatures (N 900 °C), combustion occurs within the bubbles and in the particulate phase of the bed. Non-premixed combustion was shown to allow the expansion of the safe range of operation and improve reactor performance and that an increase in the number of injection points would generally be helpful. At the injection point, 4 mixing zones were identified, ranging from poor to completing mixing, depending on the sparger type employed. The outcome of this study may also have implications for the combustion of gaseous fuels and volatile matters in the lower region of fluidized bed coal combustors. Nomenclature D Reactor diameter (m) dp Mean particle size (μm) FCH40 Initial methane molar flux (mol/s) T Temperature (°C) U Superficial gas velocity (m/s) Uj Sparger jetting velocity (m/s) XCH40 Initial methane mole fraction (−) z Height along the bed (m) τ Mean residence time (s) η Contact time index (W/FCH40) (g s/mol) Acknowledgments The University of Tehran, Iran, is gratefully acknowledged for granting sabbatical leave to R. Sotudeh. The comments and fruitful discussions with Dr. Navid Mostoufi from the University of Tehran are highly appreciated. The authors are
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