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Elemental Mercury Vapor Capture by Powdered Activated Carbon in a Fluidized Bed Reactor

F. Scala1, R. Chirone1, A. Lancia2 1. Istituto di Ricerche sulla Combustione - C.N.R., Naples – ITALY 2. Dipartimento di Ingegneria Chimica - Università Federico II, Naples - ITALY

1. Introduction

Although mercury emissions occur naturally, major pollution is caused by human activities. Recent studies recognized that about 70-85% of the total anthropogenic mercury emissions are caused by combustion sources, mainly coal-fired utilities and waste incinerators [1,2]. At typical combustion temperatures, mercury is readily vaporized and exits the combustion chamber with the flue gas mainly as elemental mercury. Contrary to other trace metals, mercury does not undergo condensation and passes through traditional air pollution control systems with very low capture efficiency [3]. Elemental mercury emitted in the atmosphere is slowly oxidized and solubilized in rainwater and transported to soil, rivers and lakes where it is transformed into more toxic organic species and assimilated by animals and plants [4]. One of the most effective techniques for the removal of elemental mercury vapor from combustion flue gas relies on the use of activated carbon as an adsorbent at relatively low temperatures (120-150°C) [1,5]. This technique requires a large gas-solid interface area and a significant contact time. However, industrial practice has shown that the most economically attractive contact design is the direct in-duct pneumatic injection of powdered activated carbon, with the subsequent collection in a particulate matter control device (fabric filter or electrostatic precipitator). This design provides a very low contact time between the flue gas and the adsorbent in the ductwork, of the order of a few seconds. In the case that the particulate matter control device of the plant is an electrostatic precipitator, this results in a low utilization of the activated carbon [5]. In the present work the possibility to use a bubbling fluidized bed upstream of a particulate matter control device to increase the utilization of activated carbon injected in the flue gas ducts is explored. The starting point is the experimental observation that a bubbling fluidized bed of coarse granular material may act as a filter capable of increasing both the residence time and the specific gas-solid contact surface of a powder carried by a gas stream [6]. On the other hand, operation of the fluidized bed, as opposed to a fixed bed, can be easily carried out in a continuous way.

2. Experimental

Mercury capture experiments have been conducted in a lab-scale pyrex reactor 65 mm ID and 1 m high (Fig. 1). The lower section of the column is filled with pyrex cylinders and acts as a gas preheater. Above this section a porous pyrex plate is placed as a gas distributor. The upper section of the column represents the mercury adsorption zone and is equipped with three sampling ports at 30, 40 and 50 cm above the distributor. The whole lower section and the first 30 cm of the upper section are heated by two electric furnaces. The temperature is measured by means of a thermocouple placed 20 mm above the distributor and is kept constant by a PID controller. The mercury containing gas stream is produced by evaporating liquid mercury into an air stream in a thermostat-regulated glass saturator. The stream is further diluted with air before entering the preheating section, to obtain the desired inlet mercury concentration.

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Fig. 1 Experimental apparatus.

The powdered activated carbon feed is generated in a separate plexiglass 40 mm ID sound-assisted bubbling fluidized bed elutriator. The fluidizing air flow in the elutriator is adjusted to obtain the desired carbon flow rate. A linear relation between the fluidizing air flow and the carbon elutriation rate was found experimentally. The elutriated flow is then diluted with air in a venturi nozzle and is fed pneumatically into the pyrex reactor. The flow enters the reactor tangentially just above the porous distributor through a semicircular Teflon duct to impress a swirling motion to the activated carbon particles. Measurement of the powdered carbon feed rate is carried out by isokinetic sampling of the flue gas exiting the reactor, by means of high capture efficiency cellulose filters. A small stream of flue gas is also sampled continuously for mercury concentration measurement through the lower sampling port (30 cm). The gas is first passed through a Teflon-coated micro-cyclone (2.5 !m cut-size at 3 lt/min) and a washing bottle containing distilled water for particulate removal and then is sent to a continuous CVAA elemental mercury analyzer (Mercury Instruments VM3000). The remaining flue gas exiting the reactor is sent to a final purification stage, consisting in a plexiglass bubble column containing a KMnO4 aqueous solution acidified by H2SO4, and subsequently to the stack. The reactor column was operated either as a fluidized bed or as an entrained bed, for comparison. In the first case the upper section was filled with 300 g of quartz particles (212 – 400 !m), corresponding to an unexpanded bed height of 6.5 cm. In the second case the column was left empty. The reactor was kept at 100 °C and the mercury inlet concentration was 90 – 95 !g m-3. The total superficial gas velocity in the column was fixed at either 0.15 or 0.32 m s-1, and the activated carbon feed rate was varied in the range 5 – 12 g h-1.!A widely used commercial powdered activated carbon (Darco FGD – lignite derived) was selected for the tests. The carbon particle size distribution was characterized by means of a laser particle size analyzer (Malvern Mastersizer 2000). The carbon powder had a relatively broad size distribution (0.1 - 80 !m) with a Sauter mean diameter of 8.2 !m. The activated carbon adsorption capacity was separately characterized at 100°C in a fixed bed apparatus under conditions similar to the fluidized/entrained bed experiments. Details of the fixed bed apparatus and experimental procedures are reported elsewhere [7]. Carbon saturation was

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relatively rapid, showing 50% residual mercury capture efficiency after about 2 min and 20% efficiency after about 20 min.

3. Results and discussion

Mercury capture in the fluidized/entrained bed experiments can be analyzed in the light of the following phenomenological framework: the activated carbon injected in the reactor accumulates in the bed and is elutriated from the bed with the flue gas. At the steady state the inlet and the outlet carbon flow rates must be equal. In these conditions there is no accumulation in the reactor, where a steady carbon loading (Wc) establishes. In principle the total carbon loading within the reactor can be divided into two contributions: a free carbon loading consisting in the activated carbon transported with the gas stream, and an adhered carbon loading consisting in the activated carbon attached by Van der Waals and/or electrostatic forces on the reactor walls, gas distributor and, if present, fluidized bed particles. Figure 2A compares the typical measured transient activated carbon elutriation rate from the reactor as a function of time in experiments carried out in either entrained bed or fluidized bed configuration, at the same operating conditions. At time zero the activated carbon feed is started. The curves show an increasing trend of the elutriation rate with time until a steady value (5.2 g h-1) is reached. This steady value (equal to the feed rate) is the same under both operating conditions, indicating that the presence of the quartz bed does not influence appreciably the carbon feed. It is noted that a 15 – 20 min time interval is needed to reach steady conditions in the entrained flow configuration, as opposed to a figure of 40 – 50 min when the fluidized bed is present. This result is clearly the consequence of the larger surface area provided by the inert bed particles and available for the activated carbon adhesion/deposition in the latter case. Figure 2B shows the measured transient activated carbon elutriation rate from the reactor as a function of time, after the steady carbon feed is suddenly stopped (at time zero). As expected, the curves show a decreasing trend with time, with a 15 – 20 min characteristic time in the entrained flow configuration, and a 40 – 50 min characteristic time when the fluidized bed is present. Notably, these figures are the same as those found for the initial transient period, when the carbon feed is started (Fig. 2A). The characteristic time is defined as the time for 98% decrease (or increase) of the carbon elutriation rate from the initial level to the steady one.

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Fig. 2 Carbon elutriation rate as a function of time after feed start up (A) and feed stop (B),

under entrained bed and fluidized bed conditions. T = 100°C; gas velocity = 0.32 m s-1; activated carbon feed rate " 5.2 g h-1.

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Even if it is not possible with the present data a separate estimation of the free Wc and adhered Wc contributions, some qualitative indications can be given. The free Wc, representing the carbon transported by the gas stream and not interacting with the reactor surfaces, should have a very short residence time within the reactor, of the order of seconds or tens of seconds at most. On the other hand, curves in Figs 2A and 2B show that the characteristic time of the elutriation process is of the order of tens of minutes. This suggests that the contribution of the adhered Wc to the total carbon loading should be important, not only in the fluidized bed case (as expected), but also in the entrained bed experiments. This indicates that for a small lab-scale reactor, as the one used here, the influence of the walls is non-negligible in determining the residence time of fine carbon particles and that the present entrained bed experiments are probably not representative of full scale conditions, where the influence of wall interactions is lower. Figure 3 reports a comparison of the typical transient mercury vapor concentration measured in the flue gas as a function of time in experiments carried out in either entrained bed or fluidized bed configuration, at the same operating conditions. After the activated carbon feed is started, the mercury concentration in the flue gas decreases reaching a steady value after a time interval comparable to that observed for the elutriation rate curves (Fig. 2A). This indicates that under both operating conditions mercury removal is strictly connected to the activated carbon adhesion/deposition dynamics on the available surfaces in the reactor. Noteworthy, under fluidized bed operating conditions the final steady mercury concentration is lower than for the entrained bed case, i.e. the mercury capture efficiency is larger. After the steady mercury concentration is reached, the activated carbon feed is suddenly stopped (Fig. 3), and the mercury concentration in the flue gas increases reaching a final value slightly lower than the inlet mercury one. The difference between the final outlet mercury concentration and the inlet one is to be ascribed to the residual mercury capture activity of part of the adhered activated carbon that is not elutriated, but remains permanently attached to the reactor surfaces. Again, the time scale of the transient process is comparable to that observed for the elutriation rate curves.

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Fig. 3 Mercury concentration as a function of time in experiments conducted under entrained bed and fluidized bed conditions. T = 100°C; gas velocity = 0.15 m s-1; carbon feed rate " 5.2 g h-1.

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Activated carbon feed rate, g/h4 6 8 10 12

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Fig. 4 Steady mercury capture efficiency as a function of the activated carbon feed rate at

U = 0.15 m/s (A) and U = 0.32 m/s (B), under entrained bed and fluidized bed conditions. T = 100°C.

Figures 4A and 4B report the mercury capture efficiency at the steady state as a function of the activated carbon feed rate in experiments carried out in either entrained bed or fluidized bed configuration, at two different gas superficial velocities. The analysis of the two figures reveals the following trends: a) the Hg capture efficiency increases with the activated carbon feed rate; b) the Hg capture efficiency increases when the gas superficial velocity decreases; c) the fluidized bed performance is always better than the entrained bed one. The first two trends are easily explained if one considers that by increasing the activated carbon feed rate (at the same gas velocity) or by decreasing the gas superficial velocity (at the same carbon feed rate) the activated carbon to mercury ratio inside the reactor increases. As regards the third trend, in the light of the elutriation rate results shown in Figs 2A and 2B, the mercury capture experiments suggest that the adhered Wc contribution is the key factor for enhancing the mercury capture efficiency in the reactor. In fact, the free Wc is characterized by a very short residence time, and it is unlikely to contribute extensively to mercury capture. The adhered Wc, instead, has a residence time of the same order of the saturation time, and it is the fraction of activated carbon that will be most extensively exploited. As a consequence, the increased surface area available for activated carbon adhesion/deposition and, in turn, the larger activated carbon loading in the reaction zone in the fluidized bed configuration, are likely to explain the better performance with respect to the entrained bed. Moreover, in examining the entrained bed data one has to keep in mind that the influence of the walls is non-negligible in determining the residence time of the carbon particles, as explained before. In a real full scale injection process (where the influence of the duct walls is lower) the mercury capture efficiency is likely to be significantly lower than in the present lab-scale apparatus.

4. Conclusions

Mercury capture experiments have been carried out in a lab-scale pyrex reactor operated either in the fluidized bed or in the entrained bed configurations. Powdered activated carbon was continuously injected in the reactor and both mercury concentration and carbon elutriation rate were followed at the outlet. Results showed that the presence of a fluidized bed led to an increase of the mercury capture efficiency with respect to the entrained bed configuration. This was explained by the larger activated carbon loading and gas/solid contact time that establishes in the reaction zone, because of the increased surface area

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available for activated carbon adhesion/deposition in the fluidized bed. The presence of both free and adhered carbon in the reactor and the interplay of phenomena like attachment and detachment of the carbon particles to/from the bed particles and wall surfaces as well as mercury saturation of the adsorbent determine the overall mercury capture efficiency in the reactor.

5. Acknowledgements

The experimental support of Ms. D. Karatza, Mr. D. Scilla, Mr. D. Foschini and Mr. A. Cammarota is gratefully acknowledged.

6. References

1. EPA: Mercury study report to congress. Report EPA-452/R-97-003 (1997). 2. Pacyna, J. M., Muench, J.: Anthropogenic mercury emission in Europe. Water, Air &

Soil Pollution, 56:51-61 (1991). 3. Sandelin, K., Backman, R.: A simple two-reactor method for predicting distribution of

trace elements in combustion systems. Environmental Science & Technology, 33:4508-4513 (1999).

4. Lindqvist, O.: Fluxes of mercury in the Swedish environment: contributions from waste incineration. Waste Management & Research, 4:35-44 (1986).

5. Scala, F.: Modeling mercury capture in coal-fired power plant flue gas. Industrial & Engineering Chemistry Research, 43:2575-2589 (2004).

6. Chirone, R., Salatino, P.: Fines residence time in ordinary or sound assisted fluidized beds. Proc. of the 3rd World Congress on Particle Technology , Brighton, UK, pp. 3257-3268 (1998).

7. Karatza, D., Lancia, A., Musmarra, D., Pepe, F.: Adsorption of metallic mercury on activated carbon. Proceedings of the Combustion Institute, 26:2439-2445 (1996).