Characteristics of Fluidized Bed Combustion with Intermittent Feeding

Article pubs.acs.org/EF

Characteristics of Fluidized Bed Combustion with Intermittent Feeding Using Woodblocks and Rubber. 2. Pollutant Emissions Feng Duan,† HouPeng Wan,‡ YunLong Han,§ ChienSong Chyang,*,∥ HsuehJung Chen,∥ and Jim Tso⊥ †

School of Metallurgy and Resource, Anhui University of Technology, Ma’anshan 243002, China Green Energy and Environment Research Laboratories, ITRI, Hsingchu, Taiwan 31040, R.O.C. § School of Civil Engineering and Architecture, Anhui University of Technology, Ma’anshan 243002, China ∥ Department of Chemical Engineering, Chung Yuan Christian University, Chungli, Taiwan 320, R.O.C. ⊥ R&D Center for Environmental Technology, Chung Yuan Christian University, Chung-Li 320, Taiwan, R.O.C. ‡

ABSTRACT: Woodblocks and rubber balls are used as the fuels to simulate the combustion behavior and pollutant emission characteristics of municipal solid wastes and hazardous wastes in a fluidized bed combustor with an intermittent feeding system. Silica sand is used as the bed material with a mean density of 2500 kg/m3 and a particle size range of 400500 μm. Effects of operating parameters, i.e., in-bed stoichiometric oxygen ratio (ISOR), static bed height, and feeding interval on the CO and NOx emission concentrations are investigated. Different fuels show different pollutant emission characteristics in this study. For rubber ball combustion, the mean CO concentration in the flue gas increases with the ISOR and feeding interval and decreases with higher static bed height. There is a reverse dependency between the CO and NOx concentrations at the furnace exit. For woodblock combustion, the mean CO concentration decreases slightly with higher ISOR and static bed height and increases slightly with higher feeding interval. The dependency between CO and NOx concentrations is unobvious.

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mass transfer efficiencies, and low pollutant emissions, etc.7−9 Batch feeding has been widely used in fluidized bed combustor.10,11 However, there are few studies about the pollutant emissions of batch fluidized bed combustion. Baron12 et al. conducted experiments on polymer pellets incineration in a lab-scale bubbling fluidized bed. They observed that emission of CO and CxHy increases with the mass of batch-fed polymer pellets. The feeding interval time has little effect on the NOx emission concentration, which is more related to fuel-N content in the fuel. Charlton13 and Haddadin14 investigated the release of SO2 from combustion of Stuart spent shale and oil shale using a fluidized bed batch technique at 600−900 °C. Some model sulfur containing compounds were combusted to give time-release data. It was shown that sulfur retention occurred at all temperatures, with the lowest sulfur dioxide release at 750 °C. The rate of sulfur release is principally reaction rate controlled above 750 °C and diffusion controlled below 700 °C. Because SO2 can be removed by limestone in fluidized bed, the sulfur content of woodblocks used in this study is close to zero. Therefore, we did not investigate the SO2 emission. The feeding materials are usually pretreated to raise burning stability before injected into a fluidized bed combustor. However, batch feeding is required for solid materials like medical waste enwrapped in packages to prevent toxins from releasing to the environment.2 There are many kinds of solid municipal wastes and medical wastes such as rubber (small particle size, high fixed carbon content) and biomass (big

INTRODUCTION The volume of municipal solid wastes (MSW) and some hazardous wastes such as medical wastes has been growing exponentially in modern times. The incineration of MSW can minimize contamination, and the heat generated from it can be a good source of renewable energy. Incineration can also be used to treat the medical wastes; however, the incineration of hazardous wastes could produce pollutants which become public health and environmental hazards if disposed improperly.1−3 Batch feeding has been widely used in dealing with different kinds of wastes to prevent pollutants from escaping the feeding devices. Song4 et al. conducted experiments of hazardous waste combustion in an intermittently fed rotary kiln. The results indicated the CO concentration rises and falls during oxygen enrichment (2−8%). Low level of oxygen enrichment has little effect on the NOx formation. Hart5 found that semibatch feeding caused localized oxygen deficiency in combustor. It was estimated that the batch-fed combustion contributes up to 7− 18 times more pollutant emissions than steady-state combustion. Jangsawang2 et al. investigated the batch combustion of medical wastes in a controlled air combustor. The result showed that the lowest CO emission exists at the optimum operating condition, which appears to be a batch size of 5 kg and a primary chamber preheating temperature at about 700 °C. Yang6 et al. investigated the transient state in a full-scale municipal solid waste combustor and assessed the effect of grating movement and waste feeding cycles. The result showed that the pollutant emission fluctuation corresponds to the waste feeding cycles. Fluidized bed combustion has many advantages over other combustion systems, i.e., good gas−solid mixing, high heat and © 2012 American Chemical Society

Received: May 23, 2012 Revised: August 13, 2012 Published: August 20, 2012 5577

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Figure 1. Diagram of CO and NOx emissions vs time with various ISORs. (Hb = 38 cm, Bt = 1 batch/10 min, EA = 80%, QT1 = 3 N m3/min, Q2nd = 1.5 N m3/min).

particle size, high volatile content). Due to the wide range of sizes of these wastes, they were pretreated simply before being fed into the incinerator. Therefore, their mean sizes are much larger than that of other fuels used in fluidized bed combustor. Different solid wastes have different combustion behavior and pollutant characteristics in batch feeding combustor. The working conditions of fluidized bed combustor should change according to the fuel properties of different wastes. Flue gas recirculation (FGR) combustion has been proven an effective and economically feasible method for reducing NOx emissions.15 However, very little work was done about the emission characteristics in batch VFBC with FGR. In Part 1 of this study, we examined the combustion behavior and temperature distribution within a vortex fluidized bed combustor. Woodblocks are used to simulate the MSW, and rubber balls are used to simulate the hazardous wastes. In this part of the study, we investigate the pollutant emissions from a pilot scale fluidized bed combustor with intermittent feeding. The effects of various operating conditions such as feeding interval, static bed height, and ISOR on the NOx and CO emissions are explored. This work can help in developing more comprehensive physical models for batch fluidized bed combustion systems and optimizing operational parameters in practical applications.

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where CMXO is the mean emission concentration of CO or NOx in ppm at different time. CXO is the sampled concentration of CO or NOx in ppm. ts is the sampling time in minutes which is 60 in this case.

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RESULTS AND DISCUSSION Effect of ISOR on the CO and NOx Emissions. Figure 1 shows the diagrams of CO and NOx emissions vs time with two different ISORs. Comparing Figure 1a with Figure 1b in the case of rubber ball combustion, the CO emission has a higher peak per cycle at higher ISOR. However, comparing Figure 1c with Figure 1d in the case of woodblocks combustion, and the effect of ISOR on CO emission peaks is not obvious. In each diagram of Figure 1, we found that the CO emission concentration fluctuates but is consistent throughout all cycles. Figure 2 and Figure 3 show the mean CO and NOx emission concentrations in three ISORs. As ISOR increases (100 to

EXPERIMENTAL SECTION

The components of the flue gas such as CO, CO2, O2, and NOx are analyzed by Anapol EU5000 gas analyzer. All the concentration values measured in this study are corrected to 11% residual oxygen on a dry basis. The total sampling time is 1 h, and sampling interval is 10 s. Experimental system, feedstock materials, and operating conditions were described in Part 1 of this study. CO and NOx emissions change periodically with the feeding cycles. The mean emission concentrations are calculated below t

C MXO =

∫0 s CXOdt ts

Figure 2. Mean CO and NOx emissions of rubber balls with various ISORs.

(1) 5578

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As ISOR increases, the mean NOx emission from woodblocks combustion increases slightly from 20.16 to 22.69 ppm while that from rubber ball combustion decreases significantly from 102.03 to 69.22 ppm. During the combustion of both feedstocks, the creation of NOx and its reduction to N2 occur simultaneously.18,19 The mean bed temperatures of both cases increase from about 700 to 825 °C when ISOR increases from 100 to 140% (Figure 8 in Part 1). The intermediate products from fuel-N cracking can be transformed easily into NOx by reacting with oxygen in the bed zone.20,21 For woodblocks, NOx emission concentration increases slightly with ISOR because of the reduced reduction reaction of NOx due to the low CO concentration in the chamber. NOx emission from rubber is much higher than that from woodblocks because rubber contains higher fuel-N content. However, unreacted char particles of rubber balls and CO are carried up to freeboard zone and reduce NOx to N2. Therefore, the NOx emission decreases significantly at higher ISOR. Effect of Static Bed Height on the CO and NOx Emissions. Figure 4 shows the diagrams of CO and NOx emissions vs time at two different static bed heights. Comparing Figure 4a with Figure 4b in rubber ball combustion, CO emission peaks are similar in all cycles at lower static bed height; however, the CO peaks change significantly at higher static bed height. Higher turbulence and more small rubber particles escaping from the bed zone at higher static bed height makes the combustion unstable, resulting in varying peak values in different cycles. Comparing Figure 4c with Figure 4d in wood combustion, the effect of static bed height on CO emission peaks is not obvious. Figure 5 and Figure 6 show the mean CO and NOx emissions at three different static bed heights. As static bed height increases, the mean CO emission from woodblocks combustion decreases a little from 30.41 to 18.73 ppm while that from rubber ball combustion decreases significantly from 80.39 to 31.15 ppm.

Figure 3. Mean CO and NOx emissions of woodblocks with various ISORs.

140%), the mean CO emission of woodblocks combustion decreases from 22.52 to 14.89 ppm while that of rubber ball combustion increases significantly from 21.25 to 64.78 ppm. The opposite trends can be attributed to the different sizes of combusting particles. Because the rubber ball particles are much smaller than woodblocks particles, small rubber particles can be carried easily by fluidizing air to the bed surface, even up to the freeboard.16 At a fixed total excess air rate condition, higher ISOR means less oxygen is introduced into the freeboard in the secondary air, which is less favorable for the rubber ball combustion; therefore, the CO emission increases with ISOR. For woodblocks combustion, large particles remain and burn completely in the bed zone. Meanwhile, the devolatilization intensifies with higher in-bed oxygen level,17 which promotes woodblocks combustion, resulting in the CO concentration decrease.

Figure 4. Diagram of CO and NOx emissions vs time with various static bed heights. (Bt = 1 batch/10 min, Cb= 120%, EA = 80%, QT1 = 3 N m3/min, Q2nd = 1.5 N m3/min). 5579

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mean CO emissions. A higher freeboard temperature conduces to the generation of NH3 and HCN (important precursors of NOx), which leads to a higher NOx emission. In addition, lower CO concentration reduces the chance of NOx reduction reaction, which leads to a higher NOx concentration. Effect of Feeding Intervals on the CO and NOx Emissions. Figure 7 shows the diagrams of CO and NOx emissions vs time with two different feeding intervals. For both woodblocks and rubber ball combustions, CO emission peaks grow tremendously as the feeding interval increases from 10 to 20 min. Because the total feed rate of fuels into the combustor is a constant, fuel quantity per batch at 20 min interval is twice that at 10 min interval. This leads to a fuel-rich condition which results in a lower devolatilization rate and incomplete combustion. This result agrees with the studies by Khraisha.11 Figure 8 and Figure 9 show the mean CO and NOx emissions at three different feeding intervals. Because the total ISOR is a constant, the CO and NOx concentration vary with different feeding intervals. As seen in Figure 8, the mean NOx emission of rubber balls combustion decreases slightly with the longer feeding intervals, while the mean CO emission increases sharply. Courtemanche22 et al. found that NOx emission rises and then falls with the increasing quantity of fuel injected. Hu23 et al. conducted coal burning experiments at different oxygen concentrations and found the NOx emission decreased monotonically with fuel equivalence ratio under both fuellean and fuel-rich condition. In this study, the fuel-rich combustion at higher feeding intervals increases the presence of HCN and NH3. In addition, it promotes NOx reduction to N2, which decreases the NOx concentration. As seen in Figure 9, the effect of the feeding interval on the mean CO and NOx emissions is not obvious for woodblocks. It can be explained that the fixed carbon and fuel-nitrogen contents of woodblocks are about half as those of rubber balls. This trend is in agreement with that of PS and PE combustion by Baron12 et al. who considered that NOx concentration depends on the nitrogen-content of the fuel and is not influenced by intermittent feeding interval. CO and NOx Relations of Different Fuel. As shown in Figure 10, the mean emission concentrations of CO and NOx from the combustion of both feeding materials are correlated for all the operating parameters. The mean NOx concentration at furnace exit decreases with the increasing CO concentration for rubber ball combustion; however, the relationship between CO and NOx is not obvious for woodblocks combustion. The reasons could be explained as follows: First, the higher content of fixed carbon of rubber balls leads to a great quantity of char generated by combustion. NO concentrations can be reduced to N2 by the following reduction reactions,24 while the lower content of fixed carbon of woodblocks leads to weak reduction reactions (Reaction 1−Reaction 3). Besides, preexisting CO2 in inlet gas reacted with char and/or fixed carbon in the beginning of combustion to produce CO in the experimental temperature region, which promoted the reduction of NO.23

Figure 5. Mean CO and NOx emissions of rubber balls with various static bed heights.

Figure 6. Mean CO and NOx emissions of woodblocks with various static bed heights.

This difference may be attributed to the different combustion mechanisms of two feedstocks in fluidized bed. Because the woodblocks have larger sizes and smaller fixed carbon content than rubber balls, their fragments mainly combust in the bed zone. The higher CO emission from a shallow bed (low static bed height) can be attributed to incomplete combustion due to the shorter residence time in the bed zone. At a higher static bed height, longer residence time in the bed zone promotes complete combustion, resulting in lower CO emission. Generally, the combustion of fixed carbon is behind the volatile combustion. The volatile matter of rubber balls releases first when the combustion starts in fluidized bed, and the fixed carbon continues to mix with the bed material. The bubbles at the bed surface burst more intensively at higher static bed height because bubble sizes increase with the bed height. Larger bubbles have bigger bubble wake and more momentum, which causes more solids to be carried to the freeboard. Therefore, more unreacted particles of rubber balls are carried up to freeboard zone at higher static bed height. In Part 1 of this study, we showed the mean freeboard temperature increases from 775 to 846 °C as static bed height rises from 26 to 50 cm. The CO emission decreases much from higher combustion efficiency due to higher freeboard temperature. As seen in Figures 5 and6, the mean NOx emissions increase with the static bed height, which is opposite to the trend of

C + 2NO → CO2 + N2

(Reaction 1)

C + NO → 1/2N2 + CO

(Reaction 2)

NO + CO → 1/2N2 + CO2

(Reaction 3)

Second, the extent of fuel nitrogen to NOx conversion is dependent on the local combustion characteristics and the 5580

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Figure 7. Diagram of CO and NOx emissions vs time with various feeding intervals. (Hb = 38 cm, Cb = 120%, EA = 80%, QT1 = 3 N m3/min, Q2nd = 1.5 N m3/min).

Figure 8. Mean CO and NOx emissions of rubber balls with various feeding intervals.

Figure 9. Mean CO and NOx emissions of woodblocks with various feeding intervals.

initial concentration of nitrogen-bound compounds. The nitrogen content of woodblocks is only 1.40%, which is much lower than that of rubber balls. Therefore, the NOx emission concentrations of woodblocks are much lower than that of rubber balls. Lastly, due to the complete combustion in the bed zone, the CO concentration from woodblocks combustion vary little during this experiment, which reduces the chance of NOx reduction reaction with CO (Reaction 3) and leads to negligible changes in NOx concentration.

1. The pollutant concentrations at the exit of the batch combustor vary periodically corresponding to the periods of waste feeding cycles. 2. For rubber ball combustion, the mean CO concentration in the flue gas increases with the ISOR and feeding interval and decreases with higher static bed height. 3. For woodblocks combustion, the mean CO concentration decreases slightly with higher ISOR and static bed height and increases slightly with higher feeding interval. 4. For rubber ball combustion, there is a dependency between the CO and NOx concentrations at the furnace exit. The mean NOx concentration decreases as the mean CO concentration increases. For woodblocks, the CO and NOx’s dependency is unobvious.

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CONCLUSIONS Pollutant emissions of woodblocks and rubber ball combustion in a votexing fluidized bed combustor with an intermittent feeding system were investigated. The results are as follows: 5581

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Figure 10. Relationship between CO and NOx emission for rubber balls and woodblocks.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +886 3 2654119. Fax: +886 3 4636242. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work by the National Science Foundation under grant NSC 95-2221-E-033-064 and the Bureau of Energy, Ministry of Economic Affairs, Taiwan is gratefully acknowledged.

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ABBREVIATIONS EA = excess air ratio (%) Hb = static bed height (cm) Cb = in-bed stoichiometric oxygen ratio, ISOR (%) Bt = feeding interval (%) QT1 = volumetric flow rate of total primary gas (Nm3/min) Q2nd = the volumetric flow rate of secondary air (N m3 min−1)

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REFERENCES

(1) Qing, S.; Wang, H.; Wu, Z. F.; Hu, J. H.; Wang, S. B. J. Combust. Sci. Technol. 2006, 12, 457−462. 5582

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