Investigation of the Vortexing Effect on Sawdust Combustion in a

Jan 6, 2016 - Energy Fuels , 2016, 30 (3), pp 1701–1707 ... However, the lowest value of CO emission is still much higher than the minimal requireme...
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Investigation of the Vortexing Effect on Sawdust Combustion in a Fluidized Bed Combustor Feng Duan,† Jia Chen,† Li-Hui Zhang,† Pin-Wei Li,‡ and Chien-Song Chyang*,‡ †

School of Energy and Environment, Anhui University of Technology, Maanshan, Anhui 243002, People’s Republic of China Department of Chemical Engineering, Chung Yuan Christian University, Chungli 32023, Taiwan, Republic of China

‡

ABSTRACT: The vortexing effect on reducing pollutant emission was verified via the combustion of three groups of sawdust with different diameters in a vortexing fluidized bed combustor (VFBC). The temperatures and pollutant emission distributions along the combustor were measured simultaneously. The combustion fraction in four zones of the VFBC were also calculated on the basis of the oxygen consumption. Additionally, the effects of operating variables, such as the in-bed stoichiometric oxygen ratio and the excess oxygen ratio, on the pollutant emissions were also investigated. The results show that the combustion fractions in the bed and freeboard zones increase with larger particle sizes. However, an inverse trend occurred in other zones. The CO emissions from the three sawdust combustions decrease with an increase in the particle size, excess oxygen ratio, and inbed stoichiometric oxygen ratio, whereas the NOx emissions exhibit an inverse trend. The vortexing effect, which is caused by secondary air injection, demonstrated its effectiveness in prolonging the residence time of sawdust in the high-temperature zone. However, the lowest value of CO emission is still much higher than the minimal requirements of the Taiwan Environmental Protection Agency regulations. Increasing the particle size or compacting the sawdust into pellets are two possible solutions for decreasing the CO emission.

1. INTRODUCTION In recent years, biomass utilization has attracted much attention because it is one of the promising solutions for energy shortage and CO2 mitigation.1,2 Sawdust is one biomass resource that is easy to obtain in many countries and areas with a forestry industry. Sawdust primarily comes from forestry sawmill and is one of the most viable biomass fuels. It can be used as a substitution for fossil fuels, which can contribute to energy conservation and environmental protection. The fluidized bed combustor (FBC) is the most commonly used tool for dealing with biomass combustion as a result of its several advantages over conventional combustion methods. The FBC enhances fuel flexibility by allowing for a variety of low-grade fuels to be burned successfully, and it has a higher combustion efficiency and produces lower air pollution.3−5 As known, CO and NOx are the major pollutants that are emitted from sawdust combustion. A FBC has lower NOx emissions compared to other combustors; this can be attributed to the lower operating temperature (700−900 °C), which significantly reduces the emission of thermal NOx and prompt NOx. For the typical bed temperature ranges and excess oxygen in the sawdust-burning FBC system, NOx in the flue gas primarily comes from the conversion of fuel N via the homogeneous oxidation of the dominant nitrogenous volatile species to fuel NOx.6−10 CO emission occurs via a function of the operating variables, such as excess air ratio, bed temperature, secondary air ratio, etc.11 The CO emission decreases with a higher bed temperature12,13 and excess oxygen.11,14,15 Additionally, the staged combustion mode can reduce the CO emission with a higher secondary air ratio.4,5 Currently, the disadvantages of fluidized bed combustion that uses sawdust are due to its higher CO emission. These CO emissions are generally larger than 2000 ppm.11,14−16 Even under good © XXXX American Chemical Society

combustion conditions that are increased by adjusting the operating variables, the CO concentration is still much higher than 100 ppm. According to the regulations of the Taiwan Environmental Protection Agency (EPA), the minimum emission of CO for an incinerator is 100 ppm and the CO emission is corrected to 11% residual oxygen on a dry basis. Because the operation velocity of a FBC is approximately higher than other combustors, the sawdust can be carried up to the freeboard zone, and this shortens the residence time in the FBC. Therefore, the concept of a vortexing fluidized bed combustor (VFBC) is introduced to create a vortex by blowing the secondary air tangentially into the freeboard to increase the combustion intensity and turndown capability. This system improves the combustion performance of the FBC.17−19 The VFBC has been used for coal combustion studies for many years, but few have focused on its application in sawdust combustion. Here, mixed sawdust was used as the original fuel. Then, it was pretreated with sieves to form two new groups of sawdust with different size distributions. These three groups of sawdust were used to investigate the effect of the fuel particle size on the combustion behavior and pollutant emissions. In our previous study,17 we compared the combustion characteristics of three combustion modes, such as direct combustion, staged combustion, and flue gas recirculation (FGR) combustion using corncob as the fuel. FGR combustion can reduce the pollutant emissions significantly. Therefore, we chose to use the FGR combustion mode in this study. Special Issue: 5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 31, 2015 Revised: January 5, 2016

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DOI: 10.1021/acs.energyfuels.5b01976 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Bed material agglomeration is a serious problem during biomass combustion. The interaction between silica sand and alkaline materials in the ash is known to cause agglomeration, and different kinds of bed materials have been tested in a normal FBC reactor.20,21 The proper combustion temperature is a key factor that can avoid the agglomeration in FBC biomass combustion. Therefore, we also used FGR to hold the bed temperature constant. We have compared the combustion behavior and pollutant emissions of sawdust and sawdust refuse-derived fuel (RDF) in a FBC.22 Results show that sawdust combustion has a higher CO emission, while sawdust RDF has different combustion characteristics because of its distinct pellet structure and burning pattern. However, the process of making sawdust RDF will increase the cost of the sawdust utilization. As discussed above, knowledge on the effect of the fuel particle sizes on the pollutant emissions in a VFBC is very limited. In this study, four different combustion fractions in the combustor are defined to interpret the sawdust combustion behavior in the VFBC and the vortexing effect on the combustion behavior and reduction of pollutant emission at different particle diameters are investigated. Besides, the optimization of operation parameters may help to decrease the CO emission of sawdust combustion in a FBC. Therefore, the effects of the operation parameters, such as the in-bed stoichiometric oxygen ratio and the excess oxygen ratio, on the pollutant emissions of three groups of sawdust combustions are also studied. Typically, different from published manuscripts, the CO and NOx concentrations along the combustor are also detected in this study to understand the generation mechanism and emission reduction of the pollutant.

Figure 1. Schematic diagram of the VFBC.

2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. The process flow diagram for the vortexing fluidized bed combustion system used in this study was introduced in a previous paper.19 The schematic configuration of the VFBC is shown in Figure 1. The combustion chamber with a crosssection of 0.8 × 0.4 m2 was constructed from 6 mm carbon steel lined with a 150 mm refractory to limit the heat loss. A total of 27 tuyeres with orifices that are 5 and 3 mm are mounted on a 6 mm stainlesssteel air distributor and are located at the bottom part of the combustor. The open area ratio of the distributor is 0.516%. The primary mixture that was introduced into the combustor from the windbox was composed of first air and FGR. The first air was supplied by a 15 horsepower (hp) Root’s blower, and the FGR was supplied by a 7.5 hp Root’s blower. The pollutant emission characteristics were varied as the hydrodynamic behavior within the VFBC changed. To eliminate the interaction of the operating parameters, the secondary mixture introduced into the freeboard zone was composed of secondary air and pure nitrogen. Nitrogen was supplied via a set of three nitrogen gas cylinders that were connected. To limit a cross effect from the operation parameters, all of the experiments were conducted at a fixed primary mixture flow rate of 3 Nm3/min and a fixed secondary mixture flow rate of 2 Nm3/min. Four equally spaced secondary mixture injection nozzles of 30 mm in diameter were installed tangentially at the level of 2.05 m above the distributor. 2.2. Fuels and Bed Materials. The mean particle diameter of the mixed sawdust is 0.744 mm, and the particle density is 600 kg/m3. Additionally, we pretreated the mixed sawdust with sieves to form two groups of sawdust with different size distributions with mean diameters of 0.354 mm (small sawdust) and 0.932 mm (large sawdust). The photos and size distributions of the three fuels are shown in Figure 2. The proximate and ultimate analysis results of the sawdust are provided in Table 1. The inert bed material was silica sand (99.5%

Figure 2. Particle size distributions of the sawdust. SiO2) with a particle density of 2600 kg/m3 and a mean diameter of 0.587 mm. 2.3. Data Acquisition. Figure 1 also presents the position of the thermocouple probes and gas sampling. Thermocouple probes (K type) were positioned at 0.45, 1.15, 1.55, 2.05, 2.55, 2.80, 3.00, and 4.50 m above the air distributor, respectively. The oxygen concentration of the flue gas at the outlet of the induced draft fan was continuously monitored by a Novatech 1632 oxygen analyzer (with ±1% precision). To interpret the sawdust combustion behavior in the VFBC, it was necessary to build a model of the combustion fraction by calculating the consumed oxygen. Four O2 concentration sampling points were located at 0.85, 2.03, 2.76, and 4.50 m above the air distributor, respectively. Therefore, the VFBC is divided into four parts, i.e., the bed zone, the splashing zone, the secondary air injection zone, and the freeboard zone. The O2 sampling points are defined as the cutoff points of two adjacent zones. The flue gas was sampled at 4.50 m above the distributor. The components of the flue gas, such as CO, CO2, O2, and NOx, were analyzed using Anapol EU5000 gas B

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3. RESULTS AND DISCUSSION 3.1. Temperature Distribution. Figure 3 shows the temperature distributions across the combustor sections for

Table 1. Proximate and Ultimate Analyses of Sawdust analysis

sawdust

proximate analysis (wt %, as received) moisture volatiles fixed carbon ash ultimate analysis (wt %, dry and ash free) C H O N S heating value (kJ/kg) LHV (WB)

7.98 68.08 20.8 3.14 52.85 7.40 39.55 0.20 0 15787.05

analyzers. The flue gas concentrations in the combustor are presented as measured concentrations, whereas the values of the emissions released from the combustor are all corrected to 11% residual oxygen on a dry basis. In this study, the bed zone, splashing zone, secondary air injection zone, and freeboard zone were the main components of the VFBC. The combustion fraction can be calculated via the following equation: Yi =

Figure 3. Temperature distributions of the particles with different diameters.

CO2,inQ in − CO2,outQ out

the sawdust with different diameters. For this figure, the bed temperature is 750 °C, the in-bed stoichiometric oxygen is 100%, and the excess oxygen ratio is 50%. No evidence of bed agglomeration was found during the 48 h combustion tests on this VFBC when silica sand was used as the bed material. The sawdust with different diameters have similar temperature distributions. The temperatures at the exit of the combustor are approximately 50 °C lower than the peak temperatures. Despite the heat loss through the combustor walls in the freeboard zone, the descending temperature gradient between the freeboard zone and the exit is small in this study. This can be attributed to the low density of the sawdust compared to other biomass fuels. This causes the combustion of many small particles and unburned carbon to begin in the freeboard zone, which partially compensates for the heat loss. Because the sawdust is light and easier to be carried up to combust in the splashing zone, there is a sharp temperature increase in the splashing zone. However, the peak temperatures of the three sawdust appear at different heights of the VFBC. The large sawdust has the closest temperature trend with that of mixed sawdust. However, the peak temperature of the large sawdust appears in the bed surface at approximately 1.15 m above the distributor, which is different from that of mixed sawdust. Most of the large sawdust combust in the bed zone because this zone had the highest proportion of large particles and because the bed temperature was held constant. Therefore, the peak temperature appeared in the bed surface as a result of the heat that was generated from the bed zone. Additionally, without the combustion of small particles in the freeboard, the large sawdust had a lower freeboard temperature than those of the other two sawdust. For the mixed sawdust, the vortexing flow that was caused by the secondary air exhibited little effect on the small particles of the mixed sawdust. The temperatures only increase slightly between the splashing zone and the freeboard zone. The peak temperatures appear in the freeboard zone, which is approximately 0.75 m above the secondary air inlet. The small sawdust temperature distribution shows a trend similar to the large sawdust distribution. However, its peak values are approximately 200 °C higher than that of the large

(0.21Q first + 0.21Q second + Q FGR CO2,ID) − Q FGCO2,OL (1)

× 100%

where CO2,in is the inlet oxygen concentration to each zone (%), Qin is the inlet gas flow rate in each zone in Nm3/min, CO2,out is the outlet oxygen concentration in each zone (%), Qout is the outlet gas flow rate in each zone in Nm3/min, Qfirst is the flow rate of the first air (Nm3/ min), Qsecond is the flow rate of the secondary air (Nm3/min), QFGR is the flow rate of the FGR (Nm3/min), CO2,ID is the oxygen concentration of the FGR at the outlet of the ID fan (%), QFG is the flow rate of the flue gas from the combustor (Nm3/min), and CO2,OL is the oxygen concentration of the flue gas at the outlet of the combustor (%). 2.4. Working Conditions. The working conditions for the experiments are shown in Table 2. The in-bed stoichiometric oxygen

Table 2. Working Conditions parameter

unit

value

bed temperature EO Sb flow rate of secondary mixture (Qsecond) flow rate of primary mixture (QPRI) feeding rate bed material density of bed material mean size of bed material static bed height weight of bed material

°C % % Nm3 min−1 Nm3 min−1 kg/h

750 40, 50, 60, and 70 80, 90, and 100 2 3 38 silica sand 2600 0.587 400 210

kg/m3 mm mm kg

ratio zone (Sb) was controlled by changing the ratio of the first air to the FGR. The second mixture was preheated to approximately 200 °C in the combustor outer jacket before it was pumped into the combustor. After preheating, the volume of the second mixture expands, which results in a higher gas velocity. The bed temperature was fixed at 750 °C and was controlled by a heat-exchange tube that was inserted in the bed to remove excess heat. C

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fractions of small sawdust in the splashing zone and in the secondary air injection zone are 22.52 and 28.43%, respectively, which are the largest among the three fuels. 3.3. Pollutant Emissions. The axial CO and NO x concentration profiles that are shown in Figures 5 and 6 were

sawdust. The small particle size accelerates the devolatilization rate in the combustor chamber as the fuel is fed into the VFBC. Additionally, its freeboard and exit temperatures are the highest among the three sawdust particles. 3.2. Combustion Fraction. Figure 4 shows the effect of the particle size on the combustion fraction. The operation

Figure 5. CO concentration distribution within the VFBC.

Figure 4. Effect of the particle size on the combustion fraction (Tbed, 750 °C; EO, 50%; and Sb, 100%).

conditions in this figure are the same as in Figure 3. As shown, the sum of the bed zone and the splashing zone combustion fraction for the three sawdust is 47.32, 47.58, and 42.32%. However, the combustion fraction in the bed and freeboard zones increases at larger particle sizes. The other two zones exhibit an inverse trend. The combustion fraction in each zone is primarily affected by the residence time of the particles in the VFBC. The combustion fraction of these three sawdust fuels in the bed zone is quite small. In the bed zone, the relatively slow devolatilization rate of the large sawdust prolongs the residence time of the particles in the combustion chamber, resulting in the highest bed zone combustion fraction (34.71%). As observed in Figure 1, the bed temperature was controlled by an adjustable heat-transfer tube that was immersed in the bed and its value is the average of the values taken from the four lateral distribution thermocouples located 0.45 m above the air distributor. The combustion fraction of small sawdust in each zone has a similar value. Its bed zone combustion fraction is the smallest, suggesting that the heat removal from the heattransfer tube is minimum. This can also explain each value in the temperature curves of the small sawdust; they are higher than those of the other two curves. Because of the lightweight and small size of sawdust, the small sawdust is easily carried up to the freeboard zone. However, the partially small sawdust are captured by the vortexing effect that is caused by injecting the secondary mixture; this prolongs the residence time of the particles in the middle zones. In this study, the combustion

Figure 6. NOx concentration distribution within the VFBC.

obtained for the EO value of 50% at different particle sizes. As shown in Figure 5, for the selected fuel options and elemental analysis (EA) values, the axial CO concentration profiles were found to have a maximum, denoted as COmax, whose location (above the air distributor) conventionally divided the combustor volume into the formation (lower) and reduction (upper) regions.23 In the lower region of the test run with the small sawdust combustion, the COmax values are significantly greater than those of the mixed and large sawdust. The main factor that is likely responsible for this effect is that most small sawdust are easily carried up from the bed zone to these zones because of their smaller particle size and lighter weight. This can also be confirmed by the combustion fractions of the splashing and secondary air zones in Figure 4. Additionally, at the same primary and secondary mixture flow rates, the volatile release and the combustion of the small sawdust are both higher than for the other two, resulting in the highest CO concentration. However, a significant reduction in the CO concentration occurred in the freeboard zone along the D

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Energy & Fuels combustor height. During this test, the excess oxygen ratio was 50% and the Sb was 100%. Therefore, the excess oxygen was introduced into the combustor from the secondary mixture. CO was oxidized in homogeneous reactions with oxygen and OH radicals because both were predominant in the freeboard region. As observed in Figure 6, all of the axial NO concentration profiles possessed a maximum, NOmax, whose location made it possible to predictably distinguish the formation and reduction regions for this pollutant. Fuel NO is the main form in biomass fuel combustion, and typically in the bottom region, it can be formed via oxidation of nitrogenous species, such as HCN and NH3, that are released from the fuel particles with the volatile matter. Meanwhile, NO reduction may likely occur in reactions with unreacted char and CO on the char surface in the freeboard zone. Figure 7 shows the effect of the particle size on the CO and NOx emissions. On the basis of this figure, the CO emissions of

emission, although its temperature in the combustor was the highest. This may be because a reducing atmosphere was created via the incomplete combustion. 3.4. Effect of Operation Parameters on the Pollutant Emissions. Figure 8 shows the effect of the excess oxygen ratio

Figure 8. Effect of the excess oxygen ratio on the CO and NOx emissions (Tbed, 750 °C; Sb, 100%).

on the CO and NOx emissions. For this experiment, the bed temperature was 750 °C and the in-bed stoichiometric oxygen ratio was 100%. The excess oxygen ratio was controlled by adjusting the flow rate of nitrogen in the secondary mixture. As observed in Figure 8, the CO emissions of the three sawdust significantly decreased as the excess oxygen ratio increased from 40 to 70%. This is because the oxygen in the secondary air increases with the excess oxygen ratio, resulting in a more complete combustion of small particles and unburned carbon in the upper combustor.24 This result is in agreement with the study by Chakritthakul and Kuprianov.6 The flow rates of the primary and secondary mixtures were held constant in this study, which ensured an equal vortexing effect that was caused by the secondary mixture and stabilized the residence time in the combustor. However, the minimum CO emissions of the small, mixed, and large sawdust combustion were 546, 291, and 187 ppm when the excess oxygen ratio was 70% in this study, which are all much higher than the highest CO emission requirement by the Taiwan EPA regulation. The NO x emissions of the three sawdust combustions exhibited an inverse trend to that of the CO emission, which can be attributed to the significant decrease in the CO emission concentration that reduces the chance of a NOx reduction reaction with CO.25 Figure 9 shows the effect of the in-bed stoichiometric oxygen ratio on the CO and NOx emissions. The bed temperature was 750 °C, and the excess oxygen ratio was 50%, during this experiment. During this test, the in-bed stoichiometric oxygen ratio was always less than 1. This is considered fuel-rich combustion, which is conducive to a higher CO emission concentration in the splashing zone. Additionally, the NOx emission shows a reverse trend to the CO emission, and its highest value appeared when the in-bed stoichiometric oxygen ratio was 100%. The reason for this is that, at a lower CO emission concentration, the probability of NOx reduction with CO also decreases. Moreover, FGR was used in this study. For a constant first air flow rate, a lower stoichiometric oxygen ratio means that more FGR is introduced into the combustor with

Figure 7. Effect of the particle size on the CO and NOx emissions (Tbed, 750 °C; EO, 50%; and Sb, 100%).

small sawdust, mixed sawdust, and large sawdust combustions are 1759, 1498, and 849 ppm, respectively. The small sawdust has the highest value among these fuels. As a result of its small particle size, its residence time in the combustor was much shorter than other fuels, resulting in incomplete combustion, although the excess air ratio was 50%. Visual observation also indicated that there was a significant amount of unburned sawdust in the quench tower. Under the same operating conditions, the residence time increased with the mean particle size. The CO emission exhibited an inverse trend with the residence time. The CO emission of large sawdust combustion was smaller than that of other fuels because of its prolonged residence time in the bed zone, which is conducive to complete combustion. Although Han et al.12 proposed that it is not feasible to use sawdust in the FBC because it is hard to improve the operation temperature and combustion efficiency, the special characteristics of VFBC in this study demonstrate its effectiveness in prolonging the residence time of sawdust in the high-temperature zone. Moreover, increasing the particle size or compacting the sawdust into pellets are two possible solutions for decreasing the CO emission. Additionally, the NOx emissions ranged from 32 to 75 ppm, as shown in this figure, which conform to the Taiwan EPA requirements. The concentration variations appear to be quite small. Among the three sawdust fuels, the small sawdust has the smallest NOx E

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Conversion and Control of the Ministry of Education is greatly acknowledged.

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(1) Werther, J.; Saenger, M.; Hartge, E. U.; Ogada, T.; Siagi, Z. Combustion of agricultural residues. Prog. Energy Combust. Sci. 2000, 26 (1), 1−27. (2) Zhang, L.; Duan, F.; Huang, Y. Effect of organic calcium compounds on combustion characteristics of rice husk, sewage sludge, and bituminous coal: Thermogravimetric investigation. Bioresour. Technol. 2015, 181 (0), 62−71. (3) Shimizu, T.; Han, J.; Choi, S.; Kim, L.; Kim, H. Fluidized-Bed Combustion Characteristics of Cedar Pellets by Using an Alternative Bed Material. Energy Fuels 2006, 20 (6), 2737−2742. (4) Salzmann, R.; Nussbaumer, T. Fuel Staging for NOx Reduction in Biomass Combustion: Experiments and Modeling. Energy Fuels 2001, 15 (3), 575−582. (5) Khan, A. A.; Aho, M.; de Jong, W.; Vainikka, P.; Jansens, P. J.; Spliethoff, H. Scale-up study on combustibility and emission formation with two biomass fuels (B quality wood and pepper plant residue) under BFB conditions. Biomass Bioenergy 2008, 32 (12), 1311−1321. (6) Chakritthakul, S.; Kuprianov, V. I. Co-firing of eucalyptus bark and rubberwood sawdust in a swirling fluidized-bed combustor using an axial flow swirler. Bioresour. Technol. 2011, 102 (17), 8268−8278. (7) Abelha, P.; Gulyurtlu, I.; Cabrita, I. Release of nitrogen precursors from coal and biomass residues in a bubbling fluidized bed. Energy Fuels 2008, 22 (1), 363−371. (8) Zhang, L.; Duan, F.; Huang, Y. Thermogravimetric investigation on characteristic of biomass combustion under the effect of organic calcium compounds. Bioresour. Technol. 2015, 175 (0), 174−181. (9) Werther, J.; Saenger, M.; Hartge, E. U.; Ogada, T.; Siagi, Z. Combustion of agricultural residues. Prog. Energy Combust. Sci. 2000, 26 (1), 1−27. (10) Winter, F.; Wartha, C.; Hofbauer, H. NO and N2O formation during the combustion of wood, straw, malt waste and peat. Bioresour. Technol. 1999, 70 (1), 39−49. (11) Permchart, W.; Kouprianov, V. I. Emission performance and combustion efficiency of a conical fluidized-bed combustor firing various biomass fuels. Bioresour. Technol. 2004, 92 (1), 83−91. (12) Han, J.; Kim, H.; Cho, S.; Shimizu, T. Fluidized bed combustion of some woody biomass fuels. Energy Sources, Part A 2008, 30 (19), 1820−1829. (13) Leckner, B.; Karlsson, M. Gaseous emissions from circulating fluidized bed combustion of wood. Biomass Bioenergy 1993, 4 (5), 379−389. (14) Kouprianov, V. I.; Permchart, W. Emissions from a conical FBC fired with a biomass fuel. Appl. Energy 2003, 74 (3−4), 383−392. (15) Youssef, M. A.; Wahid, S. S.; Mohamed, M. A.; Askalany, A. A. Experimental study on Egyptian biomass combustion in circulating fluidized bed. Appl. Energy 2009, 86 (12), 2644−2650. (16) Srinivasa Rao, K. V. N.; Venkat Reddy, G. Effect of secondary air injection on the combustion efficiency of sawdust in a fluidized bed combustor. Braz. J. Chem. Eng. 2008, 25 (1), 129−141. (17) Chyang, C.; Duan, F.; Lin, S.; Tso, J. A study on fluidized bed combustion characteristics of corncob in three different combustion modes. Bioresour. Technol. 2012, 116 (0), 184−189. (18) Duan, F.; Chyang, C.; Chin, Y.; Tso, J. Pollutant emission characteristics of rice husk combustion in a vortexing fluidized bed incinerator. J. Environ. Sci. 2013, 25 (2), 335−339. (19) Duan, F.; Chyang, C. S.; Lin, C. W.; Tso, J. Experimental study on rice husk combustion in a vortexing fluidized-bed with flue gas recirculation (FGR). Bioresour. Technol. 2013, 134 (0), 204−211. (20) Duan, F.; Chyang, C.-S.; Zhang, L.-h.; Yin, S.-F. Bed agglomeration characteristics of rice straw combustion in a vortexing fluidized-bed combustor. Bioresour. Technol. 2015, 183, 195−202. (21) Chaivatamaset, P.; Sricharoon, P.; Tia, S. Bed agglomeration characteristics of palm shell and corncob combustion in fluidized bed. Appl. Therm. Eng. 2011, 31 (14−15), 2916−2927.

Figure 9. Effect of the in-bed stoichiometric oxygen ratio on the CO and NOx emissions (Tbed, 750 °C; EO, 50%).

the first air. FGR contains more CO2 that can react with carbon in the fuel to generate CO. This is in favor of NOx reduction (reaction R1). Meanwhile, NOx emission is recirculated back to the combustor, and this increases the chance of a NOx reduction reaction with the char from the sawdust (reaction R2). 2NO + 2CO → 2CO2 + N2

(R1)

2C + 2NO → 2CO + N2

(R2)

4. CONCLUSION (1) The combustion fraction in the bed zone and in the freeboard zone increase with larger particle sizes. The other two zones show the inverse trend. In comparison to larger sawdust, the smallest sawdust has little changed combustion fractions at different zones. (2) The CO emissions of the three groups of sawdust combustion decrease with the particle diameter, excess oxygen ratio, and in-bed stoichiometric oxygen ratio, whereas the NOx emissions exhibit the inverse trend. The axial pollutant concentration profiles were found to have a maximum and conventionally divided the combustor volume into the formation (lower) and reduction (upper) regions. (3) The vortexing effect that is caused by secondary air injection demonstrated its effectiveness in prolonging the residence time of sawdust in the high-temperature zone. However, the lowest value of CO emission is still higher than the minimal requirements of the Taiwan EPA regulations. Increasing the particle size or compacting the sawdust into pellets are two possible solutions for decreasing CO emission.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the Nature Science Research Project of Anhui Province (Grant 1508085ME73), the China Postdoctoral Science Foundation (2014M560382), and the Open Foundation of Key Laboratory of Energy Thermal F

DOI: 10.1021/acs.energyfuels.5b01976 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels (22) Duan, F.; Liu, J.; Chyang, C.-S.; Hu, C.-H.; Tso, J. Combustion behavior and pollutant emission characteristics of RDF (refuse derived fuel) and sawdust in a vortexing fluidized bed combustor. Energy 2013, 57 (0), 421−426. (23) Kuprianov, V. I.; Arromdee, P. Combustion of peanut and tamarind shells in a conical fluidized-bed combustor: A comparative study. Bioresour. Technol. 2013, 140, 199−210. (24) Okasha, F. M.; El-Emam, S. H.; Mostafa, H. K. The fluidized bed combustion of a heavy liquid fuel. Exp. Therm. Fluid Sci. 2003, 27 (4), 473−480. (25) Okasha, F. Staged combustion of rice straw in a fluidized bed. Exp. Therm. Fluid Sci. 2007, 32 (1), 52−59.

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DOI: 10.1021/acs.energyfuels.5b01976 Energy Fuels XXXX, XXX, XXX−XXX