Experimental Study on Combustion of Sunflower Shells in a Pilot

and Asia, with the leading producers being Russia, Ukraine, Argentina, China, India, .... The combustor consisted of six refractory-lined steel se...
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Energy Fuels 2010, 24, 3850–3859 Published on Web 06/15/2010

: DOI:10.1021/ef100119c

Experimental Study on Combustion of Sunflower Shells in a Pilot Swirling Fluidized-Bed Combustor Porametr Arromdee,† Vladimir I. Kuprianov,*,† Rachadaporn Kaewklum,‡ and Kasama Sirisomboon§ † School of Manufacturing Systems and Mechanical Engineering, Sirindhorn International Institute of Technology, Thammasat University, Pathum Thani 12121, Thailand, ‡Department of Mechanical Engineering, Faculty of Engineering, Burapha University, Chonburi 20131, Thailand, and §Department of Mechanical Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhon Pathom 73000, Thailand

Received January 31, 2010. Revised Manuscript Received June 2, 2010

This paper reports the experimental results from a study of burning sunflower shells in an innovative swirling fluidized-bed combustor (SFBC), using an annular spiral air distributor as the swirl generator. The combustion tests were performed for two fuel feed rates;60 and 45 kg/h;while the total amount of excess air was within a range of 20%-80%. During the tests, primary air was introduced into the bed through an air distributor at the combustor bottom, while secondary air was injected tangentially into the bed splash zone, with the intention to facilitate rotational gas-solid flow and also to mitigate CO in this region. For the selected operating conditions, temperature and gas concentrations (O2, CO, CxHy as CH4, and NO) were measured along axial and radial directions in the combustor, as well as in the stack gas. As revealed by the experimental results, radial and axial temperature profiles in the SFBC were rather uniform and weakly dependent on the fuel feed rate, excess air and secondary air, whereas gas concentration profiles showed apparent effects of operating conditions in both directions. At a fuel feeding of 60 kg/h, 55% excess air seems to be an optimal value for the effective burning of sunflower shells in the SFBC, ensuring the highest combustion efficiency (∼99%) and acceptable levels of CO and NO emissions, both complying with the local emission standards, while maintaining CxHy emissions at a reasonable level.

The fluidized-bed combustion technology is proven to be the most effective and environmentally friendly technology for the conversion of energy from biomass. Several detailed research studies and reviews6-14 have addressed conventional fluidized-bed combustion techniques, such as bubbling, vortexing and circulating fluidized-bed combustion systems (combustors and boiler furnaces), firing rice husk, sugar cane bagasse, and wood residues, the most viable biomass fuels used for heat and power generation. However, some studies have highlighted the difficulties in achieving high

Introduction The production of sunflower seeds is an important agricultural sector in many countries around the world. Basically, the seeds are used as a raw material for production of sunflower oil, as well as a food product. As reported by The Food and Agriculture Organization of the United Nations,1 ∼30 million tons of sunflower seeds are produced annually in some countries of eastern Europe, the Americas, and Asia, with the leading producers being Russia, Ukraine, Argentina, China, India, and the United States. Sunflower shells, which is the major byproduct of sunflower seed processing, are characterized by excellent combustion properties, and can therefore be used as a source of energy.2-5 Based on the availability and calorific value of sunflower shells (assessed as 17 MJ/kg), the world energy potential of this biomass fuel is estimated to be ∼80  109 MJ per year.

(6) Leckner, B.; Karlsson, M. Gaseous emissions from circulating fluidized bed combustion of wood. Biomass Bioenergy 1993, 4, 379–389. (7) Natarajan, E.; Nordin, A.; Rao, A. N. Overview of combustion and gasification of rice husk in fluidized bed reactors. Biomass Bioenergy 1998, 14, 533–546. (8) Armesto, L.; Bahillo, A.; Veijonen, K.; Cabanillas, A.; Otero, J. Combustion behaviour of rice husk in bubbling fluidised bed. Biomass Bioenergy 2002, 23, 171–179. (9) Werther, J.; Saenger, M.; Hartge, E.-U.; Ogada, T.; Siagi, Z. Combustion of agricultural residues. Prog. Energy Combust. Sci. 2000, 26, 1–27. (10) Permchart, W.; Kouprianov, V. I. Emission performance and combustion efficiency of a conical fluidized-bed combustor firing various biomass fuels. Bioresour. Technol. 2004, 92, 83–91. (11) Fang, M.; Yang, L.; Chen, G..; Shi, Z.; Luo, Z.; Cen, K. F. Experimental study on rice husk combustion in a circulating fluidized bed. Fuel Process. Technol. 2004, 85, 1273–1282. (12) Chyang, C. S.; Qian, F.-P.; Lin, Y.-C.; Yang, S.-H. NO and N2O emission characteristics from a pilot scale vortexing fluidized bed combustor firing different fuels. Energy Fuels 2008, 22, 1004–1011. (13) Koornneef, J.; Junginger, M.; Faaij, A. Development of fluidized bed combustion;An overview of trends, performance and cost. Prog. Energy Combust. Sci. 2007, 33, 19–55. (14) Janvijitsakul, K.; Kuprianov, V. I. Similarity and modeling of axial CO and NO concentration profiles in a fluidized-bed combustor (co-)firing biomass fuels. Fuel 2008, 87, 1574–1584.

*To whom correspondence should be addressed. Tel.: þ66 2 986 9009, ext. 2208. Fax: þ 66 2 986 9112. E-mail: [email protected]. (1) Food and Agriculture Organization of the United Nations. Major Food and Agricultural Commodities and Producers. Available via the Internet at http://www.fao.org/es/ess/top/commodity.html, accessed on April 15, 2010. (2) Haykiri-Acma, H. Combustion characteristics of different biomass materials. Energy Convers. Manage. 2003, 44, 155–162. (3) Demirbas, A. Combustion characteristics of different biomass fuels. Prog. Energy Combust. Sci. 2004, 30, 219–230. (4) Szemmelveisz, K.; Szucs, I.; Palotas, A. B.; Winkler, L.; Eddings, E. G. Examination of the combustion conditions of herbaceous biomass. Fuel Process. Technol. 2009, 90, 839–847. (5) Zabaniotou, A. A.; Kantarelis, E. K.; Theodoropoulos, D. C. Sunflower shells utilization for energetic purposes in an integrated approach of energy crops: Laboratory study pyrolysis and kinetics. Bioresour. Technol. 2008, 99, 3174–3181. r 2010 American Chemical Society

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efficiency of 84%-96% (accounting for the effect of heat loss due to CxHy), i.e., at almost the same level as that reported for the circulating fluidized-bed combustor.21 Kaynak et al.23 conducted experimental tests on a bubbling fluidized-bed combustor firing peach and apricot stones. The combustion efficiency (including effects of CxHy-related heat loss) is reported to be 96%-97% for the peach stones, and 93%-96% for the apricot stones. Compared to olive cake, the peach and apricot stones seem to be “greener” biomass fuels. Shimizu et al.24 reported the experimental results on firing cedar pellets in a bubbling fluidized-bed combustor using two different inert bed materials;silica and alumina sands;for variable operating conditions (fuel feed rate and excess air). In both cases, the combustion efficiency was found to be very high (>99.5%). Effects of the bed material on CO emission and, correspondingly, on the combustion efficiency were substantial, while NOx emissions were at a rather low level and almost irrespective of the bed material used. Sun et al. studied the combustion of cotton stalk in bubbling25 and circulating26 fluidized-bed combustors, using silica and alumina sands. High combustion efficiency (∼99.5%) was achieved in both combustors operated under optimal conditions. No significant effects of combustor hydrodynamics on the axial temperature profiles in the reactors, as well as on the major emissions, were observed in these studies. Alumina sand was determined to provide more favorable operational conditions for the fluidized bed, mitigating agglomeration of the bed material during the combustion of this high-alkali biomass fuel. Youssef at al.27 reported the effects of operating conditions (excess air and air staging) on the axial temperature profiles, as well as on the formation and decomposition of CO, NOx, and SO2, in a circulating fluidized-bed combustor firing wheat straw, sawdust wood, cottonseed burs, and corncobs. Except for straw and sawdust (exhibiting high CO emissions), the residues showed their suitability for energy conversion with a relatively low environmental impact. At 24% excess air, the emissions from the combustor can be adjusted at minimized levels. Gulyurtlu et al.28 investigated the influence of operating conditions (temperature and excess air), as well as of fuel moisture and particle size, on the emission of polycyclic aromatic hydrocarbons (PAHs) from the combustion of coconut shells in a fluidized bed. The PAHs can be effectively reduced by increasing the fuel burnout rate. At high combustion efficiency (>99%), the emission of PAHs from the fluidized-bed combustion of biomass fuels can be ensured at a quite low level, as reported in this study and elsewhere,28,29

combustion efficiency, particularly, when firing high-ash biomass fuels,7,9,10 and also the ash-related operational problems, such as slagging, fouling, and bed agglomeration, generally caused by alkali-based compounds in biomass ashes when using silica sand as the inert bed material.9,15,16 The combustion of most biomass fuels is accompanied by substantial gaseous emissions, with the major ones being NO and CO, the rate of those depend on fuel properties, as well as the design features and operating conditions of the combustion system.6-14 Recently, two novel combustion techniques ensuring fuel oxidation in a strongly swirled flow;a vortex combustor and a cyclonic fluidized-bed combustor;have been studied for firing rice husk.17,18 Under optimum operating conditions, high (>99%) combustion efficiency is achievable in these two reactors, while controlling CO emission at a reasonable level (below 400 ppm). However, NOx emissions from the combustors are reported to be elevated, mainly because of the significant combustion intensity of these relatively “short” devices operated at high excess air. Another innovative combustion technique integrating the bed fluidization with its swirl motion;a swirling fluidized-bed combustor;has been tested for firing rice husk.19,20 In this device, a swirling fluidized bed is generated by primary air introduced into the bed through an annular spiral air distributor, while rotational gas-solid flow in the combustor freeboard is sustained by secondary air injected tangentially into the bed splash zone. High combustion efficiency at rather low major emissions is ensured in this swirling fluidized-bed combustor firing rice husk over wide ranges of fuel properties and operating conditions.20 During the past decade, a growing attention has been paid to the feasibility of effective utilization of various unconventional biomass fuels (from fibrous fuels to fruit stones and shells), mainly, by burning them in fluidized-bed combustion systems. Topal et al.21 investigated the combustion efficiency and emission performance (CO, NOx, and SO2 emissions) of a circulating fluidized-bed combustor firing olive cake. As found in this study, the combustion efficiency was strongly influenced by operating conditions, varying from 82% to 98%, depending mainly on excess air. Varol and Atimtay22 burned olive cake in a bubbling fluidized-bed combustor using air split (or air staging) and achieved the combustion (15) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Combustion properties of biomass. Fuel Process. Technol. 1998, 54, 17–46. (16) Khan, A. A.; de Jong, W.; Jansens, P. L.; Spliethoff, H. Biomass combustion in fluidized bed boilers: Potential problems and remedies. Fuel Process. Technol. 2009, 90, 21–50. (17) Eaimsa-ard, S.; Kaewkohkiet, Y.; Thianpong, C.; Promvonge, P. Combustion behavior in a dual-staging vortex rice husk combustor with snail entry. Int. Commun. Heat Mass Transfer 2008, 35, 1134–1140. (18) Madhiyanon, T.; Lapirattanakun, A.; Sathitruangsak, P.; Soponronnarit, S. A novel cyclonic fluidized-bed combustor (Ψ-FBC): Combustion and thermal efficiency, temperature distribution, combustion intensity, and emission of pollutants. Combust. Flame 2006, 146, 232–245. (19) Kaewklum, R.; Kuprianov, V. I. Experimental studies on a novel swirling fluidized-bed combustor using an annular spiral air distributor. Fuel 2010, 89, 43–52. (20) Kuprianov, V. I.; Kaewklum, R.; Sirisomboon, K.; Arromdee, P.; Chakritthakul, S. Combustion and emission characteristics of a swirling fluidized-bed combustor burning moisturized rice husk. Appl. Energy 2010, 87, 2899–2906. (21) Topal, H.; Atimtay, A. T.; Durmaz, A. Olive cake combustion in a circulating fluidized bed. Fuel 2003, 82, 1049–1056. (22) Varol, M.; Atimtay, A. T. Combustion of olive cake and coal in a bubbling fluidized bed with secondary air injection. Fuel 2007, 86, 1430– 1438.

(23) Kaynak, B.; Topal, H.; Atimtay, A. T. Peach and apricot stone combustion in a bubbling fluidized bed. Fuel Process. Technol. 2005, 86, 1175–1193. (24) Shimizu, T.; Hun, J.; Choi, S.; Kim, L.; Kim, H. Fluidized-bed combustion characteristics of cedar pellets by using an alternative bed material. Energy Fuels 2006, 20, 2737–2742. (25) Sun, Z.; Jin, B.; Zhang, M.; Liu, R.; Zhang, Y. Experimental studies on cotton stalk combustion in a fluidized bed. Energy 2008, 33, 1224–1232. (26) Sun, Z.; Jin, B.; Zhang, M.; Liu, R.; Zhang, Y. Experimental study on cotton stalk combustion in a circulating fluidized bed. Appl. Energy 2008, 85, 1027–1040. (27) 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, 2644–2650. (28) Gulyurtlu, I.; Karunaratne, D. G. G. P.; Cabrita, I. The study of effect of operating parameters on the PAH formation during the combustion of coconut shell in a fluidised bed. Fuel 2003, 82, 215–223. (29) Janvijitsakul, K.; Kuprianov, V. I. Major gaseous and PAH emissions from a fluidized-bed combustor firing rice husk with high combustion efficiency. Fuel Process. Technol. 2008, 89, 777–787.

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Figure 1. Schematic diagram of the experimental setup with the swirling fluidized-bed combustor.

in distinct reactor regions, as well as on emissions and combustion efficiency of the swirling fluidized-bed combustor, were the main focus of this study.

which indicates an extreme importance of keeping combustion efficiency at high magnitudes. As follows from the literature review, neither detail information nor technical data on the combustion and emission characteristics of firing sunflower shells in a fluidized bed are available. However, Lokare et al.30 provided important information on the ash deposition of some high-alkali biomass fuels burned in a single multifuel reactor. As reported in this study, the ash deposition rate of sunflower shells (with the K2O amount in the ash being 45.1%) turned out to be comparable to that of sawdust (20.6% K2O), but significantly (more than an order of magnitude) lower than that of straw (21.9% K2O). This fact points at the possibility of avoiding severe ash-related problems in industrial combustion systems (applications) when firing sunflower shells. This work was aimed at studying of burning sunflower shells in the swirling fluidized-bed combustor, with its potential of ensuring high combustion efficiency at minimized environmental impacts. Along with these benefits, the combustion technique provides some other advantages associated with the swirling flow of a fluidized bed (preventing the growth of large bubbles in the bed and providing flexibility of fuel particle size19) and conical shape of the bed (less amount of the bed material and lower pressure drop across the bed31). Effects of operating conditions on formation and decomposition of gaseous pollutants (CO, CxHy, and NO)

Experimental Section Experimental Facilities. The pilot swirling fluidized-bed combustor (SFBC) with a cone-shape bed, originally designed for the power output of 350 kWth and previously tested for firing rice husk,19,20 was used in this study to investigate the combustion and emission characteristics of the SFBC for firing sunflower shells under variable operating conditions. Figure 1 depicts the detail schematic diagram including the combustor with a start-up burner, auxiliary equipment for fuel and air supply, a cyclone for collecting particulates (fuel ash), and facilities for data acquisition and treatment. The combustor consisted of six refractory-lined steel sections, or modules, connected coaxially: one conical module with a cone angle of 40° and an inner diameter at the bottom plane of 0.25 m, and five cylindrical modules with a height of 0.5 m and an inner diameter of 0.9 m. In each module, the refractorycement insulation was 50 mm thick, lined inside a 4.5-mmthick metal wall. All the modules, except the upper one, were equipped with gas sampling ports and stationary chromelalumel thermocouples (of Type K), the latter being used to monitor the temperature (with the accuracy of (1%) at different locations along the centerline during start-up and combustion tests. An annular spiral air distributor arranged at the bottom of the conical module was used as the swirl generator of the fluidized gas-solid bed. It was comprised of 11 steel blades with a height of 85 mm, fixed in a radial direction, ensuring injection of primary air into the bed at an angle of 14° to the horizontal. With this angle, the tangential component of air velocity at the annular exit of the air distributor was ∼4 times greater than the superficial air velocity, which was numerically equal to the axial velocity at this horizontal plane. The cross-sectional area for air

(30) Lokare, S. S.; Dunaway, J. D.; Moulton, D.; Rogers, D.; Tree, D. R.; Baxter, L. L. Investigation of ash deposition rates for a suite of biomass fuels and fuel blends. Energy Fuels 2006, 20, 1008–1014. (31) Kaewklum, R.; Kuprianov, V. I.; Douglas, P. L. Hydrodynamics of air-sand flow in a conical swirling fluidized bed: A comparative study between tangential and axial air entries. Energy Convers. Manage. 2009, 50, 2999–3006.

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Table 1. Ultimate and Proximate Analyses and Lower Heating Value (LHV) of Sunflower Shells Used in the Experimental Tests Ultimate Analysis (wt %, as-received basis)

Proximate Analysis (wt %, as-received basis)

C

H

O

N

S

fuel moisture

fuel ash

volatile matter

fixed carbon

LHV (kJ/kg)

52.20

5.59

29.68

0.63

0.10

9.1

2.7

65.6

22.6

17200

2

secondary air, were chosen as the independent operating variables in this experimental study. The trials were performed for two fuel feed rates, 60 and 45 kg/h, while the amount of excess air ranged from ∼20% to 80%. However, to minimize the volume of experimental work, the radial temperature and gas concentration profiles were obtained only for the 60 kg/h fuel feed rate, and only the 40% excess air value, at three levels (Z) above the air distributor: 0.74, 1.55, and 2.67 m. Meanwhile, to plot the axial temperature and gas concentration profiles, the measurements were conducted along the combustor centerline at eight levels above the air distributor, for the two selected fuel feedings and a limited range of excess air: 40%-80%. To quantify the emissions of CO, CxHy, and NO, the concentrations of these species were recorded at the cyclone outlet for the two combustor loads and the entire range of excess air (20%-80%). Since the secondary air flow rate was maintained at the above constant value, 0.024 Nm3/s, the percentage of secondary air in the tests for the two fuel feedings was different: 28% at the feed rate of 60 kg/h, and 37% at 45 kg/h. Thus, at similar excess air (or for a given percentage of total air), the secondary-to-primary air ratio for the two combustor loads was somewhat different, causing corresponding effects on the behavior of temperature and gas concentrations. For the particular test run, the excess air ratio was quantified in ref 33, using the O2, CO, and CxHy concentrations in flue gas at the cyclone exit. Afterward, the corresponding percentages of total air and excess air were calculated for this trial. The heat-loss method was used to estimate the combustion efficiency as the percentage of LHV. Prior to calculation of the combustion efficiency, heat loss due to unburned carbon and that due to incomplete combustion were determined for the selected operating conditions, using standard methods.34 Fly ash was sampled from the ash collector (below the cyclone, as seen in Figure 1), to determine the content of unburned carbon in the ash required for the assessment of the associated heat loss, whereas the heat loss due to incomplete combustion was quantified using the CO and CxHy (as CH4) emissions.

flow between the blades was 0.012 m . A steel cone stabilizer with a diameter of 80 mm at the lower base was fixed on the top of the air distributor (as shown in Figure 1) with the aim of sustaining the swirling motion of the fluidized gas-solid bed. Quartz sand with a solid density of 2650 kg/m3 was used as the inert bed material in this SFBC. The static bed height in all trials was 30 cm. Sand with particle sizes of 0.5-0.6 mm along with the selected cone angle ensured stable swirling fluidized-bed regime and appropriate hydrodynamic characteristics of the bed.31 A screw-type feeder delivered sunflower shells over the bed at a level 0.6 m above the air distributor. The rotation speed of the screw feeder was adjusted using a three-phase inverter, providing the desired fuel feed rate. A 25-hp blower delivered primary air to the combustor. A diesel-fired burner (model Press G24, Riello Burners Co.) was used to preheat the sand during the combustor startup. The diesel-air flow was tangentially injected into the splash bed zone at a level 0.5 m above the air distributor, through the burner nozzle inclined at a -30° angle to the horizontal, i.e., toward the bed, as seen in Figure 1. When the bed (sand) temperature reached 700 °C, a diesel pump of the burner was turned off, at which point the required feed rate of the major (biomass) fuel was ensured by the screw feeder. However, during the combustion tests, a burner fan continued to operate, thus, supplying secondary air to the bed splash zone at a constant flow rate of Qba = 0.024 Nm3/s, to protect the burner nozzle head against overheating and impacts from solids, and to mitigate CO in this region. A model Testo-350XL gas analyzer (Testo, Germany) was used to measure the temperature and gas concentrations (O2, CO, CxHy as CH4, and NO) along the radial and axial directions in the combustor, and at the top of the ash-collecting cyclone. The measurement accuracies were (0.5% for temperature (using a Type K thermocouple), (5% for CO within the range of 100-2000 ppm, (10% for CO higher than 2000 ppm, (10% for CxHy up to 40 000 ppm (as CH4), (5% for NO, and (0.2 vol % for O2. The Fuel. Sunflower shells can be categorized as a herbaceous biomass fuel.4,16 The major chemical constituents of sunflower shells that affect fuel structure are cellulose (approximately half of the fuel mass), hemicelluloses (almost one-third of the fuel mass), and the rest is lignin.32 Table 1 shows the ultimate and proximate analyses, as well as the lower heating value (LHV) of Thai sunflower shells used in this experiment work. Table 1 shows that the proximate analysis of sunflower shells included a significant amount of volatile matter, but rather low proportions of moisture and ash, which resulted in the substantial calorific value of this biomass fuel: LHV = 17 200 kJ/kg, i.e., comparable to that of low-rank coals. Accounting for the fuel ultimate analysis, the theoretical volume of air required for burning 1 kg of “as-received” sunflower shells was estimated in ref 33 to be 5.1 Nm3/kg. Because of the extremely low fuel-sulfur content (∼0.1%), SO2 was not addressed in this study. The average dimensions of individual sunflower shells were a width of 6 mm, a thickness of 0.7 mm, and a length of 10 mm; the particle solid (or real) density was 592 ( 15 kg/m3. Operating Conditions and Procedures. The fuel feed rate and excess air, the latter being affected by both primary and

Results and Discussion Temperature and Gas Concentration Profiles in the SFBC. Figure 2 shows the radial temperature and O2 concentration profiles at three levels above the air distributor (Z) in the SFBC firing 60 kg sunflower shells/h at 40% excess air. Figure 2a shows that the temperature profiles were rather uniform, which suggested highly intensive heat and mass transfer along the radial distance. However, the O2 concentration profiles exhibited a positive radial gradient (see Figure 2b), basically caused by the tangential injection of secondary air into the reactor near the combustor wall. In the vicinity of secondary air nozzles, the radial gradient of O2 was characterized by the highest magnitude (see the curve for Z = 0.74 m) but diminished along the combustor height at a very low rate. As a result, at different levels in a freeboard region of the combustor (Z > 1 m), the concentration of O2 at the combustor wall was 2%-4% greater than that at the centerline.

(32) Demirbas, A.; Akdeniz, F. Fuel analysis of selected oilseed shells and supercritical fluid extraction in alkali medium. Energy Convers. Manage. 2002, 43, 1977–1984. (33) Bezgreshnov, A. N.; Lipov, Y. M.; Shleipher, B. M. Computations of Steam Boilers (in Russ.); Energoatomizdat: Moscow, 1991.

(34) Basu, P.; Cen, K. F.; Jestin, L. Boilers and Burners; Springer: New York, 2000.

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Figure 2. Radial (a) temperature and (b) O2 concentration profiles in the SFBC firing sunflower shells at the fuel feed rate of 60 kg/h and 40% excess air for different levels above the air distributor.

Figure 3. Effects of excess air on the axial temperature profiles in the SFBC firing sunflower shells at the fuel feed rate of (a) 60 and (b) 45 kg/h.

Figure 4. Effects of excess air on the axial O2 concentration profiles in the SFBC firing sunflower shells at the fuel feed rate of (a) 60 and (b) 45 kg/h.

the lower temperature at a given point, by ∼50 °C, for the entire range of excess air, which can be explained by the increase in heat loss across the walls while reducing the combustor load. Figure 4 shows the axial O2 concentration profiles in the SFBC for the same operating conditions, as in Figure 3. By behavior, the profiles exhibited three distinct regions. In the first region (Z < 0.5 m), the concentration of O2 turned out to be significantly reduced: from 21% (at Z = 0) to ∼5% at the feed rate of 60 kg/h, and to ∼4% at 45 kg/h. This remarkable reduction can be explained by the high proportion of volatile matter in this herbaceous biomass fuel (see Table 1), as well as by rapid fuel devolatilization and volatile oxidation in both the dense swirling-fluidized bed and the bed splash zone. Because of the overbed fuel feeding, the contribution of char-C oxidation to O2 consumption in the first region was likely insignificant. The second region

Figure 3 depicts the axial temperature profiles in the SFBC firing sunflower shells at the fuel feed rates of 60 and 45 kg/h for similar ranges of excess air. The behavior of temperature along the radial and axial directions (compare Figures 2a and 3a) indicated that the reactor was basically operated under quasi-isothermal conditions. However, because of the influence of secondary air (injected into the bed splash zone at ambient temperature), all the axial temperature profiles exhibited a small positive vertical gradient in the lower part of the combustor, whereas in the upper reactor part, the temperature was determined to be slightly reduced along the SFBC height, because of the heat loss across the combustor walls. At a fixed combustor load, an increase in excess air resulted in some reduction of the temperature at all locations in the reactor volume, mainly because of the air dilution effects. A comparison of Figures 3a and 3b shows that switching the fuel feed rate from 60 kg/h to 45 kg/h led to 3854

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Figure 5. Radial (a) CO, (b) CxHy, and (c) NO concentration profiles in the SFBC firing sunflower shells at the fuel feed rate of 60 kg/h and 40% excess air for different levels above the air distributor.

(∼3 times) lower than that at the centerline, whereas the behavior of CO along the radius was characterized by a lower radial gradient, as can be compared in Figures 5a and 5b. This fact can be explained by the contribution of CxHy oxidation near the reactor wall, leading to the formation of an intermediate product, CO, and, eventually, to elevated CO concentrations in the peripheral zone. However, in the reactor freeboard, both CO and CxHy were substantially lower, and the above radial gradients were gradually attenuated. During the combustion of biomass fuels, NO is basically expected to originate via the fuel-NO formation mechanism including the proportional effects of fuel-N, excess air, and combustion temperature, the latter being the minor factor.9,12,14 Meanwhile, NO is decomposed (reduced) through (i) reactions with CO on the fuel-chars surface,9,14,35 as well as (ii) homogeneous reactions with NH3 and light hydrocarbon radicals.36,37 Taking into consideration the behavior of O2 and CO along the radius, the concentration of NO at the central zone of the combustor might be apparently lower than that at the peripheral zone, i.e., near the wall. However, as seen in Figure 5c, the radial NO concentration profiles were rather uniform, likely because of the dilution effects of secondary air in the peripheral zone. Figure 6 compares the axial CO concentration profiles in the SFBC between the fuel feed rates for the ranges of excess air. Similar to that for O2, one can distinguish three regions in

(0.5 m < Z < 1 m) was characterized by a substantial regaining (rise) of O2, in response to the secondary air injection through the start-up burner. In the meantime, the behavior of O2 in this region was apparently influenced by oxidation of volatiles and char-C, the latter being dependent on the char residence time and also affected by operating conditions. Thus, at the highest excess air value, a rise of O2 in the second region at the 45 kg/h feed rate was substantially greater than that at 60 kg/h, mainly because of the higher proportion of secondary air. However, at the reduced load and lower excess air, the increase in O2 along the reactor height was mitigated by the enhanced rate of oxygen consumption, likely caused by an increase in the residence time of the char particles (because of the lower combustion temperatures and reduced airflow rate). In the third region (Z > 1 m), the O2 concentration gradually diminished along the combustor height (primarily because of the oxidation of char-C and volatile hydrocarbons carried over from the reactor bottom part), showing the apparent effects of excess air on the residual level of O2, particularly at the combustor top. As revealed by the experimental results, the concentration of NO2 in the flue gas at different locations in the SFBC, as well as at the cyclone exit, was negligible (0-4 ppm). Therefore, in the discussion below, NOx are treated as NO. The radial CO, CxHy, and NO concentration profiles in the SFBC are depicted in Figure 5 for the same operating conditions, as in Figure 2. At all the levels, the radial profiles of carbonaceous CO and CxHy exhibited negative gradients basically caused by the tangential injection of secondary air. However, in the bottom part of the combustor, both CO and CxHy concentrations at the centerline were determined to be very high, mainly because of the intensive fuel devolatilization; therefore, the radial gradients of CO and CxHy in this region were substantial. For instance, at Z = 0.74 m, the concentration of CxHy at the combustor wall was much

(35) Barisic, V.; Kilpinen, P.; Hupa, M. Comparison of the catalytic activity of bed materials from the combustion of biomass and waste fuels in a circulating fluidized bed boiler toward NO reduction by CO. Energy Fuels 2006, 20, 1925–1932. (36) Winter, L.; Wartha, C.; Hofbauer, H. NO and N2O formation during the combustion of wood, straw, malt waste and peat. Bioresour. Technol. 1999, 70, 39–49. (37) Turns, S. An Introduction to Combustion; McGraw-Hill: Boston, 2006.

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Figure 6. Effects of excess air on the axial CO concentration profiles in the SFBC firing sunflower shells at the fuel feed rate of (a) 60 and (b) 45 kg/h.

Figure 7. Effects of excess air on the axial CxHy concentration profiles in the SFBC firing sunflower shells at the fuel feed rate of (a) 60 and (b) 45 kg/h.

the axial CO concentration profiles where the formation and decomposition of CO occurred at different rates. For the two fuel feedings, a dramatic increase of CO along the axial distance was observed to occur in the swirling-fluidized (dense) bed, below the lowest gas sampling port, mainly because of the rapid fuel devolatilization (i.e., highly intensive release of CO and high-rate oxidation of volatile hydrocarbons to CO), while the oxidation rate of CO in this first region was substantially lower. Some contribution to formation of CO in the dense bed was likely done by the char-C oxidation. As can be seen in Figures 6a and 6b, for a given fuel feed rate, an increase in excess air led to the lower CO concentrations in the bottom part of the combustor, mainly because of the higher rate of CO oxidation. However, by reducing the fuel feed rate at a fixed excess air level, the CO concentrations became higher, especially at the lower excess air values, which can be explained by the enhanced formation of CO. Higher oxidation rates of both char-C and volatile hydrocarbons caused by the increase in the residence time (due to the reduction in both primary air and bed temperature) were responsible for this intensive formation of CO at the reduced load. The second region, up to Z = 1.0 m, was characterized by a significant negative gradient of CO along the axial distance, apparently caused by the injection of secondary air into the bed splash zone. Decomposition of CO in this region (basically via its homogeneous oxidation) was predominant. At the reduced fuel feed rate, despite the reduction in the combustion temperature, a greater axial CO gradient (reduction) was observed at all the values of excess air, because of the higher proportion of secondary air. In the third region, Z > 1 m, CO originated from the fuel chars and volatile hydrocarbons carried over from the bottom region

was oxidized by residual air, exhibiting, however, a slight reduction of CO along the combustor height for the two fuel feedings. Because of the injection of secondary air into the bed splash zone, CO concentrations in the freeboard region remained at a relatively low level, somewhat varying (reducing) at all points with increasing excess air. Thus, at the fuel feed rate of 60 kg/h, the actual CO concentrations were in the range of 1660-3300 ppm at Z = 1 m, but diminished to magnitudes of 400-780 ppm at Z = 2.67 m. However, at the reduced combustor load, because of the effects of the residence time, the CO concentrations at these locations were lower: 470-720 ppm at Z = 1 m and 170-380 ppm at Z = 2.67 m. Figure 7 shows the axial CxHy concentration profiles in the SFBC for the two fuel feed rates and variable excess air. At a first glance, these profiles seem to be quite similar to the axial CO concentration profiles. However, a closer look revealed substantial differences in the behaviors of CO and CxHy in the bottom part of the reactor, which can be explained by the differences in the formation (origin) and reactivity of these gaseous species. At the fuel feed rate of 60 kg/h, the formation rate of hydrocarbons (a primary product of thermal fuel destruction) in the dense bed was extremely high, likely because of the high bed temperature and the substantial proportion of fuel penetrated into the bed, which resulted in the significant concentrations of CxHy (over 40 000 ppm) at the lowest gas sampling port, i.e., at Z = 0.47 m. However, in the second region, CxHy was effectively mitigated (oxidized) by secondary air to relatively small values, thus causing a dramatic reduction in CxHy along the axial distance, as can be seen in Figure 7a. Similar to that for CO, a carryover of the chars and volatiles into the upper (third) region, Z > 1 m, 3856

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Figure 8. Effects of excess air on the axial NO concentration profiles in the SFBC firing sunflower shells at the fuel feed rate of (a) 60 and (b) 45 kg/h.

resulted in notable CxHy concentrations in the reactor freeboard, showing, however, some reduction along the combustor height: from 1300-2800 ppm at Z = 1 m to 520-800 ppm at Z = 2.67 m. Switching the fuel feeding rate to 45 kg/h led to the lower CxHy concentrations in the bottom part of the reactor, despite the reduction in the bed temperature. At a fixed amount of excess air, CxHy in the bed splash zone were determined to be substantially lower, compared to those for the highest load, mainly because of the higher proportion of secondary air and greater residence time, showing a significant reduction (decomposition) along the combustor height. Note that excess air had conventional effects on the axial CxHy concentration profiles. As seen in Figure 7b, the amount of CxHy in the freeboard of the SFBC operated at the reduced combustor load was at a rather low level for the range of excess air, exhibiting a slight reduction along the combustor height: from 500-890 ppm at Z = 1 m to 320-710 ppm at Z = 2.67 m. The axial NO concentration profiles in the SFBC are shown in Figure 8 for the two combustor loads and the ranges of excess air. Similar to that observed for CO and CxHy, the axial NO concentration profiles revealed three regions with apparent boundaries (Z), within which the net result (regarding NO formation/decomposition) was quite different. In the first region, Z < 0.8 m, the rate of NO formation from nitrogenous volatile species (mainly, NH39,36) prevailed over that of NO decomposition. This fact resulted in the high NO maximum, 700-950 ppm, observed at a level of ∼0.8 m above the air distributor when firing sunflower shells with elevated fuel-N at highest combustor load (see Figure 8a). The most significant axial gradient of NO was observed in the test at the highest amount of excess air (75%), despite the reduction in the bed temperature (see Figure 3a), which can be explained by (i) the higher rate of volatile oxidation, and (ii) lower CO concentrations in the dense bed (see Figure 6a). In the second region, 0.8 m < Z < 1 m, the rate of chemical reactions responsible for NO decomposition, such as catalytic reduction of NO by CO (on chars surface) and homogeneous reactions of NO with NH3 and CxHy, prevailed over the NO formation rate, resulting in the significant decomposition of NO. Note that the axial gradient of NO in this region was also subject to the dilution effects of secondary air. In the third region (Z > 1.0 m), the NO reduction along the combustor height occurred at a rather low rate, likely because of the reduction of both CO and particulate concentrations in the combustion products. Interestingly, at the levels 1.3-1.6 m above the air distributor (i.e., at a lower part of the third region), the axial gradient of

NO remained almost zero, thus, pointing at the presence of volatiles at these levels. Figure 8b shows that, at the reduced fuel feed rate, the axial gradient of NO in the first region was apparently less, compared to that at 60 kg/h, basically because of the lower bed temperature, and a lesser proportion of primary air, but higher concentrations of CO. As a result, the maximum values of NO observed at the level ∼0.8 m above the air distributor were noticeably lower (450-800 ppm), compared to those at the highest combustor load. This reduction seemed to predetermine the reduced values of NO at the exit of second and third regions (see Figure 8b). Meanwhile, the insignificant rate of NO reduction in the third region can be explained by rather low concentrations of CO and CxHy in the freeboard, as shown in Figures 6b and 7b. Emissions. Figure 9 depicts the CO, CxHy, and NO emissions (i.e., the concentrations measured at the cyclone exit), provided on the 6% O2 dry gas basis, for firing sunflower shells at the fuel feed rates of 60 and 45 kg/h for the entire range of excess air. With reduced combustor load, all these gaseous emissions were observed to be reduced, with the most significant reduction being exhibited by NO. In the meantime, these emissions, representing, in effect, the net results of formation and decomposition of the pollutants in distinct regions of the reactor, showed different trends when increasing the amount of excess air from 20% to 80%. At the lowest amount of excess air (∼20%), the CO and CxHy emissions were unreasonably high. However, these emissions can be effectively controlled by maintaining the amount of excess air at 50%-60% for the load range. Further increases in excess air (e.g., to 80%) led to an insignificant reduction in the CO emission (see Figure 9a), whereas the CxHy emissions showed some increase (see Figure 9b), likely because of the reduced combustion temperature and greater carryover of char particles and volatiles (including CxHy) into the reactor freeboard. The trends in Figure 9c confirmed the fuel-NO formation mechanism and pointed at the substantial contributions of CO and CxHy to the reduction of NO in the reactor, especially at lower magnitudes of excess air. The NO emission was determined to be increased with higher excess air and combustor load (combustion temperature). An excess air amount of ∼55% seems to be the best option for this operating variable, at which the major emissions, CO and NO, comply with the Thai emission standards for the biomass-fueled industrial applications: 690 ppm and 200 ppm, respectively, at 7% O2 dry flue gas under standard conditions (1 atm and 25 °C).38 For comparison of these limits with the corresponding emissions of 3857

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Figure 9. Effects of the fuel feed rate and excess air on the (a) CO, (b) CxHy, and (c) NO emissions from the SFBC firing sunflower shells. Table 2. Actual CO and CxHy at Stack, Unburned Carbon in Fly Ash, Heat Losses, and Combustion Efficiency of the SFBC Firing Sunflower Shells for Variable Operating Conditions Heat Loss (%) excess air (%)

secondary-to-primary air ratio

unburned carbon (wt %)

16 40 61 75

28:88 28:112 28:133 28:147

2.8 2.6 2.6 2.8

16 33 58 77

37:79 37:96 37:121 37:140

5.0 6.9 5.3 6.6

due to unburned carbon

due to incomplete combustion

combustion efficiency (%)

Fuel Feed Rate = 60 kg/h 3250 4590 1140 685 507 375 374 558

0.16 0.15 0.15 0.16

5.65 1.61 1.09 1.61

94.2 98.2 98.8 98.2

Fuel Feed Rate = 45 kg/h 2790 3500 689 297 321 232 235 504

0.27 0.38 0.29 0.36

5.49 0.76 0.57 1.11

94.2 98.9 99.1 98.5

CO (ppm)

CxHy (ppm)

the proposed technique, the above national emission standards were corrected to the 6% O2 dry flue gas to be 740 ppm for CO and 215 ppm for NO. As revealed by the results from this study, the major emissions from this SFBC firing sunflower shells at a rate of 60 kg/h in 55% excess air can be controlled at acceptable levels;740 ppm for CO and 205 ppm for NO (i.e., complying with the Thai emission standards);while maintaining the CxHy emissions at ∼400 ppm. At a reduced load, the emissions can be ensured at lower levels. Heat Losses and Combustion Efficiency. Table 2 shows the predicted heat losses that are due to unburned carbon and incomplete combustion, together with the combustion efficiency (all as percentages of the LHV) for the two fuel feed rates and the ranges of excess air and secondary-to-primary air ratio. The actual concentrations of CO and CxHy in the

stack gas, as well as the unburned carbon content in fly ashes, used in calculations of the heat losses, are also included in Table 2. It can be seen that the heat loss that is due to incomplete combustion, in response to the variation in CO and CxHy emissions (see Figures 9a and 9b), was, in effect, the major factor influencing the combustion efficiency. At the fuel feed rate of 60 kg/h, the percentages of unburned carbon in fuel ashes were rather small (2.6-2.8 wt %) and remained almost constant. Such behavior can be explained by the two opposite trends. On one hand, an increase in the air supply resulted in the higher oxidation rate of carbon during the combustion of chars, which occurred, as known, in the diffusion-flame regime.37 On the other hand, with increasing excess air, both the residence time and the combustion temperature were somewhat reduced, which likely mitigated the char oxidation. Because of insignificant fuel-ash content in sunflower shells, the magnitudes of heat loss with unburned carbon were at a rather low level (0.15%-0.16%). However, at the reduced combustor load, the content of unburned carbon in the ashes was higher

(38) Pollution Control Department, Ministry of Natural Resources and Environment, Thailand. Air Pollution Standards for Industrial Sources. Available via the Internet at http://www.pcd.go.th/info_serv/ reg_std_airsnd03.html, accessed on April 15, 2010.

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Table 3. Elemental Analysis of Fly Ashes Generated in the SFBC Firing Sunflower Shells for Some Selected Operating Conditions Ash Composition (wt % oxide in ash) excess air (%)

SiO2

Al2O3

Fe2O3

TiO2

40 61

21.9 16.0

1.2 1.3

1.4 1.0

0.05 0.07

33 58

16.3 12.0

0.9 0.9

1.2 2.0

0.05 0.04

CaO

Na2O

K2O

Mn3O4

SO3

sum

Feed Rate = 60 kg/s 15.9 9.6 24.1 13.1

0.3 0.3

22.6 23.8

0.08 0.09

1.2 1.9

74.2 81.7

Feed Rate = 45 kg/s 17.6 10.5 19.4 11.5

0.2 0.4

23.2 24.6

0.08 0.09

1.8 1.6

71.8 72.5

(5.0%-6.6%), mainly because of the lower bed temperature and the higher proportion of secondary air injected at ambient temperature into the bed splash zone, which likely increased the carryover of chars into the combustor freeboard. However, this did not significantly affect a general proportion between the two heat losses, leaving a predominant role to the heat loss due to incomplete combustion, which eventually led to the maxima of combustion efficiency: 98.8% at the 60 kg/h fuel feed rate and 99.1% at 45 kg/h, both being found for firing sunflower shells at ∼60% excess air. Thus, taking into consideration the above emission characteristics of the combustor, 55% excess air seems to be optimum with regard to ensuring the maximum combustion efficiency (∼99%) of this SFBC firing sunflower shells at levels complying with the Thai emission standards. Analysis of Ash. Table 3 shows the elemental analysis of fuel ashes (as the weight percentage of representative oxides) generated in the SFBC firing sunflower shells for some selected operating conditions. For these test runs, the total sum of the oxide percentages was rather far from 100%; this fact indicated the presence of a substantial amount of carbonates in the ashes. A comparison of the two ash compositions revealed very weak effects of the operating conditions on individual constituents in the elemental analysis. The major components in the ash of Thai sunflower shells were potassium (22.6-24.6 wt % as oxide), calcium (15.924.1 wt % as oxide), and silicon (12.0-21.9 wt % as oxide). This elevated amount of K2O may result in the relatively low fusion temperatures of the fuel ash.4 However, neither bed agglomeration nor significant ash deposit on the combustor walls was observed during the entire test period of ∼18 h for each of the two fuel feedings. As revealed by a visual inspection, sand particles removed from the combustor exhibited a normal appearance, i.e., without any features such as a sticky coating on the particle surface, which basically promotes the formation of agglomerates. This result can be explained by some factors. First, because of the injection of secondary air (at ambient temperature) through the start-up burner, the bed temperature was maintained at a level below 900 °C, or ∼50 °C lower than the maximum temperature in the freeboard, which certainly reduced the risk of the bed agglomeration. Second, along with the overbed fuel feeding, the elevated superficial velocity, as well as high gas-solid slip velocity in the conical swirling fluidized bed31 likely restrained penetration of char/ ash particles deep into the bed, thus reducing the probability of chemical reaction of alkali compounds (from fuel ash) with silica sand responsible for the particle coating. The third reason (factor) was an insignificant ash flux from the

MgO

combustion of this low-ash biomass fuel. With almost the same fuel-ash content but lower K2O concentration in the ash, Thai sunflower shells are expected to deposit the ash on the combustor wall at a rate lower than that revealed by Lokare et al. for this fuel,30 i.e., comparable to the relatively low deposition rate of wood sawdust. Conclusions An innovative swirling fluidized-bed combustor (SFBC) has been successfully tested for firing sunflower shells at fuel feed rates of 60 and 45 kg/h, while the (total) amount of excess air ranged from ∼20% to 80%. To generate a swirling gas-solid flow in the reactor, primary air is swirled and introduced into the bed using an annular spiral distributor, while secondary air is tangentially injected into the bed splash zone through a set of nozzles at a constant flow rate. This combustion technique ensures the effective control of CO, CxHy, and NO emissions in the reactor as well as high combustion efficiency when firing sunflower shells. The following specific conclusions have been derived from this experimental work: (1) The analysis of radial and axial CO, CxHy, and NO concentration profiles indicates the occurrence of three specific regions along the combustor height, showing significant differences in formation and decomposition of these pollutants at different locations in the combustor; (2) Reducing the fuel feed rate results in lower CO, CxHy, and NO emissions from the combustor, the most significant reduction being exhibited by NO; (3) CO and CxHy emissions can be effectively controlled by secondary air injection into the splash bed zone, thus ensuring the combustion efficiency at a high level; (4) The emission of NO increases sensibly with higher excess air and/or increased combustor load (accompanied by the increase in the combustion temperature); and (5) For firing sunflower shells at a fuel feed rate of 60 kg/h, 55% excess air seems to be an optimal value to ensure the highest combustion efficiency (∼99%) and acceptable levels of major gaseous emissions (740 ppm for CO and 205 ppm for NO, both complying with the local emission standards, while maintaining CxHy emissions at a level of 400 ppm). Acknowledgment. The authors would like to acknowledge the financial support from the Thailand Research Fund (Contract No. BRG 5380015) and from the Commission on Higher Education, Ministry of Education, Thailand (Contract No. 6/2551). Special thanks to Dr. Kanitta Wongyai (Electricity Generating Authority of Thailand), who performed the chemical analyses of fuel ashes.

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