A comparative study on combustion of sunflower shells in bubbling and swirling fluidized-bed combustors with a cone-shaped bed

Chemical Engineering and Processing 62 (2012) 26–38

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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

A comparative study on combustion of sunflower shells in bubbling and swirling fluidized-bed combustors with a cone-shaped bed夽 Porametr Arromdee, Vladimir I. Kuprianov ∗ School of Manufacturing Systems and Mechanical Engineering, Sirindhorn International Institute of Technology, Thammasat University, P.O. Box 22, Thammasat Rangsit Post Office, Pathum Thani 12121, Thailand

a r t i c l e

i n f o

Article history: Received 4 January 2012 Received in revised form 10 August 2012 Accepted 7 October 2012 Available online 16 October 2012 Keywords: Sunflower shells Bubbling and swirling fluidized-bed combustors Emissions Combustion efficiency

a b s t r a c t This paper compares the combustion and emission performance between the bubbling and swirling fluidized-bed combustors (FBCs) fired with sunflower shells. Morphology and thermogravimetric characteristics of this biomass fuel were investigated prior to combustion tests. In both case studies, trials were performed at two fuel feed rates, 60 kg/h and 45 kg/h, while ranging excess air from 20% to 80% at fixed combustor load. Temperature and gas concentrations (O2 , CO, Cx Hy as CH4 , and NO) were measured along axial and radial directions in both reactors, as well as at their stacks. The radial and axial temperature profiles in the two FBCs were found to be rather uniform, whereas hydrodynamics and operating conditions had apparent effects on the emission performance of the combustors. Compared to the bubbling FBC, the swirling FBC ensured a more intensive fuel burnout rate in the bed, which resulted in higher NO emission from this combustor at identical operating conditions. Excess air of ∼55% seems to be optimal to ensure high combustion efficiency (∼99%) and acceptable CO, Cx Hy and NO emissions of both combustors. At the optimal operating conditions, the emissions of both FBCs were quite similar. As follows from analysis of the radial and axial gas concentration profiles, the swirling FBC with its highly intensive burnout rate in the bottom region can be designed with a noticeably smaller height compared to the bubbling fluidized-bed combustor. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Annually, over 30 million tons of sunflower seeds are produced in Eastern Europe, Americas and Asia [1]. Sunflower shells, the major byproduct of sunflower seed processing, are characterized by excellent combustion characteristics and have therefore the potential be used as a renewable fuel [2,3]. Taking into account the availability and calorific value of sunflower shells, the world energy potential of this biomass fuel is estimated to be ∼80 PJ per year. As well known, the fluidized-bed combustion technology is one of the best options for conversion of energy from biomass. A large number of research studies have addressed conventional fluidized-bed combustion techniques, such as bubbling and circulating fluidized-bed combustion systems (combustors and boiler furnaces) firing various biomass fuels, focusing mainly on

夽 A preliminary version of this paper was presented at the 18th European Biomass Conference & Exhibition, held at Lyon Convention Center (Lyon, France), May 3–7, 2010, CD-ROM Proceedings, ISBN: 978-88-89407-56-5, ETA-Florence Renewable Energies, Florence, Italy. ∗ Corresponding author. Tel.: +66 2 986 9009x2208; fax: +66 2 986 9112. E-mail address: [email protected] (V.I. Kuprianov). 0255-2701/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cep.2012.10.002

combustion efficiency and emission performance of these systems [4–8]. The studies revealed that the combustion of most biomass fuels is accompanied by substantial emission of gaseous pollutants (the major being NO and CO), the emission rate of those depends on fuel properties as well as on design features and operating conditions of the combustor/boiler. Some authors highlighted difficulties in achieving high combustion efficiency of the systems fired with high-ash biomass fuels [8,9]. When burning biomasses with elevated content of alkali-based compounds in systems using silica sand as the bed material, there is a risk of bed agglomeration [9–11] causing eventually bed defluidization and emergent shutdown of the combustor/boiler. Apart from the conventional fluidized-bed combustion techniques, there is a group of combustors integrating a fluidized bed with strong vortex (or rotation) of gas–solid flow, which plays a significant role in intensification of fuel burnout in the reactor. Depending on hydrodynamics of the combustor, these combustion techniques for firing biomass fuel (or co-firing biomass and coal) can be categorized into three main groups: vortexing, swirling and cyclonic fluidized-bed combustors [12–14]. A vortexing fluidized-bed combustor is generally designed as a cylindrical chamber with a substantial height-to-diameter ratio comprising (i) a conventional (non-swirling) fluidized bed induced by primary air in the bottom part of the reactor and (ii) a vortexing

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Fig. 1. Various fluidized-bed combustion techniques with a cone-shaped bed: (a) a bubbling FBC, (b) a vortexing FBC and (c) a swirling FBC.

gas–solid flow in its freeboard formed by secondary air injected tangentially into the combustor [12]. Unlike in vortexing systems, a (primary) air distributor of a swirling fluidized-bed combustor is designed as an axial-flow swirler generating a fluidized and simultaneously rotating gas–solid bed [13]. However, a cyclonic fluidized-bed combustor with its moderate height-to-diameter ratio exhibits the most complicated hydrodynamics combining a non-swirling fluidized bed and a strongly swirled gas–solid flow, the latter being turned through 180◦ [14]. The usage of a centrifugal force for separation of char/ash particles from flue gas and returning solids into the fluidized bed (thus, forming an internal solid circulation) is the main feature of cyclonic devices. Vortexing, swirling and cyclonic fluidized-bed combustors are reported to ensure high (over 99%) combustion efficiency, i.e., quite low emission of products of incomplete combustion. However, burning biomasses in relatively ‘short’ reactors (i.e., with high

combustion intensity) may be accompanied by elevated emission of NO [14]. A fluidized-bed combustor with a cone-shape bed is proven to be suitable for firing or co-firing various biomass fuels [15–19]. Compared to columnar (cylindrical and prismatic) fluidized-bed reactors, this combustion technique ensures sustainable ignition and combustion of fuel using a relatively small amount of inert bed material, which is important when using costly alternative bed materials for firing high-alkali biomasses. Such a combustor can be designed with different hydrodynamic patterns, such as a bubbling [15], or vortexing [16], or swirling [17] fluidized-bed combustion technique, as illustrated in Fig. 1. Two another benefits of cone-shaped fluidized-bed combustion systems are: (i) reduced start-up time of the reactor [15–17] and (ii) lower pressure drop across the fluidized bed [20,21], both resulting in lower operational costs.

Fig. 2. Design features and geometrical characteristics of the (a) bubbling and (b) swirling fluidized-bed combustors for firing sunflower shells with heat input of up to ∼300 kWth .

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This experimental work was aimed at studying the combustion of sunflower shells in the bubbling and swirling fluidized-bed combustors of similar geometry and heat input. To facilitate interpretation of empirical findings from this study, morphology and thermogravimetric characteristics of sunflower shells were investigated prior to combustion tests. Effects of operating conditions on formation and decomposition of major gaseous pollutants (CO, Cx Hy and NO) in the reactors as well as on the emissions and combustion efficiency were compared in this work between the two combustion techniques.

2. Methods 2.1. Experimental facilities To achieve the main objectives of this work, experimental tests were carried out on two FBCs with a cone-shape bed: (1) the newly designed bubbling fluidized-bed combustor (BFBC), and (2) the swirling fluidized-bed combustor (SFBC) [13], both operated with heat input of up to ∼300 kWth when firing sunflower shells. Fig. 2 shows the schematic diagram of both combustors. It can be seen in Fig. 2 that the reactors had similar design and geometrical dimensions, both consisting of six refractory-lined steel sections (modules) connected coaxially: one conical (lowest) module with a 40◦ cone angle and 0.25 m outer diameter at the bottom plane, and five cylindrical modules of 0.5 m height and 0.9 m inner diameter. In each module, the refractory-cement insulation was 50-mm thick lined inside a 4.5-mm-thick metal wall. Gas sampling ports and stationary Chromel–Alumel thermocouples (of type K) were fixed at different levels in each reactor. Using these stationary thermocouples, temperature was monitored (with the accuracy of ±1%) along the reactor centerline during the combustors’ start up as well as in combustion tests. A bubble-cap air distributor with thirteen bubble caps (stand pipes) evenly distributed across the distributor plate was used in the BFBC to sustain fluidization of the bed. The net cross-sectional area of airflow at the distributor exit (calculated as the difference between area of the 250-mm-diameter distributor plate and total area occupied by the caps) was 0.027 m2 . Fig. 3 shows the schematic diagram of an individual stand pipe, which was modified, as compared to the design of bubble caps previously used on a 350-kWth conical FBC [22]. An annular air distributor assembled as the axial-flow swirler with eleven blades fixed at an angle of 14◦ to the horizontal was used in the SFBC to induce swirl motion of the gas–solid fluidized bed. The distance between two neighbor blades was variable (in a linear relationship with radius), thus, forming a trapezoidal crosssectional area of 0.012 m2 (total) for the airflow between the blades. Fig. 4 shows geometrical details of the air distributor for the SFBC. To sustain a rotational flow of the fluidized bed, a cone-shaped steel stabilizer was coaxially fixed on the air distributor (see Fig. 2b). Quartz sand of 2650 kg/m3 solid density was used as the bed material in both combustors, however, with different particle (sieve) size: 0.3–0.5 mm in the BFBC and 0.5–0.6 mm in the SFBC, as recommended by Refs. [15] and [13], respectively. In all combustion tests, the static bed height was 30 cm. For the selected cone angle (40◦ ), at relatively low fluidizing air velocities, the above-mentioned characteristics of the bed ensured the bubbling fluidization regime of both conventional and swirling fluidized beds [21,23]. Due to special design of the air distributor and proper sand particle size, the spouting fluidization mode (basically occurring in conical gas–solid beds at some operating conditions [20,24]) was prevented in the BFBC [21]. In both case studies, the experimental set-up included identical auxiliary equipment and facilities: a start-up burner, a fuel feeder, a

Fig. 3. Schematic diagram of an individual bubble cap of the air distributor used in the bubbling fluidized-bed combustor.

blower, an ash-collecting cyclone (installed downstream from the combustor), as well as facilities for data acquisition and treatment. The diesel-fired burner (model “Press G24” from Riello Burners Co.) was used in both FBCs to preheat sand during the combustor start up. The burner was fixed at a 0.5 m level above the air distributor and inclined at a −30◦ angle to the horizontal, i.e., toward the bed. When the bed (sand) temperature attained ∼700 ◦ C, the burner was turned off, and the desired combustor load was ensured by biomass feeding only. During the tests on the BFBC, the burner was screened (separated) from the reactor space by a 20 cm × 30 cm steel curtain. However, when conducting experiments on the SFBC, the burner fan continued to operate tangentially injecting secondary air (at a flow rate QSA = 0.024 Nm3 /s) into the conical module with the aims: (i) to sustain swirling gas–solid flow, (ii) to protect the burner-nozzle head against thermal and mechanical impacts, and also (iii) to oxidize (mitigate) CO and Cx Hy in this region. The screw-type feeder delivered sunflower shells over the bed at a 0.6 m level above the air distributor in both combustors. The fuel feed rate was controlled/adjusted using a three-phase inverter. A 25-hp blower supplied (primary) air to the combustors through the air pipe. A model “Testo-350XL” gas analyzer (Testo, Germany) was used to measure temperature and gas concentrations (O2 , CO, Cx Hy as CH4 , and NO) at sampling points inside the reactors, as well as at the exit of the ash-collecting cyclone, i.e., at stack. During measurements, a sampled gas was drawn through a standard stainless steel probe of 8 mm diameter and 700 mm length. A NiCr–Ni thermocouple fixed inside the probe shaft was employed to measure gas temperature (up to 1000 ◦ C) at each sampling point. The sampled gas was then transported into the gas analyzer through a 2.2-m long hose lined with Teflon. A built-in pump with automatic flow control provided required (optimal) gas flow to ensure the best response and accuracy of the gas sensors. Due to the thermoelectric conditioner of the gas analyzer, the gas concentrations were measured on a dry basis. The measurement accuracies were ±0.5% for temperature, ±5% for CO within the range of 100–2000 ppm, ±10% for CO

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Fig. 4. Schematic diagram of the axial flow air distributor used in the swirling fluidized-bed combustor.

higher than 2000 ppm, ±10% for Cx Hy (as CH4 ) up to 40,000 ppm, ±5% for NO, and ±0.2% (vol.) for O2 . 2.2. Fuel properties Table 1 shows the ultimate and proximate analyses, as well as the lower heating value (LHV) of sunflower shells burned in both FBCs. It can be seen in Table 1 that the proximate analysis of sunflower shells included a significant amount of volatile matter, but rather low proportions of fuel moisture and ash, which resulted in the substantial calorific value of this biomass fuel: LHV = 17,150 kJ/kg. The average dimensions of individual sunflower shells were: a width of 6 mm, a thickness of 0.5 mm, and a length of 10 mm. The fuel solid (actual) density was determined in laboratory tests to be 592 ± 15 kg/m3 . To extend awareness of fuel properties, morphology and thermogravimetric characteristics of sunflower shells fuel were investigated prior to combustion tests. A JEOL JSM-6400 scanning electron microscope configured with an energy dispersive system (SEM-EDS) was used to examine an internal surface (i.e., surface texture) of biomass particles [25,26], likely affecting biomass reactivity during the stages of fuel devolatilization and char oxidation.A Mettler-Toledo TGA/SDTA 851e thermogravimetric analyzer was used to obtain thermogravimetric characteristics (TG and DTG curves) for the tested biomass fuel. During the thermogravimetric test, a 5.2-mg sample of specially prepared sunflower shells was heated from room temperature to 900 ◦ C at a heating rate of 20 ◦ C/min. The surrounding medium for the sampled biomass was dry air supplied into the system at 60 ml/min. Some important combustion characteristics, such as ignition, peak and burnout temperatures (all indicating fuel reactivity), were obtained from the analysis of the TG and DTG curves [26–28]. Because of very small fuel S content in sunflower shells, SO2 was not addressed in this work. 2.3. Combustion test procedures

NO emissions, as well as combustion efficiency of both FBCs (to be addressed below), the gas concentrations were recorded at the cyclone exit (i.e., at stack) for the two fuel feed rates, FR = 60 kg/h and FR = 45 kg/h, when ranging EA from 20% to 80% at fixed combustor load. For the particular test run (operating conditions), the excess air coefficient (˛) was determined according to Ref. [29] using actual concentrations of O2 , CO and Cx Hy (as CH4 ) at the cyclone exit (i.e., at stack). The percentage of excess air was then calculated using its correlation with ˛: EA = 100(˛ − 1). 2.4. Determining heat losses and combustion efficiency In this work, combustion efficiency of both combustors was determined using the heat-loss method [30]. For both combustors with no ash removal through the bottom part, heat loss due to unburned carbon was predicted using unburned carbon content in the fly ash as well as fuel ash content and lower heating value: quc =

32, 866 LHV



Cfa 100 − Cfa



A

(1)

Heat loss owing to incomplete combustion was quantified as the percentage of LHV based on the CO and Cx Hy (as CH4 ) emissions from the combustors (both in ppm, on a dry gas basis and at 6% O2 ): qic = (126.4CO + 358.2CH4 )@6%O2 10−4 Vdg@6%O2

(100 − quc ) LHV

(2)

In Eq. (2), the volume of dry flue gas under standard conditions (1 atm and 0 ◦ C) and at 6% O2 was determined according to Ref. [29] using the fuel ultimate analysis (see Table 1), the theoretical volume of air (V0 ) required for burning 1 kg fuel, and also the excess air coefficient corresponding to the above-mentioned reference value of O2 (˛ref = 1.4): Vdg@6%O2 = 0.01866(C + 0.375S) + 0.008N + 0.79V 0 + (aref − 1)V 0 (3)

In this study, two key operating variables were selected as independent operating variables: the fuel feed rate (FR) and excess air (EA). During the combustion tests on both FBCs, temperature and concentrations of O2 , CO, Cx Hy (as CH4 ) and NO were measured along radial and axial directions in the two combustors operated steadily. In addition, the measurements were performed at stack of each combustor to compare combustion efficiency and emissions of the FBCs for variable operating conditions. Prior to measurements in distinct test runs, steady state conditions were verified based on stability of two temperatures: at the top of the conical module as well as at the combustor top (both measured using the above-mentioned fixed thermocouples), i.e., when their time-domain fluctuations did not exceed ±2 ◦ C. To minimize the volume of experimental work, the radial and axial temperature and gas concentration profiles of the two combustors were compared only for FR = 60 kg/h and two values of EA: ∼40% and ∼80%. However, to quantify CO, Cx Hy (as CH4 ) and

where V0 was quantified according to Ref. [30]: V 0 = 0.0889(C + 0.375S) + 0.265H − 0.033O

(4) 3

From Eqs. (3) and (4): V0 = 5.1 Nm3 /kg and Vdg@6%O2 = 7.1 Nm /kg (both at 1 atm and 0 ◦ C), the latter being independent of operating conditions. The combustion efficiency of the FBCs was then determined as: hc = 100 − (quc + qic )

(5)

3. Results and discussion 3.1. SEM images of sunflower shells Fig. 5 depicts the SEM images of an individual sunflower shell exhibiting its crosswise and lengthwise sectional views at two different magnifications. The micrographs in Fig. 5 basically

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Table 1 Ultimate and proximate analyses and lower heating value (LHV) of sunflower shells fired in the bubbling and swirling fluidized bed combustors (W = fuel moisture; A = fuel ash; VM = volatile matter; FC = fixed carbon). Ultimate analysis (wt.%, as-received)

Proximate analysis (wt.%, as-received)

LHV (kJ/kg)

C

H

O

N

S

W

A

VM

FC

52.2

5.59

29.68

0.63

0.10

9.1

2.7

65.6

22.6

reveal a cellular texture of the shell “constructed” from the tightly packed hollow cells of various sizes: from a few microns to up to 30 ␮m. Like for most shell-type biomasses, the cell wall of sunflower shells generally consists of cellulose, hemicellulose and lignin [31,32]. Cellulose microfibrils (the structural framework of a cell) are embedded in a matrix generally consisting of hemicellulose and lignin, the latter being the major binding material in the cell wall [26]. Since lignin is a highly stable aromatic polymer, the cells with heavily lignified walls (like those with relatively small diameter at the outer shell surface) increase the strength and hardness of the cell structure. It can be clearly seen in the micrographs with higher (×1500) magnification that the wall of a typical cell is very thin (2–3 ␮m) and has a large number of small holes (of 1–2 ␮m in diameter). During volatilization and further oxidation of wall material, these holes facilitate diffusion (flux) of chemical species in all directions across the shell. Thus, taking into account (i) a highly developed

17,150

internal surface of sunflower shells, (ii) a high volume to surface area ratio of biomass cells as well as (iii) a “perforated” surface of the cell wall, the thermal and chemical reactivity of this biomass fuel is expected to be significant. Yet a high level of fuel oxygen (see Table 1) certainly enhances the rate of volatile oxidation inside biomass cells. 3.2. Thermogravimetric analysis The TG and DTG curves for sunflower shells were obtained and analyzed taking into account the following empirical findings: • sunflower shells consists mainly of cellulose (approximately half of dry fuel mass) and hemicellulose (about one-third of dry fuel mass), while the rest is lignin [32], • within the temperature range of 240–450 ◦ C, volatilization of holocellulose (a mixture of cellulose and hemicellulose) is quite intensive, attaining a significant maximum rate, about

Fig. 5. SEM images of (a) crosswise and (b) lengthwise sectional views of an individual sunflower shell at different magnifications: 200× (upper images) and 1500× (lower images).

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0.000 -0.020

80

-0.040 TG DTG

60

-0.060 -0.080

40

-0.100 20

T ign

0 0

T p,1

T p,2

Tb

DTG (1/min)

Mass percent (%)

100

-0.120

-0.140 100 200 300 400 500 600 700 800 Tem perature (ºC)

Fig. 6. TG and DTG curves for the sample of sunflower shells.

30 wt.%/min, at the (holocellulose) peak temperature of 350 ◦ C [26], • unlike holocellulose, lignin decomposes within a wider temperature range, 220–650 ◦ C, exhibiting, however, a substantially lower maximum rate of decomposition, only 7 wt.%/min, at the (lignin) peak temperature of 410 ◦ C [26], • volatile matter of biomass is generally originated from holocellulose, whereas lignin is a major contributor to fuel char [26], whose oxidation rate is rather low [25,27]. Fig. 6 depicts the TG and DTG curves obtained from this study. While the TG profile represented the mass loss of the biomass sample that occurred with increasing temperature, the DTG profile showed the time derivative of the mass loss as a function of current temperature. Three distinct regions (stages) associated with thermal destruction and oxidation of biomass particles can be distinguished in the TG–DTG graph: (1) dewatering (at temperatures less than 130 ◦ C, (2) (almost) complete volatilization of holocellulose and partial volatilization of lignin (within the range of 220–500 ◦ C), (3) volatilization of the rest lignin along with combustion of the volatiles and chars (at 500–800 ◦ C). It can be seen in Fig. 6 that within the second and third regions (associated with fuel reactivity), the DTG curve had two peaks showing the highest rates of (i) holocellulose volatilization (at Tp,1 = 310 ◦ C) and (ii) lignin decomposition (at Tp,2 = 485 ◦ C). Two more combustion characteristics – ignition and burnout temperatures – were determined from the TG–DTG analysis as well: Tign = 260 ◦ C and Tb = 620 ◦ C (as shown in Fig. 6). However, at temperatures greater than Tb (up to 800 ◦ C), decomposition (oxidation) of char particles occurred at a quite low rate, likely with no flame [25]. Due to significantly higher reactivity of holocellulose (compared to lignin) and predominant proportion of holocellulose in lignocellulosic biomasses, Tp,1 is often used for comparison of reactivity of different biomass fuels [28], whereas Tb is an important indicator of fuel complete combustion. Taking into account the relatively low values of Tp,1 and Tb , sunflower shells used in this study can be categorized as a biomass fuel with high thermal and combustion reactivity. At temperatures typical for fluidized-bed combustion systems (800–900 ◦ C), this biomass is therefore expected to be burned with high combustion efficiency. 3.3. Actual operating conditions in combustion trials Table 2 summarizes the actual operating variables of the BFBC and SFBC for different test runs. In both combustors operated at design and reduced loads, the fuel was burned at four values of EA,

31

which were quantified using the actual O2 , CO and Cx Hy (as CH4 ) at stack. Note that in the combustion tests on the SFBC at fixed excess (total) air, due to the invariable flowrate of secondary air (QSA = 0.024 Nm3 /s, as mentioned above), the percentage of secondary air (SA) for the two combustor loads was different: SA = 28% for FR = 60 kg/h, and SA = 37% for FR = 45 kg/h. Correspondingly, the percentage of primary air (PA) and the secondary to total air ratio (SA/TA) were somewhat different for different loads at identical (or similar) EA values. Due to the air split (or air staging), PA in the SFBC was lower than TA in the BFBC, which resulted in different magnitudes of superficial (axial) velocity (ua ) in both combustors when burning the fuel at identical FR and EA. Note that ua had a significant influence on hydrodynamic behavior of the bed in the BFBC: at lower values of FR and EA, the bed exhibited bubbling fluidization regime, whereas transition or turbulent fluidization regimes occurred at higher fuel feeding and airflow levels, as found in observations of the bed behavior under cold-state operating conditions for a similar range of ua . For the SFBC, the ratio of tangential to axial air velocity, ut /ua , was ∼4.0, which indicated a strongly swirled flow in this combustor at the air distributor exit. However, ua in different tests on this combustor was relatively low, which likely resulted in fully swirling (bubbling) fluidization regime of the bed [23]. 3.4. Temperature and O2 concentration profiles Fig. 7 compares the radial temperature and O2 concentration profiles in the two FBCs at three levels (Z) above the air distributor when firing 60 kg/h sunflower shells at nearly the same EA value (∼40%). It can be seen in Fig. 7 that the radial temperature profiles in both reactors were quite uniform at all the levels, pointing at highly intensive heat-and-mass transfer along the radial direction. However, the radial O2 concentration profiles at these levels exhibited quite different behaviors between the combustors. It appears that in the BFBC, the radial O2 concentration profiles were rather uniform (with O2 being a bit higher at the reactor centerline, mainly due to hydrodynamic factors), whereas in the SFBC, due to the effects of secondary air (injected tangentially into the combustor), one can observe an apparent positive radial gradient of the O2 concentration across the reactor. The axial temperature and O2 concentration profiles are compared in Fig. 8 between the two FBCs firing 60 kg/h sunflower shells at EA values of ∼40% and ∼80%. The axial temperature profiles (in Fig. 8a) turned out to be fairly uniform in both reactors. However, in the lower part of each combustor (Z < 1.5–2 m), all the axial temperature profiles were found to have a slight positive gradient along the combustor height, mainly due to (i) elevated superficial velocity at the bottom plane of the conical beds, (ii) selected level of fuel feedings, and (iii) evaporation of fuel moisture. With increasing Z in the upper region of the combustors (Z > 1.5–2 m), temperature was found to be gradually reduced along the reactor height in all the test runs, and this axial diminishing of temperature was likely caused by heat loss across the walls. At a given EA level, temperature at any location in the SFBC was somewhat lower than in the BFBC, primarily due to the injection of secondary air (at ambient temperature) into the swirling fluidized bed. In the meantime, an increase in EA resulted in some reduction of temperature at all points in both combustors (basically caused by the dilution effects of excessive air). As revealed by the results in Figs. 7 and 8a, the behavior of temperature along the radial and axial directions indicated quasi-isothermal conditions of the gas–solid flow in the two FBCs. The difference in the combustion method – conventional in the BFBC and air-staging in the SFBC – predetermined a significant difference in behavior of axial O2 concentration profiles in the two

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Table 2 Actual operating conditions of the bubbling and swirling fluidized-bed combustors fired with sunflower shells at the fuel feed rates of 60 kg/h and 45 kg/h for variable excess air. Run no.

Fuel feed rate

Gas concentration at stacka

FR (kg/h)

O2 (%)

CO (%)

Cx Hy (%)

Burning sunflower shells in the bubbling fluidized-bed combustor 60 4.1 3490 2540 1 1250 670 2 60 6.2 3 60 7.9 590 310 4 60 9.2 336 280 45 3.7 1780 814 5 45 6.1 711 165 6 45 7.9 213 85 7 218 107 8 45 9.4 Burning sunflower shells in the swirling fluidized-bed combustor 60 3.9 3250 4590 9 10 60 6.2 1140 685 60 8.1 507 375 11 60 9.2 374 558 12 45 3.7 2790 3500 13 45 5.3 689 297 14 45 7.8 321 232 15 235 504 16 45 9.3 a b

Excess air

Primary (total) air

Secondary air

Velocity componentb

EA (%)

PA or TA (%)

SA (%)

ua (m/s)

20 40 59 77 20 40 60 81

120 140 159 177 120 140 160 181

0 0 0 0 0 0 0 0

4.5 5.3 6.0 6.6 3.4 3.9 4.5 5.1

0 0 0 0 0 0 0 0

16 40 61 75 16 33 58 77

88 112 133 147 79 96 121 140

28 28 28 28 37 37 37 37

2.7 3.2 3.7 4.0 2.0 2.3 2.7 3.1

10.8 12.9 14.9 16.1 8.0 9.2 10.8 12.4

ut (m/s)

On a dry gas basis. At the air distributor exit.

reactors. As seen in Fig. 8b, in the dense bed region of the BFBC (0 < Z < 0.6 m) and somewhat higher (up to Z = 1 m), the O2 concentration profiles showed a gradual diminishing of O2 along the combustor height, exhibiting, however, a substantial level of O2 in the conical module. Note that in the SFBC (with PA being 28% lower than TA), the air split led to a greater residence time of the reactants and more rapid O2 consumption in the dense bed region, which resulted in a dramatic reduction of O2 along the axial direction (to

(b)

1100

1100

1000

1000

Temperature (ºC)

Temperature (oC)

(a)

about 5%, at Z = 0.5 m) when burning this biomass with high thermal and combustion reactivity. As a response to the secondary air injection, a significant regain (increase) of the O2 concentration was observed at 0.5 < Z < 1 m in the SFBC. In the freeboard of the two FBCs (Z > 1 m), a gradual reduction of O2 was found to occur at nearly the same rate along the reactor height (as seen in Fig. 8b), showing, however, apparent effects of EA, especially at the combustor top.

900 800 Z = 2.55 m Z = 1.55 m Z = 0.6 m

700

0.2

0.4

0.6

0.8

800 Z = 2.67 m Z = 1.55 m Z = 0.74 m

700 600

600 0

900

0

1

0.2

20

15

15

O2 (vol.%)

O2 (vol.%)

20

10 5

0.4

0.6

0

1

10 5

Z = 2.55 m Z = 1.55 m Z = 0.6 m

0.8

r/R

r/R

Z = 2.67 m Z = 1.55 m Z = 0.74 m

0 0

0.2

0.4 r/R

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

r/R

Fig. 7. Radial temperature and O2 concentration profiles at three levels above the air distributor inside the (a) bubbling and (b) swirling fluidized-bed combustors when firing 60 kg/h sunflower shells at excess air of ∼40%.

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(b) 1100

25

1000

20

O2 (vol.%)

Temperature (oC)

(a)

33

900 800 BFBC: EA BFBC: EA SFBC: EA SFBC: EA

700

= 40% = 77% = 40% = 75%

600 0

0.5 1 1.5 2 2.5 3 3.5 Height above the air distributor (m)

BFBC: EA BFBC: EA SFBC: EA SFBC: EA

= 40% = 77% = 40% = 75%

15 10 5 0 0

0.5 1 1.5 2 2.5 3 3.5 Height above the air distributor (m)

Fig. 8. Comparison of the axial (a) temperature and (b) O2 concentration profiles in the bubbling and swirling fluidized-bed combustors fired with sunflower shells at the fuel feed rate of 60 kg/h for two levels of excess air.

3.5. Formation and decomposition of CO and Cx Hy The radial CO and Cx Hy (as CH4 ) concentration profiles are compared in Fig. 9 between the two combustors for the same operating conditions and locations, as in Fig. 7. In the BFBC, these profiles were fairly/rather uniform, with CO and Cx Hy being somewhat lower at the centerline, particularly in the conical module, reflecting the behavior of O2 in the reactor (see Fig. 7a). However, at the bottom region of the SFBC, the radial CO and Cx Hy (as CH4 ) concentration profiles exhibited significant (or noticeable) negative gradient along the radius, caused apparently by the tangential injection of secondary air into the reactor, whereas in the freeboard of the combustor, these profiles became quite/fairly uniform, mainly due to intensive gas–solid mixing and oxidation processes across the reactor. Fig. 10 compares the axial CO and Cx Hy (as CH4 ) concentration profiles between the two FBCs for the same operating conditions, as in Fig. 8. These profiles in both reactors exhibited three specific regions, within which formation and decomposition of CO and Cx Hy occurred at different rates. A significant increase of CO and Cx Hy along the axial distance was observed in the dense bed region (0 < Z < 0.6 m), mainly due to rapid fuel devolatilization (for CO, also due to oxidation of Cx Hy and char-C), while the rate of oxidation of these species in this region was relatively low. Note that in the SFBC, due the air split (that caused air deficiency in this region), the peaks of CO and Cx Hy were significantly higher than those in the BFBC operated at similar EA, as can be compared in Fig. 10. In the two FBCs, a significant negative gradients of CO and Cx Hy were observed along the axial distance at 0.6 m < Z < 1 m, with the gas concentrations being much greater in the SFBC, where the rate of CO and Cx Hy oxidation was enhanced by the injection of secondary air into this region. Thus, decomposition of CO and Cx Hy in the conical module of both FBCs was apparently predominant, whereas formation of these species occurred at a substantially lower rate than in the dense bed region (where the fuel burnout was significant). In the freeboard of the two FBCs (Z > 1 m), CO that mainly formed via oxidation of char-C and Cx Hy (both carried over from the combustor bottom) was decomposed by residual air, resulting eventually in rather low CO concentrations at the combustor top, which showed the trend to be reduced with increasing EA. Yet in the freeboard of the reactors, Cx Hy stayed at a relatively low level, basically exhibiting the trends and effects of EA similar to those of CO.

As can be compared in Figs. 9 and 10, the current CO concentration (at a given Z) in the freeboard region of the SFBC was substantially lower than that in the BFBC, which indicated a more intensive fuel burnout in the SFBC at similar operating conditions. For instance, at a level Z ≈ 2.6 m, the CO and Cx Hy in the SFBC were 4–6 times lower than those in the BFBC. It is therefore suggested that the SFBC can be designed with a noticeably smaller height (by ∼1 m) compared that of the BFBC for firing sunflower shells at similar fuel feeding and (total) excess air level. 3.6. Formation and decomposition of NO During the combustion of agricultural residues, NO is known to form from volatile nitrogenous species (mainly, NH3 ), via the fuelNO formation mechanism that includes effects of fuel-N, excess air and combustion temperature, the latter being the minor factor [9]. Meanwhile, NO is decomposed (reduced) through its (i) reactions with CO on the surface of fuel chars [9,12,33], as well as via (ii) homogeneous reactions with NH3 and light hydrocarbon radicals from volatiles [34,35]. Thus, NO at a given point inside the combustor represents a net result of formation and decomposition of nitric oxide at this location. The radial NO concentration profiles are compared between the two FBCs in Fig. 11 for the same operating conditions and locations, as in Figs. 7 and 9. The profiles in the BFBC (see Fig. 11a) revealed that NO in the radial direction were basically represented by quasi-uniform profiles, especially in the freeboard, which can be explained by the uniformity of temperature, as well as of O2 , CO and Cx Hy concentrations, across the reactor. Taking into account the radial behavior of temperature as well as of O2 , CO and Cx Hy (see Figs. 7 and 9), expected NO at the central zone in the SFBC in Fig. 11b might be substantially lower than that at the peripheral zone, i.e. near the combustor wall. However, the radial NO concentration profiles in this combustor were rather uniform at all levels Z, likely, because of the dilution effects of secondary air in the peripheral zone of the reactor, and also due to the intensive gas–solid mixing across the reactor. Fig. 12 compares the axial NO concentration profiles between the two FBCs for the same operating conditions, as in Figs. 8 and 10. Like CO and Cx Hy , the axial NO concentration profiles revealed distinct specific regions in the combustors, within which a net result (regarding NO formation/decomposition) was quite different. In the bottom region of both reactors (i.e., in the conical module), the rate of NO formation from nitrogenous volatile species prevailed that of NO decomposition. This apparently resulted in a high maximum

34

P. Arromdee, V.I. Kuprianov / Chemical Engineering and Processing 62 (2012) 26–38

(a)

(b) 12000

10000

10000 Z = 2.55 m Z = 1.55 m Z = 0.6 m

8000 6000

CO (ppm)

CO (ppm)

12000

4000 2000

8000 6000 4000 2000

0

0

0

0.2

0.4 r/R

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

r/R

12000

12000

Z = 2.55 m Z = 1.55 m Z = 0.6 m

10000

Z = 2.67 m Z = 1.55 m Z = 0.74 m

10000

8000

CxHy (ppm)

CxHy (ppm)

Z = 2.67 m Z = 1.55 m Z = 0.74 m

6000 4000 2000

8000 6000 4000 2000

0

0 0

0.2

0.4 r/R

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

r/R

Fig. 9. Radial CO and Cx Hy (as CH4 ) concentration profiles at three levels above the air distributor inside the (a) bubbling and (b) swirling fluidized-bed combustors when firing 60 kg/h sunflower shells at excess air of ∼40%.

of NO attained in this region in both combustors: 500–550 ppm in the BFBC and 700–950 ppm in the SFBC. The higher formation rate of NO in the SFBC can be explained by the greater residence time of the reactants, which facilitated a higher yield of nitrogenous species and, correspondingly, elevated NO in this region in spite of higher CO and Cx Hy . At upper levels in both reactors (i.e., in the freeboard), the rate of chemical reactions responsible for NO decomposition, such as catalytic reduction of NO by CO (on the surface of chars) and homogeneous reactions of NO with light hydrocarbons, prevailed the rate of NO formation, which eventually resulted in substantial decomposition of NO in this region. However, at levels 0.8 m < Z < 1 m in

(a)

(b) 20000

20000

BFBC: EA = 40% BFBC: EA = 77% SFBC: EA = 40% SFBC: EA = 75%

10000 5000

BFBC: EA = 40% BFBC: EA = 77% SFBC: EA = 40% SFBC: EA = 75%

15000

Cx H y (ppm)

15000

CO (ppm)

the SFBC, the rate of NO decomposition was apparently higher than that in the BFBC, which was likely caused by extremely high CO and Cx Hy (see Fig. 10) in this relatively short zone. Meanwhile, at Z > 1 m (where CO, Cx Hy were insignificant), the rate of NO decomposition was comparatively low in both FBCs. However, as follows from analysis of data in Figs. 11 and 12, the current NO (at fixed Z) in the freeboard region of the SFBC was apparently greater than that in the BFBC for similar operating conditions. It can be seen in Fig. 12 that effects of excess air on the behavior of NO in the axial direction were substantial, thus confirming the fuel-NO formation mechanism.

10000 5000

0

0 0

0.5 1 1.5 2 2.5 3 3.5 Height above the air distributor (m)

0

0.5 1 1.5 2 2.5 3 3.5 Height above the air distributor (m)

Fig. 10. Comparison of the axial (a) CO and (b) Cx Hy (as CH4 ) concentration profiles in the bubbling and swirling fluidized-bed combustors fired with sunflower shells at the fuel feed rate of 60 kg/h for two levels of excess air.

P. Arromdee, V.I. Kuprianov / Chemical Engineering and Processing 62 (2012) 26–38

(a)

(b)

1000

1000

Z = 2.55 m Z = 1.55 m Z = 0.6 m

800

NO (ppm)

800

NO (ppm)

35

600 400 200

600 400 Z = 2.67 m Z = 1.55 m Z = 0.74 m

200

0

0

0.2

0

0.4 r/R

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

r/R

Fig. 11. Radial NO concentration profiles at three levels above the air distributor inside the (a) bubbling and (b) swirling fluidized-bed combustors when firing 60 kg/h sunflower shells at excess air of ∼40%.

3.7. Emissions Fig. 13 depicts the CO, Cx Hy and NO emissions (presented on a dry basis and at O2 = 6%) from the two FBCs fired with sunflower shells at different fuel feed rates for a similar range of EA (see Table 2). The emission characteristics in Fig. 13 exhibit, in effect, a net result of pollutant formation and decomposition in distinct regions inside both reactors, where these processes were affected by fuel properties, operating conditions and hydrodynamics of the gas–solid bed. The CO emission can be effectively controlled by increasing excess air, as seen in Fig. 13a. In the two FBCs operated at the specified design load (60 kg/h) and relatively low excess air (about 20%), the CO emission from the combustors was rather high: nearly 3000 ppm for the BFBC and about 2700 ppm for the SFBC. However, in both combustors firing 45–60 kg/h sunflower shells, the CO emission can be controlled at a level below 500 ppm via maintaining excess air at 60–80%, the higher excess air being recommended for the 60 kg/h fuel feed rate. From Fig. 13b, the Cx Hy emissions of both FBCs showed similar trends and effects of excess air. Like CO, at excess air of about 20%, the Cx Hy emissions from the combustors operated at 60 kg/h fuel feeding were significant: nearly 2200 ppm for the BFBC and about 3700 ppm for the SFBC. It appears that at EA = 50–60%, the Cx Hy emissions from both FBCs can be controlled within 400 ppm. At

excess air over 60%, the Cx Hy emissions from the SFBC showed the trend to be increased, however, to an insignificant extent. Unlike CO and Cx Hy , the emission of NO exhibited opposite effects of EA. With increasing excess air in both reactors, the NO emission was found to be increased confirming the fuel-NO formation mechanism and pointing at substantial contribution of CO and Cx Hy to NO decomposition. When ranging EA from 20 to 80% in the combustors operated at 60 kg/h fuel feeding, the NO emission was found to be increased from 86 ppm to 250 ppm for the BFBC, and from 130 to 270 ppm for the SFBC. Note that at the design load and excess air of 50–80%, the NO emissions from the two FBCs were characterized by nearly the same values at given EA. As seen in Fig. 13, with reducing the fuel feed rate from 60 kg/h to 45 kg/h at fixed total excess air, all the gaseous emissions exhibited the trend to be reduced for the entire range of EA, which can be explained by an increase in residence time (a key factor for reducing CO and Cx Hy emissions) as well as by diminishing of combustion temperature (by 50–70 ◦ C) at all points inside the FBCs (leading to diminishing NO emission). In the SFBC, effects of the combustor load on the NO emission was found to be much stronger than that in the BFBC (see Fig. 13c), which can be explained by the reduction of PA with lowering combustor load at fixed EA (see Table 2), which led to the higher CO and Cx Hy and, consequently, higher rate of chemical reactions responsible for decomposition of NO in the bottom part of the SFBC. 3.8. Heat losses and combustion efficiency

1000

BFBC: EA BFBC: EA SFBC: EA SFBC: EA

NO (ppm)

800

= 40% = 77% = 40% = 75%

600 400 200 0 0

0.5

1

1.5

2

2.5

3

3.5

Height above the air distributor (m) Fig. 12. Comparison of the axial NO concentration profiles between the BFBC and SFBC fired with sunflower shells at the fuel feed rate of 60 kg/h for two levels of excess air.

Table 3 shows the predicted heat loss due to unburned carbon and that due to incomplete combustion together with the combustion efficiency of the BFBC and SFBC operated at two combustor loads (fuel feed rates) for a similar range of excess air. The amounts of unburned carbon content as well as of CO and Cx Hy emissions (on a dry basis and at O2 = 6%), used for determining the heat losses (by Eqs. (1) and (2), and supporting Eqs. (3) and (4)), are provided in Table 3 as well. Due to low ash content in sunflower shells, the heat loss due to unburned carbon was rather low for the entire range of EA, whereas the heat loss owing to incomplete combustion varied in proportional correlation with the CO and Cx Hy emissions of both FBCs. It can be generally concluded that high (∼99%) combustion efficiency is achievable in both combustors when firing sunflower shells at excess air of 60–80%. This result is apparently attributed to the high thermal and combustion reactivity of sunflower shells. However, for the design combustor load, excess air of ∼55% seems to be a “compromise” value ensuring high combustion

36

P. Arromdee, V.I. Kuprianov / Chemical Engineering and Processing 62 (2012) 26–38

(b)

4000

CxHy emissions (ppm, 6% O2)

CO emission (ppm, 6% O2)

(a) BFBC: FR = 60 kg/h BFBC: FR = 45 kg/h SFBC: FR = 60 kg/h SFBC: FR = 45 kg/h

3000 2000 1000 0 0

20

40 60 Excess air (%)

80

4000

BFBC: FR = 60 kg/h BFBC: FR = 45 kg/h SFBC: FR = 60 kg/h SFBC: FR = 45 kg/h

3000 2000 1000 0 0

100

20

40 60 Excess air (%)

80

100

NO emission (ppm, 6% O2)

(c) 400

BFBC: FR = 60 kg/h BFBC: FR = 45 kg/h SFBC: FR = 60 kg/h SFBC: FR = 45 kg/h

300 200 100 0 0

20

40 60 Excess air (%)

80

100

Fig. 13. Comparison of the (a) CO, (b) Cx Hy (as CH4 ) and (c) NO emissions between the BFBC and SFBC fired with sunflower shells at the fuel feed rates of 60 kg/h and 45 kg/h for similar ranges of excess air.

efficiency at minimized environmental impacts of the FBCs. At this amount of excess air, the major gaseous (CO and NO) emissions can be controlled in both combustors at acceptable levels meeting the corresponding emission limits imposed by the Thai environmental legislation for biomass-fueled industrial applications [36] – 740 ppm for CO and 215 ppm for NO (as presented

on a dry basis and at 6% O2 ) – while maintaining the Cx Hy emissions at a reasonable level, about 370 ppm, as seen in Table 3 and Fig. 13. At the reduced combustor load, all gaseous emissions of the two FBCs can be ensured at lower levels, as revealed by the data in Fig. 13.

Table 3 Unburned carbon content in ash, emissions of CO and Cx Hy (as CH4 ), heat losses and combustion efficiency of the bubbling and swirling fluidized-bed combustors firing sunflower shells for variable operating conditions. Run no.

Fuel feed rate

Excess air

Unburned carbon

Emissiona

FR (kg/h)

EA (%)

Cfa (%)

CO (ppm)

Burning sunflower shells in the bubbling fluidized-bed combustor 60 20 5.8 2963 1 60 40 5.3 1248 2 59 5.1 680 3 60 4 60 77 4.2 427 45 20 3.2 1517 5 45 40 3.5 711 6 45 60 3.8 244 7 81 3.8 272 8 45 Burning sunflower shells in the swirling fluidized-bed combustor 60 16 2.8 2681 9 60 40 2.6 1144 10 60 61 2.6 584 11 469 12 60 75 2.8 13 45 16 5.0 2303 45 33 6.9 654 14 58 5.3 360 15 45 45 77 6.6 302 16 a

At 6% O2 (on a dry gas basis).

Heat loss

Combustion efficiency

Cx Hy (ppm)

due to unburned carbon quc (%)

due to incomplete combustion qic (%)

ic (%)

2150 670 358 356 695 165 97 137

0.32 0.29 0.28 0.23 0.17 0.19 0.21 0.22

4.72 1.64 0.88 0.75 1.82 0.62 0.27 0.34

95.0 98.1 98.8 99.0 98.0 99.2 99.5 99.5

3796 685 432 701 2892 282 261 643

0.15 0.14 0.14 0.15 0.27 0.38 0.29 0.36

7.01 1.61 0.94 1.28 5.47 0.76 0.57 1.11

92.8 98.3 98.9 98.6 94.3 98.9 99.1 98.5

P. Arromdee, V.I. Kuprianov / Chemical Engineering and Processing 62 (2012) 26–38

4. Conclusions

Tp,2

Sunflower shells exhibit excellent combustion properties and reactivity, which have been revealed by scanning electron microscopy and thermogravimetric analysis of this biomass, and can therefore be easily converted into energy in fluidized-bed combustion systems. In this comparative study, sunflower shells have been successfully burned using two combustion techniques with a cone-shaped bed – the bubbling fluidized-bed combustor and the swirling fluidized-bed combustor – to investigate combustion and emission performance of these reactors when firing sunflower shells at different fuel feed rates and excess air values. The following specific conclusions have been derived from the combustion study:

ua

• the radial and axial CO, Cx Hy and NO concentration profiles in both combustors indicates an occurrence of distinct specific regions along the combustor height exhibiting significant differences in formation and decomposition of the pollutants in these regions, • for the two combustors operated at the design fuel feed rate of 60 kg/h, an excess air amount of 55% seems to be the best option for this operating variable as leading to high (∼99%) combustion efficiency and acceptable levels CO and NO emissions (complying with the Thai emission standards), while controlling the Cx Hy emissions at a reasonable level, ∼370 ppm (on a dry gas basis and at O2 = 6%), • reducing the fuel feed rate results in lower CO, Cx Hy and NO emissions of the two FBCs, • compared to the bubbling fluidized-bed combustor, the swirling fluidized-bed combustor ensures higher burnout rate in the bottom region of the reactor and can therefore be designed with a noticeably smaller height for firing sunflower shells at similar operating conditions. Acknowledgements The authors wish to acknowledge the financial support from the Thailand Research Fund and Thammasat University (Contract No. BRG 5380015), as well as from the Commission on Higher Education, Ministry of Education, Thailand (Contract No. 6/2551). Special thanks to Dr. Rachadaporn Kaewklum (from Burapha University, Thailand) and Dr. Kasama Sirisomboon (from Silpakorn University, Thailand) for the valuable advices and discussions. Appendix A. Nomenclature

A Cfa EA FC FR LHV PA QSA qic quc SA TA Tb Tign Tp,1

ash content in fuel (wt.%, on as received basis) unburned carbon content in fly ash (wt.%, on dry basis) excess air (%) fixed carbon content in fuel (wt.%, on as received basis) fuel feed rate (kg/h) lower heating value (kJ/kg) primary air (%) flow rate of secondary air (Nm3 /s) heat loss owing to incomplete combustion (%) heat loss due to unburned carbon (%) secondary air (%) total air (%) burnout temperature (◦ C) ignition temperature (◦ C) temperature at the first (holocellulose volatilization) peak of the DTG curve (◦ C)

ut V0 Vdg@6%O2 VM W Z

37

temperature at the second (lignin decomposition) peak of the DTG curve (◦ C) axial component of the velocity at the air distributor exit in the SFBC (m/s) tangential component of the velocity at the air distributor exit in the SFBC (m/s) theoretical volume of air at 1 atm and 0 ◦ C (Nm3 /kg) volume of dry flue gas at standard conditions (1 atm and 0 ◦ C) and O2 = 6% (Nm3 /kg) volatile matter content in fuel (wt.%, on as received basis) moisture content in fuel (wt.%, on as received basis) current height (or level) above the air distributor (m)

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