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Study on burning oil palm kernel shell in a conical fluidized-bed combustor using alumina as the bed material
Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
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Study on burning oil palm kernel shell in a conical fluidized-bed combustor using alumina as the bed material Pichet Ninduangdee, 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
A B S T R A C T
Article history: Received 15 January 2013 Received in revised form 4 June 2013 Accepted 4 June 2013 Available online 9 July 2013
Fluidized-bed combustion of high-alkali biomass fuels is associated with a high risk for bed agglomeration when using conventional bed material (silica sand). In this work, oil palm kernel shell was burned in the conical fluidized-bed combustor (FBC) using alumina sand as the bed material to avoid this operational problem. Thermogravimetric characteristics of the selected biomass were investigated prior to combustion experiments to assess its thermal and combustion reactivity. The combustion tests were performed at rated (45 kg/h) and reduced (30 kg/h) fuel feed rates with excess air within 20–100% for each combustor load. SEM/EDS analysis of the original/reused bed material was performed for different operating times when running the combustor at the rated load. For the ranges of operating conditions, combustion efficiency of the conical FBC was basically high, about 99%, whereas the major gaseous (CO and NO) emissions from the combustor were at levels meeting the national emission limits. No evidence of bed agglomeration was observed during the entire test period of about 45 h. A thin coating (rich with Al, Si, Ca, Mg, and Fe) was observed on external and internal surfaces of the bed material particles at all the operating times. However, the chemical composition of the bed material exhibited substantial time-domain changes indicating a gradual reduction in the bed capability to withstand bed agglomeration during the combustion of oil palm kernel shell. ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Oil palm kernel shell Fluidized-bed combustion Combustion and emission performance Bed agglomeration prevention
1. Introduction Thailand is the world’s third largest producer of palm oil. In 2012, the palm oil production in this country was about 1.7 million tons [1]. A tremendous amount of palm residues, such as oil palm kernel shell, empty fruit bunches, fronds, and fiber, are annually generated by the Thai palm oil industry. Development of highefficiency, environmentally friendly and reliable thermochemical conversion technologies for utilization of these biomass residues is therefore a problem of paramount importance for the domestic energy-related sectors. The fluidized-bed combustion technology is proven to be one of the best options for energy conversion from biomasses [2–7]. However, burning of high-alkali biomass fuels in a fluidized bed using silica sand as the bed material is often accompanied by serious operational problems, such as bed sintering and agglomeration, leading to bed defluidization and eventually abnormal shut down of the combustion system [2,3,7,8]. Pioneering studies reveal an occurrence of bed agglomeration and rather fast (within a
* Corresponding author. Tel.: +66 2 986 9009x2208; fax: +66 2 986 9112. E-mail addresses: [email protected], [email protected] (V.I. Kuprianov).
few hours) defluidization during the combustion of oil palm kernel shell in a fluidized bed with silica sand [9,10]. To prevent bed agglomeration, some alternative bed materials, such as alumina, aluminum-rich materials, dolomite, and limestone, can be employed when firing biomass fuels with elevated potassium content in fuel ash [7,11–13]. A fluidized-bed combustor with a cone-shape bed (referred to as ‘conical FBC’) seems to be the most suitable fluidized-bed combustion technique for testing new (alternative) bed materials, particularly those of relatively high cost. Compared to a columnar fluidized-bed combustion system (combustor/furnace) fired with biomass, the conical FBC exhibit some apparent benefits, such as: (i) a relatively small amount of inert bed material [5,13,14], (ii) shorter start-up time of the combustor [14], and (iii) lower pressure drop across the fluidized bed (for similar bed material and static bed height) [15], all leading to reduced operating costs of the combustor. In the meantime, as with any fluidized-bed combustion system of a cylindrical/prismatic shape, the conical FBC ensures high (99% and up) combustion efficiency and acceptable levels of gaseous emissions when burning various biomasses [5,13,16]. This study was aimed at investigating the feasibility of effective and safe burning of oil palm kernel shell in the conical FBC using
1876-1070/$ – see front matter ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jtice.2013.06.011
P. Ninduangdee, V.I. Kuprianov / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
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Table 1 Ultimate and proximate analyses, and the lower heating value of oil palm kernel shell used in experimental tests on the conical FBC. Ultimate analysis (wt.%, as-received basis)
Proximate analysis (wt.%, as-received basis)
LHV (kJ/kg)
C
H
O
N
S
W
A
VM
FC
48.06
6.38
34.10
1.27
0.09
5.4
4.7
71.1
18.8
alumina sand as the bed material for the range of operating conditions (fuel feed rate and excess air). The combustion and emission performance of the combustor, as well as the capability of the bed material to withstand bed agglomeration with time, were the main focus of this experimental study. 2. Materials and methods 2.1. The fuel Table 1 shows the ultimate and proximate analyses as well as the lower heating value of oil palm kernel shell used in this experimental study. It can be seen in Table 1 that this biomass has a quite significant content of volatile matter, a moderate proportion of fixed carbon, but rather low contents of fuel moisture and ash. Due to quite low fuel S, SO2 was not addressed in this study. Like any other agricultural waste, oil palm kernel shell consisted mainly of hemicellulose (1.2 wt.%), cellulose (38.6 wt.%) and lignin (39.0 wt.%), and also included some extractives. However, this biomass exhibited a hard texture and high solid density (about 1500 kg/m3). To achieve high combustion efficiency and stable fuel feeding, this biomass was burned as shredded fuel with individual particles of up to 9 mm in sieve size. The shape and size of the shredded shell were quite irregular: from sawdust-like fine particles to flake-shape coarse particles. Fig. 1 depicts the particle size distribution (as wt.% for selected size groups) of oil palm oil kernel shell used in this study. It can be concluded from this analysis that the biomass was basically represented by particles of 0.5–9.0 mm in sieve size. However, shell particles of 3–6 mm in sieve size were apparently predominant in the biomass sample. The standard analysis of fuel ash (wt.%, as oxides) exhibited the predominant proportion of silicon (SiO2 = 54.12%), followed by calcium (CaO = 23.21%), potassium (K2O = 8.12%), and iron (Fe2O3 = 6.14%). The elevated potassium content in the fuel ash indicated a potential problem (bed agglomeration) during the combustion of this biomass in a fluidized bed with conventional bed material (silica sand).
16,300
2.2. Experimental set up Fig. 2 depicts the schematic diagram of the conical FBC with its dimensional characteristics. The combustor consisted of a conical module with 408 cone angle and 0.25 m inner diameter at the bottom plane, and five cylindrical modules, all insulated internally. Besides the combustor, the experimental set up included a screwtype fuel feeder delivering biomass into the conical module, a 25hp blower for air supply, a cyclone for collecting the fly ash generated from biomass combustion, as well as facilities for data collection. During combustor operation, a fluidized bed of inert material was generated and sustained in the conical module by a 19bubble-cap air distributor, which ensured uniform distribution of airflow over the bed at a quite low pressure drop across the distributor. An individual bubble-cap stand pipe of the distributor had sixty four holes each of 2 mm in diameter, arranged evenly over the pipe outer surface, and also six vertical slots with 3 mm width and 15 mm height located under the cap of 47 mm diameter. Net cross-sectional area of airflow at the distributor exit (calculated as the difference between area of the 0.25-mdiameter distributor plate and total area occupied by the caps) was 0.016 m2. The combustor was equipped with stationary Chromel–Alumel thermocouples (of type K) for monitoring temperature at different locations along the combustor height (see Fig. 2) during the combustor start up and when changing operation conditions. Seven gas ports at nearly the same locations were used for sampling the flue gas across the reactor. Alumina sand with the solid density of 3500 kg/m3 and mean particle size of 0.45 mm was used as the bed material to prevent
Weight percentage
50 40 30 20 10 0 <0.3
0.3–0.5 0.5–1.0 1.0–3.0 3.0–6.0 6.0–9.0
Biomass particle size (mm) Fig. 1. Particle size distribution of oil palm kernel shell used in the combustion tests on the conical FBC.
Fig. 2. Schematic diagram of the conical fluidized-bed combustor.
P. Ninduangdee, V.I. Kuprianov / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
2.3.1. TG/DTG analysis of the biomass fuel A ‘‘Mettler Toledo’’ TGA/DSC1 thermogravimetric analyzer was employed to obtain thermogravimetric characteristics (TG and DTG curves) of oil palm kernel shell for pyrolysis and combustion processes. Nitrogen and dry air (supplied into the analyzer furnace at a flow rate of 50 ml/min) were used as the furnace medium when testing the biomass for pyrolysis and combustion, respectively. During the test, a biomass sample with the initial weight of about 10 mg was heated from room temperature to 900 8C at a heating rate of 20 8C/min. Some important thermogravimetric characteristics, such as the ignition temperature, the peak temperatures and the burnout temperature, were obtained from the TG/DTG analysis. 2.3.2. Combustion tests on the conical FBC To study the effects of operating conditions on combustion and emission performance of the conical FBC, combustion tests were performed at two fuel feed rates: 45 kg/h (rated load) and 30 kg/h (reduced load), while ranging excess air from 20% to 100% for each combustor load. In order to investigate the behavior of temperature and gas concentrations (O2, CO, CxHy, and NO) in different regions of the reactor, the variables were measured along radial and axial directions inside the reactor, as well as at the cyclone exit (i.e., at stack), using a new model ‘‘Testo-350’’ portable gas analyzer. During these measurements, a sampled gas was continuously 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 8C) at the sampling points. Due to the thermoelectric conditioner of the gas analyzer, all the gas concentrations were measured on a dry basis. Prior to the recording of variables at fixed fuel feed rate and excess air, steady-state conditions of the conical FBC were ensured by verifying the temperature stability at the exit of the conical module and at the combustor top until time-domain fluctuation of the temperatures did not exceed 2 8C. When changing the amount of excess air (at constant fuel feeding), the transition time from one steady-state regime to another was 15– 20 min. At a given point inside the reactor (or at the cyclone exit), each variable was measured at least eleven times to quantify its average value over the time interval.
2.3.3. Characterization of the bed material and fly ash After testing the combustor at the rated load for 10, 20 and 30 h, the (reused) bed material and fly ash samples were analyzed for their chemical compositions using a wavelength dispersive X-ray fluorescence spectrometer. A scanning electron microscope equipped with an energy dispersive X-ray spectrometer (SEM-EDS: JEOL, JSM-6400, Link ISIS-300) was employed to observe a physical texture of individual bed material particles (of both original alumina and reused bed material), as well as to determine an elemental analysis of the particle coating – a result of interaction between the bed material and fuel ash – after the above-mentioned operating times. For each time instant, the coating composition (as major elements: Al, Si, K, Ca, Mg, and Fe) was obtained by spot EDS analysis at five arbitrary points on the external surface of a bed material particle, as well as at five points on the internal particle surface (i.e., on the surface of a selected particle pore). In the discussion below, the coating compositions of the particle surfaces are represented by corresponding averages over all five locations on each surface for different operating times. To investigate the effects of coating on the bed particle size, the particle size distribution and average (volumetric) particle diameter of the bed material were determined with a ‘‘Mastersizer 2000’’ particle size analyzer after finishing the 30-h combustion tests at the rated combustor load. 3. Results and discussion 3.1. Thermogravimetric analysis of oil palm kernel shell Fig. 3 compares the TG/DTG curves of oil palm kernel shell between different TGA furnace environments (nitrogen and dry
100 90 80 70 60 50 40 30 20 10 0
0.00 -0.02 -0.04 TG with air TG with N DTG with air DTG with N
-0.06 -0.08 -0.10
DTG (1/min)
2.3. Experimental methods and procedures
For each test run, the amount of excess air and combustion efficiency were predicted using the standard methods [20] provided in Appendix A. As estimated by Eq. (A4) from Appendix A (using the fuel ultimate analysis from Table 1), the theoretical volume of air required for firing 1 kg oil palm kernel shell was 4.83 Nm3/kg. By taking into account the airflow rate and the net cross-sectional area of airflow, the superficial air velocity at the distributor exit (u) was expected to be within 5.0–8.4 m/s when burning 45 kg/h biomass at excess air of 20–100%. According to Ref. [19], under these conditions, the conical FBC was operated in the turbulent fluidized-bed regime characterized by a relatively high fluidization number: u/umff = 7.2–12. At the reduced combustor load, this relative parameter was lower, from 4.8 to 8, when varying the amount of excess air within the specified range.
Sample mass (%)
bed agglomeration in the combustor. The particle size distribution and average (volumetric) particle diameter of the original bed material were determined by the ‘‘Mastersizer 2000’’ particle size analyzer, whereas the chemical composition of the material was obtained using a wavelength dispersive X-ray fluorescence spectrometer. In all combustion trials, the static bed height was 30 cm. For the selected bed material (alumina sand), particle size and geometry (diameter at the lower base, cone angle, and static bed height), the minimum velocity of full fluidization (umff), i.e., the superficial air velocity at the air distributor exit sufficient to cause bubbling fluidization of the entire conical bed [17,18], is reported to be 0.7 m/s [19]. A start up burner was used to preheat the bed material prior to combustion tests. The burner was fixed at a 0.5 m level above the air distributor and inclined at a 308 angle to the horizontal, as shown in Fig. 2. When temperature of the fluidized bed reached 650–700 8C, a diesel pump of the start up burner was turned off, and the required combustor load was ensured by biomass feeding into the reactor. Basically, total start up time of the conical FBC from its cold state was 45–60 min, depending on the specified operating conditions.
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-0.12 -0.14 0
100 200 300 400 500 600 700 800 900 Temperature (ºC)
Fig. 3. TG and DTG curves of oil palm kernel shell for the tests when using nitrogen and air as the furnace medium.
P. Ninduangdee, V.I. Kuprianov / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
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air). During the pyrolysis test with nitrogen, biomass degradation was found to exhibit four stages: (i) dewatering (at temperature up to 170 8C), (ii) volatilization of a high reactive (likely, ‘‘light’’ and oily) biopolymer with the maximum volatilization rate at 274 8C (the first peak temperature), (iii) pyrolysis of cellulose and small proportion of lignin attaining the maximum volatilization rate at 354 8C (the second peak temperature), and (iv) slow volatilization of major proportion of lignin with its simultaneous conversion into char at temperatures from 370 8C to about 900 8C. Note that the peak temperature of the cellulose volatilization and the upper temperature of lignin degradation were in a good agreement with reference data [21]. Switching the furnace medium from nitrogen to dry air had a rather weak influence on the behavior of the TG/DTG curves within the first three stages; however, the effects of furnace medium on the processes at high temperatures (>500 8C) were found to be significant. Compared to the pyrolysis, the first and second peak temperatures during the combustion test turned out to be slightly lower (by about 5 8C), whereas the biomass degradation rate at these temperatures was apparently higher (see Fig. 3). This fact can be explained by increased reactivity of the biomass caused by availability of air, which oxidized biomass volatiles during the second and third degradation stages [22,23]. At temperatures of 370–490 8C (i.e., at the beginning of the fourth stage), the diminishing of the sample mass was generally due to the lignin volatilization. Within this temperature range, heat released from the volatile combustion was not high enough to ignite the char and sustain its combustion. From Fig. 3, temperature of 490 8C can be treated as the lower temperature limit for the char ignition. Rapid volatilization of lignin (major factor) and oxidation of char (minor factor) resulted in appearance of the third peak temperature (580 8C on the DTG curve), basically associated with lignin volatilization [21–23]. Two more important combustion characteristics – the ignition temperature (Tign) and the burnout temperature (Tb) – were determined based on analysis of the TG/DTG curves from the thermogravimetric (combustion) test: Tign = 250 8C and Tb = 675 8C. Taking into account rather low (first) peak and burnout temperatures of oil palm kernel shell (pointing at high thermal and combustion reactivity of the shell), one may conclude that in a real fluidized-bed combustion system with typical bed temperature (800–900 8C) this biomass fuel be burned with high combustion efficiency. 3.2. Effects of operating conditions on the distribution of temperature and O2 in the conical FBC Analysis of radial profiles of temperature and gas concentrations (O2, CO, CxHy, and NO) at different levels above the air FR = 45 kg/h, EA = 40% FR = 45 kg/h, EA = 80% FR = 30 kg/h, EA = 40% FR = 30 kg/h, EA = 80%
1000
3.3. Effects of operating conditions on formation and decomposition of CO, CxHy, and NO Fig. 5 shows the axial profiles of CO, CxHy (as CH4), and NO in the conical FBC for variable fuel feed rate and excess air. All these profiles revealed two specific regions in the reactor – the bottom region (0 < Z < 0.6 m) and the upper region (Z > 0.6 m) – exhibiting different net results of formation/decomposition of the pollutants. In the bottom region, CO exhibited a sharp increase along the axial direction caused by rapid devolatilization of fuel particles, followed by oxidation of CxHy and char carbon. For the selected level of excess air, the CO peak at the reduced load was somewhat lower compared to that at the rated load, likely due to the increased residence time. In the upper region, CO was gradually
(b) 20
FR = 45 kg/h, EA = 40% FR = 45 kg/h, EA = 80% FR = 30 kg/h, EA = 40% FR = 30 kg/h, EA = 80%
15
O2 (Vol%)
Temperature (°C)
(a) 1200
distributor in the conical FBC fired with oil palm kernel shell at variable operating conditions revealed rather good uniformity of these variables across the reactor in all combustion trials. Note that similar results were reported in previous studies on firing some shell-type biomass fuels in this combustion technique [13]. Such an appearance of the radial profiles indicated bubbling/turbulent fluidized-bed regime in the conical FBC, as well as high intensive heat-and-mass transfer along the radial direction inside the reactor. In this work, axial profiles of temperature and gas concentrations were therefore used for the analysis of combustion and emission performance of the combustor. Fig. 4 depicts the axial profiles of temperature and O2 in the combustor firing oil palm kernel shell at the rated and reduced combustor loads for the excess air values of about 40% and 80%. It can be seen in Fig. 4 that in the conical module of the reactor (Z < 0.9 m), the axial temperature profiles were quasi-uniform. This fact indicated excellent mixing of the bed material with chars and gases in the conical section, whereas in the cylindrical part of the combustor these profiles exhibited a reducing trend of temperature along the combustor height, likely due to the heat loss across reactor walls. A reduction in the combustor load led to diminishing of temperature at any point inside the reactor, because of the lower heat input to the conical FBC. Meanwhile, a local drop of temperature was observed when increasing excess air, which can be explained by the air dilution effects. In all the test runs, O2 exhibited a gradual reduction along the reactor height. The behavior of O2 indicated that a significant proportion of the biomass was oxidized in the bottom region of the conical FBC. At the reduced combustor load, despite the reduction in bed temperature, the rate of O2 consumption in this region was somewhat higher compared to that at the maximum load (for similar excess air), which can be explained by the increased residence time of reactants.
800
10
600
5
400
0 0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 Height above air distributor (m)
0.0
0.5 1.0 1.5 2.0 2.5 3.0 Height above air distributor (m)
3.5
Fig. 4. Effects of operating conditions – fuel feed rate (FR) and excess air (EA) – on the axial profiles of (a) temperature and (b) O2 in the conical FBC when firing oil palm kernel shell.
P. Ninduangdee, V.I. Kuprianov / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
(a) 20000
FR = 45 kg/h, EA = 40% FR = 45 kg/h, EA = 80% FR = 30 kg/h, EA = 40% FR = 30 kg/h, EA = 80 %
FR = 45 kg/h, EA = 40% FR = 45 kg/h, EA = 80% FR = 30 kg/h, EA = 40% FR = 30 kg/h, EA = 80%
15000
CO (ppm)
CxHy (ppm)
15000
(b) 20000
1049
10000
10000
5000
5000
0
0 0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 Height above air distributor (m)
(c) 600
0.5 1.0 1.5 2.0 2.5 3.0 3.5 Height above air distributor (m)
FR = 45 kg/h, EA = 40% FR = 45 kg/h, EA = 80% FR = 30 kg/h, EA = 40% FR = 30 kg/h, EA = 80%
500
NO (ppm)
0.0
400 300 200 100 0 0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 Height above air distributor (m)
Fig. 5. Effects of operating conditions – fuel feed rate (FR) and excess air (EA) – on the axial profiles of (a) CO, (b) CxHy, and (c) NO in the conical FBC when firing oil palm kernel shell.
reduced, mainly due to oxidation of CO by O2 and OH [24]. It should be noted that an increase in excess air (at fixed load) led to the lowering of the CO peak (observed at Z 0.6 m), mainly due to the enhanced rate of CO oxidation by O2. The axial profiles of CxHy had similar trends as those for CO and exhibited similar effects of the operating conditions. As known, CxHy are basically originated from fuel volatile matter and oxidized forming CO as an intermediate product [24]. Since CO continuously received the ‘‘feeding’’ from oxidation of CxHy and char-C, CO at different locations in the combustor was substantially higher than CxHy for the ranges of operating conditions. From the graph with the axial profiles of NO, this pollutant was found to be rapidly increased in the bottom region, basically due to biomass devolatilization and further oxidation of volatile nitrogenous species to NO via the fuel-NO formation mechanism [2]. Simultaneously, a part of NO was decomposed due to the catalytic reduction, i.e., via secondary reactions with CO and light hydrocarbons on the surface of char, ash and bed material particles [2,24]. However, in the upper region of the combustor, a significant NO reduction was observed, mainly due to the abovementioned secondary reactions. Compared to the rated combustor load, the rate of NO formation and, consequently, the NO peak observed in the fluidized bed region (at Z 0.6 m) were substantially lower in the tests at the reduced load (for similar excess air values). This result can be explained by the lower bed temperature resulting in the lower devolatilization rate and greater residence time, the latter being responsible for the higher NO decomposition rate in the bottom region. However, in accordance with the fuel-NO formation mechanism, the rate of NO formation in this region and the NO peak were higher with increasing excess air (at fixed combustor load). Based on the analysis of the axial profiles of NO in the second region, it can be concluded that the reduction rate of NO in this
region was weakly dependent on operating conditions, exhibiting however sensible effects of CO and CxHy. Thus, NO at any location along the combustor height was in apparent correlation with the NO peak for selected operating conditions. Note that the appearance of the CO, CxHy and NO peaks at Z 0.6 m, i.e., at the level of fuel injection, confirmed very high thermal (devolatilization) reactivity of this biomass, which was revealed by the thermogravimetric study. 3.4. Emissions and combustion efficiency of the conical FBC Table 2 summarizes the unburned carbon content in fly ash, the emissions of CO, CxHy (as CH4), and NO (all on a dry gas basis and at 6% O2), the actual operating conditions (fuel feed rate, excess air, and O2 at stack) as well as the combustion-related heat losses and combustion efficiency of the conical FBC for all the test runs. It appears that the CO and CxHy emissions can be effectively reduced via increasing the amount excess air delivered to the combustor, whereas the NO emission can be minimized by decreasing the excess air level. At the rated load and lowest excess air value (about 20%), the CO and CxHy emissions from the combustor were at highest levels: 1551 ppm and 1320 ppm, respectively, whereas the NO emission showed its lowest value (about 100 ppm) pointing at a significant role of CO and CxHy in the reduction of NO. With increasing excess air, both CO and CxHy emissions were substantially decreased to the quite low levels when the excess air value was maintained within 80–100%. However, unlike CO and CxHy, the emission of NO showed quite opposite effects of excess air: with increasing this operating variable, the NO emission from the combustor was found to be higher (according to the fuel-NO formation mechanism). As seen in Table 2, the combustion efficiency for both fuel feed rates exhibited some improvement with increasing the level of excess air, particularly at low excess air values. For the excess air
1050
P. Ninduangdee, V.I. Kuprianov / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
Table 2 Emissions and combustion efficiency of the conical FBC when firing with oil palm kernel shell for variable operating conditions. Excess air (%)
O2 at stack (vol.%)
Carbon in fly ash (wt.%)
COa (ppm)
CxHya (ppm)
NOa (ppm)
Heat loss (%) due to
Combustion efficiency (%)
Unburned carbon
Incomplete combustion
Burning 45 kg/h oil palm kernel shell 18 3.5 40 6.1 59 7.9 79 9.3 99 10.5
4.53 3.37 2.77 1.56 1.86
1551 631 400 246 142
1320 542 349 187 98
99 143 176 204 236
0.45 0.33 0.27 0.15 0.18
2.30 1.12 0.82 0.51 0.31
97.3 98.6 98.9 99.3 99.5
Burning 30 kg/h oil palm kernel shell 22 4.0 40 6.1 58 7.8 78 9.2 99 10.5
3.66 1.86 1.97 1.76 1.56
1230 490 330 190 121
983 412 269 148 88
68 122 178 200 213
0.36 0.18 0.19 0.17 0.15
1.80 0.86 0.64 0.40 0.27
97.8 99.0 99.2 99.4 99.6
a
At 6% O2 (on a dry gas basis).
values of 40–100%, the combustion efficiency was found to be nearly the same and characterized by highest values at distinct amounts of excess air. At the rated combustor load, combustion efficiency of 98.6–99.5% can be achieved via maintaining excess air within 40–100%. However, with reducing the combustor load at fixed excess air, all the emissions showed the trend to be reduced, whereas the combustion efficiency was improved, mainly due to the reduction in the CO and CxHy emissions. Taking into account the emission characteristics as well as the combustion efficiencies in Table 2, excess air of about 40% seems to be the best option for firing oil palm kernel shell in this conical FBC regardless of the combustor load. Under this condition, the conical FBC can be operated with high, about 99.0%, combustion efficiency, while controlling the major gaseous (CO and NO) emissions at acceptable levels meeting the corresponding domestic emission limits for the biomass-fueled industrial applications: 740 ppm for CO and 205 ppm for NO (both on a dry gas basis, as corrected to 6% O2) [25], and maintaining CxHy emissions at a reasonable level. At lower excess air values (e.g., 20%), the CO and CxHy emissions were unreasonably high, which led to a noticeable reduction in the combustion efficiency. However, with increasing excess air values over 40%, one can
observe a substantial (undesirable) increase in the emission of NO (a more harmful pollutant than the other two), as can be seen in Table 2. 3.5. Coating of bed material particles The SEM micrographs of an external surface of original bed material (i.e., unused alumina sand) revealed insignificant surface roughness, whereas cross-sectional images of sand particles exhibited the porous texture of the sand. From the SEM observation, sand pores were irregular in their size and shape, and were randomly located in the particle volume. Fig. 6 depicts the SEM images of bed material particles (upper micrographs, exhibiting an external particle surface) and those of particle cross-sectional views (lower micrographs) after different times of combustor operation (10, 20 and 30 h) at the rated combustor load. Yet the lower micrographs in Fig. 6 show the locations on the external particle surface (points 1–5) as well as those on the internal (pore) surface (points 6–10) where the coating was sampled for spot EDS analysis. The SEM images revealed the particle coating, which can be seen in Fig. 6 as a ‘‘white’’ thin layer on both surfaces of the selected particles. Due to
Fig. 6. SEM images of bed material particles (upper micrographs at 150 magnification) and their cross-sectional views (lower micrographs at 750 magnification) after different operating times: (a) 10 h, (b) 20 h and (c) 30 h.
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Fig. 7. Elemental composition of the coating on the (a) external and (b) internal surfaces of bed material particles at different time instants of combustor operation.
the coating, the external surface of the particles exhibited an apparent roughness (as seen in a box showing the surface at high magnification), which was substantially greater than that of the original alumina sand. However, the coating thickness at the two surfaces was weakly dependent on operating time. From the spot EDS analysis, the coating was rich with Al (predominant in the bed material) as well as Si, K, Ca, Mg, and Fe (the latter five from fuel ash). Fig. 7 shows the elemental composition of the coating on both external and internal surfaces of bed material particles for different operating times. The results in Fig. 7 revealed that the coating of the external particle surface was mainly represented by the ash-related elements. However, the proportions of ash-related elements in the coating on the pore surface were substantially lower than those on the external surface, due to the relatively high proportion of Al in this coating layer. As a matter of fact, timescale effects on the coating composition were rather weak. However, with increasing operating time, Si in the external coating showed a noticeable reduction, mainly due to substitution of this element by Ca from the fuel ash, whereas the diminishing of Al in the pore coating was likely caused by increased penetration of ash vapors into the sand pores. As found by visual inspections after finishing of all the tests at both rated and reduced combustor load (lasted for about 45 h), the bed material exhibited normal appearance (grains) capable to fluidize, and thus had the potential to be reused in further combustor operation. Based on the SEM/EDS analyses, it can be concluded that prevention of bed agglomeration during the combustion tests was ensured due to the adsorption of potassium salts (vaporized from fuel ash) by the external and internal surfaces of bed material particles [12,26,27]. It appears that alumina reacted with these and other salts in a quite thin layer on the external/internal surface of alumina sand particles. This layer termed ‘coating’ consisted of some eutectics (such as K2O–Al2O3– SiO2 systems) of high fusion temperature, typically 1500–1600 8C [27–29], which significantly exceeded bed temperatures and thus inhibited bed agglomeration in the combustor. Furthermore, the external particle coating included elevated Ca, Mg, and Fe, basically mitigating formation of sticky compounds on the outer surface of the particles and thus reducing the probability of adhering coarse bed material particles with char, ash, and fine bed particles [30– 32]. Another factor preventing bed agglomeration in this conical FBC was associated with the turbulent fluidization regime of the expanded conical bed [19], under which the probability of solid particle collisions was significantly lower that it could happen if the combustor were operated in bubbling fluidized-bed regime. 3.6. Effects of the coating on particle size of the bed material Table 3 shows the particle size distribution of the original bed material (alumina sand) and that of the re-used bed
material after 30 h combustion tests at the rated combustor load. It can be seen in Table 3 that the size distribution (as the volume percentage of particles for different size groups) showed an apparent increase in volumetric diameter of the re-used sand particles (575 mm) compared to that of the original alumina sand (446 mm), and this was likely caused by the coating of bed material particles. However, as follows from the analysis of the size distribution at different time instants, the predominant proportion of the bed material was represented by the Geldart-B particles, which sustained fluidizedbed regime of the combustor. Note that the increase in the bed particle size resulted apparently in a higher value of the minimum velocity of full fluidization (umff) [17,19]. However, this insignificant increase in the particle size did not lead to any noticeable changes in the hydrodynamic behavior of the bed. 3.7. Time-related changes in the composition of the bed material and fly ash Table 4 shows the composition of the bed material used/reused in the conical FBC and that of the fly ash originated from the combustion of oil palm kernel shell at the rated load for different operating times. For comparison, Table 4 includes the composition of the original alumina sand and that of the fuel ash (both obtained prior to combustion tests). The results revealed a quite intensive interaction (substance exchange) between the bed material and ash. This interaction
Table 3 Particle size distribution of the original alumina sand and that of the bed material after 30-h testing on the conical FBC for firing oil palm kernel shell at the rated combustor load. Particle size range (mm)
172–200 200–233 233–272 272–317 317–370 370–431 431–502 502–586 586–683 683–796 726–928 928–1082 1082–1262 1262–1471 Mean particle size (mm)
Volume percentage of particles in the bed material Prior to experiments
After 30-h testing
0.41 2.70 3.64 6.62 17.03 20.63 22.25 13.90 8.07 2.03 2.26 0.32 0.14 0.00 446
0.14 1.05 2.85 5.51 8.70 11.76 13.95 14.66 13.68 11.32 8.22 5.26 2.58 0.32 575
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Table 4 Composition of the bed material, as well as that of fuel ash and fly ashes originated from the combustion of oil palm kernel shell at different time instants of combustor operation at the rated load. Operating time (h)
Composition (as oxides, wt.%) Al2O3
SiO2
Bed material 0 (original alumina sand) 10 (used bed material) 20 (reused bed material) 30 (reused bed material)
99.40 64.73 57.25 46.99
0.09 27.2 29.7 34.6
Fuel/fly ash 0 (fuel ash) 10 (fly ash) 20 (fly ash) 30 (fly ash)
3.11 13.66 6.45 6.32
54.12 48.75 53.86 53.31
CaO
MgO
K2O
Na2O
Fe2O3
TiO2
P2O5
ZnO
Cl
0.05 2.41 4.44 6.97
0.03 0.56 1.27 1.66
– 2.89 4.13 5.56
0.38 0.43 0.27 0.29
0.07 0.54 0.83 1.15
0.40 0.03 0.06 0.06
0.01 1.01 1.78 2.38
0.03 – – –
– – – –
23.21 20.31 22.12 22.44
2.65 2.07 2.15 2.06
8.12 6.24 5.86 6.25
0.81 1.17 0.68 0.64
6.14 2.09 2.08 2.22
– 0.35 0.19 0.20
1.15 2.13 2.01 1.93
0.20 – – –
– 0.01 0.13 0.12
occurred likely via: (i) above-mentioned formation of the coating on the internal and external surfaces of the bed material particles, and (ii) accumulation of small proportion of ash in the bed, leading eventually to an increase of ash-related elements in the fluidized bed. As a result, SiO2, CaO and K2O in the reused bed material showed a significant/sensible increase with time, however to different extent at distinct time instants. Due to collisions, breakage and attrition of the original bed material particles, the particle coating was destructed resulting in generation of some fine particles (containing Al) in the fluidized bed, which was consequently carried out from the bed region and became a part of fly ash. These processes explain a reduction of Al2O3 in the reused bed material (roughly, by 50% for the entire experimental time), whereas Al2O3 in the fly ash was (always) substantially greater than that in the fuel ash analysis, indicating a continuous carryover of the original bed material from the bed and therefore a gradual reduction of the bed capability to prevent its agglomeration during the combustion of oil palm kernel shell.
Acknowledgement The authors wish to acknowledge the financial support from Thammasat University (Thailand) as well as the financial support from the Bangchak Petroleum Public Company Limited (Thailand). Appendix A. Determining excess air and combustion efficiency for different test runs Below are the relationships used in this study for determining the amount of excess air, as well as the combustion-related heat losses and the combustion efficiency for each test run. The excess air coefficient (ratio) for each trial was estimated by using actual O2, CO, and CxHy (as CH4) at stack (all in vol.%, on a dry gas basis), as well as by neglecting H2 and assuming N2 = 79 vol.% in the dry flue gas, as:
a¼
21 21 ðO2 0:5CO 2CH4 Þ
(A1)
The percentage of excess air at stack was then calculated as: 4. Conclusions EA ¼ 100ða 1Þ Oil palm kernel shell is a lignocellulosic biomass exhibiting high thermal and combustion reactivity despite its hard structure and unconventionally high density. The conical fluidized-bed combustor using alumina as the bed material can be employed for safe and effective utilization of this shell with elevated potassium content for energy conversion via direct combustion. Operating conditions (fuel feed rate and excess air) have sensible effects on formation and decomposition of major gaseous pollutants (CO, CxHy, and NO) inside the reactor, as well as on combustion efficiency and emissions of the combustor. Excess air of 40% seems to be the best option for firing this biomass regardless of combustor load. At these operating conditions, high (about 99%) combustion efficiency is achievable, while controlling CO and NO emissions at values below the national emission limits and maintaining CxHy emissions at a reasonable level. External and internal surfaces of the original bed material particles are covered by a coating mainly consisted of the eutectics with high fusion temperatures preventing bed agglomeration. Due to interaction between the original bed material (alumina) and fuel ash, the composition of the reused bed material and that of fly ash undergo substantial changes during the combustor operation. An apparent reduction of Al2O3 in the bed material, accompanied by an increase in K2O and SiO2, indicates a gradual loss of the bed capability to withstand bed agglomeration.
(A2)
For the reference conditions (i.e., at O2,ref = 6 vol.%, on a dry gas basis), the excess air coefficient was estimated using Eq. (A1) by neglecting products of incomplete combustion (CO and CH4) to be aref = 1.4. The theoretical (reference) volume of dry flue gas (Nm3/kg, at 0 8C and 1 atm) was determined using the fuel ultimate analysis on as-received basis (provided in Table 1) and aref as: V dg@6%O2 ¼ 0:01866ðC þ 0:375SÞ þ 0:79V 0 þ 0:008N þ ðaref 1ÞV 0
(A3)
In Eq. (A3), the theoretical volume of air (Nm3/kg, at 0 8C and 1 atm) required for firing 1 kg biomass fuel under stoichiometric conditions (i.e., at a = 1) was determined based on the ultimate fuel analysis (see Table 1): V 0 ¼ 0:0889ðC þ 0:375SÞ þ 0:265H 0:0333O
(A4)
The heat-loss method was used to estimate the combustion efficiency. For this conical FBC with no bottom ash, the heat loss due to unburned carbon (%) was as: 32; 866 C fa A (A5) quc ¼ LHV 100 C fa where Cfa is the content of unburned carbon in the fly ash, (wt.%), and A is the fuel ash content (wt.%, on as-received basis).
P. Ninduangdee, V.I. Kuprianov / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
The heat loss owing to incomplete combustion (%) was predicted by using the CO and CxHy emissions (both on a dry gas basis and at 6% O2) provided in Table 2 and neglecting H2 in flue gas as: qic ¼ ð126:4 CO þ 358:2 CH4 Þ@6%O2 104 V dg@6%O2
ð100 quc Þ LHV (A6)
The combustion efficiency was then predicted as:
hc ¼ 100 ðquc þ qic Þ
(A7)
References [1] Index Mundi Palm oil production by country. http://www.indexmundi.com/ agriculture [accessed 13.05.2013]. [2] Werther J, Saenger M, Hartge EU, Ogada T, Siagi Z. Combustion of agricultural residues. Prog Energy Combust Sci 2000;26:1–27. [3] Saenger M, Hartge EU, Werther J, Ogada T, Siagi Z. Combustion of coffee husks. Renew Energ 2001;23:103–21. [4] Fang M, Yang L, Chen G, Shi Z, Luo Z, Cen K. Experimental study on rice husk combustion in a circulating fluidized bed. Fuel Process Technol 2004;85:1273–82. [5] Permchart W, Kouprianov VI. Emission performance and combustion efficiency of a conical fluidized-bed combustor firing various biomass fuels. Bioresour Technol 2004;92:83–91. [6] 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;22:1004–11. [7] Khan AA, de Jong W, Jansens PJ, Spliethoff H. Biomass combustion in fluidized bed boilers: potential problems and remedies. Fuel Process Technol 2009;90:21–50. [8] Lin W, Dam-Johansen K, Frandsen F. Agglomeration in bio-fuel fired fluidized bed combustors. Chem Eng J 2003;96:171–85. [9] Chaivatamaset P, Sricharoon P, Tia S. Bed agglomeration characteristics of palm shell and corncob combustion in fluidized bed. App Therm Eng 2011;31:2916–27. [10] Chaivatamaset P, Sricharoon P, Tia S, Bilitewski B. A prediction of defluidization time in biomass fired fluidized bed combustion. Appl Therm Eng 2013;50:722–31. [11] Shimizu T, Han J, Choi S, Kim L, Kim H. Fluidized-bed combustion characteristics of cedar pellets by using an alternative bed material. Energy Fuels 2006;20:2737–42. [12] Ferna´ndez LMJ, Escalada CR, Murillo LJM, Carrasco GJE. Combustion in bubbling fluidised bed with bed material of limestone to reduce the biomass ash agglomeration and sintering. Fuel 2006;85:2081–92. [13] Kuprianov VI, Arromdee P. Combustion of peanut and tamarind shells in a conical fluidized-bed combustor: a comparative study. Bioresour Technol 2013;140:199–210.
1053
[14] Kouprianov VI, Permchart W. Emission from a conical FBC fired with a biomass fuel. Appl Energy 2003;74:383–92. [15] Kaewklum R, Kuprianov VI. Theoretical and experimental study on hydrodynamic characteristics of fluidization in air–sand conical beds. Chem Eng Sci 2008;63:1471–9. [16] Janvijitsakul K, Kuprianov VI. Major gaseous and PAH emissions from a fluidized-bed combustor firing rice husk with high combustion efficiency. Fuel Process Technol 2008;89:777–87. [17] Peng Y, Fan LT. Hydrodynamic characteristics of fluidization in liquid-solid tapered beds. Chem Eng Sci 1997;52:2277–90. [18] Jing S, Hu Q, Wang J, Jin Y. Fluidization of coarse particles in gas–solid conical beds. Chem Eng Process 2000;39:379–87. [19] Ninduangdee P, Kuprianov VI. Measures for reliable and effective operation of a conical fluidized-bed combustor fired with oil palm shells: a cold-state hydrodynamic study. In: Proceedings of the Third TSME International Conference on Mechanical Engineering; 2012. [20] Basu P, Cen KF, Jestin L. Boilers and Burners. New York: Springer; 2000. [21] Haykiri-Acma H, Yaman S, Kucukbayrak S. Comparison of the thermal reactivities of isolated lignin and holocellulose during pyrolysis. Fuel Process Technol 2010;91:759–64. [22] Haykırı-Ac¸ma H. Combustion characteristics of different biomass materials. Energ Convers Manage 2003;44:155–62. [23] Munir S, Daood SS, Nimmo W, Cunliffe AM, Gibbs BM. Thermal analysis and devolatilization kinetics of cotton stalk, sugar cane bagasse and shea meal under nitrogen and air atmospheres. Bioresour Technol 2009;100:1413–8. [24] Turns S. An Introduction to Combustion. Boston: McGraw-Hill; 2006. [25] Pollution Control Department, Ministry of Natural Resources and Environment, Thailand. Air pollution standards for industrial sources. http:// www.pcd.go.th/info_serv/reg_std_airsnd03.html [accessed 13.05.2013]. [26] Steenari BM, Lindqvist O. High-temperature reactions of straw ash and the anti-sintering additives kaolin and dolomite. Biomass Bioenerg 1998;14:67– 76. [27] Llorente MJF, Arocas PD, Nebot LG, Garcı´a JEC. The effect of the addition of chemical materials on the sintering of biomass ash. Fuel 2008;87:2651–8. [28] Konsomboon S, Pipatmanomai S, Madhiyanon T, Tia S. Effect of kaolin addition on ash characteristics of palm empty fruit bunch (EFB) upon combustion. Appl Energy 2011;88:298–305. [29] Tran K-Q, Iisa K, Steenari B-M, Lindqvist O. A kinetic study of gaseous alkali capture by kaolin in the fixed bed reactor equipped with an alkali detector. Fuel 2005;84:169–75. [30] Nuutinen LH, Tiainen MS, Virtanen ME, Enestam SH, Laitinen RS. Coating layers on bed particles during biomass fuel combustion in fluidized-bed boilers. Energy Fuels 2003;18:127–39. [31] Vuthaluru HB, Zhang D-K. Control methods for remediation of ash-related problems in fluidised-bed combustors. Fuel Process Technol 1999;60:145– 56. [32] Bhattacharya SP, Harttig M. Control of agglomeration and defluidization burning high-alkali, high-sulfur lignites in a small fluidized bed combustor: effect of additive size and type, and the role of calcium. Energy Fuels 2003;17:1014–21.
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Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Study on burning oil palm kernel shell in a conical fluidized-bed combustor using alumina as the bed material Pichet Ninduangdee, 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
A B S T R A C T
Article history: Received 15 January 2013 Received in revised form 4 June 2013 Accepted 4 June 2013 Available online 9 July 2013
Fluidized-bed combustion of high-alkali biomass fuels is associated with a high risk for bed agglomeration when using conventional bed material (silica sand). In this work, oil palm kernel shell was burned in the conical fluidized-bed combustor (FBC) using alumina sand as the bed material to avoid this operational problem. Thermogravimetric characteristics of the selected biomass were investigated prior to combustion experiments to assess its thermal and combustion reactivity. The combustion tests were performed at rated (45 kg/h) and reduced (30 kg/h) fuel feed rates with excess air within 20–100% for each combustor load. SEM/EDS analysis of the original/reused bed material was performed for different operating times when running the combustor at the rated load. For the ranges of operating conditions, combustion efficiency of the conical FBC was basically high, about 99%, whereas the major gaseous (CO and NO) emissions from the combustor were at levels meeting the national emission limits. No evidence of bed agglomeration was observed during the entire test period of about 45 h. A thin coating (rich with Al, Si, Ca, Mg, and Fe) was observed on external and internal surfaces of the bed material particles at all the operating times. However, the chemical composition of the bed material exhibited substantial time-domain changes indicating a gradual reduction in the bed capability to withstand bed agglomeration during the combustion of oil palm kernel shell. ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Oil palm kernel shell Fluidized-bed combustion Combustion and emission performance Bed agglomeration prevention
1. Introduction Thailand is the world’s third largest producer of palm oil. In 2012, the palm oil production in this country was about 1.7 million tons [1]. A tremendous amount of palm residues, such as oil palm kernel shell, empty fruit bunches, fronds, and fiber, are annually generated by the Thai palm oil industry. Development of highefficiency, environmentally friendly and reliable thermochemical conversion technologies for utilization of these biomass residues is therefore a problem of paramount importance for the domestic energy-related sectors. The fluidized-bed combustion technology is proven to be one of the best options for energy conversion from biomasses [2–7]. However, burning of high-alkali biomass fuels in a fluidized bed using silica sand as the bed material is often accompanied by serious operational problems, such as bed sintering and agglomeration, leading to bed defluidization and eventually abnormal shut down of the combustion system [2,3,7,8]. Pioneering studies reveal an occurrence of bed agglomeration and rather fast (within a
* Corresponding author. Tel.: +66 2 986 9009x2208; fax: +66 2 986 9112. E-mail addresses: [email protected], [email protected] (V.I. Kuprianov).
few hours) defluidization during the combustion of oil palm kernel shell in a fluidized bed with silica sand [9,10]. To prevent bed agglomeration, some alternative bed materials, such as alumina, aluminum-rich materials, dolomite, and limestone, can be employed when firing biomass fuels with elevated potassium content in fuel ash [7,11–13]. A fluidized-bed combustor with a cone-shape bed (referred to as ‘conical FBC’) seems to be the most suitable fluidized-bed combustion technique for testing new (alternative) bed materials, particularly those of relatively high cost. Compared to a columnar fluidized-bed combustion system (combustor/furnace) fired with biomass, the conical FBC exhibit some apparent benefits, such as: (i) a relatively small amount of inert bed material [5,13,14], (ii) shorter start-up time of the combustor [14], and (iii) lower pressure drop across the fluidized bed (for similar bed material and static bed height) [15], all leading to reduced operating costs of the combustor. In the meantime, as with any fluidized-bed combustion system of a cylindrical/prismatic shape, the conical FBC ensures high (99% and up) combustion efficiency and acceptable levels of gaseous emissions when burning various biomasses [5,13,16]. This study was aimed at investigating the feasibility of effective and safe burning of oil palm kernel shell in the conical FBC using
1876-1070/$ – see front matter ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jtice.2013.06.011
P. Ninduangdee, V.I. Kuprianov / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
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Table 1 Ultimate and proximate analyses, and the lower heating value of oil palm kernel shell used in experimental tests on the conical FBC. Ultimate analysis (wt.%, as-received basis)
Proximate analysis (wt.%, as-received basis)
LHV (kJ/kg)
C
H
O
N
S
W
A
VM
FC
48.06
6.38
34.10
1.27
0.09
5.4
4.7
71.1
18.8
alumina sand as the bed material for the range of operating conditions (fuel feed rate and excess air). The combustion and emission performance of the combustor, as well as the capability of the bed material to withstand bed agglomeration with time, were the main focus of this experimental study. 2. Materials and methods 2.1. The fuel Table 1 shows the ultimate and proximate analyses as well as the lower heating value of oil palm kernel shell used in this experimental study. It can be seen in Table 1 that this biomass has a quite significant content of volatile matter, a moderate proportion of fixed carbon, but rather low contents of fuel moisture and ash. Due to quite low fuel S, SO2 was not addressed in this study. Like any other agricultural waste, oil palm kernel shell consisted mainly of hemicellulose (1.2 wt.%), cellulose (38.6 wt.%) and lignin (39.0 wt.%), and also included some extractives. However, this biomass exhibited a hard texture and high solid density (about 1500 kg/m3). To achieve high combustion efficiency and stable fuel feeding, this biomass was burned as shredded fuel with individual particles of up to 9 mm in sieve size. The shape and size of the shredded shell were quite irregular: from sawdust-like fine particles to flake-shape coarse particles. Fig. 1 depicts the particle size distribution (as wt.% for selected size groups) of oil palm oil kernel shell used in this study. It can be concluded from this analysis that the biomass was basically represented by particles of 0.5–9.0 mm in sieve size. However, shell particles of 3–6 mm in sieve size were apparently predominant in the biomass sample. The standard analysis of fuel ash (wt.%, as oxides) exhibited the predominant proportion of silicon (SiO2 = 54.12%), followed by calcium (CaO = 23.21%), potassium (K2O = 8.12%), and iron (Fe2O3 = 6.14%). The elevated potassium content in the fuel ash indicated a potential problem (bed agglomeration) during the combustion of this biomass in a fluidized bed with conventional bed material (silica sand).
16,300
2.2. Experimental set up Fig. 2 depicts the schematic diagram of the conical FBC with its dimensional characteristics. The combustor consisted of a conical module with 408 cone angle and 0.25 m inner diameter at the bottom plane, and five cylindrical modules, all insulated internally. Besides the combustor, the experimental set up included a screwtype fuel feeder delivering biomass into the conical module, a 25hp blower for air supply, a cyclone for collecting the fly ash generated from biomass combustion, as well as facilities for data collection. During combustor operation, a fluidized bed of inert material was generated and sustained in the conical module by a 19bubble-cap air distributor, which ensured uniform distribution of airflow over the bed at a quite low pressure drop across the distributor. An individual bubble-cap stand pipe of the distributor had sixty four holes each of 2 mm in diameter, arranged evenly over the pipe outer surface, and also six vertical slots with 3 mm width and 15 mm height located under the cap of 47 mm diameter. Net cross-sectional area of airflow at the distributor exit (calculated as the difference between area of the 0.25-mdiameter distributor plate and total area occupied by the caps) was 0.016 m2. The combustor was equipped with stationary Chromel–Alumel thermocouples (of type K) for monitoring temperature at different locations along the combustor height (see Fig. 2) during the combustor start up and when changing operation conditions. Seven gas ports at nearly the same locations were used for sampling the flue gas across the reactor. Alumina sand with the solid density of 3500 kg/m3 and mean particle size of 0.45 mm was used as the bed material to prevent
Weight percentage
50 40 30 20 10 0 <0.3
0.3–0.5 0.5–1.0 1.0–3.0 3.0–6.0 6.0–9.0
Biomass particle size (mm) Fig. 1. Particle size distribution of oil palm kernel shell used in the combustion tests on the conical FBC.
Fig. 2. Schematic diagram of the conical fluidized-bed combustor.
P. Ninduangdee, V.I. Kuprianov / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
2.3.1. TG/DTG analysis of the biomass fuel A ‘‘Mettler Toledo’’ TGA/DSC1 thermogravimetric analyzer was employed to obtain thermogravimetric characteristics (TG and DTG curves) of oil palm kernel shell for pyrolysis and combustion processes. Nitrogen and dry air (supplied into the analyzer furnace at a flow rate of 50 ml/min) were used as the furnace medium when testing the biomass for pyrolysis and combustion, respectively. During the test, a biomass sample with the initial weight of about 10 mg was heated from room temperature to 900 8C at a heating rate of 20 8C/min. Some important thermogravimetric characteristics, such as the ignition temperature, the peak temperatures and the burnout temperature, were obtained from the TG/DTG analysis. 2.3.2. Combustion tests on the conical FBC To study the effects of operating conditions on combustion and emission performance of the conical FBC, combustion tests were performed at two fuel feed rates: 45 kg/h (rated load) and 30 kg/h (reduced load), while ranging excess air from 20% to 100% for each combustor load. In order to investigate the behavior of temperature and gas concentrations (O2, CO, CxHy, and NO) in different regions of the reactor, the variables were measured along radial and axial directions inside the reactor, as well as at the cyclone exit (i.e., at stack), using a new model ‘‘Testo-350’’ portable gas analyzer. During these measurements, a sampled gas was continuously 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 8C) at the sampling points. Due to the thermoelectric conditioner of the gas analyzer, all the gas concentrations were measured on a dry basis. Prior to the recording of variables at fixed fuel feed rate and excess air, steady-state conditions of the conical FBC were ensured by verifying the temperature stability at the exit of the conical module and at the combustor top until time-domain fluctuation of the temperatures did not exceed 2 8C. When changing the amount of excess air (at constant fuel feeding), the transition time from one steady-state regime to another was 15– 20 min. At a given point inside the reactor (or at the cyclone exit), each variable was measured at least eleven times to quantify its average value over the time interval.
2.3.3. Characterization of the bed material and fly ash After testing the combustor at the rated load for 10, 20 and 30 h, the (reused) bed material and fly ash samples were analyzed for their chemical compositions using a wavelength dispersive X-ray fluorescence spectrometer. A scanning electron microscope equipped with an energy dispersive X-ray spectrometer (SEM-EDS: JEOL, JSM-6400, Link ISIS-300) was employed to observe a physical texture of individual bed material particles (of both original alumina and reused bed material), as well as to determine an elemental analysis of the particle coating – a result of interaction between the bed material and fuel ash – after the above-mentioned operating times. For each time instant, the coating composition (as major elements: Al, Si, K, Ca, Mg, and Fe) was obtained by spot EDS analysis at five arbitrary points on the external surface of a bed material particle, as well as at five points on the internal particle surface (i.e., on the surface of a selected particle pore). In the discussion below, the coating compositions of the particle surfaces are represented by corresponding averages over all five locations on each surface for different operating times. To investigate the effects of coating on the bed particle size, the particle size distribution and average (volumetric) particle diameter of the bed material were determined with a ‘‘Mastersizer 2000’’ particle size analyzer after finishing the 30-h combustion tests at the rated combustor load. 3. Results and discussion 3.1. Thermogravimetric analysis of oil palm kernel shell Fig. 3 compares the TG/DTG curves of oil palm kernel shell between different TGA furnace environments (nitrogen and dry
100 90 80 70 60 50 40 30 20 10 0
0.00 -0.02 -0.04 TG with air TG with N DTG with air DTG with N
-0.06 -0.08 -0.10
DTG (1/min)
2.3. Experimental methods and procedures
For each test run, the amount of excess air and combustion efficiency were predicted using the standard methods [20] provided in Appendix A. As estimated by Eq. (A4) from Appendix A (using the fuel ultimate analysis from Table 1), the theoretical volume of air required for firing 1 kg oil palm kernel shell was 4.83 Nm3/kg. By taking into account the airflow rate and the net cross-sectional area of airflow, the superficial air velocity at the distributor exit (u) was expected to be within 5.0–8.4 m/s when burning 45 kg/h biomass at excess air of 20–100%. According to Ref. [19], under these conditions, the conical FBC was operated in the turbulent fluidized-bed regime characterized by a relatively high fluidization number: u/umff = 7.2–12. At the reduced combustor load, this relative parameter was lower, from 4.8 to 8, when varying the amount of excess air within the specified range.
Sample mass (%)
bed agglomeration in the combustor. The particle size distribution and average (volumetric) particle diameter of the original bed material were determined by the ‘‘Mastersizer 2000’’ particle size analyzer, whereas the chemical composition of the material was obtained using a wavelength dispersive X-ray fluorescence spectrometer. In all combustion trials, the static bed height was 30 cm. For the selected bed material (alumina sand), particle size and geometry (diameter at the lower base, cone angle, and static bed height), the minimum velocity of full fluidization (umff), i.e., the superficial air velocity at the air distributor exit sufficient to cause bubbling fluidization of the entire conical bed [17,18], is reported to be 0.7 m/s [19]. A start up burner was used to preheat the bed material prior to combustion tests. The burner was fixed at a 0.5 m level above the air distributor and inclined at a 308 angle to the horizontal, as shown in Fig. 2. When temperature of the fluidized bed reached 650–700 8C, a diesel pump of the start up burner was turned off, and the required combustor load was ensured by biomass feeding into the reactor. Basically, total start up time of the conical FBC from its cold state was 45–60 min, depending on the specified operating conditions.
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-0.12 -0.14 0
100 200 300 400 500 600 700 800 900 Temperature (ºC)
Fig. 3. TG and DTG curves of oil palm kernel shell for the tests when using nitrogen and air as the furnace medium.
P. Ninduangdee, V.I. Kuprianov / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
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air). During the pyrolysis test with nitrogen, biomass degradation was found to exhibit four stages: (i) dewatering (at temperature up to 170 8C), (ii) volatilization of a high reactive (likely, ‘‘light’’ and oily) biopolymer with the maximum volatilization rate at 274 8C (the first peak temperature), (iii) pyrolysis of cellulose and small proportion of lignin attaining the maximum volatilization rate at 354 8C (the second peak temperature), and (iv) slow volatilization of major proportion of lignin with its simultaneous conversion into char at temperatures from 370 8C to about 900 8C. Note that the peak temperature of the cellulose volatilization and the upper temperature of lignin degradation were in a good agreement with reference data [21]. Switching the furnace medium from nitrogen to dry air had a rather weak influence on the behavior of the TG/DTG curves within the first three stages; however, the effects of furnace medium on the processes at high temperatures (>500 8C) were found to be significant. Compared to the pyrolysis, the first and second peak temperatures during the combustion test turned out to be slightly lower (by about 5 8C), whereas the biomass degradation rate at these temperatures was apparently higher (see Fig. 3). This fact can be explained by increased reactivity of the biomass caused by availability of air, which oxidized biomass volatiles during the second and third degradation stages [22,23]. At temperatures of 370–490 8C (i.e., at the beginning of the fourth stage), the diminishing of the sample mass was generally due to the lignin volatilization. Within this temperature range, heat released from the volatile combustion was not high enough to ignite the char and sustain its combustion. From Fig. 3, temperature of 490 8C can be treated as the lower temperature limit for the char ignition. Rapid volatilization of lignin (major factor) and oxidation of char (minor factor) resulted in appearance of the third peak temperature (580 8C on the DTG curve), basically associated with lignin volatilization [21–23]. Two more important combustion characteristics – the ignition temperature (Tign) and the burnout temperature (Tb) – were determined based on analysis of the TG/DTG curves from the thermogravimetric (combustion) test: Tign = 250 8C and Tb = 675 8C. Taking into account rather low (first) peak and burnout temperatures of oil palm kernel shell (pointing at high thermal and combustion reactivity of the shell), one may conclude that in a real fluidized-bed combustion system with typical bed temperature (800–900 8C) this biomass fuel be burned with high combustion efficiency. 3.2. Effects of operating conditions on the distribution of temperature and O2 in the conical FBC Analysis of radial profiles of temperature and gas concentrations (O2, CO, CxHy, and NO) at different levels above the air FR = 45 kg/h, EA = 40% FR = 45 kg/h, EA = 80% FR = 30 kg/h, EA = 40% FR = 30 kg/h, EA = 80%
1000
3.3. Effects of operating conditions on formation and decomposition of CO, CxHy, and NO Fig. 5 shows the axial profiles of CO, CxHy (as CH4), and NO in the conical FBC for variable fuel feed rate and excess air. All these profiles revealed two specific regions in the reactor – the bottom region (0 < Z < 0.6 m) and the upper region (Z > 0.6 m) – exhibiting different net results of formation/decomposition of the pollutants. In the bottom region, CO exhibited a sharp increase along the axial direction caused by rapid devolatilization of fuel particles, followed by oxidation of CxHy and char carbon. For the selected level of excess air, the CO peak at the reduced load was somewhat lower compared to that at the rated load, likely due to the increased residence time. In the upper region, CO was gradually
(b) 20
FR = 45 kg/h, EA = 40% FR = 45 kg/h, EA = 80% FR = 30 kg/h, EA = 40% FR = 30 kg/h, EA = 80%
15
O2 (Vol%)
Temperature (°C)
(a) 1200
distributor in the conical FBC fired with oil palm kernel shell at variable operating conditions revealed rather good uniformity of these variables across the reactor in all combustion trials. Note that similar results were reported in previous studies on firing some shell-type biomass fuels in this combustion technique [13]. Such an appearance of the radial profiles indicated bubbling/turbulent fluidized-bed regime in the conical FBC, as well as high intensive heat-and-mass transfer along the radial direction inside the reactor. In this work, axial profiles of temperature and gas concentrations were therefore used for the analysis of combustion and emission performance of the combustor. Fig. 4 depicts the axial profiles of temperature and O2 in the combustor firing oil palm kernel shell at the rated and reduced combustor loads for the excess air values of about 40% and 80%. It can be seen in Fig. 4 that in the conical module of the reactor (Z < 0.9 m), the axial temperature profiles were quasi-uniform. This fact indicated excellent mixing of the bed material with chars and gases in the conical section, whereas in the cylindrical part of the combustor these profiles exhibited a reducing trend of temperature along the combustor height, likely due to the heat loss across reactor walls. A reduction in the combustor load led to diminishing of temperature at any point inside the reactor, because of the lower heat input to the conical FBC. Meanwhile, a local drop of temperature was observed when increasing excess air, which can be explained by the air dilution effects. In all the test runs, O2 exhibited a gradual reduction along the reactor height. The behavior of O2 indicated that a significant proportion of the biomass was oxidized in the bottom region of the conical FBC. At the reduced combustor load, despite the reduction in bed temperature, the rate of O2 consumption in this region was somewhat higher compared to that at the maximum load (for similar excess air), which can be explained by the increased residence time of reactants.
800
10
600
5
400
0 0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 Height above air distributor (m)
0.0
0.5 1.0 1.5 2.0 2.5 3.0 Height above air distributor (m)
3.5
Fig. 4. Effects of operating conditions – fuel feed rate (FR) and excess air (EA) – on the axial profiles of (a) temperature and (b) O2 in the conical FBC when firing oil palm kernel shell.
P. Ninduangdee, V.I. Kuprianov / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
(a) 20000
FR = 45 kg/h, EA = 40% FR = 45 kg/h, EA = 80% FR = 30 kg/h, EA = 40% FR = 30 kg/h, EA = 80 %
FR = 45 kg/h, EA = 40% FR = 45 kg/h, EA = 80% FR = 30 kg/h, EA = 40% FR = 30 kg/h, EA = 80%
15000
CO (ppm)
CxHy (ppm)
15000
(b) 20000
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10000
10000
5000
5000
0
0 0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 Height above air distributor (m)
(c) 600
0.5 1.0 1.5 2.0 2.5 3.0 3.5 Height above air distributor (m)
FR = 45 kg/h, EA = 40% FR = 45 kg/h, EA = 80% FR = 30 kg/h, EA = 40% FR = 30 kg/h, EA = 80%
500
NO (ppm)
0.0
400 300 200 100 0 0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 Height above air distributor (m)
Fig. 5. Effects of operating conditions – fuel feed rate (FR) and excess air (EA) – on the axial profiles of (a) CO, (b) CxHy, and (c) NO in the conical FBC when firing oil palm kernel shell.
reduced, mainly due to oxidation of CO by O2 and OH [24]. It should be noted that an increase in excess air (at fixed load) led to the lowering of the CO peak (observed at Z 0.6 m), mainly due to the enhanced rate of CO oxidation by O2. The axial profiles of CxHy had similar trends as those for CO and exhibited similar effects of the operating conditions. As known, CxHy are basically originated from fuel volatile matter and oxidized forming CO as an intermediate product [24]. Since CO continuously received the ‘‘feeding’’ from oxidation of CxHy and char-C, CO at different locations in the combustor was substantially higher than CxHy for the ranges of operating conditions. From the graph with the axial profiles of NO, this pollutant was found to be rapidly increased in the bottom region, basically due to biomass devolatilization and further oxidation of volatile nitrogenous species to NO via the fuel-NO formation mechanism [2]. Simultaneously, a part of NO was decomposed due to the catalytic reduction, i.e., via secondary reactions with CO and light hydrocarbons on the surface of char, ash and bed material particles [2,24]. However, in the upper region of the combustor, a significant NO reduction was observed, mainly due to the abovementioned secondary reactions. Compared to the rated combustor load, the rate of NO formation and, consequently, the NO peak observed in the fluidized bed region (at Z 0.6 m) were substantially lower in the tests at the reduced load (for similar excess air values). This result can be explained by the lower bed temperature resulting in the lower devolatilization rate and greater residence time, the latter being responsible for the higher NO decomposition rate in the bottom region. However, in accordance with the fuel-NO formation mechanism, the rate of NO formation in this region and the NO peak were higher with increasing excess air (at fixed combustor load). Based on the analysis of the axial profiles of NO in the second region, it can be concluded that the reduction rate of NO in this
region was weakly dependent on operating conditions, exhibiting however sensible effects of CO and CxHy. Thus, NO at any location along the combustor height was in apparent correlation with the NO peak for selected operating conditions. Note that the appearance of the CO, CxHy and NO peaks at Z 0.6 m, i.e., at the level of fuel injection, confirmed very high thermal (devolatilization) reactivity of this biomass, which was revealed by the thermogravimetric study. 3.4. Emissions and combustion efficiency of the conical FBC Table 2 summarizes the unburned carbon content in fly ash, the emissions of CO, CxHy (as CH4), and NO (all on a dry gas basis and at 6% O2), the actual operating conditions (fuel feed rate, excess air, and O2 at stack) as well as the combustion-related heat losses and combustion efficiency of the conical FBC for all the test runs. It appears that the CO and CxHy emissions can be effectively reduced via increasing the amount excess air delivered to the combustor, whereas the NO emission can be minimized by decreasing the excess air level. At the rated load and lowest excess air value (about 20%), the CO and CxHy emissions from the combustor were at highest levels: 1551 ppm and 1320 ppm, respectively, whereas the NO emission showed its lowest value (about 100 ppm) pointing at a significant role of CO and CxHy in the reduction of NO. With increasing excess air, both CO and CxHy emissions were substantially decreased to the quite low levels when the excess air value was maintained within 80–100%. However, unlike CO and CxHy, the emission of NO showed quite opposite effects of excess air: with increasing this operating variable, the NO emission from the combustor was found to be higher (according to the fuel-NO formation mechanism). As seen in Table 2, the combustion efficiency for both fuel feed rates exhibited some improvement with increasing the level of excess air, particularly at low excess air values. For the excess air
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Table 2 Emissions and combustion efficiency of the conical FBC when firing with oil palm kernel shell for variable operating conditions. Excess air (%)
O2 at stack (vol.%)
Carbon in fly ash (wt.%)
COa (ppm)
CxHya (ppm)
NOa (ppm)
Heat loss (%) due to
Combustion efficiency (%)
Unburned carbon
Incomplete combustion
Burning 45 kg/h oil palm kernel shell 18 3.5 40 6.1 59 7.9 79 9.3 99 10.5
4.53 3.37 2.77 1.56 1.86
1551 631 400 246 142
1320 542 349 187 98
99 143 176 204 236
0.45 0.33 0.27 0.15 0.18
2.30 1.12 0.82 0.51 0.31
97.3 98.6 98.9 99.3 99.5
Burning 30 kg/h oil palm kernel shell 22 4.0 40 6.1 58 7.8 78 9.2 99 10.5
3.66 1.86 1.97 1.76 1.56
1230 490 330 190 121
983 412 269 148 88
68 122 178 200 213
0.36 0.18 0.19 0.17 0.15
1.80 0.86 0.64 0.40 0.27
97.8 99.0 99.2 99.4 99.6
a
At 6% O2 (on a dry gas basis).
values of 40–100%, the combustion efficiency was found to be nearly the same and characterized by highest values at distinct amounts of excess air. At the rated combustor load, combustion efficiency of 98.6–99.5% can be achieved via maintaining excess air within 40–100%. However, with reducing the combustor load at fixed excess air, all the emissions showed the trend to be reduced, whereas the combustion efficiency was improved, mainly due to the reduction in the CO and CxHy emissions. Taking into account the emission characteristics as well as the combustion efficiencies in Table 2, excess air of about 40% seems to be the best option for firing oil palm kernel shell in this conical FBC regardless of the combustor load. Under this condition, the conical FBC can be operated with high, about 99.0%, combustion efficiency, while controlling the major gaseous (CO and NO) emissions at acceptable levels meeting the corresponding domestic emission limits for the biomass-fueled industrial applications: 740 ppm for CO and 205 ppm for NO (both on a dry gas basis, as corrected to 6% O2) [25], and maintaining CxHy emissions at a reasonable level. At lower excess air values (e.g., 20%), the CO and CxHy emissions were unreasonably high, which led to a noticeable reduction in the combustion efficiency. However, with increasing excess air values over 40%, one can
observe a substantial (undesirable) increase in the emission of NO (a more harmful pollutant than the other two), as can be seen in Table 2. 3.5. Coating of bed material particles The SEM micrographs of an external surface of original bed material (i.e., unused alumina sand) revealed insignificant surface roughness, whereas cross-sectional images of sand particles exhibited the porous texture of the sand. From the SEM observation, sand pores were irregular in their size and shape, and were randomly located in the particle volume. Fig. 6 depicts the SEM images of bed material particles (upper micrographs, exhibiting an external particle surface) and those of particle cross-sectional views (lower micrographs) after different times of combustor operation (10, 20 and 30 h) at the rated combustor load. Yet the lower micrographs in Fig. 6 show the locations on the external particle surface (points 1–5) as well as those on the internal (pore) surface (points 6–10) where the coating was sampled for spot EDS analysis. The SEM images revealed the particle coating, which can be seen in Fig. 6 as a ‘‘white’’ thin layer on both surfaces of the selected particles. Due to
Fig. 6. SEM images of bed material particles (upper micrographs at 150 magnification) and their cross-sectional views (lower micrographs at 750 magnification) after different operating times: (a) 10 h, (b) 20 h and (c) 30 h.
P. Ninduangdee, V.I. Kuprianov / Journal of the Taiwan Institute of Chemical Engineers 44 (2013) 1045–1053
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Fig. 7. Elemental composition of the coating on the (a) external and (b) internal surfaces of bed material particles at different time instants of combustor operation.
the coating, the external surface of the particles exhibited an apparent roughness (as seen in a box showing the surface at high magnification), which was substantially greater than that of the original alumina sand. However, the coating thickness at the two surfaces was weakly dependent on operating time. From the spot EDS analysis, the coating was rich with Al (predominant in the bed material) as well as Si, K, Ca, Mg, and Fe (the latter five from fuel ash). Fig. 7 shows the elemental composition of the coating on both external and internal surfaces of bed material particles for different operating times. The results in Fig. 7 revealed that the coating of the external particle surface was mainly represented by the ash-related elements. However, the proportions of ash-related elements in the coating on the pore surface were substantially lower than those on the external surface, due to the relatively high proportion of Al in this coating layer. As a matter of fact, timescale effects on the coating composition were rather weak. However, with increasing operating time, Si in the external coating showed a noticeable reduction, mainly due to substitution of this element by Ca from the fuel ash, whereas the diminishing of Al in the pore coating was likely caused by increased penetration of ash vapors into the sand pores. As found by visual inspections after finishing of all the tests at both rated and reduced combustor load (lasted for about 45 h), the bed material exhibited normal appearance (grains) capable to fluidize, and thus had the potential to be reused in further combustor operation. Based on the SEM/EDS analyses, it can be concluded that prevention of bed agglomeration during the combustion tests was ensured due to the adsorption of potassium salts (vaporized from fuel ash) by the external and internal surfaces of bed material particles [12,26,27]. It appears that alumina reacted with these and other salts in a quite thin layer on the external/internal surface of alumina sand particles. This layer termed ‘coating’ consisted of some eutectics (such as K2O–Al2O3– SiO2 systems) of high fusion temperature, typically 1500–1600 8C [27–29], which significantly exceeded bed temperatures and thus inhibited bed agglomeration in the combustor. Furthermore, the external particle coating included elevated Ca, Mg, and Fe, basically mitigating formation of sticky compounds on the outer surface of the particles and thus reducing the probability of adhering coarse bed material particles with char, ash, and fine bed particles [30– 32]. Another factor preventing bed agglomeration in this conical FBC was associated with the turbulent fluidization regime of the expanded conical bed [19], under which the probability of solid particle collisions was significantly lower that it could happen if the combustor were operated in bubbling fluidized-bed regime. 3.6. Effects of the coating on particle size of the bed material Table 3 shows the particle size distribution of the original bed material (alumina sand) and that of the re-used bed
material after 30 h combustion tests at the rated combustor load. It can be seen in Table 3 that the size distribution (as the volume percentage of particles for different size groups) showed an apparent increase in volumetric diameter of the re-used sand particles (575 mm) compared to that of the original alumina sand (446 mm), and this was likely caused by the coating of bed material particles. However, as follows from the analysis of the size distribution at different time instants, the predominant proportion of the bed material was represented by the Geldart-B particles, which sustained fluidizedbed regime of the combustor. Note that the increase in the bed particle size resulted apparently in a higher value of the minimum velocity of full fluidization (umff) [17,19]. However, this insignificant increase in the particle size did not lead to any noticeable changes in the hydrodynamic behavior of the bed. 3.7. Time-related changes in the composition of the bed material and fly ash Table 4 shows the composition of the bed material used/reused in the conical FBC and that of the fly ash originated from the combustion of oil palm kernel shell at the rated load for different operating times. For comparison, Table 4 includes the composition of the original alumina sand and that of the fuel ash (both obtained prior to combustion tests). The results revealed a quite intensive interaction (substance exchange) between the bed material and ash. This interaction
Table 3 Particle size distribution of the original alumina sand and that of the bed material after 30-h testing on the conical FBC for firing oil palm kernel shell at the rated combustor load. Particle size range (mm)
172–200 200–233 233–272 272–317 317–370 370–431 431–502 502–586 586–683 683–796 726–928 928–1082 1082–1262 1262–1471 Mean particle size (mm)
Volume percentage of particles in the bed material Prior to experiments
After 30-h testing
0.41 2.70 3.64 6.62 17.03 20.63 22.25 13.90 8.07 2.03 2.26 0.32 0.14 0.00 446
0.14 1.05 2.85 5.51 8.70 11.76 13.95 14.66 13.68 11.32 8.22 5.26 2.58 0.32 575
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Table 4 Composition of the bed material, as well as that of fuel ash and fly ashes originated from the combustion of oil palm kernel shell at different time instants of combustor operation at the rated load. Operating time (h)
Composition (as oxides, wt.%) Al2O3
SiO2
Bed material 0 (original alumina sand) 10 (used bed material) 20 (reused bed material) 30 (reused bed material)
99.40 64.73 57.25 46.99
0.09 27.2 29.7 34.6
Fuel/fly ash 0 (fuel ash) 10 (fly ash) 20 (fly ash) 30 (fly ash)
3.11 13.66 6.45 6.32
54.12 48.75 53.86 53.31
CaO
MgO
K2O
Na2O
Fe2O3
TiO2
P2O5
ZnO
Cl
0.05 2.41 4.44 6.97
0.03 0.56 1.27 1.66
– 2.89 4.13 5.56
0.38 0.43 0.27 0.29
0.07 0.54 0.83 1.15
0.40 0.03 0.06 0.06
0.01 1.01 1.78 2.38
0.03 – – –
– – – –
23.21 20.31 22.12 22.44
2.65 2.07 2.15 2.06
8.12 6.24 5.86 6.25
0.81 1.17 0.68 0.64
6.14 2.09 2.08 2.22
– 0.35 0.19 0.20
1.15 2.13 2.01 1.93
0.20 – – –
– 0.01 0.13 0.12
occurred likely via: (i) above-mentioned formation of the coating on the internal and external surfaces of the bed material particles, and (ii) accumulation of small proportion of ash in the bed, leading eventually to an increase of ash-related elements in the fluidized bed. As a result, SiO2, CaO and K2O in the reused bed material showed a significant/sensible increase with time, however to different extent at distinct time instants. Due to collisions, breakage and attrition of the original bed material particles, the particle coating was destructed resulting in generation of some fine particles (containing Al) in the fluidized bed, which was consequently carried out from the bed region and became a part of fly ash. These processes explain a reduction of Al2O3 in the reused bed material (roughly, by 50% for the entire experimental time), whereas Al2O3 in the fly ash was (always) substantially greater than that in the fuel ash analysis, indicating a continuous carryover of the original bed material from the bed and therefore a gradual reduction of the bed capability to prevent its agglomeration during the combustion of oil palm kernel shell.
Acknowledgement The authors wish to acknowledge the financial support from Thammasat University (Thailand) as well as the financial support from the Bangchak Petroleum Public Company Limited (Thailand). Appendix A. Determining excess air and combustion efficiency for different test runs Below are the relationships used in this study for determining the amount of excess air, as well as the combustion-related heat losses and the combustion efficiency for each test run. The excess air coefficient (ratio) for each trial was estimated by using actual O2, CO, and CxHy (as CH4) at stack (all in vol.%, on a dry gas basis), as well as by neglecting H2 and assuming N2 = 79 vol.% in the dry flue gas, as:
a¼
21 21 ðO2 0:5CO 2CH4 Þ
(A1)
The percentage of excess air at stack was then calculated as: 4. Conclusions EA ¼ 100ða 1Þ Oil palm kernel shell is a lignocellulosic biomass exhibiting high thermal and combustion reactivity despite its hard structure and unconventionally high density. The conical fluidized-bed combustor using alumina as the bed material can be employed for safe and effective utilization of this shell with elevated potassium content for energy conversion via direct combustion. Operating conditions (fuel feed rate and excess air) have sensible effects on formation and decomposition of major gaseous pollutants (CO, CxHy, and NO) inside the reactor, as well as on combustion efficiency and emissions of the combustor. Excess air of 40% seems to be the best option for firing this biomass regardless of combustor load. At these operating conditions, high (about 99%) combustion efficiency is achievable, while controlling CO and NO emissions at values below the national emission limits and maintaining CxHy emissions at a reasonable level. External and internal surfaces of the original bed material particles are covered by a coating mainly consisted of the eutectics with high fusion temperatures preventing bed agglomeration. Due to interaction between the original bed material (alumina) and fuel ash, the composition of the reused bed material and that of fly ash undergo substantial changes during the combustor operation. An apparent reduction of Al2O3 in the bed material, accompanied by an increase in K2O and SiO2, indicates a gradual loss of the bed capability to withstand bed agglomeration.
(A2)
For the reference conditions (i.e., at O2,ref = 6 vol.%, on a dry gas basis), the excess air coefficient was estimated using Eq. (A1) by neglecting products of incomplete combustion (CO and CH4) to be aref = 1.4. The theoretical (reference) volume of dry flue gas (Nm3/kg, at 0 8C and 1 atm) was determined using the fuel ultimate analysis on as-received basis (provided in Table 1) and aref as: V dg@6%O2 ¼ 0:01866ðC þ 0:375SÞ þ 0:79V 0 þ 0:008N þ ðaref 1ÞV 0
(A3)
In Eq. (A3), the theoretical volume of air (Nm3/kg, at 0 8C and 1 atm) required for firing 1 kg biomass fuel under stoichiometric conditions (i.e., at a = 1) was determined based on the ultimate fuel analysis (see Table 1): V 0 ¼ 0:0889ðC þ 0:375SÞ þ 0:265H 0:0333O
(A4)
The heat-loss method was used to estimate the combustion efficiency. For this conical FBC with no bottom ash, the heat loss due to unburned carbon (%) was as: 32; 866 C fa A (A5) quc ¼ LHV 100 C fa where Cfa is the content of unburned carbon in the fly ash, (wt.%), and A is the fuel ash content (wt.%, on as-received basis).
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The heat loss owing to incomplete combustion (%) was predicted by using the CO and CxHy emissions (both on a dry gas basis and at 6% O2) provided in Table 2 and neglecting H2 in flue gas as: qic ¼ ð126:4 CO þ 358:2 CH4 Þ@6%O2 104 V dg@6%O2
ð100 quc Þ LHV (A6)
The combustion efficiency was then predicted as:
hc ¼ 100 ðquc þ qic Þ
(A7)
References [1] Index Mundi Palm oil production by country. http://www.indexmundi.com/ agriculture [accessed 13.05.2013]. [2] Werther J, Saenger M, Hartge EU, Ogada T, Siagi Z. Combustion of agricultural residues. Prog Energy Combust Sci 2000;26:1–27. [3] Saenger M, Hartge EU, Werther J, Ogada T, Siagi Z. Combustion of coffee husks. Renew Energ 2001;23:103–21. [4] Fang M, Yang L, Chen G, Shi Z, Luo Z, Cen K. Experimental study on rice husk combustion in a circulating fluidized bed. Fuel Process Technol 2004;85:1273–82. [5] Permchart W, Kouprianov VI. Emission performance and combustion efficiency of a conical fluidized-bed combustor firing various biomass fuels. Bioresour Technol 2004;92:83–91. [6] 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;22:1004–11. [7] Khan AA, de Jong W, Jansens PJ, Spliethoff H. Biomass combustion in fluidized bed boilers: potential problems and remedies. Fuel Process Technol 2009;90:21–50. [8] Lin W, Dam-Johansen K, Frandsen F. Agglomeration in bio-fuel fired fluidized bed combustors. Chem Eng J 2003;96:171–85. [9] Chaivatamaset P, Sricharoon P, Tia S. Bed agglomeration characteristics of palm shell and corncob combustion in fluidized bed. App Therm Eng 2011;31:2916–27. [10] Chaivatamaset P, Sricharoon P, Tia S, Bilitewski B. A prediction of defluidization time in biomass fired fluidized bed combustion. Appl Therm Eng 2013;50:722–31. [11] Shimizu T, Han J, Choi S, Kim L, Kim H. Fluidized-bed combustion characteristics of cedar pellets by using an alternative bed material. Energy Fuels 2006;20:2737–42. [12] Ferna´ndez LMJ, Escalada CR, Murillo LJM, Carrasco GJE. Combustion in bubbling fluidised bed with bed material of limestone to reduce the biomass ash agglomeration and sintering. Fuel 2006;85:2081–92. [13] Kuprianov VI, Arromdee P. Combustion of peanut and tamarind shells in a conical fluidized-bed combustor: a comparative study. Bioresour Technol 2013;140:199–210.
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[14] Kouprianov VI, Permchart W. Emission from a conical FBC fired with a biomass fuel. Appl Energy 2003;74:383–92. [15] Kaewklum R, Kuprianov VI. Theoretical and experimental study on hydrodynamic characteristics of fluidization in air–sand conical beds. Chem Eng Sci 2008;63:1471–9. [16] Janvijitsakul K, Kuprianov VI. Major gaseous and PAH emissions from a fluidized-bed combustor firing rice husk with high combustion efficiency. Fuel Process Technol 2008;89:777–87. [17] Peng Y, Fan LT. Hydrodynamic characteristics of fluidization in liquid-solid tapered beds. Chem Eng Sci 1997;52:2277–90. [18] Jing S, Hu Q, Wang J, Jin Y. Fluidization of coarse particles in gas–solid conical beds. Chem Eng Process 2000;39:379–87. [19] Ninduangdee P, Kuprianov VI. Measures for reliable and effective operation of a conical fluidized-bed combustor fired with oil palm shells: a cold-state hydrodynamic study. In: Proceedings of the Third TSME International Conference on Mechanical Engineering; 2012. [20] Basu P, Cen KF, Jestin L. Boilers and Burners. New York: Springer; 2000. [21] Haykiri-Acma H, Yaman S, Kucukbayrak S. Comparison of the thermal reactivities of isolated lignin and holocellulose during pyrolysis. Fuel Process Technol 2010;91:759–64. [22] Haykırı-Ac¸ma H. Combustion characteristics of different biomass materials. Energ Convers Manage 2003;44:155–62. [23] Munir S, Daood SS, Nimmo W, Cunliffe AM, Gibbs BM. Thermal analysis and devolatilization kinetics of cotton stalk, sugar cane bagasse and shea meal under nitrogen and air atmospheres. Bioresour Technol 2009;100:1413–8. [24] Turns S. An Introduction to Combustion. Boston: McGraw-Hill; 2006. [25] Pollution Control Department, Ministry of Natural Resources and Environment, Thailand. Air pollution standards for industrial sources. http:// www.pcd.go.th/info_serv/reg_std_airsnd03.html [accessed 13.05.2013]. [26] Steenari BM, Lindqvist O. High-temperature reactions of straw ash and the anti-sintering additives kaolin and dolomite. Biomass Bioenerg 1998;14:67– 76. [27] Llorente MJF, Arocas PD, Nebot LG, Garcı´a JEC. The effect of the addition of chemical materials on the sintering of biomass ash. Fuel 2008;87:2651–8. [28] Konsomboon S, Pipatmanomai S, Madhiyanon T, Tia S. Effect of kaolin addition on ash characteristics of palm empty fruit bunch (EFB) upon combustion. Appl Energy 2011;88:298–305. [29] Tran K-Q, Iisa K, Steenari B-M, Lindqvist O. A kinetic study of gaseous alkali capture by kaolin in the fixed bed reactor equipped with an alkali detector. Fuel 2005;84:169–75. [30] Nuutinen LH, Tiainen MS, Virtanen ME, Enestam SH, Laitinen RS. Coating layers on bed particles during biomass fuel combustion in fluidized-bed boilers. Energy Fuels 2003;18:127–39. [31] Vuthaluru HB, Zhang D-K. Control methods for remediation of ash-related problems in fluidised-bed combustors. Fuel Process Technol 1999;60:145– 56. [32] Bhattacharya SP, Harttig M. Control of agglomeration and defluidization burning high-alkali, high-sulfur lignites in a small fluidized bed combustor: effect of additive size and type, and the role of calcium. Energy Fuels 2003;17:1014–21.