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Production of amorphous rice husk ash in a 500 kW fluidized bed combustor
Fuel 144 (2015) 214–221
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Production of amorphous rice husk ash in a 500 kW fluidized bed combustor Guanyi Chen a,⇑, Guiyue Du a, Wenchao Ma a,⇑, Beibei Yan a, Zhihua Wang b, Wenxue Gao a,c a School of Environmental Science and Engineering/State Key Laboratory of Engines/Tianjin Key Lab. of Indoor Air Environmental Quality Control, Tianjin University, Tianjin 300072, China b State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China c North China Municipal Engineering Design and Research Institute, Tianjin 300384, China
h i g h l i g h t s Rice husk is burnt in a 500 kW fluidized bed combustor. 660–720 °C and fluidization velocity of 1.0–1.2 m s
1
appear to be the optimal.
Ash with amorphous SiO2 is over 93.8 wt.% and unburnt carbon less than 5%. Combustion efficiency is 90% in all test runs.
a r t i c l e
i n f o
Article history: Received 5 June 2014 Received in revised form 24 November 2014 Accepted 4 December 2014 Available online 24 December 2014 Keywords: Rice husk Fluidized bed combustion Rice husk ash (RHA) Amorphous silicon dioxide
a b s t r a c t A 500 kW rice husk-firing fluidized bed combustor was designed for obtaining low carbon and high silicon ash in the amorphous form. The influence of operating condition (temperature, fluidizing velocity) on combustion efficiency and rice husk ash quality was experimentally investigated. Furthermore, the quality of rice husk ash collected from inner bed, two-stage cyclones and bag-house was analyzed using XRD, TEM and SEM instruments, in terms of amorphous silicon dioxide and unburnt carbon content. Comparative study was carried out between the rice husk ash produced in an electronic oven and that produced in the above-mentioned fluidized bed combustor. The results show that quality of ash from the industrial-scale fluidized bed is even better than that from electronic oven. Temperature of 660– 720 °C and fluidization velocity of 1.0–1.2 m s 1 appear to be the favoring parameters for producing high quality rice husk ash with amorphous structure of silicon dioxide content over 93.8 wt.% and unburnt carbon context less than 4.5 wt.%. The fluidized bed combustor also assures combustion efficiency 90% in all test runs. This research indicates amorphous structure of silicon dioxide may be economically promisingly produced from the self-designed rice husk combustor. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Rice husk, a renewable by-product of rice milling process, has a large annual quantity of approximately 144 million tons worldwide (2011), and attracts wide attention to its utilization and conversion [1]. The annual amount of produced rice husk is more than 44 million tons in China [1]. Rice husk was commonly combusted or gasified as an agriculture waste to produce heat and electricity or biogas, respectively [2–4]. Natarajan et al. [5] reviewed the research on combustion and gasification of rice husk in fluidized bed reactors and pointed out most results were from only small bench or lab ⇑ Corresponding authors. Tel.: +86 22 8740 2100; fax: +86 22 8740 2075. E-mail addresses: [email protected] (G. Chen), [email protected] (W. Ma). http://dx.doi.org/10.1016/j.fuel.2014.12.012 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
scale units. Recently, rice husk gasification and power generation system with a capacity of 150 t d 1 was designed and installed in China [2], and a 10 MW bubbling fluidized-bed power plant firing rice husk was established in India [6]. However, these researches only focused on combustion or gasification characteristics in terms of energy efficiency and pollutant gas emissions. Recently growing attention has been paid to rice husk ash (RHA) as it contains high silica content with amorphous characteristics of a cellular structure [7–9], which can be further purified to valueadded products, such as pure SiO2, SiCl4, Si3N4, and SiC [10,11]. Moreover, RHA can be widely used as concrete alternatives to improve the durability and performance of concrete [12], and used as a supplementary cementitious material, or used in ceramic field, e.g. whiteware manufacture [13], or as an adsorbent to absorb
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G. Chen et al. / Fuel 144 (2015) 214–221 Table 1 Proximate and ultimate analysis of rice husk. Proximate analysis (wt.%, dry basis)
*
Volatile
Ash
Fixed carbon
67.35
18.16
14.49
Ultimate analysis (wt.%,dry basis) *
*
C
H
O
38.45
5.08
37.83
Low heating value
N
S
Cl
LHV (kJ kg
0.15
0.23
0.10
14,600
1
)
Calculated by differences.
organic dye and inorganic metal, such as lead [14] and mercury [15], for synthesis of zeolites [16]. Combustion process may influence the ash formation. Therefore the effects of fluidizing velocity [11], burning temperature [17], time and cooling regime [12] on the pozzolanic properties of RHA have been studied. Rozainee et al. [11] pointed out RHA, in amorphous form with low residual carbon content (2.88 wt.%), was released in the optimum condition of fluidized velocity = 3.3 Umf. Nair et al. [12] carried out the experiments in a laboratory furnace under continuous supply of air as a function of incineration temperature, time and cooling regime. They found out the highest amounts of amorphous silica occurred in samples burnt in the range of 500–700 °C, and the most reactive RHA was produced after incineration for 12 h at 500 °C followed by quick cooling down. Huang et al. [10] studied the silica white properties produced from a lab-scale fluidized bed with inner diameter of 84 mm, and concluded that the quality of silica white sampled from the dense-phase zone at 700–800 °C was higher than that of GB-precipitated silica (GB stands for national standard). Della et al. [18] reported that the temperature of rice husk carbonization might be preferably below 700 °C to avoid any transformation of amorphous to crystalline form. Above 760 °C, silica and potassium oxide fuse on the surface of rice husk char and form a glass-like barrier which prevents further reaction of the remaining material [19]. However, RHA production and characterization has mostly been conducted at the laboratory-level. In sum, major previous studies on rice husk combustion in a fluidized bed boiler either focused on the investigation of combustion parameters (i.e. combustion intensity, combustion efficiency, carbon conversion efficiency) or on the combustion performance (i.e. pollutants emission, carbon content in the ash, heat loss), and less studies focused on RHA. There was lack of study on RHA produced in the industrial-scale fluidized bed combustion. Furthermore, the optimum operating parameters influencing on RAH quality in terms of low carbon content and amorphous silica is not clear. This research aims to consider both combustion efficiency and RHA quality in a 500 kW fluidized bed combustor. The objective of this study is to access the behavior of a self-designed fluidized bed combustor burning rice husk, and to investigate the influences of temperature and velocity on RHA production and combustion efficiency as well as to characterize the RHA quality. Finally, the RHA obtained from fluidized bed combustor is compared with that from a lab-scale furnace. The results of this study may be of interest to industry aiming at producing silicon material from active ash in an economical way which compromises energy and value-added silicon material production. 2. Material and methods 2.1. Characteristics of rice husk Rice husk, an irregular boat-like particle, is the outer cover of rice and on average it accounts for 20 wt.% of the paddy produced on the weight basis. The typical size of rice husk in this study is about 5–8 mm in length, 2–3 mm in width and 0.3 mm in thickness, and bulk density is 118.4 kg m 3. The rice husk, not suffer any previous processing before combustion, was collected in Shaoxing City, Zhejiang Province, China. Table 1 shows the calorific
value, the proximate and ultimate analysis (as received basis). Fixed carbon and Oxygen content are calculated by difference, respectively. In general, it is difficult to fluidize rice husk alone, due to its cylindrical shape, non-granular and flaky nature [3,6]. For proper fluidization and processing in the combustor, it has been strongly suggested to be mixed with other solid particles, such as silicon sand, alumina and calcite etc. forming a multi-solid system [17]. In this study, quartz sand with the diameter ranging from 0.50 mm to 0.75 mm is adopted as bed material. 2.2. Thermo gravimetric analysis (TGA) Thermo gravimetric and differential thermal analysis (TG-DTA) of rice husk was carried out in a Netzsch STA 449C Jupiter simultaneous analyzer. Its precision of temperature measurement was ±0.5 K, microbalance sensitivity less than 1.0 lg. Approximately 10 mg of the sample was placed in the microbalance and heated at rate of 50 °C min 1 under nitrogen flowing, from room temperature to a final temperature of 900 °C. 2.3. RHA production in a 500 kW fluidized bed combustor Fig. 1 shows a schematic diagram of a 500 kW fluidized bed combustor located in Shaoxing City, Zhejiang Province, China. During the design stage, several specific considerations have been taken, such as variable sectional area, staged air feeding, precisely controllable superficial velocity and temperature distribution along the height of furnace, in addition to strict retention time of ash in furnace, as well as two stages of cyclones. The system consists of a fluidized bed combustor, a screw feeder, a thermal exchanger, 2 high-temperature cyclone separators, a bag-house and an under-bed start-up burner. The inner diameter of density phase zone is 620 mm, dilutephase zone 920 mm, and total height 6.0 m. Incoming air is preheated with a gas burner during the start-up operation. The bed material of quartz sand containing over 90 wt.% silica content, is fed at the height of 0.6 m from air distributor, with a static bed height of 0.4 m. Rice husk is also fed at 0.6 m through a pressured-variable speed screw feeder which prevents high-temperature gas inside the fluidized bed from going against the screw feeder, and leading to discontinuous feeding. The fuel feeding rate, up to 125 kg h 1, is controlled by a computer-based data acquisition and controlled system. Secondary air is set at the height of 2 m, in horizontal direction. The ratio of primary air to secondary air is 7:3, excess air coefficient around 1.20–1.25. The cyclone separators are connected to the exit of disengagement section to capture the solid particles (dust, ash and char) escaping from the combustor.1 Four ash collecting points are located at the bottom of bed discharge, exits of two cyclones and bag-house, respectively. Temperatures are recorded by ten thermocouples (type K) installed along the height: 0.1 m, 0.6 m, 1.0 m (dense-phase zone), 2.0 m, 2.5 m, 3.5 m (dilute-phase zone), 4.0 m, 5.0 m, and 5.8 m (exit). 1
if without special description, in this text the height of bed is from air distributor.
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XL-30 TMP ESEM (20 kV, Holland) was used to obtain SEM of RHA samples. TEM studies were performed on a Tecnai G2 F20 instrument operated at 200 kV. The sample was suspended in alcohol and ultrasonically dispersed for 20 min. The suspension was piped onto a copper micro grid with carbon film. At least 100 particles were used to obtain a particle size distribution. Selected area diffraction (SAD) was conducted in the TEM apparatus with 14,000 enlargements to ensure state of ash samples. XRD analysis was used to assess presence of crystalline and amorphous substances in the ash. The tests were conducted in a Rigaku diffraction unit (Dmax-2500V/PC), using copper as target, with graphite-monochrome-filter at 40 kV, 200 mA. Goniometer speed was maintained at 4° min 1. To determine the unburnt carbon content in the RHA, the collected ash samples were subjected to ignition on loss test in the electronic oven at 900 °C. Minerals composition of the RHA is determined by atomic emission measurements using ICP emission spectroscopy (Perkine Elmer Optima 3300 DV). 3. Results and discussions 3.1. TG-DTA of rice husk
Fig. 1. Schematic diagram of a 500 kW fluidized bed combustor for RHA production.
The system is monitored by a microcomputer linked to a data logger system (ASAE 4118, China). In order to study the influence of different working conditions on the production of RHA, several tests regarding to velocity and temperature were carried out during steady combustion conditions, as follows: 1. Temperature along fluidized bed height; 2. Effect of temperature and velocity on combustion efficiency; 3. Chemical contents of ashes collected from bed discharge, cyclones, bag-house; and mineralogical characterization of rice husk ash with the analysis of SEM, TEM, SAD etc. 2.4. RHA production in a lab-scale electronic oven Additional experiments focusing on RHA quality were carried out in a lab scale electronic oven (SX2-2.5-10, Tianjin Zhonghuan Ltd, China), to compare with that in the fluidized bed combustor. The effects of temperature and residence time on RHA quality were studied in different procedures. During temperature investigation, ceramic crucibles with 5 g rice husk were put inside of an electronic oven with continuous air flow rate of 100 ml min 1 and a heating rate of 10 °C min 1. Each sample was held at a maximum temperature (650 °C, 675 °C, 700 °C, 725 °C, 750 °C) for 1 h and cooled down inside the oven, respectively. To study the effect of retention time, rice husk samples were fed suddenly into the furnace at a required burning temperature (680 °C or 720 °C), and the duration of rice husk samples varied from 5 s, 30 s, 1 min, 30 min to 1 h. When the rice husk was put into the furnace, it burned immediately due to relatively low ignition point, and further decarbonization took place. The ash was finally collected for further analysis.
Fig. 2 presents weight-loss curves (TG) as a function of temperature for rice husk using heating rate of 50°C min 1. The thermal degradation can be divided into three stages: moisture drying, main devolatilization and continuous slight devolatilization [20]. The moisture drying region corresponds to an initial slight weight loss between ambient temperature and nearly 105 °C, which results from the elimination of physically absorbed water in rice husk and superficial or external water bounded by surface tension. Main devolatilization stage starts at temperature of 250 °C, and the weight loss occurs promptly after that temperature until 390 °C because of volatile matter release, and a major change in the slope of curves at around 390 °C due to lignin decomposition [21]. Further loss of weight occurs until 800 °C (continuous slight devolatilization) indicating cellulose and hemi-cellulose decomposition, as CO, CO2, tar, steam etc. The whole process releases most thermal energy at peak area between 400–600 °C. The TG analysis supports the idea that most, if not all, of rice husk begins to burn even at such low temperature of about 340 °C and have been consumed in the region around 300–600 °C. 3.2. Operating performance of the fluidized bed combustor Fig. 3 shows axial temperature profile along fluidized bed height under various fluidizing velocities. Low fluidizing velocity
2.5. RHA characterization The micro-structure and quality of RHA were analyzed with scanning electron micrographs (SEM), Transmission electron microscopy (TEM), X-ray diffraction analysis (XRD). A PHILIPS
Fig. 2. Thermo gravimetric and differential thermal analysis (TG-DTA) curves of rice husk.
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of 0.6 m s 1 results in poor mixing, while temperature reduces sharply along the height of fluidized bed. When fluidizing velocity increases to 1.0–1.2 m s 1, strong combustion intensity zone will move up, where major reaction occurs at 2 m to 3.5 m. However, if fluidizing velocity increases further to 1.5 m s 1, strong combustion zone move further to dilute-phase zone, which leads to the inefficient combustion of rice husk thereby increasing the great losses in unburnt combustibles. Thus, fluidized velocity is chosen in the range of 1.0–1.2 m s 1. The temperature profile shows temperature peak moves from 1 m to 4 m of bed height, when fluidizing velocity increases from 0.6 m s 1 to 1.5 m s 1. Beyond 3.5 m, temperatures begin to fall, indicating that most of combustion is completed. This behavior can be explained by easy devolatilization process of rice husk and injection of secondary air. It may be linked to high-volatile fraction (60 wt.%) of rice husk leading to ignite even at low temperatures (340 °C). It may imply that rice husk devolatilization starts at the dense bed zone and ends just at the expansion of the reactor. Temperature is an important factor for rice husk combustion to produce amorphous silica. It has been reported that temperature higher than 700 °C promotes the silica transformation from amorphous to crystalline form [11,18,22]. Low temperature may lead to low combustion efficiency and meanwhile result in high content of unburnt carbon remained in RHA which means low quality of RHA. Previous studies also made different conclusions about the optimum temperature: Nair et al. [12] supposed the highest amounts of amorphous silica occurred in samples burnt in the range of 500–700 °C, contrarily Huang et al. [10] concluded that the quality of silica white sampled from the dense-phase zone at 700–800 °C was better. Large temperature gradient can also result in the turbulence of combustion efficiency and correspondingly poor RHA quality. Thus, test temperature here is kept in the small range of 660–720 °C, corresponding to fluidizing velocity 1.0–1.2 m s 1 (Table 2). Fig. 4 shows bed temperature profile as a function of time during actual operating conditions. It can be seen that in the first two hours, temperature of dense-phase zone is notably higher than dilute-phase zone indicating combustion mainly taking place in the dense-phase zone. This may be attributed to quick ignition of rice husk when fed to the boiler resulting in heat release. Heat released is mainly absorbed by bed at dense-phase zone due to heat transfer with bed material and surrounding atmosphere. After two-hour operation, the combustor reaches thermal equivalence. Later volatiles of rice husk move up with primary air flow and decompose under the atmosphere of secondary air in dilute-phase zone, resulting in temperature in dilute-phase zone and exit increasing slowly. The temperature throughout the combustor is stable in the range of 660–720 °C, which indicates the fluidized bed combustor working well. Due to the turbulent nature of fluidized bed, heat transfer rates are very high which leads to high combustion efficiency. In this
Table 2 Operating parameters of fluidized bed combustion experiments. Rice husk feeding rate Bed material Sand mean particle diameter Fluidizing gas velocity Bed operating temperature Excess air coefficient Ratio of primary air to secondary air
Up to125 kg h Silicon sand 0.6 mm 1.0–1.2 m s 1 660–720 °C 1.20–1.25 7:3
Fig. 4. Temperature profile in steady operating conditions.
study, the influences of fluidizing velocities and temperatures on combustion efficiency of rice husk and RHA quality were investigated, respectively. Feeding rate up to 125 kg h 1 may vary a bit, to meet each operating condition in Table 3. For the estimation of combustion efficiency, heat losses owing to incomplete combustion (accounting for the CO emission), qic, and unburnt carbon contained in particulate matter, quc,(usually in fly ash, quc-fa, and in bottom ash, quc-ba) were determined [22–24]. In each run, fly ash and bottom ash were sampled and afterwards analyzed for unburnt carbon to estimate associated heat loss. CO emission was sampled at the exit of bag-house and quantified using ‘‘Testo – 350’’ (Testo, Germany) gas analyzer. Table 3 shows these heat losses, qic and quc (quc-fa and quc-ba), along with combustion efficiencies in each operating condition. From Table 3, it can be seen that a reasonably high combustion efficiency of 90% above for the self-designed fluidized bed boiler is achieved, ranging from 88.00% to 93.10% over all tests, meaning the boiler is a good compromise of RHA and combustion efficiency. The heat loss with unburnt carbon and incomplete combustion exhibits a linear correlation with temperature and velocity. With temperature increasing at fluidizing velocity of 1.0 m s 1, heat loss with unburnt carbon and incomplete combustion decreases, corresponding to the increasing of combustion efficiency. The fluidization Table 3 Heat losses and combustion efficiency for various temperatures and velocities.
Fig. 3. Temperature profile along the bed height under various fluidizing velocities.
1
*
Temperature (°C)
Velocity (m s 1)
qic (%)
quc (%)
Combustion efficiency* (%)
670 695 710 720 720 720 720 720 720
1.0 1.0 1.0 1.0 0.9 1.1 1.2 1.3 1.4
2.05 1.88 1.69 1.39 1.01 1.58 1.75 1.84 1.89
9.95 7.61 7.06 6.81 5.89 7.23 7.45 7.90 7.99
88.00 90.51 91.25 91.80 93.10 91.19 90.80 90.26 90.12
Calculated by differences.
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Table 4 The composition of RHA by ICP (T = 720 °C, V = 1.2 m s
1
).
Components
SiO2
K2O
MgO
CaO
CuO
C
Al2O3
Fe2O3
Na2O
P2O5
wt.%
94.8
0.62
0.19
0.45
0.38
0.12
1.27
0.56
0.12
1.49
Table 5 Unburnt carbon and SiO2 content in RHA from four collecting points. Operation
Content (wt.%) Bottom 1st cyclone 2nd cyclone Bag-house
T = 720 °C V = 1.2 m s T = 670 °C V = 1.2 m s T = 670 °C V = 1.0 m s
SiO2 Ctotal SiO2 Ctotal SiO2 Ctotal
1
1
1
94.8 1.12 96.70 1.59 96.10 1.16
95.90 1.84 95.80 2.25 95.30 2.19
95.50 1.62 94.60 3.31 94.80 3.09
94.30 3.97 93.80 4.29 93.90 3.81
velocity also has a decreasing effect on combustion efficiency at a fixed temperature of 720 °C. Furthermore, the increase of fluidization velocity leads to more ash particles collected on cyclones and bag filter. At the same time, unburnt carbon content in the RHA increases. 3.3. Ash characterization 3.3.1. RHA from fluidized bed During the steady combustion tests, RHA from bottom of combustor, 2-stage cyclones, bag-house was collected, respectively. More than 75 wt.% ash was collected from 1st cyclone, followed by 20 wt.% from the bottom, 3 wt.% from 2nd cyclone, and less than 2 wt.% from bag-house. Table 4 shows the main composition of RHA by ICP analysis (sample from bottom at T = 670 °C, V = 1.2 m s 1) and Table 5 shows the contents of silicon dioxide and unburnt carbon in different operations. The quality, in terms of unburnt carbon and SiO2, of ash from combustor bottom is much better than others, the unburnt carbon as low as 1.12–1.59 wt.% and SiO2 higher than 96.10 wt.%. The RHA from bottom usually circulates inside the combustor and stays longer time, which enables
(a) section surface (100µm)
the char to be broken down into smaller fragments and thus releasing entrapped carbon to be further oxidized more completely into white ash. Whereas, still a small amount of rice husk entrain directly by the air flow to the exit and does not get sufficient heat transfer and enough time to release fixed carbon. The husk gets extensive decomposition in the ash separation system (2nd stage cyclone) and releases heat. But without attrition with sands, the rigid skeleton of rice husk does not completely broke down and some carbon still bond inside. Thus, in the air-flow direction, the unburnt carbon content increases and correspondingly the SiO2 content decreases. However, even in the worst case of bag-house collector, the unburnt carbon is still less than 4.29 wt.% and SiO2 content is above 93.90 wt.%. Furthermore, high velocity may not allow sufficient mixing of rice husk and silicon sand, which blocks heat transfer between them and leads to insufficient combustion. Since the escaped fine particles are not completely converted, this incurs a decrease in carbon conversion (unburnt carbon from 1.16 wt.% to 1.59 wt.%), when fluidizing velocity increases from 1.0 m s 1 to 1.2 m s 1. On the other hand, increasing fluidization velocity means reducing residence time that will for sure result in the presence of partially burnt rice husk in the collected ash. When temperature increases from 670 °C to 720 °C, residual carbon content in the bottom is reduced by 30% (from 1.59 wt.% to 1.12 wt.%). The microstructure analysis of sample in the condition of T = 670 °C, V = 1.2 m s 1 from bottom was performed by SEM. Few husk particles still retain their original shape due to attrition in the sand bed. Fig. 5a reveals RHA surface texture and porosity, which shows very fine particle size to the order of a millimeter or less and has pores of varying sizes within the particle. The outer surface in Fig. 5c seems more smoothing and looser, compared with the inner surface (shown in Fig. 5b).
(b) inner surface (20µm)
(c) outer surface (20µm)
Location
Si
K
Ca
Fe
a
81.2
15.6
1.9
0.8
b
93.4
3.6
2.6
0.3
c
97.5
2.6
0
0
Fig. 5. SEM micrographs of RHA from combustor, and their chemical composition by EDX (T = 670 °C, V = 1.2 m s
1
).
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G. Chen et al. / Fuel 144 (2015) 214–221
(a) burne drice-like particles
(b) Honeycomb structure
Fig. 6. TEM micrographs of RHA from combustor bottom (T = 670 °C, V = 1.2 m s
(a) amorphous cellular state
3.3.2. RHA from electronic oven To study the effect of temperature on RHA quality precisely, additional rice husk was burned from 650 °C to 750 °C with a gap of 25 °C in an electronic oven to produce RHA, which was analyzed and compared with that produced in fluidized bed. After combustion, XRD analysis was used to assess presence of crystalline and amorphous substances in the ash. The XRD spectrums of different combustion temperatures are shown in Fig. 8. It presents a broad baseline at around 22° at 650 °C, which means
).
(b) crystallized state
Fig. 7. SAD analysis of RHA from combustor bottom (T = 670 °C, V = 1.2 m s
To obtain information on morphological characteristics such as shape, density and size of silica crystallites on RHA, ash samples were subjected to a detailed transmission electron microscopy (TEM) study. As Fig. 6a illustrated, RHA is consisted of many burned rice-like particles, which have honeycomb sandwich structures and particles with 15–25 nm diameter (Fig. 6b) and agglomerate in character. To ensure state of ash samples, SAD analysis was conducted with 14,000 enlargements. From Fig. 7a, it is clearly shown that silicon dioxide is in amorphous cellular state, compared with crystallized picture where the bright point means silicon dioxide crystal (shown in Fig. 7b). These RHA characterization results prove that: the temperature in the range of 660–720 °C is the favoring condition when fluidized velocity varies from 1.0 m s 1 to 1.2 m s 1. On one hand, it guarantees the combustion efficiency as high as 93.10%; on the other hand, it produces high quality RHA with high amorphous SiO2 content of 96.7 wt.% and low unburnt carbon content of 1.12 wt.%.
1
1
).
the ash formed still has an important content of amorphous compounds. However, the wave peak is more significant with the temperature increasing from 675 °C to 725 °C, and eventually its amorphous structure transforms to crystalline phase totally at 750 °C. The crystallization of silica in the RHA is characterized first by the formation of the cristobalite crystals and then the tridymite crystals. The formation of cristobalite crystals was described in the following procedure [11]:
Fig. 8. XRD analysis of RHA from electronic oven at various temperatures (650– 750 °C).
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(a) T=680°C, t=5seconds;
(d) T =720°C, t = 1 minute;
(b) T =720°C, t=5seconds;
(e) T=720°C, t = 30 minutes;
(c) T =720°C, t =30 seconds;
(f) T =720°C, t =1 hour;
Fig. 9. TEM of the RHA from electronic oven with various calcining durations.
(i) Si–O bonds are liberated from the long polymeric chains (Si– O–C , Si–O–Si) at elevated temperatures. (ii) Subsequent cooling of the Si–O bonds has little or no tendency to crystallise. (iii) When pyrolysis temperatures are increased, more and more Si–O and C–O/C–C are released, which gradually transform to crystalline silica (cristobalite) and crystalline carbon (graphite). The optimum temperature range for RHA quality has not been in common agreement, since it was reported to be 600–800 °C [11], 500–700 °C [12] and 700–800 °C [10]. In our study, optimum combustion temperature for best quality ash in the electrical furnace is found to be 700 °C. Temperature range of 700 ± 25 °C is recommended. If temperature exceeds 725 °C, the crystallization process will take place and if temperature is lower than 650 °C, good combusting condition and high efficiency are not assured. This finding also makes good agreement with what has been found in the industrial scale fluidized bed. To study the influence of residence time on RHA, rice husk were introduced suddenly into the furnace at 660 °C and 720 °C. The duration of rice husk varied from 5 s, 30 s, 1 min, 30 min to 1 h. The TEM of the RHA is shown in Fig. 9a–f, respectively. The longer time rice husk stayed in the oven, the more crystal formatted in the ash which affected the pozzolanic properties severely. For instance, no crystal was observed in Fig. 9a–c, in comparison of growing area and stronger crystal appeared in Fig. 9d–f (marked by the red2 circle). Heating the ash takes a relatively long period of time, with the consequent effect that silica is converted to crystalline forms [18]. Therefore, the long residence time of ash 2
For interpretation of color in Fig. 9, the reader is referred to the web version of this article.
in the burning chamber seems to be the reason for the fairly low reactivity of the ash. As a consequence, it is very important to control the residence time in the furnace, so that any conceivable problem of ash as sintering could be avoided. 3.3.3. Comparison of RHA between fluidized bed and electronic oven RHA from fluidized bed were produced in 660–720 °C, 1.0– 1.2 m s 1 fluidizing air flow with the mixture of silicon sand, while RHA from electronic over were obtained between 650–750 °C and residence time of 5 s, 30 s, 1 min, 30 min, 1 h with constant air, respectively. RHA from fluidized bed shows better performance than that from electronic oven: 1. Less carbon content: Due to the air flow and attrition with the sand in the combustor, the rigid skeleton of rice husk is broke down into small fragments and releases the entrapped carbon, which will improve its carbon conversion. Contrarily, the burnout residue of rice husk in a crucible maintains its rigid skeleton and has similar shape as rice husk. Its skeleton would bond some unburnt carbon and lead to lower silicon content. 2. Less crystallization: RHA is very sensitive to the burning temperature and velocity or residence time. Higher temperature or longer residence time will result into quick crystallization of RHA. The RHA collected from combustor bottom has been proved to maintain amorphous structure; whereas, the RHA burned in the oven over 1 min has been observed with crystallization. 4. Conclusions In this study, a self-designed fluidized bed combustor was introduced to obtain active rice husk silica ash, and the experimental study on RHA has shown that:
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1. The self-designed 500 kW fluidized bed boiler successfully produce high quality of RHA. Through SEM, TEM, EDX analysis, RHA from the fly ash of fluidized bed combustor contains amorphous structure of silicon dioxide 95 wt.% and unburnt carbon less than 4.5 wt.%. Meanwhile, the combustor works well with combustion efficiency 90% in most of test runs. 2. High temperature promotes the silica transformation from amorphous to crystalline form, and low temperature may lead to low combustion efficiency as well as high content of unburnt carbon. The steady state temperature between 660–720 °C and the fluidization velocity of 1.0–1.2 m s 1 appear to be the favoring operating conditions with respect to ash quality. 3. Fluidized bed combustor shows better performance than electronic oven for producing high quality RHA in two aspects: RHA from combustor has less unburnt carbon content and less crystallization than that from electronic oven.
Acknowledgements This paper is financially supported by National Natural Science Foundation of China (51036006), National Basic Research Program of China through 973 Program (2012CB214906) and Open Fund of State Key Laboratory in Zhejiang University (ZJUCEU2011018). References [1] Rice market monitor, FAO,. [access date: 2013 May 30]. [2] Yin X, Wu C, Zheng S, Chen Y. Design and operation of a CFB gasification and power generation system for rice husk. Biomass Bioenergy 2002;23(3):181–7. [3] Mansaray KG, Ghaly AE, AlTaweel AM, Hamdullahpur F, Ugursal VI. Air gasification of rice husk in a dual distributor type fluidized bed gasifier. Biomass Bioenergy 1999;17(4):315–32. [4] Madhiyanon T, Sathitruangsak P, Soponronnarit S. Combustion characteristics of rice-husk in a short-combustion-chamber fluidized-bed combustor (SFBC). Appl Therm Eng 2010;30(4):347–53. [5] Natarajan E, Nordin A, Rao A. Overview of combustion and gasification of rice husk in fluidized bed reactors. Biomass Bioenergy 1998;14(5/6):533–46. [6] Singh RI, Mohapatra SK, Gangacharyulu D. Studies in an atmospheric bubbling fluidized-bed combustor of 10 MW power plant based on rice husk. Energy Convers Manage 2008;49(11):3086–103.
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[7] Deepa GN, Jagadish KS, Alex F. Reactive pozzolanas from rice husk ash: an alternative to cement for rural housing. Cement Concrete Res 2006;36(6):1062–71. [8] Armesto L, Bahillo A, Veijonen K, Cabanillas A, Otero J. Combustion behaviour of rice husk in a bubbling fluidized bed. Biomass Bioenergy 2002;23(3):171–9. [9] Krshnarao RV, Subrahmanyam J, Jagadish KT. Studies on the formation of black particles in rice husk silica ash. J Eur Ceram Soc 2001;21(1):99–104. [10] Huang S, Jing S, Wang J, Wang Z, Jin Y. Silica white obtained from rice husk in a fluidized bed. Powder Technol 2001;117(3):232–8. [11] Rozainee M, Ngo SP, Salema AA, Tan KG, Ariffin M, Zainura ZN. Effect of fluidising velocity on the combustion of rice husk in a bench-scale fluidised bed combustor for the production of amorphous rice husk ash. Bioresour Technol 2008;99(4):703–13. [12] Nair DG, Fraaij A, Klaassen AAK, Kentgens APM. A structural investigation relating to the pozzolanic activity of rice husk ashes. Cement Concrete Res 2008;38(6):861–9. [13] Prasada CS, Maitib KN, Venugopalc R. Effect of rice husk ash in whiteware compositions. Ceram Int 2001;27(6):629–35. [14] Naiya TK, Bhattacharya AK, Mandal S, Das SK. The sorption of lead (II) ions on rice husk ash. J Hazard Mater 2009;163:1254–64. [15] Zhao P, Guo X, Zheng C. Removal of elemental mercury by iodine-modified rice husk ash sorbents. J Environ Sci 2010;22(10):1629–36. [16] Dey KP, Ghosh S, Naskar MK. A facile synthesis of ZSM-11 zeolite particles using rice husk ash as silica source. Mater Lett 2012;87:87–9. [17] 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(11):1273–82. [18] Della VP, Kühn I, Hotza D. Rice husk ash as an alternate source for active silica production. Mater Lett 2002;57(4):818–20. [19] Yoon SJ, Son YI, Kim YK, Lee JG. Gasification and power generation characteristics of rice husk and rice husk pellet using a downdraft fixed-bed gasifier. Renew Energy 2012;42:163–7. [20] Idris SS, Rahman NA, Ismail K, Alias AB, Rashid ZA, Aris MJ. Investigation on thermochemical behaviour of low rank Malaysian coal, oil palm biomass and their blends during pyrolysis via thermogravimetric analysis (TGA). Bioresource Technol 2010;101(12):4584–92. [21] Fernandez RG, Garcia CP, Lavin AG, de las Heras JLB. Study of main combustion characteristics for biomass fuels used in boilers. Fuel Process Technol 2012;103:16–26. [22] Martínez JD, Pineda T, López JP, Betancur M. Assessment of the rice husk leancombustion in a bubbling fluidized bed for the production of amorphous silica-rich ash. Energy 2011;36(6):3846–54. [23] Kuprianov VI, Kaewklum R, Chakritthakul S. Effects of operating conditions and fuel properties on emission performance and combustion efficiency of a swirling fluidized-bed combustor fired with a biomass fuel. Energy 2011;36(4):2038–48. [24] Permchart W, Kouprianov VI. Emission performance and combustion efficiency of a conical fluidized-bed combustor firing various biomass fuels. Bioresource Technol 2004;92(1):83–91.
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Production of amorphous rice husk ash in a 500 kW fluidized bed combustor Guanyi Chen a,⇑, Guiyue Du a, Wenchao Ma a,⇑, Beibei Yan a, Zhihua Wang b, Wenxue Gao a,c a School of Environmental Science and Engineering/State Key Laboratory of Engines/Tianjin Key Lab. of Indoor Air Environmental Quality Control, Tianjin University, Tianjin 300072, China b State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China c North China Municipal Engineering Design and Research Institute, Tianjin 300384, China
h i g h l i g h t s Rice husk is burnt in a 500 kW fluidized bed combustor. 660–720 °C and fluidization velocity of 1.0–1.2 m s
1
appear to be the optimal.
Ash with amorphous SiO2 is over 93.8 wt.% and unburnt carbon less than 5%. Combustion efficiency is 90% in all test runs.
a r t i c l e
i n f o
Article history: Received 5 June 2014 Received in revised form 24 November 2014 Accepted 4 December 2014 Available online 24 December 2014 Keywords: Rice husk Fluidized bed combustion Rice husk ash (RHA) Amorphous silicon dioxide
a b s t r a c t A 500 kW rice husk-firing fluidized bed combustor was designed for obtaining low carbon and high silicon ash in the amorphous form. The influence of operating condition (temperature, fluidizing velocity) on combustion efficiency and rice husk ash quality was experimentally investigated. Furthermore, the quality of rice husk ash collected from inner bed, two-stage cyclones and bag-house was analyzed using XRD, TEM and SEM instruments, in terms of amorphous silicon dioxide and unburnt carbon content. Comparative study was carried out between the rice husk ash produced in an electronic oven and that produced in the above-mentioned fluidized bed combustor. The results show that quality of ash from the industrial-scale fluidized bed is even better than that from electronic oven. Temperature of 660– 720 °C and fluidization velocity of 1.0–1.2 m s 1 appear to be the favoring parameters for producing high quality rice husk ash with amorphous structure of silicon dioxide content over 93.8 wt.% and unburnt carbon context less than 4.5 wt.%. The fluidized bed combustor also assures combustion efficiency 90% in all test runs. This research indicates amorphous structure of silicon dioxide may be economically promisingly produced from the self-designed rice husk combustor. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Rice husk, a renewable by-product of rice milling process, has a large annual quantity of approximately 144 million tons worldwide (2011), and attracts wide attention to its utilization and conversion [1]. The annual amount of produced rice husk is more than 44 million tons in China [1]. Rice husk was commonly combusted or gasified as an agriculture waste to produce heat and electricity or biogas, respectively [2–4]. Natarajan et al. [5] reviewed the research on combustion and gasification of rice husk in fluidized bed reactors and pointed out most results were from only small bench or lab ⇑ Corresponding authors. Tel.: +86 22 8740 2100; fax: +86 22 8740 2075. E-mail addresses: [email protected] (G. Chen), [email protected] (W. Ma). http://dx.doi.org/10.1016/j.fuel.2014.12.012 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
scale units. Recently, rice husk gasification and power generation system with a capacity of 150 t d 1 was designed and installed in China [2], and a 10 MW bubbling fluidized-bed power plant firing rice husk was established in India [6]. However, these researches only focused on combustion or gasification characteristics in terms of energy efficiency and pollutant gas emissions. Recently growing attention has been paid to rice husk ash (RHA) as it contains high silica content with amorphous characteristics of a cellular structure [7–9], which can be further purified to valueadded products, such as pure SiO2, SiCl4, Si3N4, and SiC [10,11]. Moreover, RHA can be widely used as concrete alternatives to improve the durability and performance of concrete [12], and used as a supplementary cementitious material, or used in ceramic field, e.g. whiteware manufacture [13], or as an adsorbent to absorb
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G. Chen et al. / Fuel 144 (2015) 214–221 Table 1 Proximate and ultimate analysis of rice husk. Proximate analysis (wt.%, dry basis)
*
Volatile
Ash
Fixed carbon
67.35
18.16
14.49
Ultimate analysis (wt.%,dry basis) *
*
C
H
O
38.45
5.08
37.83
Low heating value
N
S
Cl
LHV (kJ kg
0.15
0.23
0.10
14,600
1
)
Calculated by differences.
organic dye and inorganic metal, such as lead [14] and mercury [15], for synthesis of zeolites [16]. Combustion process may influence the ash formation. Therefore the effects of fluidizing velocity [11], burning temperature [17], time and cooling regime [12] on the pozzolanic properties of RHA have been studied. Rozainee et al. [11] pointed out RHA, in amorphous form with low residual carbon content (2.88 wt.%), was released in the optimum condition of fluidized velocity = 3.3 Umf. Nair et al. [12] carried out the experiments in a laboratory furnace under continuous supply of air as a function of incineration temperature, time and cooling regime. They found out the highest amounts of amorphous silica occurred in samples burnt in the range of 500–700 °C, and the most reactive RHA was produced after incineration for 12 h at 500 °C followed by quick cooling down. Huang et al. [10] studied the silica white properties produced from a lab-scale fluidized bed with inner diameter of 84 mm, and concluded that the quality of silica white sampled from the dense-phase zone at 700–800 °C was higher than that of GB-precipitated silica (GB stands for national standard). Della et al. [18] reported that the temperature of rice husk carbonization might be preferably below 700 °C to avoid any transformation of amorphous to crystalline form. Above 760 °C, silica and potassium oxide fuse on the surface of rice husk char and form a glass-like barrier which prevents further reaction of the remaining material [19]. However, RHA production and characterization has mostly been conducted at the laboratory-level. In sum, major previous studies on rice husk combustion in a fluidized bed boiler either focused on the investigation of combustion parameters (i.e. combustion intensity, combustion efficiency, carbon conversion efficiency) or on the combustion performance (i.e. pollutants emission, carbon content in the ash, heat loss), and less studies focused on RHA. There was lack of study on RHA produced in the industrial-scale fluidized bed combustion. Furthermore, the optimum operating parameters influencing on RAH quality in terms of low carbon content and amorphous silica is not clear. This research aims to consider both combustion efficiency and RHA quality in a 500 kW fluidized bed combustor. The objective of this study is to access the behavior of a self-designed fluidized bed combustor burning rice husk, and to investigate the influences of temperature and velocity on RHA production and combustion efficiency as well as to characterize the RHA quality. Finally, the RHA obtained from fluidized bed combustor is compared with that from a lab-scale furnace. The results of this study may be of interest to industry aiming at producing silicon material from active ash in an economical way which compromises energy and value-added silicon material production. 2. Material and methods 2.1. Characteristics of rice husk Rice husk, an irregular boat-like particle, is the outer cover of rice and on average it accounts for 20 wt.% of the paddy produced on the weight basis. The typical size of rice husk in this study is about 5–8 mm in length, 2–3 mm in width and 0.3 mm in thickness, and bulk density is 118.4 kg m 3. The rice husk, not suffer any previous processing before combustion, was collected in Shaoxing City, Zhejiang Province, China. Table 1 shows the calorific
value, the proximate and ultimate analysis (as received basis). Fixed carbon and Oxygen content are calculated by difference, respectively. In general, it is difficult to fluidize rice husk alone, due to its cylindrical shape, non-granular and flaky nature [3,6]. For proper fluidization and processing in the combustor, it has been strongly suggested to be mixed with other solid particles, such as silicon sand, alumina and calcite etc. forming a multi-solid system [17]. In this study, quartz sand with the diameter ranging from 0.50 mm to 0.75 mm is adopted as bed material. 2.2. Thermo gravimetric analysis (TGA) Thermo gravimetric and differential thermal analysis (TG-DTA) of rice husk was carried out in a Netzsch STA 449C Jupiter simultaneous analyzer. Its precision of temperature measurement was ±0.5 K, microbalance sensitivity less than 1.0 lg. Approximately 10 mg of the sample was placed in the microbalance and heated at rate of 50 °C min 1 under nitrogen flowing, from room temperature to a final temperature of 900 °C. 2.3. RHA production in a 500 kW fluidized bed combustor Fig. 1 shows a schematic diagram of a 500 kW fluidized bed combustor located in Shaoxing City, Zhejiang Province, China. During the design stage, several specific considerations have been taken, such as variable sectional area, staged air feeding, precisely controllable superficial velocity and temperature distribution along the height of furnace, in addition to strict retention time of ash in furnace, as well as two stages of cyclones. The system consists of a fluidized bed combustor, a screw feeder, a thermal exchanger, 2 high-temperature cyclone separators, a bag-house and an under-bed start-up burner. The inner diameter of density phase zone is 620 mm, dilutephase zone 920 mm, and total height 6.0 m. Incoming air is preheated with a gas burner during the start-up operation. The bed material of quartz sand containing over 90 wt.% silica content, is fed at the height of 0.6 m from air distributor, with a static bed height of 0.4 m. Rice husk is also fed at 0.6 m through a pressured-variable speed screw feeder which prevents high-temperature gas inside the fluidized bed from going against the screw feeder, and leading to discontinuous feeding. The fuel feeding rate, up to 125 kg h 1, is controlled by a computer-based data acquisition and controlled system. Secondary air is set at the height of 2 m, in horizontal direction. The ratio of primary air to secondary air is 7:3, excess air coefficient around 1.20–1.25. The cyclone separators are connected to the exit of disengagement section to capture the solid particles (dust, ash and char) escaping from the combustor.1 Four ash collecting points are located at the bottom of bed discharge, exits of two cyclones and bag-house, respectively. Temperatures are recorded by ten thermocouples (type K) installed along the height: 0.1 m, 0.6 m, 1.0 m (dense-phase zone), 2.0 m, 2.5 m, 3.5 m (dilute-phase zone), 4.0 m, 5.0 m, and 5.8 m (exit). 1
if without special description, in this text the height of bed is from air distributor.
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XL-30 TMP ESEM (20 kV, Holland) was used to obtain SEM of RHA samples. TEM studies were performed on a Tecnai G2 F20 instrument operated at 200 kV. The sample was suspended in alcohol and ultrasonically dispersed for 20 min. The suspension was piped onto a copper micro grid with carbon film. At least 100 particles were used to obtain a particle size distribution. Selected area diffraction (SAD) was conducted in the TEM apparatus with 14,000 enlargements to ensure state of ash samples. XRD analysis was used to assess presence of crystalline and amorphous substances in the ash. The tests were conducted in a Rigaku diffraction unit (Dmax-2500V/PC), using copper as target, with graphite-monochrome-filter at 40 kV, 200 mA. Goniometer speed was maintained at 4° min 1. To determine the unburnt carbon content in the RHA, the collected ash samples were subjected to ignition on loss test in the electronic oven at 900 °C. Minerals composition of the RHA is determined by atomic emission measurements using ICP emission spectroscopy (Perkine Elmer Optima 3300 DV). 3. Results and discussions 3.1. TG-DTA of rice husk
Fig. 1. Schematic diagram of a 500 kW fluidized bed combustor for RHA production.
The system is monitored by a microcomputer linked to a data logger system (ASAE 4118, China). In order to study the influence of different working conditions on the production of RHA, several tests regarding to velocity and temperature were carried out during steady combustion conditions, as follows: 1. Temperature along fluidized bed height; 2. Effect of temperature and velocity on combustion efficiency; 3. Chemical contents of ashes collected from bed discharge, cyclones, bag-house; and mineralogical characterization of rice husk ash with the analysis of SEM, TEM, SAD etc. 2.4. RHA production in a lab-scale electronic oven Additional experiments focusing on RHA quality were carried out in a lab scale electronic oven (SX2-2.5-10, Tianjin Zhonghuan Ltd, China), to compare with that in the fluidized bed combustor. The effects of temperature and residence time on RHA quality were studied in different procedures. During temperature investigation, ceramic crucibles with 5 g rice husk were put inside of an electronic oven with continuous air flow rate of 100 ml min 1 and a heating rate of 10 °C min 1. Each sample was held at a maximum temperature (650 °C, 675 °C, 700 °C, 725 °C, 750 °C) for 1 h and cooled down inside the oven, respectively. To study the effect of retention time, rice husk samples were fed suddenly into the furnace at a required burning temperature (680 °C or 720 °C), and the duration of rice husk samples varied from 5 s, 30 s, 1 min, 30 min to 1 h. When the rice husk was put into the furnace, it burned immediately due to relatively low ignition point, and further decarbonization took place. The ash was finally collected for further analysis.
Fig. 2 presents weight-loss curves (TG) as a function of temperature for rice husk using heating rate of 50°C min 1. The thermal degradation can be divided into three stages: moisture drying, main devolatilization and continuous slight devolatilization [20]. The moisture drying region corresponds to an initial slight weight loss between ambient temperature and nearly 105 °C, which results from the elimination of physically absorbed water in rice husk and superficial or external water bounded by surface tension. Main devolatilization stage starts at temperature of 250 °C, and the weight loss occurs promptly after that temperature until 390 °C because of volatile matter release, and a major change in the slope of curves at around 390 °C due to lignin decomposition [21]. Further loss of weight occurs until 800 °C (continuous slight devolatilization) indicating cellulose and hemi-cellulose decomposition, as CO, CO2, tar, steam etc. The whole process releases most thermal energy at peak area between 400–600 °C. The TG analysis supports the idea that most, if not all, of rice husk begins to burn even at such low temperature of about 340 °C and have been consumed in the region around 300–600 °C. 3.2. Operating performance of the fluidized bed combustor Fig. 3 shows axial temperature profile along fluidized bed height under various fluidizing velocities. Low fluidizing velocity
2.5. RHA characterization The micro-structure and quality of RHA were analyzed with scanning electron micrographs (SEM), Transmission electron microscopy (TEM), X-ray diffraction analysis (XRD). A PHILIPS
Fig. 2. Thermo gravimetric and differential thermal analysis (TG-DTA) curves of rice husk.
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G. Chen et al. / Fuel 144 (2015) 214–221
of 0.6 m s 1 results in poor mixing, while temperature reduces sharply along the height of fluidized bed. When fluidizing velocity increases to 1.0–1.2 m s 1, strong combustion intensity zone will move up, where major reaction occurs at 2 m to 3.5 m. However, if fluidizing velocity increases further to 1.5 m s 1, strong combustion zone move further to dilute-phase zone, which leads to the inefficient combustion of rice husk thereby increasing the great losses in unburnt combustibles. Thus, fluidized velocity is chosen in the range of 1.0–1.2 m s 1. The temperature profile shows temperature peak moves from 1 m to 4 m of bed height, when fluidizing velocity increases from 0.6 m s 1 to 1.5 m s 1. Beyond 3.5 m, temperatures begin to fall, indicating that most of combustion is completed. This behavior can be explained by easy devolatilization process of rice husk and injection of secondary air. It may be linked to high-volatile fraction (60 wt.%) of rice husk leading to ignite even at low temperatures (340 °C). It may imply that rice husk devolatilization starts at the dense bed zone and ends just at the expansion of the reactor. Temperature is an important factor for rice husk combustion to produce amorphous silica. It has been reported that temperature higher than 700 °C promotes the silica transformation from amorphous to crystalline form [11,18,22]. Low temperature may lead to low combustion efficiency and meanwhile result in high content of unburnt carbon remained in RHA which means low quality of RHA. Previous studies also made different conclusions about the optimum temperature: Nair et al. [12] supposed the highest amounts of amorphous silica occurred in samples burnt in the range of 500–700 °C, contrarily Huang et al. [10] concluded that the quality of silica white sampled from the dense-phase zone at 700–800 °C was better. Large temperature gradient can also result in the turbulence of combustion efficiency and correspondingly poor RHA quality. Thus, test temperature here is kept in the small range of 660–720 °C, corresponding to fluidizing velocity 1.0–1.2 m s 1 (Table 2). Fig. 4 shows bed temperature profile as a function of time during actual operating conditions. It can be seen that in the first two hours, temperature of dense-phase zone is notably higher than dilute-phase zone indicating combustion mainly taking place in the dense-phase zone. This may be attributed to quick ignition of rice husk when fed to the boiler resulting in heat release. Heat released is mainly absorbed by bed at dense-phase zone due to heat transfer with bed material and surrounding atmosphere. After two-hour operation, the combustor reaches thermal equivalence. Later volatiles of rice husk move up with primary air flow and decompose under the atmosphere of secondary air in dilute-phase zone, resulting in temperature in dilute-phase zone and exit increasing slowly. The temperature throughout the combustor is stable in the range of 660–720 °C, which indicates the fluidized bed combustor working well. Due to the turbulent nature of fluidized bed, heat transfer rates are very high which leads to high combustion efficiency. In this
Table 2 Operating parameters of fluidized bed combustion experiments. Rice husk feeding rate Bed material Sand mean particle diameter Fluidizing gas velocity Bed operating temperature Excess air coefficient Ratio of primary air to secondary air
Up to125 kg h Silicon sand 0.6 mm 1.0–1.2 m s 1 660–720 °C 1.20–1.25 7:3
Fig. 4. Temperature profile in steady operating conditions.
study, the influences of fluidizing velocities and temperatures on combustion efficiency of rice husk and RHA quality were investigated, respectively. Feeding rate up to 125 kg h 1 may vary a bit, to meet each operating condition in Table 3. For the estimation of combustion efficiency, heat losses owing to incomplete combustion (accounting for the CO emission), qic, and unburnt carbon contained in particulate matter, quc,(usually in fly ash, quc-fa, and in bottom ash, quc-ba) were determined [22–24]. In each run, fly ash and bottom ash were sampled and afterwards analyzed for unburnt carbon to estimate associated heat loss. CO emission was sampled at the exit of bag-house and quantified using ‘‘Testo – 350’’ (Testo, Germany) gas analyzer. Table 3 shows these heat losses, qic and quc (quc-fa and quc-ba), along with combustion efficiencies in each operating condition. From Table 3, it can be seen that a reasonably high combustion efficiency of 90% above for the self-designed fluidized bed boiler is achieved, ranging from 88.00% to 93.10% over all tests, meaning the boiler is a good compromise of RHA and combustion efficiency. The heat loss with unburnt carbon and incomplete combustion exhibits a linear correlation with temperature and velocity. With temperature increasing at fluidizing velocity of 1.0 m s 1, heat loss with unburnt carbon and incomplete combustion decreases, corresponding to the increasing of combustion efficiency. The fluidization Table 3 Heat losses and combustion efficiency for various temperatures and velocities.
Fig. 3. Temperature profile along the bed height under various fluidizing velocities.
1
*
Temperature (°C)
Velocity (m s 1)
qic (%)
quc (%)
Combustion efficiency* (%)
670 695 710 720 720 720 720 720 720
1.0 1.0 1.0 1.0 0.9 1.1 1.2 1.3 1.4
2.05 1.88 1.69 1.39 1.01 1.58 1.75 1.84 1.89
9.95 7.61 7.06 6.81 5.89 7.23 7.45 7.90 7.99
88.00 90.51 91.25 91.80 93.10 91.19 90.80 90.26 90.12
Calculated by differences.
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G. Chen et al. / Fuel 144 (2015) 214–221
Table 4 The composition of RHA by ICP (T = 720 °C, V = 1.2 m s
1
).
Components
SiO2
K2O
MgO
CaO
CuO
C
Al2O3
Fe2O3
Na2O
P2O5
wt.%
94.8
0.62
0.19
0.45
0.38
0.12
1.27
0.56
0.12
1.49
Table 5 Unburnt carbon and SiO2 content in RHA from four collecting points. Operation
Content (wt.%) Bottom 1st cyclone 2nd cyclone Bag-house
T = 720 °C V = 1.2 m s T = 670 °C V = 1.2 m s T = 670 °C V = 1.0 m s
SiO2 Ctotal SiO2 Ctotal SiO2 Ctotal
1
1
1
94.8 1.12 96.70 1.59 96.10 1.16
95.90 1.84 95.80 2.25 95.30 2.19
95.50 1.62 94.60 3.31 94.80 3.09
94.30 3.97 93.80 4.29 93.90 3.81
velocity also has a decreasing effect on combustion efficiency at a fixed temperature of 720 °C. Furthermore, the increase of fluidization velocity leads to more ash particles collected on cyclones and bag filter. At the same time, unburnt carbon content in the RHA increases. 3.3. Ash characterization 3.3.1. RHA from fluidized bed During the steady combustion tests, RHA from bottom of combustor, 2-stage cyclones, bag-house was collected, respectively. More than 75 wt.% ash was collected from 1st cyclone, followed by 20 wt.% from the bottom, 3 wt.% from 2nd cyclone, and less than 2 wt.% from bag-house. Table 4 shows the main composition of RHA by ICP analysis (sample from bottom at T = 670 °C, V = 1.2 m s 1) and Table 5 shows the contents of silicon dioxide and unburnt carbon in different operations. The quality, in terms of unburnt carbon and SiO2, of ash from combustor bottom is much better than others, the unburnt carbon as low as 1.12–1.59 wt.% and SiO2 higher than 96.10 wt.%. The RHA from bottom usually circulates inside the combustor and stays longer time, which enables
(a) section surface (100µm)
the char to be broken down into smaller fragments and thus releasing entrapped carbon to be further oxidized more completely into white ash. Whereas, still a small amount of rice husk entrain directly by the air flow to the exit and does not get sufficient heat transfer and enough time to release fixed carbon. The husk gets extensive decomposition in the ash separation system (2nd stage cyclone) and releases heat. But without attrition with sands, the rigid skeleton of rice husk does not completely broke down and some carbon still bond inside. Thus, in the air-flow direction, the unburnt carbon content increases and correspondingly the SiO2 content decreases. However, even in the worst case of bag-house collector, the unburnt carbon is still less than 4.29 wt.% and SiO2 content is above 93.90 wt.%. Furthermore, high velocity may not allow sufficient mixing of rice husk and silicon sand, which blocks heat transfer between them and leads to insufficient combustion. Since the escaped fine particles are not completely converted, this incurs a decrease in carbon conversion (unburnt carbon from 1.16 wt.% to 1.59 wt.%), when fluidizing velocity increases from 1.0 m s 1 to 1.2 m s 1. On the other hand, increasing fluidization velocity means reducing residence time that will for sure result in the presence of partially burnt rice husk in the collected ash. When temperature increases from 670 °C to 720 °C, residual carbon content in the bottom is reduced by 30% (from 1.59 wt.% to 1.12 wt.%). The microstructure analysis of sample in the condition of T = 670 °C, V = 1.2 m s 1 from bottom was performed by SEM. Few husk particles still retain their original shape due to attrition in the sand bed. Fig. 5a reveals RHA surface texture and porosity, which shows very fine particle size to the order of a millimeter or less and has pores of varying sizes within the particle. The outer surface in Fig. 5c seems more smoothing and looser, compared with the inner surface (shown in Fig. 5b).
(b) inner surface (20µm)
(c) outer surface (20µm)
Location
Si
K
Ca
Fe
a
81.2
15.6
1.9
0.8
b
93.4
3.6
2.6
0.3
c
97.5
2.6
0
0
Fig. 5. SEM micrographs of RHA from combustor, and their chemical composition by EDX (T = 670 °C, V = 1.2 m s
1
).
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G. Chen et al. / Fuel 144 (2015) 214–221
(a) burne drice-like particles
(b) Honeycomb structure
Fig. 6. TEM micrographs of RHA from combustor bottom (T = 670 °C, V = 1.2 m s
(a) amorphous cellular state
3.3.2. RHA from electronic oven To study the effect of temperature on RHA quality precisely, additional rice husk was burned from 650 °C to 750 °C with a gap of 25 °C in an electronic oven to produce RHA, which was analyzed and compared with that produced in fluidized bed. After combustion, XRD analysis was used to assess presence of crystalline and amorphous substances in the ash. The XRD spectrums of different combustion temperatures are shown in Fig. 8. It presents a broad baseline at around 22° at 650 °C, which means
).
(b) crystallized state
Fig. 7. SAD analysis of RHA from combustor bottom (T = 670 °C, V = 1.2 m s
To obtain information on morphological characteristics such as shape, density and size of silica crystallites on RHA, ash samples were subjected to a detailed transmission electron microscopy (TEM) study. As Fig. 6a illustrated, RHA is consisted of many burned rice-like particles, which have honeycomb sandwich structures and particles with 15–25 nm diameter (Fig. 6b) and agglomerate in character. To ensure state of ash samples, SAD analysis was conducted with 14,000 enlargements. From Fig. 7a, it is clearly shown that silicon dioxide is in amorphous cellular state, compared with crystallized picture where the bright point means silicon dioxide crystal (shown in Fig. 7b). These RHA characterization results prove that: the temperature in the range of 660–720 °C is the favoring condition when fluidized velocity varies from 1.0 m s 1 to 1.2 m s 1. On one hand, it guarantees the combustion efficiency as high as 93.10%; on the other hand, it produces high quality RHA with high amorphous SiO2 content of 96.7 wt.% and low unburnt carbon content of 1.12 wt.%.
1
1
).
the ash formed still has an important content of amorphous compounds. However, the wave peak is more significant with the temperature increasing from 675 °C to 725 °C, and eventually its amorphous structure transforms to crystalline phase totally at 750 °C. The crystallization of silica in the RHA is characterized first by the formation of the cristobalite crystals and then the tridymite crystals. The formation of cristobalite crystals was described in the following procedure [11]:
Fig. 8. XRD analysis of RHA from electronic oven at various temperatures (650– 750 °C).
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(a) T=680°C, t=5seconds;
(d) T =720°C, t = 1 minute;
(b) T =720°C, t=5seconds;
(e) T=720°C, t = 30 minutes;
(c) T =720°C, t =30 seconds;
(f) T =720°C, t =1 hour;
Fig. 9. TEM of the RHA from electronic oven with various calcining durations.
(i) Si–O bonds are liberated from the long polymeric chains (Si– O–C , Si–O–Si) at elevated temperatures. (ii) Subsequent cooling of the Si–O bonds has little or no tendency to crystallise. (iii) When pyrolysis temperatures are increased, more and more Si–O and C–O/C–C are released, which gradually transform to crystalline silica (cristobalite) and crystalline carbon (graphite). The optimum temperature range for RHA quality has not been in common agreement, since it was reported to be 600–800 °C [11], 500–700 °C [12] and 700–800 °C [10]. In our study, optimum combustion temperature for best quality ash in the electrical furnace is found to be 700 °C. Temperature range of 700 ± 25 °C is recommended. If temperature exceeds 725 °C, the crystallization process will take place and if temperature is lower than 650 °C, good combusting condition and high efficiency are not assured. This finding also makes good agreement with what has been found in the industrial scale fluidized bed. To study the influence of residence time on RHA, rice husk were introduced suddenly into the furnace at 660 °C and 720 °C. The duration of rice husk varied from 5 s, 30 s, 1 min, 30 min to 1 h. The TEM of the RHA is shown in Fig. 9a–f, respectively. The longer time rice husk stayed in the oven, the more crystal formatted in the ash which affected the pozzolanic properties severely. For instance, no crystal was observed in Fig. 9a–c, in comparison of growing area and stronger crystal appeared in Fig. 9d–f (marked by the red2 circle). Heating the ash takes a relatively long period of time, with the consequent effect that silica is converted to crystalline forms [18]. Therefore, the long residence time of ash 2
For interpretation of color in Fig. 9, the reader is referred to the web version of this article.
in the burning chamber seems to be the reason for the fairly low reactivity of the ash. As a consequence, it is very important to control the residence time in the furnace, so that any conceivable problem of ash as sintering could be avoided. 3.3.3. Comparison of RHA between fluidized bed and electronic oven RHA from fluidized bed were produced in 660–720 °C, 1.0– 1.2 m s 1 fluidizing air flow with the mixture of silicon sand, while RHA from electronic over were obtained between 650–750 °C and residence time of 5 s, 30 s, 1 min, 30 min, 1 h with constant air, respectively. RHA from fluidized bed shows better performance than that from electronic oven: 1. Less carbon content: Due to the air flow and attrition with the sand in the combustor, the rigid skeleton of rice husk is broke down into small fragments and releases the entrapped carbon, which will improve its carbon conversion. Contrarily, the burnout residue of rice husk in a crucible maintains its rigid skeleton and has similar shape as rice husk. Its skeleton would bond some unburnt carbon and lead to lower silicon content. 2. Less crystallization: RHA is very sensitive to the burning temperature and velocity or residence time. Higher temperature or longer residence time will result into quick crystallization of RHA. The RHA collected from combustor bottom has been proved to maintain amorphous structure; whereas, the RHA burned in the oven over 1 min has been observed with crystallization. 4. Conclusions In this study, a self-designed fluidized bed combustor was introduced to obtain active rice husk silica ash, and the experimental study on RHA has shown that:
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1. The self-designed 500 kW fluidized bed boiler successfully produce high quality of RHA. Through SEM, TEM, EDX analysis, RHA from the fly ash of fluidized bed combustor contains amorphous structure of silicon dioxide 95 wt.% and unburnt carbon less than 4.5 wt.%. Meanwhile, the combustor works well with combustion efficiency 90% in most of test runs. 2. High temperature promotes the silica transformation from amorphous to crystalline form, and low temperature may lead to low combustion efficiency as well as high content of unburnt carbon. The steady state temperature between 660–720 °C and the fluidization velocity of 1.0–1.2 m s 1 appear to be the favoring operating conditions with respect to ash quality. 3. Fluidized bed combustor shows better performance than electronic oven for producing high quality RHA in two aspects: RHA from combustor has less unburnt carbon content and less crystallization than that from electronic oven.
Acknowledgements This paper is financially supported by National Natural Science Foundation of China (51036006), National Basic Research Program of China through 973 Program (2012CB214906) and Open Fund of State Key Laboratory in Zhejiang University (ZJUCEU2011018). References [1] Rice market monitor, FAO,
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