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Combustion characteristics of cotton stalk in FBC
biomass and bioenergy 34 (2010) 761–770
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Combustion characteristics of cotton stalk in FBC Zhiao Sun a,*, Jiezhong Shen a, Baosheng Jin b, Liyan Wei a a b
Wuxi Huaguang Boiler Co., Ltd., Wuxi 214028, P.R. China School of Energy and Environment, Southeast University, Nanjing 210096, P.R. China
article info
abstract
Article history:
The present work reports studies on the mixing and combustion characteristics of cotton
Received 10 June 2007
stalk with 10–100 mm in length in FBC. Experiments on a cold model show that cotton stalk
Received in revised form
cannot fluidize, and adding bed material can improve the fluidization condition. Cotton
10 December 2009
stalk can mix well with 0.6–1 mm alumina at fluidization number N ¼ 3–7. However, when
Accepted 6 January 2010
the fluidization number is higher more than 7, the mixing bed will exist a little segregation
Available online 2 February 2010
comparing with N ¼ 3–7. Thermogravimetric experiments show that cotton stalk can be ignited easily at a lower temperature, and its devolatilization and combustion are quick.
Keywords:
Fluidized-bed combustion of cotton stalk was tested in a 0.2 MWth test facility. According to
Cotton stalk
the temperature distribution along the bed height, when the primary and secondary air is
Fluidized bed
adapted cotton stalk can be burned stably in the fluidized bed. During pure cotton stalk
Mixing characteristics
combustion tests, silica sand and alumina are used as bed material to compare their
Combustion characteristics
agglomeration characteristics. SEM/EDX analysis on agglomerate samples after combus-
Bed material
tion about 38 h suggests that the high alkali metals content causes the formation of the coating around silica sand particles. The coating consists of compounds with low-melting temperature results in agglomeration of silica sand particles. By contrast, alumina is difficult to react with alkali metals from biomass ash, and the agglomeration of alumina was not found at 910 C. It is found that alumina is more favorable than silica sand particle for use in a fluidized bed in cotton stalk combustion. ª 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
There is an increasing concern with the environmental problems associated with the increasing CO2, NOx and SOx emissions resulting from the rising use of fossil fuels. For this reason, much attention has been drawn towards the combustion of biomass for power production [1]. Biomass is a renewable source of energy. When the biomass is burned, the released carbon goes back to the atmosphere and can be recycled into the next generation of growing plants. Therefore, the application of biomass for energy can lead to zero net CO2 emission, since carbon in the form of CO2 is fixed by photosynthesis during biomass growth.
Biomass includes a large variety of different fuels with different chemical compositions and combustion characteristics. Generally, biomass fuel has a high content of hydrogen and oxygen, a high volatile content and a low heating value. Biomass fuel has more potassium, sodium, calcium and chlorine than coal. These components are directly related to bed agglomeration, corrosion and ash deposition. Cotton stalk is one of the important and promising biomass resources. As one kind of renewable energy source, it has attracted more research and development in this field. If an efficient method is available, cotton stalk can be converted into useful energy to meet the electrical power requirements in the rural area. M. Fang et al. [2] have claimed that fluidized-
* Corresponding author. Tel.: þ86 510 8521 5556; fax: þ86 510 85219744. E-mail address: [email protected] (Z. Sun). 0961-9534/$ – see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.01.019
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bed combustion, with excellent heat and mass transfer characteristics, seems to be a suitable technology for converting biomass fuel into energy. In the literature, several studies have been reported on the combustion of cotton stalk in the fluidized-bed combustor for energy production. Cotton stalk has either been used alone or in combination with coal. However, it is pelletized to a size range 1–10 mm. This paper introduces an experimental research on the mixing and combustion characteristics of cotton stalk with 10–100 mm in length produced in China. Biomass cannot be easily fluidized due to their irregular shapes. For proper fluidization, an inert bed material such as silica sand, alumina and calcite is used to facilitate fluidization of biomass. It also acts as a heat transfer medium in the reactor [3]. However, the mixtures of solid particles of different size and different density tend to separate in vertical direction under fluidized condition. Pilar et al. [4] have reviewed several investigations reported on the fluidization of mixtures of solids with different particle sizes as well as mixtures of particles of different sizes and densities. Ekinci et al. [5] tested the density and size segregation behavior determined from temperature distributions. Segregation behavior of biomass fuel is of practical importance because the vertical location of biomass fuel influences the in-bed combustion efficiency of volatile matter. Therefore, the mixing behavior can provide the essential information necessary for efficient cotton stalk combustion in a fluidized bed. Studies have also been reported on agglomeration and defluidization phenomena in fluidized-bed combustion of biomass fuel. Grubor et al. [6] reported agglomeration and defluidization problems in a 150 kW continuously straw-fired FBC, which led to 2–3 times shutdown in a month even though the temperature was below 700 C. They concluded that the high content of potassium in straw ash was the major contributor to this phenomenon. Therefore, they suggested using alternative bed materials to avoid formation of lowmelting temperature eutectic compounds of alkali-silicates. S. Arvelakis et al. [7] studied limestone used as bed material offered advantages compared with silica sand delaying the agglomeration phenomena due to its lower reactivity with alkali metals, but the gained profits were found to vary from mediocre to marginal. In this paper, alumina is used as bed material for studying mixing behavior with cotton stalk. During pure cotton stalk combustion tests, silica sand and alumina particle are used as bed material to compare their agglomeration characteristics.
Table 1 – Cotton stalk analysis (as received). Proximate analysis (%) Moisture Ash Volatile Fixed carbon
2.1.
15 2.68 63.08 19.24
Carbon Hydrogen Sulfur Nitrogen Oxygen Ash
40.44 5.07 0.1 0.21 36.5 2.68
metals (K, Na), alkali earth metals (Ca, Mg), silicon, chlorine, and sulfur, as major ash constituents.
2.2.
Properties of bed materials
The compositions of the silica sand and alumina are listed in Table 2. The true density and bulk density of alumina used in the tests are about 2760 kg/m3 and 1180 kg/m3, respectively. By comparison, the true density and bulk density of silica sand used in the tests are about 2490 kg/m3 and 1330 kg/m3, respectively.
3. Mixing characteristics of cotton stalk with alumina particle 3.1.
Cold-state experimental apparatus
A cold-state test facility used in research of the fluidization and mixing characteristics of cotton stalk is sketched in Fig. 1. It mainly consists of an air blower, a flowmeter, a fluidized bed, a set of pressure measuring instruments, an off-gas cleanup system and an induced draft fan. The fluidized bed had an overall height of 4.4 m. The dense bed with a crosssection of 400 mm 400 mm and a height of 750 mm was partly fabricated from plexiglas where visible observations of the bed activity were possible. Four windows at the sidewall were arranged to take out the bed material in the mixed bed condition. In order to prevent air leakage and bed leakage a single piece rubber seal is used for the window gaps. The perforated-plate distributor contained 48 small nozzles to provide uniform air distribution, with 3.1% open area. The pressure drops across the distributor and the bed are measured by U-tube water manometers.
3.2.
2. Properties of cotton stalk and bed materials
Ultimate Analysis (%)
Experimental methods
The experiments showed that pure cotton stalk by itself cannot fluidize due to its peculiar shape, size and density, as well as higher void fraction. However, fluidization behavior of
Properties of cotton stalk
The typical size of cotton stalk used in experimental studies is about 10–100 mm in length. It is very light, with natural packing density of 100–130 kg/m3 and real density of about 460 kg/m3. Cotton stalk is also characterized by a high volatile content and low gross calorific value of 13.5 MJ/kg. Table 1 shows analytical data for cotton stalk. In addition, cotton stalk contains large quantities of inorganic elements such as alkali
Table 2 – Compositions of the silica sand and alumina (weight %). SiO2 Al2O3 Fe2O3 TiO2 CaO þ MgO Na2O þ K2O Silica sand 98.61 0.14 Alumina 13.07 79.24
0.08 1.75
0.01 4.10
0.06 0.45
0.11 0.53
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interior. The fluidized bed was kept at good fluidization for over 5 min to reach the steady-state condition, then rapidly stopping the fluidized gas. The mixture in each window was gently taken out, respectively, separated by a sieve and weighed. Thus, the mixed bed was divided into four sections from air distributor, and the mass fraction of cotton stalk along the bed height was obtained.
3.3.
Many trials were carried out in order to evaluate the mixing characteristics of cotton stalk and alumina mixtures, with three kinds of cotton stalk with length range 10–40 mm, 40– 70 mm and 70–100 mm, and with three kinds of alumina with diameter range 0.1–0.6 mm, 0.6–1 mm and 1–2 mm. The percentage of cotton stalk in the mixture was 2 wt%, and gas velocity was 2.5 m/s. The mixing characteristics of cotton stalk and alumina mixtures are shown in Fig. 2. The bed material size has great effect on the mixing quality. Cotton stalk with different length ranges can fluidize and mix well with 0.6–1 mm alumina particles, and the phenomena of segregation are not serious. As is evident in Fig. 2, alumina particles of diameter range 0.6–1 mm are the most suitable bed materials, which can keep good mixing with cotton stalk. According to Fig. 2 (a), when diameter range of alumina is 1– 2 mm, segregation of binary bed particles occurs. It can be explained that the Umf of binary mixture of alumina with 1– 2 mm diameter and cotton stalk is high, and fluidization number is low at U ¼ 2.5 m/s, which causes bad mixing of the binary bed particles. According to Fig. 2 (c), cotton stalk with
Fig. 1 – Schematic diagram of cold-state experimental apparatus. 1 – air blower; 2 – flowmeter; 3 – fluidized bed; 4 – cyclone; 5 – J valve; 6 – compressed air; 7 – induced draft fan; 8 – stack.
cotton stalk can be improved when it mixes with alumina particle. Good mixing is very important to keep cotton stalk stable combustion in fluidized bed. The experiments were carried out in order to know the mixing characteristics of cotton stalk with alumina particle. The experimental process was as follows: adding 2 wt% cotton stalk and 98 wt% alumina particles into the cold-state test facility which the initial unexpanded bed height was 350 mm and injecting fluidized gas into the facility. As gas velocity increased, the chamber began to expand, and larger bubbles erupting at the bed surface provided passage for cotton stalk to enter the bed
b
4.0
Cotton stalk mass fraction (%)
Cotton stalk mass fraction (%)
a
Results and discussion
3.5 3.0 2.5 2.0 1.5 1.0
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
0.5 1
2
3
4
1
2
3
4
Divided into four sections from air distributor
Divided into four sections from air distributor
Cotton stalk with length range 10-40mm.
Cotton stalk with length range 40-70mm.
Cotton stalk mass fraction (%)
c
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 1
2
3
4
Divided into four sections from air distributor
Cotton stalk with length range 70~100mm.
Fig. 2 – Cotton stalk mass fraction profiles with different cotton stalk length and alumina diameter. (a) Cotton stalk with length range 10–40 mm. (b) Cotton stalk with length range 40–70 mm. (c) Cotton stalk with length range 70–100 mm.
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4.0 U=0.6m/s, N=1.4 U=1.5m/s, N=3.4 U=2.5m/s, N=5.7 U=3.5m/s, N=8.0
Cotton stalk mass fraction (%)
3.5 3.0 2.5 2.0 1.5 1.0 0.5 1
2
3
4
Divided into four sections from air distributor
Fig. 3 – The effect of gas velocity on cotton stalk mass fraction profiles.
length range 70–100 mm has poor mixing with different diameter bed material. It can be explained with the effects of bed walls and cotton stalk by itself. As diameter range of alumina is 0.1–0.6 mm, segregation of binary bed particles is serious. It can be explained that the Umf of binary mixture of alumina with 0.1–0.6 mm diameter and cotton stalk is low, and fluidization number is very high at U ¼ 2.5 m/s, which results in poor mixing of the binary bed particles. These coincide with the experimental results of effect of fluidizing velocity on mixing behavior of cotton stalk and alumina mixture as below. The influence of fluidizing velocity on mixing of cotton stalk and alumina mixture is plotted in Fig. 3. Experimental conditions were as follows: the particle diameter range of
alumina was 0.6–1 mm; the percentage of cotton stalk in the mixture was 2 wt% and three kinds of cotton stalk with different length accounted for 1/3, respectively. As shown in Fig. 3, when the fluidizing velocity is 0.6 m/s (N ¼ 1.4), the mixing quality is poor because of low turbulence intensity. When the fluidizing velocity is larger than 1.5 m/s (N ¼ 3.4), the mass fraction of cotton stalk has more uniform distribution along the bed height. As gas velocity increases, the chamber begins to expand, and larger bubbles erupting at the bed surface provide passage for cotton stalk to enter the bed interior. Therefore, cotton stalk descends more and more into the bed as gas velocity increases. This indicates that the mixing quality will be improved at high gas velocity. Nevertheless, when the fluidizing velocity is 3.5 m/s (N ¼ 8.0), there will exist a little segregation comparing with U ¼ 2.5 m/s (N ¼ 5.7). This can be explained that when the fluidization number is higher more than 7, more light cotton stalk will be blow into dilute phase zone, which cannot mix with the bed material particles of the dense bed. Therefore, the fluidization number N ¼ 3–7 is suitable for uniform mixing. The mixing images at fluidization number 4 are shown in Fig. 4. The surface of mixture is flat after rapidly stopping the fluidized wind at N ¼ 4 which can also indicate that the binary mixture can keep good mixing. The influence of the mass percentage of cotton stalk in the mixture on mixing characteristics is shown in Fig. 5. The three kinds of the percentage of cotton stalk in the mixture were 1, 2 and 3% kg weight. Experimental conditions were as follows: the particle diameter range of alumina was 0.6–1 mm; the gas velocity was 2.5 m/s and three kinds of cotton stalk with different length accounted for 1/3, respectively. As can be observed, when the mass percentage of cotton stalk to alumina is increased there will be a little segregation. The minimum fluidization velocity, Umf, of a binary mixture is a function of two types of particles and their
Fig. 4 – Photographs of fluidizing characteristics at N [ 4. (a) Photographs of the fluidization of mixture. (b) Photograph of surface of mixture after rapidly stopping the fluidized wind.
biomass and bioenergy 34 (2010) 761–770
does not fluidize. Further, experiments show that the measured minimum fluidization velocity increases with the increase of mass percentage of cotton stalk in the bed and bed material diameter.
Cotton stalk mass fraction (%)
4.0 1 wt% 2 wt% 3 wt%
3.5 3.0 2.5
4.
Thermogravimetric studies
4.1.
Experimental apparatus
2.0 1.5 1.0 0.5 1
2
3
4
Divided into four sections from air distributor Fig. 5 – Effect of mass percentage of cotton stalk to ABM on cotton stalk mass fraction profiles.
relative concentrations. Pilar et al. [4] have concluded that no satisfactory equations are available for predicting the minimum fluidization velocity for mixture of biomass fuel and bed material. The value of Umf can be obtained by experiments [8, 9]. The plot of bed pressure drop DP across the chamber against superficial gas velocity U while decreasing bed velocity is shown in Fig. 6. The minimum fluidization velocity can be defined as the point of intersection of the line of bed pressure drop versus gas velocity at complete fluidization (horizontal line) and during the packed bed state [10]. With 2 wt% cotton stalk in the bed, the initial unfluidized bed height was 250 mm. The bed material diameter was 0.6–1 mm, and three kinds of cotton stalk with different length accounted for 1/3, respectively. The Umf of the mixture of cotton stalk and alumina from Fig. 6 is 0.44 m/s. Compared with pure 0.6–1 mm alumina of the Umf 0.33 m/s, it seems that the minimum fluidization velocity of the mixture increases. In the case of cotton stalk in a single component system, the Umf may not exist because cotton stalk with higher void fraction by itself
The knowledge of the kinetics of combustion is useful for the monitoring and improving efficiency of commercial combustors [11, 12]. The aim of thermogravimetric studies is to investigate the combustion behavior of cotton stalk, and to examine the possibility of kinetic modelling. The experiment under non-isothermal condition was carried out. Cotton stalk sample of 4.727 mg was used for the experiment. Sample was milled to 200 mm. The experiments were performed in a TG92 thermogravimetric apparatus. Alumina was used as a standard reference material. The air flow was 80 ml/min, and the low heating rate was 20 C/min. The sample was heated from room temperature up to 950 C, where it remained constant for 5 min. Weight loss and rate were continuously recorded by thermogravimetric apparatus.
4.2.
300
Theory
Data from TG and DTG curves can be used to determine the kinetic parameters. Mathematical analysis is performed by the integral method of Coats and Redfern [13, 14]. This method has been successfully used for studies on the kinetics of decomposition and combustion biomass fuels. The kinetic equation of common type can be written as follows:
da ¼ kðTÞð1 aÞn ds
(1)
Where a is calculated from the corresponding TG curve by the formula:
a¼
350
Bed pressure drop (10 Pa)
765
u0 us u0 uf
(2)
The temperature dependence of the rate constant is usually described by the Arrhenius equation:
250
E kðTÞ ¼ Aexp RT
200 150
(3)
Under constant heating rate:
100
dT ¼ q ¼ constant ds
50
(4)
After substitution Eq. (1) and some transformations:
0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
Superficial gas velocity (m/s) Fig. 6 – The DP–U curve of the binary mixture.
0.8 Za 0
da A ¼ ð1 aÞn q
ZT T0
e
E dT RT
(5)
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105 ºC
505 ºC
stage1
stage2
TG
DTG
HF
600 400
15 10
-0.4
(9)
200–360 C :
12945:98 da T ¼ 3:45 109 e ð1 aÞ ds
(14)
360–500 C :
15025:66 da T ¼ 1:45 109 e ð1 aÞ ds
(15)
2
-0.6
5
-0.8
HF
0
0 0
(8)
4
-0.2
20
T
-1.0
peak
-5 0
500
1000
1500
2000
2500
3000
Time (s)
Fig. 7 – Combustion thermogravimetric analysis curves of cotton stalk.
After taking integral Eq. (5) and some transformations: lnð1 aÞ AR 2RT E ¼ ln as n ¼ 1; ln 1 T2 qE E RT " as ns1; ln
# 1 ð1 aÞ1n AR 2RT E ¼ ln 1 2 T ð1 nÞ qE E RT
(6)
(7)
Since 2RT=E 1, lnð1 aÞ AR E ¼ ln as n ¼ 1; ln 2 T qE RT " # 1 ð1 aÞ1n AR E as ns1; ln ¼ ln T2 ð1 nÞ qE RT
Results and discussion
The thermogravimetric analysis of cotton stalk is shown in Fig. 7. As it can be observed, several events can be distinguished during the heating process of the cotton stalk sample: Stage 1, the moisture is removed and it lasts up to around 105 C. Stage 2, the volatile compounds are evolved quickly and burned. The loss of weight occurs with maximum rate between 200 C and 360 C. The ignition temperature of cotton stalk can be obtained by TG-DTG method. Firstly, the point A of intersection of the vertical line passing the DTG peak versus TG curve can be found. Next, the ignition temperature can be defined as the corresponding temperature of the intersection of the tangent of TG curve at the point A versus the horizontal line of the beginning of weight loss. Experiment shows that ignition temperature of cotton stalk is only 262 C. Stage 3, char is oxidized up to around 505 C, as shown in the oxidant atmosphere curves, being the remaining material considered as ash. Thermogravimetric studies show that cotton stalk can be ignited easily at a lower temperature, and its devolatilization and combustion are quick. Cotton stalk has higher thermochemical reactivity than coal. The combustion process of cotton stalk is interpreted in terms of two first-order reactions with Arrhenius kinetics neglecting the moisture loss process. The kinetic parameters are summarized in Table 3. As it can be seen, good values are obtained. Cotton stalk has lower activation energy than coal. The value of E for coal usually is 120–230 KJ min1. It also indicates that cotton stalk can be ignited easily. The kinetic equations of cotton stalk combustion can be written as follows:
25
800
200
6
0.0
DTG
Temperature (°C)
0.2
30
TG
1000
4.3. stage3
If Eq. (8) and Eq. (9) are denoted with: X¼
1 T
(10)
lnð1 aÞ as n ¼ 1; Y ¼ ln 2 T
(11)
" # 1 ð1 aÞ1n as ns1; Y ¼ ln T2 ð1 nÞ
(12)
Then, Eq. (8) and Eq. (9) are transformed to: AR E Y ¼ ln X qE R
(13)
Thus, If the correct n is used, the plot of Y against X should give a straight line with high correlation coefficient of the linear regression analysis, from which the values of E and A can be derived.
5.
Combustion characteristics
5.1.
Experimental facility
The cotton stalk combustion apparatus is illustrated schematically in Fig. 8. The system was basically consisted of an over-bed coal screw feeder for igniting, a fluidized bed, a hightemperature cyclone, an under-bed start-up burner and an induced draft fan. The fluidized bed had an overall height of 6 m. The fluidized bed located at height of 1.8 m was used for the secondary air. The nominal dense bed had a cross-section of 230 mm 230 mm and a height of 1.2 m. The perforated-
Table 3 – Kinematic parameters of combustion. Temperature range 200–360 C 360–500 C
Equation of linear fit
C
A/min1
E/KJ min1
Y ¼ 9.49842 12945.98023X Y ¼8.48501 15025.66381X
0.9852 0.9479
3.45e9 1.45e9
107.6 124.9
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biomass and bioenergy 34 (2010) 761–770
5.3.
7 8
3
5 10
2
9
1
Fig. 8 – Schematic diagram of the 0.2M Wth test facility. 1 – screw feeder; 2 – coal hopper; 3 – cotton stalk feeding port; 4 – fluidized bed; 5 – secondary air; 6 – cyclone; 7 – air heater; 8 – cool air; 9 – induced draft fan; 10 – stack.
plate distributor contained 88 small orifices to provide uniform air distribution, with 2.1% open area. The fluidized bed located at height of 1.3 mm from the distributor was used for cotton stalk feeding. The secondary air port was at height of 1.8 mm. The temperature distribution along the bed was measured with thermocouples. The pressure drops across the distributor and the bed were measured by U-tube water manometers. Pressure drop reduction in the bed suggests bad fluidization is a sign of agglomeration problem in the bed. Some previous researchers reported that biomass fuels presented no difficulty in handling the volume flow using a screw feeder. However, in a series of trials conducted, there were some difficulties in feeding long cotton stalk using a screw feeder. As a result, the feeding of cotton stalk in the bed was done manually during the tests, which may result in leaking the cold air into the chamber from cotton stalk feeding port.
5.2.
Experimental objective and methods
The aim of experiments is to examine if the dense bed can keep steady-state combustion for cotton stalk at different cases and observe that the bed temperature profiles during trials with different flow capacity of primary air and secondary air. The steady-state condition criteria are to have a steady temperature profile and steady pressure drop. The experimental conditions were as follows: using alumina with 0.6–1 mm diameter as bed material, cotton stalk feeding rate of about 35 kg/h on an average. The typical experimental cases are summarized in Table 4.
Table 4 – Parameters of experiments.
Results and discussion
The effects of primary and secondary air on the bed temperature profiles are shown in Fig. 9. The temperature of the dense bed can control in the range of 850–870 C. This result has indicated that the dense bed can keep steady-state combustion and there is a good mixing of cotton stalk with alumina. As it can be seen from Fig. 9, the bed temperature increases from 850 to 870 C in the dense bed to 870–910 C at height of 2.1 m which is near to the secondary air port. The results confirm that the burning of volatiles from cotton stalk mostly takes place in the upper regions of the bed due to the high volatile content of cotton stalk. In the freeboard region above 2.1 m, the temperature along the height of the combustor decreases sharply because of heat loss from fluidized bed walls to ambient atmosphere and leaking cold air into the chamber from cotton stalk feeding port. During the experiments the pressure fluctuation was very limited and the combustion was considerably stable. In addition, the experiments show that CO emission varies from 100 to 350 ppm and NOx emission ranges from 123 to 157 ppm. The combustion efficiency of cotton stalk can attain over 99%. It can be accepted that the applications of fluidized-bed boiler to burn cotton stalk with 10–100 mm length as feasible.
6. Agglomeration characteristics of bed materials: silica sand and alumina Although fluidized-bed technology has many merits, there are still some shortcomings, such as sintering, deposition and bed material agglomeration during biomass conversion. Previous studies have shown that the combustion of several typical biomass fuels results in critical agglomeration at the normal FBC temperature. The interaction between silica sand and alkaline materials such as potassium in the ash is known to cause the agglomeration [15–17]. Therefore, frequent silica sand changes are often used to avoid the accumulation of ash in the bed as a precautionary measure, which is not economically sustainable on a long-term basis. In this paper, the use of an alternative bed material is proposed. Case1 Case2 Case3
900
Bed temperiture ( °C)
6
4
800
700
600
500
Case
Primary air/m3/h
Gas velocity/ m/s
Secondary air/m3/h
Case 1 Case 2 Case 3
120 120 140
2.20 (N ¼ 5) 2.20 (N ¼ 5) 2.57 (N ¼ 5.8)
50 70 70
0
1
2
3
4
5
6
Height above air distributor (m)
Fig. 9 – Temperature profiles in combustor during trials with different cases.
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Table 5 – Analysis of cotton stalk ash. Compositions
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
TiO2
P2O5
Content (weight %)
6.33
3.36
1.61
10.33
4.45
3.36
18.76
0.61
7.12
Though the compositions vary with plant types and growth conditions, biomass ashes are normally dominated by silicon, calcium and potassium [18]. The standard analysis of the cotton stalk ash is depicted in Table 5. The K2O content in the cotton stalk ash is quite high which could be a potential problem for bed material agglomeration and heating surface fouling as in the other biomass fuels. Moreover, there are serious problems of deposition and sintering at tail convective heating surfaces after a long time operation because fly ash of cotton stalk combustion in a fluidized bed has the characteristic of strong adhesiveness [19, 20]. The duration of each experiment was about 38 h, respectively using silica sand and alumina as bed material, and the bed material diameter was 0.6–1 mm. The experimental conditions were similar with the primary air of 120 m3/h, the secondary air of 70 m3/h and the cotton stalk feeding rate of about 35 kg/h on an average. Using silica sand as bed material the bed pressure drop was observed to decrease and fluctuate during the experiment. On the contrary, using alumina as bed material the bed pressure drop was observed to remain stable. After the end of each test, bed material was removed from the combustor and was inspected in order to detect agglomeration phenomena. Different sizes of agglomerates exist using silica sand as bed material. Some agglomerates collected cannot be easily
broken by finger. The photograph of the agglomerates is shown in Fig. 10 (a). The agglomerates sampled in the course of the experiments were examined by SEM/EDX analysis. A typical SEM image of a sample is shown in Fig. 10 (b). As can be seen from SEM micrograph, the silica particles were stick together by necks. Some particle surfaces were covered with small pieces of the ash. Ash flakes linked a few particles as well. EDX analysis shows that the composition of ash flake is similar to that of the ash. But most of the particle surfaces look smooth. EDX analysis on smooth sand surface shows that the major elements are silicon and potassium. This indicates that not only ash glues the particles together, but also the potassium-rich coatings on the smooth particle surfaces contribute to the stickiness. As can be seen from Fig. 10 (c), the silica sand particle was surrounded by a coating layer. The coating layer was about 4 mm in thickness and was formed uniformly on the smooth part of the particle contour. The coating shows a solid structure, indicating that the layer has been molten in the hot bed, which is the main reason for agglomeration. EDX analysis shows that the composition of the coating is close to that of ash flake, suggesting that the coatings originate from cotton stalk ash. It provides clear evidences that the presence of alkali metals from the ash causes the formation of the sticky particle surfaces readily for agglomeration. These results are in good agreement with what others found during the experiments [21–23].
Fig. 10 – Photographs of silica sand samples after combustion. (a) Photograph showing agglomeration. (b) SEM micrograph showing agglomeration. (c) SEM micrograph showing the coating layer of silica sand particle.
biomass and bioenergy 34 (2010) 761–770
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Fig. 11 – Photographs of alumina samples after combustion. (a) Photograph of alumina particles after combustion. (b) SEM micrograph of alumina particle after combustion.
The experimental results show that, when silica sand is used as bed material, it is not possible to operate the fluidizedbed reactor above 800 C because of severe agglomeration. This phenomenon can be explained by the interaction between silica sand and biomass fuel ash that forms lowmelting point material [24]. By contrast, alumina was used as a substitute for silica sand. During the experiments the same operation conditions were held in order to give comparable results. The images of alumina samples after combustion are shown in Fig. 11. As it can be seen from Fig. 11, the combustion tests were successfully carried out for 38 h at the dense bed temperature as high as 870 C without any agglomeration. Moreover, SEM/EDX analysis on alumina particles after combustion shows that the grain is not surrounded by a coating layer. These results indicate that alumina is difficult to react with alkali metals from the ash and ABM is more favorable than silica sand for use in a FBC.
7.
Conclusions
With the increase of higher requirement for environment protection, biomass as an alternative fuel has attracted much attention in the recent years. Fluidized-bed technology is playing a more and more important role in using biomass fuel offering economical and environmental benefits compared with the conventional combustion techniques. The great potential of cotton stalk has motivated an increasing interest about the energy use of this material through FBC. The fluidizing and mixing behavior of binary mixture of cotton stalk with 10–100 mm in length and alternative bed material has been investigated. Cotton stalk alone cannot fluidize. An inert particle medium in a fluidized bed is essential to fluidize cotton stalk. The mixing behavior of cotton stalk with alumina in the fluidized bed was investigated for varying length of cotton stalk, diameter of bed material and superficial air velocity. It is found that cotton stalk can fluidize and mix well with 0.6–1 mm alumina particles at the fluidization number N ¼ 3–7. Minimum fluidization velocity for mixture of cotton stalk and alumina is experimentally determined. These studies provide the supporting data needed for cotton stalk combustion in a fluidized bed.
The combustion thermogravimetric analysis of cotton stalk occurs in three main stages, namely, drying (30–105 C), removal and combustion of organic volatile matters (180– 360 C) and combustion of carbon (360–505 ). Cotton stalk is ignited easily, and the lowest ignition temperature is only 262 C. For cotton stalk, the release and combustion of volatile matters and the combustion of carbon occur separately. The combustion of pure cotton stalk with 10–100 mm in length has been studied in a 0.2 MWth fluidized-bed combustor. The fluidizing medium was alumina with 0.6– 1 mm diameter. A fairly steady dense bed temperature has been obtained (between 850 and 870 C), which implies good fluidization and mixing during cotton stalk combustion. Due to the high volatile content of cotton stalk, a major portion of cotton stalk combustion took place in the freeboard. Adding the secondary air can improve the combustion process. The agglomeration and defluidization phenomena in FBC of cotton stalk were caused by the high content of alkali metal in the ash using silica sand as bed material. The examination by SEM/EDX of the agglomerates sampled during combustion suggests that the high alkali metal content in cotton stalk causes the formation of agglomerates and eventually defluidization. In the combustion process, alkali metal-containing compounds were prone to form low-melting alkali metal-rich ash. The molten ashes coated the surfaces of silica sand particles, promoting agglomeration and defluidization in FBC. In this paper, an alternative bed material was used to prevent defluidization. During the tests, it was observed that alumina was more difficult to agglomerate than silica sand because of its specific properties. Experimental results suggest that cotton stalk with 10–100 mm in length is a potential biomass fuel that can be utilized for clean energy production using fluidized bed.
Acknowledgement The research work was supported by National Basic Research Program of China (973 Program: 2007CB210208) and its contribution is gratefully acknowledged. Authors are also thankful to Z. Wang who carried out the thermogravimetric experiments and B. Yang who performed the SEM-EDX analyses.
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references
[1] Kristensen EF, Kristensen JK. Development and test of smallscale batch-fired straw boilers in Denmark. Biomass and Bioenergy 2004;26:561–9. [2] Fang M, Yang L, Chen G, Shi Z, Luo Z, Cen K. Experimental study on rice husk combustion in a circulating fluidized bed. Fuel Processing Technology 2004;85:1273–82. [3] Clarke KL, Pugsley T, Hill GA. Fluidization of moist sawdust in binary particle systems in a gas–solid fluidized bed. Chemical Engineering Science 2005;60:6909–18. [4] Pilar AM, Gracia-Gorria FA, Corella J. Minimum and maximum velocities for fluidization for mixtures of agricultural and forest residues with second fluidized solid. International Chemical Engineering 1992;32:103–13. [5] Ekinci E, Atakul H, Tolay M. Detection of segregation tendencies in a fluidized bed using temperature profiles. Powder Technology 1990;61:185–92. [6] Grubor BD, Oka SN, Ili MS, Daki DV, Arsi BT. Biomass FBC combustiondbed agglomeration problems. In: Heinschel KJ, editor. Proceedings of the 13th International Conference on FBC, Vol. 1. ASME; 1995. p. 515–22. [7] Arvelakis S, Vourliotis P, Kakaras E, Koukios EG. Effect of leaching on the ash behavior of wheat straw and olive residue during fluidized bed combustion. Biomass and Bioenergy 2001;20:459–70. [8] Abdullah MZ, Husain Z, Yin Pong SL. Analysis of cold flow fluidization test results for various biomass fuels. Biomass and Bioenergy 2003;24:487–94. [9] Rao TR, Ram JV. Bheemarasetti. Minimum fluidization velocities of mixtures of biomass and sands. Energy 2001;26: 633–44. [10] Coltters R, Rivas AL. Minimum fluidation velocity correlations in particulate systems. Powder Technology 2004;147:34–48. [11] Kastanaki E, Vamvuka D, Grammelis P, Kakaras E. Thermogravimetric studies of the behavior of lignite– biomass blends during devolatilization. Fuel Processing Technology 2002;77:159–66. [12] Vuthaluru HB. Investigations into the pyrolytic behaviour of coal/biomass blends using thermogravimetric analysis. Bioresource Technology 2004;92:187–95. [13] Liang XH, Kozinski JA. Numerical modeling of combustion and pyrolysis of cellulosic biomass in thermogravimetric systems. Fuel 2000;79:1477–86. [14] Vamvuka D, Kakaras E, Kastanaki E, Grammelis P. Pyrolysis characteristics and kinetics of biomass residuals mixtures with lignite. Fuel 2003;82:1949–60. [15] Fernandez Llorente MJ, Escalada Cuadrado R, Murillo Laplaza JM. Combustion in bubbling fluidized bed with bed material of limestone to reduce the biomass ash agglomeration and sintering. Fuel 2006;85:2081–92. [16] Nielsen HP, Baxter LL, Sclippab G, Morey C, Frandsen FJ, Dam-Johansen K. Deposition of potassium salts on heat transfer surfaces in straw-fired boilers: a pilot-scale study. Fuel 2000;79:131–9.
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Nomenclature FBC: fluidized-bed combustor ABM: alternative bed material N: fluidization number 3: void fraction Umf: minimum fluidization velocity (m/s) d: diameter of bed material particle (mm) U: superficial gas velocity (m/s) DP: bed pressure drop (10 Pa) TG: thermogravimetry DTG: derivative thermogravimetriy HF: heat flow a: degree of transformation u0: initial weight of the sample (mg) us: actual weight of the sample (mg) uf: final weight of the sample (mg) s: time (s) k: reaction rate constant T: absolute temperature (K) n: reaction order A: frequency factor (min1) E: activation energy (KJ/min) R: universal gas constant C: correlation coefficient SEM: Scanning Electron Microscopy EDX: Energy Dispersive X-ray Spectroscopy
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Combustion characteristics of cotton stalk in FBC Zhiao Sun a,*, Jiezhong Shen a, Baosheng Jin b, Liyan Wei a a b
Wuxi Huaguang Boiler Co., Ltd., Wuxi 214028, P.R. China School of Energy and Environment, Southeast University, Nanjing 210096, P.R. China
article info
abstract
Article history:
The present work reports studies on the mixing and combustion characteristics of cotton
Received 10 June 2007
stalk with 10–100 mm in length in FBC. Experiments on a cold model show that cotton stalk
Received in revised form
cannot fluidize, and adding bed material can improve the fluidization condition. Cotton
10 December 2009
stalk can mix well with 0.6–1 mm alumina at fluidization number N ¼ 3–7. However, when
Accepted 6 January 2010
the fluidization number is higher more than 7, the mixing bed will exist a little segregation
Available online 2 February 2010
comparing with N ¼ 3–7. Thermogravimetric experiments show that cotton stalk can be ignited easily at a lower temperature, and its devolatilization and combustion are quick.
Keywords:
Fluidized-bed combustion of cotton stalk was tested in a 0.2 MWth test facility. According to
Cotton stalk
the temperature distribution along the bed height, when the primary and secondary air is
Fluidized bed
adapted cotton stalk can be burned stably in the fluidized bed. During pure cotton stalk
Mixing characteristics
combustion tests, silica sand and alumina are used as bed material to compare their
Combustion characteristics
agglomeration characteristics. SEM/EDX analysis on agglomerate samples after combus-
Bed material
tion about 38 h suggests that the high alkali metals content causes the formation of the coating around silica sand particles. The coating consists of compounds with low-melting temperature results in agglomeration of silica sand particles. By contrast, alumina is difficult to react with alkali metals from biomass ash, and the agglomeration of alumina was not found at 910 C. It is found that alumina is more favorable than silica sand particle for use in a fluidized bed in cotton stalk combustion. ª 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
There is an increasing concern with the environmental problems associated with the increasing CO2, NOx and SOx emissions resulting from the rising use of fossil fuels. For this reason, much attention has been drawn towards the combustion of biomass for power production [1]. Biomass is a renewable source of energy. When the biomass is burned, the released carbon goes back to the atmosphere and can be recycled into the next generation of growing plants. Therefore, the application of biomass for energy can lead to zero net CO2 emission, since carbon in the form of CO2 is fixed by photosynthesis during biomass growth.
Biomass includes a large variety of different fuels with different chemical compositions and combustion characteristics. Generally, biomass fuel has a high content of hydrogen and oxygen, a high volatile content and a low heating value. Biomass fuel has more potassium, sodium, calcium and chlorine than coal. These components are directly related to bed agglomeration, corrosion and ash deposition. Cotton stalk is one of the important and promising biomass resources. As one kind of renewable energy source, it has attracted more research and development in this field. If an efficient method is available, cotton stalk can be converted into useful energy to meet the electrical power requirements in the rural area. M. Fang et al. [2] have claimed that fluidized-
* Corresponding author. Tel.: þ86 510 8521 5556; fax: þ86 510 85219744. E-mail address: [email protected] (Z. Sun). 0961-9534/$ – see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.01.019
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bed combustion, with excellent heat and mass transfer characteristics, seems to be a suitable technology for converting biomass fuel into energy. In the literature, several studies have been reported on the combustion of cotton stalk in the fluidized-bed combustor for energy production. Cotton stalk has either been used alone or in combination with coal. However, it is pelletized to a size range 1–10 mm. This paper introduces an experimental research on the mixing and combustion characteristics of cotton stalk with 10–100 mm in length produced in China. Biomass cannot be easily fluidized due to their irregular shapes. For proper fluidization, an inert bed material such as silica sand, alumina and calcite is used to facilitate fluidization of biomass. It also acts as a heat transfer medium in the reactor [3]. However, the mixtures of solid particles of different size and different density tend to separate in vertical direction under fluidized condition. Pilar et al. [4] have reviewed several investigations reported on the fluidization of mixtures of solids with different particle sizes as well as mixtures of particles of different sizes and densities. Ekinci et al. [5] tested the density and size segregation behavior determined from temperature distributions. Segregation behavior of biomass fuel is of practical importance because the vertical location of biomass fuel influences the in-bed combustion efficiency of volatile matter. Therefore, the mixing behavior can provide the essential information necessary for efficient cotton stalk combustion in a fluidized bed. Studies have also been reported on agglomeration and defluidization phenomena in fluidized-bed combustion of biomass fuel. Grubor et al. [6] reported agglomeration and defluidization problems in a 150 kW continuously straw-fired FBC, which led to 2–3 times shutdown in a month even though the temperature was below 700 C. They concluded that the high content of potassium in straw ash was the major contributor to this phenomenon. Therefore, they suggested using alternative bed materials to avoid formation of lowmelting temperature eutectic compounds of alkali-silicates. S. Arvelakis et al. [7] studied limestone used as bed material offered advantages compared with silica sand delaying the agglomeration phenomena due to its lower reactivity with alkali metals, but the gained profits were found to vary from mediocre to marginal. In this paper, alumina is used as bed material for studying mixing behavior with cotton stalk. During pure cotton stalk combustion tests, silica sand and alumina particle are used as bed material to compare their agglomeration characteristics.
Table 1 – Cotton stalk analysis (as received). Proximate analysis (%) Moisture Ash Volatile Fixed carbon
2.1.
15 2.68 63.08 19.24
Carbon Hydrogen Sulfur Nitrogen Oxygen Ash
40.44 5.07 0.1 0.21 36.5 2.68
metals (K, Na), alkali earth metals (Ca, Mg), silicon, chlorine, and sulfur, as major ash constituents.
2.2.
Properties of bed materials
The compositions of the silica sand and alumina are listed in Table 2. The true density and bulk density of alumina used in the tests are about 2760 kg/m3 and 1180 kg/m3, respectively. By comparison, the true density and bulk density of silica sand used in the tests are about 2490 kg/m3 and 1330 kg/m3, respectively.
3. Mixing characteristics of cotton stalk with alumina particle 3.1.
Cold-state experimental apparatus
A cold-state test facility used in research of the fluidization and mixing characteristics of cotton stalk is sketched in Fig. 1. It mainly consists of an air blower, a flowmeter, a fluidized bed, a set of pressure measuring instruments, an off-gas cleanup system and an induced draft fan. The fluidized bed had an overall height of 4.4 m. The dense bed with a crosssection of 400 mm 400 mm and a height of 750 mm was partly fabricated from plexiglas where visible observations of the bed activity were possible. Four windows at the sidewall were arranged to take out the bed material in the mixed bed condition. In order to prevent air leakage and bed leakage a single piece rubber seal is used for the window gaps. The perforated-plate distributor contained 48 small nozzles to provide uniform air distribution, with 3.1% open area. The pressure drops across the distributor and the bed are measured by U-tube water manometers.
3.2.
2. Properties of cotton stalk and bed materials
Ultimate Analysis (%)
Experimental methods
The experiments showed that pure cotton stalk by itself cannot fluidize due to its peculiar shape, size and density, as well as higher void fraction. However, fluidization behavior of
Properties of cotton stalk
The typical size of cotton stalk used in experimental studies is about 10–100 mm in length. It is very light, with natural packing density of 100–130 kg/m3 and real density of about 460 kg/m3. Cotton stalk is also characterized by a high volatile content and low gross calorific value of 13.5 MJ/kg. Table 1 shows analytical data for cotton stalk. In addition, cotton stalk contains large quantities of inorganic elements such as alkali
Table 2 – Compositions of the silica sand and alumina (weight %). SiO2 Al2O3 Fe2O3 TiO2 CaO þ MgO Na2O þ K2O Silica sand 98.61 0.14 Alumina 13.07 79.24
0.08 1.75
0.01 4.10
0.06 0.45
0.11 0.53
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biomass and bioenergy 34 (2010) 761–770
interior. The fluidized bed was kept at good fluidization for over 5 min to reach the steady-state condition, then rapidly stopping the fluidized gas. The mixture in each window was gently taken out, respectively, separated by a sieve and weighed. Thus, the mixed bed was divided into four sections from air distributor, and the mass fraction of cotton stalk along the bed height was obtained.
3.3.
Many trials were carried out in order to evaluate the mixing characteristics of cotton stalk and alumina mixtures, with three kinds of cotton stalk with length range 10–40 mm, 40– 70 mm and 70–100 mm, and with three kinds of alumina with diameter range 0.1–0.6 mm, 0.6–1 mm and 1–2 mm. The percentage of cotton stalk in the mixture was 2 wt%, and gas velocity was 2.5 m/s. The mixing characteristics of cotton stalk and alumina mixtures are shown in Fig. 2. The bed material size has great effect on the mixing quality. Cotton stalk with different length ranges can fluidize and mix well with 0.6–1 mm alumina particles, and the phenomena of segregation are not serious. As is evident in Fig. 2, alumina particles of diameter range 0.6–1 mm are the most suitable bed materials, which can keep good mixing with cotton stalk. According to Fig. 2 (a), when diameter range of alumina is 1– 2 mm, segregation of binary bed particles occurs. It can be explained that the Umf of binary mixture of alumina with 1– 2 mm diameter and cotton stalk is high, and fluidization number is low at U ¼ 2.5 m/s, which causes bad mixing of the binary bed particles. According to Fig. 2 (c), cotton stalk with
Fig. 1 – Schematic diagram of cold-state experimental apparatus. 1 – air blower; 2 – flowmeter; 3 – fluidized bed; 4 – cyclone; 5 – J valve; 6 – compressed air; 7 – induced draft fan; 8 – stack.
cotton stalk can be improved when it mixes with alumina particle. Good mixing is very important to keep cotton stalk stable combustion in fluidized bed. The experiments were carried out in order to know the mixing characteristics of cotton stalk with alumina particle. The experimental process was as follows: adding 2 wt% cotton stalk and 98 wt% alumina particles into the cold-state test facility which the initial unexpanded bed height was 350 mm and injecting fluidized gas into the facility. As gas velocity increased, the chamber began to expand, and larger bubbles erupting at the bed surface provided passage for cotton stalk to enter the bed
b
4.0
Cotton stalk mass fraction (%)
Cotton stalk mass fraction (%)
a
Results and discussion
3.5 3.0 2.5 2.0 1.5 1.0
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
0.5 1
2
3
4
1
2
3
4
Divided into four sections from air distributor
Divided into four sections from air distributor
Cotton stalk with length range 10-40mm.
Cotton stalk with length range 40-70mm.
Cotton stalk mass fraction (%)
c
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 1
2
3
4
Divided into four sections from air distributor
Cotton stalk with length range 70~100mm.
Fig. 2 – Cotton stalk mass fraction profiles with different cotton stalk length and alumina diameter. (a) Cotton stalk with length range 10–40 mm. (b) Cotton stalk with length range 40–70 mm. (c) Cotton stalk with length range 70–100 mm.
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biomass and bioenergy 34 (2010) 761–770
4.0 U=0.6m/s, N=1.4 U=1.5m/s, N=3.4 U=2.5m/s, N=5.7 U=3.5m/s, N=8.0
Cotton stalk mass fraction (%)
3.5 3.0 2.5 2.0 1.5 1.0 0.5 1
2
3
4
Divided into four sections from air distributor
Fig. 3 – The effect of gas velocity on cotton stalk mass fraction profiles.
length range 70–100 mm has poor mixing with different diameter bed material. It can be explained with the effects of bed walls and cotton stalk by itself. As diameter range of alumina is 0.1–0.6 mm, segregation of binary bed particles is serious. It can be explained that the Umf of binary mixture of alumina with 0.1–0.6 mm diameter and cotton stalk is low, and fluidization number is very high at U ¼ 2.5 m/s, which results in poor mixing of the binary bed particles. These coincide with the experimental results of effect of fluidizing velocity on mixing behavior of cotton stalk and alumina mixture as below. The influence of fluidizing velocity on mixing of cotton stalk and alumina mixture is plotted in Fig. 3. Experimental conditions were as follows: the particle diameter range of
alumina was 0.6–1 mm; the percentage of cotton stalk in the mixture was 2 wt% and three kinds of cotton stalk with different length accounted for 1/3, respectively. As shown in Fig. 3, when the fluidizing velocity is 0.6 m/s (N ¼ 1.4), the mixing quality is poor because of low turbulence intensity. When the fluidizing velocity is larger than 1.5 m/s (N ¼ 3.4), the mass fraction of cotton stalk has more uniform distribution along the bed height. As gas velocity increases, the chamber begins to expand, and larger bubbles erupting at the bed surface provide passage for cotton stalk to enter the bed interior. Therefore, cotton stalk descends more and more into the bed as gas velocity increases. This indicates that the mixing quality will be improved at high gas velocity. Nevertheless, when the fluidizing velocity is 3.5 m/s (N ¼ 8.0), there will exist a little segregation comparing with U ¼ 2.5 m/s (N ¼ 5.7). This can be explained that when the fluidization number is higher more than 7, more light cotton stalk will be blow into dilute phase zone, which cannot mix with the bed material particles of the dense bed. Therefore, the fluidization number N ¼ 3–7 is suitable for uniform mixing. The mixing images at fluidization number 4 are shown in Fig. 4. The surface of mixture is flat after rapidly stopping the fluidized wind at N ¼ 4 which can also indicate that the binary mixture can keep good mixing. The influence of the mass percentage of cotton stalk in the mixture on mixing characteristics is shown in Fig. 5. The three kinds of the percentage of cotton stalk in the mixture were 1, 2 and 3% kg weight. Experimental conditions were as follows: the particle diameter range of alumina was 0.6–1 mm; the gas velocity was 2.5 m/s and three kinds of cotton stalk with different length accounted for 1/3, respectively. As can be observed, when the mass percentage of cotton stalk to alumina is increased there will be a little segregation. The minimum fluidization velocity, Umf, of a binary mixture is a function of two types of particles and their
Fig. 4 – Photographs of fluidizing characteristics at N [ 4. (a) Photographs of the fluidization of mixture. (b) Photograph of surface of mixture after rapidly stopping the fluidized wind.
biomass and bioenergy 34 (2010) 761–770
does not fluidize. Further, experiments show that the measured minimum fluidization velocity increases with the increase of mass percentage of cotton stalk in the bed and bed material diameter.
Cotton stalk mass fraction (%)
4.0 1 wt% 2 wt% 3 wt%
3.5 3.0 2.5
4.
Thermogravimetric studies
4.1.
Experimental apparatus
2.0 1.5 1.0 0.5 1
2
3
4
Divided into four sections from air distributor Fig. 5 – Effect of mass percentage of cotton stalk to ABM on cotton stalk mass fraction profiles.
relative concentrations. Pilar et al. [4] have concluded that no satisfactory equations are available for predicting the minimum fluidization velocity for mixture of biomass fuel and bed material. The value of Umf can be obtained by experiments [8, 9]. The plot of bed pressure drop DP across the chamber against superficial gas velocity U while decreasing bed velocity is shown in Fig. 6. The minimum fluidization velocity can be defined as the point of intersection of the line of bed pressure drop versus gas velocity at complete fluidization (horizontal line) and during the packed bed state [10]. With 2 wt% cotton stalk in the bed, the initial unfluidized bed height was 250 mm. The bed material diameter was 0.6–1 mm, and three kinds of cotton stalk with different length accounted for 1/3, respectively. The Umf of the mixture of cotton stalk and alumina from Fig. 6 is 0.44 m/s. Compared with pure 0.6–1 mm alumina of the Umf 0.33 m/s, it seems that the minimum fluidization velocity of the mixture increases. In the case of cotton stalk in a single component system, the Umf may not exist because cotton stalk with higher void fraction by itself
The knowledge of the kinetics of combustion is useful for the monitoring and improving efficiency of commercial combustors [11, 12]. The aim of thermogravimetric studies is to investigate the combustion behavior of cotton stalk, and to examine the possibility of kinetic modelling. The experiment under non-isothermal condition was carried out. Cotton stalk sample of 4.727 mg was used for the experiment. Sample was milled to 200 mm. The experiments were performed in a TG92 thermogravimetric apparatus. Alumina was used as a standard reference material. The air flow was 80 ml/min, and the low heating rate was 20 C/min. The sample was heated from room temperature up to 950 C, where it remained constant for 5 min. Weight loss and rate were continuously recorded by thermogravimetric apparatus.
4.2.
300
Theory
Data from TG and DTG curves can be used to determine the kinetic parameters. Mathematical analysis is performed by the integral method of Coats and Redfern [13, 14]. This method has been successfully used for studies on the kinetics of decomposition and combustion biomass fuels. The kinetic equation of common type can be written as follows:
da ¼ kðTÞð1 aÞn ds
(1)
Where a is calculated from the corresponding TG curve by the formula:
a¼
350
Bed pressure drop (10 Pa)
765
u0 us u0 uf
(2)
The temperature dependence of the rate constant is usually described by the Arrhenius equation:
250
E kðTÞ ¼ Aexp RT
200 150
(3)
Under constant heating rate:
100
dT ¼ q ¼ constant ds
50
(4)
After substitution Eq. (1) and some transformations:
0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
Superficial gas velocity (m/s) Fig. 6 – The DP–U curve of the binary mixture.
0.8 Za 0
da A ¼ ð1 aÞn q
ZT T0
e
E dT RT
(5)
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biomass and bioenergy 34 (2010) 761–770
105 ºC
505 ºC
stage1
stage2
TG
DTG
HF
600 400
15 10
-0.4
(9)
200–360 C :
12945:98 da T ¼ 3:45 109 e ð1 aÞ ds
(14)
360–500 C :
15025:66 da T ¼ 1:45 109 e ð1 aÞ ds
(15)
2
-0.6
5
-0.8
HF
0
0 0
(8)
4
-0.2
20
T
-1.0
peak
-5 0
500
1000
1500
2000
2500
3000
Time (s)
Fig. 7 – Combustion thermogravimetric analysis curves of cotton stalk.
After taking integral Eq. (5) and some transformations: lnð1 aÞ AR 2RT E ¼ ln as n ¼ 1; ln 1 T2 qE E RT " as ns1; ln
# 1 ð1 aÞ1n AR 2RT E ¼ ln 1 2 T ð1 nÞ qE E RT
(6)
(7)
Since 2RT=E 1, lnð1 aÞ AR E ¼ ln as n ¼ 1; ln 2 T qE RT " # 1 ð1 aÞ1n AR E as ns1; ln ¼ ln T2 ð1 nÞ qE RT
Results and discussion
The thermogravimetric analysis of cotton stalk is shown in Fig. 7. As it can be observed, several events can be distinguished during the heating process of the cotton stalk sample: Stage 1, the moisture is removed and it lasts up to around 105 C. Stage 2, the volatile compounds are evolved quickly and burned. The loss of weight occurs with maximum rate between 200 C and 360 C. The ignition temperature of cotton stalk can be obtained by TG-DTG method. Firstly, the point A of intersection of the vertical line passing the DTG peak versus TG curve can be found. Next, the ignition temperature can be defined as the corresponding temperature of the intersection of the tangent of TG curve at the point A versus the horizontal line of the beginning of weight loss. Experiment shows that ignition temperature of cotton stalk is only 262 C. Stage 3, char is oxidized up to around 505 C, as shown in the oxidant atmosphere curves, being the remaining material considered as ash. Thermogravimetric studies show that cotton stalk can be ignited easily at a lower temperature, and its devolatilization and combustion are quick. Cotton stalk has higher thermochemical reactivity than coal. The combustion process of cotton stalk is interpreted in terms of two first-order reactions with Arrhenius kinetics neglecting the moisture loss process. The kinetic parameters are summarized in Table 3. As it can be seen, good values are obtained. Cotton stalk has lower activation energy than coal. The value of E for coal usually is 120–230 KJ min1. It also indicates that cotton stalk can be ignited easily. The kinetic equations of cotton stalk combustion can be written as follows:
25
800
200
6
0.0
DTG
Temperature (°C)
0.2
30
TG
1000
4.3. stage3
If Eq. (8) and Eq. (9) are denoted with: X¼
1 T
(10)
lnð1 aÞ as n ¼ 1; Y ¼ ln 2 T
(11)
" # 1 ð1 aÞ1n as ns1; Y ¼ ln T2 ð1 nÞ
(12)
Then, Eq. (8) and Eq. (9) are transformed to: AR E Y ¼ ln X qE R
(13)
Thus, If the correct n is used, the plot of Y against X should give a straight line with high correlation coefficient of the linear regression analysis, from which the values of E and A can be derived.
5.
Combustion characteristics
5.1.
Experimental facility
The cotton stalk combustion apparatus is illustrated schematically in Fig. 8. The system was basically consisted of an over-bed coal screw feeder for igniting, a fluidized bed, a hightemperature cyclone, an under-bed start-up burner and an induced draft fan. The fluidized bed had an overall height of 6 m. The fluidized bed located at height of 1.8 m was used for the secondary air. The nominal dense bed had a cross-section of 230 mm 230 mm and a height of 1.2 m. The perforated-
Table 3 – Kinematic parameters of combustion. Temperature range 200–360 C 360–500 C
Equation of linear fit
C
A/min1
E/KJ min1
Y ¼ 9.49842 12945.98023X Y ¼8.48501 15025.66381X
0.9852 0.9479
3.45e9 1.45e9
107.6 124.9
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biomass and bioenergy 34 (2010) 761–770
5.3.
7 8
3
5 10
2
9
1
Fig. 8 – Schematic diagram of the 0.2M Wth test facility. 1 – screw feeder; 2 – coal hopper; 3 – cotton stalk feeding port; 4 – fluidized bed; 5 – secondary air; 6 – cyclone; 7 – air heater; 8 – cool air; 9 – induced draft fan; 10 – stack.
plate distributor contained 88 small orifices to provide uniform air distribution, with 2.1% open area. The fluidized bed located at height of 1.3 mm from the distributor was used for cotton stalk feeding. The secondary air port was at height of 1.8 mm. The temperature distribution along the bed was measured with thermocouples. The pressure drops across the distributor and the bed were measured by U-tube water manometers. Pressure drop reduction in the bed suggests bad fluidization is a sign of agglomeration problem in the bed. Some previous researchers reported that biomass fuels presented no difficulty in handling the volume flow using a screw feeder. However, in a series of trials conducted, there were some difficulties in feeding long cotton stalk using a screw feeder. As a result, the feeding of cotton stalk in the bed was done manually during the tests, which may result in leaking the cold air into the chamber from cotton stalk feeding port.
5.2.
Experimental objective and methods
The aim of experiments is to examine if the dense bed can keep steady-state combustion for cotton stalk at different cases and observe that the bed temperature profiles during trials with different flow capacity of primary air and secondary air. The steady-state condition criteria are to have a steady temperature profile and steady pressure drop. The experimental conditions were as follows: using alumina with 0.6–1 mm diameter as bed material, cotton stalk feeding rate of about 35 kg/h on an average. The typical experimental cases are summarized in Table 4.
Table 4 – Parameters of experiments.
Results and discussion
The effects of primary and secondary air on the bed temperature profiles are shown in Fig. 9. The temperature of the dense bed can control in the range of 850–870 C. This result has indicated that the dense bed can keep steady-state combustion and there is a good mixing of cotton stalk with alumina. As it can be seen from Fig. 9, the bed temperature increases from 850 to 870 C in the dense bed to 870–910 C at height of 2.1 m which is near to the secondary air port. The results confirm that the burning of volatiles from cotton stalk mostly takes place in the upper regions of the bed due to the high volatile content of cotton stalk. In the freeboard region above 2.1 m, the temperature along the height of the combustor decreases sharply because of heat loss from fluidized bed walls to ambient atmosphere and leaking cold air into the chamber from cotton stalk feeding port. During the experiments the pressure fluctuation was very limited and the combustion was considerably stable. In addition, the experiments show that CO emission varies from 100 to 350 ppm and NOx emission ranges from 123 to 157 ppm. The combustion efficiency of cotton stalk can attain over 99%. It can be accepted that the applications of fluidized-bed boiler to burn cotton stalk with 10–100 mm length as feasible.
6. Agglomeration characteristics of bed materials: silica sand and alumina Although fluidized-bed technology has many merits, there are still some shortcomings, such as sintering, deposition and bed material agglomeration during biomass conversion. Previous studies have shown that the combustion of several typical biomass fuels results in critical agglomeration at the normal FBC temperature. The interaction between silica sand and alkaline materials such as potassium in the ash is known to cause the agglomeration [15–17]. Therefore, frequent silica sand changes are often used to avoid the accumulation of ash in the bed as a precautionary measure, which is not economically sustainable on a long-term basis. In this paper, the use of an alternative bed material is proposed. Case1 Case2 Case3
900
Bed temperiture ( °C)
6
4
800
700
600
500
Case
Primary air/m3/h
Gas velocity/ m/s
Secondary air/m3/h
Case 1 Case 2 Case 3
120 120 140
2.20 (N ¼ 5) 2.20 (N ¼ 5) 2.57 (N ¼ 5.8)
50 70 70
0
1
2
3
4
5
6
Height above air distributor (m)
Fig. 9 – Temperature profiles in combustor during trials with different cases.
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Table 5 – Analysis of cotton stalk ash. Compositions
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
TiO2
P2O5
Content (weight %)
6.33
3.36
1.61
10.33
4.45
3.36
18.76
0.61
7.12
Though the compositions vary with plant types and growth conditions, biomass ashes are normally dominated by silicon, calcium and potassium [18]. The standard analysis of the cotton stalk ash is depicted in Table 5. The K2O content in the cotton stalk ash is quite high which could be a potential problem for bed material agglomeration and heating surface fouling as in the other biomass fuels. Moreover, there are serious problems of deposition and sintering at tail convective heating surfaces after a long time operation because fly ash of cotton stalk combustion in a fluidized bed has the characteristic of strong adhesiveness [19, 20]. The duration of each experiment was about 38 h, respectively using silica sand and alumina as bed material, and the bed material diameter was 0.6–1 mm. The experimental conditions were similar with the primary air of 120 m3/h, the secondary air of 70 m3/h and the cotton stalk feeding rate of about 35 kg/h on an average. Using silica sand as bed material the bed pressure drop was observed to decrease and fluctuate during the experiment. On the contrary, using alumina as bed material the bed pressure drop was observed to remain stable. After the end of each test, bed material was removed from the combustor and was inspected in order to detect agglomeration phenomena. Different sizes of agglomerates exist using silica sand as bed material. Some agglomerates collected cannot be easily
broken by finger. The photograph of the agglomerates is shown in Fig. 10 (a). The agglomerates sampled in the course of the experiments were examined by SEM/EDX analysis. A typical SEM image of a sample is shown in Fig. 10 (b). As can be seen from SEM micrograph, the silica particles were stick together by necks. Some particle surfaces were covered with small pieces of the ash. Ash flakes linked a few particles as well. EDX analysis shows that the composition of ash flake is similar to that of the ash. But most of the particle surfaces look smooth. EDX analysis on smooth sand surface shows that the major elements are silicon and potassium. This indicates that not only ash glues the particles together, but also the potassium-rich coatings on the smooth particle surfaces contribute to the stickiness. As can be seen from Fig. 10 (c), the silica sand particle was surrounded by a coating layer. The coating layer was about 4 mm in thickness and was formed uniformly on the smooth part of the particle contour. The coating shows a solid structure, indicating that the layer has been molten in the hot bed, which is the main reason for agglomeration. EDX analysis shows that the composition of the coating is close to that of ash flake, suggesting that the coatings originate from cotton stalk ash. It provides clear evidences that the presence of alkali metals from the ash causes the formation of the sticky particle surfaces readily for agglomeration. These results are in good agreement with what others found during the experiments [21–23].
Fig. 10 – Photographs of silica sand samples after combustion. (a) Photograph showing agglomeration. (b) SEM micrograph showing agglomeration. (c) SEM micrograph showing the coating layer of silica sand particle.
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Fig. 11 – Photographs of alumina samples after combustion. (a) Photograph of alumina particles after combustion. (b) SEM micrograph of alumina particle after combustion.
The experimental results show that, when silica sand is used as bed material, it is not possible to operate the fluidizedbed reactor above 800 C because of severe agglomeration. This phenomenon can be explained by the interaction between silica sand and biomass fuel ash that forms lowmelting point material [24]. By contrast, alumina was used as a substitute for silica sand. During the experiments the same operation conditions were held in order to give comparable results. The images of alumina samples after combustion are shown in Fig. 11. As it can be seen from Fig. 11, the combustion tests were successfully carried out for 38 h at the dense bed temperature as high as 870 C without any agglomeration. Moreover, SEM/EDX analysis on alumina particles after combustion shows that the grain is not surrounded by a coating layer. These results indicate that alumina is difficult to react with alkali metals from the ash and ABM is more favorable than silica sand for use in a FBC.
7.
Conclusions
With the increase of higher requirement for environment protection, biomass as an alternative fuel has attracted much attention in the recent years. Fluidized-bed technology is playing a more and more important role in using biomass fuel offering economical and environmental benefits compared with the conventional combustion techniques. The great potential of cotton stalk has motivated an increasing interest about the energy use of this material through FBC. The fluidizing and mixing behavior of binary mixture of cotton stalk with 10–100 mm in length and alternative bed material has been investigated. Cotton stalk alone cannot fluidize. An inert particle medium in a fluidized bed is essential to fluidize cotton stalk. The mixing behavior of cotton stalk with alumina in the fluidized bed was investigated for varying length of cotton stalk, diameter of bed material and superficial air velocity. It is found that cotton stalk can fluidize and mix well with 0.6–1 mm alumina particles at the fluidization number N ¼ 3–7. Minimum fluidization velocity for mixture of cotton stalk and alumina is experimentally determined. These studies provide the supporting data needed for cotton stalk combustion in a fluidized bed.
The combustion thermogravimetric analysis of cotton stalk occurs in three main stages, namely, drying (30–105 C), removal and combustion of organic volatile matters (180– 360 C) and combustion of carbon (360–505 ). Cotton stalk is ignited easily, and the lowest ignition temperature is only 262 C. For cotton stalk, the release and combustion of volatile matters and the combustion of carbon occur separately. The combustion of pure cotton stalk with 10–100 mm in length has been studied in a 0.2 MWth fluidized-bed combustor. The fluidizing medium was alumina with 0.6– 1 mm diameter. A fairly steady dense bed temperature has been obtained (between 850 and 870 C), which implies good fluidization and mixing during cotton stalk combustion. Due to the high volatile content of cotton stalk, a major portion of cotton stalk combustion took place in the freeboard. Adding the secondary air can improve the combustion process. The agglomeration and defluidization phenomena in FBC of cotton stalk were caused by the high content of alkali metal in the ash using silica sand as bed material. The examination by SEM/EDX of the agglomerates sampled during combustion suggests that the high alkali metal content in cotton stalk causes the formation of agglomerates and eventually defluidization. In the combustion process, alkali metal-containing compounds were prone to form low-melting alkali metal-rich ash. The molten ashes coated the surfaces of silica sand particles, promoting agglomeration and defluidization in FBC. In this paper, an alternative bed material was used to prevent defluidization. During the tests, it was observed that alumina was more difficult to agglomerate than silica sand because of its specific properties. Experimental results suggest that cotton stalk with 10–100 mm in length is a potential biomass fuel that can be utilized for clean energy production using fluidized bed.
Acknowledgement The research work was supported by National Basic Research Program of China (973 Program: 2007CB210208) and its contribution is gratefully acknowledged. Authors are also thankful to Z. Wang who carried out the thermogravimetric experiments and B. Yang who performed the SEM-EDX analyses.
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Nomenclature FBC: fluidized-bed combustor ABM: alternative bed material N: fluidization number 3: void fraction Umf: minimum fluidization velocity (m/s) d: diameter of bed material particle (mm) U: superficial gas velocity (m/s) DP: bed pressure drop (10 Pa) TG: thermogravimetry DTG: derivative thermogravimetriy HF: heat flow a: degree of transformation u0: initial weight of the sample (mg) us: actual weight of the sample (mg) uf: final weight of the sample (mg) s: time (s) k: reaction rate constant T: absolute temperature (K) n: reaction order A: frequency factor (min1) E: activation energy (KJ/min) R: universal gas constant C: correlation coefficient SEM: Scanning Electron Microscopy EDX: Energy Dispersive X-ray Spectroscopy