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Experimental study on cotton stalk combustion in a circulating fluidized bed
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APPLIED ENERGY Applied Energy 85 (2008) 1027–1040 www.elsevier.com/locate/apenergy
Experimental study on cotton stalk combustion in a circulating fluidized bed Zhi-Ao Sun *, Bao-Sheng Jin, Ming-Yao Zhang, Ren-Ping Liu, Yong Zhang Key Laboratory of Clean Coal Power Generation, Combustion Technology of Ministry of Education, Southeast University, Nanjing 210096, China Received 20 November 2007; received in revised form 27 February 2008; accepted 28 February 2008 Available online 14 April 2008
Abstract This paper summarizes the results of an experimental study on cotton stalk (CS) combustion in a circulating fluidized bed. The mixing and fluidizing characteristics of binary mixture of CS with 10–100 mm in length and alumina bed material with a certain size distribution in a cold test facility were studied. The results show that CS by itself cannot fluidize, and adding inert bed material can improve the fluidization condition. CS can mix well with alumina at fluidization number N = 3–7. As N is more than 7, there will exist a little more segregation. The study concerning combustion characteristics of pure CS was performed on a circulating fluidized bed with a heat input of 0.5 MW. The effects of fluidizing velocity, secondary air flow and gas flow to the loop seal on the bed temperature profiles were investigated. Although there is a little more segregation at N higher than 7 in the cold tests, the hot experimental results indicate that slight segregation has little effect on the steady combustion of the dense region. In this study, the concentrations of major gaseous pollutants (CO, SO2 and NO) in flue (stack) gas were measured. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Cotton stalk; Circulating fluidized bed; Bed material; Mixing characteristics; Combustion characteristics; Pollutant emissions
1. Introduction Energy demand in the world continues to increase with the increase in population and economic development. Biomass is a potentially CO2-neutral and renewable energy resource. As an alternative fuel it has attracted much attention worldwide in the recent years [1,2]. At present, biomass is converted into heat and electricity most often by combustion. Combustors for biomass fuels are predominantly grate-fired systems, suspension burners or fluidized bed systems. All these types of combustors are being used in industry. Grate-fired combustors were the most versatile units in the mid-1980s, but the fluidized bed systems have become serious competitors in the last decade [3]. Among the combustion technologies, the fluidized bed combustion seems to be the most suitable for con*
Corresponding author. Tel./fax: +86 25 83795508. E-mail address: [email protected] (Z.-A. Sun).
0306-2619/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2008.02.018
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verting biomass into energy, because of its inherent advantages of fuel flexibility, low operating temperature and low pollution emission. A significant amount of research work has been carried out on the fluidized bed combustor (FBC) of biomass fuels [4–6]. Biomass fuels cannot easily fluidize due to their irregular shapes. For proper fluidization, inert bed material is used to facilitate fluidization of biomass. It acts as a heat transfer medium in the reactor as well. Biomass includes a large variety of different fuels with different chemical compositions and combustion characteristics. Generally, biomass fuels have a high content of hydrogen and oxygen, a high volatile content and a low heating value. Meanwhile, some biomass fuels have more potassium, sodium, calcium and chlorine than coal. The alkaline materials are directly related to bed agglomeration, sintering and deposition. Previous studies have shown that the combustion of biomass fuels results in bed agglomeration at the normal fluidized bed temperature [7–9]. Because the interaction between silica sand bed material and alkaline materials such as potassium in the ash causes the bed agglomeration, frequent bed material changes are often used to avoid the accumulation of ash in the bed as a precautionary measure. In the other hand, in order to diminish the bed agglomeration, the use of silica-free alternative bed material such as alumina, dolomite, limestone and kaolin has also been proposed [10,11]. Cotton stalk (CS) is one of the most viable biomass fuels in China. As one kind of renewable energy resource, it has attracted increasing research and development efforts. If an efficient method is available, CS can be converted to useful energy to meet the electrical power requirements in the rural area. In the literature, several studies have been reported on CS combustion in the FBC for energy production, and CS has either been used alone or in combination with coal [12,13]. However, it is pelletized to a size range less than 10 mm in length. This paper mainly introduces an experimental research on mixing and combustion characteristics of pure CS with 10–100 mm in length in a circulating fluidized bed (CFB), and the effect of the mixing behavior on combustion characteristics of pure CS in CFB hot experiments. In this study, alumina of weak acidic property was used as alternative bed material. The mixtures of solid particles of different sizes and different densities tend to separate in vertical direction under fluidized condition. The segregation behavior of biomass fuel is of practical importance because the vertical location of biomass fuel influences the combustion efficiency of volatile matter. Aznar et al. [14] 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. [15] tested the size and density on the segregation behavior determined from temperature distribution. Therefore, it is very important that the research on mixing and fluidizing characteristics of CS with bed material in the cold tests. Technical enhancements in the contribution of biomass to commercial energy needs are focused on improving the combustion efficiency and environmental impacts of biomass conversion. CO, SO2 and oxides of nitrogen (principally NO) are the major harmful pollutants emitted from biomass combustion in fluidized bed systems. CO emission strongly affecting the combustion efficiency is a function of operating variables, such as excess air ratio and air split ratio. SO2 and NO emissions which arise predominantly from nitrogen and sulfur in the fuel are weakly dependent on the combustion conditions. 2. Properties of CS and alternative bed material 2.1. Properties of CS There is a crushing unit that can crush the CS to 10–100 mm length. In this study, there are three sizes of CS in the fuel mixture with length ranges of 10–40 mm, 40–70 mm and 70–100 mm, each portion accounting for 1/3 of the total mixture by weight. CS is very light, with natural packing density of 100–130 kg/m3 and real density of about 460 kg/m3. For comparison, the natural packing density and real density of bituminous coals are in the ranges of 800–900 kg/m3 and around 1800 kg/m3, respectively. The low density of CS complicates their processing, transportation, storage and combustion. CS is characterized by a high volatile matter content and low gross calorific value of 17.3 MJ/kg. The analysis data of CS are given in Table 1. For the proximate analysis the relative errors are estimated to be below 5% for moisture, and below 3% for other components; for the ultimate analysis the relative errors are believed to be below the following values: 1% for carbon and oxygen, 3% for other elements. The ICP analysis data of CS are listed in Table 2. From the table it is observed that
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Table 1 CS analysis Proximate analysis (wt% on air dry basis) Moisture Ash Volatile Fixed carbon
Ultimate analysis (wt% on air dry basis) 8.41 1.75 67.97 21.87
Carbon Hydrogen Oxygen Nitrogen Sulfur
44.58 5.46 39.43 0.23 0.14
Table 2 ICP analysis of CS Element
K
Ca
Cl
Mg
P
Si
Na
Al
Fe
Ti
wt%
0.99
0.25
0.29
0.12
0.096
0.086
0.11
0.0086
0.0071
0.003
K, Na and Ca contents in CS are very high, which are directly related to bed agglomeration, sintering and deposition for CS combustion in CFB. 2.2. Characteristics of alternative bed material The experimental studies of Shimizu et al. [10] showed that alumina sand was more favorable for use in a fluidized bed combustor during biomass combustion. On the other hand, alumina is relatively cheap and can be easily purchased in China. Accordingly, in order to diminish bed agglomeration, alumina of weak acidic property was used as alternative bed material in the tests. The composition of silica sand and alumina is listed in Table 3. 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 are about 2490 kg/m3 and 1330 kg/m3, respectively. Preliminary experiments were conducted to choose the optimum particle size, which was based on the operational demands in the CFB combustor. The size distribution of alumina employed is given in Table 4. 3. Mixing characteristics of CS with alumina 3.1. Cold-state experimental apparatus A cold-state test facility used to research the mixing and fluidization characteristics of CS with alumina is sketched in Fig. 1. It mainly consists of an air blower, a flowmeter, a fluidized bed, an off-gas cleanup system Table 3 Composition of silica sand and alumina (wt%) Composition
SiO2
Al2O3
Fe2O3
TiO2
CaO + MgO
Na2O + K2O
Silica sand Alumina
98.61 13.07
0.14 79.24
0.08 1.75
0.01 4.10
0.06 0.45
0.11 0.53
Table 4 Particle-size distribution of alumina Tyler mesh
Sieve size range (mm)
Average size (mm)
Solids fraction retained on the sieve (wt%)
16–24 24–42 42–80 80–150 150–270
1.0–0.7 0.7–0.355 0.355–0.18 0.18–0.106 0.106–0.053
0.842 0.515 0.253 0.147 0.082
34.8 23.7 20.6 13.4 7.5
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Fig. 1. Schematic diagram of cold-state experimental apparatus.
and an induced draft fan. The fluidized bed vessel has an overall height of 4.4 m. The dense bed with a crosssection of 400 mm 400 mm and a height of 750 mm is partly fabricated from plexiglass where visible observations of the bed activity are possible. Four windows at the sidewall are arranged to take out the mixture of CS and alumina in the mixed bed condition. In order to prevent leaking air and bed material from the window gaps, a one-piece rubber was used for sealing the windows. The perforated-plate distributor with 3.1% open area contains 48 small nozzles to provide uniform air distribution. During the tests, the pressure drops across the distributor and the bed can be measured by U-tube water manometers. 3.2. Experimental methods The CS by itself cannot fluidize due to its peculiar shape, size and density. However, fluidization behavior of CS can be improved when it mixes with inert bed material. Good mixing is very important to keep CS steady combustion in fluidized bed. The experiments were carried out with the intention of researching the mixing characteristics of CS with alumina bed material. The experimental processes were as follows: adding alumina particles into the cold-state test facility. The initial unexpanded bed height was 350 mm. Then adding CS into the test facility and injecting fluidized gas slowly. As gas velocity increased, the chamber began to expand, and larger bubbles erupting at the bed surface provided passage for CS to enter the bed interior. The fluidized bed was kept at fluidization at different gas velocities for over 5 min to reach the steady state condition, then rapidly stopping the fluidized gas., The mixture in each window from up to down can be divided by a horizontal plate and be gently taken out respectively, separated by a sieve and weighted. Thus, the mixed bed was divided into four sections, and the mass fraction of CS along the bed height was obtained. The relative measurement errors were of 5% for the mass fraction of CS. 3.3. Results and discussion 3.3.1. Effect of gas velocity on mixing characteristics During the tests, the initial weight percentage of CS in the mixture was 2 wt%. The minimum fluidization velocity of the mixture is 0.44 m/s. The influence of fluidizing velocity U on the mixing characteristics of CS with alumina was investigated. It can be found that, as the gas velocity is 0.6 m/s (i.e., the fluidization number N is 1.4), the mixing quality is poor because of lower fluctuation intensity. When the fluidizing velocity is larger than 1.5 m/s (N = 3.4), the mass fraction of CS has more uniform distribution along the bed height. This
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fact can be explained that as the gas velocity increases, the fluctuation intensity goes up, and more and more CS descends into the bed interior. This indicates that the mixing quality will be improved at higher gas velocity. Nevertheless, as the gas velocity is 3.5 m/s (N = 8.0), there will exist a little segregation comparing with U = 2.5 m/s (N = 5.7). The reason is that when the fluidization number is higher than 7, more CS will be blown into the dilute phase zone, which causes that CS cannot adequately mix with bed material particles in the dense bed. As a result, the cold-state tests show that the fluidization number N = 3–7 is suitable for good mixing. 3.3.2. Effect of initial weight percentage of CS on mixing characteristics The experimental conditions were as follows: different initial weight percentages of CS in the mixture were 1 wt%, 2 wt% and 3 wt%, respectively. The gas velocity was 1.5 m/s. The influence of initial weight percentage of CS on mixing characteristics was investigated. The results show that, as the initial weight percentage of CS increases, there will have a little more segregation. It can be explained that with increasing the initial weight percentage of CS, the interactive behavior of CS by them becomes stronger, and the mixing intensity of CS with bed material declines. Therefore, in order to attain adequate mixing, the initial unexpanded bed should have more bed material to keep a certain height. However, the initial bed material cannot be more than an optimum height, which will consume more energy by air blower to overcome the resistance pressure of bed material. 3.3.3. Minimum fluidization velocity of binary mixture The minimum fluidization velocity, Umf, of a binary mixture is a function of two types of particles and their relative concentrations [16–18]. Aznar et al. [14] have concluded that no satisfactory equations are available for predicting the minimum fluidization velocity for the mixture of biomass fuel and bed material. The value of Umf can be obtained preferentially through the experiments. The curves of bed pressure drop DP across the chamber against superficial gas velocity U while decreasing bed velocity at different cases are shown in Fig. 2. The minimum fluidization velocity can be defined as the point of intersection of the lines of bed pressure drop versus gas velocity at complete fluidization (horizontal line) and during the packed bed state [19–21]. In this study, the initial bed height is 250 mm. As seen from Fig. 2, compared with pure alumina of the Umf 0.32 m/s, the Umf value of the mixture of 2 wt% CS is 0.44 m/s, and the Umf value of the mixture of 3 wt% CS is 0.52 m/s. It seems that with increasing the void fraction of the mixture corresponding to the higher weight percentage of CS, the minimum fluidization velocity of the mixture goes up. In the case of CS in a single component system, the Umf may not exist because CS with higher void fraction by itself does not fluidize. Additionally, the experiments show that the pressure drop across the bed decreases in the increase of the weight
Bed pressure drop (Pa)
3500 3000 2500 2000 1500 alumina 2 wt% CS 3 wt% CS
1000 500 0.1
0.2
0.3
0.4
0.5
0.6
0.7
Superficial gas velocity (m/s) Fig. 2. The DP–U curves at different cases.
0.8
0.9
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percentage of CS. It can be explained that as the weight percentage of CS increased, the total weight of the mixture dropped at same initial bed height. 4. Combustion characteristics of CS 4.1. Experimental CFB facility The CS combustion apparatus with a heat input of 0.5 MW is illustrated schematically in Fig. 3. The system is basically composed of a Roots blower, an under-bed start-up burner, a wind box, an air distributor, a combustor, a cyclone, a loop seal, a water cooling system, a cyclone dust collector, an induced fan, a CS feeder and an over-bed coal screw feeder for the research on biomass co-firing later. The design of biomass feeder is very important. The sketch map of CS feeder apparatus is plotted in Fig. 4. The required feed rate of CS is provided by the corresponding rotational speed of the feeder screw. In a series of trials, the set of CS input apparatus used in the tests can feed continually, adjust the feed rate flexibly and keep no leaking air or flue gas. The combustor consists of a dense phase zone of 300 mm in inner diameter and a dilute phase zone of 400 mm in inner diameter. The fluidized bed vessel has an overall height of 12 m. The residence time of gas and particles in the combustor can be kept to at least 2 s for normal fluidizing velocity in the CFB. The whole combustor was made of stainless steel 2520 with an outer insulation layer of 200 mm thick. The refractory layer of 50 mm thick is covered on the inner surface of the dense bed section. There are nine water jackets on the dilute phase section. The fluidized bed located at a height of 1.8 m is used for the secondary air. The temperature distribution along the bed height was continuously measured by shielded thermocouples of type K. The bed temperature measured in the tests is the point temperature along the central line of the
Fig. 3. Schematic diagram of the 0.5 MWth CFB test facility.
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5 6 7 4 8
3
9 2 10
1
11
Fig. 4. Sketch map of CS feeder apparatus.
combustor. The thermocouples used in the experiments are not the suction pyrometers. However, the radiation loss from the thermocouples to the furnace wall has been compensated by thermometer correction. The relative measurement errors were of about 1% for bed temperature. The pressure drops across the distributor and bed material were measured by U-tube water manometers. The pressure drop reduction in the bed suggests bad fluidization is a sign of agglomeration problem in the bed [22–24].
4.2. Experimental methods and objectives Alumina bed material was fed into the furnace in advance, and the static bed height was 500 mm. The CFB was heated first with diesel oil to 500 °C. Then CS stored in the bunker was fed into the furnace. According to the requirements of the bed temperature versus time, the input rate of CS can be controlled by adjusting rotational speed of the feeder screw. CS contains volatile matter as high as 68%, so that the ignition temperature of CS is much lower than that of coal. Thermogravimetric studies show that CS can be ignited easily at 262 °C, its devolatilization and combustion are very prompt, and char is oxidized up to around 505 °C. Hence, CS has higher thermochemical reactivity than coal. While CS was fed into the furnace, the diesel oil supply was gradually decreased. As the combustion was self-sustained at the temperature of the dense bed 700 °C, the diesel oil supply was stopped. After the steady state was reached, the temperature profiles along the bed height were continuously observed and recorded. The steady state condition criteria are to have steady temperature profile, pressure drop and circulation rate along the riser. Because the combustion apparatus has nine water jackets on the dilute phase section, based on the calculation of system energy balance, the input rate of CS with low gross calorific value 17.3 MJ/kg was about 110 kg/h on an average. According to cold experimental results as the fluidizing velocity is 4.5 m/s (N = 10.2), there will exist a little more segregation. Therefore, the important aim of combustion experiments is to examine if the dense bed can keep steady state firing for pure CS at fluidizing velocity 4.5 m/s which is the normal fluidizing velocity in the CFB. In addition, another aim of experiments is to observe the bed temperature profiles with different fluidizing velocities, secondary air flow rates and gas flow rates to the loop seal, as well as the agglomeration characteristics of alumina bed material in the CFB. Eight runs were conducted with CS combustion for various operating conditions in this study. Excess air ratios, air split ratios and gas flow rates to the loop seal are given in Table 5. Temperature along the combustor height as well as O2, CO, SO2 and NO concentrations in stack gas were measured. The relative measurement errors are believed to be below the following values: 1% for O2 and 3% for other compositions. The
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Table 5 Cases of CS combustion (CS feed rate 110 kg/h Run no.
Gas flow rate to the loop seal (Nm3/h)
Excess air ratio
Qprimary (Nm3/h)
Qsecondary (Nm3/h)
Air split ratio
1 2 3 4 5 6 7 8
5 5 5 5 5 5 0 8
1.09 1.23 1.32 1.10 1.32 1.54 1.32 1.32
216 267 300 267 267 267 300 300
200 200 200 150 235 318 200 200
1:0.93 1:0.75 1:0.67 1:0.56 1:0.88 1:1.19 1:0.67 1:0.67
effects of different operating conditions on these variables were studied. The duration of the test was about 26 h. 4.3. Results and discussion 4.3.1. Variables affecting the bed temperature profiles in combustor The hot experiments show that the burning of volatiles mostly takes place in the dilute phase region due to the high volatile content of CS. Thus the primary air and secondary air have a significant effect on the bed temperature profiles. During the tests the dense bed temperature can be controlled in the range of 830– 880 °C. Because the pressure fluctuation was very limited, there was no bottom ash drained out. It indicates that the dense bed can keep steady state firing at higher fluidizing velocity. Though there will exist a little more segregation in the mixing of CS with alumina at fluidizing velocity 4.5 m/s (N = 10.2) on the cold-state tests, but the hot experiments show that slight segregation has little effect on the steady combustion of the dense bed in the CFB. Apart from above considerations, the uniform temperature distribution along the whole bed height is also very important in the CFB. Some authors have studied that the combustion of coffee husk in a 150 mm diameter FBC. There was increased freeboard combustion during over-bed feeding which led to an increase in freeboard temperature within the region near the feed point, whereas the bed temperature fell [25]. However, for CS with 10–100 mm length, the temperature in the CFB combustor remained more or less the same in all positions, which can be explained that the mixing of long CS and bed material is relatively better. 4.3.1.1. Effect of fluidizing velocity on the temperature profiles in combustor. The experimental conditions were as follows: the flow rate of secondary air 200 Nm3/h, and the gas flow rate to the loop seal 5 Nm3/h. Because the fluidizing velocity of industrial scale CFB is around 4 m/s, in the tests the fluidizing velocities were 3.5, 4.0 and 4.5 m/s, respectively. The temperature profiles in the combustor during the trials with different fluidizing velocities are shown in Fig. 5. The sign of Qprimary denotes the flow rate of primary air, and the sign of Udense denotes the fluidizing velocity. As can be seen from Fig. 5, the dense phase temperature is ranged in 855– 870 °C. As the fluidizing velocity increases from 3.5 m/s to 4.5 m/s, it is rather clear that the dense phase temperature drops. Meanwhile, the dilute phase temperature increases and becomes more uniform. This suggests that as the fluidizing velocity increases, the strong combustion intensity zone will move to the dilute phase. Because of the heat loss across the walls, the temperature profiles over 4 m dilute phase decrease along the combustor height. 4.3.1.2. Effect of secondary air flow rate on the temperature profiles in combustor. Due to the need to maintain the temperature in the combustion furnace below the melting point of CS ash, high concentrations of the unburnt pollutants may be expected in the flue gas. The volatiles escaping from the dense phase can be burnt using the secondary air. Experimental conditions were as follows: the flow rate of the primary air 267 Nm3/h, the fluidizing velocity 4 m/s, and the gas flow rate to the loop seal 5 Nm3/h. According to different excess air ratios, the secondary air flow rates were 150 Nm3/h, 235 Nm3/h and 318 Nm3/h, respectively. The effect of the secondary air flow rate
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900
Temperature (°C)
875 850 825 800 775 Qprimary=216Nm3/h, Udense=3.5m/s
750
Qprimary=267Nm3/h, Udense=4m/s Qprimary=300Nm3/h, Udense=4.5m/s
725 700 0
4
2
6
8
10
12
Bed height above air distributor (m) Fig. 5. The effect of fluidizing velocity on the bed temperature profiles.
on the bed temperature profiles is shown in Fig. 6. The sign of Qsecondary denotes the flow rate of the secondary air, and the sign of Udilute denotes the flue gas velocity in the dilute phase. With increasing the secondary air flow rate up to 235 Nm3/h, the bed temperature obviously increases and is driven to be more uniform. Nevertheless, as the secondary air flow rate grows further from 235 Nm3/h to 318 Nm3/h, the temperature of the combustor along the bed height will significantly drop. As a result, there is an optimum secondary air flow rate at which corresponding bed temperature is relatively high and uniform as well as the combustion efficiency goes up. There are two reasons for the effect of secondary air flow rate on the bed temperature. On one hand, the oxygen flow can increase with the increase in the secondary air flow rate, which can make CS be burnt completely. On the other hand, when the flow of the secondary air is excess, it will cool the flue gas which results that the bed temperature declines and the CO emission in the flue gas increases. In conclusion, with the aim of making the boiler furnace keep the normal CFB temperature, the secondary air flow rate must be reasonable. At the experimental conditions as the air split ratio is 1: 0.88, the bed temperature is relatively high and the CO emission is lower. 900 875
Temperature (°C)
850 825 800 775 Qsecondary =150Nm3/h, Udilute =2.7m/s Qsecondary =235Nm3/h, Udilute =4.2m/s
750
Qsecondary =318Nm3/h, Udilute =4.9m/s
725 700 0
2
4
6
8
10
12
Bed height above air distributor (m) Fig. 6. The effect of secondary air on the bed temperature profiles.
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900 875
Temperature (°C)
850 825 800 775 without circulation Qloop seal =5Nm3/h Qloop seal =8Nm3/h
750 725 700 0
2
4
6
8
10
12
Bed height above air distributor (m) Fig. 7. The effect of gas flow to the loop seal on the bed temperature profiles.
4.3.1.3. Effect of air flow to the loop seal on the temperature profiles in combustor. The experimental conditions were as follows: the flow of the primary air 300 Nm3/h, and the flow of secondary air 200 Nm3/h. Fig. 7 depicts the axial temperature profiles in the combustor under different gas flow rates to the loop seal, which can reflect different solid circulation rates. The sign of Qloop seal denotes the gas flow rate to the loop seal. As shown in Fig. 7, as the gas flow rate increases to 5 Nm3/h, the temperature of the dense phase bed decreases, and the temperature of the dilute phase region sharply increases. In general, the bed temperature is driven to be uniform. Nonetheless, with the further increase to 8 Nm3/h in the gas flow rate, the whole bed temperature declines including the dense phase and dilute phase regions. The results show that the temperature profiles on the combustor are strongly affected by solid circulation rate. The reason is that the circulating solids can carry a large amount of heat, and they affect the temperature profiles in the combustor. Therefore, in order to assure combustion steady, the gas flow rate to the loop seal should be also reasonable. 4.3.2. Emission characteristics In all the test runs, due to small Cl content in CS, HCl emission from the CFB combustion system is considered to be negligible. Primary pollutants formed are CO, SO2 and oxides of nitrogen (principally NO). CO is the product of incomplete combustion which largely controlled by stoichiometry control. With the purpose of raising the combustion efficiency, reasonable measures should be taken to diminish the CO emission. In this study, the NO2 concentrations are less than 1 ppm in all tests. The N2O formation in CS combustion is relatively small or negligible. NO formation from the oxidation of fuel N is the most important mechanism in biomass combustion units. The N content of CS is relatively low, and the bed temperature of CFB is below 900 °C so that low NO emission for CS combustion in the CFB may be expected. The analysis of the flue gas was carried out on the gas stream exiting from the cyclone dust collector. After the gas was passed through a filtering system, O2, CO, CO2, SO2 and NO concentrations were measured. The emission results of CS combustion at different operating conditions are given in Table 6. The emissions of oxides of sulfur and nitrogen arise predominantly from nitrogen and sulfur in the fuel. As seen from Table 6, SO2 emission changes between 32 ppm and 55 ppm. NO emission ranges from 110 ppm to 153 ppm at the basis of oxygen concentration of 6% in volume in flue gas. The pollutant emissions for firing CS in the CFB are considered to be quite good. Additionally, from the table it is observed that there is a close link between the oxygen and NO emission. To reduce the NO emission, reasonable measures such as air staging can be taken to improve the operating conditions. 4.3.3. Combustion efficiency Based on the CO emission and unburned carbon content in fly ash, the combustion efficiency can be quantified for biomass fuels fired under different operating conditions. When firing CS in the CFB, the heat loss
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Table 6 Emissions of CS combustion (based on 6% O2) Run no.
O2 (%)
CO (%)
CO2 (%)
SO2 (ppm)
NO (ppm)
g
1 2 3 4 5 6 7 8
4.1 5.4 6.7 5.3 6.1 7.3 6.7 5.6
0.131 0.107 0.094 0.084 0.032 0.052 0.097 0.105
8.720 12.942 15.838 8.487 16.810 10.560 15.734 16.210
55 46 32 52 50 54 36 40
110 119 141 113 127 153 147 145
98.52 99.18 99.41 99.02 99.81 99.51 99.39 99.36
Table 7 Analysis of CS ash Composition
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
TiO2
P2O5
Other
wt%
6.33
3.30
1.57
17.33
9.02
4.03
33.00
0.16
7.12
18.14
owing to unburned carbon contained in the particulate matter is neglected, because carbon in the fly ash is limited. The combustion efficiency (g) is commonly calculated as follows: g¼
CO2 ½% CO½% þ CO2 ½%
ð1Þ
Table 6 shows the combustion efficiencies for CS in the CFB. The combustion efficiencies for firing CS in the CFB range from 98.52% to 99.81%. As shown in Table 6, the excess air ratio of about 1.3 and air split ratio of 1:0.88 seem to be optimum to provide highly efficient combustion of CS in the CFB. Taking into consideration the above discussion, it can be accepted that the application of CFB boiler to burn pure CS with 10– 100 mm length is feasible. 4.3.4. Bed material agglomeration, sintering and deposition Though the composition of biomass fuels varies with plant types and growth conditions, biomass ashes are normally dominated by silicon, calcium and potassium [26,27]. The standard analysis of CS ash is depicted in Table 7. It shows that the K2O content in CS ash is quite high which can cause ash sintering. Biomass ash
Fig. 8. Photographs of silica sand and agglomeration material after 3 h combustion.
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Fig. 9. Photographs of alumina bed material before and after combustion.
sintering may be serious problem in the CFB combustors. It greatly contributes to both bed agglomeration in the furnace and deposit formation in the cyclone and the tail convective heating surfaces [28,29]. Previous studies on the agglomeration characteristics of silica sand bed material in the CFB were performed. The photographs of silica sand and agglomeration material after 3 h run are shown in Fig. 8. From the figures it is observed that using silica sand as bed material, the combustion of CS results in serious bed agglomeration at the normal fluidized bed temperature. The interaction between silica sand and alkaline materials such as potassium in CS ash is known to cause the bed agglomeration. Therefore, with the aim of diminishing the bed agglomeration, silica sands are frequently changed to avoid the accumulation of CS ash in the bed. The bed agglomeration is often the reason for the unscheduled shutdown of heat and/or power boilers. The duration of the hot experiments was about 26 h using alumina as bed material. The bed pressure drop was observed to remain stable. After the end of tests the bed material was removed from the combustor and was inspected to detect the agglomeration phenomena. To attain comparable results, the photographs of alu-
Fig. 10. Photographs of CS ash.
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mina bed material prior to and after combustion are shown in Fig. 9. As seen from Fig. 9, the combustion tests were successfully carried out for 26 h at the dense bed temperature as high as 870 °C without any bed material agglomeration. The results confirm that alumina is difficult to react with alkali metals from CS ash, and alumina is favorable for firing CS in the CFB. Additionally, heat and/or power plants fired by some biomass fuels have been experiencing great problems with sintering and deposition due to the inorganic metal constituents such as alkali metals [30–32]. The potassium salts play a significant role in deposition because they can act as glue bonding the individual fly ash particles together. Furthermore, the presence of KCl in deposits is expected to play a major role in the corrosion of the superheater tubes in biomass-fired boilers. Deposit formation is considered as an important problem challenging the improvement of biomass combustion performance. After the experiments, CS ash samples were observed. The photographs of CS ash are shown in Fig. 10. It can be found that CS ash is very adhesive like other biomass fuels. Therefore, in the industrial scale plants, some precautionary measures should be taken to diminish deposit formation. 5. Conclusions The intense motion of the fluidized bed makes it possible to fire a wide range of fuels having different sizes, shapes, moisture contents and heating values. In this paper, several issues concerning CS combustion in the CFB have been discussed. The objective is to give more information related to the mixing characteristics of CS with bed material as well as the combustion characteristics of firing CS in the CFB. CS with 10–100 mm length alone cannot fluidize. The inert particle medium in the fluidized bed is essential to fluidize CS. It is found that CS can fluidize and mix well with a certain size distribution of alumina at the fluidization number 3–7. However, as the fluidizing number is more than 7, there will exist a little more segregation. These studies provide the supporting data needed for CS combustion in the fluidized bed. The combustion characteristics of pure CS with 10–100 mm length have been studied in a CFB combustor. The fluidizing medium was alumina. Although as the fluidizing velocity is 4.5 m/s (N = 10.2), there will exist a little more segregation in the cold-state tests, yet the dense bed can keep steady state combustion for pure CS in the CFB. A fairly steady dense bed temperature between 830 °C and 880 °C has been obtained. Due to the high volatile content of CS, a significant amount of combustion takes place in the dilute phase. The results show that as the fluidizing velocity increases, the temperature of the dense phase decreases. Meanwhile, the temperature of the dilute phase increases and becomes more uniform. To assure combustion steady, the secondary air flow and gas flow to the loop seal should be controlled reasonably. During the tests, the concentrations of major gaseous pollutants in flue gas were measured. The results show that SO2 emission varies from 32 ppm to 55 ppm, and NO emission ranges from 110 ppm to 153 ppm at the basis of oxygen concentration of 6% in volume in flue gas. The highly efficient combustion, over 98.5%, of CS combustion in the CFB is achieved. In this study, the excess air ratio of around 1.3 and air split ratio of 1:0.88 seem to be optimum to provide high combustion efficiency of CS. The reaction of alkali metals with silica sand can form alkali silicates that can be melt or soften at low temperatures below 700 °C. The alkali silicates could be a potential problem for bed material agglomeration. After 26 h hot experiments, alumina bed material is difficult to agglomerate due to its specific properties. The experimental results provide a reasonable database for optimal design and operation of firing pure CS with 10–100 mm length when applied to industrial scale CFB boilers. Acknowledgements The research work was supported by National Basic Research Program of China (973 Program: 2007CB210208) and its contribution is gratefully acknowledged by the authors. References [1] Gustavsson L, Bo¨rjesson P, Johansson B, Svenningsson P. Reducing CO2 emission by substituting biomass for fossil fuels. Energy 1995;20(11):1097–113.
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¨ . Thermo-chemical conversion of brown coal and biomass in a pressurized fluidized bed gasifier with hot gas ¨ nal O [2] Jong WD, U filtration using ceramic channel filters: measurements and gasifier modelling. Appl Energy 2003;74(1–3):425–37. [3] Kristensen EF, Kristensen JK. Development and test of small-scale batch-fired straw boilers in Denmark. Biomass Bioenergy 2004;26(6):561–9. [4] Clarke KL, Pugsley T, Hill GA. Fluidization of moist sawdust in binary particle systems in a gas–solid fluidized bed. Chem Eng Sci 2005;60(24):6909–18. [5] Fang M, Yang L, Chen G, Shi Z, Luo Z, Cen K. Experimental study on rice husk combustion in a circulating fluidized bed. Fuel Process Technol 2004;85:1273–82. [6] Topal H, Atimtay AT, Durmaz A. Olive cake combustion in a circulating fluidized bed. Fuel 2003;82(9):1049–56. [7] Lin W, Johansen KD, Frandsen F. Agglomeration in bio-fuel fired fluidized bed combustors. Chem Eng J 2003;96(3):171–85. [8] Jime´nez T, Turchiuli C, Dumoulin E. Particles agglomeration in a conical fluidized bed in relation with air temperature profiles. Chem Eng Sci 2006;61(18):5954–61. [9] Sjosten J, Golriz MR, Grace JR. Further study on the influence of particle coating on fluidized bed heat transfer. Int J Heat Mass Transfer 2006;49(22):3800–6. [10] Shimizu T, Han J, Choi S, Kim L, Kim H. Fluidized-bed combustion characteristics of cedar pellets by using an alternative bed material. Energy Fuels 2006;20:2737–42. [11] 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 Bioenergy 2001;20(6):459–70. [12] Fernandez L, Escalada C, Murillo L, Carrasco G. Combustion in bubbling fluidized bed with bed material of limestone to reduce the biomass ash agglomeration and sintering. Fuel 2006;85:2081–92. [13] Werther J, Saenger M, Hartge E, Ogada T, Siagi Z. Combustion of agricultural residues. Prog Energy Combust Sci 2000;26(1):1–27. [14] Aznar MP, Gracia FA, Corella J. Minimum and maximum velocities for fluidization for mixtures of agricultural and forest residues with second fluidized solid. Int Chem Eng 1992;32:103–13. [15] Ekinci E, Atakul H, Tolay M. Detection of segregation tendencies in a fluidized bed using temperature profiles. Powder Technol 1990;61:185–92. [16] Coltters R, Rivas AL. Minimum fluidation velocity correlations in particulate systems. Powder Technol 2004;147(3):34–48. [17] Wu W, Saxena SC. Mixing characteristics of light paper pellets in a sand fluidized bed. Energy 1997;22(6):615–9. [18] Rao TR, Bheemarasetti JV. Minimum fluidization velocities of mixtures of biomass and sands. Energy 2001;26(6):633–44. [19] Sun Q, Lu H, Liu W, He Y, Yang L, Gidaspow D. Simulation and experiment of segregating/mixing of rice husk–sand mixture in a bubbling fluidized bed. Fuel 2005;84(15):1739–48. [20] Chirone R, Miccio F, Scala F. On the relevance of axial and transversal fuel segregation during the FB combustion of a biomass. Energy Fuels 2004;18:1108–17. [21] Scala F, Chirone R. Characterization and early detection of bed agglomeration during the fluidized bed combustion of olive husk. Energy Fuels 2006;20:120–32. [22] Ergudenler A, Ghaly AE. Agglomeration of silica sand in a fluidized bed gasifier operation on wheat straw. Biomass Bioenergy 1993;4(2):135–47. [23] Chirone R, Miccio F, Scala F. Mechanism and prediction of bed agglomeration during fluidized bed combustion of a biomass fuel: effect of the reactor scale. Chem Eng J 2006;123(3):71–80. [24] Kate W, Peter O, Ole H. Cofiring coal and straw in a 150 MWe power boiler. Biomass Bioenergy 2000;19:395–409. [25] Permchart W, Kouprianov VI. Emission performance and combustion efficiency of a conical fluidized-bed combustor firing various biomass fuels. Bioresour Technol 2004;92(1):83–91. [26] Nielsen HP, Baxter LL, Sclippab G, Morey C, Frandsen FJ, Johansen KD. Deposition of potassium salts on heat transfer surfaces in straw-fired boiler: a pilot-scale study. Fuel 2000;79(2):131–9. [27] Skrifvars BJ, Backman R, Hupa M. Characterization of the sintering tendency of ten biomass ashes in FBC conditions by a laboratory test and by phase equilibrium calculations. Fuel Process Technol 1998;56(2):55–67. [28] Skrifvars BJ, Hupa M, Backman R, Hiltunen M. Sintering mechanisms of FBC ashes. Fuel 1994;73(2):171–6. [29] Ergudenler A, Ghaly AE. Agglomeration of alumina sand in a fluidized bed straw gasifier at elevated temperatures. Bioresour Technol 1993;43(3):259–68. [30] Vamvuka D, Zografos D. Predicting the behaviour of ash from agricultural wastes during combustion. Fuel 2004;83(14–15):2051–7. [31] Xie W, Xie Y, Pan WP, Riga A. Characterization of the ash deposits from AFBC system using thermal techniques. Thermochim Acta 2000;357–358:231–8. [32] Hansen LA, Nielsen HP, Frandsen FJ. Influence of deposit formation on corrosion at a straw-fired boiler. Fuel Process Technol 2000;64(1–3):189–209.
APPLIED ENERGY Applied Energy 85 (2008) 1027–1040 www.elsevier.com/locate/apenergy
Experimental study on cotton stalk combustion in a circulating fluidized bed Zhi-Ao Sun *, Bao-Sheng Jin, Ming-Yao Zhang, Ren-Ping Liu, Yong Zhang Key Laboratory of Clean Coal Power Generation, Combustion Technology of Ministry of Education, Southeast University, Nanjing 210096, China Received 20 November 2007; received in revised form 27 February 2008; accepted 28 February 2008 Available online 14 April 2008
Abstract This paper summarizes the results of an experimental study on cotton stalk (CS) combustion in a circulating fluidized bed. The mixing and fluidizing characteristics of binary mixture of CS with 10–100 mm in length and alumina bed material with a certain size distribution in a cold test facility were studied. The results show that CS by itself cannot fluidize, and adding inert bed material can improve the fluidization condition. CS can mix well with alumina at fluidization number N = 3–7. As N is more than 7, there will exist a little more segregation. The study concerning combustion characteristics of pure CS was performed on a circulating fluidized bed with a heat input of 0.5 MW. The effects of fluidizing velocity, secondary air flow and gas flow to the loop seal on the bed temperature profiles were investigated. Although there is a little more segregation at N higher than 7 in the cold tests, the hot experimental results indicate that slight segregation has little effect on the steady combustion of the dense region. In this study, the concentrations of major gaseous pollutants (CO, SO2 and NO) in flue (stack) gas were measured. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Cotton stalk; Circulating fluidized bed; Bed material; Mixing characteristics; Combustion characteristics; Pollutant emissions
1. Introduction Energy demand in the world continues to increase with the increase in population and economic development. Biomass is a potentially CO2-neutral and renewable energy resource. As an alternative fuel it has attracted much attention worldwide in the recent years [1,2]. At present, biomass is converted into heat and electricity most often by combustion. Combustors for biomass fuels are predominantly grate-fired systems, suspension burners or fluidized bed systems. All these types of combustors are being used in industry. Grate-fired combustors were the most versatile units in the mid-1980s, but the fluidized bed systems have become serious competitors in the last decade [3]. Among the combustion technologies, the fluidized bed combustion seems to be the most suitable for con*
Corresponding author. Tel./fax: +86 25 83795508. E-mail address: [email protected] (Z.-A. Sun).
0306-2619/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2008.02.018
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verting biomass into energy, because of its inherent advantages of fuel flexibility, low operating temperature and low pollution emission. A significant amount of research work has been carried out on the fluidized bed combustor (FBC) of biomass fuels [4–6]. Biomass fuels cannot easily fluidize due to their irregular shapes. For proper fluidization, inert bed material is used to facilitate fluidization of biomass. It acts as a heat transfer medium in the reactor as well. Biomass includes a large variety of different fuels with different chemical compositions and combustion characteristics. Generally, biomass fuels have a high content of hydrogen and oxygen, a high volatile content and a low heating value. Meanwhile, some biomass fuels have more potassium, sodium, calcium and chlorine than coal. The alkaline materials are directly related to bed agglomeration, sintering and deposition. Previous studies have shown that the combustion of biomass fuels results in bed agglomeration at the normal fluidized bed temperature [7–9]. Because the interaction between silica sand bed material and alkaline materials such as potassium in the ash causes the bed agglomeration, frequent bed material changes are often used to avoid the accumulation of ash in the bed as a precautionary measure. In the other hand, in order to diminish the bed agglomeration, the use of silica-free alternative bed material such as alumina, dolomite, limestone and kaolin has also been proposed [10,11]. Cotton stalk (CS) is one of the most viable biomass fuels in China. As one kind of renewable energy resource, it has attracted increasing research and development efforts. If an efficient method is available, CS can be converted to useful energy to meet the electrical power requirements in the rural area. In the literature, several studies have been reported on CS combustion in the FBC for energy production, and CS has either been used alone or in combination with coal [12,13]. However, it is pelletized to a size range less than 10 mm in length. This paper mainly introduces an experimental research on mixing and combustion characteristics of pure CS with 10–100 mm in length in a circulating fluidized bed (CFB), and the effect of the mixing behavior on combustion characteristics of pure CS in CFB hot experiments. In this study, alumina of weak acidic property was used as alternative bed material. The mixtures of solid particles of different sizes and different densities tend to separate in vertical direction under fluidized condition. The segregation behavior of biomass fuel is of practical importance because the vertical location of biomass fuel influences the combustion efficiency of volatile matter. Aznar et al. [14] 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. [15] tested the size and density on the segregation behavior determined from temperature distribution. Therefore, it is very important that the research on mixing and fluidizing characteristics of CS with bed material in the cold tests. Technical enhancements in the contribution of biomass to commercial energy needs are focused on improving the combustion efficiency and environmental impacts of biomass conversion. CO, SO2 and oxides of nitrogen (principally NO) are the major harmful pollutants emitted from biomass combustion in fluidized bed systems. CO emission strongly affecting the combustion efficiency is a function of operating variables, such as excess air ratio and air split ratio. SO2 and NO emissions which arise predominantly from nitrogen and sulfur in the fuel are weakly dependent on the combustion conditions. 2. Properties of CS and alternative bed material 2.1. Properties of CS There is a crushing unit that can crush the CS to 10–100 mm length. In this study, there are three sizes of CS in the fuel mixture with length ranges of 10–40 mm, 40–70 mm and 70–100 mm, each portion accounting for 1/3 of the total mixture by weight. CS is very light, with natural packing density of 100–130 kg/m3 and real density of about 460 kg/m3. For comparison, the natural packing density and real density of bituminous coals are in the ranges of 800–900 kg/m3 and around 1800 kg/m3, respectively. The low density of CS complicates their processing, transportation, storage and combustion. CS is characterized by a high volatile matter content and low gross calorific value of 17.3 MJ/kg. The analysis data of CS are given in Table 1. For the proximate analysis the relative errors are estimated to be below 5% for moisture, and below 3% for other components; for the ultimate analysis the relative errors are believed to be below the following values: 1% for carbon and oxygen, 3% for other elements. The ICP analysis data of CS are listed in Table 2. From the table it is observed that
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Table 1 CS analysis Proximate analysis (wt% on air dry basis) Moisture Ash Volatile Fixed carbon
Ultimate analysis (wt% on air dry basis) 8.41 1.75 67.97 21.87
Carbon Hydrogen Oxygen Nitrogen Sulfur
44.58 5.46 39.43 0.23 0.14
Table 2 ICP analysis of CS Element
K
Ca
Cl
Mg
P
Si
Na
Al
Fe
Ti
wt%
0.99
0.25
0.29
0.12
0.096
0.086
0.11
0.0086
0.0071
0.003
K, Na and Ca contents in CS are very high, which are directly related to bed agglomeration, sintering and deposition for CS combustion in CFB. 2.2. Characteristics of alternative bed material The experimental studies of Shimizu et al. [10] showed that alumina sand was more favorable for use in a fluidized bed combustor during biomass combustion. On the other hand, alumina is relatively cheap and can be easily purchased in China. Accordingly, in order to diminish bed agglomeration, alumina of weak acidic property was used as alternative bed material in the tests. The composition of silica sand and alumina is listed in Table 3. 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 are about 2490 kg/m3 and 1330 kg/m3, respectively. Preliminary experiments were conducted to choose the optimum particle size, which was based on the operational demands in the CFB combustor. The size distribution of alumina employed is given in Table 4. 3. Mixing characteristics of CS with alumina 3.1. Cold-state experimental apparatus A cold-state test facility used to research the mixing and fluidization characteristics of CS with alumina is sketched in Fig. 1. It mainly consists of an air blower, a flowmeter, a fluidized bed, an off-gas cleanup system Table 3 Composition of silica sand and alumina (wt%) Composition
SiO2
Al2O3
Fe2O3
TiO2
CaO + MgO
Na2O + K2O
Silica sand Alumina
98.61 13.07
0.14 79.24
0.08 1.75
0.01 4.10
0.06 0.45
0.11 0.53
Table 4 Particle-size distribution of alumina Tyler mesh
Sieve size range (mm)
Average size (mm)
Solids fraction retained on the sieve (wt%)
16–24 24–42 42–80 80–150 150–270
1.0–0.7 0.7–0.355 0.355–0.18 0.18–0.106 0.106–0.053
0.842 0.515 0.253 0.147 0.082
34.8 23.7 20.6 13.4 7.5
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Fig. 1. Schematic diagram of cold-state experimental apparatus.
and an induced draft fan. The fluidized bed vessel has an overall height of 4.4 m. The dense bed with a crosssection of 400 mm 400 mm and a height of 750 mm is partly fabricated from plexiglass where visible observations of the bed activity are possible. Four windows at the sidewall are arranged to take out the mixture of CS and alumina in the mixed bed condition. In order to prevent leaking air and bed material from the window gaps, a one-piece rubber was used for sealing the windows. The perforated-plate distributor with 3.1% open area contains 48 small nozzles to provide uniform air distribution. During the tests, the pressure drops across the distributor and the bed can be measured by U-tube water manometers. 3.2. Experimental methods The CS by itself cannot fluidize due to its peculiar shape, size and density. However, fluidization behavior of CS can be improved when it mixes with inert bed material. Good mixing is very important to keep CS steady combustion in fluidized bed. The experiments were carried out with the intention of researching the mixing characteristics of CS with alumina bed material. The experimental processes were as follows: adding alumina particles into the cold-state test facility. The initial unexpanded bed height was 350 mm. Then adding CS into the test facility and injecting fluidized gas slowly. As gas velocity increased, the chamber began to expand, and larger bubbles erupting at the bed surface provided passage for CS to enter the bed interior. The fluidized bed was kept at fluidization at different gas velocities for over 5 min to reach the steady state condition, then rapidly stopping the fluidized gas., The mixture in each window from up to down can be divided by a horizontal plate and be gently taken out respectively, separated by a sieve and weighted. Thus, the mixed bed was divided into four sections, and the mass fraction of CS along the bed height was obtained. The relative measurement errors were of 5% for the mass fraction of CS. 3.3. Results and discussion 3.3.1. Effect of gas velocity on mixing characteristics During the tests, the initial weight percentage of CS in the mixture was 2 wt%. The minimum fluidization velocity of the mixture is 0.44 m/s. The influence of fluidizing velocity U on the mixing characteristics of CS with alumina was investigated. It can be found that, as the gas velocity is 0.6 m/s (i.e., the fluidization number N is 1.4), the mixing quality is poor because of lower fluctuation intensity. When the fluidizing velocity is larger than 1.5 m/s (N = 3.4), the mass fraction of CS has more uniform distribution along the bed height. This
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fact can be explained that as the gas velocity increases, the fluctuation intensity goes up, and more and more CS descends into the bed interior. This indicates that the mixing quality will be improved at higher gas velocity. Nevertheless, as the gas velocity is 3.5 m/s (N = 8.0), there will exist a little segregation comparing with U = 2.5 m/s (N = 5.7). The reason is that when the fluidization number is higher than 7, more CS will be blown into the dilute phase zone, which causes that CS cannot adequately mix with bed material particles in the dense bed. As a result, the cold-state tests show that the fluidization number N = 3–7 is suitable for good mixing. 3.3.2. Effect of initial weight percentage of CS on mixing characteristics The experimental conditions were as follows: different initial weight percentages of CS in the mixture were 1 wt%, 2 wt% and 3 wt%, respectively. The gas velocity was 1.5 m/s. The influence of initial weight percentage of CS on mixing characteristics was investigated. The results show that, as the initial weight percentage of CS increases, there will have a little more segregation. It can be explained that with increasing the initial weight percentage of CS, the interactive behavior of CS by them becomes stronger, and the mixing intensity of CS with bed material declines. Therefore, in order to attain adequate mixing, the initial unexpanded bed should have more bed material to keep a certain height. However, the initial bed material cannot be more than an optimum height, which will consume more energy by air blower to overcome the resistance pressure of bed material. 3.3.3. Minimum fluidization velocity of binary mixture The minimum fluidization velocity, Umf, of a binary mixture is a function of two types of particles and their relative concentrations [16–18]. Aznar et al. [14] have concluded that no satisfactory equations are available for predicting the minimum fluidization velocity for the mixture of biomass fuel and bed material. The value of Umf can be obtained preferentially through the experiments. The curves of bed pressure drop DP across the chamber against superficial gas velocity U while decreasing bed velocity at different cases are shown in Fig. 2. The minimum fluidization velocity can be defined as the point of intersection of the lines of bed pressure drop versus gas velocity at complete fluidization (horizontal line) and during the packed bed state [19–21]. In this study, the initial bed height is 250 mm. As seen from Fig. 2, compared with pure alumina of the Umf 0.32 m/s, the Umf value of the mixture of 2 wt% CS is 0.44 m/s, and the Umf value of the mixture of 3 wt% CS is 0.52 m/s. It seems that with increasing the void fraction of the mixture corresponding to the higher weight percentage of CS, the minimum fluidization velocity of the mixture goes up. In the case of CS in a single component system, the Umf may not exist because CS with higher void fraction by itself does not fluidize. Additionally, the experiments show that the pressure drop across the bed decreases in the increase of the weight
Bed pressure drop (Pa)
3500 3000 2500 2000 1500 alumina 2 wt% CS 3 wt% CS
1000 500 0.1
0.2
0.3
0.4
0.5
0.6
0.7
Superficial gas velocity (m/s) Fig. 2. The DP–U curves at different cases.
0.8
0.9
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percentage of CS. It can be explained that as the weight percentage of CS increased, the total weight of the mixture dropped at same initial bed height. 4. Combustion characteristics of CS 4.1. Experimental CFB facility The CS combustion apparatus with a heat input of 0.5 MW is illustrated schematically in Fig. 3. The system is basically composed of a Roots blower, an under-bed start-up burner, a wind box, an air distributor, a combustor, a cyclone, a loop seal, a water cooling system, a cyclone dust collector, an induced fan, a CS feeder and an over-bed coal screw feeder for the research on biomass co-firing later. The design of biomass feeder is very important. The sketch map of CS feeder apparatus is plotted in Fig. 4. The required feed rate of CS is provided by the corresponding rotational speed of the feeder screw. In a series of trials, the set of CS input apparatus used in the tests can feed continually, adjust the feed rate flexibly and keep no leaking air or flue gas. The combustor consists of a dense phase zone of 300 mm in inner diameter and a dilute phase zone of 400 mm in inner diameter. The fluidized bed vessel has an overall height of 12 m. The residence time of gas and particles in the combustor can be kept to at least 2 s for normal fluidizing velocity in the CFB. The whole combustor was made of stainless steel 2520 with an outer insulation layer of 200 mm thick. The refractory layer of 50 mm thick is covered on the inner surface of the dense bed section. There are nine water jackets on the dilute phase section. The fluidized bed located at a height of 1.8 m is used for the secondary air. The temperature distribution along the bed height was continuously measured by shielded thermocouples of type K. The bed temperature measured in the tests is the point temperature along the central line of the
Fig. 3. Schematic diagram of the 0.5 MWth CFB test facility.
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5 6 7 4 8
3
9 2 10
1
11
Fig. 4. Sketch map of CS feeder apparatus.
combustor. The thermocouples used in the experiments are not the suction pyrometers. However, the radiation loss from the thermocouples to the furnace wall has been compensated by thermometer correction. The relative measurement errors were of about 1% for bed temperature. The pressure drops across the distributor and bed material were measured by U-tube water manometers. The pressure drop reduction in the bed suggests bad fluidization is a sign of agglomeration problem in the bed [22–24].
4.2. Experimental methods and objectives Alumina bed material was fed into the furnace in advance, and the static bed height was 500 mm. The CFB was heated first with diesel oil to 500 °C. Then CS stored in the bunker was fed into the furnace. According to the requirements of the bed temperature versus time, the input rate of CS can be controlled by adjusting rotational speed of the feeder screw. CS contains volatile matter as high as 68%, so that the ignition temperature of CS is much lower than that of coal. Thermogravimetric studies show that CS can be ignited easily at 262 °C, its devolatilization and combustion are very prompt, and char is oxidized up to around 505 °C. Hence, CS has higher thermochemical reactivity than coal. While CS was fed into the furnace, the diesel oil supply was gradually decreased. As the combustion was self-sustained at the temperature of the dense bed 700 °C, the diesel oil supply was stopped. After the steady state was reached, the temperature profiles along the bed height were continuously observed and recorded. The steady state condition criteria are to have steady temperature profile, pressure drop and circulation rate along the riser. Because the combustion apparatus has nine water jackets on the dilute phase section, based on the calculation of system energy balance, the input rate of CS with low gross calorific value 17.3 MJ/kg was about 110 kg/h on an average. According to cold experimental results as the fluidizing velocity is 4.5 m/s (N = 10.2), there will exist a little more segregation. Therefore, the important aim of combustion experiments is to examine if the dense bed can keep steady state firing for pure CS at fluidizing velocity 4.5 m/s which is the normal fluidizing velocity in the CFB. In addition, another aim of experiments is to observe the bed temperature profiles with different fluidizing velocities, secondary air flow rates and gas flow rates to the loop seal, as well as the agglomeration characteristics of alumina bed material in the CFB. Eight runs were conducted with CS combustion for various operating conditions in this study. Excess air ratios, air split ratios and gas flow rates to the loop seal are given in Table 5. Temperature along the combustor height as well as O2, CO, SO2 and NO concentrations in stack gas were measured. The relative measurement errors are believed to be below the following values: 1% for O2 and 3% for other compositions. The
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Table 5 Cases of CS combustion (CS feed rate 110 kg/h Run no.
Gas flow rate to the loop seal (Nm3/h)
Excess air ratio
Qprimary (Nm3/h)
Qsecondary (Nm3/h)
Air split ratio
1 2 3 4 5 6 7 8
5 5 5 5 5 5 0 8
1.09 1.23 1.32 1.10 1.32 1.54 1.32 1.32
216 267 300 267 267 267 300 300
200 200 200 150 235 318 200 200
1:0.93 1:0.75 1:0.67 1:0.56 1:0.88 1:1.19 1:0.67 1:0.67
effects of different operating conditions on these variables were studied. The duration of the test was about 26 h. 4.3. Results and discussion 4.3.1. Variables affecting the bed temperature profiles in combustor The hot experiments show that the burning of volatiles mostly takes place in the dilute phase region due to the high volatile content of CS. Thus the primary air and secondary air have a significant effect on the bed temperature profiles. During the tests the dense bed temperature can be controlled in the range of 830– 880 °C. Because the pressure fluctuation was very limited, there was no bottom ash drained out. It indicates that the dense bed can keep steady state firing at higher fluidizing velocity. Though there will exist a little more segregation in the mixing of CS with alumina at fluidizing velocity 4.5 m/s (N = 10.2) on the cold-state tests, but the hot experiments show that slight segregation has little effect on the steady combustion of the dense bed in the CFB. Apart from above considerations, the uniform temperature distribution along the whole bed height is also very important in the CFB. Some authors have studied that the combustion of coffee husk in a 150 mm diameter FBC. There was increased freeboard combustion during over-bed feeding which led to an increase in freeboard temperature within the region near the feed point, whereas the bed temperature fell [25]. However, for CS with 10–100 mm length, the temperature in the CFB combustor remained more or less the same in all positions, which can be explained that the mixing of long CS and bed material is relatively better. 4.3.1.1. Effect of fluidizing velocity on the temperature profiles in combustor. The experimental conditions were as follows: the flow rate of secondary air 200 Nm3/h, and the gas flow rate to the loop seal 5 Nm3/h. Because the fluidizing velocity of industrial scale CFB is around 4 m/s, in the tests the fluidizing velocities were 3.5, 4.0 and 4.5 m/s, respectively. The temperature profiles in the combustor during the trials with different fluidizing velocities are shown in Fig. 5. The sign of Qprimary denotes the flow rate of primary air, and the sign of Udense denotes the fluidizing velocity. As can be seen from Fig. 5, the dense phase temperature is ranged in 855– 870 °C. As the fluidizing velocity increases from 3.5 m/s to 4.5 m/s, it is rather clear that the dense phase temperature drops. Meanwhile, the dilute phase temperature increases and becomes more uniform. This suggests that as the fluidizing velocity increases, the strong combustion intensity zone will move to the dilute phase. Because of the heat loss across the walls, the temperature profiles over 4 m dilute phase decrease along the combustor height. 4.3.1.2. Effect of secondary air flow rate on the temperature profiles in combustor. Due to the need to maintain the temperature in the combustion furnace below the melting point of CS ash, high concentrations of the unburnt pollutants may be expected in the flue gas. The volatiles escaping from the dense phase can be burnt using the secondary air. Experimental conditions were as follows: the flow rate of the primary air 267 Nm3/h, the fluidizing velocity 4 m/s, and the gas flow rate to the loop seal 5 Nm3/h. According to different excess air ratios, the secondary air flow rates were 150 Nm3/h, 235 Nm3/h and 318 Nm3/h, respectively. The effect of the secondary air flow rate
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900
Temperature (°C)
875 850 825 800 775 Qprimary=216Nm3/h, Udense=3.5m/s
750
Qprimary=267Nm3/h, Udense=4m/s Qprimary=300Nm3/h, Udense=4.5m/s
725 700 0
4
2
6
8
10
12
Bed height above air distributor (m) Fig. 5. The effect of fluidizing velocity on the bed temperature profiles.
on the bed temperature profiles is shown in Fig. 6. The sign of Qsecondary denotes the flow rate of the secondary air, and the sign of Udilute denotes the flue gas velocity in the dilute phase. With increasing the secondary air flow rate up to 235 Nm3/h, the bed temperature obviously increases and is driven to be more uniform. Nevertheless, as the secondary air flow rate grows further from 235 Nm3/h to 318 Nm3/h, the temperature of the combustor along the bed height will significantly drop. As a result, there is an optimum secondary air flow rate at which corresponding bed temperature is relatively high and uniform as well as the combustion efficiency goes up. There are two reasons for the effect of secondary air flow rate on the bed temperature. On one hand, the oxygen flow can increase with the increase in the secondary air flow rate, which can make CS be burnt completely. On the other hand, when the flow of the secondary air is excess, it will cool the flue gas which results that the bed temperature declines and the CO emission in the flue gas increases. In conclusion, with the aim of making the boiler furnace keep the normal CFB temperature, the secondary air flow rate must be reasonable. At the experimental conditions as the air split ratio is 1: 0.88, the bed temperature is relatively high and the CO emission is lower. 900 875
Temperature (°C)
850 825 800 775 Qsecondary =150Nm3/h, Udilute =2.7m/s Qsecondary =235Nm3/h, Udilute =4.2m/s
750
Qsecondary =318Nm3/h, Udilute =4.9m/s
725 700 0
2
4
6
8
10
12
Bed height above air distributor (m) Fig. 6. The effect of secondary air on the bed temperature profiles.
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900 875
Temperature (°C)
850 825 800 775 without circulation Qloop seal =5Nm3/h Qloop seal =8Nm3/h
750 725 700 0
2
4
6
8
10
12
Bed height above air distributor (m) Fig. 7. The effect of gas flow to the loop seal on the bed temperature profiles.
4.3.1.3. Effect of air flow to the loop seal on the temperature profiles in combustor. The experimental conditions were as follows: the flow of the primary air 300 Nm3/h, and the flow of secondary air 200 Nm3/h. Fig. 7 depicts the axial temperature profiles in the combustor under different gas flow rates to the loop seal, which can reflect different solid circulation rates. The sign of Qloop seal denotes the gas flow rate to the loop seal. As shown in Fig. 7, as the gas flow rate increases to 5 Nm3/h, the temperature of the dense phase bed decreases, and the temperature of the dilute phase region sharply increases. In general, the bed temperature is driven to be uniform. Nonetheless, with the further increase to 8 Nm3/h in the gas flow rate, the whole bed temperature declines including the dense phase and dilute phase regions. The results show that the temperature profiles on the combustor are strongly affected by solid circulation rate. The reason is that the circulating solids can carry a large amount of heat, and they affect the temperature profiles in the combustor. Therefore, in order to assure combustion steady, the gas flow rate to the loop seal should be also reasonable. 4.3.2. Emission characteristics In all the test runs, due to small Cl content in CS, HCl emission from the CFB combustion system is considered to be negligible. Primary pollutants formed are CO, SO2 and oxides of nitrogen (principally NO). CO is the product of incomplete combustion which largely controlled by stoichiometry control. With the purpose of raising the combustion efficiency, reasonable measures should be taken to diminish the CO emission. In this study, the NO2 concentrations are less than 1 ppm in all tests. The N2O formation in CS combustion is relatively small or negligible. NO formation from the oxidation of fuel N is the most important mechanism in biomass combustion units. The N content of CS is relatively low, and the bed temperature of CFB is below 900 °C so that low NO emission for CS combustion in the CFB may be expected. The analysis of the flue gas was carried out on the gas stream exiting from the cyclone dust collector. After the gas was passed through a filtering system, O2, CO, CO2, SO2 and NO concentrations were measured. The emission results of CS combustion at different operating conditions are given in Table 6. The emissions of oxides of sulfur and nitrogen arise predominantly from nitrogen and sulfur in the fuel. As seen from Table 6, SO2 emission changes between 32 ppm and 55 ppm. NO emission ranges from 110 ppm to 153 ppm at the basis of oxygen concentration of 6% in volume in flue gas. The pollutant emissions for firing CS in the CFB are considered to be quite good. Additionally, from the table it is observed that there is a close link between the oxygen and NO emission. To reduce the NO emission, reasonable measures such as air staging can be taken to improve the operating conditions. 4.3.3. Combustion efficiency Based on the CO emission and unburned carbon content in fly ash, the combustion efficiency can be quantified for biomass fuels fired under different operating conditions. When firing CS in the CFB, the heat loss
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Table 6 Emissions of CS combustion (based on 6% O2) Run no.
O2 (%)
CO (%)
CO2 (%)
SO2 (ppm)
NO (ppm)
g
1 2 3 4 5 6 7 8
4.1 5.4 6.7 5.3 6.1 7.3 6.7 5.6
0.131 0.107 0.094 0.084 0.032 0.052 0.097 0.105
8.720 12.942 15.838 8.487 16.810 10.560 15.734 16.210
55 46 32 52 50 54 36 40
110 119 141 113 127 153 147 145
98.52 99.18 99.41 99.02 99.81 99.51 99.39 99.36
Table 7 Analysis of CS ash Composition
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
TiO2
P2O5
Other
wt%
6.33
3.30
1.57
17.33
9.02
4.03
33.00
0.16
7.12
18.14
owing to unburned carbon contained in the particulate matter is neglected, because carbon in the fly ash is limited. The combustion efficiency (g) is commonly calculated as follows: g¼
CO2 ½% CO½% þ CO2 ½%
ð1Þ
Table 6 shows the combustion efficiencies for CS in the CFB. The combustion efficiencies for firing CS in the CFB range from 98.52% to 99.81%. As shown in Table 6, the excess air ratio of about 1.3 and air split ratio of 1:0.88 seem to be optimum to provide highly efficient combustion of CS in the CFB. Taking into consideration the above discussion, it can be accepted that the application of CFB boiler to burn pure CS with 10– 100 mm length is feasible. 4.3.4. Bed material agglomeration, sintering and deposition Though the composition of biomass fuels varies with plant types and growth conditions, biomass ashes are normally dominated by silicon, calcium and potassium [26,27]. The standard analysis of CS ash is depicted in Table 7. It shows that the K2O content in CS ash is quite high which can cause ash sintering. Biomass ash
Fig. 8. Photographs of silica sand and agglomeration material after 3 h combustion.
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Fig. 9. Photographs of alumina bed material before and after combustion.
sintering may be serious problem in the CFB combustors. It greatly contributes to both bed agglomeration in the furnace and deposit formation in the cyclone and the tail convective heating surfaces [28,29]. Previous studies on the agglomeration characteristics of silica sand bed material in the CFB were performed. The photographs of silica sand and agglomeration material after 3 h run are shown in Fig. 8. From the figures it is observed that using silica sand as bed material, the combustion of CS results in serious bed agglomeration at the normal fluidized bed temperature. The interaction between silica sand and alkaline materials such as potassium in CS ash is known to cause the bed agglomeration. Therefore, with the aim of diminishing the bed agglomeration, silica sands are frequently changed to avoid the accumulation of CS ash in the bed. The bed agglomeration is often the reason for the unscheduled shutdown of heat and/or power boilers. The duration of the hot experiments was about 26 h using alumina as bed material. The bed pressure drop was observed to remain stable. After the end of tests the bed material was removed from the combustor and was inspected to detect the agglomeration phenomena. To attain comparable results, the photographs of alu-
Fig. 10. Photographs of CS ash.
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mina bed material prior to and after combustion are shown in Fig. 9. As seen from Fig. 9, the combustion tests were successfully carried out for 26 h at the dense bed temperature as high as 870 °C without any bed material agglomeration. The results confirm that alumina is difficult to react with alkali metals from CS ash, and alumina is favorable for firing CS in the CFB. Additionally, heat and/or power plants fired by some biomass fuels have been experiencing great problems with sintering and deposition due to the inorganic metal constituents such as alkali metals [30–32]. The potassium salts play a significant role in deposition because they can act as glue bonding the individual fly ash particles together. Furthermore, the presence of KCl in deposits is expected to play a major role in the corrosion of the superheater tubes in biomass-fired boilers. Deposit formation is considered as an important problem challenging the improvement of biomass combustion performance. After the experiments, CS ash samples were observed. The photographs of CS ash are shown in Fig. 10. It can be found that CS ash is very adhesive like other biomass fuels. Therefore, in the industrial scale plants, some precautionary measures should be taken to diminish deposit formation. 5. Conclusions The intense motion of the fluidized bed makes it possible to fire a wide range of fuels having different sizes, shapes, moisture contents and heating values. In this paper, several issues concerning CS combustion in the CFB have been discussed. The objective is to give more information related to the mixing characteristics of CS with bed material as well as the combustion characteristics of firing CS in the CFB. CS with 10–100 mm length alone cannot fluidize. The inert particle medium in the fluidized bed is essential to fluidize CS. It is found that CS can fluidize and mix well with a certain size distribution of alumina at the fluidization number 3–7. However, as the fluidizing number is more than 7, there will exist a little more segregation. These studies provide the supporting data needed for CS combustion in the fluidized bed. The combustion characteristics of pure CS with 10–100 mm length have been studied in a CFB combustor. The fluidizing medium was alumina. Although as the fluidizing velocity is 4.5 m/s (N = 10.2), there will exist a little more segregation in the cold-state tests, yet the dense bed can keep steady state combustion for pure CS in the CFB. A fairly steady dense bed temperature between 830 °C and 880 °C has been obtained. Due to the high volatile content of CS, a significant amount of combustion takes place in the dilute phase. The results show that as the fluidizing velocity increases, the temperature of the dense phase decreases. Meanwhile, the temperature of the dilute phase increases and becomes more uniform. To assure combustion steady, the secondary air flow and gas flow to the loop seal should be controlled reasonably. During the tests, the concentrations of major gaseous pollutants in flue gas were measured. The results show that SO2 emission varies from 32 ppm to 55 ppm, and NO emission ranges from 110 ppm to 153 ppm at the basis of oxygen concentration of 6% in volume in flue gas. The highly efficient combustion, over 98.5%, of CS combustion in the CFB is achieved. In this study, the excess air ratio of around 1.3 and air split ratio of 1:0.88 seem to be optimum to provide high combustion efficiency of CS. The reaction of alkali metals with silica sand can form alkali silicates that can be melt or soften at low temperatures below 700 °C. The alkali silicates could be a potential problem for bed material agglomeration. After 26 h hot experiments, alumina bed material is difficult to agglomerate due to its specific properties. The experimental results provide a reasonable database for optimal design and operation of firing pure CS with 10–100 mm length when applied to industrial scale CFB boilers. Acknowledgements The research work was supported by National Basic Research Program of China (973 Program: 2007CB210208) and its contribution is gratefully acknowledged by the authors. References [1] Gustavsson L, Bo¨rjesson P, Johansson B, Svenningsson P. Reducing CO2 emission by substituting biomass for fossil fuels. Energy 1995;20(11):1097–113.
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