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Articles Influence of Combustion Conditions and Coal Properties on Physical Properties of Fly Ash Generated from Pulverized Coal Combustion† Hiromi Shirai,*,‡ Hirofumi Tsuji,§ Michitaka Ikeda,§ and Toshinobu Kotsuji§ Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan, and Hokuriku Electric Power Company, 15-1 Ushijima, Toyama 930-8686, Japan ReceiVed December 1, 2008. ReVised Manuscript ReceiVed March 11, 2009
To develop combustion technology for upgrading the quality of fly ash, the influences of the coal properties, such as the size of pulverized coal particles and the two-stage combustion ratio during the combustion, on the fly ash properties were investigated using our test furnace. The particle size, density, specific surface area (obtained by the Blaine method), and shape of fly ash particles of seven types of coal were measured. It was confirmed that the size of pulverized coal particles affects the size of the ash particles. Regarding the coal properties, the fuel ratio affected the ash particle size distribution. The density and shape of the ash particles strongly depended on their ash size. Our results indicated that the shape of the ash particles and the concentration of unburned carbon affected the specific surface area. The influence of the two-stage combustion ratio was limited.
1. Introduction The amount of fly ash discharged from existing coal-fired power plants is increasing and has reached over 10 million tons per year in Japan. Over 90% of fly ash discharged in Japan is effectively used. However, over 60% of fly ash is treated so that it can be used in cement. In the Japanese electric power industry, it is desireable to reduce the cost of the treatment and expand the range of use of fly ash, such as in concrete admixtures. Therefore, it is necessary to form high-quality ash. The Japanese Industrial Standard (JIS) for the use of fly ash in cement and concrete1 classifies fly ash into three grades. The grade is determined by the ash properties (ash size, unburned carbon concentration, density, and specific surface area (obtained by the Blaine method)) and the properties of the mortar (fluidity and solidity) to which fly ash is added. To reduce NOx concentration at the outlet of a furnace and the unburned carbon concentration simultaneously, an advanced low-NOx combustion technology has been developed.2,3 To upgrade the quality of the ash, the development of combustion technology to control other ash properties has started. For this development, it is necessary to clarify the influences of † Impacts of Fuel Quality on Power Generation and Environment. * Corresponding author: phone: +81 46 856 2121; fax: +81 46 856 3346; e-mail:
[email protected]. ‡ Central Research Institute of Electric Power Industry. § Hokuriku Electric Power Company. (1) Japanese Industrial Standard (JIS) A 6201; 1999. (2) Improvement of Pulverized Coal Combustion Technology for Power Generation, CRIEPI Review No.46; Central Research INstitute of Electric Power Industry: 2002. (3) Ikeda, M.; Saito, M.; Kishi, Y.; Shirai, H.; Makino, H. CRIEPI Report No. W03016; Central Research INstitute of Electric Power Industry: 2003.
combustion conditions and coal properties on fly ash properties and to develop a method for predicting the properties of the ash. The purpose of this study is to investigate the influences of the coal properties and combustion conditions, such as the size of pulverized coal particles and the two-stage combustion ratio, on the size, density, and specific surface area of the ash particles using our test furnace. 2. Experimental Section 2.1. Coal Samples. In this study, seven types of bituminous coal were used. The properties of each type of coal are shown in Table 1. Coals with different combustibility and meltability were selected that were within the range of properties of coal that can be burned in Japanese pulverized coal-fired power plants. The fuel ratio (FR), used as the index of combustibility, is in the range of 1.0-2.2. The acid ratio, as the index of meltability, is in the range of 5-20. FR and the acid ratio are defined as follows:
FR ) acid ratio )
fixed carbon % volatiles %
SiO2 % + Al2O3 % Fe2O3 % + CaO % + MgO %
(1)
(2)
2.2. Pulverized Coal Combustion Facility. A schematic diagram of the pulverized coal combustion facility is shown in Figure 1. The facility consists of a pulverizer, a furnace with three burners, and a gas treatment system. The coal feed rate is determined by the thermal input of the coal. Here, the thermal input is 760 kW × 3 burners. The coal feed rate is approximately 100 kg/h × 3 when high-quality bituminous coal is burned. The details of the furnace are shown in Figure 2. The furnace has a width of 0.9 m, a depth of 1.9 m, and a height of 9.5 m. The mean residence time of the combustion gas is approximately 4 s on the assumption that the
10.1021/ef801028c CCC: $40.75 2009 American Chemical Society Published on Web 05/08/2009
Physical Properties of Fly Ash from Coal
Energy & Fuels, Vol. 23, 2009 3407 Table 1. Properties of Selected Coal Types
moisture [%]a ash [%]b volatile matter [%]b fixed carbon [%]b fuel ratio [-] carbon [%]b hydrogen [%]b nitrogen [%]b oxygen [%]b sulfur [%]b HHV [kJ/kg]b LHV [kJ/kg]b SiO2 Al2O3 TiO2 Fe2O3 CaO MgO P2O5 K2O Na2O SO3 acid ratio [-] a
Newlands (Australia)
Newlands-2 (Australia)
Mount Arthur
Mount Owen (Australia)
Blair Athol (Australia)
Warkworth (Australia)
Yanzhou (China)
Senakin (Indonesia)
2.4 12.1 30.1 57.8 1.92 73.2 4.5 1.6 8.4 0.3 29.8 28.8 43.7 35.4 1.7 5.0 4.9 1.1 2.0 1.4 0.4 0.9 7.2
1.9 10.9 30.5 58.6 1.92 74.5 4.7 1.6 8.1 0.4 31.0 30.0 45.8 39.3 1.5 6.0 3.7 1.4 1.5 1.1 0.5 0.2 7.7
3.3 13.1 36.9 50.0 1.36 71.1 5.1 1.8 8.1 0.7 29.7 28.5 70.4 20.9 1.5 3.7 0.3 0.5 0.1 0.8 0.3 0.2 20.7
3.3 11.2 38.0 50.8 1.34 72.8 5.2 1.7 8.7 0.6 30.6 29.5 63.4 25.9 1.5 4.8 2.7 1.0 0.9 1.1 0.4 0.8 10.7
8.4 5.7 31.3 63.0 2.01 78.1 4.6 1.7 9.6 0.3 31.4 30.3 59.4 34.5 1.9 5.7 0.4 0.2 0.4 0.1 0.1 0.3 14.9
3.0 14.9 31.6 53.5 1.69 70.5 4.6 1.6 8.0 0.4 28.7 27.7 72.7 20.1 1.0 3.7 0.6 0.4 0.2 0.8 0.2 0.2 19.7
2.8 11.5 36.3 52.2 1.44 70.8 4.8 1.3 11.4 0.3 29.5 28.5 45.9 33.2 1.6 4.6 6.7 1.8 0.4 0.5 0.6 2.6 6.0
4.2 13.7 43.1 43.2 1.00 70.0 5.6 1.4 8.7 0.7 29.0 27.7 51.7 37.0 2.8 3.2 0.8 0.5 0.1 0.5 0.5 1.1 19.6
At equilibrium humidity. b Dry basis.
Figure 1. Schematic diagram of the pulverized coal combustion facility. Table 2. Pulverized Coal Particle Size and Two-stage Combustion Ratio diameter of pulverized coal particles [µm] coal Newlands Mount Arthur Mount Owen Blair Athol Warkworth Yanzhou Senakin
median
DV-coal
two-stage combustion ratio [%]
66 44 16 46 50 37 50 43 42
91 49 18 52 55 44 56 48 45
30 0, 15, 30, 40 30 30 30 30 30 30 30
Table 3. Methods used for Measuring Properties of Ash
Figure 2. Details of the furnace.
gas temperature is 1200 °C. This time is similar to that of actual Japanese boilers. The maximum combustion temperature is more than 1400 °C. The wall has a water wall structure, the inert surface of which is covered with a refractory material. In this study, the outlet O2 concentration was set to 4%, which means that the excess air ratio is 1.24. The pulverized coal particle size and two-stage combustion ratio for each type of coal are shown
item
method
particle size distribution particle density particle shape specific surface area
laser diffraction/scattering Le Chatelier method image analysis Blaine method
in Table 2. The volumetric mean diameter of the coal particles DV-coal [µm] is defined as follows:
DV-coal )
∑ (Xi Di ∑ Xi coal
coal)
coal
(3)
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Figure 3. Influence of two-stage combustion ratio on emission of [NOx] and UC.
where Di [µm] is the diameter and Xi [-] is the weight ratio of the particles. The influence of the coal type on ash properties was investigated for a pulverized coal particle diameter (mean diameter) range from 40 to 60 µm at a two-stage combustion ratio of 30%, which are similar to conditions at actual boilers. To study the influence of the pulverized coal particle size and two-stage combustion ratio, the mean diameter range from 18 to 91 µm and a two-stage combustion range from 0% to 40% were set using Newlands coal. Before each combustion test, all ash was removed from the facility to prevent contamination of the newly formed ash. The facility was heated up for 8 h. After the furnace temperature and the NOx concentration of the outlet of the furnace became stable, the accumulated ash at the bottom of the air heater (AH), the bottom of the De-NOx unit, the bottom of the gas cooler, the bag filter, the electric static precipitator (ESP), and the bottom of the temperature controllers was removed. Then, the test was started. After approximately 8 h, the test was finished, the facility was cooled, and the ash was collected. All ash adhering to the furnace was removed, because the furnace does not have a soot-blowing system. 2.3. Ash Property Analysis. Ash was collected from all parts of the facility except the furnace, as listed above. The ash was mixed according to the collected weight, and the mixed ash was used as samples. The properties of the ash were measured by the methods shown in Table 3.
3. Results and Discussion 3.1. Characteristics of Combustion. First, the combustion characteristics of our furnace used for evaluating the ash are discussed. The influences of the two-stage combustion ratio on NOx and the emission of unburned carbon are shown in Figure 3. In our furnace, the well-known relationship between NOx, unburned carbon, and two-stage combustion ratio in the coal combustion2 was satisfied. The NOx concentration at the outlet of the furnace, [NOx], decreased and the unburned carbon concentration of fly ash, UC, increased as the two-stage combustion ratio increased. At the two-stage combustion ratio of 30%, [NOx] was minimum and UC was maximum, because the reducing atmosphere became stronger with increasing twostage combustion ratio. However, when the two-stage combustion ratio exceeded 30%, [NOx] stared to increase, because the influence of the combustion of unburned carbon at the point of air injection during the two-stage combustion became stronger and the [NOx] concentration increased. The relationship between [NOx] and UC is shown in Figure 4. [NOx] increased as UC decreased. This behavior is similar to that in the boilers of actual power plants. Furthermore, in the [NOx] range of 150-200 ppm, UC in our furnace was
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Figure 4. Relationship between [NOx] and UC.
Figure 5. Relationship between pulverized coal particle size and ash particle size.
observed to be within the range of that for actual boilers. From these results, it was found that the combustion characteristics of our furnace are similar to those in actual boilers. 3.2. Properties of Formed Ash. 3.2.1. Particle Size. The ash particle size is affected by the characteristics of minerals and combustible materials contained in the pulverized coal particles, the pulverized coal size, and the heating and formation histories of the ash particles,4.5 The influence of the pulverized coal size was investigated. The volumetric mean diameter of ash particles DV-ash increased with that of the pulverized coal particles DV-coal, as shown in Figure 5. This is due to the fact that the amount of minerals contained in coal particles increases with the pulverized coal particle size, and the size of ash particles increases by the coalescence of minerals.4 On the other hand, ash particle size generally depends on the mineral content of the coal. In this study, no such dependence was clearly observed because of the insufficient mineral content range of our selected coals. The influence of the two-stage combustion ratio was also investigated, but no clear influence was observed in this study. Next, the influence of the ash particle size distribution on the coal properties was also investigated. The pulverized coal and ash particle size distributions were approximated using the log-normal distribution. The ratio of the ash particle size distribution width to that of the pulverized coal particle size, Rw, is defined by the following equation: (4) Yan, L.; Gupta, P. R.; Wall, F. T. Fuel 2002, 81, 337–344. (5) Barta, L. E.; Toqan, M. A.; Beer, J. M.; Sarofim, A. F. 24th Int. Conf. Comb. 1992, 1135–1144.
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Figure 6. Influence of FR on ash particle size distribution.
Figure 8. Comparison of ash particle density with boilers in actual power plants.
Figure 7. Influence of Ash Particle Size on Density of Ash Particles.
Rw )
σash σcoal
(4)
where σcoal and σash are the geometric standard deviations of the pulverized coal particle size and the ash particle size distributions, respectively. The relationship between Rw and FR is shown in Figure 6. Rw is more than 1.0. This indicates that the ash particle size distribution is wider than the pulverized coal particle size distribution. Furthermore, Rw increases as FR decreases. This suggests that the number of fragmenting pulverized coal particles increases or the number of coalescing mineral particles decreases with decreasing FR. 3.2.2. Unburned Carbon. It is well-known that the unburned carbon concentration depends on FR,2 the ash content, and the combustion conditions, such as the two-stage combustion ratio. Furthermore, UC decreases with the coal particle size, because the combustibility of the coal particles increases. 3.2.3. Density. The ash density has strong relation to the ash particle size, as shown in Figure 7. The density increases as the ash particle size decreases. The ash density is affected by the true density of the component materials and the closed pore contained in the ash. The true density is affected by the formation of oxides that are affected by the chemical compositions and the heating and formation histories of the ash particles. The volume of the pore is affected by the heating and formation histories of the particle and the ash meltability (the melting point and viscosity6 of the melted ash). This result indicates that the volume of the pore increases with the ash particle size. It is thought that the volume is affected by the coalescence number (6) Rauf, R. J. Fuel 1981, 60, 1177–1179. (7) Ghosal, S.; Self, S. A. Fuel 1995, 74, 522–529.
of the mineral particles contained in the coal particle, because in the same coal, the density increases and the ash particle size decreases with decreasing pulverized coal particle size (Figure 7). Furthermore, the influence of FR and the acid ratio on the density was investigated. In this study, these influences were not clearly observed for the selected coal types. The influence of the two-stage combustion ratio also was not observed. To more effectively investigate the properties that affect the density, it is necessary that the true density and the pore are evaluated separately. Next, the ash density of our furnace was compared with that in actual boilers that have an opposed firing system with a net output range from 600 MWe to 1000 MWe, using the different samples of Newlands coals. The influence of the ash particle size in our furnace on the ash density was confirmed, as shown in Figure 8. Also, the density varied widely. This indicates that the density is affected by the heating and formation histories of the ash particles in each boiler. 3.2.4. Specific Surface Area. In JIS, the specific surface area of the ash, SAb [cm2/g-ash], is measured by the Blaine method. SAb is not the true specific surface area of the ash. However, it is an important value for evaluating the outer surface area excluding the pore surface area. The specific surface area of a powder is affected by the particle size distribution8 and particle shape. The relationship between the surface mean diameter of the DS-ash [µm] and SAb is shown in Figure 9. DS-ash [µm] is defined as follows. DS-ash )
∑ Xi Xi ∑ Di
ash
( )
(5)
ash
ash
If the shapes of the particles are similar, SAb increases as DS-ash decreases. As shown in Figure 9, no clear dependence of DS-ash on SAb was observed. Next, SAb was compared with the specific surface area, SAd [cm2/g-ash] estimated from the ash particle size distribution and ash density. Ash particles were assumed to be spherical. The relationship between DS-ash and (8) Tsivilis, S.; Tsimas, S. ZKG Int. Ed. B 1992, 45, 131–134.
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Figure 9. Relationship between specific surface area and ash particle size.
Shirai et al.
Figure 12. Comparison of SAb/SAd with that in actual power plants.
Figure 13. Circularity for selected heywood range of diameter. Figure 10. Influence of Ash Particle Size on SAb/Sad
Figure 11. Influence of unburned carbon concentration on SAb/SAd
the ratio of SAb to SAd, which is the shape factor, is shown in Figure 10. The ratio increases with the ash particle diameter. This indicates that the sphericity of the ash particles decreases as the diameter increases. It is estimated that the number of coalesced particles of minerals affects the particle shape similarly to the case for the density. Furthermore, it was found that UC, which is affected by FR and the combustion conditions such as the coal particle size and two-stage combustion ratio, affects SAb, as shown in Figure 11. The particles containing unburned carbon have a complex shape. In ash with a high unburned carbon concentration, SAb may not be correctly measured because the Blaine method cannot be efficiently applied to the particles of greatly different shapes. Next, SAb/SAd in our furnace was compared with that in actual boilers as shown in Figure 12. For a similar particle size
range, SAb/SAd in our furnace was observed to be higher than that in actual boilers. This result indicates that the shape of particles is affected by their heating and formation histories in each boiler. It is estimated that the sphericity in our facility is lower than that in actual boilers because the high-temperature zone of our furnace is smaller than that of actual boiler. To investigate the influence of the ash particle shape, the circularity and Heywood diameter of the particle are measured from images of ash particles obtained using a scanning electron microscope (SEM). The mean circularity for the selected ranges of Heywood diameter is shown in Figure 13. The circularity decreases as the Heywood diameter increases. It was confirmed directly that the increase in SAb/SAd with the ash particle size is due to the fact that the sphericity decreases. From these results, it was confirmed that SAb is affected by the size and shape of ash particles. Furthermore, the particle shape may be affected by the ash meltability, which is determined by the melting point of the ash and the viscosity of the melted ash, although this is not clearly shown in this study. 4. Conclusion In this study, the influences of the size of pulverized coal particle, the two-stage combustion ratio and coal properties during combustion on fly ash properties were investigated using our test furnace. (1) Particle size. The ash particle size is affected by the characteristics of the minerals and combustible materials contained in the pulverized coal particles, the pulverized coal size, and the heating and formation histories of the ash particles. It was confirmed that the pulverized coal particle size affected the ash particle size formed. Additionally, it was found that the width of the ash particle size distribution increased with decreasing fuel ratio. This suggests that the number of frag-
Physical Properties of Fly Ash from Coal
menting pulverized coal particles increases or the number of coalescing mineral particles decreases with decreasing fuel ratio. (2) Density. The ash density is affected by the true density of the component materials and the pore. The true density is affected by the formation of oxides, which, in turn, is affected by the chemical compositions and the heating and formation histories of the particles. The pore is affected by the heating and formation histories of the particles and the ash meltability (the melting point and viscosity of melted ash). In this study, it was confirmed that the density relates to the ash particle size and affects the heating and formation of ash particles in the boiler. In future work, to clearly investigate the factors affecting the properties of ash particles, it is necessary that the true density and the pore are evaluated separately.
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(3) Specific Surface Area (SAb). The specific surface area measured by the Blaine method is affected by the ash particle size distribution and the particle shape. It was confirmed that the ash particle size affects the shape of ash particles. Also, it was found that the unburned carbon concentration affects the specific surface area because the particles containing unburned carbon had a complex shape. Furthermore, in the comparison between SAb/SAd in our furnace and that in actual boilers, it was found that the heating and formation histories of the ash particles strongly affect their ash shape. EF801028C
