Article pubs.acs.org/EF
Particle Size Distribution as a Nonindependent Variable Affecting Pulverized-Coal Burnout in Coal-Fired Power-Plant Boilers Xinghua Xue*,†,‡ and Yunquan Wang§ †
College of Bioscience and Technology, Hubei University for Nationalities, Enshi 445000, China College of Landscape Architecture, Yangtze University, Jingzhou 434025, China § Guangzhou University, Guangzhou 510006, China ‡
ABSTRACT: The particle size distribution (PSD) is critical for pulverized-coal combustion in coal-fired power-plant boilers. However, the PSD might not necessarily be an independent variable in combustion analysis. Using industrial-scale combustion tests in a coal-fired power plant, this work aimed to explore the relationship between the PSD of pulverized coal and coal properties and to understand the role of coal properties in the PSD impacting coal burnout. The results showed that the 110− 480-μm fraction of pulverized coal strongly affects coal burnout and that this coarse fraction is closely correlated with coal properties. Therefore, it was concluded that the PSD and its influence on coal burnout are essentially dependent on coal properties. The synthetic index Fpp, combining the proximate and petrographic analyses, has a significantly high correlation with the 110−500-μm fraction of pulverized coal. Thus, Fpp can be used to predict the coarse fraction of pulverized coal from coal mills. Additionally, maceral analysis showed that coal pulverization can lead to the partitioning of macerals.
volatility bituminous coal is a single-drum, natural-circulation, pulverized-coal-fired boiler operating in a 210 MW power unit. Five types of Chinese bituminous coals (BCs) or BC blends, including Yangquan bituminous, Datong bituminous, Shenmu bituminous, Baoji bituminous, and Tongchuan bituminous, were used in this study. Table 1 shows the properties of the studied coals and the design parameters of the studied boiler. The pulverized-coal preparation system with an intermediate storage bunker was equipped with two cylindrical ball mills. The combustion conditions were controlled as follows: oxygen concentration, 4.6 ± 0.7%; primary air pressure, 2600 ± 200 Pa; exhaust gas temperature, 135 ± 5 °C; and secondary air temperature, 310 ± 5 °C. Samples of raw coal, pulverized coal, and fly ash were collected over a period of 28 days at 24-h intervals. Raw coal was sampled on the belt of the coal feeder, following Chinese standard method DL-T 567.21995. Pulverized coal was sampled at the pulverized-coal outlet tube of a cyclone collector in accordance with Chinese standard method DL-T 567.2-1995. During the sampling of raw coal and pulverized coal, 20 subsamples were collected at 1-min intervals and were then mixed into one large sample of about 1 kg. Fly ash was sampled by using a constant-velocity fly-ash sampler mounted at the rear flue of the boiler according to Chinese standard method DL-T 567.3-95. Proximate analysis and petrographic analysis were carried out for raw coals. Pulverized coal was screened by meshes at 76 and 150 μm and was divided into three parts for maceral analysis. In accordance with Chinese standard method DL/T567.6-95, the unburned carbon content in fly ash (UC) was measured by the loss-on-ignition method. UC was used to describe the burnout of pulverized coal in power-plant boilers. By using a Mastersizer 2000 particle size analyzer, the PSD of the pulverized coal was measured over a range of 0.02−2000 μm, and the wet method was employed. Water was chosen as the dispersant, the refractive index of which was set to 1.33. The refractive index and absorption rate of pulverized coal were set to 1.64 and 1.23,
1. INTRODUCTION Particle size is an important physical factor affecting coal combustion. Researchers have revealed that the particle size distribution (PSD) strongly influences the combustion reactivity and efficiency of pulverized coal,1−5 as well as pollutant emissions.6−9 On the other hand, coal properties that intrinsically have a notable impact on coal grindability are also critical for pulverized-coal combustion.10−14 Therefore, the interrelation between the PSD and coal properties should be clarified in coal combustion analysis. Coal grinding is an important technique in pulverized-coal-fired power plants. There is practical significance to exploring the relationship of PSD to coal properties and determining whether the influence of the PSD on coal combustion is independent or additionally related to coal properties. However, few studies discuss this topic. Studies have investigated the relationship between grindability and coal properties.15−17 The Hardgrove grindability index (HGI) is widely used in coal utilization, whereas it is not necessarily equated with the PSD. Impacts of coal properties on the PSD of pulverized coal from power-plant coal mills are seldom discussed. In addition, a detailed study based on industrial-scale tests is needed because of the difference between laboratory tests and the industrial boiler environment.18 According to industrial-scale combustion tests in a pulverized-coal-fired power plant, we aim to discuss the relationship between coal properties and the PSD of pulverized coal and to address the role of coal properties in the PSD affecting coal burnout. 2. EXPERIMENTAL SECTION
Received: March 7, 2013 Revised: July 4, 2013 Published: July 10, 2013
Experiments were carried out in a coal-fired power plant in Guangzhou City, China. Bituminous coal is widely utilized in Chinese coal-fired power plants. The studied 680 t/h boiler designed to burn medium© 2013 American Chemical Society
4930
dx.doi.org/10.1021/ef400406w | Energy Fuels 2013, 27, 4930−4934
Energy & Fuels
Article
Table 1. Properties of Studied Coals and the Design Parameters of Boiler proximate analysis
ultimate analysis
parameter
studied coal
designed coal
parameter
studied coal
designed coal
Var (wt %) FCar (wt %) Mar (wt %) Aar (wt %) Qnet,ar (MJ·kg−1)
20.00−27.00 43.80−48.40 10.00−15.20 13.90−22.30 19.232−21.582
21.00 52.21 8.00 29.00 20.315
Nad (wt %) Cad (wt %) Had (wt %) Sar (wt %)
1.22−2.04 63.61−70.67 3.53−4.27 0.43−0.95
1.07 52.21 3.25 0.80
Figure 1. Correlation coefficients between moisture content and the volumetric contents of different size fractions of pulverized coal.
Figure 2. Correlation coefficients between three proximate analysis parameters and the volumetric contents of different size fractions of pulverized coal. respectively. Following Chinese standard methods GB/T8899-1998, GB/T6948-1998, and GB/T16773-1997, the petrographic analyses of raw and pulverized coal were carried out at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The correlations between the PSD of the pulverized coal and the coal properties and UC were analyzed using SPSS 11.0. The volumetric contents of 75 fractions were included in the PSD of pulverized coal. The 1−600-μm pulverized coal was divided into 60 fractions at 10-μm intervals. In the size range of 600−2000 μm, there were 14 fractions at 100-μm intervals. Particles smaller than 1 μm were classed as one fraction. Correlation coefficients between the coal properties or UC and the volumetric contents of different size fractions of pulverized coal are shown in the charts of section 3 to identify (1) the size fraction of pulverized coal strongly affected by coal properties, (2) the size fraction of pulverized coal greatly impacting coal burnout, and thus (3) the correspondence between these two size fractions.
3. RESULTS AND DISCUSSION 3.1. Influence of Proximate Composition on the PSD of Pulverized Coal. The correlation coefficients between moisture and the contents of various size fractions of pulverized coal are shown in Figure 1. The correlation between total moisture (Mar) and the PSD is low and insignificant, whereas inherent moisture (Minh) and free moisture (Mf) have higher correlations with the PSD. Mf is positively correlated with the 340−510-μm fraction (α < 0.05). In contrast, a significantly negative correlation between Minh and the 100−170-μm fraction was observed (α < 0.05). Therefore, free moisture has a negative impact on coal pulverizing and can cause a larger content of coarse size fraction, whereas inherent moisture does not result in the coarsening of pulverized coal. The reason for the opposite effects on the PSD of Minh and Mf might be the difference in their occurrence states. Mf is held on the surface of coal particles or within the large pores of coal. Large amounts of Mf can lead to the transition of the coal grinding process 4931
dx.doi.org/10.1021/ef400406w | Energy Fuels 2013, 27, 4930−4934
Energy & Fuels
Article
Figure 3. Correlation coefficients between the combined indices of coal properties and the volumetric contents of different size fractions of pulverized coal.
Figure 4. Correlation coefficients between coal petrographic parameters and the volumetric contents of different size fractions of pulverized coal.
rank on coal grindability have been reported.15,17,20 However, Figure 4 shows that the correlations between coal petrography and the PSD of pulverized coal are relatively low. Only the vitrinite content (V) has significantly negative correlations with the coarse fractions of 300−460 μm. A previous study also noted that higher vitrinite contents in coal can result in higher HGI values.16 As a dimensionless quantity, the index Fpt combines six parameters of petrography as
from brittle deformation to plastic deformation, which causes an increase in the energy consumption and is unfavorable to coal pulverizing. In contrast, Minh is held by capillary action within the microfractures of coal or is combined with minerals and, therefore, is related to coal porosity, which favors coal breaking. The influences of the contents of fixed carbon (FCar) and volatiles (Var) on the PSD of pulverized coal are opposite, but the impact of ash content (Aar) is very weak (Figure 2). FCar has a positive but insignificant correlation with the coarse fractions of pulverized coal (>100 μm). The negative correlation coefficients between Var and the 120−430-μm fractions are high and significant (α < 0.05). That is, a higher content of volatiles can favor coal pulverizing. Using coal property parameters, researchers have built models to predict the grindability of coal.15,19 Based on the preceding analysis, the index Fpx, combining four important proximate analysis parameters as Fpx = (M inh + Var)/(M f + FCar )
Fpt =
V S (I + E + M ) R
(2)
where V, E, I, and M are the volumetric contents of vitrinite, exinite, inertinite, and mineral, respectively, in coal; R is the average reflectance of vitrinite; and S is the standard deviation of vitrinite reflectance. Fpt is used to describe the integrated influence of coal petrography on the particle size distribution of pulverized coal. Compared with any one parameter of coal petrography, Fpt exhibits an obviously better correlation with the PSD of pulverized coal (Figure 3). Significantly negative correlations between Fpt and the 120−480-μm fractions were observed. 3.3. Synthetic Index Used to Describe the Combined Influence of Coal Properties on the PSD of Pulverized Coal. Based on petrographic, proximate, and elemental analyses, a variety of methods for the prediction of coal grindability have been proposed.15,16,19 A proper prediction of the PSD of pulverized coal on the basis of coal properties will aid in operating-condition adjustments and combustion
(1)
is proposed to reflect the integrated influence of proximate analysis on the PSD of pulverized coal. In comparison with any one parameter of proximate analysis, Fpx has a better correlation with the PSD of pulverized coal (Figure 3). Significantly high correlations between Fpx and the 100−480-μm fractions of pulverized coal were observed. 3.2. Influence of Coal Petrography on the PSD of Pulverized Coal. The effects of the maceral content and coal 4932
dx.doi.org/10.1021/ef400406w | Energy Fuels 2013, 27, 4930−4934
Energy & Fuels
Article
Figure 5. Contents of macerals in different size fractions of pulverized coal.
Figure 6. Correlations between the unburned carbon content in fly ash and the volumetric contents of different size fractions of pulverized coal.
grinding.23 Figure 5 clearly shows the partitioning of macerals during coal pulverization in the coal-fired power plant. From the coarse to fine size fractions, the vitrinite content decreases dramatically, whereas the inertinite content increases sharply. In the ≥150-μm fraction, the content of vitrinite is much larger than that of inertinite. In contrast, in both the 150−76-μm and ≤76-μm fractions, the content of inertinite is markedly larger than that of vitrinite. The mineral content increases slightly with increasing particle size. The partitioning of exinite is not obvious because of the extremely low content of exinite in these batches of coal. Thus, coal pulverizing in coal-fired power plants can result in the partitioning of macerals. That is, vitrinite is enriched in the coarse fraction, whereas inertinite is enriched in the fine fraction. Maceral partitioning might thereby influence the combustion of pulverized coal in power boilers. 3.5. Influence of the PSD on the Burnout of Pulverized Coal. To understand the nonindependent influences of the PSD on coal burnout, it is necessary to know the size fractions of pulverized coal that are critical for coal burnout and whether these fractions are identical to the size fractions that are closely related to coal properties. Figure 6 shows that the 110−480-μm fractions of pulverized coal are closely correlated with the content of unburned carbon in fly ash. Their correlation coefficients are high and significant at the 0.05 level (2-tailed). Especially for the 130−430-μm fractions, the correlation coefficients range from 60.3% to 75.6% and are significant at the
