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Fine ash formation during combustion of pulverised coal–coal property impacts
Fuel 85 (2006) 185–193 www.fuelfirst.com
Fine ash formation during combustion of pulverised coal–coal property impacts B.J.P. Buhre a,*, J.T. Hinkley a, R.P. Gupta a, P.F. Nelson b, T.F. Wall a a
Cooperative Research Centre for Coal in Sustainable Development, Department of Chemical Engineering, University of Newcastle, Callaghan, NSW 2300, Australia b Cooperative Research Centre for Coal in Sustainable Development, Graduate School of the Environment, Macquarie University, NSW 2109, Australia Received 5 October 2004; received in revised form 24 February 2005; accepted 14 April 2005 Available online 15 September 2005
Abstract In many countries, legislation has been enacted to set guidelines for ambient concentrations and to limit the emission of fine particulates with an aerodynamic diameter less than 10 mm (PM10) and less than 2.5 mm (PM2.5). Ash particles are formed during the combustion of coal in pf boilers and fine ash particulates may potentially pass collection devices. The ash size fractions of legislative interest formed during coal combustion are the result of several ash formation mechanisms; however, the contribution of each of the mechanisms to the fine ash remains unclear. This study provides insight into the mechanisms and coal characteristics responsible for the formation of fine ash. Five well characterized Australian bituminous coals have been burned in a laminar flow drop tube furnace in two oxygen environments to determine the amount and composition of the fine ash (PM10, PM2.5 and PM1) formed. Coal characteristics have been identified that correlate with the formation of fine ash during coal combustion. The results indicate that coal selection based on (1) char characterization and (2) ash fusion temperature could play an important role in the minimization of the fine ash formed. The implications of these findings for coal selection for use in pf-fired boilers are discussed. q 2005 Elsevier Ltd. All rights reserved. Keywords: Coal combustion; Fine ash; Coal characterisation
1. Introduction The health effects of ambient fine particulates have been studied extensively and correlations have been observed between ambient fine particulate matter and human mortality rates e.g. [1]. Governments worldwide acknowledge these studies and as a result standards have been introduced to assist in reducing ambient fine particulate concentrations. In the United States, a National Ambient Air Quality Standard (NAAQS) for both ‘coarse’ particulate matter with an aerodynamic diameter less than 10 mm, PM10, as well as an NAAQS for PM2.5 is currently in effect [2]. Recently in Australia, the National Environment Protection (Ambient Air Quality) Measure (NEPM) has been modified to include advisory reporting standards for PM2.5, the monitoring of which commenced in January 2004 [3]. PM10 has been included in the NEPM Ambient Air Quality measures since, * Corresponding author. Tel.: C61 2 4921 6179; fax: C61 2 4921 6920. E-mail address: [email protected] (B.J.P. Buhre).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.04.031
1998 [4]. Around 80% of the electricity generated in Australia is generated by coal combustion [5], and coalfired power generators are increasingly being required to monitor and characterise their emissions of fine particulate matter. Air pollution control devices (APCDs) are employed at power stations to capture the ash particulates from the flue gas. Increasing the capture efficiency of these APCDs to decrease fine ash emissions can be a costly exercise. Selecting coals which reduce the formation offine ash particulates in the first place could be a cost-effective method of minimizing fine ash emissions. This paper presents the results of a study for the coal characteristics responsible for the formation of fine ash particles. Five Australian black coals have been characterised extensively using advanced analytical techniques such as QEMSCAN, SIROQUANT, and ICP-AES on ash obtained from radio-frequency ashing technique. The coals were then burned in a drop tube furnace, which simulates the combustion in a pf fired boiler. The ash was characterised and the coal characteristics responsible for the formation of the ash size fractions of legislative interest (PM10, PM2.5, and PM1) have been determined.
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2. Experimental Five Australian black coals have been selected to represent the range of ash chemistry and char swelling behaviour occurring in Australian bituminous coals; the coals were chosen on the basis of their sulphur content, other constituents and their vitrinite content. The coals were milled and a size cut between 63 and 90 mm was obtained, suitable for feeding into the drop tube furnace. The coals were subjected to proximate and ultimate analysis, reflectogram, and QEMSCAN analyses. QEMSCAN is a novel version of CCSEM technique that can directly measure the coal–mineral and mineral–mineral associations. In the technique, pulverised coal samples are mounted in carnauba wax. The sample block is sectioned and the surface is polished and coated with a thin layer of carbon to ensure electrical conductivity. The particles are thus, well separated and in random orientations with random sections exposed. Qem*SEM (quantitative evaluation of materials by scanning electron microscopy) is an earlier version of QEMSCAN and has been described by Creelman and Ward [6]. The technique is an automated image analysis system that uses Backscattered Electron (BSE) and Energy Dispersive X-ray (EDX) signals from a scanning electron microscope to create pixilated images in which each pixel represents the EDX spectrum obtained from the centre of the pixel. The light element X-ray detectors enable the detection of organic material, enabling a direct measurement of the coal-mineral associations. The pixel spacing is an operator setting and determines the resolution of the QEMSCAN images. For the analyses used in this study, a pixel spacing of 2 mm was used. Ash was obtained from the coal samples using a low temperature radio frequency ashing technique [7], after which the ash was analysed for chemical composition using ICPAES, and quantitative mineralogy using SIROQUANT. The procedure applied for the ash dissolution has been described in detail elsewhere [8]. The combination of the analytical techniques resulted in a thorough understanding of the organic and inorganic characteristics of the coal samples. After characterisation, the coals were burned in the Tetlow Model HTF375A drop tube furnace situated at the Chemical Engineering department at the University of Newcastle in
Australia. The ash was collected using a cyclone, a cascade impactor and subsequently a filter. The cyclone has a theoretical d50 of 3 mm at the operating conditions applied during the experiments. The cascade impactor is a seven stage MRI cascade impactor, model 1503. After correction for an assumed particle density, the d50’s of the seven impaction plates were 17.9, 8.2, 3.2, 1.4, 0.8, 0.4, and 0.3 mm, respectively. The experimental set-up is described elsewhere in more detail [9,10]. After combustion, the ash was characterised for its size distribution, morphology, and chemical composition. The ash collected in the cyclone was suspended in ethanol and the size distribution of the ash was determined using a Malvern Mastersizer S with a 300 mm lens. The amounts of ash collected on the different plates of the cascade impactor were combined with the size distribution obtained from the Malvern Mastersizer to obtain a complete size distribution (Table 1). The measurement of PM10 from the ash samples was based on the analysis results of the Mastersizer, while the PM2.5 and PM1 was determined from the masses collected in the cascade impactor. The amount of PM2.5 is estimated as the combined masses of the amounts collected on the filter and on the bottom five stages. The PM1 is calculated from the mass collected on the filter combined with the bottom three stages. Moisture adsorption onto the filter can significantly affect the observed mass of the material collected onto the filter [9]. The moisture is an artefact of the experimental procedure and should not be considered when determining the mass of PM. Previous publications presented PM1 measurements as the balance mass, which does not take the effect of moisture absorption into account [10]. The chemical compositions of the material collected on the filters have been determined quantitatively using a combination of Proton Induced X-Ray Emission (PIXE) and Proton Induced Gamma Ray Emission (PIGE) analysis. The PIXE and PIGE analysis results have been used to calculate the mass of the elements present (converted to oxides). This method ensures that moisture adsorbed from the combustion gas or the atmosphere does not affect the results. The repeatability of the experimental set-up to determine the amount of PM1 using this technique is described elsewhere [9,10].
Table 1 Considered coal characteristics for each size fraction and the ash formation mechanisms they could affect Fine Ash
Coal characteristic
Ash formation mechanism affected
PM10
Ash Fusion Temperature Basic to acidic oxide ratio Average mineral grain size Total ash content Char Group I content / vitrinite content Ash Fusion Temperature Ash content Char Group I content / Vitrinite content Sulphur content Ash content Char Group I content / Vitrinite content Sulphur content Average mineral grain size
Included mineral coalescence Included mineral coalescence Included mineral coalescence, Vaporization and condensation Included mineral coalescence, Char fragmentation Char fragmentation Included Mineral Coalescence Char Fragmentation, Included mineral coalescence Char fragmentation Vaporisation and condensation Included mineral coalescence, Char fragmentation Char fragmentation Vaporisation and condensation Included mineral coalescence, Vaporisation and condensation
PM2.5
PM1
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Table 2 Proximate analysis, rank and vitrinite content of the five coals used in this study (HV-Bit, high volatile bituminous; MV-Bit, medium volatile bituminous coal) Sample ID
CRC 240
CRC 272
CRC 296
CRC 297
CRC 306
Moisture (% ad) Ash (% db) Volatile matter (% db) Fixed carbon (% db) (by difference) Total sulphur content (% db) Coal rank Vitrinite content (% v/v mmf) Ash fusion temperature (deformation temperature, 8C) Average mineral size (mm)
2.14 12.96 30.41 56.63 0.60 HV-Bit 68.3 O1600
2.47 9.18 35.69 55.13 0.96 HV-Bit 50.7 1280
2.09 13.81 30.34 55.85 0.61 HV-Bit 39.0 1490
1.27 11.93 45.41 42.66 5.07 HV-Bit 57.8 1300
1.64 18.77 19.68 61.55 1.72 MV-Bit 28.1 1240
10.8
14.0
11.4
16.7
The composition of the oxidizing gas was varied during the experiments to determine its effect on fine particle formation. The coals were burned at 1400 8C in 21% O2 (air) and in 50% O2. The elevated oxygen concentration results in higher char combustion temperatures. Simulations and literature indicated that by increasing the oxygen content from 21 to 50%, the temperature of the burning char particles increase from approximately 2030–2530 8C [11]. 3. Results The coal compositions are provided in Tables 2 and 3. Table 2 provides the proximate analysis, coal rank, vitrinite content as determined from the reflectogram [12], and the deformation temperature as determined during the ash fusion test in a reducing environment [13] for all five coals. For mineral analysis, the coals have been ashed using a low temperature radio frequency ashing technique [7]. Table 3 provides the ash elemental composition, together with total sulfur and chlorine content. Tables 4 and 5 provide the total ash recovered, the PM10, PM2.5, and PM1 formed during the combustion experiments Table 3 Elemental composition of the low-temperature ash obtained from the five coals measured by ICP-AES, results presented in elemental wt% LTA ash elemental composition %
CRC 240
CRC 272
CRC 296
CRC 297
CRC 306
Si Al Fe Ca Mg Na K Ti Mn Sa(db) P Ba Sr Clb(db)
20.8 16.0 1.1 0.2 0.1 0.1 0.8 0.80 0.005 0.60 0.07 0.03 0.02 0.03
18.3 11.1 5.0 4.5 0.2 0.1 0.2 1.08 0.05 0.96 0.3 0.06 0.05 0.07
32.2 7.9 0.3 0.3 0.1 0.02 1.3 0.63 0.004 0.61 0.1 0.03 0.02 0.01
10.7 9.9 3.5 9.1 2.0 0.2 0.1 0.70 0.12 5.07 0.2 0.03 0.10 !0.01
15.6 6.8 9.7 9.1 1.2 0.1 0.6 0.48 0.13 1.72 0.2 0.04 0.04 0.05
The remainder of the ash includes oxygen, water of constitution, and carbon present as carbonates. a Sulfur as determined by [33]. b Chlorine as determined by [34].
7.7
together with the total ash collected during the experiments. The amounts are reported as percentage of the ash generated during combustion. The proportion of the ash larger than 10 mm was typically much larger in size than 10 mm and displayed a mean size in the order of tens of micrometers.
4. Ash formation mechanisms During coal combustion, ash particles are formed from the inorganic matter present in coal. The mechanisms of ash formation have been studied extensively in the literature, and are affected by the combustion conditions and the coal characteristics [14–17]. Two important coal characteristics influencing the ash formation process are † The mode of occurrence of the inorganic matter and † The combustion behaviour of coal particles containing both organic and inorganic matter. The majority of inorganic matter present in black coals occurs in the form of minerals of various types and sizes. These minerals can be closely associated with the organic matter (included minerals), or they occur excluded from the organic matter (excluded minerals). The majority of ash particles are formed from four formation mechanisms, shown in Fig. 1: † † † †
Included mineral coalescence, Char fragmentation, Excluded mineral fragmentation, and Vaporization and subsequent condensation of inorganic matter.
Table 4 Total ash collected, PM10, PM2.5, and PM1 generated during combustion in 21% O2 at 1400 8C PM10, PM2.5, and PM1 as % of ash generated during combustion in 21% O2 21% O2
CRC 240
CRC 272
CRC 296
CRC 297
CRC 306
PM10 PM2.5 PM1 (dry) Total ash collected
0.9 0.15 0.08 81.9
6.1 0.53 0.30 82.8
10.7 0.49 0.20 91.6
3.2 0.78 0.58 72.4
3.8 0.40 0.11 73.3
PM10 determined from Malvern Mastersizer, PM2.5 and PM1 determined from weight collected in cascade impactor. Results presented as % of ash generated.
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Table 5 Total ash collected, PM10, PM2.5, and PM1 generated during combustion in 50% oxygen at 1400 8C PM10, PM2.5, and PM1 as % of ash generated during combustion in 50% O2 50% O2
CRC 240
CRC 272
CRC 296
CRC 297
CRC 306
PM10 PM2.5 PM1 (dry) Total ash collected
19.8 0.72 0.34 66.7
11.7 1.41 0.44 86.1
17.4 1.06 0.39 90.0
13.3 1.60 0.85 77.5
13.3 0.44 0.19 84.0
PM10 determined from Malvern Mastersizer, PM2.5 and PM1 determined from weight collected in cascade impactor. Results presented as % of ash generated.
Typically, ash particles are characteristic in size for the mechanism via which they are formed. The first mechanism is believed to be responsible for the bulk of the supermicron ash particles formed [16]. The formation of ash particles from the fragmentation of thin-walled cenospherical char particles (i.e. the second mechanism) contributes to ash particles of only a few micrometer in size [18–20]. Excluded mineral fragmentation contributes mainly to supermicron-sized ash particles, and the last formation mechanism contributes mainly to submicron sized ash particles [14]. Coal characteristics play an important role in the extent to which the three ash formation mechanisms contribute to the total ash formed. A summary of the main characteristics affecting the formation mechanisms is provided below. 4.1. Included mineral coalescence When char particles do not fragment, the ash formed from included mineral coalescence is affected by the type, size and distribution of included minerals. The high temperatures occurring inside burning char particles cause included minerals to turn viscous and coalesce. The ash fusion temperature is a standard test done on coal ashes to characterise the various stages of melting [13]. Although the ash fusion test does not differentiate between included and excluded minerals, the test indicates the melting behaviour of the inorganic matter upon heating. The results in this paper suggest that this melting behaviour could indicate the affinity of minerals to coalesce during combustion. The ratio of basic oxides to acidic oxides is often used in the literature as an indicator of ash slag viscosity at high temperatures (e.g. [21]). If the ash fusion temperature could be used as an indicator, this ratio is likely have a similar relationship with the extent of mineral coalescence during coal combustion. 4.2. Char fragmentation The extent of char fragmentation depends on the char swelling behaviour during combustion [20]. If the char fragments are large and contain large minerals, the minerals inside these fragments will coalesce and form large ash particles. However, if the char swells extensively, the char will fragment into small pieces and minerals inside these small fragments will result in small ash particles. The two
Fig. 1. Schematic of ash formation mechanisms during pulverised coal combustion.
mechanisms shown in Fig. 1 are extreme cases and ash formed from coal combustion is a combination of the two. Monroe suggested a mathematical model to predict the coalescence behaviour of minerals as a function of cenosphere shell thickness, coal particle size, mineral grain size, and mineral volume fraction [22]. Yan validated this model by determining the char swelling behaviour of several Australian coals and its effect on the ash formation [17]. He modelled the char fragmentation based on a char classification scheme, in which the char is classified in three classes based on their swelling behaviour. Char Group I particles are defined as char particles that are highly porous and have low densities. The combustion of these thin-walled cenospherical char particles can result in more extensive fragmentation during combustion, resulting in fine ash formation. Benfell showed that the amount of Char Group I particles of Australian coals could be estimated from the coal vitrinite content and the pressure at which the char was formed [23,24]: Char Group I ðnumber%Þ Z 0:994 !Pressure ðatmÞ C 0:621 ðVitrinite; v=v% mmfÞ C 29:87 This correlation is an updated version of a correlation published earlier by the same research group [25,26]. The updated version is based on a more extensive coal database but produces similar results as the earlier correlation [24]. 4.3. Excluded mineral fragmentation The amount, size, and type of excluded minerals determine the extent of excluded mineral fragmentation. The main fragmenting mineral types in Australian black coals are calcite and pyrite [27]. Although extensive fragmentation of these
minerals has been noted, the bulk of the newly formed ash particles are generally supermicron in size [27,28]. 4.4. Vaporization and condensation The extent of vaporization of inorganic matter during coal combustion depends on (a) the temperature inside the burning char particle, and (b) the occurrence of readily vaporised material such as alkalis, sulphur, and phosphorus [14,29]. The vaporization of refractory oxides from included minerals has also been suggested to depend on the size distribution of included minerals containing refractory oxides [14,30,31]. Summarizing, the size fractions of legislative interest are the result of several ash formation mechanisms. Several coal characteristics affect the extent to which the formation mechanisms result in fine ash. This study aims at determining the dominating coal characteristics that correlates with the amounts of fine ash formed. Table 1 shows the coal characteristics considered for three size fractions (PM10, PM2.5 and PM1), together with the ash formation mechanism they could affect. 5. Discussion 5.1. PM10 The amounts of PM10 have been compared with the coal characteristics indicated in Table 1. The PM10 showed no correlation with average mineral size, ash content, or Char Group I content. The ratio of basic to acidic oxides showed moderate correlation with the amounts of PM10, and a good correlation was observed between PM10 and the ash fusion temperature. In literature, the ratio of basic to acidic oxides is frequently used to predict the slagging behaviour of ash [21]. The observed correlation between PM10 and ash fusion temperature (and to a lesser extent the correlation with the ratio of basic to acidic oxides) suggests that the heating characteristics of the ash are indicative of the fine ash formed from the coalescence of included minerals. PM10 is formed from (a) the coalescence of very small included minerals, which combined result in particles smaller than 10 mm or (b) the shedding of small minerals (!10 mm) from the burning char surface. If small included minerals are not released from the surface by shedding, these minerals adhere to the char surface and coalesce with other minerals inside the burning char particle to form large ash particles. The ash fusion temperature is a bulk ash characteristic and does not directly relate to small included minerals inside the coal. However, if we assume that the melting behaviour of the minerals is independent of size, the ash fusion temperature could indicate the likelihood of coalescence of the minerals during combustion; a high ash fusion temperature indicates that minerals originally present in the coal are less likely to coalesce during combustion. If included minerals are less likely to coalesce, they can form individual ash particles, smaller than mineral agglomerates, resulting in elevated PM10 levels. Fig. 2
PM10 (% of ash collected)
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189
25.0 20.0
LT HT
15.0 10.0 5.0 0.0 1000
1200
1400
1600
1800
Ash deformation temperature,°C Fig. 2. PM10 correlation with ash fusion temperature for the five coals (LT, low temperature; HT, high temperature experiments).
shows how the amounts of PM10 measured during the experiments correlate with the ash deformation temperatures. The PM10 of the different coals in 50% O2 correlate well with the ash deformation temperatures of the five coals, as indicated for the high temperature (HT) experiments in the figure. The amounts of PM10 formed during the low temperature experiments show one significant outlier: CRC 240. This outlier can be explained by the morphology of the ash particles formed from this particular coal. The Malvern Mastersizer calculates the size distribution of a sample in suspension based on the diffraction of a laser light through the sample. In its analysis, it assumes that all particles are dense spheres. CRC 240 displays the highest ash deformation temperature, which results in non-spherical ash particles, indicated in Figs. 3 and 4. The amount of PM10 formed in 50% O2 (high temperature experiments) is significantly higher than the amount formed in 21% O2 (low temperature experiments). The gas temperature during both temperatures was held constant, and the volatile matter release (and thus, possible coal fragmentation during its release) should not be affected significantly by the change in oxygen concentration. However, at low temperatures, the burning char surface recedes at a pace at which included minerals have enough time to coalesce, while at high temperatures, it appears that there may be more shedding from the fast receding surface. This increased shedding could result in increased fine ash particle formation. Literature has shown that coal combustion at higher
Fig. 3. Typical SEM image of ash from CRC 240 generated in 21% oxygen.
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PM2.5 (% of ash generated)
2.0 LT HT
1.5 1.0 0.5 0.0 0
20
40
60
80
100
Group I char (number %) Fig. 5. Amount of PM2.5 as a function of Char Group I content of the five coals (LT, low temperature; HT, high temperature experiments).
the same size distribution and are uniformly distributed through the maceral types. Fig. 4. Typical SEM image of ash from CRC 240 generated in 50% oxygen.
temperatures results in finer ash and elevated PM10 levels [32]. In that study, the higher combustion temperatures were achieved by elevating the gas temperatures from 1100 to 1300 8C, which could have affected the extent of coal fragmentation during volatile matter release. In this study, the elevated levels of PM10 are thought to be resulting from the decrease in time for coalescence of included minerals during the char combustion. 5.2. PM2.5 PM2.5 is formed from two mechanisms: vaporisation and condensation (the homogeneously nucleated submicron ash particles) and the release of included minerals due to char fragmentation. Depending on the char combustion temperature (oxygen concentration), the submicron material accounts for the majority of PM2.5. Several studies have suggested that the release of small included minerals by fragmentation of cenospherical char particles contributes to ash particles in the size range between one and a few micrometers [18–20]. The formation of cenospherical char particles (Char Group I particles) has been correlated with the pressure of char formation and the coal vitrinite content by Benfell [24]. The PM2.5 has been correlated to the coal characteristics indicated in Table 1. No correlation was observed between the amounts of PM2.5 and the ash fusion temperatures, the ash and sulphur contents. A poor correlation was observed between the PM2.5 and the Char Group I content estimated from the vitrinite content, shown in Fig. 5. A correlation between PM2.5 and Char Group I content could be expected, however, the experimental results shown in Fig. 5 provide no conclusive evidence. There are two possible explanations for the lack of a good correlation between the two: † During the low temperature experiments, the contribution of PM1 to PM2.5 is significantly higher (up to 74%) than that during the high temperature experiments. As a result, the PM2.5 formation is dominated by the submicron ash, and little correlation with Char Group I content is expected. † A correlation between PM2.5 and Char Group I content is only expected when the included minerals of all coals have
Experiments using more coals and more detailed coal analyses could provide more insight in the coal characteristics that correlate with PM2.5. It must be noted that the Char Group I content is estimated from the vitrinite content, and that a similar correlation between PM2.5 and vitrinite content and with the mean vitrinite reflectance can be observed. The elevated PM2.5 emissions during the experiments in 50% O2 can be attributed to: † Increased vaporisation of elements, which increase PM1 formation and † At high temperatures, there may be more shedding from the fast receding surface, similar to the mechanism resulting in elevated PM10 levels at higher temperatures.
5.3. PM1 The chemical composition of PM1 is significantly different from the bulk chemical composition, and depends on the combustion temperature and the mode of occurrence of the inorganic material in the coal. The main components in the submicron ash are sulphur, silicon, sodium, and phosphorus, with sulphur being the most abundant element detected on the filters [10]. Particles collected onto the filter were typically around 20–30 nm, much smaller than the cut-off of the last impactor stage. Although the amount of particles collected on the last stages were much smaller, they accounted for a significant proportion of the mass. This makes identification of the dominating coal characteristic correlating with the amount of PM1 a difficult task. The amount of PM1 has been compared with the average mineral size, the ash and sulphur content and the Char Group I content. No correlation was observed between PM1 and the ash content or the average mineral grain size. Fig. 6 shows how PM1 and the total sulphur content in the coal correlate. Fig. 7 shows the correlation between PM1 and Char Group I content. Of the two coal characteristics correlated with the amount of PM1, the total sulphur content is the best indicator of the amount of PM1 formed. Sulphur is the most abundant element detected on the filters and a similar correlation is observed between the amount of ash collected on the filter and the coal
PM1 (% of ash collected)
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the enhanced vaporization of refractory oxides inside burning char particles [14].
1.0 LT 0.8
191
HT
0.6
5.4. Coal selection guideline
0.4 0.2 0.0 0
2
4
6
Coal sulphur content (% db)
PM1 (% of ash collected)
Fig. 6. Amount of PM1 as a function of coal sulphur content of the five coals (LT, low temperature, HT, high temperature experiments). 1.0 LT HT
0.8 0.6 0.4 0.2 0.0 0
20
40
60
This study has characterised coals using various methods and analysis techniques and compared the results of these techniques with the amount and characteristics of the fine ash formed. Table 6 summarizes the analysis techniques and coal characteristics that have been found best to assess coals for their potential to form fine ash in this study. The guidelines can be used to assess different coals for their potential to form fine ash during combustion. The guideline for PM1 is questionable, as the correlation is skewed by the PM1 formed from one coal with high sulphur content. PM1 is formed from a wide variety of elements and the elemental mode of occurrence in the coal, which is responsible for the occurrence of these elements in the submicron ash is not always known. For example, two coals show significant amount of sodium in the submicron ash, which is not reflected in the previous guideline [10].
80
Char Group I content (number %) Fig. 7. Amount of PM1 as a function of char group I content of the five coals (LT, low temperature, HT, high temperature experiments).
sulphur content. However, no correlation could be observed between the Char Group I content and the amount of ash collected on the filter. The ash collected on the filter is very fine (around 20–30 nm) compared to the ash collected on the latter stages of the impactor (a few hundred nanometers). The ash collected on the last stages could be the result of char fragmentation, while the material collected on the filter is the result of vaporisation and condensation of inorganic material. As a first pass, the coal sulphur content could be used as an indicator of the amount of PM 1 formed during coal combustion; however, more experiments using higher sulphur coals could confirm this conclusion. The amounts of PM1 formed during the high temperature experiments are consistently higher than the amounts formed during the low temperature experiments. This observation is well documented in the literature and can be attributed to
6. Conclusion The ash size fractions of legislative interest (PM10 and PM2.5, and PM1) formed during coal combustion are the result of several ash formation mechanisms. They are formed by a combination of (1) included mineral coalescence (2) excluded mineral fragmentation, (3) char fragmentation, and (4) vaporization and subsequent condensation of inorganic matter. Ash particles are characteristics in size for the mechanism via which they are formed. Coal characteristics and combustion conditions determine the extent to which the ash formation processes contribute to the total ash formed. Five well-characterized black coals have been selected to represent the range of ash chemistry and char swelling behaviour of Australian bituminous coals. Ash was generated by combustion of the coals in a drop tube furnace, simulating combustion in a pf boiler. The amount of fine ash was determined and the coal characteristics responsible for its formation have been established.
Table 6 Guidelines for black coal assessment for their potential to form fine ash Fine ash
Analysis technique
Measured coal characteristic
Trends observed
Comments
PM10
Ash fusibility test in oxidizing environment, [13]
Ash deformation temperature
PM2.5
Maceral analysis (e.g. reflectogram [12])
Combustion in 21 and 50% O2 showed similar trends, with one outlier (Fig. 2) This correlation is expected, however, the experiments are inconclusive. (Fig. 5)
PM1
Ultimate analysis, total sulphur content [35]
Char Group I content (proportion of coal that shows swelling behaviour during char formation), determined from vitrinite content. Total amount of sulphur in the coal
The higher the ash fusion temperature, the more PM10 (% of ash) is formed during combustion The higher the Char Group I content, the more the char swells, the more the PM2.5 is expected to be formed The more sulphur is detected, the higher the amount of submicron ash formed
Sulphur has been shown to be a major contributor to PM1 and the observed correlation is mainly the result of the high sulphur presence in CRC 297 (Fig. 6)
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The amount of PM10 formed during the experiments correlates with the ash fusion temperature. A high ash fusion temperature could indicate that the minerals originally present in the coal are less likely to coalesce. If included minerals are less likely to coalesce, they can transform into individual ash particles, smaller than mineral agglomerates, resulting in elevated PM10 levels. The amounts of PM2.5 formed would be expected to correlate with the amount of char particles displaying swelling behaviour, expressed as Char Group I particles. Thin-walled cenospherical char particles can fragment during combustion, resulting in the release of fine included minerals originally present in these char particles, which transform to ash particles of a few micrometers in size. This expectation could not be confirmed conclusively from the experiments. More experiments using different coals and maceral types could confirm this expectation. The amounts of PM1 correlate slightly with the coal sulphur content. More experiments using higher sulphur coals could confirm this observation. The amount of PM10, PM2.5 and PM1 increased consistently when increasing the oxygen concentration in the combustion gas from 21 to 50%. The enhanced temperatures increase the char fragmentation and vaporization of inorganic matter. Finally, it is shown that these results can be used a guidance for coal selection for minimization of fine particle formation. Acknowledgements The authors wish to acknowledge the financial support provided by the Cooperative Research Centre for Coal in Sustainable Development, which is funded in part by the Cooperative Research Centres Program of the Commonwealth Government of Australia.
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[9] Buhre BJP. Fine ash formation from coal combustion, Doctor of philosophy in chemical engineering. Newcastle: University of Newcastle; 2004 p. 388. [10] Buhre BJP, Hinkley JT, Gupta RP, Wall TF, Nelson PF. Submicron ash formation from coal combustion. Fuel 2005;84:1206–14. [11] Timothy LD, Sarofim AF, Beer JM. Characteristics of single particle coal combustion, 19th symposium (international) on combustion 1982 p. 1123–30. [12] O’Brien G, Jenkins B, Beath H. Coal characterization by automated coal petrography. Fuel 2003;82:1067–73. [13] AS1038.15, coal and coke-analysis and testing, part 15: higher rank coal ash and coke ash — ash fusibility. Standards association of Australia, homebush, NSW, Australia; 1995 p. 15. [14] Quann RJ, Sarofim AF. Vaporisation of refractory oxides during pulverised coal combustion, nineteenth symposium (international) on combustion 1982 p. 1429–40. [15] Sarofim AF, Howard JB, Padia AS. The physical transformation of the mineral matter in pulverized coal under simulated combustion conditions. Combust Sci Technol 1977;16:187–204. [16] Flagan RC, Friedlander SK. In: Shaw DT, editor. Particle formation in pulverized coal combustion — a review, recent developments in aerosol science. London: Wiley; 1978. p. 25–59. [17] Yan L, Gupta RP, Wall TF. A mathematical model of ash formation during pulverised coal combustion. Fuel 2002;81:337–44. [18] Linak WP, Seames WS, Wendt JOL, Ishinomori T, Endo Y, Miyamae S. On trimodal particle size distributions in fly ash from pulverised-coal combustion, 29th international symposium on combustion, Sapporo, Japan 2002 p. 441–7. [19] Seames WS. An initial study on the fine fragmentation fly ash particle mode generated during pulverised coal combustion. Fuel Process Technol 2003;81:109–25. [20] Liu G, Wu H, Gupta RP, Lucas JA, Tate AG, Wall TF. Modeling the fragmentation of non-uniform porous char particles during pulverised coal combustion. Fuel 2000;79:627–33. [21] Raask E. Mineral impurities in coal combustion, behaviour, problems, and remedial measures. USA: Hemisphere Publication Corporation; 1985. [22] Monroe LS. An experimental study of residual fly ash formation in combustion of a bituminous coal Doctor of philosophy in chemical engineering. Cambridge: Massachusetts Institute of Technology; 1989. [23] Benfell KE, Liu G, Roberts DG, Harris DJ, Lucas JA, Bailey JG, et al. Modeling char combustion: the influence of parent coal petrography and pyrolysis pressure on the structure and intrinsic reactivity of its char, twenty eighth symposium (international) on combustion 2000 p. 2233–41. [24] Benfell KE. Assessment of char morphology in high pressure pyrolysis and combustion Doctor of philosophy in department of geology. Newcastle: University of Newcastle; 2001 p. 195. [25] Liu G-S, Rezaei HR, Lucas JA, Harris DJ, Wall TF. Modelling of a pressurised entrained glow coal gasifier: the effect of reaction kinetics and char structure. Fuel 2000;79:1767–79. [26] Wall TF, Liu G, Wu H, Roberts DG, Benfell KE, Gupta RP, et al. The effect of pressure on coal reactions during pulverised coal combustion and gasification. Prog Energy Combust Sci 2002;28:433. [27] Yan L, Gupta R, Wall TF. Fragmentation behavior of pyrite and calcite during high-temperature processing and mathematical simulation. Energy Fuels 2001;15:389–94. [28] Ten Brink HM, Smart JP, Vleeskens JM, Williamson J. Flame transformations and burner slagging in a 2.5 MW furnace firing pulverized coal: 1. Flame transformations. Fuel 1994;73:1706–11. [29] Quann RJ, Neville M, Sarofim AF. A laboratory study of the effect of coal selection on the amount and composition of combustion generated submicron particles. Combust Sci Technol 1990;74:245–65. [30] Eddings EG, Sarofim AF, Lee CL, Davis KA, Valentine JR. Trends in predicting and controlling ash vaporization in coal-fired utility boilers. Fuel Process Technol 2001;71:39–51. [31] Lee CM, Davis KA, Heap MP, Eddings E, Sarofim AF. Modeling the vaporization of ash constituents in a coal-fired boiler, 28th symposium (international) on combustion, Edinburgh, Scotland, UK 2000 p. 2375–82.
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[34] AS1038.8.1, coal and coke — analysis and testing, part 8.1: coal and coke — chlorine — eschka method. Standards Australia international, Strathfield, NSW; 1999. [35] AS1038.6.3.1, coal and coke — analysis and testing, part 6.3.1: higher rank coal and coke — ultimate analysis-total sulfur-eschka method. Standards association of Australia, Homebush, NSW, Australia; 1997 p 11.
Fine ash formation during combustion of pulverised coal–coal property impacts B.J.P. Buhre a,*, J.T. Hinkley a, R.P. Gupta a, P.F. Nelson b, T.F. Wall a a
Cooperative Research Centre for Coal in Sustainable Development, Department of Chemical Engineering, University of Newcastle, Callaghan, NSW 2300, Australia b Cooperative Research Centre for Coal in Sustainable Development, Graduate School of the Environment, Macquarie University, NSW 2109, Australia Received 5 October 2004; received in revised form 24 February 2005; accepted 14 April 2005 Available online 15 September 2005
Abstract In many countries, legislation has been enacted to set guidelines for ambient concentrations and to limit the emission of fine particulates with an aerodynamic diameter less than 10 mm (PM10) and less than 2.5 mm (PM2.5). Ash particles are formed during the combustion of coal in pf boilers and fine ash particulates may potentially pass collection devices. The ash size fractions of legislative interest formed during coal combustion are the result of several ash formation mechanisms; however, the contribution of each of the mechanisms to the fine ash remains unclear. This study provides insight into the mechanisms and coal characteristics responsible for the formation of fine ash. Five well characterized Australian bituminous coals have been burned in a laminar flow drop tube furnace in two oxygen environments to determine the amount and composition of the fine ash (PM10, PM2.5 and PM1) formed. Coal characteristics have been identified that correlate with the formation of fine ash during coal combustion. The results indicate that coal selection based on (1) char characterization and (2) ash fusion temperature could play an important role in the minimization of the fine ash formed. The implications of these findings for coal selection for use in pf-fired boilers are discussed. q 2005 Elsevier Ltd. All rights reserved. Keywords: Coal combustion; Fine ash; Coal characterisation
1. Introduction The health effects of ambient fine particulates have been studied extensively and correlations have been observed between ambient fine particulate matter and human mortality rates e.g. [1]. Governments worldwide acknowledge these studies and as a result standards have been introduced to assist in reducing ambient fine particulate concentrations. In the United States, a National Ambient Air Quality Standard (NAAQS) for both ‘coarse’ particulate matter with an aerodynamic diameter less than 10 mm, PM10, as well as an NAAQS for PM2.5 is currently in effect [2]. Recently in Australia, the National Environment Protection (Ambient Air Quality) Measure (NEPM) has been modified to include advisory reporting standards for PM2.5, the monitoring of which commenced in January 2004 [3]. PM10 has been included in the NEPM Ambient Air Quality measures since, * Corresponding author. Tel.: C61 2 4921 6179; fax: C61 2 4921 6920. E-mail address: [email protected] (B.J.P. Buhre).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.04.031
1998 [4]. Around 80% of the electricity generated in Australia is generated by coal combustion [5], and coalfired power generators are increasingly being required to monitor and characterise their emissions of fine particulate matter. Air pollution control devices (APCDs) are employed at power stations to capture the ash particulates from the flue gas. Increasing the capture efficiency of these APCDs to decrease fine ash emissions can be a costly exercise. Selecting coals which reduce the formation offine ash particulates in the first place could be a cost-effective method of minimizing fine ash emissions. This paper presents the results of a study for the coal characteristics responsible for the formation of fine ash particles. Five Australian black coals have been characterised extensively using advanced analytical techniques such as QEMSCAN, SIROQUANT, and ICP-AES on ash obtained from radio-frequency ashing technique. The coals were then burned in a drop tube furnace, which simulates the combustion in a pf fired boiler. The ash was characterised and the coal characteristics responsible for the formation of the ash size fractions of legislative interest (PM10, PM2.5, and PM1) have been determined.
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2. Experimental Five Australian black coals have been selected to represent the range of ash chemistry and char swelling behaviour occurring in Australian bituminous coals; the coals were chosen on the basis of their sulphur content, other constituents and their vitrinite content. The coals were milled and a size cut between 63 and 90 mm was obtained, suitable for feeding into the drop tube furnace. The coals were subjected to proximate and ultimate analysis, reflectogram, and QEMSCAN analyses. QEMSCAN is a novel version of CCSEM technique that can directly measure the coal–mineral and mineral–mineral associations. In the technique, pulverised coal samples are mounted in carnauba wax. The sample block is sectioned and the surface is polished and coated with a thin layer of carbon to ensure electrical conductivity. The particles are thus, well separated and in random orientations with random sections exposed. Qem*SEM (quantitative evaluation of materials by scanning electron microscopy) is an earlier version of QEMSCAN and has been described by Creelman and Ward [6]. The technique is an automated image analysis system that uses Backscattered Electron (BSE) and Energy Dispersive X-ray (EDX) signals from a scanning electron microscope to create pixilated images in which each pixel represents the EDX spectrum obtained from the centre of the pixel. The light element X-ray detectors enable the detection of organic material, enabling a direct measurement of the coal-mineral associations. The pixel spacing is an operator setting and determines the resolution of the QEMSCAN images. For the analyses used in this study, a pixel spacing of 2 mm was used. Ash was obtained from the coal samples using a low temperature radio frequency ashing technique [7], after which the ash was analysed for chemical composition using ICPAES, and quantitative mineralogy using SIROQUANT. The procedure applied for the ash dissolution has been described in detail elsewhere [8]. The combination of the analytical techniques resulted in a thorough understanding of the organic and inorganic characteristics of the coal samples. After characterisation, the coals were burned in the Tetlow Model HTF375A drop tube furnace situated at the Chemical Engineering department at the University of Newcastle in
Australia. The ash was collected using a cyclone, a cascade impactor and subsequently a filter. The cyclone has a theoretical d50 of 3 mm at the operating conditions applied during the experiments. The cascade impactor is a seven stage MRI cascade impactor, model 1503. After correction for an assumed particle density, the d50’s of the seven impaction plates were 17.9, 8.2, 3.2, 1.4, 0.8, 0.4, and 0.3 mm, respectively. The experimental set-up is described elsewhere in more detail [9,10]. After combustion, the ash was characterised for its size distribution, morphology, and chemical composition. The ash collected in the cyclone was suspended in ethanol and the size distribution of the ash was determined using a Malvern Mastersizer S with a 300 mm lens. The amounts of ash collected on the different plates of the cascade impactor were combined with the size distribution obtained from the Malvern Mastersizer to obtain a complete size distribution (Table 1). The measurement of PM10 from the ash samples was based on the analysis results of the Mastersizer, while the PM2.5 and PM1 was determined from the masses collected in the cascade impactor. The amount of PM2.5 is estimated as the combined masses of the amounts collected on the filter and on the bottom five stages. The PM1 is calculated from the mass collected on the filter combined with the bottom three stages. Moisture adsorption onto the filter can significantly affect the observed mass of the material collected onto the filter [9]. The moisture is an artefact of the experimental procedure and should not be considered when determining the mass of PM. Previous publications presented PM1 measurements as the balance mass, which does not take the effect of moisture absorption into account [10]. The chemical compositions of the material collected on the filters have been determined quantitatively using a combination of Proton Induced X-Ray Emission (PIXE) and Proton Induced Gamma Ray Emission (PIGE) analysis. The PIXE and PIGE analysis results have been used to calculate the mass of the elements present (converted to oxides). This method ensures that moisture adsorbed from the combustion gas or the atmosphere does not affect the results. The repeatability of the experimental set-up to determine the amount of PM1 using this technique is described elsewhere [9,10].
Table 1 Considered coal characteristics for each size fraction and the ash formation mechanisms they could affect Fine Ash
Coal characteristic
Ash formation mechanism affected
PM10
Ash Fusion Temperature Basic to acidic oxide ratio Average mineral grain size Total ash content Char Group I content / vitrinite content Ash Fusion Temperature Ash content Char Group I content / Vitrinite content Sulphur content Ash content Char Group I content / Vitrinite content Sulphur content Average mineral grain size
Included mineral coalescence Included mineral coalescence Included mineral coalescence, Vaporization and condensation Included mineral coalescence, Char fragmentation Char fragmentation Included Mineral Coalescence Char Fragmentation, Included mineral coalescence Char fragmentation Vaporisation and condensation Included mineral coalescence, Char fragmentation Char fragmentation Vaporisation and condensation Included mineral coalescence, Vaporisation and condensation
PM2.5
PM1
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Table 2 Proximate analysis, rank and vitrinite content of the five coals used in this study (HV-Bit, high volatile bituminous; MV-Bit, medium volatile bituminous coal) Sample ID
CRC 240
CRC 272
CRC 296
CRC 297
CRC 306
Moisture (% ad) Ash (% db) Volatile matter (% db) Fixed carbon (% db) (by difference) Total sulphur content (% db) Coal rank Vitrinite content (% v/v mmf) Ash fusion temperature (deformation temperature, 8C) Average mineral size (mm)
2.14 12.96 30.41 56.63 0.60 HV-Bit 68.3 O1600
2.47 9.18 35.69 55.13 0.96 HV-Bit 50.7 1280
2.09 13.81 30.34 55.85 0.61 HV-Bit 39.0 1490
1.27 11.93 45.41 42.66 5.07 HV-Bit 57.8 1300
1.64 18.77 19.68 61.55 1.72 MV-Bit 28.1 1240
10.8
14.0
11.4
16.7
The composition of the oxidizing gas was varied during the experiments to determine its effect on fine particle formation. The coals were burned at 1400 8C in 21% O2 (air) and in 50% O2. The elevated oxygen concentration results in higher char combustion temperatures. Simulations and literature indicated that by increasing the oxygen content from 21 to 50%, the temperature of the burning char particles increase from approximately 2030–2530 8C [11]. 3. Results The coal compositions are provided in Tables 2 and 3. Table 2 provides the proximate analysis, coal rank, vitrinite content as determined from the reflectogram [12], and the deformation temperature as determined during the ash fusion test in a reducing environment [13] for all five coals. For mineral analysis, the coals have been ashed using a low temperature radio frequency ashing technique [7]. Table 3 provides the ash elemental composition, together with total sulfur and chlorine content. Tables 4 and 5 provide the total ash recovered, the PM10, PM2.5, and PM1 formed during the combustion experiments Table 3 Elemental composition of the low-temperature ash obtained from the five coals measured by ICP-AES, results presented in elemental wt% LTA ash elemental composition %
CRC 240
CRC 272
CRC 296
CRC 297
CRC 306
Si Al Fe Ca Mg Na K Ti Mn Sa(db) P Ba Sr Clb(db)
20.8 16.0 1.1 0.2 0.1 0.1 0.8 0.80 0.005 0.60 0.07 0.03 0.02 0.03
18.3 11.1 5.0 4.5 0.2 0.1 0.2 1.08 0.05 0.96 0.3 0.06 0.05 0.07
32.2 7.9 0.3 0.3 0.1 0.02 1.3 0.63 0.004 0.61 0.1 0.03 0.02 0.01
10.7 9.9 3.5 9.1 2.0 0.2 0.1 0.70 0.12 5.07 0.2 0.03 0.10 !0.01
15.6 6.8 9.7 9.1 1.2 0.1 0.6 0.48 0.13 1.72 0.2 0.04 0.04 0.05
The remainder of the ash includes oxygen, water of constitution, and carbon present as carbonates. a Sulfur as determined by [33]. b Chlorine as determined by [34].
7.7
together with the total ash collected during the experiments. The amounts are reported as percentage of the ash generated during combustion. The proportion of the ash larger than 10 mm was typically much larger in size than 10 mm and displayed a mean size in the order of tens of micrometers.
4. Ash formation mechanisms During coal combustion, ash particles are formed from the inorganic matter present in coal. The mechanisms of ash formation have been studied extensively in the literature, and are affected by the combustion conditions and the coal characteristics [14–17]. Two important coal characteristics influencing the ash formation process are † The mode of occurrence of the inorganic matter and † The combustion behaviour of coal particles containing both organic and inorganic matter. The majority of inorganic matter present in black coals occurs in the form of minerals of various types and sizes. These minerals can be closely associated with the organic matter (included minerals), or they occur excluded from the organic matter (excluded minerals). The majority of ash particles are formed from four formation mechanisms, shown in Fig. 1: † † † †
Included mineral coalescence, Char fragmentation, Excluded mineral fragmentation, and Vaporization and subsequent condensation of inorganic matter.
Table 4 Total ash collected, PM10, PM2.5, and PM1 generated during combustion in 21% O2 at 1400 8C PM10, PM2.5, and PM1 as % of ash generated during combustion in 21% O2 21% O2
CRC 240
CRC 272
CRC 296
CRC 297
CRC 306
PM10 PM2.5 PM1 (dry) Total ash collected
0.9 0.15 0.08 81.9
6.1 0.53 0.30 82.8
10.7 0.49 0.20 91.6
3.2 0.78 0.58 72.4
3.8 0.40 0.11 73.3
PM10 determined from Malvern Mastersizer, PM2.5 and PM1 determined from weight collected in cascade impactor. Results presented as % of ash generated.
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Table 5 Total ash collected, PM10, PM2.5, and PM1 generated during combustion in 50% oxygen at 1400 8C PM10, PM2.5, and PM1 as % of ash generated during combustion in 50% O2 50% O2
CRC 240
CRC 272
CRC 296
CRC 297
CRC 306
PM10 PM2.5 PM1 (dry) Total ash collected
19.8 0.72 0.34 66.7
11.7 1.41 0.44 86.1
17.4 1.06 0.39 90.0
13.3 1.60 0.85 77.5
13.3 0.44 0.19 84.0
PM10 determined from Malvern Mastersizer, PM2.5 and PM1 determined from weight collected in cascade impactor. Results presented as % of ash generated.
Typically, ash particles are characteristic in size for the mechanism via which they are formed. The first mechanism is believed to be responsible for the bulk of the supermicron ash particles formed [16]. The formation of ash particles from the fragmentation of thin-walled cenospherical char particles (i.e. the second mechanism) contributes to ash particles of only a few micrometer in size [18–20]. Excluded mineral fragmentation contributes mainly to supermicron-sized ash particles, and the last formation mechanism contributes mainly to submicron sized ash particles [14]. Coal characteristics play an important role in the extent to which the three ash formation mechanisms contribute to the total ash formed. A summary of the main characteristics affecting the formation mechanisms is provided below. 4.1. Included mineral coalescence When char particles do not fragment, the ash formed from included mineral coalescence is affected by the type, size and distribution of included minerals. The high temperatures occurring inside burning char particles cause included minerals to turn viscous and coalesce. The ash fusion temperature is a standard test done on coal ashes to characterise the various stages of melting [13]. Although the ash fusion test does not differentiate between included and excluded minerals, the test indicates the melting behaviour of the inorganic matter upon heating. The results in this paper suggest that this melting behaviour could indicate the affinity of minerals to coalesce during combustion. The ratio of basic oxides to acidic oxides is often used in the literature as an indicator of ash slag viscosity at high temperatures (e.g. [21]). If the ash fusion temperature could be used as an indicator, this ratio is likely have a similar relationship with the extent of mineral coalescence during coal combustion. 4.2. Char fragmentation The extent of char fragmentation depends on the char swelling behaviour during combustion [20]. If the char fragments are large and contain large minerals, the minerals inside these fragments will coalesce and form large ash particles. However, if the char swells extensively, the char will fragment into small pieces and minerals inside these small fragments will result in small ash particles. The two
Fig. 1. Schematic of ash formation mechanisms during pulverised coal combustion.
mechanisms shown in Fig. 1 are extreme cases and ash formed from coal combustion is a combination of the two. Monroe suggested a mathematical model to predict the coalescence behaviour of minerals as a function of cenosphere shell thickness, coal particle size, mineral grain size, and mineral volume fraction [22]. Yan validated this model by determining the char swelling behaviour of several Australian coals and its effect on the ash formation [17]. He modelled the char fragmentation based on a char classification scheme, in which the char is classified in three classes based on their swelling behaviour. Char Group I particles are defined as char particles that are highly porous and have low densities. The combustion of these thin-walled cenospherical char particles can result in more extensive fragmentation during combustion, resulting in fine ash formation. Benfell showed that the amount of Char Group I particles of Australian coals could be estimated from the coal vitrinite content and the pressure at which the char was formed [23,24]: Char Group I ðnumber%Þ Z 0:994 !Pressure ðatmÞ C 0:621 ðVitrinite; v=v% mmfÞ C 29:87 This correlation is an updated version of a correlation published earlier by the same research group [25,26]. The updated version is based on a more extensive coal database but produces similar results as the earlier correlation [24]. 4.3. Excluded mineral fragmentation The amount, size, and type of excluded minerals determine the extent of excluded mineral fragmentation. The main fragmenting mineral types in Australian black coals are calcite and pyrite [27]. Although extensive fragmentation of these
minerals has been noted, the bulk of the newly formed ash particles are generally supermicron in size [27,28]. 4.4. Vaporization and condensation The extent of vaporization of inorganic matter during coal combustion depends on (a) the temperature inside the burning char particle, and (b) the occurrence of readily vaporised material such as alkalis, sulphur, and phosphorus [14,29]. The vaporization of refractory oxides from included minerals has also been suggested to depend on the size distribution of included minerals containing refractory oxides [14,30,31]. Summarizing, the size fractions of legislative interest are the result of several ash formation mechanisms. Several coal characteristics affect the extent to which the formation mechanisms result in fine ash. This study aims at determining the dominating coal characteristics that correlates with the amounts of fine ash formed. Table 1 shows the coal characteristics considered for three size fractions (PM10, PM2.5 and PM1), together with the ash formation mechanism they could affect. 5. Discussion 5.1. PM10 The amounts of PM10 have been compared with the coal characteristics indicated in Table 1. The PM10 showed no correlation with average mineral size, ash content, or Char Group I content. The ratio of basic to acidic oxides showed moderate correlation with the amounts of PM10, and a good correlation was observed between PM10 and the ash fusion temperature. In literature, the ratio of basic to acidic oxides is frequently used to predict the slagging behaviour of ash [21]. The observed correlation between PM10 and ash fusion temperature (and to a lesser extent the correlation with the ratio of basic to acidic oxides) suggests that the heating characteristics of the ash are indicative of the fine ash formed from the coalescence of included minerals. PM10 is formed from (a) the coalescence of very small included minerals, which combined result in particles smaller than 10 mm or (b) the shedding of small minerals (!10 mm) from the burning char surface. If small included minerals are not released from the surface by shedding, these minerals adhere to the char surface and coalesce with other minerals inside the burning char particle to form large ash particles. The ash fusion temperature is a bulk ash characteristic and does not directly relate to small included minerals inside the coal. However, if we assume that the melting behaviour of the minerals is independent of size, the ash fusion temperature could indicate the likelihood of coalescence of the minerals during combustion; a high ash fusion temperature indicates that minerals originally present in the coal are less likely to coalesce during combustion. If included minerals are less likely to coalesce, they can form individual ash particles, smaller than mineral agglomerates, resulting in elevated PM10 levels. Fig. 2
PM10 (% of ash collected)
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25.0 20.0
LT HT
15.0 10.0 5.0 0.0 1000
1200
1400
1600
1800
Ash deformation temperature,°C Fig. 2. PM10 correlation with ash fusion temperature for the five coals (LT, low temperature; HT, high temperature experiments).
shows how the amounts of PM10 measured during the experiments correlate with the ash deformation temperatures. The PM10 of the different coals in 50% O2 correlate well with the ash deformation temperatures of the five coals, as indicated for the high temperature (HT) experiments in the figure. The amounts of PM10 formed during the low temperature experiments show one significant outlier: CRC 240. This outlier can be explained by the morphology of the ash particles formed from this particular coal. The Malvern Mastersizer calculates the size distribution of a sample in suspension based on the diffraction of a laser light through the sample. In its analysis, it assumes that all particles are dense spheres. CRC 240 displays the highest ash deformation temperature, which results in non-spherical ash particles, indicated in Figs. 3 and 4. The amount of PM10 formed in 50% O2 (high temperature experiments) is significantly higher than the amount formed in 21% O2 (low temperature experiments). The gas temperature during both temperatures was held constant, and the volatile matter release (and thus, possible coal fragmentation during its release) should not be affected significantly by the change in oxygen concentration. However, at low temperatures, the burning char surface recedes at a pace at which included minerals have enough time to coalesce, while at high temperatures, it appears that there may be more shedding from the fast receding surface. This increased shedding could result in increased fine ash particle formation. Literature has shown that coal combustion at higher
Fig. 3. Typical SEM image of ash from CRC 240 generated in 21% oxygen.
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PM2.5 (% of ash generated)
2.0 LT HT
1.5 1.0 0.5 0.0 0
20
40
60
80
100
Group I char (number %) Fig. 5. Amount of PM2.5 as a function of Char Group I content of the five coals (LT, low temperature; HT, high temperature experiments).
the same size distribution and are uniformly distributed through the maceral types. Fig. 4. Typical SEM image of ash from CRC 240 generated in 50% oxygen.
temperatures results in finer ash and elevated PM10 levels [32]. In that study, the higher combustion temperatures were achieved by elevating the gas temperatures from 1100 to 1300 8C, which could have affected the extent of coal fragmentation during volatile matter release. In this study, the elevated levels of PM10 are thought to be resulting from the decrease in time for coalescence of included minerals during the char combustion. 5.2. PM2.5 PM2.5 is formed from two mechanisms: vaporisation and condensation (the homogeneously nucleated submicron ash particles) and the release of included minerals due to char fragmentation. Depending on the char combustion temperature (oxygen concentration), the submicron material accounts for the majority of PM2.5. Several studies have suggested that the release of small included minerals by fragmentation of cenospherical char particles contributes to ash particles in the size range between one and a few micrometers [18–20]. The formation of cenospherical char particles (Char Group I particles) has been correlated with the pressure of char formation and the coal vitrinite content by Benfell [24]. The PM2.5 has been correlated to the coal characteristics indicated in Table 1. No correlation was observed between the amounts of PM2.5 and the ash fusion temperatures, the ash and sulphur contents. A poor correlation was observed between the PM2.5 and the Char Group I content estimated from the vitrinite content, shown in Fig. 5. A correlation between PM2.5 and Char Group I content could be expected, however, the experimental results shown in Fig. 5 provide no conclusive evidence. There are two possible explanations for the lack of a good correlation between the two: † During the low temperature experiments, the contribution of PM1 to PM2.5 is significantly higher (up to 74%) than that during the high temperature experiments. As a result, the PM2.5 formation is dominated by the submicron ash, and little correlation with Char Group I content is expected. † A correlation between PM2.5 and Char Group I content is only expected when the included minerals of all coals have
Experiments using more coals and more detailed coal analyses could provide more insight in the coal characteristics that correlate with PM2.5. It must be noted that the Char Group I content is estimated from the vitrinite content, and that a similar correlation between PM2.5 and vitrinite content and with the mean vitrinite reflectance can be observed. The elevated PM2.5 emissions during the experiments in 50% O2 can be attributed to: † Increased vaporisation of elements, which increase PM1 formation and † At high temperatures, there may be more shedding from the fast receding surface, similar to the mechanism resulting in elevated PM10 levels at higher temperatures.
5.3. PM1 The chemical composition of PM1 is significantly different from the bulk chemical composition, and depends on the combustion temperature and the mode of occurrence of the inorganic material in the coal. The main components in the submicron ash are sulphur, silicon, sodium, and phosphorus, with sulphur being the most abundant element detected on the filters [10]. Particles collected onto the filter were typically around 20–30 nm, much smaller than the cut-off of the last impactor stage. Although the amount of particles collected on the last stages were much smaller, they accounted for a significant proportion of the mass. This makes identification of the dominating coal characteristic correlating with the amount of PM1 a difficult task. The amount of PM1 has been compared with the average mineral size, the ash and sulphur content and the Char Group I content. No correlation was observed between PM1 and the ash content or the average mineral grain size. Fig. 6 shows how PM1 and the total sulphur content in the coal correlate. Fig. 7 shows the correlation between PM1 and Char Group I content. Of the two coal characteristics correlated with the amount of PM1, the total sulphur content is the best indicator of the amount of PM1 formed. Sulphur is the most abundant element detected on the filters and a similar correlation is observed between the amount of ash collected on the filter and the coal
PM1 (% of ash collected)
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the enhanced vaporization of refractory oxides inside burning char particles [14].
1.0 LT 0.8
191
HT
0.6
5.4. Coal selection guideline
0.4 0.2 0.0 0
2
4
6
Coal sulphur content (% db)
PM1 (% of ash collected)
Fig. 6. Amount of PM1 as a function of coal sulphur content of the five coals (LT, low temperature, HT, high temperature experiments). 1.0 LT HT
0.8 0.6 0.4 0.2 0.0 0
20
40
60
This study has characterised coals using various methods and analysis techniques and compared the results of these techniques with the amount and characteristics of the fine ash formed. Table 6 summarizes the analysis techniques and coal characteristics that have been found best to assess coals for their potential to form fine ash in this study. The guidelines can be used to assess different coals for their potential to form fine ash during combustion. The guideline for PM1 is questionable, as the correlation is skewed by the PM1 formed from one coal with high sulphur content. PM1 is formed from a wide variety of elements and the elemental mode of occurrence in the coal, which is responsible for the occurrence of these elements in the submicron ash is not always known. For example, two coals show significant amount of sodium in the submicron ash, which is not reflected in the previous guideline [10].
80
Char Group I content (number %) Fig. 7. Amount of PM1 as a function of char group I content of the five coals (LT, low temperature, HT, high temperature experiments).
sulphur content. However, no correlation could be observed between the Char Group I content and the amount of ash collected on the filter. The ash collected on the filter is very fine (around 20–30 nm) compared to the ash collected on the latter stages of the impactor (a few hundred nanometers). The ash collected on the last stages could be the result of char fragmentation, while the material collected on the filter is the result of vaporisation and condensation of inorganic material. As a first pass, the coal sulphur content could be used as an indicator of the amount of PM 1 formed during coal combustion; however, more experiments using higher sulphur coals could confirm this conclusion. The amounts of PM1 formed during the high temperature experiments are consistently higher than the amounts formed during the low temperature experiments. This observation is well documented in the literature and can be attributed to
6. Conclusion The ash size fractions of legislative interest (PM10 and PM2.5, and PM1) formed during coal combustion are the result of several ash formation mechanisms. They are formed by a combination of (1) included mineral coalescence (2) excluded mineral fragmentation, (3) char fragmentation, and (4) vaporization and subsequent condensation of inorganic matter. Ash particles are characteristics in size for the mechanism via which they are formed. Coal characteristics and combustion conditions determine the extent to which the ash formation processes contribute to the total ash formed. Five well-characterized black coals have been selected to represent the range of ash chemistry and char swelling behaviour of Australian bituminous coals. Ash was generated by combustion of the coals in a drop tube furnace, simulating combustion in a pf boiler. The amount of fine ash was determined and the coal characteristics responsible for its formation have been established.
Table 6 Guidelines for black coal assessment for their potential to form fine ash Fine ash
Analysis technique
Measured coal characteristic
Trends observed
Comments
PM10
Ash fusibility test in oxidizing environment, [13]
Ash deformation temperature
PM2.5
Maceral analysis (e.g. reflectogram [12])
Combustion in 21 and 50% O2 showed similar trends, with one outlier (Fig. 2) This correlation is expected, however, the experiments are inconclusive. (Fig. 5)
PM1
Ultimate analysis, total sulphur content [35]
Char Group I content (proportion of coal that shows swelling behaviour during char formation), determined from vitrinite content. Total amount of sulphur in the coal
The higher the ash fusion temperature, the more PM10 (% of ash) is formed during combustion The higher the Char Group I content, the more the char swells, the more the PM2.5 is expected to be formed The more sulphur is detected, the higher the amount of submicron ash formed
Sulphur has been shown to be a major contributor to PM1 and the observed correlation is mainly the result of the high sulphur presence in CRC 297 (Fig. 6)
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The amount of PM10 formed during the experiments correlates with the ash fusion temperature. A high ash fusion temperature could indicate that the minerals originally present in the coal are less likely to coalesce. If included minerals are less likely to coalesce, they can transform into individual ash particles, smaller than mineral agglomerates, resulting in elevated PM10 levels. The amounts of PM2.5 formed would be expected to correlate with the amount of char particles displaying swelling behaviour, expressed as Char Group I particles. Thin-walled cenospherical char particles can fragment during combustion, resulting in the release of fine included minerals originally present in these char particles, which transform to ash particles of a few micrometers in size. This expectation could not be confirmed conclusively from the experiments. More experiments using different coals and maceral types could confirm this expectation. The amounts of PM1 correlate slightly with the coal sulphur content. More experiments using higher sulphur coals could confirm this observation. The amount of PM10, PM2.5 and PM1 increased consistently when increasing the oxygen concentration in the combustion gas from 21 to 50%. The enhanced temperatures increase the char fragmentation and vaporization of inorganic matter. Finally, it is shown that these results can be used a guidance for coal selection for minimization of fine particle formation. Acknowledgements The authors wish to acknowledge the financial support provided by the Cooperative Research Centre for Coal in Sustainable Development, which is funded in part by the Cooperative Research Centres Program of the Commonwealth Government of Australia.
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