Characterization of the Products of the Clay ... - ACS Publications

Sep 12, 2011 - Key Laboratory of Condition Monitoring and Control for Power Plant Equipment ... North China Electric Power University, Beijing 102206,...
0 downloads 0 Views 2MB Size
Download PDF
ARTICLE pubs.acs.org/EF

Characterization of the Products of the Clay Mineral Thermal Reactions during Pulverization Coal Combustion in Order to Study the Coal Slagging Propensity Sida Tian,*,† Yuqun Zhuo,‡ and Changhe Chen‡ †

Key Laboratory of Condition Monitoring and Control for Power Plant Equipment of Ministry of Education, School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China ‡ Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Thermal Engineering Department, Tsinghua University, Beijing 100084, China

bS Supporting Information ABSTRACT: Silicate liquid phases dominate the primary particle bonding or sintering mechanisms during boiler slagging. Clay minerals, classified as phyllosilicates in the silicate system, are the main inorganic matter in coal, with the aluminosilicates forming from them in coal ash. The high NBO/T (the ratio of nonbridge oxygens to tetrahedrally coordinated network formers) matter, rich with network modifiers resulting from the further reactions of dehydroxylated kaolinite and illite at high temperatures, is of special interest because they cause slagging. In this study, fly ash samples were prepared from two coals in a drop tube furnace system (referred as DTF ash samples) at 1050, 1150, and 1250 °C and then chemically separated using hydrochloric acid. The products of the DTF ash samples before and after chemical separation were analyzed by elemental and phase analyses to characterize the high NBO/T matter in the ash samples and to investigate the relationship between their formation and the combustion temperatures. The results show that the dissolved aluminum in the boiling acid separation could represent the soluble substances of the clay mineral reaction products. The aluminum dissolved share of 1250 °C DTF ash had a positive correlation with the content of the high NBO/T matter generated from the clay minerals, which could be used to develop a method to estimate the slagging characteristics of coals based on the silicate sintering mechanisms.

1. INTRODUCTION Slagging is well-known as one problem threatening safe, economic operation of coal-fired boilers. The source of the slagging is the mineral matter in the coal. The slagging characteristics of coal have been of great concern in coal combustion research.1 4 Today, more and more power plants use new coals or coal blends because of the low availability of the original design fuels, which has increased the demand for predictions of coal slagging characteristics. Although pyrite reaction products play a role in deposit initiation and growth for high-pyrite coals, the primary particle bonding or sintering mechanisms in slagging deposits are dominated by silicate liquid phases.1 Ash deposition in utility boilers is a nonequilibrium process, with the main ingredients of both fly ash and ash deposits being aluminosilicate, especially aluminosilicate glass.5 There have been many experimental studies of slagging deposits of coals and blended coals using pulverized coal combustion test rigs in recent years.6 9 These experimental studies illustrated the difficulty in reliably predicting the slagging characteristics of coals based only on the bulk nature of the coal ash. These studies also provided various relations for the ash deposition degrees with the metal elements such as iron and calcium, suggesting that it is impossible to summarize the deposition results based only on the metal elemental analysis. Hence, further understanding of the coal slagging characteristics needs studies of the products of the silicate minerals in the coals using the silicate structural analysis. r 2011 American Chemical Society

Clay minerals, classified as phyllosilicate, the main inorganic matter in coal, produce aluminosilicates in the coal ash.10,11 Quartz is also one of the major minerals in coal, but quartz has the best thermal stability among the silicate minerals.12 The silicate products in the coal ash which may become liquid are mainly caused by reactions involving the clay minerals. Therefore, the chemical structure characteristics of the clay mineral products from pulverized coal combustion should be investigated.

2. BACKGROUND Coals typically contain two groups of clay minerals, kaolinite and illite, which have been introduced in the literature on ash deposition.2,4,10 Their crystal structures have been well described by mineral researchers.13,14 Kaolinite has 1:1 layered structures consisting of alternate sheets of silicate tetrahedra arranged in rings and metal hydroxide (gibbsite structure) octahedra. Pure kaolinite has the formula Al4(Si4O10)(OH)8. Illite, however, has 2:1 structures made up of an octahedral layer between two layers of inward-pointing silica tetrahedra with additional interlayer cations that provide charge-balance substitutions in the octahedral and tetrahedral layers, making the compositions of the 2:1 clays more difficult to exactly define. Their structures are presented in Figure 1. Received: April 2, 2011 Revised: August 28, 2011 Published: September 12, 2011 4896

dx.doi.org/10.1021/ef200502u | Energy Fuels 2011, 25, 4896–4905

Energy & Fuels

Figure 1. Structures of kaolinite (left) and illite (right), (reprinted with permission from ref 15. Copyright 2006 China University of Geosciences (Wuhan)).

Figure 2. Reaction schemes for the phase transformations of kaolinite (reprinted with permission from ref 16. Copyright 1999 John Wiley & Sons, Inc.) and illite (data is from ref 13).

Clays are raw materials used in the ceramics industry, so there have been numerous studies of the thermal transformations of clay minerals.13,16,17 Figure 2 shows the phase transformations of kaolinite and illite during thermal treatment. When heated, kaolinite and illite both dehydroxylate below 600 °C to change into metakaolinite and dehydrated illite. Kaolinite is more easily disintegrated by heat than illite. X-ray diffraction (XRD) reflections of kaolinite are lost upon dehydroxylation below 500 °C while the crystal structure of illite is maintained until about 700 °C. At higher temperatures, their dehydroxylation products transform further to produce mullite and other substances. These thermal phase transformation studies, however, have been conducted at slow heating rates and under equilibrium conditions; i.e., the samples were maintained at fixed temperatures for one to several hours. However, residence times in the radiant zone of pulverized coal boilers are of the order of 1 s. A mineral particle taking part in the slagging, after becoming an ash particle, would undergo a short cooling process from the flame center to the furnace wall before being deposited on the ash layer of the furnace wall (water cooling pipe or refractory) where the temperature is lower than the flame center and the reaction time is much longer than the previous experience. Figure 3 illustrates

ARTICLE

Figure 3. Time temperature histories of minerals in a boiler (red line) and with slow heating (blue line).

the differences in the time temperature histories for the slow heating test and an actual boiler process, which is one of the main reasons that the slagging predictions based on thermodynamic calculations are often unreliable. The thermal transformations of clay minerals with such flash heating have been less thoroughly investigated. Slade et al.18 studied the microstructure evolution of kaolinite from 0.2 to 0.8 s during flash calcination at 1000 °C using a laminar-flow furnace, with the results showing that the kaolinite changed directly into a single product which, once formed, appeared to undergo little further chemical changes during its short time in the calciner and that the Si within the material migrated increasingly to three nonkaolinite environments (termed mullite-like, metakaolin-like, and Q4) with some Al migrating to tetrahedral sites. Srinivasachar et al.19 studied the structural changes of illite heated to 1227 °C in an entrained flow reactor, with the results indicating that the illite particles lost their crystalline structure, melted, and were transformed to a glass, with no mullite formed and no variations in the iron or potassium concentrations across the cross-section or at the particle edges, which differed from when illite was heated slow during which Fe, K, and Ca were enriched on the particle surfaces. With the situation in coals, the clay minerals may be juxtaposed with other minerals and organic matter within a coal particle, so the clay mineral reactions during coal combustion involve not only self-transformations but also further reactions of their intermediate products with other foreign metal oxides.2,4 The fired products of clay minerals in actual pulverized coal boilers are mostly glass phases with less crystalline products, mostly mullite. Spears10 pointed out that, in the fly ash, the mullite crystals are largely attributed to kaolinite, whereas illite contributes toward the glasses. Characterization of the clay mineral products in coal combustion residues is difficult. The fly ash is complex, finely dispersed mixtures of inorganic and organic constituents.20 A single analysis method, for example XRD and SEM, can not easily characterize the mineral phases in the combustion products due to the different crystallizations of the minerals in the fly ash, and the analysis of the glass substances in fly ash is more difficult because of their complex chemical compositions. Ward et al.21 studied the content and Brindle et al.11 studied the composition of the fly ash glasses as one phase to indicate that the fly ash glass composition was related to variations in the ash properties, such as the particle density. 4897

dx.doi.org/10.1021/ef200502u |Energy Fuels 2011, 25, 4896–4905

Energy & Fuels According to silicate melt polymerization theory,22,23 silicon oxygen tetrahedrons are the basic structure unit in silicate melts, and they form various anion structural groups (polymers) through the bridging and/or nonbridging oxygens. A silicate melt is a mixture of various polymers with different polymerization degrees. Silicon atoms, in tetrahedral coordinations, form highly covalent bonds with the oxygen atoms, referred to as network formers. In contrast, metal atoms are incapable of building a continuous network but weaken the network to produce nonbridge oxygens (NBOs), referred to as network modifiers. With more network modifiers, i.e., more NBOs, a silicate glass has a lower average degree of polymerization of the tetrahedral atom groups and the substance more easily melts with a lower melt viscosity and other unstable thermal physical properties. With amorphous aluminosilicates, Al3+ has a strong preference for tetrahedral coordination, with the charge deficit compensated for by association with a low field strength cation, Mn+, such as Na+ or Ca2+.24,25 Even in glasses lacking low field strength cations, Al tends to remain in a tetrahedral coordination with a network forming character through association with an oxygen in 3-fold coordination in a structure called a tricluster.24 In glass research, the parameter “NBO/T”, the ratio of NBO to tetrahedrally coordinated network formers, is usually used to characterize the polymerization degree of a melt or glass.23 26 Since the ash deposition mechanisms are dominated by silicate liquid phases, a higher NBO/T value of the amorphous aluminosilicate in the coal ash means that the amorphous matter more easily forms a liquid phase which promotes slagging. Therefore, an understanding of coal slagging characteristics must classify the actual clay mineral reaction products during pulverized coal combustion based on the network modifier ratios in their structures, that is, the NBO/T values. With the reaction characteristics of suspension firing, the clay mineral reaction products include the dehydroxylation products and their further reaction products, and the further reactions include the self-transformations of the dehydroxylation products and/or the reactions of the dehydroxylation products with foreign metal cations. The firing temperatures control all these reactions. The six-coordinate aluminums in the clay minerals change into four- or five-coordinate aluminums during dehydroxylation. In the self-transformations, the Al containing portion of metakaolinite, which lacks network modifier elements, tends to produce mullite, in which the active four- and five-coordinate aluminums change into the four- and six-coordinate aluminums in the new crystalline phases. When foreign elements interact with the metakaolinite particles, they may be involved in the further reactions of the dehydroxylated products of kaolinite to produce aluminosilicate glasses preventing the transformation to mullite.17 Since potassium and other metal ions are present between the illite tetrahedral layers, the dehydrated illite changes into an amorphous glass with the interlayer metal ions as modifiers during the further self-transformations.19 If the foreign modifier elements happen to participate in the further reactions, the NBO/T values of the illite glass products will increase again. These modifier-rich products produced by further reactions of dehydroxylated kaolinite and illite at high temperatures are important materials causing slagging, denoted as the high NBO/T matter, which need special attention. The roles of acids on the thermal reaction products of clay minerals have been investigated for various applications.27,28 Aluminum atoms form the skeletons of the clay mineral crystalline structure with silicon and oxygen atoms. In heating reactions

ARTICLE

involving the clay minerals, the aluminum atoms are included in the solid reaction products without volatilization. When some aluminosilicate products decompose during acid separations, the aluminum atoms will come into the solution. Lussier,27 in a study to prepare highly active catalytic materials, studied the mechanisms of acidic reactions on the thermal decomposition products of kaolinite. He reported that, with hydrochloric acid leaching, octahedral aluminum atoms in metakaolinite were inert to the acid attack while the four- and five-coordinate aluminum atoms, transformed from the octahedral aluminum during dehydroxylation, were extracted by the acids and that, as firing temperatures increased, mullite or Al Si spinels appeared and the extraction of Al into the acid declined sharply. These conclusions were further confirmed by Liu et al.28 Hot concentrated hydrochloric acid was widely used to extract valuable metals from fly ashes in the 1980s. Berry et al.29 studied the impact of the hot HCl on the fly ash glasses by leaching the heavy and light density fractions of high-Ca fly ash with hot concentrated hydrochloric acid. The two density fractions typically included two kinds of glasses. The glass of the light density fraction contained mainly silicon and aluminum, lack of network modifier elements, with its XRD pattern having a diffuse diffraction “hump” whose shape was close to the silica gel XRD pattern. The glass of heavy density fraction, however, contained many network modifier elements, high NBO/T glass based, with an XRD that was quite different from the silica gel XRD pattern. After the hot HCl separation, the XRD patterns of the residues of the high and light density fractions both had the same diffuse diffraction halos as silica gel, which suggested that the high NBO/ T aluminosilicate glasses were partly dissolved and that the acid separation residues of both glasses have network structures consistent with silica gel, a low NBO/T substance. Berry et al.29 described two mechanisms, ion-exchange and network hydrolysis, for the reactions between HCl and aluminosilicate glasses. The modifier cations associated with the NBO atoms in the siloxane groups are subjected to ion-exchange in the strongly acidic conditions to form silanol (Si OH) groups. The ion-exchange makes the siloxane become hydroxylated as alkali or alkaline earth cations are leached. Similar reactions may occur at the Al O 3 3 3 M+ sites. Attacks at bridging oxygen atoms by H2O, catalyzed by H+, also lead to hydroxylation. In contrast to the ion-exchange mechanism, siloxane hydrolysis does not lead directly to dissolution and no cation species enter the solution. If the reaction, however, proceeds to a major extent, the consequent depolymerization of the aluminosilicate network will result in dissolution of the monosilicate and aluminate species. Aluminate ions, in general, will be further hydrolyzed to Al3+ and other soluble species. Thus, more aluminum atoms are released from the aluminosilicate glass with the higher modifier contents (i.e., higher NBO/T value). The major minerals in the fly ash, mullite, quartz ,and feldspar, can not be dissolved by hot hydrochloric acid.30 Thus, these hydrochloric acid leaching studies have shown that the aluminum dissolved share may be a useful measurement of both dehydroxylation products and the high NBO/T matters for the clay mineral heating reaction products. Since the clay minerals are the most abundant minerals in most coals, the aluminum in coals is basically from its clay minerals. With the hot hydrochloric acid leaching, the dissolved aluminum comes from the dissolved reaction products of the clay minerals in the coal ash. Consequently, combined with the phase analyses of the coal combustion products before and after hot HCl 4898

dx.doi.org/10.1021/ef200502u |Energy Fuels 2011, 25, 4896–4905

Energy & Fuels leaching, the measurements of the aluminum dissolved shares of the coal ash samples prepared at different combustion temperatures can provide an insight into the influence of the combustion temperature on the clay mineral reactions during pulverized coal combustion and may also characterize the high NBO/T matter in the coal combustion residues. In this study, ash samples of two Chinese coals prepared in a drop tube furnace were chemically separated using boiling concentrated hydrochloric acid, referred to as boiling acid separation. The corresponding products of the ash samples before and after chemical separation were examined using a method including elemental and phase analyses to identify and measure the clay mineral products in the coals with suspension combustion.

ARTICLE

Table 1. Proximate Analyses (%) and Ash Fusion Points (°C) of the Coals Vad SH coal 40.4

Aad Cad Mad St,ad

Qgr,v

9.7 39.7 10.2 0.43 24302

WX coal 13.9 27.9 57

DT

ST

HT

FT

1130 1150 1170 1180

1.2 1.55 25092 >1450

Table 2. Chemical Composition of the Coal Ashes (%) SiO2 Al2O3 CaO Fe2O3 K2O MgO Na2O SO3 TiO2 P2O5 SH coal 48.17 18.04 14.37 6.99 1.95 2.4 WX coal 51.32 36.78 2.48 4.2 0.5 0.39

0.7 5.85 0.22 0.84 0.14 1.94 1.32 0.53

3. EXPERIMENTAL SECTION 3.1. Coal. Two Chinese coals, SH bituminous coal and WX lean coal, were used in this study. The SH coal from the Shendong mining area had a low ash-melting point and a high calcium content, while the WX coal from Shanxi province had a high ash-melting point. The proximate and ash analyses of the two coals are listed in Tables 1 and 2. The raw coals were pulverized by first crushing them to less than 3 mm with a jaw crusher and then grinding in a ball mill until 70% of the residue could pass through 100 mesh, which is similar to the coal fineness used in utility boilers. 3.2. Apparatus. Figure 4 shows a schematic diagram of the drop tube furnace (DTF) used in this study. Lv31 introduced the structure and working principle in detail. The experimental rig included a powder feeder, furnace body, sampling device, and temperature controller. The tube furnace body included inner and outer tubes. Six silicon carbide heaters were arranged evenly on the outside of the outer tube to heat the outer and inner tubes, with the pulverized coal combustion inside the inner tube. Two air flows supplied the combustion air for the experimental rig. The primary air entered the powder feeder first to form a mixture with the pulverized coal which was driven by a vibration system into the inner tube furnace via a water-cooled injector. The secondary air entered the bottom of the rig from a ring cavity between the outer and inner tubes, where it was preheated. The hot secondary air was uniformly distributed by a rectifier at the top of the rig and was mixed with the primary air within the inner tube to participate in the combustion reaction. The ash products in the flue gas came into a sampling gun at the bottom of the inner tube and were separated from the gas in a sample collector at the lower end of the sampling gun. The ash samples were removed between tests. The sampling gun was made of a casing pipe structure with cooling water flowing in the outer annular dissection. A stream of nitrogen flowed upward in the inner annular dissection and entered the central pipe via a funnel at the top inlet to dilute and cool the combustion residue particles. The primary air, secondary air, and nitrogen were supplied from compressed gas cylinders with the flue gas removed by a suction fan. The SH and WX coal samples were combusted in the drop tube furnace at 1050, 1150, and 1250 °C. The primary air flow was set to 110 g/h; the secondary air flow was 1000 g/h, and the cooling nitrogen flow was 625 g/h. The coal powder feed rate was about 0.1 g/min. The internal diameter of the inner tube was 80 mm, and the burning zone was 1230 mm long. The average particle sizes of the experimental fuels were 54 58 μm, and the residence time of the particles in the burning zone was about 1.5 s. The collected ash products are referred to as DTF ashes. In addition, a plasma asher (British EMITECH company K1050X) was used to prepare low-temperature ash samples of the two fuels to analyze the mineral composition of the fuels. The plasma asher operating parameters were an ashing power of 70 W, gray time of 13.5 h, and a

Figure 4. Schematic of the drop tube furnace. vacuum of 0.8 Mbar. Fixed bed ash samples of the two fuels were also prepared by soak heating for 1 h at 1050 °C using a muffle furnace to identify the phases in the ash samples. The muffle furnace ash samples are denoted as MF ash samples. The boiling acid separation used about 0.2 g of a DTF ash sample placed in a 100 mL beaker with 20 mL of hydrochloric acid, and the glass covered beaker was heated for 1 h on a 160 °C electric heating plate. The hydrochloric acid was prepared from deionized water and analytical grade hydrochloric acid at a volume ratio of 1:1 to give a final molar concentration of about 6.3 mol/L. After heating, the mixture in the beaker was filtered using a funnel with a 7 cm diameter quantitative filter paper. The filtered liquid was saved in a 100 mL constant volume bottle to measure the contents of the dissolved elements, and the solid residue as well as the funnel was dried for 1 h at 70 °C in a drying box for X-ray diffraction analysis. The original DTF ash sample was also subjected to the same XRD and chemical examinations as the solid boiling acid separation residue. After finishing the XRD analysis, the separation residue with its quantitative filter paper was subjected to microwave digestion in a microwave oven (CEM Corporation MARS5) for the ICP-AES analysis. The ICP-AES was used to identify the concerned elements in the residues and the filtered liquid. The boiling acid separation had good repeatability, and the good integrity of the solid residues remained after the XRD analysis. The aluminum dissolved share was defined as the ratio of dissolved aluminum in the boiling acid solution to the total aluminum in the DTF ash sample, and the aluminum residual share was the ratio of the aluminum remaining in the ash residue to the total aluminum in the DTF ash. The aluminum dissolved share plus its residual share should equal one. 4899

dx.doi.org/10.1021/ef200502u |Energy Fuels 2011, 25, 4896–4905

Energy & Fuels

ARTICLE

Table 3. Mossbauer Analyses of the Low Temperature Ash Samples of the SH and WX Coals IS-αFe/mm 3 s WX coal

SH coal

1

QS/mm 3 s

1

mass ratio/%

phases

0.293

0.614

74.3

pyrite

1.361 1.295

1.498 2.612

10.3 15.4

ankerite ferrous sulfate

0.307

0.63

73.6

pyrite

1.095

2.528

26.4

ferrous sulfate

Figure 5. XRD results for low temperature ashes of the SH and WX coals. An X-ray diffractometer (German Bruker Company D8-Adance) equipped with an array detector (LYNXEYE DETECTOR) was used to measure the XRD patterns of all the ash and fuel samples. The X-ray diffractometer used a copper target, tube power of 40 mA/40 kV, divergence slit of 2.0 mm, receiving slit of 2.0 mm, scattering slit of 2.0 mm, and scanning accuracy of 0.002°. Continuous scans were performed with a 4° (2θ)/min scan speed. Detailed phase identifications were collected for all the diffraction peaks on the XRD patterns for all samples. Only distinct diffraction peaks of the reported mineral phases are presented here to highlight the shape characteristics of the XRD patterns. The iron chemical forms in the coal dominate its effects to the slagging characteristics with aluminosilicate-bearing ferrooxides being important sources of the initial layer that occurs in deposits formed in coal-burning systems.32 Therefore, the Mossbauer spectrum analyses were performed by Lanzhou University to analyze the low-temperature and DTF ash samples to understand the iron chemical forms in these samples.

4. RESULTS AND DISCUSSION 4.1. Mineral Species in the Fuels. Both coals included kaolinite and illite clay, but their illite species differed. Figure 5 shows the XRD patterns (7 30° 2θ) of the low temperature ash samples derived from the two coals. The illite in the SH coal was muscovite (KAl2[(AlSi3)O10](OH)2), with a (001) diffraction peak at 2θ of 8.808° (d = 100.32 nm). The illite in the WX coal was tobelite, with a first basal reflection at 2θ of 8.530° (d = 103.58 nm), lower than that of the muscovite. The tobelite interlayer cations consisted mainly of ammonium ions, whose radius (0.143 nm) is bigger than that of potassium ions (0.133 nm), so that its (001) diffraction peak is lower than that of muscovite.33 Since ammonium ions replaced some or all of the potassium ions in the structural layers, the tobelite heated reaction products differed from the other illites. It is necessary for the coal slagging characteristics study to distinguish tobelite from other illites. According to the relationship between the basal spacing and the interlayer NH4 ratio summarized by Higashi,33 the interlayer cations of the tobelite in the WX coal were 100% occupied by ammonium ions, so the clay mineral had a chemical formula of NH4Al2[(AlSi3)O10](OH)2. The potassium content of the WX coal ash was very low (Table 1), which also confirmed the lack of potassium-containing minerals in this fuel. The XRD patterns also show that the two coals contained quartz (SiO2), pyrite (FeS2), and calcite (CaCO3) but no other

Figure 6. XRD results for 1050 and 1250 °C DTF ashes and 1050 °C MF ash of WX coal.

aluminum containing minerals. The low temperature ash from the SH coal contained some bassanite (CaSO4 3 0.5H2O) with low-crystallinity resulting from the oxidation of the organically bound calcium and sulfur-contained substances in the fuel during the low temperature ashing.34,35 In addition, there was a little feldspar in the SH coal. Previous chemical and density fractionations of the coals indicated that the mineral occurred as excluded minerals and was very low in the SH coal.35 The feldspar has a high melting point and is insoluble in hydrochloric acid, so it could be neglected and the aluminum in the samples could be assumed to come only from the clay minerals. Thus, the dissolved and residual shares of aluminum in the DTF ash samples can represent the dissolved and solid residues of the clay mineral reaction products generated by the boiling acid separation. The Mossbauer spectrum analysis of the two coals indicates that the iron in the two coals did not originate from the clay minerals but from pyrite, carbonate, or sulfate (Table 3). Hence, if iron existed in the clay mineral reaction products after the combustion, the new substances would have been generated by reactions of the clay minerals and other minerals containing iron in the coal samples. 4.2. Phase Components of the DTF Ash Samples. The phase components of the clay mineral reaction products were examined by comparisons of the XRD patterns of the DTF and MF samples. The phase components can indicate the reaction characteristics of the clay minerals in the coals. The XRD patterns at 1050 and 1250 °C are provided to clearly show the patterns. Figure 6 shows the XRD patterns of the 1050 and 1250 °C DTF ash samples and the 1050 °C MF ash sample from the WX coal. The clear diffuse halos on all the XRD patterns indicate the existence of amorphous materials in these ash samples. Comparison with the MF ash sample shows that the DTF samples also contained quartz and mullite (Al6Si2O13) but no cristobalite. Some dehydrated tobelite was present in the 1050 °C DTF 4900

dx.doi.org/10.1021/ef200502u |Energy Fuels 2011, 25, 4896–4905

Energy & Fuels

ARTICLE

sample with little in the 1250 °C DTF sample and in the 1050 °C MF ash. The mullite diffraction peaks are enhanced on the 1250 °C DTF ash XRD pattern compared with those on the 1050 °C DTF ash XRD pattern, which shows that the mullite increased as the temperature increased. The clay minerals decompose differently when fired in suspension compared with fixed-bed heating, where there was no cristobolite.36 The phase components of the WX coal DTF ash samples indicate that, with suspension combustion, the clay minerals in the WX coal essentially transformed themselves. Moreover, the dehydroxylation products of the clay minerals and their further reaction products both existed in the DTF samples though the increasing combustion temperature promoted decomposition of the clay minerals. Figure 7 shows the XRD patterns for 1050 and 1250 °C DTF ash samples and 1050 °C MF ash sample derived from the SH coal. The clay mineral reaction products in the DTF ash sample consisted mainly of amorphous materials with no new aluminum-containing mineral products. There was no dehydrated muscovite in the 1050 °C MF ash sample but some in the 1050 °C DTF ash, which also suggests that dehydroxylation of clay minerals occurred with the further reactions of the dehydroxylation products with the SH coal suspension combustion. As for the calcium-containing minerals in SH coal, the DTF ash samples included some undecomposed calcite and new lime without other new calcium-containing mineral products. Unlike the MF ash, the DTF ashes contained neither anhydrite (CaSO4) nor aluminosilicate minerals (anorthite or gehlenite) resulting from reactions of the clay minerals with calciumcontaining minerals. The occurrence of portlandite (Ca(OH)2) in the DTF ash samples was caused by the lime in the DTF ash absorbing moisture from the air. Thus, with suspension

Figure 7. XRD results for 1050 and 1250 °C DTF ashes and 1050 °C MF ash of SH coal.

combustion, the calcite in the SH coal basically decomposed to produce calcium oxide, i.e., lime. However, the organically bound calcium in the SH coal might react with the clay minerals to produce aluminosilicate glass. The reaction characteristics of the Fe and clay minerals in the SH coal were analyzed based on the Mossbauer spectra of its ash samples (Table 4). The iron-containing reaction products in the DTF and MF samples both included oxides and aluminum silicate glass. However, the Mossbauer spectra of the DTF ash samples were more complicated than that of the MF ash, with the oxides in the DTF ash samples including not only hematite (Fe2O3) but also magnetite (Fe3O4) and other phases. The iron contents in the aluminosilicate glass in the DTF ash samples were similar with those in MF ash sample, indicating that, as in fixedbed heating, the suspension combustion caused the iron to react strongly with the clay minerals in the SH coal. 4.3. Phase Components in the Acid Separated Residues of the DTF Ash Samples. The XRD diffuse diffraction humps of the boiling acid separation residues from the DTF ash samples were used to analyze the chemical structure of the amorphous aluminosilicate in these residues. The instrument backgrounds were subtracted from the XRD patterns for these results with the XRD patterns all having the same baseline to compare the differences among their diffusion diffracting humps. In addition, to highlight the shapes of the diffraction humps in these figures, the diffraction intensity per unit length was also reduced on the vertical direction. Figure 8 shows the XRD patterns of the boiling acid separation residues for the 1050 and 1250 °C DTF ash samples of the SH coal. The diffuse diffraction humps attributed to the glasses in the two boiling acid separation residues overlap each other, which are symmetrical with a maximum at 22.7° 2θ. These diffuse diffraction humps are the same as for the glasses of the hot HCl leached residues obtained by Berry et al.29 This shows that the aluminosilicate glasses of the boiling acid separation residues of the DTF ash samples had a chemical structure similar with silica gel, a low NBO/T amorphous substance. Quartz was the only major mineral in the boiling acid separation residues of the SH coal DTF ash samples. The diffraction peak for the dehydrated muscovite disappeared from the XRD pattern for the 1050 °C DTF ash residue sample, indicating that the acid separation caused the lattice of the dehydrated illite to dissolve. These XRD patterns also revealed that the SH coal DTF ash samples contained a small amount of mullite which could not be distinguished on the XRD patterns of the raw DTF ash samples. As expected, the quartz and mullite were not removed by the hot concentrated hydrochloric acid. Since there were few mineral products of clay mineral reactions in the residues, the acid boiling separation residues of the clay mineral reaction products in the SH coal consisted of

Table 4. Analyses of the DTF and MF Ashes of SH Coal sample MF ash 1050 DTF ash 1050

DTF ash 1250

IS-αFe/mm 3 s

1

QS/mm 3 s

0.370

0.105

0.312

0.916

1

H/T 51.40

mass ratio/%

phase

68.0

hematite

32.0

aluminosilicate

0.366

0.063

50.93

51.6

oxide

0.65

0.037

45.56

16.0

oxide

0.324 0.370

0.885 0.108

51.28

32.4 30.8

aluminosilicate oxide

0.319

0.045

48.89

0.312

0.963 4901

36.6

oxide

32.6

aluminosilicate

dx.doi.org/10.1021/ef200502u |Energy Fuels 2011, 25, 4896–4905

Energy & Fuels

Figure 8. XRD patterns for the boiling acid separation residues of the 1050 and 1250 °C DTF ashes of SH coal.

ARTICLE

Figure 9. XRD patterns for the boiling acid separation residues of the 1050 and 1250 °C DTF ashes of WX coal.

Table 5. LOI of the DTF Ashes almost all the low NBO/T matter with a similar structure to amorphous silica. The boiling acid separation residues of the WX coal DTF ash samples also retained quartz and mullite, with no diffraction peaks for the dehydrated tobelite for the 1050 °C DTF ash residue, also indicating that the acid separation destroyed the dehydrated tobelite lattice (see Figure 9). Unlike the humps in the XRD patterns for the SH coal samples, these diffuse diffraction humps had their tops skewed toward the high 2θ diffraction angle direction, which could be explained by the fact that the DTF ash from the lean WX coal might contain much unburned carbon. Each diffuse diffraction hump in Figure 9 then resulted from the superimposition of the diffuse diffractions of the unburned carbon and the amorphous aluminosilicate in the DTF ash residues. The unburned carbon in the DTF ash samples could be represented by a DTF ash loss on ignition (LOI), as shown in Table 5. The LOI values of the WX coal DTF ash samples are higher than those of the SH coal DTF samples, and a lower combustion temperature would result in more unburned carbon in the DTF sample. Figure 10 shows the diffraction hump of the boiling acid separation residue of the WX coal 1050 °C DTF ash, which had the most unburned carbon among the DTF ash samples. The upper pattern was found by subtracting the amorphous diffuse hump for the 1250 °C DTF ash residue XRD pattern for the SH coal from the XRD pattern of the WX coal 1050 °C DTF ash residue, which is a typical XRD pattern of coal char as seen in the literature.37,38 Therefore, the clay mineral dehydroxylation products and the high NBO/T matters generated by the further reactions in the DTF ash for both the SH and WX coals were partly dissolved, and the amorphous aluminosilicates in the boiling acid separation residues for the two coals’ DTF ash samples had a similar chemical structure with silica gel; so, they were the low NBO/T matter. 4.4. Characterization of the High NBO/T Matters in the DTF Ash Samples. Since the clay minerals in the WX coal essentially transformed themselves during the DTF combustion, the dissolved aluminum in the WX coal DTF ash during the boiling acid separation came from the dehydroxylation products, such as metakaolinite and dehydrated tobelite, and the change in the aluminum dissolved share with the DTF temperature reflected the phase component changes of the clay mineral decomposition for the suspension combustion. The aluminum dissolved shares for all the DTF ash samples for both coals are

sample

LOI

sample

LOI

SH 1050 SH 1150

0.11 0.05

WX 1050 WX 1150

0.29 0.23

SH 1250

0.04

WX 1250

0.19

shown in Figure 11. For the WX coal, the aluminum dissolved share was highest for the 1050 °C DTF sample, which means that the reaction product phase components at this DTF temperature were concentrated in the metakaolinite, dehydrated tobelite, and the other dehydroxylation products. The aluminum dissolved share in the DTF ash decreased from 59% to 30% with increasing DTF temperature from 1050 to 1150 °C, indicating that the major transformation products ranged rapidly from the dehydroxylation products to the further reaction products (as mullite). The aluminum dissolved share declined slightly when the DTF temperature was up from 1150 to 1250 °C, which suggests that, above 1150 °C, increases of the combustion temperature have insignificant effect on further self-transformations of the dehydroxylation products. The clay mineral reactions for the SH coal during the DTF combustion involved two pathways: (1) self-transformation and (2) reactions of the dehydroxylation products with other mineral matter. The results in Figure 11 show that the aluminum dissolved shares for the SH coal DTF ash were all higher than those of the WX coal DTF ash at the same temperature. The 1050 °C DTF ash had the maximum dissolved aluminum share of 71% among the SH coal DTF samples. With increasing temperature, the aluminum dissolved share decreased slowly to 61% at 1150 °C. The 1250 °C ash sample’s aluminum dissolved share was slightly higher than for the 1150 °C sample but still lower than for the 1050 °C sample. With reference to the analysis of WX coal DTF ash, the phase components of the clay mineral reaction products in the 1050 °C SH coal sample were also mostly in the dehydrated muscovite, metakaolinite, and other dehydroxylation products. It should be due to the difference in the forms of the illite in the SH and WX coals that the aluminum in the 1050 °C SH coal sample had a higher solubility than that of the WX coal in the boiling acid separation. Wang et al.39 reported that muscovite released potassium upon heating and was more strongly affected by acids than other mica minerals, so it should also have a stronger release of aluminum than other illites. 4902

dx.doi.org/10.1021/ef200502u |Energy Fuels 2011, 25, 4896–4905

Energy & Fuels

ARTICLE

Table 6. Acid Solubilities of Silicon and Aluminum of the DTF Ashes (mg/g)

Figure 10. XRD analysis for the boiling acid separation residue of the 1050 °C DTF ash of WX coal.

Figure 11. Dissolved aluminum share in the DTF ashes of SH and WX coals.

Although both of the 1150 and 1250 °C SH coal DTF samples had lower aluminum solubilities than at 1050 °C with the boiling acid separation, the aluminum dissolved share difference between the SH coal and WX coal samples at the same DTF temperature increased as the DTF temperature increased. The analysis of the WX coal DTF ash sample showed that, above 1150 °C, increases of the combustion temperature had insignificant effect on the further self-transformation of the dehydroxylation products, and then, the increasing difference in the aluminum dissolved share between the SH and WX coals indicates that the dehydroxylation products reacted more with other mineral matters in the SH coal to produce more high NBO/T matter as the DTF temperature increased. The aluminum dissolved share for the 1250 °C SH coal was 63% compared to 27% for the WX coal, indicating that, for the 1250 °C DTF combustion, the clay mineral reactions in the SH coal created considerable amorphous aluminosilicate with high NBO/T. The silicon solubilities during the boiling acid separation for these two coals’ ash samples differed from the aluminum solubilities, which also revealed the differences in the reaction products seen in Table 6. The silicon solubility for “WX 1050”, 2.90 mg/g, means that each gram of the 1050 °C WX coal DTF ash sample had 2.90 mg of silicon dissolved in the boiling acid solution. The silicon solubilities in the boiling acid solution were similar for all the WX coal DTF ash samples and 1 order of magnitude lower

samples

Al

Si

samples

Al

Si

WX 1050

63.69

2.90

SH1050

58.43

9.21

WX 1150

38.11

2.99

SH1150

50.29

10.64

WX 1250

36.14

3.46

SH1250

47.21

9.36

than the aluminum solutions. In contrast, the silicon solubility for the SH coal DTF ash samples was larger than those of the WX coal. The aluminum solubiltiy of the dehydroxylation products did not increase as the combustion temperature increased, while the dissolved aluminum from the high NBO/T matters increased. For the boiling acid 1250 °C DTF ash samples, although some dissolved aluminum still came from the dehydroxylation products, the change in the aluminum dissolved share mainly depended on the new-generated high NBO/T aluminosilicate. Thus, the aluminum dissolved shares for the 1250 °C DTF sample was positively correlated with the high NBO/T matter produced by the clay mineral reactions. For coals with aluminum only in the clay minerals, it would be considerably applicable to characterize the high NBO/T matters from the clay mineral reactions using the aluminum dissolved share in the boiling acid separation since the clay minerals in the two coals used here are typical among Chinese coals. For other coals whose nonclay minerals also contain some aluminum, the amount of aluminum in the clay can also be used to analyze the clay mineral reaction products during combustion after the amount of nonclay aluminum has been determined. This work only concentrated on characterization of the high NBO matter in coal ash using boiling acid separation. The reactions of clay minerals with other minerals in the coal during combustion are closely related with the mineral associations (included or excluded minerals).40,41 Further research is needed on the interaction mechanisms of the mineral matters in the coal during combustion to reduce coal slagging. The behaviors of the clay minerals which are included with other minerals in organic matter differ from discrete external minerals. This will be the focus of the authors future work.

5. CONCLUSIONS (1) During the boiling acid separation, the clay mineral dehydroxylation products and the high NBO/T matter generated by further reactions in the DTF ash were partly dissolved, and the amorphous aluminosilicate residues of both SH and WX coal DTF ashes were found to be low NBO/T matter with chemical structures similar to silica gel. The aluminum dissolved share of the DTF ash represented the proportion of soluble substances in the clay mineral reaction products. (2) For temperatures of 1050 to 1150 °C, an increasing combustion temperature significantly promoted further transformations of the clay mineral dehydroxylation products. Above 1150 °C, the temperature has less influence on the decrease of the soluble dehydroxylation products. Higher NBO/T matters were generated by the reactions of the clay minerals with other minerals as the temperature increased. (3) The aluminum dissolved share of the 1250 °C DTF ash sample correlated positively with the content of high NBO/T matters generated by the clay minerals, which could 4903

dx.doi.org/10.1021/ef200502u |Energy Fuels 2011, 25, 4896–4905

Energy & Fuels provide a simple method for estimating the slagging characteristics of coals based on the silicate sintering mechanisms.

’ ASSOCIATED CONTENT

bS

Supporting Information. Large XRD patterns forFigures 5, 6, 7, 8, 9, and 10. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This research was supported by the National Key Basic Research and Development Program of China (No. 2006CB200301) and the Fundamental Research Funds for the Central Universities of China (09QG40). Help from Prof. Xin-qian Shu of the China University of Mining and Technology on the experimental rig and comments from Dr. Yong-chun Zhao are heartily acknowledged. ’ REFERENCES (1) Benson, S. A.; Jones, M. L.; Harb, J. N. Ash formation and deposition. In Fundamentals of coal combustion for clean and efficient use; Smoot, L. D., Ed.; Elsevier: New York, 1993; pp 299 373. (2) Bryers, R. W. Fireside slagging, fouling, and high temperature corrosion of heat-transfer surface due to impurities in steam-raising fuels. Prog. Energy Combust. Sci. 1996, 22, 29–120. (3) Gupta, R. P.; Wall, T. F.; Kajigaya, I.; Miyamae, S.; Tsumita, Y. Computer-controlled scanning electron microscopy of minerals in coal-implications for ash deposition. Prog. Energy Combust. Sci. 1998, 24, 523–543 . (4) Baxter, L. L. Ash deposit formation and deposit properties: a comprehensive summary of research conducted at Sandia’s Combustion Research Facility; SAND2000-8253; Sandia National Laboratories: Albuquerque, NM, August 2000. (5) Wee, H. L.; Wu, H.; Zhang, D. Heterogeneity of Ash Deposits Formed in a Utility Boiler during PF Combustion. Energy Fuels 2007, 21, 441–450. (6) Su, S.; Pohl, J. H.; Holcombe, D.; Hart, J. A. Slagging propensities of blended coals. Fuel 2001, 80, 1351–1360. (7) Russell, N. V.; Wigley, F.; Williamson, J. The roles of lime and iron oxide on the formation of ash and deposits in PF combustion. Fuel 2002, 81, 673–681. (8) Rushdi, A.; Sharma, A.; Gupta, R. P. An experimental study of the effect of coal blending on ash deposition. Fuel 2004, 83, 495–506. (9) Barroso, J.; Ballester, J.; Pina, A. Study of coal ash deposition in an entrained flow reactor: assessment of traditional and alternative slagging indices. Fuel Process. Technol. 2007, 88, 865–876. (10) Spears, D. A. Role of clay minerals in UK coal combustion. Appl. Clay Sci. 2000, 16, 87–95. (11) Brindle, J. H.; McCarthy, M. J. Chemical constraints on fly ash glass compositions. Energy Fuels 2006, 20, 2580–2585. (12) Reifenstein, A. P.; Kahraman, H.; Coin, C. D.; Calos, N. J.; Miller, G.; Uwins, P. Behaviour of selected minerals in an improved ash fusion test-quartz, potassium feldspar, sodium feldspar, kaolinite, illite, calcite, dolomite, siderite, pyrite and apatite. Fuel 1999, 78, 1449–1461. (13) McConville., C.; Lee, W. E. Microstructural development on firing illite and smectite clays compared with that in kaolinite. J. Am. Ceram. Soc. 2005, 88, 2267–2276. (14) Fernandez, R.; Martirena, F.; Scrivener, K. L. The origin of the pozzolanic activity of calcined clay minerals: a comparison

ARTICLE

between kaolinite, illite and montmorillonite. Cem. Concr. Res. 2011, 41, 113–122. (15) China university of Geosciences national level excellent course: Crystallography& Mineralogy. http://jpkc.cug.edu.cn/09jpkc/jjxjkwx/ index2.htm (accessed June 20, 2011). (16) Lee, S.; Kim, Y. J.; Moon, H. Phase transformation sequence from kaolinite to mullite investigated by an energy-filtering transmission electron microscope. J. Am. Ceram. Soc. 1999, 82, 2841–2848. (17) Takeuchi, N.; Takahashi, H.; Ishida, S.; Horiie, F.; Wakamatsu, M. Mechanistic study of solid-state reaction between kaolinite and ferrous oxide at high temperatures. J. Ceram. Soc. Jpn. 2000, 108, 876–881. (18) Slade, R.; Davies, T. W. Evolution of structural changes during flash calcination of Kaolinite. J. Mater. Chem. 1991, 1, 361–364. (19) Srinivasachar, S.; Helble, J. J.; Boni, A. A.; Shah, N.; Huffman, G. P.; Huggins, F. E. Mineral behavior during coal combustion. 2. Illite transformations. Prog. Energy Combust. Sci. 1990, 16, 293–302. (20) Vassilev, S. V.; Vassileva, C. G. Methods for characterization of composition of fly ashes from coal-fired power stations: a critical overview. Energy Fuels 2005, 19, 1084–1098. (21) Ward, C. R.; French, D. Determination of glass content and estimation of glass composition in fly ash using quantitative X-ray diffractometry. Fuel 2006, 85, 2268–2277. (22) Lu, P. Fundamentals of inorganic materials science (Chemistry and physics of silicates); Wuhan University of Technology Press: Wuhan, China, 2006; pp 77 106. (23) Mysen, B. O. Element partitioning between mineral and melt, melt composition, and melt structure. Chem. Geol. 2004, 213, 1–16. (24) Toplis, M. J.; Dingwell, D. B. Shear viscosities of CaO-Al2O3SiO2 and MgO-Al2O3-SiO2 liquids: implications for the structural role of aluminum and the degree of polymerization of synthetic and natural aluminosilicate melts. Geochim. Cosmochim. Acta 2004, 68, 5169–5188. (25) Kuo, Y.; Huang, K.; Wang, C.; Wang, J. Effect of Al2O3 mole fraction and cooling method on vitrification of an artificial hazardous material. Part 1: Variation of crystalline phases and slag structures. J. Hazard. Mater. 2009, 169, 626–634. (26) Smedskjaer, M. M.; Jensen, M; Yue, Y. Effect of thermal history and chemical composition on hardness of silicate glasses. J. Non-Cryst. Solids 2010, 356, 893–897. (27) Lussier, R. J. A novel clay-based catalytic material—preparation and properties. J. Catal. 1991, 129, 225–237. (28) Liu, C.; Gao, X.; Zhang, Z.; Zhang, Y.; Pan, Z.; Liu, Y.; Li, S. Study on the performance of modified kaolin I. Acidity and catalytic activity. Pet. Process. Petrochem. (China) 1999, 30, 32–38. (29) Berry, E. E.; Hemmings, R. T.; Cornelus, B. J. Speciation in size and density fractionated fly ash III the influence of HCl leaching on the glassy constituents of a high-Ca fly ash. Mat . Res. Soc. Symp. Proc. 1988, 113, 55–63. (30) Ma, H. Industrial minerals and rocks; Chemical industry press: Beijing, China, 2005; pp 52 55. (31) Lv, J. Experimental Studies on Characteristics of Primary Particulate Matter after Coal Combustion on Different Conditions. In Thermal Engineering; Doctoral Thesis, Tsinghua University, Beijng, China, 2006; pp 50 53. (32) Zhao, Y.; Zhang, J.; Sun, J.; Bai, X.; Zheng, C. Mineralogy, chemical composition, and microstructure of ferrospheres in fly ashes from coal combustion. Energy Fuels 2006, 20, 1490–1497. (33) Higashi, S. Ammonium-bearing mica and mica-smectite of several pottery stone and pyrophyllite deposits in Japan: their mineralogical properties and utilization. Appl. Clay Sci. 2000, 14, 171–184. (34) Ward, C. R.; Taylor, J. C.; Matulis, C. E.; Dale, L. S. Quantification of mineral matter in Argonne Premium Coals using interactive Rietveld-based X-ray diffraction. Int. J. Coal Geol. 2001, 46, 67–82. (35) Tian, S.; Zhuo, Y.; Shu, X.; Chen, C.; Li, D. Distribution of mineral matter in Shenhua coal by combined ZnCl2 and HCl fractionation. J. Tsinghua University (Sci. Technol.) 2008, 48 (2), 240–243. (36) Tian, S.; Zhuo, Y.; Chen, C.; Li, D. Cristobalite formation in the thermal treatment of low volatile coal fly ash. J. Tsinghua University (Sci. Technol.) 2007, 47 (11), 1995–1997. 4904

dx.doi.org/10.1021/ef200502u |Energy Fuels 2011, 25, 4896–4905

Energy & Fuels

ARTICLE

(37) Gupta, S.; Dubikova, M.; French, D. Characterization of the origin and distribution of the minerals and phases in metallurgical cokes. Energy Fuels 2007, 21, 303–313. (38) Lu, L.; Sahajwalla, V.; Harris, D. Characteristics of chars prepared from various pulverized coals at different temperatures using drop-tube furnace. Energy Fuels 2000, 14, 869–876. (39) Wang, D.; Wang, F.; Zhang, H.; Feng, H. Potassium release performance of activated mica-type minerals. Geochimica (China) 1999, 28 (5), 505–512. (40) Russell, N. V.; Mendez, L. B.; Wigley, F.; Williamson, J. Ash deposition of a Spanish anthracite: effect of included and excluded mineral matter. Fuel 2002, 81, 657–663. (41) Gupta, R. P. Advanced coal characterization: a review. Energy Fuels 2007, 21, 451–460.

4905

dx.doi.org/10.1021/ef200502u |Energy Fuels 2011, 25, 4896–4905