Safely Burning High Alkali Coal with Kaolin Additive in a Pulverized

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Safely Burning High Alkali Coal with Kaolin Additive in a Pulverized Fuel Boiler Linlin Xu,† Jie Liu,† Yong Kang,*,† Yongqi Miao,‡ Wei Ren,‡ and Taotao Wang‡ †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P.R. China Center of Fuel Technology, Guodian Fuel Co., Ltd. of China Guodian Co., Nanjing 210031, P.R. China

‡

ABSTRACT: Efficient utilization of the low-rank coal has been a headachy problem, especially when firing the high alkalicontaining coal in a typical pulverized fuel boiler, where severe slagging and fouling originating from the alkali metal vapors may occur. The additives injection technology has been proved to be a promising method in combating these problems. In this study, the alkali capture mechanism of kaolin was investigated by burning a kind of high-sodium lignite in a laboratory-scale drop tube furnace. The effects of kaolin content, reaction temperature, and particle sizes of both kaolin and fuel on the sodium capture efficiency of kaolin were also investigated. It was found that kaolin could chemically adsorb NaCl, the primary sodium species proved in the flue gas, to form high-melting sodium aluminosilicates such as nepheline and albite, and the nepheline-forming reaction dominated the sorption mechanism. More kaolin addition led to more sodium fixed into the ash. However, the promotion was not that pronounced in high kaolin dosages. The sodium capture efficiency decreased as temperature was increased or larger kaolin particles were injected. Effect of the coal size on the sodium capture efficiency could be neglected in the tested size range. The sodium retention with 6 wt % kaolin addition of the fuel at 1200 °C could attain 70% of the total sodium in the combusted coal, which can considerably reduce the ash-related problems and facilitate the safe firing of high alkali coal in boilers.

1. INTRODUCTION In the last centuries, the overuse of coal, which is the most predominant energy resource in the world, has trapped the human being into an awkward situation that not only less and less coal is available, but also the difficulties in making good use of low-rank coal are distressing. Efficient utilization of low-rank coal has been a key factor in alleviating the current energy shortage. Xinjiang, one of the biggest coal reserve areas in China, contributes greatly to maintaining the sufficient fuel supply for energy-consuming industries. However, most of the Xinjiang coals are the low-rank lignite, which contains rather high contents of alkalis owing to the local alkalescent geological conditions. As is well-known, mineral matters in coal have significant influence on both the energy conversion efficiency of the fuel and the steady run of the boiler.1,2 Alkali species, in particular, are known as triggers of many intractable problems. The low-melting alkalis (primarily sodium) in coal would volatilize into the flue gas during combustion. These gaseous alkalis might exist in the form of chloride, sulfate, or hydroxide depending on the coal type and combustion conditions or react with other ash components (such as silica and calcia) to form fused compounds or eutectics. The gas-phase or fused alkali species can lead to severe fouling when depositing onto the convective boiler tubes and cause slagging when condensing on the heat transfer surface of the radiant areas. Serious erosion and corrosion of the turbine blades induced by alkali vapors in the flue gas may occur in the combined cycle power generation system where gas turbines are used.3 Additionally, the enrichment of sodium and other alkalis in the submicrometer, respirable particulates has been regarded as one of the sources of serious environmental problems.4 The above summarized problems originating from alkalis, especially the severe slagging © 2014 American Chemical Society

and fouling which can lead to the decreased heat transfer efficiency, the unscheduled shutdowns and hence the increased operational costs of boilers, limit the broad and efficient utilization of the high-sodium coal. Therefore, it is imperative and practical to have a better understanding of alkali control during pulverized coal combustion. Previous studies have provided some methods to control alkali emission during coal combustion in order to alleviate or totally prevent ash-related problems.5−18 The alkali control methods can be divided into two kinds. The one approach is to reduce the alkali contents in coal by various pretreatments such as water-washing or being beneficiated with aluminum salt solutions.5,6 These pretreatments were shown to be capable of decreasing sodium levels in the coal matrix to efficiently mitigate fouling and slagging in boilers. In addition, the coal beneficiation with aluminum salt solutions was proved to be more efficient because this method reduced the stickiness of sodium silicates. Coal pretreatments seem to radically eliminate alkali-related problems by removing sodium from the coal matrix prior to combustion. These dressing processes are, however, rather complicated and costly. Besides, their byproducts are acid or alkali wastewaters containing many heavy metal ions, which are difficult to be disposed of for discharge or recycling. Therefore, these methods are expensive and unpractical for the power plants. The other method is the use of various kinds of mineral additives to capture alkali metal compounds during coal combustion, which is a relatively simple and economical Received: May 21, 2014 Revised: August 19, 2014 Published: August 19, 2014 5640

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of kaolin for different coals may indicate the different sorption mechanisms. The temperature dependence of alkali emission and the size effect on the sorption control mechanism suggest that these factors are likely to affect the sorption process and performance of kaolin. Thus, the alkali capture mechanism needs further elucidation with deep analysis, and some firingrelated factors are also required to be investigated. The objectives of this study focus on (1) assessing kaolin as an alkali vapor sorbent for the tested high-sodium coal during combustion in a typical pulverized fuel boiler, (2) elucidating the sorption mechanism of alkali vapor by kaolin during combustion, (3) exploring the effects of additive dosage, combustion temperature, and particle sizes of both the fuel and additive on the capture performance of kaolin, (4) providing a guideline for efficiently and safely firing low-rank coal in a pulverized fuel boiler.

method in combating ash-related problems compared to coal pretreatment and has consequently been intensively investigated for the last several decades.7−18 Actually, most of these investigations were carried out in fixed-bed reactors or packedbed flow channel reactors under conditions that gaseous alkali metal compounds flow through the fixed sorbents usually at temperatures below 1000 °C to gain data about adsorption efficiency and mechanisms for some mineral additives.7−14 Among the studied minerals, clays liked kaolin minerals containing both high silica and alumina were shown to be the best candidates for sorbent use in consideration of both performance and cost. With appropriate conditions such as adequate sorbents or sufficient reaction time, alkali vapors could be completely removed from the flue gas by these additives. Both chemical fixation and physical adsorption might occur in the alkali sorption process. The prevalent alkali capture mechanisms of kaolin were the reactions between metakaolin and alkali species (chloride, sulfate, or hydroxide) to form highmelting alkali aluminosilicates.7−11 The fixed-bed (FB) studies have clearly elucidated the sorption mechanisms and verified the high sorption efficiency of kaolin. However, coal combustion in pulverized fuel boilers is quite different from FB combustion, since it is performed under conditions where both coal and additives are transported by the flue gas resulting in a short reaction time in the combustion chamber and where the fuel is fired at relatively high temperatures. Thus, the capture performance and mechanisms of kaolin would probably be different. Mechanisms of sodium capture by kaolin minerals at high temperature were investigated using a downflow combustor by Mwabe et al.15 They proposed that the actual capture process was the reaction between alkali hydroxide and activated kaolinite. The presence of chlorine and sulfur in the flue gas, affecting the occurrence mode of sodium species in the gas phase, was found to have a negative effect on the alkali capture. The change in sorbent particle size would switch the key factors (chemical kinetics or pore diffusion) dominating the controlling step of the adsorbing reaction. Temperature dependence of alkali vapor release during pulverized coal combustion and the influence of additives on alkali binding were investigated by Schürmann et al. in a drop flow reactor in the temperature range of 1100−1400 °C.16 They found a strong dependence of alkali metal release on the combustion temperature. Increasing contents of alkalis were found in the flue gas with increasing temperature. The additives with both high alumina and silica contents gave the best performance in chemically adsorbing alkalis during coal combustion. Vuthaluru et al.17 assessed different additives in alleviating fouling problems using mixtures of the candidate sorbents (kaolin, silica, and alumina) with different sodium species in a drop tube furnace, and they calculated the amounts of various additives required to mitigate fouling. Kaolin with a size range of 10−20 μm and a dosage of 2−3 wt % of the coal was found to efficiently combat fouling. Takuwa and Naruse18 reported that kaolin could trap the vapor of sodium compounds into large ash particles and depress the formation of fine particulates under coal combustion conditions. The fraction of the sodium retained by kaolin, depending on the coal type, was estimated to attain 80% for the tested coal. The capture process was proposed to be the reaction between metakaolinite and sodium hydroxide to form Na2O·Al2O3·2SiO2. Therefore, it can be seen that kaolin is of great potential in adsorbing alkali vapors during coal combustion. However, the varied sodium sorption efficiency

2. EXPERIMENTAL MATERIALS AND SCHEME 2.1. Experimental Materials. 2.1.1. High-Sodium Lignite. The high-sodium lignite used in this study was Hami coal from Shaerhu mine in Hami of Xinjiang, of northwestern China. Table 1 gives the

Table 1. Properties of the Tested Hami Coal proximate analysis

wt %, dry

ash volatile matter fixed carbon moisture, (ad) ultimate analysis

10.91 37.09 52.00 13.77 wt %, dry

C H O N Cl Total S ash composition

69.70 3.82 14.36 1.02 0.63 0.19 wt %, in ash

SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 TiO2 ash fusion temperature deformation soft hemispherical flow

35.31 16.40 6.21 25.82 2.95 0.94 5.86 0.36 0.79 °C 1240 1250 1270 1290

properties of the coal sample. The tested coal has relatively low ash but rather high alkali contents, which indicates the great possibility of fouling and slagging occurring during combustion. The size of relatively coarse parent coal was reduced to less than 1 mm by a cutting mill machine, and then the coal was divided into different sizes under 200 μm with the use of a vibrating sieve. The d50’s of the five coal samples were 13, 22, 30, 37, and 48 μm, respectively. The size of the coarsest fuel sample was chosen to be less than 50 μm which allowed for the burnout of the fuel with a short residence time in the reactor. The prepared coal particles were collected, air-dried, and stored for experiments. 5641

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2.1.2. Kaolin Additives. Kaolin, which is abundant in kaolinite (Al2O3·2SiO2·2H2O), was selected as a sorbent for its great potential in capturing alkali metal vapors in this work. Kaolin was supplied in powder form and had been manufactured to fulfill the requirements of purity and size in the experiments. The particle sizes (d50) of 3, 11, 18, and 28 μm were selected for the parametric experiments of additive granularity. The main ingredients of kaolin are the comparable amounts of SiO2 and Al2O3, together with some minor impurities as presented in Table 2. When heated at temperatures above 450 °C,

Table 2. Chemical Analysis of Kaolin composition

wt %

Al2O3 SiO2 Fe2O3 CaO Na2O

37.87 44.60 0.45 0.84 0.05

kaolin lost hydration water and was converted into metakaolin (Al2O3· 2SiO2). At a temperature about 980 °C, metakaolin decomposed to amorphous silica and alumina−silica spinel (Al2O3·SiO2). When further heated to above 1100 °C, mullite (3Al2O3·2SiO2) started to form with a relatively low formation rate.19 The structural change of kaolin upon heating would probably influence its capture performance. To investigate size effect on the capture performance of kaolin, pulverized coals and additive powders were thoroughly mixed in different size collocations before being injected into the drop tube furnace. The initial size match was determined as 30 μm for coal and 18 μm for kaolin. In addition, a series of different mass fractions of kaolin were injected to obtain information about the largest content of sodium that could be sequestered and the most enhanced utilization of kaolin with a suitable stoichiometric ratio between kaolin and the fuel. The mass ratio of kaolin in the fuel varied from 2% to 12%. The initial proportion of kaolin added in the coal was defined by a mole ratio of kaolin/sodium oxide = 3, which corresponded to approximately 8 wt % kaolin based on the tested air-dried coal. In addition, raw coal with no kaolin addition was also included in the tests to gauge the sodium fixation resulting from kaolin addition. 2.2. Experimental Apparatus and Procedure. Combustion experiments were carried out in a laboratory-scale drop tube furnace, which can perfectly simulate the combustion conditions in a pulverized fuel fired boiler. A schematic diagram of the drop tube furnace is shown in Figure 1. The combustion system mainly consists of the fuel feeding unit, an electrically heated furnace, and the ash collecting unit. The injection probe, where fuel is fed into the combustion chamber, is located at the top of the furnace. A screw feeder with a feed rate range of 20−500 g/h is used to add fuel and additive mixtures into the reaction tube. Combustion air is injected from the top of the furnace and divided into primary and secondary air. Primary air at ambient temperature delivers fuel into the reaction tube via the injection probe. The secondary air preheated by an air preheater enters the furnace through the annular clearances between the reaction tube and the injection probe. A 1900 mm long corundum reaction tube with a diameter of 100 mm is housed in the furnace, with an effective hot zone of 1700 mm. Residence time of the particles is about 2 s. The corundum tube is electrically heated by silicon−carbon rods in the adjacent heating zone, which is insulated by an adiabator cylinder and divided into three individual zones in order to maintain a constant wall temperature or adjust a temperature profile along the furnace. Reaction temperatures measured by thermocouples are monitored via the PC controller. The temperature of the furnace can reach up to as high as 1600 °C, which is able to fulfill the required temperature interval (1000−1400 °C) in the tests. An ash tray is installed at the bottom of the furnace to collect the coarse ash particles (referred to as bottom ash). The flue gas is cooled by water before entering the cyclone, and the moderate particles (referred to as cyclone ash) are collected into the ash can of the cyclone. The remaining finer particulates usually in the submicrometer size range in the flue gas

Figure 1. Schematic diagram of the drop tube furnace. leaving the cyclone are released. This study mainly focused on the bottom ash and the cyclone ash for the alkalis in the submicrometer particulates are detrimental to the environment and thus were excluded from kaolin capture performance. The ash tray and the ash can both have a smooth stainless steel inner shell which can prevent ash from condensing on the surface and makes it easier to collect the ash completely. A suction pump is connected to the cyclone to hold an appropriate air flow, maintaining an atmospheric pressure in the combustor. In each test, when the temperature of the furnace reached the set point it was left to run for another 30 min to make sure that the temperature throughout the furnace had reached a steady state. The feed rate was controlled at 200 g/h for all trials. Excess air of 25 vol % was introduced into the combustion chamber to ensure a complete combustion. The total air flow comprised 20 vol % primary air at ambient temperature and 80 vol % secondary air preheated to 700 °C. Each experiment was terminated when the sorption test lasted for 1 h. After each run, the bottom ash and the cyclone ash were collected, weighed, and stored for various analyses. All the combustion experiments were conducted in duplicate to get the averaged results. The uncertainties of the results were estimated to be within 5%. 2.3. Analytical Methods. The particle size distributions of the fuel, kaolin, and reaction products were determined by a laser diffraction particle size analyzer (Dandong Bettersize, China), which has the capability of measuring particle size in the range of 0.1−1000 μm. Complete conversion of char particles during combustion was difficult for the short residence time in the furnace, so loss on ignition (LOI) tests of the ash products were done to give more information about the ash characteristics. Mineralogical analysis of the ash samples was performed using X-ray powder diffraction (XRD, Bruker AXS, Germany) to identify the crystalline compounds presented in the examined ash samples. Quantitative element analysis of the ash samples was carried out using inductively coupled plasma optical emission spectrometer (ICPOES, Thermo Scientific, U.S.A.). Approximately 0.1 g of the collected ash mixed with 2 g fluxes (lithium tetraborate) was baked in a muffle furnace at 950 °C for 30 min. The fused mixture was dissolved into a warm hydrochloric acid and then the solution was diluted with deionized water by a factor of 10 for sodium analysis by ICP-OES. A blank solution made from the fluxes only was also prepared and analyzed to eliminate the errors caused by any sodium impurities existing in the fluxes or the acid. Then the mass fraction of sodium in the ash could be obtained by subtracting the sodium content in the 5642

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Figure 2. XRD patterns of the ash products produced at 1200 °C: (a) bottom ash of raw coal, (b) bottom ash of coal with 8% kaolin addition, (c) cyclone ash of raw coal, (d) cyclone ash of coal with 8% kaolin addition. 1-CaAl2Si2O8·4H2O (gismondine), 2-NaAlSi3O8 (albite), 3-Ca2Al2SiO7 (gehlenite), 4-NaCl (halite), 5-SiO2 (quartz), 6-NaAlSiO4 (nepheline or carnegieite), 7-Al6Si2O13 (mullite), 8-CaAl2Si2O8 (anorthite). acid at 120 °C was conducted. The temperature choice of 120 °C was aimed at completely dissolving any water-soluble sodium especially sodium silicates as shown by Kosminski.20 Water-insoluble sodium species would be sodium aluminosilicates such as nepheline (Na2O· Al2O3·2SiO2) and albite (Na2O·Al2O3·6SiO2), which could be separated by their different solubilities in acids; nepheline is soluble in sulfuric acid, whereas albite is soluble only in hydrochloric acid.21 The amount of 0.1 g ash sample was digested in deionized water at 120 °C for 1 h. Then the solution was filtrated to obtain the leachate and the residue. The leachate was analyzed for sodium by ICP-OES to get the amount of water-soluble sodium. The residue was digested in diluted sulfuric acid with the same operations as done in water and analyzed to get the content of sodium in nepheline. The rest of the total sodium would be albite. Then, the amounts of different sodium species in ash were obtained.

blank reference from that in the ash sample. The sodium capture efficiency of kaolin was calculated by comparing the sodium content in the collected ash (both the bottom and the cyclone ash) to the total sodium in the combusted coal (total sodium combusted), as defined in eq 1: capture efficiency(%) sodium content in the collected ash = × 100 total sodium combusted

(1)

In which,

sodium content in the collected ash = ash weight × sodium mass fraction in ash total sodium combusted = coal combusted × ash content (%)

3. RESULTS AND DISCUSSION 3.1. Validity and Alkali Capture Mechanism of Kaolin. With the purpose of assessing the effectiveness of kaolin as a sodium binding agent, a couple of comparative combustion tests with raw coal and the mixtures of coal and kaolin were conducted at 1200 °C. The particle sizes for coal and kaolin were 30 μm and 18 μm, respectively. The fraction of kaolin added was 8 wt % of the fuel. The sodium capture efficiency, i.e. the amount of sodium retained in the collected ash compared

× sodium mass fraction in ash The sodium capture efficiency of raw coal without kaolin addition was also calculated, which reflected the fraction of sodium bound by the minerals in raw coal. This result was used to gauge the performance originating from kaolin addition. In order to distinguish different sodium species such as chloride or aluminosilicates present in the ash products and to assess the interactions between sodium compounds and kaolin during combustion, staged leaching analysis with water and diluted sulfuric 5643

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different sodium species, e.g. water soluble and water insoluble (nepheline or albite), was needed. Thus, staged leaching analysis of the ash samples with water and diluted sulfuric acid was carried out to determine the contents of different sodium species presented in both the bottom ash and the cyclone ash for coal with and without kaolin addition. The results are presented in Table 3. As shown, the water-soluble sodium

to the total sodium in the combusted fuel (total sodium combusted), was calculated using eq 1. It was 39.14% for raw coal and 63.35% for coal with 8% kaolin addition. Clearly, another 24.21% of the total sodium combusted was retained in ash with kaolin injection, which indicated that less problematic alkali vapor was released during combustion. In this regard, it was confirmed that kaolin had a positive effect on alkali capture during pulverized coal combustion. However, the sodium capture mechanism for the tested coal needs further elucidation with various analyses. The mineralogical analysis of both the bottom ash and the cyclone ash using XRD was carried out to evaluate the interactions between kaolin and the fuel. The XRD patterns of the ash products for coal with and without kaolin addition are shown in Figure 2, with panels a and b for the bottom ash and panels c and d for cyclone ash. As for the bottom ash, the identified crystalline compounds were almost the same for coal with and without kaolin addition as displayed in Figure 2a and b, with the exception that the bottom ash of parent coal was more crystalline in comparison to that with the kaolin addition. This was probably due to the formation of amorphous metakaolin stemming from the decomposition of kaolin upon heating.11 The strongest peak identified was CaAl2Si2O8·4H2O (gismondine) for the parent coal, which consisted with the high content of CaO (25.82%) in the ash as shown in Table 1. When kaolin was added, however, the strongest peak was changed to quartz (SiO2), which could be attributed to the presence of more SiO2 resulting from kaolin decomposition during combustion. Gehlenite (Ca2Al2SiO7) was another Ca-containing crystal appearing in the patterns. The detected sodium crystalline peaks included NaCl (halite), NaAlSi3O8 (albite), and NaAlSiO4 (nepheline or carnegieite). The peak of mullite (Al6Si2O13) was also identified. The XRD patterns of the cyclone ash (Figure 2c and d) implied the amorphous feature of the ash products, with the presence of glass contents, unburnt char particles, and other uncrystallized compounds in the ash since a halo was on the baseline and a broad hump appeared in the range between 20° and 35° (2θ).22 Only several crystalline peaks such as CaAl2Si2O8·4H2O, NaCl, SiO2, and NaAlSiO4 were superposed from the amorphous areas. The amorphous character of the cyclone ash could be attributed to the following two factors. One was the rapid cooling of the ash by water before entering the cyclone, which led to the less developed crystalline structure and the formation of the glass phase. The other was the relatively fine particles and the small bulk density of the glass, which rendered these particles able to flow with the flue gas and to be collected by the cyclone. The measured particle sizes (d50) for the bottom ash and the cyclone ash were approximately 63 μm and 28 μm, respectively. Although merely several crystalline phases were identified in the XRD patterns of the cyclone ash, it was inferred that the major ingredients were similar to that in the bottom ash. Therefore, only XRD patterns of the bottom ash are shown and discussed if necessary in the following sections The sodium compounds identified using XRD were mainly crystalline NaCl, NaAlSi3O8, and NaAlSiO4 in the ash. Other sodium species such as sodium sulfate or sodium silicate (both water soluble) might also exist in the ash. The absence of these species in the XRD patterns was probably due to their low contents or amorphous forms. The mineralogical analysis results of sodium compounds were only an indication of the terminate forms of sodium retained by kaolin in the ash. To interpret the capture mechanism, quantitative analysis of

Table 3. Staged Leaching Results of Different Sodium Species Presented in Both the Bottom Ash and the Cyclone Ash for Coal with and without Kaolin Addition at 1200 °C sample bottom ash of the parent coal cyclone ash of the parent coal total ash of the parent coal bottom ash of coal with 8% kaolin cyclone ash of coal with 8% kaolin total ash of coal with 8% kaolin

water-soluble sodium (%)

nepheline (%)

albite (%)

10.01 13.43 10.86 9.44

64.17 49.63 60.55 58.52

25.82 36.94 28.59 32.04

6.36

69.51

24.13

8.21

62.88

28.91

accounted for only a small part of the total sodium bound in the ash. The rest, approximately 90%, of the sodium retained in the ash was water-insoluble sodium aluminosilicates, which suggested that the chemical interactions between sodium vapors (such as NaCl, NaSO4, and NaOH) and kaolin had happened. In order to clarify the composition of water-soluble sodium in the ash products, ion chromatography analysis of the leachates originated from the deionized water leaching was made to determine the contents of Cl− and SO42−. It gave the result that the mole ratio of Cl/Na in the leachates was equal to, actually in most situations greater than 1, while the SO42− content was negligible. With the evidence of NaCl crystalline peaks detected in the XRD patterns, it was concluded that NaCl was the primary water-soluble sodium compound in the ash. In terms of water-insoluble sodium, nepheline was the major sodium aluminosilicate in the ash, while the amount of sodium in the form of albite was only half of that in nepheline as shown in Table 3. The percentages of various sodium species normalized to the total sodium in the combusted coal (total sodium combusted) are shown in Figure 3. The small amount of water-soluble sodium, predominately NaCl, probably stemmed from the unburnt char particles or the condensed sodium vapor for both samples. Approximately 35% of the total sodium combusted was fixed in ash in the form of aluminosilicates for raw coal, which could be attributed to the moderate alumina and silica contents in the ash (Table 1). The retained water-insoluble sodium with kaolin addition was nearly 60%, indicating that another 25% of the total sodium was captured by kaolin during combustion. The amounts of nepheline and albite were both increased with a rise of 16.83% for nepheline and 7.13% for albite when compared to coal without kaolin addition, which meant that the both sodium aluminosilicates were formed as the products of reactions between kaolin and sodium species during combustion, and the nepheline-forming reaction dominated. According to Wibberley and Wall, sodium species in the gas phase were found to be mainly NaCl and NaOH during combustion, and for coal with chlorine contents above approximately 0.5 wt %, NaCl was the predominant sodium 5644

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Figure 3. Percentages of various sodium species in the total sodium combusted for different ash samples produced at 1200 °C: (1) ash of the parent coal, (2) ash of coal with 8% kaolin.

Figure 4. Sodium capture efficiency with kaolin content of the fuel.

total sodium combusted was retained in the ash when 12 wt % of kaolin was injected. The sorption efficiency increased fast at low dosages of kaolin addition until 6%, after which a gentle trend appeared and lasted as more sorbent was added. About 62% of the total sodium combusted was fixed into ash with the kaolin addition of 6%, while merely 8% more sodium was captured by kaolin with another 6% of additive being injected in the value of 12 wt % of the coal, which implied the poor utilization of kaolin in the high dosages. A possible explanation for this is the short residence time of particles in the reaction zone, making it difficult to realize the full availability of some sorbent particles that might be coated by other kaolin particles. Besides, with more kaolin added, the more unreacted amorphous metakaolin might be present in the ash reflected by a broader and higher hump in the XRD patterns similar to that shown in Figure 2b, resulting in the formation of more viscous ash and subsequently more slagging and deposition. In power stations, the cost of additives should be taken into consideration. Therefore, less kaolin addition with appropriate capture performance such as 6 wt % of the fuel should be selected for further investigations. 3.3. Effect of Reaction Temperature. Temperature, a significant factor during pulverized coal combustion, has an effect on both the structural change of kaolin and alkali vapor pressure above potentially formed alkali aluminosilicates and consequently may influence the capture efficiency. Combustion experiments in the temperature range of 1000−1400 °C with 6 wt % kaolin addition were conducted to get the temperature dependence of sodium capture efficiency. The particle sizes of kaolin and coal were the same as used in kaolin content tests. The results, presented in Figure 5, show that the sodium capture efficiency decreased as temperature was increased and only 50% of the total sodium combusted was retained in ash at 1400 °C. The inverse temperature dependence was deemed to be associated with the enhanced alkali release with increasing temperature as reported by Schürmann et al., and the alkali emission was stronger after the softening temperature of the ash was reached.16 The structural change of kaolin upon heating might be responsible for this trend. However, the deactivation of metakaolin at high temperatures to form mullite was a slow process;19 thus, the structural change effect could be neglected due to the rapid drop of kaolin in the furnace. A sharper fall occurred in the interval between 1200 and 1300 °C

vapor in gas phase up to about 1450 °C. Only above this temperature did NaOH become dominant in flue gas.23 Consequently, for the tested coal with the chlorine content of 0.63 wt % combusted at 1200 °C, NaCl would be the primary gas-phase sodium species during combustion, which was also supported by the NaCl peaks in the XRD patterns and the evidence that NaCl was the predominant water-soluble sodium compound in the ash. Therefore, the reactions between kaolin and sodium species would inherently be the interactions between the pyrolysis products of kaolin and NaCl. The formation of albite and nepheline or its polymorph carnegieite could be demonstrated by the following reaction formulas: 2NaCl + Al 2O3 ·2SiO2 (metakaolin) + H 2O → Na 2O·Al 2O3 ·2SiO2 (nepheline/carnegieite) + 2HCl (2)

2NaCl + Al 2O3 · 2SiO2 (metakaolin) + 4SiO2 + H 2O → Na 2O·Al 2O3 ·6SiO2 (albite) + 2HCl

(3)

The extra SiO2 demanded in eq 3 might derive from the coal minerals or the decomposition of metekaolin when heated at temperatures above 980 °C.19 The mechanism of sodium capture by kaolin has been clearly elucidated for firing this kind of coal. The effects of kaolin content, reaction temperature, and particle size on the sodium capture efficiency are to be discussed in the following sections. 3.2. Effect of Kaolin Content. Kaolin has been proved to be a good sorbent in capturing sodium vapor during combustion. With 8 wt % kaolin addition in coal, nearly 60% of the total sodium in the burnt fuel was bound into ash in the form of aluminosilicates. In order to obtain the best performance of kaolin, viz. the largest sodium retention in ash by kaolin during combustion, different dosages of kaolin addition were investigated under the conditions where the furnace temperature was 1200 °C with particle sizes for coal and kaolin at 30 μm and 18 μm, respectively. The sodium capture efficiency was also calculated using eq 1. The results for capture efficiency as a function of kaolin content are shown in Figure 4. It is evident that the sodium capture efficiency increased with increasing kaolin addition. About 70% of the 5645

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Figure 5. Sodium capture efficiency with the reaction temperature.

(Figure 5), which could also be related to the melting characteristic of the sodium species. The XRD patterns of the bottom ash generated at 1100 and 1300 °C are shown in Figure 6. The major crystalline phases identified at both temperatures were almost the same and similar to that shown in Figure 2b. However, the broader and higher hump in Figure 6b indicates that more glass contents were produced in the 1300 °C ash. The amorphous phenomenon occurring at the higher temperature could be ascribed to the relatively low softening temperature of the ash, which was 1250 °C for this coal as presented in Table 1. Staged leaching analysis of the ash samples collected at the both temperatures was also carried out. The results normalized to the total sodium combusted are displayed in Figure 7. As can be seen, the fractions of different sodium species retained in the ash at 1100 °C were similar to that obtained at 1200 °C with 8% kaolin addition (Figure 3). However, nepheline, composing about 75% of the total sodium fixed in ash, was the dominant sodium species identified at 1300 °C. Albite accounted for only a small part of the retained sodium with a content of 6.92% of the total sodium combusted in comparison to 16.96% at 1100 °C, which corresponded well to the nearly vanished crystalline peak of albite as shown in Figure 6b. A possible cause for the content variations of different sodium species, especially albite found at 1300 °C, was the low melting point of albite, which was merely 1120 °C. At the temperature below 1200 °C such as at 1100 °C, the well-crystallized albite was found in the ash and the content was appreciable. However, a large fraction of albite would get molten and viscous, and adhere to the reaction tube with a combustion temperature of 1300 °C. Thus, only little albite could be found in the 1300 °C ash. The percentages of nepheline at both temperatures were similar due to its high melting point of about 1526 °C. Additionally, the less watersoluble sodium, primary NaCl, at the higher temperature in this figure implies that the total sodium release increased with temperature. Finally, the stronger sodium release and the occurring of molten albite after the softening temperature (1250 °C) was reached clearly explain the sharper fall between 1200 and 1300 °C in Figure 5. 3.4. Effect of Kaolin and Coal Particle Sizes. 3.4.1. Kaolin Particle Size. The effect of additive particle size on the sorption reactions between kaolin and alkali vapors might be negligible in the fixed-bed investigations, where the

Figure 6. XRD patterns of the bottom ash yielded at different temperatures with 6% kaolin addition: (a) ash sample of 1100 °C, (b) ash sample of 1300 °C. 1-CaAl2Si2O8·4H2O (gismondine), 2NaAlSi3O8 (albite), 3-Ca2Al2SiO7 (gehlenite), 4-NaCl (halite), 5SiO2 (quartz), 6-NaAlSiO4 (nepheline or carnegieite), 7-Al6Si2O13 (mullite).

contact time was long enough and the relatively fine particles could provide adequate surface area for adsorbing alkali vapors by sorbents.10 However, the size effect of additives probably played an important role in capturing alkali vapors during combustion in a pulverized fuel boiler for the typically short residence time of merely several seconds. Combustion trails for the mixtures of 30 μm coal with 6 wt % kaolin addition in different sizes were conducted at 1200 °C to obtain the size dependence of sodium capture efficiency. The results are shown in Figure 8. It can be seen that the capture efficiency diminished as larger kaolin particles were injected. Especially when kaolin had a comparable size of 28 μm with the fuel (30 μm), the capture efficiency were considerably reduced to about 53%. A reasonable explanation for this trend could be the decreased specific surface areas with increased particle size, resulting in less available contact areas between sodium vapor and kaolin. For the particles with sizes less than 11 μm, the capture efficiency was not extensively enhanced as finer particulates such as 3 μm kaolin powders were added, which suggested that the limiting factor for the reactions between kaolin and sodium vapors might shift from surface area demand to reaction rate control due to the short residence time in the 5646

dx.doi.org/10.1021/ef501160f | Energy Fuels 2014, 28, 5640−5648

Energy & Fuels

Article

Figure 9. Sodium capture efficiency with fuel size. Figure 7. Percentages of various sodium species in the total sodium combusted for ash samples collected at different temperatures with 6% kaolin addition: (1) ash products of 1100 °C, (2) ash products of 1300 °C.

Table 4. LOI Analysis of the Ash Products from Different Coal Size Tests at 1200 °C combusted fuel size (μm)

LOI of the ash sample (wt %)

13 22 30 37 48

7.31 8.45 11.27 13.66 19.58

coal particles reflected the fact that more unburnt chars were contained in the ash, leading to more sodium being retained, which could be an explanation for the higher sodium capture efficiency of kaolin when firing larger coal particles. The staged leaching results of the ash products also gave evidence that a relatively large fraction of water-soluble sodium was found in the ash of larger coal particles. Thus, the fuel size could have little impact on the sorption efficiency when excluding the fraction of sodium contained in the unburnt chars. This was probably due to the adequate sodium vapor available for reaction in the flue gas for the tested coal sizes during combustion despite the small fraction of unburnt coal particles. The net sodium fixed into ash by kaolin was expected to attain about 70% of the total sodium combusted.

Figure 8. Sodium capture efficiency with kaolin particle size.

furnace. This coincided with the results obtained by Mwabe and Wendt.15 They proposed a simple first-order kinetic model and estimated a volumetric rate constant using multivariable regression analysis to determine the effect of sorbent sizes on the sodium capture. It was found that chemical kinetics controlled the adsorption reaction for small particles under 2 μm, whereas intraphase pore diffusion controlled the reaction for larger sorbent particles. 3.4.2. Coal Particle Size. The tests of investigating the size effect of coal on the sodium capture efficiency were also conducted with 6 wt % kaolin addition in the size of 11 μm at 1200 °C. The results are displayed in Figure 9. It is clear that the sorption efficiency was around 70%, and only a little increase appeared for coal particles less than 37 μm, while for the larger particle, i.e. 48 μm, the capture efficiency could reach about 80%. The short residence time in the furnace made it difficult to realize the complete conversion of char particles. So the loss on ignition (LOI) analysis was carried out for the ash products originating from different fuel size tests, and the results are presented in Table 4. The higher LOI for the large

4. CONCLUSIONS The investigations of sodium capture by kaolin with firing a high-sodium lignite coming from Xinjiang of China were conducted in a bench-scale drop tube furnace to have a better understanding of the validity and alkali sorption mechanism of kaolin under pulverized coal combustion conditions. The effects of kaolin content, reaction temperature, and particle sizes of both kaolin and the fuel on the sodium capture efficiency were also studied. The following conclusions are drawn from this work: (1) Kaolin could efficiently capture sodium vapor during high-sodium coal combustion. The primary reactions between kaolin and sodium species would be the interactions between metakaolin and NaCl, which was proved to be the primary sodium species in gas phase. Both nepheline and albite were formed as the reaction products, while the nepheline formation dominated. 5647

dx.doi.org/10.1021/ef501160f | Energy Fuels 2014, 28, 5640−5648

Energy & Fuels

Article

(17) Vuthaluru, H. B.; Vleeskens, J. M.; Wall, T. F. Fuel Process. Technol. 1998, 55, 161−173. (18) Takuwa, T.; Naruse, I. Proc. Combust. Inst. 2007, 31, 2863− 2870. (19) Chen, C. Y.; Lan, G. S.; Tuan, W. H. Ceram. Int. 2000, 26, 715− 720. (20) Kosminski, A.; Ross, D. P.; Agnew, J. B. Fuel Process. Technol. 2006, 87, 1037−1049. (21) Dean, J. A. Lange’s Handbook of Chemistry, 13th ed.; McGraw Hill Book Co.: New York, 1985. (22) Vassilev, S. V.; Vassileva, C. G. Fuel 2007, 86, 1490−1512. (23) Wibberley, L. J.; Wall, T. F. Fuel 1982, 61, 87−92.

(2) Increasing kaolin addition had a positive effect on improving the sodium capture efficiency. This promotion was, however, not that pronounced in the high additive dosages. (3) The inverse temperature dependence of sodium capture efficiency was related to the enhanced alkali emission with increasing temperature. The ash fusion characteristics and the low-melting albite were also responsible for this trend. (4) Sodium capture efficiency decreased when larger kaolin particles were injected, which could be attributed to the reduced surface area with increasing particle size. However, the reaction rate would be the limiting factor for sodium sorption when the kaolin particle size was reduced to about 3 μm. The fuel size had little influence on the capture efficiency in the tested size range. The net sodium retention by kaolin with 6 wt % addition in the size of 11 μm was expected to attain 70% of the total sodium combusted at 1200 °C. The results obtained in this study could be a guideline in further safely firing the problematic but relatively adequate Xinjiang high-sodium coal in the power station boilers using powder injection technology with appropriate operational conditions.

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AUTHOR INFORMATION

Corresponding Author

*Tel. /Fax: +86-022-27408813. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support received for this research from Guodian Fuel Co., Ltd. of China Guodian Group Corporation.

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REFERENCES

(1) Raask, E. Mineral Impurities in Coal Combustion: Behavior, Problems and Remedial Measures; Hemisphere: New York, 1985. (2) Bryers, R. W. Prog. Energy Combust. Sci. 1996, 22, 29−120. (3) Simms, N. J.; Oakey, J. E.; Stephenson, D. J.; Smith, P. J.; Nicholls, J. R. Wear 1995, 186−187, 247−255. (4) Takuwa, T.; Mkilaha, I. S. N.; Naruse, I. Fuel 2006, 85, 671−678. (5) Vuthaluru, H. B.; Domazetis, G.; Wall, T. F.; Vleeskens, J. M. Fuel Process. Technol. 1996, 46, 117−132. (6) Vuthaluru, H. B.; Linjewile, T. M.; Zhang, D.-K.; Manzoori, A. R. Fuel 1999, 78, 419−425. (7) Punjak, W. A.; Shadman, F. Energy Fuels 1988, 2, 702−708. (8) Punjak, W. A.; Uberoi, M.; Shadman, F. AIChE J. 1989, 35 (7), 1186−1194. (9) Uberoi, M.; Punjak, W. A.; Shadman, F. Prog. Energy Combust. Sci. 1990, 16, 205−211. (10) Kyi, S.; Chadwick, B. L. Fuel 1999, 78, 845−855. (11) Kosminski, A.; Ross, D. P.; Agnew, J. B. Fuel Process. Technol. 2006, 87, 1051−1062. (12) Tran, K.-Q.; Iisa, K.; Steenari, B.-M.; Lindqvist, O. Fuel 2005, 84, 169−175. (13) Zheng, Y. J.; Jensen, P. A.; Jensen, A. D. Fuel 2008, 87, 3304− 3312. (14) Escobar, I.; Müller, M. Energy Fuels 2007, 21, 735−743. (15) Mwabe, P. O.; Wendt, J. O. L. Proc. Combust. Inst. 1996, 26, 2447−2453. (16) Schürmann, H.; Unterberger, S.; Hein, K. R. G.; Monkhouse, P. B.; Gottwald, U. Faraday Discuss. 2001, 119, 433−444. 5648

dx.doi.org/10.1021/ef501160f | Energy Fuels 2014, 28, 5640−5648