Sintering Behavior of Agricultural Residues Ashes and Effects of

Article pubs.acs.org/EF

Sintering Behavior of Agricultural Residues Ashes and Effects of Additives Liang Wang,*,† Michael Becidan,† and Øyvind Skreiberg† †

SINTEF Energy Research, Sem Sælands vei 11, Trondheim, Norway ABSTRACT: In this work the ash sintering behaviors and effects of aluminum silicates based additives (kaolin, zeolite 24A, and zeolite Y) during combustion of wheat straw and barley husk were investigated. The sintering degrees of fuel ashes and corresponding mixtures with additives were evaluated by performing standard ash fusion tests and laboratory scale sintering tests. The ash chemistry and microstructures were investigated by a combination of X-ray diffraction (XRD) and SEM-EDX analyses. It was found that the wheat straw and barley husk ashes have high sintering and melting tendencies. At elevated temperatures, formation and fusion of low temperature melting potassium salts and potassium silicates contributed to severe sintering of the two fuel ashes. Sintering of the barley husk ash is also associated with the presence of low melting points potassium phosphates with high K/Ca ratios. The experimental results from investigating the reactions between additives and KCl showed that kaolin and zeolite 24A can both bind KCl with formation of different potassium aluminum silicates. No clear reactions between zeolite Y and KCl were observed. Both kaolin and zeolite 24A were effective to increase sintering temperatures of the wheat straw and barley husk ashes. The reactions between kaolin and zeolite 24A with potassium containing species in the two reference ashes were revealed by XRD and SEM-EDX analyses. Identification of high temperature melting potassium aluminum silicates partially explains the higher sintering and melting temperatures of the ash-additives mixtures. Zeolite Y showed a poor ability to abate sintering of the studied ashes in this work.

1. INTRODUCTION Combustion for heat and power production is the most important application of biomass materials today.1,2 Because of increasing demands of bioenergy, new biomass fuels in various forms are being introduced into the market nowadays.3 However, despite obvious advantages such as high energy generation potential and carbon dioxide neutrality, combustion of these biomass materials is challenging due to ash related operational problems in combustion systems.4 These problems are especially significant during combustion of agricultural residues that contain high amounts of ash forming elements such as potassium, chlorine, sulfur, phosphorus, and silicon.5 Most of these elements are essential nutrients for agricultural plants growth, and some are enriched due to utilization of fertilizers.6 In addition, during harvesting and storage processes, the agricultural residues are easily contaminated by sand, soil, and dust, which again enhance the concentrations of inorganic elements in these materials.1,7,8 Among these ash forming elements, potassium plays a critical role in initiating and promoting ash related problems in combustors using agricultural residues as fuel. During combustion, the potassium reacts with the other ash forming elements via the following paths: (1) to form potassium salts, i.e. KCl, K2SO4, K2CO3; (2) to form potassium phosphates especially for P rich fuels; and (3) to generate different potassium silicates.8,9 Some potassium salts, phosphates, and silicates have low melting temperatures, presenting as molten phases at normal biomass combustion temperatures.1,2,4−11 The molten potassium compounds cause sintering of ash residues and further ash slagging on the grate of a boiler.1,6,8,10 The volatilized potassium containing species in the flue gas may initiate and enhance deposition and consequent corrosion on heat transfer surfaces.2,4,8 The ash related problems heavily hamper utilization of agricultural © 2012 American Chemical Society

residues as combustion fuels. It is imperative to obtain a better understanding of ash transformation and sintering behaviors during combustion of agricultural residues and try to find ways to mitigate these problems. Using additives is a promising way to abate and prevent ash sintering and melting during combustion of agricultural residues.2,5,8,12 The additives blended with fuels and/or added into combustion systems can (1) enhance ash melting temperatures by altering and/or diluting the ash composition with refractory elements (i.e., Si, Al, and Ca), (2) bind and convert low temperature melting species into less troublesome compounds with higher melting temperatures, and (3) reduce concentrations of problematic ash species (i.e., KCl) in the combustion system by means of physical adsorption.3,4,9−11,13−24 In past decades, aluminum silicates based additives have been intensively studied, and kaolin is a wellknown representative of them.13−15,18,20,25 Kaolin, containing mainly kaolinite (Al2Si2O5(OH)4), can react with gaseous potassium compounds to form potassium aluminum silicates with high melting temperatures. Binding of potassium compounds from biomass fuels by kaolin can be described by reactions 1−4 Al 2Si 2O5(OH)4 + 2KCl → 2KAlSiO4 + H 2O + 2HCl (1)

Al 2Si 2O5(OH)4 + 2KCl + 2SiO2 → 2KAlSi 2O6 + H 2O + 2HCl

(2)

Received: March 12, 2012 Revised: July 1, 2012 Published: July 2, 2012 5917

dx.doi.org/10.1021/ef3004366 | Energy Fuels 2012, 26, 5917−5929

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cyclone ash by XRD suggests interactions between the added zeolite and K containing species from the fuel.27 The brief review above indicates that zeolites could be promising to mitigate ash related problems in biomass combustion applications. However, compared to kaolin, effects of zeolites on biomass ash sintering tendency have not been extensively studied. The aims of the present work are as follows: (1) to gain a better understanding of the reactions between zeolites and KCl, (2) to characterize sintering behaviors of agricultural residues ashes at elevated temperatures, and (3) to investigate the effects of zeolites on sintering and melting behaviors of ashes from agricultural residues.

Al 2Si 2O5(OH)4 + K 2SO4 → 2KAlSiO4 + 2H 2O + SO3 (3)

Al 2Si 2O5(OH)4 + K 2SO4 + 2SiO2 → 2KAlSi 2O6 + 2H 2O + SO3

(4)

Two main products from the above reactions are KAlSiO4 (kalsilite) and KAlSi2O6 (leucite), which have melting temperatures of about 1600 and 1500 °C, respectively.9,11 In addition, kaolin could alter the ash chemistry during agricultural residues combustion with formation of other potassium compounds that have higher melting temperatures and reduce molten phases in ash residues.13,14,20,26 Therefore, kaolin is superior to abate and even eliminate ash sintering during combustion of agricultural residues. As a member of the aluminum silicates family, zeolites are gaining interest for alkali capturing during biomass combustion.22−24,27 The term zeolite refers to a group of aluminum silicates from natural or synthetic origins, which are microporous solids with the overall formula Mx/m[(AlO2)x(SiO2)y]·zH2O.28 One of the most important applications of zeolites is to act as water softener to make a detergent functional during washing and laundering.29 Detergent zeolites play a key role in building the structure of dry washing powders during their production process.23 The most used zeolites for detergent production are zeolite A, P, and X.29 All three detergent zeolites are sodium aluminum silicates with the general formula Nax[(AlO2)x(SiO2)y]·zH2O.29 Detergent zeolites are normally characterized with a Si/Al ratio of about 1. Na+ cations are present to neutralize AlO2− in the structure and can easily exchange with other cations (i.e., Ca+).23,27 As a core component of the detergent, a large amount of detergent zeolites are carried by the wastewater entering municipal sewage systems and end up in the sewage sludge after wastewater processing.23,30 Therefore, the sewage sludge is rich in zeolites that remain as aluminum silicates and carry various cations (i.e., Ca+ and Na+).27 Results from previous studies show that sewage sludge addition can considerably reduce deposits on heat transfer surfaces during biomass combustion.21−24,27 Aluminum silicates (mainly zeolites) in the tested sewage sludge were suggested as the main compounds bonding gaseous KCl from biomass fuels and alleviating ash deposition consequently.21,23,27 In another study, sewage sludge showed the ability to increase biomass ashes melting temperatures by 100−200 °C.31 Sewage sludge addition also reduced the slagging tendency, as problematic wood waste pellets were combusted in a grate fired boiler. This occurred because sewage sludge contributed to formation of high temperature melting potassium aluminum silicates in the ash residues.32 The pure zeolite Doucil 24A was tested to abate ash deposition in a 12 MWth circulating fluidized bed boiler where a mixture of 80 wt % wood and 20 wt % straw pellets was combusted.23 Due to zeolite Doucil 24A addition, 80% of the alkali released from the reference fuel in the flue gas was captured with formation of insoluble alkali aluminum silicates.23 This process was accompanied by significant reduction of ash deposition in the convection section of the boiler. In the same boiler, another zeolite ((Na2CaO)·Al2O3·2SiO2·4H2O) was tested as an alkali getter, when mixtures of wood pellets and straw pellets were burned.27 With the zeolite addition, a large fraction of K was transported out of the furnace and ended up in the cyclone ashes. This was accompanied by considerable reductions of alkali chlorides concentrations in both flue gas and deposits.27 Identification of crystalline K−Al-silicates in

2. EXPERIMENTAL SECTION Two agricultural residues, wheat straw and barley husk, were used in this study. The received fuels were milled to particle sizes under 500 μm and were then ashed at 550 °C according to the standard ASTM D 1102. The chemical compositions of the produced ashes were analyzed by an X-ray fluorescence (XRF) spectrometer. For the two studied fuels, the ash contents and ash chemical compositions are presented in oxide form in Table 1. The three aluminum silicates based additives

Table 1. Compositions of Studied Wheat Straw and Barley Husk Ashes Produced at 550 °C barley husk ash content (wt %, d.b.a) chemical compositions SiO2 K2O CaO P2O5 Al2O3 MgO Na2O Fe2O3 SO3 Cl crystalline compounds

4.9 (as oxides wt %. d.b.) 35.77 30.77 7.61 16.21 1.55 1.05 0.01 1.01 1.76 4.26 KCl, K2SO4, SiO2, K2CaP2O7, CaCO3 K2CaP4O12, KCaPO4 Ca10K(PO4)7

a

wheat straw 5.3

39.08 29.45 9.27 7.32 0.89 0.02 0.01 0.42 5.18 8.36 KCl, SiO2, K2SO4 K2CaP2O7, KCaPO4 K2Ca(CO3)2, CaSiO3

d.b.: dry basis.

used in this study were kaolin, zeolite 24A, and zeolite Y. Kaolin was selected as a reference additive due to its well-defined antisintering effects. The kaolin powder particles are smaller than 10 μm. Zeolite 24A is a widely used detergent zeolite with the formula (Na2O·Al2O3·2SiO2)·2H2O. Zeolite Y contains mainly an aluminum silicate with a high silica to alumina (SiO2:Al2O3) molar ratio of 12. Zeolite Y is commonly used as a cracking catalyst in the petroleum refinery industry.29 Zeolite 24A and zeolite Y used in the present work have particle sizes less than 2 μm. It is well documented that KCl is directly responsible for different biomass ash related problems. One main objective of using additives is to reduce KCl amounts in biomass combustion systems.33 Therefore, abilities of the tested additives to react with KCl were investigated in the present work. Kaolin was mixed thoroughly with pure solid KCl in a molar ratio 1:2. Zeolite 24A and zeolite Y were both blended with KCl in a molar ratio 1:1, respectively. The mixtures of additive and KCl were heated for 24 h at the three final temperatures of 800 °C, 5918


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Figure 1. Melting temperatures and sintering behaviors of wheat straw ash with and without additive addition.

Figure 2. Melting temperatures and sintering behaviors of barley husk ash with and without additive addition. 900 °C, and 1000 °C in lid covered crucibles. After cooling to room temperature in ambient air, the sintering degree of the residues was evaluated by visual observation. The residues were then collected and stored for further analysis. Laboratory sintering and ash fusion tests were performed to investigate antisintering effects of the three additives on the two reference ashes. To produce ash-additive mixtures, each fuel was premixed with the additive and then heated at 550 °C for 6 h. A stoichiometric amount of kaolin was added to theoretically capture all the potassium contained in each fuel, corresponding to approximately 4% (w/w) on dry fuel basis. For comparison purpose, an equal amount of zeolite 24A and zeolite Y was added to each fuel. In addition, reference ashes from the two fuels were produced with the same heating procedure described above. The reference fuel ashes and produced ash-additive mixtures were used to determine ash fusion temperatures. Each ash sample was first shaped into a 3 × 3 mm cubic specimen. Then, this cubic ash specimen was sent into the ash fusion analyzer and heated up from room temperature to 1500 °C with a heating rate of 6 °C/min in an oxidizing atmosphere. According to recorded outer shape changes of the specimen, ash characteristic fusion temperatures were identified and determined according to the standard ISO 1995:540, including initial deformation temperature (IDT), soften temperature (ST), hemisphere temperature (HT), and flow temperature (FT). Five tests were carried out for each ash sample, and average values are shown in Figures 1 and 2. To investigate the effects of additive on biomass ashes sintering behaviors, reference fuel ashes and ash-additive mixtures were exposed for 1 h at temperatures of 700, 800, 900, and 1000 °C, respectively. After heated at the desired temperature, the residues left in the crucible were taken out of the furnace and cooled to room temperature in

ambient air. The sintering behaviors of the residues were evaluated visually and graded by a scale from 1 to 5: (1) loose ash resembling the original appearance; (2) partial sintering with fragile structures; (3) hard sintering with partial melting; (4) very hard sintered ash with slag formation; and (5) completely melted. This sintering degree grading scale was adopted from other studies, which have proved to be reliable for getting valuable information with rather quick tests.9,17 After visual observation, the residues from each sample heated at different temperatures were carefully collected and stored. The reference fuel ashes and ash-additive mixtures heated at elevated temperatures were analyzed by an X-ray diffractometry spectrometer (Bruker D8 Focus) equipped with a Cu Kalpha radiation and LynxEye detector. Main crystalline phases in each sample were identified by using the TOPAS evaluation program plus an ICDDPDF2 database. It should be noted that the cooling history may have effects on the amount of amorphous materials contained in one sample. However, the XRD analysis results are still comparable, since the same cooling procedure was performed for all ash residues after heating treatment. Residues from reference ashes and ash-additive mixtures after 1000 °C heating treatment were also examined by SEMEDX. The collected residues from one test with brittle structures were spread on a carbon tape. The sintered or melted residues were embedded into epoxy and polished to obtain flat cross sections. After coated with carbon, the unsintered samples and obtained cross sections were carefully examined by SEM-EDX. The SEM was operated in a backscattered electron mode to give a better view of the elements distribution of a scanned area. The SEM is combined with energy dispersive X-ray spectroscopy. EDX semiquantitative spot analyses and element mapping were carried out for selected samples to get more microchemistry and microstructure information. 5919


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Figure 3. SEM images of wheat straw ash (a-c) and barley husk ash (d-f) sintered at 550, 800, and 1000 °C.

3. RESULTS AND DISCUSSION

reported as one of the dominating mechanisms leading to ash sintering and agglomeration during biomass combustion.9,35,36 Different from the wheat straw ash, the barley husk ash (550 °C) is rich in potassium phosphates, and K2CaP2O7 was detected as a major crystalline compound. This observation is related to the high concentration of P in the barley husk ash. Similar crystalline phosphates were observed in other P rich fuels, i.e. barley wastes, rapeseed, and press residues.3,6,9,37−39 Previous studies have shown that phosphorus has a strong affinity to react with K, with formation of different potassium phosphates as a consequence.6,38,39 Due to the presence of alkali earth metals in biomass fuels, K−Ca and K−Mgphosphates will form as well.6,40 Some potassium phosphates with high K/Ca ratios have low melting points and may result in severe ash sintering and agglomeration in combustors.6,36 Quartz was the other main crystalline phase detected in barley husk ash together with small amounts of KCl and K2SO4. The ash sintering and melting behaviors of wheat straw and barley husk ashes are shown in Figure 1. The wheat straw ash started to melt at 700 °C and was totally fused at 910 °C within a short time interval. Swelling and shrinking of the wheat straw ash cubic specimens were clearly visible during the ash fusion

3.1. Sintering of the Two Agricultural Residue Ashes. The elemental compositions and crystalline compounds of the two reference fuel ashes are presented in Table 1. It can be seen that both wheat straw and barley husk ashes contain high amounts of K, Si, and Cl. The barley husk ash has a high P content that is two times of that in the wheat straw ash. Abundance of K, Si, P, and Cl in the two ashes strongly indicates formation of different K containing chlorides, silicates, and phosphates during combustion processes. The crystalline phases in wheat straw and barley husk ashes (550 °C) were identified by XRD. Sylvite (KCl) is a major compound observed in wheat straw ash. Small amounts of quartz (SiO2), arkanite (K2SO4), fairchildite (K2Ca(CO3)2), and a phosphate K2CaP4O12 were identified from the reference wheat straw ash (550 °C). The KCl and K2SO4 have low melting temperatures about 770 and 850 °C, respectively.4,34 Binary systems containing these two potassium salts may melt at an even lower temperature, around 550 °C.34 Hence, potassium salts and mixtures of them may present as molten phases and cause adhesion of ash particles. Melting of potassium salts has been 5920


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Figure 4. SEM-EDX analyses of wheat straw (a) and barley husk (b) ashes sintered at 1000 °C.

released K is readily to accommodate into ash residues to form molten potassium silicates.8,43 This process is accompanied by gradually sintering of the wheat straw ash at elevated temperatures.43 As shown in Figure 3-c, the wheat straw ash heated at 1000 °C melted completely with observation of voids with round rims. The voids represent the formation of bubbles as the wheat straw ash melted severely in a liquid phase with high surface tensions. One area marked by a white rectangle in Figure 3-c was examined at a higher magnification. As seen in Figure 4-a, no white KCl crystals can be found from the wheat straw ash melted at 1000 °C. EDX spot analyses (spots 1−2 and 4−5, Figure 4-a) revealed that the melted wheat straw ash contains mainly K and Si, which showed a strong correlation in the elemental maps as well (Figure 4-a). It confirmed that the potassium silicates chemistry played a crucial role during melting of the studied wheat straw ash. Figure 3 d-f are representative SEM images taken from barley husk ashes sintered at different temperatures. The barley husk ash (550 °C) has a porous structure, as shown in Figure 3-d. EDX spot and mapping analyses (not shown in the paper) revealed that K and P are two main elements in the barley husk ash (550 °C), which associates with the presence of different potassium phosphates. Different from the wheat straw ash, less amounts of KCl crystals were observed for the barley husk ash (550 °C). Again round shape grains were observed (Figure 3-d, marked with an arrow), containing melted K silicates. Figure 3-e shows that the barley husk ash has already partially melted and sintered at 800 °C, showing a continuous structure and smooth surface. The SEM image in Figure 3-f is a cross-section view of one drop-like ash grain with a hollow void and round rims. A selected area in Figure 3-f was examined with EDX, and results are shown in Figure 4-b. According to EDX spot analyses, the irregular dark gray patches (spots 3 and 4, Figure 4-b) are soil and/or sand particles trapped by melted ash, while spot 5 (lighter gray color zone) represents formation of potassium calcium phosphates. Chemical compositions detected from

tests. The ash sintering evaluation results were consistent with the ash fusion tests results. The wheat straw ash was hard sintered at 800 °C and already completely melted at 900 °C. Broken bubbles were observed from the wheat straw ashes residues after 900 and 1000 °C sintering tests. As shown in Figure 2, onset of the barley husk ash melting took place at 860 °C, and it was fully melted at 1080 °C. Sintering behaviors of the barley husk ash were in good agreement with those observed from ash fusion tests. The barley husk ash was heavily sintered after heating at 900 °C and fused as liquid at 1000 °C. After cooling, parts of the ash residues shrunk and appeared as droplets on the bottom of the crucibles. Figure 3 shows sintering behaviors of the two reference fuel ashes at elevated temperatures. From wheat straw ash produced at 550 °C, white particles of cubic and irregular shapes can be seen (Figure 3-a). The EDX analysis revealed that K and Cl are two dominating elements in these white particles. Therefore, they are KCl crystals and related to sylvite identified by XRD from the 550 °C wheat straw ash. Similar KCl crystals were also observed during thermal conversion of other K and Cl rich biomass fuels.41,42 Grains of round-shape were also found in the wheat straw ash, and one of them is pointed out by an arrow in Figure 3-a. According to EDX spot analysis, Si and K are two dominating elements (K+Si > 90 wt %) in these grains. Since SiO2−K2O eutectics have melting temperatures as low as 550 °C, these grains represent formation and melting of K silicates, which shrunk as droplike particles during the cooling process. After sintered at 800 °C, the wheat straw ash has partially melted into slag with a continuous phase as shown in Figure 3b. Compared to the wheat straw ash sintered at 550 °C (Figure 3-a), fewer white KCl crystals were observed from the wheat straw ash sintered at 800 °C. It is related to the release and breakdown of KCl from the wheat straw ash during the heating process. Previous researches have stated that a large amount of K evaporates as KCl in the temperature range 700−800 °C.8 A part of the KCl may break down at high temperatures, and 5921


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heated at 800 °C for 24 h, KAlSiO4 was observed from the mixture of kaolin and KCl, which correspond to diffraction peaks with 2 theta values of 28.7 and 34.6 degree, respectively. Formation of KAlSiO4 proved the ability of kaolin to react and capture KCl according to reaction 1. Similar XRD analyses results have been reported by B.-M. Steenari and O. Lindqvist, in which kaolin was mixed with KCl and heated in a temperature interval 700−900 °C for 12 h.11 In the present work, mixtures of additive and KCl were also heated at 1000 °C for characterizing possible high temperature reactions. For the kaolin-KCl mixture, a diffraction peak representing KAlSiO4 was identified from the XRD pattern at 2θ = 28.7° but with a lower intensity. It indicates that the temperature has an impact on the capacity of kaolin to react with KCl. Zheng et al. found that the amounts of gaseous potassium captured by kaolin decreased as the experimental temperature increased from 900 to 1300 °C.44 This observation was suggested mainly due to the reduction of active surface areas of the kaolin particles as it restrained chemical reaction intensity and rates between kaolin and potassium.44 In addition, the porosity of kaolin particles will decrease at higher temperatures. It limits pore diffusion of gaseous KCl into the kaolin particles and KCl capturing capacity consequently.15 When heated at elevated temperatures, the kaolin may lose water due to dehydroxylation above 450 °C with formation of metakaolinite (Al2O3·2SiO2).8 Metakaolinite is the main reactive compound in the kaolin reacting with KCl to form potassium aluminum silicates. However, as the temperature increases above 950 °C, the metakaolinite may dissociate into amorphous silica and alumina-silica spinel.44,45 The latter will transfer to pseudomullite at 1000 °C. Compared with metakaolinite, both the alumina silica spinel and pseudomullite have low potentials to react with KCl. Altogether, kaolin particles may become less active as the temperature becomes high enough. Less amounts of potassium will be physically adsorbed due to the reduction of surface area and porosity of the kaolin particles. Moreover, chemical incorporation of potassium into kaolin particles is more difficult because of the transformation of metakaolinite to new phases. Hence formation of potassium aluminum silicates may be depressed as the temperature becomes higher than a certain value. It may explain the decreased intensity of the diffraction peak corresponding to KAlSiO4 in Figure 5-a. The XRD patterns obtained from the sintered mixtures of zeolite 24A and KCl were more complicated as shown in Figure 5-b. Formation

spots 1 and 2 suggest that themelted ash mainly contains different K silicates. High P contents were detected from sample spots 1 and 2, which indicate that P was involved in the barley husk ash melting process. 3.2. Reactions between Additives and KCl. The chemical and mineral compositions of additives used in this study are shown in Table 2. Kaolinite (Al2Si2O5(OH)4) is a Table 2. Contents of Main Elements and Mineral Phases in the Three Additives kaolin

zeolite 24A

chemical composition (wt %) 46.12 34.26 SiO2 Al2O3 34.08 26.35 Na2O 0.04 11.74 K2O 0.2 0.03 Fe2O3 0.87 0.01 SO3 0.06 0.01 CaO 0.07 na MgO 0.16 na TiO2 0.59 0.01 main mineral phases Al2Si2O5(OH)4 (Na2O)(Al2O3) (SiO2)2·2H2O, trace SiO2 amounts SiO2 and Al2Si4O10(OH)2 a

zeolite Y 76.11 10.16 na na 0.0002 0.0012 na na 0.0003 12 (SiO2)·(Al2O3)

n: not detectable.

major component of kaolin. Zeolite 24A contains mainly (Na2O)(Al2O3)(SiO2)2·2H2O and trace amounts of SiO2 and Al2Si4O10(OH)2. Zeolite Y only contains 12(SiO2)·(Al2O3). The sintered residues from mixtures of kaolin/KCl and zeolite 24A/KCl kept rather loose structures at a heating temperature below 900 °C. Even heated at 1000 °C, no significant melting was observed from mixtures of kaolin/KCl and zeolite 24A/ KCl; and only aggregates with different sizes were found in the residues. However, the mixtures of zeolite Y and KCl sintered severely into hard aggregates and blocks at heating temperatures above 800 °C. The XRD patterns obtained from sintered mixtures of additives and KCl are shown in Figure 5. To get reliable results, three XRD analyses were performed for samples collected from residues obtained from each additive-KCl mixture sintered at the desired temperature. The XRD patterns obtained from these three samples are quite similar. After

Figure 5. X-ray diffraction results for mixtures of additives and KCl sintered at elevated temperatures. 5922


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Figure 6. SEM images of mixture of wheat straw ash and kaolin sintered at 1000 °C.

Figure 7. SEM-EDX spot and mapping analyses for mixture of wheat straw ash and kaolin sintered at 1000 °C.

Figure 8. SEM images of mixture of wheat straw ash and zeolite 24A sintered at 1000 °C.

studies are required to clarify the following: 1) influence of the structure rearrangement of kaolin and zeolite 24A on potassium binding at elevated temperatures and 2) how potassium molecules/ions incorporate into the kaolin/zeolite 24A aluminum silicates structures.9,11,44 It is surprising that only weak peaks corresponding to KCl were observed from the mixture of zeolite Y and KCl sintered at 800 °C. The mixtures of zeolite Y and KCl sintered at 900 and 1000 °C were melted and poorly crystallized, without any diffraction peaks observed. Our interpretation is that zeolite Y contains a chemical compound with a high SiO2/Al2O3 ratio, which has properties more close to silica and react with potassium containing species to form low melting temperature eutectics.17 Therefore, the KCl observed from the zeolite Y-KCl mixture sintered at 800 °C was probably due to physical condensation. As the temperature increased, a part of the KCl will be consumed in the reaction with zeolite Y, and the rest may release as vapors during the heating process. 3.3. Effects of Additives on Sintering of the Two Agricultural Residue Ashes. As shown in Figure 1, kaolin addition increased the wheat straw ash initial fusion temperature with about 300 °C. The sintering evaluation results were consistent with the ash fusion test results. The mixture of wheat straw ash and kaolin kept a brittle structure even at 900 °C. The residues from the mixture of wheat straw ash and kaolin at 1000

of KAlSiO4 and KAlSi3O8 was verified and related to reactions between the zeolite 24A and KCl. The intensity of the KAlSiO4 peak (2θ = 28.7°) increased from 800 to 900 °C and decreased again at 1000 °C. It indicates that zeolite 24A partially resembled the reaction between kaolin and KCl with forming KAlSiO4 as a product. The new crystalline phases, (K,Na)Si3AlO8, and KNa3(Al,Si)4O8 were identified. The Na in the mineral phases is originally from the zeolite 24A. It has been reported that the dehydrated zeolite 24A has a channel pore structure and intracrystalline voids.28 The vaporized KCl and/ or K+ from breakdown of KCl may diffuse into these channels and voids, followed by chemical reactions between the potassium ions and aluminum silicates. At a high enough temperature, the zeolite may lose thermal stability with the subsequent collapse of the original structure.29 It may occlude the adsorbed potassium and sodium initially in the structure as well. This process is probably preceded by high temperature reactions with aluminum silicates containing both K and Na as products. Observation of different alkali silicates from the sintered zeolite 24A and KCl mixture in this study are consistent with previous studies. During combustion of high potassium biomass fuels, added zeolite ((Na2CaO)·Al2O3·2SiO2·4H2O) sequestrated KCl(g) in the flue gas through forming products containing K−Al−Si systems.27 However, for both kaolin and zeolite 24A, more 5923


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Figure 9. SEM-EDX spot and mapping analyses for mixture of wheat straw ash and zeolite 24A sintered at 1000 °C.

Figure 10. SEM images of mixture of wheat straw ash and zeolite Y sintered at 1000 °C.

Figure 11. SEM-EDX spot and mapping analyses for mixture of wheat straw ash and zeolite Y sintered at 1000 °C.

Figure 12. SEM images of mixture of barley husk ash and kaolin sintered at 1000 °C.

wheat straw ash and zeolite Y sintered at 1000 °C. Similar enhancing effects on barley husk ash melting temperatures from kaolin and zeolite 24A were observed, as seen in Figure 2. The initial melting temperatures of barley husk ash were increased by 100−150 °C due to kaolin and zeolite 24A addition, respectively. Even when heated at 1000 °C, no severe ash sintering and slag formation were observed from the mixtures of barley husk ash with kaolin and zeolite 24A, respectively. Zeolite Y did not give any visible effect on barley husk ash sintering behaviors according to both ash fusion point determination and sintering tests.

°C are slightly sintered with the observation of ash aggregates. Similar to kaolin, zeolite 24A addition enhanced wheat straw ash characteristics fusion temperatures as shown in Figure 1. In addition, the wheat straw ash sintering behaviors at elevated temperatures were alleviated as a result of zeolite 24A addition. However, after 1000 °C heat treatment, the wheat straw ashzeolite 24A mixture was partially melted. Zeolite Y showed a poor ability to reduce sintering of the wheat straw ash. The ash characteristics fusion temperatures were somewhat increased with zeolite Y addition. However, ash melting and slag formation were still observed clearly from the mixture of 5924


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Table 3. XRD Analysis Results for Sintered Biomass Ashes with and without Additives sample temp, 1000 °C

wheat straw ash

wheat straw ash + kaolin

wheat straw ash +zeolite 24A

wheat straw ash +zeolite Y

amorphous materials trace amount of SiO2 (quartz) trace amount of SiO2 (cristobalite)

small amount of SiO2 (quartz) KAlSiO4 KAlSi2O6 KAlSi3O8 Ca3Mg(SiO)4 KAl2(Si3Al)O10(OH)2 amorphous materials

KAlSiO4 KAlSi2O6 (Na,K)AlSiO4 Na4Al2Si2O9 trace amount NaAlSi2O6 trace amount 3Al2O3·2SiO2 trace amount of SiO2 (quartz) amorphous materials

mainly amorphous materials

sample temp, 1000 °C

barley husk ash

barley husk ash + kaolin

barley husk ash + zeolite 24A

barley husk ash + zeolite Y

amorphous materials trace amount of SiO2 (quartz) trace amount of SiO2 (cristobalite)

SiO2 (quartz) SiO2 (cristobalite) KAlSi2O6 CaSiO3 trace amount of KAlSi3O8 amorphous materials

KAlSiO4 KAlSi2O6 KNa3(Al,Si)4O8 Na4Al2Si2O9 trace amount of NaSi2AlO6 trace amount of SiO2(tridymite) amorphous materials

trace amount of SiO2 (tridymite) mainly amorphous materials

completely at 1000 °C and contained mainly amorphous materials. The barley husk ash melted completely at 1000 °C. Only trace amounts of silica in quartz and cristobalite forms were observed. Addition of kaolin also hindered sintering of barley husk ash even at 1000 °C. With kaolin addition, KAlSi2O6 and KAlSi3O8 are two crystalline products detected from the ashkaolin mixture. The amount of amorphous material in the sample was lower than that of the 1000 °C barley husk ash. It was related to a reduction of ash melts formation and sintering degrees of barley husk ash with kaolin addition. The barley husk ash sintering was prevented with addition of the zeolite 24A. The high melting temperature mineral phases KAlSiO4 and KAlSi2O6 were observed from the mixture of barley husk ash and zeolite 24A sintered at 1000 °C. Sodium aluminum silicates Na4Al2SiO7 and NaAlSi2O6 are probably formed due to transformation of the precursor zeolite 24A. The main part of the residues from sintered barley husk with zeolite Y addition was amorphous with identification of a trace amount of silica in tridymite form. X-ray diffractometry (XRD) has been widely used to study the general biomass ash chemistry and to find possible reactions between the additives and biomass ashes.3,9,11,17,22,23 When applying XRD for ash chemistry characterization, two limitations of this analytical method need to be noted. First, the materials existing as amorphous can only be indirectly observed by XRD, which present as broad humps on the baseline of the diffraction pattern.9 Second, compounds in small crystal sizes may give unclear and/or weak diffraction patterns/ peaks and might be overlooked for identification.9,45 Therefore, supplementary SEM-EDX analyses were carried out on the samples to obtain more detailed microchemistry information, focusing on possible interactions between the ash and additive. For each sample, a representative area was selected for detailed spot and element mapping analyses. The semiquantitative chemical compositions of the investigated spots are summarized in Tables 4 and 5. The elemental maps were obtained to show the distribution and correlations between the main elements in the scanned area.

3.4. Study of Possible Antisintering Mechanisms. The possible mechanisms of the antisintering effects of additives were investigated by performing X-ray diffraction and SEMEDX analyses on residues from reference ashes and corresponding mixtures with additives heated at 1000 °C. The XRD analyses results are presented in Table 3. The reference wheat straw ashes after heating at 1000 °C contained mainly amorphous phases that were present as a broad hump in the obtained XRD pattern. Only trace amounts of silica in quartz and cristobalite forms were observed, and the latter partially related to transformation of low temperature DD silica to high temperature phases.9 The wheat straw ash reacted with kaolin to form KAlSiO4 and KAlSi2O6, which are also the main products of reactions between kaolin and KCl. Additionally, the other potassium aluminum silicates KAlSi3O8 and KAl2Si3AlO10(OH)2 were identified, which have high melting temperatures and may contribute to lower sintering degrees of wheat straw ash-kaolin mixtures. High temperature K containing crystalline phases KAlSiO4 and KAlSi2O6 were identified from the wheat straw ash-zeolite 24A mixture. The KAlSiO4 has been observed in fly ashes from a fuel mixture of 80% wood pellets and 20% of straw combusted with zeolite (Doucil 24A) addition in a CFB boiler.23 Together with chemical fractionation analyses performed on the fly ashes, the XRD analyses results indicate that the zeolite (Doucil 24A) captured potassium compounds in the flue gas to form potassium aluminum silicates.23 However, studies of possible reactions between zeolites and potassium containing species are scarce. It has been reported that aluminum silicates with various Si/Al ratios can react with alkali chlorides in the temperature range 800−1000 °C, with formation of different alkali aluminum silicates.46−50 Therefore, one possible route for zeolite 24A to react with potassium containing species is as follows: (1) the zeolite 24A is decomposed and transformed into different silicates and aluminum silicates at elevated temperatures and (2) some of the aluminum silicates from step (1) react with K containing species and lead to formation of potassium aluminum silicates. This assumption can also explain identification of KAlSiO4 from sintered mixtures of zeolite 24A and KCl. The mixture of wheat straw ash and zeolite Y melted 5925


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Figure 13. SEM-EDX spot and mapping analyses for mixture of barley husk ash and kaolin sintered at 1000 °C.

Figure 14. SEM images of mixture of barley husk ash and zeolite 24A sintered at 1000 °C.

Figure 15. SEM-EDX spot and mapping analyses for mixture of barley husk ash and zeolite 24A sintered at 1000 °C.

Figure 6 shows a typical image of residues from the sintered mixture of wheat straw ash and kaolin. Generally, the residues are ash aggregates in different sizes. The zone in the center of Figure 6-c illustrates the formation of melted ash having a smooth surface and a continuous phase. However, the Al content in this melted ash fraction (spots 1 and 2 in Figure 7, Table 4) was considerably higher than that of wheat straw ash melts (spots 1−2, Figure 4a). Therefore, kaolin addition introduced more Al in the wheat straw ash. It may alter ash chemistry and promote the formation of potassium silicates, instead of low melting temperature potassium silicates. In addition, the melted ash was covered and embedded by kaolin particles with irregular shapes (spots 4 and 5 in Figure 7, Table 4) and/or flake layer structure (spot 6 in Figure 7, Table 4). According to detected chemical compositions and clear correlations between K, Si, and Al (Figure 7), spots 4, 5, and 6 represent products (potassium aluminum silicates) from reactions between kaolin particles and potassium from the wheat straw ash. This finding is in agreement with the identification of potassium aluminum silicates from the sintered wheat straw ash-kaolin mixture by XRD. After heated at 1000 °C, the wheat straw ash-zeolite 24A mixture was partially melted with formation of highly sintered aggregates, shown in

Figure 8. Spots 1 and 2 (Table 4) in Figure 9 represent melted ash with Si and K as two dominating elements. In addition, high contents of Al and Na were also detected in spots 1 and 2 (Figure 9, Table 4). It suggests that portions of these two elements from zeolite 24A have been dissolved in the melted ash. High contents of K, Si, and Al were detected from spots 3− 5 (Figure 9, Table 4). Together with clear correlations for the three elements shown in the elemental maps, spots 3−5 represent interactions between these elements with formation of potassium aluminum silicates. With zeolite Y addition, the wheat straw ash melted completely at 1000 °C with depletion of individual ash particles. It should be noted that the scale bar in Figure 10-a is 1 mm. Therefore, Figure 10 shows that the mixture of wheat straw ash and zeolite Y fused as liquid and presents as big slag after cooling. The EDX analyses results (spots 1−4 in Figure 11, Table 4) revealed that the melted mixture consisted mainly of potassium silicates. Figure 12 shows an aggregate taken from the barley husk ashkaolin mixture sintered at 1000 °C. K, Si, and Al were three main elements detected from spots 1 and 2 (Figure 13, Table 5), and clear correlations can be found between the three elements from the elemental maps. It implies reactions between aluminum silicates from kaolin and potassium from barley husk 5926


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Table 4. EDX Spot Analyses Results Referring to Figures 7, 9, 11 wheat straw ash + kaolin

wheat straw ash + zeolite 24A

wheat straw ash + zeolite Y

element (wt %)

spot 1

spot 2

spot 3

spot 4

spot 5

spot 6

spot 1

spot 2

spot 3

spot 4

spot 5

spot 1

spot 2

spot 3

spot 4

K Ca Si Al P Na Mg Cl s

34.84 1.32 48.02 11.05 1.08 1.43 1.88 0.23 0.15

31.51 1.55 50.35 9.97 1.55 1.98 2.35 0.40 0.34

8.37 41.45 23.31 3.58 18.55 0.96 3.33 0.40 0.05

23.19 1.92 38.39 29.34 2.31 2.14 1.35 0.35 1.01

20.96 1.91 41.48 28.98 2.48 1.62 1.74 0.17 0.67

24.10 1.85 39.57 30.93 1.91 0.97 0.09 0.39 0.18

33.32 2.28 36.98 10.79 2.43 11.09 1.97 0.16 0.97

24.42 1.23 49.29 10.89 1.07 10.31 1.11 0.29 1.39

30.49 4.00 43.60 11.21 1.16 6.47 2.35 0.28 0.45

25.13 1.18 46.09 18.94 1.02 6.33 0.46 0.24 0.60

25.69 3.90 39.08 20.27 0.50 8.47 0.35 0.27 1.48

31.76 5.40 46.85 8.30 1.72 2.04 3.23 0.44 0.27

29.11 2.72 55.89 7.12 1.78 1.37 1.02 0.52 0.48

18.21 32.04 22.90 2.04 21.98 1.67 0.71 0.26 0.20

21.94 36.19 20.62 5.51 13.14 1.48 0.57 0.36 0.19

Table 5. EDX Spot Analyses Results Referring to Figures 13, 15, 17 barley husk ash + kaolin

barley husk ash+ zeolite 24A

barley husk ash+ zeolite Y

element ( wt %)

spot 1

spot 2

spot 3

spot 4

spot 5

spot 6

spot 1

spot 2

spot 3

spot 4

spot 5

spot 6

spot 1

spot 2

spot 3

spot 4

K Ca Si Al P Na Mg Cl S

25.68 0.78 32.84 31.04 6.71 2.24 0.48 0.14 0.09

28.00 0.95 32.73 31.18 5.84 0.85 0.12 0.29 0.05

6.25 0.94 57.83 27.43 4.06 1.03 0.65 0.58 1.25

24.49 0.85 33.35 33.32 6.25 0.72 0.51 0.36 0.14

29.36 17.81 16.86 8.56 23.98 0.94 1.99 0.27 0.23

27.58 20.71 12.98 10.41 24.96 0.55 2.37 0.25 0.19

30.39 7.36 44.29 8.30 1.43 6.15 0.80 1.22 0.05

24.28 9.26 48.07 6.91 1.55 7.48 0.98 1.41 0.07

17.22 6.71 45.75 7.75 4.03 13.98 2.91 1.24 0.40

18.66 3.95 46.86 12.43 2.62 10.75 2.66 1.50 0.57

26.88 2.54 34.77 20.51 2.32 7.27 1.18 3.27 1.27

22.23 2.81 37.37 20.54 2.14 4.71 1.26 6.07 2.86

30.48 3.23 41.82 7.03 9.64 2.80 4.13 0.41 0.46

29.83 5.17 46.33 10.02 6.75 0.68 0.69 0.31 0.22

20.85 26.27 6.23 4.14 35.15 1.50 4.64 0.50 0.70

26.93 6.66 28.19 3.80 23.56 3.43 6.32 0.46 0.65

Figure 16. SEM images of mixture of barley husk ash and zeolite Y sintered at 1000 °C.

Figure 17. SEM-EDX spot and mapping analyses for mixture of barley husk ash and zeolite Y sintered at 1000 °C.

ash. Spot 3 (Figure 13, Table 5) represents a piece of flake that contains mainly silicon and aluminum, which is plausibly from destructed kaolin particles. Melted barley husk ash was also observed as small round particles associated with the detection of high contents of K, Si, P, and Ca in the selected spots (spots 5 and 6 in Figure 13, Table 5). Figure 14 shows that the residues from the sintered mixture of barley husk ash and zeolite 24A contain both agglomerated particles and melted ash. The latter is clearly visible with a smooth surface as shown

in Figure 14-c. EDX revealed that the chemical compositions of the melted fractions were dominated by K and Si (spots 1 and 2 in Figure 15, Table 5), which is related to formation and melting of potassium silicates. Some fine particles aggregated together with rather porous structures and embed/adhere with the melted phase (Figure 14-c). The EDX element maps showed correlations between elements K, Si, and Al (Figure 15), which are in agreement with spot analyses (spots 5 and 6, Table 5). It implies that these aggregated particles contained 5927


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high temperature melting potassium aluminum silicates, which did not melt severely even after 1000 °C heating treatment. Zeolite Y addition did not give any positive effects to restrain sintering of the barley husk ash: The mixture of barley husk ash and zeolite Y melted and showed a continuous and dense phase (Figure 16 a-c). For the main portion of the melted ash, the EDX elemental maps (Figure 17) show strong correlations between elements K, Si, and Al, which were evenly distributed in the scanned area. Compared to the pure barley husk ash, zeolite Y addition enhanced the Al content (spots 1−2 in Figure 17, Table 5). High contents of Ca, P, and K were detected from spots 3 and 4 (Figure 17, Table 5), which showed a clear correlation in elemental maps. Therefore, spots 3 and 4 (Figure 17, Table 5) represent formation of potassium calcium phosphates.

4. CONCLUSIONS Severe slag formation and fusion were observed from wheat straw ash and barley husk ash as they were heated at elevated temperatures. The low melting and sintering temperatures of the two ashes are associated with the formation of potassium salts, silicates, and phosphates with high K/Ca ratio. Kaolin and zeolite 24A can react with KCl to form different potassium aluminum silicates with high melting points. Therefore, kaolin and zeolite 24A are two promising additives to abate KCl induced ash sintering and deposition during biomass combustion. No clear reactions were identified between zeolite Y and KCl in this study, and the mixture of them sintered during the heating treatment. The intensive sintering and melting of wheat straw and barley husk ashes were reduced through addition of kaolin or zeolite 24A. Kaolin and zeolite 24A reacted with the potassium compounds in the two reference ashes. High temperature melting potassium aluminum silicates are the main products from these reactions, which prevented slag formation and improved sintering characteristics of the two ashes. No evident antisintering effects were observed using zeolite Y as additive to the wheat straw and barley husk ashes.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +47 73591602. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support by the Bioenergy Innovation Centre (CenBio), which is funded by the Research Council of Norway, a large number of industry partners and seven R&D institutions.

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REFERENCES

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Sintering Behavior of Agricultural Residues Ashes and Effects of

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