Condensation Behaviors of Potassium during Biomass Combustion

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Condensation Behaviors of Potassium during Biomass Combustion Xi Jin, Jiaming Ye, Lei Deng, and Defu Che Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03381 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 23, 2017

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Condensation Behaviors of Potassium during Biomass Combustion

Xi Jin, Jiaming Ye, Lei Deng*, Defu Che

State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China

* To whom correspondence should be addressed. Tel: +86−29−82668703; Fax: +86−29−82668703. E−mail: [email protected].

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Abstract Condensation of potassium species is the origin of the slag layer formation on superheater surfaces during biomass combustion. In this paper, the condensation behaviors of potassium species were studied by a one-dimensional down-fired furnace and a novel condensation probe. Two potassium salts ((KCl and K2SO4) and two biomass fuels (wheat straw and corn stalk) were used. SEM (scanning electron microscopy), EDX (energy dispersive X-ray) and XRF (X-ray fluorescence) analyses were applied to analyze the microstructure and elemental composition of condensation products at different temperatures. The results showed that the condensation temperatures of KCl, K2SO4 and their mixture were 770±4 °C, 745±6 °C and 764±10 °C, respectively. KCl in the initial slag layer was ascribed to the heterogeneous condensation of KCl vapor and the deposition of KCl fine particles that generated by homogeneous nucleation. The pathways depended on the wall temperature of the condensation probe. K2SO4 in the initial slag layer was mainly attributed to the deposition of K2SO4-containing fine particles. K2SO4 had much less significant influence on the formation of initial slag layer compared with KCl. During combustion of biomass fuels, KCl could adhere on the surfaces of fly ash particles, and K2SO4 deposited together with KCl. The condensation behaviors of K2SO4 on fly ash particles were much less remarkable compared with KCl. The condensation mechanisms of potassium species during biomass combustion were discussed. Keywords: Potassium salts; Condensation behavior; Deposit; Biomass combustion; Grate-fired boiler.


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1. INTRODUCTION As a CO2-neutral and renewable fuel, biomass is a promising alternative to fossil fuels, and has gained much attention in recent years.1-4 In China, annual crops, such as wheat straw, corn stalk and rice husk, are quite typical and abundant.5,

6

The grate firing is the most widely used firing method.7,

8

However, due to the high potassium content of biomass fuels, high-temperature convective heating surfaces in grate-fired boiler have been suffering from serious technical problems with slagging, fouling and high-temperature corrosion during biomass combustion.9-19 These problems have become the critical restrictions in the design and operation of biomass-fired boilers.19-22 In biomass-fired boilers, main elements of the deposits collected from the superheater are K, Cl, Si, S, Ca and Mg.19, 22, 23 Several researchers11, 12, 18, 23-25 have analyzed the morphology and composition of deposit by SEM (scanning electron microscopy), EDX (energy dispersive X-ray), XRD (X-ray diffraction) and XRF (X-ray fluorescence) analyses. They found that the deposits were divided into several layers. The bottom layer was uniform and had light color. KCl11, 12, 18, 23, 24 and K2SO423, 25 were the major components of bottom layer. The outer layer had dark color and contained large particles that mainly consist of fly ash particles.11,

16

It is well known that inertial impaction, thermophoresis,

condensation, and chemical reaction are the four types of deposition mechanisms.26, 27 Inertial impaction offers the dominant mass of the deposit, but condensation and thermophoresis also play the important roles in the deposition of K, S and Cl. During combustion, the K salt vapors released from biomass fuel can condense on the surface of superheater, acting as glue to capture fly ash particles.10, 22 The K salt vapors can also condense on the surface of fly ash particles, which makes them sticky.11, 22 Hence, the condensation of K salt vapors is the origin of fouling and slagging during biomass combustion. In last decade, behaviors of potassium transformation and ash deposition during biomass combustion have drawn much attention.24,

28-31

Dayton et al.28 had measured the temporal release of K from

switchgrass during combustion by using molecular beam/mass spectrometry (MBMS). It was found that KCl was the predominant K-containing species volatilized, which was consistent with the thermodynamic equilibrium calculations by Wei et al.29 Knudsen et al.30 reported that a large fraction of 3 Environment ACS Paragon Plus

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K was released to gas phase as KCl between 700 and 800 °C during combustion of high chlorine straw. Li et al.31 found that a higher content of reactive K in biomass fuel could cause a higher deposition rate. Niu et al.24 suggested that the KCl fine particles deposited on the heating surfaces, became the initial slagging layer, and then captured large coarse particles with high concentrations of Si and Al. Thus, in the entire combustion process of biomass fuel, KCl is thought to be the most stable alkali-containing substance in gas phase, and it may play a significant role in slagging.32-34 Several other researchers32, 35, 36

pointed out that K2SO4 was the dominate K salt during biomass combustion. It nucleated first when

the flue gas temperature decreased, and then KCl condensed on the K2SO4 nucleon at low temperature. The inconsistency between the two viewpoints is mainly caused by the differences in biomass types and combustion conditions. Most researchers16,

22, 23, 31, 37, 38

speculated the condensation process by

analyzing the layered deposition gained from lab-scale experiment or full-scale test, and concluded that KCl and K2SO4 were the dominant components in initial slagging layer. However, quite few literatures have reported the condensation behavior of different potassium salts.11 To our knowledge, only Nielsen et al.11 designed a condensation probe, and vapor condensation phenomena and sulfation of KCl were investigated. The probe consisted of two small vertical plates and a larger horizontal plate. The horizontal plate could shield the vertical plates from inertial impaction of large fly ash particles. The vertical plates were used as sampling plate which kept at a constant temperature (300–400 °C) during an experiment. While the condensation of potassium salts is a complicated process. In the region of high-temperature convective heating surface, as the gas temperature decreases, potassium salts vapor may condense heterogeneously on heating surface and fly ash particle surface, or nucleate to form fine particles and then deposit on heating surface. The products and the pathways of condensation vary with temperature and salt type. To date, quite limited investigations11 were reported for the condensation behaviors of potassium salts at different temperatures during biomass combustion. In this study, a novel condensation probe is designed in a one-dimensional down-fired furnace. Two individual K salts (KCl and K2SO4), their mixture and two biomass fuels (wheat straw and corn stalk) 4 Environment ACS Paragon Plus

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are used to study the condensation behaviors of potassium salts during biomass combustion. The effects of K salt type and temperature are investigated. The condensation behaviors of different potassium salts can be evaluated through analyzing the morphology characteristics and the elemental compositions of condensation products in a wide temperature range, which can be determined by SEM-EDX and XRF. The condensation mechanisms of potassium species during biomass combustion are discussed. The fundamentals on condensation behaviors of potassium salts are beneficial to a better understanding of the dynamic behavior of biomass ash deposition.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. According to the relevant results of previous studies,14, 16, 23, 37 KCl and K2O4 were the main components of the initial slagging layer on the high-temperature convective heating surface of biomass-fired boilers. Hence, KCl and K2SO4 were used as the potassium salt samples in this study. Both salt samples were first ground in a mortar, and then sieved to 150−250 µm. After sieving, part of KCl and K2SO4 were mixed uniformly by the mass ratio of 1: 1. Two kinds of typical biomass (corn stalk and wheat straw) in North China were also selected in this study. Both of the biomass fuels were collected from the countryside area of Xi’an, Shaanxi Province. The fuel properties are listed in Table 1.6 The values and standard deviation of potassium content were obtained and calculated from the measurements of ICP-OES (Optima 7000DV, Perkin Elmer, USA). The other composition values and standard deviations were obtained from the Comprehensive Laboratory of Coalfield Geological Bureau, (Xi’an, Shaanxi Province). The biomass samples were ground in an herb shredder, air-dried at room temperature, and 150−250 µm biomass samples were selected through sieving. Before the experiments, all the samples were dried at 105 °C for 24 h and then kept in an airproof desiccator.


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Table 1. Fuel Properties of Biomass Samples (wt %, dry basis) Samples

Wheat straw

Proximate analysis Ash 6.80 Volatile matter 75.29 Fixed carbon 17.91 Ultimate analysis C 44.11 H 4.97 O 43.67 N 0.20 S 0.25 Cl 0.183 Main ash-forming elements Si 1.22 Ca 0.39 Mg 0.11 Al 0.05 P 0.04 K 1.046±0.016a a

Corn stalk

Standard deviation

7.09 74.89 18.02

0.07 0.28 0.31

46.18 4.89 40.50 1.09 0.25 0.055

0.18 0.05 0.23 0.03 0.02 0.004

1.19 0.35 0.19 0.11 0.03 1.221±0.016

0.042 0.014 0.014 0.014 0.002 -

standard deviation.

2.2. Experimental Setup. The condensing experiments were carried out in a one-dimensional downfired furnace with an axial length of 3400 mm and a maximum temperature of 1300 oC. Figure 1(a) illustrates the experimental system. The reactor was an alundum tube with an inner diameter of 70 mm. The gas flow in this experimental system was provided by an air compressor. The flow rate was controlled by a gas mass flow meter with an accuracy of 2%. The potassium salts and biomass fuels were fed into the reactor by a micro screw feeder with an accuracy of 3.5%. The condensation probe was a stainless-steel tube with an outer diameter of 20 mm. The material was ASTM SA213/TP310S which is tolerant of high temperature and corrosion to some extent. Six thermocouples were installed on the probe surface for real-time temperature monitoring. The uncertainty of temperature measurement was within ±3 °C. The condensation probe was inserted vertically at the bottom of reactor. This arrangement can avoid inertial impaction of potassium salt particles and ash particles on the probe surface. As shown in Figure 1(b), the reactor was divided into two areas. The upper area was called heating area (length of 3000 mm), and it was held at 1000 °C uniformly and stably 6 Environment ACS Paragon Plus

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during experimental runs. The vaporization of potassium salts and the combustion of biomass fuels were taken place in this area. The lower area was called condensing area (length of 400 mm). The temperature of this area decreased linearly from top to bottom (1000−580 °C). The condensation probe was installed in the condensing area.

(a) Condensing experimental system

(b) Condensation probe

Figure 1. Schematic diagram of the condensing experimental system.

In this study, the experimental setup was built up to simulate the conditions in a grate-fired boiler, especially for the conditions of combustion temperature in the fuel bed and flue gas temperature in the area of high-temperature convective heating surface. It is reported11,

19, 25, 30

that the combustion

temperature in the fuel bed is about 800−900 oC, and the flue gas temperature in the area of hightemperature convective heating surface is usually below 1000 oC. This is the main reason why the temperature of the heating area was kept at 1000 oC. During the condensation area, both the gas temperature and the wall temperature of probe decreased linearly. This would result in the condensation 7 Environment ACS Paragon Plus

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of potassium salts in the gas and on the surface of probe. Along the axis of probe, the condensation products at different wall temperatures could be obtained during the experiment. By analyzing the morphology characteristics and the elemental compositions of condensation products, the condensation mechanisms of potassium salts during biomass combustion could be obtained. A typical experimental run was composed of the following steps. First, the condensation probe was inserted at the bottom of reactor. Then, the reactor was heated to the predetermined temperature (1000 o

C). After that, the air compressor was turned on, and the flow rate was maintained at 4 L min-1. The

dried sample was put into the hopper of micro screw feeder. When the temperature of condensation probe was stable, the micro screw feeder was turned on and maintained at constant feeding rate which varied with sample type. The feeding rates of KCl, K2SO4, mixture of KCl and K2SO4 were 1.0 g min-1. To keep the excess air coefficient at 1.20, the feeding rates of wheat straw and cotton stalk were maintained at 0.96 and 0.92 g min-1, respectively. The sample was carried by carrier gas flow and went through the reactor. A fraction of potassium salts or alkali elements in biomass fuels evaporated to gas phase. When the carrier gas reached the condensing area, its temperature decreased linearly. The alkali salt gas in carrier gas would condense on the surface of condensation probe directly, or nucleate and then deposit on the probe surface. The residence time of samples in the reactor under different experimental conditions was approximately 40 s, which could ensure the burnout of biomass fuels and the vaporization of potassium salts. The flue gas flow was laminar. And the boundary layer covered the probe was also laminar. Hence, the influence of eddy impaction could be neglected. As the axis of condensation probe was parallel to the flow direction, the deposit on the probe could only be formed by condensation of potassium vapor and thermophoresis of fine particles. The pure KCl salt and the biomass samples had the shortest (around 10 min) and longest (around 30 min) formation time of condensation layer, respectively. When the condensation experiment of biomass samples was performed for a longer time, no obvious growth of condensation layer was observed, due to the low feeding rate and ash content of biomass samples. Thus, 30 min is chosen as the operation time for all the condensation experiments of different samples. 8 Environment ACS Paragon Plus

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2.3. Analysis Techniques. In condensing area, temperature of the flue gas and the probe wall decreased linearly with the flue gas flow. At the same horizontal cross-section, the wall temperature of the probe was about 30 oC lower than that of the flue gas. The wall temperature of the probe dropped from 955 to 550 oC continuously. The condensation products in different temperature range of the probe surface were collected and then analyzed through scanning electron microscopy-energy dispersive X-ray (SEM-EDX, SSX550, Shimadzu, Japan), respectively. To evaluate the repeatability, each experimental run was repeated three times to gain an experimental uncertainty of the condensation temperature. Along the circle of probe, the condensation products were uniform. While along the axis of probe, the condensation products would show different patterns at different positions, which correspond to different wall temperatures. The condensation products that had the same pattern were collected from the surface of probe and then analyzed by SEM. The typical spots in the scanned window were chosen for EDX analysis to obtain the elemental compositions. The whole scanned window was also analyzed by EDX to obtain the average values of elemental compositions. When the biomass samples were burnt during the experiment, the amount of the condensation products was limited, which was not enough for XRD analysis (X’pert Pro, Panalytical, Netherlands). The condensation products had much more complicated elemental compositions than that produced from combustion of potassium salts. Thus, these condensation products were further analyzed by XRF (S4 Pioneer, Bruker, Germany).

3. RESULTS AND DISCUSSION 3.1. Condensation Phenomena of Potassium Salts. Temperature of the condensation probe was measured by six thermocouples located on probe surface. The temperature distribution and condensation phenomena on the probe surface are shown in Figures 2 and 3. There was a clear boundary that the alkali metal vapor began to condense on the probe surface. On one side of this boundary, there were obvious condensation products. No visible condensation products were observed on the other side. The condensation temperatures of potassium salt vapor could be obtained by temperature distribution of the probe. 9 Environment ACS Paragon Plus

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Figure 2(a) shows the temperature distribution of probe and condensation phenomena of KCl. KCl vapor begins to condense at the location of 90±2 mm. The corresponding condensation temperature is 770±4 oC, which is close to the melting point of KCl (776 oC). In the area of 90−110 mm (770−727 oC) on the probe surface, white crystal condensation products with molten structure can be observed clearly. Beyond 110 mm, a uniform thin layer which consists of white powder adheres to the probe surface. As the wall temperature reduces, the condensation products become thinner. The temperature distribution and condensation phenomena of K2SO4 are shown in Figure 2(b). Uniform deposit consisted of white powder can be observed after the location of 110±3 mm on the probe surface. Nevertheless, the amount of deposition is far less than that of KCl. The condensation temperature of K2SO4 is considered to be 745±6 oC, and it is much lower than the melting point of K2SO4 (1067 oC). In this study, the heating area of the reactor was maintained at 1000 oC. This temperature is also lower than the melting point of K2SO4. According to Knudsen et al., 30 the evaporation rates of pure K2SO4 salt in atmospheric air were insignificant at 1000 oC. Thus in this study, the concentration of K2SO4 vapor in flue gas seems to be low. K2SO4 may mainly occur as fine particles. The condensed K2SO4 is formed by the deposition of K2SO4 fine particles, and no molten structure is observed. From the location 0 mm (950 oC) to 110 mm (745 oC), no condensation product is observed on the probe surface. Figure 2(c) shows the temperature distribution of probe and condensation phenomena of mixture of KCl and K2SO4. The condensation products were observed beyond the location of 95±5 mm (764±10 o

C). This condensation temperature is lower than that of KCl, and higher than that of K2SO4. The

condensation product is thinner than that of KCl, and crystal condensation products with melting structure are seldom observed. In the high-temperature convective heating surface area of boiler, the temperature of flue gas is 700−900 oC, and the wall temperature is 450−600 oC. The temperature gradient between the flue gas and tube wall is much larger than that in this study. Therefore, during the combustion of biomass fuels with high alkali metal content, the condensation phenomena of alkali metal species will be much more significant than that in this experiment.


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(a) KCl

(b) K2SO4

(c) Mixture of KCl and K2SO4 (mass ratio = 1:1) Figure 2. Temperature distribution and condensation phenomena on the probe surface (potassium salts).

The temperature distribution and condensation phenomena of wheat straw and corn stalk are shown in Figure 3. The deposit on probe surface was much less than that of potassium salts, and no visible condensation layer was observed. The reason is that the duration time is short and the feeding amount is


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quite small. As shown in Figures 3(a) and 3(b), several ash particles could be observed on the probe. A large amount of deposit was observed between 120 and 160 mm and 120 and 140 mm, respectively.

(a) Wheat straw

(b) Corn stalk Figure 3. Temperature distribution and condensation phenomena on the probe surface (biomass fuels).

3.2. SEM and EDX Analyses. 3.2.1 Individual Potassium Salt. The micro-morphology of condensed KCl in the wall temperature range of 727−770 oC is shown in Figure 4(a). Two types of structures, an innermost layer with partial melting structure, and several large-size particles with cubic and polyhedric shapes, could be observed. At this temperature, the KCl vapor is abundant in flue gas because of the higher vapor pressure.30 When the flue gas goes through this area, KCl vapor might condense heterogeneously on the probe surface. The temperature of this area is close to the melting point of KCl. Therefore, the condensation products could be partial melted. The above process can describe the formation of innermost layer. The cubic and 12 Environment ACS Paragon Plus

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polyhedric shaped particles might formed by heterogeneous condensation of KCl vapor in flue gas. Lapuerta et al.16 also reported that cubic and polyhedric shapes in micrograph of biomass deposition mainly consisted of K and Cl with a molar K/Cl ratio close to 1. Meanwhile, the KCl fine particles formed by homogeneous nucleation may also deposit on the probe surface by thermophoresis or diffusion transport. The innermost condensed layer is partial molten, and it can capture particles formed by nucleation. The new condensation layer would melt again. Hence, more and more condensation products would form. In Figure 4(b), the condensed sample in the wall temperature range of 625−658 oC is porous, and granular structure is more variable. The temperatures of flue gas and probe surface are much lower than the melting point of KCl. As the temperature decreases, the concentration of KCl vapor in flue gas declines. A lot of KCl fine particles are generated by nucleation and then deposit on the probe through thermophoresis and diffusion.27 Due to the low temperature, the molten structure is not found, and the condensation layer is hard to grow. Thus the product is in the form of smooth and uniform layer with white color. The micro-morphology of condensed K2SO4 is shown in Figures 4(c) and 4(d). The condensed products showed the flocculent structure, and no molten structure was observed. The flue gas temperature in condensing area decreases from 953 to 622 oC linearly along with the flow, which is lower than the melting point of K2SO4 (1067 oC). Due to the low temperature, the volatilization of K2SO4 in the reactor is insignificant, and much K2SO4 occurs as fine particles. These particles deposit on the probe surface by thermophoresis and diffusion transport. The morphology of condensed K2SO4 is similar to the results in the study of Nielsen et al.11 Compared with the condensation sample in the wall temperature range of 695−745 oC, the sample in the temperature range of 622−695 oC has larger size of particle and pore, due to a more sufficient process of aggregation and deposition.


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(a) KCl: 727−770 oC

(c) K2SO4: 695−745 oC

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(b) KCl: 625−658 oC

(d) K2SO4: 622−695 oC

Figure 4. SEM micrograph of the condensation samples

3.2.2 Mixture of KCl and K2SO4. The SEM-EDX analysis of condensation sample of potassium salts mixture is shown in Figure 5. The samples in the wall temperature range of 719−764 and 605−663 oC are shown in Figures 5(a) and 5(b), respectively. Two distinguishable parts, 1–2 µm granular particles (spot 1) and submicron particles (spot 2), could be observed in Figure 5(a). Iron, nickel and chromium were also detected in both spots 1 and 2. One possible reason could be that the corrosion products of the stainless steel are collected together with the condensation products. This could also be verified by the average values of elemental compositions (see the EDX data for the whole scanned window). It can be seen that the edge of granular particles is smooth. Meanwhile, the granular particles mainly consist of K, Cl and O. The molar K/Cl ratio is close to 1, and minor S is observed. From the size, shape and elemental composition of granular particles, it can be inferred that these structures are formed by heterogeneous condensation, rather than homogeneous nucleation and subsequent diffusion transport or 14 Environment ACS Paragon Plus

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thermophoresis.16 The aggregation of submicron particles forms the bottom condensation layer. The molar ratio of K/(Cl+2S) is around 1.08, indicating that the main composition of aggregation should be KCl and K2SO4. According to the previous studies32, 35, 36, during combustion of biomass fuel with high K content, K2SO4 nucleated first when the temperature of flue gas decreased, and then KCl condensed on the K2SO4 nucleon at lower temperature. In this study, the flue gas temperature of condensing area is close to the melting point of KCl. But it is lower than the melting point of K2SO4. K2SO4 mainly occurs as fine particles and then deposits on the probe surface by thermophoresis and/or diffusion transport. Meanwhile, KCl vapor may condense and melt partially on K2SO4 fine particles, which can make the particles stickier. K2SO4 particles with condensed KCl on the surface can aggregate together. According to Figures 4(a) and 4(b), it is known that KCl vapor can also condense heterogeneously on the probe surface. Hence, the condensation products (719−764 oC) of K salts mixture (KCl and K2SO4) could be mainly formed through three pathway: (1) KCl vapor condenses homogeneously and forms KCl fine particles, and then deposit on the probe surface; (2) KCl vapor condenses heterogeneously on K2SO4 fine particles, and then deposit on the probe surface; and (3) KCl vapor condenses heterogeneously on the probe surface. Nielsen et al.11 investigated condensation phenomena of potassium salts during wheat straw combustion and gained the similar results. The condensed potassium salts consisted of sponge-shaped matrix of submicron particles (primarily K2SO4) and individual angular particle (primarily KCl). Figure 5(b) shows the micro-morphology of condensation sample in the wall temperature range of 605−663 oC. The structure is quite different from that in Figure 5(a). Tabular structures (spot 3) and fibrous structures (spot 4) are investigated. K and Cl show high concentration in tabular structures, and no S is detected. Fibrous structures are rich in K, Cl and S. Because the temperature is much lower, heterogeneous condensation of KCl becomes unremarkable, and much KCl might occur as fine particles. The tabular structures might form from higher temperature area in the upstream. Then the melting structure can be observed. Fibrous structures are mainly formed by deposition of fine particles consisted mainly of KCl and K2SO4 (see spot 2). However, due to the lower temperature, the aggregation of K2SO4 and KCl in spot 4 has different shape. 15 Environment ACS Paragon Plus

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Hence, the condensation products (605−663 oC) of K salts mixture (KCl and K2SO4) could be mainly formed through two pathways: (1) KCl particles deposit on the probe surface; and (2) K2SO4 particles with KCl on the surface deposit on the probe surface. From Figure 5, it can be inferred that KCl may play a more important role in the formation of initial slagging layer than K2SO4.

(a) Temperature range: 719−764 oC EDX spot 1 spot 2 spot 3 spot 4 window (a) window (b)

K 21.34 23.44 31.47 18.46 22.73 29.14

Cl 19.45 9.97 33.92 11.59 18.31 28.05

S 0.55 5.85 5.33 2.35 1.09

(b) Temperature range: 605−663 oC O 42.56 33.28 4.35 29.58 38.51 12.17

Fe 5.95 9.73 14.39 18.25 6.16 14.57

Ni 1.88 2.67 2.95 2.73 2.04 2.81

Cr 2.35 6.54 3.48 4.3 3.37 3.55

Figure 5. SEM micrograph and EDX analysis (value in mol %) of the condensation samples (KCl: K2SO4=1:1).

3.2.3 Biomass fuels. The micro-morphology of condensation samples of wheat straw is shown in Figure 6. The average values of elemental compositions can be obtained by the EDX data for the whole scanned window and the XRF data for the condensation products. In both SEM pictures, fibrous structures (spots 1 and 3), granular structures (spot 4) and amorphous molten structures (spot 2) were observed. In spot 1, K, Cl, O and Ca show high concentrations. The molar ratio of K/Cl is close to 1, and contents of S and Si are minor. It can be inferred that KCl is the main form of potassium salt in fibrous structures. KCl vapor might also condense on the surface of fly ash particle, which can increase the stickiness of fly ash particle. Hence, in biomass-fired boilers, fly ash particles might be easier to be captured by initial slagging layer on the superheater surface, and they could also bond together easier. 16 Environment ACS Paragon Plus

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The amorphous molten structure (spot 2) is quite variable, and several common patterns can be found. In amorphous molten structure, Cl content is minor. However, contents of Si, Na and P are higher than that of other structures. Similar results were found by Bashir et al.38 and Lapuerta et al.16 This pattern indicates that amorphous structures are formed by fly ash particles. These particles may melt during combustion, and eutectic mixture could form. Because the quantity of deposition sample is quite small in this study, the amount is not enough for XRD analysis. The specific components of substance in different structures cannot be confirmed. Figure 6(b) shows the micro-morphology of deposition in the wall temperature range of 583−698 oC. Fibrous structures (spot 3) are also found. The elemental composition of spot 3 is distinguished from that of spot 1. Si and S contents are higher. Fibrous structures might contain more K2SO4. Granular structures (spot 4) cover on the surface of ash particles, and less K and Cl are detected. These structures might be formed by deposition of AAEM (alkali and alkaline earth metal) fine particles except KCl.24, 39 During combustion of wheat straw, K species enter the gas phase. When the flue gas goes through the superheater area, K2SO4 would be mainly in the form of fine particles, and KCl vapor condenses heterogeneously on the surface of fly ash particles. Meanwhile, KCl vapor can also form fine particles by homogeneous nucleation, or condense on the nucleation. Several researchers40-42 reported that KCl might be sulfated into K2SO4 by reacting with SO2, O2 and H2O. However in this study, the concentration of SO2 is extremely low. It seems that sulfation of KCl would not happen in such a short duration time. After the fly ash deposits on the probe surface, KCl and K2SO4 would continue to condense or deposit on them.24 Figure 7(a) shows the micro-morphology of condensation samples of corn stalk in the wall temperature range of 706−768 oC. The morphology is similar with that in Figure 6(a). Granular and fibrous structures partly covered on the ash particles were observed. But the elemental compositions of condensation structures are different. It can be seen that S content is quite low in granular, fibrous and amorphous molten structures. Compared with wheat straw, the concentration of K and Cl in granular and fibrous structures of corn stalk is much higher. According to Table 1, although corn stalk has the same S content with wheat straw, seldom S is observed in the condensation layer. According to our 17 Environment ACS Paragon Plus

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earlier studies,6, 8 more S might occur as organic form in corn stalk. In addition, a higher fraction of S might occur as sulfates in wheat straw.30, 43, 44 The evaporation of K2SO4 is insignificant at 1000 °C, and most KCl can release to gas phase. Hence in this study, fine particle and vapor are thought to be the main form of K2SO4 and KCl in the flue gas, respectively. Moreover, heterogeneous sulfation of condensed KCl is also negligible, due to the low level of SO2 in flue gas. Thus, the condensation layer on ash particles of wheat straw has much higher content of Cl than that of S. And S is rarely observed in the condensation layer on ash particles of corn stalk. The similar trends can also be seen from the average values of elemental compositions (see the EDX data for the whole scanned window and the XRF data for the condensation products). From Figures 6 and 7, no individual K2SO4 is observed on the surface of fly ash particle. It can be inferred that K2SO4 deposits on the surface of fly ash particles together with KCl. It seems that the deposition of KCl on fly ash is much more remarkable compared with that of K2SO4.

(a) Temperature range: 698-759 oC

EDX spot 1 spot 2 spot 3 spot 4 window (a) window (b) XRF 698-759 oC 583-698 oC

(b) Temperature range: 583-698 oC

K

Cl

S

O

Si

Al

Ca

Mg

Na

P

Fe

23.46 7.56 19.75 13.2 11.96 14.98

22.59 1.23 16.47 8.95 8.35 10.22

1.87 0.62 4.89 2.31 0.92 2.49

28.79 52.53 35.23 51.62 40.22 48.67

2.91 19.28 7.58 6.37 18.74 7.45

0.87 2.39 1.92 1.04 1.65 1.01

8.18 3.42 4.18 1.84 6.83 3.04

1.41 1.72 1.36 1.75 1.56 1.64

1.67 2.75 1.47 2.25 1.68 1.87

0.87 4.21 1.74 2.85 0.86 1.96

0.15 0.28 0.17 0.18 0.79 0.24

13.09 15.83

9.75 11.96

1.52 2.37

44.11 50.36

18.25 8.33

2.21 1.81

7.11 2.84

1.14 1.88

1.47 2.11

0.59 2.05

0.54 0.11

Figure 6. SEM micrograph and EDX analysis, and XRF analysis (value in mol %) of the condensation samples (wheat straw).


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(a) Temperature range: 706-768 oC

EDX spot 1 spot 2 spot 3 spot 4 window (a) window (b) XRF 706-768 oC 603-706 oC

(b) Temperature range: 603-706 oC

K

Cl

S

O

Si

Al

Na

Mg

Ca

P

Fe

4.78 40.32 5.32 29.68 7.44 6.81

0.34 36.97 27.98 7.09 5.77

0.31 0.72 0.17 0.36

62.68 7.12 58.45 29.46 59.26 56.28

9.37 5.47 16.55 10.19 9.16 15.84

0.69 1.33 0.51 1.16

0.72 2.13 0.24 0.22 1.35

4.64 1.65 0.76 0.71

1.60 0.09 0.12

0.76 0.70 0.43 0.59

0.16 0.21 0.13 0.14

9.76 7.89

9.15 6.95

0.31 0.57

63.22 60.93

14.25 18.73

0.94 1.21

0.56 1.45

1.01 1.17

0.18 0.25

0.51 0.67

0.11 0.18

Figure 7. SEM micrograph and EDX analysis, and XRF analysis (value in mol %) of the condensation samples (corn stalk).

3.3. Condensation Mechanisms of Potassium Salts during Biomass Combustion. Based on the results of this study and previous studies, 8, 11, 33 the condensation mechanisms of potassium salts during biomass combustion in a grate-fired boiler are illustrated in Figure 8. The temperature of fuel bed is about 800−900 oC.19, 25, 30, 45 In the combustion process of biomass fuels, the potassium species with high reactivity would release to the gas phase, which undergo complex physical transformations and chemical reactions. KCl is the most stable K-containing substance in gas phase, and it will be mainly in the form of vapor above 800 oC. When the flue gas temperature is higher than the melting point of KCl, vapor phase accounts for a major share. As shown in Figure 8, when the temperature of the convective heating surfaces area is above 700 oC, K mainly occurs as KCl vapor, fine particles of KCl, fine particles of K2SO4, and potassium silicate or potassium aluminosilicate in fly ash. KCl fine particles are generated by homogeneous nucleation of KCl vapor when the temperature decreases along with the flue gas. KCl could also adhere on fly ash 19 Environment ACS Paragon Plus

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particles, which makes the fly ash particles stickier. The vaporization of K2SO4 is insignificant compared with KCl. K2SO4 is mainly in the form of fine particles. KCl vapor can also condense on K2SO4 nucleation. The initial slagging layer is generated by heterogeneous condensation of KCl vapor and thermophoresis and diffusion of KCl and K2SO4 fine particles. The heterogeneous condensation of KCl vapor would be dominant when the temperature is higher than 700 oC. The molten content of the initial slagging layer is higher as the temperature increases. K2SO4 is aggregated with KCl in initial slagging layer.

1. Nucleation; 2. Heterogeneous condensation; 3. Thermophoresis and diffusion; 4. Inertial impaction. Figure 8. Condensation mechanisms of potassium species during biomass combustion.


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When the temperature decreases below 700 oC, KCl would be mainly in the form of fine particles. KCl fine particles can deposit on K2SO4 nucleation and fly ash particles by thermophoresis and diffusion. The initial slagging layer is mainly formed by thermophoresis and diffusion of fine particles of KCl and K2SO4. In the area of high-temperature convective heating surfaces, the initial layer performs as an adhesive which could capture the large coarse particles in flue gas. In this study, it seems that KCl plays a more important role than K2SO4 in the formation of initial slagging layer and the condensation on fly ash particles.

4. CONCLUSIONS The condensation behaviors of potassium during combustion of biomass fuel are investigated by using a one-dimensional down-fired furnace coupled with a novel condensation probe. The following conclusions can be drawn: (1) A boundary that the potassium vapor begins to condense on the condensation probe occurs clearly when potassium salts are used as samples. The condensation temperature of KCl, K2SO4, and mixture of KCl and K2SO4 are 770±4 °C, 745±6 °C and 764±10 °C, respectively. The condensation amount of KCl is much greater compared with that of K2SO4. (2) The initial slagging layer is formed by heterogeneous condensation of KCl vapor, and thermophoresis and diffusion of KCl and K2SO4 fine particles. When the wall temperature of the condensation probe is higher than 700 oC, heterogeneous condensation of KCl vapor is more significant. KCl fine particles generated by homogeneous nucleation are dominant at lower temperatures. The condensation layer can be partially molten at high temperatures. The influence of K2SO4 on the formation of initial slagging layer is much less significant compared with that of KCl. (3) During combustion of biomass fuels, KCl can condense on the surface of large fly ash particles in flue gas by heterogeneous condensation, thermophoresis and diffusion. The condensed KCl can be partially molten at high temperatures. K2SO4 fine particles deposit on the surface of fly ash particles


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together with KCl. No individual K2SO4 is observed. The condensation behaviors of KCl on the fly ash are much more remarkable compared with that of K2SO4.

AUTHOR INFORMATION Corresponding Author * Tel: +86−29−82668703. Fax: +86−29−82668703. E−mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work has been financially supported by the National Natural Science Foundation of China (51406147).


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REFERENCES (1) Frandsen, F. J. Fuel 2005, 84 (10), 1277-1294. (2) Jenkins, B. M.; Baxter, L. L.; Miles Jr, T. R.; Miles, T. R. Fuel Process. Technol. 1998, 54 (1–3),17-46. (3) Saidur, R.; Abdelaziz, E. A.; Demirbas, A.; Hossain, M. S.; Mekhilef, S. Renew. Sustainable Energy Rev. 2011, 15 (5), 2262-2289. (4) Demirbas, A. Prog. Energy Combust. 2005, 31 (2), 171-192. (5) Chang, J.; Leung, D. Y. C.; Wu, C. Z.; Yuan, Z. H. Renew. Sustainable Energy Rev. 2003, 7 (5), 453-468. (6) Deng, L.; Zhang, T.; Che, D. Fuel Process. Technol. 2013, 106 (0), 712-720. (7) Liu, Y.; Wang, X.; Xiong, Y.; Tan, H.; Niu, Y. Appl. Therm. Eng. 2014, 63 (1), 266-271. (8) Deng, L.; Che, D. Ind. Eng. Chem. Res. 2012, 51 (48), 15710-15719. (9) Werther, J.; Saenger, M.; Hartge, E.-U.; Ogada, T.; Siagi, Z. Prog. Energy Combust. 2000, 26 (1), 1-27. (10) Mu, L.; Zhao, L.; Liu, L.; Yin, H. Ind. Eng. Chem. Res. 2012, 51 (25), 8684-8694. (11) Nielsen, H. P.; Baxter, L. L.; Sclippab, G.; Morey, C.; Frandsen, F. J.; Dam-Johansen, K. Fuel 2000, 79 (2), 131139. (12) Michelsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Larsen, O. H. Fuel Process. Technol.1998, 54 (1–3), 95108. (13) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K. Energy Fuels 1999, 13 (6), 1114-1121. (14) van Lith, S. C.; Frandsen, F. J.; Montgomery, M.; Vilhelmsen, T.; Jensen, S. A. Energy Fuels 2009, 23 (7), 34573468. (15) Gruber, T.; Schulze, K.; Scharler, R.; Obernberger, I. Fuel 2015, 144, 15-24. (16) Lapuerta, M.; Acosta, A.; Pazo, A. Energy Fuels 2015, 29 (8), 5007-5017. (17) Baxter, L. L.; Miles, T. R.; Miles Jr, T. R.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol.1998, 54 (1–3), 47-78. (18) Frandsen, F. J.; Hansen, J.; Jensen, P. A.; Dam-Johansen, K.; Montgomery, M.; Fenger, L. D.; Jensen, J. N.; Karlsson, A.; Henriksen, N.; Jensen, J. P. IFRF Combust. J. 2002, (200204), 1-23. (19) Jensen, P. A.; Stenholm, M.; Hald, P. Energy Fuels 1997, 11 (5), 1048-1055. (20) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter, L. L. Prog. Energy Combust. 2000, 26 (3), 283-298. (21) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Hørlyck, S.; Karlsson, A. Fuel Process. Technol. 2000, 64 (1–3), 189-209.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

(22) Niu, Y.; Tan, H.; Ma, L.; Pourkashanian, M.; Liu, Z.; Liu, Y.; Wang, X.; Liu, H.; Xu, T. Energy Fuels 2010, 24 (9), 5222-5227. (23) Jensen, P. A.; Frandsen, F. J.; Hansen, J.; Dam-Johansen, K.; Henriksen, N.; Hörlyck, S. Energy Fuels 2004, 18 (2), 378-384. (24) Niu, Y.; Zhu, Y.; Tan, H.; Hui, S.; Jing, Z.; Xu, W. Fuel Process. Technol. 2014, 128, 499-508. (25) Miles, T. R.; Miles Jr, T. R.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Biomass Bioenergy 1996, 10 (2–3), 125-138. (26) Baxter, L. L.; DeSollar, R. W. Fuel 1993, 72 (10), 1411-1418. (27) Baxter, L. L. Biomass Bioenergy 1993, 4 (2), 85-102. (28) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9 (5), 855-865. (29) Wei, X.; Schnell, U.; Hein, K. R. G. Fuel 2005, 84 (7–8), 841-848. (30) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Energy Fuels 2004, 18 (5), 1385-1399. (31) Li, G.; Li, S.; Xu, X.; Huang, Q.; Yao, Q. Energy Fuels 2014, 28 (1), 219-227. (32) Christensen, K. A.; Stenholm, M.; Livbjerg, H. J. Aerosol Sci. 1998, 29 (4), 421-444. (33) Valmari, T.; Lind, T.; Kauppinen, E.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Energy Fuels 1999, 13 (2), 390-395. (34) Garba, M.; Ingham, D.; Ma, L.; Porter, R.; Pourkashnian, M.; Tan, H.; Williams, A. Energy Fuels 2012, 26 (11), 6501-6508. (35) Aho, M. Fuel 2001, 80 (13), 1943-1951. (36) Jiménez, S.; Ballester, J. Combust. Flame 2005, 140 (4), 346-358. (37) Michelsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Larsen, O. H. Fuel Process. Technol. 1998, 54 (1), 95-108. (38) Bashir, M. S.; Jensen, P. A.; Frandsen, F. J.; Wedel, S.; Dam-Johansen, K.; Wadenbäck, J.; Pedersen, S. T. Fuel Process. Technol. 2012, 97, 93-106. (39) Wang, X.; Xu, Z.; Wei, B.; Zhang, L.; Tan, H.; Yang, T.; Mikulčić, H.; Duić, N. Appl. Therm. Eng. 2015, 80, 150-159. (40) Hu, Z.; Wang, X.; Zhao, W.; Wang, Y.; Tan, H. Energy Fuels 2014, 28 (12). (41) Iisa, K.; Lu, Y.; Salmenoja, K. Energy Fuels 1999, 13 (6), 1184-1190. (42) Boonsongsup, L.; Iisa, K.; Frederick, W. J. Ind. Eng. Chem. Res. 1997, 36 (10), 4212-4216. (43) van Lith, S. C.; Jensen, P. A.; Frandsen, F. J.; Glarborg, P. Energy Fuels 2008, 22 (3), 1598-1609.


Page 25 of 25

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(44) van Lith, S. C.; Alonso-Ramírez, V.; Jensen, P. A.; Frandsen, F. J.; Glarborg, P. Energy Fuels 2006, 20 (3), 964978. (45) van der Lans. R. P.; Pedersen, L.; Jensen, A.; Glarborg, P.; Dam-Johansen, K. Biomass Bioenergy 2000, 19 (3), 199-208.


Condensation Behaviors of Potassium during Biomass Combustion

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