Development of online ash deposition thermogravimetric analyzer for

Article pubs.acs.org/EF

Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Development of an Online Ash-Deposition Thermogravimetric Analyzer for Pulverized Coal Combustion Denggao Chen,† Jugang Fang,‡ Minmin Zhou,§ Zhenshan Li,*,† and Ningsheng Cai† †

Downloaded via UNIV OF SUNDERLAND on November 6, 2018 at 21:56:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ‡ North China Electric Power University, Baoding, Heibei 071003, China § Institute for Clean and Secure Energy, University of Utah, Salt Lake City, Utah 84112, United States ABSTRACT: A novel system consisting of an online ash-deposition thermogravimetric analyzer coupled with an electrically heated down-fired furnace was developed for pulverized coal combustion. A steady high-temperature pulverized coal combustion was achieved in the down-fired furnace. Ash deposition weights were online measured without interference of the deposition process. The developed setup was calibrated to quantify the system error and subsequently used to investigate the ash deposition behavior. First, the gas species, coal conversions, and particle size distributions were quantitatively measured under excess air of 1.2 (oxidizing atmosphere) and 0.7 (reducing atmosphere) under furnace temperatures of 1100−1400 °C. Next, the ash deposition rates were online measured under different surface temperatures (1000−1400 °C), oxidizing and reducing atmospheres (corresponding to excess air of 1.2 and 0.7, respectively), and gas velocities (0.89 to 2.37 m/s,). The deposition samples were collected through the N2 protection method. The morphology and structural property of the sample were characterized by scanning electron microscope. From a low to a high temperature, it was found that the deposition transformed from a porous layer composed of loosely bond particles to a slag layer. The transition was found to be tightly related to the ash fusion temperature and the unburnt carbon fraction of the deposited particle. Finally, the reaction rate of unburnt carbon in the deposition was measured using a specially designed experiment. A deposition model was proposed to model the deposition process and derive the reaction rate. It was found that both the oxidation and the gasification reaction rates were a few orders of magnitude slower than those in pulverized coal combustion.

1. INTRODUCTION In the operation of a pulverized coal combustion facility, incombustible ash can deposit on the surface through slagging or fouling. Ash deposition can lead to the overall heat exchange efficiency decrease,1 erosion acceleration,2 and even emergency shutdown.3 Extensive works have been conducted to investigate depositions in pulverized coal combustion. It was found that the combustion condition,4 ash composition,5 char conversion,6 deposition surface,7 etc. had a significant effect on the deposition behavior. Numerical methods have also been used to predict ash deposition8−10 in an industrial boiler. Ash deposition is a dynamic process where the weight and property of deposition changes with time. A reliable deposition experiment is the foundation for investigating the deposition mechanism and the development of the deposition model. Among the various issues of deposition, the increase in the deposition weight or size is the most concerning. In recent publications, many methods have been developed and tested to obtain the weight change during a deposition. These methods can be summarized briefly as follows: (1) Ex situ weighting method.7,11 In this method, a deposition probe is inserted in the coal combustion facility. Ash particles deposits on the probe surface. After some time, the probe is extracted from the combustion environment and the accumulated deposition is collected and weighed. Using different accumulation times, the deposition weight change data are obtained. However, the deposition is often damaged during extraction when the deposition is loose, resulting in low-quality result. Additionally, © XXXX American Chemical Society

as this method requires a significant amount of time, the number of obtained data points is small. This may cause the loss of important information during the deposition growth. (2) In situ digital imaging monitor.12,13 This method presents a direct visual result of the deposition growth and is important for a direct understanding. However, the result can hardly be used for the model development, which often requires weight-changing data. (3) Indirect thermal conductivity or heat flux method.5,14 The deposited ash changes the heat transfer between the heat exchange surface and the high-temperature environment, which is related to the deposition thickness. Generally, the thermal conductivity decreases with deposition growth. Therefore, it is used as an indirect measurement of deposition thickness. It is noteworthy that the thermal conductivity is also related to the microstructure of the deposition and is often highly complex. This means that the thermal conductivity can only reflect the trend of the deposition growth and is not a direct measurement of the deposition weight or thickness. Namkung et al.15 developed an online weight measurement method to investigate ash deposition in coal combustion. However, the designed probe is an ash collector rather than a deposition probe, and the combustion result was not shown. Quantitative measurements of deposition weight in a combustion environment that is similar to the actual boiler is important and required. Additionally, in Received: August 8, 2018 Revised: October 29, 2018 Published: October 29, 2018 A

DOI: 10.1021/acs.energyfuels.8b02745 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

2. ONLINE ASH-DEPOSITION THERMOGRAVIMETRIC ANALYZER (AD-TGA) FOR PULVERIZED COAL COMBUSTION The motivation of the online ash-deposition thermogravimetric analyzer (AD-TGA) is to measure the real-time in situ deposition weight, without the interference of the deposition process itself. Figure 2 shows the mechanism and structure of the

published works, ash deposition in the oxidizing atmosphere is widely studied; however, the deposition in the reducing atmosphere of air-staged combustion is still insufficient. For the arch fired boiler or fluid slag boiler, many refractory belts are present on the water wall, as shown in Figure 1. These

Figure 2. Structure of the online thermogravimetric deposition setup. Figure 1. Ash deposition on refractory belts inside a furnace.

designed AD-TGA setup. It consists of a ceramic deposition probe, aluminum oxide support tube, supporter fixer, highprecision weight sensor, and data acquisition computer. The deposition probe is placed on top of the supporter and is removable. The length of the supporter can be changed to adjust the height of the deposition probe. The supporter is fixed to the weight sensor by the fixer. An electromagnetic force compensation type (MonoBloc) weight sensor from Mettler Toledo company was used. The division value is 1 mg with acquisition frequency of 1 data per second. Hence, both the deposition probe and its supporter are placed on the weight sensor; therefore, their weights can be directly recorded. In the deposition experiments, the deposition probe is inserted into a down-fired furnace. When ash particles are deposited on the probe, their total weights are directly measured by the weight sensor without the interference of the deposition process. It is noteworthy that ash particles will also deposit on the support tube and will affect the weight measurement of the deposition on the probe. To avoid this phenomenon, a shelter tube is installed around the support tube. The shelter tube is fixed to the furnace; thus, it will not affect the weight measurement. Additionally, outside the shelter tube, a flue gas cooling jacket is installed. The flue gas is induced by a vacuum pump such that air is induced into the gaps between the support tube and shelter tube. This backflow protection air could further prevent ash particles from entering the gaps between the supporter tube and shelter tube. The length of the shelter tube is also changeable according to the position of the deposition probe. The vacuum of the pump was carefully adjusted to reduce its effect on the weight measurement. The online thermogravimetric deposition setup was integrated into an electrically heated down-fired furnace (E-DFF) for pulverized coal combustion. The schematic of the E-DFF and the AD-TGA setup is shown in Figure 3. The primary part of the E-DFF is a vertical furnace, which is electric-heated by 12 uniformly distributed SiC rods. An aluminum oxide reaction

refractory belts are adiabatic with high temperatures; therefore, ash or slag will deposit on these surfaces. The slagging on the refractory belts are serious problems especially for low-NOx combustion technologies, in which a reducing atmosphere is formed near the refractory belts. This kind of deposition was simulated in our experiment setup introduced in this paper. In this work, we aim to develop an online weight measured ash deposition experiment setup for pulverized coal combustion and use this setup to obtain reliable experiment data for deposition characterization and ash deposition model development. The content of this paper includes the following: First, an online ash deposition thermogravimetric analysis setup (AD-TGA) coupled with an electrically heated down-fired furnace was developed. The combustion results, including gas concentrations of O2, CO2, CO, the coal conversion, and the particle size distribution were quantitatively measured. The developed setup was calibrated and tested to quantify the systematic error. Next, ash deposition in both oxidizing and reducing atmospheres was measured online under high surface temperature. The deposition samples in both experiments were well collected through an N2 protection method. The effect of combustion temperature on the deposition was investigated under the furnace temperature of 1000−1400 °C. The relations of deposition weight and morphology with temperature were discussed. The effect of gas velocity on the deposition rate and deposition growth at different temperatures was experimentally explored. Finally, the reaction of unburnt carbon in the deposition was investigated through a specially designed experiment. An ash deposition model was proposed to model the experimental deposition process and obtain the unburnt carbon reaction rate of the deposition. The obtained reaction rates were compared with those in the pulverized coal combustion. B


Article

Energy & Fuels

Figure 3. Schematic of the AD-TGA setup.

tube is placed inside the furnace vertically with a heated length of 2.5 m and an inner diameter of 60 mm. The outer wall temperature of the reaction tube is measured by 10 evenly distributed thermocouples and controlled automatically. A gas and coal injector is affixed on top of the furnace to inject coal and gas. As the figure shows, the primary air injection tube and burnout air injection tube are placed inside. On the outside is the secondary air injection tube. The primary air injection tube is water cooled to prevent coal reactions such as pyrolysis before injecting into the reaction tube. Burnout air or overfire air (OFA) can be injected by the aluminum oxide OFA tube that is placed inside the reaction tube vertically. The OFA injection point is adjustable. The secondary air tube is used to inject additional air when changing the velocity of the flue gas in the deposition experiment. Gaps between the injector and reaction tube are sealed to prevent air leakage that can change the combustion atmosphere. Pulverized coal is continuously fed by a micro scraper feeder whose feeding rate can be adjusted from 0.1 to 20 g/min. The coal feeding rate is monitored online by a weight sensor. The pulverized coal is dispersed and carried by the primary air from the mass flow controller. Upon injecting into the reaction tube, the coal particles start to pyrolyze and burn, thus creating the primary combustion zone. If the fed air is not sufficient to burn out the coal, after the oxygen is depleted, the reductive zone will start to form. When OFA air is injected, the residual combustible starts to burn and forms the burnout zone. Using the designs above, the three combustion zones of the actual boiler were created in the E-DFF. Coupled with the online thermogravimetric deposition setup, the AD-TGA setup can be used to explore the deposition in the three zones. When the deposits in the reductive zone, the OFA tube is removed. The gas composition and ash property after combustion are important for characterizing the atmosphere and modeling. To collect the gas and ash particles, the online deposition setup is replaced by a water-cooled sampling gun. Using this sampling gun, the gas and particles can be collected along the reaction tube vertically and subsequently measured by the analyzer. The

details of the combustion experiment using E-DFF can be found in our previous work.16 Different deposition probes were designed, as shown in Figure 4. Type A is a flat head cylinder and type B is a half circle head.

Figure 4. Deposition probes.

For type A, the deposition angle (angle of the moving direction of a particle and the deposition surface plane) is close to 90°. The rebounding of particles back to the flow can be minimized. Therefore, the deposition weight is mainly related to the incoming particle mass and the reaction of the deposition. These two factors can be investigated more clearly using this probe. In terms of type B, the deposition angle ranges from 0° to 90°, which is closer to the shape of an actual heat exchange tube.

3. EXPERIMENTS Different types of experiments were designed to test the designed ADTGA setup and to explore the different characteristics of the deposition process. 3.1. Characterization of Coal Samples. A low volatile bituminous coal was used in the experiments. The original coal was ground and repeatedly sieved by 45-μm and 106-μm sieves. The size distribution of the prepared coal sample was measured by a laser particle size analyzer and is plotted in Figure 5. The proximate and C


Article

Energy & Fuels

conducting the deposition experiments, it is important to verify the systematic error by blank tests. Blank tests using air feeding rates of 10, 20, 40, and 80 L/min were designed. Before the tests, both the supporter tube and deposition probe were heated at 1400 °C for hours to remove the possible combustibles. During the test, the furnace temperature was set from 1000 to 1400 °C with 100 °C intervals, and the distance between the coal injection and deposition probe was maintained at 1.1 m. Air was fed from the primary and secondary air injection tube using mass flow controllers. The sucking rate of the vacuum pump was adjusted at different air feeding rates to maintain nearly the same back air flow into the shelter tube. No coal was fed during the blank tests. The weight of the deposition probe and supporter was continuously recorded by a computer at a time interval of 0.5 s. Subsequently, three deposition tests were conducted at the same experimental conditions. The coal and air feeding rate were set at 3.60 g/min and 20 L/min to yield an excess air of 1.2. The furnace temperature was set from 1000 to 1400 °C with 100 °C intervals, and the deposition distance was maintained at 1.1 m. The deposition test follows the following three steps: First, a 2 min blank test was conducted to ensure that the system error was within the previous blank tests. Next, coal was fed and the deposition weight was recorded online for 10 min. Finally, upon the completion of deposition, the air was immediately switched into N2 gas to protect the deposition sample. After one test, the deposition probe was replaced by a new one. Three deposition tests were conducted at the same conditions to verify the repeatability of the AD-TGA setup. To explore the effect of temperature and atmosphere of combustion on deposition, deposition experiments under furnace temperatures of 1000, 1100, 1200, 1300, and 1400 °C were conducted. In these experiments, the type-A deposition probe was used. Excess air of 1.2 and 0.7 were adopted for the temperatures above. These tests followed the same three steps introduced above. The coal feeding rate was 3.60 g/min. The air feeding rates were 20 and 11.7 L/min for excess air of 1.2 and 0.7, respectively. The deposition distances were maintained at 1.1 m. The N2 protection method was used in each test to maintain the original morphology of the deposition. The effect of gas velocity was also explored by adjusting the air flow rate from 30 to 80 L/min at the temperatures of 1100 and 1400 °C using the type-B deposition probe. Subsequently, the thermogravimetric analysis of the deposited ash was performed. As discussed in the Introduction, the deposited ash particle that contains unburnt carbon will continue to react. The deposition of ash particle leads to an increase in weight, and the reaction of the deposition results in a decrease in weight. During deposition, these two factors are coupled. It is difficult to derive the reaction rate from the weight data. To achieve this target, a special experiment was designed to decouple the reaction from the coupled deposition process. As shown in Figure 8, the AD-TGA setup was used as a TGA after ash was deposited on the probe. The in situ reaction rate could be analyzed as such. First, the deposition experiment was conducted using the same method as the third experiment. Next, upon the completion of deposition, the fed gas was immediately switched to the synthetic gas. The composition of the synthetic gas was the same as that of the measured combustion experiment. The weight loss of the deposition was recorded online. As a cooling process is not involved, the structure and reactivity of the deposition could be maximally maintained.

Figure 5. Particle size distribution of the prepared coal. ultimate analysis of the sample were analyzed according to GB/T-21217 and GB/T-31391,18 respectively. The coal properties are listed in Table 1. The ash composition was characterized using an X-ray fluorescence spectrometer (XRF). The results are listed in Table 2. The ash fusion temperatures were measured following the ASTM D1857.19 The results are listed in Table 3. 3.2. Characterization of Temperature. As is shown in Figure 6, we also conducted an experiment to measure the temperature inside the furnace and the deposition probe surface. We conducted a measurement under the set furnace temperature of 1300 °C with excess air of 0.7. The results are plotted in Figure 7. Figure 7a shows the temperature distribution along the reaction distance. The gas temperature keeps increasing as it is being heated up by the environment and then heated up by heat released from combustion. After 0.4 m, the temperature becomes steady with max difference of 7 °C to the set temperature. The deposition tube was place at 1.1 m, where temperature is steady both upstream and downstream. Figure 7b shows the monitored temperature in 10 min. The temperature difference between the deposition surface and the furnace is within 6 °C. 3.3. Experiment Design. First, combustion experiments without OFA air were conducted to obtain both the gas species concentrations and ash properties after combustion. Hence, different excess air, i.e., the ratio of the actual fed air amount to the stoichiometric air amount, and furnace temperatures were adopted. The designed coal feeding rate was 3.60 g/min, and the air feeding rates were 11.7 L/min and 20 L/min for excess air of 0.7 and 1.2, respectively. Gas and ash samples along the reaction distance were collected. The residence time of the gas was estimated from the averaged gas velocity. The residence times at different temperatures are listed in Table 4. This value is also proportional to the reaction distance. The gas species were measured by Fourier transform infrared spectroscopy and gas chromatography. The unburnt carbon fraction (UBC) in the ash sample was measured by the TGA. Coal conversion was calculated from UBC using the ash trace method.20 The particle size distribution (PSD) was measured by the Malvern Laser Particle Size Analyzer of master sizer 2000 which covers particle size from 1 to 2000 μm. Next is the calibration of the AD-TGA setup to evaluate the systematic error. The calibration of the new setup is conducted by a blank test and a repeat deposition test. As shown in Figure 2, when the flue gas flows around the deposition probe, additional force is generated on the probe. This additional force will be transferred to the weight sensor and cause error in the deposition weight measurement. Additionally, the protection air will also generate additional force on the supporter tube and again affect the weight measurement. Before

4. RESULTS AND DISCUSSIONS 4.1. Combustion Experiment. This setup was designed to provide reliable experimental data for the development of the deposition model. This requires obtaining both high-quality combustion and deposition data. The stability of coal feeding is

Table 1. Coal Properties proximate analysis (wt %, air-dry)

ultimate analysis (wt %, dry ash free)

M

V

FC

A

C

H

O

N

S

0.76

17.46

44.48

37.30

76.29

3.21

11.67

1.60

7.23

D


Article

Energy & Fuels Table 2. Primary Ash Composition (m/m%) SiO2

Al2O3

Fe2O3

CaO

SO3

K2O

MgO

TiO2

Na2O

P2O5

49.87

25.75

14.13

3.05

1.83

0.95

0.83

2.36

1.11

0.13

Table 3. Ash Fusion Temperaturea (°C) IT

ST

HT

FT

1232

1280

1290

1304

1.2 and 0.7 are the oxidizing and reducing atmospheres, respectively. The results for 1.2 are plotted in Figure 10. With the increase in the furnace temperature, more oxygen was consumed and more CO2 was produced by the oxidation reaction. Therefore, with the increase in temperature, the concentration of O2 decreased and the concentration of CO2 increased. Figure 10d shows that the coal conversions at the reaction distance of 1.1 m are 83.8%, 89.9%, 92.0%, and 94.0%. Further, 1.1 m is the distance for the deposition experiments. The particles that hit the deposition probe would have a similar conversion. The results of 0.7 are shown in Figure 11. The changes in gas concentrations along the reaction distance are more complex than that of 1.2. At first sight, the distribution of CO2 appears disordered; however, it reveals the combustion characteristic of this coal. At 1100 °C, as the fed air was not sufficient for coal burnout, as Figure 11b shows, O2 is still present in the gas. Oxidation still dominated the char reaction. This reveals that the low excess air combustion may not generate a reductive atmosphere because of the small char oxidation rate. When the temperature was increased to 1200 °C, the char combustion became faster, and O2 was nearly depleted at 0.9 m. Near this location, gasification replaced oxidation as the primary char reaction; therefore, CO was generated. As the temperature got higher, the depletion of O2 became faster, char gasification dominated earlier, and more CO was produced. Further, the reductive atmosphere was enhanced. Under the excess air of 0.7, for temperatures of 1100−1400 °C, the coal conversions at 1.1 m are 70.9%, 73.9%, 77.4%, and 83.8%. Under the excess air of 0.7, the coal conversion is at least 10% lower than that of 1.2 at the same temperature and location. As the gasification reaction rate was obviously smaller than the oxidation rate, the insufficient air supplement reduced the burnout of char. The unburnt carbon in the deposited ash may continue to react and affect the deposition weight. Thus, the measured coal combustion data are important for developing a detailed deposition model. The particle size distributions under excess air of 1.2 and 0.7 are plotted in Figure 10e and Figure 11e, respectively. At higher temperatures, the coal conversion is

a

IT: Initial deformation temperature is the temperature at which the cone apex begins to become rounded. ST: Softening temperature is the temperature at which the base of the cone is equal to its height. HT: Hemispherical temperature is the temperature at which the base of the cone is twice its height. FT: Fluid temperature is the temperature at which the cone has spread to a fused mass no more than 1.6 mm in height.

the primary issue. Figure 9a shows the online measured fed coal weight at two different single runs. The calculated linearity of these two feeding slopes are 0.9992 and 0.9994, indicating that the feeding process is rather steady. The feeding rate was calculated as the slope of the line. The fed coal weight of every experimental point shown in Table 4 was recorded, and the total points are 32. The calculated average coal feeding rate is 3.58 g/ min, which is rather close to the designed value of 3.60 g/min. The maximum relative error of the measured coal feeding rate to the designed value is 5.01%. A good dispersion of the fed coal particles is curial for a steady combustion. This can be verified by online monitoring the gas species from the flue gas. Figure 9b shows the online monitored gas species under the furnace temperature of 1400 °C, a reaction distance of 1.1 m (OFA injected at 1.2 m), and excess air of 0.7 and 1.2. The relative variation was calculated by the following: (measured gas species value − average measured gas species value)/average measured gas species value × 100%. Under the excess air of 1.2, the maximum relative variations of CO2 and O2 are 1.44% and 5.83%, respectively. Under the excess air of 0.7, the maximum relative variations of CO2 and CO are 1.94% and 3.00%, respectively. This includes the error of the analyzer. These results show that the combustion is rather steady and establish the foundation for obtaining high-quality deposition data. The combustion experiments described in Section 3.2 were conducted. Both gas and solid samples were collected along the reaction distance under furnace temperatures of 1100, 1200, 1300, and 1400 °C and excess air of 1.2 and 0.7. The excess air of

Figure 6. Schematic of temperature characterization. E


Article

Energy & Fuels

Figure 7. (a) Monitored temperature inside furnace along the reaction distance. (b) Temperature of the probe inner surface.

Table 4. Combustion Experiment Conditions excess air

temperature (°C)

reaction distance La (m)

residence timeb (s)

measurement

1.2 0.7

1100/1200/1300/1400 1100/1200/1300/1400

0.5/0.7/0.9/1.1 0.5/0.7/0.9/1.1

1.52−1.86 @ 1.1 m 2.60−3.17 @ 1.1 m

CO, H2, CO2, O2, UBC, PSD CO, H2, CO2, O2, UBC, PSD

Distance from coal injection location to sampling location; Air flow rate was measured in standard temperature 0 °C and pressure 1 atm (STP). Residence time is calculated from averaged gas velocity according to the experiment condition. Residence time is proportional to the reaction distance.

a

b

tests, which will be discussed in the following, are in the range of 30 mg/min to 50 mg/min. Considering the effect of systematic error on the deposition rate, the maximum error is 6.7%. After completing the blank test, three repeat deposition tests were conducted at 1200 °C at the deposition distance of 1.1 m. The deposition weights were recorded online and are plotted in Figure 12b. These three tests show very similar weight growth process, revealing that the AD-TGA setup has a good repeatability. In terms of the final deposition weight, the maximum difference is between test A and test C with a value of 20 mg. It is approximately 2.7% of the total deposition weight. The calculated deposition rates are 73.08, 72.11, and 71.33 mg/ min, and the maximum relative difference is 2.39%. The results from the blank tests and repeat tests show that the developed AD-TGA setup can provide reliable deposition data. 4.3. Effect of Combustion Atmosphere and Temperature. After the calibration, the deposition experiments were conducted at five temperatures of 1000, 1100, 1200, 1300, and 1400 °C, under excess air of 1.2 and 0.7. The combustion results of these cases are provided in Section 4.1. The coal feeding rate and air feeding rate were the same with those of the combustion experiments. The deposition distance was set at 1.1 m for all the trials. The deposition weights were measured online and are plotted in Figure 13. Figure 13a shows that the difference in the deposition weight growth between temperatures is small. Generally, the deposition weight is the difference between the deposited particle weight and the char consumed by reactions. For the first factor, Weber et al.21 suggested the following two aspects to evaluate the deposition possibility of the ash particle: (1) inertial impaction and thermophoresis and (2) Rebounding and sticking. In terms of the first aspect, as the probe temperature is nearly the same with the furnace temperature, there’s no temperature gradient. Therefore, thermophoresis can be safely neglected. The inertial impaction is closely related to the particle Stokes number. First, using the parameters introduced above, the particle Reynolds number can be estimated. Both the parameters and the results are listed in Table 5. As the result shows, for the different conditions and particle sizes adopted in this work, the particle Reynolds number

Figure 8. Thermogravimetric analysis of the deposition sample.

higher, and more particles are burnt or broken into small particles. Therefore, the particles tend to become smaller. 4.2. Calibration of the AD-TGA Setup. The calibration of the AD-TGA setup was conducted by a blank test and a repeat test. The furnace temperature was set from 1000 to 1400 °C with 100 °C intervals. Results under different temperatures are rather similar. Here, the typical results of 1200 °C are shown. Four different gas flow rates were used. The calculated gas velocities in the reaction tube are 0.32, 0.64, 1.27, and 2.54 m/s. As Figure 12a shows, the gas flow has a slight effect on the weight measurement. The systematic errors in the blank tests are generally within 2 mg. The measured ash deposition rates for all F


Article

Energy & Fuels

Figure 9. (a) Online measured coal feeding rates; (b) online measured gas species concentrations.

Figure 10. Results of excess air 1.2: (a) CO2; (b) O2; (c) CO; (d) Xcoal; and (e) particle size distribution. G


Article

Energy & Fuels

Figure 11. Results of excess air 0.7: (a) CO2; (b) O2; (c) CO; (d) Xcoal; and (e) particle size distribution

Figure 12. Results of calibration: (a) weight data of blank tests; (b) weight data of repeat tests.

H


Article

Energy & Fuels

Figure 13. Online measured deposition weights and rates at different temperatures: (a) weights at excess air 1.2; (b) weights at excess air 0.7; (c) rates at excess air 1.2; and (d) rates at excess air 0.7.

Table 5. Particle Reynolds Number and Stokes Number of Typical Particle Sizes Tp (°C) 1200 1200 1200 1200 1200 1200 1000 1100 1200 1300 1400

air flow rate (L/min) 20 20 20 20 20 20 20 20 20 20 20

Dp (μm) 20 40 60 80 100 120 80 80 80 80 80

ρb (kg/m3)

Upa (m/s) 0.64 0.64 0.64 0.64 0.64 0.64 0.55 0.59 0.64 0.68 0.72

0.218 0.218 0.218 0.218 0.218 0.218 0.271 0.243 0.218 0.195 0.173

μb (Pa s) −05

5.44 × 10 5.44 × 10−05 5.44 × 10−05 5.44 × 10−05 5.44 × 10−05 5.44 × 10−05 4.85 × 10−05 5.14 × 10−05 5.44 × 10−05 5.73 × 10−05 6.02 × 10−05

Rep

Stokes number

0.05 0.10 0.15 0.20 0.26 0.31 0.25 0.22 0.20 0.18 0.17

0.01 0.08 0.18 0.32 0.51 0.73 0.39 0.35 0.32 0.30 0.28

a

Calculated average gas velocity inside the reaction tube. bData from NIST (https://webbook.nist.gov/chemistry/fluid/) Tp: particle temperature; Dp: particle size; Up: particle velocity; ρ: gas density; μ: kinetic viscosity; Rep: particle Reynolds number.

is less than 1, where the flow is a Stokes flow.22 The impaction efficiency of the ash particle is strongly related to its Stokes number.23 Using the impaction curve from Weber et al.23 and the particle size distribution from the combustion result shown in Section 4.1, a theoretical impaction fraction was calculated for each case. The calculated impaction fractions for the excess air of 1.2 at temperatures of 1100−1400 °C are 2.33%, 2.23%, 2.08%, and 1.98%. For the excess air of 0.7, they are 3.14%, 2.67%, 2.39%, and 2.29%. The theoretical impaction fraction decreases at high temperatures as the particle size decreases. In terms of the rebounding and sticking, first, as the probe temperature is rather high, alkaline vapor would not condense and form the sticky initial layer.25 Second, as is shown in Figure 4, because the

particle hitting angle is close to 90° for most particles and the particle velocity is small, particle kinetic energy can be mostly dissipated and not rebound from the surface. Besides, when the particle temperature exceeds the deformation temperature, the particle begins to melt and becomes sticky, which will further reduce the rebounding efficiency. Therefore, the rebounding fraction should be rather small for these deposition experiments. Considering the effect of the deposition reaction, although it is different from the particle reaction in the flue gas, the reaction rate should also increase with temperature, meaning that the consumption of unburnt char during deposition is faster at high temperatures. The reasons above led to the deposition weight decreasing with the increase in temperature. From the I


Article

Energy & Fuels

The samples of the experiments were collected and some of them were analyzed using scanning electron microscope (SEM). It is noteworthy that the deposition times for the excess air of 1.2 and 0.7 are 10 and 5 min, respectively. The photographs are shown in Figure 15. As is listed in Table 3, the initial deformation

experimental results, this phenomenon is more obvious for the excess air of 0.7. The relative difference in the deposition rate at 1100 and 1400 °C is approximately 40%. As is discussed above, the difference between the theoretical impact fraction for 1100 and 1400 °C is 27%. Therefore, theoretically, the rest of the 13% rate difference is caused by reaction. As for the results of 1.2, the change in the deposition rate is small owing to the high char conversion in the oxidizing environment. It also can be seen that the deposition rate decreases with time for each single test and it is more obvious under the excess air of 0.7. As is introduced in the Experiments section, during experiment, the air is first fed into reaction tube and then was followed by the coal. Although the excess air ratio was set at 0.7, for the initial fed coal stream, there’s excess air inside the tube and the coal particles could be burned out. As discussed later in this section, higher coal conversion leads to higher sticking possibility and result in higher deposition rate. Analysis of the collected samples also showed that all the bottom layers of the samples were melted. This may be the reason for the higher initial deposition rate for each single test. In the designed AD-TGA system, the weight sensor along with the deposition supporter and the probe is placed on an electric lifter. The parts above can move vertically to pull the deposition probe out of the high-temperature zone or push into it. In this process, the probe will not touch any wall, thus ensuring that the sample will not be damaged. In the experiments, after coal feeding was halted and the air was switched into N2, the deposition sample was placed in the watercooled area to cool down. It was simultaneously protected by N2. The cooling time is typically around 1 min. Hence, the morphology of the sample could be maintained. To investigate the effect of deposition time on the sample morphology, three deposition times of 5, 10, and 15 min were tested under the temperature of 1400 °C and excess air of 1.2. The photographs of these samples are shown in Figure 14. With the increase of the

Figure 15. Photos and SEM analysis of the collected deposition samples at different atmospheres and temperatures.

temperature is 1232 °C, meaning that most of the deposited ash particles will not melt under lower temperatures. Therefore, the samples from 1100 and 1200 °C are very loose. This is also shown in the first SEM photograph. The deposition is full of pores and channels. Reactants such as O2 and CO2 can easily diffuse into the deposition and consume the unburnt carbon. It is obvious that particles at the bottom and outer edges have turned into gray, which is close to the color of the pure ash particle. This is because the unburnt carbon in these early deposited particles has a longer reaction time. As the temperature increased to 1300 °C, which was close to the flow temperature, more particles were melted. The SEM photograph also shows that the particles become smoother because of melting. At 1400 °C, the temperature was near 100 °C higher than the flow temperature. Nearly all of the deposited particles had melted into slag and migrated down the deposition probe. This type of slag is difficult to remove when deposited on the heat exchange surface. For the excess air of 0.7, the trend is similar. However, as the unburnt carbon is higher than 1.2, the deposit is almost black. Additionally, because the gasification rate is smaller than the oxidation rate, the consumption of the deposited unburnt carbon is slower at 0.7. Therefore, even at 1400 °C, unmelted particles still appeared on the slag surface. These particles are trapped by the melted slag surface. 4.4. Effect of Deposition Plate Shape. As discussed in the section above, the gas velocity affects the particle Stokes number and subsequently affects the impaction efficiency. In terms of the impaction efficiency, a higher velocity leads to a higher Stokes number and subsequently a higher deposition efficiency. However, the deposition is also controlled by rebounding and sticking. To investigate the effect of gas velocity and sticking on the deposition, experiments using the type-B probe were conducted at temperatures of 1100 and 1400 °C with different gas velocities. These two temperatures were chosen because 1100 °C was lower than the initial deformation temperature, where the ash particles were still solid; 1400 °C was higher than

Figure 14. Deposition samples from different deposition times (excess air 1.2, temperature 1400 °C).

deposition time, the deposition layer is thicker and the morphology changes. As shown in Figure 10d, the deposited ash particle still contained approximately 8% unburnt carbon, and according to Li et al.,6 this type of particle will not melt as the melting temperature of carbon is high. The unburnt carbon continues to react and leaves ash on the probe. As the temperature had exceeded the flow temperature of 1304 °C, the ash melted. Because the particles on the probe outer edge were exposed to gas flow directly, the particles here reacted faster, and the melting started from the probe edge. As the deposition time became longer, more particles were deposited and a thicker melted slag layer was formed. According to Shen et al.,24 the newly deposited particle will flow on the melted slag layer surface and continues to react. The reaction will form bubbles on the slag surface. Here, it formed holes on the surface. J


Article

Energy & Fuels

Figure 16. Deposition weights (a) and photographs (b) at different gas velocities.

Figure 17. UBCs of the sample from flue gas and deposition samples: (a) excess air 1.2; (b) excess air 0.7.

the flow temperature where the particles had melted and were sticky. The gas velocity was changed by feeding different air flow rates of 30, 50, and 80 L/min at 1100 °C and 66 L/min at 1400 °C. The calculated average gas velocities in the reaction tube are 0.89, 1.48, 2.37, and 2.37 m/s. The online measured weights and photographs of these experiments are shown in Figure 16. As Figure 16a shows, at 1100 °C, with the increase in the gas velocity, the deposition rate increased slightly at first but subsequently decreased to almost zero at 2.37 m/s. At 1400 °C, the deposition rate was much larger under the velocity of 2.37 m/s. Typically, ash deposition follows the following process:25 first, condensation of alkali-based vapor and fine particle deposition due to thermophoresis; second, inertial impaction of coarse particles; third, shedding. In this experiment, the temperature of the tube is at least 1100 °C which is much higher than the alkali-based vapor condensation temperature; therefore, the condensation layer will not form. The temperature of the deposition probe is nearly the same with the furnace temperature. Therefore, there’s no thermophoresis force. The deposition of probe type B is mainly controlled by the inertial impaction. As discussed in Section 4.3, the rate of this process is controlled by the inertial impaction efficiency and rebounding efficiency. The impaction efficiency is proportional to the particle diameter and velocity. Our measurements showed that the particle size distribution at different velocities were near the same because they were close to complete conversion. Therefore, the impaction efficiency is mainly controlled by the velocity. The rebounding efficiency (or rebounding possibility)

is controlled by the particle kinetic energy and energy dissipation when hit the surface. It is related to the particle velocity, particle property, deposition angle, and deposition surface property. Large velocity leads to large kinetic energy and results in large rebounding efficiency. Hard particle and deposition surface leads to small dissipation and results in large rebounding efficiency. After the formation of the initial deposition layer, the deposition surface is the soft ash surface and the dissipation is large. Under 1100 °C, the temperature of the ash particle is lower than the initial deformation temperature and the particle is hard. For each deposition experiment, at first, the deposition surfaces are all hard ceramic; therefore, the dissipation is small. Larger particle velocity leads to larger kinetic energy and results in higher rebounding efficiency. As Figure 16 shows, the formation time of the initial deposition layer increases with velocity due to the increase of rebounding efficiency. At 2.37 m/ s, the initial deposition layer even cannot form. With the growth of the deposition layer, the deposition surface is still the same indicating that the rebounding efficiency does not change a lot. Therefore, the deposition rate does not change a lot under the same velocity. At 1400 °C, the particle is melted and sticky, and even if the ceramic surface is still hard, the kinetic energy can be nearly completely dissipated. Therefore, the formation time of the initial deposition layer is much smaller than 1100 °C. 4.5. Discussion. After completing all the analyses, the whole deposition sample was burned in oxygen at 1400 °C. The weight change before and after burning was recorded and calculated as the unburned carbon in the deposition. The results are plotted in K


Article

Energy & Fuels Figure 17. The UBC of the samples in the flue gas is directly calculated from the combustion results. As these two figures show, the UBCs in the deposition are obviously lower than those in the flue gas, meaning that the deposited carbon converted after deposition. As discussed above, the residual carbon in the deposition affects the behavior and property of the deposition. A simple deposition and reaction model was proposed to simulate the process above, and the reaction rate was obtained. The mechanism of this model is shown in Figure 18. The ash

ij E2, i yz zzP A 2, i expjjj− j RTDe zz R, i k {

ṁ RC = α UBCAshmCoal [1 − XCoal(1 − M − A)]

= ADeDDe, iT ni(Ps, i − PR, i)

(2)

The Gas release rate ṁ G is calculated according to mass balance of the chemical equation of C + CO2 /H 2OCO/CO + H 2

Here, UBCAsh is the unburnt carbon fraction of the incoming ash particle as shown in Figure 17; A2,i and E2,i are the preexponential factor and activation energy, respectively; R is the universal constant of 8.31447 J/(mol K); TDe is the deposition temperature; PR,i and Ps,i are the partial pressures of the reactant of O2 and CO2 inside the deposition and on the surface; ADei s the surface area of the deposition which is the probe surface area; DDe,i is the diffusion rate constant inside the deposition; and ni is the temperature correlation factor for reaction i. In modeling, for the excess air of 1.2, the oxidation was considered, and CO2 gasification was used for 0.7. A comparison of the reaction rate of char in combustion and after deposition would be useful. Therefore, the char reaction rate of combustion was first derived using the method of our previous work.16 Here, the apparent char reaction model was used:

Figure 18. Mechanism of the deposition and reaction.

particles generated from combustion deposited on the probe surface. As the deposited particles contained unburnt carbon, they continued to react and release gas. The measured weight is the balance of the incoming ash particles and outgoing gas and can be calculated as

R Char = A pDi(Pg, i − Ps, i) = A pkc, i(Ps, i)n (i = CO2 ,O2 ,H 2O)

ṁ Dep = αṁ Coal [1 − M − XCoal(1 − M − A)] − ṁ RC

(4)

where RChar is the char reaction rate with units of kg/s; Ap is the particle external surface area; Di is the diffusion rate constant for reactant i; Pg,i and Ps,i are the partial pressures of species i in the bulk gas and on the char surface, respectively; kc,i is the chemical reaction rate of reaction i; and n is the reaction order that is set to 1 in this work. The chemical reaction rate was calculated by

(1)

where ṁ Dep is the net deposition weight; α is the percentage of ash particles deposited on the probe; ṁ Coal is the coal feeding rate; XCoal is the coal conversion measured in the combustion experiment; M and A are the moisture and ash fraction of the coal sample, respectively; and ṁ RC is the residual carbon consumption rate. As Figure 18 shows, the reactant gas first needs to diffuse to the residual char and then react. It is assumed that the chemical reaction kinetics of the residual char is the same as that in combustion. The difference is the gas diffusion rate. The diffusion resistance of the deposition should be much larger than the bulk diffusion resistance in the combustion mode. The residual carbon consumption rate is calculated by Oxidation:

(3)

ji E1, i zyz zz kc, i = A1, i expjjjj− j RTp zz k {

(5)

Here, A1,i and E1,i are the pre-exponential factor and activation energy for reaction i, respectively, R is the universal gas constant of 8.31447 J/(mol K); and Tp is the particle temperature. Three char reactions were considered in this work. kc,O2 = A O2 exp(EO2 /RTp)

C + O2 → CO/CO2

R CO2 /CO = 0.02PO0.21 exp(3070/Tp) 2

Product ratio of CO2 /CO

kc,CO2 = A CO2 exp(ECO2 /RTp)

Gasification:

C + CO2 → 2CO

Gasification:

C + H 2O → CO + H 2 kc,H2O = A H2O exp(E H2O/RTp)

It was assumed that the kinetics of the residual char reaction was the same as that in combustion (A2 = A1, E2 = E1). The difference in consumption rate was caused by the diffusion rate. The deposition and reaction model was coded in MATLAB. A time-discrete method was used to calculate the deposition and reaction process. To obtain the reaction rate of unburnt carbon in the deposition, a special experiment was designed and

(6)

conducted. As is introduced in Section 3.2, the AD-TGA setup was used as a traditional TGA to measure the weight loss of the deposition sample after the deposition experiment without a cooling process. Synthetic gas was fed to generate a similar environment as combustion. The composition of the synthetic gas was controlled according to the combustion results shown in Figures 10 and 11. Hence, the in situ conversion of the L


Article

Energy & Fuels

Figure 19. Measured weights of deposition TGA analysis experiments (symbol: experiment; line: model).

reaction should be considered in simulating depositions in airstaged coal combustion.

deposition was recorded. The measured weights are shown in Figure 19. As shown, the depositions in the excess air of 0.7 exhibit an obvious weight loss. The modeled weights are plotted in Figure 19 as lines. As shown, the proposed model well simulated the experimental process, implying that the proposed mechanism can explain the deposition process well. The char reaction kinetics in eq 4 were derived from combustion results using the same method in our previous work.16 To convert the obtained kinetics of A1 (kg/m2/(s Pa)) and E1 (kJ/mol) into A2 (1/(s Pa)) and E2 (kJ/mol), A1 was multiplied by the average particle specific area of 207.5 m2/kg measured from the combustion sample. The kinetics are listed in Table 6. Parameters of the diffusion model in eq 2 are

5. CONCLUSIONS An online ash-deposition thermogravimetric analyzer setup was developed and coupled with an electrically heated down-fired furnace to investigate ash depositions at different conditions. Pulverized coal combustion experiments under excess air of 1.2 and 0.7 were conducted. Gas species and ash particles were collected and measured to provide the combustion results for the design and analysis of the deposition experiment. The calibration setup showed that the maximum systematic error was 6.7%. The deposition at excess air of 1.2 and 0.7 and furnace temperatures of 1000, 1100, 1200, 1300, and 1400 °C were measured online. The deposition samples were collected using the N2 protection method. Depositions with gas velocities from 0.89 m/s to 2.37 m/s were also investigated. The conversion of unburnt carbon in the deposition was also studied by experiment and modeling. Based on the studies above, the conclusions are as follows: 1. In the reductive zone of the air-staged combustion, the deposited ash particle contained unburnt carbon. For the investigated low volatile bituminous coal, the UBC could reach 32%. Even particle temperature is higher than the ash flow temperature, and only when the unburnt carbon in ash is consumed will the ash melt and form a slag on the deposition surface. 2. The combustion temperature has an obvious effect on the deposition rate at the reductive zone due to the unburnt carbon. The maximum deposition rate could reach 40% between the temperatures of 1100 and 1400 °C. Moreover, the morphology of deposition was significantly affected by temperature. When the temperature was lower than the ash deformation temperature, the deposited particles were loosely bonded to each other and formed pores and channels inside the deposit. This type of deposition could be easily removed. When the temperature was higher than the ash flow temperature, the deposited ash melted and formed a slag on the probe surface. Further, we found that the unburnt carbon in the deposit exhibited a significant effect on the deposition morphology. When the unburnt carbon in the deposit was high, even at a high temperature, the particle will not melt and form a slag. 3. Experiments at gas velocities of 0.89, 1.48, 2.37, and 2.37 m/s at temperatures of 1100 and 1400 °C revealed that when the ash particle was not melted, at a low velocity, the deposition was mainly fouling on the tube surface. With

Table 6. Kinetics of Pulverized Char Reaction and Deposited Char Reactiona reaction

A1 (1/s/Pa)

E1 (kJ/mol)

DDe

n

O2 oxidation CO2 gasification

91.92 144.42

174.2 230.6

3.80 × 10−11 1.40 × 10−11

0.98 1.00

a

Note: 1 represents the kinetics of the char reaction in the pulverized coal combustion.

determined through the modeling and also listed in Table 6. The consumption rate of char in combustion and deposition were calculated under different temperatures and plotted in Figure 20. It shows that the char reaction in the deposition is several orders of magnitude slower than that in the entrained combustion. This

Figure 20. Arrhenius plot of the char reaction rate of combustion and deposition. Note: 1 represents the kinetics of char reaction in the pulverized coal combustion. 2 represents the overall reaction rate of residual carbon in deposition. M


Article

Energy & Fuels

■

through a digital image technique. Energy Fuels 2012, 26 (11), 6824− 6833. (13) Akiyama, K.; Pak, H.; Tada, T.; Ueki, Y.; Yoshiie, R.; Naruse, I. Ash deposition behavior of upgraded brown coal and bituminous coal. Energy Fuels 2010, 24 (8), 4138−4143. (14) Zhou, H.; Zhou, B.; Zhang, H.; Li, L. Behavior of fouling deposits formed on a probe with different surface temperatures. Energy Fuels 2014, 28 (12), 7701−7711. (15) Namkung, H.; Xu, L.; Lin, K.; Yu, G.; Kim, H. Relationship between chemical components and coal ash deposition through the DTF experiments using real-time weight measurement system. Fuel Process. Technol. 2017, 158, 206−217. (16) Chen, D.; Zhang, Z.; Li, Z.; Lv, Z.; Cai, N. Optimizing in-situ char gasification kinetics in reduction zone of pulverized coal air-staged combustion. Combust. Flame 2018, 194, 52−71. (17) Proximate analysis of coal; GB/T 212-2008; Administration of Quality Supervision, Inspection and Quarantine of People’s Republic of China; Standardization Administration of China: 2008. (18) Ultimate analysis of coal; GB/T 31391-2015; Administration of Quality Supervision, Inspection and Quarantine of People’s Republic of China; Standardization Administration of China: 2015. (19) ASTM International. Standard Test Method for Fusibility of Coal and Coke Ash; ASTM D1857; 2017. (20) Costa, M.; Azevedo, J. Experimental characterization of an industrial pulverized coal-fired furnace under deep staging conditions. Combust. Sci. Technol. 2007, 179 (9), 1923−1935. (21) Weber, R.; Mancini, M.; Schaffel-Mancini, N.; Kupka, T. On predicting the ash behaviour using Computational Fluid Dynamics. Fuel Process. Technol. 2013, 105, 113−128. (22) Morsi, S.; Alexander, A. J. An investigation of particle trajectories in two-phase flow systems. J. Fluid Mech. 1972, 55 (2), 193−208. (23) Weber, R.; Schaffel-Mancini, N.; Mancini, M.; Kupka, T. Fly ash deposition modelling: Requirements for accurate predictions of particle impaction on tubes using RANS-based computational fluid dynamics. Fuel 2013, 108, 586−596. (24) Shen, Z.; Liang, Q.; Xu, J.; Zhang, B.; Han, D.; Liu, H. In situ experimental study on the combustion characteristics of captured chars on the molten slag surface. Combust. Flame 2016, 166, 333−342. (25) Yang, X.; Ingham, D.; Ma, L.; Zhou, H.; Pourkashanian, M. Understanding the ash deposition formation in Zhundong lignite combustion through dynamic CFD modelling analysis. Fuel 2017, 194, 533−543.

the increase in velocity, the deposition rate decreased and even decreased to near zero when the gas velocity increased to 2.37 m/s. This was caused by the high rebounding efficiency at high velocities. However, at high temperatures where the particle was melted, the deposition rate was much higher at high velocities. This is because the melted particle was rather sticky and would not rebound even though the velocity was high. 4. The self-designed TGA experiment of the unburnt carbon in the deposit showed that the reaction rate of the unburnt carbon in the deposit was an order of magnitude smaller than that of the pulverized combustion.

AUTHOR INFORMATION

Corresponding Author

*(Z.L.) E-mail: [email protected]. ORCID

Minmin Zhou: 0000-0003-3694-5404 Zhenshan Li: 0000-0003-3407-8079 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was funded by the National Key Research and Development Project (2016YFB0600802-A) and the National Natural Science Foundation of China (51376105, 91434124, 51561125001).

■

REFERENCES

(1) Robinson, A. L.; Buckley, S. G.; Baxter, L. L. In-situ measurements of the thermal conductivity of ash deposits. Symp. (Int.) Combust., [Proc.] 1998, 27 (2), 1727−1735. (2) Nava-Paz, J. C.; Plumley, A. L.; Chow, O. K.; Chen, W. Waterwall corrosion mechanisms in coal combustion environments. Mater. High Temp. 2002, 19 (3), 127−137. (3) Couch, G. Understanding Slagging and Fouling During PF Combustion; IEACR/72; IEA Coal Research: London, 1994. (4) Li, S.; Whitty, K. J. Investigation of coal char− slag transition during oxidation: Effect of temperature and residual carbon. Energy Fuels 2009, 23 (4), 1998−2005. (5) Shimogori, M.; Mine, T.; Ohyatsu, N.; Takarayama, N.; Matsumura, Y. Effects of fine ash particles and alkali metals on ash deposition characteristics at the initial stage of ash deposition determined in 1.5 MWth pilot plant tests. Fuel 2012, 97, 233−240. (6) Li, S.; Wu, Y.; Whitty, K. J. Ash deposition behavior during char− slag transition under simulated gasification conditions. Energy Fuels 2010, 24 (3), 1868−1876. (7) Kupka, T.; Zajac, K.; Weber, R. Effect of fuel type and deposition surface temperature on the growth and structure of an ash deposit collected during co-firing of coal with sewage sludge and sawdust. Energy Fuels 2009, 23 (7), 3429−3436. (8) Li, X.; Yu, G.; Dai, Z.; Zhou, Z.; Wang, F. Numerical Simulation of Molten Slag Deposition in Radiant Syngas Cooler with a CFD-Based Model. J. Chem. Eng. Jpn. 2016, 49 (2), 69−78. (9) Fan, J. R.; Zha, X. D.; Sun, P.; Cen, K. F. Simulation of ash deposit in a pulverized coal-fired boiler. Fuel 2001, 80 (5), 645−654. (10) Min-min Zhou, B. J. I. J. Large-Eddy Simulation of Dynamic Ash Deposition in a Pulverized Coal Boiler. Presented at the National Combustion Meeting, College Park, MD, U.S.A., 2017. (11) Theis, M.; Skrifvars, B.; Zevenhoven, M.; Hupa, M.; Tran, H. Fouling tendency of ash resulting from burning mixtures of biofuels. Part 3. Influence of probe surface temperature. Fuel 2006, 85 (14−15), 2002−2011. (12) Zhou, H.; Zhou, B.; Qu, H.; Lin, A.; Cen, K. Experimental investigation of the growth of ash deposits with and without additives N


Development of online ash deposition thermogravimetric analyzer for

Recommend Documents