Laboratory Study on Adhesive Ash Deposition Characteristics of Ammonium Bisulfate in Conditions Simulating an Air Preheater for Hard Coal Combustion

Abstract

The ash blockage of the rotary air preheater is a serious problem of the coal-fired boiler that urgently needs to be solved, which is caused by the adhesive deposition of ammonium bisulfate (ABS) and the fly ash. A comprehensive experimental study was performed to investigate the adhesive ash deposition characteristics based on an experimental platform established. The influences of the gas temperature, the gas velocity, the mass ratio of the ABS to the fly ash (R), and the ash particle size on the ash deposition characteristics were mainly analyzed and discussed under different conditions. The experimental results indicate that the liquid ABS is the root cause of the ash particles adhering to the heat transfer elements of the air preheater. The experimental results indicate that when the gas temperature is in the range of 420–493 K, the ABS ash deposition intensity and the ABS adhesion rate both increase with the increase in the gas temperature. When it is 493 K, the ABS adhesion rates of the corrugated plate and the positioning plate both reach maximum values, which are 31.7% and 27.9%, respectively. With the decrease in gas velocity, the total ash deposition intensity, the ABS ash deposition intensity, the ABS adhesion rate, and the growth rate of the ABS adhesion all increase. The content of ABS in the fly ash is also an important factor. When R rises, the ash deposition intensity and the ABS adhesion rate increase significantly. The particle size of the fly ash has little influence on the total ash deposition intensity, but has a great influence on the ABS ash deposition intensity and the ABS adhesion rate. With the increase in the particle size in the range of 30.8–100 μm, the ABS ash deposition intensity decreases by nearly 50%, and the ABS adhesion rates of plates A and B decrease by about 43.9% and 49.6%, respectively. According to the study results, some effective measures can be taken to solve the ash blocking problem of the rotary air preheater, including using the steam air heater, optimizing the operation parameters of the soot blower, and inhibiting ABS formation.

1. Introduction

In China, all the coal-fired units with 300 MW and above are required to meet the need of the stringent ultra-low emission of nitrogen oxide (NO_x). The selective catalytic reduction (SCR) denitrification system has been widely used by most coal-fired power plants due to its high denitrification efficiency [1,2]. The principle of SCR denitrification is to convert NO_x in flue gas into non-toxic and harmless N₂ and H₂O using NH₃ or urea as a reductant with the help of the catalyst and within the suitable temperature zone (573–693 K) [3,4]. But, the issue of ammonia escape can occur because of catalyst aging, catalyst poisoning, temperature changes, or an excessive amount of ammonia injected into the SCR denitrification system [2,5]. A small portion of SO₂ in the flue gas can be oxidized to SO₃ by the catalyst during the process of the flue gas denitrification [6,7]; meanwhile, the generation of SO₃ can be promoted by NO and NH₃ in the flue gas [8]. The escaped ammonia easily reacts with SO₃ and H₂O in the flue gas to form ammonium bisulfate (ABS) and ammonium sulfate (AS) [9,10,11]. The liquid ABS is very viscous and corrosive, so it easily adheres to the fly ash, the catalysts, and the heat exchange surface [12,13,14]. But, AS is a dry powder that can be easily removed using soot blowers, and its effect is negligible [14]. The cold/intermediate and cold baskets of the rotary air preheater in a coal-fired boiler is usually in the temperature range of 420–493 K, where ABS in the flue gas is in a liquid state [15,16,17]. The sticky ABS adhered to the surface of the heat transfer elements can capture numerous ash particles in the flue gas and the blend deposition then grows gradually, which leads to the blockage and the corrosion of the air preheater [18,19]. It severely affects the safe and economic operation of the coal-fired boiler. The related study results show that when the ammonia escape concentration reaches 2 ppm and 3 ppm, the air preheater operating resistance increases by about 30% and 50% after half a year, respectively [20]. The ammonia escape from the SCR denitrification system is inevitable because the amount of ammonia injected is excessive to ensure that the NO_x concentration in the discharged flue gas reaches the national emission standard, which causes the formation of ABS [21]. Therefore, it is a pressing concern to solve the blockage and the corrosion resulting from the blend deposition of ABS and fly ash at the surface of the air preheater.

In recent years, many researchers have investigated the formation mechanism and the influence factors of the ash deposition of ABS on air preheaters. The ash deposition caused by ABS is a complex process that is influenced by many factors, such as the concentrations of NH₃ and SO₃, the ash particles, and the flue gas temperature [22]. Chen et al. [16] investigated the corrosion and the viscous ash deposition characteristics of the rotary air preheater in a 300 MW coal-fired power plant through experimental research and proposed the coupling formation model of the corrosion and the viscous ash deposition. Shi et al. [23] investigated the influences of the flue gas properties, including the temperature, the volume fraction of SO₃, the water vapor content, and the dust concentration on the ash particle accumulation, through an experimental study and found that ash accumulation is greatly affected by the flue gas temperature. Zhou et al. [24] investigated the blend deposition process of the fly ash and the ABS in a drop-tube furnace using an online digital image technique and obtained the blend deposition characteristics by means of some micro-characterization techniques. Their study results indicate that the deposition surface temperature has a negative effect on the blend deposition thickness and the blend deposition characteristics are greatly affected by the distribution and the condensation behavior of the ABS. Vuthaluru et al. [25] performed a mineralogical study on the deposits on the air preheater and found that the deposit formation is caused by the sticky sulfate with a high content. Meanwhile, they pointed out that the large fluctuation of the air preheater’s operation temperature is a significant factor of the deposit formation. Zhang et al. [2] investigated the blend deposition characteristics of the ABS and the fly ash using an in situ measurement technology and found that the blend deposit with high ABS content has a more compact micro-structure, a higher strength, and a greater effective thermal conductivity. Qing et al. [26] studied the formation and the deposition of ABS/(AS) without catalytic conditions based on a test bench simulating the air preheater, and characterized the products under different conditions, finding that the formation of ABS/AS is mainly affected by the concentration of SO₃ and NH₃, the ratio of SO₃ to NH₃, and the deposition temperature. The above research results promote our understanding of the blend deposition characteristics of the ABS and the fly ash.

Generally, there are no probes installed to monitor the deposition of the fly ash inside the rotary air preheater of the power station because the rotary air preheater is much too complex in structure and is completely sealed. Therefore, the experiment method is an important means of investigating the ash deposition characteristics. In this study, based on a small simulated air preheater, the deposition characteristics of ABS and fly ash on the surface of the heat transfer element are mainly investigated by discussing the influences of the gas temperature, the flue gas velocity, the value of R, and the ash particle size on the ash deposition characteristics.

2. Materials and Methods

In order to investigate the viscous ash deposition characteristics of ABS on the surface of the heat transfer elements more visually, a small simulated air preheater was designed and established in this study. The ash samples containing different mass shares of ABS were prepared via a manual method for the experimental study. The effects of the gas temperature, the gas velocity, and the fly ash particle size on the ash deposition characteristics on the surface of the heat transfer element were studied.

2.1. Experimental Materials

2.1.1. Preparation of the Experimental Ash Sample

There is a certain amount of ABS in the fly ash in the coal-fired boiler equipped with an SCR denitrification facility, but the content of ABS is difficult to be measured. Therefore, the fly ash collected from a 600 MW coal-fired power plant and the industrial ABS powder were mixed evenly according to the different mass ratios to obtain the experimental ash samples.

(1)

Collection and characteristics of the fly ash

In order to minimize the impact of ABS in the original ash sample on the experimental results, the fly ash in this study was collected from the dust precipitator of a 600 MW coal-fired boiler. The collected fly ash was analyzed via X-ray fluorescence spectroscopy (XRF) and its particle size distribution was tested. The results are shown in

Table 1. Tested results of the fly ash via XRF.

Oxides	Mass Fraction (%)
SiO2	46.41
Al2O3	43.23
CaO	3.64
Fe2O3	2.44
TiO2	1.00
MgO	0.62
K2O	0.28
P2O5	0.25
SO3	0.24
Na2O	0.06
MnO2	0.02

. It can be seen from Table 1 that the fly ash contains various oxides, among which SiO₂ and Al₂O₃ are the main components, their contents accounting for 46.41 wt. % and 43.23 wt.% of the total oxides, respectively, and the total content of other oxides is 10.36 wt. %. In addition, the particle sizes of the collected fly ash range from 6 μm to 300 μm, and the median particle size is 45 μm.

(2)

Mixing of the ABS and the fly ash

The ABS used in this study is a product of Analytical Reagent (AR) from a company. Its particle size is about 1 mm. In order to make the ABS and the fly ash particles be completely and evenly mixed, it is necessary to grind the large-sized ABS particles. The particles of ABS can be ground to less than 100 μm using a high-speed universal grinder, which is equivalent to the particle size of fly ash particles. In this study, the dimensionless parameter R is defined as the mass ratio of the ABS to the fly ash. When the boiler is running, the value of R in the flue gas at the outlet of the SCR equipment is about between 1/50 and 1/200 [27]. In order to obtain clearer experimental results, the experimental ash samples with R values of 1/25, 1/50, 1/100, 1/150, and 0 were prepared.

2.1.2. Experimental Devices

(1)

A small-sized experimental device simulating the rotary air preheater

A small-sized self-designed experimental device used for simulating the rotary air preheater was built to investigate the deposition characteristics of the ABS adhesive ash on the surface of the heat transfer element. The system schematic diagram of the small test system is shown in Figure 1. It mainly includes 8 components and some pipelines.

As shown in Figure 1, the hot air produced by the heat gun was fully mixed with the prepared experimental ash sample to form a simulated flue gas. The ash deposition test was performed on the surface of the heat transfer element in the rectangular closed steel shell. The simulated flue gas discharged from component 4 was introduced into the dust collector bag to capture the solid particles. The experiment was performed under a sealed condition; therefore, the mass balance of the solid matter (including the ash and ABS) can be kept balanced in the test system.

In this study, some important experimental devices are necessary to be introduced. The heat gun used to heat the cold air is a high-power digital display device with a power of 2800 W. Its temperature and wind speed are adjustable. The temperature range is from 333 K to 873 K, and the wind speed range is 5–10 L/s. A combination of a large-capacity hand-push syringe and a small funnel is used as the ash injector. The maximum capacity of the ash injector is 500 mL and its diameter is 5 mm. The thermometer used is a rapid thermometer equipped with a high-precision thermal sensor. The temperature measurement accuracy is 0.1 K, and the temperature measurement range is from 223 K to 573 K. The heat transfer elements constitute the key part of the air preheater. A piece of heat transfer element is composed of a piece of corrugated plate (marked as plate A) and a piece of positioning plate (marked as plate B). There is a certain gap between the corrugated plate and the positioning plate to form a flow channel for the flue gas. In fact, there is a large number of heat transfer elements installed in the rotary air preheater and the corrugated plates, and the positioning plates are alternately installed. In this study, in order to simplify the test device, a piece of heat transfer element with a flow channel is used, and it is placed horizontally in the rectangular closed steel shell with a length of 1000 mm, a width of 200 mm, and a height of 10 mm. The heat transfer element in this study is a CU-type, as shown in Figure 2. The ripple direction of plate A is 30° to the flow direction of flue gas, and the ripple height of plate A is about 5 mm. The ripple direction of plate B is parallel to the direction of flue gas flow, and the ripple height of plate B is about 8 mm. Figure 2a shows the schematic diagram of the CU-type heat transfer element of the rotary air preheater. Figure 2b,c show the physical photos of the corrugated plate and the positioning plate in the experiment.

(2)

Processing devices of the ABS and the fly ash

These experimental devices processing the ABS and the fly ash include an electric grinder, an 8411-type electric mechanical sieve shaker and an electronic scale. The electric grinder is a high-speed universal grinder with a power of 950 W and a rotational speed of 27,000 r/min, by which the particles can be crushed to less than 100 μm. The electric grinder is mainly used to grind the ABS and mix the ABS with the fly ash. The electric mechanical sieve shaker is equipped with different-sized round-hole standard-set sieves and is mainly used for the screening of the fly ash particles. Its vibration frequency is 1400 times per minute. The accuracy of the electronic scale is 0.1 g and its measuring range is 0–10 kg, which is mainly used for weighing the ABS particles, the fly ash particles, and the fly ash deposition.

2.2. Experimental Methods

2.2.1. Experimental Procedure

The experimental procedure mainly includes four steps: the preheating, the fly ash deposition, the post-heating, and the heat dissipation and the post-processing. Each experiment step spans about one hour. The specific experimental steps are as follows:

(1)

Pre-heating

Before the experiment, it is necessary to clean up the residual ash deposition on the surface of the heat transfer element. The heat-resistant tape is used to seal the small simulated air preheater to prevent the fly ash from leaking. Then, the small simulated air preheater is preheated for about 20 min using the heat gun. The temperature is set at the experimental temperature. During heating, the asbestos is used to cover the shell surface of the air preheater for heat preservation. The air temperature (T₁) at the inlet of the simulated air preheater and the air temperature (T₂) at the outlet of the simulated air preheater are measured using a thermometer. When the difference between T₁ and T₂ is less than 10 K, the temperature of the heat transfer element is considered to be constant and the prepared ash sample can be injected into the pipe.

(2)

Fly ash deposition

An experimental ash sample of 100 g prepared in advance is placed in the ash injector. At the beginning of the experiment, the funnel nozzle of the ash injector is placed vertically down into the ash injection port of the experimental device, and the experimental ash sample is injected into the high-temperature air at a uniform speed as much as possible until the fly ash of 100 g is completely injected, which takes about 60 s. After mixing the fly ash with the high-temperature air, the temperature quickly reaches the given experimental temperature, and the deposition of the ash sample occurs in the ash deposition section.

(3)

Post-heating

When the ash deposition experiment ends, the heat gun continues to heat for about 2 min at the experimental temperature. The purpose of this step is to completely liquefy the ABS in the fly ash, and make the liquefied ABS interact with the fly ash particles and adhere to the surface of the heat transfer element to form the ABS viscous ash deposition.

(4)

Heat dissipation and post-processing

When the experimental device temperature is cooled to the room temperature, the heat transfer element is taken out from the closed steel shell and its total mass is weighed. Then, the loose ash deposition on the surface of the heat transfer element is cleaned by the brush, and the total mass of ABS adhesive ash deposition and the heat transfer element is weighed. The ash deposited on the heat transfer element surface is collected and its particle size distribution is measured using the vibrating screen machine.

2.2.2. Methods of Data Processing

The mass of the heat transfer element with clean surface used in the experiment is defined as M₁. After the ash deposition experiment, the total mass of the ash deposition and the heat transfer element is defined as M₂. After cleaning up the loose ash deposition, the total mass of the ABS adhesive ash deposition and the heat transfer element is denoted as M₃. The surface area of plate A of the heat transfer element is same as that of plate B, which is recorded as S. Some important parameters, including the ash deposition intensity, the ABS adhesion rate, and the particle size distribution, are defined to analyze the experimental results.

(1)

Ash deposition intensity (O)

The parameter of ash deposition intensity represents the mass of the fly ash particles per unit area of the heat transfer element. The total ash deposition intensity (O_tot) and the ABS ash deposition intensity (O_ABS) can be calculated using the formulas below, respectively

$$O_{tot}=\frac{M_2-M_1}{S}$$

(1)

$$O_{ABS}=\frac{M_3-M_1}{S}$$

(2)

In Formulas (1) and (2), the unit of M₁, M₂, and M₃ is grams (g) and the unit of S is square meter (m²). Thus, the unit of O_tot and O_ABS is g/m².

(2)

ABS adhesion rate (I)

In this study, the ABS adhesion rate is defined as the ratio of the mass of ABS viscous ash deposition to the total ash mass deposited on the heat transfer element surface; the calculation formula is as follows:

$$I=\frac{M_3-M_1}{M_2-M_1}$$

(3)

It can be seen from Formula (3) that parameter I is a dimensionless parameter.

(3)

Particle size distribution

The test of the fly ash particle size distribution conduces an investigation of the adhesion mechanism between the ABS and the fly ash particles. In this study, the 8411-type electric sieve shaker is used to screen the particle size, which is equipped with the standard sieves of 150 mesh (100 μm), 200 mesh (76 μm), 300 mesh (54 μm), and 500 mesh (30.8 μm). Because the amount of the ash sample deposited in each experiment is less, the particle size distribution of the ash sample on the surfaces of plates A and B is measured after uniform mixing under the same experimental conditions. In this study, the ash sample of 80 g was taken and sieved for 15 min to achieve the best sieving effect.

3. Results and Discussion

In this study, the ash deposition characteristics on the surfaces of plates A and B of the heat transfer element were studied, respectively. The experimental study focused on the effects of the gas temperature, the gas velocity, the mass ratio of the ABS to the fly ash (R), and the fly ash particle size on the ash deposition characteristics. The experimental results under different conditions were analyzed and discussed. It is worth pointing out that due to the limitations of the experimental equipment and the experimental conditions, the experimental results are not necessarily completely consistent with the ash deposition characteristics on the heat transfer element surface of the air preheater in field, but they are useful for understanding the ash deposition characteristics on the surface of the heat transfer elements in field.

3.1. Effects of the Gas Temperature on the Ash Deposition Characteristics

3.1.1. Effect of the Gas Temperature on the Ash Deposition Intensity

In order to obtain better experimental results, the gas velocity was set at 8 m/s and the value of R was set as 1/25. The experiment with R equal to 0 (not containing ABS) was used to contrast with each other. The experimental results of the total ash deposition intensity and the ABS ash deposition intensity on the surfaces of plates A and B of the heat transfer element under different temperatures are listed in

Table 2. Ash deposition intensity on the surfaces of plates A and B of the heat transfer element under different temperatures.

	Plate A of the Heat Transfer Element			Plate B of the Heat Transfer Element
	R = 1/25		R = 0	R = 1/25		R = 0
Temperature (K)	Total Ash Deposition Intensity (Qtot, A) (g/m2)	ABS Ash Deposition Intensity (QABS, A) (g/m2)	Total Ash Deposition Intensity (Qtot, A) (g/m2)	Total Ash Deposition Intensity (Qtot, B) (g/m2)	ABS Ash Deposition Intensity (QABS, B) (g/m2)	ABS Ash Deposition Intensity (Qtot, B) (g/m2)
393	229.5	0.0	231.5	171.0	0.0	170.5
418	231.0	6.0	230.5	170.5	3.5	170.5
443	266.0	47.0	232.5	213.0	32.5	174.0
468	274.5	61.5	228.5	222.5	43.5	171.0
493	283.5	90.0	229.0	242.0	67.5	169.5
518	256.0	25.5	231.0	186.5	18.0	172.5
543	257.5	24.0	233.0	186.5	15.5	173.5

.

It can be seen from Table 2 that when the value of R is 1/25, the total ash deposition intensity and the ABS ash deposition intensity on the surfaces of plates A and B increase first and then decrease with the increase in the gas temperature. When the temperature is 393 K and 418 K, correspondingly, the total ash deposition intensity is obviously smaller than that at other temperatures and the value of O_ABS is equal to or close to zero. The reason is that when the gas temperature is less than 420 K, that is, the melting temperature of pure ABS [16,17], the ABS in the fly ash is still in a solid state and has no cohesiveness. Therefore, the ash deposition on the surface of the heat transfer element is loose. The study results from Menasha show that the formation temperature of ABS under the air preheater channel conditions is in the range of 500–520 K for the typical flue gas concentrations of ammonia and sulfur oxide species, and ABS forms predominantly as an aerosol in the gas phase rather than as a condensate on the channel walls [18]. ABS in the state of aerosol easily condensates on the surface of the heat transfer elements when the temperature drops. When the gas temperature is 493 K, that is, the liquefaction temperature of the ABS, almost all of ABS is in the liquid phase; hence, the total ash deposition intensity and the ABS ash deposition intensity both reach the maximum, and the values of O_ABS on the surfaces of plates A and B are 90.0 g/m² and 67.5 g/m², respectively. When the gas temperature is 518 K and 543 K, the total ash deposition intensity and the ABS ash deposition intensity on the surface of the heat transfer element both obviously reduce because some of ABS vaporizes into the gas state. However, the value of O_ABS does not decrease to zero, which indicates that there is a certain amount of liquid ABS in fly ash particles.

By comparing the values of ash deposition intensity in Table 2, when R is equal to zero, whether plate A or plate B, the total ash deposition intensity remains basically unchanged and the average total ash deposition intensities on the surfaces of plates A and B are, respectively, 230.9 g/m² and 171.6 g/m². At a gas temperature of 393 K and at a value of R equal to 1/25, the total ash deposition intensities on the surfaces of plates A and B are 229.5 g/m² and 171.0 g/m², respectively. It is clear that the relative errors of the total ash deposition intensity on the surface of plates A and B are both less than 1% under the abovementioned conditions. The experiment data indicate that when there is no ABS in the fly ash, or when the ABS has no adhesiveness, the ash deposition intensity on the surface of the heat transfer element is not affected by the gas temperature, which further indicates that the presence of liquid ABS has a great influence on the fly ash deposition on the surface of the heat transfer element.

In addition, we can see that the variation trends of the ash deposition intensity on the surfaces of plates A and B with temperature are consistent, according to the data in Table 2. Further, whether R is equal to 1/25 or R is equal to zero, the total ash deposition intensity and the ABS ash deposition intensity on the surface of plate A are significantly greater than those on plate B at the same gas temperature. This is mainly attributed to the oblique corrugated structure of plate A, which has a greater resistance to the fly ash flow. Moreover, the oblique corrugated structure of plate A is more complex than that of plate B, and the number of ripples is more than that of plate B. As a result, the surface of plate A is more likely to cause the ash deposition.

When the gas temperature is 493 K and the value of R is equal to 1/25, the experiment pictures of the total ash deposition and the ABS ash deposition on the surface of plates A and B are shown in Figure 3. As shown in Figure 3a,c, the surface of the heat transfer element is almost completely covered by the fly ash particles. There is more ash deposition at the concave valley of the corrugated plate due to gravity, while there is less fly ash deposition at the convex peak of the ripple plate. It can be seen clearly from Figure 3b,d that the fly ash deposition resulting from ABS mainly appears at the concave valley of the corrugated heat transfer element. The deposited ash particles caused by the liquid ABS have large particle sizes and can be bonded to the surface of the heat transfer element. This is because the liquid ABS has a strong viscosity and easily bonds the fly ash particles, forming ABS adhesive ash deposition, which can adhere to the surface of the heat transfer element and is difficult to be removed.

3.1.2. Effects of the Gas Temperature on the ABS Adhesion Rate

When the value of R is 1/25, the variation curves of the ABS adhesion rate on the surfaces of plates A and B with the temperature are shown in Figure 4. Evidently, with increasing temperature, the ABS adhesion rates of plates A and B increase first and then decrease. When the temperature is 393 K, the ABS adhesion rates of plates A and B are both zero. Correspondingly, the ABS is not sticky and cannot bond the fly ash particles to cause the ABS ash deposition. At a temperature of 493 K, the ABS adhesion rates of both plates A and B reach their peak, namely 31.7% and 27.9%, respectively. According to the aforementioned discussion, the temperature of 493 K is the liquefaction temperature of the ABS, and the existence of liquid ABS in the flue gas is the critical reason for the ABS viscous ash deposition, which can result in the severe clogging of the heat transfer elements of the air preheater. Therefore, when the air preheater is operating, the cold end temperature of the heat transfer elements should be kept below 493 K as much as possible, so as to reduce the ABS adhesion rate and alleviate the ABS clogging of the air preheater.

3.1.3. Effects of the Gas Temperature on the Particle Size Distribution of the Ash Deposition

In this study, the particle size distribution of the fly ash deposited on the surface of the heat transfer element at different temperatures was tested. The testing results are shown in Figure 5. When the gas temperature is outside the range of 420–493 K, the ash particle size is in the range of 30.8–54 μm, the mass fraction of which is the largest, accounting for the total ash deposition mass. When the gas temperature is in the range of 420–493 K, the particle size of the fly ash with a maximum mass fraction accounting for the total ash deposition mass is in the range of 54–76 μm. At a temperature of 493 K, the mass fraction reaches up to 43.8 wt. %. It indicates that as the ABS is gradually liquefied, the adhesion ability of the ash deposition layer becomes stronger and can adhere to the fly ash with larger particle sizes.

3.2. Effects of the Gas Velocity on the Ash Deposition Characteristics

3.2.1. Effects of the Gas Velocity on the Ash Deposition Intensity

The effects of four gas velocities on the ash deposition characteristics have been studied when the gas temperature is at 493 K and the value of R is 1/25, as shown in

Table 3. Ash deposition intensity on the surface of plates A and B of the heat transfer element under different gas velocities.

	Plate A of the Heat Transfer Element		Plate B of the Heat Transfer Element
Gas Velocity (m/s)	Total Ash Deposition Intensity (Qtot, A) (g/m2)	ABS Ash Deposition Intensity (QABS, A) (g/m2)	Total Ash Deposition Intensity (Qtot, B) (g/m2)	ABS Ash Deposition Intensity (QABS, B) (g/m2)
5	340.0	132.5	317.5	116.0
6	311.0	108.0	288.0	94.5
7	294.5	96.0	260.5	78.0
8	283.5	90.0	242.0	67.5

.

We can see from Table 3 that as the gas velocity decreases from 8 m/s to 5 m/s, the total ash deposition intensity and the ABS ash deposition intensity on the surfaces of plates A and B of the heat transfer element both gradually increase. For plate A, the total ash deposition intensity and the ABS ash deposition intensity, respectively, increase by 19.9% and 47.2%, while for plate B, 31.2% and 71.9%, respectively. The experiment results indicate that the low gas velocity tends to cause the ash deposition, and the growth rate of the ABS ash deposition intensity is greater than that of the total ash deposition intensity with the decrease in the gas velocity. At the same time, the total ash deposition intensity also affects the ABS ash deposition intensity. This is because the lower the gas velocity, the longer the residence time of the fly ash particles in the channel, and the fly ash particles then have a greater possibility of deposition on the surface of the heat transfer elements, which makes the total ash deposition intensity larger. The longer residence time of the fly ash particles and the larger total ash deposition intensity contribute to the liquefaction of ABS in the fly ash and further increase the ABS ash deposition intensity. Overall, at different gas velocities, the ash deposition intensity on the surface of plate A is greater than that on plate B, but its growth rate of the ash deposition intensity is less than that of B plate.

3.2.2. Effects of the Gas Velocity on the ABS Adhesion Rate

The variation curves of the ABS adhesion rate on the surfaces of plates A and B under different gas velocities are shown in Figure 6. From the curves, we can see that the ABS adhesion rates on the surfaces of plates A and B both increase with decreasing gas velocity, increasing by 18.7% and 23.4%, respectively. It indicates that the gas velocity has a great influence on the ABS adhesion rate. When the gas velocity drops, the possibility of the ABS liquefaction is greater, so the ABS adhesion rate on the surface of the heat transfer element is also greater. Furthermore, the ABS adhesion rate on the surface of plate A is slightly larger than that of plate B at the same gas velocity.

3.2.3. Effects of the Gas Velocity on the Particle Size Distribution of the Ash Deposition

The particle size distribution of the fly ash deposited on the surface of the heat transfer element was tested at different gas velocities. The testing results are indicated in Figure 7. When the gas velocity is 7 m/s and 8 m/s, the mass fraction of the ash with the particle size in the range of 54–76 μm is the largest. When the gas velocity is 5 m/s and 6 m/s, the particle size of the fly ash with the maximum mass fraction is in the range of 76–100 μm. Significantly, when the gas velocity is 5 m/s, the mass fraction of the fly ash with a particle size less than 30.8 μm is close to 0. It shows that with the decrease in gas velocity, the particle size of the fly ash deposited will become larger as a whole. The reason is that when the ABS ash deposition intensity increases, the ABS adhesion rate is enhanced, and the fly ash particles agglomerate.

3.3. Effects of the Mass Ratio of the ABS to the Fly Ash (R) on the Ash Deposition Characteristics

When discussing the effects of the value of R on the ash deposition characteristics, the gas temperature and the gas velocity were separately set as 493 K and 8 m/s, respectively. Under the conditions, the values of R were set as 1/25, 1/50, 1/100, 1/150, and 0, respectively.

3.3.1. Effects of R on the Ash Deposition Intensity

The experiment results of the total ash deposition intensity and the ABS ash deposition intensity on the surfaces of plates A and B are listed in

Table 4. Ash deposition intensity on the surfaces of plates A and B of the heat transfer element under different values of R.

	Plate A of the Heat Transfer Element		Plate B of the Heat Transfer Element
Value of R	Total Ash Deposition Intensity (Qtot, A) (g/m2)	ABS Ash Deposition Intensity (QABS, A) (g/m2)	Total Ash Deposition Intensity (Qtot, B) (g/m2)	ABS Ash Deposition Intensity (QABS, B) (g/m2)
1/25	283.5	90.0	242.0	67.5
1/50	279.0	58.0	221.5	41.5
1/100	257.0	31.5	182.0	23.0
1/150	236.5	14.0	174.0	6.0
0	229.0	0.0	169.5	0.0

under different values of R. When R is reduced, the amount of adhesive ash particles decreases at the liquefaction temperature of 493 K. Therefore, as shown in Table 4, Q_tot and Q_ABS on the surfaces of plates A and B both significantly reduce with a decrease in R. When R is equal to 0, Q_tot and Q_ABS both are equal to 0. Furthermore, it can also be seen from Table 4 that Q_tot and Q_ABS on the surface of plate A are greater than those on plate B when R is fixed.

3.3.2. Effects of R on the ABS Adhesion Rate

The effects of R on the ABS adhesion rate are illustrated in Figure 8. It can be seen from Figure 8 that the ABS adhesion rate on the surfaces of plates A and B decreases with decreasing R. When R is constant, the ABS adhesion rate on the surface of plate A is basically greater than that on plate B, just as Q_tot and Q_ABS on the surface of plate A is greater than that on plate B. It indicates that the ash deposition intensity can affect the ABS adhesion rate. The greater the ash deposition intensity, the greater the ABS adhesion rate accordingly. Because the liquid ABS in the fly ash is directly related to the ABS adhesion rate and leads to the ABS ash deposition, some measures can be taken to prevent the generation of liquid ABS from the source in order to solve the air preheater clogging.

3.3.3. Effects of R on the Particle Size Distribution of the Ash Deposition

The test results of the particle size distribution of the deposited fly ash are illustrated in Figure 9. It can be seen that when R is equal to 1/25 and 1/50, respectively, the fly ash with the particle size from 54 μm to 76 μm is easily deposited on the surface of the heat transfer element, which accounts for a maximum mass fraction of the total ash deposition mass. When R is equal to 1/100, 1/150, and 0 separately, the particle size of the fly ash deposited is in the range of 30.8 μm to 54 μm, occupying the largest mass fraction of the total ash deposition. It is observed that with the increase in R, the particle size of the deposited fly ash obviously becomes larger. It indicates that the adhesive attraction for the ash particles can be enhanced with the increase in R. As a result, many larger fly ash particles adhere to the surface of the heat transfer element. Therefore, the ABS content in the fly ash is another important factor causing the clogging of the air preheater.

3.4. Effects of the Fly Ash Particle Size on the Ash Deposition Characteristics

3.4.1. Effects of the Fly Ash Particle Size on the Ash Deposition Intensity

When studying the effects of the fly ash particle size on the ash deposition characteristics, the ash sample with the value of R equal to 1/25 had been sieved using the electric mechanical sieve shaker before the experiment. The particle size of the fly ash after sieving was divided into five ranges, including less than 30.8 μm, from 30.8 μm to 54 μm, from 54 μm to 76 μm, from 76 μm to 100 μm, and greater than 100 μm, respectively. The gas temperature and the gas velocity were set at 493 K and 8 m/s, respectively. The experimental results of the total ash deposition intensity and the ABS ash deposition intensity on the surface of the heat transfer element are listed in

Table 5. Ash deposition intensity on the surfaces of plates A and B of the heat transfer element under different particle size ranges.

	Plate A of the Heat Transfer Element		Plate B of the Heat Transfer Element
Particle Size Range (μm)	Total Ash Deposition Intensity (Qtot, A) (g/m2)	ABS Ash Deposition Intensity (QABS, A) (g/m2)	Total ash Deposition Intensity (Qtot, B) (g/m2)	ABS Ash Deposition Intensity (QABS, B) (g/m2)
<30.8	286.5	162.0	244.5	135.5
30.8–54	288.0	121.5	241.5	98.0
54–76	281.0	87.5	243.0	69.0
76–100	282.0	66.5	239.0	49.0
>100	279.5	56.0	238.5	36.5
Full particle size	283.5	90.0	242.0	67.5

under different particle size ranges.

It can be seen from Table 5 that with the increase in the fly ash particle size, the total ash deposition intensity on the surfaces of plates A and B generally decreases, and the reduction is less than 5%. However, the ABS ash deposition intensity is reduced by nearly 50%. It indicates that the particle size of the fly ash has little influence on the total ash deposition intensity, but has a great influence on the ABS ash deposition intensity under certain other conditions. The reason is that the contact area among the particles gradually decreases with increasing particle size, which results in the reduction in the ABS ash deposition intensity. Additionally, as shown in Table 5, the total ash deposition intensity and the ABS ash deposition intensity under the condition of full particle size are both close to those under the condition of particle sizes from 54 μm to 76 μm.

3.4.2. Effects of the Fly Ash Particle Size on the ABS Adhesion Rate

The effects of the fly ash particle size on the ABS adhesion rate (I) is illustrated in Figure 10. It can be seen that with the increase in the fly ash particle size, I_A and I_B obviously decrease. When the particle size is less than 30.8 μm, I_A and I_B are separately 56.5% and 55.4%. However, when the particle size is larger than 100 μm, I_A and I_B drop to 20.0% and 15.3%, respectively. In the range of 30.8–100 μm, the ABS adhesion rates of plates A and B decrease by about 43.9% and 49.6%, respectively, with an increase in particle size. This is because the ash particles with smaller particle sizes have larger specific surface areas and a stronger aggregation effect. As a result, the fine ash particles are easily adhered to the surface of the heat transfer element.

4. Conclusions

In this study, a small-scale experimental device simulating the heat exchanger elements of the rotary air preheater was designed and built, and the experimental ash samples with different contents of ABS were prepared. Taking the CU-type heat transfer element, including the corrugated plate and the positioning plate, as the study object, the effects of the gas temperature, the gas velocity, the mass ratio of the ABS to the fly ash, and the fly ash particle size on the ash deposition characteristics of the heat transfer element surface were studied. Our conclusions are summarized as follows:

(1)

The gas temperature is an important factor. When the gas temperature is in the range of 420–493 K, with an increase in the gas temperature, the ABS ash deposition intensity, the ABS adhesion rate, and the mass fraction of the fly ash with the median particle size in the ash deposition all gradually increase. When the gas temperature is 493 K, the ABS adhesion rates of the corrugated plate and the positioning plate both reach their maximum values, which are 31.7% and 27.9%, respectively. When the gas temperature is lower than 420 K or higher than 493 K, the ABS adhesion rate is less than 10%. At the same temperature, the ABS ash deposition intensity of the corrugated plate is 1.3 times that of the positioning plate. Generally, the outlet temperature of the air preheater is lower than 423 K and its intermediate/cold and cold baskets are in the liquefaction temperature zone of ABS. This is the reason that the ABS adhesion ash deposition mainly appears in the intermediate/cold and cold baskets of the air preheater. Therefore, one of the feasible measures is to increase the wall temperature of the intermediate/cold and cold baskets of the air preheater using the steam air heater. The cold air before entering the air preheater is first preheated by the low-pressure bled-steam from the steam turbine in the steam air heater, but the boiler efficiency will decrease to a certain extent due to the increase in exhaust temperature.

(2)

The gas velocity is another important factor. With the decrease in the gas velocity, the total ash deposition intensity, the ABS ash deposition intensity, the ABS adhesion rate, and the growth rate of the ABS adhesion rate all increase, and the total ash deposition intensity on the surface of the heat transfer elements can promote the ABS adhesion rate. It is evident that the ABS adhesion ash deposition of the air preheater may tend to be serious due to the decrease in the flue gas velocity when the boiler load is reduced. Hence, the operation parameters of the soot blower need to be adjusted accordingly, including the time, the frequency, and the velocity of the ash blowing.

(3)

The ABS content in the fly ash has a great influence on the ABS adhesion rate. With an increase in R, the ABS adhesion rate increases significantly. The greater the ABS adhesion rate, the easier it is for the fly ash particles to agglomerate and the larger the particle size of the ash deposition. Therefore, the amount of ammonia escape from the SCR denitrification facility must be strictly controlled to reduce the formation of the ABS. The constant changes of the boiler load can cause an uneven concentration distribution of NOx in the economizer outlet [21]. Excessive ammonia is usually injected into the SCR denitrification facility for environmental protection [28,29]. As a result, the ammonia escape increases sharply. Therefore, it is recommended to implement an accurate partition ammonia injection strategy to match the NH₃ concentration well with the NOx concentration to reduce ammonia escape. Meanwhile, it is an effective method to keep the good activity of the catalyst in the SCR denitrification facility to promote denitration reaction by controlling the suitable flue gas temperature, replacing the catalyst regularly, etc. Additionally, the formation of sulfur trioxide (SO₃) can be reduced by means of the desulfurization of raw coal before burning and the low oxygen combustion technology in furnace in order to decrease the generation of ABS.

(4)

The particle size of the fly ash also has an obvious effect on the ABS ash deposition intensity and the ABS adhesion rate. With an increase in the particle size, the reduction in the total ash deposition intensity is less than 5%, but the reduction in the ABS ash deposition intensity is nearly 50%, and the ABS adhesion rate decreases significantly. When the fly ash particle size is less than 30.8 μm, the ABS adhesion rate is close to 60%. When the boiler load or the coal quality changes, the particle sizes of the pulverized coal and the fly ash all change. Under the condition of ensuring complete combustion, the larger particle sizes of the pulverized coal can be used. It not only reduces the power consumption for coal pulverization, but also helps to reduce ABS adhesion ash deposition.