Mercury emission and adsorption characteristics of fly ash in PC and CFB boilers

Abstract

The mercury emission was obtained by measuring the mercury contents in flue gas and solid samples in pulverized coal (PC) and circulating fluidized bed (CFB) utility boilers. The relationship was obtained between the mercury emission and adsorption characteristics of fly ash. The parameters included unburned carbon content, particle size, and pore structure of fly ash. The results showed that the majority of mercury released to the atmosphere with the flue gas in PC boiler, while the mercury was enriched in fly ash and captured by the precipitator in CFB boiler. The coal factor was proposed to characterize the impact of coal property on mercury emissions in this paper. As the coal factor increased, the mercury emission to the atmosphere decreased. It was also found that the mercury content of fly ash in the CFB boiler was ten times higher than that in the PC boiler. As the unburned carbon content increased, the mercury adsorbed increased. The capacity of adsorbing mercury by fly ash was directly related to the particle size. The particle size corresponding to the highest content of mercury, which was about 560 ng/g, appeared in the range from 77.5 to 106 µm. The content of mesoporous (4–6 nm) of the fly ash in the particle size of 77.5–106 µm was the highest, which was beneficial to adsorbing the mercury. The specific surface area played a more significant role than specific pore volume in the mercury adsorption process.

Keywords

mercury / combustion modes / coal property / fly ash / particle size

Introduction

Mercury is another pollutant that people have paid much attention to following SO_x and NO_x. There are two major sources of mercury in the environment, natural and man-made release, of which the latter accounts for one third of the total. Ito et al. [¹] reported that the total mercury emissions from coal-fired boilers in Japan was 0.63 t annually according to the mercury content in coal which was 0.045 mg/g, 10.3 t in South Korea [²] and 21.2 t in Poland [³]. Annually, the mercury emission is more than one fourth of the global amount in China [⁴]. The primary energy of China is coal. In 2013, the installed capacity of thermal power plants took up 69% in the total capacity [⁵]. When coals burn in the boilers, the mercury contained in coals will be released into the environment, accounting for 50% of the total mercury emission in China [⁶]. Therefore, the mercury emission from coal burning is a major source in China [⁷].

In recent years, the principle of mercury emission in the process of coal burning coal-fired power plants was studied by many researchers. The results showed that the mercury emission in coal burning was related to both the combustion modes and the coal property [⁸,⁹]. Wu et al. [¹⁰] found that the mercury in flue gas accounted for 79.4%, while the mercury content in slag was less in a 410 t/h pulverized coal (PC) boiler. Lu et al. [¹¹] indicated that 48.6% of mercury existed in flue gas in the PC boiler with the lignite, whose capacity was 860 MW. Zhu et al. [¹²], Guo et al. [¹³] and Zhou et al. [¹⁴] studied respectively the principle of mercury emission in different PC boilers with a capacity of 300 MW. The results showed that the gaseous mercury accounted for 68.3%, 83.0%, and 86.9%. A small part of mercury was adsorbed by slag and fly ash, which were respectively 19.4%, 14.0%, and 13.1%. For a PC boiler with a capacity of 600 MW, Zhou et al. [¹⁵] obtained the similar results. Besides, Zhou et al. [¹⁶] also found that most of the mercury was adsorbed by fly ash in a 35 t/h circulating fluidized bed (CFB) boiler. Wu et al. [¹⁷] studied mercury emission in a 135 MW CFB boiler and acquired the emission rule. Most of the mercury was adsorbed by fly ash and then was captured by the precipitators, accounting for 83.37%. In slag, the mercury content was only 0.55%, and the remaining mercury (16.08%) was emitted into the environment along with flue gas.

Some researchers indicated that the contents of chlorine, sulfur, and ash were important factors which influenced the mercury emission from coal-fired boilers. These factors could influence the distribution of solid products by changing the mercury forms [¹⁸–²¹].

As there is no special mercury emission control equipment, researches on the control of mercury emission in flue gas of coal-fired power plants mainly focused on removal before combustion, improvement of existing pollution control equipment, and comprehensive emission control of pollutants [²²]. Currently, the method of adsorbent injection in combination with existing typical air pollution control equipment such as electrostatic precipitator (ESP) and fabric filter (FF) may become a promising technology for mercury emission control in coal-fired flue gas [²³]. Fly ash, as an efficient, low-cost mercury adsorbent, was gradually paid attention to. The study on mercury adsorption characteristics of fly ash in utility boilers is the key issue for removing mercury emissions at this stage.

There are many factors affecting the mercury adsorption of fly ash in coal-fired boilers. Yang et al. [²⁴] found that the content and reactivity of unburned carbon in fly ash were the main factors influencing mercury enrichment coefficient. Jiang et al. [²⁵] studied the fly ash samples collected from the hopper of each electric field from an ESP in a 600 MW PC boiler. It was found that the unburned carbon content and mercury enriched content in fly ash were positively correlated. Huang and Luo [²⁶] studied three different particle sizes. It was found that the size of fly ash had some impact on the mercury transformation. The conversion ratio of Hg²⁺ to total Hg increased with increasing the size of fly ash sample. Besides, as the fly ash particle size decreased, the specific surface area increased, which was more conducive to the adsorption of mercury. However, the pore structures of fly ash with different particle sizes was not analyzed, and the particle size ranges selected were limited.

In summary, PC boilers and CFB boilers are two major combustion modes in coal-fired power plants. Although some studies have been conducted on mercury emission in coal-fired power plants, the conclusions obtained are quite different because of lack of comprehensive analysis. Therefore, the combustion modes and the factors of coal characteristics will be studied to understand the principle of mercury emission in this paper. In addition, mercury adsorption is related to the characteristics of fly ash which is a product of combustion in boilers. Since the study on the effects of particle size on the mercury adsorption properties of fly ash is especially less, the related mechanism is inadequate. Based on the comprehensive study of the effects of particle size, carbon content and pore structure on the mercury adsorption characteristics of fly ash, the corresponding reaction mechanism is systematically explored. It would provide theoretical basis for the development of future mercury removal method.

Research object and analysis method

Research object

Five coal-fired boilers were selected as the research objects in this paper. Table 1 lists the related parameters of the boilers. The combustion modes included PC boilers and CFB boilers which were equipped with ESP or FF, wet flue gas desulfurization (WFGD), and selective catalytic reduction (SCR) devices. The boiler capacity ranged from 135 MW to 600 MW. The coals included anthracite, bitumite, and lignite.

The fly ash samples in Nos. 1 and 2 PC boilers and Nos. 4 and 5 CFB boilers were taken from the 1st electric field of ESP. In each condition, the corresponding samples were collected three times, 1 kg each time. The measured mercury content of fly ash was determined by the average value obtained in three parallel experiments. In addition, the fly ash samples were screened using a crusher and a vibrating machine via quartering method in advance, to obtain the samples with eight different particle sizes, i.e.,<48 µm, 48–77.5 µm, 77.5–90 µm, 90–106 µm, 106–120 µm, 120–180 µm, 180–325 µm and>325 µm. The weights of the samples were calculated by an electronic balance to obtain the mass fractions of fly ash samples within different particle size ranges.

Analysis method

The components of the coal are very complex. Besides the common elements, there are some heavy metal elements in coals. Mercury is one of them, whose content is very low. According to the possible transition and transformation process of mercury in the boilers, the solid samples include coal, limestone, slag, fly ash, and gypsum. In the balance computation of the total mercury, the input of mercury in the system includes the mercury brought by the coal and the limestone, while the output includes slag, fly ash, gypsum and flue gas. The mercury in flue gas was measured by Portable Mercury Sampling 30B (PMS 30B) analyzer. The mercury contained in waste water of the desulfurization system was not considered due to the fact that the water recycles in the whole system were not discharged. Therefore, liquid samples were not collected. As the mercury balance calculation could be affected by some errors, the mercury balance rate in the range of 70%–130% meant that the experimental results were accurate. The results obtained all met the relevant requirements in this paper.

The coal samples were obtained at the coal feeding belt, the slag samples at the slag fetching device, the fly ash samples at the precipitators, and gypsum samples at the dehydration belt. The flue gas samples were obtained from the duct, after desulfurization equipment and in front of the chimney.

A Lumex mercury analyzer was adopted to analyze the samples. The unburned carbon in fly ash was measured by using a muffle furnace. The adsorption and desorption isotherms of fly ash samples were obtained in N₂ adsorption and desorption experiments using an ASAP 2460 analyzer. The specific surface area was obtained by using the BET equation, and the pore structure parameters of samples were obtained by using the Barrett-Joyner-Halenda (BJH) method.

Influence of combustion modes and coal property on mercury emission

Influence of combustion modes on mercury emission

The mercury emission characteristics were studied in the boilers in different combustion modes at full loads. The coal samples were named as M1, M2, M3, M4, and M5 respectively. Table 2 summarizes the proximate and ultimate analysis.

The mercury mass flow in and out of the system can be calculated with the mercury content in the sample and its mass flow. Therefore, the mercury emission characteristics in various combustion modes were tabulated in Table 3. The demercuration efficiency referred to the percentage of the mercury absorbed in solid product.

Thus, according to the percentage of mercury content in each solid product and flue gas, the mercury distribution of three PC boilers were obtained and depicted in Fig. 1. It could be seen that the most mercury was gaseous in the flue gas in the PC boilers, whose contents accounted for 86.52%, 55.79%, and 60.70% respectively. The mercury contents in gypsum were 9.57%, 38.38%, and 29.03% respectively. Only a small part of mercury was absorbed by the fly ash, which was 3.10%, 5.80%, and 10.30% respectively. The mercury in the slag was less, accounting for only 0.90%, 0.10%, and 0.01% respectively.

Figure 2 displays the mercury distribution in the CFB boilers. It could be seen that most of the mercury was absorbed by the fly ash and captured by the precipitators, which accounted for 83.71% and 81.78%. The mercury contents in the slag were 0.02% and 0.09%. The remaining mercury was gaseous in the flue gas, which was 16.27% and 18.16% respectively.

The results indicated the influence of combustion modes on the mercury emission characteristics. CFB boilers could inhibit the emission of mercury, but PC boilers could not. Compared with PC boilers, the combustion efficiency and combustion temperature in CFB boilers are lower. Therefore, the fly ash properties (such as average pore size, BET specific surface area, accumulated pore volume, and accumulated pore surface area) were different in the two combustion modes, which might influence the mercury absorption ability. Besides, the fly ash in CFB boilers did not experience the high temperature melting process due to its lower combustion temperature. Therefore, the content of unburned carbon in the fly ash was higher than that in PC boilers. The fly ash particles with a higher carbon content had a higher porosity and a larger specific surface area. As a result, its absorption ability to mercury in the flue gas was higher. The mercury absorbed by fly ash was removed by the precipitators.

Influence of coal property on mercury emission

The results obtained by other researchers show that the contents of chlorine, sulfur, ash, and mercury in coal are important factors influencing the mercury emission in coal-fired power plants. These factors can influence the mercury forms and then lead to the different distribution in solid products. The higher ash content in the coal results in the increased concentration of fly ash and the enhanced absorption to gaseous mercury. The chlorine element promotes the transformation of elementary mercury to oxidized mercury. The oxygen in the flue gas can also promote the oxidation of mercury. However, it is not as significant as the chlorine. From the perspective of chemical thermodynamics, with the flue gas temperature increasing, it is harder for gaseous mercury to be absorbed by particles. So the flue gas temperature may seriously hinder the mercury absorption in fly ash and the elementary mercury oxidation by chlorine. The sulfur in the coal restricts the oxidation of the elementary mercury and the formation of chlorine mercury, which has a negative influence on the removal of mercury [²⁷–²⁹].

According to the relativity among the content of chlorine, sulfur and ash to the mercury forms, the coal factor was put forward to quantitatively represent the influence of relevant elements on the solid product distribution of mercury. It could be rectified by the flue gas temperature and oxygen content. The coal factor, C_K, can be calculated by using Eq. (1).

(1){\mathit{C}}_{\mathrm{K}}\mathrm{=}{\displaystyle \frac{{\mathrm{Cl}}_{\mathrm{red}}\times {\mathrm{A}}_{\mathrm{red}}}{{\mathrm{S}}_{\mathrm{red}}}}\cdot {\displaystyle \frac{{\epsilon }_{\mathrm{1}}}{{\epsilon }_{\mathrm{2}}}}\times \mathrm{1000},

where Cl_red, A_red, and S_red are the reduced chlorine, ash, and sulfur in the coal; and e₁ is the correction factor considering the influence of oxygen content in the flue gas on the research results, which could be calculated by using Eq. (2).

(2){\epsilon }_{\mathrm{1}}=C_{{\mathrm{O}}_{\mathrm{2}}}/\mathrm{21},

where C_O2 is the oxygen content in the flue gas, and e₂ is the correction factor considering the influence of the flue gas temperature, which could be calculated by using Eq. (3).

(3){\epsilon }_{\mathrm{2}}=\mathrm{}{T}_{\mathrm{1}}/\mathrm{8}\mathrm{0},

where T₁ is the flue gas temperature. Since the flue gas temperature in the chimney after wet desulphurization was around 80°C, the referential temperature was chosen as 80°C.

Influence of coal property in PC boilers on mercury emission

By means of the coal factor, the mercury emission characteristics were studied in Nos. 1, 2 and 3 PC boilers. In order to ensure the preciseness of the results, four different coals and corresponding fly ash samples were collected from No. 1 boiler. The serial numbers of the coal samples were M6–M9 respectively. Besides, M10 and M11 were collected respectively from Nos. 2 and 3 PC boilers. The serial numbers of the coal were M10 and M11 respectively. The relevant coal information was given in Table 4, and Fig. 3 shows the results of the coal factor in PC boilers.

From Table 4, it could be seen that the dry basis reduced ash content was comparatively higher in M10 and M11. The ash content of the other coals was lower, which was just around 14%. The contents of S and Cl were the highest in M10 and M11. The Hg content in M6–M9 was similar to each other, which was about 130 ng/g. The Hg content in M10 was 221.65 ng/g which was the highest.

Just as shown in Fig. 3, with the coal factor (C_K) increasing, the demercuration efficiency increased. The ratio of sulfur to chlorine was relatively higher in M6, but the higher ratio of sulfur to chlorine could inhibit the forming of mercuric oxide, so that a little mercury was removed in this operation condition. The ash contents of M10 and M11 were relatively higher. If the ash contents were relatively higher, more mercury could be adsorbed. Therefore, the mercury content was comparatively higher in the fly ash.

Influence of coal property in CFB boilers on mercury emission

In this paper, CFB boilers were named Nos. 4 and 5. The influence of coal property on mercury emission was obtained from No. 4 boiler. The serial numbers of the coal type selected were M12–M14 respectively. The influence of coal property on mercury emission was also studied in No. 5 boiler in a certain operation condition. The serial number of the coal type selected was M15. The relevant coal type information was presented in Table 5. Figure 4 shows the results calculated.

From Table 5, it could be seen that reduced Hg contents were closer in M12–M14. The Hg and ash contents were the lowest in M15. Just as shown in Fig. 4, with the coal factor (C_K) increasing, the demercuration efficiency increased. It also could be seen from Table 5 that the ash content and chlorine content in M14 and M15 were comparatively lower, which could not promote mercury adsorbing. Therefore, less mercury was adsorbed and lager amount of mercury was released in the gaseous form in flue gas in their corresponding operation condition. In addition, it also could be seen that the lower ash content was not favorable for the mercury absorption, e.g., M15.

Mercury adsorption of fly ash

Influence of carbon content of fly ash on mercury adsorption

The fly ash samples in No. 1 boiler were obtained under different operating conditions, whose unit loads were 60%, 80%, 90%, and 100%, and marked as F1–F4 respectively. The samples in No. 2 boiler were obtained under unit loads of 53.83%, 56.67%, 68.33%, 80.83%, 83.33%, 90.83%, 95.83%, and 100%, and marked as F5–F12 respectively. The samples in No. 4 boiler were obtained under unit loads of 60%, 80%, and 100%, and marked as F13–F15 respectively. The samples in No. 5 boiler were obtained under unit loads of 60%, 80%, and 100%, and marked as F16–F18 respectively. The carbon content and mercury content of the fly ash samples were tabulated in Table 6.

It was found that the unburned carbon contents of fly ash in four boilers were different, caused by different kinds of coal, combustion conditions, and operation conditions. However, the carbon and mercury content of the fly ash samples in different conditions had the same trend, which was similar to the results in Ref. [³⁰]. This indicated that unburned carbon in fly ash could affect the mercury adsorption capacity of fly ash. The reason for this is that the unburned carbon provide larger pore specific surface area and richer pore structure which increase the physical mercury absorption [³¹]. Besides, unburned carbon could provide more reactive areas [³²,³³] which promote the chemical mercury adsorption. In other words, the nitrogen and oxygen-containing functional groups on the unburned carbon surface catalyzed the oxidation of elemental mercury in flue gas, and then the mercury was adsorbed on the surface of fly ash particles [³⁴,³⁵]. In addition, the mercury content of coals used in No. 1, No. 2, No. 4 and No. 5 boilers were 122.7 ng/g, 221.7 ng/g, 235.3 ng/g, and 103.0 ng/g respectively. The mercury contents of the fly ash in CFB boilers were much higher than those in PC boilers. The reason for this is that the particle diameter of coal in PC is smaller, the combustion efficiency is higher, and the combustion temperature is simultaneously higher, according to the results obtained above. In contrast, the combustion efficiency and combustion temperature were lower in CFB boilers. Due to the differences between the two combustion modes, the characteristics of the fly ash were different, leading to the different mercury adsorption capacities.

Influence of pore structure of fly ash on mercury adsorption

The pore structure parameters affecting the mercury adsorption characteristics of fly ash included specific surface area, accumulated pore volume, and relative specific pore volume [³⁶]. The low temperature N₂ adsorption/desorption experiments were conducted using the F5, F10, and F11 samples in No. 2 PC boiler to further investigate the mercury adsorption characteristics of fly ash. The experiments mainly include adsorption and desorption. The adsorption/desorption isotherms, specific surface area, pore volume and pore size distribution of samples were obtained. The effect on the mercury adsorption capacity was studied from the perspective of the pore structure of fly ash. The adsorption/desorption isotherm of F5, F10, and F11 samples were shown in Fig. 5. The three isotherms curves were morphologically different, but they basically belonged to class III isotherm, which was the anti-Langmuir curve. Besides, the adsorption and desorption curves appeared to be H3 hysteresis loop. This indicated that the pores of the three fly ash samples were slit pores formed by sheet shaped particles [³⁷]. The capillary agglomeration occurred mainly in the slits between two parallel faces of the slit hole. The curves of three isotherms were comparatively consistent. When the relative pressure was between 0.01 and 0.4, the voids between the particles were sequentially filled. When the relative pressure approached 0.4–0.45, capillary condensation occurred in some meso-pores, and the quantity adsorbed slightly increased. The pore structures of fly ash were developing continuously with the formation of many new meso-pores and macro-pores. When the relative pressure increased to 0.95–1, the capillary condensation occurred in a large number of meso-pores and macro-pores, resulting in the fact that the quantity adsorbed increased rapidly. A sharp upward curve appeared without saturated adsorption state. This indicated that there existed a large number of pores on the surface of the sample, and the pore structure showed diversity. It also could be seen that the three samples had different pore distributions due to the morphological differences of the isothermal. In comparison, the separation degree of adsorption/desorption curves of F10 sample was higher, indicating that there were a large number of meso-pores and macro-pores.

The differential and accumulated pore volume of samples were shown in Fig. 6. The curves were calculated by using the BJH method. It could be seen that all samples had a wide pore distribution and similar pore distribution curves, which were consistent with the results obtained from adsorption/desorption curves. A peak value of differential pore volume of all samples appeared at about 4–6 nm. With the peak value increasing, the corresponding pore volume increased, and the accumulated pore volume curve raised. It could be concluded that the contribution of meso-pores in this pore size range to pore volume was larger than that of pores within other pore sizes. The proportion of these meso-pores was larger.

The pore structures of fly ash samples were obtained, as shown in Table 7. In this paper, in the study of the influence of pore structure on the mercury adsorption capacity of fly ash, the specific surface area per unit volume Z was introduced to characterize the pore richness, calculated by using Eq. (4).

(4)Z={\displaystyle \frac{S_{\mathrm{0}}}{V_{\mathrm{0}}}},

where S₀ (m²/g) is the BET specific surface area of fly ash, and V₀ (cm³/g) is the sum of specific pore volume of fly ash.

It could be seen that the mercury contents of F10 and F11 were higher than that of F5. Besides, the mesoporous within 4–6 nm content of the two former was also much higher than that of F5. It could be concluded that the mesoporous within 4–6 nm increasingly developed, the mercury adsorption capacity of the fly ash increased. In other words, the content of mesoporous (4–6 nm) of the fly ash was beneficial to adsorbing the mercury. Compared with F5 and F11 samples, the BET specific surface area, accumulated pore volume, accumulated pore area, most probable pore size, and average pore size of F10 were higher. Among them, the fly ash with a higher content of the most probable pore size had a higher absorption ability, which meant the resistance of mercury entering the particle inner decreased. Besides, the S₀ and V₀ of F10 were also higher, which made the specific surface area per unit volume Z higher and the connectivity of pore structure better. As these were beneficial to the mercury adsorption, the mercury content of the F10 sample was the highest. This was consistent with the previous results about the influence of carbon content on mercury adsorption, verifying that the larger amount of unburned carbon could provide a higher content of mesoporous (4–6 nm) and more developed pore structure. In addition, F11 had a larger specific surface area and accumulated pore area, compared with F5. The pore volume of F11 was smaller, while the mercury content of F11 was three times higher than that of F5. It could be concluded that the specific surface area played a more significant role than specific pore volume in the mercury adsorption process.

Influence of particle size of fly ash on mercury adsorption

Enrichment degree of mercury in fly ash within different particle size

The fly ash samples collected from Nos. 4 and 5 CFB boilers were studied, and the mass fractions of fly ash in each particle size range were calculated, as shown in Table 8. It could be observed that the fly ash whose particle size was less than 120 µm accounted for more than 90% in two boilers. The particle size corresponding to the highest content of mercury, which was about 560 ng/g, appeared in the range of 77.5 to 106 µm in which the mass fraction was the highest when the particle size was in the range of 48 to 77.5 mm in No. 4 boiler, while the mass fraction of fly ash was the highest when the particle size was in the range of 90 to 106 mm in No. 5 boiler. As the particle size distributions of fly ash produced in the 2 boilers were quite different, the different combustion condition and coal property had some influence on the distribution.

The mercury contents of fly ash in Nos. 4 and 5 CFB boilers within different particle sizes were shown in Table 9. It could be seen that the mercury content first increased and then decreased with the increase of the particle size in the range of 24.5 to 362.5 µm, indicating that the mercury adsorption capacities of fly ash were different when the particle size changed. In this case, the appropriate particle size could make the fly ash have the best mercury adsorption capacity. Otherwise, a too large or too small size would cause the fly ash to have a low mercury adsorption efficiency. The fly ash in the range of 77.5–106 µm had the best mercury adsorption capacity. It also could be seen from Table 9 that there was a positive correlation between carbon content and mercury content, who had the same degree and trend of change, further verifying that the higher carbon content of fly ash was beneficial to mercury adsorption.

Combined with the mass fraction of fly ash within different particle sizes, the corresponding mercury contents were comprehensively analyzed. The highest mass fraction of fly ash with 48–77.5 mm was 31.99%, while the particle size corresponding to the highest mercury content appeared in the range of 90–106 mm in No. 4 boiler. In No. 5 boiler, the mass fraction was the highest, which was 40.63%, when the particle size was 90–106 mm, while the size corresponding to the highest mercury content was 77.5–90 mm. Therefore, the adsorption ability of fly ash was directly related to the particle size, which was irrelevant to the mass fraction of particle size.

Pore structure of fly ash within different particle sizes

In order to obtain the influence of particle size on mercury absorption capacity of fly ash, the fly ash samples within three different particle size ranges of 48–180 mm in No. 4 CFB boiler were studied. These ranges were 48–77.5 mm, 90–106 mm, and 120–180 mm, and marked as FL₁, FL₂, and FL₃ respectively. The low temperature N₂ adsorption/desorption experiments were conducted using these samples. The adsorption/desorption isotherms of samples were shown in Fig. 7. These samples basically belonged to class III isotherm, indicating that the pores of the samples were also slit pores formed by sheet shaped particles [³⁸,³⁹].

The differential and accumulated pore surface area of samples were shown in Fig. 8. It could be seen that the pore size of FL₁ and FL₃ corresponding to the highest differential surface area appeared in the range of 2–4 nm while the maximum value of differential surface area of FL₂ appeared in the range of 4–6 nm, and the value was much larger than those of the other two samples. This indicated that FL₂ had a large amount of mesopores (4–6 nm), reflected in the accumulated surface area curve for an apparent increase. In addition, the accumulated surface area increased slowly when the pore size was above 50 nm, indicating that the contribution of macropores to surface area was small for the 3 samples. It was mainly the accumulation of the mesoporous in the range of 2–6 nm.

The pore structure parameters of three fly ash samples were shown in Table 10. The specific surface area decreased with the increase of particle size. Moreover, the BET specific surface area of FL₁ was twice as high as that of FL₃. It was concluded that the particle size was an important factor affecting the specific surface area of fly ash. The average pore size and most probable pore size first increased and then decreased with the increase of particle size, which was consistent with the results presented in Table 8, in which, the average pore size and most probable pore sizer of FL₂ were higher than those of FL₁ and FL₃, reducing the resistance of mercury entering the particle inner, which was favorable for the mercury adsorption of fly ash [⁴⁰]. This proved the previous results that the mercury content was highest in the fly ash with 77.5–106 µm. Meanwhile, FL₂ had the highest most probable pore size, which might be related to its highest carbon content, consistent with the results in Ref. [⁴¹]. The Z of FL₂ was less than that of FL₁. However, according to the pore distribution calculated by isotherms, the pore sizes of FL₂ were more than 4 nm, and the accumulated pore volume of mesopores (4–6 nm) were higher. In contrast, the accumulated volume of pores less than 4 nm were higher for FL₁ and FL₃. According to the previous results, the more developed the mesoporous within 4–6 nm, the more conducive to the mercury adsorption of fly ash. It could be explained that the mercury content of FL₂ was highest. In addition, the specific surface area and accumulated pore area of FL₁ were larger than those of FL₃, and the mercury content of FL₁ was about twice as high as that of FL₃. It further proved that the specific surface area of fly ash had a positive influence on mercury adsorption. The micropores were not found in the three samples, which might be related to the coal used in the boilers. Moreover, it was found that the average pore size, BET specific surface area, accumulated pore volume, and accumulated pore surface area of the fly ash obtained in CFB boilers are much larger than those of the fly ash obtained in PC boilers, which verified the results of the pervious study.

Conclusions

In the 3 PC boilers, 86.52%, 55.79%, and 60.70% of the mercury exist in gaseous form in the flue gas, respectively.

In the 2 CFB boilers, 83.71% and 81.78% of the mercury are absorbed by the fly ash and captured by the precipitator, respectively. Besides, the mercury content of fly ash in CFB boiler was much higher than that in PC boiler.

The larger the coal factor, the lower the mercury emission in various combustion modes. The contents of chlorine, sulfur, and ash are important factors influencing the mercury emission by changing the mercury forms.

The unburned carbon content and pore structure of fly ash are the key factors influencing the mercury absorption. The carbon content was positively correlated to the mercury adsorption capacity of fly ash. The content of the mesoporous (4–6 nm) of the fly ash is beneficial to adsorbing the mercury. The specific surface area plays a more significant role than specific pore volume in the mercury adsorption process. The higher the specific surface area, the more conducive to the mercury adsorption of fly ash.

The mercury adsorption ability of fly ash is directly related to the particle size, which is irrelevant to the mass fraction of particle size. With the increase of the particle size of fly ash, the mercury content of fly ash increases first and then decreases. In addition, the adsorption capacity of fly ash is the strongest in the range of 77.5–106 mm.

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