A novel method of ultra-low SO2 emission for CFB boilers

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A novel method of ultra-low SO2 emission for CFB boilers: combination of limestone injection and activated carbon adsorption Yan Dong, Yuzhong Li, Liqiang Zhang, Lin Cui, Bo Zhang, and Yong Dong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01895 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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A novel method of ultra-low SO2 emission for CFB boilers: combination of limestone injection and activated carbon adsorption Yan Donga, Yuzhong Lia,*, Liqiang Zhanga, Lin Cuia, Bo Zhangb, Yong Donga a

National Engineering Laboratory of Coal-fired Pollutants Emission Reduction, School of Energy and Power Engineering, Shandong University, Jinan 250061, China, b Shandong Shenhua Shanda Energy & Environment Co. Ltd., Jinan 250061, China Graphic abstract:

Abstract: Circulating fluidized bed coal-fired boilers that adopt conventional measures of improving desulfurization efficiency exhibit difficulty in achieving ultra-low SO2 emission. To solve this problem, our team originally proposed a desulfurization process of limestone injection into furnace with desulfurization of activated carbon (AC) in the rear. In this process, most of the SO2 in furnace is initially removed. The rest is transported to the AC desulfurization device and adsorbed by AC. The utilized AC is then thermally regenerated. The SO2 released by thermal regeneration is sent back to the furnace and ultimately removed by limestone. To further complete the process, AC adsorption and desorption experiments are conducted. Furthermore, the parameters of the process are optimized, and the economic feasibility of the process is assessed. Compared with that of two-stage desulfurization, the cost of the proposed process is higher but still affordable. Therefore, the process is economically affordable and technologically feasible. Keywords: ultra-low SO2 emission, CFB boiler, activated carbon desulfurization, flue gas desulfurization Nomenclature CFB

circulated fluidized bed (-)

K

constant coefficient (-)

AC

activated carbon (-)

m

mass of the AC adsorption column (g)

FGD

flue gas desulfurization (-)

T

counting period (min)

BET

Brunauer-Emmett-Teller (-)

t

integral penetration time of the fixed-bed (min)

η

thermal regeneration efficiency (%)

C

unit added running cost (yuan/MW•h)

ηCFB

CFB desulfurization efficiency (%)

C1

maintenance cost (yuan) 1

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ηAC

AC desulfurization efficiency (%)

C2

cost of electricity and transportation (yuan)

m0

first AC adsorption capacity（mg/g）

P

power of the unit (MW)

mi

AC adsorption capacity of i（mg/g）

H

running time of the unit in a year (h)

ω

AC consumption (kg)

IPTF

integral penetration time of fixed-bed (min)

Q

flue gas flow of the CFB boiler (Kg/h)

q

experimental flow rate (L/min)

1. Introduction Recently, Chinese government agencies have raised the SO2 emission standards of coal-fired boilers and proposed ultra-low SO2 emission. TheError! Reference source not found. emission concentration of circulating fluidized bed (CFB) coal-fired boilers does not satisfy these standards because they adopt dry desulfurization of limestone injection into furnace. Therefore, the desulfurization process of CFB coal-fired boilers should be innovated. The advantages of CFB coal-fired boilers, such as economic desulfurization1, low NOX emission2, and wide fuel adaptability3, contribute to the widespread application of these boilers. Current CFB coal-fired boilers mostly adopt dry desulfurization of limestone injection into furnace4. SO2 in furnace is removed during combustion5. Desulfurization can’t often proceed in optimum condition, because dry desulfurization demonstrates low gas and solid interaction efficiency6 and is sensitively affected by many factors, such as load change, bed temperature and size distribution of particles7-9, Therefore, desulfurization efficiency is inefficient. Desulfurization efficiency can be improved by adjusting the Ca/S molar ratio, size distribution of particles, circulating ratio10-12, and so on. However, these measurements cannot effectively decrease SO2 emission concentration. Moreover, improving desulfurization efficiency by continuously increasing the Ca/S molar ratio is even more difficult13, and results in low calcium utilization efficiency and bad combustion14, 15. Our team investigated several CFB coal-fired boilers in China, such as those in the power plants of Baima and Datun. The SO2 emission concentration from CFB coal-fired boilers is beyond the standard of air pollution for thermal power plants16 and far exceeds the ultra-low SO2 emission standard (35 mg/Nm3/12.3 ppm) published by Chinese government agencies. Domestic and foreign experts have proposed many measures to reduce the SO2 emission concentration of CFB boilers. One of these measures involves changing the fuel. Alar Konist et al.17 utilized low-grade oil shale and biomass co-combustion for a CFB boiler. The SO2 emission concentration of the CFB boiler remained at 7–10.5 ppm with 100% load. Another proposed measure is two-stage desulfurization (desulfurization of limestone injection into furnace combined with flue gas desulfurization (FGD)), which has been applied in several power plants in China18,19. Two-stage desulfurization has been applied for many years in western countries (e.g., JEA power plant). The desulfurization is used to remove SO2 as well as HCl、dust、HF, and can realize low SO2 emission. However, the desulfurization is adopted in China only to remove SO2. This desulfurization technology in China has not matured enough to be used in domestic power plants. One of the processes of two-stage desulfurization is desulfurization of limestone injection into furnace united limestone–gypsum wet flue gas desulfurization, which can realize ultra-low SO2 emission of CFB coal-fired boilers. Owing to the desulfurization cost, the application of desulfurization is limited to a few CFB boilers that burn coal with high sulfur contents. The other process involves desulfurization of limestone injection into furnace combined with semi-dry flue gas desulfurization, This process is suitable for CFB boilers that burn coal with low sulfur 2


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contents. CFB boilers that burn high-sulfur coal cannot maintain stable ultra-low SO2 emission. Therefore, our team aims to innovate desulfurization process of CFB boilers and originally proposes a process of desulfurization of limestone injection into the furnace combined with desulfurization of activated carbon in the rear based on the high desulfurization efficiency and thermal regeneration efficiency of AC. This process may provide a method of ultra-low SO2 emission for CFB boilers. AC desulfurization has been extensively studied. Thus, the desulfurization mechanism is mature20, 21. AC possesses a high adsorption capacity for sulfur dioxide because of its rich porous structure22, large Brunauer–Emmett–Teller (BET) surface area23, 24, and surface functional groups25, 26. Moreover, AC can also be regenerated and reused27-29. However, the cost and consumption of AC for repeated regeneration are high30. This disadvantage leads to the high cost of sulfur removal using only AC and extremely hinders the industrial application of AC desulfurization. The desulfurization process of limestone injection into furnace combined with desulfurization of activated carbon in the rear in this study is proposed in consideration of the characteristics of AC desulfurization and limestone injection. The AC desulfurization device is installed in the flue channel of the precipitator outlet. SO2 is adsorbed and retained by AC when flue gas passes through the device. Then, the spent AC is transported to the AC regeneration device. Adsorption capacity is recovered through thermal regeneration, and AC becomes reusable. The SO2 released by thermal regeneration is sent back to the CFB boiler and ultimately removed in the furnace. In the process, AC functions as a medium adsorbent and not as a sorbent. That is, AC only adsorbs and transfers SO2. The SO2 desorbed from the AC is removed by limestone at the end of the process. Therefore, low AC consumption is achieved. One of the advantages of this process is that it exploits the high desulfurization efficiency of AC to overcome the difficulty in increasing the desulfurization efficiency of CFB boilers further, thereby improving the desulfurization efficiency of the entire process. The other advantage of the process is that it exploits the desulfurization characteristic of AC and the low cost of desulfurization of limestone injection into furnace to address the high cost of desulfurization using only AC. Therefore, the overall desulfurization cost of the process is reduced. To further complete the process, characteristics and economic feasibility of the process are explored. First, the process including the device reconstructed from the original CFB boiler desulfurization is described in detail. Second, a fixed-bed reactor model is established. AC adsorption experiments are performed based on the model. AC thermal regeneration experiments are also carried out to obtain the regeneration times and thermal regeneration efficiency. Furthermore, the process parameters for reducing AC consumption are selected. Finally, the economic feasibility of the process is assessed by using experimental data. Although the cost of the proposed process is higher than that of two-stage desulfurization, it is still affordable. Therefore, the process may achieve ultra-low SO2 emission for CFB boilers and enable the application of AC desulfurization in related industries. 2 Process description and establishment of a fixed-bed reactor model 2.1 Process description The proposed process involves the CFB boiler desulfurization system of limestone injection into furnace and the AC desulfurization system. The AC desulfurization system includes an AC desulfurization device, an AC regeneration device, material transport equipment and pipelines 3


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connecting the two systems. The process chart is shown in Fig. 1. ηCFB is CFB desulfurization efficiency and ηAC is AC desulfurization efficiency.

Fig.1. Process of desulfurization of limestone injection into furnace combined with the desulfurization of activated carbon in the rear. 2.1.1 CFB boiler desulfurization system of limestone injection into furnace. The process uses the original CFB boiler desulfurization system that adopts limestone injection into furnace. The concentration of SO2 from burning coal is assumed to be 2,000 ppm. When the Ca/S molar ratio is 2.5-3, the desulfurization efficiency of the CFB boiler is about 90%, and the SO2 concentration of the exhausted flue gas reaches 200 ppm. Obviously, the concentration exceeds ultra-low emission. In the process, the original desulfurization system of the CFB boiler needs to reconstruct itself. A gas interface and pipes are added to the secondary air duct in the CFB boiler. They are used to transfer the SO2 released from the AC regeneration device and the gas that escapes this device. Then, the mixture of SO2 and carrier gas is sent back into the furnace. Finally, SO2 is removed by limestone in the furnace. To maintain the SO2 concentration of the exhausted flue gas at the primary level (200 ppm), the desulfurization efficiency should be increased from 90% to 90.9% with the increase in the Ca/S molar ratio. According to previous studies, the increment is economically affordable and technologically feasible. 2.1.2 AC desulfurization device The AC desulfurization device is installed in the precipitator outlet, which includes a fixed bed with a fire grate. In practical operations, AC is added to the fire grate side to form an AC layer. When flue gas passes through the AC layer, AC adsorbs the SO2 from the exhausted flue gas. Then, the concentration of SO2 becomes less than 12.3 ppm. The utilized AC eventually moves to the end of the fixed bed and leaves the device. 2.1.3 AC regeneration device The AC regeneration device is installed near the AC desulfurization device, and is similar to the shell and tube heat exchangers. The utilized AC fills the tube, and high-temperature flue gas or superheated steam as a heat source flows outside of the tube. Thus, the heat from the source can be utilized to regenerate SO2. The circulated flue gas passes through the tube, and prevents the exposure of AC to air. Then, the mixture of regenerated SO2 and circulated flue gas are sent to the pipes added on the secondary air duct using the air exhauster. The SO2 from the mixture is transferred to the chamber and finally removed by the limestone in the CFB boiler. 2.2 Basis and significance of the process 4


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The mechanism of SO2 removal by AC has been known for many years21, 31. Summary of previous study results, indicates that the SO2 from flue gas is adsorbed below 200 °C and adsorbed in the form of H2SO4 at active adsorption sites in the presence of SO2, H2O, and O2. The mechanism can achieve 100% desulfurization efficiency in a short period because AC has high adsorption capacity. The utilized AC can recover its adsorption capacity by thermal regeneration to become usable32. However, AC desulfurization is uncommon in industrial applications because of its high operating cost and intensive desulfurization investigation. One of the advantages of the new process is that it can utilize the high desulfurization efficiency of AC to overcome the difficulty of increasing the desulfurization efficiency of CFB boilers. Another advantage of the process is the regeneration of AC and low cost of desulfurization of limestone injection into furnace. This advantage reduces the cost of SO2 removal by the AC alone. Therefore, the process is technically feasible and economically affordable and finally can achieve ultra-low emission of CFB boilers. The process is not only intended for the development of a perfect CFB but also enables the industrial application of AC desulfurization. 2.3 Study contents In the previous section, the new process was descripted in detail. The technical and economic feasibility was also preliminarily discussed. To further complete the process, our team established a fixed-bed reactor model for the simulation of the AC adsorption device, as shown in Fig. 2. Then, AC adsorption experiments were conducted based on the model. Furthermore, AC thermal regeneration experiments were carried out to obtain regeneration times and thermal regeneration efficiency. And parameters of the process were optimized. Finally, economic feasibility of the process was assessed.

Fig.2. Fixed bed reactor model 3 Experiments In the AC adsorption experiments, a column randomly selected from the fixed-bed reactor 5


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model was used to simulate the AC unit. The height of the column was equal to the bed thickness. 3.1 Apparatus The gas, adsorption, and flue gas analysis systems composed the entire experimental system as shown in Fig. 3. (1) Gas supply system. The components of exhaust gas, including NOx, SO2, dust and HCl, in the dust channel are complex. The existence of CO2 does not affect AC desulfurization33, and NOx exerts little influence on AC desulfurization. The concentrations of dust, HCl, and others are also low34. Therefore, the simulated flue gas consisted of pure N2, pure O2, SO2 and H2O. N2, O2 and SO2 were obtained from a gas cylinder and their mass flow rates were controlled by a mass flow controller (Sevenstar CS200). When N2 acrossed a water bottle was placed in a thermostat water bath, H2O was carried in the gaseous state. The mass flue rate of the carried H2O was determined by the temperature of the bath and the mass flow rate of N2. Comingler in Fig. 3 is a container, that mixes gases. The gases were uniformly mixed in the comingler and formed simulated flue gas. Before entering the reactor, the simulated flue gas flowed through the pipe between the comingler and reactor, which were heated to the reaction temperature by an electric heater. Then, H2O was present in the gas phase and entered the reactor. The simulated flue gas had 31% humidity and 4.1% volume fraction, and N2 functioned as the balancing gas. The mass flow of the gas and the SO2 concentration were varied according to the reaction condition. (2) Adsorption system. The adsorption system included a reactor. The reactor was a glass tube with 500 mm length and 20 mm internal diameter. Inside the reactor, a glass mesh was placed in the lower extremity of the adsorbent to support AC. The temperature of the reactor was maintained by an electric furnace located at its outer wall. The AC was placed in the reactor, which formed the AC adsorption column. (3) Flue gas analysis system The SO2 concentrations of the reactor outlet and inlet were analyzed using a Fourier infrared gas analyzer (FT-IR) and a flue gas analyzer (GasmetDXError! Reference source not found.4000). 3.2 Experimental section Four adsorption experiments were conducted to determine the effects of temperature, concentration, gas flow rate, and AC adsorption column mass. The reaction conditions are presented in Table 1. The flow rate was stabilized by adjusting the mass flow controller before the experiments. Analytical data were obtained at the end of each experiment until the readings were invariable. To increase reliability, each experiment was performed at least twice. The obtained data were drawn as breakthrough curves. According to the ultra-low emission standard, 12.3 ppm should be defined as the penetration concentration of SO2. Therefore, the time corresponding to the penetration concentration of SO2 was defined as the penetration time. Penetration times were obtained from concentration, temperature, gas flow rate, and AC adsorption column mass. Thermal regeneration experiments were conducted after the adsorption experiments. The entire system was reconstructed by the adsorption system. Only two N2 gas cylinders were used. The first cylinder was connected to the entrance of the reactor, and the other was connected to the outlet of the reactor. The utilized AC that obtained from the fourth group of experiments was 6


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utilized. The utilized AC weighed 48 g with a height of 96 mm. Analytical data were captured every minute until the readings were zero. Pure N2 was made to enter the reactor, and air swept for 5 min at ambient temperature. The reactor was heated and maintained at 400 °C. To determine the influence of high SO2 concentration on the measurement, pure N2 was connected to the inlet of the reactor at a flow rate of 1000 mL/min. The adsorption–desorption cycles were completed after the regeneration experiments. Thermal regeneration efficiency was obtained from 16 adsorption–desorption experiments. Thermal regeneration efficiency was calculated as η = (mi / m0) / m0×100% i = 1, 2…16, (1) where η is thermal regeneration efficiency (%), m0 is the first AC adsorption capacity (mg/g), and mi is the AC adsorption capacity of i (mg/g).

1.gas cylinder; 2.reducing valve; 3.;mass flow Meter; 4.water; 5.commingler; 6.thermostat water bath;7.tube furnace; 8.reactor; 9.sampling apparatus; 10.FT-IR flue gas analyzer Fig.3. Activated carbon adsorption system Table 1. Reactor conditions in the adsorption experiments Group

Variable

Inlet

Temperature

Gas flow

Mass

Humidity

Volume

concentration

（°C ）

rate (L/min)

（g）

（%）

fraction of

（ppm） 1

2

Inlet concentration

Temperature

（%）

52

137

5

32

31

4.1

70

137

5

32

31

4.1

142

137

5

32

31

4.1

382

137

5

32

31

4.1

700

137

5

32

31

4.1

70

90

5

16

31

4.1

70

105

5

16

31

4.1

7


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3

4

Gas flow rate

Mass

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70

137

5

16

31

4.1

70

150

5

16

31

4.1

70

137

2.5

32

31

4.1

70

137

5

32

31

4.1

70

137

10

32

31

4.1

70

137

15

32

31

4.1

70

137

5

16

31

4.1

70

137

5

32

31

4.1

70

137

5

48

31

4.1

4 Results and discussion 4.1 Influence of inlet concentration on desulfurization To verify the influence of concentration on desulfurization, experiments in the range of 52– 700 ppm were conducted. The breakthrough curves are shown in Fig. 4. When the SO2 inlet concentrations were 700, 382, 142, 70, and 52 ppm, the outlet SO2 concentration reached 12.3 ppm in 6, 10, 17, 31 and 80 min, respectively. Presumably, the SO2 (700 ppm) that originated from coal burning was adsorbed by AC. Then, AC was penetrated immediately, resulting in frequent regeneration and large consumption that led to costly desulfurization using only AC. According to the experimental results, the penetration time decreased as the inlet concentration of SO2 increased. Therefore, the regeneration times and consumption also decreased. In the process, most of the SO2 was initially removed by the traditional desulfurization of limestone injection into the furnace with a low cost. Subsequently, the rest was adsorbed by the AC with high efficiency. Therefore, the cost and regeneration times decreased in the process significantly. The process does not only realize ultra-low emission in CFB boilers but also promotes the industrial application of AC desulfurization. Apparently, the experimental data were applied in the AC desulfurization device. Large differences in penetration duration were observed between the AC adsorption column and the fixed-bed. In the AC adsorption experiments, the AC adsorption column was simulated with an AC micro-unit from the fixed bed. According to the first group of experimental data, the AC adsorption column was penetrated for 31 min as SO2 of 70 ppm passed through the device. However, the AC behind the AC micro-unit was not penetrated in the fixed bed. At that time, the integral average of SO2 concentration for the fixed-bed boundary was below 12.3 ppm. When the AC micro-unit was operated for 75 min in the fixed-bed, the integral average concentration increased to 12.3 ppm. The time referred to integral penetration time of the fixed-bed (IPTF). The IPTF of the four groups are shown in Table. 2. The figure shows that a short or long period of absolute adsorption (0 ppm) occurred even under variable concentrations. Therefore, the problem of temporary over-standard SO2 concentration for CFB boilers can be solved. Given the uncertain sulfur content of coal and the unstable load, the CFB boiler was likely to have inefficient desulfurization efficiency, which led to temporary over-standard SO2 concentration. If CFB boilers adopted the proposed process, SO2 could be shortly adsorbed whenever SO2 concentration exceeds the standard accidentally or inescapably. Therefore, the new process has high flexibility and stability.

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Fig.4. Effect of inlet concentration on desulfurization (reaction condition: 137 °C, 5 L/min, 32 g) 4.2 Influence of temperature on desulfurization SO2 is usually adsorbed by AC at 100 °C–200 °C according to several previous studies35-37. To confirm influence of temperature on desulfurization in the process, experiments in the range of 90 °C–150 °C were performed. The results are shown in Fig. 5. When experiments were conducted at 150 °C, 137 °C, 105 °C, and 90 °C, the penetration times were 12, 12, 48, and 72 min respectively. The penetration time increased as the reaction temperature decreased. Therefore the exhaust gas temperature was low as soon as possible. In practical operations, exhaust gas temperature is approximately 137 °C in CFB boilers. At present, low-pressure economizers are installed in several CFB boilers to reduce the exhaust gas temperature38. When the exhaust flue gas flows out from the low-pressure economizer, the outlet temperature is decreased extremely to 95 °C. Therefore, the new process is proposed to be installed low-pressure economizer with the penetration time increased from 12 min to 148 min.

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Fig. 5. Effect of temperature on desulfurization (reaction condition: 70 ppm, 5 L/min, 16 g) 4.3 Influence of gas flow rate on desulfurization Experiments were conducted with variable flow gas rates to verity the influence of gas flow rate on desulfurization. The results are shown in Fig. 6. A test was conducted before the experiments. The AC adsorption column was still below the flow rate of 20 L/min. When the flow rate exceeded 20 L/min, bubbling was observed in the AC adsorption column. Therefore, four flow rates, namely, 2.5, 5, 10, and 15 L/min, were selected in the experiments. The AC adsorption column had 20 mm diameter. Therefore, the superficial gas velocities were calculated as 0.1327, 0.265, 0.53, and 0.795 m/s at flow rates of 2.5, 5, 10, and 15 L/min, respectively. The penetration time was 7 min at the flow rate of 15 L/min. When the flow rates decreased to 10, 5, and 2.5 L/min, the penetration times became 15, 33, and 390 min, respectively. The penetration time lengthened as the flow rate or superficial gas velocity decreased. Generally, the flue gas velocity is 0.8 m/s-1.2 m/s at the precipitator outlet. To decrease the superficial gas velocity in the fixed-bed, diverging flue needs to be added. What’s more, fixed-bed areas become large as flue gas velocity decreases. Therefore, economic feasibility and fixed-bed area are simultaneously considered in the process.

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Fig. 6. Effect of gas flow rate on desulfurization (reaction condition: 70 ppm, 137 °C, 32 g) 4.4 Influence of bed thickness on desulfurization To verify the influence of bed thickness on desulfurization, experiments were conducted. The AC adsorption columns were 48, 32, and 16g, with heights of 96, 64, and 32 mm, respectively. The results are presented in Fig. 7. When the AC adsorption column heights were 96, 64, and 32 mm, the penetration times were 192, 31, and 11 min, respectively. Therefore, the penetration time increased as the AC column height increased. What’s more, selecting the appropriate bed thickness was crucial for the cost efficiency of the process.

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Fig. 7. Effect of bed thickness on desulfurization (reaction condition: 70 ppm, 137 °C, 5 L/min) 4.5 Regeneration efficiency and regeneration times To determine the thermal regeneration effect of AC, thermal regeneration experiments were carried out. The thermal regeneration efficiency curve is shown in Fig. 8. The heat source of regeneration was provided by high temperature flue gas or superheated steam. The experiments were conducted at 400 °C because AC has a comparatively high absorption capacity after thermal regeneration at this temperature. The curve showed that the adsorption capacity of AC decreased as the thermal regeneration time increased. The thermal regeneration efficiency of AC reached 65% in the 16th regeneration experiment because the surface functional groups were decomposed and porous structures were reduced during thermal regeneration. The average regeneration efficiency reached 78.5%.Therefore, AC had a high regeneration efficiency and could be thermally regenerated many times in the process. Regeneration times was selected according to the actual condition in practical applications.

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Fig.8. Thermal regeneration at 400 °C 4.6 Selection of optimum parameters According to previous research, the cost of AC desulfurization is high. Therefore，optimum parameters were selected for reducing AC consumption. First，AC consumption was defined as Formula (2). ω = m × 10-3 × (Q / 60) / (q × 10-3) × (T / t) / (16 × 0.785) = K × (m / q / t) × Q × T (2) where ω is AC consumption (kg). K is a constant coefficient determined by regeneration times and average regeneration efficiency, Q is the flue gas flow of the CFB boiler (Kg/h), and T is the counting period (min). Q and T are confirmed in practical engineering application. m, q, and t as experimental data are the mass of the AC adsorption column, the experimental flow rate, and IPTF, respectively. Therefore, m / q / t is crucial to AC consumption and proportional to ω. Secondly, m/q/t was calculated, as shown in Table 2. Table 2. Results of m / q / t and IPTF in various conditions 1

52ppm

70ppm

142ppm

382ppm

700ppm

m/q/t

0.028

0.085

0.21

0.58

75

30

12

IPTF

225

2

90 °C

105 °C

137 °C

150 °C

0.8

0.022

0.036

0.13

0.16

11

148

88

20

24

3

0.1327m/s

0.265m/s

0.53 m/s

0.795 m/s

32mm

64mm

96mm

m/q/t

0.008

0.085

0.256

0.43

4

0.13

0.085

0.126

IPTF

869

75

25

13

20

75

506

Finally, results were analyzed. AC consumption decreased as the inlet SO2 concentration, reaction temperature, and superficial gas velocity decreased. Bed thickness showed the opposite behavior. Two optimum parameters were selected: superficial gas velocity of 0.1327 m/s and bed thickness of 96 mm and bed thickness of 96 mm. In view of fixed-bed area, bed thickness of 96 mm was selected as the optimized parameter. 5. Economic assessment 13


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In order to explore economy of the process, a hypothesis is put forward that the process is applied to a 2×150MWe CFB boiler in Guangdong Xinhui Shuangshui power plant. First, field data were disposed to the obtain relation between Ca/S molar ratio and SO2 emission concentration. Second, the unit added running cost of the process was simply calculated based on experimental data. Finally, economy of the process was assessed. The CFB boiler adopts limestone injection into the furnace. The relation between Ca/S molar ratio and SO2 emission concentration was examined based on field data, as shown in Fig. 9. SO2 emission concentration obviously decreased as the Ca/S molar ratio increased in the range of 1.5–3.5. However, SO2 emission concentration almost remained invariant as the Ca/S molar ratio further increases. And it is difficult in achieving ultra-low SO2 emission by increasing Ca/S molar ratio. Therefore, it is necessary that the process is applied in the CFB boiler.

Fig. 9. Relation between Ca/S and SO2 emission concentration Then, unit added running cost was calculated. Most of the SO2 from coal burning is initially removed in the furnace. The rest was adsorbed and desorbed by the AC and its concentration decreased to 12.3 ppm. In the process, AC consumption was large. Without regard to the cost of desulfurization of limestone injection into furnace, the costs of AC consumption, electricity, and maintenance are referred to as added running cost. Therefore, the unit added running cost was calculated as Formula (3). (3) C = (H / T × ω× 4000 / 1000 + C1 + C2) / (P × H） Where C is the unit added running cost (yuan/MW•h), C1 is the maintenance cost (yuan), C2 is the cost of electricity and transportation (yuan), H is the running time of the unit in a year (h), P is the power of the unit (MW), and H / T × ω × 4000 / 1000 is the AC consumption cost (yuan). H is 14


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Energy & Fuels

5500 h, and the price of AC is 4000 yuan/ton. Therefore, C was calculated as 11.87 yuan/ MW•h. Finally, the economic feasibility of the process was assessed. The process includes the CFB boiler desulfurization system of limestone injection into the furnace and the AC desulfurization system. The cost related to the AC desulfurization system is called added cost. Added cost involves static and running added costs. Static added cost contains the cost of the AC desulfurization device, reconstruction, installation, and so on. The designed system lifespan is supposed to be 20 years. The results are shown in Table 3. Table 3. Economic comparison Project Static added cost Calcium containing

Semi-dry flue gas

wet flue gas

AC

desulfurization

desulfurization

desulfurization

13855

559

20000

662

559

-

unit

reactant cost AC consumption cost

-

-

1958

Electricity cost

2062

2079

400

ten thousand yuan

Water cost

205

114

-

Maintenance cost

346

130

400

Added cost

4740

4234

3758

Unit added cost

16.03

14.32

22.77

yuan/MW•h

In addition, the unit added cost of the process was compared with that of two-stage desulfurization39. The two-stage desulfurization is intended to apply to a 4×480 t/h CFB gangue-fired boiler. On the one hand, the benefits of the industrial application of AC desulfurization are not taken advantage of because of the high static added cost and other maintenance and installation costs. What’s more, the price of AC is higher than limestone, which further implies a high running added cost. On the other hand, the electricity cost of the process is lower than that of two-stage desulfurization. The process rarely consumes water, which can take into consideration water scarce area. Besides, the unit added cost of the process can be equal to two-stage desulfurization as AC desulfurization develops and AC price decreases. In total, the process can be economically affordable. 6. Conclusion To achieve ultra-low SO2 emission for CFB coal-fired boilers, a process of limestone injection into furnace combined with AC desulfurization in the rear was proposed. First, the process, which includes the reconstruction of the original CFB desulfurization system and introduction of the AC desulfurization device, was described in detail. Second, the characteristic of the process was determined through AC adsorption and thermal regeneration experiments. Low SO2 inlet concentration, low reaction temperature, low superficial velocity, and high bed thickness helped reduce AC consumption. The thermal regeneration experiments confirmed that the regeneration time was 16, with an average regeneration efficiency of 78.5%. The integral penetration time of the fixed bed was also put forward to assist in practical operations. Finally, the economic feasibility of the process was assessed. If the process is applied to a 2×150 MWe CFB boiler, the unit added cost would be 22.77 yuan/ MW•h. Compared with that of two-stage desulfurization, the unit added cost of the process is higher but still economically affordable. Therefore, the process can offer a method of ultra-low SO2 emission for CFB boilers and enables 15


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the industrial application of AC desulfurization. Acknowledgements The authors gratefully acknowledge the support from “The Key Research and Development Program of Shandong province（2016GGX104009） ”, “ The Fundamental Research Funds of Shandong University (2017JC012) ”, and “The R&D Project of Shandong ShenhuaShanda Energy & Environment Co., Ltd. (HXTCFB01-01) ”. References 1. Zhang, W. G.; Zhang, Y.; Sun, Y. Z.; Liu, J. Z.; Gao, M. M.; Zeng, D. L. Energy Fuels 2016, 30, 3519-3528. 2. Li, F. H.; Zhai J. P.; Fu X. R.; Sheng, G. H. Energy Fuels 2006, 20, 1411-1417. 3. Wu, Y. H.; Wang, C. B.; Tan, Y. W.; Jia, L. F.; Anthony, E. J. Characterization of ashes from a 100 kW pilot-scale circulating fluidized bed with oxy-fuel combustion. Appl. Energy 2011, 88, 2940-2948. 4. Wang. S.; Chen, J. H.; Liu, H. L.; Zhao, F. X.; Zhang, Y. N. Fuel Process. Technol. 2014, 126, 163-172. 5. Lupiáñez, C.; Guedea, I.; Bolea, I.; Díez, L. I.;Romeo, L. M. Fuel Process. Technol. 2013, 106, 587-594. 6. Li, Y.; Zheng, K.; You, C. F. Environ. Sci. Technol. 2011, 45, 9421-9426. 7. Gong, Y.; Yang, Z. G. Mater. Des. 2011, 32, 671-681. 8. Gungor, A. J. Chem. Eng. 2009, 146, 388-400. 9. Gungor, A.; Eskin, N. Int. J. Therm. Sci. 2008, 47, 157-174. 10. Krzywanski, J.; Czakiert, T.; Blaszczuk, A.;Rajczyk, R.; Muskala, W.; Nowak, W. Fuel Process. Technol. 2015, 139, 73-85. 11. Anthony, E. J.; Granatstein, D. L. Sulfation phenomena in fluidized bed combustion systems. Prog. Ener. Combust. Sci. 2001, 27(2), 215-236. 12. Ding, R. F.; Dong, J. J.; Zhang, M.; Yang, H. R.; Lv, J. F. Powder Technol. 2015, 274, 180-185. 13. Lu, J.; Zhang, J.; Zhang, H.; et al. Fuel Process. Technol. 2007, 88, 129-135. 14. Wu, Y.; Anthony, E. J. Powder Technol. 2011, 208, 237-241. 15. Gómez, M.; Fernández, A.; Llavona, I.; Kuivalainen, R. Appl. Therm. Eng. 2014, 65, 617-622. 16. Ministry of Environmental Protection. GB 13223-2011, Emission standard of air pollutants for thermal power plants; China Environmental Science Press; Beijing, China, 2011. 17. Konist, A.; Pihu, T.; Neshumayev, D.; Külaots, I. Oil Shale 2013, 30, 294-304. 18. Zhu, J.; Xie, B. C.; Jiao, X. F. Electr. Power Environ. Protec. 2015, 31, 43-45. 19. Xiao, F.; Yi, H. G. Power System and Engineer 2011, 27, 57-58. 20. Shi, L.; Yang, K.; Zhao, Q. P.; Wang, H. Y.; Cui, Q. Energy Fuels 2015, 29, 6678-6685. 21. Raymundo-Piñero, E.; Cazorla-Amorós, D.; Linares-Solano, A. Carbon 2001, 39, 231-242. 22. Seredych, M.; Bandosz, T. J. Energy Fuels, 2008, 22, 850-859. 23. Lee, Y. W.; Park, J. W.; Choung, J. H.; Choi, D. K. Environ. Sci. Technol. 2002, 36, 1086-1092. 24. Timko, M. T.; Wang, J. A.; Burgess, J.; Kracke, P.; Gonzalez, L.; Jaye, C.;Fischer, D. A. Fuel, 2016, 163, 223-231. 25. Lua, A. C.; Yang, T. J. Colloid Interface Sci. 2005, 290, 505-513. 26. Furmaniak, S.; Terzyk, A. P.; Gauden, P. A.; Kowalczyk, P.; Szymański, G. S. Chem. Phys. Lett. 16


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A novel method of ultra-low SO2 emission for CFB boilers

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