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Ind. Eng. Chem. Res. 2004, 43, 6714-6722
Studies on Flue Gas Desulfurization by Chemical Absorption Using an Ethylenediamine-Phosphoric Acid Solution Zhi-gang Tang,* Chang-cheng Zhou, and Cheng Chen Department of Chemical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China
This paper presented a novel flue gas desulfurization (FGD) process. In this method, organic amine was used to absorb sulfur dioxide in flue gas. The vapor-liquid equilibrium model of absorption of sulfur dioxide in the flue gas by an organic amine was established, and the vaporliquid equilibrium of the sulfur dioxide-ethylenediamine-phosphoric acid-water system was first predicted. For a 0.3 mol‚L-1 ethylenediamine buffer solution system, the predicted results fit the experimental results well. It is indicated that the model is preferable to predict vaporliquid equilibrium under the experimental conditions. Using an ethylenediamine-phosphoric acid aqueous solution as the absorbent, some technology conditions such as the temperature, gas-liquid ratio, pH value, concentration of the desulfurization reagent, and liquid flow rate in absorption and deabsorption were experimentally researched on the L 30 mm × 600 mm packed tower in the laboratory. The optimized conditions were finally specified after experiments. The novel FGD method presented in this paper has advantages such as high desulfurization efficiency and low investment and unit FGD expense. It is suggested that this novel method be applied to the FGD process in power plants using high sulfur coal as the fuel. Introduction SO2 emission is one of the most serious problems puzzling Chinese governments. Because natural resources are limited, coal with a high sulfur content (34%) is commonly the most important consumption resource. The percentage of coal consumption is near 75% of the total amount of energy consumption in China. Even worse, 80% consumption is from direct combustion.1-4 In recent years, SO2 emissions have seriously increased with the amazing increase of the Chinese coal consumption. In 1999, the total SO2 emission amount of China was 2.370 × 108 t, exceeding that of USA, which always had the largest SO2 emission amount before 1998 in the world. The large amount of SO2 emissions causes damage from acid rain and leads to 1000 billion RMB (¥) economic loss per year in China. Domestic research on flue gas desulfurization (FGD) is becoming the hottest topic in environmental science and engineering.5,6 Among all of the methods ever have developed, wet desulfurization methods,7-9 especially the wet limestone method, were always the first choice for Western developed countries. Compared with that of the wet desulfurization method, capital investment of the dry desulfurization method was cheaper, but the lower desulfurization efficiency of the dry desulfurization method prevented spreading of that method and the underlying problem of repollution of the dry method also existed. The limestone-gypsum method has some advantages such as a high desulfurization efficiency and the ease of obtaining a desulfurizing agent. Although limestonebased wet FGD processes have been successfully operated for a long time in Western countries, some disadvantages such as the huge magnitude of capital investment and operating cost and the single purpose of the product gypsum in the construction industry still
exist in many Chinese FGD processes because of poor technology transfer and unsteady operation, such as the Huaneng Luohuang power plant in Chongqing and the Heshan power plant in Guangxi of China. Some typical wet FGD methods are given in Table 1. With the deep concern for sustainable development, both how to decrease the cost of desulfurization and how to raise the value of the side product have same importance. In an updated report, a novel FGD method to acquire the side product SO2 liquid became more attractive because of the multiple purposes of the SO2 liquid. In the 1970s, a buffer solution of sodium sulfate and sodium sulfite was used in desulfurization and a SO2 liquid was obtained from deabsorption of a heatloaded absorption solution. This method had a high desulfurization efficiency, but loss of the absorption agent was quite large because sodium sulfite was easily oxidized in operation. Midkiff used sodium citrate and citric acid to take the place of sodium sulfate and sodium sulfite so that the loss of the absorption agent was decreased to some extent. However, some disadvantages such as the high price of citric acid and degradation of sodium citrate existed. In 1985, a patent was reported in which a sodium phosphate buffer solution was used to improve Midkiff’s method.10 Some researchers studied the absorption and deabsorption properties of the sodium phosphate buffer solution. Their studies indicated that sodium phosphate had more stability in absorption and deabsorption. Ethylenediamine was strongly recommended in other literature because of its higher efficiency for desulfurization.11,12 These papers compared the sodium phosphate buffer solution, sodium citrate buffer solution, and ethylenediamine and concluded that it was more economical to use ethylenediamine in FGD. To lower the vapor pressure, researchers of Cansolv Co. in Canada tried to add an additive in the aqueous ethylenediamine solution and obtained satisfactory results. However, they did not name the additive in their paper.13
10.1021/ie0308691 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/26/2004
Ind. Eng. Chem. Res., Vol. 43, No. 21, 2004 6715 Table 1. Comparison of Some Typical Wet FGD Methods
item
limestonegypsum method
magnesia method
dual-alkali method
amine method
WellmanLord method
Cansolv method
Elsorb method
desulfurization efficiency (%) absorbent
>90
>90
>90
>90
>90
>99
>98
limestone
magnesia
amine
sodium sulfite
gypsum
sulfur dioxide
sulfur dioxide
sulfur dioxide
investment unit FGD expense
large medium
medium large
medium medium
sulfur ammonium nitrate small medium
organic amine salt sulfur dioxide
sodium phosphate
side-product recovery
limestone sodium carbonate gypsum
small large
small small
small medium
In contrast to the application of single absorbent ethylenediamine stated in the references,11,12 phosphoric acid was tested as an additive to the ethylenediamine aqueous solution to decrease the vapor pressure in this paper. When acid HX is added to the ethylenediamine solution, the existing reaction is
NH2CH2CH2NH2 + HX T NH2CH2CH2NH3+ + X(1) The characteristics of anion X- can influence the desulfurization process. If the anion is Cl-, the causticity of the absorbent is increased. Also, SO42- is not suitable because it influences the recovery of the absorbent. Part of SO2 is oxidized into SO42- in the FGD process, which should be removed in order to keep the absorption capacity of the absorbent. Also, considering the excellent buffer effect and anti-oxidation of PO43- in the sodium phosphate method, phosphoric acid is chosen as the best additive. This phosphoric acid-ethylenediamine solution was tested in desulfurization for a simulated flue gas in laboratory experiments. Vapor-liquid equilibrium in a desulfurization process using the phosphoric acidethylenediamine solution was first studied and predicted. The effects of the temperature, pH value, and composition of the absorption agent, the initial SO2 concentration in the absorption solution, and the liquidgas ratio on the absorption-deabsorption process carried out on a packing column were also studied.
kept constant at 25 °C. At the inlet port of the bottle, the samples of the gas phase were taken and their concentrations of SO2 were determined. The samples of the liquid phase were taken every 10 min until equilibrium was reached, and their SO2 concentrations were also determined. Absorption and Deabsorption Experiments. (a) Experimental Apparatus. Parts a and b of Figure 3 show sketch maps of the absorption and deabsorption experiments. (b) Experimental Methods. (i) Absorption. Air from an air compressor and SO2 from a cylinder were introduced through the flowmeter, mixed at the buffer bottle, and flowed into the bottom of the absorption
Experimental Section
Figure 1. Experimental setup for static absorption: (1) air compressor; (2) SO2 cylinder; (3) flowmeter; (4) manostat bottle; (5) absorption bottle; (6) thermometer; (A and B) sampling; (C) exhaust gas treatment.
Materials. The simulated flue gas was prepared by mixing compressed air and an SO2 reagent. A liquid SO2 reagent (purity g99%) was purchased from Beijing Xizhong Chemical Reagent Co. An absorption solution was prepared by mixing ethylenediamine, phosphoric acid, and deionized water. Ethylenediamine (purity g99%) and phosphoric acid (purity g85%; the other component is water) were purchased from the Beijing Xizhong Chemical Regent Co. Vapor-Liquid Equilibrium Determination. (a) Experimental Apparatus and Methods. Figure 1 shows a sketch map of the gas-liquid equilibrium determination. Air from an air compressor and SO2 from a cylinder were introduced through the flowmeter, mixed at the manostat bottle and flowed through the absorption bottle as shown in Figure 2. The bath temperature was
Figure 2. Schematic diagram of the absorption bottle: (1) inlet of the water bath; (2) outlet of the water bath; (3) inlet of the flue gas; (4) outlet of the exhaust gas; (5) thermometer; (6) sampling; (7) water bath; (8) valve; (9) valve.
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Ind. Eng. Chem. Res., Vol. 43, No. 21, 2004
Figure 4. Gas-liquid equilibrium model.
concentration in the loaded solution were changed, and then the above procedure was repeated. The deabsorption tower’s dimensions were a 30 mm diameter and a 600 mm packing height, and the tower was packed with a 6 mm × 6 mm θ stainless steel ring. The major devices were also the same as those described in the Vapor-Liquid Equilibrium Determination section. Analytical Methods. The concentration of SO2 in the flue gas and in the aqueous solution was analyzed by the iodine method, as was recommended in the literature.14,15 Results and Discussion Gas-Liquid Equilibrium. SO2 is easily dissolved in water and does not obey Henry’s law. This is because the SO2 dissolved in water forms H2SO3 and is ionized as follows: Figure 3. (a) Sketch map of the absorption experiment: (1) unloaded solution tank; (2) constant-temperature tank; (3) peristaltic pump; (4) absorption tower; (5) loaded tank; (6) buffer bottle; (7) compressor; (8) SO2 cylinder; (9) flowmeter; (10) wet flowmeter. (b) Sketch map of the deabsorption experiment: (1) constanttemperature tank; (2) preheated bottle; (3) electric heater; (4) heating bottle; (5) peristaltic pump; (6) condenser; (7) deabsorption tower; (8) condenser; (9) unloaded solution tank; (10) condenser tank.
tower, countercurrently contacting the absorption solution in the packing. At the outlet port of the column, the samples of the gas phase were taken every 10 min until equilibrium was reached, and their SO2 concentrations were also determined. The ratio of the absorption solution to the simulated flue gas, the pH value, and the temperature were changed, and the above procedure was repeated. The absorption tower’s dimensions were a diameter of 30 mm and a packing height of 600 mm, and the tower was packed with a 6 mm × 6 mm θ stainless steel ring. The major devices were the same as those described in the Vapor-Liquid Equilibrium Determination section. (ii) Deabsorption. The loaded solution was heated in a preheated tank and driven into the top of the deabsorption tower using a peristaltic pump. SO2 dissolved in the loaded solution was deabsorbed using an up-vapor from the heating bottle. The temperature at the bottom of the deabsorption tower was maintained at 101-103 °C. The SO2 concentration in the bottom of the deabsorption tower was determined when the operation was steady. The temperature, the pH value, the initial ethylenediamine concentration, and the SO2
H2SO3 T H+ + HSO3- T 2H+ + SO32-
(2)
The concentrations of H2SO3 in the aqueous phase and SO2 in the gas phase are true to the relation predicted by Henry’s law. However, the total concentration of SO2 in an aqueous solution, including HSO3- and SO32-, which always has an “apparent concentration” determined by an analysis method, cannot be predicted by Henry’s law.16-18 In this paper, a model is predicted as shown in Figure 4. As described in this model, mass transfer has two steps:
(a) SO2 in gas transfer to a liquid phase: SO2(g) + H2O T H2SO3
(3)
pSO2 ) H[H2SO3]
(4)
(b) H2SO3 is ionized: H2SO3 T H+ + HSO3-
(5)
HSO3- T H+ + SO32-
(6)
Each equilibrium constant is expressed as
KS1 ) KS2 )
[H+][HSO3-] [H2SO3] [H+][SO32-] [HSO3-]
(7)
(8)
Ind. Eng. Chem. Res., Vol. 43, No. 21, 2004 6717 Table 2. Equilibrium Constants in a SO2-Water System (25 °C) reaction
equilibrium constant
SO2(g) + H2O T H2SO3 H2SO3 T H+ + HSO3HSO3- T H+ + SO32Kw ) [H+][OH-]
log H ) -0.092 log KS1 ) -1.881 log KS2 ) -7.193 log Kw ) -13.999
[H+]2CS [H+]2 + KS1[H+] + KS1KS2 KS1[H+]CS
-
[HSO3 ] )
[H+]2 + KS1[H+] + KS1KS2 KS1KS2CS
2-
[SO3 ] )
+ 2
+
[H ] + KS1[H ] + KS1KS2
[H+]2 + KS1[H+] + KS1KS2 pSO2 CS ) H [H+]2
(9)
(10)
(11)
(12)
(13)
(21)
[HPO42-] ) [H+]3 + KP1[H+]2 + KP2KP1[H+] + KP3KP2KP1
+ 3
+ 2
[H ] + KP1[H ] + KP2KP1[H+] + KP3KP2KP1
(22)
(23)
Table 3 lists the parameters in the above formulas.19,20 The ionization of ethylenediamine can be depicted as
NH2CH2CH2NH2 + H2O T NH2CH2CH2NH3+ + OH- (24) NH2CH2CH2NH3+ + H2O T NH3CH2CH2NH32+ + OH- (25) The concentrations of [NH2CH2CH2NH3]+ and [NH3CH2CH2NH3]2+ can be calculated as
KED1 )
KED2 )
[NH2CH2CH2NH3+][OH-]
(26)
[NH2CH2CH2NH2] [NH3CH2CH2NH32+][OH-]
(27)
[NH2CH2CH2NH3+]
where CED denotes the total ethylenediamine concentration in the liquid phase. The calculation is
CED ) [NH2CH2CH2NH2] + [NH2CH2CH2NH3+] +
Each equilibrium constant is +
[NH3CH2CH2NH32+] (28)
-
[H ][H2PO4 ] [H3PO4]
(17)
[H2PO4-]
(18)
3-
[H ][PO4 ] [HPO42-]
[NH2CH2CH2NH3+] ) KED1[OH-]
2-
[H ][HPO4 ]
KP3 )
[H+]3 + KP1[H+]2 + KP2KP1[H+] + KP3KP2KP1
(14)
(16)
+
KP1CP[H+]2
KP3KP2KP1CP
HPO42- T H+ + PO43-
KP2 )
CP ) [H3PO4] + [H2PO4-] + [HPO42-] + [PO43-] (20)
[PO43-] )
(15)
+
Let CP represent the total phosphoric acid concentration in the liquid phase; the concentrations of [H2PO4-], [HPO42-], and [PO43-] can be calculated as
KP2KP1CP[H+]
H2PO4- T H+ + HPO42-
KP1 )
log KP1 ) -2.148 log KP2 ) -7.199 log KP3 ) -12.35
[H2PO4-] )
Table 2 lists the parameters in the above formulas.22,23 From formula (12), the apparent SO2 concentration in the liquid phase is affected by [H+] and the partial pressure of SO2 in the gas phase at the specified temperature. [H+] has a strong relationship with the ionization of the solute. In the ethylenediamine-phosphoric acid solution, the ionization of phosphoric acid and ethylenediamine occurs. Ionization of phosphoric acid can be depicted as
H3PO4 T H+ + H2PO4-
equilibrium constant -
H3PO4 T + H2PO4 H2PO4- T H+ + HPO42HPO42- T H+ + PO43-
According to the above formulas, the following equations can be deduced:
[H2SO3] )
reaction H+
Let CS denote the total concentration of SO2 in the liquid phase:
CS ) [H2SO3] + [HSO3-] + [SO32-]
Table 3. Constants in the Ionization of H3PO4 (25 °C)
(19)
CED (29)
[OH-]2 + KED1KED2 + KED1[OH-] [NH3CH2CH2NH32+] ) KED1KED2
CED (30)
[OH-]2 + KED1KED2 + KED1[OH-]
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Figure 5. Comparison of the predicted and experimental results for gas-liquid equilibrium: (a) CED ) 0.3 mol‚L-1, CP ) 0 mol‚L-1, (b) experimental results, (s) predicted results; (b) CED ) 0.3 mol‚L-1, CP ) 0.13 mol‚L-1, (2) experimental results, (‚‚‚) predicted results. (c) CED ) 0.3 mol‚L-1, CP ) 0.26 mol‚L-1, (9) experimental results, (-‚-) predicted results. (d) CED ) 0.3 mol‚L-1, CP ) 0.39 mol‚L-1, ([) experimental results, (-‚‚-) predicted results (experimental conditions: gas flux G ) 1.5m3‚h-1; temperature t ) 25 °C; the experimental setup is as shown in Figure 2). Table 4. Relative Errors between the Predicted and Experimental Results CSO2 (mol‚L-1) CED (mol‚L-1)
CP (mol‚L-1)
0.3
0
0.3
0.13
0.3
0.26
0.3
0.39
pSO2 × (Pa)
exptl results
pred results
relative error (%)
1.43 2.04 2.94 1.12 2.32 3.01 1.08 2.98 3.73 1.18 2.52 3.37
0.561 0.5631 0.582 0.482 0.466 0.468 0.3325 0.345 0.3525 0.2235 0.2288 0.2341
0.5993 0.5996 0.6002 0.47 0.47 0.47 0.34 0.34 0.34 0.21 0.21 0.21
6.827 6.482 3.127 2.49 0.858 0.641 2.256 1.449 3.546 6.04 8.217 10.295
102
Because the net charge of the solution is equal to zero,
Figure 6. Effects of the gas-liquid ratio on absorption (experimental conditions: gas flux G ) 1.5 m3‚h-1; concentration of diamine in the absorption solution CED ) 0.3 mol‚L-1; partial pressure of SO2 in the simulated flue gas phase pSO2 ) 0.296%; temperature t ) 25 °C; the experimental setup is as shown in Figure 3a).
become very small (less than 3%) and the predicted data fit the experimental results well, as shown in Figure 5b,c. In the gas-liquid equilibrium model depicted in Figure 4, to simplify the calculation, it is assumed that the activity coefficient is equal to 1. When the concentrations of ethylenediamine and phosphoric acid are increased, the influence of the activity coefficient also increases. So, errors between the predicted data and the results of the experiments also increase, as shown in Figure 5d. Absorption and Deabsorption Experiments. In the following absorption and deabsorption experiments, AE is defined as the desulfurization percentage to evaluate the absorption effects:
AE )
-
KED1[OH ] + 2KED1KED2 CED + [H+] ) - 2 [OH ] + KED1KED2 + KED1[OH-] KP1CP[H+]2 + 2KP2KP1CP[H+] + 3KP3KP2KP1CP + [H+]3 + KP1[H+]2 + KP2KP1[H+] + KP3KP2KP1 (KS1[H+] + 2KS1KS2)pSO2 Kw + + (31) + 2 [H ] H
[H ]
If CP, CED, and pSO2 were specified from formula (30), [H+] would be calculated. Then, according to formula (12), the total SO2 content in the liquid phase also would be calculated. Figure 5 shows the predicted and experimental results of the SO2 concentration in the liquid phase compared to the partial pressure of SO2 in the gas phase at different conditions. Table 4 shows the relative error between the predicted results and the experimental results at different conditions. Because ethylenediamine is comparatively volatile, the larger error between the predicted data and experimental results in the process without phosphoric acid may be responsible for the volatile loss of ethylenediamine as shown in Figure 5a. When phosphoric acid is added until an appropriate concentration range is reached, the average errors
yi - yo × 100% yi
(32)
where yi is the inlet concentration of SO2 in the gas phase and yo is the outlet gas concentration of SO2 after absorption. DE is defined as the deabsorption percentage to evaluate the deabsorption effects:
DE )
xi - xo × 100% x i - xΦ
(33)
where xi denotes the inlet concentration of SO2 in loaded absorption solutions and xo denotes the outlet concentration of SO2 in unloaded solutions after deabsorption. When absorption is used repeatedly, xΦ represents the initial SO2 concentration before absorption. If a fresh absorption solution is prepared and used for first time, xΦ is equal to zero. (a) Effect of the Gas-Liquid Ratio. Figure 6 shows the experimental results at different gas-liquid ratios. The results indicate that AE increases with an increase of the gas-liquid ratio R. When R is greater than 0.8 L‚m-3, the desulfurization percentage’s tendency to increase is not obvious. Because the running cost is proportional to R, the suitable R was chosen as 0.61.0 L‚m-3. (b) Effect of the Ethylenediamine Concentration in an Absorption Solution. Figure 7 shows the effect
Ind. Eng. Chem. Res., Vol. 43, No. 21, 2004 6719
Figure 7. Effects of the ethylenediamine concentration on absorption (experimental conditions: gas flux G ) 1.5 m3‚h-1; gasliquid ratio R ) 0.6 L‚m-3; partial pressure of SO2 in the simulated flue gas phase pSO2 ) 0.322%; temperature t ) 25 °C; the experimental setup is as shown in Figure 3a).
Figure 8. Effects of the ethylenediamine concentration on deabsorption (experimental conditions: loaded solution flux L ) 0.45 L‚h-1; initial concentration of SO2 in the loaded solution w ) 7.4 g‚L-1; preheating temperature t ) 60 °C; the experimental setup is as shown in Figure 3b).
that the ethylenediamine concentration in the absorption solution has on the absorption process. The results show that the desulfurization efficiency AE increases with an increase of the ethylenediamine concentration in the absorption solution. However, when the concentration is higher than 1 mol‚L-1, precipitation occurs. When phosphoric acid and ethylenediamine are dissolved in water, part of the acid radical and part of the NH2 group from the ethylenediamine may form the corresponding salt, as shown in formula (33); the other NH2 group takes charge of interaction with SO2.
NH2RNH2 + H+ T NH2RNH3+
(34)
The solubility of this salt is comparatively low. When the ethylenediamine concentration increases, the activity of the salt also increases. It even leads to precipitation. According to the experimental results, the suitable ethylenediamine concentration is set as 0.3-0.5 mol‚L-1. The ethylenediamine concentration also affected the deabsorption efficiency, as shown in Figure 8. Ethylenediamine has two NH2 groups. One of them is more alkaline and may interact with SO2 to form the corresponding salt.21 This salt does not readily deabsorb by heating. Absorption and deabsorption of SO2 using an ethylenediamine-phosphoric acid buffer solution happen at the second NH2 of ethylenediamine. When a fresh buffer solution is deabsorbed for the first time, the deabsorption efficiency is comparatively low. However, when a regenerated absorption solution was reused, deabsorption became more perfect, as shown in Table 3. When the initial concentration of ethylenediamine in the absorption agent increases, the probability of SO2
Figure 9. Effects of the pH on absorption (experimental conditions: gas flux G ) 1.5 m3‚h-1; gas-liquid ratio R ) 0.6 L‚m-3; partial pressure of SO2 in the simulated flue gas phase pSO2 ) 0.308%; concentration of ethylenediamine in the absorption solution CED ) 0.3 mol‚L-1; temperature t ) 25 °C; the experimental setup is as shown in Figure 3a).
Figure 10. Effects of the pH on deabsorption (experimental conditions: loaded solution flux L ) 0.45 L‚h-1; initial concentration of SO2 in the loaded solution w ) 8.3 g‚L-1; concentration of ethylenediamine in the absorption solution CED ) 0.3 mol‚L-1; preheating temperature t ) 60 °C; the experimental setup is as shown in Figure 3b).
combining with ethylenediamine also increases. This may lead to a decrease in the deabsorption efficiency. Because a high ethylenediamine concentration is good for absorption and not good for deabsorption, a suitable value must be found. According to the experimental results, the ethylenediamine concentration is specified as 0.3 mol‚L-1; the corresponding absorption efficiency is 98.5%, and the deabsorption efficiency is 60% (the first time). (c) pH Effect. Figure 9 shows the pH effect on absorption and deabsorption. The pH value of the absorption agent is adjusted by changing the proportion between phosphoric acid and ethylenediamine. The results indicate that the desulfurization efficiency AE increases with an increase of the pH value. A pH value that is too low causes a decrease in the number of active groups for absorption, while a pH value that is too high leads to an increase of volatile loss, so a suitable pH value must be found. Figure 10 shows the pH effect on deabsorption. The results indicate that the deabsorption efficiency decreases with an increase of the pH value. This is because a decrease of [H+] with an increase of the pH value strengthens the combined power of the absorption agent with SO2. Considering both the absorption and deabsorption effects, the suitable pH value is specified as 6-7. (d) Effects of the Initial SO2 Concentration. The residual SO2 in an unloaded absorption agent after deabsorption will affect the absorption efficiency, as shown in Figure 11. The data indicate that the desulfurization efficiency increases with a decrease of the SO2 concentration in the initial absorption agent. When the SO2 concentra-
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Figure 11. Effects of the initial SO2 concentration on absorption: (9) R ) 1 L‚m-3, (0) R ) 0.6 L‚m-3 (experimental conditions: gas flux G ) 1.5 m3‚h-1; concentration of ethylenediamine in the absorption solution CED ) 0.3 mol‚L-1; partial pressure of SO2 in the simulated flue gas phase pSO2 ) 0.289%; temperature t ) 25 °C; the experimental setup is as shown in Figure 3a).
Figure 14. Effects of the liquid flow rate on deabsorption (experimental conditions: feed temperature t ) 60 °C; concentration of ethylenediamine CED ) 0.3 mol‚L-1; concentration of phosphoric acid CP ) 0.26 mol‚L-1; initial SO2 concentration w ) 16 g‚L-1). Table 5. Results of the Repeated Experiments
repeated R times (L‚m-3) 0 1 2 3 4
Figure 12. Temperature effect: (9) t ) 20 °C, (0) t ) 40 °C, (2) t ) 60 °C, (4) t ) 80 °C (experimental conditions: G ) 1.5 m3‚L-1; partial pressure of SO2 in the simulated flue gas pSO2 ) 0.320%; ethylenediamine concentration CED ) 0.3 mol‚L-1).
Figure 13. Effects of the feed temperature on deabsorption (loaded solution flux L ) 0.45 L‚h-1; concentration of ethylenediamine CED ) 0.3 mol‚L-1; initial SO2 concentration w ) 14.1 g‚L-1).
tion exceeds 6 g‚L-1, the buffer capacity declines, while too low of an SO2 concentration in the solution would result in more steam consumption in deabsorption. In this investigation, the initial SO2 concentration in the absorption agent was specified as 6 g‚L-1. (e) Temperature Effect. Figure 12 shows the effect that temperature has on absorption. The results indicate that the desulfurization efficiency will decrease when the temperature is increased. Because the absorption temperature must be controlled by means of cooling the flue gas, temperatures that are too low may cost more energy. The suitable temperature is specified as 40-60 °C. (f) Effects of the Feed Temperature on Deabsorption. Figure 13 shows the effect of the feed temperature on deabsorption. The results indicate that the
0.6 1 1 1 1
pSO2 (%) 0.312 0.296 0.288 0.324 0.294
concn of SO2 concn of SO2 in a loaded in an unloaded solution solution (g‚L-1) (g‚L-1) 13.92 13.44 13.28 14.08 13.60
4.35 4.51 4.77 5.12 4.99
AE (%)
DE (%)
99.1 68.8 96.9 98.2 95.3 97.0 93.2 96.2 92.2 101
deabsorption efficiency increases with an increase of the temperature. When the feed temperature is higher than 60 °C, the tendency of the deabsorption efficiency to increase with the temperature slows down. Too high feed temperature may cause high energy consumption for heating, so 60 °C is considered to be the suitable temperature. (g) Effect of the Liquid Flow Rate. Figure 14 shows the effect of the liquid flow rate on deabsorption. A low liquid flow rate will increase the deabsorption efficiency and decrease the throughput. According to the experimental results, the suitable rate is specified as 0.7 m3‚m-2‚h-1. Repeated Experiments. To investigate the desulfurization ability of the absorption reagent after regeneration, the absorption-deabsorption cycles were repeated several times. The experimental results are shown in Table 4. Table 5 indicates that when the absorption solution is repeatedly used several times, the desulfurization efficiency will decrease slowly. A fresh absorption reagent must be supplied every few times in order to maintain comparative desulfurization efficiency. Analysis of the Safety and Economy. The absorbent in the novel method has little toxicity, and absorption and deabsorption are operated at normal pressure and below 110 °C. Furthermore, phosphoric acid is added to decrease the volatility of ethylenediamine, so the damage of the absorbent on air can be nearly ignored. For a 200 MV power plant, several wet FGD methods have been economically analyzed and compared, as shown in Table 6, based on a primary extrapolation by the experimental results. It is indicated that the unit investment is 254 RMB‚kW-1 and the FGD cost is 426 RMB‚(t of SO2)-1, which is one-third of that of the wet limestone-gypsum method widely used in FGD and also is lower than that of the recent NADS method. Suggestions about Future Researches. In this paper, the simulated flue gas was produced by mixing the compressed air with SO2 from the cylinder. Therefore, the major objective of this work is evaluating the desulfurization effect of the ethylenediamine-phospho-
Ind. Eng. Chem. Res., Vol. 43, No. 21, 2004 6721 Table 6. Economical Comparisons among Several Wet FGD Methods in China for a 200 MW Power Plant22
item
limestonegypsum method
ammonia method
NADS method
ethylenediaminephosphoric acid method
unit FGD capital/(RMB‚kW-1) annual operation cost (104 RMB) side-product value per year (104 RMB) unit FGD expense [RMB‚(t of SO2)-1 in the flue gas]
700 3099 discarded 1299
647 4044 3024 646
300 2650 2000 450
254 5400 3824 426
ric buffer solution, while the influence of NOx on desulferization of the absorbent has not been considered. This effect will be investigated in future studies. In this experiment, the dimensions of the packed bed in the experiment were a diameter of 30 mm and a packing height of 600 mm, and the packing was a 6 mm × 6 mm θ stainless steel ring. Because the masstransfer rate is relatively large and it is easier to make the gas or liquid phase distribution in the packed column well-proportional in the laboratory scale, the influence of the dimensions of the device is minimal. When this process is extrapolated to a pilot or full scale, the dimensional effect will become significant. So, such researches on the internal forms of the kinetic data tests like the mass-transfer coefficient can be carried out in the future.
gested that this novel method be applied to the FGD process in power plants using high sulfur coal as the fuel. Nomenclature C ) concentration in the liquid phase, mol‚L-1 H ) Henry’s coefficient, Pa‚mol-1‚L-1 R ) gas-liquid ratio, L‚m-3 G ) gas flux, m3‚h-1 L ) liquid flux, L‚h-1 l ) liquid flow rate, m3‚m-2‚h-1 K ) equilibrium constant w ) initial SO2 concentration in the liquid phase, g‚L-1 x ) SO2 concentration in the liquid phase, g‚L-1 [ ] ) concentration of the ion, mol‚L-1 Subscripts
Conclusions This paper presents a novel FGD process in which an organic amine is used to absorb sulfur dioxide in the flue gas. (1) The vapor-liquid equilibrium model of absorption of sulfur dioxide in the flue gas by an organic amine was established, and vapor-liquid equilibrium of the sulfur dioxide-ethylenediamine-phosphoric acid-water system was first predicted. When the predicted results were compared with the experimental results for a 0.3 mol‚L-1 ethylenediamine-phosphoric acid solution system, the relative errors were very small at appropriate concentration ranges of ethylenediamine and phosphoric acid. This indicates that the model is able to predict vapor-liquid equilibrium under experimental conditions. (2) Using an ethylenediamine-phosphoric acid aqueous solution as the absorbent, technical conditions in absorption and deabsorption were experimentally researched in a L 30 mm × 600 mm packed tower in the laboratory. The best conditions for absorption after optimization under the laboratory scale were fixed as follows: the ratio of liquid to gas R was 0.6-1.0 L‚m-3, the concentration of ethylenediamine in the absorption solution was 0.3 mol‚L-1, the residual SO2 concentration was 4-6 g‚L-1, and the pH value was 6.0-7.5. The optimal conditions for deabsorption were as follows: the feed temperature was 60 °C, the bottom temperature of the deabsorption tower was 103 °C, the liquid flow rate was 0.7 m3‚m-2‚h-1, and the concentration of SO2 was 14-16 g‚L-1. (3) The novel FGD method presented in this paper has advantages such as high desulfurization efficiency and low investment and unit FGD expense. For a 200 MV power plant, the unit investment is 254 RMB‚kW-1 and the FGD cost is 426 RMB‚(t of SO2)-1, which is onethird of that of the wet limestone-gypsum method widely used in FGD and also is lower than that of the recent NADS method. Moreover, the side product liquid sulfur dioxide has multipurpose utilization. It is sug-
ED ) total ethylenediamine in the liquid phase ED1 ) NH2CH2CH2NH3+ ED2 ) NH3CH2CH2NH3+ i ) inlet o ) outlet P ) total phosphor in the liquid phase P1 ) H2PO3P2 ) HPO32S ) total sulfur in the liquid phase S1 ) HSO3S2 ) SO32w ) water Φ ) initial
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Received for review December 15, 2003 Revised manuscript received June 14, 2004 Accepted June 28, 2004 IE0308691
