Kinetic Study on Mercury Sorption from Fuel Gas - Energy & Fuels

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Kinetic study on mercury sorption from fuel gas Muhammad Nurul Huda, Wenhan Li, Meilin Dai, and Lvrong Lin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02784 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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Kinetic study on mercury sorption from fuel gas Muhammad Nurul Huda 1*, Wenhan Li 2*, Meilin Dai3, Lvrong Lin4 1 2

Centre for Advanced Research in Sciences, University of Dhaka, Dhaka, Bangladesh

School of Energy, Power and Mechanical Engineering North China Electric Power University, Beijing 102206, P.R. China 3

School of Renewable Energy, North China Electric Power University, Beijing 102206, P.R. China 4 *

Fujian Longking Co., Ltd., No.81,Lingyuan Road, Longyan City, Fujian, China

Corresponding author, Phone : +8801717165146, Email: [email protected]

Abstract In this paper, the adsorption performance of modified fly ash was evaluated by using fixed bed adsorption device. The influence of different inlet concentrations of mercury in simulated flue gas and different concentrations of HBr solution on the mercury absorption process were studied. The results showed that with the increase of inlet mercury concentration in the simulated flue gas, the adsorption capacity and adsorption rate of the modified fly ash increased. With the increase of the HBr concentration, the adsorption amount and adsorption rate of mercury in simulated fly ash also increased. It was also found that the adsorption process follow the pseudo-first order adsorption kinetic model. Key words: mercury; adsorption capacity; adsorption rate; kinetic model 1. Introduction China has the 1st largest coal excavator with 1243 million tons in 1998, and the 3rd largest reserves of coal, after United States and Russia. (OECD/IEA, 1999). 1 The electricity generation in China is mainly dependent upon the coal-based thermal power plants. Although the government has been putting efforts on the consumption of natural gas in replacing coal, the use of coal might still continue in the coming several decades due to its massive reserves. 2 Emissions of mercury (Hg) from coal combustion are between one and two orders of magnitude higher than emissions from oil combustion. In 2012, China has a total energy consumption of 3.6 billion tons of coal, which is creating an arising serious problem of emission of Hg, not even to mention the extremely high average Hg contamination of the coal at china. 3-4 1

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In October 2013, the United Nations Treaty Minamata Convention on Mercury was signed at Kumamoto, Japan, by over 90 countries including China, to protect human health and the environment from anthropogenic emission and releases of mercury and mercury compounds. 5-6 Previous study illustrated that mercury may be presented in flue gas as elemental mercury vapor (Hg0), as a vapor of an oxidized mercury species (Hg2+) and as particulate bound mercury (Hgp). 7-8 The latter two species are absorbable by existing pollution control devices e.g. flue gas desulfurization (FGD) and electrostatic precipitator (ESP), but the remaining elemental mercury vapor as persistent pollutant in flue gas requires for further disposal. 9-10 For the majority of existing coal-based power plants, the leading technology to comply with new mercury emission regulations is the injection of powdered activated carbon (AC) in the flue gas. The gas-phase mercury in the flue gas is sorbed on the AC. However, due to the high cost of AC and the high Hg contamination mentioned above in Chinese industries, the application of Hg sorption by AC is obviously limited, not even mention the waste of fly ash (FA) with the mixing activated carbon leads trouble to the further industrial application of FA in cement producing. 11 Meanwhile, as a cheap alternative, FA has recently been widely studied for its potential of adsorbing Hg after some certain treatments. Recently our group has focused our study on the sorption of mercury from flue gas on to FA or surface modified FA. 12-15 This paper is to report some of the mercury-FA sorption characteristics that have not been studied yet. For better understanding of how different environmental conditions influence the speed of Hg sorption on FA and the construction of mathematical models that can describe the mechanism of the sorption process, even for further study on the determination of the rate-controlling step and transition state, the investigation of sorption kinetics offers great help. In fact, not only the kinetics properties, but also the isotherms are important in sorption study that leads to an easy assumption of sorption capacity under the studied temperature. Such sorption characteristics of liquid mercury from solutions have been reported by many researchers so far, but similar report for vapor mercury has not yet been reported to our knowledge. Thus the purpose of this study is to focus on the characterization of mercury sorption on FA surface by investigating the kinetics and isotherms. 2. Method The experiment used the PSA 10.680 portable online Hg continuous emission monitoring (CEM) unit to measure, samples were put in the electric oven thermostat which was produced by Shanghai Jinghong experimental equipment Co. to complete the experiment at a constant temperature. The measurement device mainly consists of 2


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two parts, one part is the PSA 10.680 portable online Hg CEM, another part is the fixed bed adsorption device which was put in the electric oven thermostat. Fig.1 is the illustration of the experimental setup. The saturated mercury vapor flow was controlled by one mass flow controller (MFC1) from the mercury storage room and the dilute gas flow was controlled by another mass flow controller (MFC2). Two gases were mixed to get a certain concentration of mercury vapor as the simulation power plant flue gas. The simulation power plant flue gas passed the samples in the fixed bed, after that the gas troughed the shunt to make the part of simulated flue gas at the exit of the fixed bed enter the Sir Galahad Amasil adsorbed. The Amasil was heated and argon was used carrier gas, Amasil was measured by atomic fluorescence spectrum.

Figure 1. Diagram of the experiment. Experiment choose three groups of different concentrations of mercury simulated flue gas and the adsorption of three groups of different temperature as the main research content of this article, calibrated the PSA 10.680 portable online Hg CEM and verified the calibration curve of the standard or not. The PSA 10.680 portable online Hg CEM contained source of mercury, the experimental value will be set. Then the accordance and stability between the exit of mercury concentrations and set value was tested. Weighed 2g fly ash to tile on the filter paper and spread 15 g quartz sand was mixed on it evenly. To avoid the simulated flue gas blowing up samples, the test samples were put in the adsorption device of the fixed bed. Through the fixed bed, the simulated flue gas entered into the online Hg CEM to be analyzed and recorded the concentration of mercury after the activated carbon adsorption. The exhaust was bubbled into activated carbon for absorbing and discharging into the air again.

3.1 Kinetic study Adsorption kinetics study is essential for the determination of the uptake rate of adsorbate at the solid-gas interface. Several adsorption kinetic models have been developed to elucidate the adsorption kinetics and rate limiting step. In this study, two 3


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kinetic models, the pseudo-first order reaction model and the pseudo-second order reaction model, as shown below, were used to investigate their suitability to describe the behavior of Hg uptake by FA. dqt = k1 ( qe − qt ) (1) dt dqt = k1 ( qe − qt ) 2 (2) dt Where t is sampling time (min), qe is adsorbed Hg concentration (ng/g) in FA at equilibrium, and qt is the adsorbed Hg concentration (ng/g) in FA at time t. k1 and k2 are the pseudo-first order and the pseudo-second order reaction constants (min−1 and g·ng−1·min−1), respectively. By integrating Equations (1) and (2) over time t with the initial condition as qt = 0 at t = 0, the following linear equations can be obtained for the first order and the pseudo-second order models, respectively.

ln ( qe − qt ) = ln qe − k1t

(3)

t 1 1 = t+ qt qe k2 qe2

(4)

Equations (3) and (4) can be plotted as ln(qe-qt) vs. t and t/qt vs. t, respectively. A linear dependency would mean a good fit between experimental results and the model calculations. Figure 2 is plotted using a group of data according to the equations 3 and 4. The result of linear fit of experimental data with the pseudo-first order reaction model is displayed in Figure 2(a) and that with the pseudo-second order reaction model is displayed in Figure 2(b). Slope and intercept of both equations listed in Figure 1 can be used to calculate sorption parameters qe and k. By comparing the values of R2, linear correlation efficiency, a better fit model among these two can be readily identified. For all experimental results listed in Table 1, squire of correlation coefficients R2 for both models are high, nearly one. However, if qe is calculated from pseudo-second order equation, the value is far different from experimental qmax result. But qe derived from the pseudo-first order equation matches

qmax well, which indicates the pseudo-first order model interprets the sorption model better, similar with the research works for the sorption of Hg from aqueous solutions. 16 Thus the pseudo-first order equation is chosen to interpret all experimental data for all following discussions in this paper.

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Table 1.The reaction rate at different inlet concentrations equation fitting correlation coefficient qmax（experimental） pseudo-first order ng/g qe k1 R2

pseudo-second order

10

1875

2132 0.0214

0.973

3174 3.15E-06

0.972

15 20

2096 2250

2278 0.0216 2443 0.0220

0.985 0.990

3713 2.69E-06 4132 2.47E-06

0.974 0.989

c0 ug/m

3

qe

k2

R2

Temprature qmax（experimental） pseudo-first order ℃ ng/g qe k1 R2

pseudo-second order

50 140

1875 591

2132 0.0214 645 0.0372

0.973 0.988

3174 3.15E-06 1527 6.55E-06

0.972 0.984

200

357

406

0.981

758

0.994

0.0500

qe

k2

1.32E-05

R2

9 8 7 6

ln(qe-qt)

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y=-2.20E-02x+7.80 2 R =0.990

5 4 3 2 1 0

0

10

20

30

40

50

60

70

80

Time(min)

Figure 2a. linear fit of experimental data with the pseudo-first order reaction model

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0.04

0.03 y=2.42E-04x+2.37E-02 2 R =0.988

t/qt

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0.02

0.01

0.00

0

10

20

30

40

50

60

70

80

Time(min)

Figure 2b. Linear fit of experimental data with the pseudo-second order reaction model

Influence of mercury concentration (C0).

Figure 3a. The adsorption of different inlet mercury concentration curve

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Figure 3b. The different inlet concentration of mercury adsorption rate curves The general trend of concentration influence on sorption behavior in Figure 3 appears as expected. Figure 3 (a) is sorption capacity change over time and (b) is sorption rate over time. With the increasing of initial mercury concentration in flue gas, both sorption capacity and sorption rate increased slightly as well. Parameters involved here are listed in table 1, where it can be noticed that qe increased by 20% from 1875ng/g to 2250 ng/g, when initial mercury concentration in flue gas is doubled from 10ug/m3 to 20 ug/m3. The sorption rate parameter increased, even a slight, from 0.0214 to 0.0220, by only 2.7% which is negligible (lower than 5%). Results suggest that the initial concentration of Hg slightly influence both sorption capacity and rate. But with the increasing of initial Hg concentration, this influence became less remarkable.

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Influence of temperature

Figure 4a. Change of Hg concentration in fly ash over time at different temperatures

Figure 4b. Change of Hg adsorption rate over time at different temperatures Deriving from parameters in Table 1 and equation plot in Figure 4, with the increasing of temperature, both the sorption rate and capacity decreased remarkably and firmly. The reason of this phenomenon should be derived from the mechanism of vapor sorption on solid surface. When a mercury atom attracts by fly ash molecules, the adhesion among them, weather physical of chemical, decrease the distance between 8


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these two particles dramatically and the shortened distance stops the adsorbate particles from being vapor form but turns into liquid or solid state. Thus the ability of vaporization of mercury, which follows the saturated vapor pressure, should be related to the ability of mercury sorption by FA. A high temperature often leads to a higher saturated vapor pressure of mercury which results in a stronger attention of vaporization that keep the mercury atom from being closer enough with FA molecules in order to turn into a more stable solid or liquid form for the process of adsorption. Decreasing of the sorption efficiency with the increasing of temperature in this study indicate the process as physisorption. Since the increasing of temperature leads to an increasing of saturated vapor pressure which is a key factor for physisorption. Moreover with high saturated vapor pressure, more Hg molecules prefer Free State better than being adsorbed.

Adsorption isotherms Basically, two well established isotherms frequently used to describe the equilibrium of the sorption of a material at the surface at constant temperature are the Langmuir isotherm (equation 5) and the Freundlich isotherm (equation 6). Both isotherms can be interpreted as linear equations shown as equation 7 and 8 respectively. qe =

qmbCe 1 + bCe

qe =K F C

1 n e

(5)

(6)

1 1 1 1 = + qe qm bqm Ce

(7)

1 log qe = log K F + log Ce n

(8)

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3.36

3.34 y=2.60E-01x+3.03 2 R =0.998

Log qe

3.32

3.30

3.28

3.26

0.9

1.0

1.1

1.2

1.3

Log Ce

Figure 5a. Linear fitting results of adsorption isotherm model for Langmuir model.

0.00054 0.00052 y=1.58E-03x+3.58E-04 2 R =0.999

-1

0.00050 0.00048

qe

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0.00046 0.00044 0.00042 0.05

0.06

0.07

0.08

0.09

0.10

0.11

-1

Ce

Figure 5b. Linear fitting results of adsorption isotherm model for Freundlich model where b is the Langmuir constant and qm is the maximum saturation capacity; KF is the Freundlich constant and 1/n is the slope reflecting the affinity between the sorbent and adsorbate. By doing linear fit of the experimental data with the linear format of both models shown in equations 7 and 8, the parameters involved are generated and listed in Table 2. The fitted results are displayed in Figure 5, that shows both high correlation efficiencies nearly 1. Both Langmuir and Freundlich equations are capable for the interpretation of sorption isotherms. For the Langmuir fitting in Figure 5(a), according to the three assumptions on which the Langmuir isotherm is based, it can be concluded that the sorption of mercury onto FA surface is likely to be a monolayer 10


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process. Finally, the Isotherm curve is shown in Figure 6 accordingly. Two different temperatures were studied. 140oC is a classic temperature at the entrance of an electrostatic precipitator (ESP), where sorbents are injected in a power plant for the control of mercury concentration in flue gas especially in the US. Isotherm plot of it in Figure 6 shows a typical monolayer sorption trend with a max sorption concentration less than 900 ng/g which is several orders less than the adsorbate of activated carbon in our previous study. Considering temperature plays an important role in the process, isotherms of 50oC is also plotted for reference. The decreasing of qm and b values over temperature as shown in Table 2 indicates an exothermal reaction of the sorption. Therefore the adsorption process is typical physical adsorption process. The mechanism behind this should be contributed to the increasing of saturated vapor pressure along with the increasing of temperature. For physical adsorption, saturated vapor pressure certainly plays a key factor, and here its increasing limited the adsorption of Hg onto the FA surface. R2 values for Fruendlich model analysis in Table 2 are also high. So both models are suitable for our data. Noticing that n values here are both larger then 2, so the adsorption happened at both temperatures are generally considered to be difficult. Table 2. Parameters generated from two Isotherm models at two different temperatures

Temprature

℃ 50 140

langmuir isotherm

Freundlich isotherm KF((ng/g)1-1/n)

n

R2

0.999

1060

3.84

0.998

0.998

272

2.84

0.999

qm(ng/g)

b(g/ng)

R

2793

0.227

943

0.191

2

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Figure 6. Isotherm curve of adsorption process at two different temperatures

Conclusions The kinetics and thermodynamics of mercury sorption on FA surface were investigated with one kind of fly ash under different temperatures within the period of time ranging from 0 to 250 mins. The influence of inlet Hg concentration on the adsorption process was studied. Linear fitting showed the process follows the pseudo-first order sorption kinetics and revealed the adsorption as a physical sorption process. Thermodynamic study reviewed both Langmuir and Fruendlich isotherms can interpret the adsorptions at both 50 oC and 140 oC. Parameters of both isotherms are calculated and the results indicated the adsorption of Hg on fly ash surface is difficult to happen. Negative correlation between temperature and adsorption efficiency were reviewed.

Acknowledgements Financial support from the Fundamental Research Funds for the Central Universities (2014QN10 and 2015QN10) in China is gratefully acknowledged.

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Kinetic Study on Mercury Sorption from Fuel Gas - Energy & Fuels

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