Investigating Effects of Zeolites As an Agent to Improve Limestone

ARTICLE pubs.acs.org/EF

Investigating Effects of Zeolites As an Agent to Improve Limestone Reactivity toward Flue Gas Desulfurization Paul Maina* and Makame Mbarawa Tshwane University of Technology, P.O Box Private Bag X680, Pretoria, 0001 South Africa ABSTRACT: Natural zeolite was utilized as an agent to increase the reactivity of lime for desulfurization. Due to the presence of hydrous silica
aluminates in zeolite, a pozzolanic reaction ensued which ended up with an increase in the surface area of lime thus making it more reactive. Tests were done in a pH-stat apparatus and a fixed bed reactor, representing wet and dry flue gas desulfurization (FGD), respectively, to check if lime reactivity was augmented by the addition of zeolite. It was found that zeolite amplifies reactivity of lime with the highest reactivity being achieved with a lime to zeolite ratio of 1:1 (by mass). This was confirmed by Brunauer
Emmett
Teller (BET) surface area analysis (surface area increment with the reactivity) and scanning electron microscopy (SEM) imaging (rough morphology being formed). Further optimization tests were conducted in the pH-stat apparatus with the aid of design expert software to quantify linear, quadratic, and interactive effects of four variables (temperature, lime to zeolite ratio, liquid to solid ratio, and stirring speed). Temperature as a variable had the highest effect, followed by lime to zeolite ratio. Stirring speed had the least effect on the reactivity.

1. INTRODUCTION World market energy consumption is projected to grow by 50% over the next 20 years. This growth will mostly be associated with developing countries. Fossil fuels are expected to continue supplying much of the energy use worldwide. Due to sustained high prices for oil and natural gas, coal use has become very attractive economically, especially in coal-rich nations such as South Africa and China. Coal’s share of the world energy use has increased sharply over the past few years.1 In South Africa for instance, due to rapid increase in electricity demand, three old coal-fired power stations have been revived and two new ones are being built. This increase in coal consumption is accompanied by an increase in emissions of harmful pollutants such as sulfur dioxide (SO2), particulate matter, and greenhouse gas. Currently, South Africa has done well in combating particulate matter emissions. Sulfur dioxide emission control is still not very effective as yet, mostly because South African coal has low amount of sulfur in it, therefore less sulfur dioxide emission. However, due to stringent upcoming environmental regulations, South Africa will be compelled to reduce if not eliminate sulfur dioxide emissions. Almost all methods of reducing sulfur dioxide in flue gas emissions are categorized into three major groups, namely dry, semidry, and wet flue gas desulfurization (FGD). Each has specific advantages and disadvantages. Dry FGD processes are cheap to construct or retrofit, and have products which are easy to handle, but they are very inefficient. On the other hand, wet FGD processes are very efficient in reducing sulfur dioxide (reaching up to 95%) but their capital and operational costs are huge. Semidry FGD processes are in between dry and wet processes. FGD processes usually are based on the fact that sulfur dioxide is acidic in nature, therefore, it can be easily removed by reaction with a suitable alkaline sorbent. The most commonly used material for FGD is limestone due to its availability and price. Limestone is mainly made up of calcite (CaCO3) which, r 2011 American Chemical Society

although it is alkaline in nature, is not reactive enough to remove a desirable amount of sulfur dioxide from flue gases. Therefore, limestone is usually calcined to quicklime (CaO) depending on the carbon dioxide (CO2) partial pressure, in the following reaction: CaCO3 f CaO þ CO2

ð1Þ

Quick lime reactivity is usually higher than that of limestone. Many researchers attribute this increase in reactivity to the increase in surface area and porosity of the calcination product.2
5 Also, quicklime’s increase in reactivity can be associated with its superior chemical properties as compared to limestone. To further increase the reactivity, quicklime is usually hydrated to slaked lime (Ca(OH)2). CaO þ H2 O f CaðOHÞ2

ð2Þ

Slaked lime, although more reactive than quicklime and limestone, still is not reactive enough to remove enough sulfur dioxide especially from high sulfur coal, and therefore cannot beat the stringent regulations imposed on sulfur dioxide emission. This made researchers look into ways of improving the reactivity of slaked lime. Recently, the most promising result is by reacting it with a pozzolanic material. In the same way that pozzolans have been found to improve cement properties, they were also found to add to the reactivity of slaked lime. Pozzolans, being primarily made of vitreous aluminous
siliceous materials, react with slaked lime to form complex calcium alumina
silicates. These complex products have very high surface area and porosity and therefore have very high reactivity. In addition, the products of sulfation reaction with these sorbents are very stable. Received: January 28, 2011 Revised: March 29, 2011 Published: March 30, 2011 2028

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Table 1. Chemical Analysis of the Materials percentage present compound

natural limestone

calcined limestone

zeolite -

calcite (CaCO3)

85.14

5.02

quicklime (CaO)

-

75.54

-

heulandite ((Ca,Na)2
3Al3(Al,Si)2Si13O36 3 12H2O)

-

-

80.14

quartz (SiO2)

3.80

2.52

12.24

dolomite (CaMg(CO3)2)

8.99

-

-

C2S gamma mumme (Ca2SiO4)

-

7.81

-

muscovite (KAl2(AlSi3O10)(F,OH)2)

-

-

7.62

periclase (MgO) illite ((K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)])

2.06

2.81 -

-

hematite (Fe2O3)

-

1.78

-

phlogopite (KMg3AlSi3O10(F,OH)2)

-

1.74

-

portlandite (Ca(OH)2)

-

2.77

-

Several sources of pozzolanic materials have been researched, with the most common being fly ash. Although readily available and in large quantities where FGD is widely applied (coal power plants), fly ash contains some hazardous materials such as radioactive materials and mercury, which, although present in small quantities, are still dangerous.6 Also, apart from coal power plants, there are other facilities which require FGD and do not have fly ash, e.g., waste incinerator plants, pulp and paper factories, metal smelting, engines using high sulfur fluid fuels, and oil refineries. This prompted researchers to look for other sources of alumina silicates such as diatomite,7 oil palm ash,8 and rice husk ash.9,10 In this research, natural zeolite with a high content of alumina
silicates is used to enhance the reactivity of slaked lime. To the best of the authors knowledge, natural zeolite has never been used to amplify lime reactivity for FGD. With the results found in this article, natural zeolite was found to be a very important additive to lime in terms of reactivity increment. Several methods have been developed for the evaluation of the suitability of sorbents for the sulfation process. They are based on the reaction of the sorbent with gaseous sulfur dioxide or an aqueous solution of sulfur dioxide (H2SO3 or H2SO4). When strong acids like sulfuric acid or hydrochloric acid are used, the reaction mechanism is close to that on a wet FGD plant equipped with air oxidation of the bisulfite ion.11 Recently, ASTM developed a standard test method for the determination of total neutralizing capability of dissolved calcium and magnesium oxides in lime for FGD (ASTM C: 1318-95). This method recommends an acid titration procedure.

2. MATERIALS AND EQUIPMENT Both natural limestone and zeolite were mined in South Africa. The raw samples were crushed, ground, and sieved to a particle size of utmost 200 μm. The limestone was further calcined in an electric furnace at 900 °C for 4 h. The chemical analysis of the natural limestone, calcined limestone, and zeolite is shown in Table 1. The experimental work is initiated by testing if the zeolite increases lime reactivity. This was done using a pH-stat method which simulates wet FGD and a fixed bed reactor which simulates dry FGD.12 To test the increase in the reactivity of lime due to the addition of zeolite, sorbents were made up of mixtures of calcined

Figure 1. Schematic drawing of the experimental setup. (1) Peristaltic pump, (2) pH electrode, (3) pH controller, (4) acid solution beaker, (5) electronic balance, (6) stirrer, (7, 8) plastic tubing, (9) reaction vessel, (10) RS232 cable, (11) computer workstation, (12) wiring for pH electrode, (13) connection between pump and controller.

limestone and zeolite in the ratio 1:0 (full lime), 5:1, 2:1, 1:1, 1:2, 1:5, and 0:1 (full zeolite). The sorbent (1.5 g) was dissolved into 200 mL of distilled water. The solution was put in a water bath set at 60 °C with a resolution of (1 °C. The solution containing complex, high surface area, alkaline calcium aluminate silicates, was agitated by a stirrer rotating at 225 rev/min. The pH in the beaker was measured by a pH electrode inserted in the solution and connected to a pH 200 controller supplied by Eutech Instruments with a resolution of (0.01. A 1 M solution of HCl was titrated accordingly and the reactivity was determined from a recording of the volume of HCl added versus time. Each experiment was done at least twice and the average of the results was taken. A plot of the amount of acid consumed versus time taken gives the reactivity of the sorbent. Figure 1 shows the pHstat experimental apparatus used. Models used to describe the heterogeneous noncatalytic solid
fluid reaction mechanism fall into three main categories: • grain models • pore models • deactivation models. The grain model is relatively simple and largely used to describe heterogeneous noncatalytic solid
fluid reaction. This 2029

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model assumes that the porous solid is made up of small nonporous grains, and each of these grains is converted according to the shrinking unreacted core model. In shrinking unreacted core model, spherical particles making up the grains are converted at a rate depending on the limiting step on which as derived and explained in ref 13. • If chemical reaction is the rate-limiting step t ¼ ½1
ð1
xÞ1=3  τ

ð1Þ

reactivity will be given by a plot of [1
(1
x)∧(1/3)] versus time. • If diffusion through the product layer is the rate-limiting step t ¼ ½1
ð1
xÞ1=3  τ

ð2Þ

reactivity will be given by a plot of [3
3(1
x)∧(2/3)
2x] versus time . • If mass transfer through fluid film is the rate limiting step t ¼x τ

ð3Þ

reactivity will be given by a plot of x versus time. Due to the nature of the experiments, the product layer will constantly be dislodged from the surface by agitation, therefore there will be minimum resistance due to diffusion through the product layer and thus it will not be the rate limiting step. To check which between the remaining two is the rate limiting step, their respective plots were drawn. The curve which approximated a straight line curve from the origin was taken as the rate limiting step and hence the reactivity constant was calculated.

To ascertain the increase in reactivity, a laboratory-scale fixed bed reactor (Figure 2) was used to simulate dry FGD. The reaction zone is contained in a 0.009-m inner diameter stainless steel tube fitted in a furnace for isothermal operation. The sorbent material (0.2 g) was packed in the center of the reactor supported by 0.03 g of glass wool. The reactor was heated to desired temperature. A nitrogen gas (N2) stream was passed through a humidification system consisting of two 750-mL conical flasks immersed in a water bath at a set constant temperature for a specified humidity ratio depending on the partial pressure of the steam. This humidified stream was allowed through the reactor for 10 min to humidify the sorbent. Humidified sorbents are more effective for desulfurization because] SO2 is hydrated by the adsorbed water molecules on the sorbent surface before reacting.12 After humidification, nitrogen gas was mixed with a stream of 1500 ppm of sulfur dioxide gas. The experimental conditions were the typical bag filter conditions in dry/semidry FGD processes. Carbon dioxide and oxygen were not added into the gas mixture because their presence produced little effect on the sulfur dioxide uptake of sorbent at this low temperature.14 Other gases omitted in the mixture were nitrogen oxides because the current objective is to test the reactivity of the sorbent on sulfur dioxide only. The total flow rate was set at 300 mL/min. At exit, the flue gas composition was continuously monitored by an IMR 2800P flue gas analyzer with readings taken at an interval of 20 s. Sorbents for fixed bed experiments were in the same ratio as those used in pH-stat experiments. Prior to fixed bed experiments, the sorbents underwent a thorough mixing process with respect to the required lime to zeolite ratio. Hydration process then followed, in which 10 g of the sorbents was mixed with 100 mL of distilled water and placed in a water bath at 60 °C for 41/2 h. The resulting slurry was filtered and dried in an oven at 100 °C for 16 h to produce a dry solid. It was then ground, milled, and sieved to a particle size of utmost 200 μm before being used in the fixed bed. The reactor was heated to a temperature of 87 °C and a nitrogen gas of 50% humidity ratio was passed through for humidification before the actual sulfation reaction. The desulphurization activity was monitored through maximum utilization of the sorbent (mol of sulfur dioxide retained/ mol of sorbent) after it was completely exhausted. A blank run was initially tested with glass wool only in the reactor. Afterward, the sulfation test was run with different hydrated sorbents. Each experiment was done at least twice and the average of the results was taken. The fixed bed results were represented as breakthrough curves where the y-axis was the ratio of current concentration (C) to the inlet concentration (C0) of sulfur dioxide and the x-axis being the time taken. The total sorbent utilization was evaluated from the area difference between the blank run and the reaction curves. The amount of sulfur dioxide retained is given by mol SO2 retained j Msorbent ¼ ðAbl
Aexp ÞC0 10
6 υ ð4Þ mol sorbent 23652 msorbent

Figure 2. Schematic diagram of the experimental rig for laboratory fixed bed reactor.

Table 2. Range of Variables variable

units

low

high

temperature

deg C

40

80

lime to zeolite ratio (lime mass in 1.5 g sorbent) solid to liquid ratio (volume of distilled water per 1.5 g sorbent)

g mL

0 100

1.5 300

stirring speed

rev/min

100

350

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where Abl is the area under the blank run, Aexp is the area under the reaction curve, C0 is the inlet concentration of SO2 (ppm), jv is the volumetric flow rate (mL/min
1), 23652 is a volumetric constant depending on the operation conditions, Msorbent is the molar mass of the sorbent used, and msorbent is the mass of the sorbent.

Figure 3. Mass transfer through the fluid film control.

Figure 4. Chemical reaction control.

In a bid to explain the results, calcined lime, original zeolite, and one sorbent (lime to zeolite ratio 1:1) were prepared for XRD analysis using a back loading preparation method. The samples were micronized using ethanol and ground for 3 min then dried for 24 h at room temperature. Due to amorphous properties of the materials, 20% of silicon was added to each sample. The dried pellets were ground and properly mixed using mortar and pestle before packing them on sample holders. They were analyzed with a PANalytical X’Pert Pro powder diffractometer with X’Celerator detector, variable divergence and receiving slits with Fe filtered Co KR radiation in a diffraction angle range (2θ) of 5° to 89°. To further emphasize the effect of adding zeolite to lime, BET surface area analysis of all the hydrated sorbents was performed to see the effect of zeolite in the surface area and porosity of the sorbents. BJH (Barret
Joyner
Halenda), was applied to obtain the pore-size distribution from nitrogen desorption data. Adsorption measurements were performed on a micrometrics ASAP 2020 surface area and porosity analyzer by the principle of physical adsorption. High-purity nitrogen (99.99%) was used. The pore-size distribution is represented by the derivative d(Vp)/d(dp) as a function of pore diameter, where Vp is the pore volume and dp is the pore diameter. Prior to measurement, the samples were degassed and characterized using low-temperature (around
197 °C) nitrogen adsorption isotherms measured over a wide range of relative pressures. Finally SEM image scans of the calcined limestone, zeolite, and one of the hydrated sorbents (lime to zeolite ratio of 7:3) was done to evaluate the effect of zeolite on the morphological structure of the limestone. A LEO 1525 FEGSEM was used for the SEM analysis. The equipment was operated at a vacuum of 2  10
6 mbar, electron high tension of 1 kV, aperture size of 10 μm, and working distance of 4 mm using InLens detector. The imaging was done using Line Integration to reduce noise levels during scanning. The samples were sprinkled onto a double sided carbon tape and placed on aluminum stubs. After testing the effect of adding zeolite in lime, further optimization experiments were conducted. Apart from the ratio of lime to zeolite, the effects of three other variables were investigated both as single variables and in combination with others. In total all the variables investigated were temperature, lime to zeolite ratio, solid to liquid ratio, and stirring speed. Design of experiments using design expert software version 6.0.6 (Stat-Ease Inc., Minneapolis, MN) was used in these experiments for regression analysis. Response surface methodology (RSM) is a statistical method in design expert that uses quantitative data from appropriate experiments to determine regression model equations and operating conditions. A standard RSM design called the central composite design (CCD) is

Table 3. Properties of the Sorbent Varied by Lime to Zeolite Ratio sorbent (lime percentage in the sorbent)

reactivity ( 10-4/sec)

sorption capacity (mol SO2/mol sorbent)

surface area (m2/g)

100%

3.001

0.127

5.6

277.4

83.3%

3.0889

0.183

16.3

164.8

66.7%

3.1681

0.246

24.4

137.7

50% 33.3%

3.2641 3.0754

0.260 0.218

31 21.9

112.4 107.4

average pore width (Å)

16.7%

2.809

0.127

17

190.8

0%

2.5644

0.084

18.2

203.1

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Figure 5. Experimental breakthrough curves at various lime to zeolite ratios presented as the percentage of lime in every ratio.

suitable for investigating linear, quadratic, cubic, and cross product effect of variables. It also helps to optimize the effective parameters and provide a lot of information with a minimum number of experiments as well as to analyze the interaction between the parameters. In addition, the empirical model that relates the response to the variables is used to obtain information about the process. CCD comprises a two level full factorial design (24 = 16), eight axial points, and six center points. The center points were used to determine the experimental error and the reproducibility of the data. The alpha (R) value, which is the distance of axial point from the center, was fixed at 2 to make the design rotatable. The experiment sequence was randomized in order to minimize the effects of uncontrolled factors. Each response of the reactivity was used to develop a mathematical model that correlates the reactivity to the absorbent preparation variables through first-order, second-order, third-order, and interaction terms, according to the following third-order polynomial equation: Y ¼ b0 þ

þ

4

4

4

4

bj xj þ ∑ bij xi xj þ ∑ bjj x2j ∑ j¼1 i, j ¼ 1 j¼1 4

∑ bkijxkxi xj þ j∑¼ 1 bjjjx3j k, i, j ¼ 1

ð5Þ

where Y is the predicted reactivity, b0 is the first term, bj is the linear effect, bij is the first-order interaction effect, bjj is the squared effect, bkij is the second-order interaction effect, bjjj is the cubic effect, xi, xj, and xk are coded variables, and n is the number of variables, in this case 4.10 tvjmline=0.3pt?>The significance of the second-order model as shown in eq 5 was evaluated by analysis of variance (ANOVA). The insignificant coefficients were eliminated after the f (fisher)test and the final model was obtained. Additional experiments were carried out to verify the predicted model and the assosciated optimal conditions. Table 2 below shows the maximum and minimum values of each variable (values at R = þ2 and R =
2, respectively).

3. RESULTS AND DISCUSSION Figures 3 and 4 compare mass transfer through fluid film control and chemical reaction control as the rate-limiting step. It is clear from the diagrams that chemical reaction has a better linear behavior and thus is the rate-limiting step. Other researchers achieved the same result on related experiments.15
18 The values of the reactivity constant are shown in Table 3. As it can be seen, the value increases until the ratio of lime to zeolite is 1:1 then it starts to decrease. This behavior was also observed in the fixed bed (dry FGD) experiments. Figure 5 shows the breakthrough curves for the sorbents made up of different ratios of lime and zeolite, at 50% relative humidity. From the curves, the capacity of the sorbents to maintain 100% sulfur dioxide removal improved until lime to zeolite ratio 1:1, then it started to deteriorate. This is indicated by the capacity of the sorbents to maintain 100% sulfur dioxide removal. The ratio of lime to zeolite generally determines the amount of raw materials available in the preparation mixture for formation of reactive species. As zeolite is being added, it provides alumina
siliceous material which reacts with lime to form complex calcium alumina
silicates compounds. This reaction depends on the amount of the reactants available. As more zeolite is added, more alumina
siliceous material is available thus more reaction between it and available lime which leads to more reactive product. The reactivity reaches its peak with an optimum lime to zeolite ratio, which means that all the available lime reacts with all alumina
silicates. Further increase in zeolite begins to be detrimental to reactivity due to the fact that excess alumina
silicates will be available to the available lime, therefore less reactive products which ends up with less reactivity. This trend will continue until the ratio of lime to zeolite is 0:1 meaning only zeolite is available for reaction thus the least reactivity as shown in the diagram. This type of behavior has also been reported by many other researchers.7,8,10,14,19
21 To support this explanation, XRD spectra of calcined limestone, original zeolite, and one of the blended sorbents (lime/zeolite ratio 1:1) are shown in Figure 6. Only major components are symbolized in these patterns. In the blended sorbent spectrum, complex calcium alumina
silicates are generated as the major phase both in general and amorphous forms. Other reactive compounds in it are potassium/sodium alumina silicates and hydrated lime. The amount of sulfur dioxide retained per mol of the sorbent (sorption capacity of the sorbent) is shown in Table 3. As it can be seen, lime to zeolite ratio 1:1 had the highest amount of sulfur dioxide retained (0.26 mol/mol of sorbent) and lime to zeolite 0:1 had the least amount (0.084 mol/mol of sorbent). There is a lot of correlation between these results and the earlier pH-stat results. This shows that in either dry or wet FGD process, the reactivity of the sorbent approximately acts in the same manner. Also these results affirm the observations of Siagi that wet FGD can accurately be simulated by pH-stat method.12 Table 3 further shows the variation of surface area with the ratio of lime to zeolite. This affirms what has already been suggested—that zeolite increases the surface area of lime. The increase in reactivity of these sorbents is largely attributed to the increase in surface area. This is observed in the relation among the reactivity, sorption capacity, and surface area in Table 3. As the surface area increases, the reactivity and sorption capacity also increases and vice versa. The increase in surface area was attributed to the complex calcium aluminate
silicates product. 2032

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Figure 6. XRD spectra of calcined limestone, original zeolite, and the blended sorbent of lime/zeolite ratio of 1:1, where (a) is quick lime, (b) is calcium silicates, (c) is calcite, (d) is hydrated alumina silicates, (e) is quartz, (f) is dry alumina silicates, (g) is general calcium alumina silicates, (h) is amorphous calcium alumina silicates, (i) is potassium/magnesium alumina silicates, and (j) is hydrated lime.

As the amount of zeolite increases, the amount of high surface area calcium aluminate
silicates product increase until the ratio of 50% lime/50% zeolite, where the optimum amount of calcium aluminate silicates product was achieved thus maximum surface area. As the ratio was further reduced, there was less lime to react with zeolite thus less calcium aluminate
silicates product and ultimately lower surface area. Zeolite itself has a relatively high surface area but does not have reactive constituents and that is why it had a low reactivity and sorption capacity. At 16.7% lime/83.33% zeolite ratio, there was an excess of zeolite compared to lime, thus forming minute calcium aluminate
silicates products which made the overall surface area

of this blend the least. In addition, the highly reactive sorbents had very low t-plot values and pore size range of 107
278 Å as indicated in Table 3, which is evidence that the pore sizes were in the mesopore region according to International Union of Pure and Applied Chemistry (IUPAC). As stated in literature, this is the effective zone for FGD process with pozzolanic sorbents.9,19
21 Figure 7 shows the SEM images of calcined limestone, original zeolite, and the sorbent of lime to zeolite ratio 7:3. Various levels of brightness indicate difference in the surface chemistry. Calcined limestone and original zeolite surfaces (Figure 7a and b) are somehow rough with 7a showing a structure with small pores and cracks, but clear only at a high magnification (5000). 2033

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Optimization process of various variables affecting the reactivity of sorbents was analyzed by design of experiments in design expert software. Table 4 shows the experimental design matrix and response of the experiments in terms of reactivity constant. The reactivity constant ranged from 2.5644  10
4 to 3.3172  10
4 per second. From a statistical point of view, there are three tests required to evaluate the model: significance of factor test, Rsquared test, and lack of fit test. The significance test was indicated by the Fisher variance ratio (the F-test value) and its associated probability (Prob > F). The model equation was evaluated by F-test ANOVA which revealed that these regressions are statistically significant at 95% confidence level (Table 5). As a general rule the greater the Fvalue is from unity, the more certain it is that the empirical model describes the variation in the data about its mean and the estimated significant terms of the adsorbent preparation variables are real. The values of prob > F which are 0.05 or less indicate significance. Quadratic model was suggested to be the best because its prob > F is less than 0.05 ( F values of less than 0.05 ( F

block

7.21  10
12

2

model

1.09  10
8

14

7.81  10
10

1140.795