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Keywords:

  • bagasse ash;
  • hydrating agent;
  • magnesium-based sorbents;
  • flue gas desulphurization;
  • thermo gravimetric analyzer;
  • shrinking core model;
  • response surface methodology

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

This work investigates the use of a siliceous material (bagasse ash) and a hydrating agent (ammonium acetate) as additives to enhance sulphur dioxide removal efficiency in low temperature dry flue gas desulphurization (FGD) process when magnesium-based sorbent is used. Response surface methodology (RSM) was used to determine the effects of hydration temperature, hydration time, amount of ammonium acetate and amount of bagasse ash on the surface area of the sorbent. A polynomial model was developed to relate the preparation variables to the sorbent surface area. The Brunauer-Emmet-Teller (BET) surface area results show that the surface area increased from 56 m2/g to 219 m2/g when bagasse ash and ammonium acetate was used at different sorbent preparation conditions. The desulphurization experiments performed using a Thermo Gravimetric Analyzer (TGA) showed that sulphur dioxide (SO2) removal efficiency of up to 99.9% at reaction time of 2 h was achieved. The surface area results were in excellent agreement with the results obtained from the desulphurization reaction. The unreacted shrinking core model was chosen to describe the desulphurization reaction kinetics between SO2 and the magnesium hydrated sorbent. The model shows that a non-porous shielding layer forms which stops further gas(SO2)-solid (magnesium sorbent) reaction from occurring and the product layer was determined to be the rate limiting. The scanning electron microscope (SEM) results illustrates that as the sulphation reaction takes place pores become plugged with reaction products resulting to a non-porous structure. © 2014 American Institute of Chemical Engineers Environ Prog, 34: 23–31, 2015


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

Legislation to curb the emission of sulphur dioxide was introduced in early 1990 in South Africa and there is an urgent need to reduce emissions. Flue gas desulphurization (FGD) system has been identified as a process for better performance and reliability so as to ensure that sulphur dioxide emissions are within the obligatory limits [1]. There are a number of ways of eliminating sulphur dioxide from the flue gas. These comprise of semidry, wet and dry FGD process method.

Dry FGD systems have low capital and annual cost, low water usage and the waste disposal is less complex compared to other FGD processes. Other important advantages are that dry FGD system can be installed easily therefore this technology as a retrofit choice for current power plants using fired coal [2, 3]. Even though the SO2 removal efficiency in dry FGD are significantly lower than wet FGD system for calcium and magnesium-based sorbent. However studies have shown that through research and development SO2 removal efficiency can be improved to above 92% [4, 5].

It has been reported when calcium hydroxide or calcium oxide, are mixed with coal ash (bottom or fly) under optimum hydration condition they produce sorbent that can increase SO2 removal efficiency. This is because they undergo a simultaneous hydration and pozzolanic reaction producing sorbent (calcium aluminum silicate hydrate) with large surface area [5].

Several siliceous materials have been used with limestone in previous studies. Lee et al (2004) [4] used rice husk ash as it contains (SiO2) from their study they found out that the sorbent preparation process variables (hydrating temperature, period, amount of CaSO4 and rice husk fly ash have significant effect on the surface area. Karatepe et al. (2004) [5] prepared sorbents using Ca(OH)2 and diatomite and their results showed that the amount of dissolved Ca(OH)2/SiO2 and the hydration variables determined the reactivity of the sorbents.

Bagasse ash is a waste product in the sugar industry and can be used to produce sorbents which are environmentally and economically friendly. Bagasse ash as a high content of silica and can be regarded a pozzolon [6]. Bagasse fly ash and Ca(OH)2 can undergo a pozzalanic reaction to give a product with large surface area which can improve sulphur dioxide capture in dry FGD system.

Additives improve the SO2 capture in FGD. Numerous works have been reported in literature including the use of ammonium compounds and urea to improve dissolution rate of limestone in wet flue gas desulphurization [7-9]. Hydrating agents can also acts as additives to improve the surface sorbents properties. Van der Merwe et al., 2004 [10] examined the influence of magnesium acetate on the hydration of magnesium oxide; they noted that the addition of MgO (magnesium oxide) to the magnesium acetate solution resulted in the formation of sorbents with high surface area compared to sorbents hydrated in pure water.

The study of the kinetics of desulphurization by fitting it with existing model such as the shrinking core model is very important in order optimize and design dry FGD system. Several studies on reaction mechanism and kinetics have been done [11]. The gas (SO2)–solid (Ca(OH)2) reaction analysis, have been experimented using differential reactors [12] and sand-bed reactors [13]. Karatepe et al. 2004 [5] in their work prepared sorbent using calcium hydroxide and diatomite they used reacting shrinking model to explain the non-catalytic heterogeneous reaction mechanism. Their investigational results correlated successfully with the model.

The aim of this work is to research on the feasibility of using bagasse ash and ammonium acetate to improve the sorbents properties of magnesium-based sorbents. A central composite design (CCD) will be used to study concurrently the effects of hydration temperature, amount of bagasse ash, amount of ammonium acetate and hydration period on the surface area. The desulphurization will be carried out using Thermo Gravimetric Analyzer (TGA) under isothermal conditions. A mathematical and a kinetic model will be developed using the CCD and the unreacted shrinking core model, respectively.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

Material

The materials used to prepare the sorbent were bagasse ash, magnesium oxide (MgO) and ammonium acetate (NH4CH3OO). The bagasse ash was obtained from RSSC (Mhlume), Swaziland and magnesium oxide and ammonium acetate were supplied by a local chemical supplier. The chemical composition of bagasse ash was determined using X-ray Fluorescence (Rigaku RIX 300) spectrometer and is specified in Table 1. The surface area of the virgin materials used is shown in Table 2.

Table 1. Chemical composition of bagasse ash.
Chemicalwt %
SiO266.2
Al2O311.7
CaO2.3
Fe2O312.1
MgO2.1
K2O2.5
P2O50.9
Others2.1
Table 2. Surface area of material used in the study.
Virgin raw materialSpecific area (m2/g)
Bagasse ash56.12
MgO4.94
Ammonium acetate5.62

Sorbent Preparation

The technique employed for sorbent preparation can be described as follows. A given quantity of MgO (5 g) was added to 100 mL of deionized water at a given reaction temperature and stirred continuously. A specific amount of bagasse ash (0–20 g) and ammonium acetate (0–5 g) as the hydrating agent was added to the mixture. The mixture was subsequently heated to a required reaction temperature for a certain amount of time, between 1 and 10 h to facilitate the hydration of the process. The slurry mixture was then sieved and dried in oven at a temperature 300°C for 2 h. The sorbent in the solid form was further crushed and sieved to produce particle of average size of 150 µm.

Designs of Experiments

The experimental design chosen for this investigation is known as a CCD which is normally employed to investigate the linear, quadratic, cubic and cross-product effects of the four main experimental parameters on the specific surface area of the sorbent. The four experimental parameters studied are hydration time, hydration temperature, amount of bagasse ash and amount of ammonium acetate. The range and levels of the hydration period, reaction temperature, ammonium acetate and the amount of bagasse ash is shown in Table 3.

Table 3. Levels of desulphurization process variables employed for this study.
VariableCodingUnitsLevels
−2−1012
Hydration periodx1hr23.668.410
Reaction temperaturex2°C6573.18596.9105
Ammonium acetatex3g00.823.24
Amount of baggase ashx4g04.1101620

The CCD consist of a two-level full factorial design (24=16), eight axial or star points and six center points [14]. The full design of the experiments carried out for this study and the BET surface area results, sulphur dioxide removal efficiency and cost associated with each experiment are given in Table 4. A mathematical model that relates the specific surface area to the process parameters through a third polynomial equation was used as shown below.

  • display math(1)

where Y is the predicted specific surface area (m2/g), where bo, bj, bij, bjj, bkij, and bjjj represents the offset term, linear effect, first order interaction effect, squared effect, second order interaction effect and cubic effect, respectively.

Table 4. Central composite design of the experiments and surface area, SO2 removal efficiency and experimental cost results.
 Process variables      
Experiment noReaction time (h) (x1)Reaction Temperature (°C) (x2)Ammonium Acetate (g) (x3)Bagasse ash (g) (x4)Surface Area (m2/g)SO2 removal efficiency (%)Experimental cost ($)
E18.3896.893.194.05175.00.3
E28.3896.890.814.05179.099.90.12
E38.3873.113.1915.95130.10.3
E43.6296.890.8115.95205.00.12
E58.3873.110.8115.95178.00.12
E63.6273.113.194.05151.00.3
E73.6296.893.1915.95173.00.3
E83.6273.110.814.05143.091.20.12
E9285210162.00.2
E101085210218.50.2
E11665210111.00.2
E126105210202.60.2
E13685010195.90.5
E14685410191.50.41
E156852056.058.10.2
E16685220218.20.2
E17685210169.20.2
E18685210168.40.2
E19685210167.695.10.2
E20685210168.70.2
E21685210166.80.2

Model Fitting and Statistical Analysis

The regression analysis, the evaluation of the statistical significance and the fitting of model to a third order polynomial equation was done using Design expert software of version 6.0.6.

Desulphurization Experiments

The desulphurization experiments were performed using a TGA Thermax 500 (thermogravimetric analyzer) as depicted in Figure 1. The reaction zone was built using a 0.01 m inner diameter stainless steel pipe fixed in a furnace for constant temperature operation. The sample was placed into a platinum gauze basket, which is linked to and suspended from a sapphire extension rod or hang-down after opening the joint coupling, joining the furnace vessel and the pressure balance. The controlled temperature zone of the furnace is 50 mm. Thermax software monitors the temperature profile readings. The software records time, mass, pressure and temperature according to user-defined sequence to a file on the computer and enables the user to perform analytical functions on the experimental data as well as save to other formats that can be processed on other applications for various uses.The sorbent (100 mg) was filled in the center of a suspended sample holder. The stream of simulated flue gases of 2000 ppm SO2, 5.3%O2, 10%CO2, and the remaining was N2 was released at a flow rate of 150 mL/min over the sorbent. The amount of SO2 in ppm during the reaction was continuously monitored by thermal analyst, which is a computer software generating data for mass gained by the sorbent at a specific reaction period (2 h) and it is connected to the TGA unit. The conversion of SO2 captured by the sorbent can be calculated based on the raw data generated by the software.

image

Figure 1. TGA equipment used for flue gas desulphurization.

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RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

The Mechanism of Hydration and Pozzolanic Reactions

The Following Sequential Mechanism Could Explain Why Hydrating Agents Such As Nh4ch3oo Tends to Improve the Sorbent Surface Area
  • Ammonium acetate dissociation:
    • display math(2)
  • MgO dissolution by acetate complexation:
    • display math(3)
  • MgO dissolution by ammonium complexation:

    • display math(4)
    • display math(5)
    • display math(6)

    Acetic acid formed in the bulk can further dissolute MgO

    • display math(7)
    • display math(8)

    Dissociation of magnesium complexes and magnesium hydroxide precipitation in the bulk of the solution due to supersaturation [15-17]. This is the main reason why hydrating agents improves the surface area of sorbents to be used in FGD.

  • display math(9)
  • display math(10)
The Following Sequential Mechanism Could Give Explanation Why A Siliceous Additive Such As Bagasse Ash Tends to Improve the Sorbent Surface Area

Mg (OH)2 dissolution:

  • display math(11)

Reaction to form magnesium silicates:

  • display math(12)

Development of Regression Model Equation

The equation describing the model in terms of actual values after eliminating the irrelevant terms was identified by applying the Fisher's Test methodology; the final equation is shown below:

  • display math(13)

The negative sign in front of the terms specifies an antagonistic effect, while the positive sign indicates synergistic effect. The coefficient correlation (R2) can be used to evaluate the quality of the model. The R2 for Eq. (13) is 0.969. This suggests that 96.9% of the total deviation in the surface area responses is clarified by the model.

Figure 2 shows a plot of the experimental against the predicted values of sorbent surface area plotted against a unit slope. The results indicate that the regression model was very accurate in predicting the experimental data.

image

Figure 2. Predicted versus the experimental values of the sorbent surface area.

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Verification of the Model

The analysis of variance (ANOVA) was used to verify the acceptability of the model in Table 5. The cubic model was verified at a 95% confidence level and was found to be significant since the computed value (46.28) was higher than theoretical F0.05 (14,6) (3.96). This shows that the regression model is accurate in calculating the sorbent surface area. From Table 5, it was observed that generally the amount of bagasse ash (x4) had the most noticeable effect due to the large value of its sum squares; it was followed by the hydration temperature (x2) and the reaction time (x1) in respective order. The amount of ammonium acetate (x3) had a little effect on the surface area of the sorbent.

Table 5. Analysis of variance ANOVA for the regression model equation and coefficients after eliminating the irrelevant terms.
SourceSum of squaresDegrees of freedomMean of squaresF-test
Model27189.73141942.1246.28
x11596.1311596.1338.03
x24191.6214191.6299.88
x3507.061507.0612.08
x413151.18113151.18313.39
x21750.861750.8617.89
x22336.481336.488.02
x231032.8311032.8324.61
x242047.9812047.9848.8
x1x26257.0816257.08149.1
x1x397.3197.32.32
x1x4399.751399.759.53
x2x31.911.90.045
x2x41077.8311077.8325.68
x3x4879.91879.920.97
Residual251.79641.96 

The quadratic terms x4, had enormous influence on the sorbent surface area compared to the variables x1, and x3, the x2 variable has a minor influence on the sorbent surface area. The cubic terms does not affect the surface area of the sorbents. The interaction amongst the variable x2 and x4 has a huge impact on the surface area; this can be seen from Table 5.

The effect of Hydration Process Variables

Effect of Hydration Period and Temperature on the Sorbent Surface Area

Figure 3 displays the variations in sorbent surface area with the changes of the hydration temperature (x2) and time (x1). The ammonium acetate (x3) and the amount of bagasse ash (x4) were kept constant at 2.0 g and 10 g, respectively. As seen in Figure 3, low temperature levels produces sorbents with relatively low surface area as the hydration time is increased, this could be because the dissolution rate is slow at low temperature and therefore there is decrease in the degree of hydration (formation of magnesium hydroxide). However at high temperature levels the surface area of the sorbent increases as the reaction period is increased. The following explanation describes the phenomena. At higher temperature dissolution rate increases and therefore more aggregates of magnesium hydrated silicates are formed. The interaction effect between the hydration time and hydration temperature is evident in Table 5 (high F value for the term x1 and x2).

image

Figure 3. The effect of hydration temperature and time on the BET Surface area of a magnesium-based sorbent (a) Surface response plot (b) two dimensional plot where temperature is held at +96.892 and −73.108°C. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Effect of Hydration Period and Bagasse Ash on the Sorbent Surface Area

Figure 4 illustrates the response surface of sorbent surface area when amount of bagasse ash (x4) and hydration time (x1) are varied. The two parameters ammonium acetate (x3) and hydration temperature (x2) were kept constant at zero level. When amount of bagasse ash is low and at a lower reaction time the surface area is minimal but as the reaction period increases the sorbent surface area increases relatively with high bagasse ash levels. This occurrence can be explained as follows; when smaller amount of bagasse ash (4 g) is used there is inadequate silica to react wholly with MgO, and therefore yielding minimal magnesium silicate hydrated compounds. Consequently, the limitation of hydrated products formed at lower bagasse ash content (4 g) caused the surface area of the sorbent to drop at a shorter hydration period. Moreover, when more bagasse ash was used (16 g), there is more silica present in the sorbent which facilitates the formation of magnesium silicate hydrated complexes to increase the surface area of the sorbent at longer hydration period.

image

Figure 4. The effect of amount of bagasse ash and hydration time on the Surface area of a magnesium-based sorbent (a) Surface response plot (b) two dimensional plot where the amount of bagasse ash is held at + 15.046 and −4.054 g. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Effect of Amount of Ammonium Acetate and Amount of Bagasse Ash on the Sorbent Surface Area

Figure 5 depicts the combined effect of varying amount of bagasse ash and ammonium acetate on sorbent surface area. The hydration time and hydration temperature are held at 6 h and 85°C, respectively. As seen in Figure 5, when 4.054 g of bagasse ash is used, high amount of ammonium acetate increases the sorbent surface area. This clearly indicates that ammonium acetate as a hydrating agent progresses the formation of large magnesium hydroxide aggregate with high surface area [18]. Likewise when more bagasse is used (15.9 g) the sorbent surface area decreases. This could be due to poor solubility of ammonium acetate in MgO/Bagasse ash blend as more amount of ammonium acetate is used. When more bagasse is used sorbent with large surface are produced and vice versa. As explained before; there is more silica to react with magnesium oxide and hence more magnesium hydrated silicates are formed. The mechanism on how hydrating agent and pozzolonic material improve the surface area of magnesium-based sorbent can be explained from Eq. (2) to Eq. (12).

image

Figure 5. The effect of amount of bagasse ash and amount of ammonium acetate on the surface area of a magnesium-based sorbent (a) Surface response plot (b) two dimensional plot where the amount of bagasse ash is held at + 15.046 and −4.054 g. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Effect of All Hydration Reaction Process Variables

The influence of all reaction variables at a point in the design region can be explained from the perturbation plot as shown in Figure 6. Effect of one variable was evaluated and plotted against the surface area while the other variables were held at constant levels. The amount of bagasse ash displayed a greater influence on surface area than the other three parameters. It was followed by the hydration temperature and hydration period respectively. The amount of ammonium acetate showed a slight influence. It can generally be seen in Figure 6 that the surface area increases as the amount of bagasse ash, hydration temperature, hydration time increases, but a minimal increase of the surface area is observed when more ammonium acetate is used. This influence is in an excellent agreement with F Values shown in Table 5.

image

Figure 6. Individual effect of hydration process variables on surface area: A—hydration time; B—hydration temperature; C—amount of ammonium acetate; D—amount of bagasse ash.

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Desulphurization Reaction

The conversion of the sorbent Mg(OH)2 to MgSO4 versus the reaction time was plotted for a 2 h period. As shown in Figure 7 sample E15 shows the lowest sulphur dioxide removal efficiency of 58.1% at a period of 2 h since it has the lowest surface area (56 m2/g) as indicated by Table 4. This outcome can easily be explained on the basis that E15 has no bagasse ash (0 g) therefore the formation of hydrated silicate compounds was limited which further limits the BET surface area of the sorbent and eventually reducing its SO2 capture capacity. On the other hand E8 and E19 showed better SO2 removal efficiency of 91.2% and 95.1%, respectively, than E15 since these sorbents have higher surface areas (143.0 and 168.0 m2/g). It is apparent that the presence of bagasse ash improves the surface area which further facilitates the sorbent to capture additional SO2. Likewise E10 depicts a highest SO2 removal efficiency of 99.9% as shown in Figure 7 since it possesses the highest surface area (219.0 m2/g) which can be attributed to the presence of bagasse ash and the reaction conditions. In the beginning of the reaction that is in the first 20 min the rate of reaction is fast and thereafter it assumes a slightly a slower pattern of SO2 capture. The slowing down of the reaction can be attributed to the decrease in surface area of the sorbent since it is covered by the product layer (magnesium hydrated sulphate silicate).

image

Figure 7. Conversion versus time plots for samples E8, E10, E15, and E19 used in the study.

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Modeling of the Desulphurization Reaction

The kinetic parameters and the rate controlling step for the desulphurization of SO2 using sorbents prepared from bagasse ash-ammonium acetate-MgO can be determined using the experimental conversion-time curves obtained in Figure 7. The reaction kinetics was investigated using the unreacted shrinking core model (USCM). The USCM model considers the reaction to takes place first at the external surface of the sorbent [19, 20]. The region of the reaction goes into the solid and the reacting sorbents particle shrinks during the reaction. The resulting three steps are considered to take place in sequence when the reaction occurs:

  1. Diffusion of sulphur dioxide to the surface of the sorbent.
  2. Reaction on the surface between sulphur dioxide and the sorbent.
  3. Diffusion of sulphur dioxide through the product (ash) layer towards the surface of the unreacted core.

The relationship between the time and conversion of a non-catalytic heterogeneous reaction depends on the rate-limiting step. The chemical reaction, diffusion through the product layer and the mass transfer through the gas film can be the rate-limiting factors this is shown by Eqs. (14), (15), and (16), respectively:

  • display math(14)
  • display math(15)
  • display math(16)

The desulphurization rates constant were calculated from the experimental data using Eqs. (14) and (15). This is shown in Figures 8 and 9 which shows plots to determine whether the rate-limiting step between magnesium hydrated silicates and sulphur dioxide is controlled by chemical reaction at the surface or by diffusion through the product layer, respectively. The adsorption is a fast process and it can be ignored. From the R2 values in Table 6 it is clear that diffusion through the product layer is the rate limiting. This can be explained from the fact that magnesium hydrated sulphates silicates possess a greater molar volume compared to magnesium hydrated silicate; this causes a non-porous shielding layer to form at the earlier stages of reaction and therefore reducing the further contact between sulphur dioxide and the sorbent [21-23].

image

Figure 8. Plot of 1−(1 − X)1/3 versus time for samples E8, E10, E15, and E19 used in the study.

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image

Figure 9. Plot of 1−3(1 − X)1/3 + 2(1 − X) versus time samples E8, E10, E15, and E19 used in the study.

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Table 6. Kinetic constant and correlation coefficient values for samples E8, E10, E15 and E19.
 Surface chemical reaction inline image Diffusion rate Surface chemical reaction inline image 
SampleKr (min−1)R2Kd (min−1)R2
E100.00640.93610.00740.9839
E190.00450.91940.00540.9802
E80.00430.97210.00470.9971
E150.00130.98380.0020.9845

Scanning Electron Microscope (SEM)

Scanning electron microscope pictures of the sorbent prior to desulphurization and after desulphurization are shown in Figure 10. The pores of the sorbent before the reaction are observable and the particles have a needle-like appearance, after reacting (desulphurization) with SO2 the structure of the sorbent is devoid of pores or they are barely noticeable. A tangible explanation for this is the development of a product layer of magnesium hydrated silicate in the pores and as the sulphation reaction progresses the pores are shielded.

image

Figure 10. SEM analysis of sorbent E10 (a) sorbent before desulphurization and (b) sorbent after desulphurization.

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CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

This work examines the use of siliceous material and hydrating agents as additives that can be used to prepare sorbent to be used in dry FGD processes. The influence of four sorbent preparation parameters: amount of bagasse ash, hydration time, amount of ammonium acetate and the hydration temperature on the sorbent surface area was investigated using a CCD. Results indicated that sorbent preparation parameters have a substantial influence on the surface area of sorbent to be used in dry FGD. This study showed that the use of bagasse ash and hydrating agent will improve the sorbent surface area and hence augment sulphur dioxide capture in dry FGD process. The model shows that a non-porous shielding layer forms which stops further gas(SO2)-solid (magnesium sorbent) reaction from occurring and the product layer was determined to be the rate limiting. The SEM results illustrates that as the sulphation reaction takes place pores become plugged with reaction products resulting to a non-porous structure.

ACKNOWLEDGMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

The authors acknowledge the department of chemical Engineering, North West University, South Africa for their TGA facilities and the financial support from Vaal University of Technology Centre of research and Excellency.

ABBREVIATIONS
ANOVA

Analysis of variance

BET

Brunauer-Emmet-Teller

CCD

Central composite design

FGD

Flue gas desulphurization

RSM

Response surface methodology

SEM

Scanning electron microscope

TGA

Thermogravimetric analyzer

NOTATIONS
CA

Bulk concentration of the fluid (mol/cm3)

De

Effective diffusion coefficient (cm2/min)

kd

Apparent rate constant for diffusion through the product layer (min−1)

kg

Mass-transfer coefficient for the fluid film (cm/min)

kr

Apparent rate constant for the surface chemical reaction (min−1)

ks

Rate constant of surface reaction (cm/min)

R0

Average radius of solid particle (cm)

t

Reaction time (min)

X

Converted fraction

ρB

Molar density of solid reactant (mol/cm3)

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED
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