Conversion of Coal Fly Ash into Zeolite Materials - ACS Publications

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Conversion of Coal Fly Ash into Zeolite Materials: Synthesis and Characterizations, Process Design and its Cost-Benefit Analysis Jaime Li Xin Hong, Thawatchai Maneerung, Shin Nuo Koh, Sibudjing Kawi, and Chi-Hwa Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02885 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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Industrial & Engineering Chemistry Research

Conversion of Coal Fly Ash into Zeolite Materials: Synthesis and Characterizations, Process Design and its Cost-Benefit Analysis

Jaime Li Xin Hong a, Thawatchai Maneerung b, Shin Nuo Koh c, Kawi Sibudjing a, Chi-Hwa Wang a,*

a

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585, Singapore

b

NUS Environmental Research Institute, National University of Singapore, 1 Create Way, Create Tower #15-02, 138602, Singapore c

Sembcorp Industries Ltd., 30 Hill Street #05-04, 179360, Singapore

*Corresponding authors: Chi-Hwa Wang, Ph.D. Tel: +65 6516 5079, Fax: +65 6779 1936 E-mail: [email protected] 1

ACS Paragon Plus Environment


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Abstract In this work, coal fly ash (CFA) – a by-product of coal combustion, has been successfully converted into a value-added product— zeolite. This study focuses on the production of Na-A zeolite phase via the fusion method. The effects of fusion reaction temperature, hydrothermal reaction temperature, reaction time, and the concentration of alkaline were investigated. The synthesized products were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), and studied for its purity and yield. A fusion temperature of 550 °C, fusion duration of 1.5 h and a subsequent hydrothermal temperature of 100 °C for a reaction of 12 h were found to be the optimal conditions. Based on the synthesis conditions found, an up-scale production process was designed and simulated with aid from the Aspen Plus program. It was found that zeolite production via the fusion method obtained high profitability. For a 5000 kg/hr coal fly ash feed, a payback period of 7.1 years is feasible over a 20-year operation period. A cost benefit analysis was studied to compare the improved environmental performance and economics of zeolite production from CFA with current CFA disposal practices.

Keywords: Coal fly ash, Zeolite; Na-A, Waste to resource; Ash reutilization; Profitability analysis; Economic analysis. 2


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Nomenclature ANA

Analcime zeolitic phase

CAN

Cancrinite zeolitic phase

CEC

Cation exchange capacity

CFA

Coal fly ash

CSTR

Continuous stirred-tank reactor

d50

Particle size larger than 50% of total particles

LTA

Linde Type A zeolitic phase

NPV

Net Present Value

SEM

Scanning electron microscopy

SOD

Hydroxysodalite zeolitic phase

XRD

X-ray powder diffraction

3



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1. Introduction Coal has fuelled and continues to fuel the largest share of worldwide electricity production. In fact, coal consumption is expected to increase from 29.9% to 46% of world’s electricity supply by 2030 1

. In the combustion process, large quantities of coal fly ash (CFA) is produced globally at an

annual rate of 160 million tonnes 2. The current means of disposal in most countries is what causes coal fly ash production to be worrying. Most countries dump fly ash directly into landfills allowing heavy metal elements to be leached out into the environment 3. It is also unsightly, a nonproductive use of land resource, and extremely dangerous to humans when groundwater supplies get contaminated or when the tiny particles get suspended in air. Clearly, there is a need to explore alternative uses for fly ash that is more lucrative than current disposal practices. CFA has been used for the production of construction materials either as blended cements or geopolymers

8-10

. Other applications include soil amelioration 11, catalytic applications due to

its high thermal stability 12, wastewater purification 13 and the recovery of precious metals found within coal fly ash

14-16

. Due to the similarities of fly ash with volcanic material in terms of

composition, coal fly ash used for zeolite production is gaining notice amongst investors because of its potentially higher market value

17

ash have shown to exhibit high CEC

18, 19

ion exchange uses

19-21

than other applications. Zeolites produced from coal fly and promising applications in water purifications and

. It is also a more economical alternative to current synthetic zeolite

production which uses pure raw materials. Different types of zeolites exhibit vastly different applications. Hydroxysodalite is not usually considered a molecular sieve as its CEC values are lower than that of zeolites Na-A and faujasite-types due to a smaller pore size

17

and thus have

limited applications. Na-A (also referred to as LTA) have become the most important synthetic 4


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zeolite used for wastewater treatment and purification processes

22

as its widely used for drying,

as a washing builder and separation of paraffins 23. It currently has a growing market of about one million tons per annum, making the pursuit for methods of economical production a subject of practical interests 24. Many methods have been investigated for the development of zeolites from coal fly ash. Traditionally, CFA reacts with an alkaline activating agent under hydrothermal conditions—high temperature and water saturation pressure, transforming SiO2 and Al2O3 components into zeolitic crystalline phases 25. Unconventional methods such as reaction under molten-salt conditions 26, 27 and a microwave-assisted technique have also been explored

23

. However, these methods are

energy-intensive and have little potential for up-scale production. Mondragon, et al. 33 have demonstrated that an acid pretreatment process provides added performance, as it removes unwanted contaminants from CFA particles such as iron oxides and trace heavy metal elements. It also increases particle pore size diameter and specific surface area, and improves CEC values by a significant amount (~40%) especially in the case of hydrothermal method

34

. Its effects on zeolites produced from the fusion method, however, were notably less

(~10% gain). While many laboratory tests have been conducted studying the methods for converting CFA into zeolites, studies on the scale-up feasibility of the synthesis process are lacking. Wdowin, et al. 38 conducted a sub-pilot scale batch process and yielded a purity of 81 wt. % of Na-P1 zeolite. However, the original mullite and quartz phases are still prominent in the final product. A semicontinuous production process was studied by Kikuchi 39 and obtained zeolites with CEC of 250 meq/100g, which is reportedly higher than that of natural zeolites (< 150 meq/100g). Most of the 5



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scale-up processes studied are either batch or semi-continuous processes. Applications of a continuous process involving CSTR are rare despite its clear advantages over the consistency in product quality and energy-efficiency compared to batch processes. This paper discusses zeolite synthesis using a hydrothermal treatment method involving a prior fusion step with anhydrous NaOH and its performance in yielding high purity zeolites. A comparison with the one-step hydrothermal treatment method was also conducted to evaluate the different types of zeolite phases formed. The synthesized products were characterized using XRD and SEM analyses. A set of optimal synthesis conditions was obtained and a simulation model was developed for a scaled-up zeolite production process using Aspen Plus V9 Program. Details of the simulation are explained further in this paper. The simulation packages are used to evaluate the resource demands and technical performance of the process, as well as conduct economic calculations used to evaluate the profitability and feasibility of applications on an industrial scale.

2. Experimental 2.1. Materials and Reagents The samples of CFA were obtained from a coal incineration facility owned and operated by Sembcorp Industries Ltd. NaOH, in the form of anhydrous pellets, was used as an alkali activating agent. Sodium aluminate salt NaAlO2, purchased from Sigma-Aldrich, was used as a source of aluminium to adjust Si/Al ratio of the reacting mixture.

6


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2.2. Zeolite Synthesis – Hydrothermal treatment method with prior fusion step 7 g of raw CFA was mixed with a weighed amount of anhydrous NaOH, and grounded into fine powder mixture using a mortar and pestle. The dry mixture was transferred into a crucible and placed in a furnace at a chosen temperature between 400 – 650 °C for a fixed duration of 1.5 h. The resultant solid mixture was ground again into fine powder before mixing with 90 mL of deionised water and small amounts of NaAlO2 and stirred at 500 rpm. The suspended slurry mixture is then poured into a Teflon-lined stainless steel autoclave, and placed in an oven at a temperature between 100 – 160 °C, for a chosen reaction time. The reactor was then allowed to cool sufficiently in order to be handled safely before the resultant solid product was filtered via vacuum filtration and washed until a pH of 7-8 is attained. The product is lastly dried in an oven at 100 °C overnight. The effects of fusion temperature, NaOH/CFA ratio, hydrothermal treatment temperature and reaction time were explored in this study.

2.3. Characterization methods The trace elements concentration was analysed by an inductively coupled plasma mass spectrometer (ICP-MS, Agilent 7700×). Prior to the ICP-MS analysis, CFA sample was digested by hydrofluoric acid (HF) and aqua regia (HCl and HNO3 mixed in the ratio of 3:1). 0.25 g of the CFA sample was weighed into a teflon cup and then 2 mL of concentrated HF and 5 mL of aqua regia were added. The teflon cup was then put in a Parr bomb, sealed and heated to 200 °C for 2 h. After the digestant cool down, 25 mL of H3BO3 were added in order to prevent the formation of sparingly soluble species in the sample. The digestant was finally filtered through the 0.2 μm membrane filter and made up to 100 mL with ultrapure de-ionized water. On the other hand, oxide 7



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compositions of the raw CFA was analysed by X-ray fluorescence (XRF, Lab Centre XRF 1800, Shimadzu). The crystalline phases of CFA samples and the synthesized zeolites were obtained via Xray powder diffraction analysis. A small sample was grounded to fine powder form before placing it in the XRD D8 Advance equipment by Bruker. An input voltage of 40 kV and a current of 30 mA were used. The results were collected over a range of 2 Theta values of 5° to 80°. SEM micrographs of CFA and product samples were conducted using the SEM/EDX equipment JSM-5600V SEM by JEOL to analyze the morphology of the materials. Prior to analysis, samples were dried overnight, and then mounted on aluminum stubs and coated with a thin film of platinum using an auto-fine counter.

2.4. Basis of design From the experimental results yielding the optimum synthesis conditions, a conceptual process design for the zeolite production process is developed with a scale of 120 tonnes per day (or 5000 kg/h) of coal fly ash. The analysis assumes a plant located in close proximity to the Coal Incineration Facility such that it receives coal fly ash directly from local storage facilities, transportation costs are omitted.

2.5. Methods of design and analysis The process was simulated using the Solids Modelling feature in the Aspen Plus V9 Program as it offers the necessary tools to characterize and model solid streams and processes that contain both fluids and solids in the same environment. Aspen’s Activated Economics enables the possibility 8


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of running cost evaluations simultaneously with developing process models, further enabling a certain degree of cost optimisation incorporated into the process

45

through the addition of

recycling streams, heat integration and equipment design. Foremost, most of the components of the process are specified as conventional (gas/liquid) type. Components that remain in the solid state throughout the process, such as components of CFA and solid NaOH, are to be specified as conventional inert solids. Dissociation reactions of NaOH and NaAlO2 are incorporated to the simulation from the program’s database, which enables automatic enthalpy calculations of dissolution processes. Other reactions (fusion reaction, condensation and zeolite formation) which are not found in the database are specified manually using the RStoic model block which enables the computation of mass and enthalpy balances. Aspen’s stream classification is important in differentiating between the properties of different substreams in the simulation. The pre-defined stream class chosen was MIXCIPSD, suitable for handling conventional solids with a specified particle distribution. The substream classes chosen were MIXED (fluids only) and CISOLIDS (for conventional inert solids that participate in reactions). ”SOLIDS” was selected to be the property method as the simulation largely involved solids processing. Heat and mass balances and thermodynamic calculations were computed by the simulator. The design for most of the equipment blocks (i.e. furnace, pump, filters, fluidized bed heat exchange tower, cyclone, etc.) were conducted with the aid of the simulator. User-defined models were used to develop the process models for the fusion reaction, aging process and hydrothermal reaction due to lack of knowledge of the reaction mechanisms available at this stage of research. The sizing of those respective tanks was also determined manually. 9



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A user-defined Process Economics Template was set up via the Aspen Economic Analyzer V9 program to create a simulation scenario suited for a Singapore context. Table 2 lists the base parameters adjusted for this economic analysis. The in-built Activated Economics analyser within the simulator is used for conducting economic calculations of the processes to evaluate the profitability of the process and its scale-up potential. The process blocks are mapped with a chosen equipment type from the program’s database and sized according to the results of simulation and then evaluated for its cost. An investment analysis of the scenario is exported onto Microsoft Excel and analysed in detail. Activated Economics analyser within the simulator is used for conducting economic calculations of the process to evaluate profitability of the process and its scale-up potential.

3. Results and Discussion 3.1. Coal Fly ash Characterisation The chemical compositions by weight of coal fly ash are shown in Table 1. SiO2 (52.1%) and Al2O3 (20.2%) are considered as the main components in CFA sample, corresponding to the Si/Al ratio of 2.5. The XRD pattern of the CFA sample (as shown in Figure 1) suggests a predominance of quartz and mullite, and an amorphous peak at 23° likely attributed to the glassy aluminosilicate 7

. On the other hand, the SEM image of CFA (shown in a supporting information) shows the

spherical particles with particle size of < 10 μm.

10


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3.2. Zeolite Synthesis 3.2.1. Intermediary products of synthesis process Figure 1 illustrates the XRD patterns for the intermediates and zeolite product. The fusion product was formed at a fusion temperature of 550 °C. It can be observed that the quartz and mullite phases of CFA were converted into a mixture of NaAlO2 and Na2SiO3, which is in line with literature 30, 40

.

NaOH + xAl2O3 × ySiO2 ® Na2 SiO3 + NaAlO2 The soluble NaAlO2 and Na2SiO3 species dissolved into the aqueous solution during the aging process forms the aluminosilicate nutrients that eventually makeup the zeolite framework. The XRD pattern of the filtered residue from the 24-hour aging process also revealed a broad peak at 2θ = 26°, indicating a presence of an amorphous phase not initially present in the solid fusion product. This may likely indicate the formation of an amorphous aluminosilicate gel which is considered as a critical precursor to form the ordered zeolite frameworks

36

. Unfortunately, the

detailed mechanism into the formation of zeolite from the gel structure is still left uncertain. The identification of the intermediate products from the fusion reaction and aging process is relevant in developing the simulation models, explained in section 4, needed for conducting detailed economic analysis.

3.2.2. Effects of fusion temperatures The XRD patterns of zeolite formed at different fusion temperatures are shown in Figure 2. The zeolites were synthesized under the NaOH/CFA = 2, Si/Al = 1, and hydrothermal conditions of 150 °C for 24 h. The products were predominantly in the SOD and CAN phases. The SEM images 11



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of zeolite (Figure 3) shows the formation of the rod-shaped SOD crystals. It appears that the crystals formed from a fusion temperature of 400 °C were dispersed and scarce, yet they appear to be of comparable size to the other synthesized products. This could be possibly due to fewer nuclei sites that were formed due to lowered amounts of NaAlO2 and Na2SiO3 produced at a lower reaction extent 41. There were no differences in the intensities of zeolite peaks of products at 550 – 650 °C, suggesting that further increases in temperatures beyond 550 °C will not yield significant changes in product yield. Therefore, 550 °C was taken as the optimum fusion temperature.

3.2.3. Effects of hydrothermal temperatures The XRD patterns of the products formed at different hydrothermal temperatures are shown in Figure 4. The NaOH/CFA ratio = 2, Si/Al = 1, and fusion conditions were fixed at 550 °C for a 1.5 h reaction. The LTA was only observed at 100 °C. At 120 °C, SOD was the predominant phase. At higher temperatures of 140 – 160 °C, the SOD peaks were less intense, while the CAN peaks become more apparent. This likely indicates that the SOD phase have dissolved to form the CAN phase, possibly due to CAN being the more stable crystal phase of SOD. Studies conducted by Reyes, et al. 42 and Ríos, et al. 43 had also obtained similar results in which LTA was transformed to SOD and then to a co-crystallized SOD and CAN zeolite. Similarly, Barnes, et al.

44

also

suggested the transformation sequence LTA > SOD > CAN, and used it to explain why CAN was only able to be formed at higher rates of crystallization at higher hydrothermal temperatures.

12


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3.2.4. Effects of NaOH/CFA Ratio and Hydrothermal Treatment Duration In view of the importance of hydrothermal synthesis conditions on the quality and purity of zeolites formed, the effects of NaOH/CFA ratio (ranging from 1 to 2) and hydrothermal durations were evaluated. Fusion conditions are fixed at 550 °C for 1.5 h. The XRD patterns of zeolite products (Figure 5) reveals that that hydrothermal duration has to be necessarily adjusted with rate-affecting variables, i.e. hydrothermal temperature and NaOH/CFA ratio, in order to yield LTA product before its transformation to the SOD phase. Higher rates of crystallization lead to shorter reaction times and thus a smaller reactor tank size, but at the expense of high operating costs to achieve high reaction temperatures. As such, there exists an optimum set of hydrothermal conditions that is most cost-effective, and can obtain a high product quality. From the XRD results, the product formed at the NaOH/CFA ratio of 1.5, 100 °C and 12 h produced a highest intensity peak and a pure crystalline zeolite Na-A product.

3.3. Scale-up Applications 3.3.1. Brief Process Flow Description To simplify the simulation, the CFA feed was taken to consist of the major components as presented in Table 1. A particle size distribution (PSD) for the CFA feed was defined as a normal distribution function of d50 = 10 µm, and standard deviation = 5µm. Figure 6 illustrates the process flow diagram for the main process flow for zeolite production. 5000 kg/h of CFA from a storage facility is pneumatically transported to a preheater (HE-01) that recovers heat from the hot exhaust coming from the Fluidized Bed Heat Exchange Tower (FB-01/02), before heating to a temperature of 550 C in the fired heater (FH-01). 7500 13



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kg/h of anhydrous NaOH pellets are similarly heated and melted via three stages within the fired heater to 550 °C. A screw feeding system (screw feeder S-01) is used to covey the NaOH solids into FH-01. Natural gas is used as the fuel source in the fired heater and a flow of 3444.86 kg/hr is required, together with an air flow of 20000 kg/hr. The delivery of air into the heater is made possible by a forced draft fan (C-01). . Both streams are then sent to an Inconel-linedfusion reactor (R-01) to react for a residence time of 1.5 h in a semi-solid state reaction. The reactor was modelled using the RStoic block which allows the simulation of the formation of Na2SiO3 and NaAlO2 from CFA and NaOH. The block calculates the mass and enthalpy balances to fully characterize the thermodynamic properties of the product stream according to the reaction extents. A particle growth model was also included in this reactor to evaluate the increase in product particle size from d50 = 10 µm to d50 = 0.2 mm. After which, the fusion product at 550 °C is required to be sufficiently cooled before mixing with water to prevent shocks to the process, as well as preventing premature zeolite formations from taking place. The formation process of zeolites is only allowed to take place in the Hydrothermal Reactor (R-03) where the hydrothermal conditions are controlled stringently in order to maintain a high product quality, as also described in section 3. As such, the solid products are cooled in two fluidized bed towers (FB-01/02) operating in series using compressed air as a cooling medium via a direct cooling process. A cyclone system (C-01/02/03) is put in place downstream of each column to prevent the losses of solid particles entrained in the exhaust air out of the fluidized bed. The final product temperature achieved is 87.5 C, requiring a total amount of 62,400 kg/hr of compressed air at 2.1 bar, 45 C. The fluidized bed columns are also fitted with

14


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coolers to provide additional cooling using cooling water. A total of 273.2 m3/hr of cooling water is required. The cooled solid particles are subsequently mixed with preheated deionised water and NaAlO2 salts in an Aging Tank (R-02) under stirring condition for a residence time of 24 h. The deionised water is preheated to 79.1 C via heat exchanger (HE-03) with the hot zeolite product at the outlet of the Hydrothermal Reactor. The resultant mixture from the Aging Tank is heated to a temperature of 100 C (HE-02) by heat recovered from the exhaust of FB-01. The Hydrothermal Reactor (R-03), for the crystallization of zeolites, operates at 100 °C, 1 atm for a residence time of 12 h and is designed as an enclosed, agitated and jacketed reactor. In order to maintain a constant reaction temperature of 100 C, a supply of 11 750 kg/hr of Low Pressure (LP) steam is needed in a recirculating heat exchange system (HE-04). After zeolite formation, the immediate product is sufficiently cooled to 65 C and washed in the Multi-stage Washer (W-01) via 8 stages to reduce its alkalinity to a pH of 8.02, which will prevent further reactions from taking place. A flow rate of 266.9 m3/hr of water is required. After which, a solid-liquid separation via a Rotary Vacuum Drum Filter (F-01) is then used to filter out the remaining wastewater, to obtain a solid product with 2.75 wt. % moisture. The RVDF was also modelled with an additional water utility for rinsing of the filtered cake and removing contaminants, to achieve a more realistic representation of the simulation. The wastewater streams collected are sent to a wastewater treatment facility. The cost of wastewater treatment is factored into the economic calculations by taking a unit cost of $ 0.08 /m3. The solid product is lastly conveyed to a Rotary Dryer (D-01), via a belt conveyor system (B-01), to remove the remaining moisture and obtain the final product with 0.27 wt% moisture. The dryer employs ambient air, 15



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delivered by a forced draft fan (C-03), that is heated to a temperature of 110 C via heat exchange with flue gas from the fired heater, at a rate of 9500 kg/hr. A summary of all the main equipment of the simulation, their sizing and purchased equipment costs are provided in the supporting information (Table S2). Heat integration was included by using the hot exit streams, particularly from the Fusion Reactor and the Fluidized Bed coolers, to heat inlet cold streams or maintaining the temperatures of heated processes. The heat recovery systems increase the energy efficiency and thereby bringing about economic benefits to the overall process. The additional heat exchangers required (HE-01~7) have been sized for its heat exchanger surface area and heat duty by the validated simulation model on Aspen Plus.

3.3.2. Economic analysis The economic analysis was computed entirely in Singapore Dollars (1 USD = 1.4 SGD). The raw material prices, utility prices, and wastewater treatment costs are provided in the supporting information (Table S1). The prices of zeolites sold commercially are within a large price range, depending on the quality and its intended application such as wastewater treatment, separation processes or as catalysts. Panitchakarn, et al.

46

quoted a global price of US$ 83.87 per kg for

zeolite type A sold in the market. Due to the uncertainty in market price for zeolites type A produced from CFA, a low price of SGD$ 1.3 /kg (= US$ 0.9286 /kg) was first assumed as the base case to ensure a level of conservativeness in calculations. The results of the economic analysis are shown in Table 3. The total project capital cost also takes into account costs from piping, civil, instrumentation, electrical, general and administrative overhead costs, contingencies, design, 16


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engineering and procurement etc., which are calculated as a factor of the total direct costs. Similarly, the annual operating costs includes operating labour, maintenance and supervision costs on top of raw materials, utilities and wastewater treatment costs. Even with a low zeolite sale price, the production process has proven to be sufficiently profitable with a net present value of $ 126,338,000, and a profitability index of 1.23.

3.3.3. Sensitivity Analysis A sensitivity analysis was also performed to evaluate how the prices of zeolites, in the range of $ 0.80 – 1.90 /kg, affected the profits reaped. Figure 7 shows the results of the evaluation on the plant’s payback period, net present value and profitability index. It appears that a minimum sale price of $ 1.00 /kg zeolite is required for the process to be profitable, yielding a minimum profitability index of 1.00, and a payback period of 20.32 years. The product prices are to be subjected to further laboratory studies and in-depth market analysis to evaluate a suitable application for the zeolite product based on its absorbent performance, in order to derive a more realistic product pricing. A tornado diagram in Figure 8 illustrates the effects of varying certain model variables one at a time on the NPV. The upper and lower bounds of model variables used are listed in Table 4. The base-case model was taken at a product price of $ 1.30 per kg. The range of natural gas prices are according to the fluctuations in prices over the past 10 years, and did not have significant effect to the total NPV. The range in tax rates (15-40%) are in the range of the world’s corporate tax rates— 38.9% for the United States; 17% for Singapore; and a worldwide average of 22.5% 47. The electricity prices were varied based on  20% of the base price. The variable with the greatest 17



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impact is the product price, followed by interest rates and the unit cost of NaOH. Increasing the desired rate of returns on investments to 20% results in a significantly lower NPV of $ 58.9 million. Both fluctuations in electricity and natural gas prices show no significant impacts to the final NPV achieved at the end of the 20 year period.

3.3.4. Cost-benefit Analysis The production process of CFA to zeolite is compared with the costs and benefits of current synthetic zeolites production as well as current landfilling disposal of CFA. To provide a consistent basis of comparison and a more accurate analysis, a similar simulation model for the production of zeolite type-A from natural kaolinite was also developed on Aspen Plus. The process conditions follow the synthesis conditions proposed by Alkan, et al. 48. The sale price per kg of zeolite was taken to be $ 1.30 /kg, and the investment analysis parameters were consistent with that stated in Table 2. Current landfilling disposal cost for CFA is $ 77 per tonne of fly ash (US$ 55 per tonne). The cost only consists of the disposal costs among other factors such as labour, transportation and maintenance costs which are not considered. The total corresponding cost for 120 tonnes per day of CFA is $ 3,372,600 per year. The results of the cost-benefit analysis are documented in Table 5. The project capital cost for the synthetic zeolite production process is significantly lower than the process of producing zeolites from CFA due to the exclusion of the fusion process requiring less equipment and energy. As such, the process was able to achieve higher profitability with a NPV value of $ 154.4 million. However, after factoring the cost incurred for CFA not recycled, the net benefit amounted to $ 86,990,000, which remains lower than net benefit obtained from the zeolite production from reutilizing CFA. 18


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Additionally, the intangible benefits of the waste recycling attributes of this process are also a recognizable advantage. The use of CFA as a raw material greatly reduces adverse environmental impacts of landfilling. While Singapore utilises the waste ash in a safe and environmentally-benign manner in the construction of Pulau Semakau, its continual disposal will likely cause the costs for disposal to be increase as a result of Singapore’s dwindling land availability.

4. Conclusions Zeolite type-A was successfully synthesized via the fusion synthesis method. From the SEM and XRD analysis of the zeolite products conducted, the optimum conditions were found to be as such: -

Fusion reaction condition of 550 °C for 1.5 h,

-

NaOH/CFA = 1.5,

-

Si/Al = 1

-

and hydrothermal treatment conditions of 100 °C for 12 h

Based on the optimum synthesis conditions found, a simulation model was developed for a continuous process of production in an industrial scale, with a feed of 5000 kg/h of CFA. The result of the simulation defines the energy demands of the process, the amount of raw material required, the design, type and sizing of equipment required, the amount of product formed, as well as the quantity of waste produced. A detailed investment analysis was also conducted via the Aspen Plus program to evaluate the profitability of the process. The results revealed that the process was able to achieve excellent profitability with an NPV of $ 126,338,000 for a 20-year plant operating life, and a payback period of 7.1 years. Sensitivity analysis showed that the product price was the

19



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Page 20 of 42

most sensitive parameter followed by desired rate of returns and NaOH price. A cost-benefit analysis was also conducted. Acknowledgement The research is supported by the National Research Foundation Singapore, Sembcorp Industries Ltd, and National University of Singapore under the Sembcorp-NUS Corporate Laboratory.

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Table 1 Chemical properties of coal fly ash. Chemical composition a (wt. %)

SiO2

Al2O3

Fe2O3

CaO

K2O

MgO

TiO2

Others c

LOI

Moisture

52.11

20.15

6.96

4.36

2.47

0.68

4.04

5.24

3.64

0.35

Trace elemental concentration b (wt. %) Cr

Co

Mo

Cd

As

Hg

Pb

0.0065

0.01

Conversion of Coal Fly Ash into Zeolite Materials - ACS Publications

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