Temtop PMD331 Professional Particle Counter Review: Real ISO Accuracy & Field Results - YouTube

Temtop PMD331 Professional Particle Counter Review: Real ISO Accuracy & Field Results - YouTube

Bagaimana cara kerja perdagangan karbon?

Perdagangan karbon adalah sebuah sistem di mana negara-negara atau organisasi membeli atau menjual hak untuk menghasilkan emisi karbon. Ini bertujuan untuk mengurangi emisi gas rumah kaca dengan memberikan insentif keuangan bagi organisasi yang dapat menurunkan emisi mereka, sementara juga memberikan biaya tambahan bagi organisasi yang terus memproduksi emisi karbon yang tinggi.

Proses perdagangan karbon melibatkan beberapa tahap:

1. Penetapan target emisi: Pemerintah menetapkan target emisi karbon nasional yang harus dicapai dan membaginya menjadi kuota emisi yang dapat diperdagangkan.
2. Penentuan kuota: Setelah penetapan target emisi, pemerintah menentukan kuota emisi untuk organisasi dan industri tertentu. Kuota emisi ini kemudian dibagi menjadi kredit karbon yang dapat diperdagangkan.
3. Penerbitan kredit karbon: Organisasi yang mampu mengurangi emisi karbon mereka di bawah kuota yang ditetapkan dapat memperoleh kredit karbon. Kredit ini dapat diperdagangkan dengan organisasi lain yang tidak dapat memenuhi kuota mereka sendiri.
4. Perdagangan kredit karbon: Organisasi dapat memperdagangkan kredit karbon dengan organisasi lain yang membutuhkan kredit karbon tambahan untuk mencapai kuota emisi mereka.
5. Pelaporan dan verifikasi: Organisasi yang memperdagangkan kredit karbon harus melaporkan emisi karbon mereka secara teratur dan verifikasi oleh pihak ketiga untuk memastikan akurasi dan keandalan data mereka.

Tujuan dari perdagangan karbon adalah untuk memberikan insentif bagi organisasi untuk mengurangi emisi karbon mereka, dan mendorong inovasi dan investasi dalam teknologi rendah karbon dan pengurangan emisi. Dengan sistem perdagangan karbon yang efektif, diharapkan dapat membantu mengurangi emisi karbon secara global dan memerangi perubahan iklim.

ENV Finance - A New Cryptofunding Platform 2.0 on the Fantom Blockchain - YouTube

ENV Finance - A New Cryptofunding Platform 2.0 on the Fantom Blockchain - YouTube:

ENV Finance - A New Cryptofunding Platform 2.0 on the Fantom Blockchain

Description: Green Sustainable Crypto Economy. From Blockchain to Nature. Get Rewarded for Contributing to Sustainable Projects. ENV Finance aims to bring the environment and finance together, by rewarding people for contributing to sustainable projects all around the world. Our mission is to finance projects of small and medium-sized environmental organizations with cryptocurrency. The Environment and Finance are two topics that are apparently poles apart. So, it would seem impossible to merge their objectives into one. Conservation aimed to protect our planet is a difficult and usually expensive task and, more often than not, the outcome is insignificant, and generally with a negative balance. While Financial activities are not always sustainable, and typically related to assets that have a huge carbon footprint, like oil extraction and refining, raw mineral extraction, bitcoin farming, and so on. Basically, ENV Finance aims to bring the Environment and Finance together, by rewarding people for contributing to sustainable projects all around the world. ENV Finance will verify and propose suitable initiatives with increasing environmental impact, and the Fantom Opera technology will track and publish, in real time, all the contributions and financial reports of the initiatives. Contributors will be rewarded with ENV tokens, from the moment it has been financed, in full, and then again when it will be completed.

https://www.env.finance

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Why it is essential to dispose hazardous waste properly by trained people? - YouTube

Why it is essential to dispose hazardous waste properly by trained people? - YouTube:

⭐️ Natural Resources and Environment: How to Dispose of Hazardous Waste, a Serious Question That Needs to Be Resolved: Ced-79-13 - https://amzn.to/3dcoGS0

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Why it is essential to dispose hazardous waste properly by trained people?

Hazardous waste is generated by each and every industry; no matter how big or small. It has properties that make it dangerous or potentially harmful to human health and the environment which makes it vitally important to ensure that hazardous waste disposal is done with utmost care.

Hazardous waste is generally defined by one or more of these characteristics:

Ignitable: Such hazardous waste is highly inflammable. It is capable of burning at or below 140°F. Unwanted gasoline is an ideal example.

Corrosive: It burns the skin, causes irritation in the eyes and is capable of destroying living tissue when contact occurs. It can corrode metals, plastics or rubber. Automobile battery is a corrosive waste.

Explosive or Reactive: Such waste is capable of causing an explosion or releasing poisonous fumes when exposed to water, air or other chemicals. Old medicinal ether and out-dated ammunition are reactive wastes.

Toxic: These wastes are fatal if swallowed. Heavy metals like lead and mercury are toxic wastes.

Radioactive: This type of waste is capable of damaging and destroying cells and chromosomal material in the human body and other living organisms. It can also contaminate the air and the surrounding environment.

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U.S. Department of Energy Invests $72 Million to Support the Development and Advancement of Carbon Capture Technologies

Today, the U.S. Department of Energy (DOE) announced the award of approximately $72 million in federal funding to support the development and advancement of carbon capture technologies under two funding opportunity announcements (FOAs). Under this cost-shared research and development (R&D), DOE is awarding $51 million to nine new projects for coal and natural gas power and industrial sources. DOE is awarding a total of $21 million to 18 projects for technologies that remove carbon dioxide (CO2) from the atmosphere, a process known as “direct air capture.”

“The projects selected as a part of this research will help us develop the technological solutions needed to reduce greenhouse gas emissions,” said Secretary of Energy Dan Brouillette. “This is critical to balancing our Nation’s energy use while continuing to lead the world in emissions reductions.”

Through DOE’s Carbon Capture, Utilization, and Storage R&D Program, the Office of Fossil Energy has a comprehensive portfolio of technological solutions that help keep CO2 emissions out of the atmosphere. Many of these R&D efforts can be applied across both the energy and the industrial sectors.

“The primary mission of our office is to ensure that the United States can continue to rely on its fossil fuel resources for clean and secure energy. The advancement of carbon capture technologies, including direct air capture, contributes to that mission,” said Assistant Secretary for Fossil Energy Steven Winberg. “Our ultimate goal is to mature these technologies so that they can be commercialized and brought to market.”

Under the first FOA, Capture Research and Development (R&D): Engineering Scale Testing from Coal- and Natural Gas-Based Flue Gas and Initial Engineering Design for Industrial Sources, DOE selected nine projects to receive $51 million for cost-shared R&D. These efforts aim to design initial engineering studies to develop technologies to capture CO2 generated as a byproduct of manufacturing at industrial sites.

DOE also selected 18 projects to receive $21 million under the second FOA, Novel Research and Development for the Direct Capture of Carbon Dioxide from the Atmosphere. These projects will focus on the development of new materials for use in direct air capture and will also complete field testing. 

The 10 largest coal producers and exporters in Indonesia:

• Indo Tambangraya Megah (ITMG)
• Bukit Asam (PTBA)
• Baramulti Sukses Sarana (BSSR)
• Harum Energy (HRUM)
• Adaro Energy (ADRO)
• Bumi Resources (BUMI)
• Mitrabara Adiperdana (MBAP)
• Samindo Resources (MYOH)
• United Tractors (UNTR)
• Berau Coal

Click Here! Top Clean Coal Contractors for Power Plant, Gasification, Liquefaction and Emission Control System

Source: U.S. Department of Energy (DOE)

NETL Have Developed a Low-cost Membranes to Separate CO2 From Nitrogen in a High Volume of Flue Gas

National Energy Technology Laboratory (NETL) researchers have developed a method to custom-formulate low-cost membranes to more effectively separate carbon dioxide (CO2) from nitrogen in a high volume of flue gas. This ability to achieve both high selectivity and high permeability during post-combustion carbon capture operations is one of the most difficult problems facing membrane researchers today. The NETL group solved the challenge by chemically binding multiple membrane components with different critical properties into one high-performance material that can be easily scaled up to reduce the costs of large-scale carbon capture operations.

The work, which was recently featured in the journal Cell Reports Physical Science, builds upon the lab’s expertise developing mixed matrix membranes (MMMs), which combine sturdy polymers with inorganic crystalline particles that enhance selectivity and permeability. Prior work looked at integrating metal organic frameworks (MOFs) to create two-component MMMs, but this research takes the process a step further by fabricating three- and four-component MMMs, known as multicomponent mixed matrix membranes (McMMMs).

“Highly permeable membranes tend to have lower selectivity and vice versa. And very few membrane materials, even two-component MMMs, are capable of breaking the permeability-selectivity tradeoff,” NETL Research Scientist, Sameh K. Elsaidi, PhD, explained. “By formulating our membranes using multiple-component MMMs, we’ve opened a whole new realm of possibilities to enhance compatibility and gas separation performance.”

The research produced 10 different composite membranes featuring components selected for enhanced compatibility, permeability, selectivity and mechanical stability.

“Compatibility between the different components is especially important to the formulation,” Elsaidi added. “We optimised this interfacial compatibility by selecting MOFs with open metal centres that can strongly interact with the functional groups of the polymers, resulting in the formation of uniform, well-blended, defect-free MMMs.”

“We used density functional theory computations to show that the interactions between the polymer functional groups and the open metal centres were quite strong,” commented Janice Steckel, PhD, who also worked on the project. “We believe these strong interactions between the metal centre and the polymer minimises phase separation and contributes to the great separation performance of these composites.”

As part of this work, the NETL team also estimated the cost of carbon capture using each of the 10 custom-formulated membranes, with the lowest cost being US$55/t, which is a substantial cost savings from the current average of US$65/t, and represents a significant step towards meeting the US Department of Energy’s goal of US$30/t CO2 by 2030.

“While this research only encompassed 10 composite membranes, the approach we have developed has opened the door to a new exploration route for myriad MMMs with unique formulas and new state-of-the-art performance,” Elsaidi said.

This is not the first time the NETL team has tackled the permeability-selectivity tradeoff. Elsaidi and the other researchers in NETL’s Functional Materials Team have also recently developed a highly permeable carbon capture membrane through a dual-layer process.

“We believe that the membranes we are developing here at NETL represent the next generation of post-combustion carbon capture technologies,” Elsaidi said. “The work we’re doing is making a real difference in bringing down the cost of carbon capture, and that means a stronger energy foundation for the nation and better environmental sustainability.”

The research team included NETL researchers Sameh Elsaidi, PhD, Surendar Venna, PhD, Ali Sekizkardes, PhD, Janice A. Steckel, PhD, James Baker, PhD, John Baltrus, PhD, and David Hopkinson, PhD, along with Mona Mohamed, PhD, from the University of Pittsburgh.

The 10 largest coal producers and exporters in Indonesia:

• Indo Tambangraya Megah (ITMG)
• Bukit Asam (PTBA)
• Baramulti Sukses Sarana (BSSR)
• Harum Energy (HRUM)
• Adaro Energy (ADRO)
• Bumi Resources (BUMI)
• Mitrabara Adiperdana (MBAP)
• Samindo Resources (MYOH)
• United Tractors (UNTR)
• Berau Coal

Click Here! Top Clean Coal Contractors for Power Plant, Gasification, Liquefaction and Emission Control System

Source: World Coal

Yes, We Use Carbon Dioxide To Make Jet Fuel

Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam."

Daniel from Sonora, Mexico asks: “Is there a way we can use carbon dioxide, so we can get rid of global warming? Like using it for energy?”

Indeed, there are a lot of start-ups out in the world that are trying to do exactly that, and I spoke to one of them: the co-founder of a company called Opus 12, Etosha Cave. 

“At scale there would be many different ways in which we utilize CO2,” she said, “There would be CO2 being utilized for cement. We’re wearing clothing and items that were made from CO2. We’re flying 200 passenger airplanes that have carbon neutral liquid fuels in it.”

Carbon is the basic building block of life on planet earth. (Dear reader, we are called “carbon-based lifeforms” in sci-fi movies, after all.) The technical challenge is that one of the reasons carbon is the basic building block of life is that it forms very strong stable chemical bonds, and it’s energy intensive to break those apart. 

Opus 12 uses catalysts and electricity to do that. Once the CO2 is deconstructed, they plan to make it into three molecules. 

Ethylene: which is part of polyethylene, which is the most common plastic in use today. 

Methane: which is natural gas. 

Carbon Monoxide, which is the molecule that will probably be the first to be profitable.

“It’s the main molecule that can become jet fuel,” Cave explains, “It can become diesel fuel or even gasoline. It can be used to make a subset of polymers that go into consumer products. So for example, your iphone charger has a case that is made from polycarbonate. That can be made from carbon monoxide.” 

A lot of these companies are starting by aiming at fuel, because as the founder of one of these start-ups has tweeted, companies “will go where the market points, and it points to fuel.” 

The disclaimer here should be that all of these technologies are in VERY early stages. “I think in a lot of these cases, we can do it today, but we just can’t do it a cost that makes sense,” says Peter Minor, who works for Carbon180, a non-profit focused on technologies and policies that would remove CO2 from the atmosphere, “You don’t want to fill up your car for $12 a gallon or $50 a gallon.” 

However, there are markets where more expensive fuels can make sense. OPUS 12 is looking to help the Airforce make its own fuel at remote bases in order to limit fuel convoys, since they are vulnerable to attack.

This is reminiscent of how the solar industry started with markets where electricity was very expensive — offshore oil rigs and remote water pumping — in order to bridge to the point where economies of scale could drive down its cost. 

Finally, let’s not forget that there already is an incredibly common way to use CO2 that we’ve been doing for all human history: building things out of wood. As long as a wooden product is preserved and not allowed to rot, that’s captured carbon emissions, my friend.

The 10 largest coal producers and exporters in Indonesia:

• Indo Tambangraya Megah (ITMG)
• Bukit Asam (PTBA)
• Baramulti Sukses Sarana (BSSR)
• Harum Energy (HRUM)
• Adaro Energy (ADRO)
• Bumi Resources (BUMI)
• Mitrabara Adiperdana (MBAP)
• Samindo Resources (MYOH)
• United Tractors (UNTR)
• Berau Coal

Click Here! Top Clean Coal Contractors for Power Plant, Gasification, Liquefaction and Emission Control System

Source: NHPR

Steps to Control the Air Quality of Our Homes

Possibly due to the increased pollutants in the air we breathe, greater numbers of people are suffering from environmental sensitivities. Second hand smoke, dust, pollens, and molds cause millions of people to seek allergy relief products. Although we can't always control our outdoor or work environments, we can take steps to control the air quality of our homes. Furnace filters and vacuum cleaners can help indoor air pollution, but many people turn to air purifiers to make the air in their homes easier to breathe. 

Types of Air Purifiers

There are many different types of air purifiers, each designed to address a specific type of problem:

Dust Removal: Unless you keep your home spic and span and run your heating and air conditioning system 24 hours a day, seven days a week, you most likely have a build up of dust. Many people are allergic to dust, and turn to HEPA machines to filter out dust. HEPA machines usually only take care of the dust in the room where they're located, so they're not a good solution to keeping your whole house dust-free. Instead, it's best to keep your HVAC running 24/7 and invest in good furnace filters. There are several types of furnace filters, such as washable electrostatic furnace filters, that will filter out over 90 percent of dust particles over one micron in size. 

Mold Removal: If you have a problem with surface mold, it's best to consult with a professional contractor. But many people seek allergy relief products due to allergic reactions to airborne mold. Air purifiers that remove airborne mold usually raise ozone levels (like the Airfree and Air Oasis lines of products), and in the process kill odors, chemical particulates and bacteria.

Second Hand Smoke Removal: Smoke not only negatively impacts health and air quality, but it also seeps into fabrics and hard surfaces. Liquid air purifiers (such as the Air Oasis Xtreme) are the best and most effective way to remove second hand smoke.

Virus Protection: Many people buy HEPA machines that are purported to kill viruses using UV lamps. Unfortunately, small UV lamps just don't work; you need a lamp that will create at least 24,000 microwatts of UVGI.

Total Solution?

Are there any air purifiers that are complete and total allergy relief products? Unfortunately, no. To comprehensively address indoor air quality issues, you need to implement a number of approaches. First, because carpeting collects dust and releases gas, your home should have hardwood floors. Second, you should run your HVAC system 24 hours a day, seven days a week, and use a washable electrostatic furnace filter. An ozone generator air purifier near the return air duct will help tremendously. Third, you should have an induct air sanitizer to cleanse viruses, molds, and bacteria. Fourth, you should vacuum and clean your floors with a Hyla vacuum water filtration, wet cleaning, and air purification system. Fifth, you should use air purifiers in your home's bedrooms (such as Airfree or Austin Air purifiers). Sixth, you should spray surfaces with TiO2 PCO liquid, which will address gasses, viruses, and molds. 

Even considering today's air quality, respiratory illnesses aren't inevitable. By improving your home air quality through cleaning and air purifiers, you are certain to breathe easier.

Hierarchical Porous Carbons (HPCs) for Electrical Double-layer Capacitors Using Low-cost Coal-tar Pitch as a Starting Material

Abstract

A simple and effective template-free method to prepare hierarchical porous carbons (HPCs) has been developed by using low-cost coal-tar pitch as a starting material, anhydrous aluminum chloride as the Friedel–Crafts catalyst, and oxalyl chloride as the cross-linking agent. By a simple controllable Friedel–Crafts reaction, diketone-functionalized coal-tar pitch as the hierarchical porous coal-tar pitch precursor was obtained via a one-step carbonization to provide a well-developed micro–mesoporous network. Nitrogen adsorption and desorption measurements showed that the surface area, pore volume, pore size and pore size distributions of the resulting carbon materials was dependent on the usage of the cross-linking agent. The as-fabricated HPCs have a large Brunauer–Emmett–Teller specific surface area of 1394.6 m2 g−1 and exhibit an excellent electrochemical performance with the highest specific capacitance of 317 F g−1 at a current density of 1 A g−1 in a three-electrode system. A symmetric supercapacitor was fabricated from HPC-DK-1.0 in a two-electrode system, which exhibits a high specific capacitance of 276 F g−1 at a current density of 0.25 A g−1, a high rate capability and an excellent cycling stability with a capacitance retention of 92.9% after 10[thin space (1/6-em)]000 cycles. The one-step carbonization method that produced HPCs for electrical double-layer capacitors represents a new approach for high-performance energy storage.

1. Introduction

In recent years, the design and preparation of organic hierarchical porous carbons (HPCs) has attracted significant attention in academia and industry because of the HPCs unique nanoporous hierarchy, which has potential application in catalysis, gas separation, electromagnetic interface shielding, supercapacitors and fuel cells etc.1–5 To our knowledge, HPCs have been prepared by hard-soft-templating approaches or templating/corrosive-chemical-activation combination methods.6–9 These strategies have achieved great success in the preparation of various HPCs with precise pore structures. However, they have some limitations. For example, the procedure is complicated and tedious because of the required fabrication of templates with a special nanostructure or molecular structure, the removal of hard-templates or post-activation treatment, and many expensive templates are required.10–12 These limitations, result in an uncompetitive price-to-performance ratio for the HPCs compared with other materials for any given application, which limits their commercial viability. An exploration of new template-free preparation methods is urgently required in the study of HPCs.

Coal-tar pitch (CP) is the main by product of the coking process in the coal chemical industry, and is often used to prepare carbon materials because of its relatively low price, sufficient quantity and higher carbon yield. For example, CP can be used to produce needle coke, carbon fibers, mesocarbon microbeads and carbon foam.13–16 In recent years, there has been growing interest in the application of porous carbons in the fields of gas storage and EDLC.17,18 However, preparation of pitch-based HPCs often requires templates or supports, such as mesoporous silica, metal oxides or silicon wafer that caps a metallic layer.19–22 The procedure is too complicated and tedious to apply in practice, therefore, it is imperative that new methods be developed to prepare pitch-based HPC carbons. One of the most widely used strategies to prepare HPCs is by the Friedel–Crafts reaction, in which pitch is polymerized under mild conditions and uses cheap and sustainable building blocks to produce highly porous hyper-cross-linked materials.23,24 Such polymers contain various molecules from oligomers to 3D cross-linking supramolecules with significant differences in molecular size, structure, and pyrolysis behavior. Some light molecules or thermolabile groups in the pitch are removed during the controlled pyrolysis, which could generate abundant mesopores. The devolatilization of the volatile component is accompanied by bubble formation, which occurs first in the vicinity of the primary bubble nuclei. The bubbles coalesce and grow under appropriate conditions, which leads to formation of mesopore voids or macropores in carbonized products.25 Thus, the pitch polymers may be promising candidates for constructing HPCs with explicit mesopore control for high-performance supercapacitors.

We report herein the template-free fabrication of a novel type of HPC by constructing diketone (–COCO–) cross-linking bridges between polycyclic aromatic hydrocarbons (PAH) in CP to yield diketone-functionalized CP (DKCP). The polar carbonyl group has a high reaction activity and favors the modification of CP by providing it with hydrophilic properties that enhance its wettability for polar solvents, and the oxygen functional groups can be used as anchoring sites for metal particles and large molecules.26,27 Such novel bridges can provide a high crosslinking density and oxygen atoms to the hierarchical porous modified CP materials, to achieve the carbonizability of a crosslinking modified CP framework and the inheritability of a hierarchical nanoporous structure. These properties should make DKCP a promising candidate for HPC production with the characteristics of preparation simplicity and easy scalability. The overall synthetic procedure is illustrated in Fig. 1.

Fig. 1 Scheme of preparation mechanism of HPC materials in this study.

2. Experimental

2.1 Materials

The raw CP was obtained from the Anshan Iron and Steel Group Co. Ltd (Anshan, China). A refined coal tar pitch (RFCP) with a softening point of 33 °C was obtained by a mixed solvent-extraction method and its compositions. The main properties of the RFCP and DKCP are shown in Table 1. Anhydrous aluminum chloride (AlCl3) was from Tianjin Guangfu Fine Chemical Reagent Co. Ltd. (Tianjin, China). Oxalyl chloride (OC), hydrochloric acid (HCl) and dichloroethane (DCE) were from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemical reagents were of analytical grade.

Table 1 Bulk and surface elemental compositions of RFCP and DKCPs

2.2 Procedures

2.2.1 Polymerization of RFCP. In this one-step cross-linking approach, oxalyl chloride (OC) was used as an external cross-linker to react with RFCP. RFCP (50 g) was dissolved in 500 mL of 1,2-dichloroethane under argon, before a certain amount of OC and AlCl3 was added to the solution. The resultant mixture was stirred for 6 h at 40 °C to undergo the AlCl3 catalyzed Friedel–Crafts reaction of the RFCP and OC. The reaction was terminated by adding an ethanol–water solution. The product was filtered, washed with an ethanol–water solution that contained hydrochloric acid, and dried under a reduced pressure at 80 °C for 12 h. The obtained DKCPs with different OC/RFCP mass ratios are referred to as DKCP-x, where x represents the mass ratio of OC vs. RFCP. All resultant samples were dark-brown powders.

2.2.2 Carbonization. Sample carbonization was carried out in a tube furnace under atmospheric pressure according to the following procedures. Approximately 10 g of sample was carbonized at 800 °C for 2 h with a heating rate of 2 °C min−1 to yield HPCs. The obtained HPC-DKs with different OC/RFCP mass ratios are referred to as HPC-DK-x, where x represent the mass ratio of OC vs. RFCP. A N2 stream was introduced into the tube furnace throughout the carbonization.

2.3 Measurements and analyses

Fourier transform-infrared (FTIR) spectra were collected on a Thermo Nicolet-360 spectrometer (USA). The elemental content of carbon, hydrogen, and chlorine were analyzed with a Vario Macro EL analyzer (Germany). Thermogravimetric analysis (TGA) was performed to determine the pyrolysis samples by using an HCT-1 instrument (China). Samples morphologies were observed by JSM-6700F scanning electron microscope (FESEM, Japan) and by using a Tecnai-G20 transmission electron microscopy (TEM, USA). Surface chemical composition of samples was studied by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB250, USA). The surface area and porosity of the samples were estimated from the isotherms of nitrogen adsorption–desorption at 77 K by ASAP2020. The specific surface area was calculated with the Brunauer–Emmett–Teller (BET) equation. The pore size distribution of the samples was calculated based on the density functional theory (DFT) method.

2.4 Electrochemical measurements

The carbon electrode was fabricated by mixing HPCs and polytetrafluoroethylene (PTFE) with a mass ratio of 9[thin space (1/6-em)]:[thin space (1/6-em)]1. Then, the mixture was rolled into a thin film and cut into round films (12 mm in diameter). Each round film with a 2.5 mg cm−2 mass loading was dried in vacuum oven at 120 °C for 2 h, and then pressed onto nickel foams to fabricate supercapacitors electrodes. The obtained electrodes were soaked in 6 M KOH electrolyte under vacuum for 120 min. A button type-supercapacitor was assembled by two similar electrodes and separated by a polypropylene membrane. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted by using a CHI760E electrochemical workstation (Chenhua, Shanghai, China). EIS was carried out over a frequency range of 100 kHz to 0.01 Hz with an amplitude of 5 mV. The galvanostatic charge–discharge measurements and cycle life tests were conducted on a supercapacitance test system (SCTs, Arbin Instruments, USA). The specific capacitance of the working electrodes was calculated from the galvanostatic discharge process via the following equation.

        (1)

    (2)

where Cs (F g−1) is the specific capacitances of the three electrodes system, Ccell (F g−1) is the specific capacitances of the symmetric supercapacitor system, I is the discharge current (A), Δt is the discharge time (s), ΔV is the voltage change (V) that excludes the voltage drop during the discharge process, and m is the mass of the active material (g).

The energy and power density of the symmetric supercapacitor systems were calculated by using the eqn (3) and (4):

    (3)

       (4)

where Ecell (W h kg−1) is the specific energy density, Pcell (W kg−1) is the specific power density, Ccell (F g−1) is the total specific capacitance of the two-electrode cell, ΔV is the voltage change that excludes the IR drop during the discharge process, and Δt is the discharge time.

3. Results and discussion

3.1 Characteristics of RFCP and DKCPs

3.1.1 Elemental analysis and XPS analysis. The elemental analysis of the RFCP and the DKCPs is provided in Table 1. The DKCPs compared with the RFCP have a high oxygen content with an increase in OC content. The decrease of the C/O ratio indicates that the diketone-structure was successfully introduced into the RFCP (Table 1). The introduction of diketone functional groups is key to achieving a highly disordered carbon structure. The introduction of oxygen induces cross-linking of the RFCP structure, which prevents the melting and orderly rearrangement of the RFCP during the high-temperature carbonization process, and inhibits the graphitization process. The evolution of CO and CO2 during the high temperature process changes the microstructure of the carbon materials and plays a dual regulation role.

The surface chemistry of the DKCPs was studied by X-ray photoelectron spectrometric (XPS) measurement (Fig. 2a). The XPS survey spectra show two peaks at binding energies of 284.1 eV and 531.5 eV, which correspond to C1s and O1s, respectively, and suggest that all the samples contain a considerable number of oxygen-containing groups on their surfaces. During the carbonization process, the oxygen-containing groups in the pitch are unstable and can decompose to CO2 and CO during heat treatment and self-activation of the HPC,28,29 which can assist in creating additional pores.

Fig. 2 XPS spectra for RFCP derivatives (a) C1s (b) O1s, and (c) XPS spectra of DKCP-1.0

Information on the chemical state of the elements anchored to the DKCPs surface was obtained from XPS. In the C1s spectrum of the DKCP-1.0 (Fig. 2b), peaks exist for different functional groups, namely C[double bond, length as m-dash]C, C–C, and C–H bonds (284.3 eV), C[double bond, length as m-dash]O bonds (286.5 eV), and O–C[double bond, length as m-dash]O bonds (288.4 eV).30 The O1s spectrum of the DKCP-1.0 (Fig. 2c) can be fitted with two major component peaks. The peak at 531.8 eV is attributed to C[double bond, length as m-dash]O bonds and the 533.1 eV peak results from O–C[double bond, length as m-dash]O.31 These observations show that the DKCPs consists of aromatic carbon with carbonyl and carboxylic functional groups.

3.1.2 FT-IR analysis. As shown in Fig. 3, the absorption peaks at 3040 and 2910 cm−1 result from aromatic C–H stretching vibration and aliphatic C–H stretching vibration, respectively.32 The peak at 1600 cm−1 is attributed to aromatic C[double bond, length as m-dash]C stretching vibration and the peak at 1460 cm−1 is attributed to the C–H bending vibration of methyl and methylene.33 The peaks at 1720 cm−1 is attributed to C[double bond, length as m-dash]O vibration.34 The peaks at 1170 cm−1 is attributed to C–C stretching vibration. The absorption peaks of the aromatic ring skeletal vibrations move to lower wavenumbers after modification, which indicates the degree of conjugation of the aromatic rings. Hence, the degree of polymerization for the aromatic rings increases significantly. Therefore, RFCP can be bridged by OC.

Fig. 3 IR spectrum of RFCP derivatives (a), TGA curves of RFCP derivatives (b), and DTG curves of CP derivatives (c)

3.1.3 Thermogravimetric analysis. Thermogravimetric analysis (TGA) was used with derivative thermogravimetry (DTG) to study the transitions of RFCP and DKCPs at different carbonization temperatures. Fig. 3b shows that both RFCP and DKCPs decompose in a single mass loss stage from 50 to 800 °C. The mass loss results mainly from the removal of gases and light compounds that are generated via thermal polymerization and the cracking of side chains of aromatic rings.35 The carbonization yields of RFCP, DKCP-0.5, DKCP-1.0 and DKCP-1.5 are 16.3%, 41.5%, 55.9%, and 65.9%, respectively, which indicates that the carbonization yield of RFCP can be improved by cross-linking OC. This outcome can be rationalized as follows: OC can react with small molecules in RFCP to from large molecules,36 which decreases the removal of light compounds and increases the carbonization yield. The polar oxygen-containing functional groups from more thermal-resistant materials and increase the carbonization yield of RFCP. DTG curves (Fig. 3c) show that RFCP and DKCPs lose mass at varying rates, as related to the cross-linking degree by OC. The RFCP profile is characterized by a single peak that is centered at 251 °C, which indicates that the mass loss rate at this temperature reaches a maximum. However, three peaks exist in the DTG profile of the DKCPs, which indicates that the main reactions/transformations that occur during the pyrolysis process are similar. The first peak centered at 224 °C (peak I) can be ascribed to the evaporation of absorbed water. Most oligomers in the RFCP fraction are easy to gasify and/or distill below 430 °C,37 which leads to the formation of a DTG peak that is centered at approximately 367 °C (peak II). Above 450 °C, the remaining oligomers and macromolecules cross-link to form larger molecules, even solidified coke. The condensation reactions are accompanied by a release of small molecules, such as CO2, CO, H2O, and CH4, that leads to the third DTG peak that is centered at approximately. 509 °C (peak III).38,39 When the temperature exceeds 620 °C, a carbon-structure rearrangement occurs in the solidified coke and no significant mass loss appears.40 The configuration of small oligomers and large macromolecule networks in the diketone-functionalized pitch polymers leads to a stepwise pyrolysis and aggregation process, which could have a significant effect on the morphology and microstructure of the resultant carbons.

3.2 Characterization of porous structure of HPCs

3.2.1 Brunauer–Emmett–Teller (BET). Porous structures of HPC-RF and HPC-DKs are presented in Fig. 4a by measuring the N2 adsorption–desorption isotherms. All samples display mixture-type isotherms with hysteresis loops, which indicates a combination of microporous/mesoporous structure.41 The N2 adsorption isotherms of all HPC-DK samples show a steep N2 uptake at low relative pressure (P/P0 < 0.001), reflecting the existence of abundant micropores. The adsorption isotherms of HPC-DKs show an evident hysteresis loop in the medium pressure region (P/P0 = 0.4–0.9), indicating that a large number of mesopores exist in these HCPs. Therefore, the HPC-DK-0.5 shows a dominant pore size distribution of less than 2 nm. With an increase of OC mass, a more porous structure emerges in the HPC-DK-1.5 and HPC-DK-1.0. The HPC-DK-1.0 exhibits a more significant hysteresis than HPC-DK-0.5, HPC-DK-1.5, and the HPC-RF in the relative pressure (P/P0) range of 0.4–0.9, which indicates a higher amount of mesopores in HPC-DK-1.0 compared with other HPCs when using the same starting materials and for the same procedure. The porous structure was created by the crosslinking reactions and the heteroatoms in the carbonyl-functionalized pitch are chemically unstable, which provides more “active sites” for carbonization. Thus, the surface functional groups of the RFCP provide a meaningful contribution to the high BET surface area. The calculated structure parameters of a series of carbon materials, including the BET specific surface area (SSA), total and micro-mesopore volume are summarized in Table 2. For a constant sintering temperature, the SSA, pore size distribution, and pore volume of the HPCs are influenced significantly by the mass ratio of the cross-linking agent (OC). It is found that the specific surface areas and pore volume vary with the weight ratios of oxylyl chloride to refine coal-tar pitch (OC/RFCP). The highest BET surface area and pore volume (1394.6 m2 g−1, 1.54 cm3 g−1) are obtained in HPC-DK-1.0. With the increase of OC/RFCP from 0.5 to 1.0, the BET surface area increases from 410.9 m2 g−1 to 1394.6 m2 g−1, and the pore volume from 0.64 cm3 g−1 to 1.54 cm3 g−1. However, a further increase of OC/RFCP leads to a decrease in the specific surface area and pore volume. Therefore, the larger BET surface area and pore volume at large OC/RFCP can be explained as the results of higher crosslinking degree. However, a further increase of OC/RFCP leads to a decrease in the specific surface area. The reason is that when excess OC is used, only one of the two acyl chloride groups in oxalyl chloride reacts with the aromatic rings. As a result, the crosslinking degree of RFCP decreases. The other acyl chloride group on oxalyl chloride is converted into a carboxyl group, as demonstrated by the FTIR spectra of the DKCPs. Fig. 4b shows the pore-size distribution of the HPCs, which justifies the effectiveness of the micropore and mesopore introduction on the HPCs by a one-step carbonization process in the presence of a cross-linking agent (OC). The HPC-DK-1 shows a dominant pore size distribution of less than 2 nm. With an increase in OC mass, a more porous structure emerges in the HPC-DK-1.5 and HPC-DK-1.0. The amount of 3 nm mesopores also increased, which provides a low resistant ionic pathway, and improves the accessibility of the micropores to electrolytes. The number of micropores less than 2 nm increased greatly for the HPC-DK-1.5 and HPC-DK-1.0 samples, as shown in Fig. 4b. The variety of pore sizes with a high pore volume provides highly efficient mass transport through the mesopores and a large SSA from the micro-to mesopores, which achieves an excellent performance for electrical double-layer capacitor applications.

Fig. 4 (a) N2 absorption/desorption isotherms; (b) pore size distributions for the HPC samples.

Table 2 Pore structure parameters of nanoporous carbon materials

3.2.2 SEM and TEM analysis. The HPC morphologies were studied by field-emission scanning-electron microscopy and high-resolution transmission-electron microscopy (HRTEM) (Fig. 5). The (fold)block structure for the RFCP after carbonization was clearly visible, and the mesopore and macropore structures appeared in their matrix as diketone groups that were introduced into the RFCP. The decomposition of carbonyl-functional RFCP was one of the factors that affected their porous nanostructure, and the carbonyl content is the main governing index. Holes were derived from the decomposition of the carbonyl group and the self-assembly foaming process, in which gases formed by pyrolysis/gasification of active precursor molecules lead to the formation of many holes, such as in common foaming processes for preparing carbon foams.42,43 This result is consistent with the composition characteristics of pitch cross-linked discussed above, in which the rich substituent carbonyl groups and the aromatic structures affect the fusibility of the DKCPs significantly, and thus determines the structural properties of the pyrolyzed carbons. The HPC-DK-0.5 contains more oligomers with a lower cross-linking degree, which possesses a lower systemic viscosity and a higher plasticity. DKCP-1.5, in contrast, has many macromolecules with serious cross-linking and so the number of holes on the HPC-DK-1.5 surface is lower (Fig. 5d). The porous structure was verified by HRTEM. The HRTEM micrographs in Fig. 5e and f show that HPC-DK-1.0 is amorphous and has a highly disordered pore structure. Abundant micropores occur with the mesoporous channel walls, which indicates the formation of a continuous three dimensional pore network. This result is consistent with the DFT pore size distribution results.

Fig. 5 SEM images of HPC-RF (a), HPC-DK-0.5 (b), HPC-DK-1.0 (c) and HPC-DK-1.5 (d), TEM images of HPC-DK-1.0 at 100 nm (e), and 5 nm (f)

3.3 Electrochemical properties

The electrochemical performances of the obtained carbon materials were investigated in a three-electrode system in 6.0 M KOH solution. Cyclic voltammetry (CV) curves (Fig. 6a) of the carbonized material electrodes with a scanning rate of 50 mV s−1 exhibited a typical rectangular I–V curve without any redox peak with bumps −1.0 to 0 V, which suggests that all carbonized materials exhibited a pure capacitive behavior.44,45 The HPC-DK-1.0 presented the largest encircling area of the CV curve, which revealed its highest capacitance among the three HPC carbons. Galvanostatic charge–discharge (GCD) curves of HPCs obtained at 1 A g−1 (Fig. 6b) showed almost symmetrical triangles with a tiny deformation, which suggests a reversible electrochemical capacitive performance with a high charge/discharge efficiency. The capacitance of HPC-DK-1.0 calculated from the galvanostatic charge–discharge curve is 317 F g−1 at 1 A g−1 and is significantly higher than that of the carbon materials reported in previous studies.6,9,16,37 The large specific capacitance may be caused by its high accessible surface areas and rich reasonable distributed pores. More significantly, the IR-drops of the HPCs at the start of discharge are less than 7 mV, which suggests a very low equivalent series resistance, an excellent conductivity, and a high mass transfer and/or diffusion rate of ions within the electrode materials. Fig. 6c presents the rate performances of all the carbonized materials. The specific capacitances decrease with an increase in current density for all samples. The behavior is related closely to the pore-size dependent diffusion limitation of ions inside the electrode material at higher current densities.46 HPC-DK-1.0 exhibits the highest capacitance of 317 F g−1 at a current density of 1 A g−1 among the three carbonized materials, which is much higher than that of HPC-DK-0.5 (207 F g−1) and HPC-DK-1.5 (265 F g−1) at the same conditions. At a high current density of 10 A g−1, HPC-DK-1.0 exhibits a high specific capacitance of 242 F g−1, and retains 76.3% of the specific capacitance. The capacitance retentions in the same current range are 73.9% and 69.1% for HPC-DK-0.5 and HPC-DK-1.5, respectively. The high specific capacitance and excellent rate performance of the HPC-DK-1.0 is ascribed to its superhigh BET surface area (SBET = 1394.6 m2 g−1) and well-distributed hierarchical porous structures which provide a high accessible surface for electron accommodation and convenient electrolyte-ion transportation.47,48 Electrochemical impedance spectroscopy (Fig. 6d) was conducted to understand the capacitance mechanism. In the low frequency region, the line that is nearly parallel to the imaginary axis demonstrates an excellent supercapacitor capacitive behavior.49 In the medium frequency region, the inclined line with a 45° slope corresponds to the diffusive resistance of electrolyte ions within the pores of electrode materials (Warburg resistance). In the high frequency region, the semicircle diameter indicates a change in transport resistance (Rct) at the electrode/electrolyte interface.50 The intercept of the semicircle with a real axis (z′) is referred to as the internal resistance (Rs), which includes the intrinsic resistance of the electrode material, the contact resistance between the electrode material and the current collector, and the resistance of the electrolyte solution.51 Electrochemical impedance spectroscopy of the HPC-DK-1.0 presents a short Warburg region and a small semicircle diameter, which means that its hierarchical porous structure favors electrolyte ion access and rapid ion transportation. The Rs value of HPC-DK-1.0 is the smallest among the three HPC carbon electrodes, which indicates its excellent conductivity and improves its supercapacitive performance.

Fig. 6 Electrochemical performance of carbonized material based electrode measurement in a three-electrode system in 6.0 M KOH aqueous electrolyte. (a) CV curves at 50 mV s−1, (b) charge–discharge profiles at 1 A g−1, (c) capacitance at different current densities, and (d) Nyquist plots of porous carbon electrodes with inset showing plots in high frequency region.

The symmetric supercapacitor was assembled by using HPC-DK-1.0 as positive and negative electrode materials, because the three-electrode configuration may produce large errors, and lead to an overestimation of capacitance. Fig. 7a shows typical CV curves of the HPC-DK-1.0 based electrode over scanning rates of 5–200 mV s−1 in a 6.0 M KOH aqueous electrolyte. All CV curves were rectangular without obvious redox peaks at a scanning rate of 5–200 mV s−1, which is characteristic of excellent capacitive behavior.52 No significant distortions in the CV curves result when the scan rate was increased to 200 mV s−1, which suggests rapid ion/charge transport within electrodes and the near-ideal capacitive behavior with a good rate capability. Fig. 7b shows the galvanostatic charge–discharge curves of HPC-DK-1.0 based supercapacitor at different current densities from 0.25 to 10 A g−1. The symmetric linear charge and discharge curves with a negligible voltage drop demonstrate a high coulombic efficiency and a negligible internal resistance. From the discharge curve, the specific capacitance at a constant current density of 0.25 A g−1 was found to be 276 F g−1, which is much higher than that of the RGO-CMK-5 electrode (144.4 F g−1 at 0.2 A g−1),53 the curved graphene electrode (154.1 F g−1 at 1 A g−1),54 and the 3DG-MnO2-13% electrode (36 F g−1 at 0.5 A g−1)55 in a two-electrode system. The capacitance could retain a high value of 196 F g−1 even at a very high current density of 50 A g−1 (Fig. 7c), which indicates the high rate performance of the HPC-DK-1.0 based symmetric supercapacitor. The specific capacitance of HPC-DK-1.0 at different current densities is shown in Fig. 7c. The capacitance decreases rapidly from 256 to 224 F g−1 when the current density increases from 0.25 to 3 A g−1. After that, it drops slowly at high current densities from 3 to 50 A g−1, which reveals the excellent rate capability of the HPC-DK-1.0 electrodes. Such an excellent rate performance is essential for practical application involving a high-rate supercapacitor. The kinetic ion diffusion within the electrode was investigated by electrochemical impedance spectroscopy. Fig. 7d shows the dependence of the impedance phase angle on the frequency of the HPC-DK-1.0 electrode. The relaxation time constant τ0 of the supercapacitor, which is defined as 1/f0 at a phase angle of −45°, represents the point where the resistive and capacitive impedances are equal. For the symmetric supercapacitor that was fabricated from HPC-DK-1.0, the characteristic frequency f0 at a phase angle of −45° was observed to be 1.3 Hz in KOH aqueous electrolyte, which corresponds to a time constants τ0 of 0.77 s, which is nearly equal to 0.73 s of the graphene aerogel of the GA-0.5 electrode,56 and is smaller than that of a conventional activated carbon-based electrode (10 s).57 The very short time constant of HPC-DK-1.0 highlights the critical role of nanopores in promoting the ion kinetic diffusion in the interior of the electrodes.

Fig. 7 Capacitive performance of symmetric electrode for HPC-DK-1.0 in 6.0 M KOH aqueous electrolyte. (a) CV curves at different scanning rates, (b) charge–discharge profiles at different current densities, (c) capacitance retention at different current densities, and (d) Bode plot of phase angle verses frequency.

Cycling stability is one of the most important parameters for practical application of supercapacitors. The cycling stability of an HPC-DK-1.0 based supercapacitor was investigated by a consecutive charge–discharge measurement at a constant current density of 3 A g−1 for 10[thin space (1/6-em)]000 cycles. Although the specific capacitance of the HPC-DK-1.0 electrode decreased gradually with the cycling number (Fig. 8a), a capacitance retention of 92.9% was still obtained after 10[thin space (1/6-em)]000 cycles, which indicates its good electrochemical stability. The rectangular CV profile (Fig. 8a inset) and Nyquist plots (Fig. 8b) with negligible changes after 10[thin space (1/6-em)]000 cycles support this electrochemical cyclability. The small semicircle in the high-frequency region and the almost vertical line in the low-frequency region indicate that HPC-DK-1.0 has an excellent electrical conductivity (Fig. 8b inset). The capability of HPC-DK-1.0 that integrated its high rate performance with an excellent cycling stability is of great importance for high-performance supercapacitors. Fig. 8c shows the Ragone plot of the symmetric capacitor. With KOH as the electrolyte, the energy and the power density were 6.81 W h kg−1 and 25 kW kg−1 at a current density of 50 A g−1, respectively, which exhibits an outstanding power performance.

Fig. 8 (a) CV curves at different cycles, (b) Nyquist plots of different cycles with the inset showing the plots in the high frequency region, and (c) Ragone plot.

4. Conclusions

We have developed a simple and effective template-free method to prepare HPCs by constructing diketone cross-linking bridges in RFCP. The oxygen facilitated extensive cross-linking formation and prevented graphitization. The diketone structure in the pitch is unstable and can decompose to CO2 and CO during heat treatment, which can assist in creating additional pores. The SBET and Vtot of the HPCs increased with the cross-linking agent to precursor ratio. The optimum cross-linking agent to precursor ratio was found to be 1.0, which resulted in a specific surface area of 1394.6 m2 g−1 and a porosity volume of 1.54 cm3 g−1. HPC-DK-1.0 could be one of the best electrode materials for supercapacitors with a high specific capacitance of 276 F g−1 at a current density of 0.25 A g−1 and a high capacitance retention of 92.9% after 10[thin space (1/6-em)]000 cycles in a symmetric two-electrode cell because of its high surface area, small inner resistance and high electrical conductivity. These results show that these porous carbon materials are promising for use in high-performance supercapacitors.

Source: Haiyang Wangab, Hongzhe ZhuORCID logob, Shoukai Wang*b, Debang Qia and Kaihua Shen*a

a The State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian

b Sinosteel Anshan Research Institute of Thermo-Energy Company Limited

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The Present Status and Future Outlook With Respect to Up‐scaling of Zeolite Synthesis From Coal Fly Ash

Abstract

Coal fly ash has been recognised as suitable feedstock for zeolite synthesis, and with this discovery, there has been much literature produced over the years on various synthesis processes with a view to address the disposal problems associated with fly ash wastes. The different synthesis processes can be divided into a pre‐synthesis activation with an alkali solution and a hydrothermal synthesis of heating the resulting aluminosilicate gel. However, the challenge lies in up scaling the synthesis to allow production on a commercial scale. The most common pre‐synthesis involving high‐temperature fusion of fly ash with sodium hydroxide may not be feasible in an economic context, given that a large‐scale furnace that would be needed. In this review, the successes recorded in the application of acoustic cavitation by using ultrasonication technique were discussed as a possible alternative to the fusion process on one hand. On the other hand, jet‐loop reactor system provides hydrodynamic cavitation, which may be relatively scalable compared to ultrasonication and may offer an economic advantage, if systematically explored, over fusion process for large‐scale synthesis of zeolites. It is also recommended that agitation is critical during the crystallisation process and the suitability of an impeller type that offers a minimum shearing to be explored during the hydrothermal process with respect to quality and yield of the zeolite produced. The understanding gleaned from these recommendations may be useful in designing an appropriate scale‐up operation for zeolite synthesis from coal fly ash.

1. Introduction

It has been 30 years since the discovery by Holler and Wirsching that zeolites can be synthesised from coal fly ash [1]. It was found that the coal fly ash contained the necessary elements, namely silicon and aluminium, which are building blocks of the zeolitic structure. This has spawned substantial research into zeolite synthesis from coal fly ash with a view to create outlets for fly ash utilisation, thereby solving the worldwide problem of coal fly ash disposal. Coal fly ash is produced during the coal combustion process, and it is the most predominant coal combustion by‐product [2]. The electricity generated from coal combustion accounted for 29.9% of the world's electricity supply in 2011, and this is expected to increase 46% by 2030 [3]. Therefore, there are concerns with the high volumes of coal fly ash waste being produced.

In South Africa alone, approximately 36 million tonnes are produced per annum with 95% of that being dumped in ash dumps [4]. When it is seen that South Africa only accounts for 2.4% of the world coal consumption, it becomes apparent of the scale of coal fly ash produced worldwide. Especially, concerning the large‐scale world coal consumers, such as China (50.2%), United States (11.7%), and India (8%) [3], it has been reported that China produced 540 million tonnes of coal fly ash in 2011 with only 68% being reused, while about 61% of the 163.56 million tonnes of fly ash produced in India between 2012 and 2013 was reused [3]. This poses risks to the environment and surrounding populations of these ash dumps. This is due to coal fly ash having the potential to leach toxic heavy elements into the groundwater system [5] and poses a threat to person's health if inhaled [6]. Due to the problem of coal fly ash disposal, many studies investigated its use with regard to different applications, such as in agriculture, in construction, in wastewater treatment, and for the synthesis of geopolymers and zeolites as can be seen in a recent review [3]. The views of these studies were to find outlets for the utilisation of this waste, thereby addressing the disposal problems. However, these applications have not solved the problem of coal fly ash disposal, as they are not at the scale required to deal with the high volumes of coal fly ash being produced.

There is a need to investigate further into the feasibility of scaling up some of these applications. Although recent studies have been focusing on some fundamental investigations regarding scale‐up development for the synthesis of zeolite from coal fly ash [7, 8], there remains a significant amount of studies to be conducted in order to realise the end goal of synthesis on a commercial scale. This book chapter aims at giving a review of the work that has been produced regarding the synthesis of zeolites from coal fly ash with a view to recommending outstanding investigations to be carried out regarding the up scale of zeolite synthesis from coal fly ash. It will justify the necessary procedures that need to be studied in the future to realise the end goal of large‐scale zeolite synthesis from coal fly ash.

2. Coal fly ash

Coal fly ash is mainly composed of silica (SiO2), alumina (Al2O3), ferrous oxide (Fe2O3) and calcium oxide (CaO) with also some unburned carbon detected through loss on ignition tests (LOI) [9, 10]. The chemical properties depend on the type of coal being burned, along with handling and storage procedures [3, 9, 10]. This gives rise to the high variability in coal fly ash compositions obtained from different batches of coal processed in the same power station [8]. The major and minor elements in coal fly ash are expressed as oxides, with the main ones being already mentioned previously. However, it also consists of trace elements, with some being As, B, Ba, Be, Cd, Co, Cr, Cu, Ge, Hg, Li, Mo, Ni, Pb, Rb, Sb, Se, Sn, Sr, Th, U, V and Zn [10]. The major phases of coal fly ash are amorphous glass, mullite and quartz with various others in smaller amounts [10, 11]. A recent comprehensive study by Musyoka [12] on the mineral phases of South African coal fly ash reported that the ash yielded percentage amounts ranging from the more amorphous content of 57.54% to less amorphous content of 41.55%. The mullite ranged from 34.55 to 22.27% and the quartz from 21.38 to 11.92%. Another comprehensive study was performed by Vassilev and Vassileva [11] on coal fly ash from Spain, Bulgaria, the Netherlands, Italy, Turkey and Greece. The amorphous content ranged from 85 to 34%, the mullite from 40 to 0.3% and the quartz from 14 to 2%.

Furthermore, the combustion rate and temperature of the coal combustion process affects the morphology of the coal fly ash particles [10, 13]. Coal fly ash mainly consists of spherical particles, such as solid spheres and hollow spheres (cenospheres), with some irregular unburned carbon, as determined by scanning electron microscopy (SEM) analysis [3, 13]. The colour of coal fly ash ranges from water white to yellow, orange to deep red or brown to opaque and is due to the iron and unburned carbon content [3, 14]. Coal fly ash containing more than 70% SiO2+Al2O3+Fe2O3 and having a low lime content of 5% is classified as class F fly ash, while if the SiO2+Al2O3+Fe2O3 content is between 50 and 70% and a high lime content of between 10 and 35%, it is classified as class C fly ash [9, 10]. It follows from the aforementioned that class C fly ash can be produced from lignite and sub‐bituminous coal while class F from bituminous and anthracite coal [10, 11].

There have been numerous studies investigating the leachability of coal fly ash into the groundwater system that surrounds the ash dumpsites [5, 15–26]. In a more recent article by Nyale et al. [5], it was reported that the disposed ash at the dump site was able to release trace elements into the environment over time, and the study revealed that all the trace elements studied (As, Zn, Pb, Ni, Mo, Cr and Cu) were significantly leached from the labile phases (4.42–27.43%). The inhomogeneity of the ash dump was also attributed to the observed differences in the elements leached from varying core samples. Besides this type of environmental problem, the potential threat of coal fly ash to the health of the surrounding population and the mine workers exposed to the ash has been reported [6]. The leachability of genotoxic compounds from fly ash into human lungs if the ash dust is inhaled has been reported [6], and conditions such as lung cancer, asthma and many others are typical diseases attributed to fly ash inhalation [6, 27].

However, there have been several studies that investigated potential application of fly ash with a view to mitigate the disposal of this fly ash; some of the applications investigated include uses in agriculture, construction, extraction of rare earth elements, wastewater treatment and synthesis of geopolymers and zeolites [3]. Furthermore, only 5% of coal fly ash is being re‐used as reported by Eskom [4]. A comprehensive and recent review of some of the applications of coal fly ash is reported by Yao et al. [3]; however, these authors recommended the need to explore commercialisation of the applications. The following section will focus on the various synthesis methods employed in the application of coal fly ash for zeolites synthesis and assess studies aimed towards the up scale of the synthesis process.

3. Zeolites

Zeolites are crystalline aluminosilicates of which possess cavities and pores on a molecular scale [28]. They do occur as natural minerals but are more useful being synthetically produced in laboratories for uses such as sorbents, catalysts and exchange materials [28]. The zeolite consists of SiO4 and AlO4 tetrahedra of which gives it an anionic framework with the negative charge of Al being compensated by extra framework cations, some being Na+, K+, Ca2+ and Mg2+ [29]. This can be illustrated in Figure 1.

Figure 1.

(a) Basic tectosilicate structure of zeolite where dark (i.e. vertex in) and light (i.e. vertex out) shades to add three‐dimensional (3D) effect and upside down orientation of the tetrahedra for vertex sharing between two rings of the zeolite structure in its 2D view on a picture plane; (b) single‐ring tetrahedron structure and framework of a zeolitic mineral; (c) SiO⁴⁻ and AlO⁴⁻ in a ring of sodium zeolite; and (d) pictorial representation of a 3D view of a tetrahedra with centrally located Si or Al atoms, exhibited by dotted lines drawn to represent the portion within the body of the mineral [30].

The primary building units of the zeolite structure are shown in Figure 1(d), and they are TO4, tetrahedra of silicon and aluminium mentioned previously [29]. There is a concept of infinite component units with the zeolite structure regarded as being made of finite component units with infinite component unit‐like chains or layers, these are known as the secondary building units, some illustrated in Figure 2. The secondary building units can consist of single or double rings with each of those consisting of four‐, five‐, six‐ or eight‐linked tetrahedral, including the silicon and aluminium atoms [30].

Figure 2.

Secondary building units (SBU's) with SBU codes below figures [29].

The varying framework structures seen above give rise to the unique three‐letter code system to name each unique zeolite, some of these codes along with the pore sizes of some zeolites are shown in Figure 3.

Figure 3.

A comparison of the different framework pore sizes [31].

4. Synthesis of zeolites from coal fly ash

As mentioned previously, the synthesis of zeolites from coal fly ash began with the pioneering work of Holler and Wirsching in 1985 [1]. Researchers have found a number of synthesis methods to produce various types of zeolites from the coal fly ash [32–42]. All known synthetic methods can be found on the website of International Zeolite Association (IZA) at URL: http://www.iza‐online.org/synthesis/default.htm [43]. However some of the prominent methods are reviewed in this section.

All the methods developed mainly involve the dissolution Al‐Si bearing fly ash phases with alkaline solutions such as NaOH and KOH with the proceeding precipitation of zeolitic material [2]. The two most well‐known routes for fly ash conversion to zeolite are the fusion with sodium hydroxide process developed by Shigemoto et al. [32] and a two‐step process developed by Hollman et al. [35]. The two‐step process developed by Hollman et al. [35], which followed from the traditional one‐step process produced more “pure” zeolites; however, the process was relatively more costly. A comparison of the one‐step and two‐step process can be seen in Figures 4 and 5, respectively. Shigemoto et al. [32] developed the fusion process where coal fly ash is fused with sodium hydroxide using a high temperature, prior to hydrothermal treatment. A ratio of CFA:NaOH of 1:1.2 was optimum with a fusion temperature of 550°C, and this converted the fly ash particles into sodium salts. These salts (silicate and aluminate) after the hydrothermal treatment favoured the formation of zeolite Na‐X. The process is illustrated in Figure 6.

Figure 4.

Flow diagram of the traditional one‐step process [30].

Figure 5.

Flow diagram of two‐step process [30].

Figure 6.

Flow diagram of the fusion‐assisted process [30].

Murayama et al. [36] investigated the synthesis of zeolites from coal fly ash using different alkali sources, namely NaOH, Na2CO3 and KOH. It was found that regarding the reaction mechanism that three steps exist in an alkali hydrothermal reaction, namely dissolution, condensation and crystallisation. The dissolution step begins at a temperature range of 298–393 K with the amount of OH- in the alkali solution making a great contribution to the reaction. The concentration of Na+ in the alkali solution mainly determined the total reaction rate of the zeolite synthesis.

Not only the degree of zeolitisation is affected by the NaOH concentration but also the type of zeolite formed. This is according to Molina and Poole [44] who noted the difference found in the zeolite product was due to the increase in supersaturation achieved from the higher proportion of soluble species from the rise of the NaOH concentration. This phenomenon can be described by the Ostwald's rule of successive transformation with the higher the supersaturation, better the conditions are to nucleate metastable phases [44]. An example being zeolite X which later recrystallises and is replaced by the more stable zeolite hydroxysodalite. Some other factors affecting zeolite formation from coal fly ash are the SiO2/Al2O3 ratio in the feedstock, the NaOH concentration as mentioned previously, the extra framework cations, the water content, the synthesis time and temperature and the agitation used in the zeolite synthesis.

Regarding the SiO2/Al2O3 ratio Inada et al. [38] demonstrated the dependence of this ratio in zeolite synthesis. Two differing zeolites were formed from a silica rich fly ash (zeolite Na‐P1) and an alumina‐rich fly ash or silica lean fly ash (hydroxysodalite) with the ratio being controlled with SiO2 and Al2O3 aerosil powders. According to Querol et al. [45], this ratio depends on the compositions of these species within the aluminosilicate glass phase of the coal fly ash and not on their bulk composition in the fly since this phase is the first to be activated.

Murayama et al. [36] also showed how the choice of cations could significantly alter the reaction to form one zeolite crystal over another. The Na+ ions promoted the formation of zeolite P, while the K+ ions promoted the formation of chabazite. Indeed, zeolite synthesis was shown to be favoured by the use of Na⁺ ions over K⁺ ions [46, 47]. This is due to Na+ ions stabilising the secondary building units of the zeolite frameworks, while K+ ions act as a suppressor due to their promotion of slow crystallisation rates [47].

There is a problem when it comes to zeolite synthesis regarding the water content used. This is due to the increase in water content resulting in an increase in yield when using fly ash as the starting material. This is because the higher water content results in the increase in the crystalline and amorphous phases dissolution rates in the coal fly ash [46, 48]. This led to investigation of using alternate water sources, such as investigating using tap water, distilled water and acid mine drainage (AMD) in the replacement of ultra‐pure water. It was reported by Mainganye [8] that tap water and distilled water could produce similar products as the ultra‐pure water, while AMD water, however, was not suitable. Musyoka et al. [49] established that AMD could yield hydroxysodalite—a zeolite with less application. However, it was found that circumneutral mine water produced a comparable quality of Na‐P1 and X zeolites to ultra‐pure synthesis. Zeolite X and ZK‐5 were successfully synthesised using seawater at lower temperatures with hydroxysodalite also being formed; however, zeolite A could not be achieved [42]. Artificial seawater was used at room temperature (25°) to crystallise zeolites A and X in a later study [50].

The thermodynamic stability of zeolite has been reported by Boycheva et al. [51] to follow the sequence in the order of Linda --> Faujasite--> Chabazite--> Na‐P1 --> Hydroxysodalite. It has been observed previously in the transformation of faujasite into more stable phases such as chabazite and Na‐P1 at longer synthesis times in the reactant liquor [37, 51]. Due to this metastable behaviour of zeolite X, its formation is always competitive with hydroxysodalite, zeolite X favouring the lower temperatures [42, 51]. It is also known that a higher temperature produces zeolites of a greater particle size [44]. It was found that aluminium dissolution tends to be faster than silicon, of which the two need to be continuously dissolved to promote and feed crystal growth, the silicon dissolution increasing with temperature [44]. Therefore, the Si/Al ratio will favour the formation of zeolite X over A at higher temperatures. When the glassy content is high, the synthesis time drops, while a high content of quartz and mullite requires longer reaction times [2].

Inhomogeneous mixtures can result due to inadequate mixing which in turn has a dramatic effect on zeolite synthesis, this is pertaining to the viscous gel produced before the crystal growth step [52]. These inhomogeneous mixtures produce “pockets” of gel each with differing compositions and each acting like a “mini‐reactor”, producing phases based on that “mini‐reactor's” composition. The effect of agitation is not well understood and this is due to mixing having to perform the following tasks during crystallisation [52]:

• Reagent dissolution
• Initial gel formation
• Maintaining a homogenous gel
• Aiding in gel structure break‐up
• Maintaining a uniform temperature across the reactor
• Transferring “nutrients” to the growing crystals
• Keeping the zeolite crystals in complete suspension on completion of the reaction

The substantial change in viscosity during synthesis is also an important factor to consider. The solution can go from a viscous gel to something resembling water during the course of synthesis [52]. Marrot et al. [53] explained another important factor in agitation, that of shearing, of which can have harmful effects on zeolite synthesis. This can affect the stability of the zeolite and the purity of which can be synthesised. The highest crystallinity was produced by the Archimedes screw impeller due to the low rate of shearing, while higher shearing resulted in lower crystallinities [53]. Agitation during the ageing step of synthesis was investigated by Mainganye et al. [54]. It was shown that agitation during the hydrothermal step of synthesis can have a harmful effect on the stability and purity of the zeolite regarding the shearing produced [53, 54]. It was thought that, during the aging step, shearing may favour the zeolite formation due to it facilitating the dissolution of coal fly ash into the alkaline solution. The four‐blade impeller was found to optimum for zeolite synthesis, applied during the ageing step [54]. It is noted however that regarding the up scale of zeolite synthesis, the hydrothermal reactor used would also need to implement a form of agitation given the need to maintain a homogeneous mixture [55]. From the above analysis, Archimedes screw may be a favourable impeller during the hydrothermal synthesis due to its low rate of shearing; however, further research is required to ascertain this assumption. The synthesis gel that is used in the stage of hydrothermal synthesis varies considerably from the solution during aging. There is a need to understand the effect of agitation during hydrothermal synthesis. Homogenous mixing is critical if large‐scale production is envisioned, a large‐scale reactor would need to implement some form of agitation.

There have been significant studies on the use of class F fly ash for zeolite synthesis, which is pioneered Musyoka et al. [12, 56, 57]. The more recent studies have focused on using alternate techniques such as ultrasound to either used after fusing to decrease the hydrothermal synthesis time [12, 58], and the temperature needed in hydrothermal synthesis [58, 59] or complete replacement of the fusion process [60] A reduction in synthesis time was also noted by Musyoka et al. [61] when using ultrasonic synthesis using AMD and circumneutral mine waters in place of ultra‐pure water. In addition, studies have also shown that ultrasound techniques can be used to investigate the formation mechanism of zeolites from coal fly ash using in‐situ ultrasound techniques, namely of the zeolites A and X (zeolite X with novel hierarchal morphology) [57]. According to Murayama et al. [62], the three main steps involved in the mechanism of zeolite formation from coal fly ash are as follows:

• Dissolution of Si⁴⁺ and Al³⁺ in coal fly ash
• Condensation of silicate and aluminate ions in the alkali solution making the aluminosilicate gel
• Crystallisation of the aluminosilicate gel‐forming zeolites

The above process can be represented pictorially in the following Figure 7.

Another technique used is microwave‐assisted synthesis as reported by Querol et al. [64]. The activation time need during hydrothermal conversion was radically decreased from 24–48 hours to 30 minutes. It was further explained by Inada et al. [65] that continuous microwave irradiation retards the formation of zeolite in crystalline form, inhibiting the zeolite in the intermediate gel. It was found that early microwave irradiation enhances the zeolite formation. Heating during the middle stage, however, significantly inhibits zeolite formation. Therefore, it was found to be advisable to heat with microwave irradiation early on and then follow with conventional heating.

The feasibility though of scaling‐up these alternate processes, such as microwave and ultrasonic‐assisted synthesis, would prove challenging regarding the scale needed for large‐scale zeolite synthesis. Ultrasound seems promising given its use of cavitation to produce high temperatures and pressures or “hot spots” under nearly ambient conditions [66]. The main form of cavitation being studied is acoustic cavitation generated by sound waves in which ultrasound generates, grows and collapses the cavities in microseconds. These hot spots can produce temperatures greater than 5000 K and a cooling rate greater than 107 °C/s [60]. In a recent study by Ojumu et al. [60], the 90‐min high‐temperature fusion was completely replaced by a 10 minutes of high intensity ultrasound irradiation. It was shown that 24% of silicon was extracted from the fly ash, comparable to the 32% from fusion. It also reduced the crystallisation time of zeolite A and would be promising for up‐scaling. On industrial scale, there is hardly any process carried out, which allows for up scale of the process. This is due to the lack of expertise in differing fields such as material science, acoustics, chemical engineering, etc. [66]. Hydrodynamic cavitation offers a cheaper alternative. It concerns generating cavitation from the flow of a liquid under controlled conditions through obstructions such as venture tubes and orifice plates. It is generated when the pressure at the throat falls below the vapour pressure of the liquid, the liquid then flashes and generates a number of cavities. When the pressure is recovered downstream of the mechanical constriction, these cavities then collapse [66–68]. The hydrodynamic cavitation is applied on a pilot plant scale within a jet loop reactor. In a study involving the scaling‐up of a process to reduce sulphate concentrations within neutral mine drainage, a jet‐loop reactor was used [69]. The study showed that the impingement and cavitation mixing techniques applied within the jet‐loop reactor played an important role in enhancing the sulphate removal. This was due to the superior mixing in a jet‐loop reactor compared to the laboratory‐scale experiments that were also performed. The feasibility of scaling up a jet‐loop pilot plant to full‐scale seems more easily achieved than what would be needed to scale‐up the high‐intensity ultrasound process. Therefore, it may be necessary to investigate the application of a jet‐loop reacting system incorporating fly ash and sodium hydroxide to ascertain the dissolution rates of the silicon and aluminium that can be achieved. A recent study by Nyale et al. [27] showed that the intense mixing within the jet‐loop reactor increased the dissolution of the amorphous content of the fly ash used in the production of geopolymer from a slurry obtained from a jet‐loop system. This may prove favourable for zeolite synthesis; however, systematic studies that investigate the potential to use jet‐loop reactor as a process to completely replace the high‐temperature fusion need to be conducted. This would involve comparison of jet‐loop reacting system to the recent studies of involving ultrasound utilisation to determine which process might be more lucrative to up scale.

Figure 7.

Illustration of the zeolite process of a coal fly ash particle [63].

5. Up‐scaling zeolite synthesis from coal fly ash

Querol et al. [48] ran experiments at pilot scale using a 10 m3 R‐410‐A reactor vessel; the optimum conditions were generated from laboratory‐scale experiments using Parr digestion bombs: they were found to be 2 M NaOH solution, solution/fly ash ratio of 21 kg-1, T = 150°C, and time of 24 hours. It was concluded that the optimisation of synthesis yields would have to be specific for each type of fly ash from the differing power plants due to their difference in mineralogical and chemical compositions. There were reproducible results, however, in synthesising zeolite NaP1 from a specific power station fly ash. Also, high cation‐exchange capacities (CEC) were reported for ammonium and heavy metals for the varying zeolites achieved.

Moriyama et al. [70] compared the CEC values of a conventional method of synthesis to a test unit and a pilot plant. The conventional method conditions were a liquid–solid ratio of 8 dm3/kg, a temperature of 373 K and a holding time of 24 hours. The test unit consisted of a high‐pressure kneader with a volume of 5 dm3 fitted with a hot oil jacket around the pressure vessel. It was equipped with two sigma‐shaped blades for kneading and a steam exhaust line to remove the water. The pilot plant was also a high‐pressure kneader but at the volume of 0.6 m3. This process had benefits of water removal during operation eliminating wastewater treatment. It was concluded that the influence of the pressure on the CEC values obtained depends on the original coal fly ash characteristics and the type of zeolite formed was gismodine (GIS) type.

Mainganye et al. [54] investigated the effect of the impeller design concerning agitation during the ageing step of zeolite NaP1 synthesis. Three impellers were tested (four flat blade, anchor and Archimedes screw) at varying agitation speeds (150, 200 and 300 rpm). The conditions were 48 hours of aging at 47°C and static hydrothermal treatment of 48 hours at 140°C. It was shown that the phase purity of the zeolite was strongly affected by the agitation rate and type of impeller used during the ageing step of synthesis. The optimal impeller was the four‐blade impeller at an agitation rate of 200 rpm under the previous conditions mentioned, with a product yield of 0.98 ± 0.05 g zeolite/g fly ash and a space time yield of 15 ± 0.4 kg d⁻¹m⁻³.

Du Plessis et al. [71] developed waste minimisation protocols for zeolite synthesis from coal fly ash. The study effectively constructed two protocols for the minimisation of the waste generated in zeolite synthesis from South African coal fly ash. It was shown that to successfully synthesise zeolites with the waste solution, there needed to be a pH adjustment giving the opportunity to recycle 40% if the supernatant waste back in to the system. With this pH adjustment, zeolites NaP1 and analcime were successfully synthesised. Due to a high Si/Al ratio in the waste after reusing the supernatant, the more predominant phase was zeolite analcime. It was also shown that it is possible to recycle 100% of the supernatant waste without adjusting the alkalinity but by adjusting the basic synthesis process slightly.

Du Plessis et al. [72] determine the distributional fate of elements during two of the most prominent zeolite synthesis methods from South African coal fly ash, namely the fusion process and the two‐step process. An overall elemental material balance was performed around the two process routes. The results indicated that in the two‐step method, almost all the elements were concentrated in the solid zeolite product, while in the fusion‐assisted process, the elements mainly report to the solid waste. It was shown that toxic elements such as Pb, Hg, Al, As and Nb were found in both the supernatant waste and washing water within both processes.

Wdowin et al. [55] developed a technological line for converting coal fly ash to zeolites on a pilot plant scale. It could be divided into four stages: a reactor loading stage; a reaction stage; separation of the reaction products stage; and a final stage for processing the material obtained. The total volume of the reaction tank was 130 dm3 (with a working volume of 100 dm3). The tank was equipped with three 2‐kW heaters, a temperature and tank filling probe and a stirrer. The stirrer was responsible for the homogenisation of the material and prevented aggregation of the material during the reaction process. The zeolite NaP1 was synthesised with a zeolite content of 81% under the conditions of: 20 kg fly ash; 12 kg NaOH; 90 dm3 of H2O; temperature of 80°C; and duration of 36 hours. The line was fully automated and was able to synthesise zeolites Na‐X and Linde‐A after changing of the synthesis conditions.

6. Future outlook towards up‐scaling zeolite synthesis from coal fly ash

As can be seen from majority of the literature regarding zeolite up‐scale synthesis from coal fly ash, the main zeolite being investigated is Na‐P1. It is proposed that future investigation should focus on whether the faujasite zeolite (zeolite X) can be synthesised and targeted for large‐scale production. Zeolite X is important, being the most widely employed zeolites on industrial scale and used extensively as a fluid catalytic cracking catalyst for refining oil and as materials for adsorbing and removing gaseous emissions [73]. In view of the recent study by Musyoka [12] on the synthesis of zeolite X with novel hierarchical morphology from the clear extract of fused fly ash with NaOH, a significant breakthrough would be to target the zeolite with such novel morphology for up scale. The hierarchical pore structure enables the zeolites to have maximum structural functions in a limited space and volume, thus conferring a high degree of diffusion efficiency [73]. This synthesis route would be much more advantageous over the complicated procedures reported in literature to obtain the hierarchical structured zeolites.

The hydrodynamic cavitation provided in jet‐loop reacting system should be explored as suitable alternative to replace the high‐temperature fusion process for a large‐scale production process. Following from Nyale [27], who showed that the jet loop increased the amorphous content within the fly ash and reduced the amount of quartz and mullite, which would favour the synthesis of zeolites. It is reasoned that the same method could therefore be used for zeolite synthesis in the replacement of the fusion pre‐synthesis. Although in Nyale's study, the goal was to synthesise geopolymers. However, zeolites and geopolymers are very similar regarding their atomic structures of silicon and aluminium atoms. Also, the jet‐loop geopolymer produced leached significant amount of silicon which could be used for zeolite synthesis after a curing/leaching process of the slurry produced by the jet loop. This in turn could increase the yield of zeolite by creating a filtrate concentrated in the necessary silicon and aluminium.

Agitation is critical in the up scale of zeolite synthesis from coal fly ash; however, the challenge is to determine the effect of the impeller type and agitation during the hydrothermal treatment stage of the process. Although it has been shown that impeller type and agitation rate have a remarkable effect during the ageing step of zeolite synthesis [54], there is need to investigate these effects during the crystallisation process. It is postulated that impeller type that provide minimum shearing effect would be suitable during the hydrothermal process. At an industrial scale, stirring would need to be required not only to ensure homogeneity but also to ensure uniform heat distribution necessary for the crystallisation step. There have not been in depth studies to show the effect of agitation during the hydrothermal treatment process. Although the recent agitation used by Wdowin et al. [55] to synthesise zeolites from coal fly ash recorded 81% pure phase product, the purity could be due to the shear stress effect of the mechanical stirrer during the hydrothermal treatment process [54]. It is also not clear whether a pure zeolite would have been obtained if the clear filtrate was used instead of the total slurry. The aforementioned highlights the need for an in‐depth studies into agitation during the hydrothermal process. The results of such studies might hold the promise for a large‐scale synthesis that provides the highest and most pure yield of zeolite.

7. Conclusion

This review highlighted the present status and future outlook with respect to up‐scaling of zeolite synthesis from coal fly ash. Although current studies have demonstrated that the seemingly energy‐intensive fusion pre‐synthesis step can be avoided by using the acoustic cavitation technique provided by ultrasound devices, the scalability of this device for large‐scale production remains a subject of debate. The hydrodynamic cavitation provided by a jet‐loop reacting system may be a suitable alternative. However, there is a need for systematic studies that would investigate the potential of jet‐loop system, in comparison with the fusion and ultrasound pre‐synthesis steps, for the dissolution of the silicon and aluminium from the coal fly ash. The critical need for agitation and suitable impeller type, during the hydrothermal synthesis on a large scale, was outlined with an emphasis on the requirement for minimum shearing during crystallisation in large hydrothermal stirred reactors. It is expected that these recommendations would provide understanding towards the design of an appropriate scale up operation for zeolite synthesis from coal fly ash.

Source: Intech Open

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