Fly Ash Processing: Tailored solutions for the reclamation of harvested fly ash

What is Fly Ash, and Where Does It Come From?

Fly ash is a fine powder that is a byproduct of burning pulverised coal in electric generation power plants. It’s one of the most common kinds of waste produced by fossil fuel combustion. Here’s how it’s formed and what it’s made of:

Formation

When coal is combusted in a power plant, it produces gases and particles. The solid particles, including fly ash, are left behind. These particles are driven out of the boiler with the flue gases and are captured by pollution control devices like electrostatic precipitators or baghouses.

Composition

Fly ash is composed mainly of silica (silicon dioxide, SiO₂), alumina (aluminium oxide, Al₂O₃), and iron oxide (Fe₂O₃). It can also contain calcium oxide and trace amounts of other elements depending on the coal type and source. Its exact composition varies, but it typically includes a significant amount of silicon dioxide and calcium oxide, making it suitable for cement products.

Types of fly ash

There are two primary types of fly ash: 

• Class C, derived from burning lignite or sub-bituminous coal, contains higher amounts of calcium and is often used in cement and concrete. 
• Class F, produced from burning harder, older anthracite and bituminous coal, has lower calcium content and is prized for its pozzolanic properties.

Fly Ash Uses 

Fly ash is often used as a pozzolan in concrete production, replacing a portion of Portland cement. It can improve the strength and durability of concrete and can reduce the amount of cement needed in concrete mixes. Additionally, it is used in road construction, as a soil stabiliser, and in specific geotechnical applications.

Beyond its prominent role in concrete and cement production, fly ash finds versatile applications in various sectors. It is extensively used in soil stabilisation, enhancing the soil’s strength and stability, making it suitable for construction. Additionally, fly ash is a critical component in road base construction, improving the base’s durability and resilience. In asphalt production, it is an effective filler material, contributing to the overall quality of the asphalt mix. Moreover, fly ash is increasingly utilised in formulating geopolymers, offering an eco-friendly alternative to traditional construction materials. Its diverse applications demonstrate its versatility and highlight its significance in promoting sustainable practices across industries.

Fly Ash for Concrete

Utilising fly ash in construction materials is considered environmentally beneficial as it repurposes an industrial byproduct. This practice diminishes the need for landfill space and lessens the consumption of raw materials. However, it’s important to note that fly ash production directly results from burning coal, a process that significantly contributes to air pollution and greenhouse gas emissions. Therefore, utilising harvested fly ash from stored sources is an increasingly important opportunity.

In concrete production, fly ash is highly regarded as a supplementary cementitious material. It dramatically improves the mechanical properties of concrete, notably its workability, strength and longevity, which are crucial in structures requiring resilience. Moreover, incorporating fly ash into cement mixtures reduces water requirements, reducing cement production’s environmental impact. This approach aligns with global efforts towards sustainable construction, conserving natural resources while adhering to eco-friendly practices. Fly ash concrete shows increased resistance to damage from freeze-thaw cycles and chemical attacks, making it suitable for structures that withstand harsh environmental conditions or corrosive substances.

Reduction in CO2 emissions is another benefit of fly ash in concrete. The production of Portland cement contributes significantly to the CO2 in the atmosphere. By substituting a portion of cement with fly ash, we can reduce the carbon footprint of concrete production. Fly ash is often less expensive than Portland cement, so its use can result in cost savings for concrete manufacturers and construction companies.

The use of harvested fly ash in this manner contributes to efficient industrial waste management, curtails landfill usage, and fosters sustainable construction methodologies. Despite these advantages, handling and treating fly ash with care is crucial, given the potential presence of heavy metals and other hazardous elements. Proper management ensures safety and environmental compliance while maximising the material’s beneficial properties. Read more about the use of fly ash in concrete.

Harvested Fly Ash

As we grapple with the dual challenges of industrial waste and sustainable development, harvested fly ash emerges as a crucial component in addressing these issues. 

Fly Ash Shortage

The phase-out of coal-fired power plants is leading to a growing shortage of fly ash in the UK, and any coal-derived fly ash (CDFA) required for cementitious applications needs to be imported. This scarcity prompts the industry to explore alternative sources and methods, such as reclaiming and recycling fly ash that has been stored. The UK alone has over 100 million tonnes of fly ash available in single-use deposits such as lagoons and stockpiles. The UK Government has recognised the strategic importance of CDFA and the role it can play in a low-carbon economy. 

Reclaiming and recycling coal fly ash

There are several benefits to reclaiming and recycling fly ash and processing it as a by-product, including both environmental and economic advantages: 

• Securing Availability: The looming uncertainty surrounding the long-term availability of fly ash from conventional sources underlines the need for alternatives. Harvested fly ash offers a sustainable solution.

• Minimising Landfill Dependency: Recycling and processing harvested fly ash reduces reliance on landfills, preserving crucial space.

• Economic Advantages: Leveraging harvested fly ash as an alternative introduces cost efficiencies, especially in construction and concrete fabrication.

• Enhancing Concrete Quality: As with fresh fly ash, harvested fly ash improves the overall properties of concrete, boosting its resilience and durability.

• Carbon Emission Cutbacks: Integrating fly ash into concrete reduces CO2 emissions, especially when considering the carbon-intensive production of Portland cement.

• Guarding the Environment: Properly processed fly ash ensures that contaminants like mercury or arsenic don’t compromise our environment.

• Energy Retrieval: Some variants of fly ash, especially those from biomass plants, contain unburned carbon, which could be tapped for energy.

• Fostering Employment: The effort to process harvested fly ash can stimulate job creation and economic growth.
• Spearheading Sustainable Innovations: Embracing harvested fly ash in construction practices promotes a circular economy, championing industrial sustainability.

Read more about the vast harvested coal-derived fly ash (CDFA) opportunity.

Fly Ash Recovery

The process of fly ash recovery begins with meticulous extraction, followed by a series of refurbishment steps. Key among these is drying, precise classification, and thorough purification. Each step is crucial to ensure that the recovered fly ash aligns with stringent industry standards, thus guaranteeing its suitability for various applications.

Conventional Multiple-Stage Processing of Harvested CDFA

Fly ash processing encompasses intricate stages. It initiates with drying, which prepares the ash for subsequent processing. This is followed by deagglomeration to separate particle clusters, enhancing their reactivity. Classification then ensures the correct particle size distribution, with some processes also involving specific additional treatments to refine the properties of the fly ash.

• Step 1: Extraction and pre-screening
  Preparing wet fly ash for usage begins with extraction and pre-screening, both crucial steps for ensuring the quality of the resulting product. During extraction, the wet fly ash, a byproduct of the combustion process in coal-fired power plants, is carefully collected for further treatment. Pre-screening then follows, removing coarse or unwanted particles and ensuring a consistent starting material for further processing.
• Step 2: Drying
  After extraction and pre-screening, the fly ash undergoes drying, an essential step in transforming the wet byproduct into a form that for use in construction applications. The drying process reduces the moisture content of the ash, making it easier to handle and prepare for subsequent processes.
• Step 3: Deagglomeration
  The dried fly ash is deagglomerated, breaking down larger clusters of particles into individual ones. Deagglomeration is particularly important in enhancing the performance of the fly ash when mixed with cement, as deagglomeration increases the surface area of the particles, thereby improving their reactivity.
• Step 4: Classification
  The deagglomerated fly ash is then classified. Classification ensures that the fly ash particles have the required size distribution for specific uses. 

Fly ash regulations

It’s important to note that the processed fly ash must meet specific regulatory standards for construction applications. In Europe, for instance, the standard for fly ash in concrete is EN450 Class N or S. This standard outlines the properties that fly ash must have, including its chemical composition, loss of ignition, and fineness, among others.

In the United States, the regulations are outlined in ASTM C-618 and the newer ASTM E3183. These American standards similarly define the requirements for using fly ash for concrete, ensuring the resulting concrete products’ performance, durability, and safety.

Our Role in Fly Ash Processing

In June 2023, Atritor and ST Equipment and Technology showcased their potential in Coventry. Using the Atritor Dryer Pulveriser and the STET separator, we demonstrated the treatment of fly ash to meet the European EN450 standards, emphasising its role in enhancing the properties of concrete.

Fly Ash Processing Equipment and Solutions

The Atritor Dryer Pulveriser offers simultaneous drying, deagglomeration, and classification in a single stage. It can produce a product feed rate of up to 20 tonnes per hour and evaporation rates of up to 4,000 kilograms per hour, utilising inlet gas temperature as high as 550 degrees centigrade.

We collaborate with our customers in our Pilot Plant proving facility, where we test the wet fly ash using different process setups until we achieve the right end fraction for your applications. The Pilot Plant testing and our considerable knowledge of fly ash give us the knowledge to advise you on the financial viability of proceeding with a project. When specifying a project, we consider the characteristics of the fly ash as it varies depending on the source and time of year extracted. The wetter the ash, the more energy it will require to dry. We also consider capital expenditure, such as civil works and equipment. 

Our in-house design and manufacturing facilities provide the resources for a bespoke approach. Then, we install, commission and train plant users to ensure our customers achieve a profitable end product.

At Atritor, we have over 90 years of experience delivering bespoke drying and grinding solutions. We specialise in taking waste products and turning them into something valuable. From testing, design, manufacturing, installation, commissioning and training – we take care of the whole solution for fly ash processing. We have the previous experience and comprehensive testing data to apply to your feasibility study, saving you time and cost in your decision-making. Contact our team and see how we can help you with fly ash processing.