Question

What is the difference between bottom ash and fly ash in an incinerator? Describe some of the air pollution control devices found on incinerators.

Short answer

Bottom ash is heavier and remains at the bottom, while fly ash is finer and airborne. Air pollution control devices include electrostatic precipitators, baghouse filters, and scrubbers.

Step-by-step solution

Define Bottom Ash

Bottom ash is a type of ash that remains at the bottom of the incinerator after combustion. It is the non-combustible residue that does not get carried away with the gases and falls to the bottom of the chamber. It is usually heavier and contains larger particles than fly ash. In disposal or further processing, it's handled as solid waste.

Define Fly Ash

Fly ash is a fine particulate matter that gets lifted off with the flue gases during combustion. Unlike bottom ash, fly ash consists of very fine particles that escape from the bottom of the incinerator and are carried up through the stack. Fly ash often contains contaminants and can contribute to air pollution if not controlled properly.

Describe Air Pollution Control Devices - Electrostatic Precipitators

Electrostatic precipitators are used to capture fly ash and other fine particles from the exhaust gases. They work by imparting an electrical charge to the dust particles, which are then attracted to and adhere to plates or other collection devices that carry the opposite charge. This method efficiently removes particulates from the flue gas.

Describe Air Pollution Control Devices - Baghouse Filters

Baghouse filters are another method used to trap fly ash and particulates. They consist of a series of fabric filter bags through which flue gases pass. The bags capture the particulates, allowing cleaned air to pass through. The collected ash is periodically removed from the bags.

Describe Air Pollution Control Devices - Scrubbers

Scrubbers are systems that use liquid, often water, to remove pollutants from the exhaust stream. They can efficiently reduce acidic gases, such as sulfur dioxide, as well as particulates. The scrubbing liquid captures the pollutants which are then either neutralized or collected for disposal.

Key concepts

Bottom Ash

Bottom ash is a byproduct of the incinerator process, where waste materials are burned to reduce volume and weight. Upon incineration, a portion of the waste that does not combust fully settles at the bottom of the combustion chamber. Here lies the bottom ash, a solid residue consisting mainly of non-combustible materials like glass, metals, and minerals.
This ash is typically heavier with larger particle sizes compared to other ash types. Because of its weight and density, bottom ash remains confined to the lower part of the incineration system.

• Unlike other waste byproducts, it doesn't pose significant air pollution threats.
• However, it does require proper disposal or processing.
• Methods to manage bottom ash include landfilling or recycling for use in construction aggregates.

Handling this waste responsibly is essential for effective waste management and environmental protection.

Fly Ash

Fly ash is another type of residue from incineration but differs significantly from bottom ash. As the waste combusts, fine particles and gases rise through the flue, and these tiny, lighter particles are known as fly ash. Because they're carried away with the gases, they require advanced control measures.
Fly ash can be problematic due to its potential to cause air pollution. It consists of fine, powdery particles that are often rich in toxic elements like mercury and lead. If not effectively captured and treated, these particulates can enter the atmosphere, contributing to air quality degradation.

• Collection and treatment at the incinerator's emission points are critical.
• Fly ash is often captured using air pollution control devices.
• This process not only helps reduce pollution but also minimizes environmental impacts.

Efficiency in fly ash capture is crucial for both health and regulatory compliance.

Air Pollution Control Devices

Air pollution control devices are essential in managing emissions from various industrial processes, including incineration. These devices are designed to remove or neutralize pollutants from combustion gases before they are released into the atmosphere. Effective usage of these systems can significantly minimize the environmental impact of incinerator facilities.
They target several types of pollutants: particulates, acidic gases, and more. Sequence and maintenance are key factors that define their effectiveness. Some of the main control devices include:

• Electrostatic precipitators
• Baghouse filters
• Scrubbers

Each has its own mechanism of working and target pollutant types, requiring thorough understanding for effective operation.

Electrostatic Precipitators

An electrostatic precipitator is a sophisticated device used to remove particulates, such as fly ash, from exhaust gases. It functions by imparting an electric charge to dust particles suspended in the flue gas.
Once charged, these particles pass through a field created by grounded collecting plates with an opposite charge. The dust particles are drawn to, and held by, these plates.

• The process is extremely efficient in removing fine particulates.
• Collected particulates are then periodically removed through mechanical rapping or washing.
• Precipitators are highly effective in reducing particulate emissions.

Regular maintenance is essential for maintaining efficiency and preventing breakdowns, ensuring a cleaner emission process.

Baghouse Filters

Baghouse filters are another vital system for trapping particulate matter from emission gases. These devices consist of a series of fabric bags that act like a sieve for dirty flue gas.
As gases pass through the fabric, particulates are stopped and adhere to the bags, while clean gas exits on the other side. Over time, a layer of dust accumulates on these bags, which enhances its filtering efficiency.

• Periodic cleaning of the bags is essential to maintain efficacy.
• Depending on the requirement, cleaning is achieved by shaking, reverse air, or pulse-jet techniques.
• Baghouse filters are versatile and effective for different particulate sizes.

This method is straightforward and cost-effective, making it a popular choice for many industrial processes.

Scrubbers

Scrubbers are unique pollution control devices that utilize a liquid to "wash" unwanted pollutants from exhaust gases. They're particularly effective for removing acidic gases and particulates.
Commonly filled with water or other specific chemical liquids, they capture pollutants by absorbing or chemically reacting with them. The scrubbing liquid then goes through a treatment process to remove harmful elements before disposal or reuse.

• Wet scrubbers are excellent for sulfur dioxide removal.
• Scrubbers can be used in conjunction with other devices for comprehensive control.
• The efficiency of scrubbers depends on the type of liquid and pollutants involved.

Scrubbers play a crucial role in reducing harmful emissions, making them indispensable in modern incineration plants.