What is the Difference Between Bubbling and Circulating Fluidized Bed

Bubbling and circulating fluidized beds are two types of fluidized gasifiers. Gasification is a process that converts biomass or fossil fuels into gases. The resultant gases can be used as renewable energy. This process is carried through gasifiers. A fluidized bed gasifier is a type of gasifier that has a special advantage over other gasifiers. Fluidized bed gasifiers are categorized into two types as bubbling fluidized bed and circulating fluidized bed.

The key difference between bubbling and circulating fluidized bed is the type of fluidization they use. Bubbling fluidized bed utilizes bubbling fluidization while circulating fluidized bed utilizes fast, circulatory flows for fluidization.

CONTENTS

1. Overview and Key Difference
2. What is a Bubbling Fluidized Bed
3. What is a Circulating Fluidized Bed
4. Similarities – Bubbling and Circulating Fluidized Bed
5. Bubbling vs  Circulating Fluidized Bed in Tabular Form
6. Summary – Bubbling vs Circulating Fluidized Bed
7. FAQ – Bubbling and Circulating Fluidized Bed

What is a Bubbling Fluidized Bed?

Bubbling fluidized bed (BFB) gasifiers provide clean energy from a wide variety of fuels, such as biomass fuel. In a BFB gasifier, the feedstock and bed material are maintained in a fluid-like state by passing air or steam through the bed material. At a certain gas velocity, the bed starts to behave like a bubbling fluid in this gasifier.

BFB gasifier offers good temperature control and low tar content of produced gas. Furthermore, BFB is generally used for smaller-scale applications, with fuels having low heat value and high moisture content. Foster Wheeler (FW) is a company that has supplied more than 130 BFB boilers for industrial uses.

What is a Circulating Fluidized Bed?

Circulating fluidized bed (CFB) gasifier uses fast, circulatory flows. The circulating fluidized bed gasifier use higher gas velocities to fluidize the solids. In CFB, the particles are entrained with the gas and carried out of the bed. This creates a fast, upwards circulatory flow. Later, solids are separated from the gas in a cyclone and returned back to the bed, which forms a continuous circulation.

The advantages of CFB include higher capacity, scalability, and flexibility in feedstock. Furthermore, CFB is preferred for larger-scale and diverse industrial applications. Doosan Lentjes is one of the leaders in the word for circulating fluidized bed (CFB) gasifier technologies.

Similarities Between Bubbling and Circulating Fluidized Bed

1. Bubbling and circulating fluidized beds are two types of fluidized gasifiers.
2. Both use biomass fuel for the generation of renewable energy that contains gasses like CO, H2 (Syn gas), etc.
3. Both gasifiers are useful for feed materials that produce corrosive ashes.
4. These technologies have many applications.

Difference Between Bubbling and Circulating Fluidized Bed

Definition

1. Bubbling fluidized bed is a gasifier that utilizes bubbling fluidization.
2. Circulating fluidized bed is a gasifier that utilizes fast, circulatory flows for fluidization.

Suitable for

1. Bubbling fluidized bed is suitable for fuel with low fuel ratio or low heating value such as biomass fuel or waste fuel.
2. Circulating fluidized bed is suitable for fuel with high fuel ratio such as anthracite or for waste fuel firing under the high steam condition.

Ga Velocity

1. Bubbling fluidized bed uses low gas velocity.
2. Circulating fluidized bed uses high gas velocity.

Applications

1. BFB is used for smaller-scale applications, with fuels having low heat value and high moisture content.
2. CFB is preferred for larger-scale and diverse industrial applications.

Manufactured by

1. Foster Wheeler (FW) is one of the world’s leaders in manufacturing bubbling fluidized bed gasifiers.
2. Doosan Lentjes is one of the world’s leaders in manufacturing circulating fluidized bed gasifiers.

The following table summarizes the difference between bubbling and circulating fluidized bed.

Summary – Bubbling vs Circulating Fluidized Bed

In fluidized bed gasifiers, the biomass is fluidized in steam, hot gases, hot air, or oxygen. They are most suitable for biomass material gasification. Bubbling and circulating fluidized bed are two types of fluidized gasifiers. In a bubbling fluidized bed gasifier, the gas is passed through a bed of solid particles at certain velocities to suspend them but not enough to carry them away, which creates a bubbling action. However, a circulating fluidized bed gasifier utilizes higher gas velocities to carry solids out of the reactor, creating a fast, continuous circulation loop. Furthermore, BFB gasifier is particularly beneficial in smaller-scale applications, whereas CFB gasifier is particularly beneficial in large-scale and diverse industrial applications. This is the summary of the difference between bubbling and circulating fluidized bed.

FAQ: Bubbling and Circulating Fluidized Bed

1. What is the working principle of a gasifier?

• A gasifier is used to produce synthesis gas from biomass or carbonaceous material. The solid is packed in a column, and gas is passed up through the bed, producing a mixture of combustible products, which consists of methane, hydrogen, and carbon monoxide.

2. What is the mechanism of a fluidized bed?

• The fluidized bed uses gas or liquid to pass through a granular solid bed to make the solid particles rise in suspension. This enables gas-solid reactions or liquid-solid reactions to take place, ultimately leading to the production of renewable energy.

3. What is the function of a bubbling fluidized bed boiler?

• Bubbling fluidized bed (BFB) is a reliable gasifier for recovering energy from difficult fuels, such as residual biomass and industrial wastes. They can handle many high-moisture and high-ash fuels with low emissions and high operational flexibility.

4. How does a circulating fluidized bed work?

• A circulating fluidized bed (CFB) involves forced air injected beneath a bed of fuel. During the combustion process, strong drafts carry combustion products and unburned fuel into the unit’s ductwork. A cyclone is then used to separate the combustion products from the heavier, unburned fuel. The unburned fuel is returned to the bed, which creates a continuous circulation.

5. What is the application of a circulating fluidized bed?

• Circulating fluidized beds are popular for large-scale combustion operations, and diverse industrial applications. They are also becoming popular for gasification of carbonaceous feedstocks.

Reference:

1.  “Bubbling Fluidized Beds: When to Use This Technology.” SAIMM.
2.  “Circulating Fluidized Bed (CFB) Boilers.” Sumitomo Shi FW.

Image Courtesy:

1. “Packed bed and fluidized bed membrane reactors” By Michele Ongis – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Brief description of post-combustion capture technologies and the materials utilized” By Hope McLaughlin, Anna A. Littlefield, Maia Menefee, Austin Kinzer, Tobias Hull, Benjamin K. Sovacool, Morgan D. Bazilian, Jinsoo Kim, Steven Griffiths – https://www.sciencedirect.com/science/article/pii/S1364032123000710 (CC BY-SA 4.0) via Commons Wikimedia

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