CFBC boiler process and their advantage over AFBC boiler - ASKPOWERPLANT

CFBC boiler process and their advantage over AFBC boiler

CFBC boiler and its advantage over AFBC boiler
BOILER OPERATION
4

CFBC (CIRCULATING FLUIDASING BED COMBUSTION )boiler process and their advantage over AFBC ( ATMOSPHERIC FLUIDSAING BED COMBUSTION ) boiler

Fluidization bed combustion technology first introduce in 1970 in large scale in utilities boilers for the purpose of explore to new way to burning of  solids fuels as well as the high sulphure coal in an environmentally acceptable and efficient manner. In fluidization bed combustion fuel is burning the the suspension of bed particles .in fluidization boiler bed temperature is  Controlling by using reagents like Limestone as bed material, emissions of nitrogen oxides (Nox) and sulphur dioxide (SO2) . Additional benefits of fluidized bed is combustion include wide  verity of fuel & flexibility and the ability to combust Fuels such as bio-mass or waste fuels, which are difficult to burn in conventional systems because of their low heating value, low volatile matter, high moisture content or other challenging characteristics. In coal- fired systems, the

fuel is burned in an air-suspended bed of limestone and inert ash particles where SO2 is absorbed by the limestone, and Nox formation is limited by lower operating temperatures and staged combustion, when used. This technology is now used in a ‘variety of industrial and utility boiler applications now these days.

 

Fluidization process:-

The fluidizing process induces an upward flow of a gas through a stacked height of solid particles the air flow which is entering from the bottom of the furnace through a sir distributing nozzles. At high enough gas velocities (4.5 m/s ), the gas/solids mass exhibits liquid-like properties, thus the term fluidized bed. Hot primary air from PA fan is used for this .

 

Process of CFBC BOILER :-

In above figure shows that a chamber with an air supply system through the wind box chamber at the bottom of furnace , an air distributor which  promotes even distribute of air flow through the bed, and a chamber filled with sand which is called bed of the boiler which contain alumina 45-35% and silica 55-65 % or other granular material. If a small quantity of air flows through the air distributor into the sand, it will pass through the voids of an stationary mass of sand. For low velocities, the air does not apply much force on the sand particles and they remain in same  place,So the no movement take place in the boiler bed .This condition is called a Static bed and is shown in above figure (A).

CFBC boiler bed during fluidisation
CFBC boiler bed during fluidisation

 

When we increasing the air flow rate/velocity, the air exerts greater forces on the sand and reduces the contact forces between the sand particles caused by gravity. By increasing the airflow further, the drag forces on the particles will eventually counter balance the gravitational forces, and the sand particles become suspended in the upward stream. The point where the bed starts to behave as a fluid is called the minimum fluidization condition. The increase in bed volume is insignificant when compared with the non-fluidized case Fig (B).

As the airflow increases further, the bed becomes less uniform, bubbles of air start to form, and the bed becomes violent. This is called a bubbling fluidized bed (BFB), shown in Fig. (C). in this condition boiler bed behave like fluid

 

By increasing the airflow further, the bubbles become larger and begin to coalesce, forming large voids in the bed. This condition is called a turbulent Fluidized bed .

As the velocity of air is further increased (around 6 m/s), a condition is reached where in the whole of bed material is in circulation from furnace to a collection chamber (U beam hopper) and back to furnace. In u beam arrangement particles separation is done. U –beam arrangement at the exit of the furnace on the flue gas path. U-beam are a staggered steel array channels . solids particles leaving by the  furnace along with the flue gases is captured , when it pass through  the u-beam arrangement and internally recirculation  the materials to the furnace again .Thus resulting in a uniform temperature  profile throughout the furnace. Because the whole bed is in continuous circulation internally, this type of boiler is called Internal Recirculation Circulating Fluidized Bed Combustion (IR-CFBC) boiler as described in Fig. 2. In circulating beds, more particles are blown from the bed than for a bubbling bed. The particles are then collected back by a particle separate and recycled to the furnace through the loop seal cyclone . Loop seal is the heart of the CFBC boiler. this is an arrangement  which feed solids capture by cyclone to the furnace but prevents direct flow of gas from high furnace to the low- pressure cyclone . The residence time of the particle is determined by the collection efficiency of the particle separator and the solids circulation rate. As the result of the recirculation process, the effective particle residence time greatly exceeds the gas residence time

 

In CFBC boiler bed temperature is controlled by rate of recycling of fine materials. CFBC required large amount of mechanical cyclones to capture and recycle the solids particles, so normally CFBC boiler is taller than the AFBC boiler. CFBC boiler is generally more economical and its efficiency is more than 85%.

 

ADVANTAGE OF CFBC BOILER OVER AFBC BOILER:-

 

1.We get the higher temperature from CFBC boiler because of high gas velocity through the system.

2.Lower combustion temperature can be achieving constantly, which result in minimum NOx formation.

3.In CFBC boiler the combustion air is supplied at lower pressure as compared

 to the AFBC boiler .

4.CFBC boilers have more combustion efficacy as compared to the AFBC boiler.

5.Higher turndown ration ratio up to 30%.

6.Loss of ignition (LoI) in the fly ash is very less.

7.Erosion of the heat transfer surface in the combustion chamber is reduced, since the surface the surface is parallel to the flow.

 

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