Case Studies on CFBC Boilers

By

R.NAGARAJAPRASATH, B.E(Mech), BOE

SHANTI GD ISPAT & POWER PVT LTD, CHATTISGARH

CFBC Vs AFBC

HEAT TRANSFER IN CFBC WATER

WALL

o Case Study On Husk Firing In CFBC

o Importance of loop seal air nozzle arrangement.

o Panel super heater failure in CFBC.

CASE STUDIES

The CFBC boiler was supplied by a Chinese company has been

operated for a year. Boiler was designed for 80% Rice husk + 20

% Indian coal / 100% Indian coal. Boiler parameters are 75 TPH,

90 Kg/cm2, 515 Deg C with the feed water temperature of 190

Deg C.

CASE STUDY ON HUSK FIRING IN CFBC

Cracks developed in cyclone outlet cone




IMPORTANCE OF LOOP SEAL AIR NOZZLE

ARRANGEMENT

Loop is mechanical device which transfers the un-combusted

(coarser) particles to the furnace, which was collected by cyclone.

It transfers the ash particles from low pressure cyclone to the high

pressure combustor. It protects the direct flow of flue gas from

furnace to cyclone.

There are two types of air nozzles were provided in loop seal. The

purpose of nozzle types is to shift the collected ash back to

furnace and also to control the gas bye-passing from furnace to

cyclone. Down leg side nozzles were provided for aeration

purpose only and in the riser leg side nozzles were provided for

fluidization purpose.

If loopseal air nozzles were erected in wrong manner then this

cause in boiler loading because ash will not transferred from

cyclone to furnace.


ARRANGEMENT


ARRANGEMENT

The is the philosophy of loop seal design by Foster wheeler.


ARRANGEMENT

The CFBC boiler was supplied by a Chinese company has been

operated for 3 years. Boiler was designed for 100%

Imported/Indian coal. Boiler parameters are 130 TPH, 100

Kg/cm2, 540 Deg C & feed water temperature is 180 Deg C. The

arrangement is shown in below

One and half year after commissioning they were faced frequent

failure in panel super heater tubes. Due to the frequent failure in

panel super heater tubes they can’t able to run the boiler more

than a month continuously. Especially failure was occurred in

bottom of super heater. Arrangement of panel super heater was

shown in photo-02. At the time of visit boiler was in operating

condition. However failed tubes were available for inspection. In

my presence boiler was stopped due to panel super heater failure,

boiler was inspected from inside.

PANEL SUPERHEATER FAILURE IN CFBC


Reason for Failure,

1. From the available failed tube, it

was understood that the anchoring

arrangement was not proper to hold

the refractory. They do not have the

anchors at bottom and at front.

2. Tubes are provided with 6mm dia x

30mm long studs.

3. Due to that insufficient anchoring,

refractory may fail from the first

firing itself. After refractory

falling, tubes were start eroding

from bottom.


Failed tubes, shows no anchors in

bend

New studs added with modified

arrangement.


Thickness measured in the outer coil during the shutdown time. Tube

thickness was observed 2.83mm against the original thickness of 5mm.

Recommendation,

1. For the proper holding of refractory, more studs were added

additionally with the existing stud arrangement.

2. New studs also with the same dimension.

3. Phosphate bonded high alumina castable was applied over a

coil up to the tip only. Phoscast will give better life in the high

velocity & high erosive zone also. This was practically

experienced in AFBC boilers.


SEE YOU WITH NEW

CASE STUDIES

THANK YOU !!!

Engineering

Case Studies on CFBC Boilers