casestudy on ricehusk firing in cfbc by r.nagarajaprasath

8/11/2019 Casestudy on Ricehusk Firing in CFBC by R.nagarajaprasath

1/8

By, R.NAGARAJAPRASATH, B.E (Mechanical Engineering),

CASESTUDY ON RICEHUSK FIRING IN CFBC

Email ID:[email protected]

Introduction

In the past few years CFBC boiler installation in India is getting more. Among this some plant

personal wants to fire both coal and rice husk because of coal availability and its price. This paper

brought out the problems faced while firing rice husk in CFBC boiler.

About the boiler

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 & feed water temperature is 190 Deg C.

High fuel consumption while rice husk firing

Since coal price is becoming high, boiler users were forced to use the alternate fuel like biomass and

municipal wastes. Rice husk availability will be high for 3-4 month in a year. During this period we

tried to use more husk but rate of rice husk consumption was high as compared to coal.

Reason for high rice husk consumption

To cross check the boiler efficiency losses were checked and all losses were under control only but

fuel consumption is high. After comparing the combustion, steam & water circuit parameters the

problem was found.

While firing more rice husk furnace inventory is getting low. In CFBC furnace, dust availability

will change the performance.

High flue gas temperature in cyclone outlet zone. It indicates that all rise husk is burning but the

heat values were not used for steam generation.

Action plan to increase furnace inventory while firing more husk

To increase the furnace inventory additional ash feed bunker was added in this sieved bed ash from

nearby power plants was stored. When furnace inventory is getting low ash from additional bunker

was feed to increase the same.

Husk feed chute arrangement by designer

CAUSES FOR HGH CYCLONE OUTLET TEMPERATURE

Boiler was designed for both husk & coal but coal feed point is very close to bed and husk feeding

point was given 2 meter above the coal feed opening. This caused for more husk burning in furnace

top and cyclone zone. Because of more husk firing in cyclone zone, cracks were observed in cyclone

outlet cone.
mailto:[email protected]:[email protected]:[email protected]:[email protected]


2/8

Reason for High Husk feeding point

elevation by designer

In CFBC, furnace bottom will be in positive

side so there may be chance for more back

firing if husk feed chute is very close tofurnace. To avoid the back firing author has

advised to give annular air jacket

arrangement in husk feeding pipe.

Less availability of Secondary air for

Husk burning

As per design, husk feed opening was

provided above the secondary air opening,

so availability of secondary air less for

husk. After reducing the husk feed opening,

SA air was available for better combustion.

Requirement of furnace volume for husk firing

Normally husk has more volatile matters as compared to coal, so it needs more volume to burn in

furnace itself. But designer has given husk feed chute above the coal feed chute from this volume

available for husk will automatically comes low.

Husk feed opening elevation was get down to the elevation of coal feed opening. By this I have

increased the furnace volume to burn the husk in combustor itself.

MODIFICATION TO BURN HUSK IN COMBUSTOR

By this modification, husk has more air to burn better.

After reducing the husk feed point the following observations were made.

OBSERVATIONS AFTER MODIFICATION

Parameters Before modification After modification

Bed temperature while firingmore husk

Lowbecause, more firing incombustor top and cyclone

Goodbecause, husk feedingclose to bed & its burning in

bed.

Cyclone outlet temperature It was in the range of 980-1050

Deg C.

Maintaining within the limit

(930 Deg C Max)

Fuel efficiency with husk firing Poor performance Considerably OK.

Conclusion

While burning the fuel part of heat is utilised to generate the steam and part is utilised to heat thesteam. In CFBC, water wall is the only evaporator, so designer has to select the furnace volume

Cracks developed in cyclone outlet cone


3/8

accordingly. By this modification i have increased the combustor volume for husk firing. After this

modification cyclone temperature is under control and considerable amount of fuel consumption

came down.


4/8

Photographs and Commends


5/8

Photo 01: Low furnace inventory while firing more husk. While firing more husk furnace inventory

is seen with the range of 13-25 mmWC against the minimum requirement of 70-80 mmWC.

Photo 02: High furnace inventory was seen while firing more coal because coal has more ash.


6/8

Photo 03: Husk feed opening elevation is 2.5 meter above from coal feed opening. Husk feedopening elevation was lowered as the elevation of coal feed opening.

Photo 04: Annular air jacket arrangement was given to avoid back firing.

Husk feeding opening

Coal feed opening


7/8

Photo 05: New husk feed opening made in the same elevation of coal feed opening.

Photo 06: Cyclone outlet temperature is reaching 1000 Deg C while firing two feeders with husk and

two feeders with coal. This screen shot is before modification.

Photo 07: Cyclone outlet temperature is reaching 925 Deg C while firing two feeders with husk and

two feeders with coal. This screen shot is after modification.


8/8

Photo 08: Super heater and economiser zone doors were seen with heavy leakages which leads for

wrong oxygen indication.

Photo 09: Super heater & economiser doors were pulled out and the same was welded with casingplate.

casestudy on ricehusk firing in cfbc by r.nagarajaprasath

Documents