Improving the Efficiency of Afbc Boiler by Recycling Carbon Rich Fly-Ash by Group-10

BY K.ARAVIND T.JALAL THAJ R.VEERARAJAN

INTRODUCTION  Energy which is the key in moving the world in faster pace.

And this energy is generated in many ways.  Among the various methods Thermal power generation is a important way of generating energy. Day by day , the fuel which runs these Thermal plants goes on decreasing.  At a stage , it will come to stand still, but this may come earlier if we go on running our boilers in the conventional ways.  There are lot of ways have been invented by many engineers and among them recycling the carbon rich fly ash back into the boiler is one way and we are going to do that now.

WHY WE ARE RECYCLING FLY-ASH?  TO IMPROVE THE EFFICIENCY OF THE BOILER  TO DECREASE THE LOSS ON IGNITION

 TO CONSERVE THE ENERGY  TO REDUCE THE COAL CONSUMPTION WITHOUT

COMPROMISING THE LOAD  TO REDUCE THE POLLUTION

BOILER – DEFINITION  According to ASME a steam generating unit or boiler is

defined as “A combination of apparatus for producing, furnishing or recovering heat together with the apparatus for transferring the heat so made available to the fluid being heated and vapourised.”

INTRODUCTION TO FBC BOILER  Fluidization –Definition

When an evenly distributed air or gas is passed upward through a finely divided bed of solid particles on a fine mesh, the particles are undisturbed at low velocity, as air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidised” and the process is called fluidization The Boiler using this mode of combustion principle is called as Fluidized Bed Combustion Boiler(FBC).

TYPES OF FBC BOILER FBC BOILER

BUBBLING BED

ATMOSPHERIC (AFBC)

PRESSURISED (PFBC)

CIRCULATING BED

ATMOSPHERIC (ACFBC)

PRESSURISED (PCFBC)

DIFFERENCE BETWEEN AFBC AND CFBC CHARACTERISTICS

AFBC

CFBC

FLUIDIZATION VELOCITY

1.2 - 3.7 m/s

3.7 – 10 m/s

COAL SIZE

1 – 10 mm

6 – 12 mm

UNBURNT FUEL RECYCLING

NO

YES

BOILER SIZE

LARGE

SMALL

LOSSES DUE TO UNBURNT

HIGH

LOW

ASH AND FLUE GAS PATH BEFORE RECYCLING FLY ASH

ASH AND FLUE GAS PATH AFTER RECYCLING FLY ASH

CALORIFIC VALUE OF UNBURNT CARBON  The fuel has been already burnt at a temperature of the

order of 850 to 900 º C, so the volatile matter and the moisture in the un-burnts are expelled out.  The un-burnt are only the fixed carbon.  The calorific value of the fixed carbon is 8080 kcal/kg.  By means of recycling this un-burnt carbon we can get reasonable amount of energy, by which the efficiency of the boiler can be improved.

EFFICIENCY OF BOILER BY INDIRECT METHOD ηboiler = 100 - ( L1+L2+L3+L4+L5+L6+L7) Losses (L) due to

AFBC

PF Boiler

L1-Dry flue gas

4.14 %

4–5%

L2-formation of water

6.62 %

3%

L3-Moisture in fuel

0.76 %

0.5 – 1 %

L4-Moisture in Air

0.1 %

≈0

L5-Radiation and Convection

0.4 %

1%

L6- Un-burnt in Fly Ash 2.045 % (LOI)

1- 2 %

L7- Sensible heat of fly Ash

0.1 – 0.5 %

0.08 %

The losses tabulated in the AFBC column are taken from Thermax Boiler of 152 tonnes/hr @ MCL Tamilnadu.

WEIGHT OF UN-BURNT CARBON PRESENT IN ECO & APH ASH Total coal consumed per day

300 tonnes

15% Ash produced per day

45 tonnes

15% Ash collected at ECO and APH per day

6.75 tonnes

20% Weight of un-burnt carbon present in ECO and APH ash per day.

1.35 tonnes

Recycled to furnace

LOSSES DUE TO UN-BURNTS IN FLY ASH OR LOSS ON IGNITION (LOI) BEFORE RECYCLING L61 =

=

GCV of un - burnts in fly ash 100 GCV of fuel GCV of un - burnts from APH & ECO  GCV of un - burnts from ESP 100 GCV of fuel

= ((8080×0.00351) + (8080×0.009945))/5300

= 2.045%

LOSSES DUE TO UN-BURNTS IN FLY ASH OR LOSS ON IGNITION (LOI) AFTER RECYCLING L62

=

GCV of un - burnts from ESP 100 GCV of fuel

= (8080×0.009945)/5300 = 1.5 % It is found that the LOI gets decraesed by recycling the unburnt carbon in to the boiler

PERCENTAGE OF EFFICIENCY IMPROVED The percentage of efficiency improved can be calculated by the difference between LOI before and after recycling un-burnt carbons Efficiency improved = L61 -

L62

= 2.045 - 1.5 = 0.54 %

EQUIPMENTS TO BE DESIGNED FOR RECYCLING UNBURNT CARBON

METHOD OF RECYCLING UNBURNT CARBONS IN TO BOILER There are two methods to recycle UNBURNT CARBON in to the boiler, DIRECT METHOD

INDIRECT METHOD

Injecting directly in to the boiler without mixing with coal.

Injecting in to the boiler by mixing with coal.

Out of the above two method indirect method is recommended for reliable operation of plant.

FUEL SAVED PER DAY AFTER RECYCLING UNBURNTS Fuel consumed per day Total ash produced per day

= 300tonnes = 15% of fuel consumed = 45tonnes Ash collected at ECO & APH = 15% of total ash produced = 6.75tonnes Amount of un-burnt carbon produced in ECO &APH = 20% ash collected = 0.20 × 6.75 = 1.35tonnes Gross calorific value of un-burnt carbon = 8080 kcal/kg Gross calorific value of coal used = 5166 kcal/kg Total amount of energy saved from un-burnt carbon = amount of UBC × GCV of UBC = 1.35 × 1000 × 8080 = 10908000kcal/day Total amount of coal (fuel) saved per day = Energy from UBC/GCV of coal = 10908000 / 5166 = 2111.49 kg/day

TOTAL COST SAVED PER YEAR AFTER RECYCLING UNBURNTS  Cost of coal (fuel) used per ton = Rs 4100/ Total cost saved per day

= 2.11149 × 4100 = Rs 8657.10(approx)  Total amount of coal saved per year = 2111.49 × 365 = 770693.85kg = 770.693tonnes  Total cost saved per year = 8657.109 × 365 = Rs 31, 59,844.785(approx)

ADVANTAGES Thermal efficiency of the boiler increased Bulk fly ash LOI decreased Coal use decreased with no load reduction Lower fuel cost Lower ash handling cost Carbon in the fly ash burned at good efficiency over 80% Lower land fill cost Reduced pollution

FUTURE……………

FACTORS TO BE MONITORED IN BOILER TO GET MAXIMUM EFFICIENCY  Temperature of gas leaving APH should be less than

150ºC.  Increase in the excess air causes decrease in the efficiency of boiler, O2 at ECO outlet should be 3.5-4%. Excess air should be within 20-25%  Un burnt carbon loss should be kept minimum by regulating excess air, distribution of primary and secondary air.  There should not be any air leakage inside the furnace, it is achieved by closing inspection doors, openings, instrument openings, etc.,

 Keeping good feed water quality is very much important

for reliable operation of boiler  Sampling of boiler water shall be carried out regularly to monitor its quality as recommended  Removal of dissolved oxygen by deaeration in boiler

feed water is important to avoid pressure part corrosion and pitting  feed water temperature at outlet of deaerator at the saturation temperature of the water at the operating pressure and the temperature of water entering economiser at 190 ºC.

RESULTS  Improving the efficiency of boiler upto 0.5% to 0.8%  Better utilisation of fuel by recycling the ash  Fuel saved  Cost saved per day  Fuel saved  Cost saved

= 2111.49 kg/day (approx) = Rs 8657.109 /day (approx) = 770.69 tonnes/yr (approx) = Rs 31,59,844.78 (approx)

QUESTIONS..?

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