Combustion in Boilers & Energy Saving Opportunities

without investment

Presented ByP.Dharmalingam B.Tech,PG.Energy Mgt,M.S

Sr.Dy.DirectorNational Productivity Council

AIP,Ambattur,Chennai-98aipnpc@vsnl.com

Combustion in Boiler

Fuel

Air

Heat

ExhaustCombustion

Types of combustion systems• Natural gas burners are the easiest gas to burn, since one needs

only to proportion the gas-air mixture, mix with air, and ignite the mixture.

• Oil burners prepare the oil by vaporization or gasification by heating it in the burner, or by atomization in the combustion-air stream. Atomization can be accomplished with high-pressure air or steam, and is best suited for variable loads. Atomization can also be done by mechanical means (centrifugal force) which is better suited for steady loads and high capacities.

• There are several types of coal furnaces including stoker coal furnace cyclone coal furnace pulverized coal furnace fluidized-bed furnace

Coal furnaces• The stoker furnace is of limited capacity and does not lend itself to

power plants but rather it is used in industrial processes. Coal is introduced on a grate, and it is finally burned on a stationary bed. The primary air enters below the burning bed and initiates the combustion process, and also cools the grate. Secondary air is introduced over the burning bed to complete the combustion process.

• The cyclone furnace employs several independent combustion chambers. The main combustion chamber operates at a temperature of 3200°F. These were popular in the 50s and 60s but are no longer being built since they have difficulty burning low-sulfur coals and the high temperature results in significant NOx formation.

Pulverized coal furnace

• The pulverized coal furnace attempts to burn finely powdered coal and air in a gaseous torch. This is accomplished through pulverizing the coal by crushing, impact and attrition (rubbing) of the coal to a size finer than face powder (diameter<0.3 mm). The primary air dries and transports the coal.

• The advantages of a pulverized coal furnace include its ability to burn all ranks of coal from anthracitic to lignitic, and it permits combination firing (i.e., can use coal, oil and gas in same burner). Because of these advantages, there is widespread use of pulverized coal furnaces.

• The disadvantages of the pulverized coal furnace are that the coal pulverizer has a significant power demand of its own and requires more maintenance, flyash erosion and pollution complicate unit operation and increase exhaust system maintenance requirements, and pulverized systems have higher initial cost and require larger furnace volumes for the combustion process

Fluidized-bed boiler• For a fluidized-bed boiler, the velocity of combustion gas (air) entering the

bottom of the furnace is maintained such that the coal and limestone or dolomite particles are suspended (resembling a boiling liquid). The boiler tubes are immersed in the fluidized bed.

• Fluidized-bed combustion systems are categorized as pressurized vs. atmospheric bed systems, and circulating vs. stationary bed systems.

• Advantages :Higher rates of heat transfer between combustion process and boiler tubes (thus reduced furnace area and size required), and combustion temperature (1500-1600°F) is lower than in a conventional furnace. The lower furnace temperatures means reduced NOx production. In addition, the limestone (CaCO3) and dolomite (MgCO3) react with SO2 to form calcium and magnesium sulfides, respectively, solids which do not escape up the stack; however, it does require about 50% more limestone/dolomite as compared to a wet-scrubber system. This means the plant can easily use high sulfur coal. Disadvantages: 1) erosion of tubes by the particles rubbing the tubes, 2) requires more fan power to suspend the particles, and 3) system appears better suited for low-power applications.

Example of System LossThe typical steam system overall efficiency is about 35% as

follows:

Generationefficiency80%

Distribution efficiency= 83%(including conden-sate return)

Utilisationefficiency47%

Possible Boiler Losses

FLUE GAS LOSS eg,- excess air / temperature- soot / slag deposits- scale- incomplete combustion

LOADING LOSSeg.,- excess boiler capacity- variable demand

RADIATION LOSS eg., - inadequate insulation

BLOWDOWN LOSS eg., - Non-optimal water treatment

BOILERBOILERHEATHEATLOSSLOSS

Factors Affecting Heat Generation Efficiency

Equipment Design

Operating Parameters

Provision of Waste Heat Recovery

Fuel Quality

Controls

Load Management

Maintenance / Adjustments

EnergyManagement

Capital andProcessConstraints

Boiler efficiency vs Load

0102030405060708090

0 20 40 60 80 100 120

Load %

Eff

icie

ncy

%

Effect of excess air on carbon di-oxide

Fuel

Carbon di oxide in flue gas, percent, when the excess air is

0 10 20 40 100

Natural gas 12.0 10.7 9.8 8.3 5.7

Distillate oil 15.2 13.8 12.5 10.7 7.4

Residual oil 15.6 14.1 12.9 11.0 7.6

Anthracite coal

19.8 18.0 16.5 14.1 10.0

Soot deposits vs flue gas temperature

0

20

40

60

80

100

120

140

160

180

0 0.5 1 1.5 2

Soot deposits (mm)

Incr

ease

in f

lue

gas

tem

per

atu

re (

o F)

Fuel savings as feed water temperature increases

0

1

2

3

4

5

6

7

8

9

0 20 40 60 80 100

Feedwater temperature OF above ambient

% f

uel

sav

ings

Fuel savings as combustion air temperature increases

0

2

4

6

8

10

12

14

0 100 200 300 400 500 600

Combustion air temperature OF

% f

uel

sav

ings

Cleanliness of BoilersFUEL = OIL

Flue gas temperature, deg C

CO2 ,%

Air temperature, deg C

Efficiency, %

Annual Consumption, Toe

Annual losses, Toe

Clean Boiler

220

12

30

88

1000

-

After 6 months without cleaning

300

12

30

84

1048

48

Solution: brushing gas tubes of the boiler each month

Low-Cost / Short Term Opportunities

ENERGY SAVING OPPORTUNITY

1. Reduce excess combustion air to minimum

2. Maximise completeness of combustion

3. Maintain boiler cleanliness (soot / scale)

4. Repair (replace) boiler insulation and door sealing stripps

5. Insulate feedwater tank - cover tank

6. Insulate condensate return lines

ACTION TO CHECK:

• CO2/ O2 measurement

• Soot / CO measurement

• Monitor for rise in flue gas temperature

• Periodic inspection of boiler insulation condition

• Check possible feedwater temperature losses

• Check possible heat loss from condensate return lines

Case Styudy: Boiler Efficiency Improvement by damper control

Thermax Boiler

6 TPH10.75 Kg/cm2Coconut shell

fired

Hopper

Coconut shell crusher

Economiser

DustCollector

Primary Air Fan

Secondary Air Fan

Induced Draft Fan

Damper14 %

9 %

Fuel Savings due to Boiler Efficiency Improvement

• By damper control O2 in flue gas brought down from 14 to 9 %

• Correspondingly excess air from 200 to 75 %

• Savings in coconut shell consumption – 5 %

Annual Savings – Rs. 3.5 Lakhs

Case Study:Existing: High radiation loss from Moulds

Radiation from moulds

Reduced heat loss through insulation

Insulated Mould