Encon in Boiler

8/3/2019 Encon in Boiler

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ENERGY CONSERVATION

OPPORTUNITIESBoiler Operations





Typical Boiler Heat Balance

BOILER

Heat in Steam

Heat loss due to dry fluegas

Dry Flue Gas Loss

Heat loss due to steam in flue gas

Heat loss due to moisture in fuel

Heat loss due to unburnts in residue

Heat loss due to moisture in air

Heat loss due to radiation & other

unaccounted loss

12.7 %

8.1 %

1.7 %

0.3 %

2.4 %

1.0 %

73.8 %

100.0 %

Fuel



What are the energy

conservation options?



What are the energy

conservation options?



1. Reduce Stack Temperature

Stack temperatures greater than 200°C indicates

potential for recovery of waste heat.

It also indicate the scaling of heattransfer/recovery equipment and hence the

urgency of taking an early shut down for water /flue side cleaning.

22o C reduction in flue gas temperature increasesboiler efficiency by 1%





2. Feed Water Preheating usingEconomiser

For an older shell boiler, with aflue gas exit temperature of 260 o

C, an economizer could be used

to reduce it to 200o

C, Increasein overall thermal efficiencywould be in the order of 3%.

Condensing conomiser (N.Gas)Flue gas reduction up to 65oC

6

o

C raise in feed watertemperature, by economiser/condensate recovery,corresponds to a 1% saving infuel consumption



3. Combustion Air Preheating

Combustion air preheating is analternative to feed water heating.

In order to improve thermal efficiency

by 1%, the combustion air temperature

must be raised by 20 oC.





Heat Pipe



Air pre

heater wit

h heat pipe



Ecnomiser wit

h heat pipe



4. Incomplete Combustion(c c c c c + co co co co)

Incomplete combustion can arise from a shortage ofair or surplus of fuel or poor distribution of fuel.

In the case of oil and gas fired systems, CO or smokewith normal or high excess air indicates burner system

problems.

Example: Poor mixing of fuel and air at the burner.

Poor oil fires can result from improper viscosity, worntips, carbonization on tips and deterioration of

diffusers.



4. Incomplete Combustion(c c c c c + co co co co)

With coal firing: Loss occurs as grit carry-over or

carbon-in-ash (2% loss).

Example :In chain grate stokers, large lumps willnot burn out completely, while small pieces andfines may block the air passage, thus causingpoor air distribution.

Increase in the fines in pulverized coal alsoincreases carbon loss.





How to control the unburnts ?



How to control the unburnts ?





5. Control excess air(For every 1% reduction in excess air ,0.6% rise in efficiency.)

Table : Excess air levels for different fuels

Fuel Type of Furnace or Burners Excess Air

(% by wt)Completely water-cooled furnace for

slag-tap or dry-ash removal

15-20Pulverised coal

Partially water-cooled furnace for dry-

ash removal

15-40

Spreader stoker 30-60

Water-cooler vibrating-grate stokers 30-60

Chain-grate and traveling-gate stokers 15-50

Coal

Underfeed stoker 20-50

Fuel oil Oil burners, register type 5-10

Multi-fuel burners and flat-flame 10-30

Wood Dutch over (10-23% through grates) and

Hofft type

20-25

Bagasse All furnaces 25-35

Black liquor R ecovery furnaces for draft and soda-

pulping processes

5-7

The optimum excess air level varies with furnace design, type of burner,

fuel and process variables.. Install oxygen trim system.



Boiler Efficiency Improvement by damper control

Thermax Boiler

6 TPH

10.75 Kg/cm2Coconut shell

fired

Hopper

Coconut shell

crusher

Economiser

Dust

Collector

Primary Air

FanSecondary Air

Fan

Induced Draft

Fan

Damper

14 %

9 %



Annual Savings ± Rs. 3.44 Lakhs

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 %





7.Automatic Blowdown Control

Uncontrolled continuous blowdown is verywasteful.

Automatic blowdown controls can be installed

that sense and respond to boiler waterconductivity and pH.

A 10% blow down in a 15 kg/cm2 boiler results

in 3% efficiency loss.



BLOWDOWN HEAT LOSS

This loss varies between 1% and 6% and depends on a number of actors:

Total dissolved solids (TDS) allowable in boiler water

Quality of the make-up water, which depends mainly on the type of

water treatment installed(e.g. base exchange softener or demineralisation):

Amount of uncontaminated condensate returned to the boiler house

Boiler load variations.

Correct checking and maintenance of feedwater and boiler water

quality, maximising condensate return and smoothing load swingswill minimise the loss.

Add a waste heat recovery system to blowdowns

Flash steam generation



Boiler Blowdown

Blow down (%) = Feed water TDS x % Make up

Permissible TDS in Boiler - Feed water TDS

If maximum permissible limit of TDS as in a package boiler is 3000 ppm, percentage make

up water is 10% and TDS in feed water is 300 ppm, then the percentage blow down is

given as :300 x 10

=

3000 ± 300

= 1.11 %

If boiler evaporation rate is 3000 kg/hr then required blow down rate is:

3000 x 1.11

=

100

= 33 kg/hr



Blowdown Heat Recovery

Efficiency Improvement - Up to 2percentage points.

Blowdown of boilers to reducethe sludge and solid content

allows heat to go down thedrain.

The amount of blowdown shouldbe minimized by following agood water treatment program,

but installing a heat exchangerin the blowdown line allows thiswaste heat to be used inpreheating makeup and feed

water.



Blowdown Heat RecoveryHeat recovery is mostsuitable for continuousblowdown operations

which in turn provides thebest water treatmentprogram.



8.Reduction of Scaling and Soot Losses

In oil and coal-fired boilers, soot buildup on tubes acts as aninsulator against heat transfer. Any such deposits should beremoved on a regular basis. Elevated stack temperatures mayindicate excessive soot buildup. Also same result will occurdue to scaling on the water side.

High exit gas temperatures at normal excess air indicate poorheat transfer performance. This condition can result from agradual build-up of gas-side or waterside deposits. Watersidedeposits require a review of water treatment procedures and

tube cleaning to remove deposits. Stack temperature should be checked and recorded regularly

as an indicator of soot deposits. When the flue gastemperature rises about 20oC above the temperature for anewly cleaned boiler, it is time to remove the soot deposits



Cleaning :

Incorrect water treatment, poor combustion and poor

cleaning schedules can easily reduce overall thermalefficiency

However, the additional cost of maintenance andcleaning must be taken into consideration when

assessing savings.

Every millimeter thickness of soot coating increases the stack temperature by about 55oC. 3 mm of soot can cause an increase infuel consumption by 2.5%.

A 1mm thick scale (deposit) on the water side could increase fuelconsumption by 5 to 8%



9. Reduction of Boiler Steam Pressure

Lower steam pressure gives a lower saturated steamtemperature and without stack heat recovery, a similarreduction in the temperature of the flue gas temperatureresults. Potential 1 to 2% improvement.

Steam is generated at pressures normally dictated by thehighest pressure / temperature requirements for a particularprocess. In some cases, the process does not operate all thetime, and there are periods when the boiler pressure could bereduced.

Adverse effects, such as an increase in water carryover from

the boiler owing to pressure reduction, may negate anypotential saving.

Pressure should be reduced in stages, and no more than a 20percent reduction should be considered.



10.Variable Speed Control for Fans, Blowersand Pumps

Generally, combustion air control is effectedby throttling dampers fitted at forced and

induced draft fans. Though dampers aresimple means of control, they lack accuracy,giving poor control characteristics at the topand bottom of the operating range.

If the load characteristic of the boiler isvariable, the possibility of replacing thedampers by a VSD should be evaluated.



Energy saving in Boiler ID fan by speedreduction through pulley change or VSD

8´10´

6´10´

1470 RPM

1470 RPM

1181 RPM

882RPM

26 kW

14 kW

Inverter

Motor Fan

Belt can be eliminated

Fan can be Directly driven thus

eliminating the belt losses



11. Effect of Boiler Loading on Efficiency

As the load falls, so does the value of the mass flowrate of the flue gases through the tubes. Thisreduction in flow rate for the same heat transfer

area, reduced the exit flue gas temperatures by asmall extent, reducing the sensible heat loss.

Below half load, most combustion appliances needmore excess air to burn the fuel completely and

increases the sensible heat loss.

Operation of boiler below 25% should be avoided

Optimum efficiency occurs at 65-85% of full loads



Avoid Deaerator Overflow

Overflow of DM water at 40 % levelsetting

30 tons/day loss of DM water Heat Losses

Change setting to 20 %

Nil overflow

Savings - R s7.62 lakhs/year



Eliminate condensate pump

Return condensate is at high pressure

Direct the condensate to deaerator

Condensate tank can be avoided

Savings - R s.1.31 lacs/year



Improve Boiler Efficiency

CO2 in flue gas increased from 8.5to 12 %

Flue gas losses reduced from 9 to 6 %

Efficiency increased from 79 to 82 %

Savings - R s. 14.7 lacs/year



Save Electrical Energy in BoilerF

D fan Air/Fuel ratio reduced from 1.0 to

0.7

Excess air in boiler reduced

Fan power reduced by 3.4 KW

Savings - R s. 75,000/year







12.Boiler Replacement

If the existing boiler is :

Old and inefficient, not capable of firing cheapersubstitution fuel, over or under-sized for presentrequirements, not designed for ideal loading

conditions replacement option should be explored.

The feasibility study should examine all implicationsof long-term fuel availability and company growth

plans. All financial and engineering factors shouldbe considered. Since boiler plants traditionally

have a useful life of well over 25 years,replacement must be carefully studied.



Energy Conservation inBoiler Auxiliaries



Energy conservation inAuxiliaries

Fuel Handling:

Mechanical

Conveyor instead of pneumatic

Fuel Preparation:

Sieve the fines before feeding to

crusher External drying of fuel moisture using

waste heat, solar etc



Recommended sizes of coal forfiring

Type Size

Grate 100 % through 40mm mesh

Pulverise

d

70 % through 200mm mesh

Fluidised 100 % through 6mm mesh




Dust Collector:

dP across bag filter - 150 mm WC dP across ESP - 20 mm WC

For a 20 tph boiler bag filter consumes25-30 kW more than ESP




Fans:

Ex

cessive margins at design stage Damper operation is energy wasted

Downsizing, VSD..

Excess air reduction

Reducing pressure drops



Fan system efficiencies forvarious controls

% Ratedflow

Outletdamper

Inletguidevane

Variablespeeddrive

100 81 78 83

90 61 74 81

80 44 58 80

70 31 42 76

60 21 28 70

50 14 18 66



Speed vs Power

% Speed % Power

100 100

90 73

80 51

70 34

60 22

50 13



E i d t f



Energy savings due to fanspeed reduction

Existing:

N1 ± 1440 RPM , P1 ± 100 kW

Required:

N2 ± 720 RPM , P2 ± ? kW

[ 720 / 1440 ] 1/3 = P2 / 100

P2 = 12.5



Boiler feed pump

VSD for a fluctuating load

Remove a few stages in case of over-

designed pressure

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Encon in Boiler