Boiler & Furnaces Performance & Efficiency

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Note: The source of the technical material in this volume is the Professional

Boilers And Furnaces

Performance And Efficiency

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CONTENT PAGE

CALCULATING THERMAL EFFICIENCY... 1

Heat Balance (Input/Output) Method ... 1

Stack Loss Method ... 3

Calculation Procedures... 3

Simple Efficiency Equation ... 3

API RP 532 Procedure... 6

ASME Abbreviated Efficiency Test... 9

EFFICIENCY IMPROVEMENTS ... 17

Excess Air... 17

Reduce Stack Temperatures ... 17

Reduce Other Losses ... 18

FUTURE IMPROVEMENTS... 19

Excess Air Reduction ... 19

Stack Temperature Reduction ... 19

Combustion Improvements ... 22 WORK AID 1 ... 23 WORK AID 2 ... 32 WORK AID 3 ... 33 WORK AID 4 ... 35 GLOSSARY ... 37

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Calculating Thermal Efficiency

The thermal efficiency of a boiler or furnace is used to monitor its operation and determine the rate of energy usage. Changes in efficiency may indicate a deteriorating condition of the equipment, or the need to change operating conditions.

Thermal efficiency is defined as the total heat absorbed by the boiler or furnace, divided by the total heat input. Efficiency can be related to either the higher heating value (HHV) or the lower heating value (LHV) of the fuel, and it is important to state which value is being used. The difference between the two values is that the higher heating value includes the heat of evaporation of the water vapor formed in the combustion of the fuel (1059.7 Btu/lb of water). This heat is almost never recovered in boilers or furnaces. Therefore, LHV is a better

measure of achievable thermal efficiency. The HHV efficiency is several percentage points lower than the LHV efficiency. It is common practice in the furnace industry to use the lower heating value in calculations, while the boiler industry uses the higher heating value. Lower heating value is used in the following calculations.

There are two basic methods to determine thermal efficiency, the heat balance method and the stack loss method. Both of these methods are described below.

Heat Balance (Input/Output) Method

In this method, efficiency is determined by the following equation: Efficiency = Heat Absorbed

Heat Input

The following data are required to determine efficiency using this method:

• Heat Absorbed:

- Process flow rate (typical meter accuracy + 3%). - Process inlet enthalpy.

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• Heat Input:

- Fuel flow (typical meter accuracy + 3%).

- Fuel heating value (often varies with time, particularly when using plant gas as the fuel).

Typical furnace instrumentation is shown in Figure 1. The required data can be obtained using this instrumentation.

Because of the inaccuracies inherent in many of these measurements, it is very difficult to achieve a reasonably accurate efficiency estimate using this method, particularly in process furnaces. Consequently, this method is not used very much in the petroleum industry.

INSTRUMENT AND MEASUREMENT LOCATIONS

Source: API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

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Stack Loss Method

In this method, efficiency is determined by the following equation: Efficiency = Heat Input - Losses Heat Input

In this equation, the losses can be expressed as a percentage of the heat input, so it is not necessary to determine the actual rate of heat input.

The major heat loss from a boiler or furnace is the stack loss (the heat in the flue gas leaving the boiler or furnace). Methods for calculating the stack losses are described below.

A smaller, but still significant heat loss is through the walls of the boiler or furnace to the atmosphere. This is known as the radiation loss. This is difficult to calculate and is usually set as a percentage of the heat fired. Radiation loss can also include an allowance to cover other unidentified losses. The following are typical values used for radiation losses:

Heat Fired, MBtu/hr Qr, % of Net Heat Fired

<15 3

15 - 100 2

>100 1

Another heat loss from boilers is in the blowdown water. Assuming a 10% blowdown rate, this heat loss is approximately 1.9% of heat fired. For detailed calculations, the heat loss can be calculated using the blowdown flow rate and the enthalpy of the water being discharged.

Calculation Procedures

Several different calculation procedures can be used to calculate efficiency. Two procedures are described below: a simple procedure that can be used to estimate efficiency, and a detailed procedure.

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This equation requires just two measurements: stack temperature and stack excess air. The excess air and stack temperature should be measured at the same location. It is not necessary to measure any flow rates. This equation is based on typical Saudi Aramco operating

conditions and assumes an average ambient air temperature of 90ºF. Variations in ambient air temperature have a minor effect on calculated efficiency.

Excess Air. If not specified, the excess air rate can be determined from the oxygen content in the flue gas. The following equations can be used to estimate the excess air rate.

When the flue gas analysis is on a wet basis:

EA

=

111 . 4 x % O 2

20 . 95 - % O 2 ( Eqn . 2 )

where: %O2 = Percent oxygen in the flue gas. When the flue gas analysis is on a dry basis:

EA

=

91 . 2 x % O 2

20 . 95 - % O 2 ( Eqn . 3 )

It is important to know which basis is being used. When the oxygen analyzer is located directly at the stack, the flue gas analysis is usually on the wet basis. When the flue gas is extracted from the stack and is transported to an analyzer that is located some distance away, the analysis is usually on the dry basis.

The precise relationship between oxygen content and excess air is a function of the hydrogen-to-carbon ratio of the fuel. However, there is very little change in this relationship over a wide range of fuels at low excess air rates. For typical plant fuels, this calculated efficiency should be within about 1% of that calculated by more precise methods. This should be adequate for most plant applications. These results can be verified for specific operating conditions by comparing the results with one of the detailed methods presented below. Stack Temperature. Another potential source of error in all efficiency calculations is an error in stack temperature measurements. Ordinary stack temperature thermocouples can read low by as much as 100ºF, depending upon their location and the flue gas temperature being measured. If the thermocouple can "see" cold surroundings, such as the top of the convection section or the sky, the indicator will likely read low. The higher the actual stack temperature, the higher the radiation losses and thus, the higher the error.

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If precise stack temperature data are needed, such as for setting the basis for a waste heat recovery project, then the data should be taken with an aspirating ("high-velocity")

thermocouple, as illustrated in Figure 2. The tip of this thermocouple is shielded, and flue gas is continually pulled past the thermocouple.

Although it may produce inexact readings, an ordinary stack thermocouple should give representative, consistent readings, which should be satisfactory for monitoring day-to-day performance.

TYPICAL ASPIRATING (HIGH-VELOCITY) THERMOCOUPLE

6 6 5 4 3 6 6 2 2 A-A 2 2 A 7 A 1 2 ₂ 8 1

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API RP 532 Procedure

The RP 532 procedure is a detailed version of the stack loss method. In addition to the data required by the Simple Efficiency Equation, an analysis of the fuel composition is required. All sources of heat inputs and losses need to be included to make a precise efficiency

calculation. These sources are illustrated in Figure 3. This calculation requires the following additional data.

• Relative humidity of the air.

• Temperature and specific heat of the fuel.

• Temperature and rate of atomizing steam when liquid fuel is fired.

If not known, it is usually satisfactory to estimate these data, based on typical local conditions.

TYPICAL HEATER ARRANGEMENT

Ambient Air Fuel

Q _s

T _{s t}

Q _r LHV+H +H _f _m

H

a

at T

t

= T

a

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Work Aid 1 contains the work sheets required for the RP 532 procedure. An example of how it is used to calculate the efficiency of a gas-fired furnace is shown in Figure 4.

This procedure consists of the following steps:

1. Using the Lower Heating Value Work Sheet, determine the lower heating value of liquid fuel (if required). If the fuel is gas, or typical liquid fuel properties are known, it is not necessary to complete this work sheet.

2. Using the Combustion Work Sheet, determine the flue gas properties for stoichiometric combustion conditions.

3. Using the Excess Air and Relative Himidity Work Sheet, determine the amount of water vapor in the flue gas.

The vapor pressure of water at the ambient temperature can be determined from Work Aid 2.

4. Using the Stack Loss Work Sheet, determine the stack heat losses.

The enthalpy of the flue gas components can be determined from Work Aid 3. 5. The thermal efficiency can then be determined by the following equation:

e

=

100

−

100

(

Q s

+

Q r

)

LHV

+

_{H a}

+

_{H f}

+

_{H m} (Eqn.

4)

where: e = Net thermal efficiency, % (LHV).

LHV = Lower heating value of the fuel, Btu/lb of fuel. Ha = Air sensible heat correction, Btu/lb of fuel.

= Cp(air)(Ta - Td)(pounds of air per pound of fuel). Hf = Fuel sensible heat correction, Btu/lb of fuel.

= Cp(fuel)(Tf - Td).

Hm = Atomizing medium (usually steam) sensible heat correction, Btu/lb of fuel. = Cp(medium)(Tm - Td)(pounds of medium per pound of fuel).

If steam, Hm = (Enthalpy difference)(lb of steam/lb of fuel). = (hs - 1087.7)(lb of steam/lb of fuel).

Qr = Radiation heat losses, Btu/lb of fuel.

Qs = Calculated stack heat losses (from Stack Loss Work Sheet), Btu/lb of fuel. Ta = Ambient air temperature, ºF.

Td = Reference (or datum) temperature, ºF. = 60ºF (usually).

Cp = Specific heat, Btu/lb-ºF. Tf = Temperature of fuel, ºF.

Tm = Temperature of atomizing medium, ºF. hs = Enthalpy of atomizing steam, Btu/lb.

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6. The gross thermal efficiency can be determined by the following equation:

e gross = 100 - 100 Q s + Q r + latent heat_{HHV +} H a + H f + H m _{(Eqn. 5)} where: egross = Gross thermal efficiency, % (HHV).

Latent heat = (H2O formed by combustion of fuel) x1059.7.

7. The firing rate can be calculated, based on the heat absorbed in the boiler or furnace, as follows:

Q f = _e/100Q a _{(Eqn. 6)}

where: _Qf = Heat fired, MBtu/hr (LHV). Qa = Heat absorbed, MBtu/hr. e = Net thermal efficiency, %. ASME Abbreviated Efficiency Test

This procedure calculates the efficiency of boilers by both the Input/Output and Stack Loss methods. It uses the HHV of the fuel and can be used for coal-fired boilers, as well as gas-and oil-fired units. The forms for this procedure are included in Work Aid 4. Line items on these forms that do not apply to Saudi Aramco boilers have been crossed out.

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Sample Calculation - RP 532 Procedure

The following sample calculation illustrates the use of the RP 532 calculation procedure to determine thermal efficiency. (Based on Par. 3.2.2 of RP 532).

Given:

Stack temperature _{Tst = 300 ºF}

Air temperature _Ta = 28 ºF

Specific heat of air Cp(air) = 0.24 Btu/lb- ºF

Relative humidity = 50 %

Oxygen content of flue gas = 3.5 % (wet basis)

Radiation losses _Qr = 2.5 % of lower heating value of fuel Fuel temperature _{Tf = 100ºF}

Fuel specific heat Cp(fuel) = 0.525 Btu/lb- ºF Fuel composition: Methane = 75.41 vol. % Ethane = 2.33 Ethylene = 5.08 Propane = 1.54 Propylene = 1.86 Nitrogen = 9.96 Hydrogen = 3.82 Solution:

1. Complete the following work sheets from Work Aid 1

(completed copies attached).

Combustion Work Sheet.

Excess Air and Relative Humidity Work Sheet. Stack Loss Work Sheet.

2. Determine Net Thermal Efficiency, as follows: From Combustion Work Sheet, LHV = 18,120 Btu/lb

Radiation Loss Qr = 18,120 x 0.025

= 453.0 Btu/lb of fuel From Stack Loss Work Sheet, Qs = 1162.1 Btu/lb of fuel

Data extracted from API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

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Sample Calculation - RP 532 Procedure (Cont’d)

Sensible heat corrections:

Pounds of air/pound of fuel is obtained by adding the total from column 7 of the Combustion Work Sheet with the pounds of dry excess air per pound of fuel from the Excess Air and Relative Humidity Work Sheet.

Air: Ha = Cp(air) (Ta - Td)(pounds of

air/pound of fuel) = 0.24 (28 - 60)(14.322 + 3.191) = -134.5 Btu/lb of fuel Fuel: Hf = Cp(fuel) (Tf - Td) = 0.525 (100 - 60) = 21.0 Btu/lb of fuel Atomizing medium Hm = 0 (no atomizing steam required) Using Eqn. 4:

e

=

100

−

100 ( Q s

+

Q r ) LHV

+

_{H a}

+

_{H f}

+

_{H m}

e

=

100

−

100 ( 1162 . 1 + 453 . 0 ) ( 18120 - 134 . 5 + 21 . 0 )

=

91 . 03 % ( LHV ) 3. Determine Gross thermal efficiency, as follows:

From Combustion Work Sheet, H2O formed = 1.784 lb/lb of fuel. Latent heat = H2O formed x 1059.7

= 1.784 x 1059.7 = 1890.5 Btu/lb of fuel

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COMBUSTION WORK SHEET

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EXCESS AND RELATIVE HUMIDITY WORK SHEET

USE PHOTOSTAT

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EXCESS AND RELATIVE HUMIDITY WORK SHEET

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STACK LOSS WORK SHEET

USE PHOTOSTAT

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Efficiency Improvements

Excess air level and stack temperature are the two major parameters that affect boiler and furnace efficiency.

Excess Air

All the air that enters a boiler or furnace is ultimately discharged to the atmosphere at the stack temperature, and the energy it contains is lost. The primary objective of efficient boiler and furnace operations is to minimize air flow beyond that required for good combustion. The air required for combustion should enter only through the burners. The following steps can be taken to reduce excess air:

1. Seal air leaks. This is particularly important in furnaces, which operate with a draft, or negative pressure, inside and are susceptible to air infiltration. Leakage into the radiant section has the worst effect, but all leaks are wasteful. Figure 5 shows typical sources of air leaks into a furnace.

Since most boilers operate with a positive pressure inside, air leakage into boilers is usually not a problem.

2. Fire all burners at the same rate. 3. Control furnace draft.

4. Determine excess air targets for each furnace through a series of plant tests. These targets are the minimum excess air rates that have been found to be necessary for good

combustion. Since no two furnaces or boilers are exactly the same, there can be different targets for each boiler and furnace in the plant.

Reduce Stack Temperatures

Fouling of the convection section tubes is the primary cause of stack temperatures that exceed design. The extent of fouling can be determined by visual inspection of the tubes or by observing an increase in stack temperature over time. A 40°F increase in stack temperature represents a 1% efficiency loss.

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FURNACE AIR LEAKS

FIGURE 5

Reduce Other Losses

Although less important than excess air and stack temperature, several other parameters affect boiler and furnace efficiency:

• Boiler blowdown should be controlled to the rate needed to maintain boiler drum water impurities at the specified concentration. Excess blowdown wastes heat and water. Heat can be recovered from the blowdown stream.

• Insulation should be maintained to reduce heat losses. • Steam leaks should be repaired.

Inlet Outlet Leaky Covers on Observation Doors Clearance Around Tube Penetration Casing Corrosion Construction Joint Poor Seal on Access Door Idle Burner

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Future Improvements

The following methods of improving the efficiency of existing boilers and furnaces require the addition of new equipment.

Excess Air Reduction

• Add improved combustion control systems. - Automatic draft control on furnaces. - Closed loop oxygen and/or CO control.

• Replace oversized burners. It is difficult to operate burners efficiently at high turndown rates.

Stack Temperature Reduction

• Reducing the stack temperature of a furnace or boiler that is operating satisfactorily usually requires the addition of heat transfer surface.

• Additional heat transfer surface in convection section of furnaces.

• Economizers on boilers to preheat the boiler feedwater before entering the steam drum. • Combustion air preheaters. Air preheaters can transfer heat from the flue gas leaving the

stack, to the air used for combustion. Depending upon the flue gas temperature, the incoming air can be heated several hundred °F. The flue gas temperature should be kept above about 300°F to prevent corrosion of the heat exchanger due to sulfuric acid in the flue gas.

Combustion air can also be preheated using other waste heat, such as low-pressure steam. While this does not recover heat from the stack gases, it does improve the overall plant energy balance. Typical air preheater systems are shown in Figure 6.

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AIR PREHEAT SYSTEMS

a. Air Preheat System Using Regenerative, Recuperative, or Heat Pipe Unit

b. External Heat Source for Air Preheating

Source: API Standard 560, Fired Heaters for General Refinery Services, 1st Edition, January 1986. Reprinted courtesy of the American Petroleum Institute.

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Several types of air preheaters can be used, with the most common shown in Figures 7, 8, and 9. The rotary regenerative, or Ljungstrom type, preheater rotates a heat-absorbing mass from the hot flue gas duct, where it picks up heat, to the incoming air duct, where the heat is released. Another type is the tubular exchanger, in which the hot flue gas passes on one side of the tubes and cold incoming air passer on the other side of the tubes.

AIR PREHEATERS Basketed Heating Surface Shaft Axial Seal (Stationary) Housing Radial Seal (Stationary) Gas Out Gas In Air In Air Out Plate Groups Plate Groups Seal Sector

Elements of a Rotary Regenerative Air Heater Diagrammatic illustration of rotary regenerative air heater

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TYPICAL TUBULAR AIR PREHEATER Gas Inlet Gas Outlet Air Outlet Air Inlet Gas Downflow

Air Counterflow, Three-Pass

With permission from Babcock & Wilcox.

FIGURE 9

Combustion Improvements

• High-capacity, high-intensity, or axial flow forced-draft burners for improved, low excess air combustion.

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Work Aid 1

RP 532 THERMAL EFFICIENCY CALCULATION PROCEDURE

The following procedure can be used to calculate the thermal efficiency of a boiler or furnace, using the Stack Loss Method, as described in API RP 532.

1. Complete the attached work sheets:

Lower Heating Value Work Sheet (if required for liquid fuels) Combustion Work Sheet

Excess Air and Relative Humidity Work Sheet Stack Loss Work Sheet

2. Determine Net Thermal Efficiency, as follows: 2. Determine Net Thermal Efficiency, as follows:

From Combustion Work Sheet, LHV = ________________Btu/lb Radiation Loss _{Qr = LHV x %Qr/100}

= (__________)(_________) = ___________ Btu/lb of fuel

From Stack Loss Work Sheet, _{Qs = _________Btu/lb of fuel}

From Combustion Work Sheet, air required = _____________(lb of air/lb of fuel) From Excess Air Work Sheet, excess air = _____________(lb of air/lb of fuel)

Total air rate =

_____________(lb of air/lb of fuel) Sensible heat corrections:

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( RP 532 THERMAL EFFICIENCY CALCULATION PROCEDURE (CONT’D)

Atomizing medium _{Hm = Cp(medium) (Tm - Td)(lb of} medium/lb of fuel)

If steam is used: _{Hm = (Enthalpy difference)(lb of}

steam/lb of fuel)

= (hs - 1087.7)(lb of steam/lb of fuel)

Atomizing steam temperature = ___________ºF Steam enthalpy _hs = ___________Btu/lb

Hm = (__________ - 1087.7)(___________) = ___________Btu/lb of fuel Using Eqn. 4: Thermal efficiency

e

=

100

−

100 ( Q s

+

Q r ) LHV

+

_{H a}

+

_{H f}

+

_{H m} = 100 - 100 ( + ) (________ + _______ + _______ + _____) e = ___________% (LHV)

3. If gross thermal efficiency is needed, determine as follows:

From Combustion Work Sheet, H2O formed = ___________lb/lb of fuel Latent heat = H2O formed x 1059.7

= (_________) x 1059.7 = __________Btu/lb of fuel

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EXCESS AIR AND RELATIVE HUMIDITY WORK SHEET

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EXCESS AIR AND RELATIVE HUMIDITY WORK SHEET (CONT'D)

USE PHOTOSTAT

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STACK LOSS WORK SHEET

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LOWER HEATING VALUE (LIQUID FUELS) WORK SHEET

USE PHOTOSTAT

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COMBUSTION WORK SHEET

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Work Aid 2

VAPOR PRESSURE OF WATER

Source: Data taken from Steam Tables

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Work Aid 3

ENTHALPY OF FLUE GAS COMPONENTS

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ENTHALPY OF FLUE GAS COMPONENTS

USE PHOTOSTAT

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Work Aid 4

ASME TEST FORM FOR ABBREVIATED EFFICIENCY TEST

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ASME TEST FORM FOR ABBREVIATED EFFICIENCY TEST (CONT'D)

USE PHOTOSTAT

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glossary

blowdown Water removed from the boiler to control the level of dissolved impurities in the boiler water.

economizer A device for transferring heat from the flue gas to the boiler feedwater (BFW) before the BFW enters the boiler drum.

excess air The percentage of air in excess of the stoichiometric amount required for combustion.

flue gas Gaseous products from the combustion of fuel.

higher heating

value (HHV) The amount of heat released during complete combustion of fuelwhen the water formed is considered as a liquid (credit is taken for its heat of condensation.) Also called gross heating value. Usually expressed in Btu/lb.

lower heating value

(LHV) The amount of heat released during complete combustion of fuelwhen no credit is taken for heat of condensation of water in the flue gas. Also called net heating value. Usually expressed in Btu/lb.

radiation heat loss A defined percentage of the net heat of combustion of the fuel to account for heat losses through the boiler or furnace walls to the atmosphere.

stack heat loss The total sensible heat of the flue gas components, at the temperature of flue gas, when it leaves the last heat exchange surface.

thermal efficiency The total heat absorbed divided by the total heat input. Usually expressed in percent.