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Excess air in combustion Issue Incorrect amount of air in fuel

By Marjorie Davis,2014-12-23 19:48
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Excess air in combustion Issue Incorrect amount of air in fuel

Dr. Albrecht Kaupp Page 1

Excess air in combustion

Incorrect amount of air in fuel combustion Issue

accounts for the largest losses in combustion

systems.

( Understanding the technical jargon Learning

behind combustion technologies Objectives

( Knowing the very basics of combustion

fundamentals

( Appreciating the various methods to

calculate excess air levels

( Being able to relate O and CO 22

measurements to fuel properties

( Assessing fuel cost reduction potentials

( Applying quick estimates to calculate

energy losses

Excess air in combustion Page 2

; NOTES

1. A crash course in combustion principles As shown in lecture 4, all fuels consist mostly of atomic Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Sulfur (S), minerals (ash) and water (HO). 2

Fuel combustion means to let the molecular Oxygen (O) in air react 2

with the combustible components of a fuel. As an example the fuel Carbon (C) reacts with O of the air to generate Carbon Dioxide 2

(CO). If the reaction is incomplete Carbon Monoxide (CO), a 2

It is worthwhile to point out that all combustion products such as CO, CO, NO, CH, SO, SO, except for the water generated by 2xnm23

combustion of H to HO, are harmful. There is essentially nothing 2

benign in stack gas, except the water vapor. And even the water vapor, because it reacts with SO to sulfuric acid (HSO) is not 324

really harmless.

This environmental and health threat of stack gas is one more reason to reduce energy consumption per unit of product output.

2. The excess air parameter

One kg of fuel requires a certain minimum of ambient air to be fully combusted. We call this minimum amount of air the “stochiometric air” or sometimes also “the theoretical air” to combust the fuel. The stochiometric air would completely combust the fuel to Carbon Dioxide (CO), water (HO), and Sulfur Dioxide (SO) if Sulfur is 222

present. If the fuel does not get enough air for combustion it will generate smoke and a potential unhealthy mixture of stack gas products. In addition energy is wasted. The same applies if too much excess air is used for combustion. A less trivial issue in combustion technology is therefore to ensure the proper amount of air that minimizes environmental impact and fuel consumption. For convenience we define the “stochiometric air” as the air to fuel ratio, AF (kg air/kg fuel), and the excess air factor as

Mass of air (kg) to combust one kg of fuelEA = Stochiometric air (AF)

The AF is a property of a fuel that can be calculated from the ultimate chemical composition of the fuel.

Excess air in combustion Page 3

; NOTES

Table 1: Air-to-fuel ratio of various fuels

COCO2 max 2 maxFuel Phase AF wet dry

Very light fuel oil liquid 14.27 13.56

Light fuel oil liquid 14.06 13.72

Medium heavy fuel oil liquid 13.79 14.00

Heavy fuel oil liquid 13.46 14.14

Bunker C liquid 12.63 16.23

Generic Biomass (maf) solid 5.88 17.91

Coal A solid 6.97 16.09

LPG (90 P : 10 B) gas 15.55 11.65

Carbon solid 11.44 21.00

3. Terminology and equations

Excess air and the excess air factor were defined in the previous paragraph. Note, that both parameters describe the same phenomena.

For instance saying a burner requires 20 % excess air to correctly combust fuel oil, is the same as saying the burner operates at an excess air factor of 1.2. A ideal combustion process would require 0 % excess air or has an excess air factor of 1.

A combustion process requiring 100 % excess air uses twice as much air as necessary, or in other words has an excess air factor of two.

The technical literature and car industry reserves the Greek symbol Lambda () for the excess air factor. Most modern fuel efficient cars have therefore Lambda sensors (= Oxygen sensors) to control the fuel efficiency. In boilers and furnaces they are called an “oxygen trim”.

Instead of EA we will also use the symbol .

Mass of air to combust one kg of fuel = AF

Excess air in combustion Page 4

; NOTES

It is repeated, the AF ratio is a fuel specific parameter that has nothing to do with the furnace design or combustion process, while is a parameter that tells us how efficiently a fuel was combusted. The closer is to one, the more efficient is the furnace or burner design and operation. Operating a boiler very close to =1 (or 0 %

excess air) will require a “oxygen trim” that closely monitors excess air and adjusts it.

Operating very close to the minimal amount of air (= stochiometric air = theoretical air) has the inherent danger of smoke and CO generation.

Once is known it is fairly easy to calculate the mass of stack gas generated from the combustion process by

m = m (1 + AF) - m SGfash

It is worthwhile to examine the last equation. In case the boiler does not have any leaks, where stack gas escapes we can be assured that the mass entering the boiler must also leave the boiler through either the chimney or the ash bin.

In the case of oil we know m = 0. ash

Therefore m = m (1 + AF) SGf

= m + m AF ff

= fuel + combustion air

Note, that the term combustion air refers to dry air, excluding the

humidity in air that could be anything from 1 to 20 grams of moisture per kg of air.

4. Derivation of excess air factor,

The amount of excess air can not be measured directly, but is rather derived from a measurement of either the O or CO content of the 22

stack gas. Whether one measures O or CO is irrelevant for the 22

calculation of the excess air, or , as long as one has obtained an

accurate measurement of either O or CO. As previously shown in 22

lecture 6, various sensors and methods exist to measure O or CO. 22

There is no simply and also accurate equation to calculate if O or 2

Excess air in combustion Page 5

; NOTES

CO is known. The correct equation based on a CO measurement 22

is

~COV2 maxSG = 11;； ?COV?2AF

where

CO = the maximum CO content of the dry stack gas at stochiometric 2 max2

combustion. Given in volume % 3V = dry stack gas in m/kg at stochiometric condition SGn3V = air-to-fuel ratio expressed as m/kg AFno3m = normal cubic meter at 0 C and 1.01325 bar. n

VSGThe factor f = is between 0.93 to 0.97 for fuel oils. VAF

It is between 0.98 and 1 for solid fuels and between 0.9 and 1.9 for

gases.

It is best to calculate and generate appropriate charts expressing

as a function of either O or CO in the stack gas by computer 22

software.

One should appreciate the complexity involved, that has resulted in

quite a number of simplistic equations. Most commonly used equations are

~CO2max = 11;；?CO?2

21 = O212

All equations apply only if no CO and H is found in the stack gas. 2

In case of incomplete combustion, CO is found in the stack gas. In

this case is given as

COgV2maxSG = 1; gVAF

()COCO;； 1002where g = 1000515；；.. COH2

Excess air in combustion Page 6

; NOTES

Note that CO is commonly measured in ppm and 10,000 ppm = 1%. CO contents of 1,000 ppm = 0.1 % are considered high in the combustion of liquid and gaseous fuels.

5. Excess air factors found in practice

As mentioned, the excess air factor of a burner furnace or boiler is a yardstick about its efficiency as well as the skill of the operator.

Standard average figures are

( Gas burners, forced draft 1.1 - 1.3

( Atmospheric gas burners 1.25 - 1.5

( Oil burners 1.15 - 1.3

( Coal dust burners 1.2 - 1.3

( Coal firing (mechanical) 1.3 - 1.5

( Coal firing (hand) 1.5 - 2.5

These are best values that can be achieved with careful monitoring and constant adjustment of the combustion air at varying loads. In reality energy auditors may see much higher numbers.

6. Wet stack gas versus dry stack gas values There can be a considerable amount of confusion and misjudgment of the situation if one does not clearly indicate whether O or CO 22

measurements were conducted on either a wet or dry stack gas basis. As discussed in lecture 6, chemical cell sensors for O and CO 2

measure on a dry stack gas basis while Zirconium Oxide sensors measure the O on a wet and hot stack gas basis. 2

Of the four options O % (dry), O % (wet), CO % (dry) CO % 2222

(wet) only one value needs to be measured. The others are calculated based on the ultimate chemical composition of the fuel.

Excess air in combustion Page 7

; NOTES

EXERCISES

In actual field work one measures or calculates CO on a “dry” 2

basis, meaning the gas sample is cooled to almost ambient temperature and the stack gas is in a saturated state, with little moisture left.

Calculate CO in a dry stack gas and complete table 1 on page 4. 2 max

Assume the CO content of stack gas from a LPG fired boiler was 2

measured at 9 % (dry basis). However we don’t know the composition of the LPG. Does it matter for calculation of the excess

air?

Hint: Generate the “Excess Air” graphics for Butane (8)and Propane

(9) and determine the excess air at 9 % CO. 2

Excess air for Butane at 9 % CO equals _______ %. 2

Excess air for Propane at 9 % CO equals ______ %. 2

Assume the CO content of stack gas from a coal fired boiler was 2

measured at 14 % (dry basis). However we don’t know the ultimate chemical analysis of the coal. How large could the error be?

Hint: Use the “Excess Air” graphics for Carbon, Coal A and Coal B, as

well as Bunker C oil and complete the following table.

Fuel (#) % CO dry f-factor Excess air 2,

Carbon (17) 14 1 50

Coal A (12) 14

Coal B (13) 14

Bunker C (7) 14

Excess air in combustion Page 8

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Complete the following sentences:

Excess air of 23 % is equivalent to = ______________________

Excess air of 0 % is equivalent to = ______________________

Excess air of 100 % is equivalent to = _____________________

Decreasing excess air close to 0 % may have the following undesirable side effects:

a) ____________________________________________________ b) ____________________________________________________ c) ____________________________________________________

Increasing excess air beyond 100 % may have the following undesirable side effects:

a) ____________________________________________________ b) ____________________________________________________

It is fairly easy to make mistakes in the assessment of the excess air,

if it is not quite clear what basis, either “dry” or wet stack gas is

used.

Note, very rarely sensors that measure CO in the wet stack gas are 2

used. Assume the wrong CO (wet) is used. Calculate the error 2 max

for excess air at 7 % CO and 13 % CO for Bunker C oil, by using 22

the equation

~CO2max = 11;； f?CO?2

The f-factor calculated by the software is always based on a dry gas composition. The values for CO and CO refer to a dry gas 2 max2

measurement.

Excess air in combustion Page 9

; NOTES

Decide whether the following statements are true or false.

True False Statement

1. A CO content of 5 % was measured in the stack gas of 2

a coal fired boiler, but the stack looked clean.

2. The excess air of a coal fired boiler is 10 %.

3. A very low excess air means a high energy efficiency.

4. The CO content decreases whenever excess air increases.

5. The CO content increases whenever excess air increases.

6. The CO content increases whenever excess air decreases

to 0 %.

7. Smoke generation is a sign of too little excess air.

8. Smoke generation is a sign of too much excess air.

9. The excess air increases at low boiler load. 10. Increased Carbon Monoxide generation is always a sign

of too little excess air.

11. Solid fuels have always a higher CO than LPG. 2 max 12. The highest possible CO of a fuel is 25 %. 2 max 13. The CO of a fuel is changing whenever excess air 2 max is changed.

14. Excess air and excess air factor are in principle the same. 15. The air-to-fuel ratio is a fuel property that characterizes a fuel.

16. The CO (dry basis) of a fuel changes with the 2 max moisture content of the fuel.

17. Stack gas could have 21 % Oxygen. 18. Stack gas could have 0 % Oxygen. 19. The percent O content of wet stack gas is always 2 smaller than the percent O content of “dry” stack gas. 2

Excess air in combustion Page 10

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True False Statement

20. A weak CO adsorbent solution will underestimate 2 the boiler efficiency.

21. A weak O adsorbent solution will overestimate the 2 boiler efficiency.

22. Too much excess air increases corrosion.

23. Too much excess air increases the flame temperature.

24. Too much excess air decreases the pressure in a boiler.

Explore the shape of the “Excess Air” curve. What happens, if CO 2

is very close to 0?

Hint: Take any “Excess Air” printout and extend the CO axis to the left. 2

Calculate the excess air for 1 %, 0.5 %, and 0.1 % CO.. 2

Take the equation for and set f=1

~CO2max = 11;； ?CO?2

Decrease CO in steps from 1 to 0.001% for CO =21% 2 2max

CO(%dry basis) 1 0.5 0.1 2

Excess Air(%)

Often the power consumption of the forced draft fan, and occasionally an additional induced draft fan of the boiler is not

known or data is not available. On the other hand the annual fuel

consumption and annual operating hours are fairly well known.

Use the equation

; pv P in Watt MF;

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