Paper id 26201420

International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637

Effect of Excess Air on 30 TPH AFBC Boiler on Dry Flue Gas Losses and its Efficiency Harish Ghritlahre1, Tej Pratap Singh2 Assistant Professor, Mechanical Engineering Department, Parthivi College of Engineering & Management, Bhilai-3, CG, India Email: harish.ghritlahre@gmail.com1, tejpratap50@yahoo.com2 Abstract— In the present work, effect of excess air on boiler efficiency by minimizing the dry flue gas losses has been

observed. There is stoichiometric amount of air required for complete combustion of fuel. In practice combustion conditions are never ideal and excess air must be supplied to completely burn the fuel. It has been found that maximum efficiency of boiler is obtained for the range of 20-40% of excess air. For the identical range of excess air the flue gas losses are also observed to be minimum. Further boiler maximum combustion zone is found between 3.5 to 5% of O2. In this condition boiler efficiency is 81.87% & 79.41% which is closer to the deigned efficiency and the flue gas temperature is 1340C and 1350C. Indirect method technique for calculating the dry flue gas losses.

Keywords— Boiler, Boiler combustion, Excess air, Efficiency Calculation.

1.

INTRODUCTION

Power plays a great role wherever man lives and works- in industries, agriculture transportation etc. Power provides our homes with light and heat. The living standard and prosperity of a nation very directly with increase in use of power. As technology is advancing the consumption of power is steadily rising [1] .The industrial sector uses about 50% of the total commercial energy available in India. Of the commercial sources of energy, coal, lignite, and oil and natural gas are mainly used. The Indian energy sector is highly energy intensive and efficiency is well below that of other industrialized countries. There is a growing need to bring about improvement in the efficiency of energy use in the industrial sector. More efficient energy use can increase productivity and economic competitiveness as well as lower greenhouse gas emissions per unit of output. There is considerable scope for improving energy efficiency in industries dealing with iron and steel, chemicals, cement, pulp and paper, fertilizers, textiles, dairy etc. If such industries can promote energy conservation. To meet the demand – supply gap in power, the way ahead clearly is to wipe- out rampant revenue leakage and routine theft in the state-dominated power sector, and to boost capacity addition in generation and supply. Concurrently, what is needed is improvement in thermal efficiency of power station, so as to generate more electricity without proportionate increase in fuel usage. Performance of the boiler, like efficiency and evaporation ratio reduces with time, due to poor combustion, heat Globally most fossil-fuelled electricity production is from coal (63%) and in India also, we have significantly higher share of coal fired fossil - fuelled electricity generation. As per Ministry of Power website [1], coal dominates the energy mix with 82% of total primary energy consumption.

Effect of variation in air on boiler combustion and its working has been found [2].

Due to poor combustion Performance of the boiler, like efficiency and evaporation ratio reduces with time was

reported [3]. It has also shown [3] that the deterioration of fuel quality leads to poor performance of boiler.

Richard Dolezal [4] has stated that, the intention in injecting more air into furnace than is theoretically necessary for the combustion of coal is to facilitate complete combustion of coal, because it allows sufficient oxygen to be present even at the end of the flame. It also states that, the injection in injecting more air into furnace. The quantity of excess air must therefore be selected so that the sum of losses due to unburnt combustion products is a minimum.

As per Bonk D. and Freier, M [5], the solid fuel injection system approach for a natural gas –like rapid response to load changes can be minized by improving air flow measurement and control.

In essence, it is uniformity of the burner-to-burner stoichiometries-not balanced pipe-to-pipe fuel distributions-that dictates combustion uniformity.

Combustion uniformity is the key to the best boiler efficiency as well as the lowest emissions. To achieve this uniformity, pipe–to-pipe fuel distribution as part of combustion optimization.[6]

2.

ANALYSIS OF EXCESS AIR

There is stoichiometric amount of air required for complete combustion of fuel. In practice combustion conditions are never ideal and additional or excess air must be supplied to completely burn the fuel. Excess air is an expression of how much more air is used for the combustion than strictly necessary. The calculation is based on the measured level of oxygen in the flue gases and the known concentration of oxygen in the air. Excess air is generally expressed as a percentage in the form of 20 % excess air. This means that a combustion process has 120 % of the air needed for complete theoretical combustion.

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International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637 Heat loss due to radiation and convection: L5= (0.548 x [(Ts / 55.55)4- (Ta / 55.55)4] + 1.957 x (Ts -Ta) 1.25 x sq.rt of [(196.85 Vm + 68.9) / 68.9] x 0.86 x HAS /7181 x GCV) x 100 (5) Where, Vm =Wind velocity in m/s Ts= Surface temperature (K) HAS= Heating surface ce area in m2 Heat loss due to unburnt in fly ash L6 = (Df/100) x (A/100) x (Cf/ (100-C (100 f)) x CV x 100 GCV Where, Df =Distribution of fly ash in percentage CV=Calorific Value of carbon Kcal/kg

Fig:- 1 schematic diagram of stoichiometric combustion tion

In practice, a certain level of excess air is needed in all cases to ensure complete combustion. This excess air is around 3 5 % for liquid and gaseous fuels and approximately 11 % for solid fuels, depending on the preparation of the fuel. Excess air ir is basically a factor to avoid. The unneeded air simply carries heat up the stack away from the burner and is wasted.

A fine line must always be drawn between too much excess air and incomplete combustion

(6)

Heat loss due to unburnt in bottom ash L7 = (Db/100) x (A/100) x (Cb/(100-C /(100 b)) x CV x 100 GCV Where, Db=Distribution of bottom ash in percentage

(7)

By the use of (1) equation we found the heat loss by dry flue gas of boiler.

3. GOVERNING EQUATION FOR ANALYSIS The Indirect Method: The principle losses that occur in a boiler are.

Heat loss due to dry flue gas L1 = m x Cp x (Tf - Ta) x 100 GCV of fuel Where, m = Mass of dry flue gas in kg/kg of fuel Cp= Specific heat of flue gas in kCal/kg°C Tf= Flue gas temperature in °C Ta = Ambient temperature in °C GCV = Gross Calorific Value Kcal/kg

(1)

4. ULTIMATE ANALYSIS OF COAL: AIR DRIED BASIS Raigarh region coal & Korba Region coal

Coal(Raigarh) Constituents

Percentage

Coal(Korba) Constituents

C H

32.57

C H

O

3.01 4.95

O

2.82 5.28

M

13.2

M

12.0

S

0.5

S

.52

N

1.87

N

1.10

A

43.9

A

42.4

GCV Kcal/kg

3250

GCV Kcal/kg

3366

Heat loss due to evaporation of water formed due to H2 in fuel L2 = (9 x H2 x {584 + Cp (Tf - Ta)} x 100 GCV of fuel (2) Where, H2=% of Hydrogen Cp= Specific heat of water in kCal/kg°C Heat loss due to moisture present in fuel L3 = (M x {584 + Cp (Tf -Ta)} x 100 GCV of fuel Where, M % of moisture Heat loss due to moisture present in air L4 = AAS x humidity factor x Cp x (Tf - Ta) x 100 GCV of Fuel Where, AAS =Actual mass of Air Supplied Kg/kg of coal Cp= Specific heat of Air in kCal/kg°C

(3)

(4)

Percent -age 35.87

Calculation of requirement of theoretical Amount of Air Air = ((11.43 x C) + (34.5 x (H--O/8)) + (4.32 x S)) / 100 Kg/Kg of coal (8) Where, C =% of fixed carbon 20

International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637 H =% of Hydrogen S =% of Sulphur O =% of Oxygen Calculation of excess air: % Excess Air = O2/(21-O2) Calculation of Actual mass of air supplied AAS = {1+ (EA /100)} x Theoretical Air

(9) (10)

Calculation of actual mass of dry flue gas Mass of dry Flue gas = Mass of CO2+ Mass of N2 in the fuel +Mass of N2 in the combustion air supplied + Mass of oxygen in flue gas (11)

Percentage of Boiler Losses

5 4 3 2 1 0

1

2

3 4

5

6 7

8

9

Various Heat Losses in Boiler

5. CALCULATION OF DRY FLUE GAS LOSSES AND BOILER When O2 is 2-8%, By Solving the Equation (1) to (11) by use of MATLAB 7.10.0 we get following results

Fig. 4 Korba region fuel is used

Dry flue gas loss

8

Effect of excess air on Boiler dry flue gas loss

6 4 2 0 2

3.5

4

2

1 2 3 4 5 6 7 8 9 Various Heat Losses in Boiler Fig. 5 Raigarh region fuel is used

Effect of execess air on boiler efficiency

Boiler Efficiency

4

0

5 6 6.5 7 8 Oxegen Percentage Fig.2

Korba region Raigarh region

84

Percentage of Boiler Losses

6

82 80 78 76

It observed that the increase in excess air leads the dry flue gas losses. The variation in fuel quality has been depicted in figure 3 & it is clear that for Korba region coal Boiler efficiency is higher than the Raigarh region coal for the same amount of air quality, for the above is observed to be the higher CV of coal at Korba compared with Raigarh region.

For both the fuel the maximum efficiency zone lies between 3.5 to 5% O2.

74 72 70 2

3.5

4

5

6

6.5

7

Oxegen percentage

Fig. 3

Various Losses are as shown in fig. at 5% of O2

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6. DRY FLUE GAS LOSSES REDUCTION METHODOLOGY The wet flue gas losses are mainly dependent on the quality of coal & they can not be controlled. Major controllable loss is Dry Flue Gas Losses. It depends on amount of excess air and flue gas exit temperature. Excess air reduces boilers efficiency by absorbing heat that would otherwise be transferred to the boiler water and carrying it up the stack.

Excess air can be measured with a flue gas analyser.

The 30 TPH boiler operate O2% between 3 to 5%.The maximum efficiency of that boiler is 81.87& 79.41%,when the Raigarh and Korba region coal is used respectively. In that condition the flue gas temperature is 134 oC and 135 oC respectively. In that condition the dry flue gas loss is 4.83% & 4.72%.

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International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637 Dry gas loss is directly affected by the temperature of the outgoing flue gases, as well as the excess air coefficient significantly. The recommended values of excess air is 20-40%.The reduction of flue gas outlet temperature however, would require extra investment for additional surfaces in air pre heater. It should also be remembered, that fuels containing sulphur should be dealt with carefully to avoid corrosion. Corrosion (due to sulphur in fuel) can also be minimised by special alloy steels for construction of last stage heat recovery surfaces. Thus the reduction of flue gas temperature (to increase the efficiency) would be largely a trade off between initial capital cost and revenue savings of fuels cost due to higher efficiency Effect of excess air on flue gas temperature

transfer down to approximately one twentieth of that through boiler steel of an equal thickness.

Deposits of waterside scale could be more dangerous than soot although both affect heat transfer. Soot prevents heat from reaching the boiler metal but scale prevents the heat build up occurs, e.g. the metal temperature rises. d) Firing rate The flue gas temperature from any appliance will increase with an increase in firing rate and vice versa. It therefore follows that to achieve maximum thermal efficiency the firing rate should be as low as possible. The maximum heat demand should be used as a guide to firing rates required rather than the size or rating of the appliance. This would apply particularly where the boiler or air heater is oversized for the maximum duty.

Flue gas Temperature

150 145 140 135 130 125 0.00%

20.00%

40.00%

60.00%

80.00%

e) Excess Air The flue gas temperature tends to increase with an increased excess air supply due to changes in the amount of heat transferred in the combustion zone.

The dry flue gas losses are maximum in any Boiler compared to other losses.

But it depends on the above mentioned parameters like, scaling, excess air, firing rate etc. Temperature of flue gas should be controlled should not be more than 140°C.There should be corresponding variation in the air flow rate as the load on the boiler varies.

Excess Air

Fig.6

7. CONCLUSION

Dry flue gas loss The two main factors which cause this loss are the amount of excess air used and the final gas temperature. The excess air used depends mainly on the standard of the firing equipment installed and to a lesser extent on the type of fuel used.

The dry flue gas loss may be calculated knowing how much excess air is used, as indicated by the percentage oxygen in the flue gas and the outlet flue gas temperature.

The final flue gas temperature depends on the following factors.

Boiler maximum combustion zone is found between 3.5 to 5% of O2 and in this condition boiler efficiency is 81.87% & 79.41% for Korba and Raigarh region fuel respectively which is approaches the deigned efficiency and in this condition the flue gas temperature is approximately 1350C. However Increase in excess air lowers the flame temperature results in lower heat transfer and it also enhances the flue gas losses. Dry flue gas losses can be controlled by excess air within a range of 20-40 %. This paper has been successful in its purpose of reducing the dry flue gas losses in boilers and increasing the efficiency of the boiler.

a) Design of Appliance.

The shape and areas of heating surfaces affect the amount of heat absorbed by the appliance and hence the overall optimum thermal efficiency.

REFERENCES

b) Operational Temperature or Pressure.

The higher the boiler water temperature the higher will be the flue gas leaving temperature i.e. the boiler water (or air in the case of space heaters) is being used to cool the heating surfaces and hence the warmer the cooling medium, the less cooling will be achieved. c) Cleanliness of Heat Exchange Surfaces.

Both the water and combustion sides of boiler metal should be reasonably free from deposits to get rapid transfer of heat from the flame to water. Soot is as good an insulator as asbestos and on the water side, scale formation will slow heat

[1] Ministry of power website –http://www.powermin.nic.in/ [2]http://www.brighthub.com/engineering/mechanical/articl es [3] National productivity council (Bureau of Energy Efficiency), Energy efficiency in Thermal Utilities for preparation of National Certificate Examination for Energy Managers & Energy Auditors.

[4] Richard Dolezal, Large Boiler furnaces, PP-79-84.,1980 [5] Bonk D. and Freier , M,1997, Assessment of Opportunities for Advanced Technology Repowering, Proceedings of the Advanced Coal Based and Environment System Conference, Pittsburgh,july 22-24 P3,1997.

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International Journal of Research in Advent Technology, Vol.2, No.6, June 2014 E-ISSN: 2321-9637 [6] Stabilizing Influence, Case Study about advanced combustion control and artificial intelligence in a cement plant� World cement Magazine, , R.Zwick, Maerker Cement, Harburg, Germany and Dirk Tiedtke, Powitec Intelligent Technologies 2/2003 [7] P.Chattopadhyay, Boiler Operation Engineering, Tata McGraw Hill Publication, 2008.

[8] ASME Standard:PTC-19-10 Performance Test Code for flue Gas and Exhaust Gas Analysis.,1981 [9] British Standards,BS845: 1987 [10] ASME Standard: PTC-4-1[4] Performance Test Code for Steam Generating Units.,1998 [11] Thermodynamics 5th Edition by Cengel and Boles,Book.

[12] P. K. Nag, Power Plant Engineering, Book,2nd Edition.

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