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Comparison Of Combustion Performance Between Natural Gas And Medium Fuel Oil At Different Firing Settings For Industrial Boilers

W.M. W.A. Najmi* and A.M. ArhosazaniFaculty of Mechanical EngineeringUniversity of Technology MARA

Malaysia

*wanjerm3@yahoo.com.my

Abstract

Most of Malaysian industrial boilers use low grade fuel oils such as the Medium Fuel Oil (MFO) as the energy source. Due to associated problems with emission, operating cost and maintenance cost, the need to change MFO to a cheaper and cleaner fuel is desirable. This paper presents part of a study on the feasibility of using natural gas fuel to replace MFO. Tests were done on an industrial boiler with a steam capacity of 16,000 kg/hour using different firing settings which are low, medium, and high firing settings. The discussion includes the fuel characteristics, emission, energy as well as combustion efficiency. For natural gases, the average combustion efficiency is 83.9% corresponding to 43.9 MJ/kg heat energy released. The optimum efficiency was 84.3% recorded at medium fire and 30.6% excess air. For MFO, the combustion efficiency decreases from low to high fire, with slight increases as the mode shifts from medium to high fire mode. The average combustion efficiency was 85.1%, corresponding to 77.8 MJ/kg heat energy. However, a more detail analysis is required especially in terms of fuel consumption cost due to the large difference in energy densities of both fuels.

Keywords : combustion, medium fuel oil, natural gas, emission, energy.

1. Introduction

This paper publishes part of the work for a production plant in West Malaysia that has 4 units of boilers running on medium fuel oil(MFO) since the 1960s. Recently, the industry recorded tremendous increase in boiler maintenance, fuel consumption, and overall operating cost for their boilers. This is also coupled with emission problems that did not conform to the limits set by the environment act. The management decided to convert the boiler fuel from MFO to another

fuel that would allow gains in cost-savings and better emission output.

Natural gas was identified as a suitable fuel for replacing the MFO. At the time of the work which was around the middle of 2006, the cost for one liter of MFO in the Malaysian market was RM 1.39. For NG, the price for one standard meter cube (Sm3) was RM 0.49. At a glance, natural gas were relatively cheaper. However, the cost of a fuel needs to be tied up with the consumption rate and combustion

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efficiency. Emission-wise, the combustion characteristics and reactions should also be analyzed to enable a collective decision to be made [1].

The boiler type being used are package boilers, which is a self contained unit complete with feed water pump, fuel pump, burner assembly, and combustion controls. The assembly takes a relatively short period of time, and then the boiler is ready for firing. Package boilers can be either a fire tube or water tube type. The only limitations of package boilers are the pounds of steam per hour capability and the size of the unit. Mostly, package boilers are suitable for low range to medium range steam production.The boilers in the study have a rated steam output of 16000 kg/hour at 20 bar delivery pressure.

To enable the analysis of combustion behavior for both fuel types, one of the boilers were fixed with a dual fire burner, capable of firing oil and gas fuels. The dual fire burner has manufacturer rated combustion efficiencies of 83% for oil and 80% for natural gas. For oil, the fuel flow capacity is in the range between 125 to 1007 kg/hour, and for gas, 123 to 1169 Sm3/hour.

2. Fuel Properties

Medium fuel oil is also known as residual fuel oil, and is produced from the distillation of crude oil. Residual fuel oils are viscous oils that require pre-heating before it is to be used. MFO is actually a blend of gasoil and heavy fuel oil, with less gasoil than intermediate fuel oil. The density of MFO at 15oC is 0.991 kg/liter, slightly heavier than

diesel which is at 0.72 kg/liter. The amount of sulfur is also high which leads to sulfur dioxide upon combustion. However, its undesirable properties makeit the cheapest available liquid fuel. Due to the required heating process, power plants and ships are the few industries capable of using residual fuel oils.

On the other hand, natural gas is a mixture of hydrocarbons and small quantities of various non-hydrocarbons existing in the gaseous phase. It is a clean, non-polluting fuel that requires no storage and is readily available in most areas. The main combustible constituent is methane with lesser amounts of ethane, propane, butane and pentane. Natural gas is very combustible and must be carefully controlled. The use of natural gas in practical combustors such as IC engines, boilers, and furnaces for energy production is widespread not only because of its environmental benefits, but also because of its high power production per unit of fuel mass. However, when compared to other fuels like diesel and gasoline, natural gas has proven difficult to ignite due to its high auto-ignition temperature. For example, natural gas will not ignite in compression ignition engines without the assistance of additives [2].

Table 1 summarizes the composition of MFO based on verified lab analysis while table 2 lists the main constituents of the natural gas as provided by Gas Malaysia. Table 3 is the specifications of both fuels as provided by the burner manufacturer(SAACKE).

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Table 1. Medium Fuel Oil Chemical Composition

Source : NALCO lab

Table 2. Composition of Natural Gas

Source : Gas Malaysia

Substance FormulaVolumetric Percent, %

Molar Mass of components,

kg/kmol

Aluminium Al 5.8 ppmBarium Ba 1.0 ppmChromium Cr 0.15 ppmCobalt Co 0.16 ppmCopper Cu 0.92 ppmIron Fe 39 ppmManganese Mn 12 ppmNickel Ni 33 ppmPotassium K 5.7 ppmStrontium Sr 0.27 ppmVanadium V 105 ppmZinc Zn 4.7 ppmCarbon C 86 10.32Hydrogen H2 10 0.1Nitrogen N2 1 0.28Sulfur S 3 0.96Total Molar Mass 11.66

Substance FormulaMolar Mass ,

kg/kmolVolumetric Percent, %

Molar Mass of component,

kg/kmolMethane CH4 16 92.74 14.84Ethane C2H6 30 4.07 1.22Propane C3H8 44 0.77 0.3388i-Butane C4H8 56 0.08 0.0448n-Butane C4H10 58 0.06 0.0348Nitrogen N2 28 0.45 0.126Carbon Dioxide CO2 44 1.83 0.8052

Total Molar Mass 17.4096

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Table 3. Fuel Specifications

Fuel Type Medium Fuel Oil Natural GasViscosity Cst @ 50o C 180 -Specific Gravity 0.98 0.61 @ 760 mm HgMinimum available Calorific Value (Gross)

41.8 MJ/kg 38.62 MJ/Sm3

Supply Pressure 3.45 bar 250 mbarDensity @15oC , kg/l 0.991 -Vapour Density - 0.747 kg/Sm3

Flash Point , oC 66 -187

3. Data Acquisition And Sampling

The testing of both fuels were done using three different fire settings or profiles – low fire, medium fire, and high firing profiles. The different profiles are achieved using different fuel and air settings. Each profile can be differentiated visibly by the flame intensity where high firing profile produces a brighter flame. Basically, natural gas combustion shows a mixture of blue and yellow flames, whereas combustion of MFO consists only of yellow and orange flames.

Measurements for analysis are taken directly using mobile gas analyzers as well as by periodic gas sampling. Manual and direct measurements using gas analyzers assisted by pressure gages and temperature

probes are required to gather the necessary information for analysis such as in Table 4.Gas sampling are done that would later be sent to an authorized lab for further analysis. All methods of measurement would refer to the US Environmental Protection Agency (USEPA) and local environment regulations complying to ISO 14001 Environment Management System Certification that would yield information on measuring particulate matters (USEPA Method 5), metal emissions (USEPA Method 29), NOx (USEPA Method 7E ) and smoke intensity (Ringelmann Chart).

Table 4. Sample of Data for Combustion Analysis

Profile O2 , %

CO, ppm

Eff, %

CO2 , %

Stack Temp,

°C

Excess Air , %

Steam Pressure ,

Bar

Gas Inlet Press, Bar

Gas Flow

Sm³ / hr m³ / hr

1 11 8 82 5.6 193 111.1 14.2 4.4 158 31.472 7.5 3 83.7 7.6 197 57.1 14.6 4.4 235 46.813 5.3 3 84.3 8.9 200 33.9 14.5 4.4 352 70.124 4.8 3 84.3 9.1 203 30.6 14.6 4.4 478 95.22

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4. Combustion Analysis And Dicussion

The combustion performance of both fuels are presented within the scope of :

1. Determination of the stoichiometric combustion reaction and air fuel ratio.

2. Determination of the theoretical heat energy release based on the first law of thermodynamics.

3. Graphs of combustion efficiency against flue temperature, combustion efficiency against excess air and actual energyagainst excess air for both fuels.

4. Comparing the stack gas emissions against standard permissible values.

4.1 Analysis for natural gas

Stoichiometric reaction for 1 kmol of fuel,

[0.9274 CH4 + 0.0407 C2H6 + 7.7 x 10 -3 C3H8 + 8 x 10 -4 C4H8 + 6 x 10 -4 C4H10 + 4.5 x 10 -3 N2 + 0.0183 CO2]+ 2.04 (O2 + 3.76 N2) → 1.0557 CO2 + 2.0137 H20 + 7.69 N2 (1)

The Air Fuel (AF) ratio is then determined to be 16.17 kg air per kg fuel. That is, 16.17 kg of air is theoretically required to completely burn each kilogram of fuel during thecombustion process.

The energy balance based for natural gas based on the first law of thermodynamics for reacting systems can be written as [3]

Q = HP - HR or where H = heat value

Q = ∑ Ni [ h0f + ∆h ]i - ∑ Nj [ h

0f + ∆h ]j

where i = product and j = reactant

N is the number of mole of substance involved in the reaction for one kmol of fuel

h0f is the enthalpy of formation for the

substance at standard reference state (25oC and 1 atm), and

∆h is the difference of enthalpy at a specified temperature of the substance compared to standard reference state condition

Therefore,

Q = 1.056 [ h0f + ∆h ]CO2 + 2.014 [ h0

f + ∆h ]H2O + 7.445 [ h0

f + ∆h ]N2 – 0.9273 [ h0f + ∆h ]CH4 – 0.0407 [ h0

f + ∆h ]C2H6 – 7.7 x 10 -3 [ h0

f + ∆h ]C3H8 – 8 x 10 -4 [h0f + ∆h

]C4H8 - 6 x 10 -4 [ h0f + ∆h ]C4H10 – 4.5 x 10-3 [ h0

f + ∆h ]N2 – 0.0183 [ h0

f + ∆h ]CO2 – 1.98 [ h0f + ∆h ]O2 – 1.98

(3.76) [ h0f + ∆h ]N2 (2)

Solving the equation by assuming the reactants and products are both returned to thestandard reference state yields the theoretical energy released during a complete combustion process of natural gas

Qtheo = - 910.3 MJ / kmol= - 52.28 MJ/kg

4.2 Analysis for MFO

Similarly for MFO, the stoichiometric reaction for 1 kmol of fuel,

[0.86 C + 0.1 H2 + 0.01 N2 + 0.03 S] + 0.91 [02 + 3.76N2] → 0.86 CO2 + 0.1 H20 + 3.43 N2 (3)

The AF ratio for MFO was calculated at 10.77 kg of air per kg of fuel. To determine the theoretical energy release,

Q = 0.86 [ h0f + ∆h ]CO2 + 0.1 [ h0

f + ∆h ]H2O + 3.43 [ h0f

+ ∆h ]N2 – 0.86 [ h0f + ∆h ]C – 0.1[ h0

f + ∆h ] H2 – 0.01 [ h0

f +∆h ]N2 – 0.3[ h0f + ∆h ]S - 0.91 [ h0

f + ∆h ]O2 – 0.91(3.76) [ h0

f + ∆h ]N2 (4)

Solving yields Qtheo = -91.38 MJ/kg.

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4.2 Combustion characteristics

(a) Combustion efficiency trend for natural gas (b) Combustion efficiency trend for MFO

Figure 1. Combustion efficiency of both fuels

Referring to (a), the flue gas temperature for natural gas from low fire to high fire profiles is between 200oC to 240oC. The combustion efficiency rises sharply as the burner reaches steady conditions, and it is apparent that the efficiency throughout the three firing profiles for natural gas are nearly constant in the range of 84.1% to 84.3%.The optimum combustion efficiency is recorded at 84.3%, achieved at the beginning of the medium fire profile with a flue temperature of about 2000C.

For MFO, the combustion efficiency decreases during low fire, and then shows slight increases in medium fire and high fire settings. The average efficiency was 85.1%. The minimum efficiency is at 84.2% at flue temperature of 2220C, occurring at the end of the low fire setting. The maximum efficiency of 87% is recorded at the beginning of the low fire where the air-fuel mixture was probably balanced, with a flue gas temperature of 172oC.

(a) Combustion efficiency trend with increasing excess air

Actual Energy,MJ/kg against Excess Air,%

42.8

43

43.2

43.4

43.6

43.8

44

44.2

0 20 40 60 80 100 120

Excess Air,%

Actu

al

En

erg

y,M

J/k

g

Low Fire Medium Fire High Fire

Low FireMedium FireHigh Fire

Low Fire Medium Fire High Fire

High Fire Medium Fire Low Fire

(b) Energy release with increasing excess air

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Figure 2. Natural gas combustion efficiency, excess air and energy relationshipThe graph of natural gas combustion efficiency against excess air shows that the amount of excess air from low fire to high fire decreases, because more fuel (fuel rich) is available in high fire settings, but thecombustion efficiency increases about 3%.This means that burning natural gas at medium fire to high fire settings gives the optimum efficiency, at about 84.3%. Low excess air availability in medium fire to high fire settings explains why the flue temperature increases gradually to nearly 240oC due to the lack of air for effective flue gas cooling. The combustion efficiency hovers around the optimum 84% mark during medium to high fire, indicating that excess air supply during

these fire settings should not be less than 20% to maintain a good efficiency.

The actual thermal energy released during the firing process is related to the theoretical energy (52.28 MJ/kg) and the calculated combustion efficiency. It is no surprise that plotting the energy output against excess air yields a similar trend with the graph of combustion efficiency against excess air. The highest energy released is 44.07 MJ/kg, released at the highest combustion efficiency (84.3%) and excess air of 30.6%. The average energy value throughout the whole firing process was calculated at 43.9 MJ/kg.

Combustion Efficiency,% against Secondary Air,%

84

84.5

85

85.5

86

86.5

87

87.5

0 10 20 30 40 50

Secondary Air,%

Eff

icie

ncy

,%

(a) Combustion efficiency trend with increasing secondary air

Figure 3. MFO combustion efficiency, secondary air and energy relationship

The graph of combustion efficiency against secondary air can give the reason on the dropof combustion efficiency during low fire. As the secondary air feed is increased from 15% to 30%, the mixture strength changes and becomes fuel lean, causing combustion rates to decrease and the efficiency to drop. The efficiency increases again during the medium and low firing profiles as the mixture strength increases with more available fuel. It can be observed that for low fire, the secondary air

should only be limited to a maximum of 20% to acquire good combustion efficiencies. It is also observed that secondary air for medium fire should be less than 36%. In terms of energy released, the average energy value is calculated at 77.8 MJ/kg.

Table 5 summarizes the important parameters between natural gas and MFO combustion. Comparatively, MFO is better than natural gas

Low Fire Medium Fire High Fire High FireMedium FireLow Fire

(b) Energy release with increasing excess air

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in term of combustion efficiency and energy produced based on 1 kg fuel. Table 5. Summary of Combustion Parameters between Medium Fuel Oil and Natural Gas

FuelActual Energy, MJ/kg Combustion Efficiency,%

TheroreticalPractical average

By Manufacturer

Practical average

Medium Fuel oil

91.38 77.8 83 85.1

Natural Gas 52.28 43.9 80 83.9

4.3 Emission analysis

Carbon dioxides concentration in a flue gas is an indication of the combustion efficiency. As more carbon within the fuel or subsequent gaseous products is completely oxidized, CO2

will be detected in the flue gases. For MFO, low CO2 concentration even at high combustion efficiency at the beginning of the firing (low fire) is due to the lack of available fuel within the mixture. The combustion efficiency is high at this stage due to the

higher availability of oxygen in the combustion air compared to the amount of fuel in the mixture. However, the CO2

emission is only within the range of 11.3% to 12% due to the lack of fuel availability. As more fuel is introduced in the firing settings, more CO2 are formed, reaching as high as 13.5% at high fire. For the whole firing process, the average CO2 emission of medium fuel oil is 12.34%.

(a) MFO (b) Natural Gas

Figure 4. CO2 emission trend for both fuels

Natural gas combustion produces less carbon dioxide than MFO, at an average value of 9%. Contrary to MFO emission pattern, natural gas exhibits a linear relationship between CO2

emission and combustion efficiency during

low fire, and the emission increases steadily at near constant combustion efficiency (around 84%). In theory, for an equivalent amount of heat, natural gas burning produces about 30%

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less carbon dioxide than burning oil products [4].

For carbon monoxide emission, MFO combustion records an average value of 7.4 ppm, while for natural gas the average is 7 ppm. Very low CO readings indicate an excellent combustion process was capably achieved by the dual fire burner for both fuels, and the burner type can be fully recommended for this application.

Stack gas emission for both fuels were alsocollected. There are two stacks at the plant, and so both stack gases are collected and analyzed. The comparison between medium fuel oil and natural gas emission in terms of total suspended particulate and oxides of nitrogen concentration, NOx, are presented in tables 6 and 7.

Table 6. Stack Gas Emission for Medium Fuel Oil

Method Parameter UnitDetection

LimitSTD C Limit

ResultsStack 1 Stack 2

USEPA Method 5Total Suspended Particulate @Dust

gm/Nm3 0.0001 0.4 0.0506 0.0358

USEPA Method 7E NOx gm/Nm3 0.01 2 0.28 1.06

Table 7. Stack Gas Emission for Natural Gas

Method Parameter UnitDetection

LimitSTD C Limit

ResultsStack 1 Stack 2

USEPA Method 5Total Suspended Particulate @Dust

gm/Nm3 0.0001 0.4less than 0.0001

less than 0.0001

USEPA Method 7E NOx gm/Nm3 0.01 2less than

0.01less than

0.01

Medium fuel oil produced 0.0506 gm/Nm3 for stack 1 and 0.0358 gm/Nm3 for stack of total suspended particulate and 0.28 gm/Nm3 for stack 1 and 1.06 gm/Nm3 for stack 2 of nitrogen oxides. The values are still below the Standard C of USEPA limit that complies with air cleanliness regulations. For natural gas,both the dust and NOx is too small to be of significant, meaning a cleaner combustion was achieved compared to MFO firing. The NOx

emission for natural gas is also below the simulated value for NOx production in an industrial boiler, which was estimated at around 0.07% volume for gas temperatures in the range of 400K to 500K [5].

5. Conclusion

The need to change MFO to natural gas fuel was imminent for the boiler operator, but justification based on numerous factors such as economical as well as performance has to be considered before the final decision was made. From the analysis presented, the combustion behavior of both fuels iscompared mainly in terms of combustion efficiency as well as emission. It is discovered that MFO burning using the dual burner is slightly more efficient, averaging at 85.1% compared to 83.9% for natural gas, and

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produces higher practical heat energy per kg of fuel, averaging at 77.8 MJ/kg compared to 48.9 MJ/kg for natural gas. However, on the emission side, burning natural gas yields a very clean flue gas compared to MFO. Therefore, it is inadequate to replace MFO with natural gas solely based on this combustion analysis, but that decision requires an integrated approach by also considering the costs involved, especially the fuel consumption cost, as it is known that natural gas has lower energy densities which may lead to higher consumption rate.

References

[1] P Chattapadhyay, Boiler Operation Engineering: Questions and Answers,Tata McGraw-Hill Publishing Company Limited, New Delhi. TJ288 C44 1994

[2] J.A. Langille et.al., The use of OCM for ignition enhancement of natural gas

combustion for practical applications : Reactor design aspects, Chemical Engineering Science, Vol 61 Issue 20, pp 6637 - 6645, 2006.

[3] Y. A. Cengel and M. A. Boles, Thermodynamics: An Engineering Approach, Fourth Edition. McGraw Hill, 2002.

[4] A. R. Azhan, A. Rahim & W.M.W.A.Najmi, Power Output Analysis of an Internal Combustion Engine with Variable Ignition Strength and Timing,Proceedings of the Malaysian National Seminar on Advances in Mechanical Engineering (NAME) 2005, pp 55-64.

[5] M.A.Habib, M. Elshafei and M.Dajani, Influence of combustion parameters on NOx production in an industrial boiler, Computers & Fluids, Vol 37, Issue 1, pp 12-23, 2008.