improving the efficiency of low-pressured industrial ...€¦ · industrial boiler systems using...

Improving the efficiency of low-pressured industrial boiler systems using exergy analysis

M. P. Maraj Department of Petroleum Engineering, University of Trinidad and Tobago, Trinidad and Tobago

Abstract

Much of the literature surrounding the exergetic evaluation of boilers has focused primarily on large capacity and high-pressured systems, the majority of which are found in power plants. In Trinidad and Tobago however, approximately 76% of the boilers used are small low-pressured systems. This paper looks specifically at evaluating the exergetic performance of one such low-pressured boiler. The results of this evaluation confirm the large differences, described in the literature, that exist between boiler first and second law efficiencies and show that the combustion and heat transfer processes are responsible for these low exergetic efficiencies by consuming approximately 70% of the fuel’s entering exergy. The amount of excess air used for combustion significantly affects boiler performance. During this study, it was found that most of these smaller boilers were not equipped with excess air control devices and as such, excess air amounts varied significantly and was often much higher than the recommended 15%. It was also found that for every 20% decrease in excess air, combustion irreversibilities could be lowered by approximately 2–3%. Furthermore if the boiler was operated with the recommended 15% excess air, simulation shows that increasing the inlet combustion air temperatures using waste heat from the boiler’s stack could lead to a 7% reduction in combustion irreversibilities. While any improvement in boiler performance may have favourable consequences, these results indicate that dramatic decreases in boiler irreversibilities cannot be expected through air preheating and combustion control strategies and suggest that designs which minimize combustion in boiler systems should be investigated. Keywords: boiler, combustion, heat transfer, exergy analysis, irreversibilities.

Energy and Sustainability II 423

www.witpress.com, ISSN 1743-3541 (on-line)

© 2009 WIT PressWIT Transactions on Ecology and the Environment, Vol 121,

doi:10.2495/ESU090381

1 Introduction

The rapid depletion rate of non-renewable energy resources has spurred the concept of effective energy use. Boiler systems are large consumers of energy and as such it becomes necessary in the long term to investigate the effectiveness of energy use within these systems. Exergy analysis is a key tool for this purpose as it can identify and measure sources of inefficiency associated with industrial plants and their components. Figure 1 shows the boiler usage in Trinidad and Tobago for the year 2006 arranged by specific pressure ranges. These pressure ranges are important as they are a reflection of process requirements. Power plant boiler systems generally operate at high pressures normally within the range of 600 to 1500 psi (4.1–10.3 MPa). Figure 1 shows that only 7% of the total number of boilers used in Trinidad and Tobago operates within this range. It also shows that the largest percentage (76%) of boiler systems operate between 100–350 psi (0.6–2.4 MPa). These are the boilers used for food production and processing, commodity manufacturing and medical and hospitality applications. These processes will be collectively referred to as manufacturing processes in this paper. Much of the literature surroundings the exergetic evaluation of boilers however, has focused primarily on large capacity and high-pressured boiler installations, the majority of which are found in power plants. A subset of this literature can be found in studies conducted by Aljundi [1], El-Dib [2], Fungtammen [3], Gaggioli [4], Gorji-Bandpy and Ebrahimian [5], Kanoglu et al. [6], Scuibba and Su [7], Tang and Rosen [8] and Tsatsaronis and Winhold [9]. The results of such studies show that a large disparity exists between boiler first and second law efficiencies. The major reason for this discrepancy is that a first law analysis neglects to take into consideration any energy dissipations due to the combustion and heat transfer processes taking place within the boiler. These processes are highly irreversible processes account for approximately 60–70% of fuel’s entering exergy being destroyed.

5%

76%

3% 7% 2% 7%

0%

10%

20%

30%

40%

50%

60%

70%

80% Below 100 psi100 - 350 psi350 - 600 psi600 - 1500 psiAbove 1500 psiUnknown

Figure 1: The distribution of boilers operating within Trinidad and Tobago

in 2006 according to specified pressure ranges.

Similar studies involving boiler systems operating at lower pressures such as those found in manufacturing facilities are few [10, 11] but it is important that

424 Energy and Sustainability II



Table 1: The mole fractions of the components comprising the environment.

Gas Phase Substance Mole Fraction, oχ N2 0.7567 O2 0.2035 H2O 0.0303 CO2 0.0003 Other 0.0062

such studies be undertaken especially in Trinidad and Tobago given the large percentage of such boilers in use.

2 Basic concepts

2.1 Exergy

The concept of exergy is a direct outcome of the second law of thermodynamics. The exergy of a system is defined as the maximum work obtainable when the system of interest is brought to equilibrium with a reference environment. In the absence of nuclear, magnetic, electrical and surface tension effects, the total exergy flow rate of a flowing stream ‘x’ can be divided into four components: physical exergy phA , kinetic exergy ktA ,potential exergy ptA and the chemical exergy chA [12] as given by eqn. (1).

chx

ptx

ktx

phxx AAAAA +++= (1)

The sum of the kinetic, potential and physical exergies is referred to as the thermo-mechanical exergy. When it is assumed that the system under consideration is at rest mechanically with the environment, the potential and kinetic exergies are assumed to be negligible. The physical exergy is associated with the temperature and pressure of the flowing stream with respect to the temperature and pressure of the environment. The chemical exergy of a steam is related to the chemical composition of the stream as regards the composition of the environment and is the exergy required to bring the stream into chemical equilibrium with the environment. The mole fractions of the individual constituents of the environment [12] are given in table 1. The physical and chemical exergies of a flowing stream ‘x’ can be expressed as follows:

( )[ ])( oxooxphx ssThhmA −−−= (2)

ox

xxo

chx TRmA

χρχ ln

−= (3)




2.2 Irreversibility

The major causes of inefficiency in thermal and chemical processes are as a result of irreversible heat transfer, throttling, adiabatic combustion, friction and mixing [12–14]. These are not associated with any energy loss, but lead to a degradation of the energy quality, which is detected through an exergy analysis. The irreversibility associated with thermal systems can be evaluated by performing an exergy balance on the system. Assuming steady state, steady flow, this exergy balance gives:

( ) IamamWQTT

transfersexergyofrates

eeiicvjb

o −−+−

−= ∑ ∑∑

...

10 (4)

2.3 Exergetic efficiency

According to Tsatsaronis [15], a fuel and a product for the system being analyzed must be identified. The product represents the net useful or desired result produced in the system. The fuel represents the net resources which were spent to generate the product. For a system ‘X’, let:

XPA , = exergy content of the (net) product

XFA , = exergy content of the (net) fuel

XLA , = exergy losses

XI = irreversibility (rate of exergy destruction) The relationship between the four is given below:

XXLXPXF IAAA ++= ,,, (5)

The exergetic efficiency is then defined as:

+−==

XF

XL

XF

XP

AAI

AA

,

,

,

, 1ε (6)

3 An exergetic review of boiler systems and associated irreversibilities

Gaggioli [4] investigated the boiler of a coal-fired steam plant producing superheated steam at 775K, 850 psi (5.84 MPa) and found the first and second law efficiencies for the boiler to be 85% and 34% respectively. Further work to investigate the cause of irreversibility within the boiler showed that the combustion and heat transfer processes accounted for 26% and 34%,




respectively, of the exergy destroyed in the boiler. Exergy losses associated with the stack gases accounted for only 5.5% of the fuel’s entering exergy. Tang and Rosen [8] investigated a coal fired steam power plant in which the boiler produced steam at 2320 psi (16 MPa) and 540oC. The irreversibilities attributable to combustion and heat transfer were 43.6% and 40.3% respectively. Cortez [10] investigated the performance of sugarcane bagasse boilers and found the first and second law efficiencies for the boiler to be of the order of 84% and 28% respectively, with roughly 70% of the fuel’s exergy being consumed by the irreversibilities of the combustion and heat transfer processes. The above results highlight that significant potential exists for plant improvement by further investigating the combustion and heat transfer processes taking place within the boiler and modifying these where applicable. Boiler efficiencies are conventionally evaluated using a first law approach. These efficiencies indicate how much thermal energy is contained in the working fluid passing through the boiler from the stored chemical energy of the fuel. The losses that account for a decrease in this efficiency stem from un-burnt fuel, incomplete combustion, evaporation of water formed during combustion, the remaining heat content of the stack gases leaving the boiler (stack gas loss) and from radiation losses from the surface of the boiler. Combustion is the rapid oxidation of the fuel to produce heat energy. It is a complex phenomenon dependant upon a variety of factors. The second law analysis on boiler systems have shown that the combustion process can account for 15–45% of the fuel’s entering exergy [2, 4, 8, 11]. This irreversibility is present in, and has a significant impact in all combustion based energy conversion systems, such as heating and industrial furnaces, internal combustion engines and boiler combustion chambers. Excess air levels, incomplete fuel oxidation, firing rates, inlet air and flame temperatures are among the factors that affect the combustion process during regular boiler operation. Adiabatic flame temperatures are the hypothetical temperatures of the hot gases produced directly after the combustion process prior to any heat transfer. It is established that increasing adiabatic temperatures reduces combustion irreversibility as a result of the larger temperatures produced [16, 17]. Adiabatic temperatures are therefore important factors as they directly influence fuel consumption. It is generally known that preheating combustion air and reducing air fuel ratios can increase adiabatic temperatures [11, 17, 18]. Applying these methods to existing boiler installations can therefore provide opportunities for effective energy use through the minimization of combustion irreversibility. However, the practical applications of these approaches can only be realized through an economic study. Heat transfer within the boiler is defined as the process of transferring thermal energy from the hot combustion gases (at the adiabatic condition) to the working fluid. The irreversibility associated with the heat transfer process accounts for approximately 20–45% [2–4, 8, 10] of the fuel’s exergy. The major cause of this irreversibility is attributed to the temperature difference between the two streams.




During the heat transfer process, the temperature of the combustion gases decreases while the temperature of the working fluid increases. The temperature profiles of the two streams do not match because of different heat capacities and because of the intermediate phase change from water to steam. Reduction in the stream-to-stream temperature difference can be obtained using the pinch point method of analysis [12], which can result in reduced irreversibility and operating costs.

4 Boiler system investigated

This study examines the performance of a natural gas-fired water-tube boiler referred to hereafter as Boiler A. While boiler operation is dynamic in nature, the standard operating condition for any boiler constitutes what is known or regarded as ‘normal’ boiler operation. During ‘normal’ operation, Boiler A operated at 210 psi (1.55 MPa) and produced steam at a constant load of 5500 kg/hr at the saturated condition. At these conditions, the amount of excess air used for combustion generally resonated at around 140%. During this study, however the excess air to the boiler system was varied between 50–250%. The temperature of the combustion air was assumed to be constant at the ambient condition of 25oC during the variation of the excess air amounts. A zonal approach was applied to the boiler system which involved separating the boiler into an adiabatic combustor and a heat exchanger component, fig. 2. The combustor is responsible for the combustion of the fuel which produces hot combustion gases at the adiabatic temperature. The heat exchanger component facilitates heat transfer from the combustion gases to the working fluid.

FU RN ACE / CO M B USTO R

H XFUEL

AIR FLU E G ASES

STEAM

FEED W ATER

H O T CO M BUSTIO N

G ASES

Figure 2: The zonal approach applied to the boiler system.

5 Results and discussion

The results of this study show that the amount of excess air used for combustion is a primary operating variable that significantly affects boiler performance. Boiler A is not equipped with an excess air control device and as such, the excess air amount during varies significantly during regular operation and is often much higher than the optimum amount.




Optimum excess air is that percentage which gives the best compromise between incomplete combustion and excessive stack gas heat loss. It is generally recommended that boilers be operated with 15% excess air for best performance [19, 20]. Figure 3 shows the measured values of first law (energy) efficiency and second law (exergy) efficiency for Boiler A and their relationship with excess air. The figure confirms the large differences that exist between boiler first and second law efficiencies, where these differences are of the same order as those quoted in the literature. The major reason for this discrepancy is that a first law analysis neglects to take into consideration the irreversibility arising from the combustion and heat transfer processes. Boiler operation with the recommended 15% excess air amount can be achieved using an appropriate excess air control device such as an oxygen trim system (OTS). Regression analysis on figure 3 indicates that had the boiler been operated with 15% excess air, the expected first and second law efficiencies would be of the order of 90% and 35% respectively. While any attempt at improving boiler performance has favourable consequences, the above numbers indicate that incorporating an excess air control device only moderately improves first law efficiency (which itself is very high) and more significantly illustrates that dramatic improvements in exergetic efficiency cannot be achieved using such control techniques. Operating the boiler at the recommended 15% excess air achieves only a moderate increase in second law efficiency from 32% (at the standard operating condition) to 35%.

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0 20 40 60 80 100 120 140 160 180 200 220 240

Excess Air Percentage [%]

Effic

ienc

y [%

]

First Law Efficiency Second Law Efficiency

15% Optimum

Excess Air Value

Figure 3: Boiler efficiency versus excess air percentage.

Figure 4 shows how the fuel’s energy and exergy content are distributed at the standard operating condition. The diagram illustrates that stack gas losses are the largest energy loss from the boiler accounting for approximately 15% of the total’s fuel energy content. Other energy losses associated with boiler operation include radiation from the boiler surface, loss due to incomplete combustion, loss due to evaporation of hydrogen formed water and loss due to




FIRST LAW ANALYSIS

First Law Efficiency [%]Stack Gas Energy Loss [ % ]Loss due to Moisture in Air [ % ]Loss due to Evaporation of H2 formed H2O [ % ]Loss due to Radiation [ % ]

SECOND LAW ANALYSIS

Second Law Efficiency [%] Combustion Irreversibility [%]Heat Transfer Irreversibility [%] Stack Gas Exergy Loss [%]

Figure 4: Energy and exergy losses at the standard operating condition.

moisture in the inlet air. These are much smaller than stack gas energy loss with these combined losses tallying to roughly 3% of total energy supplied. In terms of exergy, figure 4 shows that the stack gas exergy losses while not insignificant represent only about 5% of the fuel’s exergetic content. This diagram also highlights the difference in magnitudes between stack gas exergy losses and the corresponding irreversibilities due to the combustion and heat transfer processes. This indicates that heat recovery from the stack gases might be thermodynamically and economically justified from the point of view of minimizing energy losses but not necessarily exergy losses. However, no cost effective opportunities to reduce boiler losses should be overlooked, but more often than none the best opportunities exist where the larger irreversibilities or losses occur. These large irreversibilities associated with boiler operation are as a result of the exergy consumed during the combustion and heat transfer processes. For the range of excess air investigated, figure 5 shows that the combustion irreversibility represents 34 to 52% of the fuel’s entering exergy while the heat transfer irreversibility is of the order of 12 to 28%. The increase in combustion irreversibility with increasing excess air can be explained by considering that with increasing excess air values, adiabatic flame temperatures are reduced as a larger amount of air will produce increasing amounts of flue gas which absorbs valuable heat. Figure 5 also shows that with every 20% decrease in excess air combustion irreversibilities can be decreased by approximately 2 to 3%. The major cause of irreversibility in the heat transfer process is due to the temperature difference between the two streams where it is known that this temperature difference is a measure of irreversibility which increases as the temperature difference increases. Figure 5 shows that with increasing excess air, heat transfer irreversibilities decrease. This is expected given that with increasing excess air, the temperature of the flue gas (at the adiabatic condition) entering the heat exchanger component of the boiler decreases. This decrease allows the temperature profiles




0

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0 20 40 60 80 100 120 140 160 180 200 220 240

EXCESS AIR [%]

IRR

EVER

SIB

ILIT

Y [%

]

Combustion Irreversibility [%] Heat Transfer Irreversibility [%]

Optimum 15% Excess Air

Figure 5: Combustion and heat transfer irreversibilities.

of the flue gas and feed-water streams to become better matched thus reducing the irreversibility associated with the process. The total boiler irreversibility associated with boiler operation can be calculated by summing the individual combustion and heat transfer irreversibilities. Figure 5 also indicates that if the boiler were operated at the recommended 15% excess air then the corresponding combustion and heat transfer irreversibilities would be 30% and 31% respectively. Even at this optimum amount, these numbers indicate that the total boiler irreversibility is still high and further suggest that no dramatic reduction in irreversibility can be expected through the use of excess air control devices. It can be seen from figure 6 that the total boiler irreversibility for boiler A is a minimum at low excess air values which may imply that it is therefore advisable to operate the boiler with a minimum of excess air to reduce overall irreversibilities. While combustion irreversibility decreases with reduced excess air, this irreversibility (even at the optimum 15% excess air) is still high being of the order of 30%. Opportunities to reduce these irreversibilities must therefore be investigated. One way to accomplish this can come from preheating the inlet air used for combustion. Preheating inlet air is known to increase adiabatic temperatures which in turn reduce combustion irreversibilities. While an air pre-heater was not installed on the boiler system being studied and it was not possible during the study to actually heat the incoming air, figure 7 simulates the effect of air preheating on combustion irreversibility. The temperatures investigated range from just above ambient conditions to 525oC. Simulating the effect of installing an appropriately sized air pre-heater (which preheats the inlet air using the exhaust flue gases from the boiler system)




produced moderate air temperatures between 125–150oC. At such elevated temperatures with an excess air amount of 15%, combustion irreversibility was of the order of 23%. This indicates that only a modest 7% reduction in combustion irreversibility is achieved when using an air pre-heater with the recommended 15% excess air amount. Greater reductions in this irreversibility are indeed possible if air temperatures are further increased as shown in figure 7. It must be stated however that achieving such high temperatures may not be practical or cost effective for small boiler systems.

60.0

60.5

61.0

61.5

62.0

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65.0

0 20 40 60 80 100 120 140 160 180 200 220 240

Excess Air Percentage [%]

Tota

l Boi

ler I

rrev

ersi

bilit

y [%

]

Figure 6: Total boiler irreversibility versus excess air.

Figure 7: The effect of air preheating on combustion irreversibility.

0

5

10

15

20

25

30

35

40

0 50 100 150 200 250 300 350 400 450 500 550

Inlet Air Temperature [oC]

Com

bust

ion

Irrev

ersi

bilit

y [%

]

15% 30% 50%

15% XS air

30% XS air

50% XS air




6 Conclusions and future implications

Combustion is intrinsically a very significant source of irreversibility and the results above show that a significant reduction by conventional means such as air preheating and combustion air control (in which optimum excess air levels are maintained) cannot be expected. The reasons underlying this result are described by Dunbar and Lior [21] who investigated a well-insulated combustion chamber with the aim of identifying and quantifying the contribution of the sub-processes taking place during the combustion process to the combustion irreversibility. They identified the sub-processes as being: (i) reactant diffusion and fuel oxidation, (ii) internal thermal energy interchange (heat transfer between gas constituents) and (iii) product mixing. Results from their studies show that the combustion irreversibility accounts for roughly a third of the fuel’s entering exergy and that approximately ¾ of this irreversibility occurs during the internal thermal energy exchange sub-process. This heat transfer element of the combustion process cannot be eliminated and therefore the irreversibility associated with it, will always be present during the combustion of fuel to produce thermal energy. As such, the use of combustion should therefore be minimized or altogether avoided in the design of new equipment. For existing equipment like boiler systems, the combustion process can be enhanced through the use of fuel cells. In the fuel cell, oxidation of the fuel by direct contact between fuel and air (oxygen) is prevented. When a fuel is burnt in air, the driving force for the reaction is the difference between the chemical potentials of the reactant and the products. This difference is the chemical affinity of the reaction. Fuel cells lower the reaction affinity by first passing appropriate oxygen carrier ions through an electrolyte. After passing through an electrolyte, the fuel oxidation is less dissipative and less irreversible as the driving force for the reaction is reduced and the repositioning of the associated electrons is achieved with greater control than in conventional combustion. Investigation into designs which combine fuel cells and boiler systems of course are subject to economic and thermodynamic feasibility studies as the cost and scale of operation of the fuel cells may not be well suited for small boiler operation. Additionally, the use of fuel cells still suffers from technical and economic drawbacks, which have hindered a wider commercial application.

References

[1] Aljundi, I., Energy and Exergy Analysis of a Steam Power Plant in Jordan. Applied Thermal Engineering, 29(2/3), pp. 324–328, 2009.

[2] El-Dib, A.F. (1998) An Exergy Analysis of Steam Power And Co-Generation Plants. Journal of Engineering and Applied Science, Vol. 45

[3] Fungtammen, B., A computer program for energy and availability analyses of thermal power plants. International Conference on Energy and Environment, November, 1990.




[4] Gaggioli, R.A. (eds). Efficiency and Costing: Second Law Analysis of Processes, American Chemical Society ACS Symposium Series 235: Washington, 1983.

[5] Gorji-Bandpy, M. & Ebrahimian, V., Exergy Analysis of a Steam Power Plant: a case study in Iran. International Journal of Exergy, 4 (1), pp. 54–73, 2007.

[6] Kanoglu, M., Dincer, I. & Rosen, M., Understanding energy and exergy efficiencies for improved energy management in power plants. Energy Policy, 35 (7), pp. 3967–3978, 2007.

[7] Scuibba, E. & Su. T.M., Second Law Analysis of the Steam Turbine Power Cycle: A Parametric Study. Computer Aided Engineering of Energy Systems Vol. 3 -Second Law Analysis and Modelling, ed. R. A. Gaggioli,, ASME Publications: California, 1986.

[8] Tang, R. & Rosen, M.A., Assessing and Improving the Efficiencies of a Steam Power Plant using Exergy Analysis: Part 1-Assessment. Proc. CSME Forum 2000, Quebec, 2000.

[9] Tsatsaronis, G. & Winhold, M., Exergoeconomic Analysis and Evaluation of Energy Conversion Plants-II. Analysis of a coal fired Steam Power Plant. Energy 10(1), 1985.

[10] Cortez, L.A.B., A Method for exergy Analysis of Sugarcane Bagasse Boilers. Brazilian Journal of Chemical Engineering, March, 1998.

[11] Maraj. M. & So’Brien G.C., Assessing the Performance of a Industrial Boiler System Using Exergy Analysis, Proc. of the 16th Annual Association of Professional Engineers of Trinidad and Tobago Conf., UWI Press: St. Augustine, 2003.

[12] Bejan, A., Tsatsaronis, G. & Moran, M., Thermal Design and Optimization, John Wiley & Sons: New York, 1996.

[13] Kotas, T.J., The Exergy Method of Thermal Plant Analysis, Butterworths: London, 1985.

[14] van Gool, W., Exergy Analysis of Industrial Processes. Energy, 17(8), 1992.

[15] Tsatsaronis, G., Thermoeconomic Analysis and Optimization of Energy Systems, Energy 18(2), 1993.

[16] Geskin, E., Second Law Analysis of Fuel Consumption in Furnaces, Energy 5, 1980.

[17] Kawai, K., High temperature air combustion boiler for low BTU gas, Energy Conversion and Management, 43, 2002.

[18] Szargut, J., Energy and Exergy Analysis of the preheating of combustion reactants, International Journal of Energy Research, 12, 1988.

[19] Boiler Efficiency Improvement Opportunities; The Natural Gas Boiler Burner Consortium, Online. http://www.energysolutionscenter.org /BoilerBurner/Eff_Improve/Index/Index_Boiler_Eff_Start.

[20] CIBO Energy Efficiency Handbook of The Council of Industrial Boiler Owners (CIBO), Online. http://cibo.org/pubs/steamhandbook.pdf

[21] Dunbar, W & Lior, N., Sources of Combustion Irreversibility, Combustion Science and Technology, 103, 1994.




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