EFFICIENCY OF STEAM GENERATOR ECOMOMIZER AT DIFFERENT LOADS

ION DOSA1, BOGDAN TOMUȘ2

Abstract: The economizer of a steam boiler is an important component that influences its efficiency. The link between steam boiler efficiency and economizer efficiency is analyzed, by computing some of the quality indicators of heat exchangers like effectiveness, thermal efficiency and gross thermal efficiency, for different boiler loads.

Key words: economizer, heat exchanger effectiveness, thermal efficiency, overall heat transfer coefficient

1. INTRODUCTION

Heating the feed water is the main purpose of economizers and this happens at the expense of the heat contained in flue gases. Some economizers, referred as steaming economizers not only heat the water but turns up to 20% by weight into steam [1].

Because the economizer operates at a higher mean temperature difference than a boiler surface arranged in the same pass it is convenient to replace the heat-recovery surfaces of a boiler with the economizer surfaces. This is a result of that the mean water temperature in the economizer is lower than the temperature of the boiler water which is equal to the saturation temperature at a given pressure. At the same temperature drop in the flue gas, the heating surface of the economizer is therefore significantly smaller than the boiler heating surface.

Economizers are made of cast-iron or steel tubes. The horizontally laid tubes of the economizer are interconnected by means of external cast-iron U-bends. Cast-iron economizers are resistant to corrosion and comparatively cheaper than steel units, but occupy much space and have a great number of flanged joints in them. To prevent the appearance of hydraulic hammer, resulting from boiling of the water in the economizer, the temperature of water at the cast-iron economizer outlet should be 40°C below the saturation temperature.

High-pressure boiler units incorporate steel economizers of the continuous loop-type whose layout is similar to that of superheaters. Non-steaming and steaming economizers of this type are available. Figure 1 shows schematically how non-steaming and steaming steel economizers are connected to a steam boiler.

1 Associate Professor, Ph. D., at the University of Petrosani, i_dosa@hotmail.com 2 Assistant Professor, Ph.D. at the University of Petrosani, tobogdan2002@yahoo.com

MultiScience - XXXII. microCAD International Multidisciplinary Scientific ConferenceUniversity of Miskolc, 5-6 September, 2018. ISBN 978-963-358-162-9

DOI: 10.26649/musci.2018.003

Shown in Figure 1 is the economizer of a high-pressure boiler, featuring horizontal staggered tube arrangement. The economizer comprises three banks of loops bent from carbon steel tubes, 32 x 4 mm in diameter. The feed water is introduced into the bottom header and, being heated, flows from the upper header in the upward direction which facilitates evolution of the gas and vapour bubbles from it. The flue gases, to which the economizer tubes are exposed, flow in the downward direction. This counter flow arrangement ensures a high mean temperature difference. The banks of loops are supported on a system of steel hollow air-cooled beams, and the economizer headers are located outside the boiler setting.

Fig. 1. Schematics of the economizer of Pp-330/140-P55 steam boiler [2]

To prevent corrosion and separation of the steam-water mixture in the outlet loops, the water velocity in the economizer tubes should be not less than 0.5 m/sec in non-steaming, and not less than 1 m/sec in steaming economizers.

In order to protect economizers from external corrosion, the temperature of the inlet water should be higher than the dew point of the flue gases. For high-moisture fuels with a low sulphur content the dew point ranges from 20 to 60 °C, and that of high-sulphur fuels reaches 130 to 140 °C.

2. CHARACTERISTICS OF INVESTIGATED ECONOMIZER

The investigated economizer is part of Pp-330/140-P55 steam boiler and its placement is shown in Figure 1.

Construction of Pp-330/140-P55 steam generator [2] is carried out in two distinct bodies, operating in parallel to the steam turbine. The steam output of generator (one body) is 330 t·h-1, at a pressure of 140 bar and 550 °C for live steam and 24.4 bar at 550 ºC temperature for reheat steam. Each steam generator body is designed with two flue gas paths - in the shape of Π - one ascending and one descending, tied together with a reverse room. Fuels used in furnace chamber can be solid (pulverized coal), liquid (heavy fuel oil) or gaseous (natural gas).

Fig. 2. Schematics of Pp-330/140-P55 steam boiler [2]

The ascending path is the furnace chamber area, where the radiation heat exchangers are located and the descending path consists in the convection heat exchange surfaces. Combustion air and the air used for the transport of pulverized coal are blown by a centrifugal air fan. The basic fuel is crushed coal, obtained in hammer mills (4 mills for each body of the steam generator). To start and support the flame, auxiliary fuel is used (natural gas or heavy fuel oil). Heavy fuel oil injector, gas burner and the pulverized coal burner have a unitary construction.

Large share of radiation heat exchange surfaces ensures that the project parameters are delivered even down to 70% of rated load.

Boiler efficiency at rated load reaches 90,07% (by project) especially by placing particular areas of regenerative convection heat exchangers (economizer and air preheater), leading to lower flue gas temperature to a value of 151 °C, when operating exclusively on pulverized coal.

Supply water parameters at steam generator rated load are: pressure 188 bar, temperature 242 °C. The unit is equipped with a data acquisition and process control system in order to track boiler operating parameters.

Also, indicator panels are located in the control room of the unit. The main specifications of the economizer are shown in Table 1.

Table 1 Economizer specifications

Nomenclature U.M. Values Inner diameter of tube di mm 32

Outer diameter of tube de mm 40 Tube length ltv m 9.75 Number of tubes 648 Transversal tube pitch st=2.1∙de mm 84 Longitudinal tube pitch sl=2∙de mm 80 Tube connection diameter dr mm 84 Flue gas channel length Lcg m 10.12 Flue gas channel width lcg m 6.42 Maximum number of tubes on a row 74 Width of tube baffles m 0.41 Heat exchange surface m2 1690 Number of pathways 4

3. MEASURED DATA AND CALCULUS

Data needed in order to perform calculus of main characteristics of the economizer overlap with what is required to perform energy auditing [3].

Algorithms and equations for energy auditing of various installations and equipment can be found in literature [4], [5].

Calculations for the economizer were performed using the ε-NTU (effectiveness - Number of heat Transfer Units) method [6], where the effectiveness is given by equation:

maxmin1

min

max

min

min

max

min

1

1

1WW

WSsk

s

eWW

e WW

WSk

(1)

where ks – overall heat transfer coefficient, W·m-2·℃-1, Wmin, Wmax – maximum and minimum heat capacitance of fluids, W·℃ -1, S heat transfer area, m2.

Quality indicators of heat exchangers can be calculated using equations [6]: - Thermal efficiency ηr:

11

2 1QQ

QQ p

r (2)

where Qp is the heat loss in W; Q1 the heat given up by the hot fluid and Q2 is the heat taken up by the cold fluid in W.

- Gross thermal efficiency ηtd:

0

111

22201111

222201

2

iiGiiG

ttcGttcG

QQ

p

ptd

(3)

where Q2 is the heat taken up by the cold fluid, W; Q01 - the heat given up by the hot fluid, W, at t0

1 - ambient temperature, °C; i01 – enthalpy of hot fluid at ambient

temperature t01, J∙kg-1; G1, G2 - the flow rates of fluids, kg∙s-1; i"2, i'2 - the enthalpies

of cold fluid at outlet and inlet J∙kg-1; i'1 – enthalpy of hot fluid at inlet, J∙kg-1. The investigated units are equipped with data acquisition and process control

systems in order to track boiler operating parameters. In order to perform a proper heat balance analysis at least 3 different loads must be considered, as a result, for the economizer too.

As the Pp-330/140-P55 steam generator has two distinct bodies, and a perfect balance between the loads it is practically impossible, for each load, two sets of measurements were performed. The two bodies will be denoted with the Pp-330 followed by A or B.

The loads for performance tests for Pp-330/140-P55 steam generator were fixed to 230 t·h-1 – 69.70%, 280 t·h-1 – 84.85% and 310 t·h-1 – 93.94%.

In addition, measurements are carried out in order to obtain more accurate data on combustion, using a TESTO 350 portable emission and combustion analyser, data on flue gas composition, flue gas temperature Tga and coefficient of excess air λ is obtained, Table 2.

During measurements coal samples were taken in order to obtain data on composition and lower heating value of the used coal.

Table 2. Flue gas composition

Pp-330/140-P55 69.70% 84.85% 93.94%

Nom. U.M.

Pp-330 A Pp-330 B Pp-330 A Pp-330 B Pp-330 A Pp-330 B O2 % 10.69 9.54 8.46 8.01 8.16 8.47 CO % 4·10-4 4·10-4 2·10-4 5·10-4 3·10-4 5·10-4

CO2 % 8.45 8.99 11.11 11.01 10.77 10.4 SO2 % 0.1 0.1 0.2 0.2 0.1 0.1 NO % 0.0 0.0 0.0 0.0 0.0 0.0 Tga °C 174.5 176.1 183.2 181 175 172.1 λ 2.01 1.79 1.66 1.6 1.61 1.65

The basic fuel is crushed coal, and in order to start and support the flame, auxiliary fuel, natural gas is used (1,762.46 to 4,958.05 m3

N·h-1). As energy audit was performed [7], in addition to existing data, in order to

calculate quality indicators for the economizer, values for feed water and flue gas flow rate and temperature are given in Table 3.

Table 3 Additional data

Pp-330/140-P55 69.70% 84.85% 93.94%

Nom.

Pp-330 A

Pp-330 B

Pp-330 A

Pp-330 B

Pp-330 A

Pp-330 B

Rated

Feed water flow rate, kg∙s-1

56.717 58.541 68.654 68.858 72.362 78.474 88.056

Feed water inlet temperature, ℃

207.49 207.63 216.35 216.71 220.93 221.22 242

Feed water outlet temperature, ℃

281.78 268.65 287.64 276.84 290.16 281.56 290

Flue gas flow rate, m3

N∙s-1141.66 159.10 146.82 161.38 164.31 164.32 108.95

Flue gas inlet temperature, ℃

454.95 440.49 481.26 456.82 485.31 472.98 493

Flue gas outlet temperature, ℃

323.61 317.29 343.78 332.16 348.21 345.76 366

4. RESULTS AND CONCLUSIONS

In order to compare the results obtained, calculations were performed at rated parameters of the boiler.

At rated operation the gross thermal efficiency of economizer ηtde is 27.81 % while the maximum value for Pp-330 A is 24.68% and for Pp-330 B is 20.86%.

The overall heat transfer coefficient ks at rated operation is 75.957 W∙m-2∙℃-

1, a small value, since maximum values for Pp-330 A and B are 89,73 and 85,63 W∙m-2∙℃-1. It looks that this value should be greater as greater heat transfer coefficient means better heat transfer. In case of rated operation, the coefficient of excess air is much smaller (1.25) than values occurring at actual operating conditions (1.6 to 2.1). As a result, flue gas flow rates are smaller, resulting smaller Reynolds numbers and heat transfer coefficient on the gas side of heat exchanger, therefore a smaller overall heat transfer coefficient.

The efficiency of the economizer ηr at rated operation is 0.983 as expected, in the range given in literature [6] while for Pp-330 A and B were 0.792 and 0.709. These values are probably a result of heat exchanger fouling on the gas side.

In Figure 3 and Figure 4 results are presented as a function of flue gas flow rate. The gross thermal efficiency of the boiler ηtdb is also represented in order to compare the variation of boiler gross thermal efficiency with the economizer’s gross thermal efficiency, effectiveness, overall heat transfer coefficient and efficiency variation. For Pp-330 A all represented values have a maximum at the maximum gross thermal efficiency of the boiler except overall heat transfer coefficient which grows

along with the flue gas flow rates. This variation is expected as higher flow rates will produces greater turbulence, higher Reynolds numbers, resulting higher heat transfer coefficients on the gas side of the economizer. On the water side, is the same thing as the feed water flow rates are also getting higher as the boiler is operating closer to the rated load. Variation of overall heat transfer coefficient has the same characteristic for the Pp-330 B.

Fig. 3. Results regarding Pp-330 A

Fig. 3. Results regarding Pp-330 B

For Pp-330 B the maximum gross thermal efficiency of economizer has a maximum at maximum gross thermal efficiency of the boiler, while the thermal efficiency and effectiveness has a minimum value pointing to a problem with the economizer, probably excessive fouling on the gas side.

Analysing the economizer operation at different loads highlights the relationship between boiler efficiency and economizer efficiency.

Conclusively, computing quality indicators for the economizer can point to problems in economizer operation, that can help maintenance teams to pinpoint boiler efficiency decrease. Solving the problem in early stages, the efficiency of the boiler can be restored, resulting significant fuel savings.

REFERENCES

[1]. NASHCHOKIN V.V., Engineering thermodynamics and heat transfer, Mir Publishers, Moscow, 1979 [2]. ***, Technical Instructions and Operation Manual for Pp-330/140-P55 boiler. [3]. CARABOGDAN I.GH., ET AL., Energy Balances. Tehnica Publishing House, Bucharest, 1986. [4]. ***, Guide to development and analysis of energy balance. M.O. of Romania, part. I, nr.792/11.11.2003. [5]. BERINDE T., ET AL., Elaboration and analysis of energy balance in the industry. Tehnica Publishing House, Bucharest, (1976) [6] STEFANESCU D., LECA A., LUCA L., Heat and mass transfer, E.D.P., Bucuresti, 1983. [7] DOSA, I., Energy Performance of Steam Generator after Its Long-Term Operation, "9th WSEAS International Conference on ENERGY, ENVIRONMENT, ECOSYSTEMS and SUSTAINABLE DEVELOPMENT" (EEESD '13), Lemesos, Cyprus, March 21-23, 22013, pag.65-70.