Efficienctly Generate Steam From Cogen Plts

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EFFICIENTLY GENERATE STEAM FROM COGENERATION PLANTS Page 1 of 6

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EFFICIENTLY GENERATE STEAM FROM

COGENERATION PLANTS

How to choose between running a steam generator or a heat recovery steamgenerator and gas turbine

As cogeneration gets more popular, some plants have two choices of equipment for generating steam.Plant engineers need to have a decision chart to split the duty efficiently between (oil-fired or gas-fired) steam generators (SGs) and heat recovery steam generators (HRSGs) using the exhaust fromgas turbines (Figures 1 and 2). Underlying the dilemma is that the load-versus-efficiencycharacteristics of both types of equipment are different.

When the limitations of each type of equipment and its capability are considered, an analysis cancome up with several selection possibilities. It is almost always more efficient to generate steam in anHRSG (designed for firing) as compared with conventional steam generators. However, other aspects,such as maintenance, availability of personnel, equipment limitations and operating costs, should alsobe considered before making a final decision. Loading each type of equipment differently also affects

the overall efficiency or the fuel consumption.

This article describes the performance aspects of representative steam generators and gas turbineHRSGs and suggests how plant engineers can generate steam efficiently. It also illustrates how toconstruct a decision chart for a typical installation.

The equipment was picked arbitrarily to show the method. The natural gas fired steam generator has amaximum capacity of 100,000 lb/h, 400-psig saturated steam, and the gas-turbine-exhaust HRSG hasthe same capacity. It is designed for supplementary firing with natural gas.

Steam generator characteristics

Table 1 (p. 190) summarizes details of the steam generator performance. The information in it is heatlosses, exit gas temperatures and efficiencies computed per the American Soc. of MechanicalEngineers Power Test Code (ASME PTC) 4.1 [1]. Gas data are in Table 1. The same gas is used forthe suplementary firing of the HRSG.

As the load increases:

1. Exit gas temperature from the boiler increases

Publication : CE

Date : May, 1997

Copyright : Copyright 1997 The McGraw-Hill Companies, Inc.

Volume : 104

Issue : 5

Page : 187

Section : OPERATION & MAINTENANCE


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2. Efficiency increases and then falls off slightly. When steam generators are the only type ofequipment available in the plant, operate them at the most efficient point, which is somewherebetween 50 and 75% of the steam capacity

3. Between loads of 25% and 100%, the difference in efficiency is small, only about 1.5%. Figure 3shows the load-vs.-performance characteristics of the steam generator capable of generating 100,000lb/h of saturated steam

Details of the major heat losses are in Table 1. The casing heat loss due to radiation and convection,in Btu/h, remains the same at all loads, as it is a function of casing temperature, ambient temperatureand wind velocity [1]. However, the loss as a percentage decreases with increasing load. The exit-gastemperature decreases at lower loads due to the lower gas flow across the boiler surfaces, thusreducing heat losses such as the dry gas loss and loss due to combustion of fuel. The combination ofthe above two trends results in a typical efficiency-versus-load curve in the shape of a flattenedparabola for the steam generator.

HRSG characteristics

The gas turbine and HRSG generate 25,000 lb/h of saturated steam (400 psig, to be consistent) in theunfired mode from hot exhaust gases. The HRSG has supplementary firing that can generate up to100,000 lb/h steam using natural gas, thus matching the steam generator's steam capabilities. AnHRSG can be unfired when energy in exhaust gases alone (usually at 900 - 1,000 degrees F) is usedfor generating steam. Since exhaust gas contains about 15% oxygen by volume, firing with additionalfuel does not require additional air. The gas temperature is hotter, 1,200 - 2,900 degrees F.

The HRSG performance simulation used ``HRSGS'' software developed by the author [2, 3]. Thisprogram computes the fuel requirements and the gas and steam temperature profiles of an HRSG atvarious loads in unfired and fired modes, and the ASME PTC 4.4 efficiency. Figure 4 shows the

HRSG performance at various loads. Figures 5 and 6 show the HRSG performance and the heatexchanger curves in unfired and fired modes. Color coding is used to distinguish the two modes inthis overlay. Table 2 shows the summary of performance at various loads, including the fuelconsumption on a Lower Heating Value basis.

HRSGs have unique performance characteristics:

1. Unlike that of the steam generator, the HRSG exit-gas temperature decreases as the load increases.Since the gas flow remains nearly constant at all loads (unlike a steam generator where the gas flowreduces as load reduces), the economizer acts as a large heat sink at higher loads, and as small heatsink at lower loads. Hence, as the water flow through the economizer decreases at lower loads, the

exit gas temperature is higher, due to the smaller duty. A more detailed explanation can be found inReference 2

2. The efficiency or the fuel utilization increases significantly as the load increases. Unlike in a steamgenerator, where we use excess air for combustion, we are, in effect, decreasing the oxygen or excessair in the exhaust gases at higher loads. (From basic combustion principles, there is a relationship thatas excess air increases, the losses increase, and vice versa.)

Reduction in excess air results in higher efficiency at higher firing rates. Combine this with the lower


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exit-gas temperature discussed above and the result is high fuel utilization. It is near 100% andcompares to 93-94% in steam generators on a Lower Heating Value basis. Verify this by computingthe incremental fuel consumption versus the incremental steam generation

3. The water temperature leaving the economizer decreases at higher loads as the ratio of water to gasflow increases. It's different in a steam generator, where the ratio is nearly the same at all loads. Thus``economizer steaming'' (box, p. 190) is not a concern at high loads in HRSGs

4. The variations in ASME PTC 4.4 efficiency versus load is large -- on the order of 10%, comparedwith steam generators, where it is about 1 to 2%

Efficient generation of steam

Develop the decision chart by adding fuel consumption rates at various loads. Four cases of steamdemand (at 400 psig) make up the complete Table 3. At 200,000 lb/h there is no choice, both are at amaximum capacity. As the load decreases the options widen.

Using wide interval steps of 25,000 lb/h, there are three choices at 150,000 lb/h, five choices at100,000 lb/h, and three choices at 50,000 lb/h. We are making the assumption that the gas turbine isoperating at its rated capacity and the HRSG's capacity may be varied by firing different amounts offuel.

Draw the following conclusions from Table 3:

1. It is naturally efficient to make use of the exhaust energy from the gas turbine to generate steambefore operating the steam generator. We generate 25,000 lb/h of steam without using additional fuel

2. It turns out to be efficient to generate the maximum amount of steam from the HRSG beforeoperating the steam generator. For example, to generate 150,000 lb/h, the combination of 100,000

lb/h from the HRSG and the balance from the steam generator requires the lowest fuel quantity of130.5 million Btu/h. The other extreme, using 50,000 lb/h from the HRSG costs 133.0 million Btu/h

3. When the amount of steam to be generated is such that either the HRSG or the steam generatorcould be used, it is better to maximize the HRSG capacity. When the amount of steam is so largesuch that both equipment sets have to be operated, then the difference in fuel consumption narrows.The difference is only about 3 million Btu/h in the case of 150,000 lb/h, while it is about 30 millionBtu/h at 100,000 lb/h and below

Not all installations are as simple as the example. If a superheater is involved, the steam temperaturein the steam generator may be achieved only between 40 and 100% load, in which case a minimum

load of 40% may be set for the steam generator. Large HRSGs may be capable of supplementaryfiring only to 1,600 degrees F, which only doubles the steam output (not quadrupling it). Remember,the goal for the particular plant is to make a decision table such as Table 3. With modification, thiscan be done for multiple-boiler installations.

A more finely tuned analysis should account for the cost of moving gases. When a gas turbine HRSGis fired to generate additional steam, the cost of moving the gas through the HRSG is very small (thegas flow does not vary much with steam production in most machines). The additional gas pressuredrop could be 2 to 4 in. water column, (due to the higher average gas temperature in the HRSG). A


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steam generator, depending on the fan efficiency and load will need to typically overcome a 6 -16-in.water column pressure drop. Hence operating costs are also a factor with steam generators.

Edited by Peter M. SilverbergV. Ganapathy ABCO Industries

[Table]

TABLE 1. STEAM GENERATOR PERFORMANCE DATA

Load, % 25 50 75 100Steam generation, lb/h 25,000 50,000 75,000 100,000Duty, million Btu/h 25.4 50.8 76.3 101.6Excess air, % 30 10 10 10Flue gas, lb/h 30,140 50,600 76,150 101,750Exit gas temperature, F 265 280 300 320Heat losses, %

- dry gas loss 3.93 3.56 3.91 4.27- air moisture loss 0.10 0.09 0.10 0.11- fuel moisture loss 10.43 10.49 10.58 10.66- radiation loss 2.00 1.00 0.70 0.50

Efficiency, %- Higher Heating Value basis 83.54 84.86 84.71 84.46- Lower Heating Value basis 92.59 94.05 93.88 93.61

Fuel fired, MM Btu/h LHV basis 27.4 54.0 81.3 108.5Data: Boiler pressure = 400 psig, Feed water temperature = 230F, BlFuel used: Natural gas, Volume %: C1 = 97, C2 = 2, C3 = 1

HHV = 1044 Btu/ft3, LHV=942 Btu/ft3Calculation Formulae: HHV Efficiency = 100 - dry gas loss - air

moisture loss - fuel moisture loss -radiation loss

LHV Efficiency = (1,044/942) (HHV Efficiency)Fuel Fired = Duty/LHV Efficiency

[Table]

TABLE 2. HRSG PERFORMANCE DATA

Load, % 25 50 75 100Steam generation, lb/h 25,000 50,000 75,000 100,000Duty, million Btu/h 25.4 50.8 76.3 101.6Exhaust gas flow, lb/h 152,000 153,140 154,330 155,570Exit gas temperature, F 319 285 273 269Fuel fired, 106 Btu/h LHV basis 0 24.5 50.0 76.5ASME PTC 4.4 efficiency, % 70.80 83.79 88.00 89.53ASME PTC 4.4 efficiency = energy absorbed by steam

exhaust gas flow 3 enthalpy + fuel input (LHV basis)

The gas turbine is operating at rated capacity. PTC is the power test code

[Table]

TABLE 3. FUEL CONSUMPTION AT VARIOUS STEAM LOADS

Total steam, HRSG steam, SG steam, HRSG fuel, SG fuel, Total fuel,lb/h lb/h lb/h 10 super- 10 super- 10 super-


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script script script6 Btu/h 6 Btu/h 6 Btu/h

200,000 100,000 100,000 76.5 108.5 185.0150,000 50,000 100,000 24.5 108.5 133.0150,000 75,000 75,000 50.0 81.3 131.3150,000 100,000 50,000 76.5 54.0 130.5100,000 0 100,000 0 108.5 108.5100,000 25,000 75,000 0 81.3 81.3100,000 50,000 50,000 24.5 54.0 78.5

100,000 75,000 25,000 50.0 27.4 77.4100,000 100,000 0 76.5 0 76.550,000 0 50,000 0 54.0 54.050,000 25,000 25,000 0 27.4 27.450,000 50,000 0 24.5 0 24.5

The gas turbine is operating at rated capacity. Fuel is calculated using LHV. SG =

[Illustration]

Illustration: FIGURE 1(left) and FIGURE 2 (right). The packaged steam generator, below, onlymakes steam

The HRSG, below, makes steam, while the attached gas turbine makes electricity

[Illustration]

Illustration: Graph: FIGURE 3. Steam generators have a peak efficiency at partial load

[Illustration]

Illustration: Graph: FIGURE 4. HRSG efficiency rises sharply as the load increases

[Illustration]

Illustration: Graph: FIGURE 5. An HRSG operating at 25% load (blue lines) and no firing canproduce 25,000 lb/h of 415-psia steam at 70.8% efficiency. The evaporator duty is 19.9 millionBtu/h and the economizer duty is 5.5 million Btu/h. The gas flow is 152,000 lb/h

[Illustration]

Illustration: Graph: FIGURE 6. A HRSG operating at 75% load (red lines) with firing can produce75,000 lb/h of 415-psia steam at 88.0% efficiency. The evaporator duty is 68.0 million Btu/h andthe economizer duty is 8.6 million Btu/h. The burner has added 49.9 Btu/h. The gas flow is 152,000lb/h

[References]

1. Ganapathy, V., ``Steam Plant Calculations Manual,'' Marcel Dekker, New York, 1994.

2. Ganapathy, V., ``Waste Heat Boiler Deskbook,'' Fairmont Press, Atlanta, 1991.

3. Ganapathy, V., Applications of HRSG Simulation, paper presented at Industrial Energy

Technology Conference, Houston, April 1995.

[Biography]


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V. Ganapathy is a heat transfer specialist with ABCO Industries (P.O. Box 268, Abilene, TX 79604;Phone: 915-677-2011, Fax: 915-677-1420,

E-mail: [email protected]), where he is responsible for thermal and process design of steamgenerators and waste heat boilers. He has over 25 years of experience in the engineering of steamgenerators and boilers. Ganapathy holds a B. Tech. degree and an M. S. in mechanical engineeringfrom I.I.T. Madras. He has written over 175 papers and 5 books. He has written chapters in theEncyclopedia of Chemical Processing and Design. He also conducts courses on waste heat boilersand steam generators.

Efficienctly Generate Steam From Cogen Plts

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