advanced boiler cycles

7/29/2019 Advanced Boiler Cycles

1/11

PDHeng ineer . com CourseM-1009

Advanced Boiler Cycles

Thisdocumentisthecoursetext.Youmayreviewthismaterialat

yourleisurebeforeorafteryoupurchasethecourse. Ifyouhavenot

alreadypurchasedthecourse,youmaydosonowbyreturningtothe

courseoverviewpagelocatedat:

http://www.pdhengineer.com/pages/M1009.htm

(Pleasebesuretocapitalizeandusedashasshownabove.)

Oncethecoursehasbeenpurchased,youcaneasilyreturntothe

courseoverview,coursedocumentandquizfromPDHengineersMy

Accountmenu.

Ifyouhaveanyquestionsorconcerns,rememberyoucancontactus

byusingtheLiveSupportChatlinklocatedonanyofourwebpages,

[email protected]

freeat1877PDHengineer.

ThankyouforchoosingPDHengineer.com.

PDHengineer.com,aservicemarkofDecaturProfessionalDevelopment,LLC. M1009C1


2/11

Advanced Boiler Cycles (1 PDH)Course No. M-1009

Introduction

Supercritical phase of steam is reached when its pressure is raised above the crititical point (3,208psia). When in this phase, there is no physical delineation between the liquid and the vaporphases. Conventional drum-type boilers have no application at supercritical pressures. There is aconsiderable increase in efficiency in cycles operating within this pressure and temperature range.There were several power plants built in the late 1950's and early 1960's that were designed tooperate in the supercritical range. They earned reputations of high efficiencies, but low reliabilityand non suitablity for cycling operation. Their popularity and utilization, however, continued inEurope and Asia. Development in metallurgy and welding techniques overcame many of the earlyproblems, and there is a resurgence of interest in supercritical technology in the United States.

Advanced Boiler Cycles

For studies involved in the variation of the basic steam cycle, it is necessary tothoroughly understand the properties of steam, the effects of pressure andtemperature, and the use of superheat. A brief review of the fundamentals thatapply to the generation of steam will be helpful.

The theoretical amount of work that can be obtained from steam used in a primemover is equivalent to the change in its total heat content from its condition at the

entering state to that at its exhaust state.

The vaporization of water occurs in two steps: First by adding heat to the water toraise it to boiling from the vapor / water interface, the continued addition of heatwill cause the steam to become superheated. The superheated steam cannotcondense as long as it is above the saturation temperature corresponding to thesaturation pressure. In a drum-type boiler, this water / vapor interface ismaintained in the steam drum. Once the saturated steam leaves the steamdrum, it passes to the superheater tube banks where it is superheated.

Basic Ideal Rankine Cycle

The basic reversible Rankine superheat cycle plotted on Temperature / Entropycoordinates is illustrated below. This is the ideal cycle upon which drum typeboilers are based.


3/11

The boiler feed pump raises the feedwater temperature isentropically from pointa to point b. Next, heat is added in the boiler at constant pressure as the two-

phase fluid circulates through the boiler generating tubes and steam drum. Afterthe saturated steam leaves the drum, it enters the superheated phase, and thetemperature rises to point c.

The area under the curve (the integral of Tds) represents heat. Heat is addedfrom point a to point d, and is rejected from point d to point a. The shaded arearepresents heat rejected from the cycle in the condenser. The net work of thecycle, therefore, is represented by the area under a b c d minus the shaded area.The heat rejected is a function of the temperture of the heat sink, which nearlyalways is a large body of water or the atmosphere. The heat sink averagetemperature is generally about 60 deg. F. This fixes the heat rejected from a

cycle as a function of a 60 deg. F. temperature. Therefore, to increase the network, and thus the efficiency of a cycle, the area under the curve a b c d must bemaximized.

Rankine Regenerative Cycle

A nearly universal variation to power plant Rankine cycles is the regenerationcycle. The Rankine regenerative cycle utilizes partially expanded steam


4/11

extracted from the turbine at various points to heat the condensate andfeedwater on its way back to the boiler or steam generator. The schematic of aregenerative cycle with two stages of feedwater heating is shown below. Highpressure steam is extracted and directed to the high pressure feedwater heater #1 (line 3). Low pressure steam is extracted at a lower pressure turbine stage and

is directed to low pressure feedwater heater #2 (line 4). For simplicity, only twofeedwater heating stages are shown here, however in large power plants asmany as eight stages may be employed. Completely reversible heat transfer cantake place only when there is no temperature difference between the heating andcooling media. This is physically impossible, of course, because a temperaturedifference is required for heat transfer to take place. Therefore, completelyreversible heat transfer is impossible. (Heat can flow only from hot to cold).

However, as the temperature difference between hot and cold are closer, theincrease in entropy is less, so the heat transfer is more efficient. This does notmean that the heat transfer is more effective when the temperature difference is

less. The rate of heat transfer is proportional to the temperature differencebetween hot and cold, but the greater the temperature difference, the greater theincrease in entropy during the process. What this means is that incrementalsteps of heat transfer to the feedwater increases the cycle efficiency over havingall of the heat transfer taking place within the boiler.

The regenerative Rankine cycle is illustrated below. It can be shown that thebest efficiency for a given number of heaters is realized when the temperaturerange per heater is approximately equal. Accordingly, for the cycle beingillustrated for additional heaters, the same plan would be followed, solving


5/11

temperature difference between the saturated vapor at the boiler pressure andthe saturated liquid at the condenser is divided into three equal parts. Theoptimum extraction pressures are the saturation pressures corresponding tothese saturation temperatures. The weight of steam to be removed at eachextraction point is calculated by setting up a heat balance across each heater.

Equating the total heat transferred from the extracted steam to heater #1 gives Qsteam = Q feedwater.

h = enthalpy in BTU / lb.

w = steam flow in lb / hr

w1( h3 - h s ) = (1-w 1 )(hs -h7 ).

In a similar manner, a heat balance may be drawn across the second heater togive:

w 2 (h4 - h7) = (1-w1 -w 2 )(h7 -h 6 )

By getting the enthalpies from the steam tables and solving for w1, the result maybe used to solve for w2 . The thermal efficiency for the cycle would be:

(h2 - h1)- (1-w1 - w2)(h5 - h6)h2 - h1

For additional heaters, the same plan would be followed, solving for each weightin order.

The increase in efficiency must result in an operating cost reduction that isgreater than the increased capital cost of the heaters and the additional piping.In a large power plant with today's fuel costs, this can economically justify eightstages of feedwater heating.


6/11

Diagram for Regenerative Rankine cycle

Reheat Cycle

Another common variation of the basic Rankine cycle is the Rankine reheatcycle. In order to take advantage of the additional heat added, as well as to gainthe practical advantage of drier steam at the turbine exhaust, most power plantsuse the Rankine Reheat cycle, illustrated in the diagrams below, in enthalpy-entropy (h-s) coordinates to the left, and temperature-entropy coordinates to theright. In the reheat cycle, the superheated steam that is passed through andexhausted from the high pressure turbine is brought to the reheat section of theboiler, where the temperature is raised to approximately the original reheattemperature. The hot reheat steam is then returned to the lower pressure turbinestages to complete its expansion. The gain in net work, therefore efficiency, isrealized as the heat added, represented by the area under the curve 3, 4, 5 on

the T-s coordinates, is greater than the additional heat rejected, represented bythe area a, 5, c, d.

The upper temperature limit of the superheated steam is limited by the currentmetallurgy of the boiler tubes, hot reheat steam lines, and turbine blading. Themajority of present-day reheat cycles operate at 1050 deg. F.


7/11

Supercritical Steam

When water is heated at a constant pressure of 3,208 psia and above, it does notboil and does not produce a two-phase mixture of water and steam. At 3,208psia and above, there is no physical differentiation between water and steam.This "critical point" is at the apex of the saturation line as shown on T-scoordinates. Above this pressure, the physical properties (density,compressibility, and viscosity) change continuously from those of a liquid to thoseof a vapor. The temperature steadily rises. The specific heat and rate of risevaries considerably during this transition.

During the boiling process at subcritical pressures, individual molecules breakout of the dense liquid clusters and form a separate vapor phase. At supercriticalpressures, as heat is added to the liquid, the clusters gradually divide into smallerclusters, and the spacing of the molecules gradually become less dense until thetransition to the wide-spaced, random molecular arrangement of vapor isattained.

Supercritical Steam Cycles

As new coal-fired plants are being considered in this country, many developers

are looking seriously at supercritical technology. The capital costs are higherthan the more conventional subcritical plants, but the cost of fuel andenvironmental concerns are playing a large role in decision making. Also, theincreased interest in efficient coal-fueled plants may be a result of thedisenchantment with the nuclear power industry. Babcock Borsig CapitalCorporation {U.S. office: Boston) maintains that coal is completely competitivewith gas-fired combined cycles when gas is priced around $3 per MBtu,assuming a 85% capacity factor and a coal price of $1.25 per MBtu.


8/11

Looking at the basic Rankine cycle once again, it can be seen that the heatadded, and thereby the net work and efficiency, can be increased by pressurizingthe liquid above the critical point and thereby eliminating the horizontal paththrough the saturated zone that occurs in subcritical boilers. This increases theheat added area. The temperature rises continuously along the constant

pressure line.

Increases in efficiency not only reduce fuel cost, but also reduces the specific(per MW) emissions of many pollutants compared to subcritical coal-firedboilers. Also, the efficiency of supercritical boilers does not fall off significantly atpart-load, particularly if the plants can operate in a sliding pressure mode. (Moreabout sliding pressure operation later.)

The earliest supercritical boilers were built in the late 50's and early 60's. Theseunits established a reputation for high efficiencies (around 35%) and lowreliabilities. 35% efficiency may not seem high when compared to claims being

made by gas-fired cogeneration plants, but we are looking at two differentdefinitions of efficiency.( Refer to the PDHengineer course "Thermodynamics ofCogeneration") The materials of the 1950's and 1960's era were not up to thedemands of high temperatures and pressures. Supercritical units becameunpopular for new construction in this country. Gas-fired and combined cyclesgot most of the attention of power plant project developers. In Europe and Asia,however, supercritical technology continued to be pursued, and by the 1990's itdominated new capacity projects. Of the new coal fired power plantscommissioned abroad between 1995 and 2000, 85% were supercritical, some20,000 mw of capacity. Equipped with advanced materials and digital controls,these new supercritical plants are delivering high efficiency (around 44% LHV)and topnotch availability factors, around 85 to 90%.

Supercritical Steam Generators

Nearly 200 supercritical steam generators are operating world-wide today, withpressures up to 4,500 psig. More advances in materials and designs introducedin the late 90's have raised steam temperatures as high as 1,150 deg.F.,achieving efficiencies of 44% LHV. As higher steam temperatures up to 1,200deg. F. come into being, efficiencies of 50% are being predicted.

The nature of supercritical steam generation rules out the use of a boiler drum toseparate steam from water. Drumless "once through" steam generators areuniversally used for supercritical operation. The term "steam generator" will beused instead of "boiler" for once-through steam production, because boiling assuch does not really take place. Turbines for subcritical systems are usuallydesigned for steam pressures of 2,520 psig. A drum operating pressure of 2,750to 2,850 is required to allow for pressure drops in the superheater and mainsteam line. The densities of steam and water rapidly approaching each other


9/11

above this pressure level represents an approximate limit for drum-type boilersincorporating steam separation and recirculation.

A significant characteristic of once through steam generators is that all impuritiesin the feedwater must either be deposited in the generating tubes or

superheating surfaces, or be carried over into the turbine. Therefore, only thebest possible feedwater treatment is acceptable because blowdown from thedrum and generating tubes is not available to remove impurities from thesystem. The makeup water and the condensate must be purified. Otherwise theconcentration of impurities in the system would gradually build up and force anoutage for cleaning or repairs.

In all boilers and steam generators, flow through the tubes must be maintained inorder to control the tube metal temperature and prevent tube failure caused bythe combustion. In a once through steam generator, circulation can only beestablished by operating the feedwater pumps and continuously admitting

feedwater to the system to cool the steam generating and superheatingsurfaces. A certain minimum water flow must be established prior to full firing ofthe steam generator. It has been normal practice to design once-throughsystems to ensure a flow rate of at least 30% at rated flow at all times. Thereforeat start up and for low loads, a turbine by-pass system is utilized to divert thesteam to flash tanks.

The fluid system start up for once through steam generators is different, andsomewhat more complicated than for drum type boilers. Once through steamgenerators come equipped with an integral start up system, which will vary indetails from manufacturer to manufacturer.

In any case, the unit is initially fired, warmed up, and brought to partial load onthe by-pass system. The boiler feed pump establishes a flow through theeconomizer and waterwall tubes. The flow continues through the flash tanks (orseparators) and on to the condenser, then through the condensate polishers andback to the steam generator. The burners are lit, and gradually the waterwalltemperature is raised to the point where the turbine can be rolled.

Supercritical Steam Generator Design Configurations

The major differences between the various once-through steam generatordesigns on the market are the configuration of the furnace enclosure circuits andthe systems used to circulate water at start up and low loads.

The three leading designs are:

Vertical tube multipass furnace, upon which early supercritical designs were based. Theseproved suitable for base load operation, but were not as well-suited for cycling because ofthe thermal stresses involved.


10/11

The spiral tube Benson furnace configuration, in combination with a boiler recirculationpump was developed to minimize thermal shock during transients.

Spiral tube Sulzer furnaces are very similar to the Benson furnaces. Both use separatorvessels for start up.

Sliding Pressure Operation

Sliding pressure operation can be utilized in certain drum type boilers as well asonce-through steam generators (with some qualifications). The main advantageis higher part load efficiencies. In a conventional drum type boiler-turbinecombination, the boiler pressure remains essentially constant throughout theoperating range, with load control accomplished by the turbine throttle valves.

Throttling steam flow is not a reversible process and therefore introducesinefficiencies to the cycle. With sliding pressure operation with drum type boilers,the turbine operates essentially with throttle valves wide open throughout most of

the load range, but admits steam to only a portion of the throttle valves (partialarc admission) rather than all of the valves (full arc admission), and the turbinepower is controlled by varying the boiler pressure. Sliding pressure operationhas the further advantage of maintaining full superheat temperature over a widerload range than conventional throttling control. This reduces the cyclic thermalstresses experienced during cycling operation. Cycling operation may be definedas rapid rates of load increase and a significantly larger number of start up andshut down cycles compared to a base load unit.

In once through steam generators, load is a function of the steam flow, which inturn is controlled by the feedwater pumps. If the pressure throughout the steam

generating tubes is allowed to drop below the critical pressure, two-phase flowand steam-water separation will result. This will result in some of the secondarysuperheater tubes having variations in the steam-water mix. Because of thedifferences in densities between steam and water, flow will be restricted in thetubes having a greater steam concentration, and uneven heat transfer couldcause tube failures.

In order to take advantage of the benefits of variable pressure operation duringload turndown, pressure control division valves are installed between the primaryand secondary superheaters. These valves keep the steam generator tubesabove the critical pressure, while permitting the turbine to operate under itsoptimum pressure for a given load. With partial arc steam admission, the fullsteam throttle pressure can be maintained down to about 60% load. Because ofthe requirement to maintain steam generator pressure, the advantage ofreducing feed pump pressure at reduced loads cannot be realized withsupercritical cycles. The steam temperature is controlled by the firing rate, and isaugmented in some units by injecting superheating water between the superheatstages. A once through steam generator requires a more precise balance of


11/11

inputs and outputs than drum type boilers because of the lack of the flywheeleffect of a boiler drum.

References:

1. The statistics on supercritical boiler use is from Power Magazine, July, 2002

2. The diagrams are taken from Elements of Applied Thermodynamics, United States NavalInstitute Press.

3. Comparisons of supercritical cycles are taken from Combustion Fossil Power Systems publishedby Combustion Engineering, Inc.

advanced boiler cycles

Documents