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Energy Analysis of Low-Rank Coal Pre-Drying PowerGeneration SystemsShangjian Hu a , Chengbo Man a , Xuezhong Gao a b , Jianwen Zhang a b , Xueyuan Xu b &Defu Che aa School of Energy and Power Engineering, Xi'an Jiaotong University , Xi'an , Chinab Shanghai Boiler Works Ltd. , Shanghai , ChinaPublished online: 25 Aug 2013.

To cite this article: Shangjian Hu , Chengbo Man , Xuezhong Gao , Jianwen Zhang , Xueyuan Xu & Defu Che (2013) EnergyAnalysis of Low-Rank Coal Pre-Drying Power Generation Systems, Drying Technology: An International Journal, 31:11,1194-1205, DOI: 10.1080/07373937.2013.775146

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Energy Analysis of Low-Rank Coal Pre-Drying PowerGeneration Systems

Shangjian Hu,1 Chengbo Man,1 Xuezhong Gao,1,2 Jianwen Zhang,1,2 Xueyuan Xu,2

and Defu Che11School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, China2Shanghai Boiler Works Ltd., Shanghai, China

Low-rank coal (LRC) is widely used for power generation inmany regions of the world. However, due to the high moisturecontent of LRC, the overall efficiency of LRC-fired power plantswithout a pre-drying system is relatively low. Studies show thatthe overall efficiency can be improved by pre-drying the coal, andthe fluidized bed drying technique is found to be a desirable choicebecause of its high drying rate, high processing capacity, and lowmaintenance cost. In this paper, two novel, fluidized-bed, LRCpre-drying systems were integrated into a 1000MW LRC-firedpower plant. Superheated steam and hot air were used as the fluidiz-ing medium. Models for each component of these power generationsystems were developed based on material and energy balances.The performances of these power plants were calculated under thetypical operating conditions, and parametric analyses were alsoperformed to evaluate the effect of operating parameters. The powergeneration efficiency is found to increase remarkably with aproperly operated LRC pre-drying system.

Keywords Coal drying; Fluidized-bed drying; Heat recovery;Low-rank coal; Power plant

INTRODUCTION

Low-rank coal (LRC), including lignite and sub-bituminous coal, occupies about 50% of the world’s coalreserve. LRC is usually easy to mine and its price is rela-tively low. In many regions of the world, LRC is the mainfuel for electric power generation.[1] However, the highmoisture content of LRC (typically 30–60% on wet basis)leads to low heating value, low thermal efficiency in com-bustion, and high transportation cost. This characteristicrestricts the widespread use of LRC.[2–4]

The moisture in LRC should be removed before com-bustion.[5] In a conventional LRC-fired power plant, rawcoal is dried with hot flue gas aspirated from the furnaceby fans or beater mills. After drying, the dried pulverizedcoal is entrained in the cooled flue gas and supplied tothe combustion system.[6,7] In such a case, a large amount

of high-grade heat is consumed to remove the coalmoisture, but it is difficult to utilize the latent heat of thewater vapor in the flue gas. Heat loss of the exhaust gasis larger due to its high water vapor content. This causesa relatively low efficiency for a conventional power plantfueled with LRC.[6] In addition to this, the direct use ofhigh-moisture LRC also leads to larger equipment size,higher capital costs, higher operational costs, and higheremission of greenhouse gases. Using pre-dried LRC isfound to be a good choice for solving these problems.[5,6,8]

After decades of research and development, numeroustechnologies for LRC drying and dewatering are now avail-able, such as rotary tube drying,[9] fluidized bed drying,[10–13]

mechanical thermal dewatering (MTE),[14–16] hydrothermaldewatering,[17,18] solvent dewatering,[19,20] etc. Althoughmany of these newly developed drying methods have beenproved to have extremely low energy consumption, mostof them remain at the laboratory scale. The most commonlyused driers for drying brown coal are the rotary drum type.However, considering energy consumption, constructioncosts, and processing capacity of the rotary-drum-typedryers, they are often still not economical enough to meetthe large demands for coal-handling in a power plant.[6]

Among these drying technologies, due to the high dryingrate, high processing capacity, and low maintenance costof fluidized bed drying, it offers a desirable choice for LRCdrying.[2,3,21–24] Experimental and theoretical investigationshave been carried out by many researchers to study thedrying process of LRC in fluidized bed dryers. Diamondet al.[25] and Calban et al.[10,26] performed experimentalstudies on the effects of drying temperature, fluidizing gasvelocity, particle size, bed height, and initial moistureconcentration on the drying rate in batch hot-air-fluidizedbed dryers. Potter et al.[12,27] measured the heat transfercoefficient for brown coal in a steam-fluidized bed dryer.Chen et al.[13] developed a model to simulate the continuousdrying of coal in a superheated steam-fluidized bed dryer.Berghel[28] reported an increase in drying efficiency byinstalling heating tubes in a spouted-bed, superheated steam

Correspondence: Defu Che, School of Energy and PowerEngineering, Xi’an Jiaotong University, Xi’an 710049, China;E-mail: dfche@mail.xjtu.edu.cn

Drying Technology, 31: 1194–1205, 2013

Copyright # 2013 Taylor & Francis Group, LLC

ISSN: 0737-3937 print=1532-2300 online

DOI: 10.1080/07373937.2013.775146

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dryer. Hoehne et al.[29] found that the overall heat transfercoefficient in a steam-fluidized bed dryer increased withincreasing operating pressure.

As drying is an energy-consuming process, the cycleefficiency of a power plant can be significantly improvedby the optimum choice of the drying process.[6] Severalstudies have been performed to evaluate the impact ofLRC pre-drying on the performance of LRC-fired powerplants. Kakaras et al.[30] carried out thermal energy analy-ses on power generation systems with integrated LRCpre-drying processes. Liu et al.[31] found that the efficiencyof a lignite-fired power plant could be improved by 1.72%–1.87% when the coal moisture was reduced from 39.5% to19.5% using a rotary-tube steam dryer. Sarunac[32]

employed an air-fluidized bed dryer to reduce coal moisturecontent from 38.5% to 23.5% and found a 1.7% increase innet unit efficiency. In the Niederaussem K. Plant inGermany, 25% of input fuel was dried by low-pressuresteam, and the latent heat of the vapor was recovered ina feed water heater, thus the power generation efficiencyof the unit was increased by around 1%.[8]

Although many studies have demonstrated that the over-all power generation efficiency can be improved by afluidized-bed LRC pre-drying system, less research has beendone concerning the effects of drying medium, heatingsource, and operating parameters on the overall power gen-eration efficiency. These are essential factors in the optimi-zation design of an LRC pre-drying power generationsystem. The objective of the present work is to conduct anenergy analysis to examine the performance of a large-scalepower plant with different fluidized-bed LRC pre-dryingsystems. A parameter analysis will be performed to evaluate

the effects of operating parameters and waste heat recoveryon the overall power generation efficiency. The results ofthis study might be helpful for providing guidance for thedesign of an effective LRC pre-drying power plant.

SYSTEM DESCRIPTION

Two novel, supercritical, pulverized LRC-fired powergeneration systems with fluidized bed LRC pre-drying arestudied in this paper. Hot air or superheated steam is usedas the fluidizing medium. An immersed heater is employedin the dryer to provide part of the heat for coal drying.Extraction steam from the steam turbine is used as the heatsource for drying. The employment of the immersed heatercan increase the drying rate, reduce the flow rate of the flui-dizing medium, and consequently reduce the operation costof the LRC pre-drying system. The conventional and thepre-drying power plants will be introduced in the followingsections.

Conventional Power Plant

Figure 1 shows a typical schematic diagram of a conven-tional, supercritical pulverized power plant. The mainsteam from the boiler is expanded in the high-pressure(HP) turbine, then reheated in the boiler, further expandedin the intermediate-pressure (IP) and low-pressure (LP)turbines, then condensed in the condenser, and finallyreturned to the boiler as water. Boiler feed water is heatedby four low-pressure feed water heaters (Nos. 1–4), onedeaerator (No. 5), and three high-pressure feed water hea-ters (Nos. 6–8). Extraction steam from the steam turbineprovides the heat for the feed water heaters. The boiler feedpump is driven by a small steam turbine.

FIG. 1. Configuration of a conventional power plant.

COAL PRE-DRYING POWER GENERATION SYSTEMS 1195

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In this conventional LRC power station, mill drying isused. Coal is milled and dried simultaneously in the beatermills. Recycling of high-temperature furnace gas is used toevaporate the large amount of moisture in the coal.

Power Plant with Superheated Steam-Fluidized BedDrying

Figure 2 shows the power plant with a superheatedsteam-fluidized bed pre-drying system. The pre-drying sys-tem is composed of a fluidized bed dryer with an immersedheat exchanger, a precipitator, heat-recovery equipment,and the auxiliary equipment, such as the fans and pipelines.

In the superheated steam-fluidized bed drying system,the exhaust gas from the dryer is approximately pure watervapor. A part of the exhaust gas is recycled as the fluidizingmedium through the recirculation fan. The recycled steamcan only obtain a finite enthalpy rise after passing through

the recirculation fan. Heat provided by the fluidized steamaccounts for merely a small proportion, which is negligible.The heat for drying is mainly supplied to the fluidized bedthrough steam condensing in the immersed heater. Heatingsteam is extracted from the low-pressure (LP) steam tur-bine in the steam cycle. Condensate from the immersedheater returns to the steam cycle. The immersed heater pro-vides most of the heat for the drying process.

Part of the latent heat of the exhaust gas is recovered inthe waste heat exchanger for heating part of the feed waterfrom the condenser. The extraction steam requirement forfeed water heaters Nos. 1 and 2 can be reduced by theemployment of exhaust heat recovery.

Power Plant with Hot-Air-Fluidized Bed Drying

Figure 3 shows the power plant with a hot-air-fluidizedbed pre-drying system. The pre-drying system is composed

FIG. 2. Configuration of a power plant with superheated steam-fluidized bed drying.

FIG. 3. Configuration of a power plant with hot-air-fluidized bed drying.

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of a fluidized bed dryer with an immersed heater, aprecipitator, an air heater for raising the temperature offluidizing air, and the auxiliary equipment, such as the fansand pipelines.

Ambient air passes through the air heater to raise thetemperature. Hot air is not only used as the fluidizedmedium of the dryer, but also simultaneously provides partof the drying heat. The immersed heater in the dryer canprovide additional heat for drying, which can improvethe drying rate. The heat for the air heater and theimmersed heater is provided by steam condensing. Heatingsteam is extracted from the low-pressure (LP) steam tur-bine in the steam cycle. Condensate from the heatersreturns to the steam cycle.

The exhausted gas from the dryer is a mixture of air andwater vapor, and the saturation temperature of the vapor isvery low (about 45�C). Hence, it is not economical torecover the waste heat of the exhausted gas.

For the sake of safety, the temperature of the air enter-ing the dryer should not be too high, and some additionalsafety measures should be used to reduce the hazard ofspontaneous combustion or exploration.[33]

MODELS

Based on material balance, energy balance, and togetherwith some assumptions, models for each component of theabove-mentioned systems are developed. Then, the methodfor calculating the overall efficiency of the power plant isintroduced.

General Assumptions

In order to assess the performance of each system on aconsistent basis, some common assumptions are made inthe simulations for all three power generation systems.These assumptions are for the convenience of comparison;they will not affect the results of our simulations.[30,31,34,35]

1. The mass flow rates of the main steam for all study casesare kept unchanged.

2. The parameters of the steam cycle and the internalefficiency of the steam turbine are identical.

3. Heat loss and steam leakage in the steam cycle areignorable.

4. The excess air coefficient and the boiler exit flue gastemperature at the outlet of air preheater are not changed.

5. The boiler heat losses remain the same, except for theheat loss due to the exhaust gas.

6. Power consumption of the auxiliary equipment remainsunchanged and is negligible.

Ultimate Analysis and Heat Value of the Coal to beDried

The drying process only changes the moisture content ofLRC, and the element composition on the dry basis

remains unchanged. A constituent content of LRC afterthe drying process can be calculated by

Xdry ¼ Xar �100�Mdry

100�Marð1Þ

where Xdry is the constituent content of LRC after thedrying process, Xar is the constituent content of LRC onthe as-received basis, Mdry is the moisture content (wetbasis) after the drying process, Mar is the moisture content(wet basis) on the as-received basis.

Reducing the moisture content by thermal drying canincrease the heat value of LRC. The available chemicalenergy of the fuel may be quantified using either the higherheating value (HHV) or the lower heating value (LHV).The overall efficiency of a power plant based on the fuelLHV will be higher than that on HHV.[8] In this study, fuelcalorific values and efficiencies are given on the basis ofHHV. The HHV of the dried coal can be obtained by thefollowing equation:

QHHV;dry ¼ QHHV;ar �100�Mdry

100�Marð2Þ

where QHHV,dry is the HHV of the coal after the drying pro-cess, and QHHV,ar is the HHV of the coal on the as-receivedbasis.

Boiler

The model of the boiler is shown in Fig. 4. A heatbalance equation of the boiler can be written as follows:

Qin ¼ Q1 þQ2 þQ3 þQ4 þQ5 þQ6 ð3Þ

where Qin is the heat entering the boiler, Q1 is the heatabsorption of water and steam in theboiler, Q2 is the heatloss due to exhaust gases, Q3 is the heat loss due tounburned gaseous combustibles, Q4 is the heat loss dueto unburned solid combustibles, Q5 is the heat loss due

FIG. 4. Model of a boiler.

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to radiation and convection, Q6 is the other heat loss of theboiler. The value of Q2 can be calculated from the proper-ties of the coal and the operating conditions of the boiler,and recommended values of Q3�Q6 are available.[36]

Thus the value of Q1 can be obtained by the followingequation:

Q1 ¼ Qin � ðQ2 þQ3 þQ4 þQ5 þQ6Þ ð4Þ

The thermal efficiency of the boiler is defined as

gB ¼ Q1=Qin � 100;% ð5Þ

The heat absorption of water and steam in the boiler canalso be obtained by

Q1 ¼ Dfw � h0 � htð Þ þDrh � rrh ð6Þ

where Dfw is the flow rate of the feed water, h0 is theenthalpy of the main steam, ht is the enthalpy of the boilerfeed water, Drh is the flow rate of the reheat steam, rrh isthe enthalpy rise of the steam passing through the reheater.

The dry coal consumption rate can be calculated by

Bdry ¼ Q1=ðQHHV;dry � gBÞ ð7Þ

The fuel consumption rate of the as-received coal isdefined as

Bar ¼ Bdry �100�Mdry

100�Marð8Þ

Feed Water Heaters

There are two kinds of feed water heaters in the feedwater heating network. One is the mixing type, which isoften used as the deaerator. As shown in Fig. 5, extractionsteam from the turbine directly joins the feed water to raisethe feed water temperature. The other is the surface type.As shown in Fig. 6, extraction steam from the turbine

condenses and heats the feed water without mixing withit. The condensate of the heating steam of a surface-typefeed water heater flows into the next heater with lowerextraction steam pressure. The flow rate of heating steamof the feed water (streams (i) in Figs. 5 and 6) can be calcu-lated using the heat and mass balance method provided inLin.[37] For the mixing-type feed water heater,

DsðiÞ ¼DtðiÞ � htðiÞ � htði�1Þ

� ��DtsðiÞ � htsðiþ1Þ � htði�1Þ

� �hsðiÞ � htði�1Þ

ð9Þ

For the surface-type feed water heater,

DsðiÞ ¼DtðiÞ � htðiÞ � htði�1Þ

� ��Dtsðiþ1Þ � htsðiþ1Þ � htsðiÞ

� �hsðiÞ � htsðiÞ

ð10Þ

where D is the mass flow rate of a stream, and h is the spe-cific enthalpy of a stream. The subscripts ‘‘s,’’ ‘‘t,’’ or ‘‘ts’’identify the specified stream which is presented in Figs. 5and 6. The flow rate of the extraction steam for each feedwater heater can be calculated from No. 8 to No. 1 sequen-tially using Eqs.(9) and (10).

Drier

Moisture removed from the wet coal during the dryingprocess can be calculated by

Mevap ¼ Bar � 1� 100�Mar

100�Mdry

� �ð11Þ

Superheated Steam-Fluidized Bed Dryer

The model of a superheated steam-fluidized bed dryer isshown in Fig. 7. Temperature rise of the recycled steampassing through the recycle fan is limited. Thus the flui-dized medium provides only a minor part of the heating

FIG. 6. Model of a surface feed water heater.

FIG. 5. Model of a mixing feed water heater.

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duty for the drying process, and it is negligible comparedto that provided by the immersed heater. The amountof the extraction steam required for the superheatedsteam-fluidized bed dryer can be calculated by

Ddry ¼IDC þ IWV � IWC

hs;dry � hts;dryð12Þ

where IDC is the enthalpy of the dry coal leaving the dryer,IWV is the enthalpy of the water vapor leaving the dryer,and IWC is the enthalpy of the wet coal entering the dryer.

Hot-Air-Fluidized Bed Dryer

The model of a hot-air-fluidized bed dryer is shownin Fig. 8. Steam flow rate of the air heater can becalculated by

Ddry;AH ¼ IHA � ICAhs;dry � hts;dry

ð13Þ

where IHA is the enthalpy of the hot air leaving the airheater, and ICA is the enthalpy of the cold air enteringthe air heater.

The flow rate of the steam in the immersed heater can becalculated by

Ddry;IH ¼ IDC þ IEG � IWC � IHA

hs;dry � hts;dryð14Þ

where IEG is the enthalpy of the exhaust gas at the outlet ofthe dryer.

The flow rate of heating steam for the hot-air-fluidizedbed dryer can be calculated as follows:

Ddry ¼ Ddry;AH þDdry;IH ð15Þ

Power Output

The power output of the steam turbine can be calculatedby

W ¼X

Di � h0 � hið Þ þX

Dj � h0 þ rrh � hj� �

þDpump � h0 þ rrh � hpump

� �þDcon � h0 þ rrh � hconð ÞþDdry � h0 þ rrh � hdry

� �ð16Þ

where Di is the flow rate of the extraction steam withoutreheating, Dj is the flow rate of extraction steam that hasbeen reheated, Dpump is the flow rate of the extractionsteam for the main feed water pump turbine, and Dcon isthe flow rate of steam entering the condenser.

Efficiency

The efficiency of the steam turbine cycle is defined as

g0 ¼ W=Q1 ð17Þ

The overall efficiency of the power generation system isdefined as

g ¼ g0 � gB � gP � gM � gGEN ð18Þ

where gP is the insulation efficiency of the pipeline, gM isthe mechanical efficiency of the turbine, and gGEN is theelectric generator efficiency.

FIG. 8. Model of a hot-air-fluidized bed dryer.

FIG. 7. Model of a superheated steam-fluidized bed dryer.

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RESULTS AND DISCUSSION

In order to evaluate the performance of the power plantswith pre-drying systems, calculations were conducted usingthe above-mentioned models. Calculations under typicaloperating conditions were carried out for each plant. Theeffects of drying degree and heating steam parameters onthe efficiency of the power plant were studied. The perfor-mance of the power plant with a hot-air pre-drying systemat different exhaust gas temperatures was also studied. Inaddition, the influence of heat recovery on the superheatedsteam pre-drying power plant was investigated. The resultswill be presented and discussed in the following.

Power Plant Performances under Typical OperatingConditions

Tables 1 and 2 present the calculation details of the typi-cal operating condition (Table 1 lists the main parameters,Table 2 lists the parameters of the feed water heater net-work). Brown coal with moisture content (wet basis) of45% on an as-received basis is employed in the study.The ultimate analysis and HHV of the coal are shown inTable 3. In typical operating conditions, final moisturecontent (wet basis) of the dried coal is specified to be 25%.

Reasonable operating conditions for each pre-dryingsystem should be applied according to the correspondingdrying characteristics of LRC. In order to maintain thesteam atmosphere in the superheated steam-fluidized beddryer, its temperature should be kept above 100�C. In con-trast, hot-air drying can be performed at a much lowertemperature. The temperature difference between the coalparticle and the immersed heater should be large enoughto keep the drying rate at an acceptable level. The mainoperating conditions of the superheated steam and thehot-air pre-drying systems are listed in Tables 4 and 5,respectively.

For the sake of comparison, performance of a conven-tional power plant using external dried LRC is studied.In this case, wet coal is first dried in a separate facilityand then delivered to the plant. The heat consumptionassociated with coal drying is not considered in the analysisof power plant efficiency, therefore the maximum efficiencycan be achieved by using external drying.

Calculation results for the typical operating conditionsof different power generation systems are summarized inTable 6. By reducing the moisture content (wet basis) from45% to 25%, HHV of the brown coal increases from12.758MJ to 17.397MJ, and the thermal efficiency of theboiler based on the HHV increases from 78.59% to84.24%. Moreover, the use of an LRC pre-drying processleads to a reduction of 6.5% of the flow rate of the raw coal.

Extraction steam reduces the efficiency of the steamcycle for both superheated steam and hot-air pre-drying

TABLE 1Main parameters of the typical operating conditions

Main steam flow rate, kg=s 721.9Main steam parameters 24.8MPa=600�CBoiler feed water parameters 30.3MPa=289�CSteam parameters before reheating 5.6MPa=364�CReheated steam parameters 5.1MPa=600�CTurbine exhaust steam parameters 4.4 kPa=2315 kJ=

kgExcess air ratio of boiler 1.15Environment temperature, �C 20Flue gas exit temperature(after the air preheater),�C

150

Heat loss due to unburnedgaseous combustibles Q3

0

Heat loss due to unburnedsolid combustibles Q4

0.01�Q

Heat loss due to radiationand convection Q5

0.003�Q

Other heat loss of boiler Q6 0.002�QHeat-supply pipe efficiency 99%Mechanical efficiency of the steamturbine

99.5%

Generator efficiency 99%

TABLE 2Parameters of the feed water heater network

Feed waterheater

Heating steamenthalpy (kJ=kg)

Drain waterenthalpy (kJ=kg)

Inlet feed waterenthalpy (kJ=kg)

Outlet feed waterenthalpy (kJ=kg)

1 2487.3 152.6 128.3 248.22 2619.3 270.5 248.2 344.23 2842.2 – 344.2 506.34 3035.8 529.6 506.3 647.15 3192.7 – 647.1 764.06 3390.6 814.0 803.8 928.57 3091.3 944.6 928.5 1179.88 3156.1 1211.3 1179.8 1272.6

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systems. The efficiency of the steam cycle with superheatedsteam-fluidized bed pre-drying is found to be slightly lowerthan that with hot-air pre-drying. This is because thesuperheated steam-fluidized bed pre-drying uses highercondition steam as the heat source of the immersed heater.

In Table 6,the electricity generation decreases from 950MW to 913.21 MW and to 919.49 MW for the superheatedsteam and the hot-air pre-drying power plant, respectively.Improvements of the overall efficiency for the superheatedsteam and the hot-air pre-drying power plants are 1.16%and 1.43%, respectively. It should be noted that the mostsignificant efficiency increase takes place in the power plantusing external LRC drying. But this is only because theenergy consumption of the external coal drying is not con-sidered in the calculation.

Effect of Drying Degree

Performances of the above-mentioned pre-drying powerplants with different drying degree (varying from 40%–10%) are analyzed to illustrate the influence of dryingdegree on the overall efficiency of these plants. Figure 9shows the variations of coal HHV and boiler efficiencywith coal moisture content. It can be seen that the HHVincreases linearly as coal moisture content decreases. Theboiler efficiency increases from 80.35% to 86.83% (HHVbasis) when the coal moisture content decreases from40% to 10%. The reason is that the heat loss of the boilerdue to exhaust gas decreases with decreasing coal moisturecontent. Figure 10 shows the overall efficiency against coalmoisture reduction for different pre-drying power

generation systems. It can be seen that the overallefficiencies of the power plants are increased linearly ascoal moisture content is reduced. The efficiency withhot-air-fluidized bed drying is slightly higher than that withsuperheated steam-fluidized bed drying. The calculatedvalues of the power plant efficiency in Fig. 10 are compara-ble with the results of Ref.[32].

Effect of Extraction Steam Parameters

In this study, extraction steam from the steam turbineprovides the heat duty during the drying process. Perfor-mances of the power plants with different extraction steamparameters are calculated to study the influence of extrac-tion steam parameters. For superheated steam drying, thepressure of the extraction steam is set equal to that of feedwater heater Nos. 4, 5, and 6, respectively, and in contrast,for the hot-air drying, which is Nos. 3, 4, and 5, respect-ively. Figures 11 and 12 present the effects of the extractionsteam parameters on the efficiency of power plants withsuperheated steam and hot-air drying, respectively. Theresults show that the overall efficiency becomes lower with

TABLE 3Ultimate analysis and HHV of the LRC on an as-received basis

C (wt.%) H (wt.%) S (wt.%) O (wt.%) N (wt.%) Ash (wt.%) H2O (wt.%) HHV (MJ=kg)

32.49 2.32 0.83 9.73 0.17 9.46 45 12.76

TABLE 4Parameters of superheated steam-fluidized bed pre-drying

system

Moisture content in the driedLRC, (%, wet basis)

25

Temperature of the dryerexhaust gas,�C

110

Parameters of heating steamof the immersed heater

0.56MPa = 3035.8 kJ=kg

Parameters of condensateof the immersed heater

126.0�C = 529.6 kJ=kg

Waste heat recoveryof dryer exhaust gas

Not Applied

TABLE 5Parameters of hot-air-fluidized bed pre-drying system

Moisture content in the driedLRC, (%, wet basis)

25

Temperature of the dryerinlet air, �C

120

Temperature of the dryerexhaust gas,�C

60

Static bed height inthe dryer, m

0.5

Superficial velocityin the dryer, m=s

3

Mean residence timeof particles, s

300

Parameters of heating steamof the air heater

0.22MPa=2842.2 kJ=kg

Parameters of condensateof the air heater

87.6�C=366.9 kJ=kg

Parameters of heating steamof the immersed heater

0.22MPa=2842.2 kJ=kg

Parameters of condensateof the immersed heater

87.6�C=366.9 kJ=kg

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higher extraction steam pressure. Similar results wereobtained in Refs.[30,32] As long as the drying duty can bemet, low-extraction steam pressure is preferred toguarantee high power plant efficiency.

Effect of Exhaust Temperature

Figure 13 shows the effect of exhaust temperature on theefficiency of a power plant using hot-air drying. The resultsshow that the power plant efficiency decreases with theincrease of the exhaust temperature. When the exhausttemperature drops from 80�C to 60�C, the efficiency risesabout 0.1%. This trend is not affected as the moisturereduction varies from 0.1 to 0.3 kg=kg. It can thereforebe concluded that changing the exhaust gas temperaturefrom 80�C to 60�C shows no significant effect on theoverall efficiency of the power plant. On the other hand,a rise in the exhaust temperature may increase the dryingrate remarkably and may further result in reductionof capital investment and operating cost. The exhaust

TABLE 6Calculation results for the typical operating conditions

Conventional LRCpower plant

Superheated steamFBD power plant

Hot-air FBDpower plant

Power plant withexternal LRC drying

Moisture content(%, wet basis)

45 25 25 25

HHV, MJ 12.758 17.397 17.397 17.397Boiler efficiency gB, % 78.59 84.24 84.24 84.24Flow rate of raw coal, kg=s 194.04 181.35 181.35 181.35Steam cycle efficiency gi, % 49.79 47.87 48.19 49.79Electricity generated, MW 950.00 913.21 919.49 950.00Overall efficiency g, % 38.16 39.32 39.59 40.90

FIG. 9. Coal HHV and boiler efficiency as a function of coal moisture

content.

FIG. 10. Effect of coal moisture reduction on power plant efficiencies

with different pre-drying systems.

FIG. 11. Effect of extract steam parameters on the efficiency of a power

plant with superheated steam pre-drying.

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temperature should therefore be chosen with caution whendesigning an LRC pre-drying system.

Effect of Heat Recovery

For the superheated steam-drying process, latent heat inthe exhaust gas can be recovered by heating a part of thefeed water from the condenser. The exhaust gas amountis small at a low degree of drying (0.1 kg=kg); recoveringall the latent heat of the exhaust gas is only enough to heat40% of the feed water. At a higher degree of LRC drying,the available latent heat and subsequently the heated feedwater amount will be larger. When the coal moisture

reduction value exceeds about 0.2 kg=kg, all feed watercan be heated, and the extraction steam for the feed waterheaters Nos.1 and 2 can be cancelled. Even then, a sub-stantial part of the energy remains unused, and the excesswater vapor has to be discharged into the environment.Figure 14 shows the ratios of heated feed water amountto the total feed water amount and the recovery rate ofwaste heat in the exhaust gas versus coal moisturereduction value. Figure 15 shows the effect of heat recov-ery on the efficiency of a power plant with superheatedsteam drying. An overall efficiency increase of about0.35% can be achieved by recovering the latent heat ofthe water vapor in the exhaust gas.

FIG. 14. Ratio of heated feed water amount to the total feed water

amount and ratio of recovered heat to the total latent heat in the exhaust

gas versus coal moisture reduction.

FIG. 13. Effect of exhaust temperature on the efficiency of a power plant

with hot-air drying.

FIG. 12. Effect of extract steam parameters on the efficiency of a power

plant with hot-air drying.

FIG. 15. Effect of heat recovery on the efficiency of a power plant with

superheated steam drying.

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CONCLUSIONS

In this study, LRC pre-drying power generation systemswere designed based on a conventional power plant with acapacity of 1000 MW, models for each component of thesepower generation systems were developed based onmaterial and energy balances, and calculations based onthese models were conducted to evaluate the performancesof these power plants. The main conclusions of this workcan be drawn as follows:

1. By pre-drying the LRC to a moisture content of 25%,the flow rate of the raw coal can be reduced by 6.5%,and the overall efficiency increases for the power plantwith steam and hot-air pre-drying are 1.16% and1.43%, respectively.

2. The boiler efficiency increases from 80.35% to 86.83%(HHV basis), and the overall efficiencies of the powerplants are increased linearly, as the coal moisture con-tent varies from 40% to 10%.

3. Using higher extraction steam parameters in theimmersed heater can cause a lower overall efficiency.

4. Changing the exhaust gas temperature has a weak effecton the overall efficiency of the power plant with hot-airpre-drying.

5. Recovering the latent heat of the exhaust water vaporbrings about a 0.35% increase in the overall efficiencyfor the superheated steam pre-drying power plant.

NOMENCLATURE

Bar Fuel consumption rate of the as-received coalBdry Dry coal consumption rateD Mass flow rateDdry Amount of the extraction steam required for

the dryerDdry, AH Flow rate of the steam in the air heaterDdry, IH Flow rate of the steam in the immersed heaterDfw Flow rate of the feed waterDrh Flow rate of the reheat steamh Specific enthalpy of streamh0 Enthalpy of the main steamht Enthalpy of the boiler feed waterICA Enthalpy of the cold air entering the air

heaterIDC Enthalpy of the dry coal leaving the dryerIEG Enthalpy of the exhaust gas at the outlet of

the dryerIHA Enthalpy of the hot air leaving the air heaterIWC Enthalpy of the wet coal entering the dryerIWV Enthalpy of the water vapor leaving the dryerMar Moisture content on an as-received basisMdry Moisture content after the drying processMevap Moisture removed from the wet coal during

the drying process

Q1 Heat absorption of water and steam in theboiler

Q2 Heat loss due to exhaust gasesQ3 Heat loss due to unburned gaseous

combustiblesQ4 Heat loss due to unburned solid combustiblesQ5 Heat loss due to radiation and convectionQ6 Other heat loss of the boilerQHHV, ar HHV of the coal on an as-received basisQHHV, dry HHV of the coal after the drying processQin Heat entering the boilerXar A constituent content of LRC on an

as-received basisXdry A constituent content of LRC after the drying

processW Power output of the steam turbine

Greek Letters

g0 Efficiency of the steam cyclegB Thermal efficiency of the boilergGEN Electric generator efficiencygM Mechanical efficiency of the turbinegP Insulation efficiency of the pipelinerrh Enthalpy rise of the steam passing through

the reheater

ACKNOWLEDGMENT

The financial support from Shanghai Boiler Works Ltd.for this research is gratefully acknowledged.

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