August 2001 ECN-C--01-075

Analysis and feasibilityof advanced gas turbine cycles

and applications

The partial oxidation gas turbineand the gas turbine with air bottoming cycle

J.W. Dijkstra

Revisions

A Final Report

Made by:

J.W. Dijkstra

Approved:

D. Jansen

Checked by:

M. Weeda

Issued:

C.A.M. van der Klein

ECN-Clean Fossil Fuels

2 ECN-C--01-075

JustificationThis study was performed under and funded by the New Energy Conversion Technologyprogram by the Netherlands Agency for Energy and Environment Novem B.V.Novem contract number : 248.101.0133.Novem project title : Assessment of the technical feasibility and future market

potential for a new concept of a gas turbine system withstepwise sub-stoichiometric combustion

Novem project manager : Dr. Ir. A.H.M. KippermanECN project number : 7.2866Contributions to this study are made by M.A. Korobitsyn, P.W. Kers (University of Twente), R.van der Ploeg (Fluor Daniel), T. Kerkhoven (Fluor Daniel), W. Altena (Fluor Daniel), F. vander Wiel (Rijksuniversiteit Groningen), R. Sepp (Rijksuniversiteit Groningen), R. Verdurmen(NIZO food research), P. Sluimer (TNO voeding), W. Rouwen (TNO voeding).The author would like to thank D. Jansen, D. Göebel (Fluor Daniel), R.M. Voncken(Rijksuniversiteit Groningen) L. Wattimena (Krachtwerktuigen Adviseurs), S. van der Wal(Jacobs Engineering Nederland) for their contributions.

This report is the technical summary of three studies, which were carried out for this project.

AbstractThe main results of three technical feasibility studies are presented concerning new concepts forgas turbine cycle schemes and possibilities for gas turbine process integration in order toachieve energy savings. The subjects and results of the consecutive parts are:(i) a gas turbine cycle with stepwise substoichiometric combustion (partial oxidation gasturbine, PO-GT cycle). Thermodynamic analysis in comparison with other gas turbine cyclesshows that the PO-GT cycle has no pronounced advantages above conventional gas turbinecycles within a feasible range of pressures and temperatures.(ii) The application of a PO-GT cycle for combined syngas and power production in a methanolproduction process. A PO-GT cycle for synthesis gas production is feasible and shows a goodoverall efficiency (82%). An integration of this system with a once-through methanol plantshows even higher overall efficiencies (92%).(iii) Application of a gas turbine equipped with an air bottoming cycle (GT-ABC) for thecombined production of electricity and clean hot air. Applications for the hot air studied arefurnace heating and for spray drying purposes in the bakery and dairy industry. For directlyfired bakery furnaces the impact on the product quality has been identified as an obstacle. Forindirectly fired furnaces a GT-ABC is feasible, is more attractive and offers the same primaryenergy reduction (12%). In the dairy industry the system is feasible and offers high potentialprimary energy savings (18%). High investment costs however are identified as problematic.

Key wordsSub-stoichiometric combustion Exergy analysisPartial oxidation gas turbine Syngas productionGas turbine Spray dryerRecuperated gas turbine Methanol productionReheat gas turbine Air bottoming cycleDairy industry Bakery furnace

ECN-C--01-075 3

CONTENTS

SUMMARY 5

1. INTRODUCTION 7

2. PART I: THE PARTIAL OXIDATION GAS TURBINE 112.1 Introduction 112.2 Method 122.3 Results 132.4 Conclusions 15

3. PART II: COMBINED SYNGAS AND POWER PRODUCTION 173.1 Introduction 173.2 Method 173.3 Results 183.4 Conclusions 20

4. PART III: ADVANCED GAS TURBINE CYCLES IN THE DAIRY AND FOOD INDUSTRY 21

4.1 Introduction 214.2 Method 214.3 Results 234.4 Conclusions 25

5. CONCLUSIONS 27

6. RECOMMENDATIONS 29

7. REFERENCES 31

4 ECN-C--01-075

ECN-C--01-075 5

SUMMARY

The gas turbine (GT) has become an important prime mover for conversion of natural gas intoelectricity. In the last decades significant improvements have been achieved in the efficiency ofgas turbines through advances in materials and turbomachinery design. However, a largeamount of energy is still contained within the gas turbine exhaust gasses. Because of thewidespread use of gas turbines for power production large energy savings can be obtained if:• This percentage can be reduced;• Better use or more use of these exhaust gasses can be made compared to current practice.In this study three possible concepts for improved energy conversion with gas turbines areassessed.(a) a gas turbine cycle with (stepwise) sub-stoichiometric combustion (or partialoxidation gas turbine 'PO-GT') and (b) a gas turbine with an air bottoming cycle. The firstalternative is evaluated for both power production and for combined syngas and powerproduction. The second alternative is evaluated for application in the dairy and bakery industry.

Gas turbine with stepwise substoichiometric combustionThe study gives a thermodynamic analysis of the gas turbine cycle with stepwisesubstoichiometric combustion in comparison with other gas turbine cycles (simple cycle, reheatcycle and recuperated cycle). The analysis is performed using analytical models, flow sheetingand exergy analysis using the thermal efficiency, specific work output power and exergeticefficiency as criteria. Based on the performance maps generated, it is concluded that a gasturbine system with partial oxidation shows no pronounced performance advantages comparedto conventional gas turbine cycles within a feasible range of pressures and temperatures.

Partial oxidation gas turbine for combined syngas and power productionThe use of a partial oxidation reactor with a hot syngas expander for combined syngas andpower production for methanol synthesis has been assessed. The assessment has been performedusing flow sheeting simulations. The criteria used were the combined syngas and powerproduction efficiency of the processes. Two cases have been developed. In the first case theconventional synthesis gas generation is replaced by a partial oxidation reactor with a hotsyngas expander. The efficiency is about 82% and the power required for the production can beproduced by the syngas. In the second case this cycle is integrated with a once-throughmethanol plant and a combined cycle. This case shows very good overall efficiencies(approximately 90-94%), higher than those of combined heat and power units.

Application of a gas turbine with air bottoming cycle in the food and dairy industryA gas turbine can be equipped with an air bottoming cycle to produce clean hot air as well aselectricity. Three cases for use of this hot air have been evaluated for technical and economicfeasibility: (i) use in directly fired bakery furnaces, (ii) use in indirectly fired bakery furnaces,and (iii) use in spray dryers in the dairy industry. A survey is made on the boundary conditionsdictated by the applications selected. In directly fired furnaces impact on the product qualitymay form a serious obstacle for the application of a GT-ABC. In indirectly fired furnaces a GT-ABC can be applied. However, a gas turbine will be preferred, having the same primary energyreduction compared to separate generation of electricity and heat (12-13%). In the dairy industryapplication of a GT-ABC is feasible. It shows a good primary energy reduction (28%), theabsolute investment however might be an obstacle for implementation.

6 ECN-C--01-075

ECN-C--01-075

1. INTRODUCTION

The gas turbine (GT) has become an important prime mover for conversion of natural gas intoelectricity. The basic concept of a gas turbine cycle is given in Figure 1.

3

4

Expander

Fuel

Compressor

2

Air 1

Combustor

Exhaust

Figure 1: Schematic presentation of a ga

In the last decades significant improveturbines through advances in materials afrom around 25% in 1960 to almost 40%the input energy is contained within the use of gas turbines for power production • This percentage can be reduced;• Better use or more use of these exhauIn this study three possible concepts foassessed. These are covered in three coprevious work in which a broad range o[9]. The concept worked out in Part II rPart I.

Further increase of gas turbine efficieIn spite of the gas turbine improvementfor further efficiency increase. The efincreasing expander inlet temperature. Inthe field of materials and turbomachinerissue, amongst others since this is closelyBesides pushing the expander inlet improvements, another possibility may bAnalysis shows that the largest losses inchamber [10].

exhaustFuelFuel

2

Air 1

6543

Figure 2: Scheme of the gas turbine cycle

Ad figure 1: working principle of a gas turbineAir at ambient conditions is compressed and fed to acombustion chamber. In the combustion chamber fuel isadded. The resulting combustion of the fuel raises thetemperature. The hot stream then enters the expander whereit is expanded to atmospheric pressure. The work deliveredby the expansion drives the compressor and a generator.

7

s turbine cycle (Simple Cycle).

ments have been achieved in the efficiency of gasnd turbomachinery design. Efficiencies have increased

today [5]. This implies however that almost 60% ofgas turbine exhaust stream. Because of the widespreadlarge energy savings can be obtained if:

st gasses can be made compared to current practice.r improved energy conversion with gas turbines arensecutive parts. Two were identified as promising in

f new energy conversion technologies was investigatedesulted from observations done during the execution of

ncys already achieved, there is still considerable potentialficiency of a gas turbine in general increases withcreasing this temperature requires further advances iny design. Besides efficiency, size is also an important related to the costs.temperature to higher limits to achieve efficiencye to use a different cycle scheme from that of Figure 1. the gas turbine process are located in the combustion

with reheat

8 ECN-C--01-075

The reheat cycle is a well-known and commercialised alternative [11]. A schematic drawing ofthe reheat cycle is given in Figure 2. Here the combustion section is divided into two steps. Inboth steps an overstoichiometric amount of air is used.

Fuel

Air

LPC HPC

Partial oxidationreactor

HPT LPT

Conventionalcombustor

1 2 3 4

bca

(1-x) kg/s

x kg/s

Figure 3: Scheme of the PO-GT cycle.

A concept derived from the reheat cycle scheme is the gas turbine with stepwisesubstoichiometric combustion, also referred to as the partial oxidation gas turbine (PO-GT). Aschematic drawing of PO-GT cycle is given in Figure 3. The combustion process is again splitup into two (or more) stages. The first stage however being substoichiometric. Air is withdrawnfrom the compressor for the conventional combustor. This system has been proposed inliterature as a method to reduce the irreversibilities in the combustion process. In literature highefficiencies of the PO-GT cycle have been reported. It was, however, not clear whether thesewere due to the PO-GT concept or to other improvements assumed. To clarify this matter astudy was performed, in which the thermodynamic performance of this cycle is analysed andcompared to alternative cycles. A summary of this study is given in Part I.

Further increase of gas turbine efficiencyCommon practice for electricity generation is to use exhaust heat of the gas turbine foradditional electricity generation with a steam bottoming cycle. Here the flue gas of the gasturbine is used to raise steam in a Heat Recovery Steam Generator (HRSG). The steam thendrives a steam turbine, coupled to a generator. As mentioned, overall power plants efficienciesof over 55% can now be reached with these Combined Cycle Power Plants (CCPP).Nevertheless, with an overall efficiency of 55%, still 45% is being disposed as waste heat. In anHRSG only part of the energy in the gas turbine exhaust heat is used for electricity production.A significant part of this energy is lost. A better use of the heat in the gas turbine exhaust gassescan be made if (part of) this heat can be applied for heating purposes, e.g. for district heating orfor industry heat supply using hot water or steam as an intermediate medium. By applyingcombined heat and power production (CHP) large energy savings can be achieved compared toseparate generation of electricity in power plants, and conventional heat generation in boilersand furnaces.

Comparable energy savings to conventional CHP may be achieved by integration of gas turbinewith other processes. Two options for this have been studied. The first option is to use theexhaust heat of a gas turbine in a second gas turbine cycle to produce additional electricity aswell as clean hot air. A schematic drawing of the system investigated, the gas turbine with airbottoming cycle (GT-ABC) is given in Figure 4.

ECN-C--01-075 9

Air

Exhaust

Air-outAir

Fuel

2122

21

3

4

2423

Intercooling(optional)

Heat exchanger

Figure 4: Scheme of the Gas Turbine with Air Bottoming Cycle

Applications for use of the hot air that are investigated are the use in bakery furnaces and inspray drying towers in the dairy industry. An assessment of the feasibility, energetic andeconomic benefits of this system is given in Part III.

The results of Part I showed a relative high exergy efficiency of the substoichiometriccombustion stage of the PO-GT cycle. This led to a new gas turbine cycle concept in which thesecond stage (the combustion stage) is omitted. The cycle exhaust gasses then consist ofsynthesis gas. This exhaust gas can then be used for chemical synthesis, e.g. methanolproduction. The resulting cycle scheme is given in Figure 5.

Generator

Turbine

POX reactor

Fuel in

Oxidizingagent

ChemicalplantCompressor

~

Figure 5: Scheme of combined syngas and power production

An assessment of the thermodynamic benefits of such a combined syngas and power plant isgiven in Part II.

10 ECN-C--01-075

ECN-C--01-075 11

2. PART I: THE PARTIAL OXIDATION GAS TURBINE

2.1 IntroductionIn spite of improvements in gas turbine efficiency in the past decades the efficiency of a gasturbine is still not at its thermodynamic maximum. With exergy analysis1 it can be shown thatthe largest losses in the gas turbine cycle are located in the combustion chamber of the gasturbine (compared to the thermodynamically ideal process) [10]. Partial oxidation (sub-stoichiometric combustion) has been proposed as a method to reduce the irreversibilities in thecombustion process.

The results of this part have been published by Kers [6], Korobitsyn and Kers [1] andKorobitsyn [7]. A full report of this study is given in [1].

A schematic representation of the 'gas turbine with stepwise sub-stoichiometric combustion'which will be further referred to as the 'partial oxidation gas turbine' or PO-GT is given inFigure 6. After compression (process 1-2), air is fed to the partial oxidation reactor, where asynthesis gas is formed. Then, the gas is expanded in the high-pressure turbine to an intermediatepressure (process 3-a), and secondary air is introduced before the final expansion stage tocomplete oxidation of the fuel (process b-4).

Fuel

Air

LPC HPC

Partial oxidationreactor

HPT LPT

Conventionalcombustor

1 2 3 4

bca

(1-x) kg/s

x kg/s

Figure 6: Basic scheme of the PO-GT cycle.

From literature very high efficiencies were reported for partial oxidation type gas turbines. (See[1] for an overview). It was however not clear whether these were due to the partial oxidation ordue to other improvements assumed.

The goal of Part I of this study is to assess the benefits of the gas turbine with stepwise sub-stoichiometric combustion, compared to alternative gas turbine cycles. The criteria used will becycle efficiency and equipment size.

The cycle with partial oxidation were compared with two other gas turbine cycles:• Simple cycle (See Figure 1);• The cycle with reheat (See Figure 2).Within these cycles recuperation can be applied. Recuperation is heat exchange betweencompressed air and gas turbine flue gas. (See Figure 7).

1 The concept of exergy analysis is not further introduced. A good discription can be found in [10].

12 ECN-C--01-075

3

5

4

6

Exhaust

2

Air1

Fuel

Gas turbine cycle with recuperation

Figure 7: Gas cycle with recuperation

The analysis of the PO-GT cycle was done using two evaluation methods: Firstly an energeticevaluation was done to assess the main characteristics of the partial oxidation cycle. Secondlyand exergetic evaluation was performed for gaining better understanding of the systems, and toaccount for the exhaust heat use. Method and results will subsequently be treated.

2.2 Method

The analysis was performed using a staged approach:1. Ideal gas analytical models were made. These models gave insight in the structure of the

performance maps generated with the process calculation the next step;2. Process calculations with the process simulation package Aspen Plus;3. The performance of the cycles in combined cycle mode has been assessed. To account for

the amount of usable exhaust heat, which can be used in combined cycle mode additionalexergy calculations have been performed. The exergy analysis has also been performed togain a better understanding of the cycles under consideration and to evaluate the effects ofstaged combustion.

Operating parameters used are:• The pressures ratio (between 8 and 40)• The turbine inlet temperature (between 1200 and 1400° C).

A detailed overview of the starting points for the simulations is presented in [1], Chapter 4.

The criteria used for comparison of the PO-GT cycle with the other cycles were:• The thermal efficiency:

[-]g value)wer heatin input (loTotal heat

WorkTotal Net LHV) ficiency (Thermal Ef =

• The thermal efficiency is a direct measure for the energy conversion efficiency from fuel towork delivered (e.g. for power production). The amount of work which can be gained fromthe exhaust gas stream is accounted for by using the exergetic efficiency, which is in principle better, but requires additional data processing;

• The Dimensionless Specific Work Output:

[-]*T*C

WorkTotal Net tput ic Work Ouess SpecifDimensionlpm,air 1φ

=

ECN-C--01-075 13

• where φm, air is the air mass flow, Cp is the heat capacity at constant pressure [J/kg k] and T1is the temperature of the environment [K]. The dimensionless specific work output is ameasure for the size of the apparatus;

• The specific work, which is also a measure for the size:

[kJ/kg]WorkTotal Net ork Specific Wm,airφ

=

.

The results are presented in tables and performance maps showing efficiency vs. dimensionlessspecific work output. The goal is to have a gas turbine, which has both a high efficiency and ahigh (dimensionless) specific work output. E.g. a cycle should be in the upper right part of theperformance map.Furthermore the exergy losses in the combustion chamber Icomb will be used for analysis.

In the exergetic evaluation the criteria used are:• The combined cycle exergy efficiency (the exergy efficiency of a cycle equipped with a

steam bottoming cycle):

AIRFUEL

EXHAUSTBCex,NETex,CC BB

BW+

⋅+=

ηη

• In which Wnet is the work done by the cycle, ηex,BC is the exergy efficiency of the bottomingcycle, Bexhaust, Bfuel, and Bair air the exergies of respectively the cycle exhaust gas, fuel andair. The exergy of a stream is the maximum work potential of the stream, with theenvironment taken as a reference. For a combined cycle an exergy efficiency of 69 % hasbeen used.

2.3 ResultsThe map resulting from the Aspen Plus simulations are given in Figure 8. Only the non-recuperated cycles are evaluated here.

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1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

28

30

32

34

36

38

40

42

44

46

Pressu

re Rati

o

TIT

Cycle with reheatCycle with PO

Simple cycle

1400o

1300o

1200o

403632

2824

20

16

12

8

Ther

mal

Effi

cien

cy (%

)

Dimensionless Specific Workoutput

Figure 8: Performance map of the simple cycle, reheat and PO cycles.

From these performance maps it can be concluded that within the working parametersconsidered in this study, the highest thermal efficiency is achieved by the simple cycle. Thecycle with reheat has the highest specific work. Below a pressure ratio of 36, the performance ofthe cycle with PO lies between that of the simple cycle and the cycle with reheat. Above thatpressure ratio, the reheat cycle yield higher efficiency and larger specific work output. Thus itcan be concluded that the cycle with partial oxidation has no pronounced benefits over the twoalternative cycles.

Table 1 gives an overview of performance values of all cycles, optimised for the maximumwork output by varying the pressure ratio, at a TIT of 1400 °C. At this condition, the exhausttemperature is rather high, which enables the use of a recuperator.

Table 1: Simulation results of the simple cycle, reheat cycle and PO cycle, with and withoutrecuperation.

Without recuperation With recuperationCase Simple

cycleReheat PO Simple

cycleReheat PO

Pressure Bara 18 6.5/40 9.5/40 13 9.8/40 14/40Exhaust temperature °C 655 887 798 438 694 489Specific work KJ/kg 501 678 579 469 626 529LHV thermal efficiency % 39.4 37.9 37.3 48.7 42.7 48.0

The gas turbine system with partial oxidation shows similar performance in terms of efficiencyand specific work as conventional gas turbine systems within a feasible range of pressures andtemperatures. This is both for cycles with, and without recuperation.

Table 2 shows the summary of the results of the exergetic evaluation, for the cases presented inTable 1. By utilising exhaust heat in a bottoming cycle, the resulting combined-cycle efficiencyranges from 57% to almost 60%. The PO-GT cycle shows similar performance in terms ofexergetic efficiency as the other cycles.

ECN-C--01-075 15

Table 2: Simulation results of the simple cycle, reheat cycle and PO cycle, with and withoutrecuperation.

Without recuperation With recuperation

Case Simplecycle

Reheat PO Simple cycle Reheat PO

WOUTPUT, kJ/kg 501 678 579 469 626 529ηηηηex, CC, % 56.91 58.98 57.60 59.68 59.46 59.80I COMB, % (1ststage/total) 28.37/28.37 15.57/24.98 12.54/27.86 26.22/26.22 25.67/24.45 12.54/25.57Woutput = specific work output, ηex,CC = exergy efficiency of combined cycle, Icomb = exergy loss in combustionprocess.

The original reason for evaluation of the PO-GT cycle was exergy loss reduction through partialoxidation. From the results of Icomb in Table 2 it can be seen that the exergy losses in the 1st stageof the combustion process of the PO cycles are indeed low compared to those of other cycles.The reduction of losses in the 1st stage however is compensated by larger losses in the secondcombustion stage, so the total exergy loss in combustion is not much different from othercycles.

From Table 2 the relative low exergy losses in the first stage of the partial oxidation cycle areobserved. The product of this sub-stoichiometric stage is synthesis gas. A possible attractiveapplication of a gas turbine cycle with a sub-stoichiometric combustor therefor could be forsyngas production. This results in a combined syngas and power plant. Possible applications ofthis syngas are:• In an existing boiler (repowering concept);• In a process plant;• In a fuel cell.The first option has already been worked out in literature (See [7] for a short overview). Thesecond option fits within the framework of this project and is further worked out in Part II. Thethird option is beyond the scope of this study.

2.4 Conclusions• A gas turbine system with partial oxidation shows similar performance as conventional gas

turbine systems within a feasible range of pressures and temperatures.• Of simple, reheat and PO-GT cycles, the reheat gas turbine has the highest value of the

specific work.• In a combined-cycle configuration, all recuperated cases show similar performance. A PO-

GT cycle here also has no pronounced advantage.• The use of sub-stoichiometric combustion in a gas turbine cycle does not result in

significant decrease of exergy losses due to combustion. The effect of recuperation onreducing the exergy losses is, by contrast, more profound.

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ECN-C--01-075 17

3. PART II: COMBINED SYNGAS AND POWER PRODUCTION

3.1 IntroductionThe exergy analysis of gas turbine cycles with partial oxidation in Part I indicated a high exergyefficiency of the sub-stoichiometric stage. The product of the sub-stoichiometric stage issynthesis gas. This combination suggests application of gas turbine cycle with a sub-stoichiometric combustor for syngas production, resulting in a combined syngas and powerplant. The possible benefits are the high efficiency of the sub-stoichiometric combustion andthose of combining syngas production with power production. The system will replace theconventional syngas generation step such as steam reforming in a chemical plant for e.g.methanol production. The system cycle will use oxygen instead of air.

The principle of combined syngas and power production is illustrated in Figure 9. Oxygen iscompressed and mixed with fuel. In a partial oxidation reactor (which is basically the same as asubstoichiometric combustor) syngas is formed. This syngas is then expanded. The expanderdrives the compressor and a generator. The actual process scheme is much more complex andinvolves air separation, multistage compression of oxygen and expansion of nitrogen, heatintegration, compression of syngas to the required pressure and optionally integration with acombined cycle or with a methanol plant.

Generator

Turbine

POX reactor

Fuel in

Oxygen

ChemicalplantCompressor

~

Figure 9: Basic scheme of combined syngas and power production

The goal of this part II of this study is to assess the feasibility of combined syngas and powerproduction and to identify a viable process flow diagram for the combined syngas and powerproduction. The criterion used will be the combined efficiency of syngas and power production.The application selected for the syngas is methanol (MeOH) synthesis, since this is the mostimportant application of syngas, and has relative mild syngas pressures demands compared toother syngas utilising processes.

3.2 MethodThe combined syngas and power production schemes have been evaluated assuming acommercial type methanol process and commercially available equipment. The currentlyavailable routes for methanol synthesis have been summarised and characterised with respect tothe required syngas quality. To serve as an alternative for conventional syngas production inmethanol synthesis, the syngas must have a pressure of at least 50 bara. The methanol plantcapacity assumed is 2000 ton/day.

18 ECN-C--01-075

Feedstock for the process is natural gas (which is assumed to be pure methane for the analysis).In the simulations integration of the water steam balance and heat integration of the MeOHproduction section and rest of the complex are excluded. The energy requirements for normalpreheat of feedstock is excluded. The syngas composition adjustment is excluded. Airseparation units are assumed to operate at 10 bara with 100% efficiency.

The feasibility of combined syngas and power production has been assessed using two cases:• Case A is a worked-out scheme of Figure 9. The conventional syngas generation step is

replaced by a PO reactor with a hot syngas expander and the air separation unit;• Case B is a more integrated scheme, which involves integration of Case A with a once-

through methanol plant and a combined cycle power plant.

These evaluations have been performed using the Thermoflex flow-sheeting program.Thermoflex showed not to be able to model water knock-out, which will affect the calculationresults. Additional sensitivity studies in Aspen Plus have been performed for comparison and tostudy the impact of input parameters.

The criteria used are:• The power efficiency

[%])(

)(MWinputfuel

MWproducedyelectricitefficiencyPower =

This can be used to compare the efficiency of the power generation section to that of aconventional gas turbine for power generation;• The "syn plus power efficiency":

[%])()(

)()()(MWinputheatnetMWinputfuel

MWexportheatMWproducedsyngasMWproducedyelectricitefficiencyPowerSyn+

++=+

in which fuel and syngas are valued through their lower heating value. The net heat input isthe small amount of heat required to close the heat balance of the models. The heat exportis the heat exported in combined heat and power operation.

3.3 ResultsSensitivity studiesSensitivity studies are performed with Aspen Plus. The process scheme is based on Figure 9. Animportant modification to be made was the addition of a syngas compressor after the syngasexpander. This compressor is required to bring the syngas to the desired pressure for methanolsynthesis. This implies that syngas is first expanded, cooled down, and then recompressed.Because of the high temperature of the syngas before expansion, this results in a net powerproduction.

Sensitivity studies have been performed on (i) expander inlet pressure, (ii) expander inlettemperature, (iii) expander outlet pressure, (iv) MeOH synthesis pressure and on (v) steamaddition rate monitoring the net shaft power production. The allowable turbo expander inletconditions had the largest impact on the power produced. Both expander inlet pressure andtemperature should be as high as possible for maximum efficiency. The feasibility of usingcommercially available expanders from various manufactures in PO application was therefor

ECN-C--01-075 19

reviewed for maximum allowable conditions. The best available expander has as maximum inletconditions a pressure of 80 bara and a temperature of 540° C. This was used in furthercalculations.

Case study simulation results

Air separationunit

PO reactor+Hot syngasexpander

MeOHplant

Air O2Syngas MeOH

~

Fuel

Figure 10: Schematic representation of Case A. Combined syngas and power plant for methanolproduction.

Case A is the worked-out scheme of the basic combined syngas and power plant, but with theair separation unit integrated in the scheme. A schematic representation of the system is given inFigure 10. Air is compressed in an air compressor and fed to an air separation unit. In the POreactor with steam addition syngas is produced which is expanded in the hot syngas expander.After cooling the hot syngas is compressed to the required pressure and fed to the MeOH plant.Not depicted in Figure 10 are the syngas compressor, nitrogen expanders, and an oxygencompressor. Intercoolers for compressors have been introduced and heaters before expanders.Furthermore a steam cycle has been introduced to lower the syngas temperature to the syngasexpander temperature limitation.

Air separationunit

PO reactor+Hot syngasexpander

Once-throughMeOHplant

O2Syngas

MeOH

~

Fuel

Combinedcycle

~

UnconvertedSyngas

FuelAir

Figure 11: Case B, integration of methanol plant with combined syngas and power plant

Case B is a further integration of the methanol plant with the syngas and power plant. Thesyngas produced is the feed for a once-through methanol process. Unconverted syngas from theMeOH process is used as fuel for a commercial scaled combined cycle, with some additionalnatural gas for making up and backup. Air from the combined cycle (25%) is withdrawn for theair separation unit.Again, the syngas compressor and oxygen compressor, steam cycle as well as the coolers andheaters are not depicted. A fuel expander is introduced in the PO reactor fuel feed stream. Thenitrogen expanders which were present in Case A can be omitted since nitrogen is expanded inthe combined cycle.

20 ECN-C--01-075

The fuel for the combined cycle is low in nitrogen and will have a high degree of CO2sequestration potential. The methanol plant has not been fully modelled, instead an (optimistic)syngas conversion of 50% has been assumed.

Table 3: Calculation results for 2000 ton MeOH/day plantCase Fuel

SourcePowerGenerated

SyngasProduction

NetHeatInput

Heatexport

PowerEfficiency

SyngasProductionEfficiency

Syn PlusPowerEfficiency

Unit MW MW MW MW MW % % %

Case A 671 9 552 11 n.a. ~1 ~82 ~82

Case B 878 274 552 0 n.a. ~57 ~82 ~94

CCPP 483 251 - n.a. n.a. ~52 - ~52Cogen 483 164 - n.a. 246 ~34 - ~85CCPP = Combined Cycle Power Plant (GE FR9)Cogen =CCPP in cogeneration mode

Table 3 presents the results of the modelling. Intermediate results presented are the fuel source,power generated, and syngas production. Since heat integration has not been fully applied thecolumn 'Net heat input' lists the amount of heat to close the heat balance. For reference theperformance of a typical combined cycle power plant (CCPP) and CCPP cogeneration planthave been listed.

It can be concluded that Case A only produces a small amount of power. This is the due to thelow turbine inlet temperature (due to the choice for a commercial available expander) and thenecessity to compress the syngas to the required MeOH plant pressure. Since the feed stream ofthe air separation unit is already pressurised, compression work for this is saved.

The results of Case B compared to the CCPP and Cogen reference values clearly indicate theadvantage of an integrated combined process and power plant. The overall efficiency of aCogen plant increases drastically when operated in Cogen mode. An even higher increase oftotal efficiency from separate electricity production to combined power and syngas and powerproduction is observed in the Case B case (approximately 90-94% total efficiency).

3.4 Conclusions• Syngas generation through partial oxidation with expansion is a good option. The

efficiency is about 82% and the power needed for syngas production can be producedthrough expansion of the syngas;

• Further integration of the partial oxidation reactor with a once-through methanol plant is afeasible option to produce both syngas and power as key products. Overall efficiencies over92% can be reached, while using technology components available today.

ECN-C--01-075 21

4. PART III: ADVANCED GAS TURBINE CYCLES IN THE DAIRYAND FOOD INDUSTRY

4.1 IntroductionIn this part the integration of gas turbine cycles with hot air production is investigated. Thesystem studied is the gas turbine with air bottoming cycle (GT-ABC). Here the exhaust heat of agas turbine is used in a gas turbine based air bottoming cycle to produce clean hot air, as well aselectricity. In a previous study [8] two promising applications for the hot air have beenidentified:1. Combined production of electricity and hot air for drying of milk or whey in a spray drying

tower;2. Combined production of electricity and hot air for use in bakery furnaces.

A full report of this part is given in Dijkstra et. al [3].

Air

Exhaust

Air-outAir

Fuel

2122

21

3

4

2423

Intercooling(optional)

Heat exchanger

SpraydryerOrBakeryfurnace

Figure 12: Gas turbine with air bottoming cycle

A schematic representation of the gas turbine with air bottoming cycle is given in Figure 12.The design consists of two gas-turbine cycles. The topping cycle (upper part of Figure 12) is aconventional gas turbine. The exhaust heat from the topping cycle is transferred through a high-temperature heat exchanger to the bottoming cycle (lower part of Figure 12). This is a gasturbine-type cycle in which the combustion chamber is replaced by a heat exchanger. Both thebottoming and the topping cycle produce electricity. The bottoming cycle additionally produceshot air with a temperature of 210° C-280° C for use in spray dryers or bakery furnaces.

The goal of part III of this study is to assess the feasibility of the GT-ABC for the applicationsmentioned above in terms of technical feasibility, and (indications of) energy efficiency andeconomics. The criteria used will be the primary energy consumption, the investment andsimple pay out time. For this, special attention is paid to the boundary conditions and technicalissues related with application of the GT-ABC.

4.2 MethodThe performance data of the GT-ABC used are those of Table 4.

22 ECN-C--01-075

Table 4: GT-ABC performance [8]Gas turbine System Total GT-

ABCpower

Electricalefficiency

Hot airtemperature

MW % °C1. Simple cycle 5.9 33.8 n.b.2. ABC, no intercooling 7 40.4 278

Allison 571-K 3. ABC, one intercooler 7.4 42.3 2204. ABC, two intercoolers 7.5 43.2 210

Starting points for the economic evaluation are:• 7000 operating hours/year;• Electricity is valued against the electricity purchasing price of 0.0835 Euro/kWh (probably a

high value);• Costs of compressor intercooling are not accounted for.

For both applications the following activities have been employed:1. An inventory of technical boundary conditions for application of the system is made;2. A list of potential users has been made and three potential users have been interviewed;3. The technical feasibility of the application of a GT-ABC is analysed;4. Additionally the feasibility of the direct application of a gas turbine, so without an air

bottoming cycle, is made;5. Taking into account the boundary conditions encountered in the activities above a typical

case for each application of the GT-ABC is selected. In the bakery industry an importantdistinction to be made is that between directly fired furnaces and indirectly fired furnaces.These will be treated separately.The resulting cases are

• A GT-ABC and GT for an indirectly fired furnace• A GT-ABC and GT for an directly fired furnace• A GT-ABC for an spray drying tower;

6. For these cases the main dimensions of a GT-ABC matching the heat demand of this case iscalculated;

7. An economic evaluation of the application of the GT-ABC.

The criteria used are the following:• For the evaluation of boundary conditions, hot air temperature, NOx level, hygienic

demands, and impact on product quality. In addition several practical aspects are taken inconsideration (space available etc.);

• For the evaluation of energy efficiency the savings on primary energy compared to separategeneration of electricity and heat are taken as the criterion using the Dutch electricity parkefficiency of 42% as a reference;

• For the evaluation of the economics the Total Process Investment and the Simple Pay OutTime (SPOT) are taken as criteria. The SPOT is defined as;

][ yrCostsOperatingVariableandFixedbenifitsElectriciy

InvestmentsssProceTotalSPOT−

=

ECN-C--01-075 23

4.3 Results

Air quality requirementsThe air quality requirements are the highest in the dairy industry. As a rule the absolute absenceof any contamination is required. The presence of NOx is not acceptable.In the bakery industry, never harmful products resulting from NOx presence were found inatmospheres that contained NOx levels up to 25 mg/kg. With respect to contamination with oilparticles no practical limits exist. A more detailed study is required to assess this issue.Table 5 gives the air temperature requirements of the applications. Comparing these to the GT-ABC exhaust gas temperature range (210-280° C) gives that the GT-ABC is capable of delivering air with a temperature sufficiently high for that of spray drying towers. However, notfor all bakery furnaces a temperature that is sufficiently high can be reached.

Table 5: Air temperature requirementsApplication Air temperature requiredDirectly fired furnace Approx. 200-300° CIndirectly fired furnace Approx. 300° CSpray drying tower Approx. 185-200° C

Technical evaluation GT-ABC and GT in the bakery industryThe number of industrial bakeries in the Netherlands amounts over 40. An inventory of theDutch Bakery industry has resulted in a list of the nine most important sites for potential use ofthe GT-ABC.

An important distinction to be made is that between directly fired furnaces, where the burnersare placed in the furnace, and indirectly fired furnaces where the heat is transferred either bycirculating air through pipes in the furnace or through heat transfer oil.

In directly fired furnaces the products come into contact with the combustion gasses. Analysisshowed that a GT-ABC could be used to supply heat to a furnace. The temperature is howevernot high enough for all furnaces. An alternative is to feed the furnace with the flue gasses from agas turbine (without an ABC).

Table 6 summarises the results of the calculations for directly fired furnaces. A typical furnacecapacity of 1 ton dough/hr has been selected. The primary energy reduction percentages of aGT-ABC amount to 20%, which is a good value. Those of a GT are even higher: 40%. A largepart of the energy savings for the GT option is achieved a lower air/fuel ratio of the GT optioncompared to the original furnace. The feasibility of this lower air/fuel ratio remains to beinvestigated. If this is acceptable then lowering the air/fuel ratio could also be an option forenergy efficiency improvement of a furnace without using a GT-ABC or GT.A complication will however be that the heat transfer is affected. For the original situation alarge part of the heat transfer takes place through radiation. With the GT-ABC or GT this willbe through convection. This will have a significant impact on the product quality (e.g. breadcolour). A significant research and development effort would be required to overcome this,which will be a serious obstacle for introduction of the GT-ABC or GT for directly firedfurnaces.

24 ECN-C--01-075

Table 6: Results direct fired furnace, 1 ton dough /hrOriginal GT-ABC GT

Air/fuel ratio1) [-]Temperature furnace in [° C]Natural gas use [Nm3/hr]

Electricity production [kWe]Energy for baking [kW]Stack losses [kW]

Primary energy savings [%]

7-53

-194271

-

-278237

879194483+436

20%

3.550546

93194118

41%1) Air/fuel ratio=1 for stoichiometric combustion

Table 7 presents the sizing results for a GT-ABC and GT for an indirectly fired furnace. Atypical capacity of 1 ton dough/hr has been selected. The primary energy savings arecomparable (respectively 12 and 13%). Therefore a GT will be preferred over a GT-ABCbecause a GT will have a lower investment.

Table 7: Results indirect fired furnace, 1 ton dough /hrOriginal GT-ABC GT

Air/fuel ratio1) [-]Temperature furnace in [° C]Natural gas use [Nm3/hr]

Electricity production [kWe]Energy for baking [kW]Stack losses [kW]

Primary energy savings [%]

2.5-28

-19451

-

-278209

897194483+436

12%

3.550546

93194118

13%1) Air/fuel ratio=1 for stoichiometric combustion

Technical evaluation GT-ABC and GT in the dairy industryThe feasibility for application of the GT-ABC for the drying of milk or whey in a spray dryer isinvestigated. An inventory of the Dutch dairy industry showed 22 locations for potential use.One quarter of the sites contacted indicated to be interested. Another quarter indicated that theywanted to be informed about the outcome of the project.

Table 8: Main Dimensions of GT-ABC for selected spray dryer case.Parameter ValueIn:Natural gasOut:AirElectricityIntercoolingMain dimensions:Gas turbineAir Bottoming CycleHeat exchangerEstimated mass rotating equipment

2520 Nm3/hr

60 000 Nm3/hr9.4 MWe1.7 MW

7.5 MWe1.8 MWe5300 m2

115000 kg

ECN-C--01-075 25

The selected GT-ABC type is the one with one intercooler. The GT-ABC main dimensions havebeen calculated for a selected whey-drying tower. The water vaporisation capacity of this toweris 2000 kg/hr. The hot air use is 60000 m3/hr at a temperature of 185° C The results arepresented in Table 8. In addition to the hot air required for the spray dryer, 9.4 MWe electricityis produced. This is about one third larger than the total site electricity demand.Taking the energy efficiency of the Dutch electricity park (42%) and the existing spray dryer asa reference, the energy savings on primary energy amount to 18%.

Economic evaluation of the GT-ABCTable 9 gives results of the project investment estimate. The estimate is based on the mainequipment dimensions. The total project costs are estimated to be 10 million Euro. This is muchlarger than the normal site investment budget (3.6 Million Euro/year). The height of theinvestment will be an obstacle for realisation.

Table 9: GT-ABC investment calculationM Euro

Rotating equipmentHeat exchangerTotal main equipmentTotal direct investmentTotal indirect investmentTotal process investment

4.90.9+5.8

8.01.7+10.0 Million-Euro

Not all electricity produced can be used inside the spray dryer site. About one third is to beexported to the grid. Nevertheless, as a best case scenario, the electricity savings benefits havebeen rated against the purchasing price of electricity. The simple pay out time based on thepurchasing price of electricity is 3.6 years. This is somewhat larger than the limit put by theindustry but sufficient to justify more detailed investigations.

Table 10: Calculation Simple Pay Out TimekEuro/year

Variable costs (additional natural gas use)Fixed costs

Total

Electricity benefits

Simple Pay Out Time

2.10.3+

2.4

5.5

3.6 Year

4.4 Conclusions• The hygienic requirements in the dairy industry are the most stringent. Minimisation of

possible oil contamination must be given appropriate attention. In the bakery industry asmall amount of NOx is acceptable, which opens the alternative of using gas turbines insteadof the GT-ABC. In the dairy industry this is not acceptable.

Dairy industry:• A GT-ABC (with one intercooler, and some small modifications) can supply air with the

quality required for a spray tower. The electricity produced for a selected spray tower is 9.4MWe, which is higher than the demand of a large site;

26 ECN-C--01-075

• The typical primary energy reduction of a GT-ABC at a spray dryer is 18%. The specificinvestment for the selected case is 1070 Euro/kWe. The absolute investment amounts to10.0 Million Euro, which is higher than the total normal site investment budget. The heightof the investment will be an obstacle for realisation;

• The simple pay back time based on the electricity-purchasing tariff is 3.6 years. This figureon itself is sufficiently low to justify further investigations of the GT-ABC cycle.

Bakery industry:• A directly fired furnace can in principle be equipped with a GT-ABC. This has however a

serious impact on the heat transfer mechanism, which shifts from radiant to convective. Thiswill affect the product quality (e.g. bread colour). This will form an obstacle forintroduction of the GT-ABC. A GT is a feasible alternative for the GT-ABC, which hashowever the same complications with heat transfer;

• For a directly fired furnace the primary energy consumption reduction amounts to 20% forthe GT-ABC and 40% for a GT. A large amount of this reduction is achieved by loweringthe air/fuel. The feasibility of this is to be investigated. If lowering the air/fuel ratio isfeasible, it also should be possible to do this in the original configuration;

• For an indirectly fired furnace the primary energy savings for a GT (13%) and GT-ABC(12%) are comparable. Firing an indirectly fired furnace with a GT is therefore the preferredoption.

ECN-C--01-075 27

5. CONCLUSIONS

The goal of this study was to assess concepts for energy consumption reduction with gas turbinecycles by means of:• Improvement of the electrical efficiency;• Make better or more use of the gas turbine exhaust gasses.

Substoichiometric combustion or partial oxidation (PO) in gas turbine cycles proved not to haveany pronounced advantages with respect to efficiency improvement.

The use of PO in gas turbine cycles is a promising option for improvement of gas turbineexhaust gas use. The syngas formed in a PO gas turbine cycle can be used for chemicalsynthesis. High overall efficiencies have been found for processes, which combine syngasproduction with power production. Further integration with a once-through methanol plant hasbeen identified as a promising option.

Producing additional electricity as well as hot air from gas turbine exhaust gasses using a gasturbine with air bottom cycle (GT-ABC) showed significant primary energy savings for all threeapplications studied. The use of the GT-ABC is feasible for applications in spray dryers in thedairy industry. The high investments may however be an obstacle. Application of this cycle fordirectly fired furnaces in industrial bakeries is possible, but possible impact on the productquality is a bottleneck. Application for directly fired furnaces is feasible but direct use of gasturbine exhaust gasses (without a bottoming cycle) is a better option offering the same primaryenergy reduction.

28 ECN-C--01-075

ECN-C--01-075 29

6. RECOMMENDATIONS

Gas turbine cycles with partial oxidation• Three applications of the syngas produced by a system with a partial oxidation reactor with

hot syngas expander have been identified: (i) chemical synthesis (ii) repowering of existingboilers and (iii) for use in a high temperature fuel cell. The first application has beenassessed within this study. The second application has been reviewed in literature. Becauseof the good syngas generation efficiency found for the first option evaluation,implementation of the third option is recommended for further study. Here the PO cycle willfunction as a syngas generating subsystem of a fuel-cell gas turbine combined cycle plant,or high temperature fuel cell system.

Combined syngas and power production• The most favourable route for further evaluation is the combined PO-electricity production

integrated with a once-through methanol process. It is recommended to investigate thefollowing improvements

• The feasibility of using a catalyst in the PO reactor• The feasibility of using a water gas membrane shift reactor for H2/CO versus

conventional CO shift reactors (not relevant for all methanol processes)• Integration of membrane technology within the methanol plant with the system

selected.• A next study phase should also cover plant-wide integration of heat (complete water steam

cycle). A detailed comparison with commercial MeOH production under equal startingpoints should be made. Investment and business models have to be assessed. Furthermore itis recommended to determine the number of hot gas expanders which could be sold yearlyfor this purpose. Once this survey is positive, further interest of (a) gas turbine supplier(s)for developing hot gas expanders, operating at higher temperatures and pressures (at least50-80 bara and 800-900° C) is to be investigated further.

Application of a gas turbine with air bottoming cycle in the food and bakery industry• A survey of the interest in the dairy industry for implementation of a GT-ABC cycle, based

on the outcome of this study, is recommended

• A survey of the interest of the bakery furnace manufacturers and the bakery industry forimplementation of a GT in indirectly fired furnaces, either with air or heat transfer oil, isrecommended.

30 ECN-C--01-075

ECN-C--01-075 31

7. REFERENCES

[1] Korobitsyn, M.A. and P.W. Kers, Assessment of the technical feasibility and future marketpotential for a new concept of a gas turbine system with stepwise sub-stoichiometriccombustion, Part I: Thermodynamic analysis, ECN-CX--98-108, October 1998.

[2] Ploeg, R.v.d., T. Kerkhoven, and W. Altena (Fluor Daniel Company), Internal project report.

[3] Dijkstra, J.W., W. Rouwen, P. Sluimer and R. Verdurmen, Geavanceerde gasturbinecycli inde brood- & banket- en in de zuivelindustrie, ECN-C-01-076, August 2001 (In Dutch).

[4] Korobitsyn, M.A., P.W. Kers and G.G. Hirs, Analysis of a gas turbine with partial oxidation,43rd ASME Gas Turbine and Aeroengine Congress, 2-5 june, 1998, Stockholm, Sweden, 1998.

[5] Anex, R., S. Velnati, M. Meo, R. Ellington, and M. Sharfman , "Innovation and theTransformation to Clean Technologies: Life Cycle Management of Gas Turbine Systems",Proceedings of the 1999 NSF Design and Manufacturing Grantees Conference, January 5-8,1999, Long Beach, CA. (http://www.ou.edu/spp/turbine/paper.html).

[6] Kers, P.W., “Partial Oxidation in Gas Turbine Cycles”, MSc Thesis, Department ofMechanical Engineering, University of Twente, 1997.

[7] Korobitsyn, M.A., New and advanced energy conversion technologies, Analysis ofCogeneration, Combined and Integrated Cycles, Thesis Univerisity of Twente, 1998.

[8] Korobitsyn, M.A., Industrial Applications of the Air Bottoming Cycle, Presented at theInternational Conference on Efficiency, Cost, Optimizations, Simulation and EnvironmentalAspects of Energy Systems and Processes ECOS '99, Tokyo, Japan, June 8-10, 1999.

[9] Korobitsyn, M.A. and A.W.M. van Wunnik, Asessment of Technological and MarketOpportunities of several Advanced Energy Conversion Processes, ECN-CX--98-122, April1999.

[10] Kotas, T.J, The exergy method of plant analysis, Krieger Publ. Comp, 1995.

[11] Cohen, H, G.F.C. Rogers, H.I.H. Saravanamuttoo, Gas Turbine Therory, Second Ed.Longman, 1972.

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