Experiences in Development and Operation of Siemens IGCC Gas

Turbines Using Gasification Products from Coal and Refinery

Residues

M. Huth, G. Gaio, A. Heilos, N. Vortmeyer, B. Schetter, J. Karg

Siemens AG Power Generation Group (KWU), Mülheim, Erlangen - Germany

Abstract

Siemens is engaged currently in 3 IGCC projects which are all placed in Europe. Buggenum/Netherlands started by the 1.1.1998 with commercial operation. In the paper an overview ispresented about the operating experience and test results with the syngas fueled V94.2 gasturbine. Special emphasis is put on the low NOx emissions of 6-30ppm(vol) (15%O2) and COemissions of lower than 5ppm (vol). Some explantions are given about early commissioningproblems with combustion induced pressure oscillations which were solved by minormodifications of the syngas nozzle design of the burners. The demonstration period furthermoreshowed that the gas turbine in the 100% air and nitrogen side integrated IGCC is capable forhandling a lot of possible transient problems in emergency cases at the interfaces like e.g. agasifier or a nitrogen compressor trip. In Puertollano/Spain an IGCC plant is equipped with aV94.3 (with enhanced output and efficiency) and will be fueled with a 50:50 mixture of local highash coal and high sulfur petroleum coke. The plant is currently under commissioning with syngas.In Priolo a new plant fueled with heavy refinery residues is under construction. In this plant twoV94.2K with modified compressor are applied in an airside non integrated concept. As the syngascomposition is considerably different from the other projects extensive combustion tests havebeen performed with a full scale burner up to half of the machine pressure. A short overviewabout the test results is given showing NOx emissions below 25ppm and excellent combustionstability behaviour.

1. Introduction

Gas turbine technology has been applied in the last years mainly in natural gas and oil-firedcombined cycle power plants. This type of plant has resulted in efficiency increases considerablyabove the 55% mark1. The addition of an integrated gasification plant also opens this combinedcycle process to coal and the other carbonaceous fuels mentioned above, at emission levels farbelow conventional plants.

Since 1994 a Siemens gas turbine model V94.2 in the Integrated Gasification Combined Cycle(IGCC) power plant Buggenum, Netherlands, has been fired with coal gas generated by a SHELLgasifier. The combustion system was modified to burn both, natural gas and coal gas and has sofar accumulated more than 11000 hours of coal gas operation. Buggenum was world’s firstcommercial IGCC Power Plant in the range above 250 MW(el.). By the end of 1997 thedemonstration period of the Buggenum power plant is finished and the plant runs as a productionunit in commercial operation2. The design data referring to overall plant efficiency of about 43%and base load power output of 284 MW (gross) and 253 MW (net) at 15°C could be confirmed.

Siemens gas turbines are also applied in two other European IGCC projects. In the Puertollanoplant in Spain, which is world’s largest IGCC plant to date, coal gas from a PRENFLO coalgasifier is applied as fuel for a V94.3 gas turbine with higher turbine inlet temperatures, higherefficiency (design efficiency: 45%) and increased net output about 300 MW. An IGCC plantbased on TEXACO residual oil gasification is currently under construction for ISAB in Sicily. Theplant which is equipped with two V94.2K gas turbines will have a net output of 512 MW.

2. Air Integration Concepts for a Syngas Power Plant

Buggenum as well as Puertollano are examples for an IGCC Power Plant with full air-side andnitrogen-side integration of the air separation unit (ASU), whilst ISAB will be operated withoutair extraction and without nitrogen return. In the fully integrated cases, the total air for the airseparation unit is extracted from the GT compressor. The nitrogen from the air separation unit isreintroduced into the gas turbine by compressing and mixing with the undiluted syngas.Consequently the turbine mass flow is nearly the same as for natural gas or fuel oil operation andthe same compressor as for standard fuels (natural gas or fuel oil) without any modification can beused for the syngas machine.

Figure 1: Simplified process flow diagram of the ISAB IGCC at Priolo/ItalyV94.2K Gas Turbine with TEXACO Oil Gasification

The plant concept with independent ASU, i.e. with no air- or nitrogen-side integration results inreduced plant complexity but introduces the necessity for compressor modifications due to theincreased turbine mass flow in comparison to natural gas or fuel oil operation. Plant descriptionsfor the fully integrated IGCC plants at Buggenum and Puertollano were given elsewhere3. Figure1 shows a simplified process flow diagram for the refinery residues based IGCC plant ISAB atPriolo/Italy as an example for an air side nonintegrated IGCC concept. It is one of the firstcommercial IGCC applications in an European refinery.

3. Fuel and combustion system design

In Table I syngas compositions and mass flow rates for the IGCC plants equipped with Siemensgas turbines are given. As a consequence of its hydrogen content the chemical kinetics of typicalsyngases is fast even after dilution with inert gases. Consequently dilution may be used to adjustmoderate burning velocities compareable to natural gas in combination with low adiabatic flametemperatures. On the other hand low adiabatic flame temperatures enable the employment of adiffusion flame combustion concept, which is easy to handle and is giving small NOx productioncompareable to a more complicated dry low NOx premix combustion system for natural gas. Dueto these advantages a diffusion type syngas flame is used for all IGCC projects with Siemens gasturbines.

Table I: Fuel characteristics of IGCC plants with Siemens gas turbines

H2 CO N2 CO2 H2O LHV base loadfuel flow

rate

fueltempe-rature

vol % MJ/kg kg/s °C

Buggenum 12 25 42 1 19 4,3 106 310

Puertollano 11 29 53 2 4 4,3 122 300

ISAB 27 32 1 5 35 8,6 53 195

Burner Design. The Siemens syngas burner design concept includes the use of natural gas or fueloil as back up fuel and is derived from the Siemens standard hybrid burner.

A sketch of the Siemens syngas burner as well as the hybrid burner is shown in Figure 2. The syn-gas passage is simply introduced at the inner cone of the outer main air swirler (diagonal swirler).The syngas burner contains all parts of the standard Siemens hybrid burner, which are necessaryfor the combustion of natural gas (in diffusion mode) or fuel oil. Only the premix nozzle fornatural gas has been left out. Since natural gas or fuel oil is only a back up fuel, it is burned indiffusion mode and steam is used as a diluent for NOx control. By using a variable nozzle design,the pressure drop across the syngas passage can be adapted to the plant specification.

Figure 2: Siemens standard hybrid burner Siemens syngas burner

4. Operation experience from Buggenum (Demkolec)

4.1 Flame induced pressure oscillations during early commissioning of the Buggenum plant

While all conventional aspects of combustion, e.g. emissions and flame stability, turned out to beas positive as predicted by test rig results and calculations, the coal gas operation was not entirelyfree of flame induced pressure oscillations, which can lead to unacceptable high combustion noiselevels. During the first tests with undiluted coal gas, flame induced pressure pulsations occurred,which were attributed to the low pressure drop across the burner syngas nozzles originallydesigned for the flow of a low BTU gas4. In Buggenum the relative pressure drop for the dilutedsyngas at base load is already lower than 10%. As a consequence of the integration design atBuggenum, dilution nitrogen can only be made available after coal gas has been introduced to thegas turbine and the air separation unit is integrated with the gas turbine. With undiluted syngas(LHV = 11MJ/kg) the pressure drop is less than 2% due to the lower mass flow rate, givingenhanced tendency for pressure pulsations. This problem was successfully solved by admixingsteam via the existing syngas purging system at times for which dilution nitrogen is not available.

Unfortunately besides the low pressure drop oscillations there occurred also oscillations at higherfuel nozzle pressure drops during the first diluted coal gas operation at high loads. ConsequentlySiemens started a burner optimization program during which the problem was solved. Thesolution consisted in a modified fuel nozzle design that disturbs the annular symmetry of thesyngas flame. Optical measurements of the flame radiation by the KEMA institute confirmed thesuppresion of oscillations by the changed syngas nozzle design5. Figure 3 shows on the left handside the symmetric flame shape of the original nozzle design and on the right hand side the shape

of the modified nozzle design. The different degrees of brightness in Figure 3 are not real but dueto the use of different optics. After introducing the modified nozzle design in September 1996,gas turbine base load operation with 291 MW (CC) at 12°C ambient temperature without flameinduced pressure oscillations could be demonstrated3,6.

Figure 3: Comparison of two different syngas flame shapes:left hand original nozzle design (test rig); right hand: modified nozzle design (machine)

4.2 Emissions at Buggenum

The adiabatic flame temperature for undiluted Buggenum coal gas is about 100 K higher than fornatural gas. Since combustion temperature is the most important parameter controlling the NOx

emission, a simple coal gas diffusion flame would give rise to gas turbine emission far beyond thelevels of standard hydrocarbon flames. So special care has to be given to NOx emission control. InBuggenum the concept of full integration of the ASU with the GT and dilution of the syngascoming from the gasifier with the nitrogen from the ASU before feeding it to the GT is used. Thisis favoured by the high flame speed of H2: highly diluted diffusion flames with comparably lowflame temperatures but nevertheless good flame stability for syngas combustion can be used6.

Meanwhile, by the end of the demonstration period in Buggenum it has been proven that thisdiffusion burner concept in combination with the high degree of dilution has excellent emissioncontrol characteristics. Figure 4 shows measured values of NOx and CO in broad range of heating

Figure 4: Emissions measured at Buggenum / V94.2 on syngas operation

values (different degrees of dilution). The NOx emission levels at base load coal gas operation are120 - 600 g/MWh NOx corresponding to 6 - 30 ppm(vol) (15%O2)

2. It has to be noticed that forless diluted syngas and other gas turbine types with higher thermal efficiency corresponding tohigher turbine inlet temperatures the NOx emissions will be higher. Within the whole operationrange the CO levels are below 5 ppm(15% O2). Even with fully diluted coal gas with heatingvalues of 4,3MJ/kg the Buggenum gas turbine is capable to operate at the lower edge (40% BaseLoad) of the designed coal gas operation range. With less diluted coal gas a further reduction ofthe minimum coal gas load seems possible.

4.3 Switch over from natural gas to syngas and vice versa

The Buggenum plant is started with natural gas. At about 50% baseload switch over from naturalgas to syngas is performed. As already mentioned dilution nitrogen is available after coal gas hasbeen introduced to the gas turbine and steam is used for dilution during and after switch overinstead of nitrogen. Normally before switch over the air separation (ASU) unit is alreadyintegrated with the gas turbine, so care has to be taken that the GT compressor delivers enoughpressure to feed the ASU. This is accomplished by regulating the inlet guide vane (IGV) positionin a way that the compressor discharge pressure is kept above the minimum pressure necessary forthe ASU. Because the natural gas with a small mass flow has to be substituted by the syngas witha high mass flow, the turbine mass flow is increasing considerably during the switch over. To keepthe power output approximately constant during this operation the turbine inlet temperature andas a consequence also the turbine outlet temperature has to decrease. A typical switch over for theV94.2 in Buggenum is shown in Figure 5. The combined cycle power output (P-CC) is layingbetween 110 and 120MW, while the turbine outlet temperature (OTC) is decreasing by about60°C. The whole procedure is fully automatic and lasts less than 4 min (cf. Figure 5). Theprocedure begins with heating up the syngas lines with steam. Then the coal gas by pass valve(CG-BV) opens to a constant position of about 57% to blow out the steam through the burnersand feeding a small mass flow of syngas to the gas turbine (GT). After about 40s the CG-BVopens to 100% and also the coal gas control valve (CG-CV) begins to open. Simultaneously thenatural gas control valve (NG-CV) closes and an increasing massflow of steam (m-steam) isadded to the increasing coal gas flow. After having reduced the natural gas control valve to 0%position the procedure ends with the closing of natural gas stop valve. The GT is now running oncoal gas diluted with steam. As soon as the dilution nitrogen is available, it is mixed to the coalgas and the steam is decreased simultaneously. Finally the steam stop valve is fully closed. Loadincrease and decrease with coal gas diluted with nitrogen and fully integrated air separation unit isnow possible.

The switch back from coal gas to natural gas is also fully automatic. The procedure starts with aload reduction, then the coal gas dilution is switched back from nitrogen to steam. Finally the coalgas is reduced to zero while the natural gas is opened. The procedure ends with two purging stepswith steam. In the first step the syngas lines to the burners are purged forward into the gasturbine, in the second step the piping system between coal gas control valves and the coal gaslock are purged backward to the flare.

4.4 Trip of the dilution nitrogen

Characteristic for this emergency case is a sudden and quick decrease of the pressure in the syngasline because the fuel mass flow is dropping down by about 50%. The consequence is a decrease ingas turbine load and after a certain time an increase in syngas heating value. In this case thecontrol system automatically reduces load and starts after a certain waiting period the steam

Figure 5: Switch over from Natural gas to Syngas at Buggenum

admixture to the coal gas. The automatic procedure ends with the GT running on coal gas dilutedwith steam with still air-side integrated ASU.

4.5 Gasifier trip

This procedure is a combination of a dilution nitrogen trip and switch back to natural gasoperation. The procedure is also fully automatic. With the trip of the gasifier, the nitrogen is shutdown, for a short time in between the coal gas is diluted with steam and the load is reduced. Theprocedure ends typically 2 min later with the GT running on natural gas without a trip of the airseparation unit.

5. Projects currently under commisioning or construction

5.1 Elcogas (Puertollano)

At Puertollano plant the combined cycle, including an advanced V94.3 gas turbine, was alreadycomissioned on natural gas in 1996. The gas turbine has accumulated over 6000 operating hourson natural gas without and with air extraction from the gas turbine compressor for ASU and

gasifier commissioning. Commissioning activities on syngas started in March 1998 and first switchover to integrated coal gas operation was achieved on 20th of March.

5.2 ISAB (Priolo)

This plant is currently under construction. Comissioning activities on syngas will start in 1999. Inopposite to the Buggenum and Puertollano plants, this plant will be operated completely air- andnitrogen-side nonintegrated (V94.2K). The heating value, adjusted by dilution with water vapor,will be about double the value compared to Buggenum and Puertollano. To check for secureoperation with the syngas burner adjusted for this fuel, extensive combustion tests wereperformed on atmospheric and pressurized conditions 7.

Lean Stability Limit tests for ISAB. Gas Turbine operation with syngas for the ISAB V94.2Kmachine is foreseen in a very wide spread load range, that implies a wide range of air excess ratiosat the burner. To check for the burners capability to burn the ISAB syngas stable under allpossible operation conditions, the weak extinction limit (lean blow off) has been investigated.Lean blow off points were measured at different air velocities, including simulated baseloadcondition and different combustion chamber pressures up to half machine baseload pressure. Tocover the worst case condition, all lean blow off points were measured with the minimum machineair temperature of 270°C. The test results are given as function of the highest stable air excessratio over a relative burner air mass flow. This relative air mass flow is defined as the ratiobetween actual air mass flow at test condition to the baseload burner air mass flow of themachine, scaled to test condition (temperature, pressure), giving the burner air velocity of themachine.

In the variation range of the relative burner air mass flow, the measured lean blow off curves -given in figure 6 - are in general flat with a small increase of the blow off air ratios at reduced

Figure 6: Measured Lean Blow Off air excess ratios at different pressue levels and variation of baseload related air mass flow

relative air mass flows (air velocity). With increasing pressure level, the stability is stronglyenhanced and an increase of the lean blow off excess air ratios with decreasing relative air massflow is more pronounced. At pressure level 5,5 bar and reduced relative air mass flow the flamecould not be blown off within the usable metering range of the smallest syngas massflowmeasuring device used. Near to machine baseload condition a very high lean blow off air excessratio above 40 was measured. This value is much higher than any expectable air ratio relevant tomachine operation, indicating that the choosen burner design covers the stability demands for aproper operation with ISAB syngas.

NOx emission test for ISAB. Due to the elevated heating value of the ISAB syngas in comparisonto Buggenum/Puertollano syngas, the combustion concept employing a diffusionburner with low NOx emissions had to be validated. The NOx emissions were measured at threepressure levels from atmospheric conditions up to a combustion chamber pressure slightly above 6bar. During all NOx emissions measurements the combustion air temperature was adjusted to thestandard baseload temperature of 340 °C. Figure 7 shows the NOx emission measurements atsimulated baseload conditions for different combustion chamber pressures corrected for 15%oxygen content in dry exhaust gas. The scatter of individual points at each pressure level indicatesthe accuracy range of the exhaust gas analysis. The results are in general agreement with theoperation experience about the dependence of NO formation in lean diffusion flames. At sufficienthigh flame temperatures a theoretical investigation8 for diffusion type flames lead to a NOformation rate beeing proportional to p0.5.

0

10

20

30

40

50

60

70

80

30% 40% 50% 60% 70% 80% 90% 100% 110% 120% 130% 140%

(m/m)BL air [%]

air

rati

o λ

λ

at

LB

O [

-]

near atm., AITca 2,5 bar, AITca 5,5 bar, AIT1 bar, GdC

ISAB LBO-Limit

blow off not reached

An approximate description of the experimental result is possible using a simple power equationgiven below. This equation has been fitted to the experimental pressure dependence of the NOx

emissions of the V94.2K ISAB syngas burner, operating with ISAB syngas. The best fit wasachieved using an exponent of 0.68. In addition numerical values of this simple equation areincluded in figure 7.

Figure 7: Measured and extrapolated NOx emissions with ISAB fuel from test rig operation with variable pressure

NO

NO

p

barx p

x bar

,( )

,( )

.

1

0 68

1= F

HGIKJ

Extrapolation of this equation indicates NOx emissions below 25 [ppmv] (15% O2) for fullmachine pressure (11 bar).

6. Conclusions

During the demonstration period in Buggenum as well as from the ISAB combustion tests it couldbe shown that the concept of diluted diffusion flames for syngas combustion leads to very lowNOx and negligible CO emissions. Furthermore the problems concerning combustion drivenpressure oscillations which occurred during the commissioning period in Buggenum were solvedsuccessfully by changes only at the coal gas fuel nozzle design. For a great number of transient

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10 11

p [bar]

NO

x at

15%

O2

[vp

pm

]

Measured ValuesCurve Fit

conditions (e.g. switch over from natural gas to coal gas and vice versa) and emergency cases(e.g. gas turbine has to control sudden trips of gasifier, air separation unit and dilution nitrogensupply) automatic control procedures have been established and successfully tested. The results ofthe Lean Blow Off tests from the ISAB test campaign indicate the possibility to operate on thissyngas on a very wide load range including idle up to base load condition.

The Buggenum plant experience constitutes a valuable basis for the design of Siemens syngasoperated gas turbines, which are available for gasification combined cycle plants with full, partialor no integration9. The use of this technology in syngas derivatives of the Annular Combustormachines of the 3A-family is intended. With these machines higher turbine inlet temperatures andhigher thermal efficiencies will be also achievable for syngas applications.

References

1. Balling, L., Joyce, J.S. and Rukes, B. - The New Generation of Advanced GUD CombinedCycle Blocks. Power-Gen Europe '95. Conference Papers: Vol. 5, pp. 409-436.Amsterdam/NL, 16-18 May, 1995.

2. De Winter, H.M.J. and Zon, G.D. - ”Commercial operation of Demkolec’s IGCC in acompetitive market”, EPRI Gasification Technologies Conference, San Francisco, California,Oct 5-8, 1997.

3. Huth, M., Vortmeyer, N., Schetter, B. and Karg, J. - "Gas Turbine Experience on and Designfor Syngas Operation". Gasification technology in Practice, Institution of ChemicalEngineers, Milan, Italy, 26-27 February 1997.

4. Vortmeyer, N., Schetter, B., Becker, B. - "Verbrennung von Kohlegas in Siemens-Gasturbinen: Erfahrungen bei der Inbetriebsetzung des Kohlevergasungs/GuD-KraftwerkesBuggenum". 17. Deutscher Flammentag, VDI Bericht Nr. 1193, VDI-Verlag, Düsseldorf,1995.

5. Verhage, A. and Stevens, P. - “Pressure Pulsations in Combustion Chambers of Large GasTurbines”. Power-Gen Europe '98., pp. 47-58. Milan/Italy, June 9 -11, 1998.

6. Vortmeyer, N., Huth, M., Schetter, B., Becker, B., Karg, J. and Emsperger, W. -"Experience in the Design and Operation of Syngas Gas Turbines". EPRI GasificationTechnologies Conference, San Francisco, California, Oct 2-4, 1996.

7. Heilos, A. Huth, M., Bonzani, F. Pollarolo G. - “Combustion of Refinery Residual Gas with aSiemens V94.2K Burner”, PowerGen Europe, Conference Papers: Vol. III, pp 187 - 197,Milan/Italy, June 9 - 11, 1998.

8. Warnatz J., Maas U.: "Technische Verbrennung", Springer , Berlin (1993).

9. Karg, J., - “Cleaning Up Power Generation”, Asian Power, p. 23-24, June 1998.

Biographical Information

Speaker: Michael HuthCompany: SIEMENS AG, Power Generation Group KWU

1. Michael Huth is the head of the group for syngas burner development of SIEMENS PowerGeneration Group. Since 1996 M.Huth is responsible for the burner development program for theBuggenum, the Puertollano and the ISAB IGCC and for the corresponding field tests.

2. He started in 1992 to work at SIEMENS Power Generation Group (gas turbine section) inthe combustion department. Until 1996 M.Huth was involved in different R & D programsconcerning the improvement of natural gas burner emissions and cold flow burner tests as well ashot burner tests.

3. He received a master degree in chemical engineering and 1992 a PhD degree withexperimental research on soot formation in technical flames at the University of Karlsruhe.