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  • 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), Mlheim, 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 worlds 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 15C could be confirmed.

    Siemens gas turbines are also applied in two other European IGCC projects. In the Puertollanoplant in Spain, which is worlds 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 12C 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 NOxemission, a simple coal gas diffusion flame would give rise to gas turbine emi

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