Gas Turbine - Steam Turbine Power Plant for Production of Electricity and Process Steam for a Chemical Industry

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1 $2.00 PER COPY $1.00 TO ASME MEMBERS The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. 70-GT-17 Discussion is printed only if the paper is published in an ASME journal or Proceedings. Released for general publication upon presentation Copyright 1970 by ASME Gas Turbine - Steam Turbine Power Plant for Production of Electricity and Process Steam for a Chemical Industry J. SAUVENIERE Technical Manager, Utilities and Power Department, Solvay & C.1 E, Brussels, Belgium G. VIDOSSICH Section Manager, Gas Turbines Application Engineering, Fiat Grandi Motori, Torino, Italy For the extension of a chemical plant it was necessary to install an additional process steam generating unit. This paper presents the selected plant. In order to obtain the best efficiency, steam and electricity production have been combined using a gas turbine and a recovery boiler. An added helper steam turbine raised even more the electricity generation efficiency. Components of the plant, cycle arrangement and performances are described. Actual performance data are not presented in this report because they were not available at the time of its printing. Contributed by the Gas Turbine Division of The American Society of Mechanical Engineers for presentation at the ASME Gas Turbine Conference & Products Show, Brussels, Belgium, May 24-28, Manuscript received at ASME Headquarters, December 10, Copies will be available until March 1, THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS, UNITED ENGINEERING CENTER, 345 EAST 47th STREET, NEW YORK, N.Y

2 Gas Turbine - Steam Turbine Power Plant for Production of Electricity and Process Steam for a Chemical Industry J. SAUVENIERE G. VIDOSSICH INTRODUCTION In 1967 Solvay planned to extend the Jemeppe plant for increasing the production and for initiating new chemical processes. Therefore, it was necessary to process more steam, about 60 t/h, 30 percent at 33 ata of pressure, the remaining at 11 ata. For a reduction of the costs it was considered worthwhile to combine electric power generation with the steam production, and therefore reduce the amount of electricity needed from outside. The use of a counterpressure steam turbine, due to the relatively high pressure of the process steam, led to a fairly low power. With steam produced at 85 ata it is possible to produce about 52 kw/t/h for 33 ata counterpressure, and 100 kw/t/h for 11 ata. The total production would be about 5300 kw, a level too low to be interesting. A power of 15 MW was then fixed as a minimum for the combined plant. For such a power/steam flow ratio, the possible solutions were H.P. boiler plus condensing-extraction steam turbine or gas turbine plus L.P. recovery boiler. From an accurate technical-economical study and the availability of natural gas it resulted that the gas turbine-recovery boiler solution was the best, in particular for the following anticipated advantages: low installed cost short delivery terms possibility of producing power independent from the boiler low cooling water requirement simple operation of the boiler low overall fuel consumption During the development of the plant design it was decided to use, for starting the set, a small steam turbine, fed by the other boilers of the plant, instead of the standard electric motor. For reason of simplicity the boiler was designed to produce all the steam at 33 ata, and the flow required at 11 ata had to pass in a pressure reducing valve. At that point it appeared interesting to combine the needs of starting the gas turbine and to reduce the steam pressure with the use of a starter-helper steam turbine, which after having started the gas turbine, is used as steam expander producing power to drive the same generator driven by the gas turbine. The power increase that can be obtained in this way is about 1400 kw (normal) with a steam flow of 35 t/h and 1800 kw (maximum) with a steam flow of 45 t/h. For this particular use it was also deemed advisable to provide a self-synchronizing clutch between steam turbine and generator, permitting the helper turbine to start and stop, without affecting the operation of the gas turbine. The steam turbine is of the impulse type with two wheels -- the first one with normal and overload nozzles, the second one with total admission. It is provided with a complete control system for controlling both the operation as starter and as helper, and in particular to permit engaging the clutch without shocks. The lubricating oil circuit is connected to that of the gas turbine. INSTALLATION After establishing the characteristics of the plant, the installation design work began. Logic asked for the new power plant to be near the old one, for easier electrical and steam connections and for operation in the center of the factory. But, as generally happens when extending existing plants, the available space was very limited, in area as well as in shape. In addition, the possibility of a further extension with a second gas turbine set had to be considered. These facts led to an unusual solution. The air inlet filters are at roof level, with the silencer vertical underneath. Previous and new pipings (steam, natural gas, etc.) run over the 1

3 FIAT T616 GAS TURBINE 2 -LOAD GEAR 3 -GENERATOR 4-OIL RESERVOIR S -TURBINE AUXILIARIES 6-STARTER HELPER TURBINE 7 -OIL COOLERS 6-INLET AIR SILENCER 9 -INLET AIR FILTERS 10-BOILER 11- DEAERATOR 12-EXHAUST STACK 13- CONTROL ROOM 14-ELECTRIC BOARDS ROOM Fig.l Plant general layout ,1111" 2

4 Fig.3 Gas turbine and auxiliaries in test room station roof. Generator breaker and electrical switches are in a ground floor room, whereas the local control panels are in a second floor room. The presence of a cable underground passage in the power station area added difficulties to the design and execution of the foundation, which is over pillars because of the low strength of the ground. The power station is situated near the office building; thus great care has been devoted to the silencing problem. For the compressor inlet noise a silencer has been fitted in the inlet duct; whereas for the exhaust noise the boiler itself acts as a silencer. The power station house walls are made of bricks, to match the existing power station. The bricks are good sound insulating material, but, for a better insulation against the engine room noise, a layer of sound insulating panels has been added to the interior of the walls. The boiler exhaust stack is 50 meters high. A general layout of the plant is indicated in Fig.l. DESCRIPTION OF THE EQUIPMENT The gas turbine installed in this plant is a type TG.16, rated kw at CIMAC reference conditions. The unit is a single-shaft simple open cycle, and, as shown in Fig.2, longitudinal section, ccnsists of a 15-stage axial compressor, a six-basket combustor, and a five-stage axial turbine. ly connected to form a single two-bearing rotor, thus making the unit compact and of easy alignment. The gas turbine is mounted over a steel bedplate, which extends at the turbine end to form an oil reservoir. Over the oil reservoir are installed some auxiliaries as the auxiliary gear (turning gear, main oil pump, etc.), the auxiliary and emergency electrically driven oil pumps, the natural gas control valves board, etc. Behind the oil reservoir, over a smaller bedplate, are assembled other auxiliaries of the unit, as the pressure switch and gage cabinet, starting air compressor, lube oil filters, etc. (Fig.3 shows a TG.lb gas turbine in Test Room.) At the compressor end there are, in order: the main reduction gear, also mounted over a steel bedplate; the 26.5 mva ACEC-Charleroi generator; and the starting-helping group. This latter, assembled over a small bedplate, consists of a reduction gear, an automatic SSS clutch, and a steam turbine for starting the gas turbine, and capable of developing up to 1800 kw in continuous duty, as helper. The bedplate-mounted subassemblies of the gas turbine are completely assembled and wired in the manufacturer's shop, and shop tested with the turbine. In this way are controlled the operation and the right connections of the auxiliaries, thus reducing the installation and checking time. The gas turbine exhausts in a Cockerill- Ougree recovery boiler for production of steam, used for the process needs. The boiler is of the two drum vertical tubes The compressor and turbine rotors are rigid- type and as shown in Fig.4 consists of: 3

5 one supplementary firing apparatus, for use of natural gas, fitted in the exhaust gas duct one steam superheater, with vertical tube loop SUPERHEATED STEAM FEED WATER 30' A _BOILER INLET DAMPER SD _ SUPPLEMENTARY BURNERS B _ BY-PASS DAMPER SN _ SUPERHEATER C _STACK BUTTERFLY DAMPER NI _ BOILER I D _ DEAF RATOR SN EC _ECONOMIZER E _GAS TURBINE B I I_BOILER II Fig.4 Recovery boiler schematic B 165 one boiler I, with vertical finned tubes expanded in the two (upper and lower) drums one economizer, with horizontal finned tubes and vertical collectors; one boiler II, of construction similar to the economizer. As it is known, to evaporate and superheat the steam, it is possible to use the heat corresponding to the cooling of the gases from the boiler inlet temperature down to a temperature higher than that of the boiling water. The difference from the minimum gas temperature at the boiler I outlet and the boiling water temperature, the so-called approach temperature difference, has been maintained to a minimum technically feasible value, i.e., 19 C for simple recovery and 37 C with supplementary firing. The fuel gas used in both gas turbine and boiler is the Dutch GrOningen gas, which is very good and contains only very little amounts of sulfur. In order to protect the boiler tubes against the possibility of condensated acid corrosion in case of a future sulfur content increase, the minimum water temperature in the tubes has been fixed at 105 C. Therefore, this is the economizer inlet temperature. The feeding water, which is at 30 C, has to be heated to the economizer inlet temperature externally to the boiler, in the deaerator tank, by - Supplementary firing Table 1 Recovery Boiler Performances - Exhaust gas flow kg/s NO YES Supplementary firing natural gas flow Nm3/h heat released Ccal/h - Exhaust gas temperature at turbine exhaust C after supplementary burners 0C at Boiler II outlet C - Pressure losses gas side mmh2o - Steam flow t/h - Temperatures water at deaerator inlet C water at economizer inlet C water and steam in boiler C superheated steam C - Superheated steam pressure ata

6 means of steam. This steam can be extracted from the boiler I outlet, but in this way the useful steam flow is reduced by about 15 percent. As the gas temperature at the economizer outlet is still high, it is possible to extract from that gas the heat needed for raising the feeding water temperature from 30 to 105 C. Water at 105 C from the deaerator is circulated in closed loop in the boiler II where it is evaporated at the same temperature. This steam-reentering the deaerator releases the condensation heat to the feeding water. This arrangement is a little more complicated but it permits obtaining the maximum utilization of the exhaust gas heat. The expected performances of the boiler are summarized in Table 1. The boiler has also a by-pass duct permitting to divert the exhaust gas directly to the stack. It is then possible to operate the turbine during the boiler overhaul period, thus maintaining the power generation. In the ducts there are three dampers as indicated in Fig.4. The dampers A and B are remote-controlled and interlocked, thus one can close only when the other is completely open. The damper C is manually operated, and it is closed only during overhaul periods, for avoiding any back flow of exahust gases from the stack to the boiler. Table 2 Plant Performances Referred to 10 C and 750 mmhg ambient - Supplementary firing - Steam helper turbine NO NO YES YES - Gas turbine consumption Gcal/h Supplementary firing consumption Gcal/h Total consumption C Gcal/h :8 P0" Gas turbine output kw 17,440 17,440 Gcal/h Steam helper turbine output kw 0 1,400 Gcal/h Total electric output E kw 17,440 18,840 Gcal/h P Useful steam at 33 ata t/h heat content referred to 30 C Gcal/h Useful steam at 11 ata t/h 0 35 heat content referred to 30 C Gcal/h Total steam useful heat $ Gcal/h Stack losses Gcal/h Radiation and oil losses Gcal/h Total losses L Gcal/h Heat utilization ratio E + S (electric energy plus steam) Heat consumption in a virtual fired boiler to produce the plant steam B = S/0.9 Gcal/h Specific heat consumption to charge to the electricity generation kcal H = (C - BO kwh Corresponding efficiency

7 1165 'C 30'C 60 T/ h 1//// / FUEL HEAT TO GAS TUOIHNE /FUEL/ /HEAT / / TO /,BOILER/ C es *C 60 3/1, C M1LC_; 130'C 33!TA 60 Jr /21 _.4_2s Tin 17.1 SCAL/h I 102 *C KG /SEC KW OIL AND RADIATION LOSSES ST/ACA LOSSES/ / ST. POWER 1 _ GAS TURBINE 6 _ REDUCTION GEAR 2 _ COMBUSTOR 7 _ AUTOMATIC CLUTCH 3 _ GAS TURBINE COMPRESSOR I _STEAM HELPER TURBINE _ MAIN REDUCTION GEAR _RECOVERY BOILER 5 _ GENERATOR 16 _DEAERATON Fig.5 Plant schematic The supplementary firing apparatus is located between the damper A and the boiler, and is of the "grid" type. Six vertical and six horizontal gas burners form a grid across the exhaust gas path. The mixing between natural gas and oxygen for the combustion is then very good, as also is the uniformity of the temperature after the burner. OVERALL PERFORMANCES This generating group has been intended for base load operation, for reducing the amount of power bought from the external grid. The gas turbine then operates at full load all the time, and the difference to the total power needed by the plant is taken from the external grid. Also the boiler is base loaded, and the minimum steam production corresponds to the boiler operating without supplementary firing. No operation with dampers half opened, and exhaust gas by-pass, is foreseen. Therefore the only interesting performances are that of full load operation. A schematic of the plant and a Sankey diagram for the maximum load condition are indicated in Figs.5 and 6. The nominal performances without and with supplementary firing, and without and with steam helper turbine in operation are indicated in Table 2. /STEAM AT/ /11"ATA, / // Fig.6 Plant Sankey diagram To determine the specific heat consumption to charge to the power generation, it was agreed to subtract from the total heat consumption of the plant, the heat that would be necessary to produce the same steam flow in a separate direct fired boiler. The resulting specific heat consumption relative to the electricity generation is rather low, and for this reason combined plants of this type are of great interest for many industries which need steam as well as power. The presence of the small helper turbine, of low price and in any case necessary for starting, contributes to further reduce the specific heat consumption. As the plant was scheduled to start operation at the end of October 1969 no indication of the actual performances were available for the present report. CONTROL As the gas turbine operates normally at full load in parallel with the external grid, its control system is used only to set the power output, and to control it for avoiding overtemperature operation. This can be done either manually by the operator or automatically by the exhaust temperature limiting system. 6

8 The control of the steam starter-helper turbine is more complex. As starter it is automatically controlled by the gas turbine sequence system by means of air pressure signals. The first pressure signal (6 psig) sets the steam turbine governor to reach the 1600 rpm speed of the steam turbine, corresponding to 20 percent of the normal speed. The group is maintained at that speed about 5 min for a period of "purge" from any fuel gas leaked from the burners of both the gas turbine and the boiler. After the purge, the second pressure signal to the governor permits the steam turbine to accelerate the unit to the self-sustaining speed. Then the steam turbine is stopped and the connecting clutch automatically disengages. When the gas turbine is at full speed and the generator in parallel with the grid, and if there is enough demand of steam at 11 ata to justify the operation of the steam turbine, it can be started. The starting is manual. By means of a pneumatic speed changer, acting on the governor, the steam turbine is gradually accelerated to the generator speed. When reaching synchronism with the generator, the connecting clutch automatically engages, and the turbine may develop power. When the operation of the turbine is stabilized, the control is switched to the inlet pressure governor which controls the steam inlet valve for maintaining the inlet pressure almost constant. As the processes fed by the 33-ata steam cannot allow interruption of flow, the 33-ata grid is considered top with respect to the 11-ata grid, and the controls of the steam system have been planned in that direction. Parallel with the steam helper turbine there is an automatic pressure reducing valve, also controlled by the 33-ata pressure, which operates when the steam turbine is stopped or when the flow to be reduced from 33 to 11 ata is higher than the capacity of the steam turbine. (See Fig.7, schematic of steam control.) Finally there is the control of the recovery boiler. Feedwater flow and water level are controlled in a standard way. When the boiler operates without supplementary firing, no steam flow control is possible. The recovery boiler produces the flow related to the gas turbine exhaust gas characteristics (flow and temperature) and the other boilers of the plant, fired, provide to control the overall flow of steam. When the supplementary firing of the recovery boiler is on, it is controlled by the pressure of the 11-ata grid. VENT S B _ SUPPLEMENTARY BURNERS PC _ PRESSURE CONTROL LER RB _ RECOVERY BOILER PR _ PRESSURE REDUCING VALVE FR _FIRED BOILER ST_ STEAM TURBINE Fig.7 Steam control schematic For instance, if there is an increase of consumption in the 11-ata grid, the pressure in the same decreases, and the natural gas flow to the boiler burners is increased. Therefore the production of the boiler at 33 ata also increases and the pressure in the 33-ata grid tends to increase. The control valve of the helper turbine or the pressure reducer opens, allowing more steam to expand to 11 ata and then satisfying the needs of steam at 11 ata. Any other variation can be controlled in a similar way. The control and monitoring devices are installed in panels differently located. In the local control room there is the gas turbine control panel with the master and auxiliaries selectors, indicating and recording instruments, and all relays for starting sequence and for protections. From that panel the gas turbine can be started and controlled on load. Near the steam turbine is the control panel for starting and loading the turbine when it operates as a helper. In a central control room, together with control panels of the previous power plant, there is a gas turbine remote control panel, with instruments for controlling the gas turbine, when on load, and for monitoring the principal operating parameters. Therefore, when the gas turbine is on load, and the steam turbine is under the inlet pressure governor control, the unit can operate without local operating personnel, remotely controlled from the central control room. 7

9 SUMMARY The number of total energy plants in the industry, using gas turbines and recovery boiler, is continuously increasing. The presented case, with the peculiar characteristic of the starterhelper steam turbine, whose installation had been suggested by the need of two different pressures for process steam, indicates that the total energy plants are very flexible in design to cater to the most different needs of industrial plants. 8