Applying Kalina Technology to a Bottoming Cycle for Utility Combined Cycles

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St.. New York, N.Y GT-35 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. Discussion is printed only if the paper Is published in an ASME Journal. Papers are available from ASME for fifteen months after the meeting. Printed in USA. Copyright 1987 by ASME Applying Kalina Technology to a Bottoming Cycle for Utility Combined Cycles A. L. KALINA President, Mem. ASME H. M. LEIBOWITZ Vice President, Mem. ASME EXERGY, INC. Houston, Texas Livermore, California ABSTRACT A new power generation technology often referred to as the Kalina cycle, is being developed as a direct replacement for the Rankine steam cycle. It may be applied to any thermal heat source, low or high temperature. Among several Kalina cycle variations there is one that is particularly well suited as a bottoming cycle for utility combined cycle applications. It is the subject of this paper. Using an ammonia/water mixture as the working fluid and a condensing system based on absorption refrigeration principles the Kalina bottoming cycle outperforms a triple pressure steam cycle by 16 percent. Additionally, this version of the Kalina cycle is characterized by an intercooling feature between turbine stages, diametrically opposite to normal reheating practice in steam plants. Energy and mass balances are presented for a 200 MWe Kalina bottoming cycle. Kalina cycle performance is compared to a triple pressure steam plant. At a peak cycle temperature of 950 F the Kalina plant produces MW vs MW for the triple pressure steam plant, an improvement of 16.0 percent. Reducing the economizer pinch point to 15 F results in a performance improvement in excess of 30 percent. Presented at the Gas Turbine Conference and Exhibition, Anaheim, California May 31-June 4, 1987

2 INTRODUCTION The vast majority of electricity produced throughout the world comes from steam-powered generating stations. Regardless of the fuel burned; oil, gas, coal or nuclear, the same thermodynamic power cycle is used. It is called the Rankine cycle, named after its inventor, the Scottish physicist William Rankine ( ). While many improvements have been made to the cycle, and to the boilers and turbines that operate within the cycle, the fact remains that the process of converting fuel into steam and steam to power has not changed during the past century. Most of the focus has been placed on materials and component development in an attempt to increase cycle temperature and turbine efficiency. This has proved to be a very expensive and slow process. The fact of the matter is the Rankine cycle's peak steam temperature has not increased at all beyond its current 1050 F limit during the past 25 years. The Kalina technology, instead, looks to the plant's thermodynamic structure. Substantial improvement is made by reducing the mismatch in temperature between the working fluid (steam) and heat source in the boiler. The mismatch in sub-critical boilers results from water being converted to steam at a temperature, typically in the F range, much lower than the temperature of the hot gases, normally greater than 1000 F. The Kalina cycle uses a working fluid comprised of two components, ammonia and water, to create a variable temperature boiling process. This narrows the gap between the temperature of the hot gases and the steam. On the condensing end absorption refrigeration principles are invoked to effect a distillation process wherein the working fluid is separated, absorbed, condensed and recombined prior to re-entry into the boiler. Combining Second Law principles with ammonia absorption practice, performance is improved by percent over the steam cycle. At this level widespread adoption of the Kalina process within the U.S. could save approximately $20 billion in annual fuel costs based on $20 bbl oil prices. An explanation of the Kalina process, how it works, its performance as a bottoming cycle relative to the steam cycle and a brief discussion of some hardware considerations are all presented in this paper. EXERGY Many engineers believe that boiler performance is solely a function of the amount of heat transferred from the source to the steam or, the extent to which the hot combustion gases are cooled before leaving the stack. This concept is wrong. Clearly, if all the steam produced from a 1000 F source were at 300 F the potential to produce work in an expansion turbine would be quite low regardless of how much heat is transferred through the boiler and how low the stack temperature is pulled down. Rather, it is the average temperature of all the steam produced, taken together with the steam flow rate, that determines real boiler performance. The example just cited introduces the concept of exergy. Exergy is a term used in conjunction with the Second Law of Thermodynamics to describe the irreversible losses that occur within a power cycle. When heat is transferred from a high temperature source to a lower one some exergy is destroyed. Exergy is defined as the availability to perform useful work from a given energy source. A more detailed explanation was presented in a previous paper (1) on the same subject. In mathematical terms: E = H [ 1- [ To (ln / T 2 ) / (T 1 -T2 )] ] where E is the exergy of the energy source, H is the initial energy (enthalpy) content of the source evaluated between T 1 and T2 T1 is the initial hot source temperature, T 2 is the final source temperature, and T o is the ambient temperature. The equation above assumes a constant sink temperature, To. In fact, however, the sink temperature e.g. cooling water, always changes within the process. As shown in [1] the exergy of a source having a variable sink temperature is: E = H [1-1 (ln ) (2-To (lp ) ) j (T1-T2 ) (T3-To ) where T ois now the initial sink temperature and T3 is the final sink temperature. The term useful work should be emphasized. Exergy differs from energy content in that energy is only a First Law consideration that does not address the quality of energy, just its quantity. To help explain this concept consider the following: One thousand cubic feet of natural gas has a heating value of approximately one million Btu's. In this state its energy content and exergy are equal. When burned in a furnace, to produce 150 F water its energy content, assuming no heat losses, is still one million Btu's. However, its exergy has been almost completely destroyed, i.e., very little work can be generated from a 150 F source. Quantitatively this is expressed as: ETAe = 1- [ [ (520) (in 610/520)] / ( ) 1 =.078 assuming T2 = To = 520 R (60 F) Taking this approach one step further the Second Law efficiency may be defined in terms of exergy as: ETA 2 = Work/Exergy where exergy is evaluated between T 1 and T2 As applied to a power plant there is a substantial loss in exergy and corresponding decrease in Second Law efficiency, that occurs in the boiler as a result of the fact that water is converted to steam at a constant temperature, typically in the F range using a heat source of 1000 F or higher. 1

3 KALINA CYCLE DESIGN STRATEGY To reduce the temperature mismatch between the combustion gases and the steam in the boiler a mixture of ammonia and water was selected as the working fluid. The distinctive property of this new fluid is that its boiling temperature is not constant. Furthermore, ammonia and water each have properties that are advantageous for current use, viz., well documented heat transport properties different boiling temperatures very soluble easily separable similar molecular weight inexpensive widely used For a utility bottoming cycle application a 70/30 mixture, ammonia/water, is considered optimum. Boiling that occurs along a non-constant temperature line improves the 'fit' between the heat source and working fluid. See Figure 1. More heat is transferred from the boiler to the working fluid, generating more mass flow and work. Less exergy is destroyed and the Second Law efficiency increases..00 OM LEGEND. HEAT GIVEN UP BY SOURCE KALINA CYCLE HEAT TRANSFER TO POWER FLUID RANKINE CYCLE HEAT TRANSFER TO POWER 11/11../11111/111/... IIMINIMEIMMIIMMUS il IIIIIIMMEINNUMEM rIMPAII. MEMEMMINIM MINEMOMM ii...12 Enommi WINIIIIISIIIIIIMMINIMEN 1111.painliginal... MEM OMMOMMEMOMMEMM SWIMMEIMEM MMIIIIIMMON MEMMEMOMEM IIM II IIMMEMMEMOMMEMMEMEMMOMME ME MMINIMMEMOMEOMMOMEMEMMEMM milmmommommummommommommilm mossemammilimmommummummumm NIIIIIIIIIIIIMEINIEMIIIIIIIIIIEIIMIIIIIMII.SMIIIMINNIIII NIMMIN E MENOCIIIIIMENIIMM 1111 MIIMENIM11 111MEN MMIIIIMMEINIMEMIIIIIEMMINIEN 111= IMMENNINIENIIIIMII BM MIIIIIIIIMIIIMMINIMEIMEIMINE HEAT DUTY MM Emu/hr Figure 1. Heat Transferred to Power Cycle NEW CONDENSER While the advantages of using mixtures in boilers has been known for some time the problem of condensing them in a Rankine cycle has been more elusive. The problem is that mixtures also condense at a variable temperature. Any attempt to condense them in a Rankine cycle using normal cooling water temperatures would result in only partial condensation. Only by raising the turbine's back pressure and thus its condensing temperature would it be possible to fully condense the entire mixture. The result is poor; all gains made in the boiler are lost in the condenser. The problem is solved by using absorption refrigeration principles with the heat remaining in the turbine exhaust to produce a 'thermal compressor'. Increasing the mixture's pressure from 24 psia at the turbine exhaust to 68 psia at the condenser inlet permits full condensation to occur. A 32 detailed heat balance of the condensing system describing the mixture concentrations is given in Figure 2 and Table 1. An explanation of its operation follows. Figure 2. Conceptual Flow Diagram of the Distillation-Condensation Subsystem 2 LEGEND Working solution Basic solution P4 lean solution 2 Enriched vapor Cooling water 6 1 I I t 110 I I - 9 I r l P3 Point# t deg F P psia h but/lb Concentration of ammonia X 2 3_ 14 M How/ Flow K 27 P2 16 Flow Rate lb/hr WATER WATER WATER WATER WATER WATER Table 1 Distillation Condensation Subsystem State Points 2

4 TO A basic solution (x = 0.42) is heated by the vapor turbine exhaust and sent into a distillation column, M. Here the basic solution is separated into an enriched vapor (x = 0.96) and a lean liquid solution (x = 0.34). The lean solution, after giving up its heat to the basic solution in heat exchangers K and J mixes with the working solution (x = 0.7) in the absorber to form the basic solution. The basic solution is condensed using cooling water. The condensed solution is then pumped through exchanger J and is mixed with the enriched vapor leaving the top of separater M. The working solution (x = 0.7) is now reconstituted and passes into condenser Q as a two phase mixture where it is finally condensed using cooling water. The condensed working mixture is then pumped up to boiler pressure through feed pump P4 and the cycle repeats. The net result of the above is that the working fluid leaving the turbine at point 38, unable to be condensed using conventional cooling sources, is elevated in pressure using only the heat remaining in the turbine exhaust to a level where complete condensation is achieved. recooler, is then expanded all the way down to condensing pressure in the low pressure turbine. A more detailed description of the process and boiler arrangement is shown in Appendix A. This recooling feature is totally consistent with the overall strategy of the Kalina technology, i.e., improve the Second Law efficiency by 'borrowing' heat from the superheater section where an excess of high temperature heat exists and `lend' it to the boiling section where there is a shortage of heat. The combustion gas to working fluid match is improved and so is the Second Law efficiency. See Figure FIP 5. L BOTTOMING CYCLE PLANT DESIGN , A combined cycle is designed using three (3) General Electric MS7001F gas turbines and one Kalina bottoming plant. The latter contains three turbines, a reheat process between the first and second stage and a recooling feature between the second and third. See Figure 3. The concept of cooling between stages is directly opposite to that of modern design practice of reheating between stages. The rationale for this major departure will become apparent as the process is described below. From Figure 3, the superheated vapor leaves the boiler and is expanded through the high pressure turbine. From there it is directed back through the boiler, reheated and then expanded through the intermediate pressure turbine to produce additional work. At this point the superheat remaining in the vapor is removed (cooled) to vaporize a portion of the working fluid that has been preheated in the economizer section of the boiler. This additional vapor is then combined with the vapor generated within the boiler proper and proceeds into the superheater. The cooled vapor, leaving the / HEAT DUTY 10, Figure 4. Boiler Heat Duty As shown in the plant's heat balance and performance summary in Appendix A, the combined cycle efficiency is 54.4% (LHV). Temperatures in the Kalina bottoming cycle do not exceed 950 F, a limit imposed by the 1000 F gas turbine exhaust temperature. PERFORMANCE COMPARISON As an indication of how this cycle configuration compares with multi-pressure, reheat steam cycle, a parametric study [2] was conducted by Stone & Webster. The results are shown in Table 2. A 200 MW triple pressure steam cycle, operating at the conditions 1800/950/900/543/2" hga, was used as the baseline with which to compare the Kalina cycle. As shown, the Kalina cycle outperforms the baseline RECOOLER Condenser Pressure %NH 3Drop Boiler Boiler Stack Cooling Working PSI Pinch% PSI Temp F Water F Fluid LP/HP Improvement Gross Compared Output To Baseline MW COMBUSTION c GAS rj HP TURBINE Figure 3. Kalina Cycle for Utility Size Bottoming Cycle Plant Schematic IP TURBINE noa,ndi NSAT ION ll LP TURBINE Triple Pressure Baseline Steam Triple Pressure Steam Kalina cycle / Kalina cycle / Kalina cycle / Kalina cycle / Source: Stone & Webster Engineering Corporation, July 1986 Table 2. Bottoming Cycle Parametric Study Kalina vs Steam 3

5 steam cycle by 16 to 32 percent, depending on the parameters specified. In terms of output the improvement was almost 31 MW; vs MW using 82 F cooling water and a 50 F pinch point. Reducing the pinch to 15 F and using cooling water at 52 F increased the Kalina cycle output to MW. A discussion on the impact of cooling water temperature and pinch point follows. IMPACT OF COOLING WATER TEMPERATURE Apart from the nominal design point performance improvement the Kalina plant offers additional off-design incentives. When the cooling water temperature is lower than the nominal 82 F shown in Table 3, the performance improvement increases still further. At 24 psia turbine exhaust pressure the Kalina condenser has the flexibility to operate at lower pressure and, correspondingly, lower temperature without affecting the turbine and condenser design. Not so with the steam plant whose condenser is already down to 2" hga. This makes it very impractical for the Rankine cycle to take advantage of lower condensing temperatures. This is apparent from Table 3. It would require at least doubling of the turbine size for the steam plant to take advantage of the 52 F cooling water to match the Kalina plant's improvement. Also note that the Kalina plant requires a single flow turbine while the steam turbine requires at least double flow and sometimes a four-flow machine. Cooling Water Inlet Temp ( F) TURBINE OUTPUT (Mw) TRIPLE PRESSURE STEAM CYCLES 2 FLOW 4 FLOW 4 FLOW 4 FLOW 33.5" 26" 30" 33.5" KALINA CYCLE , Increased Output (Mw) Source: Stone & Webster Engineering Corporation, July 1986 Table 3. Impact of Cooling Water Temperature SELECTION OF PINCH POINT In a steam plant the selection of the economizer pinch point is a trade-off between additional steam flow and boiler surface. Typically, modern plants have pinch points in the F range. Using the same selection criteria for the Kalina cycle the optimum pinch point occurs at F. Again, the Second Law dominates. Reducing the pinch to 15 F in a steam plant is not cost effective because it doesn't substantially affect the Second Law losses at the economizer entrance where the stack temperature/water temperature difference is approximately F. On the other hand, the economizer pinch in the Kalina cycle is really not a point but rather a line that extends the full length of the economizer heat exchange process. Whatever the temperature drop (pinch) selected for the economizer discharge it is the same drop that exists at the entrance. See the lower left hand portion of Figure 4. Consequently the pinch 'line' in the Kalina cycle has a much more dramatic Second Law consequence than in the steam cycle. KALINA BOTTOMING CYCLE WITH OTHER GAS TURBINES Combined cycle performance was calculated using the Kalina cycle with other gas turbines currently available. The results are shown in Figure Solar Mars 2. GEC Gas Turbines ELM Fiat TTG TG. 20L1 4. Kraftwerk Union V84 5, Kraftwerk Union V94. Rankine bottoming cycle 6. Westinghouse 501D Mitsubishi Heavy + Kalina bottoming cycle, Industries MPCP3 (501D) system 6 7. Brown, Boveri & Co. Ltd. KA13E-1 8. General Electric 7001F Combined Cycle Output-MW Figure 5. Combine-Cycle System with Kalina and Rankine Bottoming Cycles Based on published gas turbine manufacturer's data [3] the best performance was achieved with a Kraftwerk Union (KWU) V94. The combined cycle heat rate is 6150 kwh (55.4%) based on lower heating value. The results demonstrate that the best simple cycle (gas turbine only) heat rate may not be optimal for combined cycle duty, at least not with a Kalina bottoming cycle. A gas turbine with lower pressure ratio, such as the V94, (See Table 4) has a higher exhaust temperature, and correspondingly higher exergy. It is obvious that this configuration generates more output from the bottoming cycle, regardless if it is Kalina or Rankine. However, it is less obvious as to why the combined cycle performance is better. The answer lies in the Second Law. As shown in Table 4 the KWU V94 has a First Law efficiency that is lower than all the others evaluated. However, its Second Law efficiency (output/exergy) is best of all. When used in conjunction with a bottoming cycle whose Second Law efficiency is over 80% the highest combined cycle efficiency is reached. Keep in mind that the Kalina bottoming cycle Second Law efficiency is higher than either the Rankine bottoming cycle or gas turbine, both in the percent range. This suggests using the Kalina process, not only over the Rankine cycle but also to replace some of the low end Brayton cycle expansion as well; hence, lower pressure ratio gas turbines are preferable. The corollary to the above is that the new generation of gas turbines, e.g. GE 7001F, operating at elevated inlet temperatures and higher pressure ratios may not provide any substantial advantage, if at all, when applied to a Kalina 4

6 bottoming cycle. The reason for this is that less dilution air is used for higher firing temperatures so that the exergy at the turbine exhaust, at the same exit temperature, is lower by the ratio of the exit mass flows between the higher and lower firing turbines. Although less popular than drum type boilers the monotube design is not new. They are specified in all supercritical cycles and some conventional combined cycle service as well. The monotube design does impose a more rigorous specification for water quality because no blowdown or mud drums are available to remove the dissolved solids. Pressure Ratio Power Output (KW) First Law Efficiency (Output/Fuel) Second Law Efficiency (Output/Exergy) 1 Solar Turbines Mars 16 8, GEC Gas Turbines ELM , Fiat TTG 14 41, Kraftwerk Union V , Kraftwerk Union V , Westinghouse 501D 14 98, Brown, Boveri KA13E N/A 137, N/A 8 General Electric 7001F , TURBINE DESIGN Table 4. Performance of Gas Tubines One of the major features of the Kalina technology is its compatibility with existing equipment and materials. Such is the case of the turbine design where standard steam turbine practice is used. The molecular weight of ammonia and water are nearly equal; 17 vs. 18, respectively. Aerodynamic correlations based on Mach number are equivalent. This results in similar blade and nozzle shapes, tip speeds, and overall steam path geometry. Within the turbine design itself the Kalina process offers two additional features. With a normal turbine back pressure of 24 psia the Kalina turbine's volume flow is an order of magnitude less than its steam counterpart. At 2400 psia turbine inlet pressure and 24 psia exhaust pressure the turbine expansion rate is 100:1. For a comparable steam plant operating between 1800 psia and 2.0 in hga the expansion ratio is 1800:1. The Kalina turbine requires fewer stages and does not contain the expensive high volume rear end stages. Double and four flow designs are not required in the Kalina plant as they are in the steam plant, as previously discussed in the section of the impact of cooling water temperature. Along the same lines the expansion in the steam cycle normally ends in the wet region resulting in momentum losses and the need to `armor' the bucket leading edges to prevent erosion. The expansion in the Kalina cycle always ends dry, improving performance and obviating expensive leading edge treatment. The Kalina cycle turbine does require a mechanical face seal as a replacement for the labyrinth type used in the steam turbine. This is a consequence of a zero leakage specification imposed by the ammonia working fluid. BOILER DESIGN The singular characteristic of the Kalina cycle boiler is its monotube, once-through design. It is not appropriate to use a drum design because the ammonia and water are easily separable. DISSOCIATION This phenomenon has often been mentioned as a potential barrier problem. There are, however, a number of factors that substantially diminish the likelihood of dissociation and subsequent material problems. A brief discussion follows. Normally anhydrous ammonia, in the presence of an iron catalyst, will dissociate into elemental hydrogen and nitrogen at temperatures above 300 C (572 F). Extrapolating this to 950 F, which is the temperature at the boiler exit, one would reasonably conclude that dissociation is likely to occur. However, it has been demonstrated by Nielsen [4,5] that small traces of water will 'poison' the catalyst and prevent dissociation. The substantial quantity of water in the working fluid (30%) is orders of magnitude more than required to prevent dissociation. Furthermore, the substitution of stainless steel for carbon steel in the boiler and the use of CrMov in the turbine flowpath depletes the iron catalyst and diminishes the probability of dissociation even further. Tests [6] performed by M. W. Kellogg showed that no dissociation occured after subjecting a specimen containing a 70/30 mixture to 1500 psi, 1000 F, in the presence of IN 800h tubing, for 24 hours. A mass spectrometric analysis revealed no traces of elemental hydrogen or nitrogen whatsoever. Still, total confirmation of this observation awaits prototype operation. CONCLUSIONS The Kalina cycle is well suited for bottoming cycle duty in utility combined cycles. As a new technology it has major advantages/features that suggest near term commercialization. These are listed below: 1) A percent improvement in bottoming cycle output. 2) No technological barriers. Temperatures are within standard power plant practice. 3) The Kalina cycle boiler and turbine designs may be selected from state-of-the-art steam plants. REFERENCES (1) A. I. Kalina; Combined cycle System With Novel Bottoming cycle, ASME 84-GT-135, Amsterdam, (2) Kalina Bottoming Cycle Performance Information Stone & Webster Engineering Corporation; Letter Report to H.M. Leibowitz, July 3, (3) Gas Turbine World Handbook, Pequot Publishing, Southport, Connecticut,

7 Continued References (4) A. Nielson; Iron Catalysts for the Synthesis of Ammonia, Gjellerups Follag, Copenhagen, 1956, Second Edition. (5) A. Nielson, Latest Developments in Ammonia Synthesis, Advances In Catalysis, Vol. 20, (6) M.W. Kellogg Company; Ammonia Decomposition on Incoloy 800 Tubing, Houston, Texas, July (7) Y.M. El-Sayed and M. Tribus, Thermodynamic Properties of Water-Ammonia Mixtures. Center for Advanced Engineering Study, M.I.T., Cambridge, Massachusetts; ASME Winter Annual Meeting, Miami Beach, Florida, November (8) H.M. Leibowitz; Kalina cycle, A Breakthrough in Power Technology, International Cogeneration Society Technology Conference, Phoenix, April (9) Technology Assessment of Advanced Power Systems II - Kalina cycle, Electric Power Research Institute; AP-4681 November (10) Turbomachinery International Handbook, Vol. 24, No. 3, Business Journals, Inc., Norwalk, Connecticut. APPENDIX A Point# t deg F P psia h Btu/lb Concentra- Row/ Flow Rate tion of Flow 30 lb/hr ammonia WATER WATER WATER WATER WATER WATER GAS GAS GAS GAS GAS GAS GAS GAS a a Table A-1. Bottoming Cycle State Points GAS TURBINE EXHAUST SUPERHEATER S 1 EVAPORATOR B TG 1 5 / ; PREH EATER S 2 TG RECOOLER B P 22 4 TG 3 38 DISTILLATION CONDENSATION SUBSYSTEM SEE FIG. 2 Figure A-1. Kalina Bottoming Cycle Plant Schematic Gas turbine output = KWe Bottoming cycle turbine power = KWe Bottoming cycle turbine shaft power = KWe Bottominng cycle turbine electrical power = KWe Bottoming cycle output = KWe System total output = KWe Fuel consumption = x10 6 Btu/hr Overall system efficiency = (6271 Btu/KWh) Bottoming cycle efficiency = or Available exergy of exhaust gas = KW Utilized exergy of exhaust gas = KW Second Law efficiency of bottoming cycle = Bottoming cycle internal exergetical efficiency = Exergy utilization efficiency of bottoming cycle = TABLE A-2 Performance Summary ACKNOWLEDGMENT The authors wish to acknowledge the contribution of the Stone & Webster Engineering Corporation, in particular Messrs E. Zabolotny, T. Vivenzio and N. Zervos, in conducting the parametric analysis of the Kalina vs. Rankine steam plant. Sponsorship of the Kalina bottoming cycle development, as described in this paper, was provided by the Fayette Manufacturing Corporation. The authors wish to thank Mr. John Eckland for his support of this project.