Optimization of Hybrid Noncondensable Gas Removal System for a Flash Steam Geothermal Power Plant

Transcription

1 GRC Transactions, Vol. 38, 2014 Optimization of Hybrid Noncondensable Gas Removal System for a Flash Steam Geothermal Power Plant J. Chip Richardson 1, P. E., and Jason Devinney 2 1 Engineering Manager, Vooner FloGard Corporation 2 Project Engineer, Vooner FloGard Corporation Keywords Noncondensable gas, NCG removal system, steam jet air ejector, liquid ring vacuum pump, hybrid vacuum system This paper defines the required operating parameters and illustrates how these parameters are used to optimize the NCG Removal System design for a typical flash steam geothermal power plant. Abstract The energy required to operate the Noncondensable Gas (NCG) Removal System for the condenser of a flash steam geothermal power plant constitutes a significant parasitic load affecting the overall power production potential and efficiency of the generating unit. Hybrid vacuum systems, incorporating steam jet air ejectors and liquid ring vacuum pumps, have proven to be a more efficient alternative for traditional multi-stage steam jet air ejector systems in geothermal applications. The higher capital cost and installation cost of the hybrid vacuum system is more than offset by the reduced operating cost, but, for maximum benefit, the designer of the hybrid vacuum system must consider key process information to optimize the NCG Removal System design for each specific application. Both environmental and commercial factors must be considered along with the properties of the geothermal resource in the optimization of NCG Removal System design. Environmental factors such as ambient temperature and absolute barometric pressure physically limit the operating range of the liquid ring vacuum pump. The relative values of geothermal steam, cooling water and electrical power for the NCG Removal System must be considered in the division of the gas compression work between the steam jet air ejector stage and the vacuum pump stage. Figure 1. Flash Cycle Geothermal Power Plant. 713 Introduction Flash cycle geothermal steam generally includes a component of naturally occurring noncondensable gases that may amount to 1-12% or more by weight. The noncondensable gases are usually comprised primarily of carbon dioxide, with a significant component of hydrogen sulfide and smaller components of other gases. In addition, because the condenser operates under vacuum, a significant amount of air is released from the cooling water (in barometric condenser applications) and some atmospheric air will leak into the condenser system. The NCG Removal System is designed to compress these gases from condenser operating pressure to atmospheric pressure for release to atmosphere. Figure 1 illustrates how the NCG Removal System fits in the typical flash cycle power plant.

2 The design capacity of the NCG Removal System is the sum of the expected noncondensable gases in the steam reaching the condenser, the air released by the cooling water (barometric design), the expected air in-leakage and the water vapor required to saturate the noncondensable gases at the condenser operating pressure and vapor temperature. Additional noncondensable gases, from steam jet motive steam, intercondenser cooling water air release (barometric design), and gland steam condensers are usually introduced into the process. Steam Jet Air Ejector System The traditional NCG Removal System design, as shown in Figure 2, consists of a multi-stage steam jet air ejector (SJAE) system with intercondensers between stages. The Heat Exchange Institute explains The operating principle of a steam jet air ejector stage is that the pressure energy in the motive steam is converted into velocity energy in the nozzle, and, this high velocity jet of steam entrains the vapor or gas being pumped. The resulting mixture, at the resulting velocity, enters the diffuser where this velocity energy is converted to pressure energy so that the pressure of the mixture at the ejector discharge is substantially higher than the pressure Figure 2. Two Stage Steam Jet Air Ejector System. Figure 3. Liquid Ring Vacuum Pump (from Heat Exchange Institute Performance Standard for Liquid Ring Vacuum Pumps). 714 in the suction chamber. 1 Within its design operating range, the steam jet air ejector is essentially a constant mass flow device. Primary advantages of using steam jet air ejectors include reliability from the simplicity of the design that requires no moving mechanical parts, and the ability perform compression without the necessity of cooling water. However, cooling is usually employed to improve the efficiency of multi-stage systems. The primary disadvantage of using steam jet air ejectors is that the thermal efficiency is low in comparison to mechanical compression. Although no universally accepted definition of the efficiency of steam jet air ejectors exists, and the efficiency varies widely with compression ratio, the steam and cooling water requirements for a particular application can be confidently predicted by the manufacturer. Within the typical temperature and compression range of a geothermal NCG removal system, the effective efficiency of the steam jet ranges from 10 to 20 percent and peaks at compression ratios of approximately 4:1. 2 Liquid Ring Vacuum Pump Basics The Heat Exchange Institute defines a liquid ring vacuum pump as a specific form of rotary positive displacement pump utilizing liquid as the principal element in gas compression and explains that the compression is performed by the liquid ring as a result of the relative eccentricity between the casing and a rotating multi-bladed impeller. 3 Figure 3 illustrates the operating principle of the liquid ring vacuum pump. Within its design operating range, a liquid ring vacuum pump operates as a near constant volume device and absorbs most of the heat of compression and condensation in the sealing liquid. Ideal gas laws are used to convert the mass flow of noncondensable gases and condensable vapors to volumetric flow at the inlet to the vacuum pump. Vacuum pump power requirements, empirically established to Heat Exchange Institute standards for dry air performance, must be corrected for the molecular weight of the noncondensable gases and for the actual compression ratio of the application. The primary advantage of using liquid ring vacuum pumps in lieu of steam jet air ejectors is the significantly higher efficiency of compression. Although there is no universally accepted definition of efficiency for liquid ring vacuum pumps, and the overall thermal efficiency depends greatly on condensing effects, the relative efficiency of a single stage

3 pump is highest over the range of 1.5:1 to 6:1 compression. Efficiency for a particular size of liquid ring vacuum pump, however, varies significantly with the pump operating speed as well as compression ratio as shown in Figure % Typical Dry Air Isothermal Efficiency of LRVP the main cooling tower and is drained to the main condenser to be returned to the cooling tower. The actual configuration of the hybrid NCG removal system often consists of multiple parallel partial capacity trains which may be used with a common intercondenser. Configuration of the system is specified by the user based on confidence in NCG load and requirement for redundancy. Optimization of Hybrid NCG Removal Design Isothermal Efficiency 50.0% 40.0% 30.0% Compression Ra o Figure 4. Maximum Pump Opera ng Speed Application of the liquid ring vacuum pump is limited by the vapor pressure of the seal water in the vacuum pump which precludes use of vacuum pumps for the full compression range required for most geothermal condenser NCG removal systems. Typical Hybrid Steam Jet/Vacuum Pump System Figure 5 illustrates a hybrid NCG Removal System using a steam jet air ejector for the initial stage(s) of compression, with an intercondenser used to minimize the condensable vapor load to the liquid ring vacuum pump performing the final stage of compression. The system requires steam for the ejector(s), cooling water for the intercondenser and vacuum pump and electrical power for the vacuum pump. The cooling water is usually furnished from Figure 5. Hybrid NCG Removal System. Minimum pump Opera ng Speed Following is the minimum information required for design of the Hybrid NCG Removal System: 1. First stage suction pressure (condenser design operating pressure minus pressure drop in vapor piping to first stage ejector) 2. Vapor temperature at first stage suction 3. Design basis noncondensable gas mass flow rate 4. Design basis noncondensable gas composition or average molecular weight 5. Design basis air release from cooling water and in-leakage 6. Mass flow of water vapor to saturate noncondensables and air at condenser operating pressure and vapor discharge temperature 7. Design basis motive steam pressure available for steam jet(s) 8. NCG content of motive steam 9. Design basis cooling water supply temperature and pressure 10. Discharge pressure of the system (average absolute atmospheric pressure plus any pressure required for discharge vapor piping or processing) Optimization of the Hybrid NCG Removal System design is an iterative process that begins with selection of the lowest interstage pressure that allows the vacuum pump to operate efficiently with consideration of the required compression ratio, available cooling water temperature and the condensation and compression heat load to determine the expected operating temperature of the liquid ring. The preliminary volumetric capacity required for the vacuum pump must include some allowance for additional NCG from motive steam and other sources. Because liquid ring vacuum pump efficiency decreases with increasing operating speed, optimization of the system design usually favors a larger vacuum pump selection 715

4 with slower operating speed over a smaller pump operating a higher speed for the same capacity. The optimization process involves iteratively completing a mass flow and heat balance, beginning with an estimated intercondenser operating pressure and vapor temperature, as shown in Figure 6. Cooling Water Motive Steam? M3/HR Water Seal Water? KG/HR WV? KG/HR Water? M3/HR Water? KG/HR NCG? KG/HR Air? KG/HR Water Design BARA Design BARA Design BARA Design C Design C Design C From SJAE From IC? KG/HR NCG INTERCONDENSER? KG/HR NCG FIRST STAGE SJAE? KG/HR WV? BARA? KG/HR WV LIQUID RING? KG/HR Air? C? KG/HR Air VACUUM PUMP? BARA? BARA? C? C From Condenser Intercondenser Drain From LRVP Design KG/HR NCG? M3/HR Water? KG/HR NCG Design KG/HR WV? KG/HR Water? KG/HR WV Design KG/HR Air? BARA? KG/HR Air Design BARA? C Design BARA Design C? C A simple method of evaluating the operating cost of the NCG System is to value the ejector motive steam requirement, the cooling water pumping requirement and the liquid ring vacuum pump power requirement as equivalent to electrical power produced by the geothermal power plant. For example, if a 50MW net output facility requires 300,000 KG/HR of geothermal steam in the turbine, each KW of output requires 300,000/50,000=6 KG/ HR of steam. In this example, the NCG system first stage steam jet air ejector facility may require 9,000 KG/HR of motive steam, equivalent to 9,000/6 = 1,500 KW of power. The intercondenser and vacuum pump cooling water requirement may be approximately 1,000 M3/HR and require approximately 15 KW to pump the cooling water from the cooling tower to the NCG system and from the condenser to the cooling tower. The second stage vacuum pump may require 500 KW for a total equivalent power requirement of 1, = 2,015 KW equivalent total auxiliary load. For comparative purposes, a further step in the cost evaluation is to determine the auxiliary power cost per kilogram of NCG to be removed. In the above example, if the NCG content of the steam is 2% by weight, 300,000 X 0.02 = 6,000 KG/HR of NCG must be removed and the power requirement is 2,015/6,000 = 0.34 KW/ (KG/HR) NCG or 0.34 KW-HR/KG NCG. Figure 6. Mass and Heat Balance for Hybrid Vacuum System. Designing the intercondenser for the lowest practical approach temperature is important to minimize the volumetric load to the vacuum pump by minimizing the partial volume occupied by saturation water vapor. Although some water vapor can be condensed with spray nozzles installed in the vacuum pump inlet manifold, the saturation water vapor from the intercondenser is more efficiently liquefied by direct condensing than by condensing by compression in the vacuum pump. Next, the steam jet air ejector(s) must be designed to compress the design vapor load to the vacuum pump suction pressure, accounting for pressure drop through the intercondenser and associated vapor piping. When the ejector motive steam requirement is determined, the extra NCG load is calculated, and the vacuum pump operating conditions are adjusted if necessary. The result is a system designed to perform as much of the compression work as possible using the most efficient device. Evaluation of the Operating Cost of the NCG System Actual Project Examples Figure 7 illustrates the evaluation of a range of potential intercondenser pressures for a particular geothermal project, presenting the kilowatt equivalent auxiliary loads as a percentage of design output SJAE Aux KW LRVP Aux KW Total Aux KW INTERSTAGE PRESSURE BARA Figure 7. Optimization of NCG Removal System Design % 9.00% 8.00% 7.00% 6.00% 5.00% 4.00% 3.00% 2.00% 1.00% 0.00% 0.20 In this example, the power requirement of the NCG Removal System is reduced by approximately 2% of total output as the design interstage pressure is lowered from approximately 0.35 bara to approximately 0.30 bara. If this were a 50 MW plant, the difference in equivalent auxiliary power requirement would be approximately 1 MW, corresponding to a reduction in operating cost of approximately $420,000 per year based on a value of $0.05/KW-HR. Continuing to reduce the interstage pressure from AUX LOAD/NET KW 716

5 0.30 bara to 0.20 bara results in an additional reduction of 1.7% of total output corresponding to an additional reduction in annual operating cost of approximately $357,000. Table 1 provides an analysis of the NCG Removal System equivalent power requirements of several recently completed or proposed flash cycle geothermal power projects showing the effects of a range of NCG content: Table 1. Power Requirements for NCG Removal System. Project A B C D E Net KW Output Est Turbine Steam KG/HR Steam KG/HR/KW Pct NCG in Steam 1.5% 1.8% 2.0% 3.0% 6.0% Base NCG Load KG/HR SJAE Compression Ratio Est SJAE Motive KG/HR SJAE Equivalent KW LRVP Compression Ratio Vacuum Pump KW Est Cooling Water M3/HR Cooling Water Pump KW Total Auxiliary Load KW Aux KW-HR/KG NCG Aux Load/NET KW 2.86% 4.25% 4.13% 5.17% 10.54% In Examples A and B above, the engineering specifications also applied constraints to the NCG system interstage pressure or vacuum pump size, while the configuration was open for optimization in Examples C, D and E. Although the range of steam NCG content directly affects the total auxiliary power requirement, the power required to remove each kilogram of NCG was reduced by optimization of the NCG system design. Other Cost Factors The system optimization described above is based only on operating costs. Once the most efficient system design is determined, capital cost factors determined by the owner may be applied to minor variations in system design. A more rigorous evaluation would apply the site specific capitalized cost of variations in: 1. Differential NCG System cost 2. Differential NCG System installation cost 3. Incremental Drilling cost 4. Incremental Reinjection cost 5. Incremental Cooling Tower capacity 6. Incremental Auxiliary Electrical Power distribution cost 7. Maintenance costs Summary and Conclusions A. Specification of a hybrid vacuum system with arbitrary constraints on interstage pressure, cooling water use or pump power limitations does not assure optimum hybrid system design. B. Generally, optimization of the hybrid system involves assigning as much of the compression work as possible to the liquid ring vacuum pump. C. Optimum hybrid system design is project specific and requires evaluation of project specific technical and commercial considerations. D. Optimization of the Hybrid NCG System design yields energy savings significant enough to affect the overall efficiency of the power plant. References 1. Heat Exchange Institute Standards for Steam Jet Vacuum Systems, Sixth Edition. 2. Harris, L. S., and Fischer, A. S., Schutte & Koerting, Characteristics of the Steam-Jet Vacuum Pump, 3. Heat Exchange Institute Performance Standard for Liquid Ring Vacuum Pumps, Fourth Edition. 717

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