Low Emission Water/Steam Cycle A Contribution to Environment and Economics. Peter Mürau Dr. Michael Schöttler Siemens Power Generation, (PG) Germany

Transcription

1 Low Emission Water/Steam Cycle A Contribution to Environment and Economics Peter Mürau Dr. Michael Schöttler Siemens Power Generation, (PG) Germany Page 1 of 17

2 Abstract Avoiding emissions is a general goal nowadays. On one hand producing emissions affects the project profitability due to additional initial and life cycle costs like expenses for waste water disposals and the subsequent supply of demineralised water. On the other hand avoiding emissions is an important contribution to environmental protection and finally can help to ease the permitting phase of a project. This paper presents two examples how Siemens Reference Power Plants are designed to achieve the target of low emissions. Once-through BENSON developed by Siemens: Beside other important advantages of the BENSON technology like fast plant start-up without sacrificing lifetime and increased efficiency during start-up and at base load the implementation of a once-through helps to reduce water consumption of the steam/water cycle. In a power plant with drum type reasonable amounts of water need to be blown down out of the drums to achieve the necessary steam purity according to steam turbine requirements. This water needs to be replaced by demineralised water. The once through sections of a BENSON do not have a drum thus the cleaning is done by a condensate polishing plant. The resulting amount of water disposal is much lower since the salt concentration can be increased. Furthermore the enhanced average efficiency of a power plant with BENSON results in lower flue gas emissions. Zero Discharge Concept Further water losses which need to be considered are disposals coming from the clean drains system of the steam/water cycle including the. The zero discharge concept is designed to collect all kinds of blowdown and clean drains and routing it to the condenser respectively the condensate polishing plant. The regenerated condensate is fed back to the steam/water cycle. The result is a plant with a minimum water consumption. In combination with an air cooled condenser this plant is optimally adapted to arid regions. Page 2 of 17

3 1 Market Environment The energy demand worldwide is steadily growing. Not only the world population is increasing but also the energy consumption per head rises continuously mainly caused by the industrial development. Power production always has an impact on the environment. Whereas the negative impact of regenerative energy is relatively low the fossil fired power production at least affects the balance of greenhouse gases which finally is the reason for all the efforts to establish the CO 2 trading. Modern power plants are designed to reduce emissions to a minimum. Especially the gas fired combined cycle process with its high efficiency and the low content of pollutants in the fuel gas like sulphur or nitrogen helps to protect the environment. The relatively low carbon content even more reduces the amount of CO 2 emissions. Beside the flue gas, further emissions - liquid or gaseous - are leaving the power production process. One noteworthy emission is the water loss of the water/steam cycle. Considerable amounts of water have to be discharged. Water that partially contains chemical impurities. The discharge of these streams means an impact on the environment and often also spending for the effluent treatment. Furthermore the water needs to be replaced which again means additional expenditures and the water simply needs to be available. An important aspect taking into account that exceeding the natural available water resources also means to influence the environment in a negative way. This paper describes two systems which help to protect the environment by reducing emissions and at the same time contributing to the economical success of a project. Page 3 of 17

4 2 Once-through BENSON The once through BENSON was mainly designed to fulfil a customer requirement which shows a steadily increasing importance in the European market: Operating flexibility. Reasons for that importance can be found in the characteristics of deregulated and liberalized markets. Nobody knows exactly how fuel and electricity prices will develop and, as a result, in which load regime a power plant can be operated over the plant lifetime. Also competition during lifetime with other, newer plants influences the load regime and often leads to a lower dispatch rank. But these market circumstances also open up new opportunities like utilizing hourly and seasonal market arbitrage, participation in ancillary energy markets or peak shaving. To pick up the opportunities flexibility features like fast start-up or frequency response capability are very important. A highly flexible plant allows for an optimized operating profile resulting in an increased economic value of the plant. In some countries like Germany, Denmark and Spain, the significant and still growing share of wind energy causes new challenges for the operation of fossil power plants. The power output of wind turbines fluctuate heavily and thermal or hydraulic power plants must compensate these fluctuations. In this market environment, a plant capable for cycling and baseload operation is a must. The Siemens References Power Plants take this into account. Features like short start-up times, low combined cycle minimum load or full capacity steam bypass stations allowing simple cycle operation enable highest operating flexibility. Beside all these market driven influences the fast start-up saves fuel by avoiding operation at unfavourable loads and thus increases the overall efficiency of a power plant. But how can the fast start-up plant with the BENSON type affect the environmental situation? 2.1 Flue gas emissions Starting a power plant means to operate the components and systems below their designed capacity. This means that not only the power output steadily increases during start-up but also the efficiency. Comparing start-up curves of a fast start-up plant with a BENSON and a normal start-up plant with a drum type shows significant differences in the produced energy and the achievable efficiency within a certain time frame (see figure 1). Page 4 of 17

5 Within this time frame the energy output of the plant equipped with BENSON is about twice as high compared to the drum type plant. As a result the plant operator is able to sell twice as much energy during the start-up. This again improves the economical situation. Beside the increased energy output the average efficiency is also significantly higher at the BENSON plant since the areas of low power output and respectively low efficiency are passed much faster and earlier. Within the defined time frame the efficiency is improved by about 12%-points at the plant equipped with a BENSON type. Plant load [%] Data at t2 Benson Drum Energy output [MWh] Average efficiency [%] BENSON 20 Drum Time [min] t1 t2 Figure 1: Start-up curves after overnight outage Assuming a base load plant with about 30 starts per year the start-up time with reduced efficiency is negligible short compared to the full load operation. But assuming a cycling plant with about 250 starts per year the start-up sequence is a noteworthy part of the total plant operation. For this load regime the fast start-up of the BENSON plant results in an increased average efficiency of about 1%-point (see figure 2). The increased efficiency has direct influence on the flue gas emissions. With the utilized fuel a higher amount of energy can be produced. Energy that does not need to be produced by other plants and which is therefore not associated with additional emissions like CO 2. During the start-up event the formation of emissions is even more reduced by implementing the fast start-up procedure. The NO x and the CO gases primarily occur during the start-up. These emissions can significantly be reduced since the time frame of gas turbine operation at unfavorable loads with firing conditions totally differing from the optimized design point is Page 5 of 17

6 minimized. The NO x emission can be reduced by up to 10%. Reductions of more than 60% for CO emission can be achieved (see figure 3). Delta Efficiency [%-points] Increase of Efficiency Benson vs. drum type Base load Cycling Number of starts Figure 2: Influence of start-up time and load regime on average efficiency CO reduction [%] NOx reduction [%] 80 Base load Number of starts Cycling Figure 3: Reduction of flue gas emissions by enhanced start-up times Page 6 of 17

7 2.2 Water Supply and Effluents An important contribution to environment protection can be realized by saving water. This simply reduces the water consumption and on the other hand the effluents do not need to be discharged. The water/steam cycle with a once through section of BENSON has a significant advantage compared to the drum type section. To achieve the necessary steam purity according steam turbine requirements the drums which contain the highest concentration of impurities are continuously drained (see also chapter 3.1). The omission of the HP drum which is the drum with the highest flow rate saves therefore a lot of water which does not need to be drained. Instead of the blowdown the necessary cleaning of the water/steam cycle with BENSON type is done by the implementation of a condensate polishing plant. This saves about 30% of demineralised water. Exhaust Air Fuel gas Gas turbine HP separator Generator G 3~ HP-ECO HP steam HRH steam Cold reheat Clutch IP drum IP-ECO LP steam LP drum Steam turbine HP IP LP Exhaust Condensate pump Condensate polishing Feedwater pump Figure 4: Schematic of water/steam cycle with BENSON Page 7 of 17

8 2.3 Chemical Dosing To achieve appropriate steam conditions a certain amount of chemical dosing is necessary. The different types require a different boiler water chemistry to treat both s in an optimum way. For the BENSON type section the oxygenated chemistry is applied which shows advantages like the reduction of iron corrosion and iron transport to the and the formation of protection layers with much lower propensity for attack by flow-accelerated corrosion. Minimizing the deposits in the helps to maintain the heat transfer characteristics, reduces the need for chemical cleaning, lowers the risk of on load corrosion in porous deposits and reduces the pressure drop across the evaporator. Furthermore, when operating a boiler on oxygenated treatment, the dosing of ammonia can significantly be reduced between 50 and 70% which additionally helps to protect the environment. Page 8 of 17

9 3 The Zero Discharge Concept for the Water/Steam Cycle As described above the application of a BENSON once through reduces the demand for water fed to the water/steam cycle by about 30%. Depending on the water costs and simply the availability of water further reductions appear to be reasonable. Siemens developed a system to minimize the water consumption of the water/steam cycle. This system can be applied to both types of : The BENSON and the drum. The following chapters describe the Zero Discharge system in detail, taking the drum type as the basis. The combination of Zero Discharge and BENSON is described in chapter The Conventional Drain System (with drum type ) As mentioned above a considerable amount of water is removed from the water/steam cycle for several reasons and needs to be replaced by demineralised water. drum blowdown: In the drums the water/vapour mixture coming from the evaporator section is separated in the water liquid and vapour phase. The steam phase leaves the drum as saturated steam and is free of contaminations. The water phase flows back into the evaporation section of the and contains all the impurities which are fed into the drum by the feedwater flow. To limit the concentration of impurities a certain amount of water is continuously extracted from the drums as blowdown during normal operation. This blowdown is routed to the atmospheric flash tank whereas the flashed steam is blown into the atmosphere via a silencer, the water phase is dumped. Superheater blowdown: During start-up the superheater sections are also drained. This stream is free of contamination and routed to the flash tank as well. This means although this stream is suitable for rerouting it to the water/steam cycle from a chemical point of view it can not be used anymore. Steam line drains: During start-up the steam pipes and adjacent components like e.g. valves needs to be warmed up by steam. The thereby emerging condensates are routed to the atmospheric clean drains flash tank. Although the steam line drains are not contaminated the stream is not re-used. Analogue to the flash tank the water which is fed to the clean drain flash tank is either lost to atmosphere or to the cooling water line. Page 9 of 17

10 Sampling: To control the chemical conditions in the water/steam cycle the sampling system is fed from several points in the cycle like drum, superheater, condensate or feedwater system. The sampling streams are partially contaminated after the measurement and routed to the sewer. Condenser evacuation: To remove non-condensable gases in the water/steam cycle and to achieve and maintain the condenser vacuum the evacuation pumps are continuously in operation. Beside the non-condensable gases certain amounts of steam are extracted by the evacuation pumps as well. Leakages: Despite the application of high quality systems, components and maintenance a certain amount of losses caused by leakages can not be prevented. HP IP LP Steam Turbine Atmosphere Atmosphere Clean Drains Condenser Cooling Water Cooling Water Clean stream Contaminated stream Figure 5: Conventional drain system with drum type The following chapters describe the components and systems which are handled by the Zero Discharge Concept. This concept is designed to route all possible streams which leave the water/steam cycle back into the system. The Zero Discharge Concept comprises the following reduction steps: 1. Advanced Cascading Blowdown 2. Minimized Effluents 3. Treatment of Contaminations Page 10 of 17

11 3.2 Reduction Step 1: Advanced Cascading Blowdown An easy way to reduce effluents caused by drum blowdown is to route the blowdown stream to the next pressure stage. This means HP blowdown flows to the IP drum and the IP blowdown is fed into the LP drum. The disadvantage of this system is that all impurities are also carried over from drum to drum. This system is called cascading blowdown. The concept of the Advanced Cascading Blowdown system provided by Siemens hereby shows a significant advantage. The blowdowns of the HP and the IP drum is firstly routed to a separate flash pipe. Whereas the flashed clean steam is routed to the LP drum and just the contaminated condensate flows to the flash tank and will then be discharged. Beside the savings of water a noteworthy amount of energy can be recovered. The power output of a 400MW combined cycle power plant can be increased by about 130kW. HP IP LP Steam Turbine Atmosphere Atmosphere Condenser Clean Drains Cooling Water Cooling Water Clean stream Contaminated stream Figure 6: Advanced Cascading Blowdown 3.3 Reduction Step 2: Minimized Effluents A big step forward in reducing water consumption and water effluents can be achieved by the Minimized Effluents System. The goal is to route all streams back to the water/steam cycle which are free of contamination. The contaminated streams are flashed to recover at least the clean portion. Page 11 of 17

12 Therefore the clean steam phases of the flash tanks are rerouted back to the condenser. This can be done either directly for the steam line drains flash tank or indirectly for the flash tank. HP IP LP Steam Turbine No2 Condensate Storage Condenser Clean Drains Cooling Water Clean stream Contaminated stream Figure 7: Minimized effluents The following additional tanks need to be installed: tank: A flash tank (No 2) near the already existing flash tank No 1 is additionally installed. The superheater steam blowdown which is a clean stream is now directly routed to this flash tank No 2. The steam phase of the flash tank is free of contaminations and can directly be routed back to the condenser. Only the steam temperature needs to be reduced close to saturation temperature to fulfil condenser requirements. This is achieved by implementation of a heat exchanger which is installed in the condensate outlet of the flash tank No 2. The heat is transferred to the closed cooling water system. The steam phase of the flash tank No 1 is also routed to flash tank No 2 to reduce the temperature as well. The clean condensate phase of flash tank No 2 is routed to a condensate storage tank. Condensate storage tank: The condensate outlet of the flash tank No 2 and the steam line flash tank is pumped to a condensate storage tank. From there the condensate Page 12 of 17

13 is routed directly to the condenser. The storage tank is needed to compensate extraordinary high amount of condensate which can not immediately be routed to the condenser (e.g. start-up, plant outage). 3.4 Reduction Step 3: Treatment of Contaminations The final step in recovering the effluents is to avoid the discharge of condensate arising in the water phase of the flash tank No1. The discharge stream is therefore pumped to a blowdown storage tank for collecting and compensating purposes. From there the contaminated stream is pumped to condensate polishing plant. The clean condensate finally is routed back to the water/steam cycle via the condenser. Only if extraordinary high amount of condensate is collected in the storage tank the condensate is routed to the raw water tank to avoid overflow of the blowdown storage tank. But this could only happen if the condensate polishing plant is not available or the re-routing to the condenser is not possible. Further contaminated streams of the water/steam cycle like the major part of sampling streams are routed to the condensate polishing plant as well and can therefore be recovered. HP IP LP Steam Turbine No2 Condensate Storage Condenser Raw Water Blowdown Storage Clean Drains Condensate Polishing Plant Clean stream Contaminated stream Figure 8: Complete Zero Discharge system comprising all three reduction steps Page 13 of 17

14 3.5 Result Target of the Zero Discharge System is to collect the streams which leave the water/steam cycle and route them back to the cycle wherever possible. As shown in picture 4 this procedure can be applied to the drum blowdown, superheater drains and steam line drains. This already saves more than 75% water. Hugh amount of water needs to be replaced in the water/steam cycle due to extractions which are routed to the sampling system. Not all of the streams can be treated in the condensate polishing plant depending on quantity and quality of contamination. However, the recovery of the treatable part of the sampling system saves more than 15% water. The remaining water which can not be saved comprises the non-treatable sampling streams, the condenser evacuation system and leakages in the water/steam cycle. As a final result the Zero Discharge System enables a reduction of effluents and accordingly a reduction of make-up water for the water/steam cycle of more than 90% compared to a conventional drain system in a combined cycle power plant with drum type. 4 The Optimal Project Approach Taking into account all the positive influences of the before mentioned systems finally the question arises which is the best way to go. The answer depends on the project boundary conditions. Influencing factors are: Availability of cooling water Is enough water available to supply a once through cooling water system or at least to supply the make-up water system of a cooling tower? Is enough space available on site to build an air cooled condenser? Availability of make-up water for water/steam cycle Is enough water available to ensure a continuous operation of the plant? Is the power plant connected to a municipal source and therefore in competition with households in case of water shortages e.g. in summer months? Is a make-up water system connected to a well or a river which enables continuous supply? Even if the supply is currently ensured, what will be the situation in the future? What are the costs of water now and in the future? Allowable amounts of effluents Can the effluents of the water/steam cycle be dumped into the sewer system of the Page 14 of 17

15 municipal sewage plant or into the cooling water system? What amount of effluents is allowed and which quality? What are the costs for disposal? Load regime and operational flexibility Is the plant operated at base load, intermittent or daily cycling? How will a base load plant be operated in the future? Which quantity of emissions of NOx, CO and CO2 are acceptable? Not all of the combinations arising of the above mentioned questions can be examined within this paper. Nevertheless some general rules can be given. If a plant is planned to be operated in cycling mode or at least the future operation regime is unsure and the cycling operation mode can not be excluded in the near future the once through BENSON type is the best choice. The power plant owner takes advantage of the operational advantages resulting from the reduced startup time and the subsequent benefits regarding the reduction of flue gas emissions. Also the dosing of ammonia can be reduced by about 60%. Beside this the omission of the HP drum already saves about 30% make-up water for the water/steam cycle. An easy and inexpensive way to reduce the make-up water consumption of the water/steam cycle for a drum type is the application of the Advanced Cascading Blowdown system which saves more than 15% of water. An application for a BENSON type does not make sense since the blowdown of the drum is prevented anyway and the water savings of the BENSON already exceed the savings which are achievable by the Advanced Cascading Blowdown system. Reduction step 2 (Minimized Effluents) in combination with the Advanced Cascading Blowdown saves about 50% water and can of course be applied to a once through BENSON type as well. This is also valid for the complete Zero Discharge Concept which enables a reduction of water consumption of more than 90%. Page 15 of 17

16 The most favourable and economical application of Zero Discharge and the BENSON is given in arid regions in combination with an air cooled condenser. The air cooled condenser does not require make-up water for a cooling tower system. The water demand of the power plant is therefore limited to the supply of the water/steam cycle. Only the potable water supply has to be provided additionally which is normally below 5% of the overall water consumption. The Zero Discharge System ensures enormous water savings and enables a continuous operation of the plant without the threat of power reduction or plant outages due to water restrictions. Furthermore this system helps to reduce the life cycle costs which arise from the continuous expenditures for the raw water supply, the subsequent treatment in the demineralised water plant and charges for the disposal of effluents. A further cost advantage is given by a size reduction of the demineralised water tank and the raw water tank. Interruptions of the raw water supply need to be compensated by installing huge tanks for the storage of water. Since these tanks are dimensioned to ensure a save supply of the water/steam cycle for a certain amount of time, a reduction of water consumption allows for a reduction of the tank size. Of course, the water capacity must not fall below a certain limit since the activities during commissioning phase of the plant need to be ensured. About 40% of the system costs for Zero Discharge have to be spent for the polishing plant necessary for the treatment of the contaminated effluents. This is where the BENSON type comes into play. A condensate treatment plant is mandatory and available for this type of anyway. The effluents of the water/steam cycle can be fed into the polishing plant as well without having the necessity for a separate treatment plant. This improves the overall economical situation significantly. Not to forget that the high capacity of the condensate polishing plant available for the BENSON type allows for a optimization of the Zero Discharge Concept. The second flash tank and the storage tank are not necessary any more since the strict separation of contaminated and clean streams do not have to be applied. The capacity of the polishing plant is suitable to clean the higher mass flow the absolute number of contaminations remains the same. Page 16 of 17

17 HP IP LP Steam Turbine Condensate Storage Condenser Clean stream Contaminated stream Raw Water Clean Drains Condensate Polishing Plant Figure 9: Combination of BENSON and Zero Discharge system 5 Conclusion The BENSON and the Zero Discharge System help to reduce unfavorable impacts on the environment. Water savings with the consequential reduction of water supply and water discharge can be achieved by both systems: The BENSON saves about 30% the Zero Discharge system more than 90% compared to a conventional water/steam cycle with drum type. The BENSON furthermore reduces flue gas emissions of NO X up to 10% and CO up to 60% due to reduction of start-up times. The enhanced start-up times also save fuel and result in a general decrease of emissions. Beside the environmental aspect the same systems enhance the profitability of a project by the reduction of life cycle costs. Certainly, the economical benefit of the Zero Discharge system depends on the project boundary conditions. The BENSON type especially when operated in a load regime beyond pure base load is always the most economical solution. Protecting the environment and improving the economical situation of a project is not a contradiction. Page 17 of 17