A Systematic Approach Linking Condenser Performance to Heat Rate and CO2 Emissions

Peter de Graaf, Nalco Europe BV Martyn Gilbert, Rocksavage Ltd, and Andrew Hurst, Nalco Ltd

ABSTRACT A systematic approach to cooling system management was taken at InterGen’s Rocksavage Power Station with the objective of improving the overall performance of the cooling tower and condenser under varying conditions. The approach included a combination of engineering, operational and chemistry initiatives that led to measurable performance improvements. This paper will present the path taken by the Rocksavage Power Station to not only improve condenser performance, but also to quantify the impact on plant heat rate and CO2 emissions. Plant data over a multi-year timeframe was used to document the improved operating practices and control. Due to the implementation of these systematic improvements, a significant improvement in system cleanliness was achieved that resulted in reduced steam turbine backpressure and better cooling tower efficiency. Savings opportunities in terms of gas consumption and reduced carbon dioxide emissions were highlighted. These gains can help to improve the environmental performance of the Station, which would result in reduced total operating costs of the cooling system while maintaining condensing performance.

INTRODUCTION Power plant condenser cooling systems play a vital role in maintaining generation efficiency and plant heat rate. Deposition in the condenser or cooling tower fill will increase turbine backpressure and require higher steam flow to maintain power. Higher steam flow will require more fuel to be burned, assuming the boiler system has the capability to generate more steam. In some cases, the plant cannot generate at full power with a fouled condenser the result of which is de-rating. The increased fuel requirement increases plant heat rate, which translates directly to fuel cost and emissions. Even though the increase in fuel consumption may be small (< 1%), the environmental and financial impacts may be significant, as emission-trading schemes (cap-and-trade) for greenhouse gasses are directly affected. Condenser and cooling tower performance-monitoring helps to link cooling system performance to heat rate and can be expressed in terms of environmental performance and compliance. Condenser cooling operating variables can be combined with cooling system chemistry performance as well as control data and engineering calculations to help document the impact of cooling system performance on heat rate, fuel consumption and emissions of greenhouse gases. The lowest environmental impact can thus be achieved from existing plant equipment. This paper will be presented in a case history format using actual plant data. The paper builds on a previous paper presented at POWER-GEN Asia in 2006.1

THE EU EMISSION CREDIT TRADING SCHEME Signatories to the Kyoto protocol, including all Member States of the European Union (EU), have committed to control and reduce emissions of Greenhouse Gases (GHG) in order to decrease their impact on global warming.2 The European Commission has mandated an Emission Trading Scheme (ETS)3 to manage the cost-effective reduction in GHG emissions across the 27 EU countries. The ETS gives each plant an annual GHG ‘allowance’ or emission volume in metric tons of carbon dioxide & the amount of other GHG with the equivalent global warming potential to CO2. The allowance is calculated on the basis of the National Allowance Allocation Plan for GHG control agreed to by each EU Member State. Each following year, the plant has to ‘surrender’ an allowance allocation equivalent to the actual volume of GHG emitted during the previous year. If emissions are below the allowance, they become credits that may be ‘traded’ with other plants that are exceeding their

1 particular allowance. Credits may also be earned by participation in Joint Implementation (JI) or Clean Development Mechanism (CDM) projects. If a plant fails to ‘surrender’ an amount of allowance equivalent to the previous years’ emissions, meaning for example that it exceeded its maximum permitted emission volume, then a penalty is payable for the excess emissions. Up until January 1, 2008, the penalty is €40 per ton of carbon dioxide or equivalent; thereafter the amount increases to €100 per ton. Payment does not remove the legal obligation to still surrender the necessary allowance. Note that the penalty is well in excess of the expected trade value of the credits, providing an incentive for producers to meet their emission goals through performance, trading, or offsets. The first cycle of emission allowances commenced on January 1, 2005 and covers the three years to January 1, 2008. The commitment of the EU to the Kyoto Protocol drives the control of the total GHG emission volume for the EU as a whole, or, in other words, the sum of all National Allowance Allocation Plans across EU27. Plans will be reviewed every 5 years from 2008. It is expected that Emission Allowances will be progressively reduced every five years.

ROCKSAVAGE POWER STATION Rocksavage Power Station is a Combined Cycle Gas Turbine (CCGT) plant located in , , UK with a nominal output of 743 MW. The station is 100% InterGen owned and operates in a load following mode. It is one of the cleanest and most efficient power plants in the UK, with an average availability of 94% during the last four years. The Station consists of two Alstom GT26A combustion turbines, two triple-pressure unfired HRSG’s and one Alstom, 284 MW, steam turbine. It has a two pass surface condenser which is cooled using filtered river water supplied from a 12 cell plume-abated cooling tower with a system volume of approximately 5,000 m3 (1,320,000 gallons).

Photo 1 – Arial photograph of Rocksavage Power Station

The UK National Allocation Plan GHG annual allowance for Rocksavage Power Station for the trading period 2008-2012 is 1.08 million tons of CO2 equivalent, which is a considerable shortfall compared to current emissions.4 This shortfall has to be traded on the Carbon Exchange market; 5 futures (2008-2009) indicate a cost in the range of €20-25 per ton of CO2. The calculated estimate for the GHG emission cost for the Rocksavage Power Station is in excess of 10 million Euros per year. There is a clear incentive to trim GHG emissions in every possible way.

2 DETERMINATION OF CONDENSER PERFORMANCE IMPACT ON HEAT RATE AND EMISSIONS

Instead of walking the reader through the input fields required to determine condenser performance, this paper will focus on the key empirical correlations that are the backbone to the calculation model. We will first look at establishing bogey backpressure and what that means in practice. Then we will examine the relationship between heat rate and condenser backpressure, and finally how the heat rate loss is extrapolated to fuel and emissions cost, or alternatively MW de-rating. The bogey backpressure is the expected backpressure for a condenser if the tubes are clean and the condenser is operating to design specifications. We can estimate the bogey backpressure based on the cooling water outlet temperature and the design terminal temperature difference for the condenser. An alternative method is the use of condenser design graphs that contains correlations between condenser load and backpressure as function of cooling water inlet temperature and condenser cleanliness (Figure 1). Figure 1 – Condenser design graph

12.0 Condenser Cleanliness Factor is 90% 11.0

10.0 29.4oC (85oF)

9.0

8.0 24.4oC (76oF) 7.0

21.1oC (70oF) 6.0

5.0 15.6oC (60oF)

4.0 10.0oC (50oF)

3.0 4oC (39oF)

2.0 80% 90% 100% 110% 120% Condenser Load [%]

The backpressure readings from the sensors in the condenser should not be below the theoretical bogey backpressure. They can read low, however, if steam condenses in the sensing element. If apparent backpressure is below bogey, the plant must perform sensor maintenance to allow accurate performance measurements. For the Rocksavage Power Plant, with three pressure sensors on the condenser, the backpressure readings that were below bogey were corrected by establishing a correlation between the LP exhaust pressure and condenser backpressure as shown in Figure 2. The crux of the model revolves around the establishment of the correlation between heat rate and condenser backpressure. This correlation forms the basis for calculating the fuel and emissions penalties. The plant heat rate is the net efficiency in heat conversion and transfer of the various stages of the process: the gas turbine, the HRSG, the steam turbine, plus the plant operating practices. Empirical correlations between heat rate and backpressure are not straightforward, and this is shown in Figure 3. One can argue that a better correlation could be established by looking at steam turbine heat rate versus backpressure, but this brings additional complexity to the model that does not

3 necessarily have a payoff in terms of a more precise result relative to the effort that a performance engineer needs to make. Figure 2 – Empirical correlation between backpressure and exhaust steam pressure

11.00

LP Exhaust Steam Pressure = 10.50 0.7925 Back Pressure + 2.5094

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9.50

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8.50

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7.50

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6.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 Condenser Back Pressure [kPa]

Figure 3 – Empirical correlation between heat rate and bogey backpressure

7400

Heat Rate = 44.14 (Backpressure - Bogey Backpressure) + 6,930 7300

7200

7100

7000

6900

6800

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6600 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Backpressure - Bogey [kPa]

4 The heat rate penalty for Rocksavage Power Station, according to this empirical correlation, is 44 kJ/kWh per kPa change in condenser backpressure (141 BTU/kWh for 1“ Hg).6 This equals 0.63% of efficiency loss per kPa (2.15% for 1” Hg) deviation from the theoretical backpressure using the design terminal temperature difference. Increased fuel use can be estimated from the expected fuel use for the station (125,000 GJ/day, or 1.18 1011 BTU/day), the heat rate penalty (0.63% per kPa) and the difference between condenser backpressure and bogey backpressure. In this case it is 788 GJ/day per kPa (2.53 106BTU/day for 1” Hg). In terms of a de-rating, this would equate to approximately 1.8 MW per kPa (6.1 MW for 1” Hg). The expected CO2 to fuel ratio used in the model is 50.4 kg/GJ 6 (117 lb/10 BTU) thus an additional 40 ton CO2/day is emitted per kPa (135 ton CO2/day for 1” Hg) deviation from bogey backpressure. Both fuel and emission costs impact the spark spread when the Emissions Trading Scheme (ETS) is in place.

CASE STUDY In the Rocksavage Power Station case study, we will examine four areas that were part of improving condenser performance: § the mechanical cleaning of the condenser § the impact of on-line cleaning of the cooling tower fill § the implementation of a cooling water treatment monitoring and control system § the steps taken to troubleshoot air in-leakage At the end of this section we will use the model to visualize these areas of improvement and their impact on heat rate and CO2 emissions. During September 2003 an opportunity arose to perform an inspection on the waterside of the plant condenser that indicated that the cooling water had been operating in a scaling mode and deposits formed on the heat exchange surfaces would need to be removed by a condenser clean. Discovering deposits during an inspection may confirm the reason for a loss in condenser performance, but it is always a retrospective evaluation. Condenser performance monitoring will make the link between cooling system performance, heat rate, fuel consumption and environmental performance that allows you to monitor performance and take corrective action immediately, before losses mount. Pictures 2 and 3 show the condenser tubes before and after the adoption of a new treatment and control program. Several questions suggest themselves. For example, how much efficiency was lost? Did the scaling happen gradually or were there upsets in the cooling system that could have been prevented? In this case we are talking after the event, but it helped the plant focus their attention on condenser performance and cooling water chemistry control. Picture 2 – Before mechanical cleaning Picture 3 – After condenser performance optimization

5 In the UK there is a requirement to adhere to the UK Approved Code of Practice and other regulatory frameworks that direct operators of industrial cooling systems to develop a microbiological control program that includes cooling tower cleanings. An extensive monitoring and control strategy was implemented that incorporates continuous dosing of an oxidizing biocide, routine super-chlorination and periodic cell cleanings. The system stress on the condenser was managed by spreading the cell cleanings over time. The model can be used to identify and quantify the improvements generated by good microbiological control. A benefit of cooling tower cleaning and disinfection is a lower condenser inlet temperature. A potential downside is that deposits, debris and biomass that are dislodged during the cleaning operations could redeposit in the condenser and lead to a reduction of the cleanliness of the condenser. In this case, the Rocksavage plant made adjustments to the cooling water chemistry to prevent a loss of heat transfer in the condenser, while still capturing the benefits of increased cooling tower efficiency. Rocksavage Power Station did experience a loss in condenser performance that was diagnosed to be related to air in-leakage. The traditional response in power plants to a loss of condenser performance is to blame condenser fouling. Quite often, the air removal flow from the condenser is not recorded. Because Rocksavage was using the 3D TRASAR® stress management program on the cooling system, we were able to show that the increase of condenser backpressure was not cooling water related (Figure 4). This is captured in the on-line polymer measurements. The green and red lines in Figure 4 indicate the amount of polymer dispersant fed to the system, and the amount of active residual measured. The 3D TRASAR system is set to vary the amount of dispersant fed in order to maintain a constant level of 40 mg/L of active dispersant, as product, to protect the condenser and cooling tower from deposition. The dark blue line shows a sudden drop in condenser performance, but the active polymer remains steady. This means that deposition is not the cause of the performance loss. Additional data show that there was no loss in biological protection that could have attributed to the loss in performance either. The 3D TRASAR data drove the plant to investigate air in-leakage as another, often overlooked, cause of increased backpressure. The plant, with a thorough investigation, was able to find and seal large air leaks in both cases, restoring condenser performance and plant efficiency. Figure 4 – Linking condenser performance to cooling system stress and air in-leakage

100% 200 Air Inleakage Air Inleakage 90%

80% 150 70% Cleanliness Factor [%] 60% Air Leak Inert TRASAR [ppm] 50% 100 Tagged Polymer [ppm]

40%

30% 50 20%

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0% 0

Date Cleanliness factor monitoring and trending can provide a high-level view of condenser performance over a multi-year period. Figure 5 highlights the events that are presented in this case history. The events are noted by the colored squares near to the bottom of the graph. A measurable increase in condenser cleanliness was observed after the mechanical cleaning of the condenser. Although the

6 stress on the condenser increased during the fill cleaning and disinfections, the cooling water chemistry was able to cope with the additional load in silt, debris and biomass, allowing the condenser to perform close to design, a cleanliness factor of 90%. Figure 5 – Linking condenser performance to plant events

100%

Design Cleanliness Factor = 90% 90%

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Mechanical Cleaning 40% of Condenser Cell 1 Cell 3,4 Cell 8,9 Cell 7

Cell 2 Cell 5,6 Air Inleakage Air Inleakage 30% 15-Aug-02 15-Feb-03 15-Aug-03 15-Feb-04 15-Aug-04 15-Feb-05 15-Aug-05 15-Feb-06 15-Aug-06 15-Feb-07 Date

An alternative way of visualizing condenser performance uses a de-rating perspective. The model allows for the calculation of the MWs lost on the steam turbine, presented for the same period, as shown in Figure 6. Figure 6 – Linking condenser performance to plant events – de-rating perspective

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Mechanical Cleaning of Condenser Cell 1 Cell 3,4 Cell 8,9 Cell 7 5 Cell 2 Cell 5,6 Air Inleakage Air Inleakage

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0 15-Aug-02 15-Feb-03 15-Aug-03 15-Feb-04 15-Aug-04 15-Feb-05 15-Aug-05 15-Feb-06 15-Aug-06 15-Feb-07 Date

The fuel and emission penalties also can be determined for each level of condenser performance. These results are presented in Table 1. Restoring condenser performance via mechanical cleaning resulted in a reduction of fuel cost in the range of €1,200/day. This equates to a CO2 penalty of 30 tons/day, and that, depending on the trading price, could be valued at €600/day. The station was able to maintain high condenser performance by a combination of on-line fill cleaning and sanitation and

7 implementation of the 3D TRASAR stress management program for the cooling system, allowing the condenser to operate close to design. Finally, the model provides the quantification of the cost associated with an unnoticed air in-leakage. As the timeframes are quite well known, it’s possible to determine the fuel and emissions penalty, which for the two incidents adds up to €382,000 in fuel and 8,700 tons of CO2 that could potentially be valued at €174,000. Table 1 – Overview of the fuel and emission penalties

Event Days Average CF% Calculated Heat Calculated Calculated CO2 Rate Penalty Increased Fuel ton penalty kJ/kWh Use GJ/day ton/day Prior to mechanical cleaning 432 65.6% 46 825 42 After 191 80.9% 12 224 11

Cell 1 170 82.7% 12 216 11 Cell 2 48 84.9% 8 145 7 Cell 3 & 4 92 86.2% 6 116 6 Cell 5 & 6 117 83.3% 10 185 9 Cell 8 & 9 238 82.8% 10 179 9 Cell 7 138 77.4% 18 333 17

918 80.6% Maintain CF% for 918 days

Air Leak 1 39 45.7% 103 1,851 93 Air Leak fixed 155 77.7% 18 328 17 Air Leak 2 152 60.4% 60 1,084 55 Air Leak fixed 38 77.8% 19 177 17

CONCLUSION

Condenser performance monitoring can make the link between cooling system performance, heat rate, fuel consumption and environmental performance and allow power plants to take corrective actions immediately, before losses mount. Optimizing the performance of the surface condenser and cooling tower system requires a relatively low investment, is quickly implemented, and has a short payback period. The first step to improving performance should always consider the plant’s emission reduction strategy. The results are improved environmental performance in terms of emissions and emission costs plus heat rate. The result is significant fuel savings and tradable emission credits. This paper focused on a CCGT plant and the European Trading System on greenhouse gas, but the argument is even stronger for a -fired power plant. For coal-fired stations, improving condenser performance also means a lower SO2, heavy metals and solids load on the Flue Gas Desulphurization (FGD) unit, potentially resulting in lower SO2 and mercury emissions. The 0.5 to 1% improvement in heat rate that can be established with best practices for optimizing condenser performance may be perceived as being of limited financial importance to a single power plant. From a fleet management perspective, however, the reduction of the total fleet’s energy bill and emissions trading volume by 0.5 – 1.0% is significant for the company, the wider industry, and the environment.

Acknowledgement The author wants to thank Rocksavage Power for making the data available for this paper.

8 REFERENCES

1 Gehan, K. and A.J. Hook, Greenhouse Gas Reduction from Fossil Power Stations – The Effect of Surface Condenser and Cooling Tower Performance, POWER-GEN Asia conference and exhibition (Hong Kong 2006). 2 2002/358/EC: Council Decision of 25 April 2002 concerning the approval, on behalf of the European Community, of the Kyoto Protocol to the United Nations Framework Convention on Climate Change and the joint fulfillment of commitments thereunder. 3 Directive 2004/101/EC of the European Parliament and of the Council of 27 October 2004 amending Directive 2003/87/EC establishing a scheme for greenhouse gas emission allowance trading within the Community, in respect of the Kyoto Protocol's project mechanisms. 4 http://www.defra.gov.uk/environment/climatechange/trading/eu/phase2/phase2nap.htm; NAP ID=2787 5 http://www.ecx.eu/default_flash.asp , http://www.eex.com/en/, http://www.powernext.fr/index.php?newlang=eng 6 For comparison purposes: POWER Magazine, Volume 146, No. 1 (January 2002), esp. 18: 1 kPa backpressure adds 50 kJ/kWh to heat rate.

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