Performance and economic analysis of the cogeneration combined cycle under gas turbine inlet air chiller operation

Gas turbines have gained widespread acceptance in the power generation industry. The keys attractions of gas turbines are their compactness, high power to weight ratio, and ease of installation which have made them a popular prime mover. There is constant progress in terms of research and development in hot section materials, cooling technologies, and reduction of emissions to allow improvements in firing temperatures and efficiency. Consequently, thermal efficiencies are currently very attractive in both simple cycle and combined cycle operations because they are profitable in the energy market. Gas turbine output is a strong function of the ambient air temperature. The gas turbine power production is reduced with the increase of the ambient air temperature. This power loss in output presents a significant problem to energy intensive industrial facilities utilizing gas turbines for power generation when electric demands is high during hot months. Under this scenario the possibility to increase power margins augmenting power can have a notable impact on profitability. Gas turbine power loss under hot ambient conditions can be eliminated using cooling technologies. Many cooling technology solutions are available in the market to perform the objective of power augmentation. In this thesis the focus is on the absorption cooling system which Linden Cogen is evaluating to substitute the actual evaporative cooler system for power augmentation in the summer. Linden Cogen power plant is a cogeneration combined cycle located in Linden, New Jersey, US and it is close to the Bayway Refinery which benefits from the electricity and the steam generated in the power plant. Linden Cogen power plant consists of two independent power plants: Linden 5 and Linden 6. The first facility consists of five General Electric 7EA gas turbine models lined up with as many heat recovery steam generators and three single automatic extraction, direct condensing General Electric medium steam turbines. The second facility consist in one more advanced General Electric 7FA gas turbine model lined up with the corresponding heat recovery steam generators . Linden 5 is responsible to provide steam to the nearby refinery and Linden 6 is the refinery generator, whose role is to sustain its electrical load. The fogging skid presents relevant issues and Linden Cogen is considering replacing the system. First, the fogging skid can potentially cause corrosion on the filter housing. The water itself can decrease the gas turbine efficiency when it enters in liquid form into the filter housing carrying the filtered particles with it. This process accelerates the fouling process in the compressor. The main reason that the potential absorption cooling plant is taken into consideration is because it can reduce the system limitations related to the electrical power production, increasing gas turbine output and efficiency. In a combined cycle application, these effects can lead to higher plant output, lower duct burner’s fuel consumption, hence a lower heat rate. An additional positive aspect is that the absorption chiller is IP steam driven. During the summer the Bayway refinery steam demand is lower and consequentially Linden Cogeneration becomes a steam intensive plant. Due to the surplus of IP steam, the backpressure steam turbine can be driven by the IP steam at zero cost. What was an unused product that could not be utilized has been turned in the driving force to enhance the combined cycle performance and efficiency. The chiller plant is made of two absorption cooling systems of 3000 ton capacity each. The main components of the chiller plant are the backpressure steam turbine, the intermediate pressure (IP) steam turbine driven chiller, the cooling towers condenser, the chiller auxiliary water pumps, and the chemical skid. The refrigerant used is R134a, while a mixture of water (70% vol.) and ethylene-glycol (30% vol.) is adopted as the secondary loop fluid which is directly responsible for gas turbine inlet air cooling. The chiller model has been developed to characterize the humid air transformation that takes place into the equi-current heat exchange which precedes the gas turbine suction. The model calculates the gas turbine inlet air conditions in terms of temperature and relative humidity. In order to understand the concept of gas turbine power augmentation under inlet air cooling, it is important to understand how the real gas turbine Joule-Brayton open cycle changes its operational states and why gas turbine performances are enhanced. To achieve this objective, the performance analysis of the gas turbine is introduced. To consider the operation deviation from the standard conditions, the correction carvers of each gas turbine in Linden Cogen have been obtained from historical data from the site. Before analyzing the effect of the chiller over gas turbine performance, the equations that regulate the gas turbine off-design operation have been verified for each gas turbine in Linden Cogen. The combined cycle performance analysis under chiller operations is realized. The objective of the performance analysis is to compare the Linden Cogen combined cycle performance under chiller operation (chiller case) and without the chiller plant operating (base case). The performance analysis quantifies the differential total power output, the differential total fuel consumption, and the differential heat rate. The analysis looks at the four hottest months during the summer i.e. June, July, August, and September. These months have been chosen because they present the most unfavorable ambient temperatures where the chiller plants can bring the more advantages to the combined cycle. Each month is assumed as a single average day. This day is described using two average time-depending variables, namely the ambient temperature and relative humidity. A temperature and relative humidity profile have been acquired from historical data of the site. The Linden 5 and Linden 6 power plants are independent from the point of view of the power generation, but they are perceived as a unique power plant when the steam system is considered. The performance analysis has been done separately for the two power plants without introducing unnecessary complication. Additionally assumptions over the steam export, which has been made to define the steam dynamics of the power plant. In the Linden 5 combined cycle analysis, two optimal discrete dispatches have been studied in the performance analysis: the 380 MW considering three gas turbines online, and two steam turbines online with no duct burner operation, and the 645 MW where all units are online. The dispatches, and the associated configuration assigned are going to be the most frequent operating condition when the chiller plant operates. The software used to simulate the Linden 5 operation under gas turbine cooling conditions is the Intelligent Process Simulation Environment Pro (IPSEpro) or Sigma model. The chiller model is not implemented in the Sigma model, and to simulate the gas turbine inlet air temperature reduction the input ambient conditions, have been used. This aspect influences the ACC which is strongly dependent on the input ambient condition. To overcome this important inaccuracy a procedure has been implemented. The deviation between the gas turbine correction curves used in the Sigma model and the one obtained from historical off-project datasets have been quantified to see if the performance analysis favors the chiller project or not. The Linden 6 performance analysis focuses only on the top cycle. The GE 7FA has been simulated through the correction curves obtained from historical data of the site. The performance analysis does not consider discrete dispatches like in Linden 5. The dispatch available for Linden 6 will be given by the ambient temperature according to the gas turbine correction curves. The economic analysis is set out in the final part of the thesis where the benefits which Linden Cogen will gain from the installation of the absorption chiller plant are estimated. The main objective of the economic analysis is to calculate the Net Present Value (NPV) and the Payback Time (PBT) of the chiller investment. The assumption is that data cannot be estimated over a period of 10 years because of the regular uncertainty of the market. Two configurations of the Linden Cogen combined cycle performance under chiller operation (chiller case) and without the chiller plant operating (base case) are compared. The analysis will compare the differential revenue form the eclectic markets, the differential cost of the fuel for gas supply, and the incremental capacity payment for the two configurations. The economic analysis has been made during the hottest months of summer: June, July, August, and September. It has been assumed the typical year of the economic analysis described by the means of the four months. Both the electricity price and the gas price are considered time-dependent variables involved in the economic system behavior. The electricity price of an average day of each month has been calculated from the historical data of the NYISO and the PJM locational marginal price related to the location where Linden Cogen participates in the electricity market. The average locational marginal price trend of the day has been calculated through the average price of each hour of the day. The distinction between labor days and weekends to calculate the electricity sale prices has been considered. The gas price for both Linden 5 and Linden 6 power plant fuel consumption has been estimated as an average of the month from historical data of day ahead gas market. Gas price is considered constant during each month. The differential operation and maintenance cost (C_(O&M)) are assumed $500,000 per year for the all plant life considering 20 years. The expected results of the thesis are as follows. The chiller plant increases the gas turbine power output .It will eliminate the restriction on gas turbine generating capacity under summer ambient conditions, stabilizing the gas turbine power output. The chiller plant reduces the duct firing fuel consumption in the 645 MW dispatch and for this configuration a reduction of the total fuel consumption is expected. This trend is not true for the 380 MW dispatch because the fuel consumption will follow the gas turbine fuel consumption under chiller operations. The chiller decreases the heat rate and increases the power output of the GE 7FA gas turbine in the Linden 6 power plant. As a consequence of a more efficient operation using the chiller, the energy price is expected to be lower than the base case energy price for both the 645 MW dispatch and the 380 MW dispatch. The dispatch frequency increases under the chiller case because of the reduction in the energy price. The chiller case gains more revenue from the electricity market when the Linden 5 and Linden 6 power plants run more for a longer period of time. On the other hand, the chiller case increases the cost of fuel consumption as well. The differential capacity payment is the most important differential cash flow component.