Combustion Chamber. Fig Schematic diagram of a simple gas turbine

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1 Module 06: Integration and placement of equipment Lecture 37: Integration of Gas turbine with process 1 st Part Key word: Gas turbine, Acid dew temperature, after burner, specific work In its non complicated form, the gas turbine consists of a air compressor driven by a turbine mounted on the same shaft as shown in Fig Ambient air is compressed in the compressor and as a result its temperature increases. A portion of this air is sent to the combustion chamber where fuel is injected and the other part is used to cool the combustion chamber wall. The mixture is fired and the temperature of the combustion chamber increases in excess of 1500 C. This temperature is constrained by the turbine blade material. The hot mixture of air and combustion gases then enters the turbine and is expanded to generate mechanical power to drive the compressor. The hot exhaust which is around C is in general reach in oxygen ( 15% O 2 ) and can be fired second time with fuel to increase its temperature. For stand alone applications, gas turbine efficiency is a critical parameter, however, in cases where the gas turbine exhaust heat is recovered, the efficiency is less important. Combustion Chamber HOT FLUE GAS Compressor Shaft coupling Gas Ambient AIR Fig Schematic diagram of a simple gas turbine Depending upon the usage, many configurations of Gas turbine are avialble in market. HOT FLUE GAS Recuperator Combustion Chamber Compressor Shaft coupling Gas Ambient AIR Fig Schematic diagram of a simple gas turbine with recuperator

2 Fig.37.2 shows a gas turbine with recuperator ( or regenerator). The exhaust gas which is at a fairly high temperature from the turbine can be discharged to atmosphere as hsown in Fig.371. or can be sent to a recuperator to exchange heat with in coming compressed air to preheat it to increase the overall efficiency as shown in Fig It has some disadvantages too, such as it increases the pressure drop in the system thereby affecting the efficiency. The temperature of the exit gas from GT is fixed by operating constraints. However, it is possible to maniupulate the temperature of this gas using a recuperator(figs.37.2) or after burner(fig ) so that it suits the needs of the process. The exhaust gas from the turbine contains about 15% O 2 and thus can be burned second time by injecting fuel to increase its temperature as shown in Fig Combustion Chamber After burner HOT GAS Compressor Shaft coupling Ambient AIR Fig Schematic diagram of a simple gas turbine with afterburner Fig.37.4 shows an mechanically complicated arrangement called split shaft turbine. The first turbine provided necessary power to drive the compressor and the second turbine provides power for external load. Combustion Chamber HOT FLUE GAS Compressor Shaft coupling Gas Gas Ambient AIR High pressure turbine Fig Schematic diagram of a split shaft gas turbine

3 Fig.37.5 shows an integrated gas turbine which uses the exhaust from gas turbine(gt) to generate steam ina heat recovery steam generator(hrsg) before it is vented to atmosphere.it is possible to fire fuel in the gas exhaust (from GT) to increase its temperature before entering into HRSG. Combustion Chamber Heat Recovery Stream Generator Compressor Shaft coupling Gas Stream Ambient AIR Steam Boiler feed water Fig Schematic diagram of gas turbine with heat recovery steam generator(hrsg) The Gas performance is a function of a number of imporatnt parameters discussed below: Inlet temperature of GT : The power generated by a GT and its efficiency are proportional to the inlet temperature measure in absolute tempearure scale. The maximum temperature is constrained by the turbine blade material cooling of which allows higher inlet temperature ( up to 1500 C). Pressure Ratio: An increase in the power ration of the compressor first increases the power out put to a maximum point and then decreases it as shown in Fig Moreover, the optimum compression ratio increases with increasing inlet temperature.pressure ration of industrial machines are in the rage of 1o to 15. Ambient condition: The perfomace is generally specified at International standards organization (ISO) at 15 C, kg/cm 2 and 60% relative humidity. As the power consumption of compressure is proportaional to the inlet temperature air( in K) and relative humidity, the efficiency and power out put increases with decrease in ambient air temperature as well as relative humidity and vice versa.

4 Work Load: The efficiency drops as load decreases from its 100% rated capacity as is a function of machine and associated control system. Specifif work: It is defined as work out put per unit of air flow. It increases with turbine inlet temperature. It also increases with pressure ratio to a maximum limt and then decreases as given in Fig.37.6 Back pressure: Back pressure is generated by systems added between the GT exhaust and chimney. These may be heat recovery systems, another furnace for secondary burning or exhaust gas treatment units. These units cause pressure drops and back pressure created by these devices decreases the power out put. Even if these devices are not present, the change in ambient pressure also changes the machine performance. Combustion in GT creates emissions. The NOx formation in a GT can be delt with by staged combustion, steam injection and treatment of exhaaust gases Specific work kj/kg Compressor Pressure ratio Fig.37.6 Variation of Specific work with compressor pressure ratio for given turbine inlet temperature Cogeneration efforts based on GT has increased in recent years. Though generation of steam using GT exhaust is a common example, its other direct use such as process fluid heating and drying is not uncommon. The integration of GT exhaust with process GCC can be done based on

5 framework of Pinch Technology. A detaild discussion on different aspects of direct integration of GT exhaust with GCC as discussed below: For integration, the process is represented by a grad composite curve(gcc) which is the utility interface of the process. It clearly shows how cold and hot utilities and their different levels can be matched with the process to satisfy it nees. The construction of GCC is discussed in detail in Lacture 12. The hot utility available for direct integration with a process is hot gas coming out from the exhaust of GT and the process is primarily convective heat transfer. If it is assumed that the specific heat of hot gas does not cahnge with temperature within the specified range of temperature in which exchange is taking place, then it can be represented by a straight line having slope equal to the reciprocal of the heat capacity flow rate(cp) of gas as shown in Fig In this figure one can see two hot utility lines denoted by A and B. Both the hot utilities which have different CP values supply equal amount of heat to process and have the same stack temperature. However, these have different exhaust losses as can be seen from the extrapolation (dotted line) of the line to ambient temperature. The hot utility line with high temperature(t 1 ) and low CP has low exhaust loss, whereas it is reverse for high CP and low temperature(t 2 ) hot utility stream. Following conclusions can be drawn: 1. Keeping the inlet temperature of hot utility constant, the exhaust loss will increase by creasing the CP value of the hot utility line as it will drease the slope of the line. Thus exhaust losses can be minimuzed by minimizing CP. 2. Stack tempearture also pays important role in restraing exhaust losses. More is the stack temperature more will be exhaust losses. Ideally, if stack is adiabatic, stack tempearture should be equal to acid dew point temperature under the condition that pinch temperature should be below acid dew point temperature. It should be noted that stack temperatures are kept more than acid dew point tempearture for flue gasses conting oxides of sulfur(sox). Metal haet exchage services should be operated above C in flue gases containg SOx to avoid condensation of acid. The acid dew point depends on the sulfur content of flue gas and the amount of excess air. Let us discuss a different situation, given in Fig.37.8, where the hot utility line denoted by AF (slope is fixed by value of CP) cuts the process GCC denoted by B D E F G H at point C. This means that the hot utility line will bot be able to supply the heat required by CD portion of the GCC. Now what are the different options available to designer to solve this problem. 1. Introduce a second utility such as steam to service DC part of the GCC as shown in Fig Increase the CP(by increasing flowrate of GT exhaust) of the hot utility so that the slope of the line ( with H axis) decreases and the complete line is above the upper half GCC as shown in Fig Keep the hot utility flow constant and increase the inlet temperature of exhaust gas as given in Fig.37.11

6 All the above three solution will increase exhaust losses as can be seen from Figs.37.9,37.10 & Slope = 1/CP 2 CP 2 CP 1 T 1 T 2 Interval Temperatures A B CP 1 > CP 2 Stack temperature Acid dew point temperature Pinch Ambient temperature Exhaust Losses H, kw Fig.37.7 Process GCC is matched with exhaust of gas turbine profile Interval Temperatures This portion of GCC will not be serviced by the hot utility D E C A B Acid dew Temperature F Heat Loss through stack H G Ambient Temperature H, kw Fig.37.8 GCC is cut by hot utility gas (GT Exhaust) line.

7 Exhaust gas A Interval Temperatures Exhaust gas D Steam E C B I F dew Temp. H G Ambient temperature Excess heat loss through stack H, kw Fig Introducing a second utility(steam) in the hot utility plot. A Interval Temperatures I Increased gas flow rate D C E B F Acid dew Temp.Acid H G Ambient temperature Excess heat loss through stack H, kw Fig Gas flow rate is increased to supply heat to all above pinch region

8 Interval Temperatures Inlet temperature raised keeping flow constant D C E J A B I F Acid dew temp. H G Ambient temperature H, kw Excess heat loss from stack Fig Exhaust gas supply temperature increased keeping flow rate constant Now the question is how to fix up the flow rate of exhaust gas so that it is able to deliver the hot utility to the process GCC with minimum stack loss. To get answer to this question one should know how the flow rate of exhaust gas affects the pertinent parameters of the GT. In a GT based cogeneration system the exhaust gas flow rate is controlled by the required power to be generated. In fact, exhaust gas flow rate is controlled by turbine inlet temperature and compressor ratio as discussed in the earlier part of the lecture. For all practical purposes the exhaust gas canb be considered as pure air. This is because large part of the exhaust gas is air. One of the pertinent parameter worth investigating is specific work(kj/kg of air) defined as work output from GT per unit mass of air flow (kg). The specific work increases with increase in turbine inlet temperature and shows a unimodal convex shape with pressure ratio i.e. it increases with increase in pressure ratio up to certain pressure ratio and then decreases with the increase in pressure ratio. Thus, a turbine should be operated at highest possible tempearture( limited by turbine blade material) to maximize specific work. However, for a fixed turbine inlet temperature the specific work varies with turbine inlet pressure characterized by compressor pressure ratio. Thus gas turbine should work at a pressure ratio which provides maximum specific work. From Fig which is drawn to show what happens when pressure ratio is changed clealy indicates that when pressure ration is kept greater than optimum value the turbine exhaust

9 temperature drops and the stack loss also increases. Further, when it is kept lower than the optimum value, the exhaust temperature from GT increases. Q Hmin (min. heat required by process ) Decreased pressure ratio Interval Temperatures Optimum pressure ratio Increased pressure ratio Acid dew temperature Ambient temperature Stack losses H, kw Fig A departure from optimum pressure ratio results in increased stack loss The above fact may look encouraging, however, it is not so. When pressure ratio is kept lower than optimum, the gas flow need for the required power generation increases making the hot utility line slope to decrease resulting in a higher stack loss and decreased efficiency. So the conclusions are: 1. The GT should operate at maximum possible inlet temperature to maximize specific work. 2. The compressor pressure ration for operation should be selcetd so as to get maximum specific work. This compressor pressure ratio is the optimum pressor ratio. The required GT exhaust gas temperature which will meet the process heat demand can be computed from Eq The temperature of the exit gas from GT is fixed by operating constraints. However, it is possible to maniupulate the temperature of this gas using a recuperator(figs.37.2) or after burner(fig ) so that it suits the needs of the process. The exhaust gas from the turbine

10 contains about 15% O 2 and thus can be burned second time by injecting fuel to increase its temperature. The recuperator is used to transfer some of the heat available with flue gases to heat the compressed air used for burning. This way the fuel consumption is decreased at the cost of reducing the temperature of the flue gas which is used for process heating. The presence of a recuperator does not affect the specific work of the GT, nor it alters the relationship between gas flow and power generation. References 1. K.Sarabchi & G.T.Polley, Gas direct integration in the context of Pinch Technology, Chemical Eng. Technol, 25(2002)8 2. Linnhoff, B. and Flower, J.R., 1978, Synthesis of heat exchanger networks, AIChE J, 24(4): Linnhoff, B. and Hindmarsh, E., 1983, The pinch design method for heat exchanger networks, Chem Eng Sci, 38(5): Linnhoff, B., Townsend, D.W., Boland, D., Hewitt, G.F., Thomas, B.E.A., Guy, A.R. and Marsland, R.H., 1994, A User Guide on Process Integration for the Efficient Use of Energy. (The Institution of Chemical Engineers, Rugby, Warks, UK). 5. Smith, R. 2005, Chemical Process: Design and Integration (second ed.), (J. Wiley, J Wiley. Nomenclature T g = Gas supply temperature, 0 C T Stack = Stack temperature, 0 C R hp = heat to power ratio (H tpr ) w ngt = Gas turbine specific work, kj/ kg (SW gt ) c pg = Gas specific heat, (kj/ (kg 0 C))