This report focuses on the heat recovery system which use the heat from the gas turbine exhaust gas to produce steam. e report has a detailed design of the economizer and evaporator where as the steam drum, gas stack, pumps and duct burner are discussed b

3. Material balance

The flue gas comes from the gas turbine at a flow rate of 75000 kg/hr  and temperature of   500 °C. The temperature of the flue gas only allows to generate 12500 kg/hr of steam where as the design requirement is 25000 kg/hr. So through supplementary firing the temperature of the flue gas will be increased to around 900 so that the steam requirement can be met. The flue gas entering the HRSG will have a different composition than the flue gas leaving the gas turbine, because some of the oxygen content in the flue will be used to burn the fuel (natural gas). The mass balance for the supplementary is discussed in the secondary design section, the mass balance below is only for the HRSG. The composition of the flue gas will not change in the HRSG as it only used for heating. Water will change to steam in the economizer tubes.

Material balance around the evaporator

Stream 1

The composition of steam 5 will remain the same because the flue gas is only being used for heating the water and is not taking part in any reaction that would change its composition.

Stream 2

Stream 3

Stream 4

For the material balance it is assumed that the amount of water that enters the evaporator, all of it changes to steam.

Overall material balance for evaporator

Total in = 98258.729 kg/hr

Total out = 98258.729 kg/hr

Material balance around the economizer

Stream 5

Stream 6

Stream 7

Stream 8

4. Energy balance

The energy balance is very important for designing a HRSG. The main equation needed for carrying out a heat balance is

Q = m x Cp x ΔT

But there is a change in phase in the evaporator so the following equation will have to be used as well,

Q = m*l

In the equations above 'Q' is the energy content in kJ/hr, ΔT is change in temperature, 'm' is the mass flow rate in kg/hr, 'Cp' is the heat capacity in kJ/kg °C and 'l' is the latent heat of vaporization in kJ/kg.

Energy Balance around the economizer

Energy Balance around the evaporator

The energy balance around the evaporator is split in two parts. The water entering the evaporator has to gain 8 °C to reach its boiling point and the change to steam. The reason that the water entering the evaporator is not already at its boiling point is to avoid steaming the economizer. Once the water has reached 198 °C it gains more energy to change to steam but the temperature stays the same.

The table below is has figures for water gaining 8 °C to reach the boiling point.

After the water has reaching its boiling it gains energy from the flue gas to change to steam. The temperature of the steam produced is the same as the boiling point of water.

Energy lost by flue =  Energy gained to increase temperature + Energy gained for phase change

49584000 = 48750000 + 834000

49584000 kJ = 49584000 kJ

5. Design of components

1. Economizer

The purpose of an economizer is to preheat the water before it enters the drum. Water is pumped into the economizer from storage where it is heated before it passes to the steam drum.  In a vertical HRSG the best option is to have a horizontal tube economizer with counter flow configuration. The flue gas flows vertically across the economizer.

Design and data for tube side

Data and design for economizer's shell side

2. Steam Drum

The main function of the drum is to separate the steam from the water, but there are other function as well and they include chemical dosing to maintain water chemistry and continuous blow down of drum water to keep the carryover under specified limits. The top part of the drum is filled with steam where as the bottom part of the drum is filled with water. Water enters the drum after being preheated in the economizer. Water from the drum is then pumped into the evaporator where it changes to steam and the travel back to the drum.

Data and Design of steam drum

3. Evaporator

The evaporator is the most important part of the HRSG, as without the evaporator there will be no steam production. The most commonly used evaporator in heat recovery from a gas turbine exhaust for vertical HRSG is the horizontal tube evaporator. It has the advantage of being relatively cheaper than the other designs but has some transport restrictions.

Tube side data and design

Shell side data and design

4. Secondary Designs

6. HRSG stack

The stack is a part of the HRSG with a complex and detailed design, but it is a secondary unit in this report so the design would be brief. Before working out the design of the stack, a few boiler calculations will have to be done. The formula used to work out the horse power of the boiler is for different conditions, but should give a good estimate.

Boiler Horsepower = amount of steam being produced / 34.5

Using the formula above for steam production at 55125 lbs/hr, the horse power of the boiler is 1598 horse power which is equal to 1.19 megawatts. After working out the capacity of the boiler, table 1 from the appendix can be used to work out the diameter and area.. The area of the stack for boiler duty of  1.19 MW is 0.3 m2 and the diameter is 0.7 m.

7. Duct burner

The duct burner is a secondary but important part of the design. The flue gas coming from the gas turbine is at a temperature of 500 °C which has the capacity of produce 12.5 tonne/hr of steam. The requirement of the plant is 25 tonne/hr of steam, so the duct burner will be used to increase the temperature of flue gas to 900 °C to ensure that the steam production rate is       25 tonne/hr. The oxygen is the flue is used to burn the fuel so the oxygen content in the flue gas leaving the burner is less than the flue entering the burner.

Material Balance around the Duct burner

Flue gas in

Flue gas out

Flow rate in = Flow rate out

73258.74 kg/hr = 73258.74 kg/hr

The energy gained by the flue gas to increase its temperature from 500 °C to 900 °C is,

Q = m*cp*ΔT

Q = 38700000 kJ

8. Pipes and Ducts

Piping is the most essential and basic design in any type of plant. There are various piping requirements for HRSG, some of the important ones are listed below.

5. Control Strategy and Implementation  

9. Three element feed water control  

This controller is used to maintain the water level in the drum. Poor control can result in the drum overflowing and water being carried over with the steam, causing catastrophic damage. Poor control can also result in lack of water, which can cause overheating and failure of the tube banks. A three-element analog control system is used to measure and control the feed water flow, steam flow and drum level. The controller maintains the boiler’s drum level by adjusting the feed water flow and the steam flow from the boiler. The control also control minimizes the effects of ‘shrink’ and ‘swell’. The diagram below shows one of the possible implementation of the control system.

10.  Temperature control

This control is used to control the temperature of steam. There are a number of ways to do this but one the simplest and commonly used method is Attemperation; this involves fine spraying of cool water droplets into the vapour. The water droplets vaporize over time; this increases the volume of steam as well as decreasing its temperature.

11. Pressure control

The pressure of the steam has to be monitored to ensure it is correct. A pressure relief is used to adjust the pressure of the steam.

12. Valves

The valves are an important part of the control system. They are being used for different purposes throughout the plant. The control valves are being used to adjust the flow rate, pressure etc. There are valves used for isolation as well as drainage.  

13. Start-up and Shutdown

It is very important to have a accurate shutdown and start-up procedure because most of the damage from cycling comes from start-up and shutdown. It is difficult to design a HRSG for rapid temperature change, stack dampers are used to reduce the magnitude of thermal cycling.  A gas turbine HRSG is started up in 80-100 minutes, an accurate shutdown and start-up procedure will ensure that no time is wasted during start-up and shutdown.

Start up

It is recommended that the HRSG should be started manually, but the control should be switched to automatic as soon as possible. Water and flue gas side of the HRSG should be examined to ensure that there are no foreign objects. In order to prevent an explosion, the HRSG unit is purged with air by the gas turbine. The gas turbine is running at 30% of its synchronous value, with the generator working as the motor. The volume of the HRSG determines the purge time. At start-up the water level in the steam drum is kept just above the low level cause of initial expansion of- water during start-up. Until there is steam in the steam pipes, the vent on the drum is kept open. The vent is close when the pressure in the drum reaches 3.5 bar. Drum level is very important during start-up as too much water can cause the water to carry over whereas too little water can cause thermal damage to evaporator tubes. At start-up the rate of increase of temperature should be controlled by either controlling the rate at which enters the HRSG or by releasing the steam through start-up vents.

Shutdown

When the HRSG unit is being shut down, the water side should be cleaned first. Before cleaning the flue gas side, the unit should be fired to drive off the gases. If the HRSG is being shut down for a long period, the unit should be filled with an inert gas and sealed to prevent the oxidation of metal. The duct burner should be turned down until the firing is at minimum. The steam drum levels should be monitored, feed-water should be added to maintain level between normal and low level. HRSG chemical injections should be stopped as well as the blow-down from the steam drums. The feed-water control valves should be turned off when there is no more need of water.  The steam drum should be filled to the maximum to minimize the corrosion attack in the unit as well as preventing the presence of oxygen  in the system. All equipment that needs maintenance should be isolated. After shutdown the HRSG unit remains warm for around 24 hours.

14. Plant P&ID

15. HRSG  P&ID

6. Economics

This section deals with the economic appraisal of the HRSG. The cost of purchasing and operating the HRSG are estimated here. The HRSG design consists of an evaporator, an economiser and a steam drum. As the economiser and the evaporator are both primarily heat exchangers, correlations provided in C&R vol.6 (page 253, table 6.3a) are used to estimate a purchase price based on the heat transfer area. Operating costs are calculated using factors provided in C&R vol.6 (page 256, table 6.6a) these include the fixed and operating costs.

16. Capital cost of equipment

The capital costs of equipment were estimated using the graphical correlation provided in the reference mentioned above. This is the amount the company would have to pay in order to purchase the equipment during the setup of the plant before operations commence.  The table below shows the break-up of total capital cost,

17. Operating costs

This is the regular expense incurred during the operation of the HRSG. This cost is divided into two types of costs, fixed and indirect costs. These were estimated using the factors provided in the reference as mentioned above by following the simple relationship shown on the next page,

Where is the factor provided in the text

4. Fixed cost

This is the cost that is independent of the of the production level at which the plant is operating for any given period of time. This includes all costs like maintenance, operating labour, laboratory, supervision, plant overhead, capital charge, insurance, local tax and royalty. All the costs are self explanatory as what they represent but it is worth mentioning that plant overheads are the fixed costs associated with the operation of the plant like rent.

5. Indirect cost

These are the costs associated with setting the equipment up and making it part of the process. These include the equipment erection, piping, instrumentation, electrical, utilities, storage, design and engineering, buildings, contractor’s fees and contingency costs. All the costs are self explanatory as what they represent but it is worth mentioning that contingency costs are costs associated with dealing with unforeseen events in the future like a backup pump. The table on the next page is a breakup of the indirect costs.

7. Conclusion

The HRSG was selected on the basis of steam requirement and the space available. The vertical HRSG was the best option for the design. The design of the HRSG contains a evaporator and economizer and a steam drum . Forced circulation is being used in the HRSG to ensure sufficient water is supplied to the evaporator and economizer. The HRSG designed produces steam at  198 °C and 15 bar and reduced the temperature of the flue gas from 900°C to around 200 °C. The main part of the HRSG designed in this report are the economizer and evaporator. The exhaust gas available from the gas turbine didn't have high enough temperature to produce  25 tonne/hr of steam, so supplementary firing through a duct burner was used to increase the temperature of the exhaust gas, this allows production of steam at 25 tonne/hr. The data available for the design was not enough, so a basic values were assumed to work out the design of the HRSG. The capital cost of the plant is £65000 where as the operating cost of the plant is £406000.

8. References

1. Annaratone, D. (2008). Steam Generators Description and Design. Berlin: Springer.

2. Franke, J., Lenk, U., & Taud, R. (2000). Advanced Benson HRSG makes a successful debut. 

3. Ganapathy, V. (2002). Industrial Boilers and Heat Recovery Steam Generators: Design, Applications and Calculations. New York: CRC Press.

4. Ganapathy, V. (2001). Superheaters: designand performance Understand these factors to improve operation. Hydrocarbon Processing , 41-45.

5. Heselton, K. (2005). Boiler Operator's Handbook. Lilburn: The Fairmont Press.

6. Kakac, S. (1991). Boilers, Evaporators and Condensors. John Wiley and Sons.

7. Kehlhofer, R. (1997). Combined-Cycle Gas and Steam Turbine Power Plants. Oklahoma: PennWell Publishing Company.

8. Kern, D. Q. (2002). Process Heat Transfer. Tate McGraw-Hill Publishing Company.

9. Kiameh, P. (2002). Power Generation Handbook. McGraw-Hill Professional.

10. Liptak, B. G. (2005). Instrument Engineers' Handbook - Process Control and Optimization. CRC Press.

11. Ong'iro, A., Ugursal, V. I., Taweel, A. M., & Walker, J. D. (1997). MODELING OF HEAT RECOVERY STEAM GENERATOR PERFORMANCE. Applied Thermal Engineering Vol. 17 , 427-446.

12. Pinelli, M., & Bucci, G. (2009). Numerical based design of exhaust gas system in a cogeneration power plant. Applied Energy 86 , 857–866.

13. Rayaprolu, K. (2009). Boilers for Power and Process. Boca Raton: CRC Press.

14. Smith, D. (2000). Horizontal boilers make 700oC steam economic. 

15. Subrahmanyam, N., Rajaram, S., & Kamalanathan, N. (1995). HRSGs FOR COMBINED CYCLE POWER PLANTS. Heat Recowry Systems & ClIP Vol. 15, , 155-161.

16. Valdes, M. (2001). Optimisation of heat recovery steam generators for combined cycle gas turbine power plants. Applied Thermal Engineering 21 , 1149-1159.

17. http://www.engineeringtoolbox.com/boiler-capacity-d_1115.html

18. http://www.engineeringtoolbox.com/steam-thermodynamics-t_12.html

19. http://www.scribd.com/doc/19149328/Heat-Recovery-Steam-Generator-Operation-Procedure

20. http://www.hrsg.com/fundamentals.htm

9. Appendix

        Table 1

economizer calculations

 

Economizer Pressure Drop  

Shell Side

ΔPs = f’.G.De.(N+1) / 5.22 x 1010 x de x s x φs (PSI)

Where

f’ = friction factor = 0.0015 ft2 / in2 (Figure 29 page # 839)

G = Mass velocity = 630203.21 lb / hr ft2 = 3076292.043 Kg / m2 hr (Calculated)

De= Shell Equivalent Diameter= 15.75 inch = 1.3125 ft = 0.400049 m

N+1= Number of crosses= 12 x L /B, ft (L=length of tube, B = Baffle spacing) = 16 ft =16m

de= Tube equivalent diameter = 1.08 inch = 0.0027432 meter (Figure#28 P.838)

s = Specific Gravity = 1 for less viscous fluids (Kern page#157)

φs = Viscosity Factor = 1 for water and less viscous fluids

ΔPs  = 0.0015 x 630203.202 x 1.3125 x 16 / 5.22 x 1010 x 1.08/12 x 1x 1

ΔPs = 2.66 PSI < 10 PSI = 0.180 Bar

Tube Side

ΔPt = f’x G2 x L x n / 5.22 x 1010 x De x s x φt 

f’ = friction factor = 0.0035 ft2 / in2 (Figure 29 page # 839)

G = Mass velocity = 34372.15lb / hr ft2 = 167785.23 Kg / m2 hr (Calculated)

L =  Length of Tubes = 16 ft = 4.8767 m

n =  Number of passes = 1

De= Tube inner diameter =1.40 inch =0.1166 ft = 0.003556 meter (Table#10 P.843)

s = Specific Gravity  = 1 for less viscous fluids (Kern page#157)

φt = Viscosity Factor = 1 for water and less viscous fluids

ΔPt = 0.0035 x 34372.152 x 16 x 1   / 5.22 x 1010 x0.1166 x1 x 1

ΔPt = 0.0108 Psi = 0.000739 Bar < 10 PSI

Evaporator