cogeneration

Aspen Plus Cogeneration Model

Contents

1 Introduction.............................................................................................1

2 Components............................................................................1

3 Process Description...................................................................1

4 Physical Properties....................................................................3

5 Chemical Reactions...................................................................3

6 Simulation Approaches...............................................................3

7 Simulation Results.....................................................................4

8 Conclusions.............................................................................7

References.......................................................................................................8

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1 Introduction

This model simulates an Integrated Cogeneration process. It includes the following features:

A set of conventional chemical species for this process.

Typical process areas including: burning, compression, heat exchange, power generation, and the main streams connecting these units.

Property methods and unit operation models used in this process.

2 Components

The table below lists the components modeled in the simulation.

Component ID Type Component name FormulaH2O CONV WATER H2ON2 CONV NITROGEN N2O2 CONV OXYGEN O2CO CONV CARBON-MONOXIDE COCO2 CONV CARBON-DIOXIDE CO2ARGON CONV ARGON ARMETHANE CONV METHANE CH4ETHANE CONV ETHANE C2H6PROPANE CONV PROPANE C3H8

3 Process Description

An outline of the cogeneration process which includes the letdown, Gas Turbine and Steam Generation sections is shown in Figure 1.

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HIERARCHY

GASTURB

HIERARCHY

LETDOWN

HIERARCHY

STMGENNATGAS2

AIR

NOXSTEAM

HOTGAS1

POWER2

NATGAS

POWER1

WATER1

WATER14

POWER3X

STEAM-A

STEAM-B

STEAM-C

HOTGAS9

WATER24

RC

RC

W

MIXER

POWERMIXPOWEROUT

W

Figure 1: Cogeneration Overall Process

The feedstock of this cogeneration process is natural gas, which contains Methane (83.62%wt), Ethane (7.33%wt), Propane (7.25%wt) and Argon (1.8%wt).

Firstly, a turbine is used in the letdown area to utilize the internal energy of the natural gas to generate electrical power. After expanding, the gas pressure drops from 19.5 bar to 8 bar while generating 0.60MW of power.

Secondly, mixed with steam (8 bar) and compressed air (1324000kg/hr), the gas is burned completely in the burner to produce hot gas at 979 . The ℃ hot gas is passed through a gas turbine to produce 103.4 MW of electrical power. As a result, its temperature drops to 551 and its pressure drops from 8 bar to 1.1 bar.℃

Thirdly, the hot gas is passed to the steam generation area to recover heat. The gas runs through 5 heat exchangers and is cooled down by water or steam as follows:

E100 - cooled from 551to 492 ℃ E101 - cooled from 492 to 320 ℃ E102 - cooled from 320 to 238 ℃ E103 - cooled from 238 to 234 ℃ E104 - cooled from 234 to 175 ℃

Then the outlet stream HOTGAS6 from E104 is split into HOTGAS7A and HOTGAS7B. HOTGAS7A is cooled to 108 in℃ E106 and HOTGAS7B is cooled to 131℃ in E105. Afterwards these two streams are mixed again and are vented out of the process. The BFW (boiler feed water) used in this area includes two pressure grades, one at 76.5 bar and the other at 6.9 bar. Heated by the hot gas, BFW turns to steam. Then the steam is let down through a turbine to produce electrical power. Finally, three steam products, each at different pressure grades, are obtained and 37.6MW of electrical power is generated.


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Process summary

Area Purpose

Let Down Uses the internal energy of the natural gas to generate electrical power

Gas Turbine Burns the natural gas to generate electrical power using a gas turbine

Steam Generation Recovers the heat from the hot gas to generate steam and electrical power using steam turbines

4 Physical Properties

The PR-BM property method (Peng-Robinson equation of state with Boston-Mathias modifications) is used for the properties of the natural gas and combustion products. For the steam system in the steam generation area the STEAMNBS property method is used.

5 Chemical Reactions

The only reactor unit in this process is the burner modeled with RGibbs which uses the Gibbs free energy minimization method. This determines the equilibrium composition of the products resulting from the many reactions that can occur.

6 Simulation Approaches

Unit Operations – The major unit operations are represented by Aspen Plus models as shown in the following table:

Aspen Plus Unit Operation Models Used in the Model

Unit Operation Aspen Plus Model Comments / Specifications

Heat exchanger HeatX Simplified shortcut design calculations.

Flash Flash2 Rigorous simulation of gas-liquid equilibrium.

Compressor/Turbine Compr Calculates electric power required or produced.


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7 Simulation Results

The Aspen Plus simulation flowsheet is shown in Figures 2, 3, and 4.

NATGASNATGAS(IN)

NATGAS2 NATGAS2(OUT)

POWER1 POWER1(OUT)

EXP1

Figure 2: Flowsheet of Letdown area

NATGAS2NATGAS2(IN)

HOTGAS1

HOTGAS1(OUT)

POWER2 POWER2(OUT)

A IR1

A IR2

ACPOWER

NOXSTEAM

MIXGASHOTGAS

POWER2A

AIRCOMP

MIX1 BURN1

EXP2

WORKMIX

Figure 3: Flowsheet of Gas Turbine area


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HOTGAS1HOTGAS1(IN)

POWER3X

POWER3X(OUT)

STM6

HOTGAS2

STM7

WATER4HOTGAS3

STM5

WATER2

WATER3

HOTGAS4

STM19

STM20

HOTGAS5

WATER17

HOTGAS6STM18

WATER4A

HOTGAS7A

HOTGAS7B

HOTGAS8A HOTGAS8B

HOTGAS9

WATER1

WATER14

WATER15

WATER16

STM8

POWER3

STM9

STEAM-A(OUT)

STM10STM11

POWER4

STM21 STEAM-B(OUT)

STM22

STM12STM13

POWER5

STM23

STEAM-C(OUT)

WATER24

E100

E101

E102

E103

E104

V100

P101

SPLIT1

MIX1

E106 E105

V101

P103

K100

SPL102

K101

SPL103

MIX103

K102

V102

POWMIX

Water & Steam

Hot Gas

Power Generated

Figure 4: Flowsheet of Steam Generation area


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No errors occur in the simulation. Key simulation results are shown in the following table:

Key Stream Simulation Results

Flowsheet Variable Value Unit

Feed

NATGAS total 25000 kg/hr NATGAS-Methane 20905 kg/hr NATGAS-Ethane 1832.5 kg/hr NATGAS-Propane 1812.5 kg/hr NATGAS-Ar 450 kg/hrSteam for Burner 45000 kg/hrBoiler feed water (High Pressure) 180800 kg/hrBoiler feed water (Low Pressure) 42600 kg/hrAir for Burner 1324000 kg/hr

Product

Steam 9 (24bar) 27120 kg/hrSteam 21 (5bar) 6390 kg/hrSteam 23 (1bar) 185659 kg/hrElectrical Power 141689.7 kW

WasteWater 5125 kg/hrExhaust Hot Gas 1394000 kg/hr

Key Process Simulation Results

Key Process Variable Value UnitTemperature of Burner 978 ℃

Pressure of Burner 8 bar

Discharge Pressure of the NATGAS Turbine 8 bar

Discharge Pressure of the HOTGAS Turbine 1.1 bar

Discharge Pressure of High Pressure Steam Turbine 24 bar

Discharge Pressure of Medium Pressure Steam Turbine 5 bar

Discharge Pressure of Low Pressure Steam Turbine 1 bar


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Heat Balance in Steam Generation Area

Heat Balance of Steam Generation Process

Value Unit

Inlet Enthalpy of Hotgas(hotgas1) -309485 kW

Outlet Enthalpy of Hotgas(hotgas9) -495631 kW

Heat Energy Supply of Hotgas 186146 kW

Enthalpy of Inlet Water 1 -786876 kW

Enthalpy of Inlet Water 14 -185583 kW

Enthalpy of Outlet Water 24 -18290 kW

Enthalpy of Outlet Steam 9 -96704 kW

Enthalpy of Outlet Steam21 -23231 kW

Enthalpy of Outlet Steam 23 -686151 kW

Heat Energy Absorption of Water in total 148083 kW

Electrical Power Generated in STMGEN Process 38067 kW

Steam and Power Generation per 1 kg of Natural Gas

Product Name Product Quantity

Steam at 24bar pressure 1.085 kg

Steam at 5 bar pressure 0.256 kg

Steam at 1 bar pressure 7.426 kg

Electrical Power 20404.8 kJ

8 Conclusions

The Cogeneration model provides a useful description of the process. The simulation takes advantage of Aspen Plus’s capabilities for modeling. The model may be used as a guide for understanding the process and the economics, and also as a starting point for more sophisticated models for plant design and process equipment specification and purchase.


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References

V. I. Dlugosel’skii, V. E. Belyaev, N. I. Mishustin and V. P. Rybakov,

"Gas-turbine units for cogeneration", Thermal Engineering, 54:1000-1003, 2007.

Ligang Zheng and Edward Furimsky, “ASPEN simulation of cogeneration plants”,

Energy Conversion and Management, 44: 1845-1851, 2003


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cogeneration

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