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ECONOMICS OF POWER & STEAMS

KARTHICK RSENIOR ENGINEER, ONGC-MRPL

14-11-2014

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INTRODUCTION

In refinery complex, Steam Turbine Generators (STG) or Gas Turbine Generators (GTG) – cogeneration - steam levels and power. Relative efficiencies of equipments - vary inherently to capacity utilization - may have different generation costs at different loads In this paper: • Modelling of CPP including STGs and GTGs • Marginal values - calculated and presented• Gain understanding about optimal loading of

power plants

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UTILITY – AVERAGE PRICING (1/2)• The common energy sources in a refinery complex are fuel, steam and power.• Standard Refinery Fuel (SRF) as a relative scale of energy consumption for secondary sources of energy such as steam and power.

•Steam and power - total quantity generation per ton of SRF consumption.

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UTILITY – AVERAGE PRICING (2/2)Power SRF factor: The total fuel consumed in the cogeneration system is apportioned for steam & power part.

Utility SRF factor

HP Steam 14.4MP Steam 15.2LP Steam 17.1Power 3.3

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STATISTICAL MODELING OFBOILER – STG SYSTEM

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HP STEAM HEATER

BOILER – STG SYSTEM SCHEMATIC

STEAM TURBINE

DEAERATOR

POWER EXPORT

TOTAL CONDENSERBFW

MAKE UP LP STEAM

HP STEAM

HP STEAM EXPORT

LP STEAM EXPORT

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STATISTICAL MODELLING - STG• Marginal steam generation -regression of actual data, fixing -2 as intercept, to account radiation losses = 16.24 per MT LSHS

• Marginal SHP steam production – regression model:

• To begin with, marginal SHP steam cost = LSHS cost/16.24

• Iterated with STG model’s LP and HP steam production cost till convergence

Regression model for STG hence arrived is,SHP Steam = 3.44 * Power + 0.75* HPS +0.42 * LPS + 15.43

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STG: MARGINAL PRICINGUTILITY

MARGINAL VALUE DEFINITION

MARGINAL VALUE

MARGINAL PRICE

POWER 3.44 SHP PRICE * POWER MARGINAL VALUE

HP STEAM

0.75 SHP PRICE * HP MARGINAL VALUE

LP STEAM

0.42 SHP PRICE * LP MARGINAL VALUE

MP STEAM

0.71

0.47

RESPECTIVE STEAM MARGINAL VALUE / RESPECTIVE ENTHALPHY RATIOS

POWERSHP

HPsteamSHP

LPsteamSHP

PRDSMPSHPMPsteamSHP

__

PRDSMPSHPMPsteamSHP

__

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MARGINAL VALUE: ACTUAL VS MODEL (1/2)ACTUAL DISTRIBUTION MODEL PREDICTION

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MARGINAL VALUE: ACTUAL VS MODEL (2/2)ACTUAL DISTRIBUTION MODEL PREDICTION

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STG: TYPICAL MARGINAL PRICESASSUMPTION: IFO (LSHS) COST = 38000

INR/MT

NO OF ITERATIONS FOR CONVERGENCE = 7

UTILITY COST UNIT SPECIFIC GENERATION

W.R.T IFOSHP STEAM 2566 RS/MT 14.8POWER 1924 RS/MW 4.3HP STEAM 1815 RS/MT 19.7MP STEAM (HP TO MP PRDS)

1142 RS/MT 20.9

MP STEAM (MP TO LP PRDS)

1078 RS/MT 33.3

LP STEAM 8825 RS/MT 35.3

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STATISTICAL MODELING OFGTG AND HRSG SYSTEM

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STATISTICAL MODELLING – GTG & HRSG• GT- HRSG system: Marginal power from GT & marginal steam from HRSG; Marginal fuel variation independent

Data – taken from GT performance curve and modeled by regression

• GT model: Operable range: GT Fuel = 0.2528* Power Load +1.7Startup range: GT Fuel = 0.2145* Power Load +2.1Where, GT Fuel (TPH), Power Load (MWh)

• HRSG model: fa = 0.0759*Steam Load -0.9387*[fp*LHV of fp – P*3600/4.187)Where, fa =HRSG Fuel (TPH), fp = GT Fuel (TPH), P= GT Power Load (MWh)

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GTG & HRSG: MARGINAL PRICINGUTILITY

MARGINAL VALUE DEFINITION

MARGINAL VALUE

MARGINAL COST

POWER 0.2528 GT FUEL COST * POWER MARGINAL VALUE

SHP STEAM

0.0759 HRSG FUEL COST * SHPS MARGINAL VALUE

Pf p

SHPfa

• Specific power generation = 1/Power Marginal value = 1/0.2528 ~3.96 MWh/Fuel TPH (operable range)

• |||ly Specific power generation ~4.3 MWh/Fuel TPH ( for startup)

•Specific steam generation = 1/Steam Marginal value ~13.18

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RESULTS AND DISCUSSION (1/2)Hypothetical modification schemes with change

in steam

Assumption: IFO (LSHS) Cost = 38000 INR/MT

IMPACT ON STEAM DUE

TO MODIFICATIO

N

STEAM QUANTI

TY (TPH)

OPERATION

EXPENSE / SAVINGS ( INR/ANNUM)

MARGINAL COSTING

AVERAGE COSTING

MP STEAM CONSERVATION

+2 Via reduction in MP to LP letdown

+3.8 LACS +17.6 LACS

MP STEAM CONSUMPTION

-2 Via reduction in HP to MP letdown

-6.1 LACS -17.6 LACS

LP STEAM CONSUMPTION

-3 Via STG extraction

-10.77 LACS -22.1 LACS

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RESULTS AND DISCUSSION (2/2) Marginal costing brings out realistic economics

for change in steam generation or consumption relative to average pricing which is static

GT power marginal value in startup range is 4.66 MWH/Fuel TPH.

The operable range GT marginal value 3.96 MWH/Fuel TPH

STGs have almost the same marginal value (4.3 MWH/ Fuel TPH) for startup and operable range

For relative plant power load dynamics, it is better to vary the power generation from STGs

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SUMMARY• Marginal costing accounts for process and equipment efficiency for generating the additional utility • Marginal costing of utilities gives realistic economics of steam conservation or consumption from base levels.•Statistical modeling & marginal values knowledge of power plants will help in optimal utilization of equipment loading for the cogeneration of steam & power• For combination of GTGs & STGs, GTG to run at maximum power generation mode and STG to take dynamic load variations

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THANK YOU