CONVEGNO

Spinta innovativa ed Efficienza energetica Nuove proposte per un contributo reale alla competitività delle imprese

Roma - 18 aprile 2012

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How to find optimized configuration in power &

distilled water complex?

ENEL Experience

Giampiero Valenti - Enel Innovazione & Ingegneria

Technological and Feasibility Studies, Basic engineering and Cost Analysis Dpt Manager

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The following pages represent two case studies

where overall efficiency and plant flexibility found

in the past a good compromise. The two stories start

from the same point:

Once upon the time we received a request......

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When a Client ask you to provide a certain target of

water and power production, he doesn’t have a clear

idea of how his question is putting you in a

nightmare…

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In the past experience that ENEL had last decade all around

the world, several clients and partners called us to have the

right idea. And twice it has been successful.

The Barka 1 (Oman) and the Ras Laffan 1 (Qatar) power

and desalination complexes represent a compromise among

the lower cost of water and electricity, mandatory in the IPP

market, the wide flexibility required by so small networks

and the competitive bidding of the EPC.

In the following paper the highlights of the two projects have

been described

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Barka power and water desalination plant – Oman

Client requirements:

• Guaranteed Net Power Output between 400 and 440 MW

• Minimum of 3 power units

• design conditions: temperature 50°C, relative humidity 100%

• dual fuel capacity

• minimum 3 desalination units for 20 MIGD net water output

• flexibility requirements

full water production from 30% up to Guaranteed Net Power Output simple cycle operation of the GTs Net Power Output with outage of the largest desalination unit full water production with outage of the largest power unit full water production and 50% of Net Power Output with outage of one GT

• 10 scenarios with different boundary conditions

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Barka plantPlant main features:

• 2 Gas Turbines dual fuel capacity

capacity of each GT 120 MW use of the evaporative cooler

• 2 HRSGs with supplementary firing, bypass stacks and exhaust stacks

one pressure HRSG, increase of steam production of 75% with duct burner

• 1 condensing-extraction Steam Turbine

ST size 220 MW, controlled bleeding for desalination plant

• 3 Multiple Stage Flash Evaporators

• a feedwater deaerator and supply system

• seawater condensing system for steam flowing in the LP section of ST

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Barka: heat &mass balance at design conditions

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Barka: simplified main steam diagram

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Ras Laffan power and desalination complex

•Client requirements:

• Guaranteed Minimum Net Power Output 750 MW

• design conditions: temperature 48°C, relative humidity 40%

• natural gas firing

• 40 MIGD net water output

• flexibility requirements

simple cycle operation of the GTs

Net Power Output with 3 desalination out of service

full water production with outage of one HRSG

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Ras Laffan plant

Plant main features:

• 4 Gas Turbines

capacity of each GT 105 MW with evaporative cooler in design conditions

• 4 HRSGs with supplementary firing, bypass stacks and exhaust stacks

one pressure HRSG, increase of steam production of 100% with duct burner

• 2 condensing-extraction Steam Turbines

ST size 200 MW, controlled bleeding for desalination plant

• 4 Multiple Stage Flash Evaporators

• 2 feedwater deareators and supply system

• 2 condensers using closed circuit water from cooling towers

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Ras Laffan: heat &mass balance at design conditions

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Ras Laffan: simplified main steam diagram

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Plant configuration

Attention to the maximum gas temperature downstream the burner grid

Limited to 800 – 820 °C

Goal: minimize the number of GTs or their size, increasing the power in the steam part via post firing system application

Sensible power plant cost reduction due to the scale effect of the steam/water cycle

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The Efficiency

Condensate return temperature 110 °C with high co-generation

Many factors may fade the theoretical inefficiency of a huge supplementary firing:

Increase of supplementary firing means higher slope of the exhaust gas cooling curve and lower exhaust temperature at the stack

Smaller GT size means lower exhaust flow at the stack

The wide extraction for desalination plant smooth any efficiency improvement

Second pressure level not convenient with an high suppl. firing rate

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Choose the steam condenser instead of pure dumping

Power production improvement

The steam condensation

Separation of power and water generations, that means higher flexibility

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The HP inlet parameters (P,T) are direct consequence of the ST expansion curve and are strictly correlated

The main steam parameters

This is the HP turbine expansion line end point

Co-generation plant Key point in steam parameters for desalination units

Typical values at ST

extraction {2.7 bar(a)

133 ÷ 134 °C

ATTENTION!

A closer look should be given to the steam parameters whenever operating load and environmental conditions may lead the parameters falling on the left side of the ST design expansion line

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The main steam parameters

Sufficiently low exhaust gas temperature

allowBarka steam parameters { 77 bar(a)

541 °C

Choice of a certain type of axial exhaust LP steam turbine

HP ST with stop and control inlet valve directly installed on the machine

Steam flow rate to the condenser coherent with the incremental cost of the cooling water system

! Smaller layout for piping! Cheaper

! Simpler

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Possibility to install the same LP turbine of Barka plant

The main steam parameters

becauseRas Laffan steam parameters { 90 bar(a)

565 °CReduction of the steam flow to the condenser means lower costs of the cooling system

Cost and layout complications compatible with the situation

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ENEL, with the help of HRSG manufacturer, believes that this value is around 800 ÷ 820 °C

Supplementary firing may drive to cost increase when the temperature downstream the burner grid is too high

The supplementary firing system

The combustion temperature exceed this limit

! Economically convenient: to shift the position of the burner grid downstream some tubebundles, cooling down the exhaust gases before entering the burner grid and having a lowercombustion temperature with the same thermal input

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To get rid of the problems on the SH steam temperature: possible tube arrangements

The superheater

SH detached in two banks with the burner grid in the middle

A small SH upstream the burner grid

SH in three bundles: intermediate SH, burner grid, final SH and primary SH

The final SH (unfired) has a large surface due to the reduced terminal temperature difference between gas and steam

Applicable only when the required SH temperature is 15 ÷ 20 °C lower than the GT exhaust temperature

Fits particularly operating scenarios with supplementary firing system off

Maintains constant the SH temperature {HRSG steaming load very high

supplementary firing thermal input far from 100%Improves the SH temperature control of the attemperating spray water system {

low exhaust gas flow ratehigh burner thermal input

Ras Laffan

Seems to collect all the advantages of the previous solutions

Barka

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Figure 5

Different positioning of superheaters

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Condensate return temperature around 95 ÷ 100

°C

Full power production and desalination facility at part load is not a continuative operating point

The HRSG tail

Pressurized deaerator

Barka

Condensate return temperature may drop fairly lower than the designed one

Clearly foreseen an operating scenario with full power production and 25% of water production

Variable pressure deaerator

Ras Laffan

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At electrical part load the energy still

available in the exhaust gases may be

exploited

Stripping steam from the LP header

The HRSG tail

Recirculating water through HP Eco back to the

deareating tower where it flashes

Barka

No HP Eco recirculation:

! higher cost

! more complicated

Stripping steam from the LP header

No condensate preheater:

! more complicated

Ras Laffan

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Financial scenarios foresee 50% RH in

the nearby of the design ambient

temperature

The relative humidity at design

point is 100%

The HRSG tail

Producing rated power output with

evaporative cooler the plant efficiency may

be improved

Barka

The evaporative cooler is a

must to achieve the power

output target

The relative humidity at design

points is 40%

Ras Laffan

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In the Ras Laffan plant there was a further step up in the optimization of

energy exploitation.

Beyond the mechanical and thermodynamic architecture of the plant , voted

as already said to optimize the compromise, a focus has been placed also onto

the control modes by which the plant is managed.

It is not correct to limit the analysis only to the static views. There, each

scenario is calculated and optimized, but the real goal is to optimize how these

scenarios are fulfilled when plant is passing from one to another, how ramp

rates and load variation transient may be achieved without stressing the

equipment.

For these reasons a “new” control mode has been proposed and applied.

The control system of power output and water production

RAS LAFFAN PLANT

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The control system of power output and water production

RAS LAFFAN PLANT

The conceptual base of the project is:

Maximize the plant efficiency in any operating condition

• Steam turbines in sliding pressure operation

•Maximize exploitation of exhaust flue gas energy vs post firing in

the steam generation

Assure the maximum flexibility to the whole complex

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Control logics of main systems

Gas turbines are operated under electric load control (El. load following)

Post firing systems are:• Usually following electric load control• Switched to LP steam header pressure control (distiller feeder) in case of steam

Low Pressure

Steam turbines are operated in sliding pressure in turbine follow mode

HP turbine steam by passes are exploited as last chance in case of HP/LP header pressure large fluctuation

The LP steam header pressure is controlled operating as following: • As first step controlling the bleeding valves on steam turbine controlled

extraction

• Then of the post firing system managing the HP steam production

• Last on the steam by pass systems

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Simplified view of the control system concept

•P

C

•TV 1-AP •TV 1-BP •TV 2-AP •TV 2-BP

•HRSG

2

•HRSG

1

•HRSG

1

•HRSG

3

•HRSG

4

•POST

COMB

1

•POST

COMB 2

•POST

COMB 3

•POST

COMB

4

•PPLM

•GT

2

•GT

1•GT

3

•GT

4

•VAPORE BP

ai

DISSALATORI

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Power Plant Load Master (PPLM)

It is a logic of top level built in the DCS of Power plant

To control the whole plant to generate power output by local command or by remote,

it operates on: •First on the GTs load master up to its 100% MCR

•Then on the HRSG post firing system control system up to its max thermal input (coherent with the current status limitation)

It operates as “load dispatcher”, managing the four GTs and the four HRSG post

firing systems in parallel and at the same load

It does not operate directly on the steam turbine load master

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Water Plant Load Master (PWLM)

It is a logic of top level built in the DCS of Power plant

The PWLM, to operate the plant at the required water production,

provides the setp point to another master logic built in the DCS of

the Water Plant, called Water Plant Control System (WPCS)

This master logic, the WPCS, operates as “distiller load

dispatcher” controlling on demand the water production of the

four distillers in parallel.

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Connection between power generation and distilled water production

If one or more main equipment are not available

CONSTRAINT! It is not always feasible to satisfy at the same time any request of water and power output

Contractual request: Full water production from 100% Plant Reference power generation down to plant operating in island condition (power generation for auxiliaries only)

If all main equipment are available and in operation this condition is fulfilled, any request of distilled water may be satisfied indipendently from power output level.

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Net Power output and water production: operating scenarios

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Power output or water production priority ?

When both request cannot be fulfilled at the same time

Operator choice

Power output priority Water production priority

Control system duty Control system duty

the PPLM stops the opening of the PPLM modulates the HP/LP bypass opening of HP/LP by pass

the WPCS reduces the desalination plant load

the PPLM provides a feedforward signal of “lack of steam”

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The results from the site:the control system real capability

According to the client desire, initially, the control system logics were modified and the whole plant was operated in a more traditional way vs the actual design, i.e:• steam turbine s are operated in “load control mode” • the post firing system controls the HP header steam pressure (boiler follow mode). The PPLM controls only the GT load.

• There was initially a lack of reliance of the Operators vs this “new” control system approach, mailny due to the sliding pressure operation of the steam turbine , whose power is not directly under control of the Operators themselves

• very low cost of the fuel gas

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Feed back from site: the control logics really built in

The results confirmed the theory:

•Steam turbines operates all the day in sliding pressure mode with average HP steam pressure at 65 bar (instead continuously at 90 bar)

• 3 feed water pumps in operation instead of 4

•Net plant efficiency improved by 1 (one) percentage point

• But after the initial period, the control system has been reset according the original

logics, consequently the plant now operates as foreseen in the design phase

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Performances tests resultsProva GT 1 GT 1+2

GT 1+2+3

GT 1+2+3+4

Blocco 1 Blocco 2

Caso 100% 100% 100% 100% 100% 100%

Potenza netta (MW)

Risultato 108,885 219,183 329,817 440,028 398,598 391,251

Valore garantito 99,5 201,0 302,8 404,6 381,1 381,1

Consumo specifico netto (kJ/kWh)

Risultato 11290,2 11219,4 11230,0 11196,9 8783,3 9074,5

Valore garantito N.A. N.A. N.A. N.A. N.A. N.A.

Emissione NOX (ppmvd @ 15% O2)

Risultato 22,1 22,7 20,1 22,1 13,8-16,4 13,6-15,6

Valore garantito 25 25 25 25 25 25

Emissione CO (ppmvd)

Risultato 4 2,3 3,0 0,6 11,7-12,0 13,9-16,3

Valore garantito 20 20 20 20 20 20

Rumore(dBA ad un metro dal cabinato)

Risultato 81,47 82,29 82,71 81,28 - -

Valore garantito 85 85 85 85 N.A. N.A.

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These few pages cannot represent a handbook on how to build up the right plant to achieve the best compromise. Frankly speaking, the writers don’t think that technical solutions, even brilliant, may be more than only a partial contribution to the overall efficiency improvement. Others are the top key-points in this respect. But a right technical approach may contribute.

These few pages may have given you a quick overview about the solutions adopted by ENEL in two different successfully executed cases versus ambient condition and operating constraints.

If these technical solutions may have given really a sensible contribution in finding an environmentally friendly way to exploit fossil energy producing more than electric energy is let to the readers.

CONCLUSION