3 zero discharge concept

of 12 Siemens AG 2005. All rights reserved.

Zero Discharge Concept

A Contribution to Environment and

Economics

Peter Mrau

Dr. Michael Schttler

Siemens Power Generation (PG), Germany


Abstract

Reducing water consumption and avoiding effluents is a general goal nowadays. On one hand

the water consumption affects the project profitability due to additional initial and life cycle

costs like expenses for the demineralised water supply and for waste water disposal.

On the other hand avoiding high water consumption and avoiding waste water disposal is an

important contribution to environmental protection and finally can help to ease the permitting

phase of a project.

This paper presents an example how Siemens Reference Power Plants are designed to

achieve the target of low water consumption and low waste water disposal.

Zero Discharge Concept

Water losses which need to be considered are disposals coming from the clean drains system

of the water/steam cycle including the HRSG. The Zero Discharge Concept is designed to

collect all kinds of clean drains and blowdown routing it to the condenser respectively the

condensate polishing plant. The regenerated condensate is fed back to the water/steam cycle.

The result is a plant with minimum water consumption. In combination with an air cooled

condenser this plant is optimally adapted to arid regions.


1 Market Environment

The energy demand worldwide is steadily growing. Not only the world population is

increasing but also the energy consumption per head rises continuously mainly caused by the

industrial development. Power production always has an impact on the environment.

Beside the flue gas, further emissions - liquid or gaseous - are leaving the power production

process. One noteworthy emission is the water loss of the water/steam cycle.

Considerable amounts of water have to be discharged. This water contains partially chemical

impurities. The discharge of these streams means an impact on the environment and often

also spending for the effluent treatment. Furthermore the water needs to be replaced which

again means additional expenditures and consequently additional waste water from the water

treatment plant. An important aspect taking into account that exceeding the natural available

water resources also means to influence the environment in a negative way.

This paper describes a system which helps to protect the environment by reducing water

consumption and waste water emissions and at the same time contributing to the economical

success of a project.


2 The Zero Discharge Concept for the Water/Steam Cycle

Siemens PG developed a system to minimize the water consumption of the water/steam

cycle. This system can be applied to both types of HRSG: The BENSON and the drum type

HRSG. The following chapters describe the Zero Discharge system in detail, taking the drum

type HRSG as the basis. The combination of Zero Discharge and BENSON HRSG is

described in chapter 3.1.

2.1 The Conventional Drain System (with drum type HRSG) A considerable amount of water is removed from the water/steam cycle for several reasons

and needs to be replaced by demineralised water.

HRSG drum blowdown: In the HRSG drums the water/vapour mixture coming from the evaporator section is separated in the water liquid and vapour phase. The steam phase

leaves the drum as saturated steam and is nearly free of contaminations.

The water phase flows back into the evaporation section of the HRSG and contains all the

impurities which are fed into the drum by the feedwater flow. To limit the concentration

of impurities a certain amount of water is continuously extracted from the HRSG drums

as blowdown during normal operation. This blowdown is routed to the HRSG

atmospheric flash tank whereas the flashed steam is blown into the atmosphere via a

silencer, the water phase is dumped.

Superheater blowdown: During shutdown and start-up the superheater sections are also drained. This stream is free of contamination and routed to the HRSG flash tank as well.

This means although this stream is suitable for rerouting it to the water/steam cycle from

a chemical point of view it can not be used anymore.

Steam line drains: During start-up the steam pipes and adjacent components like e.g. valves needs to be warmed up by steam. The thereby emerging condensate and steam is

routed to the atmospheric clean drains flash tank. Although the steam line drains are not

contaminated the stream is not re-used. Analogue to the HRSG flash tank the water which

is fed to the clean drain flash tank is either lost to atmosphere or to the cooling water line.

Sampling: To control the chemical conditions in the water/steam cycle the sampling system is fed from several points in the cycle like drum, superheater, condensate or


feedwater system. The sampling streams are partially contaminated after the measurement

and routed to the sewer.

Condenser evacuation: To remove non-condensable gases in the water/steam cycle and to achieve and maintain the condenser vacuum the evacuation pumps are continuously in

operation. Beside the non-condensable gases certain amounts of steam are extracted by

the evacuation pumps as well.

Leakages: Despite the application of high quality systems, components and maintenance a certain amount of losses caused by leakages can not be prevented.

The following chapters describe the components and systems which are handled by the Zero

Discharge Concept. This concept is designed to route all possible streams which leave the

water/steam cycle back into the system. The Zero Discharge Concept comprises the

following reduction steps:

1. Advanced Cascading Blowdown

2. Minimized Effluents

3. Treatment of Contaminations

HP IP LP

HRSG

Condenser

Steam Turbine

HRSG Flash Tank

Clean Drains Flash Tank

Cooling Water

AtmosphereAtmosphere AtmosphereAtmosphere

Cooling WaterCooling Water

Clean streamContaminated streamClean streamContaminated stream

Figure 1: Conventional drain system with drum type HRSG


2.2 Reduction Step 1: Advanced Cascading Blowdown An easy way to reduce effluents caused by drum blowdown is to route the blowdown stream

to the next pressure stage. This means HP blowdown flows to the IP drum and the IP

blowdown is fed into the LP drum. The disadvantage of this system is that all impurities are

also carried over from drum to drum. This system is called cascading blowdown.

The concept of the Advanced Cascading Blowdown system provided by Siemens PG hereby

shows a significant advantage. The blowdowns of the HP and the IP drum is firstly routed to

a separate flash pipe. Whereas the flashed clean steam is routed to the LP drum and just the

contaminated condensate flows to the HRSG flash tank and will then be discharged.

Beside the savings of water a noteworthy amount of energy can be recovered. The power

output of a 400MW combined cycle power plant can be increased by about 130kW.

2.3 Reduction Step 2: Minimized Effluents A big step forward in reducing water consumption and water effluents can be achieved by the

Minimized Effluents System. The goal is to route all streams back to the water/steam cycle

which are free of contamination. The contaminated streams are flashed to recover at least the

clean portion.

HP IP LP

HRSG

Condenser

Steam Turbine

HRSG Flash Tank


Cooling Water

AtmosphereAtmosphere AtmosphereAtmosphere

Cooling WaterCooling Water


Figure 2: Advanced Cascading Blowdown


Therefore the clean steam phases of the flash tanks are rerouted back to the condenser. This

can be done either directly for the steam line drains flash tank or indirectly for the HRSG

flash tank.

The following additional tanks need to be installed:

Flash tank: A HRSG flash tank (No 2) near the already existing HRSG flash tank No 1 is installed additionally. The superheater steam drains which are a clean are now directly

routed to this HRSG flash tank No 2. The steam phase of the HRSG flash tank No 2 is

free of contaminations and can directly be routed back to the condenser. Only the steam

temperature needs to be reduced close to saturation temperature to fulfil condenser

requirements. This is achieved by implementation of a heat exchanger which is installed

in the condensate outlet of the flash tank No 2. The heat is transferred to the closed

cooling water system.

The steam phase of the HRSG flash tank No 1 is also free of contamination and is routed

via flash tank No 2 to the condenser as well.

The clean condensate phase of flash tank No 2 is routed to a condensate storage tank.

Condensate storage tank: The condensate outlet of the HRSG flash tank No 2 and the Clean Drains Flash Tank is pumped to a condensate storage tank. From there the

HP IP LP

HRSG

Condenser

Steam Turbine

HRSG Flash Tank


Condensate Storage Tank

HRSG Flash Tank No2

Cooling Water


Figure 3: Minimized effluents


condensate is routed directly to the condenser if demanded. The storage tank is needed to

compensate extraordinary high amount of condensate which can not immediately be

routed to the condenser (e.g. start-up, plant outage).

2.4 Reduction Step 3: Treatment of Contaminations The final step in recovering the effluents is to avoid the discharge of condensate arising in the

water phase of the HRSG flash tank No1. The discharge stream is therefore pumped to a

blowdown storage tank for collecting and compensating purposes. From there the

contaminated stream is pumped to a condensate polishing plant. The clean condensate finally

is routed back to the water/steam cycle via the condenser.

Only if extraordinary high amount of condensate is collected in the blowdown storage tank

the condensate is routed to the raw water tank to avoid overflow of the blowdown storage

tank. But this could only happen if the condensate polishing plant is not available or the

make-up demand of the condenser is very low.

Further contaminated streams of the water/steam cycle like the major part of sampling

streams are routed to the condensate polishing plant as well and can therefore be recovered.

HP IP LP

HRSG

Condenser

Steam Turbine

Raw Water Tank Condensate

Polishing Plant

HRSG Flash Tank



HRSG Flash Tank No2

Blowdown Storage Tank


Figure 4: Complete Zero Discharge system comprising all three reduction steps


2.5 Result Target of the Zero Discharge System is to collect the streams which leave the water/steam

cycle and route them back to the water/steam cycle wherever possible. As shown in Figure 4

this procedure can be applied to the HRSG drum blowdown, superheater drains and steam

line drains. This already saves more than 75% water.

Hugh amount of water needs to be replaced in the water/steam cycle due to extractions which

are routed to the sampling system. Not all of the streams can be treated in the condensate

polishing plant depending on quantity and quality of contamination. However, the recovery

of the treatable part of the sampling system saves more than 15% water.

The remaining water which can not be saved comprises the non-treatable sampling streams,

the condenser evacuation steam and leakages from the water/steam cycle.

As a final result the Zero Discharge System enables a reduction of effluents and accordingly

a reduction of make-up water for the water/steam cycle of more than 90% compared to a

conventional drain system in a combined cycle power plant with drum type HRSG.

3 The Optimal Project Approach

Taking into account all the positive influences of the before mentioned systems finally the

question arises which is the best way to go. The answer depends on the project boundary

conditions. Influencing factors are:

Availability of cooling water Is enough water available to supply a once through cooling water system or at least to

supply the make-up water system of a cooling tower? Is enough space available on

site to build an air cooled condenser?

Availability of make-up water for water/steam cycle Is enough water available to ensure a continuous operation of the plant? Is the power

plant connected to a municipal source and therefore in competition with households in

case of water shortages e.g. in summer months? Is a make-up water system connected

to a well or a river which enables continuous supply? Even if the supply is currently

ensured, what will be the situation in the future? What are the costs of water now and

in the future?


Allowable amounts of effluents Can the effluents of the water/steam cycle be dumped into the sewer system of the

municipal sewage plant or into the cooling water system? What amount of effluents is

allowed and which quality? What are the costs for disposal?

Not all of the combinations arising of the above mentioned questions can be examined within

this paper. Nevertheless some general rules can be given.

An easy and inexpensive way to reduce the make-up water consumption of the water/steam cycle for a drum type HRSG is the application of the Advanced

Cascading Blowdown system which saves more than 15% of water.

Reduction step 2 (Minimized Effluents) in combination with the Advanced Cascading Blowdown saves about 50% water and can be applied to a once through BENSON

type HRSG as well.

This is also valid for the complete Zero Discharge Concept which enables a reduction of water consumption of more than 90%.


The most favourable and economical application of the Zero Discharge Concept is given in

arid regions in combination with an air cooled condenser.

The air cooled condenser does not require make-up water for a cooling tower system. The

water demand of the power plant is therefore limited to the supply of the water/steam cycle.

Only the potable water supply has to be provided additionally which is normally below 5% of

the overall water consumption.

The Zero Discharge System ensures enormous water savings and enables a continuous

operation of the plant without the threat of power reduction or plant outages due to water

restrictions. Furthermore this system helps to reduce the life cycle costs which arise from the

continuous expenditures for the raw water supply, the subsequent treatment in the

demineralised water plant and charges for the disposal of effluents.

A further cost advantage is given by a size reduction of the demineralised water tank and the

raw water tank. Interruptions of the raw water supply need to be compensated by installing

huge tanks for the storage of water. Since these tanks are sized to ensure a save supply of the

water/steam cycle for a certain amount of time, a reduction of water consumption allows a

reduction in tank size. Of course, the water storage capacity must not fall below a certain

limit since the activities during commissioning phase of the plant need to be ensured.

3.1 Zero Discharge Concept with once trough (BENSON) type HRSG

About 40% of the system costs for the Zero Discharge Concept have to be spent for the

condensate polishing plant necessary for the treatment of the contaminated effluents. This is

where the BENSON type HRSG comes into play. A condensate polishing plant is mandatory

and available for this type of HRSG anyway. The effluents of the water/steam cycle can be

fed into the polishing plant as well without having the necessity for a separate treatment

plant. This improves the overall economical situation significantly.

Not to forget that the high capacity of the condensate polishing plant available for the

BENSON type HRSG allows for an optimization of the Zero Discharge Concept. The second

HRSG flash tank and the storage tank are not necessary any more since the strict separation

of contaminated and clean streams do not have to be applied. The capacity of the polishing


plant is suitable to clean the higher mass flow the absolute number of contaminations

remains the same.

4 Conclusion

The Zero Discharge System helps to reduce unfavorable impacts on the environment. Water

savings with the consequential reduction of water supply and water discharge can be

achieved by this system: The Zero Discharge system saves more than 90% waste water

compared to a conventional water/steam cycle.

Beside the environmental aspect the Zero Discharge Concept enhance the project profitability

by life cycle cost reduction. Certainly, the economical benefit of the Zero Discharge system

depends on the project boundary conditions. Protecting the environment and improving the

economical situation of a project is not a contradiction.

HP IP LP

HRSG

Condenser

Steam Turbine

Raw Water Tank

Condensate Polishing Plant

HRSG Flash Tank




Figure 5: Combination of BENSON HRSG and Zero Discharge system

3 zero discharge concept

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