1 Low Emission Water

Page 1 of 17 © Siemens AG 2005. All rights reserved.

Low Emission Water/Steam Cycle –

A Contribution to Environment and

Economics

Peter Mürau

Dr. Michael Schöttler

Siemens Power Generation, (PG)

Germany


Abstract

Avoiding emissions is a general goal nowadays. On one hand producing emissions affects the

project profitability due to additional initial and life cycle costs like expenses for waste water

disposals and the subsequent supply of demineralised water.

On the other hand avoiding emissions is an important contribution to environmental

protection and finally can help to ease the permitting phase of a project.

This paper presents two examples how Siemens Reference Power Plants are designed to

achieve the target of low emissions.

Once-through BENSON™ HRSG developed by Siemens:

Beside other important advantages of the BENSON technology like

• fast plant start-up without sacrificing HRSG lifetime and

• increased efficiency during start-up and at base load

the implementation of a once-through HRSG helps to reduce water consumption of the

steam/water cycle.

In a power plant with drum type HRSG reasonable amounts of water need to be blown down

out of the drums to achieve the necessary steam purity according to steam turbine

requirements. This water needs to be replaced by demineralised water.

The once through sections of a BENSON HRSG do not have a drum thus the cleaning is done

by a condensate polishing plant. The resulting amount of water disposal is much lower since

the salt concentration can be increased.

Furthermore the enhanced average efficiency of a power plant with BENSON HRSG results

in lower flue gas emissions.

Zero Discharge Concept

Further water losses which need to be considered are disposals coming from the clean drains

system of the steam/water cycle including the HRSG. The zero discharge concept is designed

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

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

cycle.

The result is a plant with a 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.

Whereas the negative impact of regenerative energy is relatively low the fossil fired power

production at least affects the balance of greenhouse gases which finally is the reason for all

the efforts to establish the CO2 trading.

Modern power plants are designed to reduce emissions to a minimum. Especially the gas

fired combined cycle process with its high efficiency and the low content of pollutants in the

fuel gas like sulphur or nitrogen helps to protect the environment. The relatively low carbon

content even more reduces the amount of CO2 emissions.

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. Water that partially contains 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 the water simply needs to be available. 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 two systems which help to protect the environment by reducing

emissions and at the same time contributing to the economical success of a project.


2 Once-through BENSON HRSG

The once through BENSON HRSG was mainly designed to fulfil a customer requirement

which shows a steadily increasing importance in the European market: Operating flexibility.

Reasons for that importance can be found in the characteristics of deregulated and liberalized

markets. Nobody knows exactly how fuel and electricity prices will develop and, as a result,

in which load regime a power plant can be operated over the plant lifetime. Also competition

during lifetime with other, newer plants influences the load regime and often leads to a lower

dispatch rank.

But these market circumstances also open up new opportunities like utilizing hourly and

seasonal market arbitrage, participation in ancillary energy markets or peak shaving. To pick

up the opportunities flexibility features like fast start-up or frequency response capability are

very important. A highly flexible plant allows for an optimized operating profile resulting in

an increased economic value of the plant.

In some countries like Germany, Denmark and Spain, the significant and still growing share

of wind energy causes new challenges for the operation of fossil power plants. The power

output of wind turbines fluctuate heavily and thermal or hydraulic power plants must

compensate these fluctuations.

In this market environment, a plant capable for cycling and baseload operation is a must. The

Siemens References Power Plants take this into account. Features like short start-up times,

low combined cycle minimum load or full capacity steam bypass stations allowing simple

cycle operation enable highest operating flexibility.

Beside all these market driven influences the fast start-up saves fuel by avoiding operation at

unfavourable loads and thus increases the overall efficiency of a power plant.

But how can the fast start-up plant with the BENSON type HRSG affect the environmental

situation?

2.1 Flue gas emissions

Starting a power plant means to operate the components and systems below their designed

capacity. This means that not only the power output steadily increases during start-up but also

the efficiency. Comparing start-up curves of a fast start-up plant with a BENSON HRSG and

a normal start-up plant with a drum type HRSG shows significant differences in the produced

energy and the achievable efficiency within a certain time frame (see figure 1).


Within this time frame the energy output of the plant equipped with BENSON HRSG is

about twice as high compared to the drum type plant. As a result the plant operator is able to

sell twice as much energy during the start-up. This again improves the economical situation.

Beside the increased energy output the average efficiency is also significantly higher at the

BENSON plant since the areas of low power output and respectively low efficiency are

passed much faster and earlier. Within the defined time frame the efficiency is improved by

about 12%-points at the plant equipped with a BENSON type HRSG.

Assuming a base load plant with about 30 starts per year the start-up time with reduced

efficiency is negligible short compared to the full load operation. But assuming a cycling

plant with about 250 starts per year the start-up sequence is a noteworthy part of the total

plant operation. For this load regime the fast start-up of the BENSON plant results in an

increased average efficiency of about 1%-point (see figure 2).

The increased efficiency has direct influence on the flue gas emissions. With the utilized fuel

a higher amount of energy can be produced. Energy that does not need to be produced by

other plants and which is therefore not associated with additional emissions like CO2.

During the start-up event the formation of emissions is even more reduced by implementing

the fast start-up procedure. The NOx and the CO gases primarily occur during the start-up.

These emissions can significantly be reduced since the time frame of gas turbine operation at

unfavorable loads with firing conditions totally differing from the optimized design point is

Figure 1: Start-up curves after overnight outage

BENSONBENSONBENSON

t2

DrumDrumDrum

DrumBenson272 140Energy output [MWh]47.0 35.0Average efficiency [%]

Data at t2 DrumBenson272 140Energy output [MWh]47.0 35.0Average efficiency [%]

DrumBenson272 140Energy output [MWh] 272 140Energy output [MWh]47.0 35.0Average efficiency [%] 47.0 35.0Average efficiency [%]

Data at t2

Time [min]

Plan

t loa

d[%

]

20

40

60

80

100

t1


minimized. The NOx emission can be reduced by up to 10%. Reductions of more than 60%

for CO emission can be achieved (see figure 3).

Figure 2: Influence of start-up time and load regime on average efficiency

Figure 3: Reduction of flue gas emissions by enhanced start-up times

Increase of Efficiency – Benson HRSG vs. drum type

0.2

0.4

0.6

0.8

1.0D

elta

Effi

cien

cy [%

-poi

nts]

Number of starts

Base load

Cycling

CO

redu

ctio

n[%

]

80

60

40

20

0

Number of starts12

10

8

6

4

2

0

NO

xre

duct

ion

[%]

Base load Cycling


2.2 Water Supply and Effluents

An important contribution to environment protection can be realized by saving water. This

simply reduces the water consumption and on the other hand the effluents do not need to be

discharged.

The water/steam cycle with a once through section of BENSON HRSG has a significant

advantage compared to the drum type section. To achieve the necessary steam purity

according steam turbine requirements the drums which contain the highest concentration of

impurities are continuously drained (see also chapter 3.1).

The omission of the HP drum which is the drum with the highest flow rate saves therefore a

lot of water which does not need to be drained. Instead of the blowdown the necessary

cleaning of the water/steam cycle with BENSON type HRSG is done by the implementation

of a condensate polishing plant. This saves about 30% of demineralised water.

Figure 4: Schematic of water/steam cycle with BENSON HRSG

Exhaust

Fuel gas

Air

Gas turbineGenerator

Clutch

Steam turbine

HRH steam

HP steam

LP steam

HP separator LP drum

HP-ECO

G3~

Cold reheat

Exhaust

Feedwater pump

IP-ECO

Condensatepolishing

Condensatepump

HP IP LP

IP drum


2.3 Chemical Dosing

To achieve appropriate steam conditions a certain amount of chemical dosing is necessary.

The different HRSG types require a different boiler water chemistry to treat both HRSGs in

an optimum way.

For the BENSON type HRSG section the oxygenated chemistry is applied which shows

advantages like the reduction of iron corrosion and iron transport to the HRSG and the

formation of protection layers with much lower propensity for attack by flow-accelerated

corrosion. Minimizing the deposits in the HRSG helps to maintain the heat transfer

characteristics, reduces the need for chemical cleaning, lowers the risk of on load corrosion in

porous deposits and reduces the pressure drop across the evaporator.

Furthermore, when operating a boiler on oxygenated treatment, the dosing of ammonia can

significantly be reduced between 50 and 70% which additionally helps to protect the

environment.


3 The Zero Discharge Concept for the Water/Steam Cycle

As described above the application of a BENSON once through HRSG reduces the demand

for water fed to the water/steam cycle by about 30%. Depending on the water costs and

simply the availability of water further reductions appear to be reasonable.

Siemens 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 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

4.

3.1 The Conventional Drain System (with drum type HRSG)

As mentioned above 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 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 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 condensates are 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 5: Conventional drain system with drum type HRSG


3.2 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 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.

3.3 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 6: 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 flash tank (No 2) near the already existing HRSG flash tank No 1 is

additionally installed. The superheater steam blowdown which is a clean stream is now

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

the temperature 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

steam line flash tank is pumped to a condensate storage tank. From there the condensate

HP IP LP

HRSG

Condenser

Steam Turbine

HRSG Flash Tank


Condensate Storage Tank

HRSG Flash Tank No2

Cooling Water


Figure 7: Minimized effluents


is routed directly to the condenser. 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).

3.4 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 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 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 re-routing

to the condenser is not possible.

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 8: Complete Zero Discharge system comprising all three reduction steps


3.5 Result

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

cycle and route them back to the cycle wherever possible. As shown in picture 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 system and leakages in 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.

4 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?

• Load regime and operational flexibility

Is the plant operated at base load, intermittent or daily cycling? How will a base load

plant be operated in the future? Which quantity of emissions of NOx, CO and CO2

are acceptable?

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

this paper. Nevertheless some general rules can be given.

• If a plant is planned to be operated in cycling mode or at least the future operation

regime is unsure and the cycling operation mode can not be excluded in the near

future the once through BENSON type HRSG is the best choice. The power plant

owner takes advantage of the operational advantages resulting from the reduced start-

up time and the subsequent benefits regarding the reduction of flue gas emissions.

Also the dosing of ammonia can be reduced by about 60%. Beside this the omission

of the HP drum already saves about 30% make-up water for the water/steam cycle.

• 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. An application

for a BENSON type HRSG does not make sense since the blowdown of the HRSG

drum is prevented anyway and the water savings of the BENSON HRSG already

exceed the savings which are achievable by the Advanced Cascading Blowdown

system.

Reduction step 2 (Minimized Effluents) in combination with the Advanced Cascading

Blowdown saves about 50% water and can of course 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 Zero Discharge and the BENSON HRSG

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 dimensioned to ensure a save

supply of the water/steam cycle for a certain amount of time, a reduction of water

consumption allows for a reduction of the tank size. Of course, the water capacity must not

fall below a certain limit since the activities during commissioning phase of the plant need to

be ensured.

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

necessary for the treatment of the contaminated effluents. This is where the BENSON type

HRSG comes into play. A condensate treatment 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 a 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.


5 Conclusion

The BENSON HRSG and the Zero Discharge System help to reduce unfavorable impacts on

the environment. Water savings with the consequential reduction of water supply and water

discharge can be achieved by both systems: The BENSON HRSG saves about 30% the Zero

Discharge system more than 90% compared to a conventional water/steam cycle with drum

type HRSG.

The BENSON HRSG furthermore reduces flue gas emissions of NOX up to 10% and CO up

to 60% due to reduction of start-up times. The enhanced start-up times also save fuel and

result in a general decrease of emissions.

Beside the environmental aspect the same systems enhance the profitability of a project by

the reduction of life cycle costs. Certainly, the economical benefit of the Zero Discharge

system depends on the project boundary conditions. The BENSON type HRSG especially

when operated in a load regime beyond pure base load is always the most economical

solution.

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 9: Combination of BENSON HRSG and Zero Discharge system

Documents

1 Low Emission Water