Low Emission Water/Steam Cycle – A Contribution to Environment and Economics (Source: Siemens Energy)
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. Rasons for that importance can be found in the characteristics of deregulated and liberalized mrkets. Nobody knows exactly how fuel and electricity prices will develop and, as a result, which load regime a power plant can be operated over the plant lifetime. Also competition dring lifetime with other, newer plants influences the load regime and often leads to a lower ispatch 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  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).

Figure 1 Start-up curves after overnight outage

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).

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

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

 

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