Contents Issue 7 (2001)

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Mike van der Walt and Abrie Wessels

Saving on Natural Resources with SRO - Desalination of Industrial Waste Water for Reuse at ESKOM Tutuka (Two Years Operating Experience)

Natural resources are protected and saved with the new spiral reverse osmosis (SRO) plant at the Eskom Tutuka power station in the Mpumalanga province of South Africa. 7,000 m3/day of saline underground mine water blended with 5,400 m3/day of cooling water blowdown is pretreated and desalinated before the product water is returned to the cooling water circuit. Weir Envig designed, constructed, installed and commissioned the plant in phases between August 1998 and April 1999 with innovative use of existing infrastructure and phased removal of the live, ageing elec-trodialysis reversal plant. The plant performance over two years of operation is presented, which demonstrates that good pretreatment and cleaning system design allows SRO to produce consistent high-quality water from this difficult and varying feed. The result is a coal mine with no effluent problems, a new source of water for the power station and a treatment plant which produces significantly better con-denser cooling water and maintains zero liquid discharge.

PowerPlant Chemistry 2001, 3 (7)

C. J. Brouckaert, Dirk Hanekom, C. Woodhouse, and C. A. Buckley

Optimal Location of a Membrane Treatment Plant in a Power Station

A water pinch analysis of a coal-fired power station was undertaken, with the objective of identifying the optimal role of an existing tubular reverse osmosis (TRO) plant in the water circuits. The analysis considered a simplified model based on Lethabo Power Station in South Africa. It was found that the TRO plant is not needed at all if the power station is considered in isolation, but is required to prevent saline pollution is the power station and the asso-ciated coal mine are considered as a whole. The financial incentive required to justify operation of the TRO was found to be of the same order as the cost to the national economy of salination of the Vaal River, from which the Lethabo Power Station draws its water. The optimal location of the power station was found to be dependent on its running costs and technical performance, so that a new RO plant would be used in a different way to the existing one.

PowerPlant Chemistry 2001, 3 (7)

Ebbe Höffner

The On-Line Measurement of Low Level Dissolved Oxygen

Theoretical bases of the measurement of low oxygen concentrations with membrane-covered polarographic-type sensors as well as differences between the most common sensors, the Clark and the Mackereth cells, are explained. The chemistry of the externally polarized Clark-type and the galvanic Mackereth-type sensors is described in detail.
The following issues are then discussed: the residual current problem, response time, calibration, membrane cleaning, and sensor renovation.

PowerPlant Chemistry 2001, 3 (7)

Maarten C. M. Bruijs, Lars P. Venhuis, Henk A. Jenner, George J. Licina, and David Daniels

Biocide Optimization Using an On-Line Biofilm Monitor

Microbial fouling, i.e., the formation of biofilms, in cooling water systems can lead to reduced heat transfer in condensers and heat exchangers. Microbiologically Influenced Corrosion (MIC) is also a result of biofilms. Piping replacements due to MIC have cost utilities millions of dollars. Biofilms also serve as nutrient rich habitats for pathogenic microorganisms such as Legionella and protect the microorganisms from biocides and strongly fluctuating environments.
In order to prevent biofilms from creating these problems, a number of oxidizing and non-oxidizing biocides are being used. Application rates and frequency are often based on general appearances or sampling techniques that focus on the number of planktonic bacteria instead of the presence of biofilms. Since the effects of undertreatment are so dire, plants often develop excessive dosing regimes, which lead to extra costs, unnecessary environmental impact, and an increased corrosion rate on equipment.
On-line monitoring of biofilm formation on metallic surfaces provides key input to automatic control equipment and system operators. Thereby, mitigation activities can be initiated well before the structural integrity of piping or components is jeopardized. Tracking biofilm activity on-line also provides feedback that is useful for evaluating the effectiveness of biocide additions and other control chemicals. This article concerns the BIoGEORGE™ probe, an on-line electrochemical biofilm monitor. It has been developed to provide information on biofilm activity in cooling systems that are common in power plants and industrial facilities. The BIoGEORGETM system makes it possible to attune dosing regimes specifically to local conditions and provides direct insight into the formation and activity of biofilms and the effectiveness of the applied dosing regimes. With BIoGEORGE™ it is possible to apply an optimal biocide dosing regime to keep biofilms under control. Two case histories of the BIoGEORGE™ systems application to monitor biofilms and the effectiveness of the applied water treatments are discussed in this paper.

PowerPlant Chemistry 2001, 3 (7)

R. Viswanathan and W. T. Bakker

Materials for Ultra Supercritical Coal Power Plants Part 1: Boiler Materials

Materials for Ultra Supercritical Coal Power Plants Part 1: Boiler Materials
The efficiency of conventional boiler/steam turbine fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide since the energy crisis in the 1970s. The need to reduce CO2 emission has recently provided an additional incentive to increase efficiency. Thus, steam temperatures of the most efficient fossil power plants are now around 600 °C (1112 °F), which represents an increase of about 60 °C (108 °F) in 30 years. It is expected that steam temperatures will rise another 50-100 °C (90-180 °F) in the next 30 years. The main enabling technology is the development of stronger high temperature materials capable of operating under high stresses at ever increasing temperatures. Recently EPRI performed a state-of-the-art review of materials technology for advanced boiler/steam turbine power plants (ultra supercritical power plants). The results of the review show that with respect to boilers, high strength ferritic 9-12Cr steels for use in thick section components are now commercially available for temperatures up to 620 °C (1150 °F). Initial data on two experimental 12Cr ferritic steels indicate that they may be capable of long-term service up to 650 °C (1112 °F), but more data are required to confirm this. For higher temperatures, austenitic steels and Ni base alloys are needed. Advanced austenitic stainless steels for use as superheater and reheater tubing are available for service temperatures up to 650 °C (1112 °F) and possibly 700 °C (1292 °F). Ni base superalloys would be needed for higher temperatures. None of these steels have been approved by the ASME Boiler Code Group so far. Higher strength materials are needed for upper water walls of boilers with steam pressure above 24 MPa (3400 psi). A high strength 2-1/2%Cr steel recently ASME code approved as T-23 is the preferred candidate material for this application. Field trials are in progress. This paper presents the results of the EPRI review relating to boiler material in detail. Results relating to turbine materials will presented in a companion paper as part II.

PowerPlant Chemistry 2001, 3 (7)