Maintenance Program: Using only BCP 5030 (Note: No other chemicals were used)
90 Day Evaluation Demonstrates BCP 5030 Prevents Hard Sulfur Fouling in a Mechanically Cleaned Geothermal Power Plant Cooling Water System
Sulfur Fouling and the Impact on Geothermal Plant Power Output
Four 400,000 gallon cooling water systems associated with a 100 MW geothermal power plant were experiencing declining cooling system efficiency due to fouling by elemental sulfur. This plant (Cerro Prieto IV) is one of four plants with a combined capacity of 720 MW operated by CFE (Compania Federal Electricidad) in the Cerro Prieto (Mexico) Geothermal Field.
Sulfur enters the cooling water system as H2S, one of a number of non-condensable gasses associated with the steam from the geothermal reservoir. Condensation of the steam in a direct contact condenser results in low levels of H2S in the [steam condensate + cooling water] mixture, which is returned to the cooling tower. In the fully oxygenated cooling tower environment the H2S is oxidized to elemental sulfur, sulfuric acid and several other oxidized forms of sulfur by a combination of microbiological and abiotic oxidation processes. Elemental sulfur accumulates in the system as hard sulfur deposits in the direct contact condenser, the hot water distribution lines, on hot water spray nozzles and in the cooling tower’s high efficiency cross-corrugated film fill. (Below)
The pattern of uneven heavy sulfur deposit accumulation in the upper fill results in channeled flow through the fill (see photos below). Channeled flow reduces air-water mixing, resulting in a loss of cooling efficiency (see temperature variation across water curtain for uniform vs. channeled flow, below).
Gradual loss of cooling efficiency due to sulfur fouling resulted in a gradual increase in the plant’s cold water temperatures (see graph, below), which in turn produces higher condenser pressures (less vacuum), resulting in less efficient steam turbine operation. Loss in turbine operating efficiency has a direct impact on the power output of the plant. Cold water temperatures and MW output for Unit 12 are shown below.
The 28-day trial of BCP 5030 in Cerro Prieto demonstrated DTEA II’s ability to disrupt, remove and disperse preformed hard sulfur deposits in the plant’s cooling water system. Based on this successful treatment program, a second trial was conducted to optimize BCP 5030 treatments to prevent development of sulfur deposits in a system which had been cleaned by mechanical methods.
3-Month Trial Demonstrates BCP 5030’s Ability to Prevent Formation of New Hard Sulfur Fouling Deposits
BCP 5030 is a highly effective deposit penetrant, dispersant and cleaner. It is used in industrial water systems to clean fouled systems, and maintain clean systems free of inorganic, organic and microbiological fouling. Based on BCP 5030’s proven abilities to disrupt and disperse difficult-to-remove deposits, a trial was conducted in one of four 25 MW units at Cerro Prieto Plant IV in order to demonstrate BCP 5030’s ability to prevent the formation of new hard sulfur fouling deposits in a mechanically cleaned cooling system.
BCP 5030 was dosed into the system in 53 slugs ranging from 137.5 ppm (1x on the first day) to 37.5 ppm. A total of 1,320 gallons of BCP 5030 was added to the 400,000 gallon system during the 3 month trial period.
CP IV operates as four independent units (Units 10, 11, 12 and 13), each supported by a cooling water system of 400,000 gallons. Units 11 and 12 were mechanically cleaned before the trial. Both units began the trial operating at approximately 1.5 °C above design cold water temperature. The remaining two units (10 and 13) were not cleaned before the trial, and began the trial operating approximately 4.2 °C and 5.5 °C above design cold water temperature, respectively. Only Unit 11 was treated with BCP 5030 during the trial.
An interim inspection of Unit 11, conducted 30 days into the trial, found no evidence of new sulfur deposit formation in the tower. All hot water spray nozzles and clean fill had remained free of sulfur deposits (see photos below).
Cold water temperatures monitored throughout the trial demonstrated that not only did BCP 5030 prevent fouling, and associated deterioration in cooling system performance, the treatments improved Unit 11’s performance, as cold water temperatures were reduced from 1.5 °C above design to 1.0 °C above design (see graph below). In contrast, cooling system performance of the untreated units (10, 12 and 13) declined during the trial, with cold water temperatures in each case increasing by approximately 3 °C above the starting temperatures (to 7.2, 3.9 and 8.6 °C above design cold water temperatures, respectively).
Lower cold water temperatures result in more efficient steam condensation and greater vacuum, as indicated by lower condenser pressures (mm HgA). Lower condenser pressures allow more efficient turbine operation, resulting in greater power output. Unit 11, treated with BCP 5030, had the lowest condenser pressure (average data for final week of the trial; see graph below), followed by Unit 10, Unit 12 and Unit 13.
The presence of sulfate, nitrite and nitrate in geothermal plant cooling waters are the result of the oxidation of H2S and ammonia, primarily by microbially-mediated processes. As in most cooling towers, and as evidenced by the accumulation of elemental sulfur (another H2S oxidation product) on surfaces in this system, a large part of the microbial activity in a cooling water system occurs in biofilms on tower surfaces. Aggressive treatment of Unit 11 in this trial minimized the accumulation of microbial biofouling populations, and by this mechanism, minimized the oxidation of reduced forms of sulfur and nitrogen to sulfate, nitrite and nitrate. As a result, sulfate, nitrite and nitrate concentrations were lowest in Unit 11 during the final week of the trial (see graphs below).
Conclusions
Hard elemental sulfur deposits which develop in certain geothermal cooling water systems impact the system in ways which result in loss of cooling efficiency, and declining plant power output.
BCP 5030 treatment prevented the formation of new hard sulfur fouling deposits in CP IV Unit 11 geothermal power plant cooling water system.
BCP 5030 treatment improved cooling efficiency in CP IV Unit 11 despite the fact that Unit 11 had been mechanically cleaned immediately before the start of the trial, suggesting additional cleaning was achieved.
In the absence of BCP 5030 treatment cooling system performance of both mechanically cleaned (Unit 12) and sulfur-fouled (Units 10 and 13) deteriorated during the trial, with cold water temperatures increasing by approximately 3 °C.
Lower cold water temperatures in Unit 11 produced the lowest condenser pressures, a condition which will allow greater turbine operating efficiency and improved power output.
Effective biofouling control with BCP 5030 minimizes the activity of microbial processes which produce sulfur and nitrogen-based acids in geothermal cooling systems, thereby reducing the demand on pH control systems.
