Feature

To reduce capital and maintenance costs of geothermal heat exchangers, a DOE-sponsored laboratory-industry team has been evaluating and field testing dozens of low-cost, high-thermal-conductivity polymer coatings applied to inexpensive, carbon-steel, shell-and-tube heat exchangers. Field testing has been conducted at two sites in California, and is due to begin at a site in Utah. The formulation showing the most value to date is ready to undergo commercial application and operation at a power plant in southern California. Initial studies indicate that the coating will allow capital cost reductions of up to 67% and significant maintenance cost reductions. The commercialization efforts are the culmination of a decade of work.

The team includes NREL, Brookhaven National Laboratory (BNL), Mammoth Pacific LP, FPL Energy, ThermoChem, and suppliers of coating materials and services, such as Ticona Corp., American Chemical Corp., Bob Curran & Sons, Applied Surface Technology, and Lauren Manufacturing. CalEnergy Operating Company (CEOC) hosted many field tests in the Salton Sea area in California.

The many coating formulations tested in the lab and field included traditional and exotic materials with varying amounts of silicon carbide, antioxidants, Teflon® particles, and other additives. Results showed that early polymer concrete coatings didn't hold up under high-pressure scale cleaning, but they did perform well thermally. Phenolic compounds were thin enough for adequate heat transfer, and exhibited a glasslike surface that resisted scale adhesion, but they didn't bond well to the steel, and their upper temperature limit was not high enough for many geothermal applications (greater than 320°F). The team even looked at a porcelain coating with a diamond powder surface finish, but the porcelain did not provide sufficient corrosion resistance in continuous contact with geothermal fluid. The right formulation came from Dr. Toshi Sugama (three-time R&D 100 award winner) of BNL, who developed a polyphenylene sulfide (PPS) coating system that was inexpensive, slick enough to enable easy scale removal, and, most important, able to withstand the high temperatures and mixed chemistry of geothermal brines and completely protect the underlying steel. Ticona Corp. supplies the custom formulation.

Over a number of years, field tests were conducted at CEOC's Hoch power plant in the Salton Sea area using the polymer concrete material, phenolics, and PPS. These tests used 20-ft.-long tubes mounted in a shell-and-tube heat exchanger, and data on pressure drop and heat transfer were available throughout the tests. The tests accessed geothermal fluid at 225°F and total dissolved and suspended solid material levels of 25% to 30%. These tests exposed the coatings to the most aggressive geothermal environment available. The most promising coating resulting from that work was the PPS.

Field tests were then begun in August 2000 using a sidestream of geothermal fluid at Mammoth Pacific's Mammoth Lakes power plant in northern California. NREL, BNL, and MPLP will experiment at this site with varying formulations of PPS, silicon carbide, and Teflon® to provide the best combination of corrosion protection, wear resistance, thermal conductivity, and ease of scale removal. The latest versions have a hard, glossy finish that will be even easier to clean. Two test rigs have been installed at Mammoth Lakes to expose the coated tubes to production and injection fluids, and their design is largely similar to those used in the Hoch experiments.

In all coating tests so far, the team has used only environmentally benign zinc-phosphate primers; however, manganese-phosphate primers, also environmentally safe, are now being considered because some priming shops already use them for many applications. BNL, together with American Chemical Corp., developed a priming method for single tubes that is faster than the flood-and-drain techniques used in the lab. In this new method, cleaning solutions and a warm zinc-phosphate solution are pumped through tubes connected in series. Sandblasting the tube interiors is no longer necessary, and large numbers of single tubes can be primed simultaneously.

To test the coating "live" and to evaluate the coating as applied by a commercial coater (Bob Curran & Sons) in another geothermal environment, a testing partner was found at FPL Energy's (a subsidiary of Florida Power & Light Co.) East Mesa binary plant in southern California. This plant will be the first test site for commercially coated tubes in an operating binary plant heat exchanger. Single tubes, retrofitted in heat exchangers, will be coated with a zinc- or manganese-phosphate primer, two layers of PPS coating, and a third layer of PPS coating toughened with silicon carbide and augmented with Teflon® particles for improved cleaning ability.

To enable these single, coated heat exchanger tubes to be retrofitted, another type of coating is used at the tube/tubesheet junction because the PPS is removed where roller expansion and seal welding occur. This fluoropolymer-based coating, being commercially developed in collaboration with Lauren Manufacturing, has excellent hydrothermal stability and good adhesion to the tube surfaces. It cures well at ambient temperatures, can easily be field-applied, and is abrasion-resistant.

The next step will be to test an entire coated tube bundle, again with the coating applied by the commercial coating partner. (The fluoropolymer coating will not be necessary for the tube bundle.)

Another geothermal testing environment is the Roosevelt Hot Springs area in Utah. Paul Hirtz of ThermoChem plans to test PPS-coated tubes along with uncoated tubes made of a variety of alloys downstream of the Roosevelt flash plant. These tests will determine how the coatings react to untreated geothermal fluid and fluid treated with silica inhibitors and acid. The coatings will be applied by a commercial applicator.

While progress is being made in tests in geothermal environments, the commercial coater has already started marketing the PPS coating to other industries operating chemical and high-temperature processes. The PPS has been shown to bond well to stainless steel without primers, and is being used for protection of stainless steel in especially harsh environments.

Power plant builders and operators will realize significant cost savings with this new PPS coating. Compared to equally corrosion-resistant titanium or stainless steel, the coated carbon-steel tubes will result in up to 67% capital cost reduction. Maintenance costs and plant downtime will be reduced because the tubes can be cleaned faster, and fewer will require replacement. The coating has high thermal performance due to thinness of the layers and the addition of silicon carbide. Commercial verification of these benefits should be available in the next several months.

For more information, please contact Dr. Keith Gawlik, NREL, 303.384.7515, keith_gawlik@nrel.gov.