December 1, 2009

Halar-Lined Chimney Remains Maintenance-Free

J. Karol Argasinski and Marcel E. Loosbergh

A fiberglass-reinforced plastic FRP/Halar stack liner was installed at Vattenfall’s 300-MW Unit 2 at the Nordjylland Power Station in 1995. The 112-meter-high (367-ft) stack, protected from corrosion by the ECTFE liner, is still in excellent condition and has not required any repairs during more than 13 years of service. (ECTFE is short for ethylene chlorotrifluoroethlyene polymer, marketed under the brand name Halar ECTFE [Halar] by Solvay Solexis.)

The original power station was built in 1977 and was designed to supply energy to Denmark’s North Jutland region. During 1991 the plant was upgraded with a SNOX-exhaust (sulfur and NO_x abatement) gas treatment system by Haldor Topsoe Engineering.

In addition, a new 112-m-high concrete stack (with an inside diameter of 4.6 m), internally protected with 100-mm-thick (3.9-inch), state-of-the-art acid-resistant brick, was erected (Figure 5).



5.    No longer a problem. Vattenfall’s 300-MW Nordjylland Power Station Unit 2 experienced significant deterioration in its brick liner caused by extremely corrosive exhaust gases. In 1995 the 112-meter-high stack was relined with an FRP/Halar liner that remains in as-new condition to this day.  Courtesy: Solvay Solexis

Tough Operating Conditions

The power plant was originally designed to burn crude refinery residue with high sulphur content — a very difficult range of operating conditions for the stack liner (Table 1). In fact, the condensation formed on the stack liner is sulfuric acid with a concentration of 70% to 80%.


Specifications for polymer stack liner. The exhaust gas constituents at Vattenfall’s 300-MW Unit 2 at the Nordjylland Power Station formed corrosive acids when condensed in the stack. Source: Solvay Solexis

Shortly after the SNOX-exhaust treatment was installed, operators discovered that the acidic condensation was attacking the "acid resistant" inner brick lining and forming large quantities of aerosol particles that detached from that lining. In addition, acidic droplets were carried up the stack by the high-speed flue gas. Those airborne solid particles, together with acidic droplets, settled over a wide rural area around the plant.

Selection of the Brick Liner Replacement

During 1995, Nordjylland decided to install additional de-misters and provide the chimney with a new trouble-free acid-resistant inner liner. Based on the maximum operating conditions of the flue gas, 110C (230F), and the presence of sulphuric acid, a high-performance thermoplastic fluoropolymer was needed to resist the extremely corrosive conditions inside the stack.

The plant owner selected Halar as the best option. Its choice as a corrosion protection liner was primarily based on its good chemical resistance and excellent permeation resistance to chemical compounds produced during the burning of crude oil. Other factors considered were the outstanding abrasion resistance and the exceptional surface smoothness of the ECTFE polymer.

The available chemical resistance data show that the ECTFE resin performs very well when exposed to concentrated inorganic acids. The Halar resin retains most of its mechanical properties and is not chemically attacked when exposed to 98% sulfuric acid at 140C or to 65% nitric acid, 49% hydrofluoric acid, and 37% hydrochloric acid at 100C after 30 days of immersion tests.

The good chemical resistance of the resin does not guarantee that the liner can provide sufficient corrosion protection. If the permeation rate is high, a substantial amount of chemicals passed through the liner will chemically attack the fabric backing and the substrate to which the liner is attached, causing failure of the structure.

The ECTFE resin was selected because on its excellent permeation resistance to water and acids, one of the best of all fluoropolymers, at temperatures as high as 100C.