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Centralia retrofit: a benchmark for wet FGD: technical and commercial innovations have contributed to making the new Centralia wet flue gas desulphurisation (WFGD) system one of the most cost effective in the world. Compared with equivalent projects of the mid 1990s, costs per kW are about half, while performance has improved.

Article Excerpt
Units 1 and 2 of the 2x700 MWe Centralia coal-fired power plant in Washington state (Figure 1) entered commercial operation in 1971 and 1972 respectively. Some 4.8 million tons of coal per year for the plant comes from the adjacent mine, with other sources, typically the Powder River Basin, providing a further 1 million tons. Prior to the decision to install flue gas desulphurisation (FGD), compliance with emission limits was accomplished by blending the Centralia coal with low sulphur PRB coal. However, in 1996, the collaborative decision making (CDM) group, which consisted of Centralia's owners, local/state environmental authorities, and several environmental intervener organisations, agreed on S[O.sub.2] emission reductions that would involve retrofitting of FGD systems.

[FIGURE 1 OMITTED]

Centralia's owners, spearheaded by PacifiCorp, the operator and owner of the largest share of Centralia at that time, decided to pursue a non-traditional partnering approach for the FGD retrofits. Alstom was selected on the basis of its ability to evaluate and offer a wide range of FGD alternatives, and the success of an on-going alliance for service work.

In January 1999, Centralia's owners contracted with Alstom to conduct a technology assessment and cost study for two FGD technologies: limestone/gypsum wet FGD (WFGD); and lime spray dry FGD. Site-specific issues strongly indicated that WFGD was the preferred approach from an overall lifecycle cost standpoint and detailed specifications and costs were developed for that alternative. On 20 May 1999 PacifiCorp, on behalf of the Centralia owners, contracted with a consortium consisting of Alstom and Stone & Webster to perform the WFGD retrofit.

In early 2000, the Centralia owners finalised an agreement to sell the plant and the mine to TransAlta. TransAlta elected to continue the project in accordance with the original partnering approach.

Flue gas path

Separate flue gas paths for each of the two units are maintained throughout the FGD system ductwork, absorbers, and stack (Figures 2, 3 and 4). The new ductwork begins after the electrostatic precipitators at a point downstream of the existing boiler induced draft (ID) fans. To provide operational flexibility during boiler start-up and shut-down, the ability to bypass the FGD system to the existing stacks was maintained and FGD system isolation and bypass dampers were installed.

[FIGURES 2-4 OMITTED]

The existing boiler ID fans (2 per unit), motors, and foundations were modified to accommodate the draft loss associated with the FGD system absorbers and ductwork. Ducts from the ID fans pass underneath existing second-stage electrostatic precipitators to the absorbers. Treated flue gas is discharged from the absorbers to a new 470 ft dual-flue stack. Since separate flues are provided for the two boiler units, absorber outlet dampers are not required.

Construction materials include: carbon steel for the hot flue gas ductwork (FGD system inlet duct and bypass duct); Alloy C-276 for the inlet transition (between absorber expansion joint and vessel wall); 317 LMN stainless steel for outlet duct, from outlet flange and the breeching flange; and roll-bond clad C-276 (1/16" thick) with carbon steel substrate for the chimney flues.

Absorber island

A high-performance spray tower absorber is used to provide the gas/liquid contact required for S[O.sub.2] removal (Figure 5). The Centralia absorbers have four installed spray levels. Each spray level is fed through an individual riser by a dedicated recycle pump....