The state's only coal-fired power plant would reduce its mercury and nitrogen oxide emissions under a proposed agreement with the state Department of Ecology.

The proposal for the Centralia-area plant owned by TransAlta, a Canada-based energy company, has been released for public review and comment through Nov. 9.

The plan would reduce NOx emissions by 20 percent below current permit limits in an enforceable order and mercury emissions 50 percent by 2012.

Nitrogen oxide is a pollutant that causes regional haze, and mercury is a neurotoxin especially harmful to children and pregnant women.

"This agreement will lead to real improvements in visibility and lower health risk from airborne mercury," said Stuart Clark, Ecology's air-quality program manager.

The Sierra Club, which has mounted a national and regional campaign to shut down coal plants and halt construction of new ones, said the agreement goes too easy on the state's largest stationary source of air pollution.

For instance, the agreement doesn't address the 9 million tons of greenhouse gases the Centralia plant spews into the air annually, Sierra Club's Ethan Bergerson said.

"It's scary to think that they're not looking at greenhouse gases in this agreement," Bergerson said.

Ecology and TransAlta started talks late last month about a plan to curb climate-change gases from the plant by 50 percent by 2020, as spelled out in an executive order issued by Gov. Chris Gregoire in May.

"The first meeting was productive and positive," Clark said.

The pollution deal for NOx and mercury was announced in April and drew criticism from the Sierra Club and other environmental groups for not being a more open, public process.

Part of the reason for the closed-door talks was that the technology TransAlta is considering to reduce mercury from its air emissions is proprietary information, TransAlta spokesman Richard DeBolt said.

"We're testing several different products and will share the information with the public when we settle on one," said DeBolt, a state legislator from Chehalis.