TransAlta, Lewis County

Comment on the environmental review documents and draft air permit (Prevention of Significant Deterioration, PSD) for TransAlta’s proposed coal-to-gas transition project. This includes reviewing a State Environmental Policy Act (SEPA) checklist and Mitigated Determination of Nonsignificance (MDNS). These documents are used by local and state agencies to help determine whether a proposed project will have negative environmental impacts.

Proposed project 
TransAlta proposes to transition one unit of its existing coal-fired electrical generation facility in Lewis County to generate electricity from natural gas. To supply natural gas to the facility, TransAlta will build a new 1,500‑foot, 12‑inch diameter natural gas pipeline. TransAlta would develop the pipeline on land they own and have used for the existing facility.

Environmental review
We reviewed the project to determine how it would impact the environment. We found that TransAlta would need to mitigate or reduce certain impacts to move forward.

The SEPA process is not a decision and does not determine whether the project moves forward. It is used as support when decisions are made about the permit and other project decisions.

Air permit
TransAlta applied for a Prevention of Significant Deterioration (PSD) permit to:

• Convert the existing Unit #2 boiler from coal to natural gas combustion
• Replace the existing oil-fired auxiliary boiler with a new natural gas-fired auxiliary boiler
• Replace the existing Unit #2’s cooling towers with two new cooling towers
• Install two new natural gas-fired water bath heaters

Documents for review

• SEPA checklist and Mitigated Determination of Nonsignificance
• Prevention of Significant Deterioration (PSD) application
• Draft Prevention of Significant Deterioration (PSD) permit
• Draft technical support document for PSD permit
• Public notice

Public hearing

Monday, September 14, 2026, 6 p.m. online (zoom link coming soon)

Background

TransAlta is a coal-fired power plant in Centralia. In 2011, the Legislature required TransAlta to shut down its coal-fired boilers in two phases. Ecology regulates air pollution from industrial facilities, including power plants. We monitor TransAlta's emissions.

A Prevention of Significant Deterioration (PSD) permit applies to new, large facilities or changes at existing large facilities that could increase air pollution.  We issue these permits, except for projects that are regulated by the Energy Facility Site Evaluation Council (EFSEC), EPA, or tribes.