Union workers will build the 470-acre hydrogen power and fertilizer plant proposed near Tupman, groups connected to the $4 billion project announced Thursday.

A newly signed labor agreement between a subsidiary of Texas-based construction giant Fluor Corp. and labor groups including the Kern, Inyo, Mono Counties Building and Construction Trades Council spells out details such as workers' wages, benefits and the project's total projected labor expenditures, according to a spokesman for the State Building & Construction Trades Council of California, AFL-CIO.

The agreement gives Hydrogen Energy California significant new support as it looks to state agencies for approval. It also takes HECA's owner, Massachusetts-based SCS Energy, a step further than the previous partners, BP and Rio Tinto, which a project spokeswoman said were in talks with construction unions but never formalized a labor agreement.

A spokesman for the trade group Associated Builders & Contractors Inc., Russell Johnson, said the labor agreement was unfortunate because it could raise the project's costs and "shut out" the 85 percent of California construction firms that are not union shops.

The coal and petroleum coke fueled plant would produce 300 megawatts of electricity for the power grid on days of peak demand but otherwise produce the fertilizer urea and other nitrogen-rich products. Backers say it will create more than 2,000 construction jobs and up to 200 permanent jobs when complete, as soon as fall 2017.

Though opposed by neighbors because of the pollution it would emit, the $4 billion project is touted as being environmentally friendly because it would bury 90 percent of its byproduct carbon dioxide and put that gas to use in nearby oil production. The U.S. Department of Energy has awarded the project a $408 million Clean Coal Power Initiative grant.

If approved by the California Energy commission and the state Division of Oil, Gas and Geothermal Resources, HECA could begin construction in spring 2013.