Bloomington Public Schools closed Jan. 16 after biodiesel fuel gelled in about a dozen school buses due to subzero temperatures. The fuel is required to be used under state law, according to the Minnesota Star-Tribune.

The problem stranded some students at bus stops on Jan. 15 for as long as 30 minutes. There have been other incidents involving cold weather's effect on biodiesel.

The Rosemount-Apple Valley-Eagan School District in the south metro area of Minnesota started school two hours late Jan. 16 after a similar problem with biodiesel fuel in about a dozen buses the day before.

Bill Walsh, director of communications for the Minnesota Department of Commerce, said the problem is with diesel, not biodiesel.

Much of the diesel fuel sold in Minnesota contains 2 percent biodiesel fuel, under legislation enacted in 2002, but that didn't essentially take effect until 2005 because of a production lag.

The Legislature presided over a tough fight, with soybean farmers pushing for the mandate and trucking and other transportation industry groups opposing it, citing concerns about costs and performance of biodiesel.

The state has mandated the biodiesel blend increase to 5 percent starting this spring. Minnesota is one of three states, besides Oregon and Washington, that have biodiesel mandates.

The state's three largest school systems didn't report major problems with their buses but said it's not uncommon for fuel filter problems to arise in extreme cold, based in part on fuel grade. In extreme weather, the schools sometimes keep buses running all night.

On Jan. 16, the Minnesota Trucking Association said members were reporting an usually high amount of fuel filter plugging and urged fleet and truck drivers to arrange to have clogged fuel filters tested. The 700-member statewide trade group also urged members to refrain from blaming biodiesel because the problem has not been confirmed.