Six companies are planning a $150 million coal-handling facility for the Port of Baltimore, the third and largest coal-related venture announced recently.

The facility, to be built on a 500-acre site on the Patapsco River, initially will handle 15 million tons of coal for export in addition to the 12 million tons handled at the port now. According to some economists, each ton of coal exported adds $18 to the local economy. The new facility is expected to provide $270 million annually to the local economy, according to state officials.

In addition, two other coal-handling facilities are under construction. The Island Creek Coal Co. is building a $60 million facility, planned for completion in 1983, to handle between 12 million and 15 million tons of coal a year, and the Consolidated Coal Co. is building a $110 million operation to add 10 million tons a year after its completion in 1985, state officials said.

The demand for coal overseas has increased, but the ability of U.S. ports to handle it hasn't caught up, according to a spokesman for the Maryland Department of Economic Development. Currently, between 50 and 60 ships are sitting in the Baltimore harbor waiting for coal and costing their companies between $15,000 and $16,000 a day, the spokesman said. Some vessels have been waiting for more than 30 days, he added.

Port officials have said they expect nearly $750 million to be spent during the next four years to retool the city's rail and harbor facilities to meet the expected high demand for coal. They also said they hope the increased foreign demand may invigorate the area's sagging economy and aid high unemployment and layoffs of their industries.

The Baltimore port is the second-largest coal exporter in the country, topped only by Virginia's ports which also are planning expansions. The new Baltimore facility can be expanded to handle 30 million tons of coal a year, state officials said.

"This will be the largest coal-exporting facility in the Port of Baltimore, which is one of the nation's most modern ports for serving the rapidly expanding U.S. coal market," Maryland Gov. Harry Hughes said yesterday in announcing the joint venture between the firms.

The coal-producing companies joining in the project are: Pittston Co.; Mapco Inc.; Elk River Resources, a unit of Sun Oil Co.; Old Ben Coal Co. of Ohio; and Utah International Inc., a unit of General Electric Co. Soros Associates, an international engineering firm, is also a partner in the venture.

The new venture is expected to provide 150 on-site jobs and additional employment in affiliated industries.