The Energy Department said 13 prospective buyers had shown interest in its Great Plains Coal Gasification Plant, the nation's largest commercial scale synthetic fuels facility. The department said the prospective buyers are interested in keeping the $2-billion plant near Beulah, N.D., running. It is now converting 16,000 tons of lignite a day into 142 million cubic feet of pipeline-quality natural gas. The government invested $500 million and loaned $1.5 billion to a holding company and four gas firms to build the complex. When gas prices fell, the firms defaulted on the loan, and the department took possession.