WASHINGTON — The nation’s leading effort to build a coal-fired electric plant that will capture and store its carbon emissions will take a shaky step forward on Tuesday, when the consortium building it announces a location for the $1.5 billion plant. But choosing a location may be the least of the problems for the builders, with the Energy Department making ambiguous statements about its commitment to the project. The department, which is supposed to pay for most of the work, called the announcement on a location “inadvisable” and seemed to distance itself from the plans, saying in a letter last...