By 2012 , that had dropped to 15 , mostly due to new rules limiting mercury emissions .
By 2012 , that limiting carbon emissions to 1,000 pounds of CO per megawatt - hour of electricity produced .
By 2012 , that had dropped to 15 , mostly due to new rules limiting mercury emissions , and limiting carbon emissions to 1,000 pounds of CO per megawatt - hour of electricity produced .