According to Inside EPA (subscription required), the Department of Justice (and the Sierra Club) have opposed Ameren’s Missouri’s motion to allow it to close its Rush Island generating facility early, rather than install otherwise required pollution control equipment. You may ask why DOJ is opposing the shutdown of a coal-fired power plant. Like Tevye in A Fiddler on the Roof, I’ll tell you. 
I don’t know.
What really blows my mind is that the government has apparently acknowledged the environmental benefits of an earlier shutdown:
The filing adds that while the United States agrees that the plant’s retirement will have environmental benefits compared to its continued operation, the motion “should be denied because it usurps the Court’s authority to set the compliance date in favor of a drawn-out process engineered by Ameren.”
The Inside EPA story did not attach or link to the DOJ brief and I have not read it. It is certainly possible that this is simply a negotiating gambit by DOJ, hoping to force Ameren Missouri to agree to an earlier shut-down. However, I fear that it is also all too possible that DOJ is simply on its moral high horse and wants Ameren Missouri to pay penance for conduct that DOJ sees as not simply violating the CAA, but as immoral.
DOJ needs to learn how to graciously accept victory.
