As you argue, the critical factor accelerating coal power plant retirements is EPA regulation. Because natural gas prices are at historic lows, it’s cheaper for some utilities to retire a coal power plant than to retrofit it with new emission control equipment. However, gas is still a more expensive electricity fuel than coal, and the price premium for gas is expected to increase over the next several years.

Specifically, regional coal prices are in the range of $2-4/MMBtu in 2012, and are assumed in Brattle’s analysis to stay at that level afterwards, whereas gas prices are currently $3-4/MMBtu and are assumed to hit $4-5/MMBtu by 2015 and $6-8/MMBtu by 2025.

Depending on the stringency of EPA air rules, Brattle projects 59 to 77 GW of coal capacity to retire rather than retrofit with emission control equipment. Retirements account for 18-27% of current coal capacity in the PJM region, 17-24% in the MISO region, and 12-16% in the SPP region.

Now consider this tidbit. If, in 2020, the EPA (or Congress) adopts a global warming mitigation policy with an implicit carbon price of $30/ton CO2, then “almost half of the U.S. coal fleet” could retire.

As discussed here (http://cei.org/regulatory-comments-and-testimony/comment-letter-epas-carbon-pollution-standard), the EPA’s proposed greenhouse gas new source performance standards for power plants set the stage for imposing implicit carbon prices on existing coal-fired power plants. This could happen as soon as 2016 or before in a second Obama administration.