No, it's not.
There's a lot of hate against this idea, variants of "but you don't understand you naieve fool" are being thrown around. Sure, this is a simplistic idea, but it's not doomed like you and the other contrarians claim. Consider:
Assuming you *did* manage to buy up 25% of the coal mines and take their produce off the market. A coal power buyout would raise the price of remaining coal, as coal supply would now be too low to meet demand from the fixed number of coal fired plants. Plants would need to bid up prices on the coal, or shut down permanently as too much demand for coal chases too little supply. Conveniently, the least efficient (i.e., the plants that emit the most unburned carbon) plants will most likely be the ones to shut down as they are the ones providing the lowest yield per unit of coal input.
Reduced total output from coal plants would reduce overall electricity generation, increasing prices overall. Increased energy prices would stimulate consumers to engage in power saving behaviour and lower the threshold of profitability on green power projects. Once equilibrium was reached again, the mix of green power gneration to dirty power generation would have improved dramatically in the economy. Part of the problem we face is that green power prices relative to dirty power prices are too high, and an initiative like this would be a shot in the arm to the green industry that desperately needs just a kick start to reach scale.
Stop hating the idea just because it wasn't presented in a 20 page brief complete with executive summary and contingency analysis. The idea holds water and the sentiment certainly is meritorious.