To pick up where renewables leave off, you want natural gas (or even petroleum) turbines that can quickly be brought on and off line.
Also: If you really are concerned about carbon dioxide, they produce a lot less of it per unit of energy.
In fossil fuels most of the energy comes from burning the hydrogen to water. Burning the carbon to carbon dioxide provides some, but it's mostly useful for packaging the hydrogen. Oil and gas is essentially long-chain-of-carbon molecules with two hydrogens per carbon and two more to cap the ends of the chain (with occasional tree-structures with the same carbon/hydrogen counts, and the odd ring-shaped or multiply-bonded impurity that''s short one or two pairs of hydrogens.)
So oil is a little over two hydrogens per carbon, gas goes from about 2.5 (butane) to 4 (methane). But coal is essentially just carbon. So gas is best, liquid oil fractions are not as good (though convenient for mobile engines), and coal is worst, on the energy/CO2 production ratio.