A correctly-structured carbon tax would make a new NG plant relatively more attractive vs. continuing to operate a coal plant without the tax. (both fossil fuels, but NG is more efficient).
Also I think it shouldn't be a straight carbon tax, but an "environmental externalities tax". I personally care far more about other forms of pollution (especially localized) than CO2; I don't think global warming is a non-issue, but it's not the #1 environmental issue. Point sources of particulates, NO2, etc. should be taxed as well as maximum emissions specified (obviously a steel mill should be allowed to emit more, and pay for it, at full production in an efficient way, than a badly adjusted oil-fired school furnace, even if the school chooses to ignore economic rationality.)
Also I think it shouldn't be a straight carbon tax, but an "environmental externalities tax". I personally care far more about other forms of pollution (especially localized) than CO2; I don't think global warming is a non-issue, but it's not the #1 environmental issue. Point sources of particulates, NO2, etc. should be taxed as well as maximum emissions specified (obviously a steel mill should be allowed to emit more, and pay for it, at full production in an efficient way, than a badly adjusted oil-fired school furnace, even if the school chooses to ignore economic rationality.)