Bob is well capitalized and business is good so he makes the capital investment in the pollution control and abatement equipment, which he will amortize over many years. By the fact that he made the capital investment, he also created pollution credit(s), which he can sell and use the proceeds against his capital cost, saving money.
Bill also owns a manufacturing company but doesn't have the money to invest, but he can buy Bob's pollution credit to tide him over and operate until business gets better and he can afford to make the capital investment, which will generate a pollution credit that he can sell. Actually Bill is paying for Bob's investment. And as you say, he can keep buying the pollution credit as long as he wants, but he is still paying for other's equipment.
Another method is taxing the pollutant(Cap & Tax), in which the govt gets the money from the emitter. In which case the emitter will buy the equipment to reduce emissions and avoid paying the tax.
The third way is to let EPA and the state agency regulate the pollutant in which the emitter, if he exceeds the regulatory threshold, is required to apply the best available control technology, which is based upon what is technically feasible and economically reasonable.
Since Congress was not able set up a cap and trade or cap and tax in 2009, EPA is regulating CO2 emissions, or in the process of setting it up to regulate.
The regulations for new plants are in place and being used while the regulations for existing plants will soon be established, and after the lawsuits are decided by SCOTUS, they will go into effect.
So if you want to put in a coal fired power plant, EPA is going to tell you that you will be exceeding the regulatory threshold of 100,000 tons per year of CO2, which means you will have to apply the best available control technology, which happens to be natural gas.
Is the natural gas alternative good or bad? I’m asking because I really have no idea.