If coal was inexhaustible how is it harnessed for energy?

do you mean "If coal were inexhaustible how is it harnessed for energy?" First, it is not inexhaustible; every bit of coal that is burned adds to the pollution burden of the atmosphere. Second, coal, whether inexhaustible or not, is harnesses for energy the way it always has been. It is burned to boil water to make steam. The steam drives a turbine, which turns a generator which produces electricity.

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I'm afraid the second sentence introduces a non-sequiteur. Coal can be burnt cleanly but its combustion products still include carbon-dioxide. However any polluting effect has nothing to do with being inexhaustible or not. SO to answer the question, coal could not be inexhaustible - it is a finite resource - and it can be harnessed only by burning it to release its potential heat energy as the last sentence details. Coal can also be distilled to give coke, gas and various chemical products.