How Is Efficiency Calculated in Coal Gasification Projects?

[mbm]

Homework Statement

For a gasification project being planned, 26000tons of coal per day will be used to produce 250000000 cubic ft per day of natural gas. if the coal is rated at 8700btu/lb and the gas at 950btu/cu ft. what will the efficiency will be?

Homework Equations

efficiency = energy output/energy input

The Attempt at a Solution

i calculated the energy content of both coal and gas and afterwards calculated the efficiency. however i have my doubts if it's the correct way to answer the question.

 

[SteamKing]

If you don't think the efficiency calculation specified in the problem is correct, what do you suggest is the correct formula?