Answer:
9.24 yr
Explanation:
The payback period refers to the amount of time it takes to recover the cost of an investment. In order to find a payback period we need to go through some calculations first
Annual savings = 5 MM Btu/hr x 8,000 hr/yr x $4/MM Btu x 14 MM Btu/hr x 8,000 hr/yr x $7/MMBtu
Annual savings = $0.944 MM/yr
TCI = 
TCI = $4.7 MM
Depreciation - Annualized fixed cost = ![\frac{[4.0 - 0] }{10}](https://tex.z-dn.net/?f=%5Cfrac%7B%5B4.0%20-%200%5D%20%7D%7B10%7D)
Depreciation - Annualized fixed cost = $0.4 MM/yr
Total cost annualized = Annualized fixed cost + Annual operating cost
Total cost annualized = 0.4 + 0.5
Total cost annualized= 0.9 MM/yr
Annual net (after-tax) profit = Annual income - Total cost annualized x (1-Tax rate + Depreciation
Annual net (after-tax) profit = $0.944 MM/yr - $0.9 MM/yr x 1 -0.25 + $0.4 MM/yr
Annual net (after-tax) profit = 0.433MM/yr
Payback period = 
Payback period = 9.24 yr