Economic Profits difficulty, determine, discount, business, economic

Matori

I’m having difficulty understanding the concept of normal profit when it comes to calculating economic profits. It is the only opportunity cost that I cannot quite put my finger on. Is it just what you would have expected to earn given the riskiness of the type of business?
Which brings me to my next question… when discounting future cash flows, we are trying to determine economic profit right? That is, profit in excess of the discount rate (normal profit)? I know every thing I read says you have to discount a project’s future cash flows by WACC…but I cannot find an explanation as to WHY. Let’s say the cash flow the first year is 100 and the WACC is 10%. This gets discounted to time 0 to $90.91. Does this mean that $90.91 is the profit in excess (economic profit) of the normal profit (WACC of 10%)?
This has been on my mind and I just cannot find an answer anywhere.