Section 21. Not less than fifteen percent of the tax imposed for severing coal shall be placed into a permanent trust fund in the state treasury to be held in trust and administered by the board of university and school lands, which shall have full authority to invest said trust funds as provided by law, and may loan moneys from the fund to political subdivisions as provided by law. The interest earned on the moneys in said trust fund shall be used first to replace uncollectable loans made from the fund, and the balance shall be credited to the general fund of the state. Up to fifty percent of the taxes collected and deposited in the permanent trust fund during a biennium may be appropriated by the legislative assembly for lignite research, development, and marketing as provided by law. An additional twenty percent of the taxes collected and deposited in the permanent trust fund during a biennium may be appropriated by the legislative assembly for clean coal demonstration projects approved by the industrial commission.