(b) Until January 1, 2005, each corporation subject to this Act shall be entitled to a credit against the tax imposed by subsections (a) and (b) of Section 201 in an amount equal to 5% of the amount spent during the taxable year by the corporation on equipment purchased for the purpose of maintaining or increasing the use of Illinois coal at any Illinois facility owned, leased or operated by the corporation. Such equipment shall be limited to direct coal combustion equipment and pollution control equipment necessary thereto. For purposes of this credit, the amount spent on qualifying equipment shall be defined as the basis of the equipment used to compute the depreciation deduction for federal income tax purposes. For tax years ending on or after December 31, 1987, the credit shall be allowed for the tax year in which the amount is donated or the equipment purchased is placed in service, or, if the amount of the credit exceeds the tax liability for that year, whether it exceeds the original liability or the liability as later amended, such excess may be carried forward and applied to the tax liability of the 5 taxable years following the excess credit years. The credit shall be applied to the earliest year for which there is a liability. If there is credit from more than one tax year that is available to offset a liability, earlier credit shall be applied first.