The Company has the following notes payable outstanding as of December 31:

As of December 31, 2018, scheduled principal payments due on convertible notes payable are as follows:

As of December 31, 2018, future amortization of discounts and debt issuance costs are as follows:

From July 30, 2013 through December 24, 2013, the Company sold convertible notes and warrants to unaffiliated accredited investors totaling $1,902,500. The notes bear interest at 10% per annum, are secured by the Company’s assets, and are convertible into one share of common stock, par value $0.001 per share, with the initial conversion ratio equal to $0.50 per share. The notes had an initial term of three years, but the maturity of the notes was extended during 2014 to match the retirement of the New AC Midwest Secured Debt. These securities were sold in reliance upon the exemption provided by Section 4(2) of the Securities Act and the safe harbor of Rule 506 under Regulation D promulgated under the Securities Act. Interest expense for the years ended December 31, 2018 and 2017, was $124,967 and $155,618, respectively. A discount on the notes payable of $841,342 was recorded based on the value of the warrants issued using a Black-Scholes options pricing model and was amortized over the initial five year life of the notes. Amortized interest expense for the years ended December 31, 2018 and 2017 on this discount was $74,447 and $152,558, respectively. As of December 31, 2018 and 2017, total principal of $990,000 and $1,550,000, respectively, was outstanding on these notes.

On June 15, 2018, the Company issued 2018 Unsecured Notes totaling $560,000 and warrants to certain holders of the 2013 Notes in exchange for their secured 2013 Notes (see description above of the private placement offering commenced during the second quarter of 2018). The 2018 Unsecured Notes have a term of five years, bear interest at 12% per annum, and are convertible into one share of common stock, par value $0.001 per share, with the initial conversion ratio equal to $0.50 per share. For each dollar exchanged, the investor received a warrant to purchase one share of common stock of the Company at an exercise price of $0.70 per share. The 2018 Unsecured Notes may be converted at any time and from time to time in whole or in part prior to the maturity date thereof. Loss on this debt exchange was $44,036. A discount on the notes payable of $89,500 was recorded based on the value of the fair value of the note and warrants exchanged. The included warrants were valued using a Black-Scholes options pricing model. From August 31, 2018 through October 30, 2018, the Company issued additional 2018 Notes totaling $300,000 and warrants to unaffiliated accredited investors. A discount on the notes payable of $40,350 was recorded based on the fair value of the warrants issued with this note using a Black-Scholes options pricing model. Amortized interest expense for the years ended December 31, 2018 and 2017 on these discounts was $8,700 and $0, respectively. These securities were issued in reliance upon the exemption provided by Section 4(a)(2) of the Securities Act and the safe harbor of Rule 506 under Regulation D promulgated under the Securities Act as well as under Section 3(a)(9) under the Securities Act. Interest expense for the years ended December 31, 2018 and 2017, was $46,587 and $0, respectively. As of December 31, 2018 and 2017, total principal of $860,000 and $0 was outstanding on the 2018 Unsecured Notes. The significant assumptions utilized for these Black Scholes calculations consist of an expected life of equal to the expiration term of the option, historical volatility of 100% respectively, and a risk free interest rate of 3%.

New AC Midwest Secured Note

The New AC Midwest Secured Note, which will mature on August 25, 2022 and is guaranteed by MES, is non-convertible and bears interest at a rate of 15.0% per annum, payable quarterly in arrears on or before the last day of each fiscal quarter. The New AC Midwest Secured Note is secured by all of the assets of the Companies. Interest expense for the years ended December 31, 2018 and 2017 was $66,694 and $267,847, respectively. As of December 31, 2018 and 2017, total principal of $271,686 and $1,146,686, respectively, was outstanding on this note. On February 25 2019, per Amendment No. 3 (“Amendment No. 3”) to the Amended and Restate Financing Agreement (See Note 13), AC Midwest agreed to waive of compliance with a certain financial covenants of the Restated Financing Agreement and strike this covenant in its entirety as of the effective date of the amendment. Also, pursuant to Amendment No. 3, the parties agreed that the maturity date for the remaining principal balance due under the AC Midwest Secured Note would be extended from December 15, 2018 to August 25, 2022.

AC Midwest Subordinated Note

The AC Midwest Subordinated Note, which will mature on December 15, 2020 and is guaranteed by MES, is non-convertible and bears interest equal to the three-month LIBOR rate plus 5.0% per annum, payable quarterly on or before the last day of each fiscal quarter beginning December 31, 2017. The interest rate shall be subject to adjustment each quarter based on the then current LIBOR rate. Commencing on June 15, 2017 and continuing on each September 15, December 15, March 15 and June 15 thereafter, the Company pays principal on the AC Midwest Subordinated Note in equal installments of (i) $500,000 per quarter for the 2017 calendar year, (ii) $625,000 per quarter for the 2018 calendar year, and (iii) thereafter $750,000 per quarter, with a final payment of all outstanding principal together with such other amounts as shall then be due and owing from the Company to AC Midwest on the maturity date. Notwithstanding the foregoing, until the New AC Midwest Secured Note is paid in full, AC Midwest will not be entitled to receive any payment on account of the AC Midwest Subordinated Note (other than regularly scheduled interest payments). Interest expense for the years ended December 31, 2018 and 2017 was $942,319 and $818,357, respectively. As of December 31, 2018 and 2017, total principal of $13,000,000 and $13,000,000 respectively, was outstanding on this note. The Company determined that the rate of interest on the AC Midwest Subordinated Note was a below market rate of interest and determined that a discount of $2,400,000 should be recorded. This discount is based on an applicable market rate for unsecured debt for the Company of 15% and will be amortized as interested expense over the life of the loan. Amortized discount recorded as interest expense for the years ended December 31, 2018 and 2017 was $593,094 and $593,094 respectively. The AC Midwest Subordinated Note was cancelled on February 25, 2019 (see Note 13).

On January 28, 2016, the Companies entered into Amendment No. 3 to Financing Agreement and Reaffirmation of Guaranty (the "Third Amended Financing Agreement") with AC Midwest Energy LLC (the “Lender”), pursuant to which Lender agreed to cause its bank to arrange for the issuance to a certain customer of the Company a standby letter of credit in the amount of $2,000,000 (the "Letter of Credit") to permit the Company to enter into a contract for mercury capture program with such customer. The Letter of Credit is to guarantee the Company's performance under its contract with such customer. Under the Third Amended Financing Agreement, and in consideration for the issuance of the Letter of Credit for the benefit of the Company, the Company shall pay AC Midwest a fee equal to 12.0% per annum of the amount available to be drawn under the Letter of Credit payable on the last day of each calendar month. No amounts were drawn on this letter of credit, which was terminated as of December 31, 2018. Fee expense for the years ended December 31, 2018 and 2017 was $29,000 and $219,333, respectively.