Operating revenue. During the nine months ended September 30, 2023, operating revenue increased $12.2 million compared to the nine months ended September 30, 2022, primarily due to sales of RNG and environmental attributes from our RNG project. Sales under our RNG project commenced in the third quarter of 2022. During the nine months ended September 30, 2023, we sold 222,906 MMBtu of RNG from our RNG project, resulting in biogas commodity sales of $0.5 million and environmental attribute sales of $10.6 million, see Key Operating Metrics above. Additionally, we recognized $1.3 million of licensing and development revenue from the agreement with LG Chem as well as $0.4 million of isooctane.
Cost of production. Cost of production increased $3.3 million during the nine months ended September 30, 2023, compared to the nine months ended September 30, 2022 mainly due to the production and sales from our RNG project, which significantly increased in 2023, after the ramp-up phase, as well as lower costs at the idling Luverne Facility in 2023.
Depreciation and amortization. Depreciation and amortization increased $9.8 million during the nine months ended September 30, 2023, compared to the nine months ended September 30, 2022, primarily due to a full three quarters of additional depreciation expense in 2023 for RNG assets placed into service in Q3 2022 and accelerated depreciation on Agri-Energy segment assets due to shorter lives stemming from the impairment assessment during the third quarter of 2022.
Research and development expense. Research and development expenses remained flat during the nine months ended September 30, 2023, compared to the nine months ended September 30, 2022. The expenses primarily include patent and personnel related costs, lab work and supplies related to our ETO and other technologies, as well as technical consulting.
General and administrative expense. General and administrative expense increased $2.7 million during the nine months ended September 30, 2023, compared to the nine months ended September 30, 2022, primarily due to increases in stock-based compensation, professional consulting fees, and personnel costs related to the hiring of highly qualified and skilled professionals.
Project development costs. Project development costs are related to our future Net-Zero Projects and Verity which consist primarily of employee expenses, preliminary engineering costs, and technical consulting costs. Project development costs increased $5.1 million during the nine months ended September 30, 2023, compared to the nine months ended September 30, 2022, primarily due to increases in personnel costs and consulting fees.
Facility idling costs. Facility idling costs are related to care and maintenance of our Luverne Facility. The increase of $0.6 million during the nine months ended September 30, 2023, compared to the nine months ended September 30, 2022, primarily due to the Luverne Facility being in production for the first half of 2022, resulting in costs being recorded as a component of production expenses.
Impairment loss. No impairment loss was recorded during the nine months ended September 30, 2023. During the nine months ended September 30, 2022, the Company recorded a $24.7 million impairment loss on long-lived assets, which reduced the carrying value of certain property, plant, and equipment, and a leased ROU asset, at the Agri-Energy segment to its fair value. The impairments recorded relate to the determination to suspend production at the Luverne Facility and shift the plant into an idled, care and maintenance status during the third quarter of 2022. See Note 3 to the Consolidated Financial Statements for additional information.
Loss from operations. Our loss from operations decreased by $15.5 million during the nine months ended September 30, 2023, compared to the nine months ended September 30, 2022, primarily due to the increased revenue from RNG operations and LG Chem licensing in 20223, as well as the prior year impairment loss, partially offset by the increase in costs for our Net-Zero, Verity, and USDA Climate-Smart Grant projects.
Interest expense. Interest expense increased $1.2 million during the nine months ended September 30, 2023, compared to the nine months ended September 30, 2022, primarily due to the interest on the 2021 Bonds, which was capitalized into construction in process during the construction phase of our RNG project in the prior periods.
Interest and investment income. Interest and investment income increased $12.9 million during the nine months ended September 30, 2023, compared to the nine months ended September 30, 2022, primarily due to an increase in interest earned on our cash equivalent investments as a result of higher interest rates.