Enhancing RNG plant efficiency: Impact of integrating a dryer and pyrolysis unit on IRR% with biochar utilization as an additive and fertilizer

Abstract

This paper investigates the impact of integrating pyrolysis and drying units into traditional renewable natural gas (RNG) facilities fed by food waste, and evaluates the potential benefits of using biochar, derived from pyrolysis, as an additive to anaerobic digesters. The study aims to determine whether the capital costs associated with pyrolysis and drying units can be offset by increased methane yields due to biochar enhancement. Initial batch testing (BMP trials) showed a decrease in methane yield with biochar addition, attributed to unacclimatized bacteria. To further investigate, semi-continuous trials were conducted, revealing that biochar-enhanced digesters (BC10) exhibited an initial decrease in methane production, followed by significant improvements over time. After three hydraulic retention time (HRT) cycles, the average methane yield in BC10 digesters reached 260.68 Nm³/tonne VS, slightly higher than the control system (AD) at 258.48 Nm³/tonne VS. Additionally, biochar addition improved volatile solids reduction (45 % in BC10) and yielded digestate with higher levels of beneficial by-products and fewer toxic compounds. Economic analysis indicated that while the advanced facility showed a modest 3.49 % increase in methane yield, it faced higher operational costs, resulting in a lower internal rate of return (IRR) of 13.71 %, compared to 17.67 % for traditional facilities. The simple payback period for the traditional RNG facility was approximately 5.44 years, whereas the advanced facility required 6.73 years to recover its initial capital investment. The findings suggest that to surpass the IRR of traditional RNG facilities, the methane yield enhancement from biochar addition should be at least 30 %. The paper emphasizes the need for careful consideration of local market conditions and technological advancements in evaluating the financial viability of RNG projects.