Operating an Anaerobic Digestion and Co-Generation System with Stringent Air Quality Regulations to Limit Nitrogen Oxides (NO_x) and Sulfur Oxides (SO_x) in Stack Gas Emissions

摘要

Performance data from a full-scale anaerobic digestion and co-generation system located at a dairy farm in central California was analyzed to determine the efficacy of a selective catalytic reduction (SCR) system to reduce engine exhaust gas nitrogen oxides (NO_x) and a biological treatment system to reduce biogas hydrogen sulfide (H_2S). Data were collected for over two years from the 710 kW capacity system to characterize power production and the performance of the emissions control strategies. Use of the SCR resulted in the exhaust gas NO_x concentration decreasing from 63.1 ± 31.9 ppmvd @ 15% O_2 to 14.2 ± 17.5 ppmvd @ 15% O_2, based on data averaged over 15 minute intervals. The regulatory limit of 44 ppmvd @ 15% O_2 was met 94% of the time and the proposed regulatory target of 11 ppmvd @ 15% O_2 was met 74% of the time. The biological H_2S removal system appears to be reducing biogas concentrations based on the measured value of 297 ± 267 ppmvd, which is lower than reported values for other similar systems. It is not possible to quantify H2S removal performance independently because the biological system is integral to the digesters. The regulatory limit of 50 ppmvd was met only 18% of the time based on 15-minute average data. Additionally, air injection into the digester headspace may be adversely affecting the 50.7 ± 3.9% biogas methane content. Overall the project provides a comprehensive dataset that demonstrates the functionality of two emissions control systems and “lessons learned” for future applications.