Low-cost process for emission abatement of biogas internal combustion engines

Biogas internal combustion (IC) engines of less than 1 MW are either equipped with an oxidative catalyst or operate at low-emission operation parameters to meet national emission standards of NOx, CO, VOCs, HCl, H2S, SO2, and formaldehyde safely. However, expensive oxidative catalysts have a limited lifetime of less than two years, and electrical efficiency at low-emission conditions is lower than that at optimal conditions, characterized by higher emission levels. While NOx levels increased seven times from 282.2 to 1965.7 mg NOx∙m−3 by changing the operational parameters of the four-stroke gas engine (Pel = 360 kW), emission levels of CO and VOC slightly changed from 540.3 to 452.5 mg CO∙m−3 and 509.6 to 329.9 mg C∙m−3. Furthermore, NOx emission levels could be reduced by a minor air to gas ratio, but levels of VOC, CO, and HCl increased at these conditions as shown for a four-stroke gas engine (Pel = 635 kW). The emissions of the dual-fuel gas compression-ignition engine (Pel = 265 kW) were generally characterized by higher emission levels. A chemical scrubber with an optional subsequent biofilter was installed as a low-cost alternative for oxidative catalysts as a waste gas treatment system. Corresponding performance levels of the chemical scrubber were in the range of 28.3–38.6% (NOx), 19.2–30.8% (CO), 22.6–23.8% (VOCs), and 33.3–70% (odor). Formaldehyde, which occurred at levels of 43.4–59.6 mg m−3, was almost completely eliminated by the scrubber after injection of ambient air, but the system failed in tests without air injection. The performance was further enhanced by addition of H2O2. Further approaches in chemical removal of formaldehyde by addition of NH3 or urea failed. The biofilter showed no treatment performance under thermophilic conditions. The amortization time of the scrubber system was less than three years, caused by economic benefits of higher electrical efficiency and substitution of the catalyst.