(702c) A Feasibility Study of Biogas Reforming to Improve Energy Efficiency and to Reduce Nitrogen Oxide Emissions

Authors: 
Tsotsis, T., University of Southern California
Dabir, S., University of Southern California
Cao, M., University of Southern California
The feasibility of employing the catalytic steam reforming of biogas to increase its energy content was determined by converting the methane content of it into a hydrogen-rich syngas mixture; and using this reformate product intermixed with raw biogas in an Internal Combustion (IC) engine to enhance combustion stability and to reduce NOX emissions. The field-testing component of the project involved catalytically reforming a side stream of biomethane from a landfill gas collection system at a California landfill via a waste-energy chemical recuperation process, in which waste heat from a gas engine was used to promote the reforming reaction of biogas. In the study, the total flow of raw biogas diverted to the engine remained constant. A fraction of that biogas was, however, separated and directed to a catalytic reactor packed with a Pt/Al2O3 catalyst. The reformer exit stream was then dehydrated (dried), blended with the remaining portion of biogas, and burned in the engine to produce electricity. When operating on the blended biogas mixture, combustion stability was enhanced and the engine ran smoothly at the full speed of 3600 rpm with a 60 Hz output frequency. On the other hand, when burning raw biogas without any reformate gas in the fuel mixture, the test engine ran poorly, sputtering and never reaching 3600 rpm. It was determined that reforming increased the energy value of the biogas by nominally 24% and estimated that a 50/50 blended reformed gas with biogas would have nominally 12% higher energy content. It was also demonstrated that the engine operating on mixtures of reformer gas had significantly lower NOx emissions than when operating on pure propane under the same load conditions. Similar comparative testing could not be performed on raw landfill gas alone because the engine would not operate at full speed, as noted above. The conducted research has validated the technical and economic feasibility of using reforming products during biogas combustion in an internal combustion engine to reduce NOX emissions and to enhance combustion stability. This, in our opinion is an important contribution to the scientific/technical literature and a significant advance for the NOx emissions reduction and the field of biogas and engine waste heat utilization.