Assessing Feasibility of Direct Drive Technology for Energy Recovery from Digester Biogas

The Regional Municipality of Waterloo completed a study, with support from Environment Canada, to evaluate the feasibility of using direct drive technology to recover energy from digester biogas. The study identified that a 150 HP engine, operating at 54 % load, could be supported by the estimated gas production of 1560 m3/d with 63 % methane by volume. Potential applications of the direct drive technology included an existing raw sewage pump, or a new air blower. Low levels of hydrogen sulfide (4–10 ppmv) and siloxanes (40–50 μg/L) reduced the require gas pretreatment to a chiller to reduce the moisture content.Total project costs for application of the direct drive engines were similar in magnitude, estimated at 1.02 Million (Cdn) and 1.04 Million for the proposed new blower and for the existing raw wastewater pump, respectively. The blower application was favored over the wastewater pump application due to ease of installation and greater flexibility for future biogas production rates. Annual operating and maintenance costs were estimated at 16,850 for both applications. Based on a cost of 0.061/kWh, the net annual savings in electricity by the direct drive engine application was estimated at 56,640, while the value of recovered thermal energy (30% efficiency) was estimated at 25,150, resulting in net annual savings of 64,900. These savings result in a payback period of 15.9 years, which although long is not unreasonable for the investment required, at the current price of electricity. The payback period for the direct drive application would decrease significantly if the price of electricity were to increase. For example, if the price of electricity increased from its current value of 0.061/kWh to 0.10/kWh, assuming 30 % thermal energy recovery, the payback period would decline from 15.9 to 10.1 years.