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

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ABSTRACT
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.

INTRODUCTION
Biogas is a renewable resource available at most wastewater treatment plants. However, a recent study has indicated that this valuable resource is not being fully utilized. Excess digester gas is normally flared during warmer months because heat recovery cannot fully utilize the digester gas produced. A recent survey of wastewater treatment plants determined that almost 20% of the total digester gas produced was flared to the atmosphere with no energy recovery (CH2M Hill, 2000).

Direct drive technology bypasses the mechanical-electrical-mechanical energy conversion inefficiencies and the power grid quality problems associated with combined heat and power (CHP) technologies. The direct drive technology is a good fit for a municipal wastewater treatment plant where there is a high demand for mechanical power. In spite of the apparent advantages, the technology has not been widely adopted.

Direct drive technology has a demonstrated advantage in that it can use biogas effectively throughout the year and has a shorter payback period when compared to boilers, micro-turbines, cogeneration units and fuel cells (Enviromega, 2004) (Figure 1). Preliminary cost estimates indicate that direct drive engines are the most economical means of recovering biogas energy, with payback periods as short as three years.

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