Green Apples, Oranges, and Bananas – Comparison and Pre-Selection of Digester Gas-Drive Advanced Cogeneration Systems for the CBFT3 Process

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Harnessing available energy from digester gas remains a relatively untapped resource in the wastewater treatment industry. Generating “Green Power” is a major component of the Columbus Biosolids Flow-Through Thermophilic Treatment (CBFT3) project being implemented by Columbus Water Works (CWW). The latest technology in lean-burn, sparkignition combustion engines, including the Advanced Reciprocating Engine Systems (ARES), is being developed by the three major engine manufacturers Caterpillar, Cummins, and Waukesha. Support for the ARES program is being provided by the US Department of Energy.

CWW evaluated the use of ARES engines for combined heat and power (CHP) generation – a.k.a. cogeneration – for its CBFT3 Class A biosolids process at the South Columbus Water Reclamation Facility (SCWRF). The proposed use of the high-efficiency engines using digester gas as a fuel source represents the first such application of this technology in the wastewater treatment industry. In addition to generating about 1.3 megawatts (MW) of electricity
(equivalent to about 40 percent of the SCWRF consumptive use), the cogeneration system will provide sufficient heat to heat the thermophilic digesters greater than 99 percent of the time. Rigorous life-cycle cost analysis presents an attractive economic payback of 8 to 10 years.

This paper describes how CWW used a combination of economic and non-economic factors to evaluate each manufacturer’s system as part of a competitive pre-selection process. In broad terms, the following major criteria were used in the selection: 1) capital cost; 2) 10-year maintenance cost (to be contracted with the manufacturer); 3) projected and guaranteed net electrical efficiencies; 4) adequacy of heat production for the thermophilic digester operation; and 5) overall capacity and level of redundancy provided by the proposed system. The results were used to execute a system pre-selection, confirm life-cycle costs, and connect design teams from the project engineering firm (Brown and Caldwell) and the high-efficiency enginegenerator manufacturer (Cummins Power Generation).

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