OLG provided the MEP design for their combined heat and power (CHP) facility. OLG along with its partners provided a design-build approach for this facility to collect their waste methane to power a Caterpillar engine generator. This engine provides power for the utility grid in addition to hot water for the plant processes, maximizing the energy and Return on Investment for the facility.
General Mills (GM), one of the world’s largest suppliers of snack foods, cereals, and dairy-based products, has two manufacturing facilities (Pillsbury & Yoplait) located in Murfreesboro, TN. Wastewater from plant operations is currently processed by dissolved air flotation to remove the solids and then undergoes aerobic biological treatment prior to discharge. Due to its strength, the yogurt whey produced by Yoplait was trucked off-site for disposal and not treated. However, a new 6.5 million gallon anaerobic digester was just completed that will be used to pretreat all plant wastewaters and the yogurt whey before they are sent to the existing treatment units for polishing and final discharge.
The startup of General Mills’ anaerobic digester occurred in 2014. The initial plans for phase 1 were to flare the off-gases. In phase 2 the plan was to capture the biogas from the process (estimated at 646 scfm, average) and utilizing it in a manner that would provide the greatest financial return. Gas utilization options investigated at a preliminary level included electric power generation via internal combustion (IC) engines with grid sale or direct use on site, including heat recovery for process use through jacket and exhaust gas heat recovery from electric power production units; direct fire to boilers; bioCNG production; micro-turbine generator(s); and preparation of the gas for pipeline sale. Based upon this preliminary analysis and internal review by GM staff, GM decided on further development of a combined heat and power (CHP) option that included electric power generation via IC engines with heat recovery (180°F hot water) for process use in the Yoplait factory.
Electricity at the GM site is provided by Tennessee Valley Authority (TVA); electric demand is approximately 13.0 MW. GM is utilizing the electricity that is produced by the proposed CHP project (± 2 MW) to maximize the internal rate of return either to offset current site demand or through direct grid sale if a more favorable rate for the electric with bundled renewable energy credits can be negotiated with TVA. Due to TVA incentives from grid sale at the time being less than using the power to offset current demand, GM chose to offset the current demand. The generator chosen was a Caterpillar 3520 4160V, 1.6MW enclosed generator feeding a 4160V paralleling gear that fed a 3750KVA, 4160Y/2400:12,470Y/7200V step-up padmount transformer tied to GM’s overhead lines. Reverse power relays were added to prevent generator power from being put onto the local utility grid system.
Project cost $18,000,000