DEVELOPMENT OF 3-D ANAEROBIC DIGESTER HEAT TRANSFER MODEL FOR COLD WEATHER APPLICATIONS

Abstract

Anaerobic digester heat losses must be minimized to reduce heating requirements in cold weather applications.Digesters must be designed with proper insulation to control manure temperature through a variety of ambient weatherconditions, but the additional insulation to mitigate cold weather operations must not impede digester economics. Thesedesign aspects are difficult to address for distributed power generation for on-farm anaerobic digestion, as these precludethe use of large-scale systems to reduce capital and operating costs. To investigate and address these issues, a 3-Dmathematical model for simulating heat transfer for anaerobic digesters for cold weather conditions is developed and usedto optimize the various geometrical parameters to achieve an adequate design that limits heat losses. An anaerobic digesterheat transfer model based on computational fluid dynamics (Fluent 6.1) is used to calculate the heat transfer through thecover, floor, and walls of a below-ground lagoon-type digester. Simulated heat transfer results are compared to a 1-Dnumerical model and validated against experimental data using an operating plug-flow anaerobic digester. The 3-Dpredictions have the advantage of avoiding space-averaged boundary conditions and can account for conduction in all threedirections in a digester. Simulated results agree reasonably well with the measurements and the one-dimensional model.Numerical simulations are performed for four digester configurations: (1) rectangular with arched top, (2) rectangular withflat top, (3) cylindrical with flat top, and (4) cylindrical with conical bottom. Sensitivity analysis has demonstrated the heatloss through the digester cover, floor, and walls for different geometrical dimensions. Comparisons of the total heat loss showthat the cylindrical digester with a flat top offers the best geometry to minimize heat losses in cold weather applications, andthat the heat-loss-to-biogas-heat ratio (HLB) is an important parameter for characterizing digester operations in cold climates.