(290h) Optimal Production of Power in a Combined Cycle from Manure Based Biogas
In this work, the production of power using a combined cycle gas turbine/steam turbine for wast heat recovery, which operates with biogas as fuel, is evaluated. The process begins with the production of biogas from pig and /or cattle slurry manure(s) using AD. Afterwards, the gas is cleaned up to remove humidity, hydrogen sulphide, carbon dioxide and ammonia. The cleaned gas (biomethane) is then used in a Brayton cycle (gas turbine) to produce energy. The flue gas that exits the Brayton cycle is typically at high temperature and it is further utilized to produce steam that generates power in a regenerative Rankine cycle (steam turbine). Two alternative steam production schemes are evaluated: either splitting the flue gas to have high temperature gas for the reheating step of the steam or sequential heating up. The model is formulated as a Mixer Integer Nonlinear Programming (MINLP) solved in GAMSÂ® for the optimal production of power and topology selection.
Â For a typical production capacity of manure in farms in Spain, 2.6 MW are produced. The hot flue gas is used in sequence for producing superheated steam and next to reheat up the stream after the first expansion in the steam turbine. While typical efficiencies of biogas combustion engines are from 25-40%, the system presented in this work reaches 65% efficiency from the biogas to electricity. The investment for the plant turns out to be 26Mâ?¬ and the production cost of the electricity is 0.35â?¬/kWh before including the credit from the conditioned digestate, that could be sold as fertilizer. The electricity cost goes down to 0.15â?¬/kWh considering a reasonable credit from the digestate, whose composition depends on the feedstock processed in the facility.
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