Cogeneration processes based on giant reed gasification combined with ORC and district heating for heat recovery: Comparative energy and exergy analysis

This research work develops a comprehensive exergy and energy assessment of a cogeneration process using Giant Reed as feedstock, which consists of a biomass dryer, a fluidized bed gasifier, an internal combustion engine operating in a cogeneration mode (CHP), and two different users for the cogenerated heat. One process layout uses cogenerated heat in a district heating network (CHP + DH layout). In the second process layout, the cogenerated heat produces additional electricity through an Organic Rankine Cycle (CHP + ORC layout). In addition to the performance of reed gasification (cold gas efficiency about 0.6), the results showed that the highest rational efficiency was reached in the cogeneration unit, while the highest relative irreversibilities were found in the gasifier and the dryer. The overall energy efficiencies are 0.46 and 0.22 for the CHP + DH and CHP + ORC layouts, respectively, while the overall exergy efficiencies are 0.21 and 0.20. The difference in the sustainability index is just 2 %. The results and methods of this research work can be used to properly design Giant Reed (or similar biomass) gasification plants and bioenergy systems for combined heat and power production, and developing case-studies considering the sustainable use of this feedstock according to a thermodynamic approach based on the second principle.