Análise energética e exergética de um processo de valorização de biogás

Abstract

Modern societies face multiple energy-related problems and challenges. In this regard, exploitation of renewable energy sources and increases in the efficiency of energy utilization provide mitigating solutions. The quantification of the quality of different energy forms, that is, their “work potential”, has resulted in the definition of the property exergy. Exergy analysis is a useful method for the design, evaluation, and improvement of energy systems, that uses the conservation of mass and conservation of energy principles, together with the Second Law of Thermodynamics. In this dissertation, a cogeneration system run with the biogas produced in a landfill is studied by First and Second-law analyses. Combined heat and power systems are a technology for power generation with high energetic efficiency, that leads to savings in primary energy sources, and to lower environmental emissions. The harnessing of landfill gas (biogas) as a fuel produces useful energy, while protects the environment from greenhouse emissions. The objectives of this study were the following: to understand the conversion of the chemical energy in the biogas into electrical energy; to perform a First and a Second Law analyses of the cogeneration plant; to identify locations where irreversibilities occur and to evaluate their magnitudes; and to assess the energy and exergy efficiencies of the system and of its constituent units. The results show that the overall-plant exergy efficiency is 36,5%, which is far from the thermodynamic ideal limit. The internal combustion engine (MCI) and one of the radiators (PC3a), are the most inefficient units, as judged by the parameters degree of thermodynamic perfection and exergy destruction quotient. Based on the rational efficiency parameter, the most inefficient devices are a subunit of a heat exchanger (PC1b) and the internal combustion chambers (MCI). This unit has a limited potential for improvement, due to the fact that combustion is a highly irreversible process. The main potential for improvement in the plant is the recovery of the energy in the exhaust gases. The recovery of the heat removed by the low-temperature refrigeration circuit seems not justified. Future work can comprehend an economic analysis using the tools of exergoeconomics.

Date of Award	2012
Original language	Portuguese
Awarding Institution	Universidade Católica Portuguesa
Supervisor	João Ferreira (Supervisor)