engleski

Techno-economic optimization of a Rankine cycle used for waste heat recovery in a biogas cogeneration plant

Biogas is one of the most important renewable energy fuels. It is produced from the animals’ manure (and other organic wastes) by anaerobic digestion. In a biogas cogeneration plant located in Croatia, a part of the exhaust gas energy is used for heating buildings within the plant, but still, a certain amount of this energy remains unused. This study aims to examine the utilization of this part of waste heat through the Rankine cycle and organic Rankine cycle using toluene as a working fluid. The unit for waste heat recovery from the exhaust gases is installed at the engine exit, before the heat exchanger of the heating system. In this paper, the technical and techno-economic optimizations of the Rankine cycle and all heat exchangers are compared, taking into account that the function of the heating system should be preserved. In technical optimization, the objective function compromises two different requirements: the maximal power output from the Rankine cycle and minimal heat exchanger surface area, and in techno-economic optimization, the maximal power output and minimal payback period. The optimization procedures are divided into two steps: in the first step is the Rankine cycle, and in the second step, all heat exchangers are optimized. Compact heat exchangers are used, and their sizing is performed by using the Engineering Equation Solver Software, which contains the experimentally measured the friction and Colburn factors. Investment costs for each component of the waste heat recovery unit are calculated using the formulae from the open literature. The optimization procedure is iterative: parameters of the optimized Rankine cycle are forwarded to the procedures for heat exchangers optimization, and heat exchangers parameters are returned to the Rankine cycle optimization procedure. The result of technical optimization is a Pareto frontier of the total outer heat exchangers surface area as a function of obtained power output from the Rankine cycle, and the result of the economic optimization is a Pareto frontier of the payback period as a function of the power output. In the considered example of a biogas power plant, there are two engines. At 100 % engine load, the outflow mass flow rate of exhaust gases from each engine is 1.77 kg/s at a temperature of 410 °C. In the analyzed case, both optimizations show similar Pareto frontiers. The waste heat recovery unit based on the Rankine cycle is more economical than the unit based on the organic Rankine cycle using toluene. When using exhaust gases from two engines, the payback period of the investment is 6.8 years, while the Levelized Cost of Electricity is 0.0419 $/kWh.

Waste heat recovery, biogas engine, Rankine cycle, compact heat exchanger sizing, multi-objective optimization

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