The usage of natural gas HHV from small cogeneration systems implemented in a 3 rd generation DH plant

Abstract

Compared with households, Serbian district heating (DH) companies pay higher natural gas and electricity prices. These facts together with the lack of incentives for DH connection and relatively mild winters encourage customers to disconnect and use split systems. To reduce the heating cost and retain customers, DH companies try to reduce operating costs. A promising way is to implement small cogeneration systems that meet their electricity demand. For a case study DH system, the paper analyzes the usage of heat from passive condensation and oil cooling in natural gas-fired cogeneration systems with capacities in the range of 100 kWe to 2 MWe. The available heat from cogeneration systems is modeled based on the manufacturers' data. The case study is a boiler room with a total installed capacity of 37.73 MW and a nominal temperature regime 130/75 ⁰C. As the plant operates with approximately constant water flow and variable supply and return water temperatures, the average temperature regime is 69.4/49.5⁰C for the average outdoor temperature of 5.4⁰C. The average regime and mild weather enable the usage of oil cooling and flue gas condensation from the cogeneration plant. The heat recuperation for variable minimum temperature differences from parallel and in-series connections of an oil cooler (OC) and a flue gas condenser (FGC) based on meteorological data are simulated. Paradoxically, the system is the least efficient during the coldest weather, when the heat from oil cooling is wasted, and FGC operates as a dry flue gas cooler.