Waste heat recovery from a landfill gas-fired power plant

Waste treatment and management is a certain challenge especially in areas with high population density. One of the options for waste treatment is landfilling, where the amount of municipal waste also produces landfill gas through anaerobic digestion. The heating value of the landfill gas is high enough to use it as a fuel in combustion processes, e.g. in internal combustion engines (ICEs) to produce electric power. In Ano Liosia, Athens (Greece) up to 6000 tons of waste are landfilled every day and the landfill gas is used in an ICE power station directly at the site of the landfill. The power station consists of 15 ICEs and has an installed capacity of 23.5 MW. The major advantages of using ICE for power generation are the high electrical efficiency of ICEs and their fast load response. However, more than 50 of the landfill gas energy content is still released to the atmosphere as engine waste heat (exhaust gas and engine cooling water). The aim of this paper is to study the possibilities of using this large amount of heat in order to increase the electricity production and efficiency of the Ano Liosia power station. Therefore, a thermodynamic and economic analysis of two different waste heat recovery (WHR) systems is conducted. The water/steam cycle and the Organic Rankine Cycle (ORC) are examined and evaluated by means of thermodynamic cycle simulation and by calculating their specific costs of power generation. Their advantages and disadvantages considering their application in landfillgas-fired ICE power stations are discussed under the consideration of maximal thermodynamic efficiency and minimal costs of power generation.