Bioenergy
Mighty River Power generates electricity at two Bioenergy sites: Greenmount and Rosedale (Auckland) and Tirohia (Paeroa). The methane gas produced within the landfills is combusted in turbo-charged gas engines to produce electricity. The process uses a potentially harmful gas that would otherwise enter the atmosphere.
When compacted, rubbish decays in the absence of oxygen, such as in a landfill with an impervious cover of soil, biogas is produced. The gas is typically 55 percent methane, 35 percent carbon dioxide with some nitrogen and other trace gases.
Methane is one of the most harmful of the greenhouse gases and, although emissions reduce over time, it can seep out of landfills for many years.
Over the last ten years, the three bioenergy power stations have produced 95 percent of New Zealand’s total biogas generation.
Electricity is produced from V12 turbo-charged reciprocating gas engines each with a displacement of 115 litres at 1000rpm. Each engine uses 560 cubic metres of Biogas per hour for a net output of 920kW.