Biogas Compressors Project in New Zealand

A Wittig ROW 120 G was installed at the Bromley WWTW in Christchurch, New Zealand to replace troublesome reciprocating compressors. The compressor package takes biogas from anaerobic digesters, via a sand filter and small gas holder and feeds the gas to the power generation room adjacent where there are 3 biogas powered generator sets, 1 large Waukesha and 2 smaller Allen generators with a total installed power output of 2.4 MW. Actual electricity output averages 1.24 MW burning 730 m3/hr of biogas.

The compressor package comprises inlet liquid separator, inlet filter, Witting ROW 120 G gas compressor with 55 kW variable speed drive motor, discharge cooler and condensate separator. A separate control panel with the Variable Speed Drive is mounted within the generator building. The gas is fed to the generator room at 3.5 bar (g) to suit the Waukesha engine and at reduced pressure to suit the 2 Allen generator sets.

Wittig Bromley Facility

This unit has run reliably since 2008 with the only unplanned shut down coming as a result of the September 2010 magnitude 7.1 Christchurch earthquake which cut the effluent flow to the WWTW. In 2010 two similar but smaller ROW 60 G compressors were installed at the Bromley WWTW, this time to take 1000 m3/hr of landfill gas from the adjacent waste site and send it via a newly installed 18 km underground pipe to a new sports centre and a civic centre / art gallery in Christchurch.

These buildings were using LPG and now burn landfill gas in cogeneration engines and boilers. At the Civic Centre waste hot water from the engines provides building heating in winter and feeds absorption chillers providing air conditioning in the summer. This replaces 2.1 million litres of LPG each year with direct savings to local taxpayers of NZ$ 3M. In addition, the City Council sells carbon credits for up to NZ$ 4M each year.