Case Study: Biosolids Drying Plant
Christchurch City Council
Biosolids Drying Plant Designed, Built And Operated For the Christchurch City Council
Long-term Partnership with Christchurch City Council.pdf
Under Christchurch City Council’s long term vision for the city’s energy future, the Council made the decision to dry and beneficially utilise the city’s biosolids instead of landfilling the dewatered sludge. In addition, it was decided too provide the heat for this drying operation from 100% renewable fuels: using the city’s surplus landfill gas, digester gas, and local wood energy resources.
Energy for Industry Ltd responded to the council’s tender to supply heat under a 15 year Design Build and Operate (DBO) arrangement with EFI providing an innovative solution using a reliability centred design approach consisting of with a two boiler configuration and integrated wood fuel supply.
Located adjacent to the Council’s drying facility, the Biosolids Energy Centre is located and consists of -
- 4.5 MWth wood fuelled three pass water tube boiler with an inclined water cooled pinhole grate, flue gas recuperator for combustion air preheating and a flue gas bag filter;
- 4.3 MWth dual fuelled conventional fire tube biogas/diesel boiler. This boiler can fired on surplus landfill gas, digester gas and diesel or biodiesel;
- A wood fuel reception and handling system with 200m3 on site storage of wood chip suitable for delivery of fuel by chip liner (high capacity) trucks; and
- High temperature hot water reticulation pipe work from the BEC to the two biosolids dryers.
The boilers are arranged in series with hot water flow through both boilers at all times to provide maximum flexibility in operation with wood, gas or a mixture of both fuel types.
To provide a highly reliable supply of good quality wood fuel, essential for meeting the council’s uninterrupted heat supply requirements, EFI has established its own local wood fuel supply chain Wood Energy New Zealand (WENZ).
As part of the supply chain EFI has secured long term access to a number of local raw wood resources including:
- forest residues from nearby Environment Canterbury forestry operations. Most of this material was previously left to decompose in the forest;
- sorted construction and demolition (C&D) wood and green wood wastes from three city transfer stations. This material was previously landfilled;
- green wood from CCC arborist tree removals and trimmings in the Christchurch City area. Most of this material was previously landfilled or hogged and then spread on land; and
- waste stem wood and low grade energy logs harvested from nearby forests.
These raw wood resources are collected and delivered to EFI’s wood fuel yard at Bromley where they are naturally dried, prepared as semi-dry wood fuel and stored ready for use.
EFI’s heat supply solution an excellent example of best practice sustainability at a local level by maximising the beneficial use of the city’s energy rich waste stream and uses these to off-set the use of non-renewable fuels for heating.
This operation is a reference project for other city councils in New Zealand demonstrating best practice for the sustainable management of city waste streams and municipal biosolids.
EFI’s Christchurch wood fuel supply chain will also act as a catalyst to encourage the uptake of renewable fuel heat plants in Christchurch.
