Kawerau switched on

The country’s largest geothermal power generation development for two decades was commissioned in August. Energy NZ was there for the final count down.  BY ALAN TITCHALL

The new 90 megawatt geothermal power plant operated by Mighty River Power on the outskirts of Kawerau township is sited so close to the mammoth Tasman Pulp and Paper Mill that it looks like its extension. 

It wasn’t meant to be that way. The first major geothermal development for over 20 years was originally envisaged to sit on the other side of highway 34 on Maori trust farmland. After a year of deep exploration drilling, which started in April 2003, the best steam resource was discovered right on the boundaries of the mill that founded Kawerau township in 1954. Plans were quickly changed and fresh negotiations entered into with surrounding landowners. It took four years to secure access to the geothermal resource via negotiations with the Crown, Ngati Tuwharetoa (Bay of Plenty) Settlement Trust, Putauaki Trust and paper mill owners, Norske Skog Tasman.

Because the location of the plant and size of the overall operation, including the steam field, changed, access agreements had to be secured with a multitude of stakeholders in the area to accommodate the eight kilometres of pipeline network weaving between the production wells behind the paper mill and the power plant, and from the plant to the re-injection wells on farmlands some two kilometres away.

“Finding a route and obtaining access for the pipe work and two kilometres of transmission lines was a huge challenge and task,” says project manager, Paul Ware.

The re-injection pipes, at one stage, dog-leg across a railway line and highway 34 via a lumberyard. Construction involved dealing with Ontrack to pull up the rail track and with Transit to tunnel under the roadway. The transmission lines also extend across highway 34 from the power station, but heads in a different direction. The transmission route to the sub station changed numerous times because of access hurdles.

“It wasn’t a straightforward process, access negotiations and obtaining and seeking resource consents continued over a four year period. Once these hurdles had passed, construction was completed in under two years,” says Ware.

Despite these challenges, construction was on time. The contract for engineering design and construction was awarded to Sumitomo Corporation after an extensive tender process and the size of the project attracted huge international interest. Work began in earnest in January 2007 with 27 subcontracted companies and around 370 workers on the project during peak construction. After extensive testing, including running at full speed (100MW) for two weeks, the plant was handed over to Mighty River Power at the end of August, a full month before the October deadline and within the $300 million budget.

Drilling deep

After extensive explorative drilling, the best resource was found in and around the paper mill.

“Drilling is a risky and expensive game,” Ware reiterates. “Drilling alone cost the project about $50 million. We finished up with six production wells extracting up to 55,000 tonnes of two-phase geothermal fluid a day. This is the maximum volume under the resource consent, which allows for a daily average of 45,000 tonnes a year.”

These wells reach down about two kilometres to geothermal resource in the country’s basement Greywacke where temperatures are between 250 and 350 degrees celsius. The steam resource used by the paper mill for the past half century has been taken from volcanic layers a lot closer to the surface through shallow wells.

“Drilling down to a depth of 2000 metres, is very complex,” Ware adds.

World’s largest dual-flash plant

There are three types of geothermal power generation. Dry steam is the oldest method, involving steam (no water) piped directly from underground to turn the turbine. Flashed-steam plants are used when water temperatures are greater than 182 degrees celsius. The resource fluid flows up through a production well under its own pressure. As the pressure decreases, the water boils or ‘flashes’ to steam – either underground or in a flash tank on the surface. Water and steam are then separated and the steam diverted into the production facility. Some geothermal plants (such as Kawerau) use a dual-flash system, where water is flashed to steam at two stages within the system to produce high and low temperature steam, extracting additional energy from geothermal resource.

The third method is by binary-cycle power plants, designed to use geothermal water at lower temperatures, ranging from about 107 to 182 degrees celsius, to heat a secondary fluid with a lower boiling point. Steam from the secondary fluid is created in a heat exchanger and is then used to turn the turbine.

Geothermal power in New Zealand uses all three technologies. Kawerau is designed as a dual-flash plant, while at Rotokawa near Taupo, Mighty River Power employs a binary cycle where low-pressure exhaust steam heats pentane, a hydrocarbon with the low boiling point of 34 degrees, in a closed circuit producing gas that spins the binary turbines. 

Custom made

The Kawerau Geothermal Power Station plant uses wet steam that is ‘flashed’ at two stages within the system before injected at two different pressures into the turbine. It is the largest such dual-flash plant in the world.

“No geothermal power station is the same,” says Ware. “It depends on the steam resource.”

Only the Fuji turbine and generator at Kawerau is bog standard, he says, the rest of the plant has been customised for the local resource and site.

“At the Rotokawa field the resource is different and for the new station Nga Awa Purua that we are developing in conjunction with the Tauhara North No2 Trust, we will be using triple-flash – three steam pressures. That’s because the resource is high pressure.”

Fluid from the Kawerau production wells is part steam and part brine and is processed through complex two-phase flow into the main steam lines (at 20 bar). The fluid enters a high-pressure separator (making up a sizable chunk of the plant) and steam comes off at about 13 bar while the brine hits the bottom of the separator. After the brine is cooled to about 115 degrees celsius, it drops down into six re-injection pumps that boosts pressure up to 15 bar, depending on projection capacity of the re-injection wells some two kilometres north east from the plant.

“Around 75 percent of the fluid resource is re-injected back, which helps to support the resource underground and avoids contaminating surface waters with dissolved chemicals,” says Ware.

After the water has been separated, high-pressure steam enters the dual-flash turbine at around 12 bar and low-pressured steam at 1.8 bar. Having driven the turbine, the steam then enters a condenser and the resulting water is pumped to the cooling tower and a ‘direct control’ cooling system.

“This a close-circuit cooling system where water from the bottom of the cooling tower (at 25 degrees celsius) is sprayed directly onto the steam coming from the turbine. The condensed steam is pumped back to the top of the cooling tower where it drops down and recycled to the condenser.”

The circuit handles a massive 23 million litres of water an hour. Vapour escapes into the atmosphere from the top of the tower and a small amount is siphoned off into the re-injection wells, Ware adds.

The four re-injection wells are up to 2.5 kilometres deep into the base greywacke strata. This is to fulfil a strategy of heat re-injection to support the resource, Ware says.

“The re-injection point is below our take point. We know the resource flow starts in the south from under Mount Putuaki and, as it travels north, it cools and sinks back down again. We know this through temperature profiles we have conducted throughout the strata. We are essentially mimicking the natural convection with the re-injection and supporting the whole resource area.”

Generation from the 90MW plus power station meets around one-third of residential and industrial demand in the eastern Bay of Plenty region, including the neighbouring pulp and paper mill, which would normally have to import electricity some distance from the centre of the North Island (losing between five and seven percent over the transmission). Mighty River Power has a ‘lease/power supply’ deal with Norske Skog Tasman extending over the next 15 years.

Ware says the operators expect the plant to generate on average 90MW. “It’s a more valuable power resource, compared to other renewables such as hydro and wind that are weather dependant. Steam is a continuous baseload resource that can operate at full capacity.”

Mighty River Power now has operations over four Volcanic Plateau geothermal fields (Kawerau, Rotokawa, Mokai and Ngatamariki), all in partnership with local Maori trusts, who often own the land and therefore access to the steam resource. Unlike mineral resources, inherently owned by the Crown, with access determined by permit, access to the underground geothermal resource is controlled by the landowner.

Mighty River Power says it has plans to have operational around 400MW of geothermal energy within the next five to ten years and says it has identified further potential resource.

“We have learnt a lot developing this Kawerau resource,” says Ware. “And we will be applying a lot of these lessons to future projects, including Nga Awa Purua (in partnership with Tauhara North No.2 Trust.”