Feeding the thermal beast

During the winter of 2008 the Huntly Power Station kept the country’s lights on, running mostly on coal and at maximum capacity to supply electricity to users in both islands. Energy NZ toured around the back of the thermal beast and explored its energy source – the local Rotowaro coalfields.   By Alan Titchall.

Originally designed as a 1000MW thermal power station that could be run on either gas or coal, Huntly Power Station spent the first 20 years of its life running on unprecedented reserves of Maui gas sold at a cheap price set by the government.

Commissioned in 1983, the power station reverted to using the low sulphur sub-bituminous coal from the northern Waikato coalfields in 2004.

Operated by Genesis Energy, Huntly Power Station is capable of producing 1448 megawatts, or 17 percent of the country’s total electricity capacity. More than 1000MW of this capacity is generated by the original steam plant fired on coal. During June 2008, when power was being sent to the South Island to compensate low hydro lake levels, Huntly was burning 10,000 tonnes of coal per day. Over May and July 2008 the weekly average was around 60,000 tonnes, and there was one seven-day week when the furnaces chewed through 70,000 tonnes of coal.

A percentage of this coal is imported from Kalimantan, Indonesia’s region of Borneo, but the lion’s share is mined closer to home – from Solid Energy’s Rotowaro coalfield located 10 kilometres west of Huntly township. The journey from pit to power station is less than 20 kilometres, but involves the heaviest machinery used in New Zealand.

Most of the world’s coals were formed in vast swamps during the Carboniferous period, about 300–350 million years ago. New Zealand coals are much younger, having been mostly formed in the late Cretaceous and Tertiary periods, 30–70 million years ago, from more-evolved vegetation types containing flowering plants that give our coal some unusual properties, including a low ash content of less than four percent, and in a few cases less than one percent.

Waikato coals are all relatively soft sub-bituminous and the seams in the north of the region are, typically, three to 10 metres thick and, unlike coalfields in other parts of the world, they are broken and divided by faults and are not easy, or cheap, to recover.

Coal production around Huntly is made up of Solid Energy’s Rotowaro Open Cast mine on the west side of the Waikato River, and the Huntly East Underground Mine on the other side of the river. At least 70 percent of Rotowaro’s coal travels by a combination of truck and overland conveyor to feed the Huntly Power Station; another 25 percent travels by train to New Zealand Steel’s Glenbrook Mill closer to Auckland. Both plants were uniquely designed and built to use this particular sub-bituminous coal.

Most of the seams at Rotowaro have now been mined out and the pits rehabilitated back into pastureland, or are in the process of being back-filled with overburden, leaving one last pit known as Awaroa 4. Having been mined since 2004, and producing between 1.5 and 1.9 million tonnes of coal a year from a number of different seams between two and five metres deep, Solid Energy estimates that about nine million tonnes of recoverable is left in Awaroa 4.

Overburden removal and coal extraction is contracted to Aussie mining company HWE Mining, a subsidiary of Leighton Contractors, who employ around 220 fulltime staff working 22 hour shifts, seven days a week. There are another 50 contractors and 45 Solid Energy staff working at Rotowaro.  

This is a deep open cast mine, expected to reach 180 metres at its deepest point, and mining involves an army of excavators ranging in size from 120 tonnes to a massive 400-tonne Komatsu PC4000 (the largest in the country), and a fleet of heavy dump trucks that include the huge 140 tonne Komatsu 730E capable of hauling 210 tonnes of load and chewing through125 litres of diesel a day.

The excavated coal is trucked to a central treatment point at the northern end of Rotowaro where it is dumped into a giant hopper feeding a rotary breaker that reduces the mined coal down into smaller pieces (50mm). At this stage, rock, old wooden pit props, roof bolts, skips and other historic debris from the old days when Rotowaro was mined underground are removed. From here, the coal is transported by conveyor belt to stockpiles at the blending plant, where coal from Rotowaro and Kalimantan is sorted and blended according to ash and sulphur content using a radioactive analyser. Out-going blends are loaded into 200 tonne trucks bins for Genesis Energy, or 1200 tonne rail bins for New Zealand Steel.

For the Huntly coal it’s short ride along a private road from the blending plant to the stockpiles at Huntly West Mine with their combined capacity of 600,000 tonnes of coal. From the 150,000 tonne ‘working stockpile’ coal is move by an overland conveyor belt over farmland to the back of the Huntly Power Station where is stockpiled again.

Heading in the opposite direction in covered trucks is a percentage of the residual coal ash from the Huntly Power Station operation, used as back-fill in the rehabilitation of old pits. Currently this ash is being compacted into the old Awaroa 3 pit at Rotowaro and sealed with a 300mm thick layer of fire clay and then covered with overburden and, finally, a layer of topsoil for planting.

Future consumption

Genesis Energy estimates that Huntly still has about 16 years of life left in it as a thermal power generator, while Solid Energy says Rotowaro only has about nine million tonnes of recoverable coal left, or about seven to eight more years of production, depending on future demand from Genesis Energy. The pit will then be returned to a mixture of pasture and forest and the final void turned into a recreational lake.

However, there’s plenty of coal resource left in the area. The Huntly East Underground Mine, for instance, has another 20 years left at least says Solid Energy. This mine has already extended north to access a further six million tonne of coal to support a new five-year supply agreement with New Zealand Steel.

“We have an ongoing programme of exploration in the area and a couple of other opencast prospects,” says Craig Smith, general manager north operations. One of these new resources is the Maramarua opencast mine about 80 kilometres north east of Huntly next to State Highway 2. Smith says mining operations are likely to resume in the Kopako 1 (K1) pit at Maramarua and possibly extend to K4 ­– a new pit in the planning.

Maramarua was originally commissioned to supply the Meremere power station built in the 1950s, mothballed in 1991 and eventually demolished. Out-putting 210MW, Meremere was the country’s first thermal power station. Around 800,000 tonnes of coal from the Maramarua field supplied the plant with coal via a six-mile long aerial ropeway.

Mining ceased following the power station’s closure, and both K1 and K2 pits were allowed to flood. Once the pits are dewatered, Solid Energy expects to start mining about 300,000 tonnes of coal a year, supplying both Genesis and New Zealand Steel.

The coal resource at Maramarua is of a slightly lower rank (reduced energy per tonne of approximately 10 percent) than that at Rotowaro, but is still a high quality coal, low in ash and sulphur.

Solid Energy is also exploring another coalfield, Ohinewai, just north of Huntly and bordering on the southern west end of Lake Waikare.

“It’s a big resource of around 40 million tonnes and very similar in quality to Rotowaro. However, a mixture of private and crown ownership, peat material on top of the seams, and the proximity of the Waikato river and Lake Waikare creating possible hydrological problems, means that we view this resource as complex and a longer term prospect.”

Then there’s the licence Solid Energy still holds to mine Huntly West, last mined about eight years ago, and the stockpile site for Rotowaro coal heading for the Huntly Power Station.

“Huntly West is a very large resource that we do have plans to mine – most likely as part of our Huntly East (underground) Mine. The licences are pretty much adjacent,” says Smith.