A beleaguered icon

It’s the first and only coal-powered thermal power station in the country and is probably going to be the last.  By Alan Titchall

On a good day, Huntly power station is capable of producing 1448 megawatts, or 17 percent of the country’s total electricity capacity: 1000 plus megawatts from coal and gas-fired steam plant; 50 megawatts from an open cycle gas turbine generator; and the balance from its new Combined Cycle Gas Turbine plant (branded e3p) that kicked into operation on 30 June 2007.

Huntly currently accounts for around for 50 percent of Genesis Energy’s total output; the rest is generated from its Tongariro and Waikaremoana hydro schemes, the Hau Nui wind farm and a number of co-generation plants.

Although it plays a critical role in supplying our electricity needs in these carbon-obsessed times, and will continue do so for many years to come, Huntly has become a target for environmentalists and an embarrassment to a Government that recently decreed to state-owned electricity generators  – no new baseload fossil fuel generation for the next decade. Huntly is a minor greenhouse sinner in a global energy world largely fuelled on coal and gas. The future economies of the US, India, China, the UK and Australia (80 percent coal-powered) are heavily dependent on future coal-fired electricity generation with an emphasis on improved technology, and gas is the preferred source of new, clean, base load generation in most countries around the world.

Carbon dioxide emissions from Huntly peaked in 2005/06 at 5.6 million tonnes (the world produces around 6.3 billion metric tons, or 6.3 gigatons, every year through the burning of fossil fuels). By the 2006/07-year, Huntly’s carbon emissions had fallen to five million tonnes. The new e3p natural gas turbine saves around 1.8 million tonnes of carbon dioxide emissions per year, says Genesis, or around 20 percent of carbon dioxide emissions from all power generation in New Zealand.

Genesis also has plans to build another gas turbine at Huntley called Unit 7, and a similar Combined Cycle Gas Turbine plant (CCGT) north of Auckland in the Rodney district to secure power to Northland. This was before the release of the New Zealand Energy Efficiency and Conservation Strategy (NZEECS) in September, which discourages base-load fossil fuel generation for the next 10 years. With Labour Party president Mike Williams on the Genesis’ board (deputy chair) those plans are at risk.

Genesis spokesperson Richard Gordon says the design of the plant has been revisited to make them “consistent with the Energy Strategy” before they go to the board for discussion.

Custom-made for Huntly

Built by the old NZED between 1973 and 1985, the original 1000MW steam power plant is made up of four identical 250MW units with combined coal/gas fired combustion engineering boilers made in Canada, and British-made Parson Turbines that generate steam at 540°C and 166 bars (2300 Psi) pressure. After the energy in the steam has been extracted in the turbine, it is condensed back to water and pumped back to the boiler in a closed cycle. Ash from the combustion of coal is removed from the flue gas by electro-static precipitators.

Commissioned as a thermal power station uniquely designed to run on either gas or coal, Huntly has spent most of its time running on cheap supplies of Maui natural gas. When the cost of that gas went up in 2004, it turned to coal. The ‘customised’ gas/coal setup makes the choice of coal supplies very selective. Coal from the South Island, for instance, has too much sulphur, and half the coal (with only three percent ash) is imported from Kalimantan, Indonesia's region of Borneo, and the other half from Solid State’s Rotomanu mine in the Waikato (eight percent ash).

The 50MW gas turbine plant, commissioned in June 2004, is made up of a General Electric gas turbine (LM6000), which drives a generator via a gearbox. It’s the same type of engine that powers a Boeing 747 aircraft; with the difference that it works at full power all of the time, not just during take-offs.

The new e3p, which stands for Energy Efficiency Enhancement Project, started construction in 2004 and is a high-efficiency combined cycle generator made up of: A 250MW industrial gas turbine made by Mitsubishi Heavy Industries; an HRSG or heat-recovery steam generator; a 135MW steam turbine; and a 385MW generator.

The gas turbine, steam turbine and the generator run together as one machine that is 57 percent efficient – compared with 36 percent for the 1000MW plant and 41 percent for the 50MW gas turbine. The unit runs hot gases from the gas turbine exhaust into the HRSG generating steam to drive the steam turbine; effectively using the heat twice.

Genesis spent $500 million on its e3p. Just getting the unit from the wharf in Auckland to Huntly 70 kilometres down SH1 proved a monumental and expensive exercise requiring resource consents from 11 different councils, and strengthening the Tainui Bridge crossing the Waikato River at a cost of $27 million.

Construction delays of six months with this CCGT were costly, as Genesis had secured gas contracts to meet an early-2007 commissioning date. The extra fuel cost reduced the SOE’s profit for the 2006/2007 financial year by $40 million.

Other recent investments have included a water-cooling system. Water from the Waikato River is drawn and used to cool the condensers in the old steam plant and discharged back into the river. Resource consent requires the temperature of the river a kilometre downstream of the station to be less than 25°C. During the summer, when hydro generation drops, the shallow river is very warm anyway. One hot summer day in February 2005 the river temperature hit 24.5 degrees and the station had to drop its production down below 40 megawatts to keep within its resource. After that, Genesis spent $30 million on a cooling tower that now enables at least one 250MW unit to run on full load, irrespective of the river upstream temperature.

Life in the old girl still

The use of cleaner gas rather than coal during the first 20 year of it life has left the original Huntly plant in good shape, particularly the mill and boilers.

The units are completely hauled over every four years with each unit out of action for 12 week during maintenance. In recent years, Genesis has used this downtime to upgrade the control system for each unit from the original analogue to a digital (DSC) system, supplied by German manufacturer Siemens. Unit 3 was completed in March 2006 and Unit 4 will be converted over between December 2007 and April 2008. The unit will be also be extensively overhauled with a life-extending programme that includes 11,000 metres of new stainless steel tubing in its condenser.

Which beggars the question – how much life is left in the old girl and will it remain on coal for the rest of its life?

Genesis’ Richard Gordon says there’s no chance that the old 1000MW plant will revert back to gas – it is just uneconomical. Huntly benefited from unprecedented reserves of Maui gas that came at a cheap price set by the government, he says, and although Genesis owns 31 percent of the Kupe gas field and 40 percent of the Cardiff gas prospect, the cost of exploration and production make gas to expensive to use in the old plant.

However, even on coal, Genesis reckons that with diligent maintenance there’s up to 18 economic years left in the original thermal plant. Retiring Huntly before this period for reserve for use in drought years is not a viable option. Genesis estimates that it will cost between $50 and $60 million a year to keep the station on stand-by, and who would be prepared to pay for its upkeep and state of preparedness?

“Huntly has a role to play in New Zealand’s power production in some shape or form to the end of its economic life,” says Gordon.