Feature

Thermal Options

The debate whether to use renewable or non-renewable resources to power New Zealand is focusing attention on the future role of thermal power stations.

By Iain Murray

Electricorp Production generates 96% of this country's electricity. Thirty hydroelectric power stations account for 75% of this. The remainder is generated by the Ohaaki and Wairakei geothermal stations and six thermal stations.

The thermal power stations -- Huntly, Marsden A, New Plymouth, Otahuhu, Stratford and Whirinaki -- are all in the North Island. They are an integral part of the whole electricity generating network, and are ranked in merit order. The lower in the merit order a station is, the more it is used for back-up purposes.

Otahuhu and Whirinaki are only used in times of extremely high electricity demand. Huntly's efficiency allows it to take a higher load along with hydro stations. This provides needed support when lake levels are low.

The increased availability of key stations -- those which generate electricity at lower cost and with less impact to the environment --  has placed fewer requirements on lower-order thermal plants. This has led to the rationalisation of thermal generating capacity.

Rationalisation has mainly taken the form of reducing plant capacity. Marsden A's generating capacity has been reduced from two boiler/turbine generating units to one. Availability of the plant has been reduced from seven days a week all year to five days a week, eight months a year. Meremere station was recently put into cold storage.

"The generating plant is in good condition. It also has a significant useful life left, so it was decided some plant would be placed in storage for future use," says Brian Cox, development manager of Electricorp Production. "Older plant may not be as efficient as new plant. However, the economics of recommissioning surplus plant make it a sound proposition relative to building brand new thermal power stations."

New Technology

If new thermal plant is built by an organisation wishing to supply electricity to the New Zealand market, it could use up-to-date non-conventional technology.

Steam-injected gas turbines (STIG) are either gas or distillate fired. Exhaust gases are used to generate steam, which is routed through the gas turbine. Using part of the exhaust heat to generate steam makes a STIG more efficient than conventional single cycle gas turbines.

The principle behind inter-cooled steam injected gas turbines is identical to that of STIG units. An intercooling stage during compression increases overall power output.

In gas-fired combined cycle units, gas is burned in a turbine which normally delivers 60-65% of the station's power output. Heat energy from the exhaust gases is then used to produce steam in a heat recovery boiler. This steam then drives a conventional steam turbine.

Gas Or Coal?

It has been calculated that gas supplies for electricity generation could last for up to 25 years.

"Such an availability is only just sufficient to meet the design-life of a new gas-fired thermal generating station," says Cox.

As New Zealand's gas resources are used up, it is likely that lobbying by interest groups against using gas for electricity generation will increase. The use of coal for electricity generation may be less of an issue. Coal is generally not perceived as being as valuable a resource as gas.

Non-conventional technology also exists for coal fired plants. In atmospheric fluidised bed combustion (AFBC), pulverised coal is fed into a bed of sand and lime. Boiler temperatures can then be much lower, 900^oC as opposed to 1250^oC. This dramatically reduces formation of nitrogen oxides and sulphur compounds.

Pressurised fluidised bed combustion (PFBC) uses technology similar to AFBC, but the boiler is kept under pressure. Hot and pressurised gases pass through a turbine. After leaving the turbine, gases go through a heat recovery boiler to produce steam. This is joined by more steam from the fluidised bed boiler, which then drives a steam turbine. Environmentally, PFBC is as acceptable as AFBC and has a higher overall efficiency.

Thermal Versus Hydro

Community concerns over carbon dioxide (CO₂₎ build-up will probably affect the ability to build coal fired power stations. While New Zealand is a very small contributor to world CO₂ build-up, people may still see the issue in a global context. Many will want the country to do its bit in reducing CO₂ emissions.

Moving away from thermal generation would mean hydroelectric stations would have to play an even more important role in electricity generation than at present. There is concern that lobbying against water use for electricity generation could increase the use of thermal power stations. This may have a negative effect on the economy and environment, as more expensive and polluting fossil fuels are burned.

"It has been quite ironical that the use of water to generate electricity has been seen as more environmentally damaging than using non-renewable resources," says Cox.

It is unlikely that new thermal plant will be built for some years. This is due to current levels of demand, the condition of existing plant and the fact that there is 600 megawatts of stored thermal plant which could be recommissioned at low marginal cost. Other factors such as gas supplies, local body resource management, concern over global warming and the price of alternatives will also affect the future role of thermal power stations.

Should renewable or non-renewable resources be used to meet future energy needs? The answer will depend on the way in which producers, consumers and other community groups consult each other to find solutions which best meet their needs.

Iain Murray is a writer for Electricorp Production.