Feature

Coal Story

Coal has been important in our past, and has an even more important, but changing, role to play in the future.

By P A Toynbee

Coal makes up 95.6% of the recoverable reserves of New Zealand's fossil fuels. If the use of all forms of primary energy continued unchanged at 1986 consumption levels, and if these were supplied from our indigenous resources, coal would last some 1,661 years, while we would run out of oil and condensate in 7 years, and natural gas in 28 years.

New Zealand's traditional use of coal has differed radically from that of most Western countries. Overseas, up to 90% of total coal consumption has been in thermal power generation and conventional steel manufacture. Most of our coal has been used as an industrial fuel and in commercial and domestic heating.

These circumstances are changing today. With all the hydro potential of the North Island fully developed -- and a seeming reluctance to develop other sites -- and with possible growth in the steel industry, we can expect to develop a pattern of energy use similar to that overseas. This change, and the increasing direct use of coal as an industrial fuel, will lead to a substantial growth in overall coal consumption.

Coal Types

The geologically older, `high-rank' coals are the bituminous coals. These are high in carbon, low in inherent moisture and high in heating value. At the other extreme are the young, low-rank lignites, which are low in carbon and heating value, and have a high natural moisture content. In between are the sub-bituminous coals, a type not common in most parts of the world, but which are by far the most-used coals in this country.

The bituminous coals of the West Coast were the first coals to be used to any extent in New Zealand. West Coast coal, especially the export coal, still forms a significant proportion of our total coal production, but emphasis has shifted towards sub-bituminous coals, which the early settlers tended to dismiss as `lignites'.

Our sub-bituminous coals are unusual compared to most overseas coals. They have a high moisture content, a low ash level, are low in sulphur and high in volatiles. They also have high reactivity, free-burning characteristics and relatively low ash-fusion temperatures. Our growing understanding of their unique properties has led to the modification of conventional firing systems and development of new ones designed to exploit the special properties of these coals.

Our most abundant coal reserves are the lignites of Southland and Otago. The lignite energy in Southland alone is equal to that of 20 Maui gas fields. The present consumption of lignite by industry, although increasing, is small and the low calorific value of the lignite makes it uneconomical to transport it very far. There is no doubt, however, that it will be on these lignites that New Zealand will ultimately depend for her energy needs.

In view of the finite and strictly limited reserves of fossil fuels -- which in New Zealand, and universally, are predominantely coal -- the use of these at maximum thermal efficiency becomes a vital issue to this and future generations.

Coal is certainly not the form of energy that suits many of our energy needs but it can be converted to a more desirable form -- to a substitute natural gas with a thermal efficiency of 70%, or to a gas of a lower heating value (80%), to petrol (50%) or to electricity (33%).

Gasification Potential

One such conversion process is gasification, likely to bring the greatest change to the way in which we utilise coal. The efficiency of the conversion process is much greater than that of converting coal to electricity in thermal power stations.

In gasification, the coal reacts at high temperature with steam. It is converted completely to a gaseous fuel, consisting mainly of hydrogen and carbon monoxide. This can be used directly as a fuel, but the low heating value is a disadvantage. More often, it is used as a synthesis gas (as that derived from Taranaki natural gas is used) in the manufacture of petrol, methanol and ammonia, or in the production of methane, as a "substitute" natural gas.

The large-scale use of coal in thermal power generation seems to be generally accepted for New Zealand's future. Such a development is relished by the coal industry and by those wishing to curtail hydro development or to postpone the advent of nuclear power. But a wise, integrated policy for the husbanding of our coal reserves would avoid such a grossly inefficient use of coal. If thermal stations had to provide the electricity currently generated from renewable resources, our coal consumption would increase five fold.

The gasification option can supply a gaseous fuel to replace electricity for domestic water and space heating. This would use half the amount of coal required by a conventional thermal power approach.

This household demand makes up over 30% of our total electricity demand. By transferring this from electricity to gas, we will ensure that the electricity we have available can be used to meet the increasing electricity demands of industry and, inevitably, of transportation, with the minimum use of thermal power generation.

There is one snag. The high efficiency and low cost of this gas manufacture depends on an operating scale far greater than the New Zealand household market could support at present. This scale of operation could be developed rapidly by the active promotion of household gas during the remaining 20-25 years of Maui gas supply. By the time natural gas resources are exhausted, New Zealand could have built up a demand for household gas sufficiently great to permit the economic operation of a large gas-from-coal project.

This is only one example of the need for inspired long-term energy planning. There is a need for co-ordination of the activities of the individual energy industries, whose narrow policies currently have no concern for the national advantage in energy utilisation and energy conservation.

Fellow of the Institute of Energy (UK), Peter Toynbee has spent a working life in energy research, with 20 years as director of the Coal Research Association.