Equipment for measuring the reactivity of chars from highly reactive coals, based on recent Norwegian developments, has been assembled. Measuring techniques are described, and the results of measurement with a range of New Zealand coals are presented. The measured reactivities are compared with results obtained by using the German standard method for reactivity, and the comparison confirms the advantages of the Norwegian method for use with coals of high reactivity. By overseas standards, New Zealand's sub bituminous coals have phenomenally high reactivities. The sub-bituminous coals found in New Zealand have unusual properties in comparison with most other coals. In the Seyler classification they are classed as ortho lignitous; in Mott's classification, as lignitic or lignite. They are high in volatiles. The inherent moisture is 15-20%, the ash 2-8%, and air-dried calorific values are 21-25 MJ kg-l.
These are free-burning coals with no tendency to swell or adhere at carbonizing temperatures. The practical effect of their high reactivity has long been recognised from the relatively low temperature at which combustion occurs, and the ease with which the coals burn as pulverized fuel.
A method developed in Norway by Okstad and Hay' for measuring the reactivity of carbon has proved especially well suited to such material of high reactivity.
In their investigations, Okstad and Hoy determined the reactivity of the char from a New Zealand sub-bituminous coal as 23 ml g-l s-l, which figure compared with 7.3 and 5.9 for two samples of charcoal, and with a range of values from 0.22 to 0.02 for six samples of coal or coke. This phenomenally high reactivity value for these local coals has prompted the interest in further reactivity measurements.
The Okstad and Hoy method relies on a high-precision temperature controller and accurate gas analysis. The modifications described in this paper were designed to avoid the reliance on such relatively sophisticated instrumentation.