Evaluate Combustion Behaviour of Australian Export and Overseas Low Rank Coal Blends

The increasing growth of the world thermal coal market is seeing more producers from Australia, Indonesia, South Africa, Colombia, USA and Canada entering the market. The coals from these exporting countries differ widely in their chemical and physical properties. These differences are significant for the properties of coals from two main coal producing countries supplying the Asia-Pacific thermal coal market, Australia and Indonesia.

Blending coals of different qualities or ranks is an important procedure at many coal-fired power plants. Carpenter's review (1995) of coal blending for power stations examined the advantages and disadvantages of coal blending. One of the disadvantages identified was that blends of component coals with significantly different petrographic and/or ash properties can cause unexpected operational problems at power stations.

The aim of this project is to investigate the claimed benefits of blending, and better inform Australian thermal coal producers and marketers of the potential impact of blending low rank coals with their coals on the performance of their coal. The specific objectives of this project were:-

• To evaluate the thermal coal performance of a blend of each of four Australian coals (A1, S2, A3, and A4) and two lower rank coals (L1 and L2), a total of eight blends, in the areas of, milling, combustion efficiency, ash deposition behaviour, electrostatic precipitator behaviour, gaseous emissions, including NOx and SO2.
• To evaluate the performance of the six candidate coals unblended, in the areas listed above, providing a reference for comparison of the results obtained for the blends

There is no strong indication that HGI values are additive for blends. There is a strong indication that the lower rank coals have lower power requirements than the (generally) higher HGI Australian coals. There is no strong indication that HGI describes the mill power requirements of the coals and the blends, particularly across a spectrum of coal rank. The mill product from the Australian coals was considerably finer than that of the lower rank overseas coals. For coals tested, a strong correlation was found between the Abrasion Index and mill wear.

While the burnout efficiency results determined for the coals and the blends were not found to be additive, the burnout performance of the Australian coals was improved by the addition of the lower fuel ratio overseas coals. The magnitude of the improvement was variable and was not able to be predicted using typical coal quality indicators, such as fuel ratio. In this study, the Australian coals all had comparatively high ash fusion temperatures and the overseas coals both had comparatively low ash fusion temperatures. The ash fusion results for all of the blends were significantly better than those of the low rank coals, and approached those of the unblended Australian coals. Furthermore, the maximum gas temperatures determined for the lower rank coals, were significantly lower than those determined for the higher rank Australian export coals. The values of ash fusion temperatures and/or predictive indices which are usually seen as being indicative of a high propensity for slagging, may be totally inappropriate for lower rank coals, due to the lower prevailing combustion temperatures with these coals.

The study also showed that adverse heat transfer behaviour associated with the formation of low absorptivity ash deposits characteristic of some Australian coals, can be substantially ameliorated by the addition of small proportions of lower rank coals.

The lower rank coals in a blend greatly improved the precipitator performance of the blend. For the blends, the ESP performance was generally found to be closer to the performance of the better coal in the blend. The poor
Precipitator performance of some Australian coals would be greatly improved through blending with lower rank coals similar to those used in this project.

The SO2 emissions of a coal or a blend can be estimated from the sulphur content of the coal. A more accurate estimate of the SO2 emissions can be made by using the additive law and the SO2 emissions of the component coals. The low sulphur content of Australian coals, especially when considered on a mass per unit energy of coal basis, is a distinct advantage of Australian coals when used in a blend. While it is recognised that equipment design and operating conditions have a significant impact on NOx emission levels, the test results showed that, even under the same operating conditions, there is little, if indeed any, correlation between coal nitrogen and NOx emission levels. The results also showed that NOx control strategy which involves blending of coals to a target coal nitrogen level would have a high risk of failure.