Use of a Laboratory Scale Method for Assessing Ash Deposition Characteristics of Australian Coals in Utility Boilers

The deposition of ash in power station boilers can lead to a loss in efficiency and availability. Also, slag deposited around low NOX burners and on furnace walls can lead to poor boiler performance, high furnace temperatures and increases in NOX emissions. Reliable means of minimising the impact of deposition are therefore of significant importance to the power industry and fuel suppliers.

To minimise the impact of ash deposition related problems most effectively it is essential to be able to predict the performance of a coal under boiler operating conditions. Various approaches have been tried with varying degrees of success. These range from simple indices based upon coal properties, through laboratory and pilot scale testing to full scale plant trials. In general moving up in scale from coal compositional measurements towards full scale has given increasingly reliable information, however the costs also increase very rapidly with scale and the flexibility in numbers and types of samples that can be tested reduces greatly. For example assessment of small samples from boreholes cannot be carried out at large scale. Some tests have been developed only with coals from restricted geographic areas and their wider applicability has not been demonstrated.

Drop Tube Furnace (DTF) based tests previously developed at CRE enabled the lower temperature deposition that occurs beyond the radiant heat transfer zones of boilers to be assessed. However, this test was developed using mainly UK coals. It was validated through testing of coals whose deposition properties had been studied at pilot scale on an ash deposition test rig. The results from this rig had been validated through parallel testing of coals studied under full-scale boiler operating conditions. Within this project it was intended to broaden the applicability of the existing testing approach in two ways. Firstly, the testing regime needed to be extended to enable deposition behaviour under conditions typical of the radiant heat transfer zone of boilers to be simulated. Secondly, the range of coals for which the tests were applicable needed to be extended to include those from other world regions. In particular Australian coals, which differ significantly in their mineralogy from the coals used to develop the test, needed to be studied and the results validated. In summary the project objectives were:

• Identification of a range of relevant Australian coals and characterise their mineralogy and fouling characteristics using an existing DTF based methodology.
• On the basis of a critical review of techniques for assessing slagging characteristics develop a small-scale slagging test by modification of the current test procedures and equipment and to validate it through comparison with pilot and full scale plant data.
• To use this test to predict the deposit characteristics of the suite of coals.

Three Australian coals were identified and samples provided by Australian coal producers. These coals were characterised by standard analyses. The test programme then consisted of three parts. First the suite of Australian coals together with standard coals from the UK were tested using the established DTF based test and the potential for fouling type deposit formation assessed.

Secondly work was carried out to develop a DTF test to simulate deposition at higher temperature (higher incident heat flux) so that fused slag deposits could be generated and investigated. This methodology was validated using available CRE 150 kW Rig measurements for a set of standard reference coals. The CRE 150 kW Rig data also provides a link to full scale.

Thirdly the suite of Australian coals was tested using the new test and their likely performance assessed.

The fouling technique employs a metal test coupon, the surface of which is maintained at a controlled temperature typically in the region of 600 ?C and deposits are built up very slowly after preconditioning the surface. Deposit growth rates normalised for coal ash content are calculated and together with simulated soot blowing data give information on the amount and strength of deposits formed. Fusibility is also assessed from photomicrographs of furnace deposit samples.

The slagging technique developed uses a refractory coupon that is assumed to be close in temperature to the gas temperature within the furnace, typically in excess of 1500 ?C. At this temperature slags form readily for those coals that are likely to give rise to problems at full scale. However, much higher coal feed rates must be used to build up sufficiently thick layers of deposit for the test to be completed in a reasonable time scale. Assessment of the deposits is carried out using a scratch test, to subjectively assess the hardness of the deposits formed, and also using Computer Controlled - Scanning Electron Microscopy (CC-SEM). This provides detailed information on the mineral species present, allowing the likely viscosity of potential deposits in a full scale boiler to be assessed, and also provides useful information on the interaction between the deposit and substrate surface.

The following has been concluded from this study:

• The suite of Australian coals could be characterised mineralogically and their fouling characteristics determined using an existing DTF-based methodology together with microscopy of the deposits formed. The DTF produces deposits comparable with those produced by a 150 kW test facility and provides information on the effect of coal quality on deposit strength and surface adhesion by comparison with data for a number of standard coals studied. Microscopic examination of coals and deposits also gives a good indication of the propensity of a coal to form deposits in a pf-fired boiler. Overall the results confirm the potential for fouling to occur, albeit that the three Australian coals were selected because they were known to give problems under some circumstances.
• Techniques for evaluating slagging propensity were assessed and the most appropriate approach for implementation to the drop tube furnace was identified.
• A slagging test has been developed that gives rise to the formation and permits the evaluation of more consolidated slagging deposits.
• The approach developed relies upon comparison of standard coals with know pilot and full scale performance and detailed microscopic analysis of the deposits shows that the methodology produces material characteristic of the advanced stages of boiler deposition.
• A simple deposit hardness test and detailed assessment by CC-SEM provide data that is in good agreement with pilot scale data for standard coals.
• The results are also consistent with available full-scale and pilot scale data for the Australian coals

Preparation techniques, such as washing and screening, offer means that coal suppliers may use to reduce ash contents and help operators minimise costs. In principle the reduction in the quantity of ash available for deposition should reduce ash deposition problems. However, there is some evidence from these studies that simply reducing ash content may have adverse effects on deposit properties. This is because coal-cleaning techniques may preferentially remove only some mineral components leaving relatively high concentrations of 'problem' minerals. This requires further investigation and a thorough evaluation of the DTF technique for studying cleaned and blended coals. If applicable it would provide a useful technique for establishing coal modifications which can be used to produce low slagging coals.