Technical Market Support » Thermal Coal
Several theoretical and analytical tools to characterise ash-related problems in pf boilers, including slagging, fouling, erosion and heat transfer, have recently been developed. This study details the evaluation of these tools, particularly in distinguishing the impact of coal properties, boiler design and operation conditions on the deposit characteristics. The project centered on six case studies, using samples collected from four domestic and two international power stations, representing a range of coal types and boiler designs.
The project has developed a sequence to apply analytical techniques in a systematic fashion, beginning with an initial evaluation using standard information, then progressing to more detailed deposit characterisation in phase one followed by the application of specific techniques in phase two as required.
The first stage is the collection of standard information on the deposit nature, location and timing, boiler type, design and operation and standard coal analyses. This information includes the coal(s) used prior to the episode, and whether operational changes such as load or air/fuel ratio were associated. In some cases, this information would be sufficient to identify the processes leading to deposit formation and enable remedial options to be identified. However, in other cases the standard information will provide little or no evidence as to the mechanism of deposit formation and deeper analysis will be required.
Application of the advanced techniques in phase one (XRF, XRD and SEM-EDS/EPMA) was of considerable benefit in all six case studies, identifying a possible deposit mechanism and thus remedial action. X-ray fluorescence analysis of the deposits showed the similarity or otherwise of the deposit chemistry to the coal ash chemistry. Quantitative X-ray diffraction analysis of the coal mineral matter was able to identify the mineral species which could give rise to slagging problems, such as potassium-bearing minerals. The application of SEM-EDS/EPMA analysis to the deposits provided valuable textural information and analytical data on glass compositions which could be used to postulate and discriminate between possible deposit mechanisms. For example, in one case study systematic variations in iron and silica were observed in the deposits suggesting initiation due to ultra-fine silica rather than iron.
Progression to phase two (QemSCAN, CCSEM, High Temperature XRD, TMA, FACT) was of further benefit in all cases, in either providing confirmatory data or enabling discrimination between competing models. QemSCAN analysis showed siderite to be the problem phase in one case study. Although quantitative XRD would also have identified siderite, QemSCAN was able to provide additional information on mineral associations that are important in assessing potential slagging behaviour. CCSEM provides similar data but has a more limited range of species which can lead to misidentification; in one case study pyrite was indicated to be an iron bearing phase by CCSEM whereas XRD analysis showed sulphate rather than sulphide to be the major phase. Thermo-mechanical analysis (TMA) proved useful in demonstrating the likelihood of deposit formation at relatively low temperatures in one case, and in another showed that a change in boiler operation was responsible for clinker formation.
The selection of the analytical techniques to be used in phase two should be made following an assessment of information following phase one. Hypotheses involving the formation of sticky ash due to the uneven distribution of minerals in pulverised coal would indicate the use of CCSEM/QemSCAN, the formation of strong deposits due to liquid formation or perhaps high furnace temperatures would indicate the use of TMA and sintering measurements. High temperature XRD is useful in identifying temperatures for formation of liquids from minerals, and TMA will indicate changes in melting when blending or temperature changes are remedial options.
The roadmap can be used to prove or disprove a hypothesis with varying degrees of certainty. The case studies highlighted the need for detailed information about the boiler operations at the time, and before, the deposit formation occurred and appropriate samples of both deposit and feed material. However, the project has demonstrated the value of applying the advanced characterisation techniques in conjunction with the standard information, which rarely results in clear identification of a deposit mechanism.