Technical Market Support » Thermal Coal
Within Europe, regulations have been introduced that reduce acceptable levels of dust emissions from coal-fired plant. Such emissions are influenced by a number of factors, but one of the main contributors is carbon in ash. Furthermore, plant operators are required to comply with environmental legislation that sets efficiency targets for plant and the disposal of ashes containing high levels of carbon to landfill is also becoming increasingly difficult. Thus carbon in ash is an important issue for those wishing to sell coals in the European market. It will be important for Australian coal producers and exporters to continue to provide coals that are capable of meeting in full the revised requirements for such plant.
A large European funded project, entitled “Development of a Carbon-in-ash Notification System” (known as the CARNO System), which is funded in part by the European Coal and Steel Community, is developing a system that will ensure dust emissions targets can be met when burning a wide range of coal types. One of the parameters that most influences carbon in ash is how well coal particles of different sizes burn. A key component of the CARNO project is therefore to develop means for evaluating the effect of particle size on combustion performance, with particular reference to predicting carbon in ash levels. The current ACARP project has studied Australian coals in parallel with other internationally traded coals that are currently being burned at utility scale. The combustion characteristics of a range of coal types are being incorporated into the CARNO System, a key feature of which is the ability to combine significant amounts of information on burnout under a range of plant conditions and reduce it to a meaningful carbon in ash reference value for each coal. This project therefore ensures that the CARNO System is capable of evaluating Australian coals, such that they are not disadvantaged by its use. The data generated also provides Australian coal producers with information on how their coals perform.
Six Australian coals, which are under consideration for use in the European power generation market and cover a range of volatile contents, were selected for study. The coals were prepared using a pf mill and aerodynamic classifier to a specification appropriate to a large scale European power plant. A detailed examination of sized fractions of the classified pf was then carried out to identify potential variations in chemical and physical characteristics for different sized particles, including petrography. With one exception for a coal that contained blend components, the variations in composition shed little light on how the different particle size fractions of the coals might perform in a boiler.
The techniques developed in this study supplement existing predictive tools developed from simple correlations based on drop tube furnace input data and potentially form a powerful general method of predicting coal burnout from laboratory scale measurements. The type of information generated by these techniques could be used to improve milling for optimum burnout performance by enabling an operator to adjust classifier settings, optimise mill load (so that the overall load is evenly distributed and individual mills are not carrying out an excessive share of the work) and optimising air to fuel ratios for individual mills.
Diagnostic software (the CARNO Adviser) was developed as part of the wider CARNO project. This software forms the basis of a diagnostic system that is encoded with a series of potential fault conditions that can arise on a full scale boiler. The software can advise an operator on whether the cause of high carbon in ash is plant or fuel related (and if plant related on the aspect of the operation most likely to be the cause). To perform this task, the CARNO Adviser has to be populated with data obtained using a carbon in ash predictor (the CARNO Predictor). The CARNO Predictor is a coal combustion model that includes the methodology adopted in the modelling of the laboratory data obtained for the Australian coals and which enables carbon in ash levels to be indicated for a given coal and furnace set-up. Whilst retaining the option to use less reliable default predictions for uncharacterised coals, the CARNO Predictor has been modified to allow input of drop tube furnace data. CARNO Advisor output has been obtained for a number of known faults and gave qualitatively correct indications of the causes of high carbon in ash when the appropriate input data for that condition was supplied.
The importance of the models forming part of the CARNO System is their ability to utilise substantial input data to provide a single figure for a predicted carbon in ash value. This figure is specific for a particular coal, when fired on a specific power plant. The fully operational CARNO System will be sensitive enough to identify the addition of unreactive components to a blend to bring the overall fuel chemical or calorific composition into specification. This is one of the commonest causes of high carbon in ash levels at full scale.