A model for pulverised fuel production in an air-swept tube mill
Highlights
► Air swept mill model is developed based on PBM grinding model, virtual internal mill classifier and physical external classifier. ► A particle transport model from grind site that is dependent on liner condition is proposed. ► After parameterisation model remarkably predicts real industrial mill operation.
Introduction
For coal power plants, the main objective of grinding is to produce a Pulverized Fuel (PF) product that meets the boiler requirement in terms of size distribution and moisture content. This is normally accomplished in vertical roller mills or tube mills. The Mineral Processing Research Group at the University of Witwatersrand in South Africa has in collaboration with a number of Eskom power plants been gathering data to help develop an air swept tube milling model.
From the data that was available from the plants and the experiments conducted in a pilot mill, key process steps that affect throughput and product size distribution were identified. Each of these process steps were put together to develop a simulator that modelled the measured behaviour well.
A number of researchers have proposed coal models for tube mills (Austin et al., 1984a, Austin et al., 1984b, Kolacz and Sandvik, 1996) which however do not address how liner condition can affect PF production in a mill. It is known that a mill loses capacity by as much as 30% as liner profile changes due to wear and can even be more dramatic if the liner type is changed. The air flow rate, temperature and variation in coal moisture are other issues that have not been addressed before but their impact is critical to the PF production process, and this necessitated the development of a temperature-momentum balance model that interacted with the grinding model. This has been discussed elsewhere (Makokha et al., 2009).
Section snippets
Milling theory
The grinding model must address both the properties of the material being ground and the role of the equipment in the size reduction process. Traditionally, in the coal industry, the Hardgrove Grindability Index (HGI) has been used as an indicator of how easy or difficult it is to grind the coal. The main problem with this parameter is that the grinding mechanism in the test equipment does not represent what is typical in a tube mill. It was thus necessary to base the grinding model on the
Modelling processes involved in PF production
To model PF production we must track the path taken by the coal once it enters the mill and identify key processes involved before the coal exits as PF via an external classifier. It should be noted that any problem with one of these processes can seriously cripple mill productivity even if the rest are functioning well. For example, a soft coal with high moisture even if it grinds easily will still result in overall poor mill performance because the moist fine coal does not transport easily
Air system
Before discussing the application of the model to an industrial mill, attention is drawn to the air flow system shown in Fig. 3.
The hot primary air is introduced to the mill with the new coal feed and the gas rate is directly proportional to the coal feed rate. The recycled oversize material from the classifier also re-enters the mill with the new coal feed. A damper is in place to ensure sufficient drying air goes into the mill while the remainder bypasses the mill and goes directly to the
Conclusion
A model based on various processes of grinding has been developed and after obtaining suitable parameters, encouraging results have been achieved. It must be appreciated that this feat is achieved with the additional burden of modelling the mill temperature and pressure. There is a need to obtain more sampling data for the reject stream before considering ways of improving the model. Testing the model on other plants with different air configuration systems and different coal properties will
Acknowledgement
The financial support from Eskom of this research work is gratefully acknowledged.
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