Prediction of Ash Phase Equilibria using FACT Models

The behaviour of mineral matter at high temperatures in coal utilisation technologies is a major issue that influences coal value, marketing and resource utilisation. A wide variety of phenomena, such as the formation of slagging and fouling deposits in pf combustion, agglomeration in fluidised beds, ash slag flow in IGCC and other slagging reactors, and a number of other key high temperature phenomena are directly related to the formation of liquid slag (molten oxides) and to the stabilities of solid crystalline phases. The traditional methods used to characterise the high temperature behaviour of the coal inorganic material are becoming increasingly outdated and are unable to accurately predict the behaviour of ash and slag in coals and coal blends in existing and new power generation technologies.

This project has developed new fundamentally-based, predictive tools for application in coal utilisation technologies. These tools involve the use of the FactSage computer program and new thermodynamic databases. FactSage is an extremely powerful tool that is used to predict thermodynamic equilibrium in these systems. This highly sophisticated tool provides information on the phases formed, their proportions and compositions, the activities of individual chemical components and the thermodynamic properties for all compositions, oxygen partial pressures and temperatures.

For accurate predictions the FactSage program requires accurate and self-consistent thermodynamic databases. The new, state-of-the-art CCSD thermodynamic database for the system Al2O3-CaO-FeO-Fe2O3-Fe2O3-SiO2-MgO-K2O-Na2O has been completed. These are the principle chemical components present in mineral matter in coals. Development of the new CCSD thermodynamic database is a significant step in modelling of coal ash /slag systems. The new database, used in conjunction with the FactSage computer package, can describe the melting behaviour of ash in coals from around the globe, making it a key tool for future coal selection and blending by coal suppliers and utilities, coal mine planning and resource optimisation. The use of the tool will assist the development of new more efficient power generation technologies. The FactSage program and the use of the new databases having a significant impact on coal selection internationally (e.g. CRIEPI, ECN- Energy Research Centre of the Netherlands, EERC- North Dakota Energy & Environmental Research Center, Hongik University in South Korea, Chubu University Japan). FactSage is already being used in the design of step-change technologies in the evaluation and selection of coals for entrained flow coal gasification and blast furnace iron making by research teams in Japan and Korea (CRIEPI, Japan; Ishii Project, Japan; Posco, South Korea). These technologies represent the next generation of energy efficient processes with targets to greatly reduce greenhouse gas emissions and significantly improve energy efficiencies.

The phase equilibrium predictions are now also being used to solve longstanding problems in relation to the physical properties of complex oxide melts in coal ash systems. In particular, it is now possible to interpret and systematically analyse slag viscosity data, and the new thermodynamic models have been used in new advanced viscosity models enabling the viscosities of fully liquid and partially crystallised slags to be predicted.

The CCSD research team has also demonstrated how these advanced tools can be packaged in the form of customised computer software. The software retains accuracy over a wide range of compositions and was developed to provide assistance to coal marketing, sales and mine planning applications. The report describes

• Important ash characteristics that can be predicted using FactSage,
• The new CCSD database, and
• Real and potential applications of the database to problems in coal utilisation.