Advanced Models for Predicting Carbon Burn Out and NOx Formation in Coal Combustion

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Paper No. 15 ADVANCED MODELS FOR PREDICTING CARBON BURN-OUT AND NOX FORMATION IN COAL COMBUSTION J.M. Jones, P.M. Patterson, M. Pourkashanian, L. Rowlands and A. Williams* Department of Fuel & Energy, Leeds University, Leeds LS2 9JT, U.K. Tel: [phone], Fax: [phone], email: [e-mail] Abstract Current computational fluid dynamic models of coal combustion in industrial combustors usually reduce the complex chemistry into a small number of simplified global reaction steps. The reason for this has been due to the difficulties associated with detailed coal combustion models in computer codes. Thus, predictions of important charactenstics such as carbon burn-out and NOx concentrations were greatly simplified within the models. However, with the advent of more powerful computers and better physicochemical models it is now possible to increase the predictive power of CFD codes. This paper assesses typical CFD models for coal combustion chemistry and identifies approaches towards improved predictions of the following physical models: a) coal pyrolysis behaviour denved from detailed knowledge of the coal structure, in particular: (i) the nature and rate of release of speciated volatiles and soot precursors and their combustion; (ii) the evolution of porosity of the chars (iii) the distribution of fuel nitrogen between char and volatiles b) rate of burn-out of the chars c) unburned carbon in the ash d) NOx formation Such models can be used in both staged and unstaged burners as well as in reburn situations. be given of their mathematical formulation and some examples of their application to the CFD modelling of industrial combustors. 1. Current C F D coal combustion models In the U K at the present time there is considerable interest in modelling coal combustion to improve low N O x pf coal burners. This is the DTI N O x programme which includes both modelling and experimental studies. For C F D modelling of coal combustion the process is usually simplified13 to the following reactions: Step 1: Coal = Char + Volatiles Step 2: Volatiles (HC) + 02 -> CO + H20 Step 3: CO + Vi02 = C02 Step 4: <t>C(char) + 02 -> 2(<\>-\)CO + (2-<j))C02 1.1 Devolatilisation In the first step, the global devolatilisation rate is often simplified to a first order reaction with a single Arrhenius expression: l