Numerical investigation of Solid Recovered Fuels’ co-firing with brown coal in large scale boilers – Evaluation of different co-combustion modes Michalis Agraniotis a, * , Nikos Nikolopoulos b , Aris Nikolopoulos a,b , Panagiotis Grammelis b , Emmanuel Kakaras a,b a Laboratory of Steam Boilers and Thermal Plants/National Technical University of Athens, Heroon Polytechniou 9, 15780 Athens, Greece b Institute of Solid Fuels Technology and Applications/Centre for Research & Technology (CERTH/ISFTA) Hellas, 502 00 Ptolemais, Greece article info Article history: Received 10 September 2009 Received in revised form 13 July 2010 Accepted 15 July 2010 Available online 30 July 2010 Keywords: Solid Recovered Fuel (SRF) Co-firing CFD Pulverised coal (PC) boiler abstract In the current work the co-combustion of Solid Recovered Fuels’ (SRFs’) with brown coal in large scale pulverised coal boilers under different operational conditions is numerically investigated. In order to overcome the difficulty of the complex, inhomogeneous nature of waste recovered fuels, SRF is modelled as a mixture of two different fractions, the biogenic and the plastic one. For each fraction different com- bustion mechanisms are presented, whilst for the first time the proposed combustion mechanism of the plastic fraction is incorporated in a commercial CFD code and validated against available experimental data. A 600 MW e brown coal boiler is simulated as a reference and its operational characteristics are com- pared with parameterised scenarios of SRF co-firing conditions. Based on the numerical results, the opti- mum co-firing concepts regarding the more efficient operation of the boiler (hot spots and fuel’s burnout) are identified, decreasing the environmental impact of the boiler’s emissions. Ó 2010 Elsevier Ltd. All rights reserved. Introduction The reduction of waste amount disposed off at landfills and the promotion of more sustainable waste treatment methods are con- sidered as key elements in the current European waste manage- ment policy. The European Community Strategy for Waste Management, COM (96)/399, sets materials’ and energy recovery from waste as the second most favourable option – after preven- tion of waste production – and the European Waste Landfill Directive (1999/31 EC) sets specific reduction targets of the biode- gradable waste quantities disposed off at landfills. In this frame- work, mechanical treatment of waste and thermal utilisation of waste derived fuels produced in the waste treatment plants be- come of prime importance. In order to promote the standardisation of waste derived fuels and assure fuel quality, CEN and the accord- ing Technical Committee 343 [1] prepares specific standards cover- ing aspects of fuel production and thermal utilisation. All types of waste derived fuels that are in compliance with these under prep- aration standards are characterised as Solid Recovered Fuels (SRF). The SRF term refers therefore to a wide range of waste recovered materials from high calorific fractions of Municipal Solid Waste (MSW) to sewage sludge and shredded tyres. Thermal utilisation of Solid Recovered Fuels from MSW through their co-combustion with brown coal in a large scale pulverised fuel boiler is investigated in the present work. Solid Recovered Fuels that derive from MSW have a high biogenic fraction (50– 70%), since they mainly consist of biogenic components like paper, cardboard, wood and of plastic. The substitution of brown coal by SRF in existing boilers is an advantageous practice in environmen- tal, technical and economic terms. Some of the main advantages of the SRF co-firing practice are: (a) the satisfying environmental behaviour proven in large scale co-firing campaigns, (b) the fossil fuel savings achieved through coal substitution, (c) the potential co-utilisation of increased SRF quantities in large scale boilers even by keeping low co-firing shares, (d) the low installation and oper- ating costs, by using the existing feeding and dosing system for the main fuel, and finally (e) the potential CO 2 emissions savings, due to the biogenic content of SRF. The above parameters were investi- gated in a large scale SRF co-firing campaign taken place in a 600 MW e brown coal boiler in Germany, where satisfying results were generally obtained [2]. One important issue for every co-firing project is keeping stable combustion conditions in the boiler and assuring in the same time complete burnout of the alternative fuel. In the case of Solid Recov- ered Fuels this is a challenging task, due to their inhomogeneous 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.07.030 Abbreviations: HDPE, High Density Polyethylene; LDPE, Low Density Polyeth- ylene; MSW, Municipal Solid Waste; PP, Polypropylene; PSD, Particle Size Distri- bution; PVC, Polyvinylchloride; SRF, Solid Recovered Fuels; UDF, User Defined Functions. * Corresponding author. Tel.: +30 210 7722865; fax: +30 210 7723663. E-mail address: magran@central.ntua.gr (M. Agraniotis). Fuel 89 (2010) 3693–3709 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel