Advanced modelling and testing of a 13 MW th waste wood-fired grate boiler with recycled flue gas Boštjan Rajh a,⇑ , Chungen Yin b,⇑ , Niko Samec a , Matjaz ˇ Hriberšek a , Matej Zadravec a a Faculty of Mechanical Engineering, University of Maribor, 2000 Maribor, Slovenia b Department of Energy Technology, Aalborg University, 9220 Aalborg East, Denmark article info Article history: Available online xxxx Keywords: Waste-fired grate boiler Bed model Radiation modelling Computational Fluid Dynamics (CFD) Flue gas recycling abstract Numerical modelling is widely used in industry for detailed understanding of the combustion process and for appropriate design and optimization of biomass/waste-fired boilers. This paper presents a numer- ical study of a 13 MW th waste wood-fired grate boiler, based on the coupled in-bed fuel conversion mod- elling and freeboard combustion modelling methodology. A 1D model is developed for the conversion of the waste wood in the fuel bed on the grate, providing the appropriate grate inlet condition for the 3D simulation of the freeboard region. Since part of the flue gas is recycled into the boiler as an innovative attempt to improve the boiler performance, a refined weighted-sum-of-grey-gases-model of greater accuracy is developed to better address the impacts of the elevated CO 2 and H 2 O vapour concentrations on radiative heat transfer in the boiler. The impacts of full buoyancy on the turbulent flow are also inves- tigated. The temperature profiles at different ports in the furnace are measured to shed some light on the flow and combustion characteristics in the boiler and also to collect some in-flame data for modelling val- idation. The overall modelling strategy, the new sub-models and the use of recycled flue gas are all of great benefit or reference for modelling and design of grate-fired boilers. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Grate-firing is one of the main combustion technologies that are widely used in Waste-to-Energy (WtE) plants for combined heat and power (CHP) production from waste combustion [1]. However, some WtE plants are still reported to suffer from comparatively high loss of ignition, relatively low plant efficiencies due to the high internal energy consumption caused by biomass/waste han- dling [2] and the high excess air often required to attain an accept- able burnout [3], and high emissions. These problems are further complicated by the great diversity in waste and biomass due to the fact that combustion and plant performance are largely affected by fuel properties [4]. Computational Fluid Dynamics (CFD) is a cost-effective tool to achieve an in-depth understanding of biomass boilers and to opti- mize their design and operation for higher efficiencies and lower emissions. Many CFD simulations are successfully performed for biomass conversion for various issues or purposes. For instance, heat transfer of a fouled fin is modelled with the aim to develop a procedure for design of the convective section in a biomass boiler [5]. Biomass co-firing in a cement calciner is modelled to optimize the operation conditions and reduce pollutant emissions [6]. CFD is used as a valuable tool in the development of modern fuel injection systems [7]. A control approach for potential performance improvement of a 75 kW th co-current, fixed bed biomass gasifica- tion process is developed [8]. A generalized numerical framework for biomass fast pyrolysis in fluidized-bed reactors is developed [9], and is formulated into an open-source CFD code based on a multi-fluid model and a global kinetics mechanism [10] in which the predicted tar yields, gas production and unreacted biomass are found to be sensitive to drag coefficient models [11]. In terms of the CFD modelling strategy for biomass/waste-fired grate boilers, two different approaches prevail in literature. The first commonly used way is the coupled in-bed fuel conversion modelling and freeboard combustion CFD, in which a separate model for in-bed biomass conversion, subject to the air or gas flow entering from beneath the grate and incident radiation heat trans- fer onto the top of the fuel bed, needs to be developed to provide the grate inlet conditions to the freeboard CFD. This approach has been successfully used in quite some CFD studies of grate- fired boilers. For instance, the combustion behaviour of a full- scale 10 tonne/h waste incineration furnace is simulated with the focus placed on particle mixing in the fuel bed [12]. Wheat straw combustion in a 108 MW th grate boiler is simulated, in which the results are compared against the in-flame measurement data and http://dx.doi.org/10.1016/j.enconman.2016.02.036 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors. E-mail addresses: bostjan.rajh@gmail.com (B. Rajh), chy@et.aau.dk (C. Yin). Energy Conversion and Management xxx (2016) xxx–xxx Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Please cite this article in press as: Rajh B et al. Advanced modelling and testing of a 13 MW th waste wood-fired grate boiler with recycled flue gas. Energy Convers Manage (2016), http://dx.doi.org/10.1016/j.enconman.2016.02.036