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Grate boilers are widely applied in biomass combustion for heat and energy generation. However, grate-firing systems often suffer from low efficiency and high emission. Computational fluid dynamic (CFD) becomes increasingly popular as a flexible method to obtain the detailed combustion behaviors in the furnace, and to optimize the performance of existing grate boilers. This paper presents efforts toward a reliable baseline CFD model for an industrial biomass-fired grate boiler, where the structure of the fuel-bed was sufficiently considered. The combustion performances in the furnace were investigated experimentally and numerically by coupling the fuel bed and freeboard, to clarify the impact of bed structure on the simulation accuracy. Results show that when the bed structure has been taken into account, the predictions are in better agreement with the various measurements, as the deviations between them are all within 10 %. Moreover, the flame center drops, the total heat transferred to the water-cooled walls increases, and the residence time of flue gas is greatly prolonged from 2.76 s to 3.90 s, resulting in a distinct difference of the temperature and species patterns in the freeboard. The simulations will provide valuable theoretical guidance for the accurate modeling of grate-fired boiler. [Display omitted] • A baseline model is proposed for accurate modeling of biomass-fired grate boiler. • Effect of fuel-bed structure on the simulation of grate boiler is clarified in detail. • Bed structure greatly affects the predicted temperature and species patterns. • Flue gas residence time and heat transfer are also highly impacted by bed structure. [ABSTRACT FROM AUTHOR]

Abstract The subject of this paper is the steam boiler nonlinear mathematical model. The furnace chamber heat transfer is modelled assuming that the flue gas temperature across the entire furnace chamber is constant. Only radiation heat transfer to the boiler waterwalls is modelled. Distributed parameter mathematical models are developed of the steam superheater individual stages: the superheater first stage, the platen superheater, the second and third stage of the superheater. In the same way, the boiler two-stage economizer is modelled. The flue gas and air temperatures downstream the air heater individual stages are determined using the ε-NTU method (effectiveness ε - the number of transfer units NTU). Due to such arrangement of the boiler heating surfaces and the heat transfer nonlinear character, iterative calculations of the steam, flue gas, and air temperatures are necessary. The developed mathematical model makes it also possible to determine temperatures of the tube outer and inner surfaces and external fouling for all heating surfaces of the boiler. The developed non-linear mathematical model of the entire boiler has high practical importance. The proposed mathematical model can be used to simulate the boiler operation in various conditions, allowing to determine and correct selection the parameters of boiler operation. A non-linear mathematical model of the boiler can also be used to design the individual stages of the superheater properly so as to achieve the desired temperature behind the individual stages. Determination of the local temperature of the tube wall allows selecting an appropriate grade of steel for the individual stages of the superheater. Also, the developed non-linear mathematical model can be used for boiler operation monitoring in on-line mode. Highlights • A nonlinear mathematical model of the entire steam boiler was presented. • Mathematical models of individual heating surfaces in the boiler were presented. • Results of the boiler operation simulations were presented. • Results of the boiler economizer operation were presented. [ABSTRACT FROM AUTHOR]