Numerical Study of Biomass Grate Boiler with Coupled Time-Dependent Fuel Bed Model and Computational Fluid Dynamics Based Freeboard Model

Abstract

In this paper, a coupled numerical model is developed to provide an advanced simulation for industrial scale grate boilers. The detailed process of biomass conversion in the grate bed is captured using the developed one-dimensional fuel bed model, taking into consideration the separately controlled residence time and primary air supply in different zones. The distributions of gas concentrations, temperature, and bed height along the grate bed are described by the transient simulation results, which are then coupled to the three-dimensional freeboard simulation model as boundary conditions. The results from the coupled model are compared with the on-site measurement data from a wood-chip grate boiler for steam generation, and good agreements are achieved. The in-furnace combustion processes for different quality fuels are evaluated using the model. The results show that higher quality fuel has a larger high temperature combustion zone together with a lower bottom ash temperature, which suggests a higher efficiency. Moreover, a comparison between the standard grate speed control scheme and the modified one shows that increasing the fuel residence time in the first zone has the potential for improving the boiler's combustion efficiency.

Uncertainty Quantification of Biomass Composition Variability Effect on Moving-Grate Bed Combustion: An Experiment-Based Approach

“…A similar method was employed in order to study the effect of variant fuel types on thermal efficiency, and it resulted in a wider high-temperature zone in the overbed region. 10 The fuel bed is the heart of the biomass boiler; hence, the whole operation improvement is strictly subjected to the bed's robust performance. For example, to maximize the flame temperature and minimize emitting CO fraction by regulation of equivalent air ratio, an independent packed bed combustion model was used.…”

“…Meanwhile, it was concluded that a thick layer of biomass bed under the reaction front remains at around 370 K. A 1D fuel bed combustion model was exploited and combined with the overbed combustion model for a complete system analysis. 10 Longer residence time in the front of grate length caused early combustion completion with lower ash temperature and higher overbed temperature. Maẗzing et al 13 employed a quite similar methodology for the investigation of low-rank fuel combustion.…”

“…The ignition front rate speeded up (almost doubled) with CO 2 -rich air feed conditions. A similar method was employed in order to study the effect of variant fuel types on thermal efficiency, and it resulted in a wider high-temperature zone in the overbed region . The fuel bed is the heart of the biomass boiler; hence, the whole operation improvement is strictly subjected to the bed’s robust performance.…”

“…They showed that higher primary air flow rate leads to more CO mass fraction and inversely less CO 2 mass fraction. Meanwhile, it was concluded that a thick layer of biomass bed under the reaction front remains at around 370 K. A 1D fuel bed combustion model was exploited and combined with the overbed combustion model for a complete system analysis . Longer residence time in the front of grate length caused early combustion completion with lower ash temperature and higher overbed temperature.…”

Operation Adaptation of Moving Bed Biomass Combustors under Various Waste Fuel Conditions

“…Larger high-temperature combustion zones were observed as a result of higher quality fuel, and, thus, consequently higher system efficiency. The effect of biomass fuel compositions variability on combustion properties in a moving grate combustor was studied through an independent bed model coupled with a simplified overbed combustion model [19]. The results demonstrated a major impact of composition variability on ignition front rate, conversion time, and output heat flux, as well as a minor impact on the flame temperature.…”