Three-dimensional simulation of the co-firing of coal and biomass in an oxy-fuel fluidized bed
Graphical abstract
Introduction
The oxy-fuel co-firing of coal and biomass in fluidized beds, which combines the CO2 absorbed from the atmosphere during the growth of the biomass with CO2 enrichment via oxy-fuel combustion, is considered a new, clean combustion technology that could be significant for the reduction of carbon emissions. Tan et al. [1,2] called it carbon dioxide negative emission technology. This means of combustion can preserve fossil fuel resources such as coal and deal with the solid waste produced by agriculture and forestry.
Many experiments have been carried out over recent years in order to understand the characteristics of this combustion, including the basic dynamics of combustion (using devices such as thermogravimetric analyzer and tubular furnaces [[3], [4], [5]]) and the behaviour of the combustion in fluidized beds [[6], [7], [8], [9], [10], [11]]. Some interesting results have been obtained via basic research into combustion kinetics. For example, the characteristics of oxy-fuel combustion using coal have been significantly improved by the addition of biomass, including acceleration of the combustion rate [12], an increase in the ignition and burnout properties [13], and a reduction in the pollutants emitted [14]. Experiments that have been carried out using fluidized bed units with different scales have further validated the feasibility of coal and biomass oxy-fuel co-firing technology for stable combustion and CO2 enrichment. Experiments that describe these results include a 0.1 MWth fluidized bed combustion test that was carried out in China [9], a 30 MWth fluidized bed combustion test conducted in Spain [11], and a 0.8 MWth fluidized bed combustion that took place in Canada [1,2]. These studies provide a useful basis for scaling-up the design optimizing the structural parameters of these industrial devices. At present, the demand for numerical simulation as an important “numerical test technology”, matches that of experimental research in the development of fluidized bed oxy-fuel combustion technology.
Until recently, the numerical simulation of oxy-fuel combustion in fluidized beds has been limited [15]. Previous studies have mostly used empirical models (e.g., [[16], [17], [18]]) or Euler–Euler numerical simulations (e.g., [[19], [20], [21]]). These methods of numerical simulation do not fully consider the complex gas-solid flow, mass and heat transfer, and chemical reactions that take place within fluidized beds as dense gas–solid reactors. For example, Euler-Euler approach treats the fluid and solid phases as interpenetrating continua, so it can not consider the collision between particles, the effect of particle size distribution, and so on. To overcome this problem, we recently developed a numerical simulation of oxy-fuel combustion in fluidized beds based on the multiphase particle-in-cell (MP-PIC) scheme (see Ref. [22]), which was successfully applied in the study of the oxy-fuel combustion of coal in fluidized beds of different sizes (e.g., [[23], [24], [25], [26]]).
Following our previous research on the oxy-fuel combustion of coal in a fluidized bed (Powder Technol. 2019, 349, 40–51) [22], this paper presents a 3D Eulerian-Lagrangian model, which is computationally facilitated by MP-PIC, to simulate the co-firing of coal and biomass in an oxy-fuel fluidized bed. In this model, the particle phase is described by the discrete particle method (DPM), while the gas turbulence is solved by large eddy simulation (LES). Different chemical reaction models are used for coal and biomass respectively, including devolatilization, char combustion, volatile combustion and the production of pollutants. The numerical simulation was verified by combustion experiments carried out in a micro-fluidized bed reactor with an online mass spectrometer. It was then used to study the effect of biomass mass ratio, combustion temperature, combustion atmosphere and oxygen concentration on the co-firing of coal and biomass.
The results are beneficial to the structural enlargement and parameter optimization of oxy-fuel fluidized beds. This work should be helpful not only for the development of numerical simulation methods, but also for understanding the complex characteristics of oxy-fuel combustion, especially when co-firing coal and biomass in fluidized beds. Also, the developed simulation method could be used for the fluidized bed oxy-fuel combustion with two different fuels, and even multiple fuels.
Section snippets
Description of numerical simulation
The numerical simulation of co-firing coal and biomass in an oxy-fuel fluidized bed is difficult as it includes complex systems of dense particles (i.e., fluidized media, coal particles and biomass particles), chemical reactions (such as pyrolysis, char combustion and the formation of pollutants), and the transfer of both heat and mass. For numerical simulation of such a complex system, two important aspects need to be considered in the simulation strategy: one, the simulation needs to be able
Validation of simulation
The numerical simulation was verified by combustion experiments carried out in a micro-fluidized bed reactor with an online mass spectrometer. The experimental device and process are described in detail in our previous paper (see Ref. [22]). The entire fluidized bed was surrounded by an annular electric heater and an insulation layer, for maintaining the ambient temperature. The combustor in fluidized bed had a height of 106 mm and a diameter of 10 mm. The fluidized gas (air or O2/N2) was blown
Conclusions
Following our previous research on the oxy-fuel combustion of single fuel particles (coal particles), an MP-PIC scheme-based 3D Eulerian–Lagrangian model was developed for the co-firing of coal and biomass in an oxy-fuel fluidized bed. This model was used to simulate the effects of biomass mass ratio, combustion atmosphere, oxygen concentration and combustion temperature on the co-firing of coal and biomass in oxy-fuel fluidized beds. The main findings are summarized below:
- (1)
The proposed model
Author statement
Qinwen Liu: model building and simulations.
Wenqi Zhong: corresponding author.
Jinrao Gu: data analysis.
Aibing Yu: guide model building, language modification.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Key Program of the National Natural Science Foundation of China (51736002). ABY is also grateful to the Australian Research Council (IH140100035) for financial support.
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