Oscillating transient flame propagation of biochar dust cloud considering thermal losses and particles porosity

Abstract

This research investigates the transient flame propagation and oscillation phenomenon in the flame speed of porous biochar dust cloud. Time-dependent mass, momentum and energy equations are solved in the spherical coordinate. The gas phase reaction includes the chemical reactions, thermodynamic properties, and multi-element transition properties. To account for the porosity effects of particles, the biochar dusts are modeled as spherical particles with unlimited number of pores (semi sphere) on the surface. The particle trajectory is governed by the equation of motion. The thermophoretic, gravitational, buoyancy and drag forces are employed in this model. In the energy equation, the absorption and radiation emissions by particles is considered. The results reveal that the inertia differences between the particles and gas causes a difference in the velocities of these two phases at the flame front—which is more evident at the early stages of flame propagation when there is a significant change in the density of dust particles. Moreover, the oscillation is further intensified by enhancing the oxygen concentration due to a higher reaction rate, and, as a result, higher velocity difference between the two phases.

Introduction

A key challenge in front of the researchers is to meet the growing energy demand through a sustainable energy supply pathway as a replacement of fossil-fuel technologies. It has been argued that biomass as an environmentally-friendly energy source may help to abate the undeniable impact of greenhouse gas emissions [1], [2], [3]. However, the direct combustion of biomass for power generation is facing a number of drawbacks including low energy density, low bulk density and high moisture content [4], [5], [6], [7], [8]. Biomass upgrading through torrefaction and pyrolysis are effective approaches to address these challenges [9], [10], [11], [12], [13]. The biochar solid products from these processes are suitable feedstocks for power generation [14]. Therefore, understanding the combustion characteristics of biochar particles is of great importance for a successful transition to a low-carbon energy economy.

A major bottleneck in this system is a strong desire to capture the two-phase reacting flow phenomena given the multiphase nature of such a medium. Gremyachkin [15] introduced a combustion model for a high-porosity carbon particle in oxygen, at which the authors considered the heterogenous and homogenous chemical reaction in particles as well as radiative heat transfer. In a subsequent research, Krainov and Moiseeva [16] conducted a numerical study to simulate the flame front propagation mechanism in coal dust-methane-air mixture in an enclosed volume with an ignition source in the center. Rosas et al. [17] proposed carbon/carbon and zeolite/carbon composites by pyrolytic carbon infiltration of organic and inorganic substrates and different porosity properties. The authors examined the chemical vapor infiltration kinetics of the substrates using a thermogravimetric system at atmospheric pressure. A novel system called hybrid flame analyzer (HFA) was introduced by Rockwell and Rangwala [18] to examine the effect of design variables including the particle size, particle concentration, turbulent intensity and equivalence ratio on the premixed turbulent dust-air flames. In a more recent study by Wang et al. [19] the characteristics of pyrolysis, gasification, and oxy-fuel combustion of coal particles and char in premixed structure using a non-isothermal thermogravimetric analysis was determined experimentally.

Another barrier involved in the combustion of solid fuel particles is to obtain the critical condition at which the solid particles can be completely consumed throughout the process. To circumvent this, Makino [20] found that this critical point highly depends on the heat loss and a comprehensive parameter—which is a function of the combustion rate, oxygen mass fraction and pressure ratio. That was a main initiative for Bidabadi et al. [21] who studied the effects of convective and radiative heat losses on the premixed flames propagation of lycopodium particles in counterflow configuration. In another study, Tufano et al. [22] performed a direct numerical simulation (DNS) of coal particle combustion in laminar and turbulent regimes. The authors analysed the influence of locally turbulent flow on volatile flame interaction and devolatilization. Blake [23] developed a theory for the steady combustion of a spherical carbon particle in the slow viscous flow of an oxidizing ambient. A thorough review paper by Krazinski et al. [24] focused on the flame propagation mechanisms of coal dust particles.

After carefully reviewing the literature, it is apparent that the combustion behavior of biochar particles is relatively less understood in the literature. Moreover, the majority of the numerical and experimental works have assumed equal velocities for the two phases. It is also understood that the current literature has not investigated the radiative emission, absorption, and scattering by the gas and particles. The force interaction between the two phases (i.e. drag, buoyance, gravity, Stocks, and thermophoretic forces) seems to be under-explored in the preceding papers. Taken together, this research aims at proposing an effective, comprehensive, and detailed numerical simulation of oscillating transient flame propagation mechanism of a porous biochar dust cloud under non-adiabatic condition to further enhance the current knowledge in this realm. Mass and species transfer between the two phases are modelled as a result of both gas-phase and particle surface reactions. Moreover, energy transfer between the two phases includes convective, conductive, and radiative heat transfer. The effect of both radiative and convective heat losses on the structure of biochar-fuelled flame and flow strain rate is thoroughly analysed in this study. This model is also equipped with the flame-speed oscillation phenomenon that is observed in the experimental studies of combustion in the two-phase medium.