Oscillating transient flame propagation of biochar dust cloud considering thermal losses and particles porosity
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.
Section snippets
Model description
The schematic diagram of biochar dust cloud is shown in Fig. 1. As seen in this figure, the carbon dust cloud has a radius of and number density of (particles in volume unit). The particles are uniformly distributed in the medium. A hot spot, located in the centre of medium, is used to ignite the two-phase mixture. The particles close to the ignition source are preheated first during this process. This is followed by the oxygen diffusion and, as a result, the heterogenous reaction
Governing equations in particle phase
This section is dedicated to the particle-phase governing equations. It is believed that the particle porosity affects the combustion characteristics of organic dust particles. As such, it is assumed that the spherical biochar particles possess unlimited semi-sphere pores uniformly distributed on the surface. The volume porosity and porosity factor are written as follows [25,26]:
Where , , , , and are volume porosity, particle void
Numerical procedure
The ignition of dust flow is more difficult than a gaseous mixture because the distance between particles are much larger than the mean free path of gases, necessitating a much larger ignition kernel volume and ignition time [41]. A hot spot located at the centre of sphere with 0.8 cm diameter generates 1.6 J energy to ignite the dust cloud. The boundary and initial conditions employed in this research are listed in Table 2. In order to avoid repeating the same boundary and initial conditions,
Results and discussion
The main objective of this section is to analyze the flame propagation mechanism, the structure of a transient flame, the absorption and radiation emission by particles, the thermal losses and porosity of particles in the combustion characteristics of biochar dust particles. For this purpose, it is assumed that the distribution of particles is homogenous, and the oxidizer contains 40% oxygen and 60% nitrogen at atmospheric pressure and ambient temperature. The fuel equivalence ratio and
Conclusion
Transient flame propagation in biochar spherical dust cloud was numerically analysed in this research. Unstable conservation equations for gas and particle phases in a spherical coordinate system, thermodynamic specifications, and gaseous phase reactions with multi-factor transfer specifications were written. Particle phase equations including two-phase interactions in momentum equations were solved by taking into account buoyancy, gravity, Stocks, drag, and thermophoretic (obtained from gas
Declaration of Competing Interest
The authors declare that there is no conflict of interest.
References (42)
- et al.
Reduction of carbon emissions from China’s coal-fired power industry: insights from the province-level data
J. Clean. Prod.
(2020) - et al.
Solid-state dechlorination pathway for the synthesis of few layered functionalized carbon nanosheets and their greenhouse gas adsorptivity over CO and N2
Carbon N. Y.
(2014) - et al.
Reducing greenhouse gas emissions in Sandia methane-air flame by using a biofuel
Renew. Energy
(2018) - et al.
Flexible and portable graphene on carbon cloth as a power generator for electricity generation
Carbon N. Y.
(2018) - et al.
Advanced power generation using biomass wastes from palm oil mills
Appl. Therm. Eng.
(2017) - et al.
Performance assessment of a biomass fuelled advanced hybrid power generation system
Renew. Energy
(2020) - et al.
Logistics cost analysis of rice straw for biomass power generation in Thailand
Energy
(2011) - et al.
Biomass derived porous carbon for CO2 capture
Carbon N. Y.
(2019) - et al.
Thermodynamics analysis of a novel steam/air biomass gasification combined cooling, heating and power system with solar energy
Appl. Therm. Eng.
(2020) - et al.
Combustion behaviour of biochars thermally pretreated via torrefaction, slow pyrolysis, or hydrothermal carbonisation and co-fired with pulverised coal
Renew. Energy
(2020)