Elucidating the characteristic energy balance evolution in applied smouldering systems

Applied smouldering systems are emerging to solve a range of environmental challenges, such as remediation, sludge treatment, off-grid sanitation, and resource recovery. In many cases, these systems use smouldering to drive an efficient waste-to-energy process. While engineers and researchers are making strides in developing these systems, the characteristic energy balance trends have not yet been well-defined. This study addresses this topic and presents a detailed framework to uncover the characteristic energy balance evolution in applied smouldering systems. This work provides new experimental results; a new, validated analytical description of the cooling zone temperature profile at steady-state conditions; insight into the characteristic temperature changes over time; a re-analysis of published data; and a robust framework to contextualize the global energy balance results from applied smouldering systems. Altogether, this study is aimed to support researchers and engineers to better understand smouldering system performance to further the development of environmentally beneficial applications

Modelling oxygen-limited and self-sustained smoldering propagation: Thermochemical treatment of food waste in an inert porous medium

Smoldering treatment is emerging as a valuable engineering tool for many processes, including food waste treatment. However, smoldering systems are currently not well-understood nor optimized. Therefore, numerical models provide invaluable insight into the process dynamics, which improves our understanding and supports the development of novel systems. These smoldering models couple heat, mass, and momentum transfer with pyrolysis and oxidation chemical reactions within porous media. While recent models have untangled many aspects of these systems, local oxygen transport rates from bulk flow to the fuel surface are still not well-resolved. In this work, local oxygen-transport equation was approximated by an analytic derivation based on the gas–solid oxygen non-equilibrium hypothesis. With the improved oxygen-transport equation, a 2D model with five-step reaction scheme for smoldering propagation of food waste in sand was developed. Kinetic parameters obtained from TG experiments were incorporated into the bed-scale smoldering propagation model. The developed model was validated with experimental data that stretched from robust to weak smoldering propagation. It was demonstrated that the developed model matches well with experiments. Furthermore, this model revealed: (i) the emergence of non-uniform gas flow in the reactor, (ii) the evolution of the kinetic- and oxygen-transport-limiting regimes, and (iii) valuable insight into the fundamental changes with smoldering robustness

Smouldering combustion as an emerging technology for contaminated site clean-up: computational simulations

Smouldering, a flameless form of combustion is the governing process of an innovative environmental technology called Self-sustaining Treatment for Active Remediation (STAR). STAR involves the destruction of organic liquids in soil through self-sustained smouldering. A one-dimensional numerical model was developed to simulate the smouldering remediation of bitumen-contaminated sand. A one-step char oxidation mechanism was employed with Arrhenius parameters calculated from thermogravimetric experiments for bitumen under air. Local thermal equilibrium between sand and air was assumed and radial heat losses were considered. Model results were compared to a smouldering experiment using bitumen-contaminated sand. It was found that this simple model reasonably predicted the self-sustaining process: the peak temperatures, the smouldering front velocity, the complete destruction of bitumen, and the temperature decline due to heat losses behind the front. However, it failed to accurately predict the thickness of the front and heat transfer processes in the clean sand behind the reaction zone. This work suggests that more detailed chemical kinetic schemes and local thermal non-equilibrium processes may also be necessary to accurately simulate smouldering remediation of liquid hydrocarbons

Thermal and oxidative decomposition of bitumen at the Microscale: kinetic inverse modelling

Understanding the thermal decomposition of fuels and estimating their kinetic parameters are essential for simulating chemical reactions in numerical models. In this work, 2-step, 3-step, 4-step, and 5-step kinetic mechanisms for bitumen combustion were developed. The kinetic parameters were optimized via inverse modelling (genetic algorithm) by coupling thermogravimetry (TG) and differential thermogravimetry (DTG), conducted at 5, 10, 20, and 40 °C min under nitrogen and air atmospheres. A 3-step mechanism that includes competing pyrolysis and oxidation reactions was identified as the simplest mechanism able to appropriately simulate all TG experiments, thus avoiding the need for more complex mechanisms. A unique set of kinetic parameters was found by averaging all the parameters optimized at different heating rates and atmospheres, resulting in an average error of 6% when compared with experimental data. This is the first time that averaged optimized parameters were employed, providing similar results as optimizing against all experiments at once. Differential scanning calorimetry experiments were used to calculate the heat of pyrolysis and oxidation, and showed that char oxidation provided the highest energy release, whereas bitumen and asphaltene oxidation represented a 30–110 times lower heat of reaction. This is the first time that thermogravimetry and differential scanning calorimetry experiments were used to optimize kinetic parameters for bitumen combustion

Multi-step scheme and thermal effects of coal smouldering under various oxygen-limited conditions

Coal smouldering fires are global disasters and notoriously challenging to characterize. To better understand combustion and chemistry within these coal fires, TG-DSC experiments were carried out in two different coal samples in nitrogen and oxygen-limited (including ambient air) atmospheres. A mechanism with 8-step reactions simulating coal combustion was proposed, including one drying, two pyrolysis, two oxygen adsorption, and three high-temperature oxidations. The kinetic parameters were optimized via a Genetic Algorithm (GA). It was found that this 8-step mechanism created unnecessary overfitting to the TGA data. Therefore, two alternative schemes were developed with 5- and 4-step, which included and neglected oxygen adsorption, respectively. To compare the two schemes, GA and Gaussian multi-modal fitting were used to quantify the mass change rates in each step and their thermal effects as functions of oxygen concentrations. Moreover, the heat flow rates of coal samples were calculated and compared with measured DSC curves. The results indicated that oxygen adsorption could play a critical role in the transition from water evaporation to high-temperature combustion, and the 5-step scheme could reflect coal's intrinsic oxygen-limited combustion characteristics more accurately. Altogether, this study provides novel insight into coal smouldering under oxygen-limited conditions

Investigation of multi-dimensional transfer effects in applied smouldering systems: A 2D numerical modelling approach

Applied smouldering systems are gaining popularity for a variety of energy conversion applications. Radial heat loss plays a crucial role in these systems, as they cause multi-dimensional effects (e.g., in temperature, airflow, and chemical activity). These effects can control system operation limits and performance; therefore, a robust understanding of these multi-dimensional effects is crucial for design engineers. A multi-dimensional applied smouldering numerical model was developed that couples key physics and chemistry. The model was validated against highly instrumented smouldering experiments. The model was then used to qualitatively investigate the multi-dimensional effects and quantitatively analyze the energy balance that dictates the limits of the self-sustaining process. Moreover, a sensitivity analysis of the system energy efficiency, air flow, fuel concentration, and porous medium permeability was completed. The results provide insight into the interconnected nature of key physical (e.g., temperature, air flow, permeability) and chemical (e.g., oxygen concentration, reaction intensity) qualities. Altogether, this work provides a novel tool for investigating, designing, and optimizing smouldering reactors for a range of applications such as soil remediation, waste-to-energy, and improving sanitation in the developing world

Investigation of applied smouldering in different conditions: The effect of oxygen mass flux

Self-sustained smouldering combustion can be a fire hazard, but also a valuable waste-to-energy tool and an effective means for the destruction of organic contaminants. In all contexts, smouldering is an oxygen-limited phenomenon and therefore oxygen mass flux plays a major role. In this study, a multidimensional, thermodynamic-based smouldering model was developed and validated against experiments to quantify the complex interplay between chemical reactions and heat and mass transfer processes. Oxygen supply was independently varied by diluting oxygen mass fraction feeding the smouldering reactions. Smouldering robustness was quantified by local and global energy analyses establishing when a negative net energy balance indicated the onset of quenching. It was confirmed that a diluted oxygen mass fraction resulted in increased heat transfer efficiency driving the smouldering front towards quasi-super-adiabatic conditions. The centreline peak temperature remained almost constant despite a decreasing smouldering front velocity and the weakening local energy balance. Under these unique conditions, key multidimensional heat and mass transfer effects could be explored systematically, showing the displacement of air towards the periphery of the reactor leading to lower peak temperatures and eventually localized quenching. The visual manifestation of localized quenching was an unburnt crust near the reactor wall

Processes defining smouldering combustion: Integrated review and synthesis

Smouldering combustion is an important and complex phenomenon that is central to a wide range of problems (hazards) and solutions (applications). A rich history of research in the context of fire safety has yet to be integrated with the more recent, rapidly growing body of work in engineered smouldering solutions. The variety of disciplines, materials involved, and perspectives on smouldering have resulted in a lack of unity in the expression of key concepts, terminology used, interpretation of results, and conclusions extracted. This review brings together theoretical, experimental, and modelling studies across both fire safety and applied smouldering research to produce a unified conceptual understanding of smouldering combustion. The review includes (i) an overview of the fundamental processes with a synthesis of nomenclature to generate a consistent set of terms for these fundamental processes, (ii) a distillation of ignition, extinction, and transition to flaming research, (iii) a review of the temporal and spatial distribution of heat and mass transfer processes as well as their solution using analytical and numerical methods, (iv) an overview of smouldering emissions and emission treatment systems, and (v) a summary of key gaps and opportunities for future research. Beyond merely review, a new conceptual model is provided that articulates similarities and critical differences between the two main smouldering systems: porous solid fuels and condensed fuels in inert porous media. A quantitative analysis of this conceptual model reveals that the evolution of a smouldering front, while a local process, is determined by a global energy balance that is cumulative in time and has to be integrated in space. As such, the fate of a smouldering reaction can be predicted before the effects of global heat exchange have impacted the reaction. This approach is relevant to all forms of smouldering (including fire safety), but it is particularly important when using smouldering as an engineered process that results in the positive use of the energy released by the smouldering reaction (applied smouldering). In applied smouldering, predicting the fate of a reaction ahead of time allows operators to modify the conditions of the process to maintain self-sustained smouldering propagation and thus fully harness the benefits of the reaction