Heat losses in a smouldering system: The key role of non-uniform air flux

Smouldering combustion is emerging as a valuable tool for energy conversion purposes. However, the effects of radial/lateral heat losses, while critical to its viability, are not well understood. It is known that heat losses weaken the smouldering reaction near the walls. It is less known that these losses generate non-uniform air flux across the system cross-section, potentially changing conversion rates and quenching limits. This study integrated: (i) highly instrumented smouldering experiments across numerous scales, (ii) a novel method of estimating non-uniform air flux in the experiments, (iii) analytical modelling to predict non-uniform cooling, and (iv) energy balance calculations to quantify the non-uniform heat of smouldering. Altogether, this work demonstrates that heat loss-induced non-uniform air flux is significant, affecting key smouldering propagation and cooling characteristics. The uniform air flux injected at the base became redistributed with a ~50% decrease at the centreline and a ~50% increase at the wall. This was shown to cause a concave (in the direction of air flow) smouldering front and a concave cooling front. The former was shown to cause radial heat transfer inwards, leading to super-adiabatic heating towards the centre of the reactor. The latter was shown to inhibit cooling along the centreline, which progressed ~40% slower than expected during propagation. Altogether, the multiple and integrated analyses used reveal the magnitude and significance of heat losses in smouldering systems. This insight is valuable to better harness smouldering for engineering applications

Heat losses in applied smouldering systems: Sensitivity analysis via analytical modelling

As applied smouldering systems gain popularity for a variety of energy conversion purposes, there is a strong interest in optimizing the reactor design to support robust smouldering. Heat losses play a critical role in the energy balance of smouldering systems, and therefore have strong implications toward understanding propagation limits and reactor design. Heat losses in an applied smouldering system were approximated by adapting the analytical model from Kuznetsov (1996), originally developed for unsteady local thermal non-equilibrium heat transfer in a porous cylinder, to simulate the cooling zone trailing the smouldering front. The analytical model was adapted to a smouldering system by solving on a domain that lengthens as the cooling zone expands at the rate of the smouldering velocity. The results are incorporated into a global energy balance on the smouldering system, thereby providing an inexpensive and rapid method to estimate the system energy efficiency. Confidence in the analytical model was provided by demonstrating its predictions compare well with existing experimental and numerical estimates of heat losses from similar smouldering systems. The model was then used to quantify the sensitivity of the heat losses to two key reactor design parameters: radius and insulation quality. The system energy efficiency was shown to be highly sensitive to improved insulation and increased radius up to ~0.1 m (i.e., laboratory-sized reactors). However, this sensitivity diminished with size. Beyond 0.4 m radius, the predicted system energy efficiency was high (~85-95%) and relatively insensitive to reactor radius and insulation quality. Therefore, commercial, batch treatment smouldering reactors do not need to be larger than 0.4 m in radius to protect against heat losses and maximize their energy efficiency. This threshold design radius is considerably less than used in current reactors and therefore can provide valuable cost savings

Scaling up self-sustained smouldering of sewage sludge for waste-to-energy

Self-sustained smouldering combustion presents strong potential as a green waste-to-energy technique for a range of wastes, especially those with high moisture content like wastewater sewage sludge. While well-demonstrated in laboratory experiments, there is little known about scaling up this process to larger, commercial reactors. This paper addresses this knowledge gap by systematically conducting and analyzing experiments in a variety of reactors extending beyond the laboratory scale. This work reveals a robust treatment regime; however, it also identifies potential complications associated with perimeter heat losses at scale. Two key impacts, on the smouldering reactions and the air flow patterns, are shown to potentially degrade treatment if not properly understood and managed. Altogether, this study provides novel insight and guidance for scaling up smouldering science into practical, waste-to-energy systems

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

Mortality from gastrointestinal congenital anomalies at 264 hospitals in 74 low-income, middle-income, and high-income countries: a multicentre, international, prospective cohort study

Background: Congenital anomalies are the fifth leading cause of mortality in children younger than 5 years globally. Many gastrointestinal congenital anomalies are fatal without timely access to neonatal surgical care, but few studies have been done on these conditions in low-income and middle-income countries (LMICs). We compared outcomes of the seven most common gastrointestinal congenital anomalies in low-income, middle-income, and high-income countries globally, and identified factors associated with mortality. // Methods: We did a multicentre, international prospective cohort study of patients younger than 16 years, presenting to hospital for the first time with oesophageal atresia, congenital diaphragmatic hernia, intestinal atresia, gastroschisis, exomphalos, anorectal malformation, and Hirschsprung's disease. Recruitment was of consecutive patients for a minimum of 1 month between October, 2018, and April, 2019. We collected data on patient demographics, clinical status, interventions, and outcomes using the REDCap platform. Patients were followed up for 30 days after primary intervention, or 30 days after admission if they did not receive an intervention. The primary outcome was all-cause, in-hospital mortality for all conditions combined and each condition individually, stratified by country income status. We did a complete case analysis. // Findings: We included 3849 patients with 3975 study conditions (560 with oesophageal atresia, 448 with congenital diaphragmatic hernia, 681 with intestinal atresia, 453 with gastroschisis, 325 with exomphalos, 991 with anorectal malformation, and 517 with Hirschsprung's disease) from 264 hospitals (89 in high-income countries, 166 in middle-income countries, and nine in low-income countries) in 74 countries. Of the 3849 patients, 2231 (58·0%) were male. Median gestational age at birth was 38 weeks (IQR 36–39) and median bodyweight at presentation was 2·8 kg (2·3–3·3). Mortality among all patients was 37 (39·8%) of 93 in low-income countries, 583 (20·4%) of 2860 in middle-income countries, and 50 (5·6%) of 896 in high-income countries (p<0·0001 between all country income groups). Gastroschisis had the greatest difference in mortality between country income strata (nine [90·0%] of ten in low-income countries, 97 [31·9%] of 304 in middle-income countries, and two [1·4%] of 139 in high-income countries; p≤0·0001 between all country income groups). Factors significantly associated with higher mortality for all patients combined included country income status (low-income vs high-income countries, risk ratio 2·78 [95% CI 1·88–4·11], p<0·0001; middle-income vs high-income countries, 2·11 [1·59–2·79], p<0·0001), sepsis at presentation (1·20 [1·04–1·40], p=0·016), higher American Society of Anesthesiologists (ASA) score at primary intervention (ASA 4–5 vs ASA 1–2, 1·82 [1·40–2·35], p<0·0001; ASA 3 vs ASA 1–2, 1·58, [1·30–1·92], p<0·0001]), surgical safety checklist not used (1·39 [1·02–1·90], p=0·035), and ventilation or parenteral nutrition unavailable when needed (ventilation 1·96, [1·41–2·71], p=0·0001; parenteral nutrition 1·35, [1·05–1·74], p=0·018). Administration of parenteral nutrition (0·61, [0·47–0·79], p=0·0002) and use of a peripherally inserted central catheter (0·65 [0·50–0·86], p=0·0024) or percutaneous central line (0·69 [0·48–1·00], p=0·049) were associated with lower mortality. // Interpretation: Unacceptable differences in mortality exist for gastrointestinal congenital anomalies between low-income, middle-income, and high-income countries. Improving access to quality neonatal surgical care in LMICs will be vital to achieve Sustainable Development Goal 3.2 of ending preventable deaths in neonates and children younger than 5 years by 2030

Global variation in anastomosis and end colostomy formation following left-sided colorectal resection

Background End colostomy rates following colorectal resection vary across institutions in high-income settings, being influenced by patient, disease, surgeon and system factors. This study aimed to assess global variation in end colostomy rates after left-sided colorectal resection. Methods This study comprised an analysis of GlobalSurg-1 and -2 international, prospective, observational cohort studies (2014, 2016), including consecutive adult patients undergoing elective or emergency left-sided colorectal resection within discrete 2-week windows. Countries were grouped into high-, middle- and low-income tertiles according to the United Nations Human Development Index (HDI). Factors associated with colostomy formation versus primary anastomosis were explored using a multilevel, multivariable logistic regression model. Results In total, 1635 patients from 242 hospitals in 57 countries undergoing left-sided colorectal resection were included: 113 (6·9 per cent) from low-HDI, 254 (15·5 per cent) from middle-HDI and 1268 (77·6 per cent) from high-HDI countries. There was a higher proportion of patients with perforated disease (57·5, 40·9 and 35·4 per cent; P < 0·001) and subsequent use of end colostomy (52·2, 24·8 and 18·9 per cent; P < 0·001) in low- compared with middle- and high-HDI settings. The association with colostomy use in low-HDI settings persisted (odds ratio (OR) 3·20, 95 per cent c.i. 1·35 to 7·57; P = 0·008) after risk adjustment for malignant disease (OR 2·34, 1·65 to 3·32; P < 0·001), emergency surgery (OR 4·08, 2·73 to 6·10; P < 0·001), time to operation at least 48 h (OR 1·99, 1·28 to 3·09; P = 0·002) and disease perforation (OR 4·00, 2·81 to 5·69; P < 0·001). Conclusion Global differences existed in the proportion of patients receiving end stomas after left-sided colorectal resection based on income, which went beyond case mix alone

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

Quantifying oxygen diffusion during thermal degradation of combustible porous media

Oxygen diffusion controlled combustion occurs when local oxygen transport is slower than the chemistry, commonly found in porous combustible material or combustible material embedded within an inert porous medium. This mode of combustion, such as smouldering, can pose dangerous fire risks and also be harnessed in environmentally beneficial applications. However, the oxygen diffusion limitation is poorly understood in all contexts and persists as a key knowledge gap. Quantitative analysis of oxygen diffusion effects is therefore crucial for understanding the combustion behavior of combustible porous media and developing precise smouldering simulation models. In this paper, a reactive transport model incorporating both oxygen diffusion and chemical consumption was developed. Using coal as the model fuel, the impacts of key parameters on global mass loss during the one-dimensional diffusion combustion of coal samples were simulated and compared with TGA experiments conducted within a range of oxygen concentrations between 3–21%. Using this method, key kinetic and oxygen diffusion parameters were obtained within reasonable ranges by using a genetic algorithm optimization method. With these optimized parameters, the local oxygen distribution profiles in the samples at different inlet oxygen concentrations were simulated. The results indicate that oxygen diffusion can lead to large oxygen concentration differences within the coal samples, exceeding 63% of the inlet oxygen concentration. These oxygen differences can impact the local chemistry throughout the sample, and lead to fundamental errors in analyzing global kinetic analyses, if the transport effects are not considered. Altogether, this study delivers new insights into a potentially rate-limiting phenomenon that is relevant in progressing knowledge on many fire problems and engineering applications

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

Downward water mobility in applied smoldering

Applied forward smoldering is used in energy-efficient combustion systems to treat high moisture content waste. However, these systems must be operated in a robust manner far from quenching conditions. Quenching can lead to process failure; therefore, it is critical to accurately resolve water transport in different phases throughout space and time to optimize these smoldering systems. In this work, liquid mobility was integrated into a validated smoldering model that previously only included immobile water. The model was applied to a vertical reactor with an upward propagating forward smoldering reaction. Comparisons between mobility and non-mobility models indicate that the water mobility must be considered to accurately simulate both smoldering ignition and propagation in systems with high water saturations. Water mobility during smoldering can lead to two opposing effects: i) water accumulation at the bottom of the pack, which results in large ignition times or ignition failure; and ii) downward displacement of re-condensed water ahead of the smoldering front, which results in robust propagation with high peak temperatures and front velocities. Finally, a sensitivity analysis revealed the influence of operational parameters on water mobility, which can control the fate of smoldering. Tall fuel pack, high permeability, well-sorted particles, and low capillary-bound saturation were found to significantly accelerate water downward mobility and inhibit ignition. Nevertheless, extending the duration of the initial convective heating process and reducing the packing height favored ignition. These results are highly relevant to current industry applications and address a key knowledge gap in smoldering research