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Smouldering peat fires, the largest fires on Earth in terms of fuel consumption, are reported in six continents and are responsible for regional haze episodes. Haze is the large-scale accumulation of smoke at low altitudes in the atmosphere. It decreases air quality, disrupts transportation and causes health emergencies. Research on peat emissions and haze is modest at best and many key aspects remain poorly understood. Here, we compile an up-to-date inter-study of peat fire emission factors (EFs) found in the literature both from laboratory and from field studies. Tropical peat fires yield larger EFs for the prominent organic compounds than boreal and temperate peat fires, possibly due to the higher fuel carbon content (56.0 vs 44.2%). In contrast, tropical peat fires present slightly lower EFs for particulate matter with diameter ≤2.5 μm (PM2.5) for unknown reasons but are probably related to combustion dynamics. An analysis of the modified combustion efficiency, a parameter widely used for determining the combustion regime of wildfires, shows it is partially misunderstood and highly sensitive to unknown field variables. This is the first review of the literature on smouldering peat emissions. Our integration of the existing literature allows the identification of existing gaps in knowledge and is expected to accelerate progress towards mitigation strategies.

A new method is described for calculating flare combustion efficiency (CE) and destruction and removal efficiency (DRE) using a numerical parametric model. The method combines key variables that affect flare performance including the flare vent gas net heating value (NHV), flare design, flow rate, exit velocity, and inert gas composition, alongside the environmental influence of crosswind speed. Each effect is characterized using a parametric model derived from experimental testing data and computational fluid dynamics (CFD). The inclusion of CFD allows the model to be extended into the high-wind conditions that cannot be adequately controlled for in empirical testing yet represent some of the most challenging conditions in which to maintain good combustion. This new parametric model method (PMM) is coupled with ultrasonic flowmeters from which the molecular weight and net heating value of the flare gas can be derived using the vent gas speed of sound measurement. In doing so, this method provides a reliable continuous flare combustion efficiency measure that can be deployed at scale with minimum hardware updates. The system was verified using an extractive sampling method with tests conducted on three full-scale industrial flares including non-assisted, single-arm pressure-assisted, and multi-arm pressure-assisted flare designs. A total of seventy valid test points were carried out with varying flow rate and flare gas heating value, covering a CE range from 46–100%. The uncertainty of the method was assessed using both traditional error propagation and Monte Carlo methodology. The results from the new method agree with the extractive method to within 0.8% in the ≥98% DRE region where flares are expected to operate to limit the impacts of flaring as a source of methane as a greenhouse gas. Uncertainty analysis revealed that the larger DRE discrepancy for DRE ≤ 98% correlates to the measurement uncertainties for both methods.

BackgroundAir quality modelling of smoke from wildfires requires knowledge of emission factors and how these vary.AimsExperimental fires were used to test the variation of emission factors with fuel load to improve a smoke forecasting model.MethodsGas and particle-phase chemical composition of smoke plumes from laboratory-scale fires was measured with different fuel loads and at different stages of fire progression.Key resultsDifferent fine woody debris loads had no significant effect on the emission factors. In contrast, different stages of the fire produced significant differences in emission factors. The lowest emission factors for most species (except carbon dioxide) were observed during the flaming fire front, which accounted for half the total emissions. Importantly, we found that emission ratios relative to carbon monoxide were consistent across different fuel loads and throughout the fire’s progression.ConclusionsBy modelling the modified combustion efficiency, it is possible to simultaneously model the emissions of carbon monoxide and the emissions of nearly all other pollutants throughout the progression of a fire.ImplicationsThe emissions of carbon monoxide, fine particulate matter and other key pollutants all scale with the intensity of the fire, simplifying the task of modelling these emitted pollutants downwind of prescribed fires.

Reactivity controlled compression ignition (RCCI) is a potential low-temperature combustion (LTC) technique for running intrinsically efficient compression ignition engines while reducing the oxides of nitrogen (NOx) and particulate matter (PM) emissions. However, poor low-load combustion efficiency is a major challenge in the RCCI strategy. In this work, a combination of injection strategy and cold and hot exhaust gas recirculation (EGR) strategies were investigated to improve the low-load combustion efficiency of a production light-duty compression ignition engine operating in the gasoline-diesel dual-fuel RCCI mode. The engine was operated at a low load of 3 bar gross indicated mean effective pressure and at an engine speed of 1500 rpm with wide ranges of single and multiple fuel injection strategies. Significant improvement in combustion efficiency was achieved by targeting the directly injected diesel fuel in the piston lip region. Multiple fuel injection strategy in which more than 50% of the diesel fuel was targeted in the squish region was beneficial in terms of NOx, total hydrocarbon (THC), and soot emissions. RCCI operation with cold EGR, at the optimum injection timing, resulted in more than 96% reduction in engine-out NOx emissions (<20 ppm, 0.4 g/kWh) with near-zero soot (0.001 g/kWh) emissions with indicated thermal efficiency (46%), similar to conventional diesel combustion (CDC). Increasing the reactivity of the gasoline-air mixture, with the optimum distribution of the diesel fuel between the piston bowl and squish regions, reduced THC emissions by 75% and carbon monoxide (CO) emissions by 30% and improved the combustion efficiency by ~25.3% points compared to the baseline dual-fuel operation.

Thermochemical recuperation heat recovery is an advanced waste heat utilization technology that can effectively recover exhaust waste heat from oxy-fuel Stirling engines. The novel combustor of a Stirling engine with thermochemical recuperation heat recovery system is expected to utilize both reformed gas and diesel fuels as sources of combustion. In this research, the effects of various factors, including the H 2 O addition, fuel distribution ratio (FDR), excess oxygen coefficient, and cyclone structure on the temperature distribution in the combustor, combustion emissions, and external combustion system efficiency of the Stirling engine were experimentally investigated. With the increase of steam-to-carbon ratio (S/C), the temperature difference between the upper and lower heating tubes reduces and the circumferential temperature fluctuation decreases, and the combustion of diesel and reformed gas remains close to complete combustion. At S/C=2, the external combustion efficiency is 80.6%, indicating a 1.6% decrease compared to conventional combustion. With the increase of FDR, the temperature uniformity of the heater tube is improved, and the CO and HC emissions decrease. However, the impact of the FDR on the maximum temperature difference and temperature fluctuation across the heater is insignificant. When the FDR rises from 21% to 38%, the external combustion efficiency increases from 87.4% to 92.3%. The excess oxygen coefficient plays a secondary role in influencing temperature uniformity and temperature difference, and the reformed gas and diesel fuel can be burned efficiently at a low excess oxygen coefficient of 1.04. With an increase in the cyclone angle, the heater tube temperature increases, while the maximum temperature difference at the lower part decreases, and the temperature fluctuation increases. Simultaneously, the CO and HC emissions increase, and the external combustion efficiency experiences a decrease. A cyclone angle of 30° is found to be an appropriate value for achieving optimal mixing between reformed gas and diesel fuel. The research findings present valuable new insights that can be utilized to enhance the performance optimization of Stirling engines.

Knowing the optimal operating parameters of Stirling engines is important for efficient combustion through adaptability to changed pressures and oxygen atmospheres. In this study, the optimum operating conditions for efficient combustion in a singular Stirling engine combustor at different oxygen atmospheres were investigated and determined. Numerical simulations were performed to investigate the effects of ejection ratio and pressure on combustion performance. In an oxygen/carbon dioxide atmosphere, the results show that increasing the ejection ratio substantially alters the flame distribution in the Stirling engine combustor, increasing heat transfer and external combustion efficiency. In contrast, increasing the ejection ratio reduces the average and maximum temperatures of the Stirling engine combustor. Increased pressure affects the flame distribution in the Stirling engine combustor and impedes the flow and convective heat transfer in the combustor, reducing the overall external combustion efficiency at pressures above 6.5 MPa. In an air/carbon dioxide atmosphere, an increased ejection ratio reduces the average and maximum temperatures in the Stirling engine combustor. However, the overall flame distribution does not change substantially. The external combustion efficiency tends to increase and then decrease because of two opposing factors: the increase in the convective heat transfer coefficient and the decrease in the temperature difference. Increasing pressure inhibits forced convection heat transfer in the Stirling engine combustor, reducing external combustion efficiency, which drops from 78% to 65% when pressure increases from 0.2 MPa to 0.5 MPa.

Improving the NOx conversion efficiency and particulate combustion efficiency under cold-start conditions (low-temperature conditions) is still the main challenge faced by catalytic gasoline particulate filter systems (CGPFs). In this study, the physical and mathematical models of novel CGPFs are proposed based on the computational fluid dynamics software. Then, the models are validated based on experiments, and the performances of conventional and novel CGPFs are analyzed comparatively. The comparison conclusions indicate that the NOx conversion efficiency of the novel CGPFs increases by 3.2% and the particulate combustion efficiency increases by 2.7% under the same operating condition. Finally, the effects of exhaust flow v f , exhaust oxygen mass fraction C o , exhaust NO mass fraction C NO , and electric heating power P e on the NOx conversion efficiency and particulate combustion efficiency are investigated. The weights of each influencing parameter on the NOx conversion efficiency and particulate combustion efficiency are explored by orthogonal tests. The conclusions show that the NOx conversion efficiency is increased by 3.6% and the particulate combustion efficiency is increased by 16.7% compared to the initial condition. This study has an important reference value for improving the purification efficiency of vehicle emission under cold-start conditions.

The mechanism of bonding in biomass pellets is such a complex event to comprehend, as the nature of the bonds formed between combining particles and their relevance to pellet quality are not completely understood. In this study, pure and blended biomass pellets made from Norway spruce and pea starch were characterized using advanced analytical instruments able to provide information beyond what is visible to the human eye, with intent to investigate differences in bonding mechanism relevant to quality. The results, which were comprehensively interpreted from a structural chemistry perspective, indicated that, at a molecular level, the major disparity in bonding mechanism between particles of the pellets and the quality of the pellets, defined in terms of strength and burning efficiency, were determined by variation in the concentration of polar functional groups emanating from the major organic and elemental components of the pellets, as well as the strength of the bonds between atoms of these groups. Microscopic-level analysis, which did not provide any clear morphological features that could be linked to incongruity in quality, showed fracture surfaces of the pellets and patterns of surface roughness, as well as the mode of interconnectivity of particles, which were evidence of the production of pellets with dissimilarities in particle bonding mechanism and visual appearance.

As the main engineering power plant, diesel engines are irreplaceable in the future. However, the stringent emission regulations impose many tough requirements to their developments. Recently, flexible fuel injection strategy has been recognized as an effective technology in creating an advanced spray and mixture formation and improving combustion efficiency indirectly. However, the detailed combustion and emission behaviors under flexible fuel injection are still unknown. Therefore, this paper aims to investigate the combustion and emission characteristics under flexible fuel injection and explore an optimal injection strategy for high-efficiency combustion. A numerical simulation method is conducted by coupling the large-eddy simulation (LES) model and the SAGE combustion model. Then, the spray mixing, combustion flame propagation and emissions formation under various multiple-injection strategies are investigated. Results reveal that initial an ultrahigh injection pressure has a significant influence on the spray’s axial penetration while dwell time mainly affects the spray’s radial expansion. Under an initial ultrahigh injection pressure, the turbulence kinetic energy (TKE) becomes larger, and the vortex motions are stronger, contributing to a better spray turbulent mixing. Meanwhile, a snatchier flame structure with a favorable level of equivalence ratio and a homogeneous temperature distribution is obtained. In this way, the peak heat release rate (HRR) could increase by 46.7% with a 16.7% reduction in soot formation and a 31.4% reduction in NOx formation.

AbstractThis study experimentally confirms the intensification of kerosene combustion in the case where a mixture simulating steam reformation products in a model combustion chamber with a supersonic velocity of the flow at the inlet. It is shown that the used mixture has a higher chemical activity than ethylene. The use of steam reformation products of hydrocarbon or synthetic propellants in schemes with pulse-periodic combustion control increases combustion efficiency without the use of special design solutions for organizing initiation and stable combustion.