Methane

Natural gas is increasingly used in stationary spark-ignition engines because of its abundance, high octane rating, and lower carbon dioxide emissions than conventional liquid fuels. Blending hydrogen into natural gas pipelines enables large-scale transport, but pipeline compressor engines must run directly on this blended gas. Many prior hydrogen blending studies on such engines have predominantly relied on idealized indoor testing platforms coupled with dynamometer loading to isolate the impact of hydrogen addition; however, this study experimentally evaluated hydrogen blends on a six-cylinder, four-stroke industrially representative engine-compressor unit loaded by a closed natural gas compression loop operating under open-air ambient conditions to analyze the system-level response to hydrogen addition. Additionally, emissions were characterized by using a high-resolution FTIR MKS 2030 MultiGas analyzer. Natural gas–hydrogen blends containing 0–30% hydrogen by volume were tested in 5% increments. Natural gas flow generally decreased with hydrogen addition, while brake thermal efficiency increased at all operating conditions. Relative to the natural gas baseline, methane emissions decreased by as much as 47%, volatile organic compounds by 62%, and carbon monoxide by 94%; however, NOx emissions increased by as much as 324%. Carbon dioxide emissions increased by up to 12% at lower loads and decreased by up to 13% at higher loads. At 30% hydrogen, the equivalence ratio decreased by 6.25% at the lowest tested load and 8.28% at the highest tested load. Hydrogen blending can, therefore, improve efficiency and reduce carbon-based emissions in industrial, rich-burn engines, but the accompanying NOx increase and leaner operation warrant further testing with adjusted spark timing and controlled equivalence ratio.

Atmospheric methane (CH₄) growth reflects both emissions and chemical removal, but their contributions remain difficult to separate because the dominant sink, reaction with hydroxyl radicals (OH), is incompletely constrained. This review links methane sources and attribution with OH variability, methane lifetime, atmospheric oxidation capacity (AOC), and climate. Tropospheric OH accounts for roughly 90% of methane removal. Methane oxidation consumes OH, contributing to a nonlinear feedback that extends its perturbation lifetime. Fire co-emissions, low-NO_x biogenic chemistry, reactive halogens, and climate variability add further spatial and temporal variability. Comparisons of methyl chloroform and hydrofluorocarbon inversions with chemistry models show that inferred OH trends and spatial patterns depend on tracer emissions, transport, and chemical mechanisms. The Amazon and boreal–Arctic regions show contrasting examples of coupled source and oxidant variability. The 2020–2023 methane surge further illustrates the difficulty of distinguishing enhanced wetland emissions from changes in OH-mediated removal. Future methane concentrations and the benefits of mitigation will depend on coordinated changes in methane and reactive precursor emissions. Better constraints on AOC, particularly OH variability and methane–oxidant feedbacks, would improve methane attribution and future projections.

Landfill and dumpsite fires are a recurring but structurally under-addressed dimension of municipal solid waste management, releasing toxic compounds and uncontrolled methane emissions while damaging the infrastructure that would otherwise capture that methane. This study aimed to develop and validate a replicable Fire Risk Index (FRI) for landfills and dumpsites, integrating Hazard, Vulnerability, and Exposure across 22 factors weighted using the Rank Order Centroid method. Vulnerability emerged as the dominant component (combined weight of 0.45), ahead of Hazard (0.33) and Exposure (0.22), pointing to site management, rather than climate alone, as the most actionable lever for risk reduction. The methodology was validated at eleven pilot sites in Chile and Ghana (FRI 1.04–2.06, no site below the medium-risk threshold) and subsequently applied to Chile’s complete national inventory of 104 disposal sites using a three-tiered data collection approach. Under the SSP5-8.5 climate scenario, the climate-driven increment in risk is projected to be uneven across Chile, most pronounced in the Northern Macrozone. A dedicated sensitivity analysis found that three methane-related factors, waste composition, leachate control, and gas capture status, jointly account for 18.9% of the FRI’s total weight and can alone shift a site’s risk classification by a full category. These findings position the FRI as an evidence-based tool linking fire risk management to methane mitigation, with direct applications for landfill licensing, incident reporting, and climate adaptation planning across Latin America and Sub-Saharan Africa.