Temporal Variability of Emissions Revealed by Continuous, Long-Term Monitoring of an Underground Natural Gas Storage Facility

Study of spatiotemporal evolution of coupled airflow-gas-dust multi-field diffusion at low-gas tunnel

The increasing level of mechanization in coal mining means more dust and gas are generated during excavation operations in tunnels. The high concentrations of dust and gas severely affect production efficiency and the physical and mental health of workers. Here, Ansys Fluent simulations were performed to derive the spatiotemporal evolution of coupled airflow-dust-gas diffusion in a low-gas excavation face. The aim was to optimize pollution control by determining the optimal duct distance, L, from the working face in the excavation tunnel. Our results showed that the airflow field affects the coupled diffusion and transport of dust and gas. According to a comparison of the effects of different duct distances from the working face, when L = 6 m, the average dust concentration in the tunnel is low (257.6 mg/m3), and the average gas concentration in the tunnel is 0.28 %, which does not exceed the safety limit. Accordingly, the optimal distance of the duct for pollution control is 6 m. The results of field measurements supported the validity of the simulation. Our findings can be used to improve the air quality in tunnels, thereby keeping miners safe and the working area clean.

Pulse interaction induced systematic errors in dual comb spectroscopy

Systematic errors are observed in dual comb spectroscopy when pulses from the two sources travel in a common fiber before interrogating the sample of interest. When sounding a molecular gas, these errors distort both the line shapes and retrieved concentrations. Simulations of dual comb interferograms based on a generalized nonlinear Schrodinger equation highlight two processes for these systematic errors. Self-phase modulation changes the spectral content of the field interrogating the molecular response but affects the recorded spectral baseline and absorption features differently, leading to line intensity errors. Cross-phase modulation modifies the relative inter-pulse delay, thus introducing interferogram sampling errors and creating a characteristic asymmetric distortion on spectral lines. Simulations capture the shape and amplitude of experimental errors which are around 0.1% on spectral transmittance residuals for 10 mW of total average power in 10 meters of common fiber, scaling up to above 0.6% for 20 mW and 60 m.

Open-path dual-comb spectroscopy of methane and VOC emissions from an unconventional oil well development in Northern Colorado

We present results from a field study monitoring methane and volatile organic compound emissions near an unconventional oil well development in Northern Colorado from September 2019 to May 2020 using a mid-infrared dual-comb spectrometer. This instrument allowed quantification of methane, ethane, and propane in a single measurement with high time resolution and integrated path sampling. Using ethane and propane as tracer gases for methane from oil and gas activity, we observed emissions during the drilling, hydraulic fracturing, millout, and flowback phases of well development. Large emissions were seen in drilling and millout phases and emissions decreased to background levels during the flowback phase. Ethane/methane and propane/methane ratios varied widely throughout the observations.

Study on dust-gas coupling pollution law and selection of optimal purification distance of air duct during tunneling process

During the excavation of high gas mine, gas and dust often exist at the same time. In order to ensure that the gas concentration remains within a safe range and minimize the risk of workers' pneumoconiosis, we simulated the interaction mechanism of airflow, gas, and dust, explored the pollution law of gas and dust, and obtained the optimal purification distance (L_p) by the CFD method. The reliability of the numerical simulation was verified by field measurements. Firstly, the properties of the gas and dust affected the structure of the airflow field. At the same time, the change in the airflow field affected the concentration distributions of the gas and dust. During the diffusion process, some high-risk regions in which the gas or dust concentrations exceeded 0.80% or 200 mg/m³, respectively, were discovered. Moreover, we have found that the airflow velocity in the top region of the tunnel and at the intersection corner between the cutting face and tunnel wall was the main factor affecting the purification effects. When L_p = 5-8 m, the gas concentration remained below 0.50%. When L_p = 6 m, the dust concentration reached a minimum of 287.5 mg/m³. Therefore, the optimal purification distance was determined to be 6 m; in which case, the gas and dust concentrations decreased by 32.84% and 47.02%, respectively.

Influence of carrier effect on Pd/Al2O3 for methane complete catalytic oxidation

Pd/Al2O3 catalysts modified by different chemical elements (Mg, Si, Ce, and Zr) were tested for methane (CH4) catalytic combustion, and PdO nanoparticles loaded on modified Al2O3 were systematically studied. These conditions assess the carrier effects of Pd/Al2O3 and acid strength influences on CH4 combustion. We observed carrier effects on activation energy through tuning Pd 3d binding energies (BEs) and on pre-exponential factors (A) through Pd dispersion and acidity on supports. When the BE of Pd 3d5/2 is 337.3 eV, PdO nanoparticles loaded on modified Al2O3 have excellent activity in cracking the C-H bond of CH4, which leads to the lowest activation energy (E a ), regardless of the size effect of the PdO nanoparticle. Furthermore, a theoretical construction that acid sites on catalysts promote the reversible elementary step (2Pd-OH ↔ Pd-O* + Pd* + H2O) right shifts improving the A dependency on the quantity of exposed Pd* and Pd-O*. As a result, Al2O3, as the carrier, not only modifies the electronic characteristics and size of supported PdO nanoparticles but also participates in the reaction process via acid sites on the surface of Al2O3.

On the Long-Term Temporal Variations in Methane Emissions from an Unconventional Natural Gas Well Site

Understanding methane emissions from the natural gas supply chain continues to be of interest. Previous studies identified that measurements are skewed due to "super-emitters", and recently, researchers identified temporal variability as another contributor to discrepancies among studies. We focused on the latter by performing 17 methane audits at a single production site over 4 years, from 2016 to 2020. Source detection was similar to Method 21 but augmented with accurate methane mass rate quantification. Audit results varied from ∼78 g/h to over 43 kg/h with a mean emissions rate of 4.2 kg/h and a geometric mean of 821 g/h. Such high variability sheds light that even quarterly measurement programs will likely yield highly variable results. Total emissions were typically dominated by those from the produced water storage tank. Of 213 sources quantified, a single tank measurement represented 60% of the cumulative emission rate. Measurements were separated into four categories: wellheads (n = 78), tank (n = 17), enclosed gas process units (n = 31), and others (n = 97). Each subgroup of measurements was skewed and fat-tailed, with the skewness ranging from 2.4 to 5.7 and kurtosis values ranging from 6.5 to 33.7. Analyses found no significant correlations between methane emissions and temperature, whole gas production, or water production. Since measurement results were highly variable and daily production values were known, we completed a Monte Carlo analysis to estimate average throughput-normalized methane emissions which yielded an estimate of 0.093 ± 0.013%.