Continuous OTM 33A Analysis of Controlled Releases of Methane with Various Time Periods, Data Rates and Wind Filters

Abstract

Other test method (OTM) 33A has been used to quantify emissions from natural gas sites since it was introduced by the Environmental Protection Agency (EPA). The method relies on point source Gaussian (PSG) assumptions to estimate emissions rates from a targeted site or source. However, the method often results in low accuracy (typically ±70%, even under conducive conditions). These accuracies were verified with controlled-release experiments. Typically, controlled releases were performed for short periods (15–20 min) under atmospheric conditions that were ideal for effective plume transport. We examined three methane release rates from three distances over various periods of time ranging from seven hours to seven days. Data were recorded continuously from a stationary tower. Atmospheric conditions were highly variable and not always conducive to conventional OTM 33A calculations. OTM 33A estimates were made for 20-min periods when the mean wind direction corresponded to ±90° of the direction from the controlled release to the tower. Further analyses were performed by varying the frequency of the data, the length of the individual OTM 33A periods and the size of the wind angle used to filter data. The results suggested that different (than conventionally used) period lengths, wind filters, data acquisition frequencies and data quality filters impacted the accuracy of OTM 33A when applied to long term measurements.

Temporal variation and grade categorization of methane emission from LNG fueling stations

“…The Gaussian point source method, OTM33A issued by the U.S. Environmental Protection Agency, is used to calculate the methane emission rate. Usually 20 min concentration measurement is required to acquire an emission rate estimation 12 . In our study, the measurement time is between 2 and 7 h. Hence in our analysis, we break down the whole measuring period to several sequential 20 min sections, and data within each section is analyzed to obtain an average emission rate within that section.…”

“…Their capacity and configurations were listed in Table 1 . The following factors were considered when selecting these stations for effective application of OTM 33A method in the EPA official website, which are used as the main basis for the selection of LNG fueling stations 11 , 12 . An accessible and safe downwind site for instrument installation; A relatively consistent wind condition blowing from the source to the point of measurement; Zero or few obstructions between the source and the measurement point; No other nearby emission sources.…”

“…Following the standard practice of OTM 33A 11 , 12 , the raw methane concentration data and meteorological data require the implementation of certain quality control criteria before performing the next step of the calculation. These criteria mainly include wind speed greater than or equal to 1 m/s, turbulence intensity less than or equal to 0.22, atmospheric stability greater than 1.…”

Methane Emission Estimation of Oil and Gas Sector: A Review of Measurement Technologies, Data Analysis Methods and Uncertainty Estimation

“…Third, the distance between the measuring position and the objective has to be clearly known, and there are no other nearby emission sources besides the target source. Fourth, the emission source has to located close to the ground [18,98,99]. There are three important parameters in OTM33a models: meteorological parameters, position information, and methane concentration.…”

“…Heltzel et al [99] pointed out that the uncertainty of an uncontrolled OTM33a method can reach ±70%. In order to solve this problem, Heltzel et al used controlled release experiments to explore the effect of sampling frequency, sampling time, selected wind direction range, and other factors on the uncertainty of the estimation result [99]. They found that the above factors had a significant impact on the accuracy of the OTM33a method.…”