The United States produces more natural gas than any other country in the world and is now the leading global exporter of this resource. As natural gas continues to emerge as a reliable, low-cost, low-carbon alternative energy source, investment in the infrastructure necessary to increase the U.S.’ exporting capacity has hit a fever pitch.
Today, natural gas is exported most efficiently in the form of LNG (liquified natural gas) which is a hydrocarbon mixture made primarily of methane. To make LNG, the feed gas must be super-cooled to a temperature of -160ºC. However, cooling a feed gas to produce LNG can cause contaminants to drop out (become liquid) before the rest of the gas stream, resulting in a slew of issues that can complicate the process. As a result, the purity of natural gas used for LNG is paramount to a successful process. A low-level contaminant that has become a point of interest in natural gas is mercury.
Mercury is present in all organic matter including crude oil (2-100 ppb) and natural gas (0.001-0.2 ppb). Despite the very low concentrations, trace amounts of mercury in natural gas can present a couple of complications in the gathering, processing, and liquefaction of the resource. One complication is mercury causes significant corrosion of anything aluminum. For midstream applications, the many aluminum components including valves and sensors can quickly corrode by these trace levels of mercury. In downstream applications, aluminum constructed cryogenic heat exchangers that cool and liquify the natural gas are particularly susceptible. Another complication is mercury can also deactivate impurity catalysts that are supposed to remove sulfur from the gas. Mercury interferes with the catalyst by adhering to the active sites designed to attract sulfur compounds which decreases efficiency and leads to more untimely replacements. The need for the installation of a mercury removal unit (MRU) at the entrance to these facilities adds increased costs and facility design complexities, making it important to know the mercury levels of your gas.
The most accurate means of testing for mercury in natural gas is by ASTM D6350 – Standard Test Method for Mercury Sampling and Analysis in Natural Gas by Atomic Fluorescence Spectroscopy. This method is written with procedures for both obtaining a representative sample in the field as well as analyzing the sample in the lab using atomic fluorescence spectroscopy for volume concentrations of 0.0001 ppb Hg. The sampling method utilizes mercury’s ability to form amalgams with gold which extracts the mercury from the gas stream as it travels through gold coated sorbent filters.
The method procedures are as follows:
- First, regulate the gas stream to be sampled to 100-200 ml/min and purge through the sampling system for 30 minutes to prevent any cross contamination from previous sampling events.
- After purging of the system, the gas stream is then directed to flow through two filters containing gold coated silica that are attached in series. The gold silica filters trap the mercury efficiently and effectively allowing the mercury to be removed from the gas stream and safely transported back to the lab for analysis.
- The lab should then analyze the mercury content from the filters using an analytical instrument with AFS (atomic fluorescence spectroscopy). The filters are attached one at a time to the instrument and as argon carrier gas flows through the filters, the gold/mercury amalgam is heated to 500°C allowing the mercury to release from the gold sorbent and enter the detector in a gaseous phase. The AFS raw data looks like a chromatogram peak and utilizes peak area and response factor to determine the mercury concentration on the filter. The sum of the concentration from both filters is reported in uG/M^3.
Another comparable method to ASTM D6350 is ASTM D5954 which most notably implores the use of atomic absorption spectroscopy (AAS) in place of AFS. AFS is superior to AAS because it is not susceptible to the interferences present within AAS that render the requirement of detector compensation schemes. AFS does not need compensation schemes mainly due to the fluorescence process which re-emits the absorbed light energy as an omnidirectional photon allowing the detector to be installed at a 90-degree angle from the UV light source suppling energy to the mercury atoms. AAS detectors are installed directly in line with the UV light source which requires the need for Zeeman correction and/or split reference beam compensation schemes. In essence, AFS is actually measuring the amount of mercury present via fluoresced photons where AAS is inferring mercury presence by difference using the previously mentioned correction factors.
SPL is committed to being at the forefront of emerging clean energy technologies and has invested in outfitting all of its facilities with state-of-the-art mercury in natural gas sampling devices adhering to ASTM D6350 guidelines. Look for us at the ASTM D03 Gaseous Fuels committee meetings in New Orleans on Dec 5-6 that governs this method ensuring SPL has a voice in its development.
Andy Hartman
Laboratory Director
Andy Hartman is the Laboratory Director at SPL’s flagship hydrocarbon lab in Houston, TX. Over the 9 years he has been with SPL, he has held various lab management and business development roles. Andy has also spearheaded laboratory startups for SPL in CO, TX, and NM. Prior to his employment with SPL, he attained his BS in Microbiology from Texas State University in 2012 where he performed undergraduate chemistry research for several years. When not at work, Andy enjoys spending time with his wife and infant son. He also has a passion for cars and fulfills this desire by attending car shows, wrenching on his own cars, and maintaining various RC vehicles.