Stable Isotopes for CCUS Applications: H2S Treatment at Lab Scale

Hydrogen Sulfide (H2S) gas may be found among the products of industrial processes which are desirable candidates for carbon sequestration programs. With regard to such programs, the coexistence of H2S with CO2 in an effluent stream to be sequestered may pose operational threats (i.e., corrosion and safety hazards). According to the Center for Disease Control and Prevention (CDC) National Institute for Occupational Safety and Health (NIOSH), exposure to hydrogen sulfide may cause irritation to the eyes and respiratory system. Workers may be harmed from exposure to hydrogen sulfide. The level of exposure depends upon the concentration duration, and work being done. (https://www.cdc.gov/niosh/topics/hydrogensulfide/default.html). Generally, these challenges may be overlooked in favor of sequestering H2S in conjunction with CO2. The reactivity of H2S assures short-lived residence time in sequestration reservoirs and clastic lithologies in general. Setting a chemical baseline for the CO2 will be required by many programs that involve the Environmental Protection Agency (EPA) or primacy regulatory states (e.g., North Dakota, Wyoming, Louisiana) permitting Class VI, CO2 injection operations. As a result, measurement labs and operators of Class VI injection wells must consider preparatory procedures for the safe handling, transportation, and analysis of gases containing high concentrations of H2S.

Through a wider lens, careful monitoring of carbon sequestration programs may be viewed as a means of mitigating risk. Therefore, one should consider the scientific rigor of their monitoring program as a cornerstone of their successful implementation. One investigative technique that applies especially well to this sort of risk mitigation is the use of a conservative tracer like stable carbon isotopes for differentiating the materials associated with a given project from extraneous materials that may be present in the subsurface. Stable isotopic compositions are very common examples of inherent (or, conservative) tracers that are used in this way. For instance, the composition of the stable isotopes of carbon (often expressed in ‰ δ13C) measured in the CO2 to be sequestered may be of great interest to the project manager, as that composition can be adduced in the case of a suspected leak. Obviously, in such a case, a favorable outcome will depend on the integrity of analytical results. Therefore, if the best interests of all stakeholders are to be accounted for in a monitoring program, harmful impurities like H2S need to be dealt with effectively while preserving key conservative chemical tracers, like stable carbon isotopes of CO2.

The removal of H2S may be necessary for the success of many monitoring programs. One reason for this necessity is the general unwillingness and/or incapacity of analytical laboratories–forensic laboratories that track the chemical properties of relevant injection streams and monitoring sites over time–to handle H2S while taking their measurements. Fortuitously, the removal of H2S in these systems can be designed on a small-scale using batch or semi-batch processes, since relevant analytical techniques (i.e., gas chromatography, Isotope Ratio Mass Spectrometer (IRMS)) require
very small quantities of fluid. The presence of highly concentrated H2S in a gas sample may damage expensive and delicate instrumentation. Scrubbing high concentrations of H2S from a sample, depending on the technique employed, may affect (through fractionation effects) the stable carbon isotopic composition of the CO2. It is important to employ techniques that do not adversely affect the isotopic composition of the CO2 component of the gas.

With the foregoing considerations in mind, DIG Labs designed two different “scrubbing” processes for the removal of H2S from gaseous CO2. Both designs apply to a scale in accordance with the IRMS analysis of fixed gases that produce between 100 and 500 mL of scrubbed gas at standard pressure, in which the concentration of H2S does not exceed 100 ppm. Along with the requirements of scale imposed by analytical testing, field sampling constraints and project-specific details formed a basis for the designs. Specifically, DIG Labs settled on sets of process parameters that could handle the intake of a dry gaseous stream comprising ~98 vol% CO2 and ~2 vol% H2S, contained at ~100 psi of pressure in a 500-cc stainless steel cylinder (treated with Sulfinert technology).
One method involves a continuous-flow, packed-column “dry” scrubber filled with iron oxide pellets. The other, “wet” scrubbing method involves a semi-batch system of packed columns filled with caustic solution.
Perhaps the most important constraint on the design choices of an H2S scrubbing system built for the purposes outlined above, is the need to conserve the chemical signals that serve us as conservative tracers. Many physical, chemical, and biological processes, which are well studied and documented, demonstrate a potential to fractionate the isotopes of a natural system (i.e., the relative partitioning of heavier and lighter isotopes across two coinciding phases). As DIG Labs has suggested two chemical means of separating H2S from CO2 for this application, special attention was paid to the preservation of the isotopic character of CO2.
This investigative process revealed that dry field scrubbing techniques used in oil and gas which employed carbon steel (iron pellets or shavings) did not preserve the isotopic character of the CO2 gas at low inlet concentrations of CO2 (< 5 %). However, our results show that, given a relatively pure effluent stream (> 90 vol% CO2), δ13CCO2 of the effluent gas was consistent and preserved across both wet and dry scrubbing techniques.
Further investigations are needed to confirm and apply these observations across different feed compositions and operating conditions (removal efficiency over time or breakthrough time in a continuous run). However, the initial results suggest that both the dry (iron chelate) and wet (caustic solution) treatment methods may be suitable for applications where preserving the δ13C signature of the CO2 stream is important.

For its design, DIG Labs utilized scrubbing technologies that are accepted, and even considered standard, for refining industry practices. The primary benefit of selecting this technology is the abundance of scientific studies–spanning several decades–which develop and refine its application to the treatment of sour gases both with and without CO2. DIG Labs was able to utilize
the wealth of wisdom and heuristics embodied by these studies to tune its own design parameters; the final design incorporated principles and ideas from numerous historical applications of this technology, all of which proceeded from the special goal of scrubbing H2S preferentially in the presence of significant amounts of CO2. The objective was achieved without changing the stable isotopic character of the CO2, so a meaningful baseline can be set in this important sequestration system.
This work is valuable for various applications, including:
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Tracing CO2 sources: If the δ¹³C value remains unchanged after treatment, it can still be used as a reliable fingerprint to identify the origin of the CO2 stream.
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Carbon Capture and Storage (CCS): Consistent δ¹³C values throughout the treatment process simplify the accounting of captured carbon in CCS projects. The δ¹³C signal can be used to track the CO2 without needing to account for isotopic fractionation by the treatment methods.