June 2020 - Technical Article

Hydrogen Sulfide (H₂S) Removal Using Caustic Scrubbing Technology

Reduce caustic consumables and OPEX for Renewable Natural Gas (RNG) Projects

Author: Graham Hartlett, Missy Lee

Keywords: renewable natural gas (RNG), biogas upgrading, hydrogen sulfide removal, H₂S removal, H₂S treatment methods, hydrogen sulfide removal from natural gas, desulfurization, biogas desulfurization, caustic scrubber

Industries: WWTP, SSO, Paper, Agriculture/Dairy

Summary: Hydrogen sulfide (H2S) removal for biogas upgrading plants is performed during the pre-treatment phase, regardless of whether the renewable natural gas (RNG) is being used in low or high BTU applications. DMT specializes in a range of desulfurization techniques under the Sulfurex® umbrella, including the Sulfurex® CR which harnesses caustic scrubbing technology for H2S removal. This article covers 1) the process of the Sulfurex® CR, 2) how its Sulfurex® SDS (Smart Dosing System) reduces operational expenses (OPEX), 3) how the Sulfurex® SDS compares to other H2S sensors on the market, and 4) how the Sulfurex® SDS can affect the bottom-line.

Hydrogen Sulfide (H2S) Removal in Renewable Natural Gas

With increasing awareness of renewable natural gas (RNG) as a major climate change abatement strategy, biogas upgrading is taking center stage. There are currently hundreds of RNG projects under development in the United States and Canada alone and thousands more in untapped biogas upgrading potential. Biogas is generated through the decomposition of organic matter from various feedstocks such as wastewater treatment plants (WWTP), separated source organics (SSO), agriculture (e.g., dairy manure), or paper mills. To transform biogas into RNG, it requires conditioning through process units designed to remove certain contaminants from the raw gas. A usual pollutant in biogas is hydrogen sulfide (H2S), which is typically removed regardless of whether the RNG is being used in low or high BTU applications. H2S removal is important because H2S is a highly corrosive and toxic gas that can damage CHP engines and feed gas compressors, thereby, dramatically increasing maintenance costs on RNG projects.

DMT specializes in H2S removal through a range of desulfurization technologies under the Sulfurex® umbrella. Each H2S removal treatment method has its own specific advantages that if leveraged correctly, can help maximize a project’s ROI. DMT’s Sulfurex® CR (Chemical Reaction) uses caustic scrubbing technology for H2S removal and has proven in the field to provide operational ease and a small footprint at remarkably low capital expenditure (CAPEX). The system ingeniously combines desulfurization with gas cooling and drying through the integrated dehydration system. Cooling the gas also increases the selectivity for H2S over CO2. So how does the Sulfurex® CR work?

Desulfurization Using Caustic Scrubbing Technology

The Sulfurex® CR operates as a counter-current scrubber where raw biogas containing H2S interacts with a caustic solution in a crossflow manner over packing media in the scrubber tower. This desulfurization technique ensures maximum contact between the incoming biogas and caustic solution, thereby, providing high-efficient H2S removal. During this process, H2S is absorbed by the caustic solution and converted into sodium hydrosulfide (NaHS). It is the Sulfurex®’s Smart Dosing System (SDS), however, that keeps operating expenses (OPEX) low.

What does “SDS” mean? One of the main concerns our clients have regarding biogas desulfurization is the high cost of consumables, which in this case is caustic solution. To dose the right amount of caustic requires knowing exactly how much H2S is in the biogas. The ‘Smart’ in SDS means intelligently controlling the amount of chemicals being dosed in response to real-time information on H2S loading in the raw biogas entering the plant. With the Sulfurex® CR, only the amount of caustic needed at any point in time is dosed, keeping OPEX under control.

DMT has been commissioning the Sulfurex® CR since 1989.

Sensors in H2S Removal Affect OPEX

With other H2S removal systems in the RNG market, an electro-chemical sensor is installed which allows H2S to oxidise on a pair of enclosed electrodes, changing the electrical current passing from one electrode to another. Once calibrated, the change in H2S levels (i.e. the amount of H2S being oxidized) is proportionate to the change in current. The problem with this technique, however, is that the oxidized H2S needs to be removed from the electrodes with air for it to accurately calculate H2S levels on the next sample. This process can take up to one hour to complete. By then, H2S levels may have drastically changed, causing the current caustic dosing amount to be out of sync with the reality of the H2S load.

In contrast, the Sulfurex® CR’s H2S sensor is always online due to the innovative way DMT mixes a specific combination of raw biogas with air in the sample line. This H2S treatment method allows measuring and cleaning the electrode simultaneously which means the sensor is continuously feeding the dosing pump information on how much caustic solution to dose at any moment in time. This real-time information provides a better performing plant and a massive reduction in OPEX.

Sulfurex® SDS, a Bottom Line Solution

The Sulfurex® SDS is for the RNG customer who no longer wants to constantly worry about whether they are dosing caustic correctly, or to decide dosing constantly for peak events versus for average loadings (e.g., having off spec gas during peak events). What can a customer expect to save in OPEX as a result of DMT’s Sulfurex® SDS? Field studies indicate that these H2S removal systems are capable of responding to real-time changes in the feed gas which results in a saving of 30 to 35% in caustic consumption. For a RNG plant with a biogas inlet concentration of 350 standard cubic feet per minute (scfm), receiving 10,000 ppm of H2S as an example, the investment on the smart system is paid back within 24 months on caustic purchases alone. Now that’s Smart!