Market and product

Compiled by Bao Hien

Chinese researchers have developed an electrochemical process that converts hydrogen sulfide (H₂S) in natural gas into potassium sulfate fertilizer, instead of recovering sulfur through conventional methods. The experimental system operated continuously for 100 hours, offering a potential way to turn a toxic gas into a higher-value product.

The research was conducted by a team led by Associate Professor Haohong Duan of Tsinghua University and Hua Zhou of Beijing University of Chemical Technology. The findings were published in Nature Communications in August 2025.

The technology combines toxic gas treatment with the production of a commercially valuable chemical. Instead of burning H₂S to recover sulfur, the researchers use electricity to drive oxidation reactions that produce potassium sulfate (K₂SO₄), a fertilizer used in agriculture.

H₂S is commonly found in natural gas and crude oil. It can cause poisoning, corrode pipelines and damage catalysts in oil refineries. Concentrations in raw natural gas can reach 1–10% by volume, making its removal an important step before the gas can be used.

Most industrial H₂S is currently treated using the Claus process. This method burns part of the H₂S to produce sulfur dioxide (SO₂), which then reacts with the remaining H₂S to recover sulfur. Although the process accounts for more than 95% of global sulfur production, it requires temperatures above 900°C and additional treatment steps to control residual toxic gases.

Generating oxidants directly inside the reactor

According to the researchers, recovered sulfur sells for around US$110 per tonne, while potassium sulfate is valued at approximately US$500 per tonne. Directly converting H₂S into potassium sulfate could therefore serve two purposes: reducing pollution and increasing the value of the resulting product.

The technical challenge lies in oxidizing H₂S into sulfate ions. The reaction requires a strong oxidant, such as hydrogen peroxide (H₂O₂), but supplying this chemical externally increases costs.

To address this problem, the team developed an electrochemical system that generates H₂O₂ directly at the cathode through an oxygen reduction reaction, using oxidized carbon nanotubes as the catalyst.

In the system, H₂S is absorbed into a potassium hydroxide solution to form sulfide ions. These ions are then oxidized through intermediate compounds, including thiosulfate and sulfite, before being converted into sulfate, the key component needed to produce potassium sulfate.

Initial experiments showed relatively low conversion efficiency because the H₂O₂ generated was diluted in the solution. Only about 45% of the H₂S was converted, while the sulfate yield remained below 25%.

The researchers subsequently switched to a flow reactor design that delivers gaseous oxygen directly to the electrode surface, improving H₂O₂ generation in the reaction zone.

Simulations showed that the H₂O₂ concentration in the solution layer approximately 10 micrometres from the electrode surface reached 1.04%, more than 86 times the concentration in the bulk solution. As a result, a reactor with an electrode area of 4 cm² achieved an H₂S conversion rate of 98% and a potassium sulfate yield of 90%.

When tested with a mixture of H₂S and methane designed to simulate natural gas, the researchers found that H₂O₂ did not oxidize methane under the conditions studied. A small amount of CO₂, at 1,000 ppm, also had little effect on system performance.

System operates continuously for 100 hours

Building on the small-scale results, the team increased the electrode area to 100 cm² to assess the system's ability to process a larger gas stream.

In tests using simulated gas containing 10% H₂S at a flow rate of 300 ml per minute, the H₂S concentration fell from 100,000 ppm to below 15 ppm. The conversion rate exceeded 90%, while the potassium sulfate yield reached approximately 70%.

After 100 hours of continuous operation, the system produced 397 g of potassium sulfate. Its operating voltage remained between 2.52 and 2.94 V, indicating stable performance throughout the test period.

The researchers also developed an alternative reactor design incorporating a solid electrolyte layer to produce sulfuric acid directly. However, the resulting solution had a concentration of only 1.4%, with an electricity utilization efficiency of approximately 30%. Sulfur deposits also required the reactor to be flushed with sodium sulfite solution every 10 hours.

Environmental benefits depend on clean electricity

Despite its technological promise, the process requires further improvements before it can compete with established industrial methods.

A life-cycle assessment found that greenhouse gas emissions from the system under initial experimental conditions were higher than those of existing industrial processes. According to the researchers' model, using wind power, increasing the concentration of the alkaline solution to reduce the energy required for concentration, and sourcing H₂S directly from natural gas could lower emissions to approximately 0.4 kg of CO₂ per kilogram of potassium sulfate, below the level associated with the industrial method used for comparison.

Economically, production costs were estimated at around US$0.44 per kilogram of potassium sulfate, compared with a market price of approximately US$0.50 per kilogram. This relatively narrow margin remains sensitive to electricity prices, potassium hydroxide costs, current density and conversion efficiency.

The current results have been demonstrated only in a reactor with an area of 100 cm², rather than an industrial production line. Further assessment is needed to establish equipment durability, energy consumption, long-term operating performance and economic viability at larger scales.

The researchers suggest that the electrochemical principle could also be adapted to treat other sulfur-containing industrial gases and wastewater. If further developed, the technology could help transform hazardous waste into useful industrial feedstocks while creating new opportunities to recover value from by-products in the oil and chemical industries.