New process for removing hydrogen sulphide from sour gas

ADI International Inc. of Fredericton, New Brunswick, has just completed a major R & D effort, under the Industrial Assistance Research Program (IRAP) of the National Research Council Canada, which helped develop a new process for removing hydrogen sulphide from gas streams.

Hydrogen sulphide can be present in various air streams and is often found in gas streams associated with petroleum storage and transfer facilities, anaerobic digesters, sewage treatment plants and pulp and paper mills. In many cases, the hydrogen sulphide has to be removed because of its toxicity, corrosive properties and unpleasant odour.

An ADI installation in Australia.	A pilot-scale MEDIA G2@ filter for H2S scrubbing.

MEDIA G2^® adsorption media consists of an inorganic, natural mineral impregnated with a substantial quantity of iron hydroxide. It was originally developed for removing arsenic from contaminated drinking water supplies, having been awarded a US patent, certified by NSF Standard 61, and with a performance claim verified by Canada's Environmental Technology Verification Program.

Virtually every wastewater treatment plant produces off-gas (i.e., "biogas"); many anaerobic digesters produce a biogas which is high in hydrogen sulphide concentration. The simplest means of dealing with this is to flare the biogas, which converts hydrogen sulphide to sulphur dioxide; however, typical conversion rates are only 90%-95%, which in many cases is not sufficient to meet stringent air quality regulations. In addition, sulphur dioxide emissions may be excessive.

Where greater H₂S removals are required, or in cases where flaring is not possible because the biogas is to be used as fuel in boilers or electrical generators, other treatment methods must be used. These include adsorptive media (e.g., iron sponge, or other proprietary media), caustic scrubbers, or water scrubbers. These all work well, but have drawbacks. For example, iron sponge and other media are typically used only once and then thrown away (iron sponge can theoretically be regenerated, but the procedure can result in ignition of the media), caustic scrubbers are expensive and use large quantities of chemicals, and water scrubbers typically achieve only 85% H₂S reduction.

MEDIA G2^® consists of a natural mineral, calcined to increase hardness, porosity, and surface area, with an iron oxide coating applied. It has proven capable of providing up to 99.98% reduction of H₂S in biogas. The media may be generated and reused repeatedly, and the regeneration process is simple, safe, and produces no hazardous wastes.

This media is a simple, low-cost option for use in wastewater treatment facilities requiring scrubbing of odorous off-gases from digesters. The high level of treatment should allow compliance with the strictest of air emissions standards. It could also result in significant economic impact in the protection of boiler or electrical co-generation facilities utilizing biogas as fuel.

In some cases, where the cost of scrubbing gas upstream of such equipment previously made biogas uneconomical as fuel, use of this product may now make it practical, thereby resulting in use of "free" fuel that would otherwise be wasted, and contributing to the conservation of natural resources. Having reduced hydrogen sulphide to less than 0.2 mg/L, sulphur dioxide emissions from combustion are not an issue.

MEDIA G2^® can handle H₂S up to 30,000 ppm; reduces H₂S to non-detectable levels (0.2 ppm); and reduction is achieved in 60 seconds or less contact time. It can be regenerated, simply with air (residuals include water and sulphur). Up to 560 mg H₂S is adsorbed per gram of media, 50% of media weight.