From Sour to Sweet - Treating Biogas with High H2S Concentrations

Biogas pre-treatment is almost always necessary ahead of biogas utilization systems, like CHP and RNG, to reduce corrosion causing hydrogen sulfide (H2S) compound exposure in downstream systems. The cost effectiveness of treating hydrogen sulfide compounds becomes challenging for applications with high levels of H2S concentrations (over 1000ppm) and high biogas production rates. While there are multiple technologies available to treat H2S (i.e. absorptive media, biological, and chemical systems), the optimal H2S treatment strategy can differ for different utilities depending on the biogas utilization application, H2S concentrations, biogas production and O&M costs. This presentation will provide a technical and financial overview of the technology alternatives for removing hydrogen sulfide (H2S) from biogas (also known as 'biogas sweetening') as well as real-world results from two recent life cycle cost evaluations for the City of Tampa's H.F. Curren Advanced Wastewater Treatment Plant (AWTP and El Paso Water's Bustamante WWTP. These two case studies provide examples across a range of flow and hydrogen sulfide load conditions to illustrate how choice of biogas treatment technologies can have vastly different lifecycle costs and, ultimately, impact the net present value of a biogas utilization project. The world of options for biogas treatment evaluated in these case studies includes in-situ iron salts, media adsorption, caustic scrubbing, biologically-regenerated caustic scrubbing, catalytically regenerated caustic scrubbing, and biological filters. Average biogas H2S levels can vary on the order of 10 - 10,000 ppmv from plant to plant, and cost effectiveness of the H2S treatment technology will depend on the balance between capital costs, material costs, and annual operating and maintenance costs. This balance was assessed during the preliminary design phase of two recent projects that included biogas pretreatment, HFC AWTP and Bustamante WWTP. HFC AWTP is a 96 MGD plant in Tampa FL whose digester gas was tested during the preliminary design phase of a CHP system. The average hydrogen sulfide level of four samples was 9,500 ppmv, which is at the extreme end of the expected range for wastewater digester gas. With the high hydrogen sulfide levels, the cost of a typical media adsorption system and the annual cost for media replacement became cost prohibitive, so an evaluation of other bulk H2S treatment technologies was completed. Life cycle costs were evaluated, as well as other considerations like hazardous chemical inputs and storage, sensitivity to hydrogen sulfide fluctuations, outlet hydrogen sulfide levels, and operations and maintenance challenges. It was determined that a biotrickling filter system (which relies solely on biology in the filter to remove hydrogen sulfide) followed by lag-lead media adsorption vessels to 'polish' the gas after bulk treatment was the most cost effective, environmentally friendly, and, overall, preferred solution. The second case study covers an evaluation of H2S treatment technologies for Bustamante WWTP, a 28 MGD plant in El Paso TX, whose digester gas was found to have an average of 6,500 ppmv of hydrogen sulfide. The plant is undergoing improvements including expansion of digester capacity, new digester mixing systems, new covers on existing digesters, rehabilitation of existing CHP system, and upgrade of biogas pretreatment system, all of which are currently under design. The plant's high hydrogen sulfide levels shortened the useful life of existing digester covers, so as part of the H2S treatment strategy, a recommendation was to add ferric to the digesters to affix hydrogen sulfide. After adding ferric, the hydrogen sulfide load is lowered, such that media-based systems are more cost effective than bulk systems, like chemical or biological scrubbing. Additionally, the reduction of hydrogen sulfide with ferric allowed the plant to avoid a Title V air permit modification. After evaluating different media alternatives, it was determined that the plant could modify their existing iron sponge vessels to double adsorptive capacity by switching to granular media and incorporating a continuous air injection system. These case studies demonstrate how lifecycle cost analysis can be applied to assess H2S treatment alternatives and how technology selection depends on plant specific factors, especially hydrogen sulfide loading. These case studies will also show the relationship between capital costs, raw material costs and O&M costs for each alternative to gain an understanding of the factors that contribute to the overall lifecycle costs for each alternative. The economic results for the HF Curren and the Bustamante facilities are shown in the papers' Figures.