Cleaned Biogas

Application Summary

Analysis Point Analyte Typ. Range Suitable Analyzer
Biogas After Scrubber H2S 0-50 PPM OMA-300 Process Analyzer

Introduction

Biogas can be generated through a few different processes. The more common production methods are capturing gas from the natural decomposition in landfills, and through decomposition of biomass collected specifically for use in biogas generation. Landfill gas (LFG) is a useful product of solid waste disposal. Landfills naturally generate CH4 and CO2 as waste decomposes over time. Biogas generation from a biomass feedstock, by contrast, is a dedicated process through which either waste biomass or biomass specifically grown for biogas production is digested to methane. The gas from both sources can be collected and, once it is purified, it can either be used to supplement natural gas, or it can be used as its own alternative fuel source.

Methane in a landfill gas process is generated through anaerobic digestion. Waste material is added to the landfill and trapped. The available oxygen is slowly consumed during the aerobic digestion stage. Once the oxygen is consumed, anerobic digestion begins. Typically, the system will come to steady state around 1 year after the material is added. The main products of the process are CO2 and CH4. Biogas production from biomass is a more streamlined process with better defined parameters. The biomass feedstock must first go through a pretreatment step (homogenizer). This step typically consists of a physical and chemical pretreatment. This feed is then broken down further into easily consumable molecules in the pre-digestor. The products are fed to the anaerobic digester where they are broken down again, yielding high levels of methane and CO2. Finally, the used biomaterials are sent to the post-digestion stage where they can be sold as fertilizer or biomass for other uses. Depending on the process, the CH4 and CO2 are collected off each of these stages.

The methane streams from each process can be combined into one biogas stream containing mostly CH4, CO2, and CO. The process of anaerobic digestion also generates H2S as a byproduct. H2S is an extremely dangerous chemical. Exposure can be lethal at around 500 PPM, and it is explosive at higher concentrations. H2S is also corrosive, and can lead to sulfur stress cracking. For these reasons, it is a closely monitored contaminant that must be removed before the gas can be stored, sold, or used. H2S is closely monitored at the outlet of the scrubber to ensure that H2S is removed down to safe levels for storage and transport.

The OMA-300 process analyzer continuously measures 0-50 PPM H2S in the combined biogas stream at the outlet of the scrubber. This information along with the analyzer at AT2 inform the control room on the efficacy of the sulfur removal process. Response time is critical in the desulfurization treatment of the gas to keep the system running as efficiently as possible and maintain product biogas specifications.

System Benefits

  • Continuously measures 0-50 PPM H2S in the combined biogas stream.
  • Totally solid-state build with no moving parts — modern design for low maintenance.
  • Fast response time, analyzer updates reading in under 5 seconds.
  • Combined sampling system and controller allows for a cost-effective solution.

Example Biogas Sample Conditioner

0-50 PPM H2S in the combined biogas stream. Key features for this application include:

  • Stainless steel components/fittings & Viton or Kalrez gaskets for corrosion resistance.
  • Particulate filtration to remove solids that could influence the measurement.
  • Liquid filtration to remove entrained liquid that could influence the measurement.

Further Reading