Raw Biogas

Application Summary

Analysis Point Analyte Typ. Range Suitable Analyzer
Biogas from Anerobic Digestion H2S 0-30,000 PPM OMA-300 Process Analyzer
CH4 0-50% MicroSpec MCP-200 Infrared Analyzer
CO2 0-50%
CO 0-10%

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 methane 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.

The methane in a landfill is generated through the process of anaerobic digestion. As solid waste is added to a landfill, it initially goes through an aerobic digestion stage where the available oxygen is consumed. This step creates mostly CO2 gas. Once the oxygen has been depleted, the process turns over to anaerobic digestion. At the beginning of this process, compounds are converted to acids and alcohols, releasing CO2 and hydrogen. The acids are then consumed to produce acetate and organic acids, and eventually the landfill begins to produce methane. When the system has reached steady state, about 1 year after the material is added, the main products of the process are CO2 and methane. The concentrations generally remain steady at around 50% methane and 45% CO2. The production can remain at this level for 20 years before slowly tapering off as the available organic material is consumed.

The 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. The specifics depend directly on the feedstocks used, but the goal is to create a homogeneous feed that is easily biodegradable. This feed is then broken down into the component lipids, carbohydrates, and proteins, and then finally broken into short chain carboxylic acids and alcohols in the pre-digestor. The products are fed to the anaerobic digester where they are broken down further into mainly methane and CO2. When the methane concentration drops, the remaining biomaterial (digestate) is removed from the digester. Depending on the conditions, this can be put into an open-air storage facility, where it can be used to produce a variety of products, such as fertilizer. If the digestate is still producing an appreciable amount of methane, it can be stored in a closed system, and the methane extraction can be continued at a lower rate.

The different methane streams 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. The components of this stream are measured to ensure that the system is functioning properly. The OMA-300 process analyzer continuously measures 0-30,000 PPM H2S in the combined biogas stream.

The MCP-200 process analyzer continuously measures 0-50% CH4, 0-50% CO2, and 0-10% CO in the biogas stream from the digesters. The readings provide real-time information on how well the digesters are working and, most importantly, the level of methane output from the process. Using online control for the anaerobic digesters allows for the digester parameters to be optimized for peak performance and ensures that the product methane will meet the minimum-BTU gas methane concentration specification. This information, along with the analyzer at AT3, inform the control room on the efficacy of the sulfur removal. Response time is critical in the desulfurization treatment of the gas to keep the system running as efficiently as possible, and to maintain product biogas specifications.

System Benefits

  • Continuously measures 0-50% CH4, 0-50% CO2, 0-10% CO, and 0-30,000 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% CH4, 0-50% CO2, 0-10% CO, and 0-30,000 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.
  • Integrated MCP-200 analyzers housed in explosion-proof enclosures.

Further Reading