February 26, 2015

Effective Gas Analysis Critical in Biogas Production

By BonYook Koo, Emerson Process Management, Rosemount Analytical


image001Biogas is a mixture of different gases produced by the breakdown of organic matter and is used as a renewable energy source that exerts a very small carbon footprint. Globally, biogas production serves as a good energy recovery initiative. It helps to reduce global warming by reducing the discharge of methane (CH4) from wastes. And the global production of biogas is increasing as can be seen in the chart (Fig. 1). However, there are some important process challenges biogas producers face and accurate gas analysis is critical for efficient production and to prevent corrosion and dangerous pipe leaks.

The composition of biogas is typically 40-60 percent methane, 40-50 percent carbon dioxide, and the remainder being water, hydrogen sulfide, and other impurities. The composition of a specific biogas depends on the type of waste it is being obtained from, what it’s being purified for, as well as the purpose for which the biogas is being used.

Biogas producers need to know what the purpose and use of the biogas is for end users. They need to know the exact composition of the biogas. It’s especially important that they know the carbon dioxide levels.  It’s usually difficult to predict the amount of methane and carbon dioxide because it really depends on the source and type of wastes the biogas is being produced from and the means by which it’s being purified.

For example, biogas contains hydrogen sulfide, which is usually toxic, and it can be high in biogas that is produced from animal wastes, such as from chicken, or from certain vegetables. Biogas is also usually very wet, and when carbon dioxide and hydrogen sulfide are mixed with water, it can be quite corrosive. For these reasons, it’s usually good to remove or lower the amount of hydrogen sulfide, water, carbon dioxide and other impurities to minimal levels before the biogas is processed. If this isn’t done, then the engines that generate electricity from biogas will experience high rates of corrosion, which leads to inefficient running of the engines to produce electricity. Leaks can be dangerous to plant personnel as hydrogen sulfide can be released. In addition, when the wastes used to process biogas are varied, the amount of methane will change due to the behavior of the anaerobic bacteria in the digester used to generate the biogas. For these reasons, it’s critical that plants obtain accurate analysis of the composition of the biogas being produced.

370XAA natural gas chromatograph such as the 370XA can help address these operational and maintenance needs for biogas producers. Plants require a cost-effective, online gas chromatograph (GC) that can measure the biogas composition within a four-minute cycle time for ongoing real-time analysis. It’s also critical that the GC is easy to use and has low maintenance requirements to reduce time personnel spend on maintenance. Unlike larger gas processing plants, petrochemical plants and refineries that are able to employ skilled personnel for maintenance of the gas chromatographs, biogas plants usually will only have technicians who are skilled in the operation and maintenance of the typical instrumentation required in the plant. In addition to low maintenance requirements on an ongoing basis, the 370XA also includes maintainable module technology that allows operators to easily replace the GC module in the field in under two hours, reducing downtime and repair costs.

Effective measurement of biogas composition is a critical step to ensure a high quality product and reduce the risk of corrosion and potentially dangerous toxic gas leaks, a cost-effective, easy-to-use gas chromatograph is a key element in biogas processing.