With depressed electrical prices for produced biogas, many projects are now moving towards business models predicated on production of renewable natural gas (RNG). In order to produce RNG, projects must first clean and upgrade raw biogas to pipeline and/or transportation fuel quality through the use of various engineering approaches. In this presentation, an overview of available and emerging biogas cleaning and upgrading technologies are discussed, highlighting positives, negatives and costs.
The aim of this fact sheet is to provide farmers, third party project developers, regulatory agencies, and other stakeholders with a basic understanding of the chemical composition of renewable natural gas, the most appropriate end use options for dairy digesters, and some of the more common techniques used to clean biogas to RNG quality at dairy digesters.
The authors utilized years of research and industry expertise as well as thorough literature search describe the concept of renewable natural gas and the technologies to clean the biogas. The authors aimed to provide information based on the current literature, but not to favor one technology over another.
When CHP is the end-use of biogas, the most common biogas purification approach for dairy digesters in the US is to remove water vapor and hydrogen sulfide. Existing projects use a variety of approaches, ranging from biological processes (both post digestion and via oxygen injection into the digester) to physical-chemical absorption processes such as iron type-sponge or activated carbon.
However, if RNG is the end-use a higher degree of purity is required. Often times a dedicated water vapor removal unit and hydrogen sulfide scrubbing unit is still required for removal of the bulk of the hydrogen sulfide mass. Thereafter, water scrubbing or PSA are often used to remove carbon dioxide from biogas, producing an RNG fuel that can be utilized in a variety of different ways. Other technologies exist, however their application on dairy digesters has been rather limited due to concerns related to maturity, cost, and complexity. The best technique is also situation-specific, and therefore, it is critical to understand the mechanics of each purification process, its limitations, and its economics before making a decision.
As electrical rates continue to drop throughout the PNW and US, current and new AD project developers are strongly considering a shift from CHP towards higher value end-uses for biogas, particularly RNG. Interest is increasing due to a growing CNG industry in the US, the decoupling of CNG and diesel prices, and the potential for competitive pricing and high revenues in comparison to fossil-CNG, given existing government incentives. Projects are presently limited and business models must still be proven before wide-scale adoption of biogas upgrading technologies within a dairy digester platform. In addition, concerns historically plaguing CHP projects, related to power purchase agreement pricing, interconnection fees, and scaling are still potentially present within a pipeline fuel model. Nonetheless, the potential exists for a new business model approach to AD projects on US farms.
No future plans.
Craig Frear, Assistant Professor, Washington State University email@example.com
Nick Kennedy, Associate in Research WSU; Georgine Yorgey, Associate in Research WSU; Dan Evans, President Promus Energy; Jim Jensen, Associate in Research, WSU Energy; Chad Kruger, DIrector WSU CSANR
For those seeking additional detail, or information about other technologies, more comprehensive reports and reviews are available (Jensen, 2011; Krich et al., 2005; Ryckebosch et al., 2011). This publication is part of the Anaerobic Digestion Systems Series, which aims to provide information that improves decision-making for anaerobic digestion systems.
This research was supported by funding from USDA National Institute of Food and Agriculture, Contract #2012-6800219814; National Resources Conservation Service, Conservation Innovation Grants #69-3A75-10-152; Biomass Research Funds from the WSU Agricultural Research Center; and the Washington State Department of Ecology, Waste 2 Resources Program.
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