11/1/2020 0 Comments Biogas Plant Design
Landfills operate similarly to avoid pulling a vacuum on the headspace.
![]() The major differences between biogas and natural gas piping systems is the amount of treatment that each gas typically undergoes before conveyance, and the way the piping systems operate. Natural gas pipelines are heavily regulated, inspected and maintained. These regulations fall into two categories: major conveyance piping for distribution, and end-user (after the meter) gas piping. End-user gas piping design and regulations will be used for this article as a comparison to biogas piping because of the similarities smaller diameter and operating at lower pressures. Table 1 provides an overview of regulations typically applied to end-use natural gas piping in the United States. Major distribution network pipelines have separate regulations (ANSI B31.8), which are not addressed within this article. ![]() Limited treatment often allows significant concentrations of moisture and hydrogen sulfide (H 2 S) to be present, which can present both safety and gas utilization concerns. Since biogas piping operates close to atmospheric pressure there is a risk of air entrainment, which is also a potential safety concern. Specifics of H 2 S, moisture and air are discussed later in this article. There are no codes or regulations specific to biogas piping in the United States. Depending on the owner of the system, biogas piping can be a secondary consideration during routine inspections and maintenance of the anaerobic digester system until a significant safety or performance issue has arisen. However, there are codes, regulations and guidelines (Table 2) that can be referenced for specific aspects of biogas piping installations. ![]() At the DRANCO Hengelo plant in the Netherlands biogas piping connects to the flare in the foreground, then the storage balloon in the rear, and the gensets on the left in the green containers. Photo courtesy of Organic Waste Systems (OWS) Biogas And Natural Gas Comparison To understand the potential safety and performance issues of biogas, it is important to review the property differences between natural gas and biogas (Table 3). Natural gas piping can be operated at pressures much higher than biogas piping. There are many charts and guides for selecting the size of natural gas piping based on flow rate, temperature and pressure. In comparison, biogas piping prior to conveyance (with a blower, compressor), or gas storage, is considered low pressure piping and is designed differently. The piping is typically sized to use the pressure of the anaerobic digester to push the biogas to the conveyance system and avoid pulling biogas out of an anaerobic digester at a faster rate than it is produced (and consequently, risk letting in undesirable air). Since most anaerobic tank vessels, storage systems and covers have physical limitations to hold high pressures, a majority of the biogas piping from these reactors operate at pressures slightly above atmospheric pressure (14.7 pounds per square inch (PSI)) from the reactor to the conveyance system.
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