Biogas is a renewable energy source that can be used as an alternative to natural gas. The production of biogas involves the utilization of microorganisms to break down biomass (organic matter such as food or animal waste) in an oxygen-free environment. This process is known as anaerobic digestion. Because the main components of biogas are methane (CH4) and carbon dioxide (CO2), it is used to produce sales-quality fuel (known as biomethane) that can replace natural gas.
The production of biogas comes with challenges. Biogas can often contain significant amounts of hydrogen sulfide (H2S), an extremely toxic and explosive gas with an odor much like rotten eggs. Exposure to H2S at levels as low as 500 PPM can be fatal. In order to safely and responsibly produce sales-quality fuel from biogas, the H2S must be monitored and removed.
The incoming landfill gas typically consists of about 50% methane and 45% CO2. In order to maintain compliance with local municipalities, odors and emissions from landfills must be controlled. This can be done by ensuring the system is always under a slight vacuum. The methane concentration can be used as an indicator that the system is under vacuum. Keeping the concentration of methane under the known production of methane from the landfill typically means that some air is being pulled into the system, ensuring that there is minimal leakage of methane into the surrounding atmosphere. The readings provide real-time control over the methane content of the landfill gas.
The biogas stream coming from the landfill inlet is combined with the biogas stream coming from the predigester, anaerobic digester, and post digester. The components in this combined stream are monitored before they are sent to a scrubbing unit. To achieve efficient control of the scrubbing unit, the plant operators must monitor the concentration of the contaminants in the inlet stream. This analyzer is usually used in conjunction with AT3.
The biogas that leaves the scrubber is typically sent to storage. Before the biogas can be stored, the level of H2S in the gas must be measured. The level of H2S gas is used to monitor the efficiency of the scrubbing unit. Moreover, it is essential that the H2S concentration is minimized to ensure it is safe for storage. Iron drums are typically used for biogas storage, which can be quickly destroyed by corrosion if the H2S level is not closely monitored. This analyzer is usually used in conjunction with AT2.
The gas leaving the Biogas Storage unit is monitored for H2S and methane before being sent to the Combined Heater and Power Generation unit. H2S is closely monitored to ensure the concentration is kept low enough to minimize the hazard to local personnel, and to prevent damage to down-stream equipment. Methane is measured to monitor the heating value of the gas going to the generators, and to ensure a consistent concentration of methane is being supplied to the CHP plant. The heating value and concentration of the methane are two key factors in optimal CHP plant operation.
The gas is monitored at AT5 after running through various purification and drying processes. The gas that leaves the scrubber typically still contains small levels of H2S and CO2, along with moisture. All three of these components must be removed before the gas can be considered high enough quality to meet stringent standards for pipeline injection. The readings at AT5 provide real-time control over the biogas purification and drying processes.
The upgraded biomethane leaving AT5 is ready to inject into the natural gas distribution network and be sent to a customer. Different customers have different requirements and guidelines that must be met before accepting the final biomethane product. For this reason, the biomethane must be analyzed at AT6 to ensure it meets the customer’s specifications. Common measurements at AT6 include the composition, heating value, and level of contaminants such as BTEX and H2S.
The sales-quality natural gas must be injected with an odorant before being sent to the customer. The purpose of odorizing natural gas is to give it a distinct smell, so that it can be detected in the event of a leak. On-line odorant analysis allows for the optimization of the odorant distribution system in real time. Secondary benefits allow for the optimization of pipeline performance by studying lag times, odorizer performance, fading effects, and other odorant issues.