landfill gas or covered lagoon applications, separating nitrogen from methane is challenging with membranes; therefore, further gas polishing is required when there is a significant amount of nitrogen in the raw biogas. In addition, biogas oxygen levels can cause performance issues as membranes are not selective for separating oxygen. For example, sources of oxygen can be a less-than-airtight landfill or leaky covered lagoon, as well as leaking fittings on the suction side of compressing equipment.
A typical pretreatment option uses an activated carbon filter to remove hydrogen sulfide to a level <100 parts per million (ppm) and removal of volatile organic compounds (VOCs). Drying can be achieved by refrigerating the gas to 4 to 15°C (40 to 60°F) and capturing the resulting condensate. The volatile organic compounds should be free of any volatile organic compounds which can irreversibly foul or compromise some membrane types.
Pressure Swing Adsorption
The pressure swing adsorption (PSA) system is a batch process utilizing several vessels running in parallel under pressure. The heart of the process is an adsorptive medium, similar to activated carbon, which separates the constituents of the gas stream based on the molecular weight and size of the constituents. Predrying the gas ahead of the adsorbers to approximately 5°C (41°F) is required to keep the humidity out of the vessels to maximize their performance as dry adsorbers.
Carbon dioxide is preferentially adsorbed onto the media because it is a smaller molecule than methane and can permeate into the tiny pores in the carbon bed more easily and deeply. The methane goes through adsorber process columns relatively untouched, while the carbon dioxide does not. The adsorption process is reversible; thus, the carbon dioxide is eliminated during the regeneration cycle.
When a vessel has been saturated with contaminant gas, it is regenerated by reducing the pressure. At lower pressure, compounds are removed and desorbed from the media in a separate gas stream called “tail gas.” The common cycle time to saturation is 2 to 4 minutes. The tail gas is a low-quality gas stream that contains the previously adsorbed compound (such as carbon dioxide, hydrogen sulfide, and methane). Each vessel alternates its mode of operation from service, depressurization, regeneration, and finally repressurization before it returns to service mode.
Amine Scrubbing
The amine scrubbing system uses a two-step approach to upgrading biogas. The first step is adsorption followed by a second step of stripping or desorption. The amine portion of the scrubbing solvent molecule chemically reacts to the carbon dioxide in the biogas to retain it in solution. A common chemical deployed as the scrubbing solvent is monodiethanolamine (MDEA). The methane fraction of the biogas passes through the packed tower reactor untouched by the scrubbing chemical. High methane purities can be achieved in the recovered natural gas (>99.9% v/v).
In the second step, the scrubbing solution is heated to boiling to reverse the chemical reaction. The carbon dioxide in the fully packed stripper tower is disassociated from the scrubbing solution and discharged. High carbon dioxide purity in the off-gas can be achieved to accommodate potential for reuse of the carbon dioxide. The regenerated amine scrubbing solution is then cooled and reused back in the scrubbing tower in a closed loop system. Systems run at a relatively low operating pressure of approximately 0.5 to 3 psi. This low-pressure design affords equipment and operational savings as compared to systems that run at high pressure. Raw biogas with a high content (>300 ppm) of hydrogen sulfide is recommended to be pretreated by any one of several desulfurization techniques.
The recovered methane is dehumidified through a desiccant dryer and then pressurized to supply the natural gas grid. Pressurization power consumption is lower than for systems that use gas compression ahead of the scrubbing system since just the recovered methane is compressed versus the total raw biogas flow in other systems.
Water Wash
The water wash system is a two-stage process much like amine systems. The first step is a high-pressure reactor column that works in a countercurrent fashion. Chilled water flows downward and biogas flows upward under high pressure (150 psig). Soluble gases such as carbon dioxide dissolve in the water. The second tower serves as a depressurization tower where pressure is released from the solution and carbon dioxide comes out of solution. Makeup water is added when needed. Blow-down water is purged from the system to maintain the desired pH and water quality.
This system is run as a wet process; thus, no predrying of the biogas is formally required but is recommended. Any hydrogen sulfide in the biogas stream is adsorbed in the process after which the hydrogen sulfide (H2S) is purged from the system into the wastewater blow-down.
See also: Biogas, Gas Cleaning, Gas Processing, Gas Treating.
Biogenic Coal Bed Methane
All coals contain biogenic gas, but only the higher rank coal (bituminous) contains thermogenic gas. Biogenic methane is formed in coal seams by naturally occurring bacteria that are associated with meteoric water recharge at outcrop or sub-crop. Biogenic methane is a term used to describe natural gas derived from the reduction of carbon dioxide via biogeochemical processes. The processes which contribute to the production are complex, poorly understood, but pervasive in nature and vary from site to site. Biological processes include methanogenesis and hydrogenogenesis, but there are also geochemical processes that have been identified. Methanogenic bacteria generate methane by several pathways, principally the fermentation of acetate and the reduction of carbon dioxide. In coal seams, methanogens may increase coalbed methane production.
See also: Biogas.
Biogeochemical Cycles
A biogeochemical cycle (also called the substance turnover or cycling of substances) is a pathway by which a chemical substance moves through biotic compartments the biosphere) and through abiotic compartments of the Earth (such as the atmosphere, the hydrosphere, and the lithosphere).
There are biogeochemical cycles for the chemical elements (such as calcium, carbon, hydrogen, mercury, nitrogen, oxygen, phosphorus, selenium, and sulfur) as well as ; molecular cycles for water and silica; macroscopic cycles such as the rock cycle as well as human-induced cycles for synthetic compounds such as polychlorinated biphenyl derivatives (PCBs). In some cycles, there are reservoirs where a substance remains for a long period of time.
The biogeochemical cycles operate at the global scale and involve all of the main components of the Earth system in which materials are transferred continually between the atmosphere, the aquasphere, and the geosphere. However, since the biogeochemical cycles involve elements that are essential for life, organisms play a vital part in those cycles. Typically then, the biogeochemical cycles involve an inorganic component (the abiotic part of the cycle, including sedimentary and atmospheric phases) and an organic component (comprising plants and animals, both living and dead). Like other environmental systems, biogeochemical cycles involve the flow of substances between stores (also known as reservoirs) in the geosphere, atmosphere, hydrosphere, and biosphere. Water plays a vital role in mediating many of the flows between stores. Three of the key biogeochemical cycles are the nitrogen, carbon, and sulfur cycles.
Finally, it must be noted that the biogeochemical cycles have been modified substantially by human activities which is necessary for consideration to understand the various environmental issues related to human activities.
Biohydrogen
Biohydrogen is hydrogen produced
