Biogas Cleaning

Modern living affords conveniences our ancestors would scarcely have imagined. Because of transportation networks and technology, fruit, for example, is never out of season. Just as milk and wine are pasteurized to remove unhealthy organisms and render them fit for transport, storage and sale, natural gas and biogas must be subject to specific regimens of cleansing and purification.

Why does gas need "cleansing"?

Natural gas is extracted from shale and other formations of sedimentary rock. What emerges from wells is often referred to as "wet natural gas" because it is accompanied by water vapor and NGLs, or natural gas liquids. Other, less welcome elements and compounds come along for the ride: sulfur, helium, nitrogen, hydrogen sulfide, and carbon dioxide. Among the most problematic of the mix are hydrogen sulfide (H2S) and carbon dioxide (CO2). Even in small amounts H2S produces health and respiratory problems in those near to it. Higher concentrations can even kill you. Just as bad for natural gas providers is the fact that H2S can deteriorate the lining of pipelines and storage tanks. Carbon dioxide poses a different problem. It reduces the heating value of the gas thereby lowering its quality and efficiency.

Are harmful compounds present in biogas?

Biogas, the renewable natural gas derived from organic matter, is not immune to the permeation of H2S, CO2 and other extraneous gases. Since it is the end product of a series of chemical reactions that take place in the absence of oxygen, one might believe that methane (CH4) is the only compound left standing after anaerobic digestion is complete. Not so. The bacteria that break down the organic materials — e.g. manure, food scraps, dead leaves, sewage — generate activity that yield other chemical substances, CO2 and H2S among them. The phases of anaerobic digestion each produce different substances. Hydrolysis ends with simple sugars, amino acids and volatile fatty acids. Acidogenesis also creates volatile fatty acids, along with ammonia, CO2 and H2S. Acetogenesis brings about acetic acid, hydrogen and CO2. Finally, methane, CO2 and water come from methanogenesis.

How are contaminants removed?

When natural gas is sent to the processing plant, cleaning gas of its H2S and CO2 is task number one. These "acid gases" are, more often than not, treated with amine solution as the natural gas flows through a column. Derived from ammonia, the amine molecules in this liquid draw the hydrogen sulfide and carbon dioxide from the gas flow. This is said to "sweeten" the "sour gas." Another reliable gas cleaner is membrane technology. The basic concept is that the acid gases will flow through the synthetic membrane that separates high-permeability gases, i.e. the acid gases, from lower permeability ones — the methane. Biogas cleaning is also workable with varying techniques. A very effective method is the use of materials containing high amounts of iron oxide: allophanic soil, brown soil and black sand. Moving the gas flow through these materials has demonstrated a nearly complete elimination of H2S from the biogas.