Biogas  [ Return towards  Chemicals/Pharmaceuticals & Environment  ]

Biogas is produced by the decomposition of organic matter in an oxygen free atmosphere (anaerobic fermentation). This natural biological process can takes place in swamps or landfills. It can also be controlled in digesting plants supplied with organic waste. Comprised mainly of methane and carbon dioxide, biogas is a major greenhouse gas and a potential renewable energy source. Today, it is flared off or collected through a pipeline grid to power plants producing heat and electricity. Driven by the context of fossil fuel scarcity, biogas, through purification and enrichment, is finding new valorisation processes. Thus, for a country such as France, which imports more than 90% of its gas, it is estimated that biogas could cover 10 to 15% of the nation’s consumption.

By combining several technological blocks, Air Liquide is developing a global solution that includes all stages involved, from the collection of biogas to the compression or liquefaction of biomethane. Before purification begins, the first stage consists of compressing the biogas supplied at atmospheric pressure. To do so, Air Liquide is proposing to set up a lubricated screw compressor, a proven technology. The second stage is intended to remove corrosive compounds from the biogas, meaning hydrogen sulphide, volatile organic compounds (VOCs), siloxans, dust, etc. At this point, the technologies required are a regenerative adsorption system and a trapping by activated carbon bed. The next stage consists in the separation of the two main components of biogas: methane and carbon dioxide. To do so, MEDAL* has developed an innovative process based on the use of hollow fiber polymer membranes. Air Liquide has patented the “methane recovery process” system designed. If biogas is produced thanks to an anaerobic process (digestion plant), Biomethane purity turns out to be nearly 100%. In addition, in the case of sanitary landfills, if the biogas is sucked out to avoid olfactive nuisance and methane slip to the atmosphere, it will contain some nitrogen and oxygen. In order to separate them from the methane, the gas flows through a distillation column. This cryogenic process is very efficient for the separation of CH₄ and N₂. Lastly, the final stage may vary depending on the commercial end-use specified by the customer. A compression stage may serve as input for a pipeline network or a vehicle filling station. A second option is liquefaction, where the objective is to store the biomethane in the Liquefied Natural Gas state. To this end, Air Liquide employs a technology commonly used by DTA, especially for hydrogen and helium applications: the Brayton cycle.

	Left side : the 2 stages membrane allowing CO2 / CH4 separation.  Center: regenerative COV removal system. Right side : 2 activited carbon bed column allowing gas polishing.

Developed for prototypes and applications where the gas flow rate to be processed was relatively low, these technologies are associated to an industrial scale for several years. Thanks to a modular, upgradeable system, DTA can meet needs ranging from 100 to 10,000 m3 of processed biogas per hour. Medal has already set up major industrial plants: in Johnstown, Pennsylvania, the installation for Keystone Renewable Energy (32 extraction wells) has a capacity of 3,600m3/h, and the capacity at Energy System Group in Johnson City, Tennessee, is 4,100m3/h.

In addition to the reliability of the technology and the fact that no manual operation is required

–         the man-machine interface provides easy operation of the equipment

–         the very low operational expenditure is a notable benefit due to a very high energetic efficiency

In fact, thanks to the reduced number of rotating machines and the limiting of contaminating effluents, maintenance is kept to a minimum and basically involves the compressor placed at the head of the chain…

* Medal is part of the Air Liquid Group