Cleaning Raw Biogas Into Usable Natural Gas

Biogas Upgrading Technology - Cleaning Raw Biogas Into Usable Natural Gas

Digester systems that change waste material into biogas are becoming more prevalent throughout the world. Rural farmers now have a means to yield good potential fertilizer and biogas from waste materials like manure in a cheap renewable way. 

The question is that this biogas produced is almost 60% methane and 29% Co2 with trace elements of H2S, and is not up to the potential of 99% pure methane natural gas if the owner was planning on selling this gas or using it as fuel gas for machinery. The corrosive nature of H2S alone is adequate to destroy the internals of expensive plant.

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The solution is the implementation of a biogas upgrading or purification system. Biogas upgrading is a series of processes where the gas is first cleaned from contaminants and then dried, so that what is left at the end of the process is 98%+ methane fuel gas. Manufacturers that yield biogas purification systems each have their own different processes and technology that they employ to yield the sales potential gas. A few of them are detailed below.
Water Washing
This is the most coarse recipe of purifying biogas. Here raw biogas from the digester is compressed and fed into the scrubber vessel where passing water streams adsorb the gas contaminants leaving near pure methane. The gas is then dried by dessicant in the drier columns and exit the theory as 98%+ pure natural gas.
Pressure Swing Adsorption
Otherwise known as Psa, this purification recipe separates the Co2, Nitrogen, Oxygen and Water from the raw biogas stream by adsorbing gases at high pressure and desorbing them at low pressure as waste. The Psa theory ordinarily consists of 4 different adsorption columns working in sequence; Adsorption, depressurizing, desorption and repressurizing.
The raw biogas is compressed and fed into the bottom of the adsorption column where it is purified. During this time the remaining columns regenerate, such that there is all the time 1 absorber column actively cleaning gas. Psa does not scrub hydrogen sulphide so this most be removed before it enters the compressor.
Polyglycol
Using polyglycol (Tradename Selexol)to purify biogas is similar to the water washing recipe with regeneration. Selexol can adsorb hydrogen sulphide, carbon dioxide and water. However the energy required to regenerate the solution after adsorbing H2S is high, so hydrogen sulphide is removed before the process.
Chemical Reaction
Raw biogas can be upgraded by discrete chemical reactions that remove the C02 and other contaminants from the gas stream. The chemicals such as Alkanolamines react at atmospheric pressure in an adsorption column with the Co2 and are regenerated afterwards with steam. The hydrogen sulphide must first be removed to avoid toxifying the chemicals.
Advantages and Disadvantages
Each plant type fulfills its purpose of supplying high potential natural gas for grid injection. However depending on the site location, discrete environmental and economic factors might make choosing a definite type of theory a more sensible choice. For areas where water is an expensive reserved supply a more thorough option would be a Psa or Selexol theory which regenerate the adsorbent, However this has to be offset against the heat input required in regeneration.
Another foremost factor to think is the methane loss linked with each design. The methane loss is measured using gas analyzers and flowmeters at the suction and extraction sides of the plant. Most plants are guaranteed by manufacturers to perform a maximum 2% methane loss. Some up-to-date studies However have measured between 8-10% methane loss at Psa and Selexol plant sites, perhaps due to leaks and poor maintenance. Chemical systems have even lower guaranteed losses since the chemicals selectively react with the Co2 in the gas stream instead of adsorbing.
Energy Demands
For a biogas upgrading plant the auxiliary power required to drive the compressors, pumps etc is in any place between 3-6% of the total energy production in the form of natural gas. The cost linked with upgrading biogas also decreases with larger plant size, a smallish plant of 100 metres cubed per hour will upgrade gas at more than twice the cost of a plant outputting 200 - 300 metres cubed per hour.
Conclusions
A Digestor is only the beginning of the process to change biomass into beneficial high potential natural gas. A biogas purification theory takes the raw biogas at nearby 60% methane from the digester and straight through a extra process outputs 98% methane for ether use as fuel gas or supplied to the grid. The four main upgrading processes are water washing, pressure swing adsorption, polyglycol adsorption and chemical treatment. Water washing and Psa are the most predominantly used systems in the world today. Typical energy requirements for a biogas purification theory are between 3-6% of the total methane output, with smaller plants cost more to run than larger ones. As digester systems become more coarse nearby the world and citizen begin to catch on to biogas as a renewable source of energy, no doubt we will see more of these systems become ready and more innovative designs.

Source: http://2011technology-news.blogspot.com/2011/09/biogas-upgrading-technology-cleaning.html