How To Clean Up Raw Biogas

Biogas Upgrading Technology - Cleaning Raw Biogas Into Usable Natural Gas
By Richard Belcher

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

The problem is that this biogas produced is roughly 60% methane and 29% Co2 with trace elements of H2S, and is not up to the quality 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 enough to destroy the internals of expensive plant.

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 produce biogas purification systems each have their own different processes and technology that they employ to produce the sales quality gas. A few of them are detailed below.

Water Washing

This is the most common method 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 system as 98%+ pure natural gas.

Pressure Swing Adsorption

Otherwise known as PSA, this purification method 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 system usually 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 always 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 method 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 various 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 quality natural gas for grid injection. However depending on the site location, various environmental and economic factors might make selecting a certain type of system a more sensible choice. For areas where water is an expensive resource a more appropriate choice would be a PSA or Selexol system which regenerate the adsorbent, however this has to be offset against the heat input required in regeneration.

Another important factor to consider is the methane loss associated with each design. The methane loss is measured using gas analyzers and flowmeters at the suction and discharge sides of the plant. Most plants are guaranteed by manufacturers to achieve a maximum 2% methane loss. Some recent studies however have measured between 8-10% methane loss at PSA and Selexol plant sites, possibly 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 anywhere between 3-6% of the total energy output in the form of natural gas. The cost associated 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 convert biomass into useful high quality natural gas. A biogas purification system takes the raw biogas at around 60% methane from the digester and through a special 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 system are between 3-6% of the total methane output, with smaller plants cost more to run than larger ones. As digester systems become more common around the world and people begin to catch on to biogas as a renewable source of energy, no doubt we will see more of these systems become available and more innovative designs.

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