Biogas Upgrading Technology - Cleaning Raw Biogas Into Usable Natural Gas
Digestion systems that convert waste material to biogas are becoming more frequent worldwide. Rural farmers now have a means to produce good quality fertilizers and biogas from waste materials such as manure in a cheap and renewable way.
The problem is that this biogas produced is approximately 60% methane and 29% Co2 with trace elements of H2S, and it does not reach 99% pure methane natural gas quality if the owner plans to sell this gas or use it as fuel gas for machinery. The corrosive nature of H2S is only enough to destroy the internal parts of the expensive plant.
The solution is the implementation of a biogas improvement or purification system. Biogas upgrading is a series of processes where the gas is first cleaned of contaminants and then dried, so that what remains at the end of the process is 98% + methane fuel gas. Manufacturers that produce biogas purification systems have their own different processes and technology that they use to produce the quality gas for sale. Some of them are detailed below.
This is the most common method of purifying biogas. Here, the raw biogas from the digester is compressed and fed into the wash container where the water streams adsorb the gas contaminants and leave pure methane nearby. The gas is then desiccant dried in the drier columns and exits the system as 98% + pure natural gas.
Pressure swing adsorption
Also known as PSA, this purification method separates CO2, nitrogen, oxygen, and water from the raw biogas stream by absorbing high-pressure gases and desorbing them at low pressure as waste. The PSA system generally consists of 4 different adsorption columns that operate in sequence; Adsorption, depressurization, desorption and repressurization.
The raw biogas is compressed and fed to the bottom of the adsorption column where it is purified. During this time, the remaining columns are regenerated, so there is always 1 absorbent column that actively cleans the gas. PSA does not scrub hydrogen sulfide, so it must be removed before it enters the compressor.
The use of polyglycol (trade name Selexol) to purify biogas is similar to the regeneration water wash method. Selexol can adsorb hydrogen sulfide, carbon dioxide and water. However, the energy required to regenerate the solution after adsorbing H2S is high, so the hydrogen sulfide is removed before the process.
Crude biogas can be enhanced through various chemical reactions that remove CO2 and other contaminants from the gas stream. Chemicals such as alkanolamines react to atmospheric pressure in an adsorption column with Co2 and are then regenerated with steam. Hydrogen sulfide must first be removed to avoid toxicity from chemicals.
Advantages and disadvantages
Each type of plant fulfills its purpose of supplying high quality natural gas for injection into the network. However, depending on the location of the site, various environmental and economic factors may make selecting a certain type of system a more sensible option. For areas where water is a costly resource, a more appropriate option would be a PSA or Selexol system that regenerates the adsorbent, however this must be compensated for by the input of heat required in the regeneration.
Another important factor to consider is the loss of methane associated with each design. Methane loss is measured using gas analyzers and flow meters on the suction and discharge sides of the plant. Most plants are guaranteed by manufacturers to achieve a maximum methane loss of 2%. However, some recent studies have measured between 8-10% of methane loss in PSA and Selexol plants, possibly due to leaks and poor maintenance. Chemical systems have even lower guaranteed losses as chemicals selectively react with Co2 in the gas stream rather than adsorb.
For a biogas improvement plant, the auxiliary power required to drive the compressors, pumps, etc., is between 3-6% of total energy production in the form of natural gas. The cost associated with upgrading biogas also decreases with the size of the larger plant, a smaller 100 meter bucket per hour plant will upgrade gas to more than double the cost of a plant that produces 200 - 300 cube meters per hour.
A digester is just the beginning of the process to convert biomass into high quality useful natural gas. A biogas purification system extracts the raw biogas with around 60% methane from the digester
