| Digester systems that convert waste material into | | | | is similar to the water washing method with |
| biogas are becoming more prevalent throughout the | | | | regeneration. Selexol can adsorb hydrogen sulphide, |
| world. Rural farmers now have a means to produce | | | | carbon dioxide and water. However the energy |
| good quality fertilizer and biogas from waste | | | | required to regenerate the solution after adsorbing |
| materials like manure in a cheap renewable way. | | | | H2S is high, so hydrogen sulphide is removed before |
| The problem is that this biogas produced is roughly | | | | the process. |
| 60% methane and 29% Co2 with trace elements of | | | | Chemical Reaction |
| H2S, and is not up to the quality of 99% pure | | | | Raw biogas can be upgraded by various chemical |
| methane natural gas if the owner was planning on | | | | reactions that remove the C02 and other |
| selling this gas or using it as fuel gas for machinery. | | | | contaminants from the gas stream. The chemicals |
| The corrosive nature of H2S alone is enough to | | | | such as Alkanolamines react at atmospheric pressure |
| destroy the internals of expensive plant. | | | | in an adsorption column with the Co2 and are |
| The solution is the implementation of a biogas | | | | regenerated afterwards with steam. The hydrogen |
| upgrading or purification system. Biogas upgrading is a | | | | sulphide must first be removed to avoid toxifying the |
| series of processes where the gas is first cleaned | | | | chemicals. |
| from contaminants and then dried, so that what is | | | | Advantages and Disadvantages |
| left at the end of the process is 98%+ methane fuel | | | | Each plant type fulfills its purpose of supplying high |
| gas. Manufacturers that produce biogas purification | | | | quality natural gas for grid injection. However |
| systems each have their own different processes | | | | depending on the site location, various environmental |
| and technology that they employ to produce the | | | | and economic factors might make selecting a certain |
| sales quality gas. A few of them are detailed below. | | | | type of system a more sensible choice. For areas |
| Water Washing | | | | where water is an expensive resource a more |
| This is the most common method of purifying biogas. | | | | appropriate choice would be a PSA or Selexol system |
| Here raw biogas from the digester is compressed | | | | which regenerate the adsorbent, however this has to |
| and fed into the scrubber vessel where passing | | | | be offset against the heat input required in |
| water streams adsorb the gas contaminants leaving | | | | regeneration. |
| near pure methane. The gas is then dried by | | | | Another important factor to consider is the methane |
| dessicant in the drier columns and exit the system as | | | | loss associated with each design. The methane loss is |
| 98%+ pure natural gas. | | | | measured using gas analyzers and flowmeters at the |
| Pressure Swing Adsorption | | | | suction and discharge sides of the plant. Most plants |
| Otherwise known as PSA, this purification method | | | | are guaranteed by manufacturers to achieve a |
| separates the Co2, Nitrogen, Oxygen and Water | | | | maximum 2% methane loss. Some recent studies |
| from the raw biogas stream by adsorbing gases at | | | | however have measured between 8-10% methane |
| high pressure and desorbing them at low pressure as | | | | loss at PSA and Selexol plant sites, possibly due to |
| waste. The PSA system usually consists of 4 | | | | leaks and poor maintenance. Chemical systems have |
| different adsorption columns working in sequence; | | | | even lower guaranteed losses since the chemicals |
| Adsorption, depressurizing, desorption and | | | | selectively react with the Co2 in the gas stream |
| repressurizing. | | | | instead of adsorbing. |
| The raw biogas is compressed and fed into the | | | | Energy Demands |
| bottom of the adsorption column where it is purified. | | | | For a biogas upgrading plant the auxiliary power |
| during this time the remaining columns regenerate, | | | | required to drive the compressors, pumps etc is |
| such that there is always 1 absorber column actively | | | | anywhere between 3-6% of the total energy output |
| cleaning gas. PSA does not scrub hydrogen sulphide | | | | in the form of natural gas. The cost associated with |
| so this most be removed before it enters the | | | | upgrading biogas also decreases with larger plant size, |
| compressor. | | | | a smallish plant of 100 metres cubed per hour will |
| Polyglycol | | | | upgrade gas at more than twice the cost of a plant |
| Using polyglycol (Tradename Selexol)to purify biogas | | | | outputting 200 - 300 metres cubed per hour. |