Biogas Treatment 

Landfill sites can produce H2S in the presence of wastes plasterboard or other products that contain sulphur. It can also be present if there is the presence of gypsum (CaSo4.2H20).

In anaerobic digestion, H2S is produced by the microbial breakdown of organic material under anaerobic conditions and produces biogas that must be treated before using in vulnerable machinery.

In humans, prolonged exposure to high enough levels of H2S can cause unconsciousness and those affected may continue to experience headaches, reduced attention span and altered motor functions. Effects of H2S gas exposure may not become apparent for up to 72 hours following removal from the affected environment.

As H2S is heavier than air it usually accumulates in low lying areas of poorly ventilated spaces. During the production and collection of biogas in landfill sites or anaerobic digestion plant, H2S gas in the presence of air and moisture can form sulphuric acid which is capable of corroding metals within machinery that is often not seen.

There are several types of treatment system that involve high initial investment together with high ongoing costs of operation.

Organics has developed bio-scrubber technology, the main advantage of which is that, in most circumstances, no additional costs for chemical-additions are incurred. The bacteria involved in the process are ubiquitous and, as long as correct environmental conditions are maintained, the bacteria will function reliably and predictably in the removal of hydrogen sulphide from gas streams. The system is ideal for use with biogas producing projects that rely on anaerobic digestion such as bespoke AD projects or biogas generated in landfill sites.

A gas chiller can reduce the temperature of a biogas stream to around 2ºC. Temperatures below this can also be achieved but will result in freezing of the heat exchanger tubes. Where temperatures of less than zero are required it will be necessary to run tandem heat exchangers, switching the flow from one to the other as they become blocked with ice.

By reducing the temperature to close to freezing point the relative humidity of the gas can also be reduced, thus minimising the possibility of condensate formation in pipes down-stream of the chiller. The unit is usually placed before or after the booster unit on biogas collection systems either on landfill gas or bespoke anaerobic digesters. 

Chiller packages are, as with other items of purpose built process plant supplied by Organics, unlimited in practical size.

Flow rates of up to 10,000 Normal cubic metres per hour can be effectively “chilled” to the target levels.

It is often the case that suppliers of certain types of equipment, such as internal combustion engines and gas turbines, will specify a maximum moisture content in the gas to be fed to their plant.

The options for dealing with such requirements are limited. Hydroscopic filters will reduce dew points by up to 6ºC but this may provide inadequate protection, especially as the dew point of the feed gas often varies significantly.

Where properly designed and built a packaged chiller will reliably ensure the target dew point is continuously met and the equipment is most effective on landfill biogas or biogas generated within AD plants.

By means of a high-voltage applied to an ultra-violet light generating, catalytic wire mesh, molecules in ambient air are split and energised, converting the moisture and air into a flow consisting of a mixture of unstable atoms, ions and radicals, with elevated electron activity levels. These Activated Air molecules may then be injected into a gas carrying target compounds. The result is the high-rate oxidation of odorous molecules.

• Highly efficient Odour Control by means of the high-rate oxidation of target substances,
• No moving parts, mechanical wear,
• No chemical additions required,
• No water consumption and no waste products
• Low energy requirements

It is now well acknowledged that landfill biogas or biogas from AD plants can be employed as a unique fuel that does not normally require back-up from deisel or natural gas injection. However, biogas possesses characteristics that are not matched by any other similar fuel gas, such as natural gas.

For this reason, a biogas fuel skid must be built specifically for this application. Of particular note are the high-levels of moisture drop out and the possibility of a wide range of potentially corrosive trace gases. These range from halogens to ammonia and mercaptans.

• High-reliability gas pumping sets
• Optional air blast cooling and refrigerated chilling for dew-point management
  site-specific design
• Automatic moisture removal systems for collected condensate
• Integrated control systems with facilities to link into main control centre.

• Flow rates available: 50 to 20,000 cubic metres per hour
• Suction pressure head rarely needs to exceed 150 mbar. Deep-vacuum systems can also be considered for particular situations.
• Delivery pressure head: 300 to 600 mbar g are normally achieved with twin-stage centrifugal blowers. Higher pressure heads may be achieved with screw compressors, rotary vane compressors and multi-stage reciprocating compressors.
• Organics fuel skids have a well-established record in providing biogas to generators with up times in excess of 99%.