Landfill Gas Emission

LANDFILL GAS EMISSION

Landfill gas contains a high percentage of methane due to the anaerobic decomposition of organic matter, which can be utilised as a source of energy. Previously, we will explain the composition and properties, risks, migration and control of landfill gas.

1      Composition and properties

Climatic and environmental conditions also influence gas composition. Due to the heterogeneous nature of the landfill, some acid-phase anaerobic decomposition occurs along with the methanogenic decomposition. Since aerobic and acid-phase degradation give rise

to carbon dioxide and not methane, there may be a higher carbon dioxide content in the gas generated than what would otherwise be expected. Furthermore, depending on the moisture distribution, some  carbon  dioxide  goes  into  solution. This  may  appear  to increase (artificially) the methane content of the gas measured in the landfill. A typical landfill gas contains a number of components such as the following, which tend to occur within a characteristic range:

Methane: This is a colourless, odourless  and flammable gas  with a density lighter than air, typically making up 50 - 60% of the landfill gas. 

Carbon dioxide: This  is a colourless,  odourless and non-inflammable  gas that is denser than air, typically accounting for 30 - 40%. 

Oxygen: The flammability  of methane  depends  on the  percentage  of oxygen.  It   is, therefore,  important to control  oxygen  levels,  where gas abstraction  is undertaken.

Nitrogen: This is essentially inert and will have little effect, except to modify t explosive range of methane.

2 Hazards

Landfill gas consists of a mixture of flammable, asphyxiating and noxious gases and may be hazardous to health and safety, and hence the need for precautions.

Some of the major hazards are listed below:

Explosion and fire: Methane is flammable in air within the range of 5 - 15% by volume, while hydrogen is flammable within the range of 4.1 - 7.5% (in the presence of oxygen) and potentially explosive.  Fire, occurring  within the waste, can be difficult  to  extinguish and can  lead to  unpredictable and uncontrolled subsidence as well as production of smoke and toxic fumes.    

Trace  components:  These comprise  mostly alkanes  and alkenes,  and  their oxidation products such  as  aldehydes,  alcohols  and  esters .  Many of them  are  recognised as toxicants, when    present in air    atconcentrations  above occupational  exposure standards.     

Global warming: Known also as greenhouse effect, it is the warming of the earth's atmosphere by the accumulation of gases (methane, carbon dioxide and chlorofluorocarbons) that absorbs reflected solar radiation.

3 Migration

During  landfill  development,  most  of  the  gas  produced  is  vented  to  the atmosphere, provided the permeable intermediate cover has been used. While biological and chemical processes affect gas composition through methane oxidation, which converts methane to carbon dioxide, physical factors affect gas migration. The physical factors that affect gas migration include:

Environmental  conditions:  These  affect  the rate  of degradation  and  gas pressure build up.

Geophysical conditions: These affect migration pathways. In the presence of fractured geological strata or a mineshaft, the gas may travel large distances, unless restricted by the water table.

Climatic conditions:  Falling   atmospheric   pressure,  rainfall   and  water infiltration rate affect landfill gas migration.

The  proportion  of  void  space  in  the  ground,  rather  than  permeability,  determines  the variability   of  gas  emission. If  the  escape  of  landfill   gas  is  controlled and  proper extraction system  is  designed,  this  gas  can be utilised  as a source of energy.  If  landfill gas  is  not  utilised,  it  should  be  burnt by means of flaring. However,  landfill  gas utilisation can  save  on  the  use of  fossil  fuels  since  its heating  value is  approximately  6 kWh/m³ and can be  utilised in internal combustion engines for production of electricity and heat.

It  is  important that landfill  gas is extracted during  the operation  phase.  It  is extracted out of the landfill by means of gas wells, which are normally drilled by auger and are driven into the landfill at a spacing of 40 - 70 m. In addition, horizontal systems can be installed during operation of the landfill. The gas wells consist mainly of perforated plastic pipes surrounded by coarse gravel and are connected with the gas transportation pipe with flexible tubing.

The vacuum necessary for gas extraction and transportation is created by means of a blower. The most important factors influencing planning and construction of landfill gas extraction systems are settling of waste, water tables in landfills and gas quality.