Technology Outline

1) Production of landfill gas
2) Utilization of landfill gas

In Hong Kong, landfilling has been the main disposal practice for municipal solid wastes since 1960s. Currently, there are three operating strategic landfills and 13 closed landfills with a total capacity of 135 million tons and 72.5 million tons respectively.

In a landfill, decomposition of organic portions of the solid wastes produces carbon dioxide, methane gas and other organic compounds and gaseous products. In the advanced landfills, gas collection systems are installed to collect the landfill gas (see diagram below).

landfill. The text above describes the image.
(Source for above: Environmental Protection Department, HKSAR)

1) Production of landfill gas

Landfill gas is a mixture of gases. It contains 45-60% of methane by volume, with the rest being mainly carbon dioxide and a trace amount of nitrogen, hydrogen, ammonia, sulfur dioxide and carbon monoxide. When municipal solid wastes are deposited in a landfill, the wastes undergo a number of simultaneous biological, physical and chemical processes to give out landfill gas and other compounds. These processes include decomposition in which wastes are decomposed by bacteria under aerobic and anaerobic conditions, and volatilization in which wastes in liquid or solid form are vaporized into gaseous form. The decomposition of wastes in a landfill can be divided into 4 phases with different types of bacterial activities and various landfill gas compositions in each phase. Methane is produced in the third phase by methanogenic bacteria under anaerobic conditions (see figure below).

Production of landfill gas. The text above describes the image.

The rate and volume of landfill gas production are site specific and are determined by the waste characteristics (such as the types and ages of wastes buried in the landfill) and the environmental factors like temperature, moisture, and oxygen availability in the landfill. Field experience has suggested that landfill gas appears within 3 months to 1 year after waste disposition, reaching a peak at 5 to 10 years after the waste is buried, and then tapering off for a period of up to 25 years or more. It is possible for a landfill to give off landfill gas for a total of 30 years or more. Take Gin Drinker's Bay Landfill as an example, it was closed in 1979 and still giving off landfill gas at a rate of 85 cubic meter per hour (EPD website, 2005) and the gas is currently used for heating in the leachate treatment process.

2) Utilization of landfill gas

Landfill gas is widely regarded as a renewable energy resource. It can be used for generating electricity, used as medium calorific value fuel, or processed into high calorific value fuel by vapor-liquid separation and by solvent scrubbing to remove the contaminating liquid condensate and vapor contaminants.

Utilization of landfill gas. The text above describes the image.

A summary of landfill gas utilization technologies is listed below:

  1. For power generation - internal combustion engines, gas turbines, fuel cells.
  2. As a medium-CV (calorific value) fuel - direct combustion of the gas in a boiler or industrial furnace, gas-fired chiller, industrial process heating, leachate processing
  3. As a high-CV fuel - pipeline-quality gas, compressed gas vehicles

In Hong Kong, landfill gas is used in mainly three ways, including

  • for power generation with engine gensets to meet on-site electricity needs;
  • as a fuel in the treatment process of the leachate arising from the landfill;
  • as a fuel for process heating in towngas production.

In Hong Kong, the landfill sites are under the Environmental Protection Department (EPD). For more detailed information about the applications of landfill gas in Hong Kong, please visit the relevant page at the EPD website:
This web page has hyperlinks which may transfer you to third-party website.http://www.epd.gov.hk/epd/english/environmentinhk/waste/ prob_solutions/msw_lgu.html

   
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