Environmental Engineering Reference
In-Depth Information
B. LFG Pretreatment System
A condensate removal system is required, since the gas is at temperatures above ambient
and is saturated with water vapour and organic vapours. As the gas cools the water vapour
condenses to form water in the pipe, which reduces the efficiency of gas collection and
transport. The condensate system used to remove the water vapour consists of baffled or
expansion chambers which cool and condense the water. Condensate systems both below and
above ground may be required to de-water the gas. A filter would also be included to remove
fine particulate material from the gas flow. The gas is then compressed and possibly passed to
a pre-treatment section if a greater degree of clean-up is required, for example, to remove
corrosive trace gases and vapor from the gas stream. Such possible pre-treatments may
include further filtration, gas chilling to condense certain constituents, absorption and
adsorption systems to scrub the gases, and other gas clean-up systems such as membranes and
molecular sieves to remove trace contaminants. A large proportion of the landfill gas consists
of carbon dioxide which is non-combustible and therefore reduces the overall calorific value
of the gas. Therefore, for utilization systems requiring a high specification gas or a high
calorific value, then clean-up systems to remove carbon dioxide may be required. Such
systems include water scrubbing, absorption on zeolites and membrane separation and are
expensive to install and maintain (Brown and Maunder 1994; Stegmann 1996).
C. LFG Utilization System
1. Combustion technologies (Flaring Practices) LFG flaring
Combustion is the most common technique for controlling and treating landfill gas.
Combustion technologies such as flares, incinerators, boilers, gas turbines, and internal
combustion engines thermally destroy the compounds in landfill gas. Over 98% destruction of
organic compounds is typically achieved. Methane is converted to carbon dioxide, resulting in
a large greenhouse gas impact reduction. Combustion or flaring is most efficient when the
landfill gas contains at least 20% methane by volume. At this methane concentration, the
landfill gas will readily form a combustible mixture with ambient air, so that only an ignition
source is needed for operation. At landfills with less than 20% methane by volume,
supplemental fuel (e.g., natural gas) is required to operate flares, greatly increasing operating
costs. When combustion is used, two different types of flares can be chosen: open or enclosed
flares.
1.1. Open flame flares
Open flame flares
(e.g., candle or pipe flares), the simplest flaring technology, consist of
a pipe through which the gas is pumped, a pilot light to spark the gas, and a means to regulate
the gas flow. The simplicity of the design and operation of an open flame flare is an
advantage of this technology. Disadvantages include inefficient combustion, aesthetic
complaints, and monitoring difficulties. Sometimes, open flame flares are partially covered to
hide the flame from view and improve monitoring accuracy.
