BIOGAS RECOVERY FROM LANDFILLS - Energy Recovery - page 50

1.2. Enclosed flame flares
Enclosed flame flares are more complex and expensive than open flame flares.
Nevertheless, most flares designed today are enclosed, because this design eliminates some of
the disadvantages associated with open flame flares. Enclosed flame flares consist of multiple
burners enclosed within fire- resistant walls that extend above the flame. Unlike open flame
flares, the amount of gas and air entering an enclosed flame flare can be controlled, making
combustion more reliable and more efficient.
1.3. Other enclosed combustion technologies
Other enclosed combustion technologies such as boilers, process heaters, gas turbines,
and internal combustion engines can be used not only to efficiently destroy organic
compounds in landfill gas, but also to generate useful energy or electricity, as described later
in this chapter. In addition to flaring, the other options for dealing with landfill gas (once
collected) are as follows:

• boilers for making thermal energy

• Internal combustion engine for generating electricity

• Gas turbine for generating electricity

• Fuel cell for generating electricity

• Conversion of the methane to methyl alcohol

There are limited data comparing emissions from landfill gas flares to energy producing
combustion devices (which includes boilers, turbines and internal combustion engines).
According to very limited data in a USEPA 1995 report, carbon monoxide and NO x emissions
are highest from internal combustion engines and lowest from boilers. Flares and gas turbines
are somewhere in the middle.
Flaring of landfill gas is done either in a candle flare or a shrouded flare. A candle flare is
an open air flame. With such, there is no reliable means to monitor for dioxins or other toxic
emissions. Shrouded flares involve enclosing the flame in an insulated cylindrical shroud
which can be anywhere from 5 to 18 m tall. While dioxins can be tested for in such flares, it is
possible that enclosing the flare will keep the post combustion temperature in dioxin-
formation range, resulting in increased dioxin emissions. Essentially, this is a loose-loose
situation. Most shrouded landfill gas flares have exit temperatures of around 760 0 C, well
above the dioxin formation range (which end around 400 0 C). In such cases, dioxins will be
formed in midair as the exhaust hits the cooler background air after leaving the stack.
Dioxin emissions data are also very sparse. Flares are known to generate more dioxin
than internal combustion engines or boiler mufflers. There is high variability in dioxin
emissions from landfill gas burners (based on composition of waste dumped and also on the
combustion technology - internal combustion engines are much more variable). Burning
landfill gas is dirtier than burning natural gas. Whether using an internal combustion engine
or a gas turbine, burning landfill gas to produce energy emits more pollution per kilowatt hour
than natural gas does.
Some public concerns have been raised about whether the combustion of landfill gas may
create toxic chemicals. Combustion can create acid gases such as SO 2 and NO x . The generation
of dioxins has also been questioned. EPA investigated the issue of dioxin formation and