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Re: Dioxins and Burning Landfill Gas
>Burning with internal combustion or turbine engines means that you can
>get rid of the landfill gas and generate electricity at the same time and
>lower use of other fossil fuels.
Intuitively this sounds reasonable, albeit still grossly inefficient overall, but looking at the data from the UK dioxin inventory indicates that combusting the gas may be by far the worst option so far as dioxin production goes:
A review of Dioxin Emissions in the UK - HMIP
DOE Research report DOE/HMIP/RR/95/004
4.8 LANDFILL GAS
There are limited data on emissions of PCDD/Fs from landfill gas. The gas generated may be collected for use in engines, turbines or in industrial processes. In some cases gas is flared. At all sites some gas escapes directly to the atmosphere.
Bremmer et al (1994) report emission concentrations of 0.022 and 0.07 ng I-TEQ m3 for gas that is flared and combusted in a turbine respectively. Fiedler (1994) reports a range of 0.006 to 0.07 ng I-TEQ m3 in the exit gas. Mitchell et al (1993) report concentrations of 0.078 and 0.097 ng I-TEQ m3 in the exhaust of a landfill gas engine in the UK, and 0.32-0.36 ng I-TEQ m3 in raw gas. One data set (Warren Spring Laboratory, 1991) provided an emission concentration of 1.2 ng I-TEQ m3 for a gas engine. The following emission factors are obtained:
€ Escaping gas: 0.32-0.36 ng m3 (raw gas)
€ Flared gas: 0.022 ng m3 (flue gas at 11% oxygen)
€ Combusted in engines: 0.006-1.2 ng m3 (flue gas at 11% oxygen)
An estimated 2 M tonnes of methane are emitted from UK landfill sites annually, of which 0.36 M tonnes are collected and flared, while 0.11 M tonnes are used for engines or heat ([Department of the Environment, 1993a). Assuming that landfill gas comprises 50% methane and approximately 10 m3 of flue gas is produced per m3 of gas burned, the following methane quantities are obtained:
€ Escaping gas: 4,384 M m3 (STP) p.a.
€ Flared gas: 1,008 M m3 p.a. (raw) = 10,039 M m~3 p.a.
€ Combusted in engines: 308 M m3 p.a. (raw) = 3,068 M m3 p.a. (flue gas)
The flared gas and combusted gas emission factors are expressed in terms of the flue gas produced during combustion.
If you believe these figures (and I keep an open mind) then the dioxin implications of the different scenarios are, by my calculations:
1) Vent all UK Gas releases 1.8 - 2.05 g TEQ to air
2) Flaring all gas releases 1.25 g TEQ to air
3) Combusting in engines all gas releases 0.34 to 68.4 g to air
Considering that the total predicted future releases to air by 2000 is 110-350 g TEQ then the third option looks like it could potentially be a (the?) major contributor to UK emissions if we went down the gas burning option.
Surely this is another case where the only real answers are:
1) to avoid the dioxin precursors and
2) reduce the landfill gas emissions?
If the UK could break dangerously misplaced affection for landfill and support the Landfill Directive currently being discussed in Europe then the putrescible wastes could be stopped from going to landfill in the first place. New Labour has done (another) environmental U turn on this and, rather than attacking dodgy landfill and supporting composting and digestion as alternatives for putrescible waste, now votes with the conservative opposition.
_\\|//_ Alan Watson C.Eng
(' O^O ') Oakleigh
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