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USE OF LANDFILL GAS FOR POWER GENERATION IN THE UNITED KINGDOM
by Steve Last
It is a well known fact that the gas generated in landfills can be tapped after the completion of the landfill, and when burnt as fuel has an energy value of about half that of natural gas. It is of lower calorific due to the fact that there is only about 60%, at most, combustible methane in the gas created and as methane in the gas provides almost all of its energy this has a calorific value of 37 GJ/t compared to 50 GJ/t for natural gas.
Therefore, for all calculations for the potential landfill gas energy yield, and economic value, this must be remembered.
Trying to estimate countrywide landfill gas emissions is not straightforward, as there are a great many variables which will affect yield rates.
When estimating landfill gas yields, there are two complimentary forms of landfill gas emission prediction normally used. These are modeling and field measurement.
Modeling is a useful predictive tool but it needs validation by actual measurements. It is normal for calibration (verification) of models to be carried out before major investment commences on landfill gas utilisation schemes, and verification is carried out by physical test pumping of landfill gas from a temporary gas pump and falring rig, placed in a number of well chosen locations in the landfill.
United Kingdom Research in this field has been carried out by:
Golder Associates
Enviros Consulting
AEA Technology Environment under contract to the Department of Environment Transport and the Regions (DETR).
In order to make an estimate by modeling, data must be input, or estimates made based upon assumptions, on the composition of the waste arisings and dates disposed. Particularly important will be the composition data over recent years and predictions for the future. This is not easy given that so much more waste will be directed away from landfills over the next few years, and hence current waste composition and input rates cannot be readily extrapolated into the future with any certainty at all.
Recent studies which combined as much of the available data as possible with the first order exponential decay model as outlined by the models supported for IPCC permitting, has produced results which reportedly suggest that emissions of methane from landfills for the year 2000 were likely to be in the order of 660,000 t/y (K A Brown et. al.).
The researchers are understood to also have reported that:
“Although the system is in equilibrium one year to the next, it shall be assumed that only 75% of this can usefully be captured because of that which will escapes before a landfill can be capped. The energy available through generation would then be 0.75´660,000´(calorific value of methane)´(efficiency)´(load factor). Typical efficiency of this process is 75%, there is a load factor of 0.9 (all load factors from www.bwea.com/old/fs4cap.htm ), and the calorific value of methane is 37GJ/t. This gives a potential to generate 392 MW of power. Add this to the 232 MW already existing from capped and trapped landfill gas in the UK (from www.biogas.org.uk) gives the total potential of just over 0.6GW. Putting this into perspective, the total installed capacity of the UK as a whole is about 73GW (figure from Electricity Association, year ending 1998) so ideally landfill gas alone could satisfy about 0.9% of the UK's energy requirements. This kind of analysis shall also be conducted upon waste incineration, gasification and anaerobic digestion (in Sections 2, 3 and 5 respectively) to draw comparisons between the processes.”
Generating electricity from landfill gas is clearly a better sustainable solution than merely trapping and flaring it in a completely unproductive manner, although flaring by burning should always be installed and maintained, as is required in the UK and also by most other EU Landfill Regulations. (Flaring reduces the Greenhouse Gas producing potential by about 20 fold.)
So, landfill gas is a resource that should be employed and by so doing a small but significant reduction in demand for non-renewable energy use can be achieved.
For the site operators there is the added bonus of an additional financial revenue stream from the electricity sales, when EfW schemes are implemented. In recent years the incentives provided by the UK government have also provided a significant boost by raising the economic viability of these projects both cost and reducing risk.
This article courtesy of "http://www.directory.landfill-gas.com.
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