At present, much of the methane gas leaking out of rubbish tips comes
from biological materials, especially waste food. So, as long as we keep
throwing away things like food and newspapers, landfill gas is a sustainable
energy source – plus, burning that methane might be a good idea from
a climate-change perspective, since methane is a stronger greenhouse-gas
than CO2. A landfill site receiving 7.5 million tons of household waste per
year can generate 50 000 m3 per hour of methane.
In 1994, landfill methane emissions were estimated to be 0.05 m3 per
person per day, which has a chemical energy of 0.5 kWh/d per person,
and would generate 0.2 kWh(e)/d per person, if it were all converted to
electricity with 40% efficiency. Landfill gas emissions are declining because
of changes in legislation, and are now roughly 50% lower.
SELCHP (“South East London Combined Heat and Power”) [www.selchp.
com] is a 35 MW power station that is paid to burn 420 kt per year of black-
bag waste from the London area. They burn the waste as a whole, without
sorting. After burning, ferrous metals are removed for recycling, hazardous
wastes are filtered out and sent to a special landfill site, and the remaining
ash is sent for reprocessing into recycled material for road building
or construction use. The calorific value of the waste is 2.5 kWh/kg,
and the thermal efficiency of the power station is about 21%, so each 1 kg
of waste gets turned into 0.5 kWh of electricity. The carbon emissions are
about 1000 g CO2 per kWh. Of the 35 MW generated, about 4 MW is used
by the plant itself to run its machinery and filtering processes.
Scaling this idea up, if every borough had one of these, and if everyone
sent 1 kg per day of waste, then we’d get 0.5 kWh(e) per day per person
from waste incineration.
This is similar to the figure estimated above for methane capture at
landfill sites. And remember, we can’t have both. More waste incineration
means less methane gas leaking out of landfill sites. See figure 27.2, p206,
and figure 27.3, p207, for further data on waste incineration.
283The power per unit area of using willow, miscanthus, or poplar, for electricity
is 0.2 W/m2. Source: Select Committee on Science and Technology Minutes
of Evidence – Memorandum from the Biotechnology & Biological Sciences
Research Council [www.publications.parliament.uk/pa/ld200304/
ldselect/ldsctech/126/4032413.htm]. “Typically a sustainable crop of 10