The result is sensitive to some input data, where of one can
mention the price of biogas, the investment costs for anaerobic
treatment and waste incineration, the transport costs for wastewater
sludge and sludge from anaerobic treatment to the agricultural
sector and the costs for the source separation system.
Table 2 shows that the local scenario “Maximized electricity
production” receives the highest scores for emissions of climate
gases. Also “Maximized return of nutrients” receives good results.
The local scenario “Maximized return of nutrients” receives better
results compared to “Business-as-usual” partly because this
scenario includes production of biogas that keep out the use of
fossil fuels in the transport sector. A second reason is that more
electricity is generated within the system that keeps out electricity
production outside the system with higher emissions of climate
gases. The reason for the increased electricity production is that
new waste treatment capacity partly consists of a facility for
anaerobic digestion, which decreases the need for investments in
waste incineration. With less waste incineration, more district heat
will be produced from other combined heat and power plants
within the district heating system. These units have a higher
electrical efficiency compared to waste incineration, thus more
electricity will be produced within the system. The local scenario
“Maximized electricity production” receives even better results for
emissions of climate gases. The reason is that in this scenario the
electricity production from the system increases even further. This
is partly because in this scenario the effects of investing in a waste
incineration facility with higher electrical efficiency compared to
existing ones is studied. The result is also because this scenario
includes investments in a sludge dryer, heated by district heat. This
causes an increase in the heat demand, thereby combined heat and
power plants within the district heating system may run during
a longer period of time.
The local scenarios “Maximized return of nutrients” and
“Maximized electricity production” reduces the emissions of
greenhouse gases by 4e7% (17 000e29 000 tonnes CO2-eq./year)
compared to “Business-as-usual”. Another figure to relate to is the
total emissions of CO2 from Göteborg Energi, which in 2009
amounted to 545 000 tonnes CO2 [7]. As seen before, the local
scenario “Maximized return of nutrients” yield a decrease in the
system cost, in other words the cost for reducing the emissions of
greenhouse gases by choosing this path is negative. In contrast the
system cost increases by the local scenario “Maximized electricity
production”, hence the cost for reducing greenhouse gases by the
measures stapled in this scenario can be calculated to between 80
and 550 SEK/tonne CO2 (depending on which external scenario you
choose to study).
4. Discussion and conclusions
The following ten conclusions have been drawn from the
project.
I. From an economic point of view, new waste treatment
technologies have difficulties in the competition with the