Page 28 - Materials Chemistry, Second Edition

14 E. I. Wiloso and R. Heijungs

convert energy at rates that are responsible for climate change over extended areas.
With 81 % of recent global energy use originating from fossil fuels, 6 % from
nuclear, and 13 % from renewable energy (IEA-Bioenergy 2009), it is under-
standable that human societies have recently begun to reconsider the use of
renewable sources. In light of this development, we are now, along with other
environmental impacts, facing two major problems: depletion of fossil resources
and an increase in anthropogenic levels of carbon dioxide.
Alternative options that are available to reduce our dependence on non-
renewable sources and simultaneously mitigate climate change are already in
development. The use of bio-based renewable energy (bioenergy) is now deemed
to be one of the most promising renewable energy alternatives. Reasons typically
given for why bioenergy should be promoted are diverse. Bioenergy is considered
carbon neutral, it is made from renewable resources, it stimulates the agricultural
sector, and it may be produced domestically in many countries, hence diminishing
political and economic dependency on other countries (Guinée et al. 2009).
However, criticisms have also developed against biofuels, particularly on their role
in the food price spikes and the nature of land-use change. A specific example of
this case is the maize to bioethanol for transportation fuel in the United States that
induced land-use impact, direct and indirect (Harvey and Pilgrim 2011). WRI
(2005) indicated that land use (18.2 %) and agriculture’s (13.5 %) contribution to
greenhouse gas emissions (GHGs, including N 2 O and CH 4 in addition to CO 2 ) are
globally estimated to be at least twice the amount of the total emissions from
global transport (13.5 %). This assessment indicates the importance of the
potential contribution of the land-use aspect to the overall environmental burden of
bioenergy systems. Major activities related to these land-use-related impacts are
deforestation that releases carbon dioxide from burning or decomposing biomass
and oxidizing uncovered humus. In addition to other impact categories such as
biodiversity loss and soil quality degradation, all these emissions may negate any
GHG benefits of biofuel systems for decades to centuries (Tilman et al. 2009). In
this regard, these same authors proposed that biofuels should receive policy sup-
port as substitutes for fossil energy only when they make a positive impact on four
important objectives: energy security, GHG emissions, biodiversity, and the sus-
tainability of the food supply.
Bioenergy is presently the largest global contributor (77 %) to renewable
energy and has contributed significantly to the production of heat, electricity, and
fuels for transport (IEA-Bioenergy 2009). Therefore, in the following parts of this
chapter, discussion will be focused on bioenergy as the dominant fraction of
renewable energy. The main feedstocks for bioenergy are biomass residues from
forestry, agriculture, and municipal waste. Only a small portion of sugar, grain,
and vegetable oil are used for the production of liquid biofuels (IEA-Bioenergy
2009). There are many technological routes available to convert biomass feedstock
into final bioenergy products. Several conversion technologies have been devel-
oped to adapt to the unique physical nature and chemical composition of various
biomass feedstocks. These include direct combustion (heat), co-firing/combustion
(heat/power), gasification (heat/power), anaerobic digestion (heat/power/fuel: