Page 155 - New Trends In Coal Conversion

118 New Trends in Coal Conversion

well below the levels required within IEA long-term climate models. Acceleration is
urgently needed to ramp up the contribution of bioenergy across all sectors. Moreover,
bioenergy is a complex subject with many potential feedstocks, conversion processes,
and energy applications. It interacts strongly with the agriculture, forestry, and waste
management sectors, and its prospects are linked to the growth of a broader bio-
economy (IEA, 2017).
The use of renewable energy is nowadays an unavoidable measure to attain sustain-
able development in the world. However, combustion of fossil fuels is still the main
source of energy on the earth and a major contributor to atmospheric carbon dioxide
emissions, which are directly related with the global warming and climate change con-
cerns. Coal is a cheaper and more abundant resource than other fossil fuels, such as oil
and natural gas, while at the same time it is a reliable fuel for power production (Tof-
tegaard et al., 2010). Over 40% of the worldwide electricity is produced from coal
(IEA, 2016), and it is expected that coal plays an important role on the energy supply
if the global energy demand continues to rise in the near future.
Biomass is considered as a renewable energy source for mitigating greenhouse
gases (GHGs), nitrogen oxides, and sulfur oxide emissions. Biomass is carbon neutral,
and it has low contents of nitrogen and sulfur. Combustion of biomass is the most inex-
pensive option of converting biomass fuels to energy. The carbon dioxide generated
from the combustion of biomass has been previously removed from the atmosphere
by the photosynthesis process while the plant grows, hence net carbon emissions
are null. Biomass can be derived from different organic matter resources such as dedi-
cated energy crops, forestry and agriculture residues, seaweed, animal manure, and
organic wastes. Thus, biomass can be classified based on its origin into the following:
(1) primary residues such as wood, straw, cereals, maize, etc., obtained from the by-
products of forest products and food crops; (2) secondary residues such as saw and pa-
per mills, food and beverage industries, apricot seed, etc., derived from processing
biomass material for industrial and food production; (3) tertiary residues such as
wastes and demolition wood, etc., that are derived from other used biomass materials;
and (4) energy crops (Bhuiyan et al., 2018).
Biomass cofiring consists of burning biomass along with coal in coal-fired power
plants to generate electricity. Biomass cofiring with coal is recognized as one of the
most attractive short- to medium-term options for using biomass in the power gener-
ation industry. Solid biomass cofiring involves the combustion of wood chips or pel-
lets in coal-fired power plants, whereas gas biomass cofiring means the firing of
gasified biomass with natural gas or pulverized coal (PC) in gas power plants (indirect
cofiring) (Agbor et al., 2014). In this context, cofiring of biomass with coal may be
considered a bridge between the energy production systems based on fossil fuels
and those based on renewable energy sources, which would contribute to reduce
CO 2 emissions and the dependency on fossil fuels. In addition, the use of biomass
in combination with coal in the same power plant would avoid the typical problems
associated with small biomass-fired power plants, i.e., high specific cost (due to the
larger size of coal power plants) and low efficiency, while at the same time it would
reduce the risk of a biomass shortage (Valero and Us on, 2006). Modern coal power
plants are more efficient than smaller-scale dedicated biomass power plants. There