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3 Advanced Technologies (Biological and Thermochemical) …
of its wood pellets demand is met through import from Canada and some Eastern
European countries [22].
Other European countries are also using biomass co-firing technologies in their
power plants that make a significant portion of a total of 230 power and CHP co-firing
plants around the world. For example, 14 biomass co-firing plants are operating in
Finland using wood and peat pellets and around 5 plants are in Austria utilizing wood
chips (mostly bark) co-fired with pulverized coal [22].
Canada and the U.S. are among the top ten countries producing GHG emissions
mainly CO 2 based on 2008 data [22], amounting to 6673 megatonnes (Mt) (2013)
and 722 megatonnes (Mt) (2015) carbon dioxide equivalent (CO 2 eq) in the United
States and Canada, respectively. Thus, these two countries have specific policies to
drop their GHG emissions by around 28–30% by 2030 [22]. All the biomass co-firing
plants in the US and most of them in Canada use direct co-firing technology. Almost
half of the US plants use wood products and railroad ties in co-firing while Canada
uses agricultural and forestry residues, domestic and municipal wastes, and energy
crops [22].
3.1.2 Pyrolysis
Pyrolysis or Destructive Distillation System is the thermal decomposition of wastes in
the absence of oxygen at the temperature range of 300–1300 °C [6, 12]. This method
operates at mild pressure (1–5 bar) but high temperatures. Pyrolysis converts MSW
into gaseous, liquid and solid fuels. As mentioned previously, the waste feed needs
pre-treatments such as drying and mechanical separation of glass, metals and inert
materials [23]. Moisture of biomass has a negative effect on pyrolysis due to the high
vaporization heat requirement. Thus, the feedstock needs pre-drying to a moisture
content of below 40 wt% [24]. Pyrolysis reaction occurs in three stages [11, 25]:
– The first stage, pre-pyrolysis, occurs at temperatures below 300 °C and includes
internal rearrangements such as bond breakage, formation of free radicals and
carbonyl groups, reduction in molecular weight and formation of small amounts
of water, CO, CO 2 and char.
– During the second stage, which is the main pyrolysis stage, solid decomposition occurs. At this stage, the biomass monomers undergo re-polymerization
and further fragmentation reactions (such as cracking of C–C bonds, reforming
and further cleavage of C–H and C–O bonds) or may react with the free radicals. The re-polymerization of the primary products results in the formation of
secondary tars. A longer reaction time and low to moderate temperature favor the
re-polymerization reactions and promote the formation of char, water and CO 2
from the secondary tars.
– In the last stage, char devolatilization, caused by the further cleavage of C–C,
C–H and C–O bonds results in the production of hydrocarbons, hydrogen, CO
and CO 2 as well as carbon (char or coke).
3 Advanced Technologies (Biological and Thermochemical) …
of its wood pellets demand is met through import from Canada and some Eastern
European countries [22].
Other European countries are also using biomass co-firing technologies in their
power plants that make a significant portion of a total of 230 power and CHP co-firing
plants around the world. For example, 14 biomass co-firing plants are operating in
Finland using wood and peat pellets and around 5 plants are in Austria utilizing wood
chips (mostly bark) co-fired with pulverized coal [22].
Canada and the U.S. are among the top ten countries producing GHG emissions
mainly CO 2 based on 2008 data [22], amounting to 6673 megatonnes (Mt) (2013)
and 722 megatonnes (Mt) (2015) carbon dioxide equivalent (CO 2 eq) in the United
States and Canada, respectively. Thus, these two countries have specific policies to
drop their GHG emissions by around 28–30% by 2030 [22]. All the biomass co-firing
plants in the US and most of them in Canada use direct co-firing technology. Almost
half of the US plants use wood products and railroad ties in co-firing while Canada
uses agricultural and forestry residues, domestic and municipal wastes, and energy
crops [22].
3.1.2 Pyrolysis
Pyrolysis or Destructive Distillation System is the thermal decomposition of wastes in
the absence of oxygen at the temperature range of 300–1300 °C [6, 12]. This method
operates at mild pressure (1–5 bar) but high temperatures. Pyrolysis converts MSW
into gaseous, liquid and solid fuels. As mentioned previously, the waste feed needs
pre-treatments such as drying and mechanical separation of glass, metals and inert
materials [23]. Moisture of biomass has a negative effect on pyrolysis due to the high
vaporization heat requirement. Thus, the feedstock needs pre-drying to a moisture
content of below 40 wt% [24]. Pyrolysis reaction occurs in three stages [11, 25]:
– The first stage, pre-pyrolysis, occurs at temperatures below 300 °C and includes
internal rearrangements such as bond breakage, formation of free radicals and
carbonyl groups, reduction in molecular weight and formation of small amounts
of water, CO, CO 2 and char.
– During the second stage, which is the main pyrolysis stage, solid decomposition occurs. At this stage, the biomass monomers undergo re-polymerization
and further fragmentation reactions (such as cracking of C–C bonds, reforming
and further cleavage of C–H and C–O bonds) or may react with the free radicals. The re-polymerization of the primary products results in the formation of
secondary tars. A longer reaction time and low to moderate temperature favor the
re-polymerization reactions and promote the formation of char, water and CO 2
from the secondary tars.
– In the last stage, char devolatilization, caused by the further cleavage of C–C,
C–H and C–O bonds results in the production of hydrocarbons, hydrogen, CO
and CO 2 as well as carbon (char or coke).