54
There are many ash-related issues encountered in combusting rice straw and
other herbaceous biomass including: accumulation, slagging, fouling, and corrosion
of the boiler due to the chlorine and alkali content (Zafar 2018). Although there are
no reported large-scale direct combustion power plants using rice straw as of 2018,
large-scale application could be highly feasible—as demonstrated by combined
heat and power plants (CHPs) in Europe that operate using wheat and oat straw.
Traditionally, European power plants feed straw bales directly in combustion chambers (also known as “Vølund cigar feeding”), to save on additional energy input for
dissolving the bales; however, more advanced power plants use shredded straw,
combining it with coal (i.e., coal and straw co-firing) in fluidized bed systems (Zafar
2018) for higher efficiencies. Thus, rice straw pretreatment may be necessary before
energy conversion but may have a negative effect in the overall energy balance.
In terms of pretreatment, particle size may be reduced after 2 weeks of air drying.
Chou et al. (2009) reported that rice straw should be reduced to about 2–5 mm
length for energy conversion. Rice straw may also be compressed into cubes with
sides of 50 mm
3
using hot press machines to increase its density and energy content
for a more uniform combustion (Kargbo et al. 2010). In order to solve slagging and
fouling problems, leaching may be used as a pretreatment method. In a study by
Kargbo et al. (2010), pretreatment was done by spraying water on spread rice straw
(not more than 30 cm thick) over a steel mesh, which resulted in reduced concentration of K, Na, and Cl.
The size of currently available biomass combustion systems ranges from a few
kW up to more than 100 MW (Nussbaumer 2003). Biomass combustion systems
can be classified according to capacity: small–scale (less than 100 kWth), mediumscale (from 100 kWth to 10 MWth), and large-scale (greater than 10 MWth)
(Obernberger 2008). However, because of logistical problems in gathering rice
straw and transporting it to power plants, small-scale rice straw bioenergy applications near farming fields are more common.
4.4 Nonenergy Thermal Conversion Byproducts
This section focuses on the nonenergy applications of thermal conversion products,
such as biochar from pyrolysis and ash residues from gasification and combustion
systems.
4.4.1 Biochar
Biochar is the charcoal-like carbon-rich residue produced during biomass pyrolysis.
Generally, it must contain at least 50% wt carbon, 75% of which is fixed carbon
(Jonsson 2016). Due to the release of volatile matter from the biomass during pyrolysis, biochar contains relatively high carbon and lower oxygen content than the raw
M. C. Maguyon-Detras et al.
There are many ash-related issues encountered in combusting rice straw and
other herbaceous biomass including: accumulation, slagging, fouling, and corrosion
of the boiler due to the chlorine and alkali content (Zafar 2018). Although there are
no reported large-scale direct combustion power plants using rice straw as of 2018,
large-scale application could be highly feasible—as demonstrated by combined
heat and power plants (CHPs) in Europe that operate using wheat and oat straw.
Traditionally, European power plants feed straw bales directly in combustion chambers (also known as “Vølund cigar feeding”), to save on additional energy input for
dissolving the bales; however, more advanced power plants use shredded straw,
combining it with coal (i.e., coal and straw co-firing) in fluidized bed systems (Zafar
2018) for higher efficiencies. Thus, rice straw pretreatment may be necessary before
energy conversion but may have a negative effect in the overall energy balance.
In terms of pretreatment, particle size may be reduced after 2 weeks of air drying.
Chou et al. (2009) reported that rice straw should be reduced to about 2–5 mm
length for energy conversion. Rice straw may also be compressed into cubes with
sides of 50 mm
3
using hot press machines to increase its density and energy content
for a more uniform combustion (Kargbo et al. 2010). In order to solve slagging and
fouling problems, leaching may be used as a pretreatment method. In a study by
Kargbo et al. (2010), pretreatment was done by spraying water on spread rice straw
(not more than 30 cm thick) over a steel mesh, which resulted in reduced concentration of K, Na, and Cl.
The size of currently available biomass combustion systems ranges from a few
kW up to more than 100 MW (Nussbaumer 2003). Biomass combustion systems
can be classified according to capacity: small–scale (less than 100 kWth), mediumscale (from 100 kWth to 10 MWth), and large-scale (greater than 10 MWth)
(Obernberger 2008). However, because of logistical problems in gathering rice
straw and transporting it to power plants, small-scale rice straw bioenergy applications near farming fields are more common.
4.4 Nonenergy Thermal Conversion Byproducts
This section focuses on the nonenergy applications of thermal conversion products,
such as biochar from pyrolysis and ash residues from gasification and combustion
systems.
4.4.1 Biochar
Biochar is the charcoal-like carbon-rich residue produced during biomass pyrolysis.
Generally, it must contain at least 50% wt carbon, 75% of which is fixed carbon
(Jonsson 2016). Due to the release of volatile matter from the biomass during pyrolysis, biochar contains relatively high carbon and lower oxygen content than the raw
M. C. Maguyon-Detras et al.