45
than the theoretical oxygen) is introduced, the process shifts to gasification. Any
further increases in temperature and oxygen supply lead to combustion reactions.
The composition of the products produced during thermal conversion depends
on the severity of process conditions and oxygen supply. Thermal processes tend to
shift the biomass components into different materials in terms of their carbon,
hydrogen, and oxygen contents. Slow pyrolysis, for example, tends to increase the
carbon content of the feedstock producing char, while rapid or fast pyrolysis produces more hydrocarbons. Oxidation, on the other hand, produces gaseous products, such as CO 2 and H 2 O. Steam and hydrogasification tend to produce gaseous
products enriched with hydrogen.
4.2 Properties of Rice Straw for Thermal Conversion
Biomass properties and chemical composition are important in determining a material’s suitability for thermal conversion. Chapter 1 presented rice straw properties
and composition. The moisture content of the biomass is one of the characteristics
initially considered and is an important criterion in selecting the appropriate technology for conversion. Biomass feedstocks with moisture contents usually above 60
to 65%, by weight, have very low calorific values to be considered for combustion.
Hence, biogas production is more feasible if drying is not considered (IFC 2017).
For gasification, moisture contents of about 50 and 60%, by weight, can be handled
using updraft bed gasifiers and fluidized bed gasifiers, respectively (Roos 2009). For
pyrolysis, the moisture content of the biomass should be below 20–25% (Dong
et al. 2016). Some reactor designs, however, may be amenable to higher moisture
contents such as the multi-sectional rotary kilns used for pyrolysis of wastes with
high organic content (Chen et al. 2015). As mentioned in Chap. 1, however, rice
straw moisture content widely varies depending on the handling, collection and
storage methods, and duration.
Physical properties such as particle size, specific heat capacity, and thermal conductivity of the biomass, on the other hand, affect the rate at which heat and oxygen
penetrates the biomass particles during thermal processing. Particle size or particle
size distribution (PSD) is the measure of the physical dimensions of the biomass
material and can be obtained using various standard sieves. Biomass materials (i.e.,
agricultural residues) vary in sizes and need to be ground to less than 10 mm in size
for various conversion processes (Capareda 2014). Loose rice straw is typically
long and needs to be chopped into smaller pieces. To do so, an additional chopper
or shredder is needed, entailing additional energy input for the process. Particle size
is particularly important in the pyrolysis process for it can control the rate of heat
transfer in the biomass, making it a major factor in the rate of drying and primary
pyrolysis reaction (Tripathi et al. 2016; Isahak et al. 2012; Demirbas 2004). Specific
heat capacity is defined as the ratio of the amount of heat energy transferred to or
from the material to the resulting increase in temperature of this material per unit
mass (i.e., J kg-K
−1
). Thermal conductivity, on the other hand, is the ability of the
4 Thermochemical Conversion of Rice Straw
than the theoretical oxygen) is introduced, the process shifts to gasification. Any
further increases in temperature and oxygen supply lead to combustion reactions.
The composition of the products produced during thermal conversion depends
on the severity of process conditions and oxygen supply. Thermal processes tend to
shift the biomass components into different materials in terms of their carbon,
hydrogen, and oxygen contents. Slow pyrolysis, for example, tends to increase the
carbon content of the feedstock producing char, while rapid or fast pyrolysis produces more hydrocarbons. Oxidation, on the other hand, produces gaseous products, such as CO 2 and H 2 O. Steam and hydrogasification tend to produce gaseous
products enriched with hydrogen.
4.2 Properties of Rice Straw for Thermal Conversion
Biomass properties and chemical composition are important in determining a material’s suitability for thermal conversion. Chapter 1 presented rice straw properties
and composition. The moisture content of the biomass is one of the characteristics
initially considered and is an important criterion in selecting the appropriate technology for conversion. Biomass feedstocks with moisture contents usually above 60
to 65%, by weight, have very low calorific values to be considered for combustion.
Hence, biogas production is more feasible if drying is not considered (IFC 2017).
For gasification, moisture contents of about 50 and 60%, by weight, can be handled
using updraft bed gasifiers and fluidized bed gasifiers, respectively (Roos 2009). For
pyrolysis, the moisture content of the biomass should be below 20–25% (Dong
et al. 2016). Some reactor designs, however, may be amenable to higher moisture
contents such as the multi-sectional rotary kilns used for pyrolysis of wastes with
high organic content (Chen et al. 2015). As mentioned in Chap. 1, however, rice
straw moisture content widely varies depending on the handling, collection and
storage methods, and duration.
Physical properties such as particle size, specific heat capacity, and thermal conductivity of the biomass, on the other hand, affect the rate at which heat and oxygen
penetrates the biomass particles during thermal processing. Particle size or particle
size distribution (PSD) is the measure of the physical dimensions of the biomass
material and can be obtained using various standard sieves. Biomass materials (i.e.,
agricultural residues) vary in sizes and need to be ground to less than 10 mm in size
for various conversion processes (Capareda 2014). Loose rice straw is typically
long and needs to be chopped into smaller pieces. To do so, an additional chopper
or shredder is needed, entailing additional energy input for the process. Particle size
is particularly important in the pyrolysis process for it can control the rate of heat
transfer in the biomass, making it a major factor in the rate of drying and primary
pyrolysis reaction (Tripathi et al. 2016; Isahak et al. 2012; Demirbas 2004). Specific
heat capacity is defined as the ratio of the amount of heat energy transferred to or
from the material to the resulting increase in temperature of this material per unit
mass (i.e., J kg-K
−1
). Thermal conductivity, on the other hand, is the ability of the
4 Thermochemical Conversion of Rice Straw