3.1 Thermochemical Conversion Technologies
59
use optical and magnetic separators to separate waste materials prior to combustion
for better resource recovery [13].
The effectiveness of the WTE process is determined based on the thermal potential
of the waste, effectiveness of the system and the type of produced energy. The energy
efficiency for the production of heat, combined heat and electricity (cogeneration)
and pure electricity from the combustion process has been reported to be 80%, 20–
30% and 20%, respectively [12]. It should be noted that the thermal efficiency of
a combustion process is always much higher than power generation efficiencies in
natural gas and coal-fired boilers, typically between 25 and 50% [14]. The selection
and design of biomass combustion systems shall be based on the characteristics
of biomass fuels such as particle size, specific gravity, ash and moisture content,
extractive contents such as fats, proteins, simple sugars, phenolics and inorganics,
elemental contents (C, H, N, S, etc.) and textural composition of biomass (lignin,
cellulose, hemicellulose) [9, 15]. High moisture content of biomass makes it less
desirable as a fuel for combustion, due to the energy requirement for drying [9, 11],
and the low heating value of the fuel due to the high latent heat requirement for
evaporating water in the fuel. Ash content and inorganic compounds composition
are other parameters that negatively affect the heating value of the biomass. In most
cases, a biomass fuel requires some pre-treatments such as drying, size reduction,
additives and thermal upgrading to improve its performance in combustion [15].
3.1.1.1 Co-firing
Combustion of solid fossil fuels such as coal could result in release of air pollutants including sulfur dioxide (SO 2 ), nitrogen oxides (NO x ), mercury and particulate
matters as well as GHG such as CO 2 [16]. Co-firing biomass with coal has been
increasingly popular as a cost-effective and environmentally friendly technology
for conversion of biomass and coal into energy with the potential to abate most of
the above emissions as the biomass fuel is carbon neutral, and essentially free of
sulfur, with a lower fuel-N content [11, 16]. Compared with the biomass combustion
that is associated with some operating challenges related to the adverse effects of
biomass ash (of a lower melting point), co-firing of biomass and coal is considered
as a more advantageous option for electricity production from biomass [17]. Cofiring of biomass and coal has higher conversion efficiency and reduces the common
issues such as fouling and corrosion that result from the high ash content of biomass.
However, there are some new operational problems that may arise from co-firing at
high co-firing ratios. These include the negative impact on the boiler performance
such as ash deposition and tube corrosion. Compared with other thermochemical
processes, co-firing requires the least modifications to the original coal-based power
plants for biomass utilization [11]. Various types of biomass such as sawdust, forestry
wood waste, switchgrass, herbaceous crops and various types of manure could be
co-fired with different types of coal.
Co-firing can be generally be classified into three categories: direct, indirect and
parallel co-firing [18]: In the direct process, biomass is either blended with coal
59
use optical and magnetic separators to separate waste materials prior to combustion
for better resource recovery [13].
The effectiveness of the WTE process is determined based on the thermal potential
of the waste, effectiveness of the system and the type of produced energy. The energy
efficiency for the production of heat, combined heat and electricity (cogeneration)
and pure electricity from the combustion process has been reported to be 80%, 20–
30% and 20%, respectively [12]. It should be noted that the thermal efficiency of
a combustion process is always much higher than power generation efficiencies in
natural gas and coal-fired boilers, typically between 25 and 50% [14]. The selection
and design of biomass combustion systems shall be based on the characteristics
of biomass fuels such as particle size, specific gravity, ash and moisture content,
extractive contents such as fats, proteins, simple sugars, phenolics and inorganics,
elemental contents (C, H, N, S, etc.) and textural composition of biomass (lignin,
cellulose, hemicellulose) [9, 15]. High moisture content of biomass makes it less
desirable as a fuel for combustion, due to the energy requirement for drying [9, 11],
and the low heating value of the fuel due to the high latent heat requirement for
evaporating water in the fuel. Ash content and inorganic compounds composition
are other parameters that negatively affect the heating value of the biomass. In most
cases, a biomass fuel requires some pre-treatments such as drying, size reduction,
additives and thermal upgrading to improve its performance in combustion [15].
3.1.1.1 Co-firing
Combustion of solid fossil fuels such as coal could result in release of air pollutants including sulfur dioxide (SO 2 ), nitrogen oxides (NO x ), mercury and particulate
matters as well as GHG such as CO 2 [16]. Co-firing biomass with coal has been
increasingly popular as a cost-effective and environmentally friendly technology
for conversion of biomass and coal into energy with the potential to abate most of
the above emissions as the biomass fuel is carbon neutral, and essentially free of
sulfur, with a lower fuel-N content [11, 16]. Compared with the biomass combustion
that is associated with some operating challenges related to the adverse effects of
biomass ash (of a lower melting point), co-firing of biomass and coal is considered
as a more advantageous option for electricity production from biomass [17]. Cofiring of biomass and coal has higher conversion efficiency and reduces the common
issues such as fouling and corrosion that result from the high ash content of biomass.
However, there are some new operational problems that may arise from co-firing at
high co-firing ratios. These include the negative impact on the boiler performance
such as ash deposition and tube corrosion. Compared with other thermochemical
processes, co-firing requires the least modifications to the original coal-based power
plants for biomass utilization [11]. Various types of biomass such as sawdust, forestry
wood waste, switchgrass, herbaceous crops and various types of manure could be
co-fired with different types of coal.
Co-firing can be generally be classified into three categories: direct, indirect and
parallel co-firing [18]: In the direct process, biomass is either blended with coal