130
Water for Energy and Fuel Production
decomposition products were not the same at low and high residence times for both
low-temperature (180°C) and high-temperature (250°C) operations. In general, the
composition and yield of bio-oil can be optimized by suitably adjusting the temperature and the residence time such that heavy residues containing asphaltenes
and pre-asphaltenes are converted selectively to oil (and not gas). Often an addition of a reducing agent such as CO or H 2 prevents polymerization reactions and
stabilizes the active free radicals.
5.3.2.2 Biomass Particle size, heating rate, and Concentration
While a reduction in a particle size of biomass (particularly for biomass such as
wheat straw, barley straw, and switchgrass) is energy intensive, in general, a smaller
particle size results in higher degree of hydrolysis and fragmentation. However, the
literature data show that the particle size has a secondary effect on biomass conversion and product distribution in an HTL process because of high solvation and
extraction powers of water at high temperature and pressure. Zhang et al. [64] found
no effect of the particle size variation from 0.5 to 2 mm of grass perennials on the
yield of bio-oils. Akhtar and Amin [41] recommended that the particle size between
4 and 10 mm should be suitable to overcome the heat and mass transfer limitations
at a reasonable grinding cost.
While the studies mentioned earlier indicated the marginal effects of particle
size on the herbaceous biomass liquefaction process, Kobayashi et al. [68] showed
a significant effect of particle size of woody biomass on the HTL process. Wood
powder pulverized by the vibration mill, cutter mill, and grinder was used as a liquefaction material. The wood powder was sieved between 212 and 500 μm. Based
on the results of water solubles and specific surface areas for three different milling
processes obtained in this study, it was concluded that an increase in specific surface
area increased the production of water soluble (saccharine); however, the difference
in water solubles between the grinder and the cutter mill was only marginal. This
indicated that the crystallinity of the wood powder also affects the water solubles
yield.
Bio-oil production during the HTL process generally occurs at moderate heating
rates. Slow heating rates usually lead to the formation of char residue due to secondary condensation and polymerization reactions. Very high heating rates also promote
secondary reactions that generally result in more gas production. The heating rate is
important for both dry and wet pyrolyses, although it is less important in the HTL
process due to the better dissolution and stabilization of fragmented species in hotcompressed water medium. Zhang [45] and Zhang et al. [64] observed that for a heating rate range of 5°C–140°C/min during the HTL process for grassland perennials,
bio-oil yield increased from 63% to 76% with an increase in the heating rate.
The solids concentration also affects the bio-oil production [41]. In general, high
amount of water favors the production of liquids largely due to enhanced extraction and higher degree of solvation of biomass. The solvent enhances the stability
and solubility of fragmented components, thereby reducing the production of solids residues and gases. At high biomass concentration, the interactions between the
fragmented biomass components and the water decrease, the reactions among various fragments increase, and thus the influence of water on the product distributions
Water for Energy and Fuel Production
decomposition products were not the same at low and high residence times for both
low-temperature (180°C) and high-temperature (250°C) operations. In general, the
composition and yield of bio-oil can be optimized by suitably adjusting the temperature and the residence time such that heavy residues containing asphaltenes
and pre-asphaltenes are converted selectively to oil (and not gas). Often an addition of a reducing agent such as CO or H 2 prevents polymerization reactions and
stabilizes the active free radicals.
5.3.2.2 Biomass Particle size, heating rate, and Concentration
While a reduction in a particle size of biomass (particularly for biomass such as
wheat straw, barley straw, and switchgrass) is energy intensive, in general, a smaller
particle size results in higher degree of hydrolysis and fragmentation. However, the
literature data show that the particle size has a secondary effect on biomass conversion and product distribution in an HTL process because of high solvation and
extraction powers of water at high temperature and pressure. Zhang et al. [64] found
no effect of the particle size variation from 0.5 to 2 mm of grass perennials on the
yield of bio-oils. Akhtar and Amin [41] recommended that the particle size between
4 and 10 mm should be suitable to overcome the heat and mass transfer limitations
at a reasonable grinding cost.
While the studies mentioned earlier indicated the marginal effects of particle
size on the herbaceous biomass liquefaction process, Kobayashi et al. [68] showed
a significant effect of particle size of woody biomass on the HTL process. Wood
powder pulverized by the vibration mill, cutter mill, and grinder was used as a liquefaction material. The wood powder was sieved between 212 and 500 μm. Based
on the results of water solubles and specific surface areas for three different milling
processes obtained in this study, it was concluded that an increase in specific surface
area increased the production of water soluble (saccharine); however, the difference
in water solubles between the grinder and the cutter mill was only marginal. This
indicated that the crystallinity of the wood powder also affects the water solubles
yield.
Bio-oil production during the HTL process generally occurs at moderate heating
rates. Slow heating rates usually lead to the formation of char residue due to secondary condensation and polymerization reactions. Very high heating rates also promote
secondary reactions that generally result in more gas production. The heating rate is
important for both dry and wet pyrolyses, although it is less important in the HTL
process due to the better dissolution and stabilization of fragmented species in hotcompressed water medium. Zhang [45] and Zhang et al. [64] observed that for a heating rate range of 5°C–140°C/min during the HTL process for grassland perennials,
bio-oil yield increased from 63% to 76% with an increase in the heating rate.
The solids concentration also affects the bio-oil production [41]. In general, high
amount of water favors the production of liquids largely due to enhanced extraction and higher degree of solvation of biomass. The solvent enhances the stability
and solubility of fragmented components, thereby reducing the production of solids residues and gases. At high biomass concentration, the interactions between the
fragmented biomass components and the water decrease, the reactions among various fragments increase, and thus the influence of water on the product distributions