Page 50 - Energy from Toxic Organic Waste for Heat and Power Generation

38 Energy from Toxic Organic Waste for Heat and Power Generation

of carbonyl, carboxyl, and hydroperoxide groups. In the second stage, the
solid biomass decomposition starts at a higher rate and leads to the forma-
tion of pyrolytic products. During third stage, char decomposes at a very
slow rate and forms carbon-rich solid residues [73].

Fast Pyrolysis: Fast pyrolysis is a process in which biomass is rapidly
heated to high temperatures in the absence of air (specifically oxygen) [74].
It occurs in a high-temperature range of 300–700°C at a faster heating
rate of 10–200°C/s, with a short solid resistance time of 0.5–10 s and with
fine particle size (< 1 mm) feedstock. In the fast pyrolytic process, bio-
mass decomposes to generate vapors, aerosol, and some charcoal like char.
After cooling and condensation of vapors and aerosol, dark brown liquid
oil is formed [75]. Fast pyrolysis is recommended when the desired out-
put is mainly liquid and gaseous products. Fast pyrolytic processes produce
60–75 wt% of liquid bio-oil, 15–25 wt% of solid char, and 10–20 wt% of
noncondensable gases, depending on the feedstock used [76].

Flash Pyrolysis: Flash pyrolysis occurs in a very high-temperature
range of 750–1000°C at a faster heating rate > 1000°C/s with a short
resistance time < 0.5 s and with very fine particles < 0.2 mm. Biomass
conversion to crude oil can have efficiency up to 70% in the flash pyrolysis
process [77]. Pyrolysis oil differs greatly from petroleum base fuels and, as a
result, presents some difficulties, if it is used as a replacement for petroleum
fuels. Major issues to be tackled are high-water content, high-oxygen con-
tent and viscosity, and acidic nature [78].

3.5.4.2 Gasification
Biomass gasification is the conversion of an organically derived carbona-
ceous feedstock by partial oxidation into a gaseous product, i.e., syngas,
consisting primarily of hydrogen (H 2 ) and carbon monoxide (CO), with
lesser amounts of carbon dioxide (CO 2 ), water (H 2 O), methane (CH 4 ),
higher hydrocarbons, and nitrogen (N 2 ). In a gasifier, the reactions are car-
ried out at elevated temperatures, 500–1400°C, and atmospheric or elevated
pressures of up to 33 bar. The oxidant used can be air, pure oxygen, or a
mixture of these gases. Air-based gasifiers typically produce a gas containing
a relatively high concentration of nitrogen with a low heating value be-
3
tween 4 and 6 MJ/m . Oxygen and steam-based gasifiers produce a product
gas containing a relatively high concentration of hydrogen and CO with a
3
heating value between 10 and 20 MJ/m [79].