Page 250 - Synthetic Fuels Handbook

236 CHAPTER EIGHT

The calorific value of the gas is the prime factor for power generation—the higher the
value, the better. Hence, the availability in the gas of any compounds that increase calorific
value is generally welcomed—product gas, which contains carbon monoxide (CO), hydro-
gen (H ), various hydrocarbons [methane (CH ), ethylene (C H ), ethane (C H ) tars, and
4
2
2
4
2
6
chars]. The presence of inert components [water (H O), carbon dioxide (CO ), and nitrogen
2
2
(N )] is also acceptable, provided it is kept within certain limits.
2
Environmental concerns include disposal of associated tars and ashes, particularly for
the fluidized bed reactors, where these substances must be separated from the flue gas
stream (in contrast to the pyrolysis plants, where most tar and ash deposits at the bottom
of the reactor). Concerns over biomass transportation are similar to those mentioned above
for fermentation, and a positive fertilizer effect can also in many cases be derived from the
gasification residues.
Biomass ash has also the potential to be used as a clarifying agent in water treatment,
as a wastewater adsorbent, as a liquid waste adsorbent, as a hazardous waste solidification
agent, as a lightweight fill for roadways, parking areas, and structures, as asphalt mineral
filler, or as a mine spoil amendment.
Direct conversion processes, that is, combustion, of biomass encounters the same
problems as those in case of coal or other solid fuels. The conversion of biomass into
other useful forms such as synthetic gas or liquid fuels is considered as an alternative way
to make use of biomass energy. Gasification is an effective and well-known technology
which has been applied to coal but the technology developed for coal is not exactly suit
to biomass utilization because of the different physical and chemical properties of coal
and biomass.
Combustion. Combustion offers the most direct route for energy recovery, and is
an effective means of utilizing the total energy content of whole wood and other bio-
mass. Technology for the combustion of carbonaceous feedstocks is well developed
and feasible.
Gasification results in only a partial oxidation of the carbon constituent. Much of the calo-
rific value of the original fuel leaves the gasifier in the form of chemical energy in carbon
monoxide, methane, and hydrogen.
In combustion, air containing sufficient oxygen to completely oxidize the hydrocarbon
is used. The resulting gas (flue gas) is essentially all carbon dioxide, water vapor, and dilu-
ent nitrogen, and has no calorific value. The energy of the original fuel leaves the system as
sensible heat that can be used for steam generation and, if required, electricity generation.
Thus, combustion offers a practical means for recovering the energy in biomass
materials, using currently available technology.
However, due to the large land area over which biomass must be harvested, the seasonal
nature of the supply, and the large volume of material to be transported and stored, the reli-
able provision of biomass to a large combustion plant can present considerable problems.
Pyrolysis. Pyrolysis is the thermal decomposition of materials in the absence of oxygen
or when significantly less oxygen is present than required for complete combustion.
Pyrolysis dates back to at least ancient Egyptian times, when tar for caulking boats and cer-
tain embalming agents were made by pyrolysis. In the 1980s, researchers found that the pyrol-
ysis liquid yield could be increased using fast pyrolysis where a biomass feedstock is heated at
a rapid rate and the vapors produced are also condensed rapidly (Mohan et al., 2006).
Pyrolysis is the basic thermochemical process for converting biomass to a more useful
fuel. Biomass is heated in the absence of oxygen, or partially combusted in a limited oxy-
gen supply, to produce a hydrocarbon-rich gas mixture, an oil-like liquid, and a carbon-rich
solid residue. The pyrolysis of biomass to produce hydrocarbons has been studied since the
1930s, and while relatively simple to perform, typically such processes are nonselective,