Page 257 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

8.2 Fuel Substitution 233

Replacing gasoline with natural gas or propane can reduce the emissions of
VOCs. This replacement is not limited to stationary combustion sources, and some
engines can be powered by compressed natural gas or propane. Extensive research
and development being conducted in the petroleum industry to improve the com-
bustion properties, handling, and use of natural gas as a substitute of gasoline to
dominate the auto fuel market, although it is still not the case yet.
Biodiesel, which will be introduced shortly as one type of alternative fuels, is
being used as a substitute for petroleum diesel fuels for diesel engines. Intensive
experimental and computational studies on biodiesel have been carried out in the
past decades. B20, a mixture of 20 % of biodiesel and 80 % of petroleum diesel, is
now widely adapted by the US government for transportation industry. It is char-
acterized by near-zero emissions of sulfur and net-nitrogen. But there is controversy
data in the literature about the emissions of particulate matter.

8.3 Thermochemical Conversion of Fuels

Solid fuels such as coal or biomass may be converted to liquid or gaseous fuels by a
thermochemical conversion (TCC) process. A TCC process is a chemical reforming
process in which the depolymerization and reforming reactions of organic matter
take place. There are many TCC processes that convert a wide variety of feedstock
into different fuels through different types of reactions. They can be small or large
in scale. Overall, TCC processes can be divided into three categories.
• Pyrolysis
• Gasification, and
• Liquefaction

8.3.1 Pyrolysis

Pyrolysis is a process where organic matter is degraded by thermal reactions in the
absence of added oxidizing agents. Pyrolysis, being as old as civilization, was
widely employed in the seventeenth and eighteenth centuries for conversion of
charcoal to fuels for the smelting industry.
With wood as a feedstock, pyrolysis is also referred to as carbonization or
destructive distillation. Pyrolysis decomposes a fuel with larger molecules into
smaller fractions such as C 2 H 6 , CO, CO 2 ,H 2 ,H 2 O, oily liquids, and a solid car-
bonaceous char residue. The actual products depend on the TCC process. The
mixture of gases, liquids, and solids require further separation in order to produce
the final fuel products. The principles introduced in Chaps. 5 and 6 sets the
foundation of the engineering designs of the related equipment and processes.