Page 169 - Biofuels Refining and Performance

152 Chapter Five

5.2 Cetane Number and Exhaust Emissions
The cetane number (CN), which is related to the ignition properties, is
a prime indicator of fuel quality in the realm of diesel engines. It is con-
ceptually similar to the octane number used for gasoline. Generally, a
compound that has a high octane number tends to have a low CN and
vice versa. The CN of a DF is related to the ignition delay (ID) time, i.e.,
the time between injection of the fuel into the cylinder and onset of
ignition. The shorter the ID time, the higher the CN, and vice versa.
Standards have been established worldwide for CN determination,
e.g., ASTM D613 in the United States, and internationally the
International Organization for Standardization (ISO) standard ISO
5165. A long straight-chain hydrocarbon, hexadecane (C H ; trivial
34
16
name cetane, giving the cetane scale its name) is the high-quality stan-
dard on the cetane scale with an assigned CN of 100. A highly branched
isomer of hexadecane, 2,2,4,4,6,8,8-heptamethylnonane (HMN), a com-
pound with poor ignition quality, is the low-quality standard with an
assigned CN of 15. The two reference compounds on the cetane scale
show that CN decreases with decreasing chain length and increasing
branching. Aromatic compounds that are present in significant amounts
in petrodiesel have low CNs but their CNs increase with increasing size
of n-alkyl side chains [12, 13]. The cetane scale is arbitrary, and com-
pounds with CN 100 or CN 15 have been identified. The American
standard for petrodiesel (ASTM D975) prescribes a minimum CN of 40,
while the standards for biodiesel prescribe a minimum of 47 (ASTM
D6751) or 51 (European standard EN 14214). Due to the high CNs of
many fatty compounds, which can exceed the cetane scale, the lipid
combustion quality number for these compounds has been suggested [14].
The use of biodiesel reduces most regulated exhaust emissions from
a diesel engine. The species reduced include carbon monoxide, hydro-
carbons, and particulate matter (PM). Nitrogen oxide (NO ) emissions
x
are slightly increased, however. When blending biodiesel with
petrodiesel, the effect of biodiesel is approximately linear to the blend
level. A report summarizing exhaust emissions tests with biodiesel is
available [15], and other summaries are given in Refs. [16, 17].
The structure of the fatty esters in biodiesel affects the levels of exhaust
emissions. When using a 1991-model, 6-cylinder, 345-bhp (257-kW),
exhaust
direct-injection, turbocharged, and intercooled diesel engine, NO x
emission increased with increasing number of double bonds and decreas-
ing chain length for saturated chains [18]. Although often a trade-off is
and PM exhaust emissions, no trade-off has been
observed between NO x
observed in this work when varying the chain length [18]. The CN and
density were correlated with emission levels [18]. However, emissions are
likely affected by the technology level of the engine. When conducting
tests on a 2003-model, 6-cylinder, 14–L, direct-injection, turbocharged,