Environmental Engineering Reference
In-Depth Information
However further work is required in specifying acceptable fuel characteristics, con-
firming the long-term effects on engine durability, and ensuring safety in handling
and storing ethanol-diesel blends [177].
8.3.2.3 Engine Exhaust Emissions Using Bioethanol
Bioethanol used in combustion engines has a tremendous potential for a net reduc-
tion in the emissions of greenhouse gases. Life-cycle emissions predict the great
environmental benefit that can be achieved from the use of bioethanol as transport
fuel. Ethanol and others biofuels are considered as “climate friendly”, even when
considered on a life-cycle basis [191, 192].
CO
2
is released into the atmosphere when a fuel is burned in the engine.
However, it is recycled into organic tissues during plant growth. Only about 40% or
less of the organic matter is actually removed from farm fields for ethanol produc-
tion [174]. Bioethanol is believed to give a 70% carbon dioxide reduction compared
to petrol [51].
CO is formed by the incomplete combustion of fuels, most readily produced from
petroleum fuels, which contain no oxygen in their molecular structure. Since ethanol
and other oxygenated compounds contain oxygen, their combustion in automobile
engines is more complete. The result is a substantial reduction in CO emissions
(up to 30%), depending on the type and age of engine/vehicle, the emission control
system used, and the atmospheric conditions in which the vehicle operates.
The addition of bioethanol to diesel fuel has also a beneficial effect in reduc-
ing particulate matter (PM) emissions [193]. The degree of improvement varies
from engine to engine and also within the working range of the engine itself. While
there is considerable value in being able to use the fuel directly in an unmodified
engine, small adjustments to fuel injection characteristics may result in further gains
in reducing emissions [177].
Because of its high octane number, the addition of bioethanol to gasoline leads
to the reduction or removal of aromatic hydrocarbons (e.g. benzene), and other
hazardous high-octane additives commonly used to replace tetraethyl lead in gaso-
line [194]. Clear trends of reduced hydrocarbons and CO emissions and increased
NO
x
emissions have been observed with increasing percentages of ethanol in the
blend (from 0 to 20%). A standard vehicle operates at air/fuel ratios significantly
richer than stoichiometric, with an average air/fuel ratio running on gasoline of
approximately 12.2:1. For leaner base conditions, the trend could be the opposite,
with increasing hydrocarbon emissions and reduced NO
x
emissions with increasing
ethanol contents [195]. Acetaldehyde emissions are also superior with increasing
ethanol contents in the blend as this compound can be produced from ethanol via
oxidation under certain operating conditions. Interestingly, such emissions have also
a close relationship with the engine load and the ethanol content in the blend. With
increasing loadings from idling, the acetaldehyde emissions gradually decrease to
their minimum at medium loads, then increase again at high engine loads [192].
Toxic unregulated emissions (i.e. formaldehyde, propionaldehyde, 1,3-butadiene,
acrolein, linear alkenes and aromatics) and fine particulate should be considered to
