Page 269 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

8.4 Biofuels 245

black soot portion, are greatly reduced for biodiesel engines, NOx emissions are
usually high.
The NOx emissions from biodiesel may be 10 % more than that produced by
petroleum diesel. Biodiesel combustion usually has more widespread high-
temperature distribution areas than petroleum diesel fuel [53]. As we have learned
from combustion chemistry, higher combustion temperature leads to more thermal
NOx formation. This partially explains the reason behind the high NOx emissions
typically measured from the engine exhaust produced by biodiesel fuels. However,
ethanol could act as an effective NOx reducing additive [20]. Strategies for reducing
NOx emissions from biodiesel combustion include increasing spray cone angle,
retarding start of injection, exhaust gas recirculation (EGR), and charge air cooling
[55].

8.4.2.2 Challenges to Biodiesel

The specific gravity of biodiesel ranges from 0.86–0.90, which is higher than that of
No. 2 diesel. This may result in a higher fuel mass of biodiesel injection through an
unmodified diesel injection system than does a No. 2 petroleum diesel. The high
heating value of biodiesel is slightly above 17,000 Btu/lb, and it is lower than that
of No. 2 petroleum diesel (19,300 Btu/lb). As a result, the total thermal energy
delivered from biodiesel is less than that of No. 2 diesel.
The main barrier for commercialization of biodiesel is its higher cost than
petroleum diesel. The price of biodiesel is almost double that of petroleum diesel if
subsides are not taken into account.
Fuel stability is another major concern to biodiesel. Biodiesel is less saturated
and normally has poorer thermal stability, oxidative stability, and storage stability
than petroleum. A study performed at the University of Idaho showed that biodiesel
placed in water degraded 95 % by microorganisms in 28 days, while petroleum
diesel fuel degraded only 40 % in same time [56]. At low temperatures, biodiesel
will gel or crystallize into a solid mass that cannot be pumped; as a result the engine
cannot run.
Another emerging challenge is the food crisis as it is unethical to convert edible
food to biodiesel to feed the SUVs instead of the starving people. There was a great
rise in the price of food (especially corn) and fertilizer from 2006–2008, partially
because quite a lot of corn in North American was used for biodiesel production.
Research and development should be focused on waste to biodiesel instead.

8.4.3 Bioethanol

Bioethanol (C 2 H 5 OH) can be produced from starch in corn, sugar in sugar cane, or
cellulose from the cellulosic biomass. Starch and sugar based bioethanol is con-
sidered as the first generation bioethanol, while cellulosic bioethanol the second.