Page 267 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

8.4 Biofuels 243

• Pelletization is a process that reforms the wood powder into certain shapes. As
depicted in Fig. 8.3, the powder is pressed down into the die block and is
compressed into cylindrical the pellets.
• The last step is to cool the pellets to increase their durability.
The environmental impact assessment for biomass pellet production should be
conducted from feedstock harvesting to the delivery of pellets to the end user.
Magelli et al. [32] conducted an assessment for wood pellet production in Van-
couver, Canada with end users in Sweden by streamline life-cycle analysis. They
compared the total emission factors (in g/kg fuel) between wood pellet and natural
gas as fuels. The results in Table 8.4 show that wood pellets as a fuel has a great
potential in reducing air emissions. The reduction could be greater if the pellets
were produced and consumed locally without international transportation. On the
other hand, we have to realize that wood pellets cannot replace natural gas because
each of them has its uniqueness due to the phase difference.

8.4.2 Biodiesel

Biodiesel has some substantially different properties than petroleum diesel because
of its different structure. Biodiesel is described as a fuel comprised of mono-alkyl
esters of long chain fatty acids derived from vegetable oils or animal fats (in ASTM
Standard D6751-03). Typical biodiesel has the cetane number that is close to the
high end No. 2 petroleum diesel. For example, the biodiesel viscosity–temperature
relationship is similar to that of No. 2 diesel fuel, which followed the Vogel
equation [54].

Table 8.4 Emission factors for pellet and natural gas production
Air emissions Pellet Natural gas (NG) Emission factor
(g/1,000 kg fuel) (g/1,000 kg fuel) ratio (Pellet:NG)
Pollutant CO 1,196 1,213 0.986
NOx 6,420 6,452 0.995
VOC 169 384 0.440
PM 496 505 0.982
SOx 2,958 3,040 0.973
6 9.8 0.612
NH 3
GHGs CO 2 281,550 446,750 0.630
N 2 O 7.8 10.7 0.729
53 972 0.055
CH 4
Based on the data in Magelli et al. [32]