Environmental Engineering Reference
In-Depth Information
(and oxidation) to convert metallic impurities into the corresponding sulfates (Prankl et al. 2004),
e.g., the remaining alkaline catalysts into alkali sulfates (Mittelbach 1996). With this process, the
sodium and potassium sulfate produced from catalyst residues are less volatile than the correspond-
ing oxides, and as a result the material loss at higher temperatures is eliminated. At the same time,
the ash determination method that is used for fossil diesel samples is considered sufficient and does
not need to change (Prankl et al. 2004). Most international fuel quality standards require the sul-
fured ash content in the biodiesel to be equal to or below 0.02% m/m and the oxide ash content of
diesel to be equal to or below 0.01% m/m (Prankl et al. 2004).
9.2.4 S
ulfur
Sulfur is one of the “heteroatoms” of (bio-)diesel that, although present only in small amounts, plays
a catalytic role in the environmental performance of the fuel (Bacha et al. 2007) as well as in the
engine durability. The combustion of high-sulfur fuel, which emits more sulfur dioxide (Bacha et al.
2007; White Paper 2007) and particulate matter (PM), is harmful to the environment by contribut-
ing to atmospheric pollution and acid rain (Foon et al. 2005) and to human health, given that these
emissions have high mutagenic potential (Prankl et al. 2004; White Paper 2007). At the same time,
the use of high-sulfur fuel can wear the engine [i.e., the sulfuric acid produced corrodes the cylinder
liner and piston (Foon et al. 2005)) and reduce the efficiency and durability of the emissions control
system (White Paper 2007), although this effect is highly correlated with the operating conditions
(WWFC 2006; Rilett and Gagnon 2008).
By nature, biodiesel is an ultra-low-sulfur fuel (ULSF) (Körbitza et al. 2003; Costenoble 2006;
White Paper 2007), that is, <0.001% for biodiesel delivered from palm oil (Foon et al. 2005) because
it contains just traces of sulfur.* Thus, it can be used in a diesel engine with a catalyst without the
risk of reducing the efficiency of the catalyst, unlike sulfur-containing diesel (Prankl and Wörgetter
et al. 2000). The “poisoning” of the catalyst from the sulfur results in increased emissions of carbon
monoxide (CO), hydrocarbons (HCs), and particles but is avoided by the use of a ULSF such as
biodiesel (Prankl and Wörgetter 2000; WWFC 2006).
Moreover, biodiesel could be used as an additive for ultra-low-sulfur diesel (Prankl and Wörgetter
2000). Using hydrotreating process (an upgrading process of desulfurization), the sulfur can be
removed from the diesel to meet the required levels of a ULSF
†
(Bacha et al. 2007). However,
hydrotreating also tends to destroy minor constituents, mainly nitrogen and oxygen compounds,
that provide lubricity to the fuel (Prankl et al. 2004) as well as naturally occurring antioxidants by
increasing the risk of peroxide formation (Bacha et al. 2007). Hence, an additive will need to be
added to ULSF diesel (Prankl et al. 2004). A blending of 2% v/v biodiesel can play the role of this
additive by substituting fossil additives and significantly improving the lubricating properties of
diesel fuels with low sulfur content (Prankl and Wörgetter et al. 2000).
These multiple and serious impacts from the combustion of sulfur increase the need for strict
limits on the sulfur contamination of (bio)diesel. Hence, the EU defined the maximum sulfur con-
tent as 10.0 mg/kg
‡
and the United States as 15/500 mg/kg
§
(on road/off road fuel). Moreover,
although many countries that have rapidly increased their share of the biodiesel market have not yet
set limits for sulfur content, such as Brazil, they are planning to set them in the near future (White
*
Although B100 from most feedstocks is essentially sulfur free, this parameter is feedstock and process dependent, i.e.,
used restaurant grease and tallow-derived biodiesel can contribute to higher than typical sulfur levels in biodiesel (Rilett
2008).
†
The term “ultralow sulfur diesel” may refer to different levels of sulfur in different parts of the world; that is, in the
United States it means less than 15 ppm sulfur and in Europe and the Asia-Pacific region it means less than 10 ppm sulfur
(Bacha et al. 2007).
‡
The standardized analytical methods either involve ultraviolet fluorescence spectrometry (EN ISO 20846)—applicable
to sulfur concentrations of 3-500 ppm—or wavelength-dispersive X-ray fluorescence spectrometry (EN ISO 20884).
§
The standardized analytical methods either involve ultraviolet fluorescence spectrometry (ASTM D 5453)—applicable
to sulfur concentrations of 3-500 ppm—or wavelength-dispersive X-ray fluorescence spectrometry (ASTM D 4294).
