CHAPTER 2 LITERATURE REVIEW The relevant literature to the work presented in this thesis is reviewed in this chapter. The first section presents a review of the reasons for the biodiesel NO x effect. In the next section, the influence of the composition of fuel and its properties on biodiesel NO x emissions is reviewed. The third section then reviews the NO x control strategies applicable to biodiesel fuelled engines. In the last section, a summary of the findings from the literature survey and the objectives of the research work are presented. 2.1 REASONS FOR BIODIESEL NO x EFFECT Many researchers have proposed some possible reasons for the increase of NO x in biodiesel fuel; however, the exact cause of the biodiesel NO x effect is still under investigation. An overwhelming number of studies have shown that high isentropic bulk modulus of biodiesel causes an artificial advance in the injection timing relative to petrodiesel, and higher NO x emissions [13-16]. In the pump-line- nozzle (PLN) injection system, earlier injection timing causes an increase in the fuel mass delivery and residence time, which results in very high reaction temperature and more NO x formation. For example, in the experiment carried out by Monyem et al. [14] the actual injection timing occurred earlier with biodiesel, in spite of no changes to the injection timing setting. In their experiment, B100 fuel injects about 2.3earlier than the studied petroleum diesel fuel. The observed arti ficial advance of injection timing is attributed to differences in the fuels’ densities, bulk modulus of compressibility, and speed of sound. The bulk moduli are 1967 MPa, and 2087 MPa and the speeds of sound are 1511 m/s and 1526 m/s for petroleum diesel and biodiesel, respectively. Essentially, these property differences cause a faster pressure rise within the fuel injector; since the diesel fuel injector needle valves are
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CHAPTER 2
LITERATURE REVIEW
The relevant literature to the work presented in this thesis is reviewed in this
chapter. The first section presents a review of the reasons for the biodiesel NOx effect.
In the next section, the influence of the composition of fuel and its properties on
biodiesel NOx emissions is reviewed. The third section then reviews the NOx control
strategies applicable to biodiesel fuelled engines. In the last section, a summary of the
findings from the literature survey and the objectives of the research work are
presented.
2.1 REASONS FOR BIODIESEL NOx EFFECT
Many researchers have proposed some possible reasons for the increase of
NOx in biodiesel fuel; however, the exact cause of the biodiesel NOx effect is still
under investigation. An overwhelming number of studies have shown that high
isentropic bulk modulus of biodiesel causes an artificial advance in the injection
timing relative to petrodiesel, and higher NOx emissions [13-16]. In the pump-line-
nozzle (PLN) injection system, earlier injection timing causes an increase in the fuel
mass delivery and residence time, which results in very high reaction temperature and
more NOx formation. For example, in the experiment carried out by Monyem et al.
[14] the actual injection timing occurred earlier with biodiesel, in spite of no changes
to the injection timing setting. In their experiment, B100 fuel injects about 2.3� earlier
than the studied petroleum diesel fuel. The observed artificial advance of injection
timing is attributed to differences in the fuels’ densities, bulk modulus of
compressibility, and speed of sound. The bulk moduli are 1967 MPa, and 2087 MPa
and the speeds of sound are 1511 m/s and 1526 m/s for petroleum diesel and
biodiesel, respectively. Essentially, these property differences cause a faster
pressure rise within the fuel injector; since the diesel fuel injector needle valves are
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hydraulically opened, a faster rise in fuel pressure will cause the needle to open
earlier which results in more quantity of fuel entering the combustion chamber, and
produces thermal NOx. McCormick et al. [17] did not find any increase in NOx
emissions with common rail injection system equipped engines. However, Zhang and
Boehman [18] found much higher NOx emissions with the common rail system, and
concluded that injection timing shift alone could not be the reason for biodiesel NOx
effect.
In spite of its lower heating value, biodiesel also has a lower stoichiometric
air-fuel ratio. Thermodynamically, adiabatic flame temperature is a function of
heating value and stoichiometric air/fuel ratio. Benajes et al. [19] show significant
effects of adiabatic flame temperature, heat release rate and stoichiometric burning on
NOx formation in diesel engines. The adiabatic flame temperature of biodiesel is
reported to have slightly higher than petro-diesel due to complete combustion
resulting from fuel bound oxygen [20]. In addition to that, fuel-bound oxygen,
aromatics, and double bonds are the lower order parameters affecting adiabatic flame
temperature. Thus, when computing the stoichiometric adiabatic flame temperatures
for petroleum diesel and biodiesel, they may closely match. In contrast, Monyem et
al. [14] have shown that biodiesel has lower adiabatic flame temperature than
petrodiesel.
Some studies [21-23] report the higher heat release rate of biodiesel as a
possible cause for the biodiesel NOx effect. Szibist et al. [16], however, observe a
lower rate of heat release for biodiesel fuel at all loads. This conclusion is further
supported by extensive experimental work by Bittle et al. [24]. The reduced soot
formation of biodiesel leads to decreased radiative heat transfer which results in
increased combustion temperature and NOx. Several mechanisms are reported to
contribute to lower soot formation in biodiesel: increased fuel-bound oxygen, lower
stoichiometric air-fuel ratio (decreased equivalence ratio for same actual air-fuel
ratio), decreased concentration of aromatics, advanced start of combustion (due to
artificial advance and shorter ignition delay), changed soot particle structure,
reduction of fuel-bound sulphur, decreased boiling point, and different sooting
tendencies of various biodiesel esters [25]. Mueller et al. [26] observed increased
stoichiometric burning of biodiesel combustion which could lead to rise in
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temperature and NOx. Many researchers [27 - 29] reported the radiative heat transfer
from in-cylinder soot can significantly influence NOx formation rate. The oxygen
content in biodiesel could cause reduced soot formation [30], which results in
lessened radiative heat transfer and thus increase of temperature [25]. On the contrary,
Agarwal et al. [31] observed increased formation of particulate matter with biodiesel
fuel than diesel fuel.
A change in biodiesel fuel properties might lead to an increased ignition delay
period. Generally, longer ignition delay increases the premixed burn fraction which is
responsible for increased NOx generation [32]. The peak fuel injection pressure is
directly proportional to the bulk modulus of the fuel. In conventional mechanical fuel
systems, biodiesel exhibits higher injection pressures due to its higher bulk modulus.
For a fixed rack position where injection duration remains constant, however, an
increase in injection pressure and density results in increased mass delivery and
thermal NOx. Choi and Reitz [33] reported that fuel mass delivery rates and spray tip
penetrations increase when biodiesel is used as a diesel engine fuel. Yuan and Hansen
[34] conducted computational study using Zeldovich NOx formation model together
with a Kelvine Helmholtz Rayleighe Taylor (KH-RT) spray break-up model and
concluded that decreased spray cone angle and advanced start of injection of biodiesel
influences NOx formation. Biodiesel requires longer pulse-width than diesel in
electronic controlled engines due to its low calorific value causing more quantity of
fuel entry into the cylinder which results in high temperature and NOx [35].
The higher boiling point, viscosity, and surface tension of biodiesel fuel may
contribute to the increased NOx emissions [36]. Yuan et al. [37] have claimed that
biodiesel has more widespread high-temperature distribution areas than diesel that
could contribute higher NOx. Varuvel et al. [38] concluded that the increased
premixed combustion of biodiesel fuel is one of the reasons for biodiesel NOx effect.
Since biodiesel contains oxygen, it premixes more fully during the ignition delay, and
a larger fraction of its heat release occurs during the premixed-burn phase of
combustion at ignition. Combustion that is more premixed has higher oxygen
concentrations and therefore produces more NOx. Allen and Watts [39] state that the
Sauter mean diameter of methyl ester biodiesel varies from 5 to 40% higher than
petroleum diesel fuel. An increase in the Sauter mean diameter reduces the premix
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phase of combustion, causing an increase in the diffusion phase of combustion and
NOx.
Ignition delay, conventionally defined as the time between the start of
injection and start of combustion, for biodiesel is generally shorter than that for
petroleum diesel due to a higher cetane value [40]. A short ignition delay generally
results in a smaller fraction of premixed burn and a correspondingly higher
fraction of diffusion burn. Decreases in biodiesel ignition delay reported by others
include 1.06° by Canakci [41] and 1.1° by Tat et al. [42], both reporting differences
between soy methyl ester and petroleum diesel. Through numerical simulation, Ra et
al. [43] show that certain physical properties namely, vapour pressure, surface
tension, and heat capacity have a significant influence on differences in ignition delay
between biodiesel and petroleum diesel fuels. The lower vapour pressure of biodiesel
results in slower evaporation rate, which tends to increase ignition delay.
CH and OH radicals are continuously formed during combustion reactions.
The formation of CH-radicals is an indicator of low temperature pre-combustion
reactions, which is the first step for the combustion process, once fuel is evaporated.
OH radicals are formed during high temperature reactions and are located in the flame
front, where vaporized fuel reaches the highest temperatures [44]. Brezinsky et al.
[45] claimed that the high rate of acetylene production from the unsaturated fatty
acids of biodiesel is the major cause of increased NOx formation. The acetylene is,
responsible for hydrocarbon CH radical generation and prompt NOx. Recently, Violi
et al. [46] conducted an experiment to analyse biodiesel combustion using shock tube
and detected reduced formation of OH radicals. Moreover, they observed decreased
reactivity of biodiesel over a certain low to intermediate temperature range (Negative
temperature coefficient (NTC) behaviour) [47]. The increased rate of CH radical
formation, lower rate of OH radical generation and NTC behaviour of biodiesel,
indicates that biodiesel combustion is a low temperature reaction when compared to
mineral diesel combustion. Therefore factors such as elevated adiabatic flame
temperature, higher heat release rate and stoichiometric burning might not be the
major reasons for biodiesel NOx effect. The US National Renewable Energy
Laboratory (NREL) also reported the increased formation of prompt NOx as the main
reason for biodiesel NOx effect and they suggested the use of antioxidants as a
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prompt NOx control strategy [8]. The biodiesel NOx effect has been reviewed in detail
by Jacobs et al. [48] and Hoekman and Robbins [49]. The NOx mitigation techniques
applicable to biodiesel fuels are reviewed by Rajasekar et al. [50].
2.2 EFFECT OF FUEL PROPERTIES ON BIODIESEL NOx EMISSIONS
There is widespread agreement that no single factor is responsible for the
biodiesel NOx effect. The physical and chemical properties of biodiesel may influence
combustion temperature, residence time and injection pattern and thus NOx emissions.
The fuel properties such as bulk modulus, fuel bound oxygen, degree of saturation,