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With the growing capacity in biodiesel production and the resulting glut of the glycerol by-product, there is increasing interest in finding alternative uses for crude glycerol. One option may be to burn it locally for combined process heat and power, replacing fossil fuels and improving the economics of biodiesel production. However, due to its low energy density, high viscosity, and high auto-ignition temperature, glycerol is difficult to burn. Additionally, the composition of the glycerol by-product can change dramatically depending upon the biodiesel feedstock (e.g., vegetable oils or rendered animal fats), the catalyst used, and the degree of post-reaction cleanup (e.g., acidulation and demethylization). This paper reports the results of experiments to (1) develop a prototype high-swirl refractory burner designed for retrofit applications in commercial-scale fire-tube package boilers, and (2) provide an initial characterization of emissions generated during combustion of crude glycerol in a laboratory-scale moderate-swirl refractory-lined furnace. We report a range of emissions measurements, including nitrogen oxides, total hydrocarbons, and particle mass for two grades of crude glycerol (methylated and demethylated) and compare these to No. 2 fuel oil and propane. We also present preliminary data on the emissions of select carbonyls (by cartridge DNPH). Results indicate that a properly designed refractory burner can provide the thermal environment to effectively combust glycerol, but that high particulate emissions due to residual catalysts are likely to be an issue for crude glycerol combustion.
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Glycerol combustion and emissions
Myles D. Bohon
, Brian A. Metzger
, William P. Linak
Charly J. King
, William L. Roberts
Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695, USA
Air Pollution Prevention and Control Division, National Risk Management Research Laboratory,
U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, USA
ARCADIS Geraghty and Miller Inc., Durham, NC 27709, USA
Available online 25 September 2010
With the growing capacity in biodiesel production and the resulting glut of the glycerol by-product,
there is increasing interest in finding alternative uses for crude glycerol. One option may be to burn it
locally for combined process heat and power, replacing fossil fuels and improving the economics of biodie-
sel production. However, due to its low energy density, high viscosity, and high auto-ignition temperature,
glycerol is difficult to burn. Additionally, the composition of the glycerol by-product can change dramat-
ically depending upon the biodiesel feedstock (e.g., vegetable oils or rendered animal fats), the catalyst
used, and the degree of post-reaction cleanup (e.g., acidulation and demethylization). This paper reports
the results of experiments to (1) develop a prototype high-swirl refractory burner designed for retrofit
applications in commercial-scale fire-tube package boilers, and (2) provide an initial characterization of
emissions generated during combustion of crude glycerol in a laboratory-scale moderate-swirl refrac-
tory-lined furnace. We report a range of emissions measurements, including nitrogen oxides, total hydro-
carbons, and particle mass for two grades of crude glycerol (methylated and demethylated) and compare
these to No. 2 fuel oil and propane. We also present preliminary data on the emissions of select carbonyls
(by cartridge DNPH). Results indicate that a properly designed refractory burner can provide the thermal
environment to effectively combust glycerol, but that high particulate emissions due to residual catalysts
are likely to be an issue for crude glycerol combustion.
Ó2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Keywords: Glycerol combustion; Emission characterization; Bio-fuels; Burner development; Waste fuels
1. Introduction
Biodiesel fuels are produced through the
transesterification of triglycerides (fats, oils, or lip-
ids) into fatty acid methyl esters (FAME). During
this process, the triglyceride raw material reacts
with an alcohol (almost always methanol) and a
base catalyst (typically sodium or potassium
hydroxide) to produce FAME (biodiesel) and
glycerol (propane-1,2,3-triol) by-product. On a
molar basis, one mole of glycerol is produced
for every three moles of FAME, and volumetri-
cally, approximately 10% of the initial reactants
are converted to glycerol. Depending upon the
feedstock and the process specifics, the glycerol
1540-7489/$ - see front matter Ó2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Corresponding author. Address: Engineering Bldg 3,
Campus Box 7910, 911 Oval Drive, Raleigh, NC 27695,
E-mail address: (M.D. Bohon).
Available online at
Proceedings of the Combustion Institute 33 (2011) 2717–2724
of the
waste may also contain significant and variable
amounts of excess alcohol, water, catalyst, and a
mixture of other organic material (soaps and
unreacted fats and oils) collectively characterized
as MONG (matiere organique non-glycerol). In
2009, the U.S. biodiesel production capacity
exceeded 10.2 10
gal) and is expected
to increase another 1.63 10
L (4.3 10
gal) by
2011 [1]. Recent decreases in the price of crude oil
and the slim margins associated with biodiesel
production have negatively affected the industry
such that it is estimated to be currently operating
at less than 25% of this capacity. However, even at
this reduced utilization, the market for the result-
ing glycerol (primarily cosmetics and food and
beverage industries) is nearly saturated [2]. The
current price of crude glycerol is so low ($0.04–
0.11/kg, $0.02–0.05/lb) that many biodiesel pro-
ducers are stockpiling it while they wait for better
markets to materialize [3].
As biodiesel production evolves from a cottage
industry to become a viable alternative transpor-
tation fuel, there will be a growing need to find
new value-added uses for the glycerol waste. This
presents opportunities for the industry to optimize
or transform the process to increase efficiencies,
and reduce or reuse wastes. Numerous alternative
uses of glycerol are currently being investigated.
These include conversion to commodity chemicals
(such as propylene glycol, propionic acid, and iso-
propanol) with higher market values, use as fertil-
izers, and as extenders in animal feeds [4].
Another possible use is as a boiler fuel to produce
process steam and co-generate electricity. If com-
bined with biodiesel production, this has the
added advantages of optimizing energy integra-
tion, eliminating transportation costs, and dis-
placing the need for fossil fuels. Utilizing
glycerol as a fuel has the potential of replacing
3.8 10
L/year (1.0 10
gal/year) of fuel oil,
equivalent to 1.8 10
kg/year (1.3 10
year) of carbon dioxide. Even though glycerol is
known to have a moderate heating value
(16 MJ/kg), it has not previously been used as
a fuel. Glycerol has a very high activation energy
resulting in an auto-ignition temperature of
370 °C as compared to 210 and 280 °C for kero-
sene and gasoline, respectively [5]. Pure glycerol
is also highly viscous (1030 cP at 22 °C), making
it difficult to pump and atomize. Finally, its chem-
ical similarity to acrolein (propenal) causes con-
cern if this toxic but normally unstable carbonyl
compound is formed as a product of combustion.
Patzer et al. [6] investigated glycerol combustion
in an unmodified package boiler, but were unable
to achieve stable glycerol flames until they co-fired
smaller amounts of glycerol with yellow grease.
This is consistent with the apparent issues of glyc-
erol ignition and flame stability in package boilers
designed for high rates of heat transfer, cold walls,
short residence times (2 s), and high gas-quench-
ing rates (500 K/s) [7]. In contrast, the research
presented here describes efforts to (1) develop a
prototype high-swirl refractory burner designed
for retrofit applications in commercial-scale fire-
tube package boilers and capable of stable opera-
tion with 100% glycerol fuels, and (2) provide an
initial characterization of emissions generated
during combustion of crude glycerol fuels in a
similar laboratory-scale moderate-swirl refrac-
tory-lined furnace. While data collected at the
U.S. EPA/NRMRL from the refractory-lined fur-
nace was performed under an approved quality
assurance project plan, the data pertaining to
the prototype burner were not subjected to the
Agency’s required peer and policy reviews.
2. Materials and methods
2.1. Prototype 7 kW refractory burner
This study examined glycerol combustion in
two experimental systems. The first is a prototype
7 kW refractory burner based on a design
described by Chen et al. [8]. This burner, shown
in Fig. 1a, consists of an air swirl chamber, a ven-
turi restriction, and a refractory-lined combustion
chamber. An air-assisted glycerol atomizing noz-
zle (Delavan model 30609-3) is located along the
centerline of the swirl chamber and venturi result-
ing in an annular space for the swirling air. The
14.2 L combustion chamber is lined with cast
refractory 1.9 cm thick. In its current configura-
tion, the prototype burner is equipped with a steel
restrictor at the exit of the combustion chamber to
promote internal recirculation. The assembly is
designed to be inserted into the first pass of a com-
mercial fire-tube boiler, and provide an insulated
environment to promote ignition and flame stabil-
ity. Axial and tangential air flowing through the
venturi at the inlet of the combustion chamber
produces a swirling pressure gradient, creating
an intense recirculation zone into which the glyc-
erol is sprayed. While a large range of swirl is
possible, swirl numbers between 2 and 10 were
examined and adjusted by varying the axial and
tangential air flow rates. The burner refractory is
first preheated with a traditional fuel, in this case
propane, and then transitioned to glycerol. Fig-
ure 1b illustrates the burner operating on 100%
glycerol. Stable flames are possible through a
combination of the hot refractory walls and
intense gas recirculation. The prototype burner
was mounted on a test stand under a large fume
hood. U.S. Pharmacopeia (USP) grade glycerol
was pumped (28 g/min and 276 kPa) to the
spray nozzle and atomized with air (32 SLPM
and 172 kPa). Gas samples were collected through
an uncooled quartz probe at the throat of the
exhaust cap, cooled through an ice bath to remove
condensing water, and directed to a California
2718 M.D. Bohon et al. / Proceedings of the Combustion Institute 33 (2011) 2717–2724
Instruments (model 400 HCLD, Orange, CA)
analyzer and an Infrared Industries (model
FGA-4000 XDS, Hayward, CA) exhaust gas ana-
lyzer (O
, CO, and CO
). Exhaust temperatures
were measured using a bare type B thermocouple.
The prototype burner experiments included preli-
minary measurements of selected volatile carbon-
yls. A known volume (1.1 L) of burner exhaust
gas was drawn through a cartridge containing
2,4-dinitrophenylhydrazine (DNPH) impregnated
material purchased (Waters Corp., model 37500,
Milford, MA) for the purpose. DNPH reacts with
the carbonyls to create DNPH-carbonyl deriva-
tives that were later dissolved in acetonitrile and
analyzed by high performance liquid chromatog-
raphy. DNPH is also selective for NO
and care must be taken so as not to saturate the
DNPH. DNPH saturation, however, is evident
during chromatography. In addition to USP
grade glycerol and an air blank, methane, pro-
pane, and kerosene fuels were also characterized
for carbonyl emissions.
2.2. 82 kW refractory-lined furnace
The second experimental system, illustrated in
Fig. 1c, is a laboratory-scale refractory-lined fur-
nace. This system was equipped with an 82 kW
rated International Flame Research Foundation
(IFRF) movable-block, variable-air swirl burner
which incorporated an air assisted atomizing noz-
zle positioned along its center axis. Swirl numbers
up to 1.8 are possible. Additional details regard-
ing this experimental system are presented else-
where [9,10]. Based on its similar design to the
prototype burner (refractory-lined, adjustable
swirl, and air atomization), this system was also
used to examine operational issues (fuel delivery,
atomization, flame ignition and stability) as well
as provide a preliminary assessment of several
emissions. Like the prototype burner, the labora-
tory furnace was preheated (using natural gas)
and then transitioned to glycerol fuel. However,
unlike the prototype burner, the laboratory fur-
nace burned two formulations of crude glycerol
received from Foothills Bio-Energies Inc. (Lenoir,
NC). These glycerol fuels were fed from drums
using an in-barrel heating system, insulated fuel
lines, and continuous circulation within a fuel
loop with a portion directed to a Spraying Sys-
tems Co. (model Air Atom 1/4-JSS) air-atomizing
nozzle. Fuel temperature, air pressure, air flow,
and air temperature were maintained at 93 °C,
204 kPa, 30 SLPM, and 150 °C, respectively. Pre-
heating the crude glycerol in this manner reduced
its viscosity significantly (20 cP for pure glycerol
at 100 °C) allowing it to be handled similarly to
other fuel oils. Gas samples were extracted from
an exhaust location (see Fig. 1c) and directed to
a set of continuous emission monitors (CEMs).
These samples were conditioned and analyzed
for CO
(Beckman Corp., model 755, La Habra,
Fig. 1. Experimental facilities used for glycerol combustion: (a) cross-sectional view of 7 kW prototype burner; (b)
prototype burner operating on 100% USP grade glycerol; and (c) 82 kW refractory-lined furnace with sampling locations
M.D. Bohon et al. / Proceedings of the Combustion Institute 33 (2011) 2717–2724 2719
CA), O
(Beckman Corp., model 755, La Habra,
CA), CO (Thermo Electron Corp., model 48,
Franklin, MA), NO and NO
(Teledyne Technol-
ogy Co., model 200A4, San Diego, CA), and total
hydrocarbons (THC, Thermo Electron Corp.,
model 43c, Franklin, MA), in accordance with
Methods 3A, 7E, 10, and 25A [11]. Once steady-
state operation with glycerol fuels was achieved,
particulate matter (PM) samples were collected
(in triplicate) on filters (Method 5) for mass deter-
mination and limited chemistry [11]. PM samples
were also directed to a scanning mobility particle
sizer (SMPS, TSI Inc., model 3080/3022a, Shore-
view, MN) and an aerodynamic particle sizer
(APS, TSI Inc., model 3321, Shoreview, MN) to
determine particle size distributions. Filter sam-
ples were later examined for elemental carbon
and organic carbon (EC/OC) using a thermal/
optical carbon analyzer (Sunset Laboratory Inc.,
model 107, Tigard, OR) and inorganic elements
by wavelength dispersive X-ray fluorescence spec-
troscopy (WD-XRF, Philips, model 2404 Panalyt-
ical, Natick, MA). WD-XRF data were collected
by Panalytical’s SuperQ software and analyzed
using UniQuant 5 (Omega Data Systems, Veldho-
ven, The Netherlands).
2.3. Crude glycerol fuels
There is a great deal of variability in the feed
stocks and processes used to make and purify
the FAME product, recover useful reactants for
recycle, and process the glycerol by-product [12].
Important glycerol post-processes include acidula-
tion/neutralization to adjust the pH, and evapora-
tion/distillation to separate the water and excess
methanol for reuse. Biodiesel manufacturers typi-
cally make efforts to reclaim the excess unreacted
methanol. However, based on the relative costs of
methanol recovery and purchasing new methanol,
this is not always the case. Further, as it may be
advantageous to utilize the methanol as a fuel
component, we decided to examine both methyl-
ated and demethylated crude glycerol fuels. Meth-
ylated crude glycerol typically contains (by
weight) 50–70% glycerol, 10–20% methanol, 5–
10% salts, <3–10% water, <1–5% free fatty acids,
and <1–5% MONG. Demethylated crude glycerol
typically contains (by weight) 70–88% glycerol,
<1% methanol, 5–15% salts, <5–15% water, <1–
5% free fatty acids, and <1–5% MONG [13].
Table 1 presents an analysis of both crude glycerol
fuels as received from Foothills Bio-Energies pro-
duced from the transesterification of low free fatty
acid chicken grease. Values for USP grade glyc-
erol are included for comparison. All three fuels
(USP, methylated, and demethylated) contain sig-
nificant amounts of oxygen (52, 43, and 17 wt%,
respectively). However, the low value measured
for the demethylated glycerol may indicate a fairly
low glycerol concentration and larger quantities
of MONG. Also notable are the low nitrogen
(<0.05 wt%) and sulfur (<0.05–0.08 wt%) contents
and high ash (2–3 wt%) contents of the two crude
glycerol fuels. This ash corresponds to very high
sodium levels (1.2–1.8 wt%) consistent with the
use of NaOH catalyst. Other ash elements in nota-
ble concentrations include phosphorus and potas-
sium. The heating values determined for the
methylated and demethylated fuels (21.8 and
20.6 MJ/kg, respectively) are also notably higher
than that for pure glycerol (16.0 MJ/kg).
3. Results and discussion
3.1. Prototype 7 kW refractory burner
USP grade glycerol combustion was examined
in the prototype refractory burner over a range of
swirl numbers and equivalence ratios and com-
pared to operation with propane and No. 2 fuel
oil. All three fuels generated no or negligible
ash, as we wanted to avoid ash formation and
deposition in these tests and concentrate on flame
ignition and stability issues. Propane was chosen
as it represents a similar (but non-oxygenated)
three-carbon alkane, similar to glycerol. No. 2 fuel
oil was chosen to examine and compare atomiza-
tion using a common liquid fossil fuel. Both pro-
pane and No. 2 fuel oil have heating values
(46.2 and 42.5 MJ/kg, respectively) significantly
greater than glycerol. We decided to match the
burner load for all three fuels (7 kW). This corre-
sponds to fuel feed rates of 28.0, 9.6, and 10.3 g/
min for USP glycerol, propane, and No. 2 fuel
oil, respectively. Glycerol combustion was exam-
ined over a wide range of air flows and swirl.
Equivalence ratios were evaluated by using a pre-
determined glycerol flow rate based on desired
power output and then airflow was adjusted (both
total and swirl) to achieve a stable flame, whereby
the flame was entirely contained within the cham-
Table 1
Analysis of three glycerol fuels.
Methylated Demethylated
C (%) 39.1 42.05 67.27
H (%) 8.7 10.14 11.43
N (%) 0 <0.05 <0.05
O (%) 52.2 43.32 17.06
S (%) 0 0.078 <0.05
O (%) 0 1.03 1.47
Ash (%) 0 3.06 2.23
Ca (ppm) <23 119
Na (ppm) 11,600 17,500
K (ppm) 628 541
Cl (ppm) 124 154
Mg (ppm) <8 29
P (ppm) 2220 1750
HHV (MJ/kg) 16.0 21.8 20.6
2720 M.D. Bohon et al. / Proceedings of the Combustion Institute 33 (2011) 2717–2724
ber through the full range of swirl. The highest air
flow was chosen where the glycerol could burn
through the full range of swirl without blowout.
Equivalence ratios were determined based on
measured air and fuel flows and confirmed from
measurements of exhaust O
. Stable and optimum
operation was achieved over a range of three air
flow rates (210, 227, and 243 SLPM) for swirl
numbers from 2 to 10. Interestingly, accounting
for fuel oxygen, these conditions correspond to
low global equivalence ratios (U= 0.37–0.44).
Corresponding air flows and swirl using both pro-
pane and No. 2 fuel oil were not possible as the
flames tended to blowout. This was due to the
flow rate of air through the atomizing nozzle
required to atomize the highly viscous glycerol.
When the fuel was switched to a less viscous fuel,
the high air flow rate through the small orifice cre-
ated too great a velocity which blew out the pro-
pane and diesel flames. Stable operations were
achieved at air flow rates of 180 and 202 SLPM
for propane and 172 and 195 SLPM for No. 2 fuel
oil for all swirl numbers (2–10) examined. These
conditions correspond to global equivalence ratios
between 0.48 and 0.65. Lower air flow rates for
the glycerol case did produce stable flames for
some swirl conditions, but not for the full range,
and thus it was difficult to resolve the disparity
in the equivalence ratios. The recirculation zone
strength will scale with the swirl number. For all
swirl numbers investigated here, the flame was sta-
ble. The mean exhaust gas temperature decreased
with decreasing swirl number, and was fairly
insensitive to swirl number at high swirl.
Table 2 summarizes emission measurements
made at the burner exit. Emissions from USP
glycerol combustion compare favorably with
those from the other two traditional fuels. Emis-
sions of CO were undetectable, and O
and CO
were consistent with corresponding stoichiome-
tries and mass balances for complete combustion.
Interestingly, NO
emissions for the glycerol
flames were exceedingly low (7–10 ppm, 0% O
compared to those for the two fossil fuels (110–
140 ppm, 0% O
). This was true even though O
levels during glycerol combustion were very high.
Except for NO
, these emissions did not have a
notable dependence on the swirl number over
the range examined. Figure 2 shows a slight influ-
ence of increasing swirl number on NO
tion. Temperatures measured at the burner exit
were fairly comparable, with those for glycerol
perhaps somewhat lower than propane and No.
2 fuel oil. All three flames are predominately diffu-
sion controlled, where peak flame temperatures
occur at near stoichiometric equivalence ratios.
Calculated stoichiometric adiabatic flame temper-
atures for glycerol, propane, and No. 2 fuel oil are
2201, 2394, and 2413 K, respectively. The adia-
batic flame temperature for glycerol is slightly
lower, which may contribute to the reduced NO
Table 2
Emissions measured from 7 kW prototype burner and 82 kW refractory-lined furnace.
7 kW prototype burner 82 kW furnace
USP glycerol Propane No. 2 fuel oil Methylated Demethylated
Load (kW) 7.3 7.3 7.3 7.4 7.4 7.3 7.3 80.5 53.9
0.444 0.392 0.370 0.562 0.488 0.645 0.488 0.63 0.77
2.25 2.55 2.70 1.78 2.05 1.55 2.05 1.58 1.30
(ppm) 3.0 3.5 3.6 60.2 62.8 74.7 62.5 146.5 118.3
at 0% O
(ppm) 6.9 9.1 9.6 110.5 135.4 117.8 128.6 235.2 155.5
(%) 11.8 12.9 13.3 9.6 11.3 7.7 10.8 7.9 5.1
(%) 7.3 6.7 6.3 6.8 5.9 7.0 6.2 12.5 15.4
CO (%) 0.0 0.01 0.0 0.01 0.00 0.0 0.0 –
THC (ppm) – – – – – – – 4.7 7.1
Exit temp. (°C)
958 901 877 1001 974 986 946 1041 1075
Flame temp. (°C)
1201 1103 1060 1359 1213 1628 1343 1782 1716
Equivalence and stoichiometric ratios determined by excess O
in the exhaust.
Temperature measured at the throat of the exhaust for the 7 kW prototype burner and at the exit of the 82 kW
refractory-lined furnace.
Adiabatic flame temperature calculated at stoichiometric ratios listed above.
Fig. 2. NOxemissions corrected to 0% O
vs. swirl
number for the 7 kW prototype burner.
M.D. Bohon et al. / Proceedings of the Combustion Institute 33 (2011) 2717–2724 2721
formation. However, these differences in tempera-
ture are not large enough to account for all the
disparity. This may indicate that the thermal
mechanism is not the dominate mechanism,
but NO
formation is rather a combination of
thermal and prompt mechanisms, both of which
may be suppressed in the glycerol case. It is unli-
kely that there is any significant contribution of
fuel NO
formation due to very low levels of
nitrogen in all three fuels. One possibility is that
there is greater partial premixing in the glycerol
case which may contribute to reduced thermal
formation. Appleton and Heywood [14]
show that with better atomization, NO
tion is reduced as global equivalence ratios
decrease. However, the glycerol case should exhi-
bit lower partial-premixing due to its higher boil-
ing point compared with No. 2 fuel oil.
Additionally, propane should exhibit the most
premixing due to being a gaseous fuel. Therefore,
it seems unlikely that glycerol has greater partial
premixing. If large differences in peak flame tem-
perature and partial premixing cannot explain
the dramatically different NO
levels, one possible
explanation is the very large fuel-bound oxygen
content of the glycerol (52% by mass). Unfortu-
nately, there is no work in the literature with fuels
with such high fuel-bound oxygen contents and
what effect this may have on NO
formation is
not well understood. However, the presence of
so much oxygen within the fuel may contribute
to a broadening of the flame front, thereby reduc-
ing peak temperatures. The presence of so much
fuel-bound oxygen may also inhibit the prompt
mechanism. The NO
formation is not inhib-
ited by the presence of fuel-bound oxygen in the
propane and diesel flames and may proceed
through a combination of both prompt [15] and
thermal mechanisms, while both mechanisms
could be inhibited in the glycerol flame. It was
attempted to examine this effect by mixing glyc-
erol with other non-oxygenated fuels. However
this effort failed due to the high polarity of glyc-
erol and its immiscibility with most fuels. Further
work needs to be done to understand the effect of
high fuel oxygen content on NO
Table 3 presents preliminary emission mea-
surements of formaldehyde, acetaldehyde, and
acetone from the prototype burner operating with
USP glycerol. As can be seen, aldehyde concentra-
tions in the glycerol emissions were approximately
10 times those measured in the ambient air blank,
slightly higher than those measured from methane
and propane, and comparable to those from ker-
osene. In no test was acrolein detected above
17.5 ppb. These preliminary data indicate that pri-
mary carbonyl emissions from glycerol combus-
tion may be comparable to those from other
conventional fossil fuels.
3.2. 82 kW refractory-lined furnace
Both methylated and demethylated crude glyc-
erol fuels burned reasonably well in the refrac-
tory-lined furnace without fossil fuel co-firing. In
fact, the warmed demethylated glycerol fed more
consistently through the air atomizer than the
methylated fuel which, due to its lower viscosity,
required larger amounts of fuel and atomizing
air to produce a stable spray. This difference in
viscosity accounts for the higher load and excess
air reported in Table 2 for this fuel. The required
high fuel feed rates (due to low heating values)
produced long flames which were shortened by
maximizing the IFRF burner swirl (1.8). Interest-
ingly, the refractory-lined furnace uses a UV-
based flame safety system, and although both
fuels produced stable flames (base on visual obser-
vations) the UV detector had difficulties establish-
ing a stable signal. Eventually, a flame rod was
substituted and stable flame signals were estab-
lished. The equivalence ratios were determined
based on exhaust O
Table 2 presents the results of the gas-phase
emission measurements averaged over the course
of three replicate experiments. These results indi-
cate that glycerol combustion in a refractory-lined
furnace produced gas-phase emissions compara-
ble to previous experiences with fossil fuels (natu-
ral gas and No. 2 fuel oil). Unfortunately,
accurate CO emissions could not be determined
due to instrument malfunction. However, both
total hydrocarbon concentrations as well as total
carbon (TC) concentrations in the fly ash (see
Table 4) were consistently low and typical of emis-
sions indicating reasonably complete combustion.
Oxygen levels were slightly elevated, but this was a
consequence of maintaining proper fuel atomiza-
tion and the high inherent oxygen contents of
the glycerol fuels. Concentrations of NO
(150–240 ppm, 0% O
) were typical of the rela-
tively high combustion temperatures and low fuel
nitrogen contents. The data suggest that the deme-
thylated glycerol produced slightly less NO
the methylated fuel. It is notable that the proto-
type burner produces NO
emissions significantly
lower (6 ppm, 0% O
) than those measured in
the refractory-lined furnace. This difference in
emissions may be related to the variation
in swirl (1.8 compared to 2–10), but is most likely
related to the longer residence times in the refrac-
Table 3
Preliminary measurements of several carbonyls (ppm).
Formaldehyde Acetaldehyde Acetone
Air 1.0 0.25 0.45
Methane 1.5 0.50 1.70
Propane 6.0 1.50 0.75
Glycerol 15.0 2.25 1.25
Kerosene 10.0 0.625 1.00
2722 M.D. Bohon et al. / Proceedings of the Combustion Institute 33 (2011) 2717–2724
tory-lined furnace. The prototype burner was able
to maintain stable glycerol flames at global equiv-
alence ratios beyond the operating range for the
other fuels examined (propane and diesel). It
should be noted, however, that the crude glycerol
fuels examined in the refractory-lined furnace
were not the same as the USP glycerol examined
in the prototype burner. The presence of MONG
and other process by-products in the crude glyc-
erol fuels reduces the fuel oxygen and may well
affect NO
formation. Because of the large differ-
ences in fluid dynamics, swirls, and residence
times between the two experimental systems, it is
difficult to compare NO
emissions. However,
comparisons within their individual systems is
valid and of interest.
Mass concentrations of fly ash determined
gravimetrically indicate average emissions of
3380 and 2200 mg/m
for the demethylated and
methylated glycerol fuels, respectively. These are
very high values and are consistent with the high
ash concentrations of these fuels. These values
can be compared to concentrations of 90 mg/
measured in the same combustor burning a
No.6 fuel oil with an ash content of 0.1% [7].In
fact, concentrations of 3000 mg/m
those for coal combustion before particulate con-
trol. Particle size distributions measured from
emissions of the two fuels indicated a large dis-
tinct accumulation mode (100–110 nm) suggest-
ing vaporization, nucleation, and coagulation of a
significant amount of ash. These results are con-
sistent with the very high alkali metal content of
the fuels. These data also indicate the presence
of a substantial coarse mode (>5 lm), especially
for the demethylated fuel.
Table 4 presents a summary of the elemental
analyses performed on the filter samples. For
these measurements, it was assumed that total car-
bon (TC) is the sum of organic carbon (OC) and
elemental carbon (EC). Other elements with
atomic numbers >9 (fluorine) were determined
by WD-XRF. Carbon analyses indicate that
approximately 1% of the PM is organic carbon,
and another 2–3% is elemental carbon. These val-
ues are comparable to those measured from tradi-
tional fossil fuels and are consistent with the low
levels of hydrocarbons measured. Elements deter-
mined by XRF (and presented as stable oxides)
accounted for approximately 80% and 89% of
the particulate mass for the demethylated and
methylated fuels, respectively. Major elements
include Na, P, Cl, and K. Sodium specifically
accounts for over 40% of the fly ash and its pres-
ence is the results of the NaOH catalyst used dur-
ing the transesterification process. The other
major elements (P, Cl, and K) are typical of bio-
fuels. Between the unburned carbon and the inor-
ganic elements measured, the majority of the par-
ticulate mass composition is identified.
4. Conclusions
Waste glycerol is produced in significant quan-
tities during the transesterification of triglycerides
to produce biodiesel fuels, and new value-added
uses for this waste are needed to optimize process
efficiencies and reduce the impacts of disposal.
Although not an ideal fuel, waste glycerol might
be used in boilers to produce process steam and
co-generate electricity with the added advantages
of optimizing energy integration, eliminating
transportation costs, and displacing the need for
fossil fuels. This work examined efforts to develop
a prototype high-swirl refractory burner designed
for retrofit applications in package boilers and
provide an initial characterization of emissions
generated during combustion of crude glycerol
fuels. These results represent important first steps
toward characterizing the use of waste glycerol as
a boiler fuel. Study conclusions can be summa-
rized as follows:
(1) Stable 100% glycerol combustion was
achieved for both a 7 kW prototype high
swirl burner (using USP grade glycerol)
and an 82 kW (rated) refractory-lined fur-
nace (using crude methylated and demethy-
lated glycerol wastes).
(2) For the prototype burner, optimum glyc-
erol combustion corresponded to operation
at very high swirls (2–10) and excess air
(U= 0.37–0.44). In contrast to glycerol,
propane and No. 2 fuel oil combustion
became unstable at high excess air
(U< 0.45).
(3) With the exception of NO
, both combus-
tors produced gas-phase emissions similar
to natural gas and distillate fuel oils indicat-
ing low total hydrocarbon emissions and
efficient combustion.
Table 4
Fly ash elemental analyzes (wt%).
Methylated Demethylated
C 4.88 2.74
OC 1.31 0.53
EC 3.56 2.21
O 23.1 27.3
Na 41.8 45.8
Mg 0.033 0.067
P 4.56 5.98
S 0.99 1.48
Cl 0.96 1.45
K 1.45 1.53
Ca 0.338 0.46
Fe 0.114 0.143
Cu 0.013 0.009
Zn 0.781 0.688
Trace 1.850 1.940
Undetermined 19.2 10.4
M.D. Bohon et al. / Proceedings of the Combustion Institute 33 (2011) 2717–2724 2723
(4) Interestingly, NO
emissions from the pro-
totype burner (7–10 ppm, 0% O
) were 20
times lower than those from the refrac-
tory-lined furnace (160–240 ppm, 0% O
Differences in burner swirl and excess air,
as well as differences in compositions
between pure glycerol and actual crude
glycerol wastes are believed to be
(5) Preliminary measurements of carbonyl
emissions (by DNPH cartridges) indicate
formaldehyde and acetaldehyde emissions
only 10 times larger than an air blank,
and comparable to several common fossil
fuels. Acrolein was measured at less than
17.5 ppb. Continued measurements of car-
bonyls are one priority for a second phase
of this work.
(6) Combustion of crude methylated and deme-
thylated glycerol wastes produces fly ash
concentrations (2200–3400 mg/m
) much
larger than residual fuel oils and compara-
ble to coal combustion before particulate
control. Particulate size distributions indi-
cate a large accumulation mode suggesting
significant Na vaporization.
(7) WD-XRF analysis indicates that 40–50% of
the fly ash (determined as stable oxides) is
composed of Na with smaller amounts of
phosphorus (4–6%), potassium (1–2%),
chlorine (1–2%), and sulfur (1%). This is
consistent with unrecovered NaOH used
as a process catalyst. Approximately 3–4%
of the fly ash is unburned carbon. The large
concentrations of fly ash formed during
glycerol combustion combined with the
high alkali metal content of this ash pre-
sents a significant issue that needs to be
addressed before crude glycerol fuels can
be utilized in boilers.
Portion of this work were sponsored under
Contract EP-C-09-027 with Arcadis G&M Inc.,
the NCSU/EPA Cooperative Training Program
in Environmental Sciences Research, Training
Agreement CT8333235-01-0 with North Carolina
State University, and funds from the Diversified
Energy Corporation. The authors would like to
thank Seung-Hyun Cho and Daniel Janek for their
contributions. The U.S. Environmental Protection
Agency through its Office of Research and Devel-
opment partially funded and collaborated in the
research described here. The views expressed by
the individual authors, however, are their own
and do not necessarily reflect those of the U.S.
Environmental Protection Agency.
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2724 M.D. Bohon et al. / Proceedings of the Combustion Institute 33 (2011) 2717–2724
... Using crude glycerol and glycerol as fuels and fuel additives is the most straightforward way with various benefits. First, they can serve as independent liquid fuel [22], liquid fuel additives [23], liquid and solid fuel slurries [24], and additives to solid fuel such as coal [25] and biomass pellets [26]. Moreover, crude glycerol and glycerol are compatible with different types of fossil fuels, such as diesel [27], biodiesel [28] and light fuel oil [29]. ...
... Moreover, crude glycerol and glycerol are compatible with different types of fossil fuels, such as diesel [27], biodiesel [28] and light fuel oil [29]. Therefore, they can work in existing combustion systems without requiring significant modifications, including the furnace [30], boiler [22], compression and spark-ignition engines [31], gas turbine [32], Stirling engine cogeneration system [33], fluidized bed [34], and microturbine [35]. ...
... For instance, when being used in a micro-gas-turbine, preheating glycerol increased the combustion efficiency significantly by improving atomization due to the decrease in viscosity [72]. Because of the long ignition delay and slow burning rate of glycerol, preheating the combustor to a high temperature prior to crude glycerol injection ensures thorough evaporation of glycerol and consequently stable combustion [22]. More promisingly, stable glycerol combustion is achieved even without preheating by atomization optimization through a flow-blurring burner [53]. ...
Biodiesels as renewable fuels are important in approaching low-carbon transportation. One critical challenge associated with biodiesel production is the utilization of crude glycerol, an abundant waste byproduct from which glycerol can be obtained via refinement, to maximize economic and environmental benefits. Efforts on crude glycerol and glycerol utilization include direct use (e.g., combustion fuels) and indirect use (e.g., reformation for value-added chemicals). Among different approaches, the direct use as fuels and fuel additives is straightforward and affordable. Recently, significant progress has been made to understand their combustion performance and explore new applications, including fuel additives for reducing emissions and improving the combustion performance of traditional fuels, and fuel for combustion synthesis of valuable materials. The current review discusses these achievements and future outlook on using crude glycerol and glycerol as fuels and fuel additives for combustion applications. Overall, crude glycerol can achieve high-efficiency combustion with the improved design, but ash deposition is still the main challenge. Fortunately, integrating crude glycerol with solid fuels could mitigate ash issues and improve the combustion performance of low-rank solid fuels. In contrast, glycerol is an oxygenated fuel additive for lowering NOx emissions. Furthermore, glycerol is capable as a fuel in combustion material synthesis for manufacturing the raw materials for catalysts, sensors, and battery applications.
... Bohon et al. [88] studied the emission characteristics of combustion of glycerol in a prototype 7 kW refractory-lined burner and 82 kW refractory-lined furnace. There were three different types of glycerol feedstock namely US pharmacopeia, methylated, and demethylated glycerol. ...
Glycerol is an oversupplied commodity from biodiesel production with beneficial properties for the synthesis of versatile utility biochemicals. The functional properties of glycerol with three hydroxyl groups could be tailored toward producing fuels such as hydrogen, syngas, C1-C3 hydrocarbon fuel, bio-oil, methane, etc. This study elucidates the reported thermochemical pathways such as pyrolysis, gasification, combustion, steam and aqueous reforming, and supercritical water gasification for fuel production from biodiesel-derived glycerol. The mechanism of these pathways, process conditions, catalytic integration, and limitations were investigated. Also, reactor strategies such as fixed bed reactors, fluidized bed, and solar reactor strategies used for the thermochemical valorization of glycerol was discussed. The studies revealed that hydrogen up to 70% yield could be generated from glycerol using noble metals and nickel-based catalysts. Catalyst deactivation due to coking could be minimized by the addition of alkaline metals, which discourages methanation reaction. Reaction parameters such as temperature, catalyst amount, time on stream, and glycerol/water or steam ratio influence the product distribution. The glycerol steam reforming is more energy-intensive and requires temperatures in the range of 450-1000 oC, whereas the aqueous reforming is propagated in the range of 180-250 oC. The circulating fluidized bed reactors limit coking due to self-regeneration of the catalysts in situ, however, they are cost-intensive. Life cycle assessment analysis revealed that supercritical water reforming (SCWR) of glycerol offers a sustainable pathway to reduce CO2 by 95% and integration of SCWR in biodiesel plants to produce hydrogen for heating can realize a net present value between 7.70 to 15.70 million USD. Further studies to analyze the economic impact of the individual pathways to optimize the production of fuels from glycerol are required. It is hoped that this study will engage industries and researchers to increasingly use glycerol as a substrate for the production of fuels for transportation.
... To utilise glycerol as fuel source in a furnace, Bohon et al. [19] studied the combustion of pure glycerol, methylated and demethylated glycerol in two refractory furnaces of 7 kW and 82 kW. The results showed that NO x emission of crude glycerol were 20 times higher than that of pure glycerol, but a large quantity of ashes and residues was found in the refractory-lined furnace. ...
The growing production of biodiesel results in the supply glut of crude glycerol byproduct. One option is to utilise the glycerol as supplementary fuel for combined heat and power generation, thereby partially substituting conventional fossil fuel while improving the economics of biodiesel production. In the present study, the spray combustion and emissions characteristics of glycerol were examined using a swirl flame burner. Due to the inherent low heating value of glycerol, the swirling air was premixed with methane to form a co-fired globally lean flame. Both the crude glycerol and pure glycerol spray flames emitted strong spectral intensity of yellowish-orange in the flame brush, overwhelming the bluish flame of fuel-lean methane. The dual heat release zone indicated by the OH* chemiluminescence marks the spatial locations of the swirling premixed flame and centrally injected spray flame. The sooting region of the flame was found to extend downstream beyond the heat release zone. The presence of spray reduced the intensity of the central reverse flow, while the size of the central recirculation zone was also affected. By employing a globally lean-flame and preheating approach, the NO emission was kept low while the higher atomising air-to-liquid ratio led to lower CO emissions. By employing a dual-fuel injection strategy, the glycerol can be effectively burned under swirl and moderately heated condition.
... Researchers are working on the combustion of glycerol to overcome some of the limitations in its direct utilization. Bohon et al. [153] conducted an experiment on the combustion of crude glycerol. They concluded that crude glycerol combustion as a source of heat and power has the limitations of lower energy density, lower auto-ignition temperature, and high viscosity, which need to be addressed. ...
Full-text available
Biodiesel is an emerging substitute for petroleum-based products. It is considered an ecologically safe and sustainable fuel. The high cost of biodiesel production is linearly related to its feedstock. Crude glycerol, which is a by-product of the biodiesel industry, is also a major challenge that must be addressed. A large volume of crude glycerol needs to be disposed of, and this involves processing, dumping, and land requirements. This increases the cost of biodiesel production. One way to decrease the cost of biodiesel production is to utilize its by-product to make valuable products. Crude glycerol can be processed to produce a variety of chemicals and products. The present utilization of crude glycerol is not enough to bring down its surplus availability. Thermochemical conversion processes can utilize crude glycerol as a starting feedstock and convert it into solid, liquid, and gaseous fuels. The utilization of crude glycerol through integrated thermochemical conversion processes could lead to an integrated biorefinery. This review paper highlights the research scope for areas where crude glycerol could be utilized as a feedstock or co-feedstock in thermochemical conversion technology. Various thermochemical conversion processes, namely, gasification, pyrolysis, combustion, catalytic steam reforming, liquefaction, and supercritical water reforming, are discussed and shown to be highly suitable for the use of crude glycerol as an economical feedstock. It is found that the integration of crude glycerol with other thermochemical conversion processes for energy production is a promising option to overcome the challenges related to biodiesel production costs. Hence, this paper provides all the necessary information on the present utilization status of crude glycerol in thermochemical conversion processes, as well as identifying possible research gaps that could be filled by future research studies.
The article presents the test results of the co-firing process of a glycerine fraction derived from the production of liquid biofuels (fatty acid methyl esters) with coal. The test was performed in industrial conditions using a steam boiler with a capacity of approx. 2 MW in one of the building materials manufacturing facilities. The process of co-firing a mixture of a 3% glycerine fraction and eco-pea coal was evaluated. The reference fuel was eco-pea coal. The combustion process, composition and temperature of exhaust gases were analyzed. Incorrect combustion of glycerine fraction may result in the emission of toxic, mutagenic, and carcinogenic substances, including polycyclic aromatic hydrocarbons. During the test of the combustion process of a mixture of glycerine fraction and eco-pea coal, a decrease in the content of O2, CO, and NOx was observed as well as an increase in the content of H2, CO2, and SO2 in the fumes and growth of temperature of exhaust gases in relation to the results of combustion to eco-pea coal. Reduced content of carbon monoxide in exhaust gases produced in the combustion could be caused by the high temperature of the grate or by an excessive amount of oxygen in the grate. The higher content of oxygen in glycerine changes the value of excess air coefficient and the combustion process is more effective. The bigger content of sulfur dioxide in burnt fuels containing the glycerine fraction could be caused by the presence of reactive ingredients contained in the glycerine fraction. The reduced content of nitrogen oxides in exhaust gases originating from the combustion of a fuel mixture containing a fraction of glycerine could be caused by lower content of nitrogen in the glycerine fraction submitted to co-firing with coal and also higher combustion temperature and amount of air in the combustion chamber. The increased content of carbon dioxide in exhaust gases originating from the combustion of fuel mixture containing glycerine fraction could be caused by the influence of glycerine on the combustion process. The increase of hydrogen in the glycerine fraction causes the flame temperature to grow and makes the combustion process more efficient.
Large quantities of glycerol are produced as a somewhat useless, industrial by-product, when producing biodiesel. Thus, the combustion of this waste (containing glycerol, less volatile, non-glyceride oils, ash and water) in a fluidised bed has been investigated. The fuel entered the bottom of the bed (on its axis) as bubbles of vapour, which rose up the bed, surrounded by bubbles of fluidising air. While more difficult to burn than medicinal glycerol, continuous burning of the waste was sustained for a total of ∼ 4 h in a bed of silica sand (500 – 710 μm) at 750°C, fluidised by air. However, after ∼ 4 h, fluidisation ceased, because the silica sand agglomerated into globules a few mm wide, probably cemented by a eutectic of K2SO4 and KOH; this industrial glycerol did originally contain potassium and sulphate ions, from its manufacture. Under similar conditions, when burning the waste in a bed of fluidised alumina (Al2O3) particles (355 – 425 μm), the bed de-fluidised after almost ½ h, and then sintered into a cake, again possibly cemented by the potassium salts K2SO4 and KOH. As for combustion, there was evidence that waste glycerol can be burned in a fluidised bed of SiO2 particles autonomously, without supplying heat. In such a fluidised bed, it appeared that glycerol vapour, inside a bubble, first decomposes thermally, yielding CO and H2. The less volatile oils were slower to evaporate and decompose. Combustion of the waste fuel with air occurred in a bed of SiO2 particles only to a limited extent in rising bubbles, depending on the bed’s depth. Otherwise, burning occurred above the fluidised particles, just as a mixture of methane or propane in air burns, when fluidising a hot bed of silica particles. The role of the particles is to inhibit combustion by scavenging radicals.
Full-text available
According to the United States Energy Information Administration (US EIA), the annual pure biodiesel production in the USA was 6.88 million cubic meters in 2020. Similarly, significant biodiesel production was reported across the world with countries like Brazil, Germany, and Indonesia estimated to have produced 5.9, 3.2, and 6.2 million cubic meters in 2019. This enormous biodiesel production, which is widespread globally, has led to huge amount of waste in the form of glycerol as a product of transesterification. It was estimated that crude glycerol production was 189.27 million cubic meters in 2021. Thermochemical conversion processes, such as combustion, gasification, and pyrolysis, are viable ways to utilize the waste glycerol. In this study, the thermogravimetric analyzer was used to investigate and compare the characteristics of crude and pure glycerol combustion. In particular, the kinetic and thermodynamic parameters as well as the ignition and burnout temperatures were evaluated for the two fuels. The glycerol samples were subjected to a temperature range of 50–700 °C under dissimilar heating rates, i.e., 5, 10, 15, and 20 °C/min. Results of the thermal decomposition process indicate that while there is a single stage in pure glycerol, the crude glycerol is characterized with three decomposition stages. In addition, the experimental results showed that the main combustion process in both samples occurred at about 150–(325 ± 25) °C. The effect of heating rate on TG and DTG curves showed that at higher heating rates, the degradation curves shifts to higher temperature values. The crude glycerol ignition temperature is in the range of 195–218 °C compared to 180–212 °C for pure glycerol, and their burnout temperatures were 463–508 °C and 238–276 °C, respectively. The activation energies were evaluated with Kissinger method and found to be 75 kJ/mol for the devolatilization event in the crude glycerol and 79.6 kJ/mol for the pure glycerol. Graphical abstract
This study proposes an efficient technique to burn ethanol spray in an intense swirling air flow and investigates the mechanism of flame lift-off. On the basis of typical tubular burners for gaseous fuels, ethanol spray was axially induced and mixed with the tangentially injected air under room temperature, yielding the ethanol spray tubular flames under various operating conditions. The results show that from an ultra-lean condition of global equivalence ratio of 0.1 to the rich condition, two typical flames, namely attached and lifted tubular flames, were established. Then, the structure of the ethanol spray tubular flame was specified by temperature measurements, OH-PLIF imaging, indicating its overwhelming aerodynamic and thermal stability with low pollutant emissions. Meanwhile, a flue gas analyzer was used to characterize the gas composition in the hot exhaust. It is found that the burner can achieve very low emissions of both CO and NOx, illustrating high combustion completeness under both attached and lifted flame conditions. To give a better understanding and make fully utilization of spray tubular flames, a parametric study was carried out to investigate the effects of the tangentially swirling air flow, the flow rate and oxygen concentration of atomizing gas, and the flow rate of liquid ethanol. Generally, an attached flame is lifted under a higher tangential velocity of air flow; increasing oxygen concentration of the atomizing gas flow leads to the height decrease of the flame lift-off, even the reattachment of the lifted flame; by raising the ethanol flow rate to exceed a critical value, the flame also lifts off. Furthermore, the evaporation time of droplets, residence time and chemical reaction time were calculated to quantify the flame lift-off behavior. The lifted flame can be established only when the evaporation time is larger than both the reaction time and the residence time.
Full-text available
Hydrogen produced by water decomposition caused high diffusible hydrogen content in deposited metal, but also produced numerous spatter and unstable fragments which were considered as the main reason that deteriorated the weld formation during underwater wet flux cored welding (FCAW). Drawn inspiration from onshore gas shielded welding, in this paper, a liquid shielded welding method was invented. As a by-product of biodiesel, glycerol was chosen as liquid protectant. The results suggested that the diffusible hydrogen content in deposited metal was reduced by 62.4% with glycerol shielded due to its higher decomposition threshold and less hydrogen in decomposition products. Arc bubbles were attached to the molten pool surface and their expansion rate and rising velocity were slowed down resulted in more stable droplet transfer process and less spatter owing to high viscosity glycerol protectant. Almost porosity-free joints were obtained and the mechanical properties of the welded joints were also improved.
Full-text available
This report identifies twelve building block chemicals that can be produced from sugars via biological or chemical conversions. The twelve building blocks can be subsequently converted to a number of high-value bio-based chemicals or materials. Building block chemicals as considered for this analysis are molecules with multiple functional groups that possess the potential to be transformed into new families of useful molecules. The twelve sugar-based building blocks are 1,4-diacids (succinic fumaric and malic), 2,5-furan dicarboxylic acid, 3-hydroxy propionic acid, aspartic acid, glucaric acid, glutamic acid, itaconic acid, levulinic acid, 3-hydroxybutyrolactone, glycerol, sotitol, and xylitol/arabinitol.
Full-text available
The potential use of sorbents to manage ultrafine ash aerosol emissions from residual oil combustion was investigated using a downfired 82 kW laboratory-scale refractory-lined combustor. The major constituents were vanadium (V), nickel (Ni), iron (Fe), and zinc (Zn). The overall ash content of residual oil is very low, resulting in total ash vaporization at 1725 K with appreciable vaporization occurring at temperatures as low as 1400 K. Therefore, the possibility of interactions between ash vapor and sorbent substrates exists. Kaolinite powder was injected at various locations in the combustor. Ash scavenging was determined from particle size distributions (PSDs) measured by a Scanning Mobility Particle Sizer. Impactor samples and X-ray fluorescence (XRF) analyses supported these data. Injection of kaolinite sorbent was able to capture up to 60% of all the ash in the residual fuel oil. However, captures of 30% were more common when sorbent injection occurred downstream of the combustion zone, rather than with the combustion air into the main flame. Without sorbent addition, baseline measurements of the fly ash PSD and chemical composition indicate that under the practical combustion conditions examined here, essentially all of the metals contained in the residual oil form ultrafine particles (0.1 μ m diameter). Theoretical calculations showed that coagulation between the oil ash nuclei and the kaolinite sorbent could account for, at most, 17% of the metal capture which was always less than that measured. The data suggest that kaolinite powders reactively capture a portion of the vapor phase metals. Mechanisms and rates still remain to be quantified.
Bluff-body and swirl-stabilized flames are similar in that they represent, in simplest terms, the fundamental interaction between a fuel jet and a surrounding toroidal vortex. The vortex in this case is the recirculation vortex which affects the properties of the flames. It is found, not surprisingly, that the two most important fundamental parameters that govern both types of flames are (1) the vortex circulation (Γ), and (2) the fuel jet momentum. Comparisons are made of the properties of the two types of flames using the proper nondimensional parameters, including the fuel-to-air momentum flux ratio and the properly nondimensionalized vortex strength. Such comparisons can help to illustrate the tradeoffs between the degree of swirl and the choice of bluff-body size in devices such as industrial burners, gas turbines, and ramjets. The data also show how one can control flame properties by controlling the vortex strength Γ and fuel momentum and thus gain a degree of control that is not provided by simple jet flames. For flames that extend beyond the recirculation zone, the flame lengths of both types of flames are found to scale with the square root of fuel-to-air momentum flux ratio and with the inverse of vortex circulation. Thus, the length of these flames can be easily controlled and are found to scale in a different manner than the length of a simple jet flame. The location of the forward stagnation point of the recirculation zone, which is important in determining flame stability and the heat transfer to the fuel nozzle, also scales with the square root of fuel-to-air momentum flux ratio for both flames. For the same vortex strength and jet momentum, both flames display remarkably similar flow structure, including the presence of counter-rotating vortices. One vortex is driven by the air flow while the other is driven by the fuel flow. For both flames increasing the vortex strength significantly increases the fuel-air mixing rate (i.e., shortens flame length), which is desired in certain applications.
The NO concentration profiles of premixed hydrocarbon-air flames were measured using probe sampling and ultraviolet absorption of NO. The flames were stabilized on a Meker-type burner of 7 cm diameter at a pressure of 1 atm. Investigated fuels were CH4, C2H5, C2H4, C2H2, C3H8, n-C4H10, n-C5H14, i-C8H18, C6H6, C6H12 and gasoline. The measurements show the formation of nitric oxide according to the Zeldovich mechanismwith k1 = 5·10 exp(−75400/RT) cm/mol.sec in fuel lean flames. In fuel rich flames the NO formation due to the Zeldovich mechanism would require unreasonably high concentrations of O atoms and evidence is given for another way of NO formation in these flames. HCN has been found as an intermediate species and exceeds the NO concentration in very fuel rich flames.
Dramatic increases in the price of crude oil, and consequently, transportation fuels, coupled with increased environmental concerns have resulted in rapid growth in biodiesel production, both in the United States and worldwide. As biodiesel production increases, so does production of the primary coproduct, glycerol. Since the existing glycerol supply and demand market was tight, recent increases in glycerol production from biodiesel refining has created a glut in the glycerol market. As a result, the price of glycerol has fallen significantly and biodiesel refiners are faced with limited options for managing the glycerol by-product, which in the biodiesel industry, has essentially become a waste stream. This article is a review of promising options for both the catalytic and biological conversion of glycerol into various value-added products, many of which are bio-based alternatives to petroleum-derived chemicals. © 2007 American Institute of Chemical Engineers Environ Prog, 26: 338–348, 2007
In continuous-flow combustors burning atomized liquid fuels, inhomogeneities exist withinthe flow due to imperfect fuel-air mixing. These fuel-air ratio nonuniformities affect the formation of NO, both from the nitrogen in the air and from elemental nitrogen contained in the fuel. Experiments to demonstrate these effects have been carried out in a cylindrical atmospheric-pressure burner fueled with kerosene and kerosene doped with pyridine. Mixing and kinetic models are developed to explain how the measured concentrations of NO formed via the Zel'dovich mechanism and from fuel nitrogen vary with mean fuel-air ratio and initial degree of mixedness.
The characteristics of particulate matter (PM) emitted from residual fuel oil combustion in two types of combustion equipment were compared. A small commercial 732 kW rated fire-tube boiler yielded a weakly bimodal particulate size distribution (PSD) with over 99% of the mass contained in a broad coarse mode and only a small fraction of the mass in an accumulation mode consistent with ash vaporization. Bulk smaples collected and classified by a cyclone indicate that 30% to 40% of the total particulate emissions were less than 2.5μm aerodynamic diameter (PM2.5). The coarse mode, PM was rich in char, indicating relatively poor carbon burnout, although calculated combustion efficiencies exceeded 99%. This characteristic behavior is typical of small fire-tube boilers.Larger utility-scale units firing residual oil were simulated using an 82 kW rated laboratory-scale refractory-lined combustor. Particulate matter emissions from this unit were in good agreement with published data including published emission factors. These data indicated that the refractory-lined combustor produced less total but more fine particulate emissions, as evident from a single unimodal PSD centered at ⊂0.1 μm diameter. Bulk cyclone segregated samples indicated that here all the PM were smaller than 2.5 μm aerodynamic diameter, and loss on ignition (LOI) measurments suggested almost complete char burnout. These findings are interpreted in the light of possible mechanisms governing the release of organically bound refractory metals and may have particular significance in considering the effects of fuel oil combustion equipment type on the characteristic attributes of the fine PM emitted into the atmosphere and their ensuing health effects.