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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
a,
, Brian A. Metzger
a
, William P. Linak
b
,
Charly J. King
c
, William L. Roberts
a
a
Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695, USA
b
Air Pollution Prevention and Control Division, National Risk Management Research Laboratory,
U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, USA
c
ARCADIS Geraghty and Miller Inc., Durham, NC 27709, USA
Available online 25 September 2010
Abstract
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.
doi:10.1016/j.proci.2010.06.154
Corresponding author. Address: Engineering Bldg 3,
Campus Box 7910, 911 Oval Drive, Raleigh, NC 27695,
USA.
E-mail address: mdbohon@ncsu.edu (M.D. Bohon).
Available online at www.sciencedirect.com
Proceedings of the Combustion Institute 33 (2011) 2717–2724
www.elsevier.com/locate/proci
Proceedings
of the
Combustion
Institute
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
9
L(2.710
9
gal) and is expected
to increase another 1.63 10
9
L (4.3 10
8
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
8
L/year (1.0 10
8
gal/year) of fuel oil,
equivalent to 1.8 10
9
kg/year (1.3 10
6
tons/
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)
NO
x
analyzer and an Infrared Industries (model
FGA-4000 XDS, Hayward, CA) exhaust gas ana-
lyzer (O
2
, CO, and CO
2
). 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
x
species
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
2
(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
noted.
M.D. Bohon et al. / Proceedings of the Combustion Institute 33 (2011) 2717–2724 2719
CA), O
2
(Beckman Corp., model 755, La Habra,
CA), CO (Thermo Electron Corp., model 48,
Franklin, MA), NO and NO
2
(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.
USP
glycerol
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
H
2
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
2
. 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
2
and CO
2
were consistent with corresponding stoichiome-
tries and mass balances for complete combustion.
Interestingly, NO
x
emissions for the glycerol
flames were exceedingly low (7–10 ppm, 0% O
2
)
compared to those for the two fossil fuels (110–
140 ppm, 0% O
2
). This was true even though O
2
levels during glycerol combustion were very high.
Except for NO
x
, 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
x
forma-
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
x
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
U
a
0.444 0.392 0.370 0.562 0.488 0.645 0.488 0.63 0.77
SR
a
2.25 2.55 2.70 1.78 2.05 1.55 2.05 1.58 1.30
NO
x
(ppm) 3.0 3.5 3.6 60.2 62.8 74.7 62.5 146.5 118.3
NO
x
at 0% O
2
(ppm) 6.9 9.1 9.6 110.5 135.4 117.8 128.6 235.2 155.5
O
2
(%) 11.8 12.9 13.3 9.6 11.3 7.7 10.8 7.9 5.1
CO
2
(%) 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)
b
958 901 877 1001 974 986 946 1041 1075
Flame temp. (°C)
c
1201 1103 1060 1359 1213 1628 1343 1782 1716
a
Equivalence and stoichiometric ratios determined by excess O
2
in the exhaust.
b
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.
c
Adiabatic flame temperature calculated at stoichiometric ratios listed above.
Fig. 2. NOxemissions corrected to 0% O
2
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
NO
x
mechanism is not the dominate mechanism,
but NO
x
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
x
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
NO
x
formation. Appleton and Heywood [14]
show that with better atomization, NO
x
forma-
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
x
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
x
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
NO
x
mechanism. The NO
x
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
x
emissions.
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
2
.
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
x
(150–240 ppm, 0% O
2
) 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
x
than
the methylated fuel. It is notable that the proto-
type burner produces NO
x
emissions significantly
lower (6 ppm, 0% O
2
) than those measured in
the refractory-lined furnace. This difference in
NO
x
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
x
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
x
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
3
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/
m
3
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
3
approach
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
x
, 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
x
emissions from the pro-
totype burner (7–10 ppm, 0% O
2
) were 20
times lower than those from the refrac-
tory-lined furnace (160–240 ppm, 0% O
2
).
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
responsible.
(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
3
) 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.
Acknowledgements
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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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.