Nitrogen-based alternative fuel: An environmentally friendly combustion approach

Article abstract of DOI:10.1039/c3ra47890d

We report here on a continuous combustion of a low carbon nitrogen-based alternative fuel. The investigated fuel, an aqueous solution of urea and ammonium nitrate, consists of common fertilizer commodities. This nonflammable, nontoxic and nonexplosive fuel underwent combustion at a pressure range of 1 to 25 MPa. The pressure was found to greatly affect pollutant levels in the effluent gas. Molecular nitrogen yield was 99.89% at 25 MPa, and the lowest NO_x level was 128 mg MJ^-1, below the regulation standard for power generation. The alternative fuel described herein is an excellent gas generator, producing an environmentally friendly working fluid consisting of 73.0% H₂O, 21.6% N₂, and 5.4% CO₂. This fuel has the potential to be a sustainable future renewable energy storage medium as well as an energy carrier.

Full text of DOI:10.1039/c3ra47890d

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Nitrogen-based alternative fuel: an
environmentally friendly combustion approach
Cite this: RSC Adv., 2014, 4, 10051
a
b
b
*
Alon Grinberg Dana, Gennady E. Shter and Gideon S. Grader
We report here on a continuous combustion of a low carbon nitrogen-based alternative fuel. The
investigated fuel, an aqueous solution of urea and ammonium nitrate, consists of common fertilizer
commodities. This nonflammable, nontoxic and nonexplosive fuel underwent combustion at a pressure
range of 1 to 25 MPa. The pressure was found to greatly affect pollutant levels in the effluent gas.
Molecular nitrogen yield was 99.89% at 25 MPa, and the lowest NOx level was 128 mg MJꢀ1, below the
regulation standard for power generation. The alternative fuel described herein is an excellent gas
generator, producing an environmentally friendly working fluid consisting of 73.0% H2O, 21.6% N2, and
5.4% CO2. This fuel has the potential to be a sustainable future renewable energy storage medium as
well as an energy carrier.
Received 22nd December 2013
Accepted 30th January 2014
DOI: 10.1039/c3ra47890d
www.rsc.org/advances
scarce levels of atmospheric CO2. Although some schemes to
trap atmospheric CO2 are being discussed, the logistic chal-
1. Introduction
lenges in production and transport are formidable when glob-
ally relevant quantities are considered.2 On the other hand, the
global accessibility to atmospheric nitrogen, coupled to the
future availability of hydrogen, will enable a large scale
production of ammonia and its fertilizer derivatives. In addi-
tion, future photocatalytic water splitting processes can poten-
tially be a low-cost alternative to the contemporary fossil fuel
derived hydrogen.
Large scale integration of intermittent renewable energies, such
as solar and wind, introduces considerable uncertainty into an
electric power system operation.1 Energy storage media and
energy carriers are the missing links required for any sustain-
able and robust energy system.2 Currently, pumped hydroelec-
tricity storage (PHS) and compressed air energy storage (CAES)
are the only commercially available technologies capable of
providing large scale (above 100 MW) energy storage.1,3
However, both of these technologies can be implemented only
where the appropriate geographical or geological conditions are
available,4 and they do not provide an energy carrier solution.
The chemical bonds within fuels are perhaps the most
attractive form of large scale energy storage.5 For example,
nature uses photosynthesis to convert sunlight into useful
chemical energy in the form of biomass, and humans have long
ago discovered the convenience of utilizing a variety of chemical
fuels. Hydrogen is oen considered as the cleanest fuel,2,6,7
emitting only water during utilization. However, the feasibility
of a pure hydrogen economy is questionable due to safety issues
associated with the transport of hydrogen, as well as the
extremely low volumetric energy density of hydrogen, even
when compresses or liquied.7
In this work, we suggest a three component low carbon
nitrogen-based alternative fuel as a chemical hydrogen storage
medium and energy carrier. The fuel consists of an aqueous
solution of Ammonium Nitrate (AN) with a reducing agent
additive. As a global commodity, AN is already manufactured in
millions of tons annually, and accounts for more than 24% of
the world's nitrogen fertilizer market.9 It is considered to be an
inexpensive condensed source of oxygen,10 and it is chemically
stable over a wide range of temperatures and pressures.11 The
solubility of AN in water is relatively high (66 wt% at 20 ꢁC), and
its aqueous form is chemically stable, nontoxic, nonammable,
and nonexplosive, making it safe to handle and transport.11
Urea was chosen as a reducing additive to react with the
excess oxidizing species that are released during AN decompo-
sition.12 Urea, the most important nitrogenous fertilizer world-
wide,13 is also used to reduce the Nitrogen Oxides (NOx) levels in
various stationary and mobile sources.14,15 Urea is favored for
the reduction of AN decomposition products as it is a nontoxic
and nonvolatile source of ammonia, and it is soluble in an
aqueous AN solution. In addition, the total number of hydrogen
bonds per water molecule is almost independent of the urea
concentration, since urea ts structurally and sterically very well
into the water hydrogen bond network.16
Nonetheless, chemical hydrogen storage in the form of other
stable compounds is advantageous.5,8 Once available, hydrogen
could be chemically stored via two major fuel carriers: carbon or
nitrogen. However, entrapment of carbon is a challenge due to
aThe Nancy and Stephen Grand Technion Energy Program, Technion – Israel Institute
of Technology, Haifa 3200003, Israel. Fax: +972-77-887-5099
bThe Wolfson Department of Chemical Engineering, Technion – Israel Institute of
Technology, Haifa 3200003, Israel. E-mail: grader@technion.ac.il; Fax: +972-4-829-
5672
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The ideal effluent composition of aqueous Urea Ammonium ($99%, Sigma-Aldrich), urea ($99.5%, Sigma-Aldrich) and
Nitrate (UAN) combustion is about 73.0% H2O, 21.6% N2, and water (Milli-Q® ultrapure water). The monofuel composition
5.4% CO2 on a mole basis (Reaction (R1)). On an energy was 60% AN, 15% urea and 25% water by weight.
equivalent basis, the CO2 emission from aqueous UAN
The monofuel was pumped by a high pressure pump (SSI
combustion is only about one forth than CO2 emission from Series III 10 ml minꢀ1) into a continuous 0.5 L cylindrical
natural gas combustion.
reactor (Parr N4652) composed of stainless steel 316T. The ow
rate was set with 2% accuracy. The pressure inside the reactor
was controlled by a Programmable Logic Controller (PLC) via a
pneumatic valve and oscillated about ꢂ2% in magnitude. The
reactor was externally heated to 450 ꢁC, and the temperature
inside the reactor was measured using two thermocouples at
different heights from the bottom of the reactor.
AN
zfflfflfflfflffl}|fflfflfflfflffl{
urea
zfflfflfflfflffl}|fflfflfflfflffl{
3NH4NO3 ðaq:Þ þ N2H4CO ðaq:Þ þ 5:56H2O
ðlÞ
/ 4N2ðgÞ þ 13:56H2OðlÞ þ CO2ðgÞ
;
o
HRxn
¼ ꢀ437 kJ molꢀ1 ¼ ꢀ3:28 MJ kgꢀ1 (R1)
^
D
The 170 mm long fuel inlet tube with an internal diameter of
3.175 mm entered the reactor vertically from the top, and was
extended towards the bottom of the reactor oor. The effluent
gas owed through ꢁa pneumatic actuator valve and entered an
11.5 L heated (180 C) gas receiver in which the pressure was
kept constant at 0.20 ꢂ 0.01 MPa.
In this paper we demonstrate the feasibility of the aqueous
UAN continuous combustion process, and report on the depen-
dence of this process on pressure, residence time, and fuel ow
rate. Being common fertilizers, a clean usage of these materials as
future fuels would expand their market as well as the production
for these already common chemicals. As the economy of scale
teaches, an increased production of a global mass product would
inevitably bring down its cost – for everyone's benet.
The reaction system was monitored and controlled by the
PLC which also recorded the reactor's temperatures and pres-
sure at one second intervals.
Sampled gas from the gas receiver was diluted by 99.995% N2
at a 1 : 40 ratio using a gas dilution unit (M&C DIL-1/H). The
diluted gas was analyzed by a multicomponent online Fourier
Transform Infrared (FTIR) gas spectrometer (Gasmet CX-4000O2)
2. Experimental
Aqueous UAN combustion was carried out using a continuous
high pressure reaction system (Scheme 1). The reagents were AN
Scheme 1 Experimental system setup. Bold lines indicate heating mantles. Red lines indicate heaters. Vessels are not to scale. LI: level indicator,
TC: thermocouple, PT: pressure transmitter, PLC: programmable logic controller. FTIR: Fourier transform infrared.
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calibrated with the necessary standards. The selected species for with the main combustion process essentially occurring within
analysis were H2O, CO2, CO, N2O, NO, NO2, NH3, HNO3 and the fuel inlet tube. Unreacted species can further react in the
HNCO. Homonuclear diatomic molecules (N2, O2, and H2) were reactor volume, yet at a lower rate due to the colder environment
not detected by the FTIR spectrometer since they are IR trans- and lower concentration. The only species detected by FTIR
parent. The spectrometer's sampling frequency was 10 Hz with analysis were NO2, NO, N2O, NH3, CO2, CO and H2O.
automatic averaging over 20 second segments. Subsequently, 15
consecutive data points were averaged.
3.2. The ow rate effect
The dependency of the residence time inside the fuel inlet
tube on pressure and fuel ow rate (eqn (E1)) was calculated The decomposition of AN is known to be dependent on various
under the following assumptions: (1) the reaction occurs factors such as pressure, temperature, sample volume and
immediately as the uid enters the hot fuel inlet tube, (2) no reactor design.10 As shown in Fig. 1B and C, the product
condensation occurs inside the reactor, and all species exit the composition of aqueous UAN continuous combustion was found
reactor in the gas phase, (3) the molar ratio of products to to be affected by the ow rate. Although the observed levels of
reactants is identical to that of the ideal reaction (Reaction H2O and CO2 were not signicantly affected by changes in the
(R1)), (4) the reactor is homogeneous, (5) the process is at a ow rate, levels of N2O and NH3 dropped considerably as the fuel
steady state, and (6) the ideal gas law applies. In addition, the ow rate decreased from 8 ml minꢀ1 to 4 ml minꢀ1 at 5 MPa.
following parameters were used: an effective tube volume of
1.35 ml, a fuel density of 1.33 g mlꢀ1, and an average combus- fuel ow rate increased. As shown in Fig. 2, minimum levels in
tion temperature of 1000 ꢁC.
N2O, NH3, and CO were observed, while the NO levels exhibited
In eqn (E1) s indicates the residence time inside the tube, V a maximum in the same fuel ow rate range.
At a constant pressure, the pollutant levels changed as the
indicates the free volume of the tube, Q indicates the gas ow
In order to overcome the high pressure inside the reactor,
rate out of the reactor, P indicates the absolute pressure inside the fuel was injected by a piston pump. As a consequence, at low
the reactor, and r indicates the fuel ow rate into the reactor.
fuel ow rates the ow became intermittent due to the long
delay between repeated piston strokes. Such intermittent ow
caused frequent initiation and termination of the combustion.
As further outlined below, the aqueous UAN combustion
process proceeds through a radical reaction mechanism. Since
intermittent ow of reactants diminishes the radical pool,
desirable reaction pathways are interrupted, and relatively high
pollutant levels are emitted at such low ow rate conditions.
On the other hand, at high fuel ow rates and relatively low
pressures, the Exo-TS residence time can amount to several tens
of milliseconds at the working conditions described in this
work (eqn (E1)). The critical Exo-TS is where the highest
concentration of radicals occurs as well as the highest temper-
ature in the system. Albeit the high temperature in the inlet
V
P
s h y120
(E1)
ms ml
MPa min r
Q
Experiments were carried out under pressures ranging from
1 to 25 MPa and fuel ow rates ranging from 1.0 to 10.0 ml
minꢀ1. Gas from the receiver was sampled for online analysis
aer 10 residence times in the reactor have elapsed. For the
calculation of the N2 yield, 11/3 moles of N2 per 1 mole of AN was
used as the maximum value (Reaction (R1)).
3. Results and discussion
3.1. Continuous combustion
In order to examine the feasibility of an aqueous nitrogen-based tube, the relatively short residence time is insufficient for the
fuel combustion, continuous combustion experiment series chain reaction process, and the gases exiting the fuel inlet tube
were conducted under pressure. During a typical experiment are subsequently diluted and quenched in a colder gas envi-
conducted at 5 MPa, the average temperature measured inside ronment inside the reactor. As a consequence, reaction path-
the reactor 60 mm from the fuel inlet tube by TC2 (Scheme 1) ways are slowed down or altered at high ow rate conditions,
was unaffected by the ow rate. However, the average temper- leading to incomplete combustion and to the observed higher
atures measured 15 mm from the tip of the fuel inlet tube by pollutant levels.
TC3 were 535 ꢁC and 660 ꢁC at fuel ow rates of 4 ml minꢀ1 and
At a representative constant pressure of 5 MPa, the ow
intermittency phenomenon was dominant at fuel ow rates of
8 ml minꢀ1, respectively (Fig. 1A).
The aqueous UAN monofuel does not require an additional up to about 3 ml minꢀ1, giving rise to the high N2O, NH3 and CO
oxidizer, and it ignites once the required thermal conditions are levels. At fuel ow rates above 8 ml minꢀ1, which correspond to
met; hence the ignition may occur within the fuel inlet tube if its residence times below 75 ms (eqn (E1)), the insufficient resi-
temperature is sufficiently high. We ensured this condition in dence time caused pollutant levels to rise (Fig. 2).
this work, and consequently, the hottest zone in the reactor is
Levels of NH3, for example, may rise once the radical pool
expected to occur along this tube. As the gases pass the depletes due to a deciency in OH radicals. The presence of
Exothermic Tube Segment (Exo-TS), they subsequently cool these radicals is critical for the conversion of NH3 to higher
down due to the relatively colder reactor environment. The reactive species such as NH2 radicals by hydrogen abstraction
reported rising temperature at increased ow rate exclusively in (Reaction (R2)).17
the vicinity of the tube tip indicates that the Exo-TS advances
At a constant pressure, the NO levels were inconsistent with
further downstream towards the tube tip as the ow rate the minimum phenomenon described above for the other
increases. Therefore, the data shown in Fig. 1A is consistent pollutants (Fig. 2). A possible explanation of this effect is based
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Fig. 1 Aqueous UAN continuous combustion at 5 MPa. (A) Measured temperatures as a function of time at fuel flow rates of 8.00 ml minꢀ1 and
4.00 ml minꢀ1. TC2: temperature at 80 mm from the bottom of the reactor and 60 mm from the tip of the fuel inlet tube, TC3: temperature at 35
mm from the bottom of the reactor and 15 mm from the tip of the fuel inlet tube, TC4: temperature within the reactor's heater. (B and C) FTIR
spectra of combustion products as a function of wavenumber at selected wavenumber ranges at different flow rates.
on the opposite manner in which the NO and N2O species
interact. Isocyanic acid (HNCO) from urea decomposition
(Reaction (R3))18 reacts with OH radicals to give NCO radicals
(Reaction (R4)).19 The presence of NCO contributes to the elimi-
nation of NO and the formation of N2O (Reaction (R5)), which is
an important chain-terminating reaction.20 If the radical pool is
sustained, O radicals compete with NO over NCO (Reactions (R5–
R7)), thus hindering the removal of NO. Furthermore, the reac-
tion of O with NCO yields NO (Reaction (R7)).17 These O radicals
may also react with N2O, giving yet more NO (Reaction (R8)).17
Therefore, at the ow rate window where minimum N2O levels
were obtained, relatively high NO levels were detected (Fig. 2).
NH2CONH2 % HNCO + NH3
HNCO + OHc % NCOc + H2O
NCOc + NOc % N2O + CO
NCOc + NOc % N2 + CO2
NCOc + O: % NOc + CO
N2O + O: % 2NO
(R3)
(R4)
(R5)
(R6)
(R7)
(R8)
The intermittent reactant ow effect is prominent in a three
dimensional representation of the species levels as a function of
fuel ow rate and pressure as shown in Fig. 3. This effect is
NH3 + OHc % NH2c + H2O
(R2)
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Below 5 MPa, the NO species was the major detected pollutant,
accounting for about 60% of the total nitrogen-containing
pollutants. However, above 5 MPa the relative amount of NO
decreased signicantly, and at 25 MPa it was as low as 6.7% of
the overall nitrogen-containing pollutants (Fig. 4B). Although
the relative amount of N2O increased at higher pressures,
accounting for about 70% of all nitrogen-containing pollutants
(Fig. 4B), its absolute level decreased monotonically (Fig. 4A).
Higher pressures result in an increased volumetric concen-
tration, which directly increases the reaction kinetics. A
threshold pressure of 5 MPa was found to separate between two
distinct pressure regions. While the product distribution
generally remained constant at the low pressure region, the
decrease in absolute pollutant levels indicates higher reaction
rates with no principal change in reaction pathways. On the
other hand, in the high pressure region the considerable vari-
ation in product composition suggests that a critical minimum
intermediate concentration was reached, and that reaction
pathways were altered. A possible radical reaction mechanism is
postulated below.
Fig. 2 Detected pollutants at a constant pressure of 5 MPa as a function
of the fuel flow rate. The fuel flow rate was experimentally set, and the
residence time inside the fuel inlet tube (Exo-TS) was theoretically
calculated (eqn (E1)). Error bars indicate Standard Deviation (SD).
mainly illustrated by high levels of N2O and NH3, and by low N2
yield at fuel ow rates below 4 ml minꢀ1 at relatively low pres-
sures (Fig. 3C, D and F). The effect of the residence time is seen
as a decrease in N2O, NH3, and CO levels (Fig. 3C–E) and as an
increase in N2 yield (Fig. 3F). This effect is observed as the low
pressure and high ow rate conditions change towards higher
pressures and lower ow rates, reecting short and long resi-
dence times, respectively.
3.4. Nitrogen transformation routes
Generally, combustion of aqueous UAN is characterized by
endothermic processes followed by exothermic reactions. While
a conventional liquid fuel (e.g., gasoline) only vaporizes prior to
its combustion in air, aqueous UAN involves signicant endo-
thermic processes in addition to water vaporization. These
processes include the dissociation of AN to ammonia and nitric
acid mediated by a proton transfer (Reaction (R9)),21 nitric acid
homolysis (Reaction (R10)),22 urea decomposition into HNCO
and ammonia (Reaction (R3)), and urea hydrolysis (Reaction
(R11)).18 Other endothermic high temperature decomposition
pathways have also been proposed for urea.18,22
3.3. The pressure effect
The effect of pressure on the pollutant levels involves two
distinct regions. In the low pressure region of 1 to 5 MPa, a
sharp and signicant decrease in pollutant levels, as well as an
increase in N2 yield, was observed as the pressure increased. In
the high pressure region of 5 to 25 MPa, these trends proceeded
to a lesser extent (Fig. 3). The lowest N2 yield, 72.73%, was
obtained at a pressure of 1 MPa and a fuel ow rate of 1.2 ml
minꢀ1. The highest N2 yield, 99.89%, was obtained at a pressure
NH4NO3(l) % NH3(g) + HNO3(g)
,
DHRxn ¼ +169.0 kJ molꢀ1
(R9)
ˆ o
RDS
ƒƒƒƒƒ!
o
HRxn
^
HNO3ðgÞ
OHc þ NO ; D
¼ þ206:4 kJ molꢀ1
2ðgÞ
of 25 MPa and a fuel ow rate of 10 ml minꢀ1
.
(R10)
The ow rate effect described above (Section 3.2.) was found
to be pressure-dependent, as the minimum and maximum
trends of species levels seemed to reach an asymptotic value as
the pressure increases (Fig. 3).
NH2CONH2(aq.) + H2O(l) % CO2(g) + 2NH3(g)
,
ˆ o
DHRxn ¼ +77.1 kJ molꢀ1
(R11)
At a representative constant fuel ow rate of 6 ml minꢀ1, the
The prevailing intermediate species in aqueous UAN
pollutant levels decreased drastically as pressure and residence combustion are analogous to those identied in several NOx
time increased. Specically, NO levels dropped by three orders removal processes20 and in combustion of nitrogen that is
of magnitude from 193.4 millimole per AN mole at 1 MPa, to 0.3 chemically bound to carbon-based fuels.23 Reaction pathways
millimole per AN mole at 25 MPa (Fig. 4A). With the exception reported in the literature for AN thermal decomposition21,24,25
of NO2, the distribution of the nitrogen-containing pollutants and nitrogen chemistry in combustion20,23 can be used to better
was approximately constant at the low pressure region of 1 to understand the aqueous UAN combustion mechanism subse-
5 MPa (Fig. 4B). The levels of NO2 decreased only slightly from quent to the endothermic processes reported above.
5.6 millimole per AN mole at 1 MPa to 2.5 millimole per AN
The initial oxidized nitrogen species in the aqueous UAN
mole at 2.5 MPa. However, at 5 MPa levels of NO2 decreased combustion system is HNO3 from AN dissociation (Reaction
abruptly below the detection limit (the detection limit is (R9)), which has the highest nitrogen oxidation state in the
equivalent to about 0.1 millimole per AN mole). Levels of NO2 system of +5 (Fig. 5). This species was not detected in the effluent
remained under detection limit at higher pressures as well. gas since it readily decomposes at relatively low temperatures;
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Fig. 3 Emission levels of major combustion pollutants and N2 yield. (A–E) Levels of NO2, NO, N2O, NH3, and CO in millimole per AN mole. Red
dots indicate experimental data. (F) Yield of N2 was calculated using an atomic mass balance. Blue dots indicate calculated values. Surfaces were
numerically interpolated using a cubic interpolation method with 300 steps in both x and y directions. The x and y axes of the N2 yield showing
the pressure and fuel flow rate were inverted to better demonstrate the effect graphically.
HNO3 was assumed to completely decompose inside the hot other oxidizing species to form the reactive NH2 (Reaction (R2)),
reactor. Above 290 C, AN decomposition is known to progress NH, N2H, and N radicals26 (Fig. 5).
ꢁ
through a radical reaction pathway, where the HNO3 homolysis
As the pressure increases, the rst nitrogen-containing
into NO2 and OH radicals constitutes the Rate Determining Step pollutant to decrease was found to be NO2. Subsequently, a
(RDS) (Reaction (R10)).21 Dissociation of AN (Reaction (R9)), urea decrease in NO levels was observed above 5 MPa (Fig. 4B). The
decomposition (Reaction (R3)), and urea hydrolysis (Reactions absolute levels of N2O decreased as well with increasing
(R11)) produce NH3, which further reacts with OH radicals or pressure (Fig. 4A). Still, N2O is the main nitrogen-containing
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Fig.
5
Schematic outline of the principal reaction pathways of
aqueous UAN combustion. Reactions are reversible, the arrows indi-
cate the prominent reaction direction. Several reactions are duplicated
in the scheme for clarity. The nitrogen oxidation state is indicated for
selected species.
pollutant detected at 25 MPa (Fig. 4B). As the reaction coordi-
nate progresses, the nitrogen oxidation states gradually
decrease from +5 (HNO3), to +4 (NO2), then to +2 (NO), and
eventually +1 (N2O). This pathway eventually leads to nitrogen
atom formation with a zero oxidation number – namely N2. This
transformation route of the nitrogen-containing pollutants is
consistent with the nitrogen oxidation state of each pollutant
(Fig. 5).
3.5. Regulation view point
Emission regulation and standards need to be considered
before any novel fuel such as the one presented in this work is
utilized. To put the current results into perspective, the US
Environmental Protection Agency (EPA) regulation for
stationary power generation turbines was used.
The identied pollutants of aqueous UAN combustion were
NOx, CO and NH3. The NOx family includes seven compounds,27
out of which only N2O, NO and NO2 were detected in this work.
For power generation using natural gas and fuels other than
natural gas, the NOx emission standards are 290 mg MJꢀ1 and
700 mg MJꢀ1, respectively.28 These values are attributed to new
stationary power generation turbines producing useful output
of less than 53 GJ hꢀ1 (measured as high heat value of heat
input at peak load). The corresponding emission standard of
CO is 6.4 mg MJꢀ1 with regard to natural gas-red turbines.29 An
Fig. 4 Detected nitrogen-containing pollutants at a constant fuel flow
rate of 6 ml minꢀ1. (A) Absolute levels of detected nitrogen pollutants NH3 emission standard is not available for stationary power
at varied pressures and Exo-TS residence times. (B) Distribution of
nitrogen-containing pollutants at varied pressures and Exo-TS resi-
dence times. Pressure was experimentally set, while residence time
was theoretically calculated. Error bars indicate SD. The error bars in B
generation because it is not emitted by fossil fuel combustion.
The emissions of aqueous UAN combustion in the laboratory
scale reactor described here were calculated using the collected
data at 25 MPa and a fuel ow rate of 10 ml minꢀ1. These
are relatively large at higher pressures and short residence times due to
the relatively low pollutant levels under these conditions, and the emissions were found to be 127 mg MJꢀ1 of NOx, 8 mg MJꢀ1 of
CO, and 17 mg MJꢀ1 of NH3. While the CO emission was slightly
limited resolution of the FTIR detection system.
above the standard level, the NOx emission was signicantly
below the regulation standard.
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The current reactor used in this work was not ideal due to a
nonuniform temperature prole and a large distribution of the
residence times. An improved reactor design and an imple-
mentation of a catalytic converter are expected to further reduce
all of the pollutants much below the regulation levels.
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4. Conclusions
In this paper we demonstrated a continuous and stable
combustion of aqueous UAN at different pressures and fuel ow
rates. The ignition of the monofuel took place inside the fuel
inlet tube within the reactor. Residence time, uctuations in
fuel feed, and working pressure strongly affected the effluent
gas composition.
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Up to pressures of 5 MPa, a substantial decrease in absolute
levels of all pollutants was observed, yet the distribution of the
nitrogen-containing pollutants was approximately constant. At
higher pressures, the absolute pollutant levels further
decreased, yet the ratio of the nitrogen-containing pollutants
changed signicantly. Specically, above 2.5 MPa NO2 levels fell
below the detection limit.
We proposed a radical reaction mechanism which correlates
with the experimental data. The nitrogen transformation route
in the mechanism was consistent with the decreasing nitrogen
oxidation state during the process. We found that a sustainable
and efficient combustion of aqueous UAN requires an uninter-
rupted reactant ow in order to continuously replenish the
radical pool.
Emissions of CO were slightly above regulation standards.
However, the maximal N2 yield was 99.89%, and laboratory
scale NOx emissions were below the regulation standards for
stationary power generation turbines. An improved reactor
design and catalytic treatment of effluent gases should be
considered in order to further reduce NOx, NH3 and CO levels.
In summary, the aqueous UAN low carbon nitrogen-based
alternative fuel seems to offer a very compelling technology for
chemical hydrogen storage due to its relatively high energy
density and ease of clean operation. Coupled to the future
availability of abundant hydrogen from water splitting, this fuel
facilitates the development of low carbon and non-carbon
chemical hydrogen storage. Additional complete life cycle
analysis work of these fuels is required.
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