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Gasification of unsymmetrical dimethylhydrazine
in supercritical water: Reaction pathway and
kinetics
Lei Yi, Liejin Guo
*
, Hui Jin, Jiajing Kou, Deming Zhang, Runyu Wang
State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF), Xi'an Jiaotong University, Shaanxi, 710049,
China
article info
Article history:
Received 14 January 2018
Received in revised form
25 February 2018
Accepted 13 March 2018
Available online xxx
Keywords:
UDMH
SCWG
Resource utilization
Harmless treatment
Reaction pathway
Kinetics
abstract
Unsymmetrical dimethylhydrazine (UDMH) is a high N-containing (as much as nearly 50%)
substance. Traditional treatment methods such as incineration will inevitably cause the
formation of nitric oxide and secondary pollution. Supercritical water is a preferred
transformation medium due to its unique physicochemical properties. However, at present
most of studies are limited to supercritical water oxidation (SCWO) which tends to produce
hydrogen nitrate resulting in corrosion to the reactor. To conquer this problem, we propose
supercritical water gasification (SCWG) technology which is in a reducing environment,
realizing both harmless treatment and resource utilization. In order to promote its
industrialization process, the reaction pathways and kinetic parameters should be studied.
In this paper, the reaction pathways and kinetics of UDMH in supercritical water were
conducted under the conditions of 400 Ce550 C in quartz reactor, which avoids the
catalytic effect on the reaction kinetics. From the resource utilization perspective, the most
abundant quantitatively detectable gaseous product is methane, together with less
hydrogen, carbon monoxide and ethane orderly. All these gaseous products are combus-
tible. The maximum of carbon efficiency is 90.25% at 550 C, 10 min. In the point of view of
harmless treatment, the organic compounds contained in the residual liquid are detected
with
1
H NMR, FTIR and GC/MS. Results show that UDMH could be fully degraded within
3 min and the ultimate organic compounds in the residual liquid are mainly dimethyla-
mino acetonitrile and trimethylamine. In addition, a reaction pathway for UDMH disposed
in supercritical water is developed. Finally, the quantitative kinetic model for describing
the gaseous products and ammonia-nitrogen in the residual liquid is brought forward. The
pyrolysis activation energy for UDMH in supercritical water is 49.98 ±7.38 kJ/mol.
©2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
Pollution is the price we pay for economic growth in an ever-
increasing pace. Much effort has been made to deal with it,
which is still a worldwide threat to overwhelm us. Hazardous
waste is one of the most dangerous varieties of pollution that
will do a great harm to human beings and surroundings if
cannot be properly treated. Unsymmetrical dimethylhydra-
zine (UDMH) is considered to be a kind of hazardous waste
*Corresponding author.
E-mail address: lj-guo@mail.xjtu.edu.cn (L. Guo).
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/he
international journal of hydrogen energy xxx (2018) 1e11
https://doi.org/10.1016/j.ijhydene.2018.03.092
0360-3199/©2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Yi L, et al., Gasification of unsymmetrical dimethylhydrazine in supercritical water: Reaction pathway
and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.092
because of its high toxicity. Owning the advantages of high
specific impulses, high thrust levels and better thrust control
[1], UDMH is used as a source of propellant for military and
space programs [2] in many countries, such as Russia, China
and the U.S [3]. However, it has a high potential to exacerbate
the environment and the human health [4].
Table 1 gives a brief list with respect to comparison of
different approaches to decompose UDMH waste water. Apart
from traditional methods (such as adsorption, incineration),
the mostly used method is oxidation, which has the disad-
vantage of forming the known highly carcinogenic N-dime-
thylhydroxylamine (NDMA) for the reason of UDMH oxidation
[5]. As such, new way of treatment for UDMH still needs
exploring. It is reported that SCWG technology can deal with
various organic waste, such as sewage sludge [6], waste paper
[7], vinasses generated during the alcohol process [8], garbage
and industrial wastes [9], with the advantage of high reaction
rate, harmless treatment and resource utilization. So it is
worth making an attempt to dispose UMDH with SCWG.
The properties of water above its critical point are quite
different from those of liquid water under ambient conditions.
The number and persistence of hydrogen bonds are both
diminished and the dielectric constant is much lower, which
is one of the reasons why organic wastes enjoy complete
miscibility in SCW. On the other hand, the dissociation con-
stant (K
w
) for SCW is much higher than it is for ambient liquid
water, which makes it an ideal solvent for organic compounds
[18e23]. Supercritical water has been proved to be a good
medium to treat many kinds of organic substances [24e31].
Research concerning the transformation of nitrogen in
supercritical water has been studied by many researchers.
Killilea's group [32] reportedly studied the distribution of ni-
trogen under the conditions of SCWO, finding that most of
nitrogen in any oxidation state is converted to N
2
, as well as
trace of N
2
O that could be eliminated via catalytic reduction or
operating the SCWO process at higher temperature. Quinoline
used as a model nitrogen-containing compound, also under
SCWO environment, Yuan [33] obtained a reduction of total
nitrogen to 85% with the presence of sulfided NiMo catalyst,
and put forward the denitrogenation pathway. In Liu's[34]
work, the transformation mechanism of nitrogen is given.
With no nitrogen-containing gaseous products being detec-
ted, the concentration of inorganic nitrogen in the residual
liquid, mainly in the form of ammonia-nitrogen, increased
with the increase of temperature and prolonging of reaction
time.
Although many attempts have been made to dispose
wastes containing UDMH, there are still several problems to
be tackled. For example, conventional thermal methods, such
as incineration in flame, consume much energy and lead to
the formation of large amounts of secondary pollutants [35].
SCWO process has accomplished many challenges in recent
years, but in terms of UDMH disposal, highly carcinogenic
substance such as NDMA could be formed in the process of
SCWO. Beyond that, corrosion imposed by hydrogen nitrate is
one of bottlenecks remaining to be solved. Compared to
SCWO, SCWG can slow down the rate of corrosion and recycle
part of energy contained in the wastes, realizing resource
utilization. However, the gasification mechanism and kinetic
model of UDMH in SCWG process are not readily accessible.
Table 1 eComparison of different approaches to decompose UDMH waste water.
Method Experimental condition Advantages Disadvantages Reference
Activated carbon fiber adsorption Temperature: 25 C, processing time: 3 h No toxic compound formed Low processing efficiency [10]
Catalytic fenton oxidation Room temperature, reaction time: 1.5 h No toxic substance formed Long reaction time, adsorption will cause
secondary pollution
[11]
Catalytic oxidation Temperature: 25e75 C, half time: 1e300 min,
pH: 7,9
The iron-containing catalysts are stable
in neutral media
Formation of toxic compounds, low
handling capacity
[12]
Microwave catalytic oxidation Microwave power 490 W, irradiation time 9 min,
H
2
O
2
dosage 1 mL and activated carbon
dosage 1 g per 100 mL UDMH sewage
High degradation rate Long reaction time, low handling capacity [13]
Ultraviolet induced chlorination Temperature: 40e125 C, degree of irradiation:
0.1e1.0 W per liter, pH 5
Suitable for multiple pollutants,
clean effluent
Long reaction time, complex system,
operating at certain pH
[14]
Magnetic carbon nanocomposite Room temperature, contact time: 30 min, pH 6 High removal efficiency Long reaction time, secondary pollution,
limited pH range
[15]
Chemical oxidation Room temperature, reaction time 30 days,
adding oxidant
High effectiveness with strong oxidant Mostly produce toxic compounds,
long reaction time
[16]
Hybrid cavitation Near room temperature, pressure: 7 bar, reaction
time: 40 min, pH: 2e7.4
High removal efficiency, no oxidant needed,
no toxic products
Low processing efficiency [17]
international journal of hydrogen energy xxx (2018) 1e112
Please cite this article in press as: Yi L, et al., Gasification of unsymmetrical dimethylhydrazine in supercritical water: Reaction pathway
and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.092
In previous studies, researches on reaction mechanisms
are mostly under the condition of oxidation process. Seldom
have any researchers conducted under reducing environment
where NDMA or hydrogen nitrate cannot be formed during the
process. In this paper, study on SCWG of unsymmetrical
dimethylhydrazine was conducted under the conditions of
400 Ce550 C in quartz reactor. Firstly, from the resource
utilization perspective, the effect of temperature and reaction
time on gasification as well as ammoniaenitrogen in residual
liquid was studied. Secondly, in the point of view of harmless
treatment, the characteristics of residual liquid and trans-
formation of nitrogen were performed. And then we proposed
a reaction pathway and developed a kinetic model to describe
the gaseous products and nitrogen transformation.
Experiment
Apparatus and methods
The quartz tubes were produced by Lianyungang Quartz Ce-
ramics Co., Ltd with one end sealed by hydro-oxygen flame.
Reagents used in our work were AR grade purchased from
Sinopharm Chemical Reagent Co., Ltd.
To make the kinetics more specific and free from the in-
fluence of the catalytic effect of the metal wall, quartz tube
reactor was employed [36]. All experiments were carried out in
quartz tube reactors (I.D. ¼3 mm, O.D. ¼5mm,
length ¼200 mm). In order to purge the air in the reactor, argon
was insufflated into the quartz tube through a 200 mm long
needle inserted down to the bottom of the tube. Then the
needle was elevated slowly, filling the tube with argon. After
the mixture of UDMH and deionized water of certain amount
had been loaded into the reactor, the open end of the tube was
sealed with hydro-oxygen flame. Afterwards, the tube was
placed into the isothermal tube furnace, which had been
heated up to the desired reaction temperature before each
experiment. The reactor could be heated with high rate (e.g.,
up to 700 C in 2 min). The reactor was keeping in the furnace
until the target reaction time ended, followed by cooling down
to the room temperature in the air. The schematic diagram of
the electrothermal furnace heating system can refer to the
literature [37]. Then in two ways was the reactor processed in
accordance with the analytic methods. For the gaseous prod-
ucts, the reactor was put into a one-end-sealed steel gaseous
tube collector (I.D. ¼10 mm, O.D. ¼14 mm, length ¼229 mm),
which had been filled with argon using the same method
mentioned above. A pressure gauge was attached to the col-
lector to measure the instant pressure. After that, the open end
was sealed with a replaceable rubber plug pushed tightly by a
steel sleeve which had a hole for insertion of the sampling
syringe. In order to obtain a homogeneous mixture of gases,
the steel tube collector was smashed hard down to the ground
and shaken upside down for many times. The sample gas was
then injected into a gas chromatograph with a sampling sy-
ringe to determine its composition. For the sake of making the
results more convincing, each experiment under same con-
ditions was repeated at least three times. The overall standard
deviation of the entire sample is no more than 7%. For the
liquid products, the reactor was cut in half with a diamond
knife to collect the residual liquid, part of which employed for
TOC analysis, part for ammonia-nitrogen. The tendency of
transformation of different organic compounds in residual
liquid at certain reaction time and temperature was analyzed
with GC/MS.
Sample analysis
The composition of the gaseous product was detected by Agi-
lent 7890A gas chromatograph (GC) equipped with a thermal
conductivity detector (TCD) and capillary column C-2000 pur-
chased from Lanzhou Institute of Chemical Physics in China.
The carrier gas was argon (99.999%) with a flow rate of 5 mL/
min. A standard gas mixture of H
2
,CO,CO
2
and CH
4
was used
for calibration.
1
H NMR spectra were analyzed on a Bruker
Advance Ⅲ-400. The FTIR spectra were obtained by a Bruker
Vertex 70 spectrometer and peaks are reported in cm
1
.The
liquid organic products were extracted by dichloromethane
and qualitatively detected by GC-MS (Agilent 6890Ae5973N
MSD) and the total organic carbon (TOC) of the residual liquid
was determined by Elementar vario TOC. Nitrate and nitrite
were measured by Metrohm 930 Compact IC Flex. Ammonia-
nitrogen concentration was quantitatively analyzed by multi-
controller (Lovibond-ET99722). Each time before being
analyzed, the residualliquid must be diluted to a certain extent.
Results and discussion
This part is divided into 4 subsections. Firstly, from the
resource utilization perspective, attention should be drawn on
the gas products. Secondly, in the point of view of harmless
treatment, we should focus on the degradation rule of the
toxic substance in the way of different approaches (e.g.,
1
H
NMR analysis, FTIR, GC/MS) to the detailed study on the re-
sidual liquid. And then based on them, we proposed a reaction
pathway for UDMH processed in supercritical water. Finally a
kinetic model was constructed and the kinetic parameters
were determined.
Resource utilization
Results were obtained at four different temperatures (400 C,
450 C, 500 C, 550 C) and reaction time of different time in-
terval from 1 min to 10 min (1, 1.5, 2, 3, 4, 5, 10 min). Constant
amount (130 mL) of UDMH solution with a concentration of
10 wt% was loaded into the reactor in accordance with the
water density of 0.083 g/mL.
The most abundant quantitatively detectable gaseous
product is methane, together with less hydrogen, carbon
monoxide and ethane orderly. All these gaseous products are
combustible. Nitrogen increases with the rise of temperature
and the prolonging of reaction time.
The variation of different gaseous products yield, together
with the concentration of ammonia -nitrogen, along with
temperature and reaction time is shown in Fig. 1(a)e(f),
wherein ethane is not presented due to its little content. As it
can be seen, all indicators tend to be stable after 4 min. The
maximum of CE and HE could be attained at 550 C, 10 min,
which are 90.25% and 88.54%, respectively. CO and hydrogen
international journal of hydrogen energy xxx (2018) 1e11 3
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and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.092
molar yield, CE and ammoniaenitrogen have a ladder-type
increase with both temperature and reaction time, reaching
their maximum at the highest temperature and longest re-
action time while CH
4
molar yield and HE are not the case.
They are only a function of reaction time but not temperature,
seemingly at the same level after 4 min. The tendency of the
CH
4
yield could be attributed to two aspects: low temperature
favors the formation of methane in terms of thermodynamics.
On the other hand, high temperature promotes the reaction
rate. Given the two reasons, the higher the temperature, the
higher the methane yield.
Harmless treatment
It could be seen from the above analysis that UDMH could be
degraded in the first few minutes. In order to obtain a better
insight of how UDMH was decomposed and the reaction
pathway, the study of detailed characteristics of products is
necessary.
1
H NMR analysis
Before analyzing, all samples were extracted by CDCl
3
(
1
HNMR:
d¼7.26). The spectra of UDMH (98 wt%) is in good accordance
with the spectra data obtained from Advanced Chemistry
Development,Inc. As shown in Figs. 2 and 3, there are two kinds
of proton, one connected to nitrogen (d
1
¼2.95) and the other
connected to carbon (d
2
¼2.41). The difference of the chemical
shift is due to the distinction of electronegativity between ni-
trogen and carbon. The difference of the electronegative of the
element connected to the proton would result in the difference
of the chemical shift. The weak peaks of UDMH may be the
result of impurity. It could be seen that UDMH could be fully
degraded within 3 min when the characteristic peaks of UDMH
(d
1
and d
2
) are totally disappeared, transforming to
Fig. 1 eVariation of FTIR spectra at different reaction time, 400 C.
international journal of hydrogen energy xxx (2018) 1e114
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and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.092
intermediate products. These results are consistent with the
results of gaseous products (Fig. 1).
FTIR analysis
All samples were extracted by CCl
4
before being analyzed. The
wavenumbers are in the range of 1000e4000 cm
1
due to the
high peak of CCl
4
making the peaks of extracted samples too
weak to be seen clearly. The preserved range also can offer
most important information to identify different groups.
Peaks (Fig. 3) at 2786, 2831, 2870 and 2964 cm
1
attributed to
symmetric and asymmetric stretching vibrations demon-
strate the existence of CeH bond in UDMH [38]. The peak band
between 3300 and 3500 cm
1
resulted from stretching vibra-
tions indicates the presence of NeH bond in UDMH. The as-
sociation is formed because of the high concentration of
UDMH, resulting in a wide peak in the NeH range. Peak at
1635 cm
1
indicates the existence of amide Ⅱ[39] and peaks
below 1500 cm
1
are due to the bending vibrations of CeH
bonds which are relatively weak compared to other kinds
mentioned above.
The FTIR spectra of residual liquid show that almost all Ce
H stretching and bending vibrations have disappeared at
1 min. One of the two left peaks at 1384 cm
1
attributed to the
vibration of methyl may be resulted from trimethylamine or
dimethylamino acetonitrile containing methyl. The other
peak at 3430 cm
1
is mostly a result of vibration by NeH bond
in ammonia which is the main existing form of nitrogen in
residual liquid (see later analysis). These two peaks become
weaker and weaker as reaction time prolongs, indicating the
decomposition of the intermediate products. The FTIR anal-
ysis is consistent with the analysis by
1
H NMR illustrating that
UMDH could be effectively decomposed in supercritical water.
GC/MS analysis
Relative percentage content at reaction time of different time
interval was used to deduce the reaction pathway of the
degradation of UDMH in supercritical water, and that at
different temperature to identify its conversion along with the
increase of temperature. Table 2 shows the identified organic
compounds with their retention time, name, chemical for-
mula, structural formula and relative percentage content.
It has been reported [3,16,40,41] that formaldehyde dime-
thylhydrazone (FDMH) is one of the oxidation products of
UDMH if it is exposed to air for some time. Apparently as
shown in Table 2, FDMH is the main oxidation product, ac-
counting for over half of the initial reactant, and could be
formed through being deprived of the two hydrogen atoms
attached to the nitrogen by oxygen and further reaction [42].
That no monomethylhydrazine (MMH) had been found also
supports their idea about the initial step of the oxidation of
UDMH in the air. Notably, neither N-nitroso dimethylamine
(the most highly toxic, known as NDMA) nor other less carci-
nogenic and mutagenic compounds, such as 1,1,4,4-
tetramethyl tetrazene, had been detected in the residual
liquid. The reason may be due to the high reaction rate of su-
percritical water or the chemical bonds cracked by high tem-
perature. Interestingly, the main organic products in the
residual liquid are trimethylamine (TMA) and dimethylamino
acetonitrile (MSDS) that has not been reported in other litera-
ture. Apart from these two products, others only account for a
small part, consumed within 5 min. From the ascending of
relative content of TMA and descending of MSDS as well as
their linked structural formula, it could also be deduced that
the former is derived from the latter at least large partly and
these two compounds are the last hurdles to be fully degraded.
Fig. 3 eVariation of
1
H NMR spectra at different reaction
time, 400 C.
Fig. 2 eVariation of different indicators with reaction time
and temperature (a) CH
4
(b) H
2
(c) CO (d) CE (e) HE (f)
ammonia-nitrogen.
international journal of hydrogen energy xxx (2018) 1e11 5
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and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.092
Table 2 eIdentities and relative amounts of compounds in residual liquid.
RT (min) Name Chemical
formula
Structural
formula
Reactants 400 C 450 C500
C
1 min 1.5 min 2 min 5min 5 min 5 min
Area %
1.46 Trimethylamine C
3
H
9
Ne0.42 1.62 1.49 24.34 30.38 40.22
1.56 2-Butanone C
4
H
8
Oe0.64 eeeee
2.29 Hydrazine,trimethyl- C
3
H
10
N
2
e1.02 3.72 1.34 ee e
2.50 Hydrazine,2-ethyl-1,
1-dimethyl-
C
4
H
12
N
2
e0.86 1.32 0.68 ee e
3.13 Formaldehyde,
dimethylhydrazone
C
3
H
8
N
2
55.01 69.58 37.34 30.08 ee e
3.19 Hydrazine,1,1-dimethyl- C
2
H
8
N
2
44.99 23.99 38.66 19.74 ee e
4.04 Acetaldehyde,
dimethylhydrazone
C
4
H
10
N
2
e1.03 1.90 6.08 ee e
13.05 Acetonitrile,
(dimethylamino)-
C
4
H
8
N
2
e2.46 15.44 40.58 75.66 69.62 59.78
“-”: not detect.
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Transformation of nitrogen
To follow the tracks of migration and transformation of ni-
trogen in UDMH in the process of disposal, concentration of
total nitrogen (TNb) and ammonia-nitrogen also have been
measured. The results are shown in Fig. 4 (seven different
reaction times at each temperature). It had been detected
that there were no nitrate and nitrite in the residual liquid in
all experiments so the rest of the nitrogen atoms were
reasonably regarded as nitrogen. This idea is also supported
in literature [3]. The absence of nitrate and nitrite demon-
strated that there is no strongly corrosive substance formed.
As it is can be seen, TNb has a tendency to decrease with the
increase of reaction time and temperature, indicating more
nitrogen atoms are converted to gas nitrogen, while
ammonia-nitrogen has a trend of the other way around,
suggesting more intermediates or initial reactions are trans-
formed to ammonia, resolved in residual liquid. Ultimately,
TNb is at the same level regardless of either reaction time or
temperature, illustrating nitrogen atoms in the liquid could
not be further transformed to gas but to ammonia. Note,
actually the concentration of ammonia-nitrogen may be
underestimated because of the test method that each time
before inhaling a certain amount of residual liquid, the sy-
ringe have been flushed with deionized water in advance, so
the inhaled liquid may have been diluted in some degree. As
such, ammonia is the main existence form for inorganic ni-
trogen. The way to lower the concentration of it is readily
accessible [43e46].
Reaction pathway
Having presented the liquid products and the migration and
transformation of nitrogen in UDMH, we now discuss their
pathway.
After the hydrogen atoms attached to nitrogen being took
away by oxygen, the free electrons of the nitrogen could form
a double-bond with the other one, which is not a stable
structure as an intermediate product. As a result, the nitrogen
with two methyl charges negative while the other positive.
Then two molecules of the intermediate product could form
one molecule of FDMH and the precursor of methane and
nitrogen.
It is reported [47] that there are two competing reaction
pathways in supercritical water, ionic reaction, preferred at
higher pressure and/or lower temperature, and free radical
steps, dominating at lower pressure and/or higher tempera-
ture. K
w
of water above its critical point is much lower than
that at ambient conditions, leading to a poor medium for ionic
chemistry [18]. Accordingly, free radical reaction pathway
must play an important role in the UDMH degradation process
in supercritical water. B. Al-duri [48] investigated the removal
of N-containing heterocyclic hydrocarbon wastes, concluding
that the destruction of N-containing hydrocarbons occurs by
free radical mechanism.
Fig. 5 gives a brief possible reaction pathway for gasifica-
tion of UDMH in supercritical water concerning only the
organic compounds. Compound (1) could form FDMH in the
above mentioned way, or be converted to azomethane
through rearrangement due to its relatively stable structure
and azomethane could be oxidized by hydroxyl radical which
is abundant in supercritical water immediately to form for-
monitrile. And then associated with compound (2) and (1),
formonitrile could be transformed to MSDS. On the other
hand, the formation of trimethylhydrazine, 2-ethyl-1,1-
dimethylhydrazine and acetaldehyde dimethylhydrazone
could be explained by FDMH's reaction with compound (1) or
(2) or its addition reaction of hydrogen.
Fig. 4 eVariation of concentration of TNb and ammonia-
nitrogen with reaction time and temperature.
Fig. 5 eReaction pathways for gasification of UDMH in
supercritical water.
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Kinetic model
Model development
This section develops a kinetic model for the formation of
gaseous products and ammonia-nitrogen, using the lumped
parameter method which has been used by many researchers
[34,49,50], to substitute for the intermediate products. Ac-
cording to Table 2 and Fig. 5, the main intermediate product is
MSDS, so we use “Int”to represent all intermediate products
whose chemical formula is regarded as C
4
H
8
N
2
to balance the
chemical reaction equations.
The reaction of UDMH in supercritical water is complex and
there is no need to figure out every detail of the intermediate
reaction step at the cost of large amount of computing re-
sources considering the purpose of this work. The lumped
parameter method can simplify the reaction process so that
we can focus on obtaining the quantitative kinetics of the gas
production and the nitrogen transformation.
UDMH could form methane and nitrogen or Int and gas
products via pyrolysis. Gas could also be generated through
steam-reforming reaction in which nitrogen atom could be
transferred to gaseous nitrogen or ammonia-nitrogen. It is
reported that water-gas shift reaction starts at a very high
temperature, basically above 550 C[51]. On the other hand,
carbon dioxide has not been detected throughout the experi-
ments, so it seems reasonable to take it out of consideration in
the reaction equations. Besides, the intermediates could be
decomposed to form gases and other intermediates. As all
intermediates are lumped as “Int”, there is no consumption of
intermediates in these steps. Once the gaseous products
formed, reaction among them is inevitable. So we introduce
the reaction of methanation which is reversible.
According to the reaction pathways and kinetic model,
gasification of UDMH in supercritical water can be described
as following reactions.
C2H8N2!
k12CH4þN2R1¼k1CUDMH (1)
2C2H8N2!
k2Int þ4H2þN2R2¼k2CUDMH (2)
2C2H8N2!
k3Int þH2þ2NH3R3¼k3CUDMH (3)
C2H8N2þ2H2O!
k42CO þ6H2þN2R4¼k4CUDMHCW(4)
C2H8N2þ2H2O!
k52CO þ3H2þ2NH3R5¼k5CUDMHCW(5)
Int!
k6CH4þInt R6¼k6CInt (6)
Int!
k7H2þInt R7¼k7CInt (7)
Int!
k8CO þInt R8¼k8CInt (8)
Int!
k9NH3þInt R9¼k9CInt (9)
CO þH2%
k10 CH4þH2OR
10 ¼k10CCO CH2R10r¼k10rCCH4CW(10)
The rate equation for each reaction is assumed to be of first
order in concentrations (C) of each species. The differential
material balance equations for each species are as follows.
dCCH4
dt ¼2R1þR6þR10 R10r(11)
dCH2
dt ¼4R2þR3þ6R4þ3R5þR7R10 þR10r(12)
dCCO
dt ¼2R4þ2R5þR8R10 þR10r(13)
dCNH3
dt ¼2R3þ2R5þR9(14)
dCInt
dt ¼R2þR3(15)
dCUDMH
dt ¼R12R22R3R4R5(16)
dCW
dt ¼2R42R5þR10 R10r(17)
the subscript W represents water.
The relation between the rate constant of the reverse re-
action and that of the forward reaction is determined by the
equilibrium constant K, which could be calculated with the
REQUIL reactor block in ASPEN software.
kr¼k
K(18)
For methanation, the equilibrium constants are 2.96
10
4
L
2
/mol
2
, 8.53 10
3
L
2
/mol
2
, 2.81 10
3
L
2
/mol
2
, 1.04 10
3
L
2
/
mol
2
at 400 C (17.9 MPa), 450 C (20.9 MPa), 500 C (23.7 MPa),
550 C (26.3 MPa), respectively. The pressure of the reactor was
calculated through the property of water.
Berkeley Madonna was used to fit the experimental data
to the model we developed. Table 3 shows the estimated
parameters value of rate constants and corresponding
Arrhenius parameters, through which it could be seen that
the reaction rate constants increase as temperature in-
creases. E
a
(apparent activation energy) and A (apparent pre-
exponential factor) were obtained through the following
equation:
ln k¼ln AEa
RT (19)
wherein k is the rate constant and R is the gas constant.
In the process of fitting, we found that reaction (4) is
impossible to occur, whereas reaction (6) and (7) are not sen-
sitive to the numerical value of the rate constant, indicating
that methane and hydrogen generated from intermediate
products account for only small part of the corresponding gas
yield. The main source of methane and ammonia-nitrogen is
pyrolysis of UDMH. Compared to other compounds, such as
indole [34,52], algae [53] and phenol [54], activation energy in
this work is smaller illustrating that UDMH is easier to be
gasified in supercritical water. By comparing the activation
energy of reaction (1) with others containing intermediate
products, such as reaction (2), (3), (7), (9), it indicates that the
international journal of hydrogen energy xxx (2018) 1e118
Please cite this article in press as: Yi L, et al., Gasification of unsymmetrical dimethylhydrazine in supercritical water: Reaction pathway
and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.092
final products could be obtained more difficultly via the in-
termediate products. Therefore, the key way to increase the
reaction rate might be reducing the formation of the inter-
mediate products. Once again, note that the concentration of
ammonia - nitrogen is probably underestimated.
Model validation
Fig. 6(a)e(c) shows the experimental data of gas products as
well as ammonia contained in the residual liquid and the
fitted curve of the model at the range of 400e550 C, 1e10 min.
In Fig. 6(e), all of the points would fall on the diagonal if the
Table 3 eEstimated parameters value of rate constants and corresponding Arrhenius parameters.
Rate constant 400 C 450 C 500 C 550 CE
a
(kJ/mol) ln A
k
1
(min
1
) 0.3015 0.5423 0.7429 1.6825 49.98 ±7.38 7.69 ±1.20
k
2
(min
1
) 0.0089 0.022 0.0464 0.0971 72.88 ±1.07 8.31 ±0.17
k
3
(min
1
) 0.0289 0.0665 0.1098 0.2283 61.67 ±3.72 7.49 ±0.60
k
4
(L mol
1
min
1
)0000ee
k
5
(L mol
1
min
1
)0000ee
k
6
(min
1
) 0000ee
k
7
(min
1
) 0.0081 0.0104 0.0289 0.0462 57.03 ±10.37 5.22 ±1.68
k
8
(min
1
) 0.1156 0.1329 0.1676 0.2028 17.54 ±1.74 0.95 ±0.28
k
9
(min
1
) 0.0032 0.0087 0.0173 0.0225 60.93 ±8.20 5.27 ±1.33
k
10
(L mol
1
min
1
) 0.0925 0.1792 0.1676 0.0925 28.78 ±2.20 3.09 ±0.36
Fig. 6 eCurve fitting: (a) 400 C (b) 450 C (c) 500 C (d) 550 C (e) parity plot for experimental and calculated results.
international journal of hydrogen energy xxx (2018) 1e11 9
Please cite this article in press as: Yi L, et al., Gasification of unsymmetrical dimethylhydrazine in supercritical water: Reaction pathway
and kinetics, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.092
model provides a perfect correlation of the experimental re-
sults. It could be seen from Fig. 6 that our model does a
satisfying job of fitting the experimental data expect that a few
points deviate due to the unstable yield of the gas products in
the first minute.
Conclusions
Supercritical water gasification (SCWG) technology used for
processing UDMH proved to be a harmless, resourceful utili-
zation technology. In this paper, gasification of unsymmetri-
cal dimethylhydrazine in supercritical water was studied in
the range of 400 Ce550 C. Here are the main conclusions:
(1) Methane-rich gas products (molar fraction 70e90%
depending on the reaction condition) could be obtained
in SCWG process. Carbon gasification efficiency could
reach as high as 90.25% at 550 C, reaction time of 10 min.
(2) UDMH couldbe fully degraded at 400C, within 3 min from
the comprehensive analysis by
1
H HMR, FTIR and GC/MC.
(3) In the supercritical water gasification process, neither
nitrate nor nitrite was found which will reduce the
requirement for reactor materials. The main existence
form for inorganic nitrogen is ammonia whose way of
elimination is mature at a low cost.
(4) A reaction pathway of UDMH processed in supercritical
water also has been put forward. The proposed kinetic
model suggests that UDMH (pyrolysis activation energy:
49.98 ±7.38 kJ/mol) is relatively easier to be gasified
under the condition of supercritical water.
Acknowledgment
This work is financially supported by National Key R&D Pro-
gram of China (Contract No. 2016YFB0600100) and the Na-
tional Key R&D Program of China (Contract No.
2016ZX05025004-001).
Nomenclature
Gas yield mole of certain gaseous product/mass of UDMH
initially loaded into the reactor, mol/kg
CE carbon gasification efficiency, mass of C atom in
gaseous product/mass of C atom in UDMH initially
loaded into the reactor, %
HE hydrogen gasification efficiency, mass of H atom in
gaseous product/mass of H atom in UDMH initially
loaded into the reactor, %
TOC total organic carbon
dchemical shift
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