REACTION CHARACTERISTICS OF SNCR DE-NOx IN MODERATE TEMPERATURE
RANGE WITH UREA SPIKED HYDRAZINE AS REDUCING AGENTS
Chen H1, Hong L1, Chen D Z1*, Yin Lijie1, Huang S2
1. Thermal & Environmental Engineering Institute, Tongji University, China
2. Shanghai Environment Group Co., Ltd., China
Introduction
It has been well known that NOx can be reduced to N2 and H2O in selective non-catalytic
reduction (SNCR) process by using ammonia, urea and isocyanic acid as reducing agents which are
known as Thermal de-NOx process[1], NOxOUT process[2] and RAPRENOx process[3]. The SNCR
is a conceptually simple process of NOx control which is attractive due to its simplicity, catalyst-free
system, ease of installation on existing plants, applicability to all types of stationary-fired equipment,
lower investment and operating cost. It is more and more popular to use urea as reductant because of
its convenient transport, secure storage and relatively low price. However, the NOxOUT process
requires a high temperature for effective NO reduction, but for many low temperatures’ flue gas,
such as sintering flue gas with no obvious high temperature area in iron and steel industry [4]. Also
by adopting high-temperatured SNCR de-NOx alone the exhaust gas cannot meet up the
requirements of the standards, for example, NOx in flue gas from municipal solid wastes incinerators
is required to be lower than 200mg/Nm3 from the present 400mg/Nm3; and the De-NOx efficiency of
high-temperatured SNCR process is just around 30~40% in practice. To reduce the effective
temperature range for SNCR process provides another chance for De-NOx without expensive
catalytic. Many scholars tried to add H2, H2O2, hydrocarbons, sodium salts, biomass gasification gas,
etc. as addictives into SNCR process to move the reaction temperature window to medium
temperature[1, 5-9]，but the effective temperature window cannot reduce to below 650℃. A set of
new reducing agents, namely hydrazine compounds were investigated and they were reported to shift
the effective SNCR temperature window to the moderate temperature range of 450~600℃[10-11];
Especially hydrazine hydrate was more effective[12]. However, hydrazine hydrate is very expensive
compared to urea. The purpose of this paper is to obtain a cheaper hydrazine-based reductant for
moderate temperature SNCR de- NOx.
Materials & Methods
Materials
Pure urea (CO(NH2)2, ≥99%, Sinopharm Chemical Reagent Co., Ltd) and hydrazine hydrate
solution (50% N2H4·H2O, Sinopharm Chemical Reagent Co., Ltd) were used to prepare the reductant
reagents. The tested solutions are listed in Tab.1.
Tab.1 Blended hydrazine hydrate & urea solutions
Solution
Composition
Molar ratio of urea N to hydrant N
1
Urea solution + hydrazine hydrate
5:1 (16.7% hydrazine N in total)
2
3
4
Hydrazine hydrate solution + urea
Hydrazine hydrate solution + urea
Hydrazine hydrate solution + urea
1:10 (9.1% urea N in total)
1:5 (16.7% urea N in total)
1:3 (25% urea N in total)
SNCR de-NOx process
All the SNCR experiments have been carried out in a pilot-scale system which has been
described elsewhere [13]. The flue gas was generated by a natural gas burner. A NO cylinder and a
N2 cylinder were adopted to adjust the initial NO concentration and O2 partial pressure. The flow of
the gas was measured by a micro-flute-shaped tube manometer and de-NOx reductants were sprayed
into the hot flue with an atomizer. Velocity of flue gas was in the range of 12 ~ 15 m/s. A KM9106
type NO/NOx gas analyzer (Kane International Limited, UK) was adopted to measure NO/NOx
before and after reductants sprayed.
Investigation of thermal decomposition and oxidation characteristics of reductants
To explore thermal decomposition and oxidation reaction of the hydrazine hydrate in N2 and in
air, the hydrazine hydrate solutions with various mass concentrations were sprayed into the
electricity heated pipe at the flow rate of 18ml/min with N2 or air (21% O2 and 79%N2) as carrying
gas; as shown in Fig.1, after four passes of flow in the heated pipe the temperature of the gas flue
was increased from ambient to 700℃. NH3 produced in this process was detected with a PGM-7800
type gas analyzer (RAE, USA) at the outlet; hydrazine emitted from evaporation was measured with
a PGM-7240 type gas analyzer (RAE, USA); and the oxidation products (NO, NOx) were measured
with the KM9106 type NO/NOx gas analyzer.
Fig. 1 Schematic diagram for reductant solution thermal decomposition
In addition, urea and the crystals resulted from drying of solution 1, 2, 3 and 4 listed in Tab.1
were subjected to thermogravimetric analysis with a WRT-3P type thermal analyzer (Shanghai
Precision Scientific Instrument Co., Ltd, SH) in the nitrogen atmosphere. The crystal samples were
heated from 30℃ to 600℃ at a heating rate of 10℃/min to study their thermal decomposition
stability. Again the NH3 emitted during the process was measured.
Results and discussion
Efficiency of SNCR de-NOx with different reducing agents
When urea and hydrazine-spiked urea is used as reducing agent, the de-NOx efficiencies are
shown in Fig.2(a), it can be seen that the de-NOx efficiencies of both urea and hydrazine-spiked urea
are zero or even negative when temperature is below 930℃~950℃; which indicates that adding
hydrazine into urea cannot reduce NOx in moderate temperature range. The hydrazine-based
reductant did broaden the reaction temperature window compared to urea solution; but the peak deNOx efficiency is reduced. This is because whenin the high temperature zone, the trend of oxidation
of hydrazine hydrate is enhanced[11]. The results in Fig. 2(a) shows that adding hydrazine hydrate
into urea can not provide an effective reagent for SNCR de-NOx in the moderate temperature range.
Single hydrazine(NSR=2), PO2=16.6~9.9%
Single Urea(NSR=2), PO2=9.8~13.1
60
Solution 1
Solution 2
Solution 3
Solution 4
50
40
20
0
500
-20
600
700
800
900
1000
1100
Temperature/℃
NO Reduction/%
NO Reduction/%
40
30
20
10
0
500
-40
-10
600
700
800
900
Temperature/℃
(a) Hydrazine-spiked urea
(b) Urea-spiked hydrazine
Fig.2 The efficiency of SNCR de-NOx with blended hydrazine & hydrazine as reducing agent
However, as the results shown in Fig.2(b), when hydrazine hydrate was adopted as a basis
reductant and urea was spiked into the hydrazine hydrate to produce a relatively cheaper reductant,
the corresponded SNCR de-NOx efficiencies were remarkable in the temperature range of
500℃~600℃ and comparable to hydrazine hydrate. When urea was spiked by different ratio
(Solution 2, Solution 3 and Solution 4), the de-NOx efficiency changed accordingly. Of which
Solution 3 with 16.7% of urea N substituting hydrazine N has the very close peak de-NOx efficiency
to that of hydrazine hydrate (93.3% of its peak value). But for Solution 4, in which urea N increases
to 25% of the total N, the de-NOx efficiency begins to decrease sharply, its peak de-NOx efficiency is
only 23% of that of hydrazine hydrate. Presently solution 3 provided the best solution for a cheaper
SNCR de-NOx reductant at moderate temperature range.
Hydrazine hydrate provides lots of NH2 in moderate temperature range, some dominant
reactions for NO reduction are as follows:
NH2 + NO = N2 + H2O
(1)
N2H2 + NO = N2O + NH2
(2)
NH2 + NO = NNH + OH
(3)
H + O2 + M = HO2 + M
(4)
N2H4 (+M) = NH2 + NH2 (+M)
(5)
When urea is spiked, it will decompose in the temperature range of 500~600℃:
NH2CONH2 + H2O = 2NH3 + CO2
(6)
With substitution ratio of hydrazine hydrate N by urea N increasing from 9.1% to 25%, the
effects on promoting NO reduction by reactions (1), (2), (3) become weaker, while the effects played
by reactions (4), (5), (6) become stronger, urea N would change into NH3 which has no ability to
reduce NOx in temperature range of 500~600℃.
Decomposition of urea and hydrazine hydrate
In order to understand the reaction mechanism of SNCR de-NOx process with urea spiked
hydrazine, thermal decomposition of urea, hydrazine hydrate and the urea spiked hydrazine were
studied.
(1) Urea decomposition and NH3 emission in N2 and air
When urea was put into the thermogravimetric analyzer under the N2 flush or air flush, NH3
release can be detected, as shown in Fig.3.
NH3 Production
200
60
100
40
50
20
0
200
300
400
500
Temperature/℃
0
600
150
TG Wt/W0 /%
150
NH3 production/ppm
80
80
TG Wt/W0 /%
Urea Decomposition
NH3 Production
100
200
60
100
40
NH3 production/ppm
Urea Decomposition
100
50
20
0
200
300
400
500
0
600
Temperature/℃
(a) N2
(b)Air
Fig. 3 Urea decomposition and NH3 emission in N2 and air
The data of urea decomposition in pure N2 and air shown in Fig.3 indicate that urea starts to
decompose at the temperature around 200℃, and completes at the temperature of 500℃. Comparing
data in Fig. 3 (a) and Fig.3 (b) it can be seen that there is a little difference between decomposition of
urea in N2 and in air, the start and termination temperatures of NH3 release in air are 50℃ and 30℃
earlier than that in N2. Although NH3 was largely generated after 200℃, but in Fig.2(a) the de-NOx
efficiencies are zero or even negative in lower temperature range, which prove that NH3 is inactive to
reduce NOx in lower temperature range; and reagents releasing NH3 at temperatures lower than 900℃
is just a waste of reductant. When the temperature rises to 400℃ in N2, chemical reaction (7)
becomes the main reaction which produces NH2, H2, and NCO rather than NH3.
N2H4CO = NH2 + H2 + NCO
(7)
N2H4CO = NH3 + HNCO
(8)
N2H4CO = NH2 + H + HNCO
(9)
The reaction (7), (8), (9)[14-18] can meanwhile generate components like HNCO and NCO
which is the main components to reduce NO, however, in low temperature, these decomposition
components are easily oxidized to NO through reactions (10)~(12)[17], this is another reason of the
negative De-NOx effect under temperature 900℃ in Fig.2(a).
HNCO + O = NH + CO2
(10)
NH + O = NO + H
(11)
NCO + O = NO + CO
(12)
(2) Hydrazine hydrate decomposition
When hydrazine hydrate solution was sprayed into the heating pipe as shown in Fig.1 in N2
and air respectively, the NH3 release and NOx emissions at the outlet are shown in Fig.4.
Low mass fraction of hydrazine hydrate
High mass fraction of hydrazine hydrate
NH3 Production
NOx Emission
6.5
500
200
6.0
160
140
120
100
80
60
40
400
5.5
300
5.0
200
4.5
100
4.0
0
20
0
500
NO x Emission /ppm
NH 3 Concentration/ppm
NH 3 Concentration/ppm
180
520
540
560
580
600
620
Temperature/℃
640
660
680
3.5
500
550
600
650
700
750
Temperature/℃
(a) N2
(b)Air
Fig.4 Hydrazine decomposition and NH3 emission in N2 and air
It can be seen that NH3 concentrations increase with the increase of temperature in N2, and the
higher mass fraction of hydrazine, the higher concentration of NH3. Comparing Fig.4(a) and Fig.4(b),
it is found that NH3 concentration in N2 is much higher than that in air, the reason is that, in air flow,
oxygen takes part in the reaciton, and a large amount of oxygen primitives such as O, OH, HNO and
HO2 exist in the reaction system, so the hydrazine hydrate not only decompose themselves, but also
react with the oxygen primitives. The possible elementary reactions suggested by Shen are listed in
Table.2[19]. Combining Fig.4 and Tab.2, it can be determined that NH3 is not the significant
compound to reduce NOx at moderate temperature.
Tab.2 Elementary reactions of hydrazine hydrate decomposition in aerobic condition[19]
NH2 + O2 = HNO + OH
HNO + O2 = NO + HO2
OH + HO2 = H2O + O2
N2H4 + O = N2H3 + OH
N2H4 + OH = N2H3 + H2O N2H3 + O = N2H2 + OH
N2H3 + OH = N2H2 + H2O N2H2 + O = NH2 + NO
N2H2 + O = NNH + OH
N2H2 + OH = NNH + H2O N2H2 + NO = N2O + NH2 NH + O2 = HNO + O
NH + O2 = NO + OH
In air, the concentration of NO/NOx increases with the rising of temperature as shown in
Fig.4(b), which demonstrates that, in high temperature region, adding hydrazine hydrate into urea
has no effect to remove NO/NOx.
(3) Blended hydrazine hydrate & urea decomposition
Three crystal samples of different spiked ratios (Solution 2, Solution 3 and Solution 4) were
dried for 24hs at 50℃ for TG analysis, the TG curves and NH3 emission are shown in Fig. 5.
150
100
9.1% urea N
16.7% urea N
25% urea N
Single Urea
60
100
NH3/ppm
TG Wt/W0 /%
80
Solution 2
Solution 3
Solution 4
40
50
20
0
0
100
200
300
400
500
600
100
200
300
400
500
600
Temperature/°C
Temperature/°C
(a) TG
(b) NH3 emission
Fig.5 Decomposition of the blended hydrazine & urea and NH3 emssion during the process
From Fig.5(a), it is found that there are three mass loss stages for single urea decomposition,
while four mass loss stages for the blends, this is because that some accompanying reactions such as
evaporation of water and decomposition of hydrazine occur during the blend decomposition. The TG
curves of solution 2 with 9.1% urea N are similar to that of Solution 3 with 16.7% urea N, and that of
Solution 4 with 25% urea N is almost coincided with that of the single urea except the first mass loss
stage. The NH3 emission are shown in Fig.5(b): when the temperature is below 350℃, the NH3
concentration gradually increases with the increase of temperature; when the temperature is great
than 350℃, the NH3 concentration gradually decreases for some components like NH2, H2, and NCO
are formed. With the increase of urea N to the total N, the NH3 concentration rises. It can be found
that some new substance formed for Solution 3 and it can resist NH3 emission during temperature
range of 200~350℃.
When the temperature reaches about 400℃, the decomposition reaction stops. The residues of
different spiked ratios are different from each other. More residues are left when the proportion of
hydrazine is higher. The residues may affect the reduction of NOx. According to Li et.al[20],
Hydrazine hydrate can react with urea to form semicarbazide through reaction (13), and the
derivatives of urea such as isocyanuric acid will enhance the polymerization of hydrazine, and form
some branched polymers such as polyurea polyol if the proportion of hydrazine is high enough[21].
H2NNH2·H2O + NH2CONH2 → H2NCONHNH2 + NH3·H2O
(13)
So it can be determined that a most reasonable substitution ratio is existed between 16.7% urea
N and 25.0% urea N, with this ratio reducing agents will not only save cost but also remain good
effect on denitration at moderate-temperature.
Conclusion
In order to obtain a cheaper hydrazine-based reducing reagent for moderate temperature SNCR
de-NOx, and research the detailed de-NOx mechanism, the efficiency corresponding to SNCR de-NOx
with different reducing agents were studied in a pilot-scale system and the decompositions of urea
and hydrazine hydrate were studied by TG. The main conclusions are as follows:
1) Spiking urea into hydrazine hydrate can broaden the temperature window and shift the
temperature window to the lower side. When the N substitution rate of hydrazine hydrate N by urea N
is 16.7%, the solution keeps a good de-NOx effect in moderate temperature range.
2) NH3 is proved not the significant compound to reduce NOx at moderate temperature in air
condition.
3) TG analysis of crystals resulted from drying of the blended urea and hydrazine solutions
showed that when urea is spiked into hydrazine at a proper N substitution rate, new substance is
formed with ability to resist NH3 emission at temperature range of 200~350℃.
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