Fuel 361 (2024) 130671
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Full Length Article
Theoretical analysis on applying steam reforming to the primary
combustion of the boron-based fuel-rich propellant
Chaolong Li , Zhixun Xia , Likun Ma *, Binbin Chen , Yunchao Feng , Jiarui Zhang ,
Pengnian Yang
National University of Defense Technology, Changsha 410073, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Theoretical analysis
Steam reforming
Primary combustion
Boron-based fuel-rich propellant
The primary combustion products of boron-based fuel-rich propellant contain a significant amount of condensed
boron, which poses challenges for complete burnout of the primary combustion products during the secondary
combustion process. To enhance the ignition and combustion characteristics of these primary combustion
products, a pioneering theoretical analysis on applying steam reforming to the primary combustion of the boronbased fuel-rich propellant was conducted. The results demonstrate that steam reforming improves the ignition
and combustion characteristics of the primary combustion products through three main aspects: optimizing the
composition of combustion products, improving energy distribution between the primary combustion and sec­
ondary combustion process, and elevating combustion temperature, respectively. It theoretically confirms the
feasibility of this innovative combustion organization method for solid rocket scramjet. Furthermore, it has been
found that there is an optimal steam-fuel ratio (the value is 1.0 for B35), which facilitates the conversion of
combustible components in the primary combustion products from a gas–solid state to a pure gaseous phase.
Simultaneously, the temperature of combustion products after steam reforming reaches the highest. Therefore,
theoretically speaking, the steam-fuel ratio of solid rocket scramjet with steam reforming should not exceed the
optimal steam-fuel ratio to ensure better ignition and combustion characteristics of the combustion products
after steam reforming. This analysis provides valuable theoretical support for the design of solid rocket scramjets
with steam reforming.
1. Introduction
Boron exhibits exceptional energetic properties attributed to its high
bulk and mass heat values of 136 kJ/cm3 and 58 MJ/kg, respectively
[1]. Its role as the primary component in fuel-rich propellants signifi­
cantly elevates its heat value [2], thereby theoretically offering immense
potential for achieving superior performance in solid rocket scramjets
fueled by boron-based fuel-rich propellants. However, the challenges of
ignition difficulties and insufficient combustion of boron in the super­
sonic combustor impede its practical engineering applications [3].
The specific combustion process of the boron-based fuel-rich pro­
pellant in the solid rocket scramjet can be divided into two stages: the
primary combustion process occurring in the gas generator and the
secondary combustion process occurring in the supersonic combustor
[4]. The primary combustion products contain a significant amount of
unreacted boron due to the fuel-rich nature of the boron-based
propellant [5]. Current findings indicate that the combustion efficiency
of these primary combustion products in the supersonic combustor is
below 80 %, with even lower efficiency for boron particles [6]. As these
primary combustion products serve as initial inputs for the secondary
combustion process, they play a crucial role in influencing the overall
performance of solid rocket scramjets. Therefore, it is imperative to
conduct a comprehensive study on the primary combustion process of
boron-based fuel-rich propellants.
Wen et al. [7] conducted experimental investigations on the impact
of pressure on the primary combustion products of the boron-based fuelrich propellant. The results show that the pressure in the gas generator
significantly influences the physicochemical properties of primary
combustion products, with higher pressure favoring secondary ignition
and combustion of the primary combustion products, particularly for
boron. Yuan et al. [2] experimentally examined the agglomeration
characteristics of boron particles in the primary combustion process. It
has been found that the severe self-conglomeration of boron particles
* Corresponding author.
E-mail address: malikun@nudt.edu.cn (L. Ma).
https://doi.org/10.1016/j.fuel.2023.130671
Received 4 April 2023; Received in revised form 14 December 2023; Accepted 15 December 2023
Available online 20 December 2023
0016-2361/© 2023 Elsevier Ltd. All rights reserved.
C. Li et al.
Fuel 361 (2024) 130671
steam reforming reaction into the thermal cracking process of hydro­
carbon fuel (kerosene) for scramjet applications, revealing a reduction
in coke formation rate with an optimal concentration of steam. Su et al.
[12] proposed a novel approach combining methane steam reforming
technology and carbon capture to effectively mitigate carbon emissions
during hydrogen production, showcasing its potential for achieving lowcarbon emission levels. Considering the main condensed phase products
of the primary combustion products consist of boron and carbon, the
steam reforming reactions for the boron-based fuel-rich propellant
involve the following two hydrolysis reactions as presented in Eq.(1)
[13] and Eq.(2) [14]. If the two reaction equations were to occur within
the reforming chamber, it would enable the conversion of the boron and
carbon in the primary combustion products into hydrogen and carbon
monoxide. It is well-known that the ignition and combustion charac­
teristics of these gaseous combustible products (hydrogen and carbon
monoxide) are significantly superior to those of condensed boron and
carbon. Therefore, the novel combustion organization method may have
great potential for enhancing the combustion performance of the pri­
mary combustion products in the supersonic combustor.
Nomenclature
Abbreviation
AP
Ammonium perchlorate
KP
Potassium perchlorate
HTPB
Hydroxyl-terminated polybutadiene
b0k
atomic amount of chemical element k in per kilogram
reactants
bk
atomic amount of chemical element k in per kilogram
combustion products
Cp
specific heat at constant pressure, J/(kg⋅K)
La
Lagrange constant
l
number of chemical elements
nj
moles of components j
n
moles of gas phase components
R
gas constant, J/(kg⋅K)
S
entropy, J/K
T
temperature, K
Greek symbols
λ
steam-fuel ratio
μj
chemical potential of components j
Boron Hydrolysis Reaction : 2B(s) + 3H2 O(g)→B2 O3 (s) + 3H2 (g)ΔH
= − 506.6kJ/mol
(1)
Carbon Hydrolysis Reaction : C(s) + H2 O(g)→CO(g) + H2 (g)ΔH
= 131.3kJ/mol
occurred on the burning surface of the propellant. The phenomenon can
increase the radius of the boron, which would affect the combustion rate
of the unreacted boron of the primary combustion products [8]. Xu et al.
[5] further qualitatively and quantitatively analyzed the composition of
condensed and gaseous primary combustion products of boron-based
fuel-rich propellant by experimental methods. The findings indicate
that the main chemical energy of the primary combustion products ex­
ists in the condensed combustible components, concretely including
boron and carbon. Ma et al. [9] conducted a numerical study on the solid
rocket scramjet fueled by boron-based fuel-rich propellant, concluding
that achieving sufficient burnout of condensed boron from primary
combustion products is crucial for overall combustion efficiency. Based
on the above discussions, it can be concluded that current combustion
organization technology faces challenges in achieving sufficient com­
bustion of primary combustion products in the supersonic combustor. To
enhance the ignition and combustion characteristics of the primary
combustion products, we propose for the first time to incorporate steam
reforming technology into the primary combustion process of the boronbased fuel-rich propellant for the solid rocket scramjet. The schematic of
the solid rocket scramjet with steam reforming is presented in Fig. 1. In
comparison to conventional solid rocket scramjet, an extra reforming
chamber is installed between the gas generator and supersonic
combustor to facilitate the reforming reaction between steam and pri­
mary combustion products.
Previously, steam reforming technology has been employed for hy­
drocarbon fuel. Zheng et al. [10] experimentally demonstrated that the
endothermic nature of the steam reforming process significantly en­
hances the heat sink of hydrocarbon fuel. Hou et al. [11] incorporated
(2)
However, there is a lack of relevant research on the applying steam
reforming to the primary combustion process for solid rocket scramjet.
In order to study the feasibility of this innovative combustion organi­
zation method, a preliminary theoretical analysis on incorporating
steam reforming into the primary combustion process of the boronbased fuel-rich propellant was conducted for the first time in this
study, assuming thermochemical equilibrium. Furthermore, the effects
of steam-fuel ratio, steam temperature, and fuel-rich degree of the
boron-based propellant on the primary combustion products were
examined. Ultimately, valuable conclusions derived from the theoretical
analysis were presented to guide designing solid rocket scramjet with
steam reforming.
2. Methodology
In this work, steam reforming is incorporated into the primary
combustion process of the boron-based fuel-rich propellant for the first
time. The main task is to study the feasibility of this innovative com­
bustion organization method from the point of view assuming the
chemical reactions are infinitely fast. Therefore, the particles’ size,
phase transitions [15], and chemical transformations [16] on the par­
ticles’ surface are not taken into account. The mathematical model used
in this paper is based on thermochemical equilibrium [17] instead of
chemical reaction kinetics [18]. The principle of minimum Gibbs free
energy is employed to confirm thermochemical equilibrium. It is
postulated that combustion products consist of NS species, gas phase
products have NG species, and condensed phase products from NG + 1 to
NS species. The governing equations are given as follows:
Fig. 1. Schematic of the solid rocket scramjet with steam reforming.
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C. Li et al.
Fuel 361 (2024) 130671
Table 1
Compositions of the boron-based fuel-rich propellant.
Compositions
Chemical formula
Mass fraction/%
AP
KP
HTPB
Boron
Magnesium
Aluminum
NH4ClO4
KClO4
(C4H6(OH))n
B
Mg
Al
28
4
26
35
3.5
3.5
l
∑
i
i=1
NG
∑
×
j=1
(
NS
∑
akj aij nj πi +
j=NG+1
= b0k − bk +
NG
∑
akj nj μj
RT
j=1
)
NG
∑
(
akj nj Δln(n)+
akj Δnj +
j=1
NG
∑
akj nj Hj0
j=1
RT
)
Δln(T)
dddd(k = 1,⋯,l)
(3)
NG
∑
i=1
Hj0
μj
aij πi +
Δln(T) =
dddd(j = NG + 1, ⋯, NS)
RT
RT
l
NG
∑
∑
i
aij nj πi +
i=1
(
j=1
NG
∑
(
i=1
j=1
NG
∑
aij nj Hj0
j=1
j=1
)
RT
i
i=1
NG
∑
aij nj Sj0
j=1
R
Δln(T)
(5)
(
)
NS
NG
∑
∑
Hj0
nj Hj0
Δnj +
Δln(n)
RT
RT
j=NG+1
j=1
⎡
⎤
( )2
0
0
NG
NG
h0 − h
⎢ ∑ nj Cp,j ∑ nj Hj ⎥
+⎣
+
⎦Δln(T) =
RT
R
R2 T 2
j=1
j=1
πi +
(6)
NG
∑
nj Hj0 μj
j=1
(
RT
)
RT
+
l
∑
NG
∑
nj Hj0
(4)
NG
∑
n j μj
nj +
j=1
i
(
nj − n Δln(n) +
j=1
= n−
l
∑
)
NG
∑
Fig. 2. Mass fractions of the major combustion products with and without
steam reforming (λ = 0.5).
R2 T 2
(
)
NS
NG
∑
∑
Sj0
nj Sj0
πi +
Δnj +
Δln(n)
R
R
j=NG+1
j=1
[
]
0
l
NG
∑
∑
nj Cp,j
nj Hj0 Sj0
s0 − s
+
+
Δln(T) =
2T
R
R
R
j=1
j=1
)
NG
∑
+n−
nj +
j=1
Fig. 3. Energy fractions of combustible products with and without steam
reforming (λ = 0.5).
(7)
NG
∑
nj Sj0 μj
j=1
R2 T
where nj represents the moles of combustion product j, while n de­
notes the moles of gas phase components.πi is equal to the value of
-Lai /RT, where the Lai signifies the Lagrange constant of combustion
product j. l refers to the number of chemical elements. b0k and bk repre­
sent the atomic amount of chemical element k in per kilogram reactants
and combustion products, respectively. H0j stands for the standard mole
Cp0
= a1 T − 2 + a2 T − 1 + a3 + a4 T + a5 T 2 + a6 T 3 + a7 T 4
R
(8)
H0
T
T2
T3
T 4 a8
= − a1 T − 2 + a2 T − 1 ln(T) + a3 + a4 + a5 + a6 + a7 +
2
RT
3
4
5 T
(9)
S0
T− 2
T2
T3
T4
= − a1
− a2 T − 1 + a3 ln(T) + a4 T + a5 + a6 + a7 + a9
RT
2
2
3
4
(10)
3. Results and discussion
enthalpy of component j. S0j represents the standard mole entropy of
C0p,j
component j, and
indicates the specific heat at constant pressure for
component j in the standard state. The μj symbolizes chemical potential
for both gas phase (j = 1 NG) and condensed phase products (j > NG). R
is the universal gas constant, while T denotes the temperature of the
combustion products.
The thermodynamic functions for specific heat C0p , enthalpy H0 , and
3.1. Effect of steam reforming on the primary combustion
The boron-based fuel-rich propellant is a commonly used fuel for
solid rocket scramjet [2]. In this paper, the baseline compositions of the
studied boron-based fuel-rich propellant are presented in Table 1. The
oxidizing ingredients of the fuel-rich propellant include AP (Ammonium
perchlorate) and KP (Potassium perchlorate), while HTPB (Hydroxylterminated polybutadiene) serves as an energetic binder to shape the
grain [20–22]. Boron is the main fuel ingredient, and the baseline pro­
pellant is named B35 in this work due to its 35 % mass fraction of boron.
Metal additives such as Mg and Al are utilized to enhance the
entropy S0 for each component can be expressed as temperature func­
tions using least squares coefficients. The coefficients a1 a9 are readily
available on NIST Chemistry WebBook [19].
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C. Li et al.
Fuel 361 (2024) 130671
Fig. 4. Mass fractions of combustion products for various steam-fuel ratios.
Fig. 5. Energy fractions of combustible products for various steam-fuel ratios.
Fig. 6. Mass fractions of combustion products for various steam temperatures.
Table 2
Mass fractions of condensed phase products and combustion temperature for
various steam-fuel ratios.
Steam-fuel ratio
λ = 0.5
λ = 1.0
λ = 1.5
Mass fractions of condensed phase products
Combustion temperature
0.527
2045 K
0.467
2168 K
0.353
1999 K
Table 3
Mass fractions of condensed phase products and combustion temperature for
various temperature.
combustion performance of boron [23,24]. Fig. 2 shows the mass frac­
tions of the major combustion products with and without steam
reforming, respectively. The steam-fuel ratio λ is defined as the mass
flow rate ratio of the steam to the fuel. In this case, the temperature of
the steam is set to 800 K, which corresponds to the typical temperature
of the scramjet combustor wall[25]. The steam pressure is set to 2 MPa,
which closely approximates the reference pressure in the gas generator
of the solid rocket scramjet [3]. Due to the steam reforming, the mass
fractions of gas phase products increased significantly from 30.24 % to
44.74 %, while the mass fractions of condensed phase products
decreased correspondingly from 69.76 % to 55.26 %. Consequently,
steam reforming leads to an enhancement in the mass fractions of gas
phase products and a reduction in the mass fractions of condensed phase
Steam temperature
500 K
800 K
1000 K
Mass fractions of condensed phase products
Combustion temperature
0.547
2039 K
0.527
2045 K
0.512
2049 K
products.
Additionally, Fig. 3 presents the energy fractions of combustible
products with and without steam reforming, respectively. Steam
reforming significantly enhances the energy fractions of gas phase
components in combustible products, while simultaneously reducing the
energy fractions of the condensed phase components in the combustible
products. Moreover, the temperature of combustion products with steam
reforming is higher (2045 K) compared to those without (2015 K). This
observation suggests that the overall steam reforming process is
exothermic, involving hydrolysis of boron and carbon for hydrogen
generation. It also indicates that a portion of the chemical energy con­
tained in primary combustion products is released in the reforming
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Fuel 361 (2024) 130671
temperature of 800 K. The mass fractions of condensed phase products
decrease as the steam-fuel ratio increases, while the gas phase products
exhibit an opposite trend. The consumption of boron and carbon in the
condensed combustible products is sufficient when the steam-fuel ratio λ
is equal to 1.0, as depicted in Fig. 4(b). Consequently, during the sec­
ondary combustion process, only hydrogen (H2) and carbon monoxide
(CO) remain as combustible products. Simultaneously, no excess steam
is observed in the gas phase product, in which case the steam-fuel ratio λ
is defined as the optimal steam-fuel ratio λopt . Therefore, the optimal
steam-fuel ratio λopt for B35 is 1.0. Fig. 5 displays the energy fractions of
combustible products for various steam-fuel ratios. While the steam-fuel
ratio reaches the optimal steam-fuel ratio λopt , the combustible products
only exist in the gas phase products. This conversion of combustible
components from a gas–solid state to a pure gaseous phase can signifi­
cantly alleviate the challenges associated with heat release for primary
combustion products in supersonic combustors.
Table 2 presents the mass fractions of condensed phase products and
combustion temperature for various steam-fuel ratios. As the steam-fuel
ratio λ increases, there is a gradual decrease in the mass fractions of
condensed phase product, while the combustion temperature initially
increases and then decreases. This is because when the steam-fuel ratio λ
exceeds the optimal steam-fuel ratio λopt , the excess steam cannot un­
dergo steam reforming reactions to release heat and elevate the com­
bustion temperature. Conversely, it would further reduce the
combustion temperature since the temperature of steam is lower than
the temperature of primary combustion products. Theoretically
speaking, the ignition and combustion characteristics of the combustion
products may be compromised when the steam-fuel ratio λ exceeds the
optimal steam-fuel ratio λopt , potentially leading to the failure of solid
rocket scramjet with steam reforming. Therefore, the steam-fuel ratio λ
for the solid rocket scramjet with steam reforming should not surpass the
optimal steam-fuel ratio λopt , which provides theoretical support for the
design of the solid rocket scramjet with steam reforming.
Table 4
Compositions for B25, B35, and B45.
Propellants
B25
B35
B45
AP
KP
HTPB
Boron
Mg
Al
38
4
26
25
3.5
3.5
28
4
26
35
3.5
3.5
18
4
26
45
3.5
3.5
chamber due to the steam reforming process.
In summary, the steam reforming process enhances the ignition and
combustion characteristics of the primary combustion products mainly
through three aspects: optimizing the composition of combustion
products, improving energy distribution between the primary combus­
tion and secondary combustion process, and elevating combustion
temperature, respectively. Firstly, the change in the composition of
combustion products is primarily manifested by an increase in the mass
fractions of hydrogen (H2) and carbon monoxide (CO) and a decrease in
the mass fractions of boron and carbon. It is acknowledged that the
ignition delay time of these gaseous combustible products is signifi­
cantly shorter compared to boron and carbon particles in the supersonic
combustor [26]. Secondly, due to the exothermic nature of the overall
steam reforming process, there is a reduction in chemical energy
required to be released during the secondary combustion process. This
reduction leads to decreased chemical reaction time while maintaining a
constant reaction rate. Finally, the temperature of combustion products
will be elevated through the overall steam reforming process. It can
decrease the activation energy for reactants and increase the chemical
reaction rates according to Arrhenius’s formula. The aforementioned
analysis provides theoretical evidence confirming the feasibility of this
novel combustion organization method for the first time.
3.2. Steam-fuel ratio
3.3. Steam temperature
The steam-fuel ratio is a significant parameter for the design of the
solid rocket scramjet with steam reforming. Fig. 4 presents the mass
fractions of combustion products for various steam-fuel ratios at a steam
The steam is generated by the active cooling cycle through the hightemperature combustor wall during the operation of the solid rocket
Fig. 7. Mass fractions of combustion products with steam reforming for various fuel-rich degrees.
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C. Li et al.
Fuel 361 (2024) 130671
Fig. 8. Energy fractions of combustible products with steam reforming for various fuel-rich degrees.
scramjet with steam reforming. Hence, the steam temperature is also an
important aspect for the design of the solid rocket scramjet with steam
reforming. Fig. 6 presents the mass fractions of combustion products for
various steam temperatures. And the steam-fuel ratio λ is set to 0.5 in
this part. It can be observed from Fig. 6 that an increase in steam tem­
perature leads to higher mass fractions of gas phase products and lower
mass fractions of condensed phase products. Specifically, the mass
fractions of carbon monoxide (CO) and boron (B) increase, while those
of carbon (C) and boron oxide (B2O3) decrease. According to Le Cha­
telier’s principle [27], the chemical reaction should proceed in an
endothermic direction as the steam temperature rises. Therefore, the
boron hydrolysis reaction should proceed in the reverse direction, while
the carbon hydrolysis reaction continues in the forward direction.
Table 3 lists the mass fractions of condensed phase products and
combustion temperature for various steam temperatures. As the steam
temperature increases, the temperature of combustion products in­
creases a little, less than 10 K. It suggests that raising the steam tem­
perature has a limited effect on enhancing the temperature of
combustion products. Based on the assumption of thermochemical
equilibrium, the increase in steam temperature causes the chemical
equilibrium to proceed in an endothermic direction. This direction
promotes the generation of boron, which is challenging to sufficiently
burn in the supersonic combustor. Hence, it is theoretically difficult to
judge whether higher steam temperature results in improved ignition
and combustion characteristics of the primary combustion products.
Based on the above analysis, experimental verification is necessary to
assess the actual influence of steam temperature on the steam reforming
process for the primary combustion products.
3.4. Fuel-rich degree
The fuel-rich degree of the solid propellant is closely correlated with
the characteristics of its primary combustion products. Table 4 gives
three compositions of the boron-based propellant with varying fuel-rich
degrees, achieved by adjusting the mass fraction of oxidant AP and fuel
B. Moreover, the fuel-rich degree of the propellant increases with an
increase in the mass fraction of boron. Fig. 7 shows the mass fractions of
combustion products for various fuel-rich degrees with a steam-fuel
ratio of 0.5 and steam temperature of 800 K. It demonstrates that the
mass fractions of condensed phase products increase as the fuel-rich
degree of the solid propellant increases. This can be explained that the
solid propellant with a higher fuel-rich degree contains a lower pro­
portion of oxidant, which causes fewer fuel ingredients such as boron to
be consumed in the primary combustion process. Fig. 8 displays the
energy fractions of combustible products for various fuel-rich degrees.
As the fuel-rich degree of the solid propellant increases, the fractions of
combustible condensed products including boron and carbon increase.
However, this phenomenon hinders achieving highly efficient combus­
tion of the primary combustion products in the secondary combustion
process. Consequently, employing a higher fuel-rich degree of propel­
lant exacerbates the challenge of heat release for primary combustion
products in the supersonic combustor.
Table 5 presents the heat values and combustion temperature for the
solid propellant with various fuel-rich degrees. The increasing fuel-rich
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C. Li et al.
Fuel 361 (2024) 130671
understand the impact of steam reforming on primary combustion
processes, which is the main focus of the following study.
Table 5
Heat values and combustion temperature for the solid propellant with various
fuel-rich degrees.
Fuel-rich degree
B25
B35
B45
Heat value
Combustion temperature
28.7 MJ/kg
2067 K
33.5 MJ/kg
2045 K
39.3 MJ/kg
2014 K
CRediT authorship contribution statement
Chaolong Li: Writing – original draft, Software, Methodology,
Formal analysis, Data curation, Conceptualization. Zhixun Xia: Writing
– review & editing, Supervision. Likun Ma: Writing – review & editing,
Validation, Supervision, Funding acquisition. Binbin Chen: Investiga­
tion. Yunchao Feng: Data curation. Jiarui Zhang: Data curation.
Pengnian Yang: Data curation.
degrees can raise the heat value of the solid propellant, thereby
improving the performance potential of the solid rocket scramjet under
sufficient combustion conditions. Nevertheless, a higher fuel-rich degree
of propellant results in primary combustion products containing more
combustible condensed substances, like boron and carbon. At the same
time, its combustion temperature is lower. These factors are not
conducive to the ignition and combustion of combustion products after
steam reforming in the supersonic combustor. On the other hand, a
higher fuel-rich degree necessitates a larger optimal steam-fuel ratio,
which occupies more space within hypersonic vehicles for the water
tank. Consequently, this increases vehicle weight and reduces available
space for effective payload placement, adversely impacting overall hy­
personic vehicle design considerations. Therefore, comprehensive
consideration should be given to both the heat value of solid propellant
and the steam-fuel ratio when designing a fuel-rich degree.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgement
4. Conclusions
This work was supported by the National Natural Science Foundation
of China (Grant No. 52006240, No. 12272409).
In this paper, a novel theoretical analysis on applying steam
reforming to the primary combustion of the boron-based fuel-rich pro­
pellant was carried out. The influence of steam reforming on the primary
combustion of the boron-based fuel-rich propellant was studied. The
findings indicate that the steam reforming process significantly en­
hances the ignition and combustion characteristics of the primary
combustion products, mainly through the three aspects: optimizing the
composition of combustion products, improving energy distribution
between the primary combustion and secondary combustion process,
and elevating combustion temperature, respectively. It theoretically
validates the feasibility of this innovative combustion organization
method.
Moreover, the impacts of steam-fuel ratio, steam temperature, and
fuel-rich degree on the primary combustion products have been pre­
sented. As the steam-fuel ratio increases, there is a gradual decrease in
the mass fractions of the condensed phase product decrease gradually,
while the combustion temperature initially increases first and then de­
creases. It was found that the optimal steam-fuel ratio (the value is 1.0
for B35) can ensure better ignition and combustion characteristics of the
combustion products after steam reforming. On the one hand, it facili­
tates the conversion of combustible components in primary combustion
products from a gas–solid state to a pure gaseous phase. Simultaneously,
the temperature of combustion products after steam reforming reaches
the highest. Raising the steam temperature has a limited effect on
increasing the temperature of combustion products. Due to the
assumption of thermochemical equilibrium, the increase in steam tem­
perature causes the chemical equilibrium to proceed in an endothermic
direction. This direction promotes the generation of boron, which is
challenging to sufficiently burn in the secondary combustion process.
The heat value and optimum steam-fuel ratio of the solid propellant both
are higher when using a higher fuel-rich degree of propellant.
Comprehensive consideration should be given to both the heat value of
solid propellant and the steam-fuel ratio when designing a fuel-rich
degree.
Although the theoretical investigation based on thermochemical
equilibrium can obtain the compositions of combustion products under
any conditions, the results drawn by this method have a deviation from
the actual engineering results since the thermochemical equilibrium
may not be realized due to the limited residence time in the reforming
chamber. Therefore, experimental verification is necessary to further
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