DOI: 10.3303/CET2291055 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 9 January 2022; Revised: 30 March 2022; Accepted: 4 May 2022 
Please cite this article as: Campari A., Pio G., Ustolin F., Paltrinieri N., Salzano E., 2022, Design and Optimization of an Emergency Auto-
thermal Burner for Liquid Hydrogen, Chemical Engineering Transactions, 91, 325-330  DOI:10.3303/CET2291055 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 91, 2022 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Valerio Cozzani, Bruno Fabiano, Genserik Reniers 

Copyright © 2022, AIDIC Servizi S.r.l. 
ISBN 978-88-95608-89-1; ISSN 2283-9216 

Design and Optimization of an Emergency Auto-thermal 

Burner for Liquid Hydrogen 

Alessandro Camparia,b, Gianmaria Piob, Federico Ustolina, Nicola Paltrinieria, 

Ernesto Salzanob,* 

a Department of Mechanical and Industrial Engineering, NTNU, Trondheim, Norway 
b Dipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali, Università di Bologna, Bologna, Italy 

 ernesto.salzano@unibo.it 

The adoption of hydrogen has been largely indicated as a feasible solution to support society in achieving its 

ambitious targets for a low-carbon future. The increased knowledge in liquefaction techniques has encouraged 

the study of technological solutions based on liquid hydrogen (LH2). Moreover, handling and distribution under 

cryogenic conditions represent attractive options due to the elevated energy density of LH2. Despite these 

advantages, the bottleneck for its widespread adoption is represented by safety aspects. Considering that an 

LH2 tank truck has a probability of suffering a car accident like all other road vehicles, an emergency auto-

thermal burner has been designed in this work. This safety system has the purpose to dispose of the content of 

a tank truck to avoid the loss of containment. The disposal process includes the vaporization and pre-heating of 

the LH2, the mixing with ambient air, and its combustion. This device is completely self-supporting since the 

heat required to vaporize the LH2 is entirely provided by the combustion of the fuel itself. Firstly, the equation of 

energy balance around the burner was numerically solved to estimate the temperature of flue gases. Then, 

inner and outer heat transfer coefficients were determined for each section of the coiled-tube heat exchanger. 

Finally, the heat transfer surface was calculated. The spontaneous conversion between two spin isomers of 

hydrogen was considered. From the perspective of the heating process, the enthalpy of conversion represents 

an additional energy request. If the decrease of para-hydrogen fraction was neglected, the length of the heat 

exchanger would be significantly underestimated. Based on the obtained results, it is possible to conclude that 

the designed device can be transported on-site and started up easily, making it suitable for emergency response. 

1. Introduction 

The rising demand for clean and sustainable energy sources all over the world is stimulating research about 

alternative fuels for power generation, aviation, and road transport. The adoption of hydrogen has been largely 

indicated as a possible solution for social and environmental issues (IEA, 2019). The simultaneously increased 

knowledge in cryogenic liquefaction techniques and the development of proper large-scale tanks have recently 

encouraged the study of technological solutions based on liquid hydrogen (Valenti, 2016). The main advantage 

of storing by liquefaction is the density of LH2, which is around 70.9 kg/m3. On a mass basis, hydrogen shows 

the highest LHV, equal to 119.93 MJ/kg. In contrast, liquid hydrogen is characterized by a relatively low heating 

value on a volume basis, i.e., 8.49 GJ/m3 (NIST, 2021). Cryogenic liquid fuels can be stored and transported in 

double-walled vessels. The purpose of these special containers is to protect the cryogen from the conductive, 

convective, and radiative heat transfer from the outside. An efficient insulation system is crucial for storing and 

transporting LH2 since makes it possible to minimize the boil-off. Vacuum-insulated vessels are the great 

majority of containers for cryogenic fuels. In particular, multi-layer insulation (MLI) is used for its extremely low 

thermal conductivity, radiation heat transfer, and density (Valenti, 2016). On the other hand, in case of loss of 

vacuum, a near-catastrophic failure of the containment system may occur (Molkov, 2012). Super-insulated 

tanker trucks have a capacity ranging from 20 to 50 m3 and can store up to 4,000 kg of LH2 at a pressure that 

usually ranges from 6 to 10 bar (Usolin et al., 2019). These trucks are commonly used for journeys of up to 

325



4,000 km, but they are not suitable for transport above this distance as the hydrogen heats up and the pressure 

inside the tank tends to rise considerably (Decker, 2016). Accidental leaks of liquid hydrogen might occur after 

severe car accidents. Holes and fractures in the containment system may release large amounts of cryogenic 

fuel in the environment, that rapidly vaporizes, mixes with air, and forms a flammable mixture. Several categories 

of fire and explosion events, such as jet fires, pool fires, flash fires, vapor cloud explosions (VCE), and boiling 

liquid expanding vapor explosion (BLEVE), may occur in presence of ignition sources (Ustolin et al., 2019). 

Moreover, a loss of integrity of the super-insulated vessel may expose personnel to direct contact with an ultra-

low temperature fluid. Besides the extreme temperature, the LH2 has low viscosity, which enables it to penetrate 

through porous material very quickly. If LH2 gets in direct contact with the human skin, can cause severe 

damages and potentially lethal injuries (Molkov, 2012). Considering that an LH2 tank truck has a probability of 

suffering a car accident comparable with that of all other road vehicles, it is of the utmost importance to design 

safety devices able to guarantee an acceptable level of security during transport. The emergency auto-thermal 

burner designed in this work has the purpose to dispose of the content of a large-size tank truck in case of 

accidents that may lead to failure of the containment system and accidental release of LH2 in the environment.  

2. Emergency auto-thermal burner 

The emergency system is mainly composed of three coiled-tube heat exchangers: the economizer, the 

vaporizer, and the superheater. The chemical reactor upstream of the superheater represents the transition (i.e., 

the para-ortho conversion) taking place within the pipe. The proper amount of air is fed to the premix burner by 

a fan. The by-pass line allows overriding the economizer if there is no liquid phase left within the tank. After a 

car accident, if the containment system does not show leaks or fractures, the emergency burner should be 

connected to the vessel through a flange. The liquid hydrogen spilled from the vessel passes through the 

economizer, the vaporizer, and the superheater; all these heat exchangers are tube-shaped and directly flame 

heated. When the LH2 has turned to gaseous hydrogen, its temperature rises sharply, then its chemical 

composition tends to correspondingly change (due to the para-to-ortho conversion); in particular, the ortho-para 

ratio rises with increasing temperature (Milenko et al., 1997). It is noteworthy that this emergency device is 

completely self-supporting. The heat required to vaporize the LH2 is entirely provided by the combustion of the 

fuel itself. A simplified process flow diagram of this safety device is represented in Figure 1 below. 

 

Figure 1: Process flow diagram of the emergency burner for liquid hydrogen 

3. Methodology 

Information about the equivalence ratio of the flammable mixture, the temperature of exhaust gases, the heat 

transfer coefficients, and the para-to-ortho conversion are provided in the following subsections. 

3.1 Assumptions 

The technical characteristics of the super-insulated truck trailer have been hypothesized based on the projects 

of the major companies which operate in the field of cryo-liquefied gases (Decker, 2016). The volume of the 

vessel is the maximum currently being used for road transport of liquefied gases. The main technical 

specifications of the LH2 tank truck are summarized in Table 1. 

326



Table 1: Technical specifications of the LH2 truck trailer (Decker, 2016) 

 Symbols Set values 

Volume of the vessel (m3) 𝑉𝑡𝑎𝑛𝑘 50
 

Operating pressure (bar) 𝑝𝐿𝐻2 8 

Operating temperature (°C) 𝑇𝐿𝐻2 -253 

Mass of LH2 (kg/s) 𝑚𝐿𝐻2 3,51 

Maximum time for discharge (h) 𝑡𝑚𝑎𝑥  3 

Hydrogen has a laminar burning velocity one order of magnitude higher than conventional fuels’ one (Pio and 

Salzano, 2018). To minimize the flame instability, it is important to lower the burning velocity of hydrogen. Since 

the purpose of the emergency burner is to dispose of the largest amount of fuel in the shortest possible time, a 

feasible solution is to burn a very rich air-fuel mixture. If the equivalence ratio of a hydrogen-air mixture ranges 

from 7 to 8, the burning velocity is of the same order of magnitude as that of a methane-air stoichiometric 

mixture. In fact, at 0 °C a hydrogen-air mixture with an equivalence ratio of 7, and a methane-air mixture with 

an equivalence ratio of 1.1 have laminar burning velocities equal to 50 cm/s and 35.5 cm/s, respectively (Pio 

and Salzano, 2018). 

3.2 Design of the burner 

Considering a rich hydrogen-air mixture (𝜑 = 7), the major components of flue gases are nitrogen, hydrogen, 

and water. The energy balance around the burner has the following expression: 

𝜀 �̇�𝐻2 𝐿𝐻𝑉 − �̇�𝑟𝑎𝑑 − �̇�𝑔 ∫ 𝑐𝑝 𝑔 (𝑇) 𝑑𝑇 + �̇�𝐻2 ∫ 𝑐𝑝 𝐻2 (𝑇) 𝑑𝑇 + �̇�𝑎 ∫ 𝑐𝑝 𝑎 (𝑇)  𝑑𝑇 = 0
𝑇𝑎

𝑇0

𝑇𝐻2

𝑇0

𝑇𝑔

𝑇0

 (1) 

where 𝜀 is the combustion efficiency, �̇� the mass flow (kg/s), 𝐿𝐻𝑉 the lower heating value (kJ/kg), �̇�𝑟𝑎𝑑 the 

radiative losses (kW), 𝑐𝑝  the heat capacity (kJ/kg K); subscript 𝐻2 represents hydrogen, 𝑔 the exhaust gases, 

and 𝑎 the ambient air. The heat capacity of a gas can be expressed through a polynomial function of the 

temperature (Perry et al., 1997). The numerical coefficients are supplied in the Reaction Mechanism Generator 

(RMG) database (Green and West, 2021). Given the coefficients for each chemical species, the polynomial 

function can be easily integrated. The unknown exhaust gases temperature 𝑇𝑔 is calculated through Newton-

Raphson’s algorithm in MATLAB. Since the energy required to preheat the liquid hydrogen, vaporize it, and 

superheat the GH2 is entirely provided by the enthalpy of combustion of the fuel itself, the flue gases are cooled 

by the hydrogen which flows into the tube. The heat exchanged is given by Eq. 2: 

�̇�𝐻2 [∫ 𝑐𝐿𝐻2 (𝑇) 𝑑𝑇 + ∆𝐻𝑒𝑣𝑎 𝐻2 (𝑇, 𝑝) + ∫ 𝑐𝑝 𝐻2 (𝑇) 𝑑𝑇
𝑇𝐻2

𝑇𝐻2 𝑒𝑣𝑎

𝑇𝐻2 𝑒𝑣𝑎

𝑇𝐿𝐻2

] = −�̇�𝑔 ∫ 𝑐𝑝 𝑔 (𝑇) 𝑑𝑇
𝑇𝑔 2

𝑇𝑔

 (2) 

where ∆𝐻𝑒𝑣𝑎 is the latent heat of vaporization (kJ/kg); 𝑇𝐿𝐻2 and 𝑇𝐻2 𝑒𝑣𝑎 are the storage temperatures of liquid 

hydrogen and its boiling point (°C), respectively; 𝑇𝑔 and 𝑇𝑔 2 are the temperatures of exhaust gases before and 

after heat transfer (°C), respectively. Exhaust gases move vertically thanks to the natural draft; they transfer 

heat to the hydrogen, which flows within the coiled tube. The total heat transferred is given by three additive 

terms: the thermal powers exchanged within the economizer, the vaporizer, and the superheater. 

Table 2: Transferred thermal power within each section of coiled-tube heat exchanger 

Heat exchanger Transferred thermal power  

Economizer 
�̇�𝑒𝑐𝑜 = 𝑈𝑒𝑐𝑜  𝐴𝑒𝑐𝑜  (𝑇𝑔 − 𝑇𝐿𝐻2 ) = �̇�𝐻2 ∫ 𝑐𝐿𝐻2 (𝑇) 𝑑𝑇

𝑇𝐻2 𝑒𝑣𝑎

𝑇𝐿𝐻2

 
(3) 

Vaporizer �̇�𝑣𝑎𝑝 = 𝑈𝑣𝑎𝑝 𝐴𝑣𝑎𝑝 (𝑇𝑔 − 𝑇𝐻2 𝑒𝑣𝑎 ) = �̇�𝐻2  ∆𝐻𝑒𝑣𝑎 𝐻2 (𝑇, 𝑝) (4) 

Superheater 
�̇�𝑠𝑠ℎ = 𝑈𝑠𝑠ℎ  𝐴𝑠𝑠ℎ  (𝑇𝑔 − 𝑇𝐻2 ) = �̇�𝐻2 ∫ 𝑐𝑝 𝐻2 (𝑇) 𝑑𝑇

𝑇𝐻2

𝑇𝐻2 𝑒𝑣𝑎

 
 

(5) 

where �̇� is the thermal power transferred (kW), 𝑈 is the overall heat transfer coefficient (kW/m2 K) and 𝐴 is the 

heat transfer surface (m2). Both the heat transfer surface and the overall heat transfer coefficient are unknown. 

Then, inner and outer heat transfer coefficients should be determined for each heat exchanger, depending on 

the flow regime within the pipe. 

327



Since the exhaust gases are a mixture, the thermophysical properties are calculated as a weighted average of 

the properties of pure compounds. The coil can be considered a circular duct with high roughness in which 

exhaust gases flow in a turbulent regime. The outer heat transfer coefficient is calculated through the correlation 

developed by Martinelli (1947), while the Darcy friction factor is determined by the correlation proposed by Von 

Kármán (1939): 

ℎ𝑒𝑥𝑡 =
𝑅𝑒 𝑃𝑟 [(3.36 − 1.767 ln(𝜀 𝐷𝑐𝑜𝑖𝑙⁄ ))

−2 2⁄ ]0.5

5 {𝑃𝑟 + ln(1 + 5 𝑃𝑟) + 0.5 ln
𝑅𝑒 [(3.36 − 1.767 ln(𝜀 𝐷𝑐𝑜𝑖𝑙⁄ ))

−2 2⁄ ]0.5

60
}

∙
𝑘𝑔

𝐷𝑐𝑜𝑖𝑙
 

(6) 

where 𝑅𝑒 and 𝑃𝑟 are Reynolds and Prandtl numbers, respectively, 𝜀 is the roughness of the duct (µm), 𝐷𝑐𝑜𝑖𝑙  the 

coil diameter (m), and 𝑘𝑔 the thermal conductivity of exhaust gases (W/m K). 

Liquid hydrogen flowing along the economizer is a sub-cooled liquid. The heat transfer coefficient for a fully 

developed turbulent flow within the pipe can be estimated by the following correlation for heating (Dittus and 

Boelter, 1985): 

ℎ𝑒𝑐𝑜 = 0.023 𝑅𝑒
0.8𝑃𝑟 0.4 ∙

𝑘𝐿𝐻2
𝐷

 (7) 

where 𝐷 is the pipe diameter (m) and 𝑘𝐿𝐻2  the thermal conductivity of liquid hydrogen (W/m K). 

The hydrogen enters the vaporizer as a saturated liquid. Assuming that the convective and nucleate boiling 

contributions are additive, the following correlation (Chen, 1966) can be used to determine the heat transfer 

coefficient: 

ℎ𝑣𝑎𝑝 =
1

1 + 2.53 ∙ 10−6[𝑅𝑒𝐿  2.35 (𝑋𝑡𝑡
−1 + 0.213)0.736]1.17

∙  ℎ𝑛𝑏 + 2.35 (𝑋𝑡𝑡
−1 + 0.213)0.736  ∙ ℎ𝑒𝑐𝑜 (8) 

where 𝑅𝑒𝐿 is the Reynolds number for liquid-phase only, 𝑋𝑡𝑡 the Lockhart-Martinelli parameter, and ℎ𝑛𝑏 the 

nucleate boiling heat transfer coefficient W/m2 K), calculated through the correlation proposed by Forster and 

Zuber (1955). 

The sharp increase in hydrogen temperature taking place within the superheater leads to a variation of its 

thermophysical properties. In addition, it is important to consider the variable concentration of para- and ortho-

hydrogen along the tube. All the thermophysical properties can be determined as an average of the properties 

of the pure fluids, weighted on the ortho-hydrogen fraction. The heat transfer coefficient is calculated by the 

correlation proposed by Gnielinsky (1976), while the friction factor is calculated by the Colebrook one (Brkić, 

2011): 

ℎ𝑠𝑠ℎ =  
[(0.782 ln(𝑅𝑒) − 1.51)−2 8⁄ ] (𝑅𝑒 − 1000) 𝑃𝑟

1 + 12.7 √(0.782 ln(𝑅𝑒) − 1.51)−2 8⁄  (𝑃𝑟 2/3 − 1)
∙

𝑘𝐻2
𝐷

 (9) 

The equilibrium concentration of para- and ortho-hydrogen is a function of the temperature: at the liquefaction 

temperature, almost all the hydrogen is in para-form, while at the ambient temperature it shows a 3:1 ortho-to-

para ratio. Therefore, a spontaneous para-ortho conversion occurs along the pipe. From the perspective of the 

heating process, the enthalpy of conversion of this endothermic reaction is an additional heating load. The 

conversion rate is calculated through a second-order rate equation (Milenko et al., 1997): 

𝑑𝑐𝑜𝐻2
𝑑𝑡

= 𝑘𝑝→𝑜 ∙ 𝑐𝑜𝐻2 ∙ (1 − 𝑐𝑜𝐻2 ) − 𝑘𝑜→𝑝 ∙ 𝑐𝑜𝐻2
2  (10) 

where 𝑘𝑝→𝑜 and 𝑘o→𝑝 are the conversion constants of the direct and reverse reaction, respectively, and 𝑐𝑜𝐻2  is 

the concentration of ortho-hydrogen. 

The conversion constant of the ortho-para transformation is given by (Milenko et al., 1997): 

𝑘𝑜→𝑝 = (18.2 + 1.6) ∙ 𝑇
0.56±0.02 ∙ 𝜌 + 5 ∙ 104 ∙ [0.77 + 0.03 + (921 + 91) ∙ 𝑇 −2.5±0.2] ∙ 𝜌3.6 (11) 

where 𝑇 is the hydrogen temperature (K) and 𝜌 is the density (g/cm3). 

The difference in hydrogen internal energy between the inlet and outlet of the pipe shall be equal to the heat 

transferred from exhaust gases to the cryogenic fuel. The heat transfer surface is unknown. Setting the diameter 

of the pipe, the length is calculated through the following differential equation: 

𝑑 𝑙𝑝𝑖𝑝𝑒

𝑑 𝑇
=

2

𝜋 𝐷
∙

�̇�𝐻2 𝑐𝑝 𝐻2 (𝑇)

𝑈(𝑇)
∙

1

𝑇𝑔 − 𝑇
 (12) 

328



This equation can be used for the stretches of pipe where sensible heat is transferred. However, hydrogen does 

not vary its temperature into the vaporizer but gradually increases the vapor fraction. Hence, Eq. 13 can be 

integrated with respect to the variable 𝑥, i.e., the vapor fraction (kgvap/kg): 

𝑑 𝑙𝑝𝑖𝑝𝑒

𝑑 𝑥
=

2

𝜋 𝐷
∙

�̇�𝐻2 ∆𝐻𝑒𝑣𝑎 𝐻2
𝑈(𝑥)

∙
1

𝑇𝑔 − 𝑇𝐻2 𝑒𝑣𝑎
 (13) 

The equations above were solved through the Runge-Kutta fourth-order method. 

4. Results and discussion 

Temperatures of combustion products before and after heat transfer with cryogenic hydrogen are represented 

as functions of equivalence ratio in Figure 2. 

 
Figure 2: Temperature of exhaust gases before and after heat transfer as functions of equivalence ratio 

For a stoichiometric mixture, the calculated adiabatic flame temperature is 2254 °C; this result matches with 

experimental data for the combustion of hydrogen in air (Molkov, 2012). The decrease in temperature of flue 

gases after heat transfer with hydrogen becomes more significant for the higher equivalence ratios. In fact, the 

greater 𝜑 is, the greater becomes the flow rate of hydrogen that must be heated up to near-ambient temperature. 

Radiative losses tend to reduce the temperature of exhaust gases, consequently increasing the total length of 

the pipe. In Figure 3 (a) the heat transfer coefficient within the vaporizer is represented as a function of the vapor 

fraction, while Figure 3 (b) shows the heat transfer coefficient into the superheater as a function of temperature. 

 
Figure 3: Inner heat transfer coefficients within the vaporizer (a) and the superheater (b) 

At the evaporation temperature, the boiling regime onsets within the vaporizer, and the heat transfer coefficient 

rises sharply. As the vapor fraction increases, the heat transferred tends to enhance gradually and then falls 

abruptly when the boiling regime ends. During the vaporization, the temperature difference between the pipe 

wall and the boiling hydrogen remains almost constant (approximately 6.5 °C); the resulting wall temperature is 

ultra-low since the inner heat transfer coefficient is dramatically greater than the outer. The gaseous hydrogen 

temperature varies from −250.9 °C to 0 °C through the superheater. All the thermophysical properties of the 

fluid vary sharply with temperature; moreover, its chemical composition is not constant since the fraction of 

ortho-hydrogen increases as the fuel approaches the outlet of the pipe. The minimum heat transfer coefficient 

can be observed for a temperature around −240 °C; within the range −240 ÷ −100 °C, it rises to 2530 W/m2 K. 

329



For temperatures higher than −100 °C, the heat transfer coefficient remains approximately constant, since the 

decreases in density, heat capacity, and dynamic viscosity are counterbalanced by the sharp increases in fluid 

speed and thermal conductivity. In equilibrium conditions, at 0 °C the fraction of ortho-hydrogen would be 

approximately 75 %. Nevertheless, a complete para-to-ortho conversion cannot occur because the residence 

time of hydrogen at high temperature is too short to reach the equilibrium. Eq. (7), solved through the Runge-

Kutta method, shows that the maximum ortho-hydrogen fraction is slightly lower than 50 %. Setting the diameter 

of the coil and the pipe equal to 1 m and 3.8 cm, respectively, the characteristic dimensions of the emergency 

auto-thermal burner are summarized in Table 3:  

Table 3: Technical specifications of the emergency burner 

 Symbols Economizer Vaporizer Superheater 

Mass flow rate (kg/s) �̇�𝐻2  0.1625 0.162 0.1625 

Operating pressure (bar) 𝑝 1.75 1.74 1.70 

Temperature difference (°C) ∆𝑇 4.1 - 250.9 

Pipe length (m) 𝑙𝑝 0.85 13.2 104.5 

Coil height (cm) 𝐻𝑐𝑜𝑖𝑙  3.8 17.6 140 

5. Conclusions 

An emergency system capable of safely disposing of the LH2 tank truck content was designed. This device is 

completely self-supporting, lightweight, and does not require any heat transfer fluid; it is specifically designed to 

avoid a near-catastrophic failure of the containment system in case of a car accident. Given the high flow rate 

of hydrogen to burn in a limited time, two burners can be connected in parallel to speed up the process. This 

allows using small-sized heat exchangers and fans, making also possible a modular control of the process. The 

spontaneous para-to-ortho conversion was considered. The results show that, if the decrease of para-hydrogen 

fraction was neglected, the length of the superheater’s pipe would be underestimated by almost 5 m. The main 

disadvantage of this device is the difficulty to bypass the economizer and the vaporizer when remains gaseous 

fuel only within the tank; in the final stage of the discharge process, the emergency burner can operate in off-

design conditions. It is possible to conclude that the designed device can be transported on-site and started up 

easily, making it suitable for emergency response in case LH2 is involved. 

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	Design and Optimization of an Emergency Auto-thermal Burner for Liquid Hydrogen