INDIAN INSTITUTE OF SPACE SCIENCE AND

TECHNOLOGY, THIRUVANANTHAPURAM

A STUDY REPORT ON COKING OF KEROSENE

Submitted by

Rahul Anand

Rahul_anand_gunjan@yahoo.com

In

4th

Semester, BTECH-AEROSPACE ENGINEERING

In

Liquid Propulsion System Center (LPSC)

Trivandrum

December-January, 2009

ABSTRACT

Kerosene is used as a fuel in semi cryogenic engines which also serves the purpose of coolant in

regenerative passage. At high temperatures, kerosene gives carbonaceous deposits commonly known as

coke. Coking reduces the heat transfer across the chamber wall of the engine as it sticks to the inner

walls of the passage, creating an insulating layer between the coolant and the chamber wall. Studies on

coking of hydrocarbon rocket fuel have been carried out worldwide but the mechanism of its formation

is still uncertain. In this report, we discuss the various ways in which coking can occur and the measures

which can be taken to suppress coking.

INTRODUCTION

Semi cryogenic liquid rocket engines use

kerosene as the fuel along with liquid oxygen as

oxidizer. They have the advantage of being

relatively low cost, creating low pollution,

reusable and having high performance. Safety

and cost factors associated with the storage and

handling of kerosene also gives it an edge over

liquid hydrogen.

Kerosene is used as regenerative coolant in the

semi cryogenic engine to keep the chamber wall

material at safe operating temperatures. Prior to

combustion, fuel is circulated through channels

in the chamber wall to carry the heat away.

Engine performance increases by operating at

high chamber pressures, which results in

proportionally higher heat fluxes to the chamber

walls. As the coolant travels along the

regenerative passage, its temperature increases.

At elevated temperatures, hydrocarbon fuels

can decompose and leave behind solid deposits

on the wetted surfaces in a process called

‘coking’. These deposits decrease the

effectiveness of cooling by forming an

insulating layer over the inner wall of the pipe.

They act as a thermal barrier, which increases

the wall temperature of the combustion

chamber and can eventually cause material

failure. Coking is a major challenge associated

with the use of kerosene as fuel in aerospace

applications, and we will discuss its formation

and ways to reduce it in this paper.

Coke is a carbonaceous substance formed from

kerosene in the flow passage when the

temperature goes beyond a certain limit, known

as the ‘coking limit’. This temperature rise has

to be controlled so as to eliminate the coke

formation in the regenerative passage. The

thermal stability of kerosene depends upon

many parameters like its chemical composition,

amount of non hydrocarbon compounds like

sulphur, nitrogen and oxygen present in

kerosene, material of the piping, flow

conditions and residence time of the kerosene in

the passage. The deposition starts at around

NOMENCLATURE

SEM- Scanning Electron Microscopy

GC- Gas Chromatography

MS- Mass Spectroscopy

PAH- polyaromatic hydrocarbons

Isp- Specific Impulse

100°C and continues to increase as the

temperature increases as a result of pyrolytic

reactions.

The sulphur and the oxygen molecules present

in the fuel can facilitate coking. Therefore,

studying the mechanism of formation of coke

becomes essential to deduce techniques for

suppressing it. There are many models for the

coking mechanism but it varies with the

composition of the kerosene and system

parameters.

OBSERVATIONS ON COKE

FORMATION

1. Depositions from fuels at high

temperatures is the agglutination of

carbonaceous solid pellets cohered by

colloid material [1]

. It occurs mainly due

to the agglomeration of oxidative non

hydrocarbon chemicals in the kerosene.

This is inferred by studying the

deposits, which show that sulphur,

nitrogen and oxygen content of the

deposits is always higher than the

original fuel. These impurities can react

with thermally generated free radicals

during the course of the reaction to

form stable solids. Studies conducted in

this field have shown that removing

these impurities improve the thermal

stability of kerosene.

2. It has been found that the rate of

deposition increases if copper is used as

the wall material. The walls start to

degrade with the deposition. This is

because of the reaction with the sulphur

present in the kerosene, which leads to

the formation of brittle copper sulphide.

3. Kerosene is a mixture of more than 100

hydrocarbons, with n-dodecane and its

derivatives being the prominent

components. Sulphur is present in trace

amounts (<30ppm) along with

aromatic(5%), olefins, dienes and

naphthenes. Coke can be formed in

different ways from all these

components at different temperatures.

4. At low temperatures (~200°C), coking

occurs mainly due to rearrangement

and condensation. At high temperatures

(>350°C), Coke formation involves

dehydrogenation, cyclisation,

isomerization and hydrogen transfer in

addition to condensation.

5. The mechanism for coke formation is a

free radical consecutive reaction:

aliphatic-polycyclic-resins-asphaltenes-

coke. Isolation of the intermediate has

shown a continuous increase in the

molecular weight, degree of aromaticity

and C:H ratio.

6. Asphaltenes are complex molecules,

believed to consist of associated system

of polyaromatic sheets, bearing alkyl

side chains. They are highly polar and

surface active. Preheating of fuel prior

to their burning encourages

precipitation of asphaltenes, which

ultimately break down to give coke.

Asphaltene deposition is the

consequence of the thermal instability

of kerosene. Kerosene is thought to be a

colloidal system and asphaltenes are the

dispersed phase. They are stabilized by

resins, formed as intermediates during

the course of reaction.

Fig: Coke obtained from asphaltene on heating

kerosene.

7. The SEM results of the copper sample

after coking showed heterogeneous

deposition. The maximum deposits

were found at the middle portion,

which is the hottest part. This can be

explained by the adsorption properties

of the copper which acts as a catalyst.

The coke formed in this region is a

mixture of pyrolytic and asphaltic coke.

8. Formation of such huge rings from

condensation of long chain paraffins

and aromatics can be explained by the

Diels- Alder reaction, shown by the

dienes present in the kerosene. [9]

These

dienes on cycloadditions can increase

the size of the rings considerably. It

may be noted that the rate of reaction

increases with temperature.

Fig: butadiene undergoing Diels-Alder reaction

with ethane.

RECOMMENDED ACTIONS

TO CONTROL COKING

There are a few measures which can be

adopted to suppress coking. They are as

follows-:

(i) By plating the inner surface of the

regenerative coolant passage with some

inert material would affect the rate of

coking, as copper acts as a catalyst for

pyrolysis of the hydrocarbons, which

ultimately lead to the formation of coke.

Some eligible materials are Nickel, Gold,

Silver, Zirconium or some other noble

metals.

(ii) By increasing the flow rate, the

residence time of the fuel in the

regenerative passage would decrease and

hence, the deposition may decrease.

(iii) By distillation of the kerosene fuel by

special methods like Deer Distillation[1]

to

remove suspended impurities. The oxidative

coke, formed during the initial stages of

heating is mainly due to the presence of

oxygen, sulphur and nitrogen atoms in the

fuel.

(iv) Precooling the fuel before it is sent to

the regenerative passage, will increase the

margin of the coking limit.

(v) By hydrogenating the fuel before it is

kept in the storage tank, we can suppress

coking to a large extent as the ring

propagation is believed to be caused due to

the presence of dienes. [9]

Hydrogenation

prior to heating would reduce the amount of

unsaturation compounds, thus preventing

coking and would also result in an increase

in the calorific value of the fuel.

(vi) By adding metal particles like

Aluminum to the fuel would increase the

Isp of the fuel. If we add a composite of

boron (or boron hydride) and a suitable

transition metal, it could reduce coking by

forming organometallic complexes with the

active compounds like asphaltene, formed

during the course of the reaction. Boron

hydride addition would inhibit coke

formation to some extent. This has to be

further studied to understand completely. A

similar method has been employed in the

aviation industry and NASA has also used

aluminium gelled RP-1 for spaceflight.

Laboratory experiments and some more

research in this field may help us find a

suitable element which would wash away

the coke without affecting the combustion.

(vii) The use of gelled propellants is another

area of research which can lead to a

solution to the coking problem. Here, an

external gellant is used which creates a

cross linked structure in the liquid fuel, like

a long chain polymer. This would increase

the thermal stability of the hydrocarbons

present in kerosene, thereby reducing

coking. The other advantages of using

gelled propellants are safer handling,

reduced slosh, reduced the O/F ratio leading

to lighter exhaust gases, less leakage and

greater Isp than normal liquid propellants.

(viii) The use of nanometal additives or

nanocomposites is another alternative. Once

the precursors to the coking reactions are

known completely, we can design a

nanocomposite can be designed which

would increase the Isp as smaller particles

undergo more efficient combustion. It

would also trap the coking process by

reacting with the precursor element. As of

now, availability and the cost of

nanoparticles is an issue so it can be

pursued later.

(ix) According to literature, semi cryogenic

technology was used by both the US and

USSR. Though the US engines have

reported coking, the Russians didn’t face

this problem. It is not surprising since the

kerosene used by Russia (RG-1) was

superior in quality compared to RP-1, used

by US. RG-1 was low in sulphur content

(<1ppm), aromatic content (approx 5%) and

low dienes content, as these are thought to

be the major facilitations for coking. RG-1

was 3% denser than the RP-1.

(x) Boriding the inner surface of wall of the

cooling passage can help in reducing coking

as Boron being a trivalent element has

capability to form complexes readily. It can

form complexes with the depositing

asphaltene and dissolve away. Boriding the

surface will also increase the hardness and

smoothness of the surface.

CONCLUSION, DISCUSSIONS &

RECOMMENDATIONS

To understand the coking phenomenon, we

must have the knowledge of the major

components of the kerosene before heating

as well as during the course of heating at

different temperatures. The study of the

intermediates can help us to understand the

precursors and mechanisms of coke

formation. For this GC/MS can be

employed to analyze the samples of

kerosene as it has been successfully used to

study volatile mixtures with more than

hundred components. X-ray spectroscopy or

13C NMR spectroscopy of the coke samples

can help us elucidate on the actual structure

of the coke formed at that particular

temperature (given in appendix). The

structure of the coke may help for

conducting a retro-organic analysis of the

coking mechanism. Study of this

mechanism is essential for the development

of a semi cryogenic engine indigenously,

and the design of a synthetic kerosene to

suit the needs of ISRO.

APPENDIX 1

Coke formed at different temperatures has

different structure. At low temperatures

(<473K), if some oxygen is present in the

liquid kerosene, oxidative coke is formed.

At high temperatures, coke is mainly

pyrolytic because of cracking. It is due to

the formation of acetylene, benzene and

other PAHs. A catalytic coke is formed

because the wall material acts as a catalyst.

When the pyrolysed kerosene is cooled

down, asphaltic coke can appear. It is

because of condensation of PAHs. These

different types of coke may have

amorphous, tubular or filamentous structure

depending upon the temperature at which

they are formed.

REFERENCES

[1] Liang, Yang, Zhang, ‘Investigation of

heat transfer and coking characteristics of

hydrocarbon fuels’, Journal of Propulsion

and Power, Vol.14 No.5, pp. 789-796,

September-October 1998.

[2] Giovanetti, Anthony J., ‘Deposit

formation and heat transfer in hydrocarbon

rocket fuels’, NASA Report 168277,

October 1983.

[3] Wickam D.T., Alptekin G.O., Engel

J.R., Karpuk M.E., ‘Additives to reduce

coking in endothermic heat exchangers’,

35th AIAA/ASME/ASEE Joint Propulsion

Conference and Exhibit, 20-24 June 1999

[4] Goodger E.M., Hydrocarbon Fuels,

Mcmillan publication, 1960, pp. 483-486.

[5] Zhiming Fan, Watkinson Paula,

‘Formation and characteristics of

carbonaceous deposits from heavy

hydrocarbon coking vapours’, Industrial &

Engineering Chemistry Research,

Vol.45,No.19, September 13, 2006, pp-

6428 to 6435.

[6] Evaluation of coking limits of kerosene-

preliminary results, Report no-

LPSC/SCED/TR/051/08

[8] Brown Sarah, Frederick Robert A.,

‘Laboratory scale thermal stability

experiments on RP-1 and RP-2’, Journal of

Propulsion and Power, Vol.5 No.2,

October-November 2007.

[9] Wickham D.T., Alptekin G.O.,. Engel

J.R and Karpuk M.E., TDA Research Inc,

‘Additives to reduce coking in endothermic

heat exchangers’, AIAA 99-2215