Combustion of Liquid Fuel Sprays

290 Pages·1990·9.503 MB·English

by  Alan Williams (Auth.)

Combustion of Liquid Fuel Sprays
Alan Williams
BSC, PhD, CEng, FInstE, FRSC, FInstPet, FInstGasE
Livesey Professor and
Head of Department of Fuel and Energy,
The University of Leeds,
Leeds, UK
Butterworths
London Boston Singapore Sydney Toronto Wellington
φ PART OF REED INTERNATIONAL RLX.
All rights reserved. No part of this publication may be reproduced in any material
form (including photocopying or storing it in any medium by electronic means and
whether or not transiently or incidentally to some other use of this pubhcation)
without the written permission of the copyright owner except in accordance with
the provisions of the Copyright, Designs and Patents Act 1988 or under the terms
of a licence issued by the Copyright Licensing Agency Ltd, 33-34 Alfred Place,
London, England WCIE 7DP. Applications for the copyright owner's written
permission to reproduce any part of this publication should be addressed to the
Publishers.
Warning: The doing of an unauthorised act in relation to a copyright work may
result in both a civil claim for damages and criminal prosecution.
This book is sold subject to the Standard Conditions of Sale of Net Books and may
not be re-sold in the UK below the net price given by the Publishers in their current
price list.
First published, 1990
© Butterworth & Co (Publishers) Ltd, 1990
British Library Cataloguing in Publication Data
Williams, Alan
Combustion of liquid fuel sprays
1. Liquid fuels: Sprays. Combustion
I. Title
621.402'3
ISBN 0-408-04113-7
Library of Congress Cataloging-in-Publication Data
Williams, Alan, 1935- June 26-
Combustion of liquid fuel sprays/by Alan Williams.
p. cm.
Includes bibliographical references
ISBN 0-408-04113-7:
1. Combustion. 2. Liquid fuels. 3. Atomization.
I. Title
QD516.W49 1989 89-23939
621.402'3-^c20
Composition by Genesis Typesetting, Borough Green, Sevenoaks, Kent
Printed and bound in Great Britain by Courier International Ltd, Tiptree, Essex
PREFACE
The development of spray combustion has effectively followed two
separate paths, one concerned with engine applications in which the
aerospace applications have dominated the field, the second has been
concerned with stationary equipment such as furnaces and boilers.
Generally textbooks have been concerned with one area or the other and the
object of this book is to outline the fundamentals of the combustion of
sprays in a unified way which may be applied to any existing or future
technological application. During the last decade the necessity of
controlling the emission of pollutants has asstimed greater significance in
all aspects of combustion. More recently the requirements have been for
pollution control and increased combustion efficiency. In the future it
will be necessary to burn fuels which are of low or variable quality or
synthetic fuels having properties differing greatly from present-day fuels.
For all these developments a greater understanding of spray combustion is
required.
This book has been written at a level suitable for those entering the
subject and for students undertaking courses or research in fuel,
combustion or energy studies. The emphasis has been towards the
fundamental aspects although some industrial applications have been
outlined, and where possible the text is augmented by Appendices expanding
certain quantitative aspects of the book.
I am indebted to a number of individuals and organisations for
assistance and the provision of material, namely Mr. P. Beard, Professor N.
Chigier, Mr. J.B. Champion, Professor I. Fells, Mr. R.A. Freeman, Mr. D.
Gunn, Dr. G. Masdin, Dr. K.J. Matthews, Dr. K. Salooja, Mr. P. Sharp, Dr.
P.J. Street, Professor J. Swithenbank, Dr. J. Sykes, Mr. S. Turner, Airoil
Flaregas, Babcock Energy Ltd., CEGB, Foster-Wheeler Power, Hamworthy
Combustion Systems Ltd., Malvern Instruments, NEI (International Combustion
Ltd.) Peabody-Holmes Ltd., Stein Atkinson Stordy, Stordy Engineering, The
Institute of Energy, Weishaupt (UK) Ltd.
CHAPTER 1
SPRAY COMBUSTION AS A SOURCE OF ENERGY
1.1 The General Nature of Spray Combustion
The combustion of sprays of liquid fuels is of considerable
technological importance to a diversity of applications ranging from steam
raising, furnaces, space heating, diesel engines to space rockets. Because
of the importance of these applications spray combustion alone is
responsible for a considerable proportion of the total energy requirements
of the world, this being about 25% in 1988.
Spray combustion was first used in the 1880's as a powerful method of
burning relatively involatile liquid fuels, indeed it remains the major way
of burning heavy fuel oils today even though they can now be burned in
fluidised bed combustors. The basic process involved is the disintegration
or atomisation of the liquid fuel to produce a spray of small droplets in
order to increase the surface area so that the rates of heat and mass
transfer during combustion are greatly enhanced. Thus the atomisation of a
1 cm diameter droplet of liquid into droplets of 100 μm diameter produces
10^ droplets and increases the surface area by a factor of 10,000.
A burning spray differs from a premixed, combustible gaseous system
in that it is not uniform in composition. The fuel is present in the form
of discrete liquid droplets which may have a range of sizes and they may
move in different directions with different velocities to that of the main
stream of gas. This lack of uniformity in the unburnt mixture results in
irregularities in the propagation of the flame through the spray and thus
the combustion zone is geometrically poorly defined.
Flames used in industrial applications are highly complex systems
because of various complicating factors such as the complex flow and mixing
pattern in the combustor chamber, heat transfer during the combustion
process, and the non-uniform size of the spray droplets. For purposes of
demonstration the assumption is often made that the system is uni­
dimensional. Thus the flame can be considered as a flowing reaction system
in which the properties of flow, temperature, etc. vary only in the
direction of flow and are constant in any cross section perpendicular to
the direction of flow.
The general nature of the processes involved in spray combustion in
such an idealised case for the combustion of a dilute spray is shown in
Figure 1.1. Such systems, containing droplet diffusion flames are
characterised by their yellow nature. In this one-dimensional case the
individual droplets that make up the spray burn as discrete droplets in a
surrounding oxidising atmosphere which is most commonly air. This
heterogeneous spray combustion is also clearly shown in Plate 1.1 which
shows a simplified (flat) spray system. This plate also demonstrates the
other major features of a spray flame, namely atomisation, air entrainment,
flame stabilisation and the irregular nature of atomisation. The flame
front is visible as a diffuse zone across the upper part of the plate.
Sprays can also burn in what is effectively a homogeneous way in that the
droplets can evaporate as they approach the flame zone so that in fact
clouds of fuel vapour are actually burning rather than droplets. This is
illustrated diagrammatically in Figure 1.2. Such flames are characterised
by their blue colour and are usually associated with sprays of volatile
fuels (such as aviation kerosine) which have small initial droplet
diameters (eg. 10 μm).
It is clear that for any detailed understanding of the process of
spray combustion it is necessary to have an adequate knowledge of the
combustion of the individual droplets that make up the spray so that a
burning spray may be regarded as an ensemble of individual burning or
evaporating particles. However it is also necessary to have a statistical
description of the droplets that make up the spray with regard to droplet
size and distribution in space.
The essential stages involved in spray combustion are outlined in
Figure 1.3. The fuel is transmitted from the fuel storage tank by a fuel
handling system that incorporates pumps, filters and some means of control
to an atomiser by means of which the fuel is atomised into small droplets,
these droplets are usually injected directly into the combustion chamber
where they burn. The combustion process is very complicated because, to a
large extent, the mixing of the fuel and oxidant takes place inside the
chamber and thus the mechanics of the mixing process play an important
role. This mixing process is controlled by the geometry of the combustion
chamber, the spatial distribution and momentum of the injected spray, the
direction and momentum of the air flow and the influence of any flame
stabilisation devices. Consequently the atomiser and combustion chamber
should be designed as an integrated unit rather than as independent items.
1.2 Sources of Liquid Fuels Used In Spray Combustion
The liquid fuels used at the present time are almost exclusively the
fuel oils. The term 'fuel oil' means different things in different
countries but essentially it covers the range of products from gas oil to
extremely viscous products of high molecular weight. This group
encompasses both diesel fuels and industrial fuels used in boilers and
furnaces. Their major source is crude oil but fuel oils can also be
produced from coal by solvent extraction as well as by the pyrolysis of oil
shale and tar sands. Alternative liquid fuels can also be produced from
plants, such as vegetable oils (soya bean, palm oil etc.), or by slurries
of finely powdered coal (ca 70 wt %) or bitumen particles dispersed in
water.
In view of the increase in world energy consumption and the
importance of the contribution of oil as illustrated in Figures 1.4 and
1.5, there has been considerable interest in the security of oil supplies
as well as in its price. As far as the global supply of oil on a long term
basis is concerned then the general indications are that the total world
ultimately recoverable is of the order of 250 Gt (10^ tonnes) (Masters et
al, 1987). This includes 40 Gt tonnes already recovered, some 90 Gt tonnes
in proved identifiable reserves whilst the rest is an undiscovered
resource. In addition there are recoverable reserves of extra heavy oil of
about 8 Gt of which 7.6 Gt are in Venezuela, together with 10 Gt of natural
bitumen distributed mainly in Canada (4 Gt) and USA (6 Gt) . Table 1.1
lists the present-day fossil fuel reserves of oil, gas, coal and uranium
and the dominance of the coal reserves is clear (Appendix 1 lists some
energy interconversion factors). It is interesting to note that in the
case of crude oil that the proven reserves have not changed over the last
25 years and indeed the ratio of (proven reserves) / (production rate) has
remained effectively invariant with a value of about 32, and thus oil has
behaved over that time as if it is an infinite resource, which is clearly
not the case. Table 1.2 indicates the possible lifetimes of crude oil
based on the assumptions of (a) the present-day demand, and (b) a continued
expansion of oil demand assuming it increases at the rate of 5% per annum.
The difference in lifetime of oil and coal is illustrated in Figure 1.6.
Whilst future growth rates always remain uncertain it is clear that by the
early part of the next century alternative sources of liquid fuels must be
developed, these may be based on coal in view of its extensive resource
base, or on biomass or synthetic fuels generated by some other renewable
energy source such as solar.
1.2.1 Fuel Oils from Crude Oil
The main source of liquid fuels at the present time is crude
petroleum which occurs naturally in sedimentary basins in the earth's
strata. They are derived from vegetable and animal remains such as
proteins, lipid, polyisoprene hydrocarbons such as terpenes and derivatives
of porphyrins, since chlorophyl is an important plant ingredient. Loss of
oxygen and nitrogen leaves a largely hydrocarbon mixture. The
hydrocarbons, pristane and phytane, have been used as biological markers
and the pristane/phytane ratio has been used as an indicator of the
oxidising conditions under which the sediments were deposited. As a result
crude oil consists essentially of hydrocarbons together with smaller
quantities of sulphur, oxygen, nitrogen-containing hydrocarbons and some
órgano-metallic compounds particularly of vanadium and iron, sodium is also
present as sodium chloride. Gaseous, liquid and solid (or semi-solid)
compounds may be present in crude oil and these are separated at the well­
head and during the refining processes to give a range of liquid products,
some of which are used as fuels for spray combustion. The properties of
the liquid fuels produced are markedly dependent upon the source of the
fuel, the nature of the refining operations and method of blending used to
produce the final product.
The hydrocarbons present in crude oil have differing boiling points
and are separated by the process of distillation into a range of primary
products. The nature of many of the final products, particularly the fuel
oils, is determined by the chemical composition of the crude oil that is
distilled. Because of the multiplicity of the molecular species present,
crude oils, as well as the products, may be classified in terms of the
concentrations of broad chemical groupings, namely paraffinic, naphthenic,
aromatic or asphaltic. During the process of distillation and other
refining operations these products are distributed amongst the final
products according to their properties as are the sulphur and nitrogen-
containing hydrocarbons, etc. and inorganic components.
A typical but simplified refinery flowsheet is illustrated in Figure
1.7. The raw crude oil is fractionated in the crude distillation unit into
distillate and residuum streams. Typically in a two-stage unit, an
atmospheric tower produces middle distillates and lighter fractions with a
vacuum section producing a heavy gas oil cracking stock and other streams
for lubricating oil production, etc. Before processing the salt and water
concentrations are reduced and then the crude is flash distilled, thus
producing gases and gasoline and a number of sidestreams which after
additional purification stages yield directly aviation kerosines, diesel
fuels and gas oils. The major finishing process used for these products is
hydrotreating in which mild hydrogenation conditions are employed to reduce
the concentrations of sulphur, oxygen and nitrogen compounds as well as the
unsaturated compounds. A number of properties are improved by this
process, particularly odour and stability. The stripped atmospheric
bottoms are reheated and passed to the vacuum tower. Here further yields
of (vacuum) gas oils are produced whilst the vacuum residuum is used
directly or if further processed. In the latter case the residuum may be
processed in a visbreaker to produce essentially a gas oil product directly
available for sale. Alternatively it may be charged to a delayed coker to
produce gas oil and lighter fractions as well as petroleum coke. Thus,
apart from these two processes, the distillation process provides the whole
range of fuel oils since the final products are blended from straight
distillates, residues and by-products from other refinery operations. A
number of additives may be incorporated into the final product; in the case
of aviation fuels used for spray combustion the additives normally present
are anti-oxidants whilst in the case of engine fuels pour-point
depressants, antiwear additives and antirust additives may be present.
Other additives to reduce pollutant formation may be added prior to use;
these are discussed in Chapter 6. Further details of these processes are
outlined in standard textbooks on petroleum, e.g. Modern Petroleum
Technology, Hobson and Pohl, 1984.
1.2.2 Oil from Shale and Oil-Sands
Extensive deposits of oil shales and oil sands (tar sands) exist
throughout the world and the reserves of oil from such sources are
considerable as indicated in Table 1.1. Of major significance are the oil-
sands in Canada (Alberta) , USA and Venezuela and the oil shale deposits in
the USA and Brazil. Oil sands consist of deposits containing heavy
hydrocarbons which are essentially the same as in conventional oil but
having high viscosities, ie tar-like. Oil-shales are significantly
different since they contain no free oil, but it is in the form of a solid
mixture of organic compounds called "Kerogen" which decomposes on heating,
yielding a light shale-oil. The elemental composition of the shale in
Green River, Colorado, is approximately by weight 56% C, 7% Η, 13% O, 2.5%
Ν and 2.6% S. It consists of a number of multi-ring structures
(isoprenoids, porphyrins) held together by cross linking groups such as
-0-, -SS- and -CH2-.
Production of oil from shale or oil sand involves extracting the raw
material, processing it to extract the hydrocarbons and then converting the
crude oil so produced into a form in which it can be used. The methods
used depend upon the nature of the deposit. In the case of oil shale,
extraction involves a mining operation and the product is then heated to
about 400*0. The liquid product obtained has many of the properties of a
conventional crude oil except that it has a high viscosity and high
nitrogen content. An alternative method involves the in situ conversion of
the oil shale to oil, this would obviate the mining operation but so far
these techniques have been unsuccessful.
In the case of oil-sands two methods are generally applicable,
depending upon the nature of the deposit. Firstly, hot water or steam may
be introduced into the formation to reduce the viscosity of the oil which
is pumped out in the conventional way. This is particularly suitable for
the deeper oil-sand deposits in which the deposits have fissures or are
sufficiently permeable for the oil to flow to the production well. The
second technique is similar to the oil-shale operation in which the oil-
sand is recovered by a mining operation and then processed using hot water
or steam and diluents. Even so the recovered oil has a low specific
gravity, high viscosity and high sulphur content necessitating upgrading
before use.
1.2.3 Liquid Fuels from Coal
The production of oil from coal has been the subject of considerable
research effort, particularly during the Second World War and is receiving
considerable interest at the present time. There are three basic processes
available:
(a) Methods based on the Fischer-Tropsch process.
Here the coal is gasified by conventional processes (these are
outlined in Merrick, 1984) to a mixture of carbon monoxide and hydrogen.
These are reacted catalytically in the Fischer-Tropsch process producing
hydrocarbons thus:
η CO + (2η+1)Η2 - C^H2^^.2 + η Η2Ο
2 η CO + η Η2 - (^^2^η ^ ^^2
together with alcohols
η CO + 2η Η2 - ^η"2η+1^^ (η-1)Η2θ.
Such an oil into coal process is currently operated by the South African
Coal, Oil and Gas Corporation (SASOL) in which the CO/H2 mixture is reacted
in two ways, in the Lurgi (Arge) process and the Synthol process. In the
former a pelletized iron fixed-bed catalyst is used and a wide range of
hydrocarbons is produced including gasoline and furnace oil (a gas oil
equivalent) together with a range of waxes. In the Synthol process, a
fluidised iron catalyst is used which produces gasoline, gas oil and
alcohols as well as other products. The Synthol process operates at higher
temperatures resulting in comparatively low yields of heavier oils and
waxes. The process is very flexible and by changing the catalyst
composition the product spectrum can be varied to suit the demand of the
end use. In addition, if the sole objective were to produce liquid fuels
for spray combustion the liquid hydrocarbons and the alcohols do not need
to be separated before use.
(b) Coal Pyrolysis.
During the carbonisation processes in the manufacture of coke and the
older coal gas processes the products include coal tar. Some of this is
used directly as a fuel and the rest is distilled to give a series of fuel
oils, the coal tar fuels, and also a benzole fraction which is blended with
gasoline. The coal tar fuels are designated CTF 50, 100, 200, 250, 300 and
400, the number being its recommended atomisation temperature in degrees
Fahrenheit. These fuels can be burned as spray flames in the usual way
although they are highly aromatic fuels and produce highly luminous flames.
The yield of liquid fuels by this route is small, about 8% wt, and
this can be increased to some 75% by the use of hydrogenation. The process
relies on the rapid heating of finely divided coal to drive off the
volatile components, both liquid and gaseous. Often fluidised bed reactors
are employed and staged so that the pyrolysis conditions become
increasingly more severe with each stage. The best known method here is
the COED process where a low sulphur fuel oil is produced by hydrotreating
the liquid product. This, and a number of other similar processes, have
been discussed by Merrick (1984). The major disadvantage of coal pyrolysis