The learning objectives of this chapter are that the learner will know the
requirements for a combustion process to occur have a basic understanding of the
combustion process and have a basic understanding of the chemistry of
combustion
essentially there are three elements required for combustion and you should
be familiar with them from your firefighting training these form the
fire triangle showing the relationship between fuel oxygen and heat a more
sophisticated model is now used to take account of those combustion processes
which can be stopped by interrupting the chemical or chain reaction that occurs
this is referred to as the fire tetrahedron a four-sided triangular
pyramid rather than the simpler triangle for our purposes we need only look at
the simple model we have seen earlier that the various fuels used on board
ship include liquid hydrocarbon fuels and cargo boil off gas the fuels that we
will consider here are marine distillate and residual fuels these fuels cannot be
burnt in the liquid form and it is necessary to treat and process these
fuels prior to combustion the fuels must be delivered to the engine cylinder or
boiler furnace in a way which generates vapor this vapor can be ignited if the
other combustion conditions of oxygen and heat are present it is the purpose
of the fuel system to achieve this
to allow combustion of fuel to take place there must be oxygen present
oxygen is supplied to the combustion chamber by filling it with air the air
being referred to as combustion air for most modern diesel engines this is done
by a turbocharger which forces compressed air into the engine cylinder
for a steam boiler the combustion air is usually supplied by a forced draft fan
the amount of air supplied along with other requirements will dictate how good
the combustion will be you would have seen black smoke from the funnel when
the engine or boiler is first started this is usually due to the fact that
insufficient air is being supplied at this stage for complete combustion when
the theoretical ideal amount of air to give complete combustion is supplied and
mixed with the quantity of fuel the mixture is referred to as the
stoichiometric mixture we will say more about this later in this chapter
even when oxygen and fuel are present as a combustible mixture combustion cannot
take place until sufficient heat is present to ignite the mixture ignition
can be achieved by raising the temperature of a combustible mixture -
its flashpoint and providing an external means of ignition for example a spark
ignition can also be achieved by raising the temperature of the mixture above its
self ignition temperature diesel engines rely on the temperature of the air
charged in the cylinder after compression being above the self
ignition temperature of the fuel to provide the ignition source they are
classed as compression ignition engines combustion in a diesel engine is a non
continuous process steam boilers rely on an external ignition source such as a
spark igniter or pilot flame once ignition takes place the combustion will
be continuous as the main flame vaporizes the fuel and heats it to above
the flashpoint click on the buttons to read the definition
the flash point of a fuel is the temperature at which the fuel will give
off combustible vapors which can be ignited by an external ignition source
in the presence of air the combustion may stop
if the ignition source is removed if the fuel is just at the flashpoint the
slightly higher temperature the fire point is that temperature which will
allow combustion to continue even when the ignition source is removed
the self ignition temperature of a fuel is the temperature
which a combustible mixture will ignite without an external source of ignition
it is also referred to as the auto ignition temperature or the temperature
of spontaneous combustion
we have referred to the term combustible mixtures in the previous pages and now
it is time to say more about them a mixture of fuel and air will only be
combustible if the fuel is vaporized and the ratio of fuel to air is within the
flammable range it is easier to vaporize the fuel if the surface area exposed to
the heat source is large compared to the mass of fuel it is therefore necessary
to break the fuel up into very small droplets this process is referred to as
atomization in a diesel engine it is desirable that the whole volume of the
combustion chamber is used
it is therefore necessary to distribute the fuels throughout the combustion
chamber this distribution is referred to as penetration the fuel and air must be
mixed so that the fuel and oxygen are evenly distributed to ensure that
combustion is as complete as possible to achieve this it is necessary to create
the correct mixing conditions in particular to control the air movement
or turbulence click on the buttons to learn more about some of the terms we
have just used
uh tamas ation is the term used to describe the breakup of a liquid fuel
into very small droplets which when mixed with air form a mist in this state
the surface area of the fuel is much greater than that of the solid liquid
stream and the fuel will therefore heat up rapidly and vaporize in the hot
cylinder or boiler furnace the purpose of the diesel engine fuel injection
valve is to generate conditions for atomization to take place as the fuel
enters the combustion chamber similarly a boiler burner tip will help create the
conditions for atomization often using compressed air or steam to assist the
process
penetration can be thought or distance traveled by the fuel once it
enters the combustion chamber or furnace insufficient penetration will lead to
the fuel air mixture being over rich in the vicinity of the injector or burner
tip leading to incomplete combustion excess penetration may lead to fuel
reaching cylinder components or furnace walls large fuel droplets tend to
penetrate further than small droplets over atomization may therefore lead to
lack of penetration
turbulence is the term used to describe movement of air in the cylinder prior to
and during the combustion process the movement has to be predictable if
correct mixing of air and fuel is to be achieved air turbulence is generated in
the diesel engine cylinder by the shape and position of scavenged parts and
valves and the shape of the piston crown and other combustion chamber components
in the boiler furnace this is achieved by the swirl plate of the burner unit
correct penetration of fuel into turbulent air will ensure good
distribution and an even mixture
when the three elements fuel oxygen and heat are present in the right conditions
then ignition will occur and combustion will follow the whole purpose of the
combustion process is to convert the chemical energy that the fuel contains
into heat energy which can then be converted into useful work and power in
the diesel engine the released heat energy from combustion of the fuel and
the associated pressure rise act directly on the piston to produce the
power to turn the crankshaft and in turn drive a propeller or other equipment in
a steam boiler the released energy is used to generate high-energy steam which
is normally used to power a turbine we have already been introduced to the term
stoichiometric mixture earlier in the chapter
we have already stated that a stoichiometric mixture is the mixture of
fuel and air that would theoretically result in complete combustion of the
fuel the amount can be calculated by considering exactly how much air is
required for the complete combustion of each of the elements within the fuel for
the fuels that we are considering the main elements will be hydrogen carbon
and sulfur although other elements and contaminants are also taken into account
in practice however the combustion process requires more air than this
ideal minimum in the diesel engine cycle the short period available for the whole
process of fuel injection mixing with air ignition and combustion means that
complete combustion is impossible for the fuel combustion to be as complete as
practically possible air in excess of the stoichiometry camonte is needed this
may be as much as 30 percent more than the ideal minimum in a steam boiler
furnace it is easier to mix the fuel and air at entry and a much smaller amount
of excess air is required
the heat energy equivalent released during combustion of the hydrogen and
carbon in moline fuel is the major source of energy for both the diesel
engine and the steam boiler we normally refer to the amount of energy equivalent
as the higher or lower value of the heat of combustion which is measured in mega
joules per kilogram taking into account the heat energy required to produce
water vapor gives the lower value of heat of combustion for those substances
where water vapor is a product of combustion the other elements found in
marine fuels have much lower energy equivalents than hydrogen and carbon as
the concentration of these other elements increases the overall heat of
combustion of the fuel reduces marine fuels typically have an energy
equivalent of between 39 and 42 mega joules per kilogram
click on the buttons to learn more about the combustion of hydrogen
hydrogen has an energy equivalent for the higher value of heat of combustion
of approximately 142 mega joules per kilogram and a lower value of
approximately 121 mega joules per kilogram the chemical equation for the
complete combustion of hydrogen in oxygen shows the products of combustion
are water vapor and heat
you
sulfur has an energy equivalent for he bastion of approximately nine mega
joules per kilogram the chemical reaction of sulphur with an oxidant can
produce a number of different oxides of sulfur generally referred to as Sox for
our purposes we will consider these as simple reactions during combustion the
chemical equation for the complete combustion of sulfur in oxygen can be
shown in stages and depends on the amount of oxygen available the products
of combustion are sulfur dioxide and heat or sulfur trioxide and heat when
combined with water or water vapor the sulfur dioxide and sulfur trioxide can
also form either sulfurous or sulfuric acid these products contribute to acid
rain when present in exhaust emissions
we have seen that the products of complete combustion
the carbon fuels would be only water and carbon dioxide if we use the
stoichiometric amount of pure oxygen however the combustion process is not
supplied with oxygen directly but with atmospheric air which contains a high
proportion of nitrogen as well as other trace elements also marine fuels often
contain sulfur which will burn and produce its own products of combustion
obviously any other elements that are present in the fuel and any excess
lubricating oil will produce other products of combustion all of these
additional products will be emitted to the atmosphere in the exhaust gas some
of these emissions are regarded as atmospheric pollutants which are harmful
to the environment and are considered undesirable components of the exhaust
gas recent legislation particularly the introduction of Marple and x6 regulates
the permitted levels of these undesirable components in exhaust gas
emissions click on the button to find out more about diesel engine exhaust
emissions
you
the exhaust gas from a diesel engine will contain all of the expected
products of combustion plus a high amount of the excess air not used during
combustion process in addition to those already discussed there will also be
products of incomplete combustion since complete combustion is impossible to
achieve in practice then combustion of marine fuel results in the formation of
other products of combustion these will include unburned hydrocarbons unburned
carbon in the form of soot and partly burnt carbon as carbon monoxide there
will be incomplete combustion of some of the elements in the fuel resulting in
emission components as shown in the list we have already seen that sulfur in the
fuel leads to the formation of oxides of sulfur referred to as Sox the nitrogen
in the combustion air can form a number of different oxides which are referred
to as NOx the NOx and Sox can combine with moisture in the atmosphere
to produce acids which contribute to acid rain non combustible elements such
as ash and particulate matter originating from the fuel or air supply
will pass through the combustion process unchanged or partly oxidized all of
these will pass out with the exhaust gas and can be hazardous to health and the
environment
in this chapter we have looked at the basic requirements for combustion of
marine fuels by considering the elements that make up the fire triangle we have
also looked at the basic chemistry of combustion for the main elements
normally found in marine fuels and the products from combustion of each we have
also discussed the relationship between the air and fuel to form a combustible
mixture and the condition of the fuel for ignition and combustion to be
possible we have also considered the environmental impact of burning
hydrocarbon fuels particularly in these legends try the questions in the
following self-test to check your understanding so far