Exhaust Emissions from the Combustion of Fuels

Our planet over its life span of a few billion years has been developed essentially via states of quasi-equilibrium that took a long time to produce many changes to its states� However, our exceptionally rapid growth and development in every aspect of our lives through rapid exploitation and harnessing of our natural resources are becoming much too rapid to maintain the ecological equilibrium we have come to depend on and enjoy� This is a serious problem, and it appears not to have had an effective remedial solution so far� Rapidly accelerating consumption of fuel and other resources, our increasing expectation of a more comfortable existence and a higher standard of living, and the ever-rapid increase in the world’s population are major current concerns to us all� These are being supplemented by the growing problem of global warming and the increased accumulation of greenhouse gases in the atmosphere, production of acid rain, and depletion of the protective ozone layer� No doubt, lengthening of the average life span of humans is an additional concern while considering the future availability of sufficient natural resources, water, and food�

Ideally, the mixing of air in excess proportions with a hydrocarbon fuel should result, after combustion, in all the carbon in the fuel appearing as CO2 and all hydrogen appearing as H2O, together with unutilized oxygen and unchanged nitrogen� If too much excess air is supplied, then combustion would produce only low temperatures that may lead to incomplete combustion and even ignition failure altogether� An excess supply of fuel will produce incomplete combustion products, with some of the fuel remaining unconverted� Much of the hydrogen in the fuel will be oxidized to H2O, with some appearing as molecular H2� Because of the insufficient amount of oxygen available, the carbon in the converted fuel will produce CO and CO2, with hardly any unutilized oxygen remaining� However, a chemically correct mixture with a stoichiometric supply of air will be sufficient, in principle, to convert the hydrogen into H2O and the carbon into CO2 with no

unconsumed O2 or unconverted fuel in the products� In practice, due to the operational conditions and the fuels employed, the ideal composition is difficult to attain for a variety of reasons, producing a range of other products, although often in small concentrations� Some of these survive and become final products, whereas others are transiently unstable and would not reach the exhaust stage; however, in reality, they play a critical role in the combustion processes� Some of the main reasons for this behavior are the following:

1� Inhomogeneous mixing of the fuel and air 2� Insufficient time given to complete combustion 3� Excessive heat loss, which quenches flames, and associated reac-

tions, such as contact with cold surfaces or mixing with colder air 4� Dissociation effects at high temperatures 5� Nitrogen and other elements present in the fuel becoming reactive at

high temperatures 6� Flames not propagating throughout the mixture, for example, due to

insufficient time given followed by a rapid drop in temperatures

Accordingly, the products of combustion in reality, as shown schematically in Figure 7�1, will contain not only CO2, H2O, O2, and N2, but also CO, H2, unburned fuel, oxides of nitrogen and sulfur, and other partial oxidation products� Controlling the composition of the combustion products and the corresponding species present is a great challenge� However, this is

necessary not only to ensure optimum efficiency but also to minimize the emission levels of undesirable pollutants from fuel combustion processes�

With the recent emphasis on the reduction in the emission levels of greenhouse gases such as carbon dioxide, even complete and perfect combustion will yield products that are considered potentially harmful to the environment, and their releases need curtailing�

This is of course not easy� Much research, resources, and ingenuity from present and, no doubt, future generations are required to develop clean renewable forms of energy that can replace our dependence on consumable carbonaceous-type fuels�

Up to the end of the first half of the twentieth century, only little attention was given to deal effectively with the problems of air pollution arising from the exploitation of fuel resources� The focus of control measures was initially on reducing the emissions of a toxic and particulate matter-smoke� This was followed later on, driven by government legislation, by attempts to control and reduce the emissions of unburned hydrocarbons, carbon monoxide, and oxides of nitrogen and sulfur, the main ingredients for the formation of smog, acid rain, and ozone layer depletion� Only relatively recently, the emission level of greenhouse gases, where carbon dioxide is a main agent, was recognized as a potentially serious problem requiring urgent control and reduction� It is now well acknowledged that emissions are a long-term problem requiring strict control measures not only locally but also globally�

Any material emitted that can change the concentration and composition of the normal atmosphere is considered as a pollutant and its release must be controlled� Fuel combustion is a prime source of air pollution� The main pollutants emitted after the combustion of fuels vary in both concentration and quantity very considerably depending on the fuel used and the mode of operation of the system� These are the following:

• Unburned hydrocarbons • Carbon monoxide • Oxides of nitrogen • Oxides of sulfur • Particulates/smoke • Carbon dioxide and other greenhouse gases • Others, for example, aldehydes, toxins, and ozone depleters

The action of unburned hydrocarbons and oxides of nitrogen in the presence of sunlight in the open atmosphere leads to the formation of the undesirable photochemical smog, which has been identified to be associated with serious negative health and materials effects (Figure 7�2)� Through improvements in science and technology and progressively tightening preventive measures, including through legislation worldwide, the emissions of both unburned hydrocarbons and oxides of nitrogen have been severely restricted, with a consequent reduction in the production of smog�

Carbon monoxide is emitted from the combustion of fuels as a result of incomplete combustion, resulting either from oxygen deficiency or from the quenching of combustion processes due to, for example, contact with cold surfaces, rapid expansion, or a relatively excessive air supply� Wellestablished methods are available for successful oxidation of carbon monoxide and its removal from exhaust gases before its release into the atmosphere� This is primarily achieved with the aid of suitable catalysts in the presence of additional oxygen�

Oxides of nitrogen, commonly described as NOx, are produced by combustion of fuels in air� They arise from the reaction of oxygen with some of the nitrogen in the air or that may be present in the fuel at high temperatures� Although the amount of NOx produced is relatively small, it contributes negatively not only to the production of smog but also to the production of acid rain and greenhouse gases and depletion of the ozone layer; further, NOx is also toxic� The formation of the oxides of nitrogen increases essentially logarithmically with temperature� Their production is also aided by having slightly fuel-lean mixtures with excess oxygen and permitting relatively

long periods for their formation reactions to proceed to completion� Hence, measures to curb the emission of these oxides tend to focus mostly on reducing the peak local temperature values encountered in combustion�

Oxides of sulfur have serious negative health and materials effects� They arise primarily from the oxidation of any sulfur contained in the fuel� These oxides are serious producers of acid rain and can affect health� They undermine the functioning of catalytic converters, such as those fitted to remove exhaust pollutants from automobile exhaust gases� Recently, in the continuing effort to combat the undesirable effects of these oxides, strict measures have been enacted to reduce the sulfur content in processed liquid fuels such as gasoline and diesel fuels to ultralow levels through advanced and complex refining methods�

Particulates and smoke are encountered in significant concentrations in fuel combustion processes where fuel-rich mixture zones and incomplete combustion may be present such as in nonhomogeneously premixed diffusion-type combustion processes� Minerals and other solid impurities in the fuel will inevitably appear also as particulates� With some fuels and mainly due to quenching effects, it is likely that aldehydes and other undesirable compounds can be emitted, but in relatively very small concentrations�

Compliance with exhaust emissions regulations is validated via being categorized based on a realistic set of operating conditions representing average operating conditions� In the transport sector, different driving cycles have been proposed and adopted� On a prescribed driving pattern, an integrated representative sample of the exhaust gases is collected and considered a realistic average representation of the emissions from a vehicle� Figure 7�3 shows details of a driving cycle displaying the speed variation of the vehicle with driving time for collection of a representative sample of the products for evaluation�

In transport engine applications, the extent of exhaust gas emissions will depend mainly on the following factors:

• Fuel being used • Engine applications • Design of the engine and its associated vehicle • Operating conditions • Exhaust gas after treatment, such as through the employment of

suitable catalysts, special filters, or exhaust gas recirculation

Other considerations that control the total amount of emissions pro duced are

• Average number and size of vehicles, their distribution, and mode of use within a specified region

• Distances traveled, traffic movement, road conditions, load carried, and passengers per car

Some remedial measures to control and reduce emissions can be enacted through

• Choice and processing of the fuels employed • Suitable exhaust gas treatment before releasing into the atmosphere • Optimum choice of operating conditions • Controlling and optimizing the course of the combustion process • Employing appropriately some exhaust gas recirculation

In evaluating exhaust gas composition data, it is important to establish whether the concentrations quoted are on

• Volume or mass • Dry or wet basis • For steady or transient operation • Relative to power produced or energy supplied basis • Whether they are plotted based on fuel-to-air or air-to-fuel mass or

volume ratios

It is necessary also to watch whether the plots are based on a logarithmic or linear scale�

Some of the recent measures that are being developed to improve engine performance and reduce associated emissions are the following:

• Resorting whenever possible to fuel sufficiently lean mixture with excess air operation

• Employment of direct fuel injection rather than carburetion for better control of fuel introduction and distribution among the different sections of the engine or device�

• Using optimized exhaust gas recirculation, especially for NOx emission control through measures that mainly reduce the peak temperature values, without seriously undermining the performance of the device�

• Employment, after further development and improvement yet to be made, of homogeneous charge compression ignition operation that could permit better lean mixture operation�

• Employment of stratified mixture combustion where relatively rich mixtures are arranged near the ignition source, while the remaining mixture is fuel lean such that the overall mixture is still fuel lean� This approach avoids the high temperatures associated around the stoichiometric mixture region�

• Optimizing variable valve timing mainly to achieve effective exhaust gas recirculation�

• Using alternative fuels such as compressed natural gas and hydrogen�

• Employment of optimized exhaust gas turbocharging such as through variable geometry or exhaust gas bypass under selective conditions�

• Employing quasi-adiabatic engines that will reduce heat losses and improve overall fuel conversion�

• Employing engines with a high compression ratio to enhance the efficiency and output, subject to the successful avoidance of the undesirable onset of knock�

• Using sufficiently high-quality clean fuels� • Improving engine controls, especially under transient conditions

such as during starting, load changes, and acceleration� • Using hybrid operation with mixed staged sources for power pro-

duction and storage�

The reduction of the impact of exhaust gas emissions in general and that of greenhouse gases is closely linked to reducing fuel consumption in combustion devices� Some of the main measures for improving the fuel consumption of combustion engines are the following:

• Employing a fuel having the right characteristics and properties for the application

• Increasing the compression and expansion ratios of the engine

• Running the engine at the lowest speed at which the required power can be produced, thus reducing frictional losses that increase markedly with speed

• Enhancing the rate and completeness of the combustion processes • Avoiding the onset of knock • Using lightweight yet high-strength materials • Employing, whenever possible, design features that minimize

engine size for a specific engine power output requirement • Incorporating design and operational features that will reduce throt-

tling and motoring losses • Reducing engine friction, parasitic losses, and auxiliary power

requirements and usage (e�g�, air conditioning) • Reducing heat losses to the surroundings • Exploiting usefully the exhaust energy such as via cogeneration and

turbocharging

As indicated earlier, oxides of nitrogen are highly objectionable pollutants and acid rain producers; in addition, they are very strong greenhouse agents whose emissions need to be increasingly restricted� Changes in the following operational factors are generally accepted as means for reducing NOx emissions from fuel combustion processes:

• A decrease in the peak level of combustion temperature� • A decrease or elimination of fuel nitrogen since the bulk of it will be

released after combustion as oxides of nitrogen and not molecular nitrogen�

• A decrease in excess oxygen for combustion� • A decrease in the time available for reaction� • An increase in exhaust gas and inert gas recirculation� • Catalytically and/or chemically treating the exhaust gases before

releasing them into the atmosphere� • The presence of diluents such as gaseous nitrogen or water injection

decreases the release of NOx from combustion processes, although this can affect engine performance adversely�

• A reducing agent such as ammonia or urea can be added to the exhaust gas to reduce NOx emissions by converting them into nitrogen and oxygen� However, ammonia is toxic, corrosive, and relatively expensive�

Figure 7�4 shows a schematic representation of the variations in the concentrations of the exhaust gases emitted (not to scale) by a diesel engine with changes in the mass of fuel consumed: (A) oxygen, (B) oxides of nitrogen, (C) carbon dioxide, (D) particulates/smoke, and (E) carbon monoxide�

In general, primary controls of emissions are applied when the operating variables of the combustion device are adjusted and controlled to reduce the emission of undesirable exhaust products� This may include the application of exhaust gas recirculation or water injection� However, the extent of this reduction in undesirable emissions is limited and may lead to undermining the energy/power output and increasing the specific fuel consumption, as well as other negative effects� Accordingly, when emission constraints are sufficiently severe or restrictive, secondary treatment methods are applied� These increase the complexity of control and increase operating, capital, and maintenance costs� These secondary methods rely on exhaust gas treatment approaches such as the catalytic treatment of the exhaust gases beyond the device but before the rejection of the gases into the outside atmosphere� This way the performance of the device would not be penalized at the expense of increased complexity�

There are mainly two types of catalytic converters that can suitably process the exhaust gases of engines and reduce their harmful effects before their release into the open atmosphere� These are of the two-way oxidation and the three-way oxidation and reduction types� The two-way catalytic converter deals with the oxidation of carbon monoxide and any unconverted hydrocarbons� When required, NOx is removed by separate and additional measures� The three-way catalysts deal at the same time with the simultaneous conversion of carbon monoxide, unconverted hydrocarbons, and oxides

of nitrogen (Figure 7�5)� This type of converter is used almost universally in most current automobiles Figure 7�6� However, these converters require stoichiometric mixtures that are ensured through special engine control systems that include a sensor for measuring the oxygen concentration in the exhaust gases, and these converters are made up of an aluminum oxide frame with platinum and rhodium coatings� Platinum is mainly responsible for aiding in the oxidation of the carbon monoxide and unburned hydrocarbons, while

rhodium is responsible for the reduction of the NOx� Such a device, which requires operation on stoichiometric mixtures for its effective functioning, results in engines that are not operationally flexible� They are incapable of operating on mixtures with different equivalence ratio values�

Changes in the conditions of the earth’s atmosphere are not necessarily a recent event� Numerous changes of varying severity did occur over the millennia� However, it is increasingly acknowledged that changes in the concentrations of greenhouse gases in the earth’s atmosphere are a reality, and the warming effects caused by these changes need urgent attention and control� Much of these gases have been attributed to the combustion of carbon-containing fuels releasing carbon dioxide as well as various discharges of fugitive greenhouse gas emissions such as those of methane, whether deliberately or inadvertently released� These additions to the atmosphere most probably have contributed to the gradual warming of our planet due to their associated greenhouse effect� It is suggested that carbon dioxide has been removed regularly from the atmosphere by numerous physical and chemical factors over millions of years, including being stored underground in the form of fossil fuels� The gas, however, is being released in recent times at rates that far exceed those for its removal, thus contributing to the rapid buildup of its concentration in the earth’s atmosphere�

Greenhouse gases such as carbon dioxide, methane, and nitrous oxide are transparent to certain wavelengths of the sun’s radiant energy� Thus, thermal radiation energy penetrates the earth’s atmosphere� The presence of such gases effectively prevents some of this thermal radiation from escaping, trapping some of the heat within the atmosphere, as shown schematically in Figure 7�7� In time, the mean global temperature can gradually increase,

leading to gradual global warming� Thus, the commonly discussed greenhouse effect can be defined as a warming of the atmosphere and surface of the earth by complex combined contributions from processes involving sunlight and atmospheric gases and minute particles�

Human activity in general and the combustion and release of fuels, especially in recent decades, as shown in Figures 7�8 and 7�9, have been identified as significant potential contributors to global warming� There is an urgent

need to develop effective measures to control such emissions and reduce the intensity of this effect to limit the consequent potential for increased global warming, which has been recognized as having potentially alarming associated long-term consequences� Figure 7�8 shows the estimated rapid increase in the amount of carbon dioxide released into the atmosphere throughout the twentieth century arising from the activities of the different global geographical regions� It can be seen that countries in North America have been the biggest contributors to these releases� In addition, as can be seen in Table 7�1, the extent of emissions of carbon dioxide per unit of energy produced varies very widely depending on the fuel source and its mode of processing� An example is the combustion of hydrogen, which appears not to produce directly any carbon dioxide� However, it was produced through processes involving the gasification of coal or the reforming of natural gas; nevertheless, it will be associated indirectly with relatively large greenhouse emissions at its source of manufacture� Accordingly, great care is needed in assigning realistically the extent of overall greenhouse gas emissions of the different fuels� On this basis, the hydrogen manufactured in these ways has been described as “black hydrogen” to distinguish it from the gas produced from renewable energy sources such as those through water electrolysis while employing wind or solar energy; it is described then as green hydrogen� The burning of liquid hydrogen is especially associated with serious indirect emissions of greenhouse gases since the liquefaction process is highly energy intensive�

A wide variety of sources contribute to the observed increased presence of carbon dioxide in the atmosphere� However, increased human activity, primarily through carbonaceous material oxidation via combustion and through increased releases of other greenhouse gases such as methane, is an obvious contributor� A decrease in overall fuel consumption, when it can be achieved, will lead to less total mass of carbon dioxide emitted� According to the Kyoto Protocol, which was negotiated in 1997, industrialized countries

were supposed to commit to reducing their total greenhouse gas emissions to an average of 5% below the 1990 levels over the period of 2008 to 2012� This was meant to represent a first step� However, this and other more intensive measures are urgently needed and await full and comprehensive implementation�

The amount of carbon dioxide produced depends of course on the amount of carbon fuel consumed� For example, for every unit of energy produced, natural gas will produce about half that of carbon dioxide generated by the burning of coal� However, methane is a much stronger greenhouse gas than carbon dioxide� Its release into the atmosphere comes from a number of sources, including oil and gas industrial installations, livestock releases, organic waste decomposition, coal mining, natural gas flaring, and seepages, as well as the incomplete burning of vegetation and natural gas�

Some greenhouse gases are more effective in trapping solar radiation than others� The Global Warming Potential (GWP) is a measure of how much a given mass of a greenhouse gas is estimated to contribute to global warming� It is a relative scale compared to that of the same mass of carbon dioxide that has a GWP value of one� Methane is 23 times more potent as a greenhouse gas than carbon dioxide over a presence of 100 years�

Some of the measures that may be employed to reduce greenhouse gas emissions can be the following:

1� Improving the conduct of combustion processes to reduce fuel consumption per unit energy produced

2� Improving throughout the utilization of the energy produced in various devices to economize on the use of fuel

3� Fuel switching by substituting fuels with lower carbon-to-hydrogen ratio such as natural gas for liquid fuels or coal

4� Developing effective methods for the long-term disposal and usage of carbon dioxide

5� Reducing the release of greenhouse gases such as methane into the atmosphere through leakages or flaring

6� Reducing or eliminating exhaust gas pollutants, especially oxides of nitrogen, that are strong greenhouse gases also

The International Energy Agency’s estimates of the emissions of greenhouse gases by sector, for example, in North America are as follows:

• Industrial processes: 17% • Electric power generation stations: 22% • Waste disposal and treatment: 3%

• Land use and biomass burning: 10% • Residential, commercial, and other sources: 10% • Fossil fuel retrieval, processing, and distribution: 11% • Agricultural by-products: 13% • Transportation fuels: 14%

Since most living matter contains some sulfur compounds, sulfur organic compounds occur naturally in fossil fuels� Consequently, unless removed, they would appear in their products, such as natural gas, coal, gasoline, and diesel fuel� Sulfur is a major source of air pollution and has been shown to react with the catalysts in catalytic converters of current design automobiles, reducing greatly their effectiveness�

The burning of coal and various petroleum products by industry, power plants, and vehicular engines liberates huge amounts of sulfur dioxide, which reacts with atmospheric humidity and oxygen to produce sulfuric acid, contributing to acid rain� The sulfur oxides, the fine particulates of sulfates and acid aerosols, produce a variety of negative effects on people’s health and the environment� Sulfur dioxide, together with oxides of nitrogen, which are the major sources of acid rain, contribute to increased respiratory problems, especially in the elderly and children� Emissions of sulfur products should be eliminated to reduce the amount of greenhouse gas emissions and to lessen the damage or injury caused to many plant species� Moreover, increases in the concentrations of these oxides in the atmosphere accelerate the corrosion of metals and may damage stone, masonry, paint, some fibers, leather, and electrical components�

It is technically challenging and costly to remove sulfur from fuels� Furthermore, high-sulfur-content crude oils and coals are becoming increasingly available now, whereas the availability of low-sulfur-content crude is becoming increasingly limited and is more costly�

In recent years, the sulfur content of gasoline and diesel have been reduced in most countries, with a continuing tendency to reduce them further to very low levels� Sulfur in gasoline in recent years has been reduced to levels lower than 30 ppm by mass and in diesel lower than 15 ppm� To arrive at such ultralow concentrations of sulfur in fuels adds to the complexities of refining petroleum and results in increased costs� Heavier and cracked oil feedstocks usually contain high concentrations of sulfur, and it is more difficult to reduce to the sulfur concentration to very low values, which is increasingly required�

Corrosion represents a continuing challenge for the fuel and energy industry with huge economical losses and costs� It is directly associated not only with the type of metal or steel involved but also with the quality of fuels being used� In addition, the presence of moisture enhances the rate of corrosion, producing damage to metals in the form of pitting, cracking, or erosion� The major concern of corrosion originating from fuels is the presence of sulfur and other materials in the fuel such as vanadium� Improving the fuel quality through processing, refining, and using additives is vital for reducing the incidence, intensity, and rate of corrosion� Coatings, inhibiters, and electrochemical methods have been widely used in industry to combat corrosion�

Hydrogen tends to attack various industrial materials with different intensities� This attack not only involves the formation of an oxide or sulfide but also proceeds via a reaction of hydrogen forming minute gas bubbles� These can cause metal damage by producing small local ruptures, changing the surface texture, forming blisters, reducing ductility, and inducing embrittlement�

1� Indicate very briefly, with reference to their modes of combustion, why gas turbines tend to have higher exhaust emissions of NOx but lower concentrations of carbon monoxide than automotive spark ignition engines do, even when operating on the same fuel�

Answer:

• Gas turbines usually involve a steady, partially mixed diffusion type of combustion, where the bulk of the combustion will be at the mixture regions not very far from the stoichiometric ratio� These are associated with high temperatures�

• Overall, excess air is used in the gas turbine, providing excess oxygen, which aids in the production of NOx�

• The typical spark ignition engine operates on homogeneous mixtures of fuel and air, often throttled with exhaust gases and catalytic converters that reduce the extent of emitted NOx� The intermittent nature of its combustion represents very short residence times per operating cycle� Such intermittent combustion may impede slightly the formation reactions for NOx to go to completion within the combustion time available�

• The fuel used in gas turbines more widely varies in quality than that used in spark ignition engines and may include less-controlled concentrations of combined nitrogen�

• Land-based gas turbines often work steadily at high output for very long periods, whereas the automotive engine is rarely operated at full load for long� The continuous nature of gas turbine combustion would result in the production of NOx, which increases virtually logarithmically with peak temperatures�

1� What do you consider the main factors that control the extent of exhaust gas emissions in transport engine applications? Very briefly list some of the key measures that are being developed recently to reduce emissions while improving associated engine performance�

2� Why are there intense efforts to reduce the exhaust emissions of oxides of nitrogen after the combustion of common fuels in practical systems? Outline briefly why diffusion-type flames burning in air produce more oxides of nitrogen per mass of fuel than when the combustion takes place in a homogeneously premixed mode with excess air�

3� In Figure 7�10 relating to the concentration of exhaust emissions from a combustion process, identify the curves shown (not drawn to scale) from within the list of the following species: (a) carbon dioxide, (b) carbon monoxide, (c) oxygen, (d) nitrogen, (e) unburned fuel, (f) oxides of nitrogen, and (g) particulates� Briefly indicate the basis for your choice�

4� Suggest why the exhaust gas of power devices may contain a significant amount of carbon monoxide even when more than sufficient air to oxidize the fuel fully is supplied�

5� With reference to their mode of combustion and the fuels they use, briefly contrast and compare the characteristics of exhaust emissions from industrial gas turbines, spark ignition engines, power plants operating on the vapor power cycle, and diesel engines�

6� There is increased emphasis on reducing the emission of greenhouse gases, notably carbon dioxide, from mobile power sources through reducing specific fuel consumption� Outline very briefly some of the important measures that can be incorporated in the design and operation of vehicles to achieve such reductions�

7� Exhaust gas recirculation, both cooled and uncooled, is being increasingly adapted to various degrees to influence the combustion process and control the extent of exhaust emissions in combustion devices� Very briefly outline the basis for this application�

8� A vehicle consumes 7�5 liters of gasoline per 100 km traveled� Estimate the mass of carbon dioxide produced per kilometer traveled� Take iso-octane to represent gasoline�

9� Where do you consider that further reductions in exhaust emissions may come from in relation to industrial-type furnaces and how they may be achieved?