**6. The diesel engine exhaust emissions**

Regulated exhaust emissions from the diesel engine include HC, CO, NOx and PM. The conditions under which each of these is generated are interrelated in a complex web of factors; and much effort have focused on measures to improve the combustion process in order to counter their generation. These 'internal measures' target the highlighted emission factors like high injection pressures, injection rate shaping, combustion chamber designs, turbocharging and intercooling, transient control, spray-air interactions etc. Measures that target actual emissions in the exhaust system are considered as after-treatments. These measures deal with reduction or elimination of the emissions variables along the exhaust pipe prior to their release into the atmosphere.

#### **6.1 The hydrocarbon (HC)**

It is conventional in the domain of engine emission to refer the unburnt hydrocarbons (UHC) simply as 'hydrocarbons' (HC). The concentrations could range from 20 to 300 ppm; and for control purpose, distinction is often made of total hydrocarbons (THC) and non-methane hydrocarbons (NMHCs) by excluding methane gas from the total hydrocarbons. HC emissions contain several compounds, the variety and concentrations differ as the engine operating factors differ and vary. Small fractions of total unburnt hydrocarbons emerge as the originally sprayed fuel that survived oxidation during combustion, while higher proportions emerge as different intermediates due to pyrolysis and partial oxidation of fuel and lubricants. The diversity includes alkanes, alkene, alkynes and aromatic, meaning that HC emissions occur as single, double and triple bonds or in combinations; straight and branched chains or ringed chain structure as in aromatics. Aromatics with benzene basic ring structure are the building blocks for higher cyclic-structures of PAHs which can occur from fuel breakdowns and also from of incomplete combustion of fuel. The emission of hydrocarbons in diesel engines is significantly influenced by engine operating conditions of which, idling to light load modes are most culprits. The prevalent reasons include over-mixing of fuel with air which leads to over-leaning and therefore difficult to support combustion particularly at low temperatures; under-mixing which causes over-rich mixture and so difficult to ignite; and flame quenching at low temperature walls which causes partial burning. During ignition delay, the equivalence ratios attained by fuel-air mixture varies within a wide range. Some portions of the mixture are favorable to auto-ignition; some are locally too rich to burn while others are too lean to support combustion. For fuel injected when combustion has commenced, high temperature favors

#### *Diesel Exhaust Emissions and Mitigations DOI: http://dx.doi.org/10.5772/intechopen.85248*

rapid oxidation of fuel-air charge towards complete combustion. However, there is over-rich portion of the mixture due to slow mixing of fuel and high quantity of intermediate products of pyrolysis due to high temperature and local starvation of air, fuel portions left in the nozzle sac, crevices volumes, as well as the effect of wall quenching as pointed. These lead to products of incomplete combustion and fuel in the exhaust. Many forms of intermediate products of incomplete combustion involve 'oxygenates' which are not pure hydrocarbons. Along with hydrocarbons, they constitute volatile components (VOCs) which condense as particulate matter as temperature drops on the exhaust line.

#### **6.2 Carbon monoxide (CO)**

During oxygen deficient combustion process, fuel is not fully oxidized; CO (which is an intermediate product on the fuel reaction-path), is produced along with HC as products of incomplete combustion. However combustion in most diesel engines are practically lean beyond stoichiometric, therefore CO emission is normally low except during transient operation and is easily oxidized to CO2 upon release to the atmosphere.

## **6.3 Nitrogen oxides (NOx)**

The oxides of nitrogen relevant to diesel engine emission are nitrogen oxide (NO) and nitrogen dioxide (NO2). Nitrous oxide (N2O) is not a regulated emission; they are collectively referred to as oxides of nitrogen (NOx) of which, NO is dominantly emitted during combustion and NO2 is formed as a consequence of further oxidation. In the presence of ultra-violet light, NO2 forms a photochemical smog with non-methane hydrocarbon. During combustion, the main oxidant is oxygen, a constituent of air drawn together with nitrogen, which is technically non-reacting to fuel. However at high temperatures, nitrogen reacts with oxygen; and the mechanism follow many pathways. As adapted from [30] the NO formation mechanisms are: the thermal or Zeldovich mechanism, the Fenimore or prompt mechanism, the N2O-intermediate mechanism and the NNH mechanism. The thermal mechanism is dominant in high temperature combustion over wide equivalent ratios while Fenimore mechanism is predominant in combustion of rich fuel mixture. The N2O-intermmediate is considered to play an important role in the production of NO in very lean, low temperature combustion and the NNH is a new introduction to NO formation mechanisms.

*Thermal or Zeldovich mechanism* consists of two chain reactions.

$$\bullet \text{ + N}\_2 \leftrightarrow \text{NO} \star \text{N} \tag{7}$$

$$\text{N} \star \text{O}\_2 \leftrightarrow \text{NO} \star \text{O} \tag{8}$$

with an extended reaction.

$$\text{N} \star \text{OH} \leftrightarrow \text{NO} \star \text{H} \tag{9}$$

The forward (f) and reverse (r) reaction rate coefficients as in [31] are as follows:

$$\begin{aligned} k\_{x1f} &= 1.8 \times 10^{11} \exp\left[-38, 370/\text{T}\text{(K)}\right] \text{ [=} \text{] } \text{m}^3/\text{kmol} - \text{s} \\\\ k\_{x1,r} &= 3.8 \times 10^{10} \exp\left[-425/\text{T}\text{(K)}\right] \text{ [=} \text{ ${}^\text{m}$ } \text{/kmol} - \text{s}\end{aligned} $$

$$\begin{aligned} k\_{x2,f} &= 1.8 \times 10^7 \text{T} \exp\left[-4, 680/\text{T}\left(\text{K}\right)\right] \text{ [=] m}^3\text{/kmol} - \text{ s} \\\\ k\_{x2,r} &= 3.8 \times 10^6 \text{T} \exp\left[-20, 820/\text{T}\left(\text{K}\right)\right] \text{ [=] m}^3\text{/kmol} - \text{ s} \\\\ k\_{x3,f} &= 7.1 \times 10^{10} \exp\left[-450/\text{T}\left(\text{K}\right)\right] \text{ [=] m}^3\text{/kmol} - \text{ s} \\\\ k\_{x3,r} &= 1.7 \times 10^{11} \exp\left[-24, 560/\text{T}\left(\text{K}\right)\right] \text{ [=] m}^3\text{/kmol} - \text{ s} \end{aligned}$$

The reaction set are generally coupled to combustion chemistry through the O2, O and OH species. However depending on the time scale, it is also possible that combustion is completed before NO formation becomes significant, then the two processes can be decoupled and the formation rate of NO is expressed as:

$$\frac{d[NO]}{dt} = \text{2}\,\text{k}\_{N,\text{M}}[O]\_{eq} [N\_2]\_{eq} \tag{10}$$

Thermal mechanism is highly dependent on temperature; as typified by equation (z1), the activation energy is very large (about 319,050 kJ/kmol), giving insignificant NO formation in reactions where temperatures are below 1800 K.

The NO formation by *Fenimore mechanism* is linked to the combustion chemistry of rapidly oxidizing flame zone of premixed hydrocarbon flames ahead of the NO formation through thermal mechanism. Fenimore [32] discovered this and gave it the appellation of *prompt NO*. The scheme is generally described as the reaction between hydrocarbon radicals and with molecular nitrogen leading to the formation of *amines or cyano-compounds*. These amines or cyano-compounds are then converted into intermediate compounds that yields NO. Starting from CH radicals in the formation process, the Fenimore mechanism can be expressed as:

$$\text{CH} + \text{N}\_2 \leftrightarrow \text{HCN} + \text{N} \tag{11}$$

$$\text{C} \star \text{N}\_2 \leftrightarrow \text{CN} \star \text{N} \tag{12}$$

For air-fuel mixture where equivalence ratios (Φ) are less than 1.2, hydrogen cyanide (HCN) forms NO through the following the sequence:

$$\text{HCN} \star \text{O} \leftrightarrow \text{NCO} \star \text{H} \tag{13}$$

$$\text{NCO} \star \text{H} \leftrightarrow \text{NH} \star \text{CO} \tag{14}$$

$$\text{NH} + \text{H} \leftrightarrow \text{N} + \text{H}\_2\tag{15}$$

$$\text{N} \star \text{OH} \leftrightarrow \text{NO} \star \text{H} \tag{16}$$

When (Φ) are greater than 1.2, other reaction pathways open up with more complex chemistry. As stated [33], the NO formation will cease to be rapid instead, it is recycled to HCN. In addition, the Zeldovich reaction that couples to prompt mechanism further reduces the formed NO to elemental nitrogen as N <sup>+</sup> NO <sup>→</sup> N2 <sup>+</sup> O.

The *N2O-intermediate* mechanism is relevant in fuel-lean premixed combustion schemes where (Φ < 0.8) and at low temperatures. Reaction steps for this mechanism are:

$$\text{O} + \text{N}\_2 + \text{M} \leftrightarrow \text{N}\_2\text{O} + \text{M} \tag{17}$$

$$\text{H} + \text{N}\_2\text{O} \leftrightarrow \text{NO} + \text{NH} \tag{18}$$

$$\text{O} + \text{N}\_2\text{O} \leftrightarrow \text{NO} + \text{NO} \tag{19}$$

In equations (17), 'M' represents a general third body; and by implication, the involvement of third bodies means that the mechanism is favored at elevated pressures. Similarly, equations (17) and (19) involve oxygen radical O, signifying that the mechanism favor oxygen rich conditions.

The NO formation by *NNH mechanism* is the most recently discovered reaction pathway and the steps involved are:

$$\text{N}\_2 \star \text{H} \to \text{NNH} \tag{20}$$

and

$$\text{NNH} + \text{O} \rightarrow \text{NO} + \text{NH} \tag{21}$$

This route to NO formation is linked to combustion of hydrogen, hydrocarbon fuels with high carbon-to-hydrogen ratios and certain fuels that contain nitrogen in their molecular structure (fuel nitrogen). The amount of fuel borne nitrogen is quite negligible with respect to diesel fuel and therefore insignificant.

Irrespective of the mechanism of formation, the NO in diesel engines is a byproduct of combustion and not a compound in transit like CO. Although NO formation is possible through any mechanism, it can either be sustained or destroyed depending on the prevailing equilibrium direction. It is expedient that in mitigation strategy, more emphasis is laid on the formation through thermal or Zeldovich mechanism since high temperature formation is related to the time of post-flame reactions. This is the reason why the NO formation in diesel combustion are associated with the extended Zeldovich mechanism, favored at high (local) temperature equilibrium diffusion flames.

#### **6.4 Particulate matter (PM)**

The term particulate matter (PM) is used to describe any matter that could be trapped on a sample filter paper when the exhaust gases are cooled to 52°C or less. It includes all condensates formerly in vapor state from various substances like sulfates, nitrates, organics, as well as solid black carbon particles (soot) and ash. The sources and reasons for PM emissions are immense when the variety of species are considered; however the HC and the soot particles are of more interest. The multi-phase and the complex nature of PM are characterized by its chemical composition and its physical characteristics. Physically, the sizes at which PM occur bears greatest attribute to the way it is characterized. Three customary sizes used to describe PM are the nucleation mode, accumulation mode and coarse mode particles. The nucleation mode particles are the smallest particles that occur by nucleation of species from its vapor state. The sizes vary from the small nuclei at formation, to identifiable film drops; and they usually attach to aggregates of solid

**Figure 9.** *Diesel engine exhaust PM size distributions expressed in number and mass metrics [34] as modified in [35].*

particle if present. They are characterized by high number-concentrations, which are affected by heat, which cause vaporization and leads to drastic reduction in number concentration. The accumulation mode particles are usually aggregates of primary particles that form as solid carbonaceous core upon which condensates of volatile particles adhere. Coarse mode particles are larger, perceived as accretion of accumulation mode particles that were deposited at the walls and latter carried back to the aerosol bulk stream. These size distributions can be expressed either in number or mass metrics as shown in **Figure 9**. Chemically, the composition of PM are described as volatile or soluble; and the non-volatile or insoluble components (depending on method of separation). These are further classified according to their chemical speciation in assay as prescribed by various regulatory authorities that prescribe the acceptable test protocols and procedures.

**Figure 10** gives conceptualization of composite fractions, typical of diesel PM emissions which include: sulfates, nitrates, organics carbonaceous and ash [1, 29, 34].

*Sulfate ions SO4 2−* commonly emitted from diesel engine is water-soluble sulfuric acid, therefore assayed through solubility. Other possible sulfate particulates include metallic salt like calcium sulfate CaSO4. The metal ion may originate from lubricant additives which actively react with sulfuric acid. They are less hydrophilic compared to sulfuric acid. Sulfate particulates have been drastically reduced in modern automotive engines as result of improvements in fuels technology leading to achievement of ultra-low sulfur diesels blended with synthetic lubricity additives.

*Conceptualization of the composite fraction of particulate matter [6].*

*Nitrates* are similarly water soluble and this also dictates the assaying technique. The major component formed by NO3<sup>−</sup> ions is nitric acid (HNO3) in the reaction perceived to be due to NO2 and water.

$$2\text{NO}\_2 + \text{H}\_2\text{O} \rightarrow \text{HNO}\_2 + \text{HNO}\_3\tag{22}$$

$$\text{\textbulletHNO}\_2 \rightarrow \text{HNO}\_3 \downarrow \text{2NO} + \text{2H}\_2\text{O} \tag{23}$$

Although the formation mechanism has not been well reported, the presence of HNO3 in diesel exhaust is not in doubt. Nitrate particulates are more researched in atmospheric science where acid-ammonia reactions dominate.

*Organic fractions* assayed by heating or vacuum evaporation are referred to as volatile organic fraction (VOF); and if by dissolution in organic solvent, it is called soluble organic fraction (SOF). In either approach, the masses obtained are closely equal if proper adjustments are made for the non-organic compound in the case of heating. During heating process, evaporated fraction may include water-bound species, and are therefore called volatile organic components (VOC) to distinguish pure organic and from the presence of non-organic particles. The VOF is by far the most complex of the diesel PM because many organic compounds are present. Typical diesel emitted VOF are from unburnt or partially combusted hydrocarbons arising from fuels and lubricants emitted during low engine loads when the exhaust temperatures are low [6].

The *carbonaceous fractions* are also referred in many ways as elemental carbon (EC), soot, black carbon (BC) and graphite carbon. Diesel soot particles are generated mainly during diffusion regimes of the heterogeneous combustion in engines. It basically occur as primary particles in sizes of 20–50 nm which accretes to form identifiable aggregate structures, whose size depends on material availability and exhaust line temperature [29, 34, 36].

*Ash fraction* denotes the burnt or incombustible ashes that arise from metals. Substances such as oxides, sulfates and phosphates of metals used in lubricants additives are likely to be found as well as burnt material of the worn engine components.

#### *6.4.1 PM formation process in the engine*

The PM formation process in the engine is complex and majorly originate from fuel and lubricant. The spray combustion process presented earlier, is indeed the story of soot formation process. When fuel is only partially oxidized, it results in complex intermediates which constitute the volatile organic fractions in the exhaust stream and later condense as particles of organic compound along the exhaust line. The soot process in engine involves conversion of liquid fuel to vapor phase and then to solid particles which are oxidized back to gaseous products. The un-oxidized particles are emitted as the visible black soot. The identifiable steps in soot process could be outlined as pyrolysis, nucleation, coalescence, surface growth,

**Figure 11.** *Steps in soot formation process [37].*

agglomeration and oxidation. The first five steps are outlined as in **Figure 11** but, any of these activities in the formation process could be terminated through oxidation.

The soot oxidation process does not necessarily follow in the sequence but recognized as the final step in the combustion process which could occur anytime and convert hydrocarbons to CO, CO2 and H2O. This means that the precursor feedstock, the nuclei, the primary and agglomerated particles could be consumed in the oxidation process into gaseous products at any stage. In effect, this highlights soot emission as an intermediate product due to incomplete combustion in the engine. Therefore reduction of soot emission through engine combustion process will involve prevention of its formation, promotion of its complete oxidation or both.

### **7. Mitigations of diesel engine emissions**

Many strategies are used to eliminate or reduce the amount of diesel engine exhaust emissions released to the atmospheric environment. For long, a wholesome approach of integrating '*internal factors*' which aims to achieve better engine combustion and '*after-treatments*' which aims at reducing already borne pollutants in the exhaust stream are considered in application. These internal factors include improvements in combustion chamber design, air cooling and boosting; and fuel delivery systems. They are usually modified according to how each parameter is viewed to influence the generation of particular emission variable. Management of these factors can be challenging because some conditions that favor reduction of one variable may be opposed to the other as is typically the case of PM and NOx reductions. The after-treatment technologies are also used in a way that, each or closely related emission variables are targeted for mitigation. These include the use of diesel oxidation catalysts to convert CO and HCs into CO2 and water; diesel particulate filter (DPF) to trap soot; and suitable reductant like urea or ammonia to reduce NO to elemental nitrogen. Integration of these technologies for smooth engine operation is also a big challenge in modern engine technology. In operation, sensors and control loops are used to monitor and coordinate related issues for example, catalysts in the exhaust system operate within a reference temperature window and good thermal management is an issue; the need for additional air requirement to strike the right balance for exhaust-gas quality control or the need for right dosage of urea based on limiting trade-offs. Equipment manufacturers usually describe these peculiarities depending on their compromise path to achieve attainment of the prevailing regulatory emissions levels. Many texts approach the complexities of diesel after-treatments with specialized details [3, 29, 38]. For simplicity, the way some factors affect diesel exhaust emissions and how the mitigation strategies are pursued by targeting those factors are itemized as follows:

#### **7.1 Mitigations by internal factors (air intake, engine-flow and injection related issues)**

The importance of achieving stoichiometric air-fuel ratio has been highlighted to be necessary towards ensuring quality combustion. This is usually achieved through design and operating considerations for *intake port/injector profiles* and *orientations*, *swirl motion* in relation to *fuel injection timing and duration*.

For *naturally aspirated engines*, fixed cylinder volume means air charge per cycle is constant; and for a constant engine speed, the load is controlled through fuel injection. Increase in fuel injection in order to increase the load will affect

#### *Diesel Exhaust Emissions and Mitigations DOI: http://dx.doi.org/10.5772/intechopen.85248*

the mixture composition in many ways. Some portion of fuel is injected during ignition delay period and greater quantity injected as large droplets towards the end of injection. These trigger wide range of equivalence ratios during the combustion phase. For the pre-mixed portion, some will attain stoichiometric proportion, some too lean and others may even be locally too rich to support combustion. The portion injected later will need high temperature to vaporize, fresh air or lean mixture to mix with, and which must be at a fast rate in order to undergo complete combustion. Different emission scenario are possible here depending on engine operating factors. For engine started from cold, rich mixture is inevitable, therefore, exhaust stream will be smoky due to initial richness; allowing the engine to idle for warm up and gradual load increase during the transient period before engine attain its operating temperature, this will lead to more emission of unburnt hydrocarbons and intermediates. Similarly, if combustion rate is not fast enough, lately sprayed fuel and some portion still undergoing pyrolysis will be caught up by fast cooling as expansion stroke progresses and this will lead to emissions of unburnt hydrocarbon and intermediates.

Some of these drawbacks are usually overcome with the *use of turbocharger* to increase the mass of air inducted into the cylinder. This permits proportional increase in fuel injection to suit required power output of the engine. As air velocity will be higher through the intake ports, whether as in swirl-supported or quiescent combustion system, air-fuel mixing is enhanced. This leads to better fuel oxidation and reduction in emission of HC, PM and CO; however, it is responsible for increase in NO formation.

The influence of *fuel injection* scheme on emissions and possible improvements that could be achieved through their modifications are also interesting. Increase in injection pressure can be used to improve entrainment rate which leads to improved combustion efficiency. This reduces emission of hydrocarbon and intermediates; while increasing combustion temperature. Higher temperature regime is a veritable platform for NO formation. On the other hand, injection timing has its own influence on these variables. Advance of injection timing promotes mixture formation which similarly enhances combustion and increase in temperature with attendant NO formation, retard in injection timing has a reverse effect as the premixed portion is reduced and ultimately NO formation is reduced.

One technique that has been used to reduce NO formation is *intake charge dilution*. This may be accomplished through methods like recirculation of exhaust gas, introduction of water spray or nitrogen. The mechanism behind these is that as diluents, they possess high specific heat capacity which make them sources of thermal sink. Since their presence reduce oxygen concentration of the fresh air charge, they will slow down the rate of combustion and thereby lower the peak pressures and temperatures that promote the formation of NO.

#### **7.2 After-treatment mitigation technologies**

As has been highlighted, three after-treatment technologies used in diesel engine exhaust emissions mitigations are use of *oxidation catalysts* that oxidizes CO, HCs (which includes PAHs) and SOF; catalyzed and non-catalyzed *diesel particulate filters* (DPF) to filter and regenerate soot and use of *NOx reductants* like selective catalytic reduction (SCR) with ammonia, SCR with hydrocarbons (deNOx or lean NOx catalyst) and NOx adsorber-catalyst system.

#### *7.2.1 Diesel oxidation catalysts (DOCs)*

These catalysts further oxidizes CO, HC, SOF and PAH into CO2 and water as schematically represented in **Figure 12**.

The following chemistry are involved with the compounds:

$$\text{HC} \star \text{O}\_2 \rightarrow \text{CO}\_2 \star \text{H}\_2\text{O} \tag{24}$$

$$\text{\textbullet{\text{?CO}}} \star \text{O}\_2 \rightarrow \text{\textbullet{\textbullet{?CO}}\_2} \tag{25}$$

It is pertinent to note that DOC oxidizes all compounds with reducing character, and not all these are beneficial. The oxidation of sulfur dioxide to sulfur trioxide is an example; it is highly soluble in water and consequently it leads to formation of sulfuric acid which is emitted in the tail pipe.

$$\text{2SO}\_2\text{+ O}\_2 \rightarrow \text{2SO}\_3\tag{26}$$

$$\text{SO}\_3 \star \text{H}\_2\text{O} \rightarrow \text{H}\_2\text{SO}\_4\tag{27}$$

Similar incident occurs with the oxidation of NO to NO2 which is more toxic than NO.

$$\text{2NO} \star \text{O}\_2 \rightarrow \text{2NO}\_2 \tag{28}$$

This was initially an undesirable development with the use of DOCs until it was discovered that NO2 is a beneficial oxidizer in the catalytic regeneration of DPF where it donates one oxygen atom and reduces back to NO. Platinum, a highly acclaimed noble metal enables a high conversion efficiency of up to 90% to be achieved at a sufficiently high exhaust temperature. It is highly durable in operation and commercially available, DOCs can also remove diesel odor.

#### **Figure 12.**

*Conversion chemistry with the use of DOC.*

**Figure 13.** *Wall-flow diesel particulate filter.*

### *7.2.2 Diesel particulate filter (DPF)*

Diesel particulate filters are used to trap particles of micron and sub-micron sizes carried in the exhaust stream. In effect, condensed SOF, water, soot and ash are contained in the particulate soup. In application, the most commonly used is wallflow DPF which has the cells alternately plug at each end, **Figure 13**. In this way, exhaust gas permeates through walls of the filter while the particles are trapped. The intricacies of PM depositions, measurements and removal are immense and beyond the scope of this piece, and the references already cited on 'after-treatments' can be further consulted. However, the key points to be noted are that PM filtration in the engine system has a technical problem of pressure drop across the filter when it is clogged with accumulated matter. The pressure drop across the DPF due to buildup of PM is critical for continuous safe engine operation. This is conventionally regarded as exhaust back pressure in a relaxed scientific sense.

The gas flow mechanics of the exhaust line is such that exhaust gas is driven by positive compression pressure of the engine, sufficiently high to overcome any obstruction along the exhaust flow line. In a strict sense therefore, the term 'back pressure' is more suitable to express the pressure drop across the entire exhaust line which is numerically equal to the exhaust pressure at the turbo (or exhaust manifold) outlet, not just the pressure drop across a component of the exhaust system [29, 39]. The factors that contribute to pressure drop across the filter can be classified into four perspectives: (a) the geometrical properties where the length, frontal area, wall thickness and channel dimensions are considered; (b) the substrate material properties whereby the porosity, permeability and pore size are considered; (c) the exhaust gas flow characteristics where the temperature, flow rate and viscosity are considered; and (d) the nature of PM membrane, where the particle size distribution, density and permeability are considered [38, 40]. These considerations are normally used to study pressure drop across DPF in both modelling and experimental efforts. With regards to engine operations, engine load and speed determines the exhaust flow rate. Increasing the load and speed increases the pressure upstream of the filter; this means that the quantity of soot accumulated by the filter depends on the usage of the engine. By implication, the pressure drop across the filter set at a pre-determined value has become a parameter to determine when to burn-off accumulated soot (to regenerate the filter). Many techniques are employed in regenerating DPF; these include the use of electric heaters, injection of fast burning fuel into the exhaust line upstream of the filter and use of microwave heating. Modern application adopt the use of catalyzed filters to lower the oxidation temperature of the trapped soot. This ensures continuous regeneration of DPF as the light-off temperature of the catalyst is reached. This is usually in the neighborhood of the exhaust line temperature. Further reading on filter loading and pressure drop across it can be seen in [6, 41–43].

### *7.2.3 NOx reduction*

Reduction of NOx has been successful with reduction catalysts both for lean burn gasoline engines and diesel engines. The reduction chemistry of NO which is favored thermodynamically under the prevailing engine temperatures and pressure can be simply describe as follows:

$$\text{2NO} \rightarrow \text{N}\_2 + \text{O}\_2 \tag{29}$$

One of the best described catalysts is the copper exchange zeolite, Cu/ZSM5 and the decomposition is very sensitive to water and SO2, and operates well at low

#### **Figure 14.**

*Operations of NOx adsorber. Adapted from [3].*

space velocities. Ammonia and Urea offer good selective reduction in the presence of a catalyst over the competitively reaction process with oxygen. By far, this is the predominant technique in application. The chemistry can as well be simplified as:

$$\text{4NO} \star \text{4NH}\_3 \star \text{O}\_2 \rightarrow \text{4N}\_2 \star \text{6H}\_2\text{O} \tag{30}$$

*SCR technique* has a high NOx conversion efficiency of about 90% and this has made it the choice for large diesel engine, co-generation plants and non-engine installations. However, it has a major disadvantage of high installation, space and operating cost. In addition, during operation, there is problem of ammonia slip which leads to equipment fouling with ammonia sulfate.

*Lean NOx or deNOx catalysts* act by the replacement of ammonia with hydrocarbon. Platinum-based catalysts and base-metal catalyst like Cu/ZsM5 have proved successful but with the drawback of operating within a narrow temperature window and about 50% conversion efficiency in active systems and 10–20% in passive systems tested on regulatory test cycles.

*NOx adsorber catalysts* have up to 80% reduction efficiency but depends on drive cycle through two stages of operation. The first stage is that of NOx storage on the catalyst wash coat during lean operation also referred to as chemisorption stage. The second is regeneration of the trap through desorption and non-selective catalytic reduction of NOx during periods of rich operation. These phases of operation are shown in **Figure 14** using upper and lower halves respectively; and a metal nitrate indicated as MeNO3 used to store NOx [3].

**Figure 15.** *Complete train of after-treatment technologies in the exhaust line.*
