L06 - AIR POLLUTIONS, their FORMATION and IMPACTS - NOx

Question 1

When it comes to air pollution, there are two categories:

Answer 1

Primary and Secondary

Question 2

What are Primary Air pollutants

Answer 2

those that are sent to atmosphere by
the original processes (burning,
etc.)

ex: CO2, SO2, CO, NO, NO2

Question 3

What are Secondary air pollutants?

Answer 3

Formed from the reactions of
primary pollutants (helped by
sunlight and rain)

Question 4

How are primary air pollutants formed?

Emission: Nitrogen
oxides ->

1. Formula(s)
2. Method and condition of formation?
3. Main negative impacts
4. One method of reduction

Question 5

How are primary air pollutants formed?

Emission: Sulphur
oxides ->

1. Formula(s)
2. Method and condition of formation?
3. Main negative impacts
4. One method of reduction

Question 6

How are primary air pollutants formed?

Emission: Carbon dioxides ->

1. Formula(s)
2. Method and condition of formation?
3. Main negative impacts
4. One method of reduction

Question 7

How are primary air pollutants formed?

Emission: Particulate Matters ->

1. Formula(s)
2. Method and condition of formation?
3. Main negative impacts
4. One method of reduction

Question 8

What is NOx?

Answer 8

1. NOx refer to oxides of nitrogen including NO2 and NO.
2. NOx is formed where O2 and N2 are meeting at high temperatures. (reacts to form smog and acid rain).
3. The higher the temperature, the more NOx is formed.
4. In engines, the higher the temperature, the more efficient the engine.
5. Marine diesel engines are most efficient engines and also produce highest NOx compared to other types of engines and combustion systems.
6. Engines with lower speed generally produce higher NOx.
7. NOx from other sources (e.g. boilers) are relatively small.

Question 9

how does a 4-stroke diesel engine work?

Answer 9

4 stroke so 4 steps:

1. Intake:

Air (light blue) is drawn into the cylinder through the open green air inlet valve on the right as the piston
moves down.

2. Compression:

The inlet valve closes, the piston moves up, and compresses the air mixture, heating it up. Fuel (dark blue) is injected into the hot gas through the central fuel injection valve at the end of compression.

3. Power:

As the air-fuel mixture ignites and burns, it pushes the piston down, driving the crankshaft (red wheel
at bottom) that sends power to the propeller or generator

4. Exhaust:

Green outlet valve on the left opens to let out the exhaust gases, pushed out by the returning piston.

Question 10

What is the reaction mechanism of NOx?

Answer 10

The reaction mechanism of NOx can be expressed using the Zeldovich formula:

N2 + O = NO + N (k1/k2)
N + O2 = NO + O (k2/k/3)
N + OH = NO + H (k4/k5)

k = the reaction rate and is a function of temperate. And k is high at a high temperature.

Question 11

What is the unit for NOx?

Answer 11

• ppm (part per million)
• g/kWh
• g/kg fuel
• % mass or volume

Question 12

How can you convert from one unit to the other?

Answer 12

???

Question 13

What are the negative impacts of NOx?

Answer 13

1. Human health: NOx is an irritant gas to human lungs and could cause lung inflammation.

NOx is a reactive gas, at the presence of sunlight:

2. NOx together with VOC (Volatile organic compounds are organic compounds that have a high vapor pressure at room temperature)
   and other reacting gases in the atmosphere, could lead to smog (smoky fog) phenomenon.
3. Additionally, NOx contributes to global warming, as well as to acid rain and ground-level Ozone (O3)

Question 14

What is photochemical smog?

Answer 14

Photochemical smog is the chemical reaction of ground level ozone, nitrogen oxides, VOC and other gases and particles in the atmosphere with help from sunlight, which leaves airborne particles and ground-level ozone.

Question 15

How can NOx levels be reduced?

Answer 15

1. EGR is an option. Exhaust Gas Recirculation technology on vessels is an effective strategy to control NOx emissions from diesel engines. The EGR reduces NOx through lowering the oxygen concentration in the combustion chamber, as well as through heat absorption
2. LNG is also an option as an alternative fuel.

LNG engines are tuned to either emit low nitrogen oxide (NOx) emissions—at the cost of higher methane emissions in some cases—or to incorporate NOx reduction technologies such as exhaust gas recirculation (EGR) or selective catalytic reduction (SCR).

• Lean burn, low temperature combustion.
• SI combustion

3. Selective Catalytic Reduction - is a means of converting nitrous oxides in the exhaust with the help of a catalyst into diatomic nitrogen and water. This exhaust gas after-treatment technology has a NOx abatement capability Of more than 80%. The SCR concept involves injecting a Urea-Water solution into the exhaust gas stream in combination with a special catalyst unit.

The SCR is considered as an additional and independent exhaust treatment system and as such does not interfere with the basic engine design or combustion process.

Other methods:

• Injection contrtol
• Water-fuel emulsions.
• Humid Air Manifold (HAM or water
  injection).
• Miller cycle: Valve timing.
• Low NOx combustion.
• Etc.

The best way to minimize NOx formation is to reduce flame temperature, reduce excess oxygen, and/or to burn low nitrogen-containing fuels.

Question 16

Pros and Cons of LNG to reduce NOx emissions:

Answer 16

LNG (Liquefied Natural Gas)
Advantages:

1. Cleaner Burning Fuel: LNG produces lower NOx emissions compared to conventional marine fuels like heavy fuel oil.
2. Other Emissions: LNG also reduces SOx (sulfur oxides) and particulate matter emissions significantly.
3. Efficiency: Modern LNG engines are highly efficient and can meet strict emission standards.

Disadvantages:

1. Infrastructure: Requires substantial investment in LNG infrastructure for bunkering (fuel supply) and storage.
2. Methane Slip: There’s a potential for methane slip (unburned methane), which is a potent greenhouse gas.
3. Initial Costs: High initial investment in LNG engines and retrofitting existing ships.

Question 17

Pro and Cons of EGR to reduce NOx emissions:

Answer 17

Advantages:

1. NOx Reduction: EGR can reduce NOx emissions by up to 40-50% by recirculating a portion of the exhaust gas back into the engine’s intake, lowering combustion temperatures.
2. Fuel Flexibility: Can be used with different types of fuels, including heavy fuel oil and diesel.

Disadvantages:

1. Complexity and Maintenance: Adds complexity to the engine system and increases maintenance requirements.
2. Potential for Increased Soot and PM: Can lead to increased particulate matter (PM) and soot formation.
3. Limited Reduction: Not as effective as SCR in reducing NOx emissions to the lowest levels.

Question 18

Pros and Cons of SCR to reduce NOx emissions:

Answer 18

Advantages:

1. Highest NOx Reduction: Can achieve NOx reductions of 90-95%, making it the most effective technology for NOx control.
2. Established Technology: Widely used and proven technology in various applications, including marine and industrial sectors.
3. Compliance: Meets the most stringent NOx emission regulations, such as IMO Tier III standards.

Disadvantages:

1. Urea Supply: Requires a continuous supply of urea (ammonia) for the catalytic process, adding to operational costs.
2. Space Requirements: Needs additional space for the SCR system and urea tanks, which might be a constraint on some ships.
   Installation Costs: High installation and retrofitting costs, particularly for existing vessels.

Question 19

Out of LNG, EGR and SCR, which option is the best for NOx reduction?

Answer 19

SCR is generally the best option for NOx reduction due to its ability to achieve the highest levels of reduction, meeting the strictest environmental regulations.

However, the choice between LNG, EGR, and SCR will depend on specific circumstances, such as existing infrastructure, budget, operational requirements, and overall environmental strategy.

1. Best for Highest NOx Reduction: SCR
2. Best for Overall Emission Reduction (NOx, SOx, PM): LNG
3. Best for Flexibility with Different Fuels: EGR

In practice, many operators might use a combination of these technologies to optimize emission reductions and comply with various regulations.

Question 20

What is the single most important physical factor that causes NOx
formation?

Answer 20

high combustion temperature

Question 21

Name 3 impacts of NOx emissions?

Answer 21

1. human health - lung inflammation
2. contributes to acid rain, ground level ozone layer, formation of smog when reacting with VOC
3. global warming

Question 22

Describe the term “smog”?

Question 23

In which of the following combustion systems, NOx is formed most and
why:
– Marine medium speed diesel engines?
– Gas turbines?
– Steam boilers?
– Marine slow speed diesel engine?
– Dual fuel engines?

Answer 23

1. higher combustion temperatures
2. longer combustion duration
3. high compression ratios

Question 24

Name three methods by which diesel engine’s NOx can be reduced.

Answer 24

1. Selective Catalytic Reduction
2. Exhaust Gas Recirculation
3. Humid Air Motor