Chapter 2 Combustion Flashcards
What is the figure for the specific enthalpy of vapourisation of steam at 15°C?
2.465
What is the amount of sulphur likely to be found in residual oils found today?
Around 6%
What is the problem with SO3 (sulphur trioxide) being found in the flue gas?
It has been shown to raise the acid dew points of the gases. This results in the condensation of acid vapours on colder metal surfaces and the formation of sulphurous and sulphuric acids resulting in corrosion.
What temperature are mild steels and cast irons most severely attacked by dilute acids?
Mild steels :28°C below the dew point
Cast irons: 50°C below the dew point
Where is the problem of acid formation most relevant?
Air heaters and economisers of boiler plants.
In IC engines the problem can affect cylinder liners and exhaust valves, especially where the exhaust valve has a water-cooled cage.
What can the on-board engineer do to help prevent acid corrosion?
Maintain the scavenge air temperature at the correct value to stop the corresponding exhaust temperatures decreasing. This should stop the formation of acids at an earlier stage than what the designers intended and the components will not be able to withstand this attack for long.
What catalysts are known to accelerate the formation of sulphur trioxide?
Vanadium pentoxide (present in the fuel) Platinum
What are the two types of acid attacks?
Dilute acids attack the meal continuously
Strong acids cause an initial attack, which is then stifled by a passive gas layer or skin developed on the metal surface
In practice combustion can only be achieved by supplying excess air.
What percentage excess air should you expect for:
- boiler
- IC engine
Boiler = 30% excess air, around 12kg of air/kg of fuel
IC engine= 100% excess air, around 40kg of air/kg of fuel
What are the parameters for stp and npt?
Stp = 760mm mercury (1.013bar) and 15.5°C Ntp= 760mm mercury (1.013bar) and 0°C
stp and npt are not rigid definitions and the assumptions made should be clearly stated: %CO2 should always be quoted by volume, %CO2 by mass is higher
What is the problem with nitrogen in the combustion process?
At very high peak temperatures nitrogen is oxidising and nitrous oxide (NOx) is being released to atmosphere.
What are the associated problems with NOx?
Increased acidity of particles, cloud water and precipitation, causing damage to marine and plant life.
They are also an important part of the formation of forming photochemical smog and atmospheric oxidants.
What ways are there of reducing NOx emissions IC engines?
Very technically challenging.
Replacing some excess air with recirculated exhaust gas reduces the overall content of nitrogen and excess oxygen available in any one combustion cycle and reduced heat loss to the fresh nitrogen drawn in from the air
When do IC engines need to meet the IMO Tier III standard?
1st January 2016
What engine processes/modifications have been developed to meet lower NOx emissions?
Reducing peak temperatures that are causing the formation
Exhaust gas recirculation and the Miller cycle linked with two stage turbocharging are the two main strategies.
Exhaust gas scrubbing technology could also be employed
