DF 10 - the trouble with emissions Flashcards
specification reference - (k) (n)
what goes and comes out of a car engine
fuel goes into a car engine and CO2, CO, H2O, unburnt hydrocarbons, nitrogen, NOx and SOx and particulates come out of a car engine
what is NOx, SOx, and CxHy
NOx - oxides of nitrogen
SOx - oxides of sulfur
CxHy - various hydrocarbons
what is evaporative emission
vaporized fuel being emitted into the atmosphere from the fuel system of a motor vehicle.
how much does evaporative emission account for the emissions of volatile organic compounds from petrol vehicles
10%
where do the oxides of sulfur come from in vehicle exhausts
comes from sulfur compounds in the fuel. these combine with the oxygen from the air in heat of the engine
where do the oxides of nitrogen come from
they’re formed mainly from the components of the air itself. at the high temperatures in vehicle engines, nitrogen and oxygen react in the air to form nitrogen oxide. some of this reacts with more oxygen to form nitrogen oxide
why do NOx and SOx give rise to acid rain
they’re both acidic
what does acid rain do
causes health problems, corrodes limestone buildings and damages forests and lakes
how is carbon monoxide formed
by the incomplete combustion of hydrocarbon fuels.
what is the problem with carbon monoxide
it is very toxic to humans and is oxidised to carbon dioxide in the atmosphere
what are particulates
small carbon particles not visible to naked eye that can get into our lungs and cause irritation and disease. they are also produced by the incomplete combustion of the hydrocarbon fuels in diesel
why is ozone a secondary pollutant
it is not released directly into the atmosphere.
how is ozone formed
from chemical reactions that occur when sunlight shines on a mixture of primary pollutants, oxygen and water vapour
give examples of primary pollutants
nitrogen oxides and hydrocarbons
what is the difference between a primary and secondary pollutant
A primary pollutant is an air pollutant emitted directly from a source. A secondary pollutant is not directly emitted as such, but forms when other pollutants (primary pollutants) react in the atmosphere.
what is photochemical smog
haze in the atmosphere accompanied by high levels of ozone and nitrogen oxides, caused by the action of sunlight on pollutants.
when does a photochemical reaction occur
when a molecule absorbs light energy and then undergoes a chemical reaction
what reactions occur in photochemical smogs
other irritating compounds are formed by the breakdown and further reactions of the hydrocarbons
what is the problems with ozone
it can be an irritating toxic gas and high concentrations near ground level are damaging to human health. it weakens the body’s immune system and attacks lung tissue
how does ozone contribute to global warming
it acts as a greenhouse gas
what does the composition of primary and secondary pollutants in ozone depend on
the nature of the primary pollutants, weather conditions, time of day and length of smog
when do photochemical smogs normally occur
in the summer during high pressure (anticyclonic) conditions
what do photochemical smogs cause
haziness and reduced visibility in the air close to the ground. it can cause eye and nose irritation and some difficulty in breathing. in vulnerable groups such as asthmatics who already have respiratory problems and many older people, this effect can be enhanced
how can high ozone concentrations affect plants and animals
ozone is a highly reactive substance that attacks most organic matter. compounds with carbon-carbon double bonds are particularly vulnerable so many materials such as plastics, rubbers, textiles and paints can be damaged
what do the products of combustion depend on
the amount of oxygen available
what is the equation for complete combustion (in a plentiful supply of air/oxygen)
hydrocarbon + oxygen —> carbon dioxide + water
what is the equation for incomplete combustion (in a limited supply of air/oxygen)
hydrocarbon +oxygen (limited) —> carbon + carbon monoxide + water
give a balanced equation for the complete combustion of heptane
C7H16 + 11O2 —> 7CO2 + 8H2O
what else can be used as fuels
alcohols
what does the burning of sulfur compounds produce
sulfur dioxide
S + O2 —-> SO2
how do nitrogen and oxygen react to form nitrous oxide
nitrogen and oxygen in the air react in the high temperatures of the vehicle engines
N2 + O2 —> 2NO
how are other nitrogen oxides formed
by burning nitrogen compounds in fuels but these are present in very low proportions especially in vehicle fuels
what is a key fact to note about NOx and SOx
the sulfur is present in the fuel but the nitrogen isn’t
how is sulfuric acid formed
sulfur dioxide reacts with water in the atmosphere to form sulfuric acid, a weak acid
SO2 + H2O —> H2SO4
how is sulfur dioxide oxidised to sulfur trioxide
it occurs in the stratosphere. the sulfur trioxide then reacts with water in the atmosphere to form sulfuric acid, a strong acid
SO3 + H2O —-> H2SO4
how is nitric acid formed
nitrogen oxide and nitrogen dioxide react with water and oxygen in the atmosphere to form nitric acid, a strong acid
2NO + H2O + 1 1/2O2 —-> 2HNO3
2NO + H2O + 1/2 O2 —-> 2HNO3
what is the major source and polluting effects of particulates
major sources - volcanoes, burning fuels and burning coal
polluting effects - penetrate deep into the human body causing heart attacks and lung cancer
what is the major source and polluting effect of volatile organic compounds
major sources - plants, unburnt fuels from petrol vehicles
polluting effects - photochemical smog
what is the major source and polluting effect of carbon monoxide
major sources - incomplete combustion of hydrocarbons in fossil fuels, burning biomass
polluting effects - toxic gas, photochemical smog
what is the major source and polluting effect of carbon dioxide
major sources - combustion of fossil fuels
polluting effect - greenhouse effect
what is the major source and polluting effect of nitrogen oxides
major sources - combustion of fuels in power stations and vehicles
polluting effect - acid rain, photochemical smog
what is the major source and polluting effect of sulfur oxides
major sources - volcanoes, burning of fuels containing sulfur
polluting effect - toxic gas and acid rain
how can we tackle the problems of emission
one involves changing the design of the car and the other involves changing the fuel used by the car.
also we can limit the traffic entering towns and encouraging car sharing schemes of tackling the problems directly
which 3 reactions naturally occur to convert pollutants into carbon dioxide, water and nitrogen
1) using oxygen to turn carbon monoxide to carbon dioxide
2) using oxygen to turn hydrocarbons to carbon dioxide and water
3) reacting nitrogen oxide with carbon monoxide to form carbon dioxide and nitrogen
why is the way that pollutants are converted a problem
the reactions that convert pollutants into carbon dioxide, water and nitrogen all occur naturally. however, under the conditions inside an exhaust system they go too slowly to get rid of the pollutants
what are catalytic converters
they are in petrol cars and contain catalysts of platinum or rhodium on a honeycomb structure
what do catalytic converters do
they speed up the 3 reactions that convert the pollutants into less harmful products
what is another name for a catalytic converter
a 3-way catalysts as they speed up the 3 reactions that convert pollutants
when do catalytic converters only work
if the air-petrol mixture is carefully controlled so that it is exactly the stoichiometric mixture for the fuel
what does it mean by the “stoichiometric mixture of the fuel”
it has the exact calculated ratio of hydrocarbon to oxygen needed for complete combustion
what would happen if the air-petrol mixture inside a car had too much fuel
there is not enough oxygen in the exhaust fumes to remove carbon monoxide and the hydrocarbons
what do cars fitted with 3-way catalyst systems need to have
oxygen sensors in the exhaust gases, linked back to electronically controlled fuel injection systems
when do catalytic converters work
when they are hot
at what temperature do platinum catalysts start working at
around 240° but you can get the catalyst to start working at around 150° by alloying the platinum with rhodium
why can converters only be used with free-lead fuel
because the catalysts are poisoned with lead
how is the catalyst given a large surface area
its used in the form of a fine powder spread over a ceramic support with a surface that has a network of tiny holes
why would any attempt to reduce NOx to nitrogen in a diesel engine fail
diesel engines have a higher concentration of oxygen so any attempt to reduce NOx to nitrogen would fail as the reducing agent would be oxidised by the oxygen
what do diesel engines have to overcome this problem
they have oxidation catalysts that turn carbon monoxide to carbon dioxide and hydrocarbons to carbon dioxide and water
how have the amount of particulates in diesel exhaust gases been reduced
by burning them at high temperatures
what is the problem with burning particulates at high temperatures to get rid of them
it reduces fuel efficiency
what are the main pollutants in petrol engines
carbon monoxide, hydrocarbons and nitrogen monoxide
what are the pollutants in petrol engines converted to
carbon monoxide
2CO + O2 —> 2CO2
hydrocarbons
C7H16 + 11O2 —-> 7CO2 + 8H2O
nitrogen monoxide
2NO + CO —–> N2 + 2CO2
what catalyst converts the pollutants in a petrol engine
platinum or rhodium in a porous support
how are sulfur oxide pollutants best avoided in petrol engines
by removing sulfur impurities from the fuels before being made available to the motorist
what are the main pollutants in diesel engines and how are they removed
carbon monoxide, hydrocarbons, particulates and nitrogen oxide compounds.
CO and CxHy are removed in the same way as in petrol engines
particulates are removed by diesel particulate filters that contain a variety of materials, most commonly ceramic
what is regeneration in a catalyst
burning off the carbon particles
how is regeneration of a catalyst accomplised
by increasing the temperatures at times decided by the vehicles computer. this increases fuel consumption
how are nitrogen oxides reduced in a diesel engine
by recycling some of the exhaust gases through the cylinder, lowering the temperature and thus the amount of NOx formed.
what is an alternative to reduce NOx in diesel engines
alternatively a reagent such as ammonia is used in the presense of a catalyst
4NO + 4NH3 + O2 —> 4N2 + 6H2O
how is SOx reduced in diesel engines
sulfur dioxide is best avoided in the first place by using ultra low sulfur fuels
