∙ Describe how primary pollutants are formed in the combustion of the fuel in a coal-fired power station.
Oxides of sulphur, (SOx):
Every living organism contains sulphur, this is where the sulphur in coal comes from, but every living organism has different amounts of sulphur, so the emissions of sulphur oxides may vary.
Oxides of nitrogen, (NOx):
Flue gases are combustion products and excess air formed during the combustion of coal, in these flue gases are oxides of nitrogen, this is because coal contains compounds of nitrogen formed from protein of organisms that the coal (200 WORDS) is formed from. During combustion the nitrogen compounds are oxidised and forms a substance called fuel NOx. Also during combustion, atmospheric nitrogen and oxygen combine and form a substance called thermal NOx.
∙ Describe and explain the most favourable conditions for forming photochemical smog, and how high concentrations of tropospheric ozone are produced.
The best conditions to form photochemical smog are; a nice clear sunny day, this is because UV-rays play a big part in the development of the smog. Ozone, O3, is one of the substances that makes up photochemical smog, In order for ozone to be formed NO2 particles must be broken down into NO and O, this process uses UV-light:
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NO2 -→ NO + O
Without this reaction taking place the formation of ozone would not be able to take place:
O2 + O → O3
Also the time of day is important, this is because:
- Early morning traffic increases the emissions of both nitrogen oxides and VOCs as people drive to work.
- Later in the morning, traffic dies down and the nitrogen oxides and volatile organic compounds begin to be react forming nitrogen dioxide, increasing its concentration.
- As the sunlight becomes more intense later in the day, nitrogen dioxide is broken down and its by-products form increasing concentrations of ozone.
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At the same (400 WORDS) time, some of the nitrogen dioxide can react with the volatile organic compounds to produce toxic chemicals such as PAN.
- As the sun goes down, the production of ozone is halted. The ozone that remains in the atmosphere is then consumed by several different reactions.
( internet
Also temperature rises can carry pollutants to a high level, but if there is a temperature inversion it can leave the pollutants close to the surface of the earth.
Ozone occurs naturally at ground level at low concentrations, the sources for these are hydrocarbons and the odd few molecules that have come down from the stratosphere. Tropospheric ozone is formed when O2 reacts with O, in order for this to happen NO2 must be broken down into NO and O. The reaction in which NO2 is broken down uses UV-radiation. However, once the ozone is formed it can react with NO to form NO2 again:
O3 + NO → O2 + NO2
(http://cdiac.esd.ornl.gov/pns/…) - internet
∙ Describe the chemistry of the processes chosen as BPEOs at Longannet for minimising sulphur dioxide and NOx emissions, and suggest why the Longannet management made those choices.
The BPEO, (best practical enviroment option), use two ways to minimise sulphur dioxide, one of them is the limestone process , this is passing the flue gases through a slurry of limestone, (calcium carbonate), the product formed, (600 WORDS), from this is calcium sulphite. This is then aerated to oxidise the sulphite ions to sulphate ions.
The other method used is called the sea water scrubbing process, this is not used as much as the limestone process. This is where the flue gases are passed through sea water, which approximately has a pH of 7.5, this makes it slightly alkaline. The sulphur dioxide dissolves forming sulphite ions, once again these are aerated to oxidise the sulphite ions to sulphate ions.
Methods for minimising NOx could be; using low NOx burners, because rate of reaction increases as the temperature increases, the temperature of the combusting fuel is lowered, this then decreases the rate of production of NOx.
Another method for Minimising NOx emissions is the most recent method which is called gas reburn, (the diagram below), this is in three steps, the first step is the primary combustion zone, this is where coal dust is oxidised in less air giving a lower combustion rate which means less production of NOx, the second step is called the reburning zone, this is where natural gases, (mostly methane and ethane), are injected into the boiler, the NOx reacts with the alkanes to give, (800 WORDS), nitrogen, carbon dioxide and water vapour, the final step is the burning zone, this is where any excess alkanes or carbon monoxide are oxidised.
I believe the managers at Longannet made these choices because they are the healthiest options for the enviroment, and to humans, it also produces cheap power.
∙ Outline the part played by chemists in the research on photochemical smog formation.
Research has showed that photochemical smog only develops over urban areas. Research has also been conducted to discover what pollutants are in the troposphere, (and that it is in the troposphere that reactions take place). Also what helps it to develop better, for example very clear and sunny days, (because of the UV-light). A lot of research has taken place in the lab, conducting experiments looking into the conditions it needs to develop, etc…, and also on rates of reaction.
Sources
- webpage title: Photochemical smog, by Tracey Gow and Michael Pidwirny, October 17 1996.
webpage title: Current Greenhouse Gas concentrations, by T.J Blasing and Sonja Jones, Revised and updated October 2002.
Longannet: clean coal power?, adapted from Enviromental Pressure by Donald Miller, Chemistry Review, Volume 9, Number 4, March 2000.
Photochemical smog: the killer on a summer’s day, adapted from what is photochemical smog by Gwen and Mike Pilling, Chemistry Review, Volume 5, Number 5, May 1996.
