CH3CH3· + O2 ---> CH3CH2OO· (this is called a peroxy radical)
CH3CH2OO· + NO ---> CH3CH2O· +NO2
(here, NO2 is formed, and the resulting radical goes into the next reaction.
CH3CH2O· + O2 ---> CH3CHO + HO2·
HO2· + NO ---> OH· + NO2
(these reactions form more NO2)
Describe and explain the most favourable conditions for forming photochemical smog, and how the high concentrations of tropospheric ozone are produced
The most favourable conditions for photochemical smog to form are hot humid days, for example summer days. “In summer 1995 the first smog came early. The bright sunshine and still air in early May provided ideal conditions” [2]. Ozone is formed by the reaction of volatile organic compounds (VOCs) and nitrogen oxides (NOX) in the presence of sunlight and warm temperatures. A stagnant air mass, the result of high atmospheric pressure and light winds, keeps the pollution from being dispersed. The ideal conditions for ozone build up occur from May to August in the UK
The formation of photochemical smog is due to reactions carried out in the troposphere by the reaction
O2 + O O3
The oxygen needed for this reaction are formed by the action of ultraviolet light absorbed by nitrogen dioxide at wavelengths less than 435nm, this cause the nitrogen dioxide molecule to dissociate into NO molecules and O atoms
hv, λ < 435nm
.NO2 . NO + .O.
But once formed these radicals go on to react with NO to form NO2..
O3 +NO3 O2 + NO2.
The more NO2 is available to produce more oxygen atoms and more ozone will therefore be formed. Further reactions with hydrocarbons and other volatile organic compounds can lead to an increase of ozone in the troposphere. (Fig 1)
O. + H2O 2.OH
These .OH radicals can abstract H from the hardocarbons to form alkyl radicals
.OH + RCH3 H2O + RCH2
(Fig 1)
Photochemical smog occurs in the lower portion of our atmosphere, known as the troposphere. (Fig 2.)
If there is light prevailing wind, the polluted air mass is transported from the urban areas, where it was created, to rural regions. In fact, the highest ozone readings are often recorded over rural areas because the chemical reactions producing the secondary pollutants take place quite slowly.
Fig 2
This graph shows the ozone concentration peaks in the middle of the day.
Fig 3
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 environmental option) for minimising waste material from power staions varies from one to another from research carried out at individual plants.
Controlling SOx emissions
One way of reacting sulphur dioxide in the power stations flue gas with limestone forming calcium sulphate (CaSO4). Another option is to dissolve the sulphur dioxide in seawater.
Sea water scrubbing process - In this process, the flue gases are passed through sea water, which is naturally alkaline with a pH of 7.5. the sulphur dioxide dissolves in this alkaline solution forming sulphite ions. The water is aerated to oxidise the sulphite to more harmless sulphate for disposal at sea.
Fig 4
Controlling NOx production
The most recent arrangement for lowering NOx emissions is gas reburn. not only is less NOX produced, but any NOX formed is chemically removed. There are three burning zones in the boiler furnace. In the primary combustion zone, powdered coal is oxidised in less air than normal, with the consequence that the combustion rate is lowered and less NOX is formed in a given time. What makes this a novel process is the injection of natural gas (mainly methane and ethane) into the boiler just above the flame in the reburning zone. The NOx reacts with the alkanes to produce nitrogen, carbon dioxide and water vapour:
CH4(g) + 4NO(g) 2N2(g) + CO2(g) + 2H2O(g)
Fig 5
Low-NOx burners are designed to "stage" combustion (see Figure 6). In this technology, a fuel-rich combustion zone is created by forcing additional air to the outside of the firing zone (auxiliary air) and by delaying the combustion of coal. Reduction of 30 to 55 percent of NOx can be achieved with low-NOx burners. Advanced stage combustion technologies use overfire air and gas or coal reburning to achieve even greater reductions of NOx .
Fig 6
Longannet chose to use sea water scrubbing as a BPEO because there was no solid waste created plus at Longannet the key raw material is sea water and is readily available because it is situated on the bank of the Fifth of Forth. As far as the reduction of NOx is concerned Longannet is fitted with both low NOx burners and gas reburn, therefore making it one of the cleanest power stations in the world.
Outline the part played by chemists in the research on photochemical smog formation.
There are currently several fronts of research to discover and explain the
complexity of the chemistry involved in photochemical smog.
Monitoring of tropospheric pollutants is an important first step is to know which pollutants are present in the troposphere
Fig 7
There are now several monitoring stations across the country recording pollutant concentrations.
Studying individual reactions in the laboratory allows chemists to make predictions about pollution how they occur.
Modelling studies on the computer allow chemists to predict the behaviour of pollutants in a photochemical episode, a build up of data would allow a more accurate prediction
Bibliography
- Chemical Ideas & Chemical Story lines – Salters Advanced Chemisrty - p79
- Article 1 - Longannet: clean coal power? – by Donald Miller
- Article 2 – Photochemical smog: the killer on a summers day – by Gwen and Mike Pilling
- Chemistry - Molecules, Matter and Change - Loretta Jones & Peter Atkins – p342-343
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LOW NOx EQUIPMENT - www.parkerboiler.com/low-nox/
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- www.colby.edu/sts/st215/8.1view/sld013.htm
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- www.unc.edu/~tracyde/morechemistry.html
Fig 1 – The formation of Photochemical smog
Fig 2 - Ozone concentration peaks in the middle of the day during a smog episode
Fig 3 – Structure of the atmosphere
Fig 4 – Sea water scrubbing of flue gases
Fig 5 – Gas reburn furnace
Fig 6 – A low NOx burner
Fig 7 – Variation of the concentration of ozone and oxides of nitrogen on a summer weekday in London