Acid Gases Flashcards
What is the significance of the National Emissions Ceiling Directive?
It imposed strict limits on SO2, NOX, VOC and NH3
What are the effects of SOx and NOx which require the National Emissions Ceiling Directive?
- SOx can come down quickly as acid rain or slowly as acid mist or smog
- They cause respiratory diseases
- They inhibit plant growth and damage stonework and buildings
- They acidify bodies of water and hurt aquatic life
- NOx combine with hydrocarbon residues and ground level ozone to produce smog
- Emissions travel far
What is the major source of SOx naturally?
Volcanoes
What is the major source of NOx naturally?
Thunderstorms
Are the naturally sources of SOx and NOx negligible compared to human activity?
No
What are other ‘natural’ acidifying mechanisms?
The formation of peat
Planting conifer forests
In which country does plumes from UK stacks contribute to the acidification of lakes?
Norway
What are the three classifications for industrially-produced SOx and NOx?
- Large scale power stations connected to the electricity transmission grid
- Smaller scale Combined Heat and Power Stations as found in district heating, industrial complexes, refineries
- Other sources e.g. furnaces, chemical plants, reactors where periodical de-coking takes place by oxidation with hot air
What should be the first solution to minimise the production of SOx and NOx?
Minimising the causes of pollution by using fuels which are intrinsically cleaner
Give an example of fuel sources which emit less CO2 and S
Burning natural gas will emit only half as much CO2 as coal or coke.
Burning low sulphur coal rather than coal also helps decreasing SOx
What percentages of NOx’s are produced from combustion?
95 % is NO
5 % is NO2
What are the three origins for NOx from combustion and other high temperature processes?
- Thermal NOx
- Fuel NOx
- Prompt NOx
What is thermal NOx?
Combination of N2 and O2 of air at high temperature by free radical chain reaction produces thermal NOx.
What is fuel NOx?
From the oxidation of chemically combined N in the fuel (many solid or heavy liquid fuels contain organic impurities that have nitrogen)
What is prompt NOx?
From reactions between atmospheric N2 and the partiallyoxidised hydrocarbons that form in the early stages of combustion?
How much does thermal NOx account for in the NO2 emission from coal-fired boilers?
Thermal NOx: 20 %
Fuel and Prompt NOx: 80 %
Which types of NOx is it thus important to try and reduce emissions?
Fuel and Prompt NOx
What is the typical method employed for controlling NOx’s?
Reducing them to N2 using a reducing agent such as NH3, urea or partially oxidised hydrocarbons. These can be added to the process or the combustion conditions can be controlled to produce them at an intermediate stage.
What are the stages in reducing NOx to N2 in coal?
- Treat coal so that the mixing of air is delayed
- Partial combustion desorbs or produces reducing species (hydrocarbons, CO, NH3) which react with NO to produce N2
- The N2 can still produce thermal NO later but to a lesser extent that its more reactive, organic precursors
What is the flame type in a conventional burner?
Short well-mixed flame. It is diverging.
In which way is the primary air and fuel mixed with secondary air in a conventional burner?
Radially.
There is vigorous mixing which results in the short flame.
What is the final product of burning fuel N in a conventional burner?
NOx
What is the flame type in a low NOx burner?
The flame is longer and more cylindrical in shape.
There is a core reducing zone and combustion then occurs on the outside zone of the flame
In which way is the primary air and fuel mixed with secondary air in a low NOx burner?
Axially.
