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.
What happens in the core region of the flame of a low NOx burner?
The coal particles are volatilised by radiant energy thus supply H2, CH4 and CO which can reduce fuel N and NOx to N2. This is called the reducing zone.
What about for low NOx burners for liquids and gases?
Similar strategies for reducing NOx as those for solid fuels.
Is there an alternative strategy for low NOx burners of liquids and gases?
Decreasing the maximum combustion temperature. This could be achieved by lean operation with excess air or steam injection
What is the consideration that must still be made despite low NOx burners and cleaner fuel?
Some NOx will still be formed
What is a frequent ‘end-of-the-pipe’ solution?
Reacting the oxides with reduced forms of nitrogen e.g. NH3, urea to reduce them to N2
What are the two end-of-pipe solutions adopted for the reduction of NOx with NH3 to produce N2 and water vapour?
- Selective Non-Catalytic Reduction (SNCR)
- Selective Catalytic Reduction (SCR)
What are the temperatures and efficiency for SNCR?
At high temperatures (800 - 1200 oC)
40 - 80 % of NOx removed
What are the temperatures and efficiency for SCR?
At lower temperatures (150 - 500 oC)
80 - 95 % NOx removed
Is SCR or SNCR more effective?
SCR
What is the main drawback of SCR and SNCR?
Ammonia may be let to slip through if poorly operated or during an upset in the process
Does proper burner design mitigate the conversion of S to SO2 and SO3 as in the low NOx burners?
No
What are the stages of conversion of S?
S –> SOx –> HSO3 - –> SO4 2-
Do most BATs absorb SO2 before or after combustion?
After
Generally how do ‘end-of-pipe’ solutions remove SO2?
Alkaline absorbent containing Ca or Mg or Na ions which result in the formation of solid particulates of sulphates e.g. gypsum
What are the two end-of-pipe solutions for the removal of SO2 from stack gas?
- Spray Flue Gas Desulphurization
- Wet FGD scrubber
Described Spray FGD
A slurry of alkaline absorbent (Ca (OH)2, NaHCO2, Na2CO3) is pumped around and sprayed into the gas effluent
Describe wet FGD scrubber
Using an alkaline solution to absorb acid gases (mostly HCl, HF, H2SO4)
What is the one type of BAT that attempts to remover sulphur in-situ before entering gas?
Dry Sorbent injection - typically CaCO2 mixed with fuel in the furnace beforehand
