Gaseous Pollutant Control Questions and Answers

Q: Arrange the causal chain by which a poisoned catalyst raises emissions: 1. Halogens or sulphur compounds reach the catalyst surface 2. Active sites are blocked and activity falls 3. The reaction becomes incomplete at the design temperature 4. Outlet concentration rises above the permitted level Select the correct sequence.

Correct answer: 1, 2, 3, 4The chain runs from cause to consequence. A poison - halogen, sulphur compound, or a heavy metal such as lead or phosphorus - reaches the catalyst; it occupies or destroys the active sites, so the activity falls; the reaction can no longer go to completion in the residence time available at the design temperature; and the outlet concentration rises until the permit is breached. Recognising the order matters, because rising outlet concentration at unchanged temperature and flow is the symptom that points back to the catalyst.

Q: Arrange these methods in order of increasing pollutant concentration at which they are appropriate: 1. Biofiltration of a large dilute odorous stream 2. Adsorption on activated carbon 3. Thermal or catalytic incineration 4. Condensation for recovery Select the correct sequence.

Correct answer: 1, 2, 3, 4The methods are chosen by concentration. Biofiltration handles the very dilute, high-volume streams where combustion would cost prohibitive fuel. Adsorption suits low to moderate concentrations and can concentrate the pollutant for recovery or destruction. Incineration becomes attractive at moderate to high concentrations, where the stream approaches self-sustaining combustion. Condensation is for the highest concentrations, where the recovered liquid has value in its own right.

Q: Consider the following pairs of method and the mechanism it uses: Absorption - Dissolution into a liquid Adsorption - Retention on a solid surface Incineration - Oxidation to carbon dioxide and water Condensation - Chemical reaction with a reagent How many of the pairs given above are correctly matched?

Correct answer: Only threeAbsorption, adsorption and incineration are correctly described. The fourth is wrong: condensation removes a vapour by cooling it below its dew point so that it becomes a liquid - a purely physical change of phase involving no reagent and no chemical reaction. That is precisely why it recovers the material in usable form.

Q: Consider the following pairs of desulphurisation route and its characteristic: Wet limestone - Highest removal, produces gypsum Spray dryer absorption - Dry product, no liquid effluent Dry sorbent injection - Lowest capital cost, poorest reagent use Wet limestone - No reheat required How many of the pairs given above are correctly matched?

Correct answer: Only threeThe first three pairs are correct. The fourth is wrong: wet limestone scrubbing leaves the gas cooled and saturated with water vapour, so it commonly does require reheat before the stack, both to restore buoyancy for plume dispersion and to prevent acid condensation in the ductwork. Avoiding that reheat is one of the advantages claimed for the semi-dry and dry routes.

Q: Consider the following pairs of nitrogen oxide control and its feature: Selective catalytic reduction - Eighty to ninety per cent removal Selective non-catalytic reduction - Thirty to fifty per cent removal Both routes - Ammonia or urea as the reagent Selective non-catalytic reduction - Requires a catalyst bed How many of the pairs given above are correctly matched?

Correct answer: Only threeThe first three are correct. The fourth contradicts the name: non-catalytic reduction uses no catalyst at all, which is exactly what distinguishes it - it injects the reagent into the furnace and relies on a narrow high-temperature window to drive the reaction. Its low capital cost follows from having no catalyst bed to buy, house and eventually replace.

41.

Statement I: Ammonia slip can be eliminated by injecting more reagent.

Statement II: Removal efficiency rises with the ammonia to nitrogen oxide molar ratio.

In the light of the above statements, choose the correct answer.

A.Statement I is incorrect but Statement II is correct
B.Both Statement I and Statement II are correct
C.Both Statement I and Statement II are incorrect
D.Statement I is correct but Statement II is incorrect

Answer: A

The second is correct but it produces the opposite of the assertion: more reagent means more unreacted ammonia leaving with the gas. The balance between removal and slip is the central control problem of both reduction processes.

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42.

Statement I: A packed tower is designed to operate below its flooding velocity.

Statement II: Pressure drop rises very sharply as the flooding point is approached.

In the light of the above statements, choose the correct answer.

A.Statement I is incorrect but Statement II is correct
B.Both Statement I and Statement II are correct
C.Both Statement I and Statement II are incorrect
D.Statement I is correct but Statement II is incorrect

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both are true and the second is the reason for the first. Below flooding the pressure drop rises gently with gas rate; approaching it the curve turns almost vertical as liquid accumulates in the packing, and beyond it the tower carries liquid out of the top and ceases to work. Designing at 60 to 70 per cent of the flooding velocity keeps the tower on the flat part of that curve with margin for load variation.

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43.

Arrange the stages of an adsorption cycle in order:

1. Fresh bed adsorbs the pollutant with a clean outlet

2. The mass transfer zone advances through the bed

3. Breakthrough occurs and the outlet concentration rises

4. The bed is regenerated with steam and returned to service

Select the correct sequence.

A.1, 3, 2, 4
B.2, 3, 1, 4
C.1, 2, 3, 4
D.2, 1, 3, 4

Answer: C

Two beds are normally provided so that one adsorbs while the other regenerates, giving continuous treatment. Switching at breakthrough rather than saturation is what keeps the outlet clean throughout the adsorbing half of the cycle.

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44.

Arrange these desulphurisation routes in order of increasing removal efficiency:

1. Dry sorbent injection

2. Spray dryer absorption

3. Wet limestone with forced oxidation

4. Wet limestone with a polishing stage

Select the correct sequence.

A.2, 1, 3, 4
B.1, 3, 2, 4
C.2, 3, 1, 4
D.1, 2, 3, 4

Answer: D

Capital cost and complexity rise along the same sequence, as does the difficulty of handling the by-product. Which route is chosen depends on the sulphur in the fuel, the standard to be met and whether a gypsum market exists nearby.

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45.

Arrange the steps of selective catalytic reduction in order:

1. Flue gas is brought to the catalyst's working temperature window

2. Ammonia is injected and mixed into the gas

3. The mixture passes over the catalyst and the oxides are reduced

4. Nitrogen and water leave with the treated gas

Select the correct sequence.

A.1, 2, 3, 4
B.2, 1, 3, 4
C.1, 3, 2, 4
D.2, 3, 1, 4

Answer: A

Mixing must be complete before the catalyst is reached, since a maldistributed reagent gives both poor removal and local slip. The temperature window is narrow and the plant's arrangement is dictated by finding gas at the right temperature.

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46.

Arrange the causal chain by which a poisoned catalyst raises emissions:

1. Halogens or sulphur compounds reach the catalyst surface

2. Active sites are blocked and activity falls

3. The reaction becomes incomplete at the design temperature

4. Outlet concentration rises above the permitted level

Select the correct sequence.

A.2, 3, 1, 4
B.1, 2, 3, 4
C.2, 1, 3, 4
D.1, 3, 2, 4

Answer: B

Correct answer: 1, 2, 3, 4

The chain runs from cause to consequence. A poison - halogen, sulphur compound, or a heavy metal such as lead or phosphorus - reaches the catalyst; it occupies or destroys the active sites, so the activity falls; the reaction can no longer go to completion in the residence time available at the design temperature; and the outlet concentration rises until the permit is breached. Recognising the order matters, because rising outlet concentration at unchanged temperature and flow is the symptom that points back to the catalyst.

Discuss in Forum

47.

Arrange these methods in order of increasing pollutant concentration at which they are appropriate:

1. Biofiltration of a large dilute odorous stream

2. Adsorption on activated carbon

3. Thermal or catalytic incineration

4. Condensation for recovery

Select the correct sequence.

A.1, 3, 2, 4
B.2, 3, 1, 4
C.1, 2, 3, 4
D.2, 1, 3, 4

Answer: C

Correct answer: 1, 2, 3, 4

The methods are chosen by concentration. Biofiltration handles the very dilute, high-volume streams where combustion would cost prohibitive fuel. Adsorption suits low to moderate concentrations and can concentrate the pollutant for recovery or destruction. Incineration becomes attractive at moderate to high concentrations, where the stream approaches self-sustaining combustion. Condensation is for the highest concentrations, where the recovered liquid has value in its own right.

Discuss in Forum

48.

Consider the following pairs of method and the mechanism it uses:

Absorption - Dissolution into a liquid

Adsorption - Retention on a solid surface

Incineration - Oxidation to carbon dioxide and water

Condensation - Chemical reaction with a reagent

How many of the pairs given above are correctly matched?

A.Only one
B.Only two
C.All four
D.Only three

Answer: D

Correct answer: Only three

Absorption, adsorption and incineration are correctly described. The fourth is wrong: condensation removes a vapour by cooling it below its dew point so that it becomes a liquid - a purely physical change of phase involving no reagent and no chemical reaction. That is precisely why it recovers the material in usable form.

Discuss in Forum

49.

Consider the following pairs of desulphurisation route and its characteristic:

Wet limestone - Highest removal, produces gypsum

Spray dryer absorption - Dry product, no liquid effluent

Dry sorbent injection - Lowest capital cost, poorest reagent use

Wet limestone - No reheat required

How many of the pairs given above are correctly matched?

A.Only three
B.Only one
C.Only two
D.All four

Answer: A

Correct answer: Only three

The first three pairs are correct. The fourth is wrong: wet limestone scrubbing leaves the gas cooled and saturated with water vapour, so it commonly does require reheat before the stack, both to restore buoyancy for plume dispersion and to prevent acid condensation in the ductwork. Avoiding that reheat is one of the advantages claimed for the semi-dry and dry routes.

Discuss in Forum

50.

Consider the following pairs of nitrogen oxide control and its feature:

Selective catalytic reduction - Eighty to ninety per cent removal

Selective non-catalytic reduction - Thirty to fifty per cent removal

Both routes - Ammonia or urea as the reagent

Selective non-catalytic reduction - Requires a catalyst bed

How many of the pairs given above are correctly matched?

A.All four
B.Only three
C.Only one
D.Only two

Answer: B

Correct answer: Only three

The first three are correct. The fourth contradicts the name: non-catalytic reduction uses no catalyst at all, which is exactly what distinguishes it - it injects the reagent into the furnace and relies on a narrow high-temperature window to drive the reaction. Its low capital cost follows from having no catalyst bed to buy, house and eventually replace.

Discuss in Forum

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