Flue gas desulfurization by sorbent injection: During pulverized coal firing in boilers, limestone powder is injected primarily to achieve which effect on the flue gases?

Introduction / Context:
Sorbent injection is a practical approach to control sulfur emissions from coal combustion. Finely divided limestone or lime reacts with sulfur dioxide to form stable calcium sulfite/sulfate solids that can be captured downstream, thereby lowering gaseous SO2 released to the atmosphere.

Given Data / Assumptions:

• Pulverized coal boiler with in-furnace or duct injection of calcium-based sorbent.
• Goal is emission control, not catalytic promotion of SO3.
• Particulate capture devices (e.g., ESP or baghouse) are present to remove reaction products.

Concept / Approach:
Calcium carbonate (CaCO3) calcines to CaO at high temperature; CaO then reacts with SO2 (and O2) to form CaSO3/CaSO4. The overall effect is chemical sequestration of sulfur species from the gas phase, reducing stack SO2. This is the basis of furnace sorbent injection and circulating dry scrubbers.

Step-by-Step Solution:
Recognize limestone’s role as a sulfur sorbent rather than a catalyst.Recall reaction pathway: CaCO3 → CaO + CO2; CaO + SO2 (+ 1/2 O2) → CaSO3/CaSO4.Choose the option stating “Reduce SO2 content.”

Verification / Alternative check:
Performance metrics are often expressed as SO2 removal efficiency (%) versus Ca/S molar ratio, confirming the objective is SO2 reduction.

Why Other Options Are Wrong:
Catalytically convert SO2 to SO3: not desirable; SO3 can cause acid mist and fouling.Increase dew point: not an aim; higher acid dew points can worsen corrosion.Increase NOx formation: the opposite of typical goals; combustion tuning seeks NOx reduction.

Common Pitfalls:
Assuming limestone acts as a catalyst; it functions as a reactive sorbent forming solid salts.

Final Answer:
Reduce SO2 content in the flue gas