Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: The output gases from downdraft gasifier are relatively cleaner compared to gases from updraft gasifier
Reason R: In downdraft gasifier the products from the pyrolysis zone and drying zone pass through oxidation zone causing thermal cracking of volatiles
In light of the above statements, choose the correct answer from the options given below

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

To remember the difference:
- Downdraft: Tar flows through the fire $\rightarrow$ Tar is burned (cleaner gas).
- Updraft: Tar flows away from the fire $\rightarrow$ Tar escapes (dirtier gas).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Gasification converts carbonaceous materials into gaseous fuels (producer gas or syngas) through thermochemical reactions.
The direction of air and fuel flow determines the temperature profile and tar content of the resulting gas.
Detailed Explanation:
Let us analyze Assertion A:
Downdraft and updraft gasifiers are two common types of fixed-bed gasifiers.
In a downdraft gasifier, the fuel and air move downwards together.
This design produces a gas with a very low tar content, making it much cleaner than the gas from an updraft gasifier.
Thus, Assertion A is true.
Let us analyze Reason R:
In a downdraft gasifier, air is injected at the level of the hearth, and the gas is drawn out from the bottom.
As the biomass descends, it undergoes drying and pyrolysis, releasing heavy volatile organic compounds (tars).
These pyrolysis products are forced to flow downward through the high-temperature combustion (oxidation) zone.
As they pass through this zone (which reaches temperatures up to 1000\(^\circ\)C to 1200\(^\circ\)C), the heavy tars undergo thermal cracking.
This cracking breaks them down into simpler, non-condensable gases like carbon monoxide (\(CO\)), hydrogen (\(H_2\)), and methane (\(CH_4\)).
In contrast, in an updraft gasifier, the pyrolysis gases escape directly from the top without passing through the combustion zone, leaving the tar intact.
Thus, Reason R is true.
Since the thermal cracking of volatiles in the oxidation zone (R) is the direct cause of the cleaner gas output (A), the reason is the correct explanation of the assertion.

Step 2: Final Answer:
Both Assertion A and Reason R are true, and Reason R is the correct explanation of Assertion A.