ANS:B - cryogenic operations (low temperature cooling).

Hydrogen recovery from coke oven gas on a commercial scale, as practiced in fertiliser plants like Rourkela, is typically achieved through cryogenic operations (low-temperature cooling). Explanation:

1. Coke Oven Gas Composition: Coke oven gas is a by-product of the coking process in steel manufacturing and typically contains hydrogen (H2), methane (CH4), carbon monoxide (CO), and other gases.
2. Hydrogen Recovery: Hydrogen recovery from coke oven gas is economically significant because hydrogen is a valuable resource used in various industrial processes, including ammonia synthesis for fertiliser production.
3. Cryogenic Operations: Cryogenic operations involve cooling the coke oven gas to very low temperatures, typically below the boiling point of hydrogen (-252.87°C or -423.17°F). At such low temperatures, hydrogen can be liquefied and separated from other gases present in the coke oven gas stream.
4. Separation Process: The cryogenic separation process typically involves cooling the gas stream using refrigeration units or liquefaction processes. As the temperature decreases, hydrogen condenses into a liquid phase, allowing it to be separated and collected from the gas mixture.
5. Benefits of Cryogenic Recovery: Cryogenic hydrogen recovery offers high efficiency and purity of hydrogen extraction from coke oven gas. It is a well-established and commercially viable method used in various industrial applications, including fertiliser production plants like Rourkela.
6. Alternative Methods: While adsorption on materials like palladium and absorption using solutions like ethanolamine or pyragallol can also be used for hydrogen recovery in specific applications, cryogenic operations are more commonly employed on a commercial scale due to their effectiveness and efficiency.