High- vs. low-temperature carbonisation of coal: Compared to low-temperature carbonisation, which statement is NOT correct for high-temperature carbonisation?

Introduction / Context:
Coal carbonisation thermally decomposes coal to produce coke, coke-oven gas (COG), tar, and byproducts like ammonia. Operating temperature profoundly affects product distribution and properties, which is critical for metallurgical coke quality and byproduct recovery.

Given Data / Assumptions:

• High-temperature carbonisation: around 900–1100 °C.
• Low-temperature carbonisation: around 500–700 °C.
• Focus is on relative changes in ammonia yield, gas composition, tar character, and COG calorific value.

Concept / Approach:
At higher temperatures, stronger cracking and dehydrogenation occur. This elevates the hydrogen content in COG (more H2, less heavy hydrocarbons), increases aromaticity of the remaining tar (due to severe cracking and polycondensation), and typically reduces the gas calorific value because H2 has a lower volumetric heating value than heavier hydrocarbons and CO. Ammonia is thermally unstable at such temperatures, so its net yield declines.

Step-by-Step Analysis:
Ammonia: less at high T → statement (a) is correct.Tar aromaticity: high T → tar becomes more aromatic, not less → statement (b) is incorrect.COG composition: high T → more H2 → statement (c) is correct.COG calorific value: with cracked, lighter gas and more H2, CV tends to be lower than low-T process → statement (d) is correct.

Verification / Alternative check:
Process texts document that low-temperature carbonisation yields more tar (less aromatic, more oxygenated compounds) and richer gas in higher hydrocarbons, whereas high-temperature regimes favor aromatics in tar and H2-rich COG.

Why Other Options Are Wrong:
(a), (c), and (d) reflect well-established trends for high vs. low temperature carbonisation.

Common Pitfalls:

• Assuming more severe heating always increases calorific value; composition matters.
• Confusing tar yield (quantity) with tar character (aromaticity).

Final Answer:
aromatic content of tar is low