Coal storage and safety – effects of weathering and spontaneous combustion Improper storage of coal can lead to weathering and even spontaneous combustion. These processes most notably increase the coal's:

Introduction / Context:
Coal stockpiles are reactive. Oxidation, moisture cycling, and microbial action can cause weathering and, in severe cases, self-heating leading to spontaneous combustion. Understanding property changes aids in designing safe storage and minimizing quality loss.

Given Data / Assumptions:

• Coal stored with inadequate compaction, drainage, or monitoring.
• Exposure to air and moisture over time.
• Ambient temperatures that allow low-temperature oxidation.

Concept / Approach:
Weathering oxidizes coal, increasing oxygen functional groups and making the material more friable (crumbly). Volatile matter may polymerize or oxidize, and the net calorific value decreases due to partial oxidation and moisture gain. Caking properties deteriorate; yields of valuable carbonised products (tar, gas) fall in carbonisation. Thus, the most characteristic increases are in oxygen content and friability.

Step-by-Step Solution:

Verification / Alternative check:
Field experience shows weathered coal exhibits higher H/C- and O/C-ratios (more oxygenated groups) and higher fines generation on handling.

Why Other Options Are Wrong:

• Caking index: Generally declines with oxidation.
• Yield of carbonised products: Decreases, not increases.
• Calorific value: Decreases due to oxidation and moisture.

Common Pitfalls:
Assuming spontaneous heating always leads to ignition; often it stops at weathering but quality losses remain significant.

Final Answer:
friability and oxygen content