What is the reason compact fuels tend to burn slowly?

Compact fuels tend to burn slowly primarily because air cannot circulate as freely among them. When fuels are tightly packed or compacted, the spaces between the fuel particles are reduced. This limited airflow restricts the supply of oxygen necessary for combustion, leading to a slower burning process. For efficient combustion, a sufficient amount of oxygen must reach the fuel; therefore, when air circulation is hindered, the rate of combustion decreases, and the fire burns more slowly.

The other options do not adequately explain the slow burning characteristics of compact fuels. For example, while retaining moisture can affect burning, it is not the main reason for the slow combustion in compact fuels. Similarly, being less continuous does not directly relate to burning speed, and having less surface area would typically lead to a slower burn rate only if sufficient airflow was available, which is not the case in compacted situations. Ultimately, the critical factor is the restriction of oxygen flow due to the compact arrangement of the fuels.