NFI Core Knowledge Practice Exam – All-in-One Study Guide to Master the National Fireplace Institute Certification! 2026

The primary combustible material remaining in Stage IV of wood combustion is carbon in the charcoal. During the burning process, wood undergoes several stages of combustion. Initially, the wood releases volatile gases, which are combustible and burn off. As heating continues, these gases burn away, leaving behind carbon-rich materials.

In Stage IV, the combustion process has progressed to the point where most of the volatile content has been emitted, and what remains is primarily charcoal, which consists largely of carbon. This carbon can burn and continue to release energy, albeit at a much slower rate compared to the earlier stages.

The presence of unburned gases is more characteristic of earlier stages of combustion, as they are only present until they are fully consumed. Wood remnants may still be around, but they are no longer the main source of combustion products. Catalysts typically play a role in enhancing chemical reactions, often in a different context, and do not represent a significant material left over from wood combustion in this stage. Therefore, carbon in the charcoal stands out as the focus of Stage IV of wood combustion.