Short answers, pulled from the story.
Combustion is a high-temperature exothermic redox chemical reaction between a fuel and an oxidant, usually atmospheric oxygen. It proceeds as a chain reaction involving radical intermediates including singlet oxygen, hydroxyl, monatomic oxygen, and hydroperoxyl. A flame is visible only when substances undergoing combustion vaporize.
Complete combustion requires every fuel molecule to receive exactly the right amount of oxygen, a condition called the stoichiometric ratio. In practice, chemical equilibrium is rarely fully reached, leaving carbon monoxide, hydrogen, and carbon as residual products. The chemical equilibrium of combustion in air strongly favors the products, but achieving it completely is nearly impossible.
Carbon monoxide binds with hemoglobin in red blood cells, preventing them from carrying oxygen. Breathing it causes headache, dizziness, vomiting, and nausea. At high concentrations it causes unconsciousness or death, and long-term exposure at moderate and high levels is positively correlated with heart disease risk.
Nitrogen oxides and sulfur oxides produced during combustion combine with water and oxygen in the atmosphere to form nitric acid and sulfuric acids. These acids return to Earth as acid deposition, commonly called acid rain. Acid rain harms aquatic organisms, kills trees, and reduces nutrient availability to plants.
Smoldering is the slow, low-temperature, flameless form of combustion sustained by heat released when oxygen attacks a condensed-phase fuel surface directly. Materials that can sustain it include coal, wood, cotton, tobacco, peat, synthetic foams, and polyurethane foam. Residential fires often start this way from weak heat sources such as a cigarette or a short-circuited wire.
Combustion instabilities in the F1 engine used in the Saturn V program produced violent pressure oscillations that caused massive damage to the combustion chamber and surrounding components. The problem was solved by redesigning the fuel injector. Such instabilities can reach pressure oscillations as high as 180 decibels.