Short answers, pulled from the story.
Combustion is the high-temperature exothermic redox chemical reaction that occurs when a fuel reacts with atmospheric oxygen. This process produces oxidized products often in gaseous form creating a mixture termed smoke.
A flame appears only when substances undergoing combustion vaporize. Without this vaporization fire remains invisible even if the reaction proceeds.
Hydrogen burning with oxygen into water vapor releases 242 kilojoules per mole of heat. This specific reaction reduces enthalpy at constant temperature and pressure.
Incomplete combustion happens when there is insufficient oxygen to allow fuel to react completely or when combustion is quenched by a heat sink like a solid surface. Water is produced but carbon and carbon monoxide appear instead of carbon dioxide.
These oxides combine with water and oxygen in the atmosphere creating nitric acid and sulfuric acids. They return to Earth's surface as acid deposition commonly called acid rain which harms aquatic organisms and kills trees.