What is a by-product of 90% combustion devices?

In the context of combustion processes, particularly those that involve the burning of hydrocarbon fuels, the primary by-products can vary depending on the efficiency of the combustion. When fuel is burned efficiently, as in a 90% combustion device, water is produced alongside other gases.

When hydrocarbons combust completely, they typically produce carbon dioxide and water vapor as the two main products of the reaction. In a high-efficiency combustion device operating at 90% efficiency, the combustion is more complete, leading to a greater production of water. Water vapor is a significant by-product because it results from the hydrogen in the fuel combining with oxygen during the combustion process.

While carbon monoxide can emerge from incomplete combustion, and carbon dioxide is produced as well, in the case of well-regulated and efficient combustion, the production of water vapor becomes notable. Therefore, the correct answer highlights the role of water as a key by-product of a combustion process operating at high efficiency, such as 90%, aligning with principles of thermodynamics and combustion chemistry.