The products of the gas generator are also a thermal processing process of solid fuel (coal or coke) through gas, that is, oxygen or oxygen compounds (steam, carbon dioxide) are passed through a high-temperature solid fuel (coal, coke) layer, and the organic matter (air, water vapor) that acts as an oxidizer is called a gasification agent, and a mixed gas containing hydrogen, carbon monoxide, methane, etc. is generated, which is called coal gas. According to different gas agents, they are divided into the following three types:
       1. Air gas: air is used as the gasifying agent.
       2. Water gas: water vapor is used as the gasifying agent.
       3. Mixed gas: air and water vapor are used as gasifying agents.