Steam generators produce steam by utilizing heat from a fuel source. The primary method involves burning fossil fuels such as coal, natural gas, or oil to heat water, which then evaporates into steam. This process is known as the Rankine cycle, where heat is transferred from the fuel source to the water, causing it to boil and produce steam. The steam is then used for various applications, including power generation in power plants and heating in industrial processes.

The efficiency of a steam generator is highly dependent on the type of fuel used. For instance, natural gas-fueled steam generators are known for their high efficiency, with some reaching up to 90% thermal efficiency. Coal-fired steam generators, on the other hand, typically have lower efficiency, ranging from 30% to 40%. This discrepancy in efficiency is due to the higher energy content of natural gas compared to coal, as well as the more efficient combustion process of natural gas.

According to the U.S. Energy Information Administration (EIA), natural gas accounted for approximately 35% of the total electricity generation in the United States in 2020. This highlights the significant role of natural gas in power generation, particularly in steam generators. Additionally, the EIA reports that coal's share of electricity generation has been declining over the years, from a peak of 50% in the 1970s to around 20% in 2020. This shift in fuel preference is primarily driven by environmental concerns and the pursuit of higher efficiency in power generation.

In summary, steam generators primarily use fossil fuels like coal and natural gas to produce steam. The choice of fuel significantly impacts the efficiency and environmental impact of the steam generation process.