There are a lot of factors to consider when determining the real efficiency of boilers. These factors become more critical when changes occur in the economic landscape, such as the rising cost of gas. While gas is certainly more environmentally friendly than other fuels, the current price of gas may cause you to take a new look at burning oil or coal. Because gas fuels contain a lot more hydrogen, boilers burning gas typically average 3% less efficiency than those firing oil or pulverized coal. Plus, the fuel price per Btu of gas may be considerably more than for oil or coal. This difference is actually causing some larger industrial companies to allocate more production to plants that use fuels other than gas.
Whatever fuel you choose to burn will certainly affect the overall efficiency of your boilers, but not as much perhaps as other factors, such as not compensating for pressure and temperature changes in your plant's environment.