Fuels have the most profound effect on the size and shape of a boiler, because all boilers have to be primarily built around the characteristics of the fuel to be burnt. When the fuels for the project are understood, the resultant customization of the boiler is better. Fuel flex­ibility in a boiler is most desirable, for multiple fuels as in the case of industrial boilers or a wide range of the same fuel as in the case of utility boilers; as it enables plant owners to optimize the fuel cost. But every flexibility brings a performance penalty and a trade-off is necessary.

Prime or principal or main fuel is the fuel around which the boiler is built and the perfor­mance is guaranteed.

Start-up fuel is the fuel with which the boiler is started up and loaded until it is ready for taking the main fuel. Oil firing in case of PF boilers, gas firing for CFBC boilers, and oil/charcoal for BFBC boilers are some examples of start-up fuels.

Alternate fuel is available as an alternative to the main fuel, and usually full generation is obtained on both fuels with appropriate guarantees. Alternate fuel is planned when disruption of main fuel is a distinct possibility. In bagasse-based cogeneration (cogen) plants, coal or oil is planned as an alternate fuel during the off-season. Oil — and gas — fired boilers, coal — and lignite-fired boilers, bagasse and husk-fired boilers are some of the examples of employing alternate fuels where the fuels are somewhat similar and use the same firing equipment. Dissimilar fuels and firing equipment are also employed to meet such demands when burners are used along with stokers, FBC, or PF to cite a few examples. This type of operation is expensive in both start-up costs and running costs. Auxiliary equipment in the boiler has to be sized for the condition that governs it. The performance is generally lower, as there is usually some intentional tramp air ingress through the idle equipment to keep it cool, and the power consumption of the oversized auxiliaries increases.

Auxiliary or additional fuel is available in the plant and is to be burnt along with the main fuel in cofiring mode. When the quantities are small, there are no specific requirements other than merely a disposal in the boiler in a safe and profitable manner. Pith firing in paper plants along with coal is an example.

However, the performance demands of combined firing of main and auxiliary fuels would be more rigorous. In steel plants, coke oven gas or blast furnace gas firing along with coal illustrates the case in point.

Supplementary fuel is available in case of an emergency. In HRSGs behind GTs in cogen projects, oil or gas firing is done to get additional steam to the extent of even 100% for pro­cess needs. Of course the boiler would need to be modified suitably, mainly in upgrading the metallurgy and installing additional combustion equipment and safety devices. In GT-based power plants, supplementary firing is done for meeting the peak requirements to the extent of 10% of power. During this period the efficiency is lower as the fuel passes through the steam cycle and not the GT cycle.

Cofiring or cocombustion is the simultaneous firing of two or more fuels. This is useful when the auxiliary fuel is difficult to burn due to high moisture or ash. Many biofuels are disposed of in the cofiring mode as long as they do not create slagging and fouling problems.