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Show WASTE GAS FIRING IN PROCESS HEATERS AND BOILERS . Increasing energy costs have led to heightened interest in energy conservation and more recently in the use of "alternate" fuels. Alternate fuels are considered to be fuels not normally encountered in plant operation and in addition have a low cost. Many alternate fuels meet the criteria of low cost but create other problems. An example is the use of coke as a fuel. Sulfur, fixed nitrogen and ash are often present in quantities that create significant environmental problems. These impurities coupled with incomplete combustion will create significant fouling of the heat transfer surfaces which in turn will reduce efficiency. Alternate fuels that usually do not create the above mentioned problems are gases that have high volume percentages of inert components such as water vapor, carbon dioxide and nitrogen. Often gases of this type have been considered to be wastes and have been vented to the atmosphere, flared or burned in an incinerator. Since many of these gases do not contain impurities that lead to pollution and heat transfer surface fouling, they can be used to fire conventional process heaters and boilers. However when these gases are used as a fuel, certain characteristics must be considered and properly compensated for in the design of the combustion equipment and process heater or boiler. Some characteristics that must be considered are: high volume flow rate of fuel and combustion products, low flame temperature, limits of flammability, and increased combustion volume. TYPICAL ALTERNATE GASEOUS FUELS Although the possible combinations of gaseous fuel components is limitless, the four examples given in Table I are representative of typical combinations. The gases listed in the table have a range of inert percentages of from 50 to 88% and a range of hydrogen percentage from 0 to 15.7%. As will be discussed later, the amount of hydrogen contained in the mixture is an important factor in determining the ability of the fuel to be burned. The four fuels listed are: flex gas, shale oil off-gas from retorting, landfill recovery gas and carbon black off-gas. Flex gas is a by-product of the petroleum flexicoking process and the composition listed is for a flex gas with relatively high volume percent of combustible components. The composition listed for the shale oil off-gas is an "average" gas composition and as the name indicates is the by-product of shale oil retorting. The composition for the landfill gas is typical of gases created and given off as organic material decays in landfills. The carbon black off-gas composition listed is for a gas with minimum combustible composition. Mixtures of carbon black off-gas with 50% more combustibles are possible. 1.1.1 |