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Show WASTE GAS FIRING IN PROCESS HEATERS AND BOILERS FUEL AND AIR PRESSURE REQUIREMENTS Table II lists some of the operating characteristics and requirements for the four fuels. The fuel flow required for each million BTU per hour input varies from 2198 standard cubic feet per hour to 25641 standard cubic feet per hour. The flow of methane required for the same heat input is 1099 cubic feet per hour. Thus for a given heat input, the flow of these "low" BTU gases is much greater than the flow required for conventional fuels. This fact can create several design problems. Proper burner operation requires complete mixing of the fuel and combustion air. There are two sources of mixing energy, the fuel pressure available and the air pressure available. Many burners used in the refinery and petrochemical industries utilize natural draft or low air pressure drop, thus the primary source of mixing energy is the fuel pressure available. In most cases it is advantageous to design the combustion equipment for use with both conventional and alternate fuels. The pressure available for the alternate fuel then governs the burner tip design. For example, if landfill off-gas is used for a given tip design the fuel pressure required is about 7.5 times greater than the pressure required for natural gas. Thus, if there is approximately 35 psig available for the landfill gas it may be possible to operate a low air pressure drop burner that is normally used in process heaters and boilers designed for landfill gas with methane as the fuel. It is obvious that pressure requirements of the other alternate fuels prohibit the use of the same gas nozzles, when alternate fuels are used in combination with conventional fuels. Thus the burner design must incorporate a dual nozzle system. It also is obvious that because of the large volume differences, separate piping systems must be utilized. The necessity of this requirement is increased in many cases when the alternate fuels are only available at very low pressures. Although it is not as critical, the fact that different air flows are required for the various fuels needs to be considered in the system design. Table II indicates that the air flow, with 15% excess air will vary from 9449 ft3 per million BTU for the flex gas to 12030 ft3 per million BTU for the landfill gas which is also the requirement for methane. This means that air pressure drop requirements may be as much as 60% greater for conventional fuels when compared to these alternate fuels. The differences in air flow requirements must also be considered in combustion control system design. A final pressure loss consideration is the loss in the combustion product stream. For a given heater or boiler design, the pressure losses through the convection section and stack will vary depending on the fuel burned. Table II indicates the quantity of wet flue gases for each fuel when the fuel is burned with 15% excess air. The quantity ranges from 14228 standard cubic feet for landfill gas to 34212 standard cubic feet for the carbon black off-gas for each million BTU fuel consumption. The quantity of wet flue products for methane firing is 13130 standard cubic feet when firing with 15% excess air at 1.1.2 |
