Natural gas-fired heater

A major fire erupted in a nonflammable solvents manufacturing unit in a U.S. Gulf Coast chemical complex. A furnace tube in a natural-gas-fired heater ruptured due to overheating. At least 1,800 gallons (6,800 liters) of a combustible heat transfer fluid spilled and burned intensely. Within about 25 minutes, the intense hot fire damaged four levels of structure and associated process equipment. The plant on-site emergency squad quickly and properly responded. However, the price tag for short-lived incident was over 1.5 million in direct property damage and over 4 million in business interruption (U.S. 1979). Fortunately, there were no injuries. [2]... 

Plans were under way to return a reactor to service. This gaseous phase reactor requires a startup temperature in excess of 500° F (260° C). A natural gas—fired heater provides the heat energy to a circulating combustible heat transfer fluid and the heat transfer fluid flows through the reactor tubes until startup temperature is reached. 

Pyrolysis furnace. This unit also performs two operations It preheats the vapor to its reaction temperature, 500°C, and it carries out the pyrolysis reaction. The unit is constructed of refractory brick, with natural gas-fired heaters, and a large bundle of Nickel, Monel, or Inconel tubes, within which the reaction occurs. The tube bundle enters the coolest part of the furnace, the so-called economizer at the top, where the preheating occurs. 

A major fire erupted in a nonflammable solvents manufacturing unit in a US Gulf Coast chemical complex. A furnace tube in a natural-gas-fired heater ruptirred due to overbeating the heat transfer fluid trapped in a stagnant tube. At least 1800 gallons (6800 L) of a combustible heat transfer fluid spilled and burned irrtensely. Within... 

By using a traditional form of heat energy, such as natural gas, cycle economics will benefit from an increase in hot water temperature. Using a heat transfer fluid from a gas fired heater at 130°C, Carnot efficiency is almost 25 percent, and hydride compressor efficiency increases to 15 percent. While 15 percent is about 1/2 that for a mechanical compressor, electricity is about 6 times costlier than natural gas, so the hydride compressor will enjoy a 67% lower energy cost. A simplified schematic of a natural gas powered hydride compressor appears in Figure 3. 

Difluoroethane is reacted to form vinyl fluoride and hydrogen fluoride. A natural gas fired process heater supplies molten salt used to maintain the reactor temperature. The gaseous reaction products are separated and the hydrogen fluoride and difluoroethane are recycled back to the DFE process. The crude vinyl fluoride is purified and stored until shipped by railcar or truck tanker. Acidic vent gases from this process are controlled by the emergency scrubber of the DFE process. 

Fresh reducing gas is generated by reforming natural gas with steam. The natural gas is heated in a recuperator, desulfurized to less than 1 ppm sulfur, mixed with superheated steam, further preheated to 620°C in another recuperator, then reformed in alloy tubes filled with nickel-based catalyst at a temperature of 830°C. The reformed gas is quenched to remove water vapor, mixed with clean recycled top gas from the shaft furnace, reheated to 925°C in an indirect fired heater, and injected into the shaft furnace. For high (above 92%) metallization a CO2 removal unit is added in the top gas recycle line in order to upgrade the quaUty of the recycled top gas and reducing gas. 

Reducing gas is generated from natural gas in a conventional steam reformer. The natural gas is preheated, desulfurized, mixed with steam, further heated, and reformed in catalyst-filled reformer tubes at 760°C. The reformed gas is cooled to 350°C in a waste heat boiler, passed through a shift converter to increase the content, mixed with clean recycled top gas, heated to 830°C in an indirect-fired heater, then injected into reactor 4. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

Gas-Fired water heaters are also made more efficient by a variety of designs that increase the recov-ei y efficiency. These can be better flue baffles multiple, smaller-diameter flues submerged combustion chambers and improved combustion chamber geometry. All of these methods increase the heat transfer from the flame and flue gases to the water in the tank. Because natural draft systems rely on the buoyancy of combustion products, there is a limit to the recovery efficiency. If too much heat is removed from the flue gases, the water heater won t vent properly. Another problem, if the flue gases are too cool, is that the water vapor in the combustion products will condense in the venting system. This will lead to corrosion in the chimney and possible safety problems. 

The numerous process heaters used in refineries to heat process streams or to generate steam (boilers) for heating or steam stripping can be potential sources of sulfur oxides (SO2, and SO3), nitrogen oxides (NO and NO2), carbon monoxide (CO), particulates, and hydrocarbons emissions. When operating properly and when burning cleaner fuels such as refinery fuel gas, fuel oil, or natural gas, these emissions are relatively low. If, however, combustion is not complete, or heaters are fired with refinery fuel pitch or residuals, emissions can be significant. 

Many processes are heat driven, take place at elevated temperatures, or require product drying. As a result, process heaters and dryers are common equipment in processing facilities. Many of these are fired units fueled by a variety of gas or liquid fuels frequently by natural gas. They may be used to heat a process stream directly, to heat an intermediate heat transfer fluid, or to... 

A typical natural-draft gas-fired process heater is shown in Fig. 20.1. Suppose we gradually close either the stack damper or the air register the flow of air into the firebox will then be reduced. If both the process-side flow and the fuel-gas rate are held constant, the following sequence of events occurs ... 

Figure 20.1 Typical natural draft gas-fired process heater.

The fire investigators could find no one, including the four operators, who would admit to increasing the natural gas flow between 8 30 A.M. and 9 30 A.M. Regrettably, the flow recorder on the natural gas supply to the heater was not properly inking. Therefore none of the flow rates nor exact times of gas flow rate change could be established with certainty. 

Description Natural gas is preheated and desulfurized. After desulfurization, the gas is saturated with a mixture of preheated process water from the distillation section and process condensate in the saturator. The gas is further preheated and mixed with steam as required for the pre-reforming process. In the pre-reformer, the gas is converted to H2, C02 and CH4. Final preheating of the gas is achieved in the fired heater. In the autothermal reformer, the gas is reformed with steam and 02. The product gas contains H2, CO, C02 and a small amount of unconverted CH4 and inerts together with undercomposed steam. The reformed gas leaving the autothermal reformer represents a considerable amount of... 

Improve fuel quality. Switching to a cleaner-burning fuel, such as natural gas, reduces the emissions from fired heaters. This must be traded off against the higher cost of natural gas relative to heating oil and coal. 

Boilers normally are forced draft (FD) versus most fired heaters that are natural draft. It is cheaper to combine FGR with an FD system because the FD fan can be used to induce the flue gas and the plenums already exist to distribute the air. 

Effect of Fuel Heat Content. Because process heaters often bum two or more fuels—natural gas, process plant off-gas, heavy fuel oil, or light fuel oil (each having a different Btu content)—a change in fuel to the fired heater can dramatically change the heat developed. Some time will elapse before the change in process fluid temperature is registered on the heater outlet thermocouple. 

Oxygen and natural gas, separately preheated in fuel-fired heaters, are/ thoroughly premixed prior to ignitiqn by a jet burner.. The burner gas is/ quenched by water spray to 90 C and further cooled by water washing in a column. Carbon soot is partly removed at each of these stages. Final j carbon removal is by filtration through a moving bed of wet coke. Burner gas composition is shown in Table 9-1. 

---