Originally appeared in: October 1990 issue, pgs 217-225. CHEMICAL ENGINEERING. Reprinted with publisher’s permission. Fired heaters and boilers have been made more efficient in the past decade, but further gains can be expected Ashutosh Garg MAKE variations to the burners of a fired heater to + 2% of the mean air flow. Kinetics Technology International Such flow modeling of flue ducts can EVERY Corp., and also help achieve uniform flue gas H Ghosh distribution in air preheaters, avoid- Engineers India Limited ing the dead pockets that cause local- ized dew point corrosion. Although the world has been awash with oil, recent events in the Middle East show how tenuous that condition BTU Firing Heavier Fuels With improved methods, petroleum refiners are extracting more products from each barrel of crude oil, leaving COUNT can be. Forecasters had predicted that energy prices would be climbing by the a much poorer quality residue to middle of the decade, if not sooner, burn in heaters. even without the spur of Iraq’s invasion of Kuwait. Energy conservation is ex- Fuel specifications in one refinery pected to again become popular and before and after recent expansions heat-recovery equipment and how fuels for fired heaters to become heav- are noted in Table 2. The differences they can be realized are listed in Table ier, dirtier and more difficult to burn. are typical of changes in fuel specifi- 1. Complicating matters, pollution control cations. laws will further restrict both gaseous At the Heart of Fired Heaters and solids emissions. In conventional atomizers, fuels of When fuel was cheap, most fired heat- inferior quality tend to convert oil ers were fitted with natural-draft burn- Heat-recovery equipment is, of course, droplets into small coke particles. ers, which were operated with high vital energy conservation. In addition The droplets burn slowly and are not excess air (30-50%) and long flames. to seeing larger numbers of air pre- totally consumed by the flames. En- heaters and waste-heat boilers, opera- trapped in flue gas, the particles are They have always been bottom fired, tions and maintenance supervisors and carried out through the stack. The oil with steam atomizing guns. Boilers, engineers are likely to encounter im- fuel should be filtered if it contains on the other hand, were mostly fitted proved burners and soot blowers in solids. with forced-draft mechanically atom- fired heaters. They will also see many ized oil burners that worked under more exchangers for recovering low- Three things can be done to enable positive pressure. They have usually level heat at faster paybacks. Potential fired heaters to burn heavier fuels accurately scaled transparent model of improvements in energy efficiency for more efficiently: (1) raise the pres- the ducts and windbox. One study sure and quality of the atomizing based on such a model reduced flow steam to improve atomization; (2) Furnace Improvements Services www.heatflux.com 1 MAKE EVERY BTU COUNT of cast-iron tubes, the recuperative exchanger has been proven in service as a combustion air preheater. The cast tubes may have fins inside and outside, and be located and be placed at the outlet to avoid dewpoint con- densation. The heat transfer fluid in the circulat- ing liquid preheater is warmed in the fired heater’s convection section and passes through a liquid to air ex- changer, which heats the combustion air. Such a unit is the preheater of choice when space is limited (because it does not need an induced-draft fan and flue gas ducts) or when a heat transfer fluid is available in the plant. This preheater can also be used to preheat process and utility streams. mix the fuel oil and atomizing fluid circulating-liquid system. Exchangers having a transfer surface thoroughly; (3) and reduce the size of Pyrex are gaining acceptance. This of the spray droplets so that they will Frequent in steam-generation service, borosilicate glass makes an ideal sur- burn quickly without producing un- the regenerative exchanger has been face for exchangers that are designed burned carbon. These changes are successfully adopted for fired heaters. for cooling a flue gas below its dew- limiting particle emissions to less Seals have been improved in design point. Chemically inert and resistant than 50 mg/m3, even when heavy and in material to reduce leakage of air to scaling, the glass is also inexpen- fuel oils and vacuum residues are to flue gas, a hazard. sive. Glass tubes will be increasing burned. installed in recuperative exchangers A recent trend is to use fuel gas, if A newer approach to leakage control in for recovering energy now vented out available at a high enough pressure, large preheaters involves automatically of stacks. as the atomizing fluid. Not only is activated deflecting sector plates at the gas inexpensive, it also enriches the hot end. Plate heat exchangers, which can op- flame. erate below the flue gas dewpoint, can Different heat-transfer shapes have also lower flue gas temperatures to about Preheat Combustion Air been adopted to suit alternative fuels. 65oC because condensation occurs Heat is recovered from the stack of Other improvements pertain to modular below 65oC, this exchanger recovers fired heaters and used to warm up motors, support bearings and soot both sensible and latent heat. combustion air. The most common blowers. air preheaters are the regenerative A static exchanger consisting of circu- Protected by an acid-resistant coating, and recuperative exchangers and the lar tubes, or a rectangular cross-section the heat transfer metal surface is not contacted by condensate. Even when the flue gas temperature drops to 55oC, the flue gas leaves the air pre- heater superheated by about 20oC. This prevents downstream condensa- tion and corrosion. Waster-heat boilers represent a good choice for raising fired-heater effi- ciency if steam is needed, because heat recovered as steam is more valu- able than heat saved as fuel. A boiler can be installed in convection section of a fired heater. The stack tempera- ture depends on the boiler steam pres- sure and feed water temperature. The Furnace Improvements Services www.heatflux.com 2 MAKE EVERY BTU COUNT inlet temperature must be controlled to avoid dewpoint corrosion. Direct-contact recovery of condensate heat holds promise for recovering low- level heat. In this new system, hot flue gas is passed through a cold water stream. This cools the flue gases to between 32 oC -43oC, and heats the water stream to about 55oC, recovering both latent and sensible heat from the flue gas, and boost- ing the efficiency of the fired heater from 5% to 8%, one side benefit of the scrub- bing is a reduction of SO2 and SO3. The limitation of this equipment is that low-level heat is useful mostly for warm- ing water. Sootblower Advances Soot deposits on tube surfaces can reduce heat-transfer efficiency by as much as 10%. Conventionally, high pressure steam blown across tube surfaces, (usually, about once per shift) and re- moves ash deposits. Steam blowers re- quire continuous maintenance. Micro- processor-based control has boosted Heat recovered from fluegas can be used to warm up the combus- blower flexibility, making it possible tion air (below); Convection section of fired heater can be adapted to blow areas where heat transfer has to generate steam (right). dropped below a critical level and to operate the blowers in varying se- quences. A new method of keeping a heat transfer surface clean is to suspend the ash and soot in a gas stream by means of nearly continuous sound pressure waves. These sonic sootblowers gen- erate sound by means of a flexible diaphragm or a motor connected to a frequency converter. The major advantages of sonic soot- blowers are that they can be mounted anywhere, cover a large area, provide continuous cleaning and be fully auto- mated. They also need little mainte- nance, because they have only one moving part. Their chief limitation is that they can remove only light, friable deposits of slag or soot. If the fuel oil being burned contains sodium and vana- dium, the deposits in the convection section will be sticky because of their low melting point. Such deposits are Furnace Improvements Services www.heatflux.com 3 MAKE EVERY BTU COUNT difficult to remove by steam or ide provides a sensitive, reliable guide fouls the convection section, boosting sound. Fuel treatment can make the to combustion efficiency. With oil-fired the emission of particles. The fouling deposits friable. furnaces, CO contents range from 150 of the convection section lowers the to 200 ppm. Flue gas analyzers now draft, and this progressively decreases Closer Combustion Control measure both oxygen, carbon monox- the air flow, increasing the quantity of Little attention was paid to efficient ide and other combustible gases. With particles. At turndown operation, heat- combustion control when fuel was the cost of microprocessor based con- ers emit greater quantities of solids cheap. With the development of trollers continuing to decline, even because of poor mixing of air and forced draft firing and air pre- small heaters will be equipped with fuel. heaters, controlling excess air be- advanced control systems. Also influential is fuel quality, par- came important. Microprocessors ticularly the content of asphaltenes in have made efficient control easy. Nitrogen oxides, major air polluters, liquid fuels. Particle emissions from range from 100 to 500 ppm in flue gas, modern forced-draft burners being Onstream oxygen measurement predominantly as nitric dioxide (NOx). fed, preheater air, can be controlled to provides the dynamic feedback Factors that influence the quantity of 50 mg/m3 even when heavy fuels are needed for closely controlling com- NOx in flue gas are: quantity of nitro- fired with 5% excess air. bustion in fired heaters. The oxygen gen in the fuel, furnace temperature, content in exhaust gases represents excess air and the temperature of the air a direct measure of combustion entering the heater. The authors efficiency. The formation of NOx can be reduced Ashutosh Garg When the oxygen content is low, by re-circulating the flue gas, conduct- the measurement of carbon monox- ing staged combustion, decreasing the Senior thermal quantity of excess air, and lowering the engineer in Ki- temperature of the combustion air. netics Technol- ogy International Reducing the excess air to a heater Corp. Formerly, he was a deputy manager with the Heat can be recovered from fluegas via direct Heat & Mass Transfer Div. of -contact spraying (left); sonic sootblowers Engineers India Ltd. His career prevent tube deposits by means of pressure responsibilities have included waves (below) the design, sales and commis- sioning of fired heaters and com- bustion systems. He holds a degree in chemical engineering from the Indian Institute of Technology. H. Ghosh Chief consultant with Engineers Indian ltd. He is responsible for the design of all heat-transfer equipment. He received a degree in chemical engineering from Jadavpur Uni- versity, and worked for Fertilizer Corp. of India and A.P.V. Cal- cutta, before joining Engineers India Ltd. Furnace Improvements Services www.heatflux.com 4 *Reproduced with the permission of Chemical Engineering.