Page 1 STEAM-HYDROCARBON REFORMER FURNACE DESIGN Introduction Direct-fired steam hydrocarbon reforming furnace is the “work-horse” of gas production processes. Steam reforming process is a well-established catalytic process that convert natural gas or light hydrocarbons in a mixture containing a major portion of Hydrogen. The Steam reforming process has gained more and more importance with the increasing demand of various type of syngases for the chemical and petrochemical industries. Its application are in the production of: - Ammonia - Methanol - OXO Alcool - Hydrogen In particular Hydrogen has become a very important product for the refinery desulphurisation and hydrocracking process units. The furnace may “stand alone”, or operate in conjunction with a pre- reformer, post-reformer, or other schemes. In the furnace, the reforming of steam-hydrocarbon mixtures is accomplished in catalyst-filled tubes. In hydrogen plants, in-tube fluid pressures are typically 25 ÷ 30 kg/cm2 with outlet temperatures up to 860°C (and even higher) depending on the process requirements. The reformer reaction process is endothermic, requiring high level heat input. A variety of catalyst (nickel-based) are available for a given feed and product requirement. Safe, reliable and efficient operation is needed to meet the user’s product demands. Radiant section arrangement As the process requires high heat input levels, the catalyst-filled tubes are placed vertically in the radiant firebox section of the furnace. The steam- hydrocarbon mixture is typically preheated outside the radiant section to 500°C ÷ 650°C to minimize the radiant heat load and, therefore, the furnace fuels requirement. Excessive preheat will effect coke formation of
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Hydrogen Plant Steam-hydrocarbon Reformer Glob Ale
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STEAM-HYDROCARBON REFORMER
FURNACE DESIGN Introduct ion Direct-f i red steam hydrocarbon reforming furnace is the “work-horse” of gas product ion processes. Steam reforming process is a wel l-establ ished catalyt ic process that convert natural gas or l ight hydrocarbons in a mixture containing a major port ion of Hydrogen. The Steam reforming process has gained more and more importance with the increasing demand of various type of syngases for the chemical and petrochemical industr ies. I ts appl icat ion are in the product ion of: - Ammonia - Methanol - OXO Alcool - Hydrogen In part icular Hydrogen has become a very important product for the ref inery desulphurisat ion and hydrocracking process units. The furnace may “stand alone”, or operate in conjunction with a pre-reformer, post-reformer, or other schemes. In the furnace, the reforming of steam-hydrocarbon mixtures is accomplished in catalyst- f i l led tubes. In hydrogen plants, in-tube f luid pressures are typical ly 25 ÷ 30 kg/cm2 with out let temperatures up to 860°C (and even higher) depending on the process requirements. The reformer react ion process is endothermic, requir ing high level heat input. A variety of catalyst (nickel-based) are avai lable for a given feed and product requirement. Safe, rel iable and eff ic ient operat ion is needed to meet the user’s product demands. Radiant sect ion arrangement As the process requires high heat input levels, the catalyst- f i l led tubes are placed vert ical ly in the radiant f i rebox sect ion of the furnace. The steam-hydrocarbon mixture is typical ly preheated outside the radiant sect ion to 500°C ÷ 650°C to minimize the radiant heat load and, therefore, the furnace fuels requirement. Excessive preheat wi l l ef fect coke formation of
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the feed, result ing in carbon deposits on the catalyst causing degradation and/or pluggage, and also potent ial tube fai lure in the preheat sect ion. Properly located combustion equipment (burners) assures heat input as the mixture passes through the catalyst tubes and is reformed to the required out let condit ions. The catalyst tube arrangement consists of a mult iple of “once-through” paral lel “passes”, typical ly with the preheated inlet mixture enter ing the top of each catalyst tube, and exit ing at the bottom. Once the reformed gas exits the catalyst tubes, i t is col lected in a header system and cooled in an external process gas waste heat exchanger. The eff luent from this equipment is typical ly cooled to 320°C ÷ 370°C to permit further react ion in downstream equipment. Safe and rel iable operat ion of the reformer furnace depends on the disposit ion of the catalyst tubes and the burners that supply the heat to the catalyst tubes. In theory, complete control of heat input along the vert ical catalyst tube length wi l l maximize catalyst react ivi ty, minimize tube temperatures, and minimize tube or catalyst damage during operat ing upsets such as process steam interrupt ion, or wide load swings. Such a design requires an excessive number of burners and be di f f icul t to operate. Several wel l-proven configurat ions are avai lable which, each in their own way, provide a pract ical approach towards meeting the requirements of this process. Two part icular designs are considered.
FIGURE 1
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Side Fired (Terrace Wall™) Foster Wheeler Fired Heater Division developed i ts patented Terrace Wall™ reformer furnace in the early 1960’s and cont inuosly improves i t to incorporate the desired process requirements and provide a safe, operable, and economic design. This design (Figure 1) typical ly locates a single, in- l ine row of catalyst tubes in the middle of the radiant f i rebox, and locates burners on both sides to provide uniform heat distr ibut ion around the catalyst tube circumference. The burners f i re vert ical ly upward along the refractory-l ined wal ls of the radiant sect ion, essential ly paral lel to the catalyst tubes, to assure f lame stabi l i ty and avoid f lame impingement. The burners provide a f lat-shaped f lame and are suitably spaced along the length of the f i rebox, assuring uniform heat input to the catalyst tubes; essential ly the refractory wal l becomes a uniform heat radiat ing plane. (See f igure 2) Operat ing f lexibi l i ty is “bui l t in” to al low tr imming of burners in specif ic areas where minor hot spots may occur, since the burners “serve” a single row of tubes. With catalyst tubes typical ly 11 to 14 meters long, control of vert ical heat distr ibut ion along the tube lengths is typical ly obtained by providing two (2) levels of burners. This permits control led heat input as process condit ions, catalyst act iv i ty or other factors varies during operat ion. As is the case with al l f i red process furnaces, the radiant sect ion heat transfer is augmented by a convect ion component as hot f lue gases recirculate, drawn downwards by the relat ively colder tubes. In this design, the recirculat ion is essential ly “contained” by the sidewal ls on both sides of the tubes, and “reheating” (as the gases return upwards along the sidewal ls past the up-f i red burners) is predictable, result ing in eff ic ient overal l heat transfer.
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The upflowing hot f lue gases exhaust “natural ly” at the top of the f i rebox, enter ing the heat recovery sect ion where feed preheat, steam generat ion, air preheat or post-combustion Nox reduct ion (SCR’s) may be instal led.
FIGURE 2
Flat-shaped f lame conf igurat ion The top of the f i rebox is the point of highest pressure ( lowest negative pressure, or “draft”) . This area must be control led to be maintained at least 2.5 mm water column below atmospheric pressure to keep the furnace at negative pressure throughout, avoiding hot f lue gas leakage through the various openings (tube penetrat ions, sight doors, etc.) and to prevent hot f lue gas from contact ing the furnace casing plate. The Terrace Wall™ design can frequently ut i l ize simply a natural draft stack. Where very high fuel eff ic iency is needed (e.g., air preheat) or an SCR is instal led, the pressure loss through this equipment usual ly dictates the use of mechanical draft equipment ( induced draft fan). With f i r ing at more than one level to reduce the vert ical heat f lux variat ion, and with a uniform radiat ing plane effected by f i r ing along the side wal ls, the catalyst tubes can be spaced typical ly at a 1.4 to 1.7 rat io (center-to-center divided by outside diameter) to obtain an opt imal distr ibut ion of heat around the tube, minimizing peak tube metal temperatures. (using the API RP-530 curve, the circumferent ial heat f lux factor – or variat ion from average f lux – for calculat ing tube temperature is 1.31 to 1.25; see Figure 3).
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FIGURE 3
Ratio of maximum local to average heat f lux.
Single row of tubes with equal radiat ion from both sides. Source: API RP-530
Each catalyst tube is f langed on top to permit catalyst loading from walkways at the f i rebox roof (or arch). The f i rebox sidewal ls are slopped at a small angle to opt imize radiant heat transfer. This also creates a “terrace” shape, which provides a mounting space for the individual burner levels. Burner access for operat ion and maintenance is from essential ly unrestr icted platforms located along the sidewal ls at each f i r ing level. Burner noise plenums, ductwork supplying preheated combustion air or, in some cases, hot gas turbine exhaust, can be readi ly instal led. Various fuels such as natural gas, ref inery gas, or even l iquid fuels (and associated atomizing steam) can be readi ly piped to the burners. The off-gas from a PSA (Pressure Swing Absorpt ion) hydrogen puri f icat ion system is used as fuel for the reformer furnace. This is low BTU fuel, and usual ly avai lable at low pressures. When properly integrated with hydrogen plant design i tsel f , the PSA fuel can provide most – or al l – of the fuel needed in the reformer furnace. Typical ly, the plant design prefers to l imit the PSA off-gas to “base-load” at 90% or so of the total reformer fuel requirement, al lowing the balance (ref inery gas or natural gas) to be used for control l ing the heat input.
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The burner arrangement on the Terrace Wall™ design (f i r ing along the refractory sidewal ls) al lows stable burning 100% of PSA off-gas once the f i rebox is heated. Top Fired (“Downfired”) The down-f ired design (Figure 4) locates from one to as many as ten or more rows (or “ lanes”) of catalyst tubes ( in- l ine) in a single radiant f i rebox enclosure, with rows of burners located in the roof (or arch) of the f i rebox between the tube lanes. The burners f i re downwards, paral lel to the hydrocarbon-steam mixture f low direct ion through the catalyst tubes. Burner f lame and hot gas radiat ion provide heat input to the tubes. The combustion of low calor i f ic value PSA gas produce long, lazy and uncontrol lable f lame patterns which wi l l be creat ing down-f lowing as wel l as side turning f lames with impingement on catalyst tubes, since no hot refractory l in ing is present to retain the f lame away from the catalyst tubes.
FIGURE 4
Typical Downfired design
This arrangement effects somewhat higher heat f luxes at the top of the tube (coldest f luid).
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The concentrat ion of heat f lux at catalyst tube inlet might result in local overheating of both tubes and catalyst in part icular when operat ing at part ial loads. Each row of burners provides heat input affect ing two rows of catalyst tubes. The two lateral rows are subject – in addit ion to f lue gas radiat ion – also to the radiat ion of the hot unshielded wal l facing the tubes. This fact results in an overheating of one side of the lateral catalyst tube with consequent heat maldistr ibut ion. Flue gases are col lected at the bottom of the f i rebox in refractory “ tunnels”, properly sized and arranged to maintain a uniform f low pattern in the f i rebox. The f lue gases exit the “tunnels” and are directed to the heat recovery sect ion for process coi l heat ing, steam generat ion services, and air preheating exchanger. To assure negative pressure at the f i rebox, mechanical draft equipment ( induced draft fan) must be instal led to overcome the “draft gain” in the f i rebox and the pressure losses in the various heat recovery coi ls and/or equipment. Having f i r ing only at one level, there is no possibi l i ty of control of the heat input along the catalyst tubes, and the heat transfer mechanism more dependent on burner spacing (not by uniformly heated sidewal ls). The catalyst tubes are spaced at a 2.0 to 2.5 rat io (center-to-center divided by outside diameter) to minimize peak tube metal temperatures. (Using the API RP-530 curve, the circumferent ial heat f lux factor for calculat ing tube temperature is 1.20 to 1.15 at this spacing). Lane spacing (versus tube length) is establ ished to assure proper heat transfer. Access for burner operat ing maintenance, is from the walkways located between the tube lanes and burner rows at the f i rebox arch.
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Also in case of a generous lane spacing part icular ly with mult iple fuel headers and/or hot duct ing to the burners, this area, which is very hot during natural operat ion, might be dangerous for operator. In case of fai lure of the induced draft f lue gas fan, hot f lue gases wi l l be trapped at the top of the down f i red radiant box since no draft is avai lable and therefore the excessive heat concentrat ion and the possibi l i ty to have sl ight posit ive pressure at the top of the downfired reformer is a r isk of injury for operating personnel, present on top of radiant sect ion under the penthouse. The downfired arrangement is more di f f icul t to be operated since an uneven heat f lux distr ibut ion caused by a maldistr ibut ion of the heat f i red on the various lane of burners might effect heavi ly tube l i fe. In addit ion during start-up and warm-up of steam reformer al l the heat l iberated by the downfir ing burners wi l l remain at the top determining a very hot area at arch level since the remaining radiant zone, st i l l in cold condit ion and without vert ical wal ls, are not suitable to provide the heat downwards. This can result in uncontrol led f lame and detr imental after burning condit ions between the catalyst tubes arranged in paral lel lanes. Catalyst Tubes At a specif ied design point, a comparison can be shown (between the two design conf igurat ions) of the in-tube f luid and tube metal temperature prof i les along a catalyst tube (Figure 5). The comparat ive prof i les for typical hydrogen reformer condit ions indicate the higher heat f lux at the top of the tube on the downfired design, as evidenced from a steeper f luid temperature prof i le (and relat ively hotter tube metal temperature). The Terrace Wall™ design has the advantage that with a proper spl i t of f i r ing between the two burner level the f luid/metal temperature prof i le can be modif ied and opt imized in accordance with the actual operat ing condit ion whi le in the Downfir ing design the temperature prof i le is only a consequence of the operat ing condit ions. Operat ional upsets such as interrupt ion of process steam or unexpected impuri t ies in the hydrocarbon feed tends to result in greater catalyst temperature with possibi l i ty of tubes damages in the higher f lux inlet zone of the downfired unit .
FIGURE 5
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Temperature prof i le vs. catalyst tube length
Terrace Wall™ against Downfired design
Outlet Header System The reformed gas out let from the bottom of each catalyst tube is directed to the out let col lector header system, and then to the process gas heat exchange train ( typical ly a waste heat boi ler which generates steam). In the Terrace Wall™ design, each catalyst tube out let is connected by an Incoloy 800H out let pigtai l , which is then connected to the out let header. The out let header is Incoloy 800H (or centr i fugal ly cast equivalent material) . This system is ful ly contained in an insulated enclosure to minimize heat loss and provide for expansion (see Figure 6). The out let header is direct ly connected to the process gas waste heat boi ler inlet channel in most cases. This arrangement also permits “pinching” of the individual pigtai ls ( top inlet and bottom out let) to isolate a fai led tube without shutt ing down the whole unit . Experts ski l led in this procedure have the equipment and know-how to safely pinch-off the tubes.
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FIGURE 6
Outlet pigtai l and hot out let header Mechanical Features Proper instal lat ion and support systems for the catalyst tubes are cr i t ical to the successful long-term operat ion of the reformer furnace. Much work has been done over the years in learning the “do’s” and “don’ts” of the systems. Experience is the best teacher, and use of that experience in today’s reformer furnaces assures the most rel iable product. In the Terrace Wall™ design with out let pigtai ls and hot out let header (Figure 6), the system provides ful l load top support (catalyst tube weight plus catalyst weight) with expansion of the catalyst tube upwards through the arch (typical ly 200 ÷ 250 mm). Top support is provided with a simple, posit ive counterweights system, which al lows for the necessary variat ion in expansion between adjacent tubes. (Figure 7). The hot outlet header expands along the furnace length, “pul l ing” the out let pigtai ls and the catalyst tubes with pract ical ly no stress since al l the weight is supported from the top.
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Although a single row of tubes f i red equal ly from both sides should have l i t t le – i f any – temperature di f ference from one side to the other, the out let pigtai l does provide f lexibi l i ty to reduce any bending stresses which might develop due to tube bowing.
FIGURE 7
Top support ing system
Heat Recovery Arrangement With the Terrace Wall™ reformer, the f lue gas heat recovery sect ion (convect ion section) is placed on top of radiant f i rebox. This minimizes the plot requirements, and provides cont inued upf low of the f lue gases. The convect ion coi ls are horizontal ly mounted with al l the services: mixed feed preheat, prereformer preheat, feed gas preheat, steam superheater and steam generat ion, feedwater preheater in a proper sequence to opt imize the heat recovery. Steam generat ion coi ls are designed for forced circulat ion to assure posit ive f low throughout start-up and off- load operat ion. The steam drum is mounted on the reformer.
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Combustion air preheat exchanger can be mounted either on top of the upf low convect ion sect ion, or mounted alongside the reformer furnace. Figure 8 shows a typical 2-cel l Terrace Wall™ reformer with the close-coupled process gas waste heat boi ler, steam drum, and hot air ducts to the burners. On the downfired reformer design, the hot f lue gas exit ing the radiant sect ion “tunnels” can be directed to a grade-mounted heat recovery sect ion with ei ther vert ical or horizontal f lue gas f low depending on coi l services and auxi l iary equipment.
FIGURE 8
Typical Terrace-Wall design
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Miscel laneous Refractor ies Radiant sect ion l inings are exposed to f i rebox temperatures of 1000°C and higher, and therefore necessitat ing high qual i ty insulat ing refractory materials to withstand the environment and reduce the heat loss ( lower the casing temperature). Insulat ing f i rebricks backed by l ightweight insulat ing blanket is used. Convect ion sect ions are l ined with insulat ing castable. Assembly Where shipping clearance is adequate, the Terrace Wall™ radiant sect ion design lends i tsel f to ful l modular izat ion (steel and l inings, catalyst tubes and out let col lectors, shop instal led). (See f igure 9). This feature is not possible with the Downfired design. Convect ion sect ion is usual ly ful ly modular ized with steel, l in ings and coi ls shop instal led.
FIGURE 9
Environmental
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Burner Nox levels can be effect ively reduced using current low Nox burner designs. The low calor i f ic value of PSA gas and the staged air design effectively reduces the Nox generated by the burner. This design is possible in the Terrace Wall™ reformer since the shape of the f lames is control led by the sloped wal l design. Conclusion The steam-hydrocarbon reformer furnace can be designed to meet the specif ic needs of a hydrogen plant. Optimal design conf igurat ions are avai lable; one wi l l provide the best solut ion for a part icular purpose. Based on the considerat ion mentioned above i t is clear the Terrace Wall™ design has several advantages i f compared with the Downfir ing design for what concerns safety, rel iabi l i ty and operabi l i ty, along with design experience and qual i ty. These are important factors to be considered when select ing this important component in a hydrogen plant. Foster Wheeler Experience The attached pages show Foster Wheeler experience in the steam reformer heater design and the photos of some steam reformer heaters are herewith attached.
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Partial l ist of Hydrogen Steam Reformers built by Foster Wheeler
JOB YEAR CLIENT CONTRACTOR
OWNER COUNTRY SIZE (MMSCFD)
NOTES
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