I.- Ping Chung, PhD, is a senior development engineer in the Technology and Commercial Development Group at
6.3 Common Heater Types .1 general
6.3.5 Steam Cracking Furnaces .1 Cracking Furnace Design
The typical firebox layout is shown in Figure 6.13. Radiant coils are placed in the center of the firebox with burners placed on both sides in order to provide uniform heating.
Several variations of this design are shown in Figure 6.14.
Figure 6.12
Bottom-fired reformer.
TaBle 6.2
Favored Flame Shape for Various Reformer Types Reformer Type
Burner Capacity
(Typical), MM Btu/h Flame Characteristics Top fired 8–10 (2.35–2.93 MW) Round, compact flame Side fired 1.2–2.0 (0.35–0.6 MW) Flat flame, sweeping
along the radiant wall Bottom fired 6–8 (1.75–2.35 MW) Round, narrow
“pencil” flame
TaBle 6.1
Typical Firing Splits Application
Firing Split (PSA:
Makeup Gas) Ambient air 40%:60%
Preheated air 80%:20%
6.3.5.2 Burners for Cracking Furnaces
In order to understand the critical role of burners in a steam cracking furnace, it is important to keep in mind that, unlike most process heaters, the steam cracking furnace is the main reactor in the plant. A deviation from the ideal process temperature profile has a direct impact on plant throughput and profitability since it increases
coking rates and lowers the coil selectivity. The process temperature profile inside the cracking coils is, to a cer- tain degree, determined by the layout of the cracking coils, but the heat distribution in the box, determined by the number, type, location, and heat release of the burn- ers, remains the key parameter.
Cracking furnaces typically come in four different burner layouts:
1. 100% radiant wall fired 2. 100% floor fired
3. Combination floor + wall fired 4. Floor + terrace fired
The layout depends on licensor, age of the furnace, type of radiant coil, size of the firebox, and NOx emission limitations. In early firebox designs, the fired duty was mostly provided by radiant wall burners (see Figure 6.15).
Since the heat release of a single burner was limited to about 1 MMBtu/h (0.3 MW), many burners were needed to provide the total firing duty. The number of burners in a naphtha cracking furnace would typically range from 160 in the 1970s up to 240–300 in the late 1980s. The low firing duty meant that a single burner could be considered as a point source of heat. The heat distribution could be adjusted by varying the fuel pres- sure between fuel headers (zone firing was, historically, a popular feature), by adding more or less secondary air to individual burners, or simply by turning burners off.
The ongoing tendency to reduce investment cost and maintenance cost has significantly changed the character of these furnaces since the late 1980s. The firing capac- ity of a single firebox increased tremendously, which made the use of 100% sidewall burners very costly and maintenance-intensive. Although all the main licensors have moved away from the use of 100% wall firing, there is no clear consensus on the ideal system. The use of large heat
Cracked gas Steam drum
TLE
Convection banks
Convection banks
Firebox
Radiant coils
Burners
Flue gas
HC feed
Dilution steam HP super- heated steam
HP water
Figure 6.13
Typical cracking furnace firebox layout.
(a) (b) (c)
Figure 6.14
Variations of typical cracking furnace firebox layout. (a) Single cell. (b) Twin cell. (c) Twin cell without inner walls.
release floor burners, typically in the range 5–15 MMBtu (1.5–4 MW), is common (see Figure 6.16). Sometimes a few rows of wall burners are specified as well; sometimes the furnace is all-floor fired; sometimes a second row of burn- ers (“terrace”) is added, situated about halfway up the wall, consisting of burners of similar design to the floor burners.
A key parameter that is commonly specified by the fur- nace designer, and demonstrated by the burner supplier, is the vertical normalized incident heat flux profile.6
While the firebox duty has increased, the firebox size has not kept equal pace due to efforts to reduce invest- ment costs. As a result, the firing density (expressed in fired duty per firebox volume or floor surface area) has increased faster than the firebox duty, and burners have been placed so close together that they have started influ- encing each other in a negative sense. These burner–
burner interactions have been the reason for a number of designs for which at start-up the combustion system has exhibited poor flame quality—severe rollover of the flames into the cracking coils (see Figure 6.17). The con- tinuous flame impingement resulted in excessive local coking of the process tubes, short run-lengths, carbu- rization of the material, and coil cracks. Even the NOx emissions have been higher than anticipated based on results measured during the burner test (more correctly termed “witnessed performance demonstration”).
Like all other process furnaces, cracking furnaces have become the subject of tighter NOx and CO emissions limits. Cracking furnaces, however, are especially being scrutinized because of their high firing rates, which make them very large producers of NOx in an absolute sense.
Moreover, the firebox temperatures are the highest in the petrochemical industry, with bridgewall tempera- tures up to 2300°F (1260°C) compared to the typical fire- box temperatures in refinery furnaces of 1400°F–1600°F (760°C–870°C). This means that even on a relative basis, the thermal NOx production of a cracking furnace is often two to three times higher than that of a refinery furnace.
There are other factors that make the design of burn- ers for cracking furnaces especially challenging, such as fuel flexibility. The normal (design) fuel is usually a by- product of the process, referred to as residue gas.
Figure 6.15 Radiant wall burners.
Figure 6.16
Large heat release floor burners.
Figure 6.17
Example of flame rollover in a pilot-scale cracking furnace.
The main components are methane and hydrogen.
Depending on the design of the plant, the hydrogen content can be as low as 5 vol% or as high as 80 vol%.
The burners also must have the flexibility to burn other fuels in other scenarios such as “start-up,” “backup,” and
“emergency backup.” Sometimes the alternate supply is natural gas (mostly methane). More often, the other fuel supply is LPG. The plant has an integrated fuel system, the consequence of which is that the design fuel can be contaminated with higher molecular weight species (C4s, C5s), which might be olefinic, diolefinic, or worse.
The result is often tip plugging during these incidents.
For reasons of energy efficiency, the burner is occa- sionally integrated with either a combustion air preheat system or with TEG.
In order to properly account for all of the factors men- tioned earlier, single-burner testing has almost completely been superseded by multiple-burner testing. Because problems at start-up are so damaging and costly,7,8 the test is often complemented by computational fluid dynamics (CFD) simulations (see Volume 1, Chapter 13), the accu- racy and sophistication of which is developing rapidly.9 6.3.5.3 Integration with Turbine Exhaust Gas
Gas turbines typically run at a high excess air. Turbine material temperature limits require a very lean combus- tion, and the turbine exhaust gas (TEG) also contains cool- ing air for the turbine blades. As a result, the TEG contains high levels of oxygen, typically between 13 and 17 vol%.
In power plants, the waste heat in the TEG is used to gen- erate steam in a heat recovery steam generator, reheating it using burners that consume this excess oxygen. As an alter- native for such a combined cycle power generation system, the TEG can be used as an oxidant for industrial burners.
Although the TEG oxygen content is less than that of air, it is still sufficient for stable combustion, especially since its temperature is typically 950°F–1050°F (510°C–565°C).
Steam cracking furnaces and steam methane reformers are very good candidates for integration with gas turbines, since the amount of flue gas from a single gas turbine is com- patible with the firing requirements of a world-scale plant.
While firing with TEG requires some investments such as forced draft fans for makeup/backup air and the ducting between furnaces and gas turbine, reduction in firing rates due to its high temperature is in the range of 15%–20%. In addition, firing with TEG increases the amount of flue gas relative to firing with air, due to its low oxygen content.
A higher amount of flue gas through the convection sec- tion of a cracking furnace results in a higher production of superheated high-pressure steam (1800 psi, 980°F) (124 bar, 527°C), which further improves the energy balance of the plant. Finally, the high overall thermal efficiency that is achieved since the flue gas in a cracking furnace is typically
cooled down to ∼250°F (120°C) makes this a very attractive concept, especially when energy costs are high.
Combustion tests with ultralow NOx burners firing 1000°F (540°C) TEG have shown a 20% reduction in NOx (on a concentration basis) compared to the ambient air case.10 However, on an absolute basis (expressed in lb/h), the combined NOx emissions of the gas turbine and cracking furnace are only ∼10% higher than the cracking furnace firing ambient air.
6.4 Process Heater Design