Improving the flow, installation experience with a new reformer flue gas tunnel system
For decades, down-fired steam methane reformer furnaces have used flue gas tunnels (or ‘coffins’) along the radiant section floor to collect and improve flue gas flow uniformity.
Blasch Precision Ceramics
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These tunnels range from 4-10 ft high, 2-3 ft wide, and 40-100 ft long, depending on the unit design capacity. However, the conventional refractory firebrick or tongue- and groove-firebrick construction has always constrained the flue gas to a non-uniform flow, which has been correlated to non-uniform catalyst-tube temperatures and accelerated tube aging. Due to tunnel size and refractory volume, traditional brick design uses only basic shapes. Typical brick-and-mortar installations require several physical features that severely limit tunnel effectiveness, making uniform flue gas flow unachievable. The ability to design and construct tunnels using highly-engineered refractory shapes could be the answer to improving flue gas tunnel effectiveness and improving catalyst tube reliability and longevity.
A system to improve flue gas flow
BD Energy Systems and Blasch Precision Ceramics have co-developed an improved reformer tunnel system to achieve flue gas flow uniformity among and along the tunnels. This system combines BD Energy Systems steam methane reforming (SMR) experience with Blasch’s customised refractory shapes. The result is unparalleled flue gas flow control, using the patented Blasch StaBloxTM reformer tunnel system in conjunction with the patent pending BD Energy Systems’ Tunnel Optimal Performance (TOP) system. Because of this new ability to fine-tune flue gas flow, these tunnels can be adapted to other applications and can open the door to previously unexplored SMR process possibilities.
This paper provides an overview of the first two installations of the Blasch StaBlox Tunnel System, which occurred in the summer of 2018 at a pair of unnamed US ammonia plants with the installation performed by ParFab Companies and overseen by BD Energy Systems and Blasch Precision Ceramics. Both installations involved removal and replacement of all tunnels.
As described above, conventional brick-and-mortar tunnels are typically 4 ft to 10 ft high, 2 ft to 3 ft wide, and 40 ft to 100 ft long. Nominal brick dimensions are 9 in. x 3 in. x 6 in. (L x H x W). Buttresses extend from the outer wall surface for support and stability. Discrete expansion gaps (expansion joints) are intended to accommodate thermal expansion to avoid wall distortion (‘snaking’). Tunnel wall openings are created by removing ½ bricks (4½ in. x 3 in.) from the tunnel wall. Columns of openings are arranged at least 1½ to 2 brick lengths apart (centre-to-centre) and are distributed along the length of the tunnel to control flue gas flow into the tunnel. These brick tunnels have historically been a significant source of reliability concerns, with partial failure or even full tunnel collapse being common. The causes of these failures can be classified into three main sources: mass, material selection, and expansion management.
The StaBlox system uses five core tunnel construction components. The construction begins with a base component that spans the bottom of the tunnel and mates to the side wall blocks. The side wall blocks are 18 in. long x 9 in. high. Each block contains two 4.5 in. dia. openings. An orifice insert with an engineered inside diameter is installed in each opening. In addition to orifice inserts, these openings can hold ancillary parts, such as tie-rods inside the tunnels, which connect the two tunnel walls and replace buttresses as an additional support structure. These tie-rods can be easily removed and replaced during outages to allow easy access down the tunnel interior for inspection or repairs. At the top of the tunnel is a lightweight slab that mates to the side walls. In order to allow for a completely mortar-free construction, the slabs also employ a shiplap joint to prevent any gas bypass through the tunnel slabs. Every component of this system is designed to accommodate its own thermal expansion. In order to eliminate any gas bypass through the expansion joints, a fibre gasketing system is employed.
Levelling the floor
Once the conventional brick tunnels were removed, the first and arguably most important task is to carefully prepare the floor for installation. The StaBlox Tunnel System requires a flat surface on which to build. Assembly tolerances and thermal expansion allowances are carefully calculated and quite tight, so any irregularities could create issues at assembly. The StaBlox Tunnel System is considerably lighter than a conventionally constructed tunnel, but if the structure under the floor of the furnace is unstable or badly warped, this should ideally be corrected prior to installation. This proved necessary at one of the sites, where a badly warped I-Beam was cut out and replaced with a straight one. Once the structure was deemed to be acceptable, a levelling castable was used to even out any low spots in the area in which the tunnels will stand. Forms were placed slightly further apart than the tunnel was wide, and the castable was poured into that space. Once that was allowed to cure, an IFB layer was put down followed by wall assembly.
Placement of base blocks
Assembly began by laying down all of the base blocks. A string was run along the desired edge of the tunnel, in order to ensure that the base blocks are laid down straight. Further, it is imperative that the base blocks be properly gapped in order to ensure that the side blocks will have the proper allowances for thermal growth. The side blocks have tight tolerances and self-align as they fit together, but if the base blocks are too close together or too far apart, the side blocks will eventually run out of room to adjust within the space allotted to them.
The first tunnel was constructed by laying down the 9 in. wide base blocks as described and gapping them at the calculated value between every other block (to mirror the anticipated expansion of the 18 in. long side blocks). It was subsequently determined when the tunnel was complete, that the gaps between the base blocks were excessive, leading to rather large gaps between the side blocks and lids. The overall assembled length of the base blocks was then measured and found to be well over the design length. In retrospect, it became clear that in the real world, there was enough debris even on a freshly swept floor to ensure that the theoretical gap was large enough. The next two attempts yielded assemblies that were actually a bit short, creating issues with side brick and lid clearances (Figure 3). Eventually, the proper gapping was determined and verified through an accurate overall length measurement of the assembly and through subsequent assembly of the tunnels (Figure 4). As most would understand, initial installations of new technology inevitably present some unknowns at the time of installation.
However, assembly and disassembly of three of these walls were still completed within hours. The experience garnered in the actual assembly of these tunnels strongly suggests that they can be disassembled and moved for major maintenance and then replaced without substantial effort.
Building the walls
Once the base blocks were in place, installation began on the walls. Due to the blocks possessing a “tongue” on one side and “groove” on the other, it was important to remain mindful that all blocks were to be oriented in the same direction. The vertical groove arrives with a strip of refractory ceramic fiber cemented into place. The entire first row of side blocks were placed on the base blocks first just to ensure that the gaps between the base blocks were correct and, consequently, that the gaps between the side blocks were consistent and of the proper thickness. Each side block covers two mating features on the base blocks and has only minimal movement end to end. The blocks were each centred on their range of limited movement (Figure 5). The installation partners, ParFab Field Services, started by assigning specific individuals to transport blocks; temporary installation of rollers proved to expedite this process considerably. The ParFab crew nominated one individual to place the horizontal ceramic fiber gaskets neatly on the blocks being placed while everyone else continued building (Figure 6). The combination of construction method and optimised task management resulted in approximately 4 hours/tunnel by the end.
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