A summary of the 2016 Nitrogen+Syngas article on tunnel open area, flow ratios and catalyst tube life.
A model of one 12 m tunnel, before and after.
How open area along the wall decides which tubes run hot.
In a reformer furnace revamp, replacing brick flue gas tunnels with engineered blocks lets you place an opening in every course and size each one, so flue gas enters evenly along and among the tunnels. The 2016 Nitrogen+Syngas article calculated a conventional 12 m tunnel at an average outer to inner flow ratio of 77.4% with a spread of ±55.1%. Redesigned with StaBlox and the TOP method from BD Energy Systems, the same tunnel came to 65.64% ±1.8% against a target of 65%.
| Publication | Nitrogen+Syngas, issue 345, July and August 2016 (excerpt) |
| Title | New Blasch StaBlox flue gas reformer tunnel system |
| Authors | Not named in the excerpt; written by Blasch Precision Ceramics with BD Energy Systems |
| Topic | Flue gas flow uniformity in down-fired steam methane reformers |
Catalyst tubes are typically designed for 100,000 hours. Some stay in service for 20 years, others fail far ahead of schedule, often in regions of the furnace where nothing obvious is wrong. Inspection reports link this to tube temperatures above average, and those to regions where the flue gas moves faster than average.
Ideally the flue gas flows straight down at one velocity everywhere. That means each tunnel should take flue gas in proportion to the firing of its burner row, and evenly along its length. Outer burner rows heat one row of tubes, inner rows heat two, so outer tunnels should carry less. If outer burners fire at 65% of the inner rate, an outer tunnel should carry 65% of the flow of an inner one.
Along a tunnel, the open area should shrink gradually toward the open end. Near the closed end the gas inside barely moves, the pressure difference across the wall is small, and large openings are needed. Toward the exit the velocity and the pressure difference rise, so less open area draws the same flow.
The ideal open area falls in a smooth line toward the tunnel exit, and a wall of half bricks can only follow it in steps.
Brick tunnel: the outer tunnel carried about 77% of the inner tunnel flow in total, but the ratio varied from column to column, with a standard deviation of 21.3%. Near the closed end the outer tunnel drew 110% of the inner flow per column, near the open end only 56%. Behind every step change in open area the flow climbed again, and buttresses and expansion joints left regions with no flow at all. Flue gas moved sideways across the furnace to make up the difference.
Engineered tunnel: the wall was split into 9 inch sections with one column of openings each, without buttresses and with the expansion gaps spread over all blocks. Each opening received an orifice insert with a bore chosen for its position. The flow ratio held at about 65.6% along the whole length, with a standard deviation of 0.7%, both at the closed end and at the exit.
The article compares refractory data for the tunnel parts. Super duty brick deflects 7.86% under creep load at 1,427 °C, mullite bonded alumina 1.11% at 1,649 °C. The hot modulus of rupture is below 600 psi for super duty brick, up to almost 500 psi for a high grade castable and 1,700 psi for mullite bonded alumina.
Without buttresses, the wall footprint is about half as wide, so the inside of the tunnel can be wider in the same space. The authors estimate about one third more flow area and a lower pressure drop. The weight of the tunnel system falls by up to 60%.
If the plant later raises capacity, the opening pattern of a brick tunnel no longer fits and may need a rebuild. Here only the inserts are changed, and the walls stay.
If your revamp scope includes the tunnels, treat them as part of the heat distribution, not just as refractory to replace. A tunnel rebuilt brick for brick keeps the same steps and no-flow zones.
The design described here is the StaBlox flue gas tunnel system. The earlier article covers why brick tunnels fail mechanically, and the 2020 article shows what measured tube temperatures did after an installation.
Outer burner rows heat only one row of catalyst tubes and fire less than inner rows, so less flue gas is produced above the outer tunnels.
All half brick openings have one size, so open area can only change by leaving out more or fewer bricks per section. Buttresses and expansion joints add zones without openings.
In the design described, yes. The orifice inserts are replaced in place, and the tunnel walls are reused.
Calculated, for a 12 m tunnel. Field data from an ammonia plant came later and are summarized in the 2020 World Fertilizer article.
Send the tunnel layout and the firing pattern. A Blasch engineer can show you where your present tunnels draw too much or too little.
