A summary of the 2019 Blasch white paper on what running without one tunnel costs in hydrogen.
Repair now, or wait for the next turnaround?
A model puts a number on the wait.
A collapsed outer flue gas tunnel in a top-fired steam methane reformer lowers the furnace temperature around it and with it the hydrogen yield. The Blasch CFD white paper from 2019 puts the local hydrogen loss at up to 9.7% and the furnace-wide loss at 0.95%. If the primary reformer sets the output of a 2,000 t/d ammonia plant, that comes to about 19 t/d, or roughly 2.07 million USD over a year at 300 USD/t.
| Publication | White paper, Blasch Precision Ceramics, 2019 |
| Title | A computational fluid dynamic study on the effect of operating a steam methane reforming furnace without one of the flue gas tunnels |
| Authors | Not named in the paper |
| Published version | Nitrogen+Syngas 360, July and August 2019 |
A tunnel collapses during a campaign. At the next maintenance outage the debris is cleared, but there is no time to build a new tunnel, so the rebuild is planned for a later turnaround. The furnace runs on with seven of eight tunnels.
The paper compares that furnace with the same furnace and all eight tunnels in place. The modeled box is 42.5 ft high and 52.5 ft wide and long, with tunnels 72 inches high and 26 inches wide inside, 96 burners and 336 catalyst tubes. The published article on the same study explains the model and the flow field in detail.
Clearing the debris and running on looks free, yet the furnace pays for the missing tunnel every day until it is rebuilt.
Temperature: the model gives an average of 1,753 °F inside the furnace with all tunnels, 1,681 °F without the outer one, and 1,537 °F in the area where that tunnel would stand.
Hydrogen: reformer output depends on temperature. Treating the flue gas temperature as the process gas temperature, the paper estimates up to 9.7% less hydrogen in the area next to the missing tunnel and 0.95% less across the whole furnace.
Production: for a plant making 2,000 t/d of ammonia, where the primary reformer limits output, that means about 19 t/d less, or 6,916 t over a year of running before the rebuild.
Money: at a bulk price of 300 USD/t, the paper puts the theoretical annual impact at 2,074,800 USD.
The authors are open about the limits. The reaction inside the tubes is not modeled, the tubes act as heat sinks, temperatures near the roof are assumed equal in both cases, and the gas composition rests on several simplifying assumptions. The figure is a theoretical estimate for a modeled furnace. Your own plant will differ in size, layout, price and in whether the reformer limits production at all.
Plants often keep running with a damaged tunnel, and with brick that is understandable: a rebuild takes masons, mortar and curing time an outage rarely has. The paper changes the comparison. Once the wait has a daily price, you can set it against the cost of rebuilding one tunnel in a short outage.
StaBlox blocks, lids and bases in common sizes are kept for exactly this case. See StaBlox compromised tunnel replacement, and the recorded webinar on the collapsed tunnel study.
Many plants do. The study does not assess safety, but it estimates a steady hydrogen loss for as long as the tunnel is missing.
No. The 19 t/d assume a 2,000 t/d ammonia plant whose primary reformer limits production. A plant with spare reformer capacity would lose less.
Both report the same results. The white paper gives the figures in US units and closes with the annual cost estimate.
With precast blocks set dry, without mortar or curing, a single tunnel goes up in hours instead of days.
Tell us your tunnel size and your next outage window. A Blasch engineer will tell you what a single tunnel rebuild needs.
