Ceramic coating application in a refinery steam methane reformer furnace
Increase an SMR’s thermal efficiency by applying a high emissivity ceramic coating on the furnace box walls.
Yahya AktaS, Muammer Sever and Metin Becer
TÃ¼pras, Izmir refinery
Viewed : 1394
Ceramic nowadays is a very popular material, thanks to its heat and corrosion-resistant properties. Ceramic material is inorganic and in the form of non-metallic oxide, nitride, or carbide. In the 20th century, new ceramic materials were developed for usage in advanced ceramic engineering, such as in semiconductors and space technology. Ceramic coating has a well-known thickness, emissivity, and heat transfer coefficient. Ceramic coatings can be applied to both tubes (preventing oxidation and scale information) and refractory (maximising heat transfer to the process) surfaces.
By increasing the emissivity — a material’s ability to absorb and reradiate energy — of a refractory lining in a furnace, these specialised ceramic coatings can provide energy savings depending on zthe fuel being used. High emissivity ceramic coatings in industrial furnaces are widely used for this purpose.Â¹
Ceramic coating applications are one of the best ways to improve radiant and conductive heat transfer efficiency in high temperature industrial furnaces.
Benefits of the application can be listed as:
- Increased efficiency, leading to energy savings
- Reduced stack gas emissions of NOx and CO2
- Preventing oxidation, carburisation, and various types of corrosion
- Extended equipment life
In a refinery with a hydrogen production unit, most of the hydrogen production takes place on the nickel based catalyst in the steam methane reformer (SMR) furnace tubes. The reactions inside the 188 vertical catalytic tubes in two rows (94+94) in the SMR furnace are endothermic. Continuous and homogeneous heat flow is critical for the reaction kinetics.
The SMR heater is a terrace-wall type SMR furnace with two radiant chambers. The heat required for the endothermic reactions is provided by 144 burners on two floors (72 burners each) and on both sides of the tubes. The fuel sources in the furnace are the off-gas mixture from the pressure swing adsorption (PSA) system and the refinery fuel gas.
The hydrocarbon feed mixture (natural gas and/or LPG and/or light naphtha) combined with high pressure (HP) steam is separated into two and enters the SMR furnace tubes from the upper region.
During the downward flow, CxHy and H2O react on the catalyst surface and turn into H2, CO, and CO2. The heat required for the reactions is transferred continuously from the combustion chamber to the catalytical tubes. The hydrogen-rich high temperature process gas leaves the tubes at the bottom through hairpins and is routed down to the transfer line to downstream equipment, a waste heat boiler.
The furnace operates with a natural draft, and the airflow in the combustion chamber is regulated by the stack damper opening. While the hot combustion gas in the radiant region transfers its heat to the catalytic tubes, some of the heat is absorbed by the refractory surface and reflected to the tubes. The combustion gas leaves some of its heat in the radiant zone and flows towards the convection zone. In the convection zone, it passes through the preheat and economiser coils, transferring its heat energy to the unit feed, boiler feed water, and steam streams.
The thermal efficiency of the furnace box can be increased by applying a high emissivity ceramic coating on the furnace walls, which enhances the radiative heat transfer contribution from the hot surface walls to the catalyst tubes.2 Thanks to the high emissivity coating material to be applied to the refractory surface, it aims to increase the amount of heat absorbed in the radiant region and reflected to the tube surfaces. In this way, while the heat transferred from the combustion gas to the radiant zone increases, the heat transferred to the convection decreases, and therefore the steam production decreases. In this way, it aims to achieve the same production capacity with less fuel consumption values or to increase the production amount with the same fuel consumption values.
The application was discussed with a company experienced in this field. In line with the information received about the application, it was decided to make the refractory coating during the unit planned turnaround. The application was conducted by company employees, with the chemicals provided by them.
Floor-to-ceiling scaffolding was installed in the two radiant chambers of the SMR furnace for the application. Burner mouths are closed with planks, and it is ensured that no dust or chemicals get into them. The existing refractory surface has been sandblasted to obtain a smooth surface on which the coating will adhere.
All of the catalytic tubes are covered with paper and the ceramic coating solution is prevented from contaminating the tubes.
The application was made by spraying the prepared water based ceramic coating solution on the refractory surface with pressurised guns.
After the application was completed, the sawdust remaining after sandblasting on the floor was vacuumed and cleaned. Afterwards, the papers wrapped on the tubes were removed and the scaffold in the radiant region was taken out.
In the post-application period, with the unit start-up, it has been observed that the adsorbents loaded in the PSA unit were entrained by the off-gas flow and reached the burners of the SMR furnace. While a portion of the adsorbent reaching the burners has burned, the remaining part caused some damage to the ceramic coating application surface by spraying from the burners.
However, the adsorbent particles reached the convection zone, causing a decrease in the heat transfer and an increase in temperatures in this zone. This made it difficult to separately examine the positive effect of the newly made ceramic coating application.
Increasing PSA unit efficiency with adsorbent change during the turnaround and different operating conditions between the selected two days make it difficult to examine the effect of the ceramic coating. However, two days at the furnace exit temperature at the same H2 production capacity from the pre- and post-start-up periods were selected for comparison purposes. While these days were selected, a peer based comparison was made with the operation values at the same H2 production capacity, furnace exit temperature, and steam/carbon ratio.
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