Thermodynamic and Material Challenges in High-Temperature Reforming Processes

A threshold temperature of 700 °C is required for operating the catalyst once applied to Steam Methane Reforming (SMR) and Autothermal Reforming (ATR) for converting methane gas to hydrogen gas. There is a severe drawback to increasing by 700 °C, beyond the additional energy required. In this blog post, I will outline possible reasons we should use low-temperature catalysts from an engineering perspective. 

1. Increasing the amount of energy/heat loss: 

According to the Stefan-Boltzmann law, for a real surface with emissivity ε (between 0 and 1), the radiant exitance, j(power per unit area), becomes: 

j=εσT⁴ 

, where σ = 5.670374419×10⁻⁸ W m⁻²K⁻⁴. 

Since the constants ε and σ are independent of temperature, the radiant exitance is proportional to the temperature raised to the fourth power. In this context, ultra-low-temperature catalysts, often Manganese (Mn)- or cerium (Ce)-based, are used for low-temperature methane oxidation at 150-350 °C. This temperature value is approximately two- to seven-fold lower than 700 °C for a nickel-based catalyst, indicating that ultra-low-temperature catalysts reduce wasted heat energy exponentially. On top of that, the furnace temperature should be maintained well above the part used for the catalytic reaction, due to the temperature difference caused by the distance between the furnace and the catalysts. This situation is also dominated by the Stefan-Boltzmann law; therefore, heat loss by radiation will increase. 

2. Discouraging catalyst activity (sintering, coking) 

The unique perspectives of the catalyst reaction are all attributed to the catalyst's active sites. However, according to research, the sintering process, which merges grain boundaries, thereby decreasing the number of grains and increasing the size of each grain, is carried out at approximately 700–800 °C. This phenomenon directly reduces the surface density of Ni-based catalysts, thereby deactivating them. 

While the sintering process reduces catalyst activity by directly decreasing the surface density (active sites), coking also undermines catalytic activity by covering the Ni surface or blocking pores, and, even worse, whisker-like carbon atoms can form cracks in the catalyst substrate, ultimately breaking the catalysts. This deactivation caused by carbon deposition is due to methane cracking and the Boudouard reaction. Methane cracking is a process in which methane gas is dissociated over a nickel-based catalyst into carbon atoms and hydrogen gas.

The Boudouard reaction is a process in which carbon monoxide is dissociated into carbon atoms and carbon dioxide. Both generated carbons are attached to the catalyst and hinder catalytic performance over the approximate range of 500-700 °C. In ultra-low-temperature catalysts, however, these temperature-dependent reactions can be avoided. 

3. Demand high-temperature-resistant materials for the reformer tube. 

Considering that the nickel-based catalyst’s typical operation temperature is approximately 700 °C, the consideration of furnace material choice is important. The reasons the reformer tube fails are creep, carburization, and oxidation. 

Creep is defined as time-dependent strain under constant load and elevated temperature. Its rate never reaches zero. Carburization is the formation of metal carbide corrosion products that occurs when metals are exposed to temperatures above approximately 700 °C in carburizing environments (those containing carbon monoxide, hydrocarbons, etc.). Oxidation of reformer tube surfaces is responsible for metal loss of a few mm/year, which limits tube life, and repetitive thermal cycling causes spalling and accelerates metal surface damage. 

Even though heat-resistant alloys, such as HK-40, HP-50, Nb-modified HP, and micro-alloyed HP, have been significantly developed to provide high creep strength with good oxidation and carburization resistance, long-term high-temperature operation will eventually lead to some creep or deformation. Here again, simply lowering the operating temperature with ultra-low-temperature catalysts can prevent these predictable problems, as all three are temperature-dependent.  

How could this temperature disparity happen? 

Note that these ultra-low-temperature catalysts generally activate methane at lower temperatures and do not supersede the high-temperature SMR or ATR pathways. Methane activation can take place between 150 and 350 °C without breaking the C-H bond when oxygen becomes more mobile, e.g., from defects/vacancies in MnCeOx, resulting in a low activation energy for the catalyst. For example, C-H bonds have an average bond dissociation energy of 413 kJ/mol, while the cost of bypass mediated by oxygen through H-bonding is 70-90 kJ/mol.

The same bypassing principle could be applied to the selective catalytic reduction (SCR) system. Because vanadium(V)-based catalysts are usually used to convert NO2 to N2 and H2O, the energy requirement to operate the system is approximately 200-400 °C, which is higher than for Mn- or Ce-based catalysts. If ultra-low-temperature catalysts could be commercialized at scale, energy costs and waste heat could be significantly reduced, thereby making the process more cost-effective.