**3.7 Ammonia plant operating condition**

The operating conditions of ammonia production units can be summed up as follows:


#### **3.8 Refractories for ammonia production unit**

As per conventional wisdom, the ammonia production unit operating temperature is not a matter of concern since it is moderate and does not exceed 1350°C. Considering only the operating temperature, hence, it can be stated that even the lowest grade of aluminosilicate refractories, except for the secondary reformer dome, would suffice. Only the strength of the refractories should be high enough to withstand the operating pressure. But is this the case when we consider the gaseous environment in ammonia production units?

The discussion in the previous sections revealed that even the "inert" gases, like nitrogen and steam, influence the refractory properties and hence, performance. It also is evident that it is not only the high temperature that damages the refractories. Rather for certain working environments, for example, destruction of refractories by CO, low, rather than high, temperature causes greater damage to refractories. It is not only operating temperature, but the impact of the gaseous environment also needs to be considered for refractory selection.

The gases from the working environment can permeate through channels as well as open pores (**Figure 3**) to the interior section of the refractories. In the specific case of reformers, where the operating pressure is of the order of 35 bar, gas permeation is a very likely possibility. Additionally, refractories have finite thermal conductivities, i. e. the temperature along with the thickness of refractory lining decreases.

The silica reduction rate, by hydrogen, increases with the increase of temperature and becomes significant when the temperature exceeds 1100°C (**Figure 7**). This implies that silica reduction by hydrogen is expected to occur on the refractory hot face, not in the interior part. Refractory destruction by CO, on the other hand, will not occur on the refractory hot face since the reformer operating temperature is >1000°C. But at a certain distance from the hot face, along with the thickness of the refractory, where the temperature is in the 500 to 700°C range, the extent of CO decomposition and as a consequence refractory destruction would be high.

In short, the refractories remain vulnerable to destruction by CO even if the operating temperature of the unit is >1000°C. In the context of ammonia production units, thus, refractories are at risk of destruction by CO and the destruction process may commence at the interior part of the refractory, where the temperature is conducive for CO decomposition.

During the interaction of aluminosilicate refractories with hydrogen, silica is preferentially attacked and this leaves a porous alumina network [15]. Both Hydrogen, as well as CO, permeate through the residual porous alumina structure and thus, silica reduction by hydrogen as well as CO attack continue in the interior part of the refractory. The SEM analysis of the used refractories from an ammonia plant transfer line shows that the loss of silica occurs from the aggregate and not the matrix [19]. This observation contradicts the conventional belief that the matrix, being a finer part of the refractory formulation, has higher reactivity and thus, is more vulnerable to chemical attack.

Since primary as well as secondary reformers and transfer lines operate at high pressure, at 35 bar, it is prudent to see the impact of pressure on Hydrogen and CO attack of refractories. An increase in pressure reduces the rate of silica reduction by hydrogen (**Figure 10**). Higher operating pressure, thus, is favorable for the prevention of refractory degradation by silica reduction. Le chatelier principle, on the contrary, predicts that an increase of pressure would enhance the decomposition rate of

#### *Refractories for Ammonia Production in Fertilizer Unit DOI: http://dx.doi.org/10.5772/intechopen.104934*

CO into carbon (Eq. 8). Such contradicting situations make the refractory selection for ammonia plants more convoluted.

Based on the discussion of the previous section it is obvious that the combined effect of Steam - Nitrogen - CO and H2 on the aluminosilicate refractory is fairly contradicting as well as complex since:


Apart from the opposing conditions of refractory-gaseous environment interactions, marginal volatilization of silica from the refractory may poison the catalysts downstream. Refractories, hence, are selected so that even the slightest silica reduction in the process is mitigated. This is the reason why the lowest possible iron oxide as well as silica-containing refractories, are selected for primary as well as secondary reformers. This is because, in the absence of iron, the CO decomposition rate is reduced, whereas the reductant effect of hydrogen is prevented by the absence of silica in the refractories. Needless to mention that such high purity raw materials are not available in nature and thus, the refractories for primary and secondary reformers should be based on synthetic raw materials like WTA or White Fused Alumina (WFA), which are low in both iron oxide as well as silica.

Refractories, based on synthetic raw materials like WTA or WFA, are typically recommended only for locations where the operating temperature exceeds 1650°C. Primary and secondary reformers are examples, where although the operating temperature is low, still synthetic raw material-based refractories are recommended due to the gas composition in their operating environment as well as the possibility of catalyst poisoning. In other units of ammonia making plant, that is, other than primary and secondary reformers, the conditions are not as severe due to the operating temperature being lower than 500°C, which is the temperature of CO decomposition initiation, and hence, lower alumina aluminosilicate refractories with lesser purity would suffice.

### **4. Brick or monolithic for ammonia plant**

Owing to the progress made on the material development front, there is no significant difference between brick and monolithic chemistry. The gap between brick and monolithic has been bridged primarily by reducing the flux concentration in monolithic formulations. For example, the fluxing component in castable and majority of aluminosilicate gunning formulations is CaO originating from the cement. With the progress in materials technology, aluminosilicate castables and gunning materials can be produced without any cement, that is, without CaO. And thus, in the majority of industrial applications, refractory bricks can be replaced by monolithic.

The aforementioned progress in materials technology can be exploited and monolithic, instead of bricks, based on WTA, can be used in ammonia plants. The primary advantage of monolithic, that is, mechanized installation, thus, can be capitalized for the refractory lining of ammonia plants. The fallout of mechanized installation of monolithic is the requirement of skilled bricklayers is eliminated and simultaneously, the refractory installation rate also is enhanced. Monolithics, depending on its type, the installation rate is 2.5 to 10 times higher than those for the bricks. The substitution of bricks by monolithics, with similar chemistry, would reduce the inventory cost, delivery time, dependence on human skill, and above all the installation time. All these factors put together would reduce the plant downtime when a brick is replaced by monolithic.

**Figure 20** illustrates the installation rates of various classes of monolithics and their characteristics. Castables and gunning materials typically have 1.5 and 8% CaO, respectively. Apart from the high concentration of CaO, the strength characteristic of the gunning formulations is not very favorable. But the time required for gunning material installation is significantly lower than that for the castable. New generation gunning formulations, which also are known as wet gunning or shotcreting, have strength characteristics as well as CaO concentration similar to that of castable. On the contrary, the time required for shotcrete installation is comparable to that for gunning material. An additional advantage of shotcrete material is extremely low rebound loss, that is, lower dust generation during installation compared to that of gunning material. This makes shotcrete significantly more user as well as environment-friendly

compared to gunning material. In short, shotcrete materials have the advantages of both castable as well as gunning materials, that is, faster installation like gunning material and installed material property as well as chemistry similar to that of castable.

In short, low iron - low silica-alumina based bricks and monolithics are recommended for primary as well as secondary reformers. Lower alumina products with lesser purity would suffice for the rest of the ammonia plant.

### **5. Conclusion**

Going by conventional perception 45% alumina refractories would suffice for the fertilizer industry since the operating temperature does not exceed 1350°C. This, however, is not the case owing to the simultaneous presence of CO, H2, CO2, Steam, and N2 in the environment of ammonia production units. Generally, steam and N2 are treated as inert gases in regard to their interaction with refractory materials. But steam reduces the reduction rate of SiO2 of Aluminosilicate refractories by Hydrogen gas. Steam, on the other hand, enhances the decomposition of CO as per the Boudouard reaction. The favorable effect of steam for silica loss by its reduction, hence, can not be exploited owing to the simultaneous presence of CO and Hydrogen in the ammonia production unit environment. The aluminosilicate refractories recommended for ammonia production units, mainly for primary and secondary reformers, should, thus, be low in iron oxide as well as silica content so that the adverse impact of both Hydrogen and CO is minimized. Such formulation can be either in the form of bricks or monolithic. Refractories with such stringent chemistry requirements can only be met by those based on synthetic raw materials like WTA or WFA.

The fertilizer industry is one good example that epitomizes that the alumina content of aluminosilicate refractories is not decided only by the operating temperature. Owing to the simultaneous presence of Hydrogen, CO, N2, and steam in the operating environment, there is no option but to use aluminosilicate refractories with a low concentration of iron oxide as well as Silica through the operating temperature of the reformers barely exceeds 1100°C. Based on this analysis it is evident that operating temperature is not the only determinant of refractory quality for an industrial process but the gaseous environment of the unit also plays a significant role in the refractory selection process.

Currently, the reduction of greenhouse gas emissions is a major focus of all industrial processes. To achieve this goal, replacement of fossil fuels by hydrogen for iron production as well as other industrial processes are being targeted. The analyses presented in this paper also will provide direction for refractory selection for the industrial processes where hydrogen is being used or hydrogen is a yield, e.g. Gasification.
