Use of nitrogen-methanol endothermic atmospheres

With careful attention to injector design, precise control of the atmosphere can be achieved.

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In this column, I will discuss the use of nitrogen-methanol endothermic atmospheres for the heat treatment of steel.

Introduction

When heat treating steels in a typical atmosphere furnace, an endothermic atmosphere is used. This atmosphere is generated in an endothermic generator using natural gas. It produces an atmosphere with the nominal composition 40% H2, 20% CO, and 40% N2. There are trace amounts of CO2, CH4 and water vapor formed (less than 1 percent total).

If natural gas isn’t available, propane can be used in an endothermic generator, or a mixture of nitrogen and methanol can be used. The nitrogen-methanol can be injected directly into the furnace (most used), or the methanol can be heated to a vapor and mixed with nitrogen and injected into the furnace or an endothermic generator (less common).

Dissociation of Methanol

The constraints imposed by the thermodynamics and kinetics of methanol dissociation are a major consideration in the use of nitrogen-methanol used to produce an endothermic atmosphere, regardless whether injected directly into the furnace or an endothermic generator is used.

Thermodynamics

Methanol is reported to dissociate by the following reactions:

Of these, only reaction [E1] is appropriate for the carburizing or neutral hardening. Reactions [E2], [E3], and [E5] produce soot. In addition, reactions [E2] – [E5] are non-equilibrium reactions. Based on thermodynamics, the dissociation of methanol is controlled by temperature. Reaction [E1] is likely to prevail above 700°C (1,292°F), so is appropriate to use as the basis for an endothermic atmosphere. Figure 1 shows the change in free energy as a function of temperature for the likely reactions of methanol. From this evaluation, methanol dissociates to CO and H2 at temperatures above 700°C, and results in sooting below that temperature.

The calculated gas composition as a function of temperature is shown in Figure 2.

Figure 1: The change in free energy, as a function of temperature, according to methanol decomposition reactions [E1]-[E5].
Figure 2: Calculated gas composition based on a 60/40 methanol-nitrogen mix.

Kinetics

The kinetics of methanol dissociation have not been extensively studied in relation to endothermic atmosphere formation. However, Aronowitz et al [1] offers some guidance for temperatures in the range of 800-1,000°C. In his paper, methanol dissociation was impeded by the presence in methane, and enhanced in the presence of hydrogen. In another paper [2], it was suggested that the dissociation of methanol in the temperature range of 1,000-1,120°K (727-847°C) was:

Alternatively, the kinetic rate constant for methanol dissociation has been determined using a plug flow reactor [3], and found to be:

Methanol starts decomposing at 910°K (637°C), and complete dissociation at 1,150°K (877°C), with the major constituents of hydrogen and carbon monoxide.

Methanol evaporation and dissociation is not rapid at austenization temperatures. This was illustrated dramatically in a vertical drop-bottom furnace operating at 870°C using a drip system at 1.5 liters per hour, with no fan circulation [4]. Because of the slow evaporation and dissociation kinetics, methanol was observed dripping through the furnace work zone and hitting the refractory door at the base. The total fall distance was approximately three meters.

Furnace Injection/Delivery Systems

For nitrogen-methanol atmospheres, the calculation for methanol required is a bit more difficult as methanol is a liquid. To obtain the proper atmosphere, the total volume will be 40 percent nitrogen. One gallon of methanol dissociates to form 240 standard cubic feet of CO and H2, so for 1,000 SCF of atmosphere needed for a furnace, then 400 standard cubic feet of nitrogen is required, and 600 standard cubic feet of CO and H2 are required. Therefore, 2.5 gallons per hour (600 SCF needed/240 SCF per gallon CH3OH = 2.5 gallons per hour) are needed.

There are several different methods to inject methanol into the furnace work zone. In many early systems, methanol was delivered by nitrogen pressure, and injected directly into the furnace. This injection was often done near an internal fan to allow the fan to help catalyze the reaction. In other systems, methanol has been injected into the furnace using a small 12.5-25mm diameter tube, with the methanol dripping onto a drip pan. However, either of these systems can cause problems with methanol dripping directly on parts, resulting in staining.

Since the thermodynamics and kinetics dictate that methanol must be heated to the process temperature rapidly above 700°C to avoid the formation of soot (Figure3) [5], it is important to ensure a large surface to volume ratio of the methanol on injection into the furnace hot zone.

Figure 3: Calculated soot formation as a function of temperature [5].

Drip systems have a large methanol droplet size, so it is important to reduce the droplet size for rapid heating, evaporation, and dissociation. Spargers can reduce droplet size; however, they are prone to sooting. Round cone spray nozzles have been found to produce a fine enough spray for proper reaction [4]. Vertical nozzles have been found to perform better than horizontal nozzles, with reduced warpage. The use of nozzles with a manual clean-out is always preferred.

Nozzle insertion depth is very critical. If nozzles are inserted too far into the furnace, warpage can occur due to differential thermal gradients. If the nozzles are inserted back further into the refractory, then the surrounding refractory would become wet with methanol, and sooting and clogging of the nozzles would occur. Empirically, the optimal distance has been found [4] to be approximately 50-100 mm inside the refractory. Generally, for flow rates from 4 to 16 liters per hour, an optimal tip size of 1.25 works well, with a round spray pattern with a 30° exit angle. This is at a delivery pressure of 103 kPa (15 psi).

The use of single nozzles in a furnace is generally limited to approximately six liters/hour of methanol. For flows greater than this, multiple nozzles are preferred. It is recommended that each nozzle have precision needle values and flow meters so that the flow between the nozzles can be balanced.

Conclusions

In this article, we have described the use of nitrogen-methanol atmospheres for heat treating steel. With careful attention to injector design, precise control of the atmosphere can be achieved. In the next column, we will discuss the storage and control problems of a nitrogen-methanol installation.

Should you have any questions regarding this column, or suggestions for any future columns, please contact the author or the editor. 

References

  1. D. Aronowitz, D. W. Naegeli and I. Glassman, “Kinetics of the Pyrolysis of Methanol,” J. Phys. Chem., vol. 81, no. 25, pp. 2555-25559, 1977.
  2. D. Aronowitz, R. J. Santoro, F. L. Dryer and I. Glassman, “Kinetics of the oxidation of methanol: Experimental results semi-global modeling and mechanistic concepts,” Symposium (International) on Combustion, vol. 17, no. 1, pp. 633-644, 1979.
  3. M. Ran, J. Shi, J. Niu, C. Qin and J. Ran, “Investigation and improvement of the kinetic mechanism for methanol pyrolysis,” International Journal of Hydrogen Energy, vol. 42, no. 26, pp. 16345-16354, 2017.
  4. D. S. MacKenzie, “The Dissociation of Methanol Used for the Neutral Hardening of Steel,” Heat Treatment of Metals, vol. 1, pp. 1-6, 1995.
  5. M. Akuh, T. Nitsch, G. Plicht and F. Steinbach, “Smart nitrogen-methanol injection lance for carburizing and hardening atmospheres, a case study,” Heat Processing, no. 1-2, pp. 1-6, 2022.