Introduction

As the supply of fossil fuels decreases, it is quite possible that future stationary or mobile energy systems will use hydrogen fuel cells. Natural gas has been proposed as a transition fuel as it is currently plentiful and has an existing infrastructure.

The process of generating hydrogen from natural gas (mostly methane) is outlined in Figure 1. After sulfur removal, steam and methane are combined and reacted at high temperature in a “steam reformer.” The effluent contains some carbon monoxide and water, which are reacted in a “water-gas shift reactor” to form carbon dioxide and additional hydrogen. The gases exiting the shift reactor are separated, with pure hydrogen product.

Figure 1: Process Steps in Hydrogen Production from Methane

This paper will focus on the reactions that occur within the steam reformer and determine the equilibrium conversion of methane and water to hydrogen, carbon monoxide, and carbon dioxide. It is important to know the chemical compositions to determine natural gas requirements for a fueling station with an on-site reformer as well as size the reactor and separation units.

Methane Steam-Reforming Reactions

The steam reforming reaction is given as

CH4 + H2O ↔ 3 H2 + CO

This reaction is reversible. The methane conversion is determined from the equilibrium constant, which is given below as a function of the number of moles n of the individual components:

Note that the equilibrium constant is always a function of temperature (for this reaction a higher temperature leads to increased hydrogen formation). For this reaction, the number of moles increases with the methane conversion, and therefore the equilibrium constant also depends on the total pressure in atm.

In the steam reformer, the water gas shift reaction also takes place as

CO + H2O ↔ H2 + CO2

For this reaction, the equilibrium constant is given by

Since the number of moles is the same for the products and the reactants, this equilibrium constant is independent of total pressure. Furthermore, a lower temperature favors conversion to hydrogen.

The two chemical reactions state that methane (CH4), carbon monoxide (CO), water (H2O), carbon dioxide (CO2), and hydrogen (H2) are present in the system. There is a simple process for determining the equilibrium compositions of these gases.

Determining the Amount of Chemicals Formed in the Steam Reformer

Consider a feed of 1 mole CH4 and Nw moles H2O to a steam reforming reactor that operates at 900 K and 1 atm total pressure. It is helpful to set up a table and write equations to determine the equilibrium number of moles of CH4, H2O, CO2, CO, and H2. If x1 is defined as the steam reforming reaction conversion and x2 as the water-gas shift reaction conversion, it is possible to construct Table 1.

Table 1. Equilibrium Numbers of Moles

With the final number of moles from this table, Equations 1 and 2 become at 1 atm total pressure and a temperature of 900 K:

With these two equations and two unknowns, the following process can be used to solve for x1 and x2 at high temperatures:

1. Start with x2 = 0 and use Equation 3 to solve for x1.
2. Using the x1 value that was just calculated, use Equation 4 to solve for x2.
3. Using the x2 value that was just calculated, use Equation 3 to solve for x1.
4. Repeat steps 2 and 3 until the values for x1 and x2 do not change. This could take 10 or more loops to converge to the solution.
5. Check to make sure the final answer satisfies Equations 3 and 4.

For the case of Nw = 1, with x2 = 0, x1 = 0.5561 is obtained from Equation 3. Then, using this value for x1 in Equation 4, x2 = 0.1498 is obtained. After a few loops, the numbers converge to x1 = 0.5248 and x2 = 0.1556. With these values, the reactor output will be 0.4752 mol CH4, 0.3196 mol H2O, 0.1556 mol CO2, 0.3692 mol CO, and 1.7300 mol H2.

Estimating the Ratio of Water to Carbon

The example above (with Nw = 1) gives a low hydrogen conversion. Table 2 shows the CH4 conversion x1 as a function of Nw at 900K.

Table 2: CH4 conversion

Thus, to obtain over 90% methane conversion, there should be about a 3.7:1 molar ratio of water to methane. Alternatively, the reaction can be carried out at a higher temperature.

After the steam reforming reaction, a low-temperature water-gas shift reactor is used to covert almost all of the CO into CO2 and H2 (since at 400 K KWGS = 736). These calculations could be performed by setting up a new table similar to Table 1 for only the water-gas shift reaction and solving for the unknown compositions of H2, CO, CO2, and H2O.

Fuel Station: Determining the Natural Gas Requirements

For Nw = 3,  the steam reforming reactor output will be 0.1449 mol CH4, 1.6695 mol H2O, 0.4754 mol CO2, 0.3797 mol CO, and 3.0407 mol H2. In a low-temperature water-gas shift reactor, the 0.3797 mol CO is converted into 0.3766 mol H2. Thus, 1 mol of CH4 has produced 3.4173 mol H2.

During the transition to a hydrogen economy, a typical hydrogen station would be required to produce 200 kg of hydrogen per day. A kilogram of hydrogen has the same energy as a gallon of gasoline. A fuel cell vehicle would need 4 kilograms of hydrogen to fill the fuel tank and be able to travel over 60 miles per kilogram of hydrogen.

Assuming a constant production rate, the following would be required:Since 1 mol CH4 is required to produce 3.4173 mol H2, the methane requirement is 20.3 mol CH4/min. Converting to standard liters per minute (at 1 atm pressure and 0°C):

Although it is beyond the scope of this case study about 50% more methane is needed for a combustion reaction to provide heat for the endothermic steam reforming reaction. Thus, the total methane requirement is 683 standard liters per minute (SLPM).

This process produces 0.855 mol CO2 per mol of CH4. This carbon dioxide will have to be separated from the hydrogen. However, with hydrogen produced at the service station, the carbon dioxide could be captured and sequestered instead of being a vehicle emission.

This article has shown how to estimate the conversion of CH4 gas into H2 for use in fuel cell vehicles and determine the CH4 requirement for a transition-era fueling station.