The Haber-Bosch Process

The overall reaction

The overall reaction of the Haber-Bosch process is:

N_2(g) + 3 H_2(g) ⇋ NH_3(g)       ΔH = -92 kJ/mol

The reaction is exothermic and combines a molecule of nitrogen with 3 molecules of hydrogen to form 2 molecules of ammonia. The exothermicity of the reaction and the reduction in the number of molecules during conversion explain the difficulty to promote it.

Because the reaction is exothermic, the equilibrium will favour the reverse reaction of ammonia separation into nitrogen and hydrogen under high-temperature conditions. Yet, reaction rates are also temperature-dependent and a sufficiently high temperature is required to reach a reasonable conversion rate. Accordingly, most industrial reactor will operate between 400 and 450 °C in order to have a reasonable production rate while limiting the impact of the reverse reaction. This compromise means that only ~15% of the N₂/H₂ mixture is converted into NH₃ through a single pass in the reactor, imposing multiple recycling to reach full conversion.

Because the reaction reduces the molecule count, the equilibrium will move towards the smallest molecule count under higher pressures. Furthermore, increasing pressure also favours chemical reaction rates, so that higher and higher pressures should lead to higher and higher yields. Yet, yield gains are not increasing as quickly as installation and running costs if extremely high pressures are mandated (high pressure piping and gas compression are both expensive), so that an economic trade-off is responsible for pressure selection. In most modern industrial installations, the Haber-Bosch process operates at ~200 atm.

Catalysis, separation and recycling

While 450 °C and 200 atm should insure the conversion of nitrogen and hydrogen into ammonia, left alone this process would be extremely slow and incompatible with industrial expectations. To accelerate reaction rate, the gases are pumped into a reactor filled with an iron-based catalyst, which does not change the equilibrium but accelerates the reactions towards this equilibrium.

The most frequent catalyst is iron-based, with some potassium hydroxide added as a promoter.

Because the conversion rate during a single pass through the reactor is only ~15%, while hydrogen production is expensive and energy-demanding, there must be a mechanism to reuse unconverted hydrogen. For this purpose, recycling of unconverted gases (H₂ and N₂) towards the reactor inlet is used once they have been separated from produced ammonia.

At reactor outlet, all species (nitrogen, hydrogen and ammonia) are gaseous and separation of gases is quite difficult. In the Haber-Bosch process, the largely different liquefaction temperatures of all 3 gases is exploited: ammonia liquifies at -33 °C, nitrogen at -196 °C and hydrogen at -253 °C at atmospheric pressure. Yet, because gases are at high pressure, the mixture does not have to be chilled as cold as -33 °C before ammonia turns liquid, is drained from the bottom of the chiller, and nitrogen and hydrogen are recycled.

This separation step is important in the energy balance of the installation because chilling produces relatively cold and low-pressure gases that must be reheated and recompressed before recycling to the reactor inlet. A significant part of the improvement of Haber-Bosch installations with time has been the integration of each step with heat energy coming out of hydrogen production loop and ammonia rector reused to heat up and compress gases out of the ammonia separation unit.

Conventional Haber-Bosch process

The Haber-Bosch process as currently exploited results from an optimisation in the context of fossil fuels as only viable source of energy without any consideration for the reduction in greenhouse gas emissions. In fact, the process became more and more efficient during the 20^th century, but only for economic reasons. Consequently, the process (1) produces hydrogen by converting fossil fuels into syngas, followed by a water gas shift reaction, (2) compresses and heats with fossil fuels the stream nitrogen and hydrogen entering the ammonia reactor, and (3) powers the separation of ammonia with energy extracted from the previous 2 steps (therefore indirectly with fossil fuels).

The most efficient conventional Haber-Bosch installations run on methane. The key steps are:

• Hydrogen production from methane reforming
• Ammonia synthesis with the Haber-Bosch process

Hydrogen is produced through a multiple step process: (1) methane is converted into syngas, (2) a water gas shift reaction increases the hydrogen content and eliminates any carbon monoxide, and (3) CO₂ is extracted.

The conversion into syngas is itself split into 2 sub-steps occurring in the primary and secondary steam methane reforming reactors. The primary reactor operates in an oxygen-free environment that is saturated with steam (allothermal conditions) at 850-900°C and 25-35 bar. Because of the lack of oxygen, reactions cannot be exothermic and the process can only be sustained if an external source of heat is provided. Such a heat source is burning methane in an auxiliary burner. The secondary reactor operates with products of the primary reactor combined with an air stream. The oxygen from air reacts with primary products to release heat, so that the secondary reactor is autothermal (operation at 900-1000°C without external heat source). Beyond the reactivity created by oxygen, the air injection is also critical to provide the necessary source of nitrogen for the Haber-Bosch process.

The products of the 2-step methane reforming are a mixture of hydrogen, steam, carbon monoxide, carbon dioxide, nitrogen, and some unconverted methane, while only hydrogen and nitrogen are desirable. Methane reforming is therefore followed by a water gas shift reaction that is an equilibrium between steam and carbon monoxide on the one hand, and hydrogen and carbon dioxide on the other hand. The water gas shift reaction operating towards hydrogen releases heat, so that active cooling is required to convert as much CO as possible. Such a heat source is later used for gas heating or compression.

After the water gas shift reactor, the stream is mainly made of hydrogen, carbon dioxide and nitrogen. Carbon dioxide is extracted through the Benfield or Selexol process. If traces of carbon monoxide are still present, the stream is run through a methanation reactor to convert it because CO is a poison to the Haber-Bosch catalyst.

The ammonia production stage incudes (1) the Haber-Bosch reactor, (2) an ammonia separation unit, as well as (3) a gas recycling loop for unconverted nitrogen and hydrogen. The Haber-Bosch reactor operates at 15-25 MPa and 400-450 °C and reactivity is promoted with an iron-based catalyst (either magnetite or wustite). Despite the high-pressure/high-temperature environment and the use of catalysis, the single-pass conversion is only ~15%, so that significant amounts of nitrogen and hydrogen exit the reactor. Ammonia is separated from H₂/N₂ by exploiting its properties: it is easier to liquify at moderately low temperature/high pressure while other gases in the stream remain gaseous. The unconverted H₂/N₂ are then recycled to the inlet of the Haber-Bosch reactor but reheating and recompression to 15-25 MPa and 400-450 °C is required.

External heating for the allothermal reaction and necessary cooling during the water gas shift reaction means that a significant amount of wasted heat exist. This heat is used to produce high pressure steam, later expanded in turbines that propel the compressors for (1) the air stream to the autothermal methane reforming reactor and (2) the hydrogen/nitrogen stream entering the Haber-Bosch reactor.

Overall consumption of methane (CH₄) and consequent CO₂ emissions results both from conversion of CH₄ into H₂ and from burning methane to provide heat to some reactors. The specific CO₂ emissions are in the range of 1.5-1.6 tons of CO₂ per ton of produce NH₃. The large worldwide consumption of NH₃ (140 Mt/y in 2014) mainly for fertilising purposes explains why NH₃ production alone is responsible for 1.2% of worldwide CO₂ emissions.

The process CO₂ emissions can be split between CH₄ consumption to produce H₂ with steam reforming and CH₄ consumption to provide heat to the process. About ¾ of CH₄ consumption is the feedstock to produce H₂, so that switching to electrolysis would reduce the impact of NH₃ production by at least 75%.

Alternative approach

An alternative approach is possible while retaining the same key Haber-Bosch process of hydrogen fixation on nitrogen. Yet, this requires to rethink each step. The production of hydrogen needs being shift from consuming fossil fuels to exploiting renewable electricity and producing hydrogen through electrolysis. The gas compression (and induced heating) must also be powered by renewable electricity. The ammonia separation uses alternative method requiring less cooling and therefore less energy.

Any alternative approach to the conventional Haber-Bosch process aims at using renewable energy sources rather than fossil fuels.

The first step of producing hydrogen is obtained by running an electrolyser with renewable electricity, therefore dramatically cutting CO₂ emissions. Yet, while H₂ production is a lot cleaner, the resulting stream is lacking nitrogen content. An additional separation step of O₂/N₂ from air (also powered with renewable electricity) is therefore required. It generally is a pressure swing adsorption (PSA) method, but cryogenic distillation or membrane separation could also be considered. (Note that both H₂ and N₂ production steps have O₂ as a by-product).

The second step of combining hydrogen and nitrogen also uses the Haber-Bosch process but all compression efforts rely on electric compressors run on renewable electricity.

The production of hydrogen with electrolysis is the main reason for CO₂ emission reduction when using an alternative approach based on renewables. Assuming an electrolysis efficiency of 66% for an alkaline electrolyser, the specific energy consumption to produce required H₂ is 215 GJ/tH₂, which translates into 37.9 GJ/tNH₃. Then, the energy consumption for N₂ separation from air and for compressors in the Haber-Bosch process is ~2.7 GJ/tNH₃. As for the conventional fossil-fuel-based, the production of hydrogen is by far the dominant factor. Said otherwise, once hydrogen is produced, conversion into NH₃ is only consuming a marginal amount of additional energy (~7% of H₂ production), notably less than liquefying the same hydrogen (~20% of H₂ production).

The equivalent CO₂ emissions of NH₃ production with alternative methods depend on the specific impact of renewable electricity production:

• From wind power: 7-38 gCO₂/kWh (see wind turbine LCA), which translates into 0.08-0.43 tCO₂/tNH₃
• From solar power: 9-45 gCO₂/kWh, which translates into 0.10-0.51 tCO₂/tNH₃

Accordingly, even with the least favourable source of renewable energy, ammonia can be produced with only 1/3 of CO₂ emissions of the current best fossil fuel approach. With the most favourable source of renewable energy, emissions are only 5% of the state-of-the-art methane-based method.

It is important to point out that the electricity-powered method of ammonia production has some drawbacks compared to the conventional one:

• Hydrogen production does not simultaneously produce N₂, so that N₂ must be separately extracted from air at an energy cost
• The electrolysis process produces hydrogen at relatively low pressure (10-30 bar) while the Haber-Bosch process requires at least 100 bar, increasing compression needs as the conventional approach can deliver hydrogen at high pressure
• The alternative NH₃ production requires a lot of water to produce hydrogen, at about 1.6 tH₂O/tNH₃

But it also has some advantages:

• Hydrogen from electrolysis is very pure, removing the purification step required after the steam reforming step of the conventional approach
• Compressors are entrained by electric motors that are more efficient (eff=90+%) than turbines propelled by steam (eff~45%), while renewable electricity production is cleaner than steam production from methane
• The process can be more easily scaled from small local units producing ammonia at consumption point to large centralised industrial operations similar to current fossil-based installations
• As both electrolysis and air separation produce pure O₂, this process could be a significant source of industrial high-grade oxygen with about 1.4 tO₂/tNH₃