The Colors of Ammonia
Additional information to do with ‘colors’ is provided in the post Colors of Hydrogen.
Most discussions to do with climate change focus on the emissions from the fossil fuels in transportation. Yet there are many other sources of CO2 emissions, including those from the chemicals industry. For example, the Haber-Bosch (H-B) process, which is used to “fix” atmospheric nitrogen to make ammonia, generates a significant percentage of the world’s CO2. The ammonia is then made into a wide range of chemical products, of which nitrate fertilizers are the most important. Without these fertilizers the world’s population would be much smaller than it is now.
Unfortunately the H-B process (which includes the steam reformation of methane) is a major source of CO2. If we are to control climate change and not have a population crash, it will be necessary to develop a different type of process for “fixing” nitrogen in the air and for making “green” ammonia.
This post starts with a very brief overview of the Haber-Bosch process. We then look at “greener” alternatives by describing different “colors” of ammonia.
The Haber-Bosch Process
For those who are technically inclined, the following sketch provides a highly simplified overview of the current ammonia manufacturing process.
There are three main sections to the process:
The separation of nitrogen from air. The by-product, oxygen, has commercial value. This is a well-established process. It uses energy, and so is indirectly responsible for some CO2 emissions, but these are relatively minor compared with other steps in the process.
The steam-reformation, synthesis gas (SynGas) process. Methane (natural gas) is reacted with steam to make hydrogen. The by-product from this process is carbon dioxide, which is discarded to the atmosphere. This is the principal source of CO2 from the overall process.
In the Haber-Bosch process itself nitrogen and hydrogen are reacted with one another to create ammonia according to the following equation.
N2 + 3H2 ↔ 2 NH3
When talking about the manufacture of “green” ammonia, it is important to understand which phase of the process is being modified. that the challenge lies not with the H-B step but with a need to not use methane as a source of hydrogen. In other words, green ammonia requires the use of green hydrogen.
Four Categories or Colors
Technologies for manufacturing ammonia can be placed into one of four categories, sometimes referred to as Gen 0 to 3, or by different “colors”: brown, blue and green. (The use of color names is for convenience only — ammonia gas has no color at all, although it does form a misty cloud when released in moist air due to the formation of ammonium hydroxide).
Gen 0 — Brown Ammonia
This is the conventional Haber-Bosch (H-B) process shown in the sketch. It has been been used world-wide for more than a century. If coal is used then the hydrogen so generated is given the color black.
Although there have been many advances in the technology since Haber and Bosch did their work in the first part of the 20th century, the required conversion of methane to hydrogen and carbon dioxide has not changed.
Gen 1 — Blue Ammonia
Some companies are developing processes for manufacturing “blue” hydrogen. The basic process remains the same as for brown hydrogen except that the generated CO2 is not released to the atmosphere. Instead it is sequestered, i.e., stored underground, either directly or as a carbonate. When blue hydrogen is used in the H-B process the ammonia product is also referred to as “blue”.
Gen 2 — Green Ammonia
Generation 2 processes for manufacturing ammonia continue to use the H-B process. However, the hydrogen used in the reaction is generated by electrolysis of water using renewable energy sources. Methane is not used. The “green” hydrogen is used to create green ammonia.
The solid oxide electrolysis can also generate pure nitrogen, thus eliminating the need for cryogenic separation of air into nitrogen and oxygen.
Gen 3 — Direct Manufacture
The technologies for producing ammonia that have been discussed up to this point rely on the high temperatures and pressures of the Haber-Bosch process to break the tight triple bond that holds the dinitrogen (N2) molecule together.
Currently there is much research going on into processes that can create ammonia directly from nitrogen and water without having to generate hydrogen as an intermediate, and without needing high pressures and temperatures. Electrochemical techniques can be used. Another approach is to duplicate the enzymatic reactions that we see in nature. The roots of many legumes are particularly good at “fixing” nitrogen.
None of these Gen 3 processes are close to being commercialized, so it appears for now as if the most realistic prospect for making “green” ammonia is with the Gen 2 technology.
Ammonia as a Fuel / Energy Storage
Ammonia has been used as an industrial chemical for over a century. It is now being evaluated as a possible fuel, particularly in the shipping industry. This is an important topic that we will discuss later. The ABS Whitepaper ‘Ammonia as a Marine Fuel’ from ABS provides background information.
Ammonia is also being considered as a means of storing energy — a function that will be absolutely crucial in a world where so many power sources are highly intermittent.
The sketch shows how ammonia may fit into the energy grid of the future.
