(220b) Investigating Strategies for Decarbonizing the Ammonia Production Using Biomass Based Chemical Looping

While the demand for energy consumption is rising rapidly, the sources of energy generation have been depleting significantly. Traditionally fossil fuels have been the major source of energy for humankind, but on the contrary, they also have contributed to the majority of CO₂ emissions, leading to global warming. Consequently, scientists and industrialists throughout the globe have shifted their attention towards alternative sources of energy such as solar energy, hydroelectricity, wind power, geothermal, renewable fuels, etc. The challenge amongst many of these sources lies in their higher cost of energy generation, which makes them impractical for implementation in existing industries. Amongst these alternative sources, biomass stands out distinctively due to its tremendous availability and carbon neutral nature. Biomass has the capability to blend into the existing infrastructure of energy generation to a great extent and hence has received a lot of attraction.

Ammonia industry is one of the prominent contributors to the global CO₂ emissions with up to 500 million tons of CO₂ (≈1.8% of global emissions). Traditionally ammonia is produced using the Haber-Bosch process, wherein high-purity H₂ and N₂ catalytically react with each other at around 350-500 ^oC and 150-300 bar. This process requires a supply of high-purity H₂ and N₂ in a 3:1 ratio. The H₂ required is generated using the Steam Methane Reforming (SMR) process wherein natural gas is converted to syngas, which further undergoes Water Gas shift (WGS) reaction followed by CO₂ removal using amine separation. On the other hand, the N₂ is generated using an Air Separation Unit (ASU). Both these processes are highly energy intensive and thus contribute significantly to the CO₂ emissions. The utilization of other renewable energy strategies such as solar energy, electrolysis of water for H₂ production, etc. increases the cost of ammonia production considerably.

Chemical looping technology is a clean technology that utilizes different feedstocks to produce high-purity hydrogen and sequestration-ready CO₂ using the redox reactions of metal oxides. It consists of 3 reactors namely a reducer, an oxidizer, and a combustor. The fuel feedstock reacts with the oxygen carrier (OC) particles in the reducer to produce a sequestration-ready CO₂ stream, simultaneously reducing the OC particles. The reduced OCs are partially oxidized using steam to produce high-purity H₂. The OCs are further sent to the combustor where they are completely oxidized back to their original form, thus completing the chemical and the thermal loop.

Using the concept of chemical looping technology, a process is developed to inherently produce a high-purity mixture of H₂ and N₂, which is directly sent to the Haber-Bosch process for ammonia generation. In the proposed system, a carbon-neutral fuel source i.e. waste biomass is sent into the reducer reactor where it reduces the OC particles to form a sequestration-ready steam of CO₂. The reduced OCs are sent to the oxidizer reactor where they are partially oxidized using a mixture of steam and depleted air. The partially oxidized OCs are further sent to the combustor reactor wherein they react with the air and get completely oxidized back to their original state. The air exiting the combustor is depleted in oxygen, thus containing majorly nitrogen with a slight fraction of oxygen. A fraction of this oxygen-depleted air is sent to the oxidizer reactor. The gaseous stream exiting the oxidizer reactor is a highly pure mixture of H₂ and N₂ in the ratio of 3:1.

To demonstrate the system, process simulations were performed using ASPEN plus software to understand the effect of different parameters on the process throughput. Further optimization was done using simulation, in order to obtain the gaseous mixture in required ratio. A heat exchanger network is designed to improve the overall efficiency of the process by effectively using the hot and cold streams within the process. To demonstrate the applicability of the process, bench scale experiments are conducted in 2.5 kW_th moving bed reactor. In the reducer reactor, the oxygen carriers flow from the top to the bottom, while the biomass is injected in the middle. Due to middle injection, the volatiles from biomass travel upward and react with the OCs to generate CO₂ and water. The char flows downward along with OCs, where it reacts with enhancer gas to form CO, which further reacts with OCs to form CO₂. In the oxidizer reactor, a countercurrent flow of OCs and oxidizing gases is established to ensure maximum conversion.

The proposed process is autothermal in nature, implying that it does not require any external source of heat energy, thus significantly lowering the energy input of the ammonia production process. This process not only generates renewable hydrogen but also inherently employs air separation giving rise to a greatly energy-efficient process to produce the reaction mixture for ammonia synthesis. This reduces the overall cost for ammonia production whilst also reducing CO₂ emissions significantly due to inherent CO₂ capture by generation of sequestration ready CO₂ stream. When employed in the current state of the art ammonia production process, the proposed process replaces the Steam Methane Reformer (SMR), Water Gas Shift (WGS) reactor, Air Separation Unit (ASU), and Amine scrubbing unit for CO₂ capture, together using a single chemical looping system. Furthermore, the pure stream of CO₂ generated in the reducer reactor can be utilized with ammonia to produce urea, an important compound in the fertilizer industry. Thus, chemical looping provides a highly energy efficient alternative to traditional technologies used in ammonia production processes and significantly reduces the carbon footprint of the ammonia industry.