Energy consumption optimization of a synthetic ammonia process based on oxygen purity

Energy optimization of ammonia synthesis processes based on oxygen purity under different purification technologies

Ammonia can be produced from a wide range of raw materials such as coal, natural gas, coke and oil. Coal gasification is a process that converts biomass or fossil fuel-based carbonaceous materials into CO, H2 and CO2. A cryogenic air separation process was used to obtain oxygen from air because of high purity and high amount of oxygen, which will be used for coal gasification. For an ammonia synthesis process using pure oxygen gasification, the energy consumption of cryogenic air separation occupies a large proportion. The aim is to reduce energy consumed in the ammonia plant. The models of the process were developed with the aid of Aspen Plus. The energy consumption of the different processes was obtained through energy analysis, economic analysis and sensitivity analysis. From the three simulations, it can be seen that Simulation 3 produced oxygen with the highest purity of 0.979. From the energy analysis, the energy consumed on the total utilities in Simulation 1 was 5.626×1010 BTU/h with an energy savings of 1.55%, the energy consumed in Simulation 2 was 5.286×1010 BTU/h with an energy savings of 1.53% while the energy consumed on the total utilities in Simulation 3 was 1.425×109 BTU/h with an energy savings of 74.90%. Simulation 3 consumed the least energy. The economic analysis showed the total cost of each plant for a 10-year duration. Simulation 1 had a total operating cost of 42.083 billion USD/year, Simulation 2 had a total operating cost of 41.9615 billion USD/year and Simulation 3 had a total operating cost of 918.841 million USD/year. Therefore, Simulation 3 consumed the least cost of total operation. It can also be seen that the higher the energy consumption in a plant, the higher the total cost of the plant as Simulation 3 consumed the least energy, which justified that. Simulation 3 is the air separation plant that optimises the energy consumption, thereby reducing the energy consumed in the whole ammonia plant.

Optimization of Cryogenic CO2 Purification for Oxy-coal Combustion

This study analyzes and compares the economics and sustainability aspects of two hydrogenation processes for producing renewable methanol and ammonia by using wind-power based electrolytic hydrogen. Carbon dioxide from an ethanol plant is used for producing methanol, while the nitrogen is supplied by an Air Separation Unit (ASU) for producing ammonia. The capacities are 99.96 mt/day methanol and 1202.55 mt/day anhydrous ammonia. The methanol plant requires 138.37 mt CO2/day and 19.08 mt H2/day. The ammonia is synthesized by using 217.72 mt H2/day and 1009.15 mt N2/day. The production costs and the carbon equivalent emissions (CO2e) associated with the methanol and ammonia processes, electrolytic hydrogen production, carbon capture and compression, and ASU are estimated. The integral facilities of both the methanol and ammonia productions are evaluated by introducing a multi-criteria decision matrix containing economics and sustainability metrics. Discounted cash flow diagrams are established to estimate the economic constraints, unit product costs, and unit costs of hydrogen. The hydrogen cost is the largest contributor to the economics of the plants. For the methanol, the values of emissions are -0.85 kg CO2e/kg methanol as a chemical feedstock and +0.53 kg CO2e/kg methanol as a fuel with complete combustion. For the ammonia, the value of emission is around 0.97 kg CO2e/kg ammonia. The electrolytic hydrogen from wind power helps reduce the emissions; however, the cost of hydrogen at the current level adversely affects the feasibility of the plants. A multi-criteria decision matrix shows that renewable methanol and ammonia with wind power-based hydrogen may be feasible compared with the nonrenewable ones and the renewable methanol may be more favorable than the ammonia.

Ammonia, as a bulk chemical, is widely used in fertilizers and fuels and is also a promising hydrogen carrier. Aiming at reducing the high energy consumption of the ammonia synthesis process, this study innovatively proposed a low-carbon ammonia synthesis process based on the cold energy utilization of liquified natural gas regasification. In this process, the cold energy released from the regasification of liquified natural gas was used as the refrigeration source of the air separation unit and carbon capture and storage unit, which improves the comprehensive energy utilization efficiency of the whole process of synthetic ammonia. Based on Aspen Plus simulations, the process economy and exergy were comprehensively analyzed. The total efficiency of the low-carbon ammonia synthesis process was 42.69%, of which the ammonia production capacity was 35.07 t/h, and the total capital investment of the integrated process was 3.4 × 107 USD, and the product cost was 387.86 USD. Compared with three different ammonia plants, the levelized cost of ammonia in the low-carbon ammonia synthesis process was reduced to 324.03 USD/ton. The proposed process can well solve the problems of high energy consumption in the ammonia synthesis process and provide a valuable production route for the low-carbon ammonia synthesis process.

Investigation of novel integrated air separation processes, cold energy recovery of liquefied natural gas and carbon dioxide power cycle

Process modelling and optimization of a 250 MW IGCC system: ASU optimization and thermodynamic analysis

The global agreement on the reduction of greenhouses entails increased use of intermittent renewable energy sources such as wind and solar. In this contest, there is a huge need for efficient and cost-effective energy storage systems. Currently, different types of grid energy storage systems are under operation worldwide. However, few of them have the capacity of storing several MWhs of electricity. Large-scale energy storage systems will play a vital role in grid stabilization by reducing imbalances between energy production and demand. Solid oxide cells (SOC) are promising and efficient technologies operating both on electrolyzer and fuel cell modes applicable for energy storage and production. The efficiency of SOC can be further increase by utilization of excess high temperature steam from other processes like ammonia synthesis when it is operating as electrolyzer. In this way, part of the energy needed for electrlozer is covered and less input energy is required, which leads to higher efficiency. This is the main reason behind combining solid oxide cells with the ammonia synthesis process. In the present work, we propose a large-scale electricity storage system operating with reversible solid oxide cells (RSOC) to store electricity as synthetic ammonia. Ammonia can be easily produced, when RSOC operates as an electrolyzer, and stored in the liquid form during off-peak demand. During the peak demand, stored ammonia can be fed into the RSOC, when RSOC operates as a fuel cell, to produce green electricity. Besides the RSOC, the proposed system mainly consists of an air separation unit (ASU) and a Haber-Bosch loop (HBL). Besides large-scale storing of grid electricity, such a system can create an efficient link between electricity and ammonia markets. The system performance is evaluated at a component level thermodynamic analysis showing that a round-trip storage efficiency higher than 50% is achievable.

The ignition of aluminum foils in gaseous oxygen was experimentally tested using a diode laser as the energy source, which provided a well-controlled, accurate, and reproducible method of ignition. The tests were conducted under different conditions in terms of oxygen pressure, oxygen purity, aluminum thickness, and gas velocity. The aluminum foils tested were between 0.2 mm and 0.45 mm thick, a range typical of fins contained in brazed aluminum heat exchangers (BAHXs) used in air separation units (ASUs). The experimental apparatus was composed of a pressure vessel in which a single aluminum test sample was placed. The vessel contained an optical window that allowed a short laser pulse of known power to be applied to the aluminum sample. The energy dose was systematically varied in order to identify the threshold ignition energy, defined as the point at which the probability of aluminum combustion with propagation beyond the laser spot was 50 %. The experimental results show that O2 pressure has no significant effect on the ignition energy of aluminum over the pressure range tested (10 bar to 120 bar). This conclusion holds for both standard commercial grade purity O2 (99.8 %) and high purity O2 (99.99 %), as well as for gas velocities higher than typically encountered in ASU BAHXs. Heat conduction calculations indicate that aluminum ignition occurs when the laser spot temperature reaches the melting point of the passivating oxide layer (about 2200 K to 2300 K). The heat conduction model accurately explains the dependence of the ignition energy on the aluminum sample thickness. These test results have been used in assessing the risk of ignition of BAHXs used in high pressure oxygen service in ASUs.

Thermodynamic evaluation of the novel distillation column of the air separation unit with integration of liquefied natural gas (LNG) regasification

Techno-economic comparison of green ammonia production processes

Thermodynamic analysis of the air separation unit and CO2 purification unit in the semi-closed supercritical CO2 cycle with nearly zero emission

Thermodynamic analysis of an organic Rankine–vapor compression cycle (ORVC) assisted air compression system for cryogenic air separation units

Ammonia is an important chemical raw material and the main hydrogen energy carrier. In the context of “carbon neutrality”, green ammonia produced using renewable energy is cleaner and produces less carbon than traditional ammonia production. Raw hydrogen dynamically fluctuates during green ammonia production because it is affected by the instability and intermittency of renewable energy; the green ammonia production process has frequent variable working conditions to take into account. Therefore, studying the transition state process of green ammonia is critical to the processing device’s stable operation. In this study, a natural gas ammonia production process was modified using green ammonia, and steady-state and dynamic models were established using UniSim. The model was calibrated using actual factory data to ensure the model’s reliability. Based on the steady-state model, hydrogen feed flow disturbance was added to the dynamic model to simulate the transition state process under variable working conditions. The change in system energy consumption in the transition state process was analyzed based on the data analysis method. The proportional-integral-derivative (PID) parameter optimization method was developed to optimize energy consumption under variable conditions of green ammonia’s production process. Based on this method, process control parameters were adjusted to shorten fluctuation time and reduce energy consumption.

A cryogenic air separation unit (ASU) with double column configuration produces pressurized oxygen (PGOX) at purity of 99.6% and above. A single column configuration is sufficient to produce at a purity of 95%, which is required for carbon capture power plants (CCPP). It requires less capital and consumes lower power than a double column configuration. It further makes an economic sense if the unutilized bone-dry low purity (waste) nitrogen, also called pressurised impure gaseous nitrogen (PiGAN), may be heated by the waste heat available in CCPP. It is expanded in a gas-based reheat cycle to produce electricity. This may partially offset the power consumed in the ASU. Using exergy analysis, pressure of PiGAN is determined in such a way that the net power consumed by ASU (power consumed minus power generation in power cycle) is minimized. At this pressure of PiGAN, single column ASU consumes 12.5% less power than double column ASU.