Overview-The commercial production of oxygen and nitrogen supported the industrial revolution in the late 19th century and up to the mid-20th. Oxy-acetylene cutting and welding were important for projects like the Eiffel Tower, the Panama Canal and the Central Valley Project Corporation hydropower plants. Oxygen, nitrogen and argon refined steelmaking and metal heat treatment, from Bessemer steel converters to cryogenic tempering. Food conservation has benefited from nitrogen, argon and CO2 with the controlled atmosphere conservation of produce.
Since the 1980s, higher purities and lower power consumption have been sought, challenging the conventional designs of air separation units (ASUs). Air gases have transformed the paper industry, ore smelting, glass plants, medical and oil refineries. With the increasing demand across industry sectors, oxygen and nitrogen have become commoditized and are now commonly used in home care, welding, food, and, for a while, water treatment.
Over the last 30 years, the pharmaceutical and semiconductor industries pushed the envelope for producing ultrahigh purity gas requirements further. Gas analyzers could not even keep up with the new air separation plants producing at the part-per-billion purity level. Simultaneously, extra-large air separation plants with capacities above 2,000 tons of oxygen per day were developed to serve the energy and chemical industries, such as coal-to-liquid, gas-to-liquid, and various gasification processes for producing liquid fuels, chemicals and electricity. The world’s largest air separation plant in the world today was commissioned in 2016 and produces 5,000 metric tons per day of oxygen.[1] Restraints to the cryogenic oxygen and nitrogen market entail small on-site vacuum and pressure swing adsorption plants, membrane generators, and oxygen concentrators for home care.
The global market growth valuation for air gases[2] is:
Killing a Gentle Giant-Air separation plants, particularly cryogenic plants, are large power and energy consumers. A cryogenic plant producing 1,000 tons of oxygen per day can consume the same monthly energy as 8,000 homes and uses the same power as 2,400 homes. However, the power consumption of cryogenic plants is very steady, with a capacity factor upwards of 95%, meaning that they operate at a nearly constant power level around the clock. They are an ideal load on an electric grid because they guarantee a constant utilization of the transmission network and provide a steady and predictable base load for electric generators.
Cryogenic ASUs require steady-state operations, 24/7 year-round, because of the fractional distillation process. Although ASUs can turn down their production and power consumption to some limited extent, it can take hours to ramp up and stabilize the plant. If attempting to reduce power below a certain threshold, typically around 60% of the design production, the distillation columns will suddenly dump their liquid inventory and the plant will shut down on a fault. Another cause for a plant trip is a power blackout, however short. Restarting the plant, attaining product purity and resuming production takes several hours to a day, significantly stressing the operators on shift. On the contrary, gas liquefiers are fast to start and shut down because they do not have distillation columns.
The cost to produce oxygen is largely based on electricity, and the steady operation and high capacity factor were advantageous to electric utilities. With the deregulation of electricity and the transition to regional transmission operators and independent system operators such as MISO and CAISO, the cost of market electricity has become variable, even volatile.
The surge of intermittent renewable generation, such as wind and photovoltaic solar, has lessened the grid’s ability to maintain capacity (frequency and voltage firming) by displacing conventional power generation. Grid operators and state regulators have addressed this issue by allowing market electricity prices to vary widely and by encouraging electric loads to follow the pricing trends: when electricity is cheap, increase your consumption, and when it is expensive, reduce it. Figure 1 (on the next page) exemplifies this price volatility with California’s Trading Hub NP15 on the Day-Ahead Market between August 29 and September 12, 2022 (divide the price by 10 to obtain cents/kWh).
Unfortunately, this leaves ASUs vulnerable to high energy prices, particularly during evening hours when the sun sets and gas turbines ramp up, since they have very limited turndown capabilities and much operational inertia. Figure 1 shows that oxygen or nitrogen production costs can fluctuate as much as 800% from hour to hour. Consequently, air gas costs become more volatile, affecting many industry sectors. Coincidental factors affecting the air gas industry include the cost of bulk transportation and current supply chain issues for parts.
ASUs do not generate CO2 except for the carbon footprint of the plant’s electricity. So, is the current regulatory approach to energy decarbonization, a nail in the coffin of air separation plants and the industries they serve? Or does it matter, since manufacturing has been offshored since the 1990s anyway?
A New Challenge-Are ASUs bad for the environment? Or are they collateral damage by environmental regulations? From Antofagasta, Chile, to Eureka, CA and across the globe, air gases have performed environmental miracles with the paper, glass, and mining industries. Oxygen has successfully displaced or mitigated chlorine and sulfur emissions and reduced NOx from furnaces while allowing client industries to improve recoveries, improve air quality and reduce emissions. Even liquid nitrogen converts piles of used tires into shoes and road-silencing substrates. Strawberries, pears, and green beans are available year-round in the supermarkets. Lithium, which combusts spontaneously in the air, can be safely recycled from used electric vehicle batteries under argon blanketing.
Today, the air separation industry is at a new crossroads. It is not the time to look back and ask for recognition. That ship has sailed with the commoditization of gases. The new challenge going forward is to partner with energy decarbonization while evolving into nimble operations that can absorb the volatility of energy prices and availability. This is a tall order because nothing is cheap in cryogenic equipment.
What is the future of air separation, of ASUs capable of fast operation flexibility, participating in demand response without affecting production, and even perhaps running off the electric grid? Is the time ripe for scaling up Sterling acoustic liquefiers and expecting more from adsorbents? RIX Industries (CSA CSM, Benicia, Calif.) must think so with their advanced liquid oxygen plant producing LOX within 20 minutes of a warm start. This small air separation plant no longer uses fractional distillation. Still, the flexibility of operation and fast start comes at the cost of product purity and production capacity.
NET Power is developing its serial number 1 (SN1) utility-scale project, which combusts fuel with oxygen instead of air and uses supercritical carbon dioxide as a working fluid to drive a turbine instead of steam. This eliminates nearly all emissions, including air pollution and CO2, and inherently produces pipeline-quality CO2 that can be sequestered, all while operating at competitive costs and efficiencies compared to traditional gas power plants.[3]
We are not talking about painting the plants green and adding a few solar panels for PR. Turning the PURPA-1978 adage of “combined heat and power” to “combined cooling and power,” Liquid Air Energy Storage (LAES) shows the promise of storing significant amounts of energy in a small footprint with a roundtrip efficiency near 50% when accounting for the massive cooling potential for server farms, cold storage facilities, and even small river streams. Despite three years of drought in California, hydropower plants had to be bypassed for river temperature control for two months in 2022. The price to pay was 15,000 MWh of carbon-free hydroelectric generation forfeited, US$1.4 million of electricity revenues not realized, and 11,760 tons of additional CO2 emissions from replacement generation. Is this a shining opportunity for LAES? Absolutely! But who should take the initiative? Not the national labs, which are focused on lithium-ion battery technology. 2023 is a wet year, and all the reservoirs had filled up in California by March. The Bureau of Reclamation and the Department of Energy lost interest in LAES as early as April 2023. Still, the opportunity is real for the cryogenic industry, and the Inflation Reduction Act may provide financial feasibility to what used to be just great ideas.
References
[1] Air Liquide Sasol T17 plant.
[2] GlobeNewswire’s Market Study Report
[3] www.netpower.com.
Image: 500-ton-per-day, 1981 cryogenic oxygen plant with reversing exchangers. Credit: Nils Tellier
Figure 1: California TH_NP15 hourly price of electricity between August 28 and September 12, 2022. Credit: EPSIM Corp.
Image 2: Removing an ASU cold box. Credit: Nils Tellier; Advanced LOX plant. Credit: RIX Industries
