By Rud Istvan
ctm asked what I thought about the 7/31/2019 Forbes article by James Conca headed, “Net Zero Natural Gas Plant Game Changer.” My quick answer after reading the piece was, not much. He asked me to write up a WUWT guest post. I decided to research it rather than rely on Forbes; turns out Forbes omitted ‘fun’ facts, and got others just wrong.
Carbon Capture and Sequestration (CCS) has been a green dream for a long time. (There is a now slightly dated review in essay Clean Coal in ebook Blowing Smoke.) There are two very fundamental problems.
1. CC. It is very expensive and technically difficult. There is today exactly one power plant in the world with partial CC, SaskPower’s Boundary Dam project in Canada, which began operation in the fall of 2014 at a CC only cost of $1.5 billion. The plan was to convert coal fired generating unit #3 to CC, capturing 800,000 metric tons of CO2 annually to sell for tertiary oil recovery in Saskatchewan’s nearby Weyburn oil field. For the two most recent complete years (2017-18) the average Unit 3 uptime was only 70% (severe maintenance problems), while the CC parasitic electrical load has run about 30% rather than the planned 25%. On July 10, 2018 SaskPower announced it would not be expanding CC to units 1 and 2 as originally planned.
2. S. There is exactly one US sequestration only demonstration, the ADM/DOE project in Decatur, Illinois. The plan was to sequester CO2 from ADM’s Decatur ethanol operations (no CC problem) in the brine filled Mt. Simon sandstone formation 7000 feet below Decatur. The demonstration project cost $208 million, with DOE contributing $141 million. The goal was 1 million tons sequestered over 3 years, at an injection rate of ~1000tpd. The goal was never reached, because the injected CO2 reacted with the brine to form mineralization that slowly plugged the sandstone injection sites, necessitating ever more injection wells. ADM announced in 2018 that it was delaying its more ambitious next phase despite the neat little provision (26 USC 45Q) tucked into the February 2018 tax bill providing an inflation indexed $20/CCS CO2 ton tax credit (tertiary oil recovery deemed S).
The Forbes article describes a zero net emissions 25MWe natural gas fired electricity generating demonstration plant in La Porte, Texas (near Houston). It uses a new ‘Allam’ cycle using ‘oxyfuel and supercritical CO2’, where the effective turbine working fluid is CO2. The idea is to burn the natural gas in a high pressure ‘so supercritical CO2’ pure oxygen ‘so oxyfuel’ environment, thereby eliminating the CC problem. The demonstration plans to sell the resulting pure CO2 either to industry or for Texas tertiary oil recovery. Startup was last month. Hence Forbes.
For those interested in details, the ‘Allam’ cycle was invented by North Carolina startup Net Power. They provide the usual overly optimistic stuff (examples follow); a March 2019 presentation is available via www.NetPower.com. I obtained my 33 page copy from University of North Carolina Institute for the Environment, where the greens are as excited as Forbes.
Combusting natural gas with pure oxygen requires an air separation unit (ASU). Sure enough, the demonstration plant has its own cryogenic ASU. At any real volume, ASU is always cryogenic, meaning big compressors, heat exchanger fans, and lots of electricity. Since N2 boiling point is -195.8C while O2 boiling point is -218.8C, for every liter of LOx you must also make about four liters of liquid nitrogen—which is why liquid nitrogen is cheap. Air Liquide and Linde sell prepackaged containerized ASU to industry, hospitals, and the like. They also build massive custom plants to supply bulk tonnage liquid oxygen (LOx) to big users like the basic oxygen furnaces in modern steel mills. As a benchmark, an Air Liquide unit designed to support aircraft operations at 2 tons/day uses 3600KWh per LOx ton according to the device spec.
The Net Power materials say the 25MWe demonstration plant costs $140 million. That implies the capacity costs $5600/KW. A standard CCGT (capacity about 600-800MW) costs between $2000 and $2500/KW. Even allowing that this only a first demonstrator, Net Power has a long way to go on capital cost, AND has to find a lot of high value CO2 sales. Sequestration alone suggests the economics don’t work.
The demonstration plant is 50MWth and 25MWe. (Forbes confused thermal and electrical capacity). That means it runs 50% thermal efficiency. Standard CCGT runs 61% at full load, and still 58% at 40% load (and shuts down below 40%). That means the Allam cycle has a 20% fuel cost disadvantage, perhaps made up by selling CO2.
The demonstration plant is 25MWe, but Net Power page 7 says the parasitic ASU load is planned to be 29%, in the ballpark of Boundary Dam. So its salable electrical output is only 17MW, meaning its true capital cost is $8235/KW, NOT $5600/KW as Net Power implied (but did not say).
Finally, through ‘magical’ financial calculations on presentation page 22, Net Power says the 26 USC 45Q $20/ton CCS tax credit is really worth about $40/ton to them. Except a tax credit is only useful if you have taxable income, which Net Power doesn’t. I leave grokking their math to the reader, because I can’t.
Thus the title of this guest post: NOT a Game Changer.