Safe operations, environmental compliance and product quality for refinery heavy residue gasification
Petcoke gasification. The chemistry of gasification fits somewhere between combustion and pyrolysis. Combustion is a high temperature reaction in an excess of oxygen. It produces heat, carbon dioxide and steam. Pyrolysis is the high temperature decomposition of a hydrocarbon to form solid carbon in the absence of oxygen. It is used to produce coke from coal in steel making.
Stephen B Harrison
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Gasification is a high temperature process which needs a precisely controlled concentration of oxygen which is often supplied from an air separation unit (ASU). Gasification is increasingly being used on refineries to process petcoke, a heavy residue from coking units.
Today, one of the world’s largest gasification projects is at the Jazan refinery in Saudi Arabia where more than a dozen gasifiers built by Técnicas Reunidas will produce syngas from petcoke.
One of the drivers behind refinery residue gasification projects has been the IMO 2020 regulation which has increased the demand for low sulphur marine fuels. Furthermore, petcoke has recently been banned in India for use as cheap feed to coal fired power plants due to its high sulphur content. Instead, gasifying it to produce syngas and hydrogen, which is used to desulphurise or hydrogenate fuels, can create value from the petcoke.
Gasification projects require some of the world’s largest ASUs
At Jazan, the gasifiers will produce enough syngas to generate 4 GW of power and steam. The syngas is fired directly in gas turbines which produce 2,400 MW of electricity in an integrated gasification combined cycle (IGCC) power plant. The syngas-island will also export hydrogen and steam to the refinery. The gasification process consumes vast quantities of oxygen. In order to feed the gasifiers at Jazan, the process requires six mega ASUs, each one rated at 3000 Tonnes per day of oxygen. Air Products has been instrumental in the Jazan refinery heavy residue gasification project and has secured a strong gasification technology position through the acquisition of GE’s Gasification business and Shell’s coal gasification technology.
Coal and petcoke have similar properties as gasification feedstocks. Both can be used to yield hydrogen for the refinery or chemicals production. The Lu’an coal to chemicals project at Changzhi in China’s Shanxi province, is large coal gasification project. Four gasification reactors have been constructed to supply syngas to a chemical complex and four large Air Products ASUs feed the gasifiers with oxygen. A methanol-based syngas clean up plant sits downstream of the gasifiers.
Switching from China to India, the ten gasification reactors at the Reliance refineries at Jamnagar are designed to run on petcoke and for flexibility can also operate on a mixture of 35% coal and 65% petcoke. They are fed by five of the world’s largest ASUs, each rated at 5,250 Tonnes per day of oxygen. The target products from the project include hydrogen for clean fuels processing and syngas.
In the past, coal gasification and steel blast furnaces were the two largest consumers of oxygen from ASUs. The tide is turning, and refineries are now joining the super-league of oxygen consumers with catalyst regeneration and gasification applications driving oxygen demand.
Gas quality - oxygen purity matters
“The ASUs at Jamnagar are incredible” says Bodhistaya Das, an ex-Linde ASU Process Design and Commissioning Engineer who has operational experience at the Reliance Jamnagar ASU complex. He is now a Process Control Engineer working in India. “The diameter of the High-Pressure distillation column trays is huge at 7m. To work at this scale, we had to overcome some technical challenges in designing the six-stage cascade reboiler-condenser unit between the low-pressure and high-pressure distillation columns. In addition to process effectiveness, safety is a major concern for this piece of equipment because hydrocarbon accumulation in the liquid oxygen sump must be avoided”.
The oxygen produced on the Reliance ASUs is at 99.5% purity and the argon from the process is used to improve the overall energy efficiency. This contrasts to a standard ASU design which is optimised to produce oxygen at between 99.6 and 99.8% purity and export argon as a liquid product. This higher oxygen purity means that most of the argon from the air can be captured for sale as a valuable co-product.
Commenting on the implications of argon recovery from air separation, Stephen Gibbons, Head of Product Management for the continuous gas analyser product range within ABB’s Measurement & Analytics business line says that “paramagnetic oxygen gas analysers have been used for decades by ASU operators to measure the amount of oxygen in the feed to the argon column and to analyse the final oxygen purity. It is the final oxygen purity where repeatable and accurate oxygen gas analysis is essential to minimise argon losses and maximise profits”.
The monetary value of an inaccurate oxygen reading at 99.7% when the reality is an oxygen purity at 99.6% means that 0.1% of the oxygen flow is high value argon which is being sold at the lower price of oxygen. That’s a discount that few ASU operators can afford. Gibbons adds that “the new Magnos28 incorporates our innovative microwing. This is a digitalised, solid-state version of the gas-filled dumbbell that has been used in paramagnetic gas analysers for decades. It results in less drift on the measurement and better accuracy, which means that ASU operators can know exactly where their argon is going. Upgrading to the new Magnos28 enables ASU operators to minimise argon losses and maximise their profits”.
Oxygen sump hydrocarbon analysis and process safety
In 1997 ASUs at the Fushun Ethylene complex in China and the Shell Gas to Liquids plant at Bintulu on the island of Sarawak suffered catastrophic explosions. In each case the root cause was traced back to abnormally high levels of contamination in the ambient air: soot particles from forest fires in one case and ethylene in the other.
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