Raising boiler efficiency (TIA)
When you are processing 100000 b/d or more of crude, reliability is of the utmost importance. There is no justification for unscheduled shutdowns due to a lack of process steam.
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When Valero Energy Corporation needed to replace a boiler at its Memphis plant four years ago, it selected one from Rentech Boiler Systems, Inc. of Abilene, Texas. It has since ordered three other boilers from the company for other refineries.
Over the years, Valero has purchased many refineries. When it does so, it typically makes upgrades to the equipment. For example, as part of its purchase of Premcor, Valero acquired four refineries including one located on 250 acres alongside the Mississippi River’s Lake McKellar, just outside of Memphis. Originally built in 1941, the Valero Memphis Refinery has a rated capacity of 195 000 b/d of light, low sulphur crude oil, and produces light products including gasoline, diesel, jet fuel and petrochemicals. After acquiring the refinery, Valero invested about $250 million upgrading the now 70-year-old plant to modernise it and increase its efficiency. Among those improvements was a new boiler. Valero put the job out for competitive bid and selected Rentech based on both the economic and technical proposal.
“Valero’s corporate philosophy is to be very conservative with the boiler design, since a conservative boiler design translates into greater reliability,” says Paul Brown, a Sales Engineer operating out of Rentech’s office in Lincoln, Nebraska. “They request things like very large furnaces, very large steam drums, and we are able to accommodate that with some very big boilers.”
Typically there are two choices in selecting a boiler. One can get a package boiler, fabricated in the factory and shipped to the site for installation or one can get a ‘stick-built’ boiler which is built on site. Package boilers are the less expensive option, but they come in standard sizes and are not optimised for specific site conditions and requirements. Stick-built boilers are far more costly and take longer to install, but are designed for that particular application.
Rentech takes the best of both approaches. Each boiler is custom built to the customer’s specifications, preassembled and tested at an in-house 100 000 square foot manufacturing plant in Abilene, Texas. The boiler can be shipped to the site in one or more pieces for final installation and commissioning. Since the boilers are factory built, they do not require the extensive site work required for stick-built boilers.
“Valero is very concerned about being environmentally friendly,” says Brown. “That is why we keep the furnaces as big as we can, which makes it easier to achieve lower NOx emissions. By integrating the boiler and SCR, we can make sure all the components work together to reduce emissions.”
The Memphis boiler was a 250 000 lb/hr unit and a similar sized unit was installed at Valero’s Three Rivers Refinery – a 100 000 b/d plant located midway between Houston and San Antonio – at around â€¨the same time. The company has since installed two more Rentech boilers, a 250 000 lb/hr unit at the Texas City Refinery, and a 350 000 lb/hr unit in Corpus Christi.
All four boilers are designed to follow steam load, firing up and down with the demand, and typically produce 750° steam at 450-650 psi. But there are other differences unique to the needs of each site. For example, with refineries dating back to 1908 and acquired from different companies, the control systems vary from one location to the next. Typically the boilers are controlled separately though a distributed control system (DCS), with the plant management system distributing the load among the boilers. But this varies from refinery to refinery with some plants being more manual and others relying more on their control system.
The functions of the boilers are also different. Two of the units are designed as high turndown units that can run reliably at low fire and then ramp up when needed. Even though the new boilers are more efficient than the other units, since the older units cannot be turned down as reliably, the boilers are kept at the minimum level until needed.
Walker Garrison, Technical Advisor for Utility Infrastructure at Valero, says that the boilers have been operating reliably in an environmentally friendly manner. “The older boilers have about 85% to 87% lower heating value efficiency, but the newer ones have 91-92% efficiency,” he says. “And, while we were EPA compliant before, these take us from an older emissions control technology to Best Available Control Technology (BACT).”
This short case study originally appeared in PTQ's Technology In Action feature - Q2 2015 issue.
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