Power Engineering

by Joe Zwers, contributing writer

Typically there are two choices in selecting a boiler. One approach is to order a package boiler fabricated in the factory and shipped to the site for installation. The alternative is to obtain a “stick-built” boiler that is built on site. Package boilers are the less expensive option, but they come in standard sizes and are not optimized for specific site conditions and requirements. Stick-built boilers are far more costly and take longer to install. On the plus side, they are designed for that particular application.

Alternatively, there is a hybrid that combines the best of both approaches. Each boiler is custom built to customer specifications, preassembled and tested. It can then be shipped to the site in one or more pieces for final installation and commissioning. Since they are factory built, they are less expensive and don’t require the extensive site work associated with stick-built units.

Valero Energy Corporation needed to replace an aging 250,000 lb/hour boiler at its Memphis plant. It opted for a hybrid design.

“For the first boilers, we went through a bid process and selected RENTECH based on both the economic and technical proposals they submitted,” said Walker Garrison, technical advisor for utility infrastructure at Valero who serves as subject matter expert for water treatment, boilers and compressed air systems.

Valero followed this up with two more 250,000 lb/hour boilers for other plants and is now installing a 350,000 lb/hour boiler in Corpus Christi. Each is supplied by RENTECH Boiler Systems, Inc., which favors the hybrid boiler approach.

The Corpus Christi boiler uses a non-traditional design. Typically the components, including the steam drum and boiler, are prefabricated in the shop and shipped to the site. But because of the large steam capacity of the Corpus Christi boiler, coupled with Valero’s interest in a conservative design, it is too large to ship in one piece so a modular design was used. The furnace and convection section will be preassembled in the factory, but the steam drum is being shipped separately. A crew then came to the site to assemble the components, mounting the steam drum along with the risers and downcomers.

A total of four new Valero boilers have been designed to follow steam load, firing up and down with the demand, and typically produce 750 degree steam at 450 to 650 psi. But there are differences specific to the needs of each site. For example, the control systems vary from one location to the next. Generally the boilers are controlled separately through a distributed control system (DCS) with the plant management system distributing the load among the boilers. But the details vary from site to site – with some plants being more manual and others relying more on their control systems.

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 these new boilers are more efficient than the other units, since the older units can’t be turned down as reliably, the new boilers are kept at the minimum level until needed.

Garrison says that the boilers have operated well, reliably providing the required steam, avoiding unplanned shutdowns and doing it in an efficient and environmentally friendly manner. It’s now four years since the first unit at Memphis went online. The others are more recent.

“The older boilers have about 85 percent to 87 percent lower heating value efficiency, but the newer ones have 91 to 92 percent efficiency,” Garrison said. “And while we were EPA compliant before, these take us from an older emissions control technology to Best Available Control Technology (BACT).”

Custom vs. Off the Shelf

There are times when selecting a pre-designed, package boiler is the best option. If price is more important than reliability or efficiency, such as when there are multiple boilers on site with excess steam capacity, then it can make economic sense to purchase a lower cost boiler.

But when reliability, efficiency and lower emissions are primary criteria, it is better to purchase a boiler custom designed and built for the exact needs of the site. This can include features such as larger furnaces to minimize problems with flames or air circulation and larger steam drums to provide more time to ride through short-term feedwater issues. In non-attainment areas, lower emissions can be achieved by integrating the boiler, the SCR and low-NOx burners.

In such as case, having the boiler built and tested in the factory before shipping to site results in lower costs and shorter construction and commissioning times than having the unit built from scratch on site.

Mark Otteman, owner of Power System Sales and Service, a manufacturers’ representative specializing in boiler applications and pollution equipment, prefers the conservative approach to boiler selection and operation rather than dealing with firms specializing in small, hot rod boilers.

He provided an example:

If you allow water to carry over from the steam drum into the superheater, it will eventually blow out the superheater. Having a large steam drum allows better separation for getting water out of the steam before it does to the superheater. In addition, if you lose feedwater flow to the boiler, the larger steam drum allows more time to correct the water issue before the steam levels drop.

“With some boilers, you have a little over a minute from normal operating level on the steam drum to low level trip,” Otteman said. “If you have a problem with your feedwater, there isn’t a lot of time to react to correct the problem. With RENTECH boilers, you are sitting in that five-minute interval, which gives someone a lot more time to react to a feedwater situation.”

This design concept minimizes the amount of refractory so operators don’t need to worry about replacing refractory seals or rebuilding refractory walls. According to Otteman, the end result is improved reliability.

“The superheaters are buried back in the convection section so they are protected from the temperature of the exhaust gas going into them,” Otteman said. “This helps extend their life.”

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