Best practices in boiler replacement (TIA)
When you are processing 100 000 barrels or more of crude in a day, reliability is of the utmost importance. There is no justification for unscheduled shutdowns due to a lack of process steam.
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Boiler replacement, then, must be done smoothly and efficiently. Here are best practices observed in the field from several refinery and petrochemical projects conducted by Rentech Boiler Systems of Abilene, Texas.
Several boiler options are available. Package boilers are those fabricated in a factory and shipped to be installed on site. There are also stick-built boilers which are built at the site itself. There are also hybrid approaches which combine elements of package and stick-built boilers.
Package boilers are standardised, come in specific sizes, and are the less expensive option. If the design fits exactly the needs of a facility, they are a good choice. However, with refineries this is rarely the case. Often, a package boiler will not be sized appropriately for the specific needs of the facility. Further, large refineries require large boilers and a packaged unit of that size can be difficult, if not impossible, to transport.
Some refineries seek to get around that by specifying a stick-built boiler. They are designed specifically for the petrochemical application. Further, the size of the boiler is less of an issue as there is no need to transport it from a factory to the refinery. Components are shipped to the site and everything is manufactured and assembled there. The down side includes the fact that they are more costly and take a lot longer to install. But where huge boilers are required, this may be the only option.
A hybrid approach, favoured by Rentech, takes elements from both approaches. Each boiler is custom built to customer specifications, pre-assembled and then tested. The boiler can be shipped to the site in one or more pieces for final installation and commissioning. Smaller boilers can be shipped in one piece. Larger units are shipped in two or more pieces. As each part is pre-assembled and tested before shipping, only a small amount of work is needed on site to combine the parts at the refinery. Since these boilers are factory built, they are less expensive and do not require the extensive site work required for stick-built boilers.
Some companies size boilers with little margin, others conservatively. Rentech advises the latter. Those building smaller, hot rod boilers may be a little cheaper. However, this approach can impact reliability. By building conservatively, warranty problems are avoided and petrochemical sites gain a more reliable boiler in the end. This manifests in ways such as a design with a slightly larger steam drum.
A good example is water to carry over from the steam drum into the superheater. If the boiler allows that, it will eventually blow out the superheater. Having a larger steam drum, on the other hand, allows better separation to remove water from the steam before it arrives at the superheater. Additionally, if feed water flow is lost to the boiler, a larger steam drum allows more time to correct the water issue before steam levels fall. This is far more desirable than scrambling with only a minute or so from normal operating level on the steam drum to low level trip. That can be a disaster for a refinery in terms of lost production as it can sometimes leave too little time to correct the situation. With a larger steam drum, a five minute interval should offer enough time to react to any feed water situation that may crop up.
Achieving lower emissions
Running a smaller boiler flat out is going to raise the emissions profile of any facility. And that could mean running foul of legislators and air quality control authorities. By sizing for a larger replacement furnace, a platform is provided to lower emissions and minimise problems with flame impingement. Plants that keep their furnaces larger find that it is easier to achieve lower NOx.
Additionally, by augmenting the new boiler with selective catalytic reduction (SCR) technology, emissions can be lowered further. Coen LoNOx burners too can help to minimise NOx production from boilers. One university campus in California, for example, attained NOx levels of 3 ppm from its Rentech boilers. This kept them well below the 5 ppm demanded by the local air quality management district.
The combination of the right burners and SCR can keep emissions down without the mass flow and parasitic losses of flue gas recirculation. As a result, not as much reactant is required. This allows the use of a more compact SCR since it does not need to deal with the higher mass flow or higher NOx levels.
By observing these best practices, replacement boilers will be more efficient, will have lower emissions and will require much less maintenance.
This short case study originally appeared in PTQ's Technology In Action feature - Q4 2018 issue.
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