HRSG design, installation, and O&M: lessons learned

Many facilities have switched from ageing boilers to modern combined heat and power systems (CHP) or combined cycle power generation systems that utilise gas turbines, steam turbines and heat recovery steam generators (HRSGs). Other facilities are planning to switch as they see the advantage of onsite power generation and being able to control their own power costs.

Whether the project is in the planning stages, is ready to be installed, or is now operating, here are some lessons learned in addressing challenges in both new builds and retrofits when an HRSG is involved.

Space

When retrofitting an HRSG into an existing facility, space is almost always an issue. The existing boiler is generally going to have a smaller footprint than a combustion turbine and HRSG package. Combustion turbines are generally pre-engineered as part of a standard package. That leaves only a certain amount of space to integrate the HRSG into the building or site layout. Before anything is designed or built, therefore, it is vital to scope out the layout of the facility, how much room will be consumed by the combustion turbine package and what room is left for the HRSGs. That gives you enough data to either tailor the design to the available space or to figure out plant modifications needed to allow for the addition of HRSGs.

When it comes to space, it is also important to pay attention to how the HRSGs are going to be delivered to the facility. There may be tight angles for trucks to negotiate, headway restrictions and other factors that make it challenging to get the HRSG into the facility. Decisions should be made in advance as to the maximum size of HRSG parts that can be brought in, where they can be staged and how they are to be assembled. All of this takes careful planning and co-ordination with plant personnel.

Design customisation

You can either try to fit the facility to the HRSG or fit the HRSG to the exact needs of the facility. The former approach is often problematic. Yes, it is cheaper if you can specify standardised HRSG designs. But you can run into challenges when the HRSG does not align well with the space or the workflows of the facility.

Customised designs cut down on-site rework and minimise disruption during assembly and installation. Custom models are built, preassembled, and tested in the factory then shipped in pieces to the site. Final installation and commissioning are much faster. In any case, large HRSGs are impossible to transport unless they are broken down into several parts. A relatively small amount of work is required to combine the parts at the site. Design customisation, then, is a smart way to circumvent space limitations, while providing the facility with equipment that is more closely suited to its needs.

It makes long-term financial sense, therefore, to specify an HRSG that exactly fits site requirements. By matching and integrating the system to the plant layout and the combustion turbines selected, it is possible to maximise the efficiency of the units.

Many end users embrace the customised approach. At the outset of a university project, the college engineers contacted us to go over their exact needs as to how to couple new HRSGs to Solar Titan turbines. They wanted to know exactly how they would be sized, how they would operate and perform. There were many meetings and design revisions. They ended up with HRSGs that precisely fitted their needs.

Above: Many facilities are congested and access may be difficult

Design conservatively

A conservative design approach is also recommended. By making the drum sizes and wall thicknesses a little thicker than those of standardised units, you save on maintenance in the long run and enhance the longevity of the unit. Those that size their HRSGs with little margin may pay a little less but at the risk of lowerv overall reliability. By building conservatively, warranty problems are avoided, and sites gain a more reliable unit in the end.

A slightly larger steam drum, for example, prevents the possibility of water carrying over into the superheater. A larger steam drum allows better separation for removing water from the steam before it arrives at the superheater. Additionally, if feedwater flow is lost, 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 before the normal operating level on the steam drum falls enough to result in a low-level trip. That can be a disaster in terms of lost production as it can sometimes leave too little time to correct the situation.

Water quality and chemistry

HRSGs are all about steam and water, with the exhaust gas from a turbine used to produce steam in the HRSG, which is then used to run a steam turbine, for heating or for other facility processes. Therefore, it goes without saying that water quality and water chemistry play an essential role in HRSG operations. The number one thing you must do to keep the HRSG operating properly is maintain good water quality. An entire chapter or small book could be devoted to this subject. Suffice to say, water chemistry should always be maintained within the parameters laid out in HRSG manufacturer guidelines. Thus, inspection and water treatment steps should be a part of maintenance schedules as a way to guarantee the performance and longevity of the equipment.

Maintenance

Water quality is just one aspect of HRSG maintenance. Based on our experience, the most poorly maintained part of any HRSG tends to be the pressure vessel. As you are dealing with high pressures, this poses a serious safety issue. Failure to keep the pressure vessel in good shape is going to come back and bite you in performance, downtime, and longevity.

Further maintenance steps to take include:

• Regular testing, analysis, and tuning to counter gradual degradation due to issues such as corrosion or deposition, which affect efficiency and output.
• Thoroughly inspect the HRSG, as well as related piping, valves, vents, and fittings on a regular basis.
• Make all this part of a maintenance schedule which is adhered to.
• Keep heat transfer surfaces clean as scale and deposits such as calcium, magnesium and silica can coat the water sides of the heat transfer tubes and have lower thermal conductivity than bare metal, retarding heat transfer and producing overheating and eventually leading to tube failures. Bear in mind that as little as a 1/64-inch layer of iron plus silica scale formed by high pressure steam can produce a 3.5% fuel loss.

Emissions

Whenever combustion turbines are involved, emissions must be closely monitored. And if HRSGs add another burner or make use of supplemental firing, emissions levels will be impacted. This must be addressed using emissions monitoring and deploying emissions reduction measures, such as selective catalytic reduction to lower NOx and oxidation catalysts to reduce CO, to meet overall site permitted emissions requirements.

Emissions limitations should be discussed upfront so that designs incorporate mitigation measures as needed.

The last thing you want is to have to retrofit selective catalytic reduction technology and other emissions reduction systems once the heat recovery steam generator is built. Good designs put emissions reduction systems in the proper location so they operate at the highest efficiency.

Burners are also available that are specifically designed to lower the output of NOx. One university campus in California, for example, had to deal with the 5 ppm demanded by the local Air Quality Management District. It attained NOx levels of 3 ppm from its Rentech equipment.

Note that the combination of the right burners and SCR equipment can keep NOx emissions down. As a result, not as much reagent for the SCR is required. This allows the use of a more compact SCR system since it doesn’t need to deal with the higher mass flow or higher NOx levels.

The benefits

By observing these best practices, the heat recovery steam generator will be more efficient, will have lower emissions and will require much less maintenance.

For more information, visit www.RentechBoilers.com

Author: Kevin Slepicka Vice President of Heat Recovery Boilers, Rentech Boiler Systems