HRSGs for small combined-cycle and cogen plants
2005 case study from Combined Cycle Journal
Size matters, or so it seems. Take heat-recovery steam generators, for example. Mention “HRSG” and most people in the power business think of triple-pressure units in service at large combined-cycle plants. Articles in the trade press and papers presented at major industry conferences focus on these units.
During the recent building boom associated with gas-fired generating facilities, so many HRSGs were built by two or three manufacturers for service behind only two or three gas-turbine (GT) models, you could almost say they were “mass produced.” Not much to talk about there.
Drop down in size to the HRSGs serving in small combined-cycle and cogeneration plants — those powered by GTs producing up to about 60 MW — and it’s another world in design variation. The reason is simple: large high-pressure, high-temperature reheat HRSGs burning natural gas — and possibly some distillate — are optimized for maximum electricity production and, in most cases, base-load service.
Small HRSGs often are unique — custom-designed to (1) serve a specific process, (2) satisfy a specific operating requirement and (3) burn a waste or intermittent fuel. Some engineers mistakenly believe that HRSGs for the smallest GTs in industrial energy systems (assumption here is machines in the range of 1 to 5 MW) are simply “light-duty, commercial-type boilers.”
That’s not accurate. Public utilities and industrial companies that buy many small HRSGs specify watertube boilers built to Section I of the ASME Boiler and Pressure Vessel Code that are as robust as much larger units.
Perhaps such thinking prevails because many specifiers and purchasers of small HRSGs are contractors or engineering firms that do not have boiler specialists on staff.
The small HRSG market is served by several firms offering conventional drum-type and once-through steam generators (OTSGs). A leader in the supply of drum-type boilers for GTs rated up to 60 MW is Rentech Boiler Systems of Abilene, Texas.
Rentech specializes in natural-circulation steam and hot-water HRSGs designed to Section I of the ASME Code for GTs up to 25 MW, according to Rentech senior sales engineer Kevin Slepicka.
The company’s largest unit in terms of exhaust-gas flow is the HRSG supplied for the Corona combined cycle. All boilers are made in Rentech’s large, modern shops. Slepicka, who has been involved in HRSG design for more than a decade, says U.S. manufacturing is a competitive advantage when customers want to maintain tight control of their projects. Rentech’s shops are easy to access, and the HRSGs in the size range offered are all transportable by truck or rail in modules of convenient size.
A typical small HRSG supplied by Rentech has ceramic fiber insulation sandwiched between an outer carbon steel casing and inner stainless liner. The evaporator section, consisting of carbon steel tubes and downcomers, is integral with steam and mud drums.
Supplementary firing sometimes is installed to boost steam flow, such as on a boiler designed to recover heat from a 1.5-MW GT installed at a college on the West Coast. The unit makes 10,000 lb/hr of steam on exhaust gas alone, double that with the duct burner in operation. The majority of Rentech’s jobs are for saturated-steam service.
When superheated steam is required — for example, in combined-cycle service — steam pressures go to more than 800 psig, temperatures to well over 900F. T22 tubes are used in the superheater for such applications.
Boiler designers must have accurate information on expected ambient temperature and electrical and thermal outputs over a typical operating year to do the best job for the owner. Slepicka says that the need for supplemental firing can have a tremendous impact on boiler design. For example, under certain operating conditions there is the possibility that the flue gas may not have enough oxygen to support firing of the duct burner to the extent needed to meet the thermal requirement.
To illustrate how dramatically temperatures and flows can vary under different operating regimes, Slepicka says consider this example: 5.3-MW Solar Taurus 60 produces about 165,000 lb/hr of 950F exhaust gas when operating at full load. At 50% of the full-load rating, you get about 100,000 lb/hr of 1100F exhaust. The owner must ask, “Do I need full steam capacity when the turbine is operating at half load?”
If the answer is “no,” supplementary firing is not required. For a boiler producing saturated steam, half-load operation demands no special consideration. However, for a HRSG designed to produce superheated steam, the gas flow and temperature at reduced load might dictate a change in superheater arrangement, a different tube material, and revised thinking with respect to the superheater.
If you can’t achieve the necessary thermal output when the GT is reduced to half load, a duct burner must be installed. This is where the boiler designer earns his salary. The variable that really makes a difference in boiler design is the maximum gas temperature downstream of the duct burner.
Rentech designs its units for high reliability and long life, says Slepicka, meaning it wants to hold the gas temperature at the burner exit to 1,600F in normal operation. At this temperature, the company’s standard sandwich casing of stainless liner, ceramic fiber insulation and carbon steel exterior noted earlier works well, and the owner benefits from the cost-effective design. But 1,600F is not always possible because of the thermal demand.
Consider the following
An unfired HRSG on the back end of a Taurus 60 produces about 25,000 lb/hr of saturated steam. With supplemental firing and a final gas temperature of 1600F, 60,000 lb/hr is possible. But these numbers are for STP conditions (59F ambient at sea level). If the outside air goes to 100F, and there’s no turbine inlet cooling system installed to keep exhaust flow constant over the ambient temperature range, exhaust gas flow will drop. So maintaining 1,600F at the burner outlet means that steam output will probably drop by a few thousand pounds per hour.
You can push the burner to make up the shortfall, but this is not a recommended operating strategy. Rentech has a safety shutoff when the burner exit temperature increases to the 1,700F-1,750F range to protect firing chamber integrity, evaporator tubes and fins, etc.
Of course, you can get more steam from a HRSG downstream of a Taurus 60, but the design would change dramatically. Let’s say you need 100,000 lb/hr. This could be achieved by designing for a burner exit temperature in excess of 2,000F. To accommodate such a high firing temperature, Rentech would offer a design that includes 100% membrane waterwall construction. The resulting unit looks very similar to a conventional fired packaged boiler, such as this O-type boiler that sits behind a Taurus 60 at a military base and produces 80,000 lb/hr of 250-psig saturated steam.
Proper sizing of the duct burner is important to ensure the desired thermal output under all operating conditions. The designer’s greatest challenge, perhaps, is to produce rated steam output when the GT is not operating. This requirement is not unusual for cogen systems installed in process plants where a drop in steam pressure and/or flow can have severe financial consequences.
A rule of thumb
A “standard” duct burner used in supplementary firing duty is only capable of producing about half the rated steam flow when the GT is out of service. Thus, if rated thermal output was critical, you would need a much larger burner than normally would be installed, and it would operate in the so-called “fresh-air-firing” mode when the GT is shut down.
However, when the GT is in service, a standard 10:1 turndown burner cannot provide the operating flexibility normally required (its maximum firing level with GT exhaust gas available would be 50% of the burner’s rated output). A burner and the control system required to provide a 10:1 turndown with the GT at full output must have a wider operating range, and it and the companion control system cost more than a standard supplementary firing system.
Slepicka points out that designers can build boilers to accommodate virtually any operating conditions specified by the owner. It’s just a matter of cost to achieve the flexibility desired. Consider the situation where a transfer to fresh-air firing must occur seamlessly — without a decrease in steam pressure and output.
The most common approach is to install forced-draft fans and have them start on a GT trip. However, the challenge here is to get sufficient fresh air into the furnace quickly enough to keep the burner operating in accordance with National Fire Protection Association (NFPA) and other safety codes. A fan failure on start would trigger a shutdown, and a furnace purge would be required before restart. It is unlikely that a critical process could tolerate such a transient.
Another option is to install louvers upstream of the duct burner and an induced-draft fan just ahead of the stack. The fan runs continuously — an obvious operating cost. With this arrangement, when the GT trips, fast-acting air inlet louvers open automatically and burner output is ramped up quickly.
Keep in mind that selection of the larger burner for fresh air firing has implications for the emissions control system as well. It will require more CO and NOx catalyst than normally would be installed if the exhaust heat from a natural-gas-fired GT equipped with a state-of-the-art low-NOx combustor were producing half the thermal output.
Slepicka says that integrating emissions control into HRSG design and achieving the owner’s goals for both CO/NOx and thermal energy is a difficult job, one characterized by tradeoffs with almost every decision. For HRSGs in process plants, this is particularly challenging given the variety of fuels and operating conditions that often characterize that industry sector.
On occasion it is impossible to install catalyst in a “conventional” boiler layout and achieve all design goals. The optimum solution sometimes involves splitting the evaporator section. This allows positioning of the catalyst in the gas stream for maximum reactivity, thereby reducing the amount of catalyst required.