Advanced Boiler and HRSG Designs Offer Improved Performance

Innovative boiler and heat recovery steam generator (HRSG) designs are improving efficiency, reducing emissions, and lowering maintenance costs. Some well-proven technology, such as circulating fluidized bed designs, allow greater fuel flexibility, which is vitally important as plants struggle to compete.

Competition is fierce in the power industry. That means finding the most economical way to produce power can mean the difference between a plant being dispatched or not.

Market conditions haven't gone unnoticed by original equipment manufacturers (OEMs). To make things easier on power companies-and to make their own products saleable-OEMs are constantly trying to improve designs. Stretching traditional boundaries can be challenging, but often pays off with major benefits.

Ultrasupercritical CFB

Advances in ultrasupercritical boiler designs continue to be made and proven. Tom Steitz, vice president of Wood Aftermarket Services (see sidebar "An Evolving Brand"), offered the Samcheok Green Power Project in South Korea as a shining example of a plant employing the latest ultrasupercritical boiler technology. The plant includes four 550-MW circulating fluidized bed (CFB) boilers powering two 1,100-MW generators.

An Evolving Brand Most readers probably recognize the Foster Wheeler brand name. The company has been a staple of the power industry nearly as long as POWER magazine. The Foster Wheeler tag originated in 1927 when two U.S. companies-Power Specialty Co. and Wheeler Condenser & Engineering Co.-merged. In November 2014, AMEC, a UK-based company with a storied history of its own, and Foster Wheeler combined to form a global engineering, project delivery, asset support, power equipment, and consultancy conglomerate. Wood Group, with headquarters in Aberdeen, Scotland, acquired Amec Foster Wheeler in October 2017. The combined company now goes by the name Wood and is a global leader of project, engineering, and technical services in the energy, utility, and process markets. Wood operates in more than 60 countries, employing about 55,000 people.

The unique low-temperature CFB combustion process, coupled with ultrasupercritical steam technology, offers high net plant efficiency (reportedly 42.4% LHV [lower heating value] and 38.8% HHV [higher heating value] see sidebar "What's the Difference Between LHV and HHV?"). The low combustion temperature also reduces emissions, but it doesn't limit the plant's ability to achieve ultrasupercritical steam conditions. The Samcheok plant operates with a steam temperature of 603C (1,117F) and a superheated steam pressure of 257 bar (3,728 psi).

What's the Difference Between LHV and HHV? Understanding plant efficiency claims can be confusing. That's because some hydrogen-rich fuels release water during the combustion process. The water is subsequently evaporated during combustion with the process using some of the heat released by the fuel. This latent heat of vaporization is temporarily lost and therefore does not contribute to net power generation. If the water vapor released by fuel combustion is simply discharged to the environment via the exhaust stream, the latent heat of vaporization is permanently lost. That is the case, for example, with many internal-combustion engines and simple cycle gas turbines. On the other hand, some advanced boilers have a secondary condensation process downstream of the combustion step, which condenses the water vapor in the exhaust stream and recovers at least some of the latent heat being carried with it. The recovered heat can then be used productively. The numerical difference between the lower heating value (LHV) and the higher heating value (HHV) of a fuel is roughly equivalent to the amount of latent heat of vaporization that can be practically recovered in a secondary condenser per unit of fuel burned. The numerical value of a fuel's HHV is always greater than or equal to the LHV. Therefore, a power plant's net efficiency on an LHV basis is always higher or equal to the HHV-based efficiency. Comparing the efficiency of power units without knowing which calorific heating value was used to calculate the efficiency can be misleading.

Most conventional coal technologies require the fuel to be finely ground and dried before entering the furnace. The CFB does not require those steps. Instead, the fuel is coarsely crushed and dropped into fuel chutes, which lead to ports in the lower section of the furnace. The CFB's wide fuel range allows excellent fuel procurement flexibility. The Samcheok plant is sourcing high-moisture Indonesian coal and biomass to fuel its units, which is expected to save millions of dollars in fuel costs over the plant's life.

Unlike conventional boilers that burn the fuel in a massive, high-temperature flame, CFBs utilize circulating hot solids to cleanly and efficiently burn the fuel in a flameless combustion process. The low, uniform combustion temperature minimizes the formation of NO_x and allows the injection of limestone to capture acid gases as the fuel burns, giving the CFB some of the lowest furnace emissions currently achievable. Because CFBs don't require back-end flue gas desulfurization equipment for SO_x control, Samcheok's owner-Korea Southern Power Co.-saved hundreds of millions of dollars in construction costs.

Solid particles in the furnace are collected by steam-cooled solids separators, which recycle most of them back to the furnace. The particles pass through a high-performance Intrex heat exchanger where superheated steam is produced in steam coils submerged in the bubbling bed of hot solids.

Because ash doesn't melt and slag in the CFB boiler, fouling and corrosion are minimized. With clean tube surfaces, the hot solids are better able to conduct heat efficiently throughout the entire boiler. In addition to reduced emissions, other benefits include decreased maintenance and increased reliability.

1. State-of-the-art. This image showing a triumphant group of power plant workers with fists raised was taken to mark Samcheok Unit 2 entering commercial operation in June 2017. Courtesy: Korea Southern Power Co.

The Samcheok facility entered full commercial operation in June 2017 (Figure 1). Despite using first-of-its-kind technology, the commissioning process reportedly went very smoothly.

Poland Embraces CFB Technology

In Poland, coal continues to be the go-to resource for power generation (see "King Coal Is Alive and Kicking in Poland" in the March 2018 issue), and the country was an early adopter of the CFB design. At the Turow power plant in western Poland near the country's border with Germany, pulverized coal units began burning low-quality brown coal in 1962. Due to deteriorating environmental conditions, among other things, in the 1990s the directorate decided to upgrade to CFB technology as part of a $1.6 billion modernization effort.

Six boilers were replaced with the Foster Wheeler design between 1998 and 2004. The environmental impact was significant. SO_x emissions decreased 72%, NO_x was cut to less than half, and particulate emissions dropped 90%. Furthermore, the units have improved availability and greater fuel flexibility.

The Aagisza plant located near Bedzin in south-central Poland offers another success story. It is home to what was the first supercritical CFB boiler, constructed between 2005 and 2008. The unit has lost some of its bragging rights now that the ultrasupercritical Samcheok plant is in commercial operation, but it is still a notable project.

The CFB design is suitable not only for coal-fired plants, but also for plants burning petcoke, municipal solid waste, refuse derived fuels, and wood and agricultural biomass. Amec Foster Wheeler sold its CFB technology to Sumitomo SHI FW last year. Sumitomo licensed the technology back to the company (now Wood), giving it aftermarket servicing rights for North American units.

HRSG Upgrades

With the large amount of renewable energy being added to the power grid, combined cycle plants are increasingly being asked to vary output. That can be a challenge, and in many cases, heat recovery steam generators (HRSGs) are the limiting factor. Tony Cirillo, senior program director for strategic development with AECOM, said many OEMs are improving designs to make HRSGs more responsive to load swings and ramping.

"Basically, it's all about the material thicknesses," Cirillo said.

When you weld tubes to headers or tubes to drums, the welds become a source of fatigue and fracture. Cirillo said the tube could be x thick and the drum could be 5x thick. Correspondingly, there are differences in expansion and contraction where the pipe joins the header or where the pipe joins the drum, which presents the potential for corrosion and cracks, and ultimately failure. To account for this, warmup times must be extended.

"From a design standpoint, instead of welding a bunch of tubes from multi-rows to a header, manufacturers are going to singular rows for collection," he said. "New designs now have a very gradual tube-to-header-to-drum transition rather than simply a tube-to-drum transition."

Analogous to conventional, natural circulation fossil-fired power boilers, some OEMs are going so far as to eliminate the drum altogether. Called a once-through design, the HRSG essentially has no superheater steam drum and uses vertical steam separators to enable faster start-ups. This is an improvement in many respects, but the HRSG loses most of its surge capacity.

"While that allows you to be responsive, the range over which you can respond is not that great because the drum acts like a surge damper, if you would, to moderate between rapid changes between the demand and the supply," Cirillo pointed out.

One company that has made some meaningful improvements is Siemens Heat Transfer Technology (HTT). Formerly NEM Energy, the Siemens subsidiary was rebranded and renamed Siemens HTT in January 2018. The company offers two types of fast-start HRSG technologies.

2. Mega project. In this image one of the heat recovery steam generator (HRSG) modules destined for Egypt is loaded onto a transport vessel in South Korea. Siemens will supply a total of 24 HRSGs for three 4.8-GW plants in Beni Suef, Burullus, and New Capital. Courtesy: Siemens

One is its Benson HRSG, which is a once-through design built without a high-pressure (HP) steam drum. The Benson model is typically a vertical HRSG with a modular design (Figure 2), which allows a smaller footprint and reduces the on-site construction time.

The other fast-start design is called the DrumPlus HRSG. It has an HP drum, but it has a relatively small diameter and correspondingly small wall thickness. As a result, peak stresses are significantly reduced.

To reduce the size of the DrumPlus steam drum, the water/steam separators, known as bottles, were relocated outside of the drum. This allowed the separator to be optimized without the limits set by space restrictions inside the drum. Of course, that means there are a number of separation bottles that must be installed in something similar to an instrumentation rack during erection, but it does allow gas turbines to ramp-up without restrictions, which improves flexibility, reduces startup costs, speeds power delivery to the grid, and lowers NO_x emissions during startup. a-

-Aaron Larson is POWER's executive editor.

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