Flexibility is the name of the game in the modern energy environment

Credit: Alstom

CFB technology has been typically associated with small-scale industrial boilers using difficult fuels. But with the emergence of utility-scale commercial installations, David Appleyard finds that this is changing

Given the multi-decade lifespan of the average power generating asset, security of fuel supply is a key issue for the industry. However, it is becoming increasingly evident that, as ever, flexibility is the name of the game in the modern energy environment. A number of factors, such as the changing characteristics and availability of fuels and the increasing opportunities available from generating energy from waste streams, has prompted the development of fuel-flexible technologies, in particular Circulating Fluidized Bed (CFB) boilers.

Challenge of PF boilers

Much of the focus of development in once-through boilers running on pulverized fuel/coal (PF or PC) has been on raising steam temperatures to improve thermal efficiency rates. This presents a number of challenges. For example, higher temperatures typically demand the use of more expensive materials such as alloy metals. Such materials are capable of withstanding the conditions within a boiler, where flame temperatures may reach 1200oC or more, but come at a cost, both in terms of the materials required and the sheer scale of the design.

Another key challenge associated with high temperatures is slagging and corrosion as ash products within the fuel melt. These molten materials can then adhere to the boiler tubes and the walls, which inevitably affects heat transfer between the boiler gases and the steam/water working fluid. Robert Giglio, Vice President of Foster Wheeler’s Global Power Group, observes: “In a PC furnace the fuel burns at a very high temperature, causing the fuel’s ash to melt, which fouls and corrodes the boiler’s heat transfer surfaces. This is the primary root cause behind boiler maintenance.”

In order to resolve this slagging issue, boilers running high ash or lower-quality fuels tend to be larger to enable these ash particles to cool before encountering boiler surfaces. Nonetheless, while there are engineering solutions, the conventional once-through is typically restricted to a relatively narrow range of fuel types for which it is designed.

Giglio says: “The quality of coal has been declining steadily in steam coal markets around the world. If you go back 10 years, 6000 kcal/kg was the benchmark for nearly all seaborne coal trades. Today most coal trades are below 5500 kcal/kg and the fastest growing trades volumes are for coals between 4200-5500 kcal/kg. Why? Because sellers of these lower quality coals are offering huge discounts that buyers can’t ignore.”

Referring to the emergence of low-quality Indonesian coals that are now outpacing the supply of high-quality Australian, Russian and US coals, Giglio notes: “When existing PF plants burn these cheap fuels they get a huge savings in fuel cost, but at the expense of lower plant reliability, more plant maintenance and even a loss of power output. Power producers could build new PF plants designed to fire these fuels with much less impact to reliability and maintenance, but they lock themselves into a narrow fuel range, making it difficult to adapt to further declines in coal quality or to fire higher-quality fuels if this low-quality market trend reverses. Considering a power plants operates for 30 to 40 years, no one’s fuel procurement plan is safe from error.” He concludes: “CFB boilers avoid these issues, that’s why utilities are looking more at CFBs for power generation.”

This is a point echoed by Radue, who says: “One thing is fuel flexibility, the fuel dictates our boiler choice of technology, and secondly the size of the boilers – it’s the main driver of our design process. Because of the fuel being king, we see a demand for fuel flexibility, we see a lot wider fuel specs than we did in the past, and because operators want to hedge their bets where they get the fuel from, these boilers are designed for a lifetime of 30+ years.”

A route to fuel security of supply

CFB technology has evolved over the last four decades or so from industrial boiler technology used to burn difficult fuels in the late 1970s to a utility-scale product supplied by a growing number of major OEMs.

Larry Hiner, manager of boiler product lines at Babcock & Wilcox Power Generation Group, says: “We do see CFB as a more favourable product for burning multiple fuels like biomass and petcoke, either in combination with coal or alone. We see a lot more of the market moving towards renewable fuels like biomass, or projects like petcoke, or lower-value coal. The lower-value coal is easier to combust with a CFB than with PC: the coal would burn too hot in PC firing so you have slagging issues in those. In a controlled environment like a CFB these lower-value fuels are more suitable.” He points to Turkey and sub-Saharan Africa as examples of the increasing use of domestic low-quality coals: “We are seeing more countries looking to utilize their lower-value coals, when they can utilize local coals, rather than importing.”

CFB boiler

Credit: Alstom

Hiner also points to the increasing use of wood and biomass, notably in co-firing applications in Europe. “We see a lot of wood-fired units that are going into Europe and certain countries. People are looking to burn biomass and this would favour fluid bed boilers, because you can’t pulverize wood easily. If you’re looking at fuel flexibility, like firing wood in combination with coal, CFB is sometimes the best – and one of the only – choices.”

Giglio further defines the business case: “Unlike PF technology, CFBs have no flame stability issue since it utilizes a flameless combustion process, you get a lot of solids at the right temperature, and no matter what fuel you add it will burn because fuel particles stay in the system for a long time. You could use a CFB boiler to fire a lousy sub-bituminous coal with heating values lower than 4000 kcal/kg with not a lot of change in design. As the market goes up and down you can buy fuels based on business, and economic decisions without limitations on what your boiler can burn.”

Longer transit times

Within a CFB boiler the circulating bed material flows through the furnace together with the fuel and air. Combustion gases are then separated from the solid material with cyclones and any solids are returned to the lower part of the furnace while the gases pass on through the boiler.

One of the key differences between PF and CFB technologies is the boiler transit time. In a PF system a particle of fuel must reach a high enough temperature and be supplied with sufficient oxygen for complete combustion in a transit time of perhaps three or four seconds. In a CFB any unburned material is returned to the furnace and a larger particle can spend as much as 30 minutes within the furnace for complete combustion to occur.

Longer transit times also enable lower temperatures, which presents a number of attractive advantages.

“The CFB process does not allow the fuel ash to melt. We use the ash to conduct heat, instead of letting it melt and becoming a problem – melting ash is the number one cause for maintenance in boilers due to the resultant corrosion and fouling – because CFBs’ combustion temperature (850oC) is well below the ash melting point,” Giglio explains.

CFB provides an even combustion temperature profile throughout, which supports the use of a wide variety of fuel properties such as heating value, moisture, ash content and a number of low melting point ash components.

Furthermore, the fact that solids are circulating means they can actually clean the surfaces of the boiler if any slag does start to form. Thus CFB offers high combustion efficiency, but also lower primary emissions of pollutants such as NOx and SOx.

The formation of NOx is a function of the temperature of the combustion process in air, with lower temperatures producing lower NOx emissions. Because a CFB furnace combustion temperature is about 850oC – significantly below that of a PF which can easily exceed 1200oC – NOx emissions for the CFB are inherently low.

Furthermore, the lower temperatures allow limestone to be added to the circulating solids bed to capture sulphur as the fuel burns. This is not possible in a PF boiler where control of SOx requires the installation of a back-end flue gas scrubber such as wet FGD (flue gas desulphurization). “For most fuels the CFB needs no back-end scrubbing, which saves hundreds of millions of dollars on the installed cost of the plant,” says Giglio.

Pierre Gauvillé, CFB Product Manager at Alstom, explains: “CFB technology was developed some 35 years ago to use low-quality coals with a high sulphur content because the CFB technology allows, by introducing limestone into the boiler, capture in the furnace without the need for additional back-end treatment.”

“The first feature of the CFB is that it is a clean combustion technology,” adds Gauvillé, stating that in a typical PF boiler, thermal NOx emissions can be up to five to 10 times the level seen in a CFB, according to the coal rank and PF technology and due to the lower operating temperatures in a CFB boiler.

Comparing CFB efficiencies

Although CFB systems operate at lower temperatures than PF boilers, their heat transfer rates and thermal efficiencies are comparable. This is due to the heat transfer characteristics of the solid material, as Giglio explains: “If you’re trying to conduct heat in a gas, which is what PF does, it does this through conduction and radiation – which has a very low heat transfer coefficient.” This is one of the reasons why such high temperatures are required in PF boilers. Within a CFB, as the solid material is actually touching the boiler surfaces, the conduction coefficients are some 20 times higher, Giglio adds.

As a result of the improved heat transfer, CFB boilers are capable of producing supercritical and ultra-supercritical steam conditions. For example, the Łagisza 460 MWe CFB Power Plant in Poland, owned by Poludniowy Koncern Energetyczny SA (PKE), has been in commercial operation since June 2009 and produces supercritical steam at 560/580oC.

Besides being one of the largest operating CFB steam generators in the world, Lagisza has vertical-tube technology and a low-temperature flue gas heat extraction which allows the plant to achieve an efficiency of over 43 per cent.

“The Lagisza original international tender was for OTSC PC [once-through supercritical pulverized coal] technology so the alternative selection of CFB OTSC technology over conventional PF technology is of historic significance,” says Giglio.

Scaling CFB technology

One of the key technical developments within CFB has been the drive to scale, pushing the technology from industrial to full utility-scale projects. This is a point picked up by Hiner, who says: “A lot of what we see is CFBs on a smaller scale displacing PC, but on the larger scales the PCs dominate. We do see CFBs moving into that market as they scale up in size.” He points to the introduction of B&W’s IBHX (in-bed heat exchanger) technology. “That’s integral to our furnace design that allows us to scale CFBs up the utility scale to 300, 400, 500 MW and there is nothing to preclude us from going up to about the 600 MW-700 MW size range.”

At an even larger scale, Amec Foster Wheeler is supplying Korean Southern Power Company (KOSPO) with four 550 MWe ultra-supercritical CFBs for the 2200 MWe Green Power Plant project in Samcheok, Korea. Utilizing a two-boiler-on-one-steam-turbine configuration, the plant is now under construction and is expected to start up in 2016. In addition, the CFBs didn’t require back-end flue gas desulphurization equipment for SOx control.

In April this year, Amec Foster Wheeler followed this up with the announcement of a contract from Hyundai Engineering Co Ltd for the design and supply of two 150 MWe CFB steam generators for Therma Visayas, a subsidiary of Aboitiz Power. The new plant will be located in the central province of Cebu in the Philippines, and commercial operation is scheduled for the first quarter of 2018.

Similarly, Alstom’s €950 million contract with Estonia’s Narva Elektrijaamad AS, for the supply of two 300 MW CFB units for a power plant in Narva, is fuelled with local oil shale and will be able to operate with 50 per cent wood, cheap on a heat input basis. Its connection to the grid took place in early May 2015.

More recently, Alstom has signed a contract to deliver equipment for the 135 MW Concepcion Circulating Fluidized Bed (CFB) Power Plant, which will be constructed in the Philippines and is expected to be completed by the end of 2015.

Future CFB development

Although CFB technology offers fuel flexibility, low emissions and high efficiency at utility scale, there are nonetheless limitations. In particular, the use of waste streams can prove problematic with, for example, glues, plastics and other such substances, potentially causing agglomerations within the fluidized bed material.

Says Hiner: “One of the Achilles’ heels for a CFB-type boiler where you have a sand medium is, if you have low eutectic melting points in the fuels and a lot of man-made materials tend to be poor fuels because they have high alkalinity, high sodium and potassium. We would tend to use grate systems for those types of facilities. We know there are some people who have promoted use of CFBs but we know those units tend not to operate as reliably.”

Indeed, reliability is another perennial consideration for all plant owners and operators and a key area of interest for OEMs working on the development of CFB technology.

As Tero Luomaharju, Product Manager for CFB boilers at Valmet Technologies – formerly part of Metso – explains: “We have paid attention to availability and tried to get better. We have developed superheaters for high corrosive fuels. We can burn high chlorine fuels with high steam parameters compared to early days. Now we have water-cooled construction for cyclones it means we have fewer problems; now we have higher temperatures in superheating even if we have high chlorine in fuel, and we can get more electric output from the same unit.”

Luomaharju also picks up on extending the fuel flexibility capabilities of CFB technology: “There have been some waste firing areas; we are doing material tests and other things to decrease corrosion potential. We are developing boilers for demanding fuels like biomass and waste, and corrosion is one focus in this key development area. We are starting to explore the limits with different materials in different places of the boiler, and if there is any potential to increase temperatures with demanding fuels.”

He also highlights the role of forecasting in CFB technology and the predictive maintenance opportunity. “It’s much more complicated in CFB: we can predict the influence of fewer properties for need of maintenance. Regarding how much maintenance is needed, we are developing some new measurement devices to analyze flue gas and its corrosive qualities. I think online measurement of flue gas quality is going to be more common in future.”

Hiner also considers further fuel flexibility as a key avenue of future development in CFB technology: “We are extending the ranges of fuels that are suitable for fluid beds. It’s how we control combustion, but it’s also to avoid agglomeration,” he says.

With all its advantages, and the likelihood of further technical development yielding greater fuel flexibility, it seems that utility-scale CFB is now a commercial reality. All that is really required for its widespread adoption is to overcome the naturally conservative nature of power plant owners and investors.

David Appleyard is a freelance journalist specialising in the energy and technology sectors