Environmental pressures and associated uncertainties have discouraged investment in new coal plants – to the extent that more than 90 per cent of currently planned generating capacity in the United States is gas-fired.

However, recently the outlook for coal has changed dramatically, thanks to, among other things, escalation and instability in gas prices, burgeoning electricity demand and the recognition that system reserve margins have been getting dangerously low (as exemplified in California’s energy crisis).

Coal is abundant in the United States, with recoverable reserves amounting to more than 296 billion tons. The 450 existing coal-fired power plants account for over 50 per cent of US electricity generation. It has enjoyed a long run of low and stable prices and is seen by many as playing a potentially critical role in the much needed expansion of electric power generation in the United States. A number of US utilities have now announced plans to upgrade and/or enlarge existing coal plants and build new ones.

A notable recent example is the 80 MWe Wygen unit 1 project of Black Hills Corporation. This is not a large power plant by modern standards, but it has the distinction of being the only new pulverised-coal-fired power station currently on order in the USA. It is scheduled for operation in July 2003.

Black Hills Corporation is an energy and communications company with three business units: a regulated utility, Black Hills Power and Light; an independent power business; and a communications subsidiary.

In January 2001, Babcock & Wilcox was awarded a contract valued in excess of $100 million by the independent power subsidiary of Black Hills Corporation, to engineer, design, procure and construct the Wygen unit 1 power plant. The Wygen unit 1 pulverised-coal-fired boiler will produce 778 000 lb/h of superheated steam at 1005°F and a pressure of 1620 psi. B&W’s extended scope with Black Hills Corporation includes all auxiliary boiler island equipment, the turbine-generator, all turbine island equipment, air-cooled condenser, coal and ash handling systems, and the plant control system.

B&W will design, manufacture and construct all major emissions control equipment, including its environmentally effective and cost-efficient dry scrubbing system that features a patented spray absorption process for sulphur dioxide removal.

Additionally, B&W will design and supply the selective catalytic reduction system for post-combustion nitrogen oxides control, and a fabric filter baghouse for particulate removal. The boiler features B&W’s latest design of ultra low-NOx burners, the DRB-4Z, and overfire air systems.

Understandably, with new coal projects, in particular pulverised coal plants, still thin on the ground, it is seen as a significant development and a vital project by Babcock & Wilcox. Company president, James F. Wood, says he expects “Wygen unit I to be a strong reference plant for future power development projects in North America and the world.”

Wygen unit 1, which is part of the company’s strategy of “investing in low-cost generation”, is located at Black Hills Corporation’s energy complex near Gillette, Wyoming, in the Powder River Basin. The new unit will consume approximately 500 000 tons per year of low-sulphur sub-bituminous coal from the company’s adjacent Wyodak mine. Black Hills has been operating the Wyodak coal mine since acquiring it in 1955. Coal production is around 3 million tons per year, and the mine contains an estimated 275 million tons of reserves.

Wygen unit 1 will be the fourth coal-fired plant at Black Hill’s Gillette energy complex. The other three are Black Hills’ Neil Simpson I and II, 20 MWe and 80 MWe respectively, and the 362 MWe Wyodak unit 1. The latter, also a B&W plant, was completed in 1978. It is 20 per cent owned by Black Hills and 80 per cent by Pacificorp. There are also two 40 MWe gas fired plants at the complex, one, operated by the regulated part of Black Hills, entered service in June 2000, and the other, operated by the independent power subsidiary, is due to start up in June 2001.

Neil Simpson unit 2, also incorporating a B&W boiler, was completed in 1995. Wygen unit 1 will share control facilities and administration with Neil Simpson unit 2, cutting costs for both plants.

Minimising water consumption is a priority in this arid region of north eastern Wyoming. So, for example, Black Hills has been a pioneer of air-cooled condensing technology in the western hemisphere. In 1969 Neil Simpson I was the first utility plant to use an air-cooled condenser on a steam turbine for power generation. The Wyodak plant was for a time the largest air-cooled power plant in the world, subsequently surpassed by power stations in South Africa.

The DRB-4Z burner

A major contributor to Wygen unit 1’s low NOx emissions will be the DRB-4Z burners – the most advanced coal burner yet developed by Babcock & Wilcox. The DRB-4Z burner features a patented triple air zone design. Low NOx burners of the 1990s were developed using single and dual air zones.

The DRB-4Z has advanced this design concept with the addition of a third air zone. This third air zone, the transition zone, is located adjacent to the coal nozzle. The transition zone acts as a buffer between the fuel rich flame core and the inner and outer secondary air streams. The flow field produced by the transition zone draws gases from the outer portions of the flame inward toward the flame core. NOx formed in the oxygen-rich outer flame region is reduced to nitrogen in the process.

This new design results in significantly lower NOx emissions compared with what is achievable using traditional single or dual zone low NOx burners.

The DRB-4Z burner was conceptually developed using computational fluid dynamic (CFD) modelling. Modelling confirmed the advantages of an additional air zone surrounding the burner nozzle. This additional air zone helped accomplish critical aspects of mixing around the flame core. A prototype burner was constructed and refined through an extensive programme of large-scale combustion tests performed in B&W’s 100 million Btu/hour Clean Environment Development Facility (CEDF) located in Alliance, Ohio.

The mechanical design of the DRB-4Z burner draws heavily on past B&W low NOx burner designs, which have demonstrated excellent mechanical reliability in the severe conditions prevalent in utility boiler service. The burners are constructed with heavy, high quality, stainless steel plate for portions of the burner exposed to high temperatures. This is combined with a system of stiffeners to maintain the structural integrity while accommodating thermal expansion.

Adjustable spin vanes provide online tuning capability. Each burner has individual airflow control capability by a manually adjustable slide damper. Relative airflow measurement to each burner is provided by a multi-point pitot grid.

The burner is shop assembled as a single shipping unit ready to install using provided hardware.

In addition to Wygen, there are currently seven other projects (new and retrofit) using the DRB-4Z burner, all in the USA. The first commercial application was a retrofit at unit 6 of Reliant Energy’s Parish plant. Also running on Powder River Basin coal, these burners went into operation in May 2000. DRB-4Z burners are also being installed at Kansas City Power and Light’s Hawthorn unit 5, which is in the process of being rebuilt following the February 1999 explosion, and is due to be back on line by June 2001.

500 MW plant planned

Meanwhile, on 26 February 2001, the independent power subsidiary of Black Hills Corporation announced that it had initiated the permitting process to build a 500 MW coal-fired power plant at the Gillette energy complex. This new power plant could be operational in mid-2005. The project is contingent upon three primary factors: an expedited regulatory approval timetable, the availability of adequate electric transmission access to multiple markets, and the securing of long-term power contracts for the energy produced by the project.

Daniel P. Landguth, Chairman and CEO of Black Hills Corporation, said, “Our mine-mouth coal-fired power plants near Gillette are some of the most reliable and low-cost units in the nation, and we believe the time is right to significantly expand our capabilities there.”

“We are hopeful that our ambitious response to market opportunity is matched by the commitment of others so that we might proceed rapidly,” Landguth continued. “We appreciate the need for regulatory review and compliance, but we are hopeful that the review process can be expedited. We also believe that the State of Wyoming can position itself as an energy leader by facilitating private sector construction of additional electric transmission line infrastructure.”