Natural Gas Conversions of Existing Coal-Fired Boilers
By F.J. Binkiewicz Jr., PE; R.J. Kleisley; B.E. McMahon; J.E. Monacelli; D.A. Roth; and D.K. Wong

Electric utilities are always searching for ways to minimize costs, improve availability, and reduce emissions. Recent changes in the price of natural gas have made that fuel economically attractive, and it has the added benefit of reduced air emissions. For utilities with existing coal-fired units, conversion from coal firing to natural gas firing might be an option worth considering.

Why Consider Fuel Switching?

The first step in the process is to identify the forces that drive the decision to convert from coal to gas. The key forces are regulatory (both in terms of emissions and as an offset for a new unit), fuel costs, the age of the plant, and the need for plant output.

Regulatory forces are currently in a state of flux, and there are a wide range of proposed rules and legislative efforts that could have a far-reaching impact on coal-fired operation. Carbon dioxide controls seem to be coming in the near future that will require some additional operating restrictions placed on power plant owners. There also may be other regulatory issues to evaluate, such as New Source Review and offsets for other emissions regulated by state and federal laws.

The price of natural gas has recently become more attractive as a baseload fuel due to additional supply and reduced demand from general industry. There are many different projections of where gas prices might be in the near future, all of which are based on the forces of supply and demand. The current price of natural gas is relatively low and stable compared to previous years.

Utilities should be aware that natural gas prices are much more sensitive than coal prices to short-term changes in supply and demand. While current economic conditions favor natural gas usage, Babcock & Wilcox Power Generation Group Inc. (B&W PGG) strongly advises its customers to evaluate potential price volatility as a key component in the decision-making process.

A plant may be considered for fuel switching based on its age and how close it would be to a possible retirement or major rebuild. The timing for fuel switching may be ideal if the boiler in question is already under consideration for major projects like superheater replacement, burner modifications, air system changes and/or the addition of back-end emissions control equipment.

Utilities must also factor in the future need for electrical power generation—either because of market demand projections or to replace a unit that might be approaching the limit of its useful service.

One of the other key factors to consider is the need for plant output, including a potential for derate and/or increased turn-down capability. A unit’s continued usefulness might involve its ability to operate or be on standby during periods of low load.

As utilities look at their long-term forecasts, plants that operate efficiently and with high availability will play a key role in meeting future demand. Consequently, these plants will need to be evaluated for projects that will extend their useful life. Those projects might be targeted for efficiency improvements with coal as a fuel (such as burner upgrades and emissions control equipment) or as fuel-switch projects that take advantage of the benefits of natural gas.

Identifying Options

The first step is to perform an engineering study to help determine the best options for your specific application. Among the many options to consider are:

  • Fuel switch with modifications to the existing boiler.
  • Fuel switch for the existing boiler and the addition of a gas turbine to the existing boiler cycle, such as adding a simple cycle to the existing system, repowering the hot windbox, or combined cycle repowering. 
  • New combined cycle plant (elemental review) with retirement of the existing coal plant.

Each option has advantages and disadvantages, including cost and operational considerations. Potential owners should:

  • Compare modification costs vs. the capital cost of a new gas turbine.
  • Estimate the impact of future changes in fuel prices and the potential risk associated with natural gas price volatility.
  • Predict the life expectancy of gas turbines and heat recovery steam generators (HRSG) compared to steam boilers.
  • Estimate the acceptable plant derate, if applicable.

Because no two plants are identical, it is important that utilities work with an experienced supplier like B&W PGG to evaluate the best solutions for their needs.

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