Coal, Gas

Repowering with Gas

Issue 2 and Volume 118.

Jeff Brehm   By Jeff Brehm, electric power research institute

Nearly three-fourths of coal-fired power plants in the U.S. are now more than 30 years old, and they face a pincer action between new, more stringent environmental regulation on one side and age/technological obsolescence on the other. Whether an aging coal-fired power plant should be retired, converted to gas, upgraded environmentally or repowered is one of the most challenging decisions facing power plant owners today. It’s estimated that as much as 30 gigawatts (GW) of existing U.S. power generation capacity could be lost through plant closings due to new U.S. Environmental Protection Agency (EPA) regulations.

Short-term options include retiring the plant or quick conversion to direct firing of the boiler with natural gas. But increasingly, utilities are exploring repowering plants with combined-cycle technology. EPRI has studied the key factors involved in such conversions and found that repowering can save about 20 percent of the capital cost compared with a brand-new power plant, on a dollar-per-kilowatt basis, and it can save about 5 percent on a cost-of-electricity basis.

The most likely plant configuration involves replacing the coal boiler with a gas-fired turbine (GT) that feeds its exhaust into a heat recovery steam generator (HRSG), whose output is run through a steam turbine to increase both electricity production and overall plant efficiency. Where feasible, the existing steam turbine would be retained and refurbished to add another 20 to 30 years of operation. To further reduce capital costs, utilities could also attempt to retain as much as possible of the original balance-of-plant equipment, including the switchyard, administrative buildings, condenser and source of cooling water.

The concept is not new; in the last 20 years, more than a dozen such repowering projects in the United States have utilized GT/HRSG technology. But recent changes in the economics, including low fuel prices and increasingly stringent environmental regulations, have made it a more attractive option. Technology advancements also have changed the equation. Modern gas turbines operate with higher efficiencies and adding a heat recovery steam generator (HRSG) raises overall plant efficiency to 40 percent to 50 percent (HHV) above the range of most coal-fired plants, reducing fuel consumption and lowering plant emissions.

Operational flexibility is another key benefit. Advances in gas turbines provide the capability to fast-start the engine, achieving full capacity from a cold start in about ten minutes, at least in simple-cycle operation. And combined-cycle repowering can boost capacity. Compared to an existing coal plant, which uses a boiler to produce steam, which in turn drives the steam turbine—the only source of electric power generation—replacement with one or more trains of GT/HRSG can double or triple plant capacity.

EPRI’s research found that the ideal candidate plant for repowering is relatively young—roughly 20 to 30 years old—with plenty of life left in the steam turbine, auxiliaries and balance of plant. Sites also would have in place critical subsystems, such as once-through cooling with sufficient margin to handle the additional heat rejection from the repowered plant and/or space to add cooling towers; an existing gas supply line with sufficient capacity in close proximity; no transmission limitations; a spare bay in the switchyard or space to expand; enough real estate to conveniently site the gas turbine and the HRSG; and an HRSG site close enough to the steam turbine to minimize piping runs.

A recent EPRI study considered the economics of a hypothetical redeveloped plant site, comparing the cost and performance of two repowering options. The objective was to provide a framework and methodology for utilities to use in a case-by-case assessment of options at various sites.

In the study, both options entailed repowering an old coal plant with a new GT/HRSG power train, but they differed in the extent to which original downstream equipment was replaced. The first option included procuring a new steam turbine, other process equipment and auxiliaries, while the second retained considerably more original equipment, including the steam turbine, auxiliaries, condenser and cooling system, electrical switchyard, foundations and buildings. The first option offered projected greater savings in capital investment, levelized cost of electricity, and project scheduling, but those differences can change drastically if initial project cost estimates prove too optimistic.

The inventory of possible candidates for repowering is large; worldwide, the number of potential candidates is estimated at more than 1,500 units. But realistically, the opportunities are more modest. Many utilities already have made the environmental investment in their bigger, more efficient coal plants. The ones under consideration for repowering tend to be the ones run on the margin. The question facing the owners of these assets is their long-term value, and that’s always a tough call only they can make.

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