For decades, coal has dominated global power generation. Yet, its share of power generation is declining. The International Energy Agency predicts that global coal consumption has peaked and will not return to former levels, as cited in an article appearing in the Dec. 3, 2020, issue of The Economist.
Many factors are driving coal’s decline. Environmental concerns, economically competitive renewable energy, and declining profitability are only a few. To successfully navigate these trends, power plants are increasingly making the switch from coal to natural gas to produce electricity.
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Nearly all coal-fired power plants use steam turbines to produce electricity. To drive turbine blades, boilers burn coal to produce pressurized steam. Coal has remained the most dominant source for electricity generation because of its abundant supply, low cost, and efficient output.
While these advantages remain, mining and burning coal impact the environment and quality of life in nearby communities. The health and environmental effects of coal add regulatory hurdles and indirect costs that make alternatives more attractive, driving power plant conversions from coal to natural gas.
Coal’s decline is likely to continue for the foreseeable future, despite recent growth in some global markets. China, for instance, produces half of the world’s coal-fired electricity. As a result, it is the world’s largest emitter of CO².
Yet despite the fact China grew its number of coal-fired plants five-fold between 2000 and 2019, it has begun canceling planned capacity additions and investing in renewable energy. China is expected to continue its emphasis on clean, low-carbon energy. Its plans to reach peak carbon emissions by 2030 and to become carbon neutral by 2060 will require the replacement of coal-fired power plants with renewable energy for decades to effectively phase out the world’s biggest market for coal-fired boilers.
U.S. environmental regulations also have contributed to the decline in coal-fired generating capacity by increasing operational costs. The Mercury and Air Toxics Standards implemented by the Environmental Protection Agency in 2015 set new limits on air pollutants associated with coal combustion, including mercury, arsenic, and heavy metals. This resulted in most coal-fired plants having to install activated carbon injection technology to treat their coal-fired boiler flue gas at an average cost of $5.8 million per generator, according to a U.S. Senate Committee hearing report.
In the United States, another headwind of its aging coal plants is a loss of efficiency. About 74% of coal plants today have been in operation for at least 30 years. While coal can be inexpensive, boiler inefficiency is costly. For a power plant spending $100 million annually on fuel, a 1% improvement in boiler efficiency by converting to an alternative fuel source can result in significant fuel savings, as well as a reduction of CO² and other emissions that contribute to global warming.
Combustion technologies vary in coal-fired utility boilers. Each achieves a different level of fuel efficiency, measured as the amount of coal needed to produce the same amount of electricity. The least efficient boilers in operation today are aging subcritical units that convert less than 35% of coal energy into electricity. Newer subcritical units are more efficient electricity generators, achieving about 38% efficiency.
Supercritical units operate at higher temperatures to generate hotter steam under higher pressure. These units produce electricity with an efficiency rate of approximately 42%.
The most efficient coal-fired boilers, called ultra-supercritical units or high-efficiency low emissions (HELE) units, approach 48% energy efficiency. Still, even HELE units emit about twice the CO² as electricity generated from natural gas.
To remain competitive and increase efficiency, many coal-fired operations are switching plants to natural gas. Between 2011 and 2019, 103 coal-fired plants were converted to, or replaced by, natural gas-fired plants. The U.S. Energy Information Administration predicts coal to gas conversions will continue.
In most cases, when a plant switches from coal to become a gas-fired plant, its equipment is either converted to burn natural gas or it adopts new technologies to become a natural gas-fired combined-cycle plant.
Natural gas combined cycle power plants can reach 60% efficient power generation by utilizing both gas turbines and a special type of boiler called a heat recovery steam generator. In a gas turbine, a continuous blast of hot gasses is mixed with air in a combustion chamber to drive turbine blades. The heat recovery steam generator repurposes waste heat from burning natural gas to heat water and operates a steam turbine, boosting the plant’s total output.
Simple-cycle combustion turbines use the same process to produce electricity as combined cycle plants, but without incorporating the heat recovery steam generator. These operations cost less and can be constructed faster. However, without the benefit of repurposing waste heat, simple cycle plants can reach only 35%-40% efficiency. These attributes make simple-cycle combustion turbines appropriate for supplying peak-load demand.
Another emerging gas turbine technology is a smaller, lighter aero-derivative turbine. Because aero-derivative gas turbines can quickly reach maximum production and change power levels, they are becoming increasingly popular with electric utilities for providing peak and intermittent power generation.
These advances in turbine technology, along with environmentally friendly attributes of natural gas as a fuel source, have placed conversion to natural gas-fired plants at the forefront of efficient power generation. As of 2018, the natural gas combined cycle is the technology with the most electricity generating capacity in the United States. The EIA predicts natural gas combined cycle plants will remain the top source of electricity generation in the United States for the foreseeable future.
To learn more about advancements in power generation, read our Power Generation and Renewable Energy Trends White Paper – Part 1.
This article was contributed by Fluid and Gas Handling and Parker Energy Teams.
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