Coal to Natural Gas Conversions – A Bridge to a Lower Carbon Future

Climate change has made reducing greenhouse gas (GHG) emissions an urgent priority worldwide. The power sector, decreasingly so, but still heavily reliant on coal as fuel, accounts for nearly 30% of energy-related emissions from generating electricity. This has necessitated an accelerated transition from high-carbon fuels towards a mix of lower-carbon energy systems. While renewables such as solar and wind are integral to building a sustainable energy future, natural gas is serving a crucial role in building today’s and tomorrow’s energy mix.

Converting existing coal plants to run on natural gas offers a pragmatic pathway for significantly curbing emissions from power generation. Pound for pound, natural gas emits nearly 50% less carbon dioxide when burned compared to coal. Retrofitting coal plants to utilize gas combustion turbines or combined cycle systems can thus provide substantial short-term emissions reductions.

Not only is natural gas more efficient, but advanced gas power generation technologies are also highly reliable, providing base power generation support during times of the day when the wind is not blowing and/or the sun is not shining. The abundant supply and low pricing of natural gas in regions like the U.S. further add to the economic viability of fuel-switching.

With favorable market conditions and policy incentives, coal-to-gas plant conversions can pave the way for cleaner electricity production. As an intermediate solution and as a backstop of reliability and consistency, natural gas power enables the continued displacement of high-carbon coal while also supporting the base power generation needs required during a large-scale buildout of zero-carbon renewable clean energy. Further, due to the intermittency of renewables, natural gas holds a key role in providing the base fuel for the nation’s grid stability and reliability. This article explores the technologies, benefits, and implications of transitioning from coal to lower-emitting natural gas generation.

Understanding Power Plant Fuel Coal to Gas Conversion

Coal-to-gas conversion involves modifying existing coal-fired power plants to use natural gas instead as the main fuel for generating electricity. Converting coal plants to burn gas typically requires adding new advanced gas turbines and heat recovery steam generators while retaining the existing steam turbine and generator. However, a key component of the economic attractiveness of converting existing coal plants to natural gas infrastructure is that the electric transmission grid is already in place, thus eliminating the additional capital required to build out the electric transmission grid to a new power plant at another location, and there is no need to wait for the regulatory study and approval associated with power generation placed at a new grid location.

There are two primary technological pathways for converting coal power plants to natural gas:

Combined Cycle Conversion

This configuration utilizes both a natural gas combustion turbine and a steam turbine together in a highly efficient combined cycle system. The gas turbine’s exhaust heat is captured to produce steam for the steam turbine. Combined cycle plants achieve efficiencies of over 60%.

Simple Cycle Conversion

Here only a natural gas turbine is installed to generate power. While simpler and cheaper than combined cycle conversion, simple cycle systems have lower efficiencies around 40%, and are typically used for peaker plant power generation needs due to their ease of startup and shut down as compared to combine cycle power generation

Other equipment like gas compressors, valves, pipelines, and control systems are also added to integrate natural gas handling and combustion.

Environmental Impact

Switching power generation from coal to natural gas offers considerable reductions in greenhouse gas emissions. Gas also emits far lower levels of criteria for air pollutants like sulfur dioxide, nitrogen oxides, and particulate matter.

Converting a typical coal plant to run on natural gas would far reduce its lifetime carbon dioxide emissions. Natural gas conversions also avoid the continued environmental impacts on our land resources of coal mining and coal transportation.

Looking towards the future, emerging technologies in turbine manufacture will allow for the increasing use of hydrogen fuel or hydrogen blended into natural gas, which will progressively even further lower the carbon intensity of gas generation, as the supply and economic viability of hydrogen increases in time. While not by nature renewable, natural gas conversion of power plants from coal serves as a pragmatic stepping stone towards more deeply decarbonized power systems, as well as serving to provide required reliability for power consumers during times of the day when the sun is not shining or the wind is not blowing.

Economic and Policy Drivers

Favorable economics and policy incentives are accelerating the transition from coal to natural gas power. In addition, the natural gas market’s current and future conditions include low natural gas prices. These factors, along with environmental regulations, are encouraging utilities to convert existing coal plants.

Key policy drivers include:

• Emission performance standards that limit carbon dioxide emissions from new and existing power plants. This encourages switching to lower-carbon natural gas.
• Renewable portfolio standards mandating increased generation from renewables. This reduces run times for coal plants, favoring conversion to more flexible gas plants.
• Carbon emissions pricing programs increase the operating costs of high-emitting coal plants relative to gas plants.
• Government grants and tax credits to subsidize gas plant conversion costs.

For energy producers, converting existing coal plants to natural gas is often cheaper than building new renewable or gas capacity. Older coal plants also face significant maintenance costs to meet emission standards. Switching to gas turbines powered by relatively inexpensive shale gas can provide solid returns on investment. In addition and as mentioned, converting an existing power plant enables the usage of existing electric power transmission equipment in place, which is quicker from a regulatory approval process, and much less capital intensive than building a new plant in an alternate location.

Challenges and Considerations

While promising, coal-to-gas conversion also poses some technical and economic challenges:

• Older coal plants may require extensive modifications and reinforcements to integrate new gas turbine systems. Space constraints can also limit options.
• Mothballing usable coal power assets early creates stranded cost liabilities for generators. However, ongoing operation also needs to be economically viable.
• Natural gas supply and transport infrastructure must be assessed to ensure sufficient on-site gas availability. Pipeline expansions may be needed.
• While gas prices are currently competitive, future volatility could impact short-term operating profits. Hedging instruments can help mitigate this gas price risk.
• Conversion costs can be prohibitive for some marginal coal plants nearing retirement age.
• Renewable energy continues to improve its cost-effectiveness but remains an intermittent source of power.

Overall though, the drivers for conversion outweigh the challenges. Continued technology advancements and supportive policies will further improve the investment case for gas conversion projects.

The Future of Energy–Conversion Landscape

Energy industry experts forecast that renewable energy generation is projected to increase within the energy mix of future global electricity production. However, the large-scale integration of variable solar and wind generation creates its own reliability challenges, which require the deployment of natural gas pipelines, storage, and gas power generation capacity in order to complement renewable energy deployment.

As introduced earlier, renewable energy is not able to provide base power to the electric grid due to its intermittent nature. This manifests as the so-called “Duck Curve” issue, where the net power load from renewable power generation fluctuates tremendously through the day as solar radiation ramps up and down, and average wind speeds fall during the day. Output from renewables also cannot be scheduled as needed. Balancing the massive intermittent generation nature of renewables requires the highly flexible complementary power generation capacity of gas power generation and the natural gas pipelines and storage network behind them.

Here natural gas peaking plants, with their fast ramping abilities, can fill the gaps cost-effectively. Converted gas plants can provide crucial support to avoid instability as renewables expand. Gas capacity also supplements when renewable generation falters.

Meanwhile, advancing technologies like carbon capture, utilization, and storage (CCUS) will increasingly allow natural gas generation with zero on-site emissions. As turbine technologies advance, blending hydrogen into the natural gas supply of gas turbines can further lower their carbon intensity over time. Although currently extremely economically challenging to provide long-term power storage, in the future emerging battery energy storage solutions may help integrate renewables with increasing reliability.

Coal-to-gas switching paves the way for the renewable revolution by providing grid reliability and peak capacity. In time, next-gen technologies may enable gas generation with zero emissions. But for now, natural gas conversion allows the continued displacement of coal while enabling the exponential growth of solar, wind, and other renewables. This paradigm is key to building an increasingly decarbonized future.

Frequently Asked Questions

1. What are the main drivers for converting coal power plants to natural gas?

The primary drivers for converting coal power plants to natural gas include environmental concerns, economic factors, and policy initiatives. Environmental concerns focus on reducing greenhouse gas emissions and air pollutants associated with coal combustion. Economically, a conversion to natural gas can be more cost-effective due to lower fuel prices and higher efficiency of gas-fired plants, and less capitally intensive than building a new plant in an alternative location. Policy drivers involve governmental regulations and incentives aimed at reducing carbon emissions and promoting cleaner energy sources.

2. How does natural gas conversion reduce a power plant’s carbon footprint?

Converting a power plant from coal to natural gas significantly reduces its carbon footprint because natural gas combustion emits substantially lower levels of carbon dioxide (CO2) compared to coal. Additionally, natural gas burns more cleanly, resulting in lower emissions of sulfur dioxide (SO2), nitrous oxides (NOx), and particulate matter. These reductions contribute to a decrease in overall greenhouse gas emissions and a smaller carbon footprint for the power plant.

3. What are some of the success stories in coal-to-gas power plant conversions?

There have been numerous successful conversions of coal power plants to natural gas around the world. For instance, in the United States, the conversion of the Big Cajun II power plant in Louisiana helped reduce CO2 emissions. This success story demonstrates the feasibility and environmental benefits of transitioning from coal to natural gas.

4. How does natural gas fit into the future of renewable energy?

Natural gas is considered a bridging fuel in the transition to a renewable energy future. Its ability to provide reliable and flexible power generation makes it a required complement to the increasing installation of intermittent renewable sources like solar and wind. Natural gas-fired plants can quickly adjust output to balance the grid when renewable energy availability fluctuates each day, ensuring a stable energy supply during the transition to a more renewable-centric energy system.