Coal, Gas and Conventional Thermal Generation

Learning objectives

By the end of this lesson you should be able to:

Understand how conventional thermal power stations generate electricity.
Distinguish between coal- and gas-fired generation.
Explain the strengths and weaknesses of conventional thermal technologies.
Recognise the different system services provided by thermal generators.
Appreciate how the role of thermal generation is changing as electricity systems decarbonise.

Introduction

For more than a century, conventional thermal power stations formed the backbone of electricity systems around the world.

Coal, oil and later natural gas supplied reliable, controllable electricity that could be generated whenever consumers required it.

Although renewable energy has grown rapidly in recent decades, conventional thermal generation continues to play a major role in many electricity systems. In addition to producing electricity, these generators provide many of the engineering services required to keep the grid stable, including inertia, frequency response, voltage support and reserves.

Understanding thermal generation is therefore essential, not only because of its historical importance, but because it remains a benchmark against which many newer technologies are compared.

What is conventional thermal generation?

Conventional thermal generation produces electricity by converting heat into mechanical energy, and mechanical energy into electrical energy.

The process follows the same basic sequence regardless of fuel.

Fuel is burned.
Heat is released.
Water is converted into high-pressure steam (or hot gases drive a turbine directly).
A turbine rotates.
The turbine drives a synchronous generator.
Electrical energy is produced.

Although the engineering has become more sophisticated over time, the underlying principle remains largely unchanged from the first commercial power stations of the nineteenth century.

Coal-fired power stations

Coal power stations burn pulverised coal inside a boiler.

The heat produced converts water into high-pressure steam.

The steam passes through several turbine stages before being condensed back into water and reused.

Coal plants are generally characterised by:

Large generating capacities.
Long operating lifetimes.
High capital costs.
Relatively slow start-up times.
Significant carbon dioxide emissions.

Historically, coal provided inexpensive and dependable electricity because fuel could be stored on site for long periods.

However, concerns about air pollution and climate change have led many countries to phase out coal generation.

Gas-fired power stations

Natural gas is now the dominant fossil fuel in many electricity systems.

Several different technologies exist.

Open-cycle gas turbines (OCGT)

These operate similarly to aircraft jet engines.

Air is compressed.

Natural gas is burned.

The hot gases drive a turbine connected to a generator.

OCGT plants:

Start very quickly.
Respond rapidly to changing demand.
Have relatively low capital costs.
Are less fuel efficient than combined-cycle plants.

They are often used during periods of peak demand or to provide reserves.

Combined-cycle gas turbines (CCGT)

Combined-cycle plants improve efficiency by making use of waste heat.

After passing through the gas turbine, the hot exhaust gases are used to produce steam.

The steam drives a second turbine.

This allows significantly more electricity to be extracted from the same quantity of fuel.

CCGT stations typically offer:

Higher efficiency.
Lower emissions than coal.
Large generating capacity.
Good operational flexibility.

Today they provide much of the flexible generation needed to complement variable renewable energy.

Why thermal generators became dominant

Conventional thermal generation possesses several characteristics that made it particularly attractive throughout the twentieth century.

Fuel can generally be stored.

Output can be controlled.

Large amounts of electricity can be generated from relatively compact sites.

Perhaps most importantly, thermal generators are dispatchable.

This means operators can instruct them to increase or decrease output according to system requirements, subject to their technical limits.

This controllability made conventional generators well suited to balancing electricity systems before the widespread deployment of renewable generation.

Operational characteristics

Thermal generators cannot change output instantaneously.

Several technical constraints influence how they operate.

These include:

Minimum stable generation

Many plants cannot operate below a certain output without risking unstable combustion or inefficient operation.

Ramp rates

Generators can only increase or decrease output at a finite rate.

For example, a generator may increase output by only a few megawatts per minute.

Start-up time

Some units require only a few minutes to start.

Others require many hours.

Minimum up and down times

Once started, some generators must remain online for several hours before shutting down.

Similarly, after shutting down they may require time before restarting.

These characteristics influence how generators participate in electricity markets and system operation.

Beyond energy production

Conventional thermal generators provide much more than energy.

Because they are synchronous machines directly connected to the electricity network, they naturally provide several important system services.

These include:

Inertia.
Frequency response.
Voltage support.
Reactive power.
Fault current.
Operating reserves.

Historically, many of these services were provided automatically whenever the generator was connected to the grid.

As thermal generation retires, electricity systems increasingly need alternative technologies to provide these services.

Strengths

Conventional thermal generation offers several important advantages.

Dispatchability

Output can usually be adjusted to match demand.

High availability

Fuel availability is generally independent of weather conditions.

Large generating capacity

Individual power stations can supply hundreds or even thousands of megawatts.

Grid support

Synchronous generators naturally contribute inertia and system strength.

Mature technology

Thermal generation is well understood and supported by decades of engineering experience.

Limitations

Despite these strengths, thermal generation also has significant disadvantages.

Carbon emissions

Coal and natural gas release carbon dioxide during combustion.

Coal is particularly carbon intensive.

Air pollution

Combustion produces pollutants such as nitrogen oxides, sulphur dioxide and particulate matter, although modern emission controls have reduced these significantly.

Fuel costs

Unlike wind and solar generation, fossil fuel generators require continuous fuel purchases.

Operating costs therefore depend heavily upon fuel prices.

Efficiency losses

No thermal power station converts all fuel energy into electricity.

A significant proportion is lost as waste heat.

Water requirements

Many thermal stations require substantial cooling systems, increasing water consumption.

Thermal generation in a low-carbon electricity system

As countries pursue net-zero emissions, the role of conventional thermal generation is changing.

Coal generation has declined rapidly in many countries due to its high emissions.

Natural gas often remains important because it provides:

Flexible generation.
Reserve capacity.
Reliability during periods of low renewable output.

Over time, new technologies such as batteries, long-duration storage, hydrogen, carbon capture and demand response may replace some of these functions.

However, this transition presents an engineering challenge.

Many services historically supplied automatically by thermal generators must continue to be provided if electricity systems are to remain secure.

Comparing coal and gas

Characteristic	Coal	Natural Gas
Carbon emissions	High	Lower
Fuel flexibility	High	High
Start-up speed	Slow	Fast to moderate
Operational flexibility	Limited	Good
Efficiency	Moderate	High (especially CCGT)
Typical role today	Declining baseload	Flexible generation and balancing

Although natural gas emits less carbon dioxide than coal, both remain fossil fuels and contribute to greenhouse gas emissions.

Why £/MWh is not enough

Suppose two generators each produce electricity for £60/MWh.

One can:

Start within five minutes.
Respond rapidly to changing demand.
Provide inertia and reserves.

The other:

Requires twelve hours to start.
Changes output only slowly.

Although their energy costs appear identical, their contribution to the electricity system is very different.

This illustrates an important principle that will recur throughout this module:

Electricity technologies should not be evaluated solely by the cost of producing energy.

They should also be evaluated according to:

When they can generate.
How quickly they can respond.
Whether they support system stability.
How reliably they are available.
What additional services they provide.

A key insight

Conventional thermal generators have historically provided many services simultaneously.

When operating, they produced:

Energy.
Capacity.
Flexibility.
Inertia.
Frequency response.
Voltage support.
System strength.
Reserves.

As electricity systems evolve, these services do not disappear simply because thermal generation declines.

Instead, they must be provided by other technologies or combinations of technologies.

Key takeaways

Conventional thermal generation converts heat into electricity using turbines and synchronous generators.
Coal and natural gas remain important contributors to many electricity systems, although their roles are changing.
Gas-fired generation is generally more flexible and less carbon intensive than coal.
Thermal generators provide many valuable system services beyond energy production.
Their operational characteristics include ramp rates, start-up times and minimum operating levels.
Carbon emissions and fuel costs are significant disadvantages of fossil-fuel generation.
Evaluating thermal generation solely by its energy cost ignores many of the services it contributes to system reliability.
Modern electricity systems must replace both the energy and the engineering services historically provided by conventional thermal generation.

Looking ahead

Conventional thermal generation has dominated electricity systems for much of the past century.

The next lesson examines a very different technology:

nuclear power, which offers extremely low operational carbon emissions and high reliability, but introduces a different set of engineering, economic and policy challenges.