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Energy Required for Steam Production: Enthalpy and Entropy

This data file quantifies the energy required for steam production across various industrial applications, from heating to power generation, chemical processing, and hydrogen reforming. For instance, low-pressure saturated steam at 100°C demands roughly 2.6 GJ/ton (720 kWh/ton), while medium-pressure dry steam at 6-bar and 300°C requires about 3 GJ/ton (830 kWh/ton), and supercritical steam at 250-bar and 600°C calls for around 4 GJ/ton (1,200 kWh/ton).

Steam serves numerous industrial processes observed in manufacturing and energy sectors, raising an important inquiry: what is the energy necessary to produce steam?

What is the Energy Content of Steam at 1 Atmosphere's Pressure?

At a constant pressure of 1-bar (1 atmosphere), it takes about 4.2 kJ of energy to heat each kg of water by 1°C (the specific heat capacity of water). Therefore, heating 20°C water to its boiling point of 100°C might require roughly 0.34 GJ/ton of energy.

Saturation Point? Once water reaches its boiling point at 100°C, it becomes saturated with heat, meaning that further heat will convert the liquid into steam without increasing its temperature. This saturated steam cannot exceed 100°C until all water has evaporated.

Transforming water into steam necessitates overcoming the bonds that bind water molecules in liquid form, termed the latent heat of vaporization, estimated at 2,256 kJ/kg or approximately 2.3 GJ/ton.

Only after all water has vaporized can the remaining steam’s temperature rise further. Each kg of steam increases by 1.9 kJ for every 1°C (the specific heat capacity of steam, Cp). Hence, the enthalpy per ton of 1-bar steam is presented in the chart below.

Specific heat of steam at 1-bar pressure starts at 2.7 GJ per ton. For reference, 1 ton of steam equals 1,000 kg, and 1 GJ equals 1,000 MJ or 1,000 kJ, which translates to 0.278 kWh. For additional conversions, including those into tons of coal, mcf of gas, or barrels of oil, please refer to our energy units overview.

Amazingly, this analysis reveals that less than 20% of the thermal energy used to generate steam is for heating room-temperature water to a boiling point, while over 80% is required to convert 100°C water into 100°C steam. This gradual boiling characteristic allows for cooking pasta efficiently, especially under atmospheric pressure.

What is the Energy Content of Pressurized Steam?

The previous discussion made basic assumptions about 1-bar pressure, indicating that the steamed water could instantly expand into an open area.

In practice, industrial settings prefer generating pressurized steam, which is beneficial as it can flow wherever directed without the need for costly, high-maintenance compressors. This changes the dynamics of energy economics.

Does pressure change the water's boiling point? As water transitions into steam at 1-bar pressure, it expands approximately 1,600 times its volume, resulting in significant pressure increases in closed vessels. Higher pressure leads to an increase in water's saturation temperature, pushing the boiling point higher; for example, at 100-bar (100 atmospheres), water won’t boil until it reaches 300°C.

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How Does This Impact Steam Production Costs?

Contrary to intuition, higher pressure and temperature steam production does not always require more energy. When gases expand, they cool. Therefore, the more gases expand, the more energy needs to be added to regain the desired temperature. High-pressure steam, by nature, has less expansion than low-pressure steam, as shown in the following chart. At 1-bar, boiling 100°C water into 100°C steam necessitates a 1,600x expansion, while boiling water at 100-bar and 500°C requires only a 33x expansion. Creating higher density steam is thus less energy-intensive.

In terms of energy, the latent heat of vaporization decreases as pressure and temperature increase, suggesting that at 1-bar, the latent heat required to vaporize water is 2,256 kJ/kg. As illustrated in the charts, the latent heat of vaporization diminishes at higher temperatures and pressures, reflecting increased enthalpy in the water phase.

Understanding the Critical Point of Steam

There exists a threshold, termed the "critical point," typically at 220-bar and 375°C, where the latent heat of vaporization reaches zero. At this critical pressure and temperature, steam and water are indistinguishable, forming a dense mixture of water particles. Supercritical fluids above their critical temperatures and pressures can achieve greater heat and density, providing efficiency in power generation.

The correlation between steam's enthalpy and temperature is plotted below, displaying measured values ranging from 0-600°C and stress levels from 1-500 bars. Solid lines indicate specific heat, where energy is added to raise the fluid's enthalpy and temperature. The dashed lines signify latent heat vaporation for saturated steam, indicating where additional energy must be supplied for further vaporization before temperature increases.

How Does Entropy Influence Steam Energy Content?

A seeming paradox in the charts suggests energy could be created and destroyed, seemingly contravening thermodynamic laws. For example, both supercritical steam at 460°C and 250-bar, and medium-pressure steam at 280°C and 10-bar share identical energy content of 3.0 GJ/ton. This raises questions about the practicality of compressing a gas stream from lower to higher pressure without requiring incremental energy for heating.

Examining Enthalpy vs. Entropy, it's noted that the two steam samples, despite sharing similar enthalpy, have differing entropy levels. Steam at 280°C and 10-bar carries an entropy of 7.0 kJ/kg-K while steam at 460°C and 250-bar has an entropy of 5.7 kJ/kg-K. The entropy remains constant unless heat exchange occurs.

In practical terms, compressing the gas from 10-bar to 250-bar yields about 1.2 GJ/ton of compression work, naturally raising the steam's temperature to approximately 850°C. This phenomenon is understood through steam data tables that track density, volume, enthalpy, internal energy, and entropy across various temperate and pressure points.

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