Decarbonizing Industrial Heat: Compression Solutions for Waste Heat Recovery and Steam Utilization

According to the International Energy Agency (IEA), heat is the world’s largest energy end-use and accounted for almost half of total energy consumption in 2021. Process heat makes up two-thirds of industrial energy demand and nearly one-fifth of global energy consumption. It also constitutes the majority of direct CO2?emissions from industry, as most heat is generated via fossil fuels (i.e., gas- or oil-fired boilers)[1].

“Decarbonized Heat and Industrial Processes” is one of five fields that Siemens Energy has identified as being critical to achieving net-zero. In this post, I outline some of the innovative solutions we are deploying within the compression business to advance this initiative by helping our customers recover and reuse valuable waste heat.

Heat Pumps with Steam Compression

Heat pumps are a proven technology that date as far back as the 1850s and have the inherent advantage of being able to convert usable waste heat into steam using only electricity. This allows for heat generation to be decarbonized, and potentially emissions-free if the electricity comes from a green source. While electric boilers/heaters also offer this advantage, heat pumps are around 4x more efficient.?

Most of the installed base of heat pumps today are classified as “low-temperature” and designed for temperatures up to 100°C (typically for district heating applications). Siemens Energy has installed more than 50 low-temperature heat pumps, most of which are in Scandinavia.

High-temperature heat pumps (HTHPS), on the other hand, can achieve temperature levels up to 150°C at pressures up to 5 bar (steam). These are typically used for providing process steam in industrial processes and facilitate efficient heat supply using relatively small amounts of drive energy.

At the core of the heat pump is a specially designed compressor capable of compressing high-mole weight refrigerant gases from the temperature and pressure level of the heat source to the level of the heat sink.

?Centrifugal compressors, both integrally geared compressors (IGCs) and in-line machines, typically are the best compressor technology for industrial heat pumps. Siemens Energy’s HTHPs can deliver up to 70 MWth per unit and are engineered to achieve coefficients of performance (COPs) above 4. In simple terms, this means 4x more heat energy is provided to the heat sink than the amount of electricity consumed.

At Siemens Energy, we are also working to apply IGCs for steam compression downstream of heat pump systems. Such configurations allow for output temperatures in excess of 300°C at 60 bara pressure. These levels fall within the heat requirements of many important processes used across the downstream oil & gas, chemicals, and pulp & paper industries.

When IGCs are used as the heat pump drive, it is possible to combine the heat pump and steam compressor in one machine/package, optimizing the footprint and cost of such a system. In certain cases, if even higher temperatures than 300°C are required, it is possible to install a small electric superheater downstream of the steam compressor.

Mechanical Vapor Recompression (MVR) Cycles

Another method for converting waste heat into steam is the Mechanical Vapor Recompression (MVR) cycle.

In such cases, thermal energy from a mid- or low-temperature waste stream is used to directly evaporate feedwater. The steam is then compressed to the required temperature and pressure level by the process. The entire latent heat is provided by the free-of-charge waste heat and only the sensible heat, which is significantly lower than the latent/evaporation heat, is provided by adding energy to drive the steam compressor.

One advantage of the MVR cycle vs. the heat pump cycle is that no refrigerant is required. In many cases, CAPEX and footprint can also be reduced by eliminating the need for an expansion valve, the flash tank, the refrigerant condenser, and reducing interconnecting piping.

The figure below shows an example where 68 MWth of waste heat at 75°C could be utilized to produce 100 MWth of steam at 6 bar and 180°C. The resulting system has a COP > 3.

If the heat source temperature is increased, the COP can be increased and CAPEX further lowered. In another example where the waste heat source is ~105°C and the steam is required at 3 bar, a COP > 7 is possible.

Both heat pumps and MVR cycles represent viable methods of meeting industrial heat demands in a sustainable way. The MVR cycle is typically more advantageous for heat sources with higher temperatures because the steam evaporation temperature and pressure are increased, which leads to a higher COP and smaller footprint by a reduction in volume flow.

Recovering Heat from CO2 Compressors

Another area where we are applying innovative compressor and waste heat recovery solutions is in carbon, capture, utilization, and storage (CCUS) applications.

?For transporting CO2 in pipelines after sequestration, compression to supercritical pressure (between 100 and 200 bara) is often required. Typically, the compressor(s) would be designed for high efficiency and minimized power consumption. This is achieved in several ways, including through interstage cooling.

?However, the heat released during intercooling typically remains unused. Siemens Energy has developed a novel approach to utilize the captured heat to generate low-pressure steam for the carbon capture process, reducing the duty on the reboiler and subsequently lowering CO2 emissions.

?For a typical amine-based capture system, up to 80% of the required steam and heat energy can be provided, while only requiring between 1/4 - 1/6 of that heat as additional mechanical power for the compressor at comparable CAPEX and footprint. This results in a COP between 4 and 6.

?Depending on the alternative heat source and the value of heat versus mechanical or electrical energy, this results in a significant reduction in OPEX of the capture system. Furthermore, if the heat is alternatively produced by fossil fuel combustion, this also reduces CO2 production and thus CAPEX of the capture system, as less CO2 needs to be captured.

?Initial calculations show that the specific energy demand per ton of CO2 captured could be reduced by up to 1.3 GJ/t. Furthermore, if the alternative heat source is a fossil fuel, it may be possible to reduce the size of the amine system (and its associated CAPEX), as there will be less CO2 that needs to be captured.

?The application of the heat recovered is not limited to steam production of amine systems. However, the synergies are obvious, as most carbon capture processes apply amine systems for sequestrating CO2, which requires large amounts of low-pressure steam. Alternatively, the heat could be used for process or district heating

Conclusion: Finding the Right Solution

These are just a few examples of how we are applying innovative compression solutions at Siemens Energy to accelerate the decarbonization of industrial heat. Other areas where we are working collaboratively with customers to integrate waste heat solutions include electrolyzer plants and clean ammonia production facilities (among others).

Ultimately, facility constraints and the unique objectives of the operator will determine if waste heat recovery is economical, and also the specific type of technology that should be applied.

In all cases, it is beneficial to engage with equipment providers in the earliest phases of the project timeline to identify synergies between processes, eliminate bottlenecks, and select a solution that balances system performance, reliability, and cost with emissions reductions.

[1] IEA