How heat exchange tech can help Europe’s large energy users
Growing opportunities for energy recovery and re-use
Author: Jamie Zachary
Europe’s ongoing energy crunch brought about by Russia's war on Ukraine has led to growing efforts by government policymakers and legislators to reduce energy consumption in energy-intensive industries.
The Netherlands recently has already announced that large energy consumers will need to cut back on their usage beginning at the start of next year. As part of the regulations, these consumers will be forced to invest in energy-saving measures, provided the investment can be recouped within a five-year span.
The Polish government, meanwhile, is discussing measures that will include offering support to high-energy-use industries in the form of subsidies tied to renewable energy usage.
Other governments throughout Europe are all considering similar types of measures.
One solution available to Europe’s large energy users is the opportunity to recover and re-use energy that is inherent in bulk granular solids during production processes such as cement, steel and many others.
For example, the cement-making process involves heating limestone and materials such as clay to high temperatures (up to 1,300oC) in large kilns. There, the combined mixture transforms into small nodes (e.g., clinkers) that are then ground up into a fine powder.
This calcination process has come under scrutiny due to the substantial amounts of energy needed – one tonne of cement takes around 110kWh of energy to produce – as well as the significant volume of carbon dioxide it releases. However, the clinker produced must be cooled, and this offers a largely unrealized source of recoverable energy.
Likewise, it is estimated that the iron and steel manufacturing process generates hundreds of kilograms of blast furnace and steel slag for every tonne of finished product. This extremely hot (e.g., 1,200 °C to 1,600 °C) slag is typically quenched with water to cool it down, representing millions of gigajoules of energy that is lost to the environment as the granulation water is cooled before it is re-used.
However, this slag also has significant energy recovery potential if dry granulation methods are used. This could save as much as 10 to 15% of the total energy input of the industry today – which, in turn, can lead to reduced fossil fuel use and CO2 emissions.
Moving bed heat exchangers
Moving bed heat exchangers (MBHEs) that are based on vertical plate or vertical tube technology offer metallurgical-type industries the functionality to transfer energy simply and efficiently to and from solid granular materials with minimal energy needs and a low environmental footprint.
How it works is a high-temperature solid enters into the MBHE and then passes between a series of parallel stainless-steel plates or through vertical oriented tubes. A relatively cold working fluid (e.g., water, thermal oil, air) is simultaneously circulated counter-currently within those plates or around the tubes, with a relatively hot working fluid coming out.
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As the granular solids flow by gravity through the unit in a carefully controlled manner using a designed-for-purpose discharge feeder, there is no contact between the solids and the working fluid. Instead, it relies on conduction through the plate or tube wall.
No different than in any other heat exchanger, it is the heat transfer surface area that defines the amount of energy that can be transferred.
The recovered thermal energy from granular solids can be made available for re-use in diverse ways, including being exchanged to working fluids such as water, steam, thermal oil, air or even supercritical carbon dioxide (sCO2).
In the case of a thermal processing step such as a calciner, it can then be looped back to pre-heat the feed material going to the thermal processor. Or it could be used to pre-heat the combustion air for the kiln. The amount of energy recovered by preheated combustion air is directly reducing the needed primary energy demand. Pre-heating either the feedstock and/or the combustion air will reduce the amount of fuel needed for the process.
By capturing and re-using the energy, the thermal efficiency of this processing step can be improved; it may also help to achieve higher rates of clinker production with limited equipment modifications.
The recovery opportunities of an MBHE in metallurgical-type industries is significant. To illustrate, at slag production rate of 100 tph, a vertical tube MBHE used to cool a “dry” granulated slag from 1,000° C to 350° C can recover about 20 megawatts of thermal energy. When combined with a heat recovery steam generator and steam turbine operating with an overall efficiency of about 65%, close to 13 MW of electrical energy can be produced.
MBHEs that use vertical plate or vertical tube technology to transfer energy to and from solid granular materials are just beginning to open the doors to what is considered possible in energy intensive industries.
Solex Thermal Science has decades of experience in recovering energy from bulk solids. Learn more about how our technology works by visiting the Heating, Cooling and Drying section of our website. Or explore our many different solutions.
Ready to talk specifics? Contact a Solex team member today.
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