By July 27, 2016 Read More →

Many industries use combined heat and power to improve energy efficiency

Iron, steel, glass, and cement industries have high-temperature waste streams that can provide input to generate electricity

combined heat

Source: U.S. Energy Information Administration, Annual Electric Generation Survey EIA-860 (2015 early release)

Combined heat and power systems have long been used to reduce the overall energy intensity of industrial systems according to the US Energy Information Administration.

There are two types of combined heat and power, depending on whether the system produces power first, then heat, or heat first, then power.

In topping cycles, the hot exhaust of an electricity generator such as a natural gas turbine or reciprocating engine is used to provide process heat, hot water, or space heating for the site.

According to preliminary 2015 data, topping cycles are used by 89 per cetof total combined heat and power capacity.

In bottoming cycles, also referred to as waste heat to power, wasted heat from a furnace or other high-temperature industrial processes is recovered and used for power production.

Bottoming cycles typically use waste heat boilers or steam turbine systems.

Ongoing research, development, and deployment efforts are focused on these systems as a way to reduce wasted heat and increase industrial energy efficiency.

Bottoming cycles are mostly used in industrial facilities in the chemical, paper, and primary metals sectors, as these industries often have high-temperature waste streams that are favorable for waste heat recovery.

As much as 20 per cent to 50 per cent of the energy consumed in some industrial processes is ultimately lost through waste heat contained in streams of hot exhaust gas and liquids and through heat conduction, convection, and radiation from hot equipment surfaces and heated product streams.

The overall energy efficiency of some industrial processes can be improved by capturing and reusing the waste heat.

In some cases, such as industrial furnaces, efficiency improvements resulting from waste heat recovery can improve energy efficiency 10 per cent–50 per cent.

A study by the US Department of Energy’s Energy Efficiency and Renewable Energy office identified research, development, and demonstration efforts to expand waste heat recovery practices in the US industrial sector.

The waste steams analyzed in this study showed that roughly 60 per cent of unrecovered waste heat is low quality (i.e., temperatures below 450 degrees Fahrenheit).

According to the study, the greatest potential for expanding bottoming-cycle combined heat and power is in energy-intensive industries, such as iron and steel, glass, and cement.

These industries have high-temperature waste streams that can provide the input to generate electricity.

Technological advances that allow the use of lower temperature waste streams can increase the potential for bottoming-cycle combined heat and power.

These new technologies with lower temperature requirements can also help to expand the bottoming cycle for nonenergy-intensive industries such as wood products, transportation equipment, and fabricated metal products.

Determining the applicability of the bottoming cycle requires more complex analysis than the topping cycle.

One approach, called pinch analysis, examines the temperature and heat flow rates of hot and cold streams and attempts to optimize the heat exchange between streams.

Even though effectively implementing bottoming-cycle combined heat and power is complex, the energy that is converted to electricity would otherwise be wasted.


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