Efficient Gas Heating Industrial Furnaces

By Martin Schoenfelder posted 02-22-2016 13:54

  

M. Schönfelder, J. G. Wünning, Steven R. Mickey

Even at times when fuel prices are very low, energy costs represent a large fraction of operating costs for an industrial furnace. This is especially true for electrically heated furnaces. Electricity is widely used in industrial applications, however, it is far too expensive to provide process heat if it could also be provided by a gas heating system at less than a third of the energy costs. New natural gas sources are currently being discovered and exploited. However, even though the prices for natural gas are close to an all-time low, using it as efficiently as possible is still favorable both economically and environmentally.

Exhaust-Gas Losses

The energy content of natural gas is usually expressed as HHV (higher heating value), including the latent heat of water content in the exhaust. Condensation of water vapor is not desired in industrial furnaces, which is why approx. 10% of the HHV is not available for heating the furnace. In some parts of the world the LHV (lower heating value) is used to define a fuel, not considering its latent heat.

If gas burners are being used to heat high-temperature industrial furnaces, hot exhaust gases usually represent the largest losses since the products of combustion are exhausted at a higher-than-ambient temperature. For a well-adjusted system with around 3-5% O2 in the exhaust, the losses can be estimated at approximately 3% per 100 °F of exhaust-gas temperature. [1]

For example, in a radiant tube heated hardening furnace exhaust gas temperatures without heat recovery are typically 1,700 °F or higher, leading to exhaust gas losses in excess of 50%. Even though this is just a rough estimate, it already shows that heat recovery measures can lead to huge energy savings, thus paying for themselves within a short period of time and having a significant positive environmental impact. A thorough and systematic analysis should therefore always be performed when planning an installation [2].

Heat Recovery

In a gas-heated system, the most effective way to use the energy bound in hot exhaust gases is to preheat the combustion air [3]. It is favorable to perform the heat exchange directly at the burner in order to avoid complicated, inefficient, and potentially dangerous transportation of hot gases outside of the furnace.

This led to the development of so-called self-recuperative burners using counter-flow heat exchangers to preheat the combustion air. The main characteristic determining the achievable efficiency of a burner is the heat exchanger surface area of the recuperator. It needs to be appropriate for the burner capacity used, and made of a material capable of withstanding the maximum exhaust gas inlet temperatures. Standard self-recuperative burners, if well designed, can cut the exhaust gas losses in half. The newest generation with high-end heat exchanger is even more efficient, achieving efficiencies of >80% LHV [4].

Regenerative burners usually provide a very large heat-exchanger surface area, but are more complicated due to the switching mechanisms necessary. They must be operated in a clean environment and achieve efficiencies of up to >85% LHV, and are therefore best suited for large-scale continuous applications.

High air preheat temperatures require special measures to prevent excessive NOx emissions. Combustion research has been quite successful in recent decades in analyzing the causes for NOx formation and thereby has enabled engineers to develop proper measures to avoid it. Examples of low-NOx strategies are staged combustion and flameless oxidation (FLOX®), which provide low-NOx emissions without expensive secondary measures like selective catalytic reduction (SCR). [1]

In summary, selecting the best burner requires a thorough analysis of the application in question. Amongst others, the following points need to be considered:

  • Radiant tube or open firing,
  • Maximum and most common process temperature,
  • Continuous or batch operation,
  • Yearly operating hours and fuel cost,
  • Emission limits (esp. NOx),
  • Safety and maintenance requirements.

Economics

When considering any energy efficiency technology these days, the low energy prices come to mind. However, even at these low prices an investment in efficient burner technology is still highly rewarding. Due to the long runtime of industrial furnaces per year and the huge efficiency gain of modern gas burner systems, the payback time is often shorter than 2 years. With many systems being in operation for 20 or more years, the long-term savings then become a huge competitive advantage, especially once the energy prices begin to increase again or new CO2 regulations come into play.

For example, by using state of the art self-recuperative burner technology in a hardening furnace with a net heat input requirement of 3 mmBTU/hr and a yearly operating time of 6,000 hours, fuel savings in excess of $40,000 per year are possible at a natural gas price of $3.5/mmBTU. Should the natural gas price increase to only $5/mmBTU, the yearly savings would be as high as $60,000. After the initial investment is paid back within 1-2 years, these savings can be considered yearly income for the company. In addition, many utilities offer additional incentives for investments in energy efficient combustion systems [5].

Conclusion

State of the art burner technology provides clean and efficient heat to industrial furnaces. Professional service and honest communication is necessary to select the best heating system, to install and operate it properly, and to build a long-term, trustworthy relationship between end user and burner supplier.

Even at today’s low energy prices, the investment in efficient burner technology pays for itself within a short period of time. Especially family owned businesses, not driven by shortsighted bonus maximization, but by long-term investment strategies, should consider modern combustion technology. It maximizes the long-term profitability of their furnaces as strategic assets, and is therefore crucial for businesses to remain competitive and successful no matter how the energy prices fluctuate in the future.

References

[1] Wuenning J.G., “Clean and Efficient Gas Heating of Industrial Furnaces”, Industrial Heating, 2014

[2] Wuenning J.G., Milani A., “Handbook of Burner Technology for Industrial Furnaces,” Vulkan Verlag, ISBN 978-3-8027-2950-8, 2011

[3] Morris, A., “Improving Energy Efficiency with Thermal-Capture Technology”, Industrial Heating, 2015

[4] WS Thermal Process Technology, “Product Overview – REKUMAT S”, www.flox.com

[5] Office of Energy Efficiency, http://energy.gov/eere/femp/energy-incentive-programs

0 comments
670 views

Permalink