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