Clean and Efficient Gas Heating of Industrial Furnaces
Some benefits of gaseous fuel:
Energy costs usually represent a large fraction of operating cost for an industrial furnace. Most of these furnaces use natural gas as an energy source for heating.
Electricity is a good source of energy for lighting, electronics or drives, but it is usually too valuable for providing process heat. Fuels are available as solid, liquid or gaseous fuels. These fuels have different characteristics that make them a preferred option for different applications.
Since solid fuels – like coal or waste – are cheap, they are used for electricity generation in large power plants. Solid fuels require preparation like grinding, however, which makes them difficult to handle for use in smaller burners. Another drawback of solid fuels is the fact that the combustion products contain ashes, which are not welcome in industrial furnaces.
Liquid fuels like gasoline have a high energy content per volume, which make them the ideal fuel for cars and trucks. Gasoline is too expensive for heating industrial furnaces, however, and can cause safety issues when distributed in furnace environments.
Gaseous Fuel Benefits
The following characteristics make gaseous fuels the ideal energy source for furnace applications.
Gaseous fuels can be transported and distributed in gas pipelines and grids, which are widely available. In remote areas that are not connected to the natural gas grid, LPG (propane) could be an option.
The existing grid and gas reservoirs can be used to store gaseous fuels and ensure safe supply. Electricity cannot be stored easily, and future energy supply relying on renewable electricity faces this problem.
Combustion systems have improved considerably in the last decades, and modern burner systems can provide clean combustion with low NOx emissions.
Natural gas is available in most parts of industrialized regions worldwide. New natural gas discoveries will ensure a cost-effective supply of future energy. The natural gas grid can be used even in future times by feeding in renewable fuels like biogas and hydrogen, which are derived from excess renewable electricity.
When using fuels for heating high-temperature furnaces, exhaust gas represents the major losses of a heating system. There are some rules of thumb that enable a user to estimate the losses, efficiencies and possible savings of a fuel-fired heating system.
The energy content of natural gas is usually given as HHV (higher heating value), including the latent heat of water content of the exhaust. Modern condensing boilers for domestic heating use this latent heat, but condensation of the water vapor is not desired in industrial furnaces. For that reason, 10% of the HHV is not available for heating the furnace. In some parts of the world LHV (lower heating value) is used to define a fuel where the latent heat is not considered.
Combustion products from industrial furnaces are exhausted at a higher-than-ambient temperature. The losses can be estimated by taking the amount of excess air of the combustion into account. For reasonably well-adjusted systems with around 15-20% excess air, the sensible exhaust heat losses are around 3% per 100°F of exhaust-gas temperature.
An example of this would be the following. In a radiant-tube-heated hardening furnace, exhaust-gas temperatures without heat recovery are typically in a range of 1700°F or more. From the fuel input, 10% must be subtracted due to latent heat. From the remaining 90%, 17 times 3% (~50%) is lost due to the sensible heat of the exhaust. That means only 45% of the fuel heat input is available to heat the furnace.
These numbers give just a rough estimate, and a more thorough analysis should be performed when planning an installation. These losses can be partially reduced by exhaust-gas heat-recovery measures.
In some cases, exhaust is used to heat other processes or is fed into a steam grid. Generally, however, the most effective way to use the exhaust energy is to preheat the combustion air. Preheated combustion air is required when and where the exhaust energy is available.
State-of-the-art heat exchangers can use half of the sensible heat corresponding to heating the combustion air. Information like “up to 90% efficient” is not really helpful because even a burner without heat recovery will be 90% efficient when heating a cold furnace. If possible savings are claimed, it must be clearly stated under what conditions these savings could be achieved.
The main characteristic that should be looked for is the heat-exchanger surface area of the recuperator and the flow configuration. A burner with a simple-shaped recuperator of limited size combined with a high-capacity burner will not be capable of recovering a lot of energy. Heating up the combustion to temperatures close to the process temperature is only possible with counterflow heat exchangers where the combustion air and the exhaust are flowing in opposing directions.
Regenerators usually provide a very large heat-exchanger surface area but for the price of a switching mechanism. A thorough analysis has to be carried out to determine the right burner for the right application. The following should be considered:
• Radiant tube/open firing
• Yearly operating hours
• Fuel cost
Self-recuperative burners operated in pulse-firing mode have become quite popular in recent years and are offered by several manufacturers for direct and indirect (radiant-tube) heating. The recuperator is generally of cylindrical shape, and the heat-exchanger surface area is enhanced by fins on metal recuperators and a pointed or corrugated surface on ceramic heat exchangers.
Self-recuperative burners of that style can cut the sensible exhaust losses approximately in half if they are correctly designed. Referring to the example above, the exhaust-gas temperature would be reduced from 1700 to 850°F (927 to 454°C), correlating to sensible heat losses of ~25% or an available heat of 75% (LHV) or 67.5% (HHV).
Regenerative burner designs provide considerably more heat-exchanger surface area and can therefore recover more heat from the exhaust. Typically, the sensible exhaust heat losses can be cut in half compared to standard recuperative burners or to a quarter compared to cold-air burners.
For an example that leads to a reduction of the exhaust-gas temperatures from 1700 to 425°F, correlating to sensible heat losses of ~12.5% or an available heat of >85% (LHV) or >77% (HHV) depending on the burner capacity, regenerative heat exchangers can be integrated into the burner design (Fig. 3) or are attached to a pair of burners.
For taking advantage of high efficiency even for small burner capacity, where the application of regenerative burners is not justified, a new type of recuperator was developed to combine the simplicity of a recuperative burner with the performance of a regenerative burner. To provide a large heat-exchanger surface but not extend the overall burner size (to allow retrofits), the recuperator is divided up into a large number of small recuperators. Small dimensions and low velocities guarantee a high transfer without increasing the pressure drop. This flow configuration is called gap flow, and the recuperator is named after that. Heat recovery of regenerators are almost reached, and available heat of >83% (LHV) or >75% (HHV) is achieved for furnaces operating in a range of 1600-1800°F (871-982°C).
High air preheat temperatures require special precautions to avoid excessive NOx emissions. Combustion research was quite successful in recent decades to analyze the causes for NOx formation and thereby enabled engineers to develop proper measures to avoid it. Examples for low-NOx strategies are staged combustion and flameless oxidation, which provide low-NOx emissions without expensive secondary measures like selective catalytic reduction (SCR).
Modern burner systems can provide clean and efficient heat to high-temperature industrial furnaces. Thorough, honest and professional information is required to choose the suitable system. Magical solutions often do not work, so a seller should be able to explain what he is selling. Clean combustion and energy efficiency should remain the focus even when current energy prices are low. IH
1. Wuenning J.G., Milani A., “Handbook of Burner Technology for Industrial Furnaces,” Vulkan Verlag, ISBN 978-3-8027-2950-8