Capturing heat from the furnace
Natural gas-fired process furnaces are widely used in industry, particularly in applications involving metallurgy, heat-treating, glass and ceramics. Even at today’s attractive rates for natural gas as an industrial fuel, it makes complete sense to recover as much of the waste heat from the furnace as possible and use it for beneficial purposes.
Sources of Heat Loss
In any furnace operation, heat is lost from the furnace skin, from heated product leaving the treatment area, from conveyer or pusher equipment, from excessive draft and from open doors or other access points. But easily the largest and most concentrated type of heat loss is from the furnace exhaust, some with temperatures of 1000° F or higher. Fortunately, this is usually also the most practical heat to recover and reuse.
Two types of heat recovery systems that are commonly used with industrial furnaces are recuperators and regenerators. According to the U.S. Department of Energy, the recuperator is the most widely used heat recovery device. A recuperator is a gas-to-gas heat exchanger that is installed on the furnace exhaust that preheats incoming combustion air.
With relatively clean natural gas combustion exhaust, these heat exchange surfaces may even be finned or dimpled to capture the maximum amount of heat. Although the combustion exhaust itself is clean, the heating process may be producing corrosive or particulate byproducts that could damage or clog a high efficiency heat exchanger such as one with fins. Be sure your recuperator design takes into account the specific characteristics of your furnace operation.
Technology is Available
John Sultzbaugh is the Director of Engineering for Hauck Manufacturing Company of Lebanon, Pennsylvania. Hauck both manufactures furnace combustion equipment, including recuperative burners, and offers custom engineering services to industrial furnace users. Sultzbaugh points out that the current attractive pricing of natural gas from domestic sources encourages the use of gas-fired furnaces, but in some cases may give recuperation retrofits a longer payback. However, he also points out that for operators with site emission permits written on a lb/MMBtu basis, recuperation allows the use of more furnace capacity, hence increased productivity.
Hauck offers a range of burner types for industrial furnaces, including its Ecomax direct-fired self-recuperative burner for high temperature furnaces in the North American market. The company also provides assistance to customers looking to add recuperation to an existing burner system.
He notes that items to consider include existing burner construction to determine exposed metal parts and insulation that may have to be upgraded to accommodate higher temperature combustion air and flame temperatures. Air supply piping may need to be increased in size or redesigned to allow higher air pressure. Sultzbaugh adds, “We also need to look at the air-fuel ratio, which will require adjustment.”
Asked whether existing burner controls can be adapted to the addition of recuperation, Sultzbaugh explains, “It depends on the type of control method being used. Older type technology using relay logic is not readily adaptable. If an electronic control is used, such as one using mass flow control, it can more readily be adapted via programmable logic controller (PLC) programming changes.“
Firebridge, Inc is a combustion engineering firm headquartered in Burlington, Ontario with wide experience in industrial furnace engineering. Russ Chapman from that firm points out that flue gas recuperators are generally restricted to about 1800° F flue gas temperatures. His company is working on designs that might allow systems to operate at temperatures as high as 2200° F, but for the time being the limitation still stands.
Eclipse, Inc. is a long-time leader in industrial furnace burner technology. According to Jim Roberts from Eclipse, in recent years an important improvement in furnace heat recovery is development of self-recuperative burners, such as Eclipse’s ThermaJet design. He says, “This is the culmination of 20 years of development in the marketplace to get burners that not only eject a hot stream of gases, but bring the exhaust gases back through the burner to recover heat normally lost to the flue.”
Regenerators: Another Approach
Another potential tool for salvaging a significant amount of the heat energy from a furnace exhaust is the regenerator. Although less commonly used than recuperators, regenerators are still widely used in high temperature furnaces such as glass and steel reheat furnaces. A regenerator uses two or more vessels or vessel sections containing a high thermal conductivity matrix. The matrices may be ceramic or metallic. Exhaust passes through the matrix giving up a large portion of its heat before discharging.
After the matrix is heated, the stream is mechanically directed to another section or vessel and incoming combustion air is drawn through the hot section and heated. The regenerator alternates the hot and cold flows so heat recovery is continuous. The advantage of a regenerator over a recuperator is that it presents a much larger amount of heat exchange surface to the hot exhaust flow.
A disadvantage is that the heat content of the incoming air has some variation, making precise combustion control more difficult. This can be reduced by faster cycle times or the use of multiple vessels at various stages of cooling. Another consideration is that some exhaust products inevitably remain in the vessel to combine with incoming combustion air.
In the Future
There are still a good many plants with major energy reduction potential, and the level of adoption is uneven. Russ Chapman from Firebridge notes that larger companies are starting to benchmark their large plants against each other, and against the competition, in terms of sustainability of the plants. However he feels that a single-minded focus on short term profitability sometimes means a lack of incentives for long-term energy saving projects.
As an example he cites a Tier 2 automotive manufacturer that has a plant with a monthly energy bill of $200,000 per month. He feels this company could reduce that bill by 25%. However the plant is on its third plant manager in three years, and no action is taking place in making improvements. Clearly, energy savings are not always enough.
Global Competition Driven
Chapman notes that smelting in the copper mining industry is an example of an industry starting to move in this direction. “It’s because they are competing worldwide and are comparing key performance indicators, making efforts to get in line.”
Asked if effective heat recovery can lower greenhouse gas emissions from industrial furnace applications, Chapman replies, “Absolutely! Energy reduction equals emission reduction. The only inhibitor is scale.” He explains, “The cost of retrofitting little furnaces is disproportionately higher than one operating at, say, 45 MMBtu/hr or larger, so the economic case is more difficult to make.”
Taking the Step
Unquestionably, heat recovery strategies have huge potential for reducing energy bills and plant emissions.
Jim Roberts from Eclipse notes, “Sometimes the cost looks staggering, but remember that most furnaces have a very active running schedule, so the paybacks, even at low gas costs, can be very fast. Even if paybacks are in the two-to-three year range, the long-term savings are worth it.”