Gas Furnace Heat Recovery: From Economizers to Regenerative Burners
The hot gas leaving a gas-fired furnace carries a significant amount of energy. At 800 to 1200 degrees C, the flue gas enthalpy is 50 to 70 percent of the heat released by the burner. Most of that energy is wasted if the flue gas goes straight up the stack at 200 to 300 degrees C. Heat recovery captures some of that energy and uses it to preheat the combustion air, preheat the charge, or generate steam. The result is lower fuel consumption, lower emissions, and a lower operating cost.
Here is how the various heat recovery technologies actually work.
Start with the basic energy balance.
A gas-fired furnace firing natural gas at 10 percent excess air has a flue gas composition of roughly 8 percent CO2, 12 percent H2O, 5 percent O2, and 75 percent N2. The flue gas exits the furnace at a temperature close to the furnace temperature (minus the heat loss through the walls). The enthalpy of the flue gas at 1000 degrees C is about 1.6 kWh per cubic meter of gas burned.
The minimum flue gas temperature is set by the acid dew point - the temperature at which the water vapor and the sulfur compounds in the flue gas condense and form corrosive acids. For natural gas with very low sulfur, the acid dew point is around 60 to 80 degrees C. For fuels with higher sulfur, the dew point can be 120 to 150 degrees C.
The stack temperature of a furnace without heat recovery is typically 200 to 300 degrees C above the acid dew point, to avoid corrosion in the stack and the breeching. The energy lost up the stack is 20 to 40 percent of the input energy, depending on the furnace temperature and the stack temperature.
Heat recovery captures some of this energy.
Economizer is the simplest technology.
An economizer is a heat exchanger in the flue gas path that heats the combustion air or the boiler feedwater using the flue gas heat. The economizer is typically a finned tube heat exchanger - the flue gas flows over the outside of the tubes, and the air or water flows through the inside.
For a gas furnace, the most common application is combustion air preheating. The economizer heats the combustion air from ambient (20 to 30 degrees C) to 150 to 300 degrees C using the flue gas heat. The preheated air enters the burner at a higher temperature, so less fuel is needed to reach the flame temperature.
The fuel saving from combustion air preheating depends on the preheat temperature and the furnace temperature. A rough rule of thumb: for every 100 degrees C of air preheat, the fuel consumption drops by 4 to 6 percent. So heating the air from 25 degrees C to 200 degrees C (a 175 degrees C rise) saves 7 to 10 percent of the fuel.
An economizer for a 5 MMBtu/hr furnace costs $30,000 to $80,000 installed. The payback is typically 1 to 3 years, depending on the fuel price and the operating hours.
The economizer has limitations. The flue gas temperature drop is limited to about 200 degrees C - going lower requires more heat transfer area and creates condensation issues. So an economizer typically takes the flue gas from 600 to 800 degrees C down to 400 to 600 degrees C, and the air preheat is limited to 200 to 300 degrees C.
Recuperative burners go further.
A recuperative burner is a burner with a built-in heat exchanger. The flue gas passes through the burner body, transferring heat to the incoming combustion air through a metallic or ceramic heat exchanger. The air is preheated inside the burner, close to the flame.
A metallic recuperator can preheat the air to 400 to 500 degrees C. A ceramic recuperator can preheat to 600 to 800 degrees C. The higher preheat temperature gives more fuel savings - 15 to 25 percent for metallic, 25 to 40 percent for ceramic.
A recuperative burner costs 3 to 5 times more than a standard burner, but the fuel savings are large. The payback is typically 6 to 18 months for high-utilization furnaces.
The recuperative burner has limitations too. The metallic recuperator has a temperature limit (typically 700 degrees C max flue gas) and a life limit (typically 5 to 10 years before the metal fatigues). The ceramic recuperator is more temperature-tolerant but more fragile and more expensive.
Regenerative burners are the most efficient.
A regenerative burner system uses two burners that alternate between firing and exhausting. Each burner has a ceramic regenerator (a bed of ceramic balls or structured ceramic media) that absorbs heat from the flue gas during the exhaust phase and then transfers the heat to the incoming air during the firing phase.
The cycle time is typically 30 to 120 seconds. During the exhaust phase, the flue gas at 1000 to 1300 degrees C passes through the regenerator, heating the ceramic to 800 to 1100 degrees C. During the firing phase, the combustion air passes through the hot regenerator and is preheated to 800 to 1000 degrees C. The preheated air goes to the burner, mixes with the gas, and fires into the furnace.
The fuel savings are 40 to 55 percent compared to a cold-air burner. The air preheat temperature of 800 to 1000 degrees C is much higher than what an economizer or a recuperative burner can achieve.
A regenerative burner system costs $500,000 to $3,000,000 installed for a large furnace. The payback is 1 to 3 years for high-utilization furnaces with high fuel prices.
Regenerative burners are common on large continuous furnaces (pusher, walking beam, rotary hearth) but are also used on some large batch furnaces. The system is complex - the burner cycling, the valve controls, the regenerator maintenance - and the operator needs training to run it.
Heat recovery for the charge is a different approach.
Instead of preheating the combustion air, some furnaces use the flue gas to preheat the charge. A preheat chamber at the entry end of a continuous furnace uses the flue gas to heat the incoming charge from ambient to 300 to 600 degrees C before the charge enters the main heating zone. The fuel savings are 10 to 20 percent, and the heat-up time in the main zone is reduced.
Charge preheating is common on continuous heat treatment furnaces (pusher, walking beam, roller hearth) but is not used on batch furnaces.
Heat recovery for steam or hot water is a third option.
A waste heat boiler (WHB) in the flue gas path generates steam or hot water from the flue gas heat. The steam can be used in the plant for process heating, cleaning, or space heating. The hot water can be used for similar purposes or for absorption chilling.
A WHB is typically used on large furnaces (above 10 MMBtu/hr) where the steam demand is meaningful. The payback is 1 to 4 years depending on the alternative fuel cost for the steam generation.
The right heat recovery technology depends on the furnace size, the operating hours, the fuel price, and the plant's energy integration.
For a small batch furnace (under 5 MMBtu/hr) with moderate operating hours, an economizer is the right choice. Simple, cheap, effective.
For a medium furnace (5 to 20 MMBtu/hr) with high operating hours, a recuperative burner is the right choice. Higher efficiency, longer payback but still good.
For a large continuous furnace (above 20 MMBtu/hr) with high operating hours, a regenerative burner is the right choice. Highest efficiency, longest payback but the best return on investment.
A hybrid approach is also possible. Some operators install an economizer for the bulk heat recovery and a small regenerative system for the final trim. The combination can be more cost-effective than either alone.
Author: MONTE INTELLIGENCE heat recovery engineering team. For heat recovery system selection and economic analysis, contact helenxu@cnlymonte.com.
