Gas-on-gas heating is a heat exchange process where one gas stream transfers thermal energy to another gas stream, typically without the two streams mixing. The most common application is in coal-fired power plants, where a device called a gas-gas heater (GGH) recovers heat from hot, untreated flue gas and uses it to rewarm cleaned flue gas before it exits the smokestack. This saves energy and prevents environmental and structural problems that would occur if the cooled, cleaned gas were released directly.
How a Gas-Gas Heater Works
In a power plant equipped with pollution controls, flue gas follows a two-stage path. First, hot exhaust gas leaves the boiler at high temperatures and passes through a gas-gas heater, where it gives up some of its heat. The now-cooled gas enters a scrubber (part of a flue gas desulfurization system) that removes sulfur dioxide and other pollutants using a wet chemical process. This scrubbing drops the gas temperature significantly and saturates it with moisture.
That cold, wet gas can’t simply go up the stack. It would cause corrosion, visible steam plumes, and poor dispersal of any remaining pollutants. So the gas-gas heater uses the thermal energy it captured from the untreated gas in the first stage to reheat the cleaned gas before it reaches the chimney. The two gas streams never directly contact each other. Heat moves between them through metal tubes or a rotating heat-transfer element, depending on the design.
Why Reheating Matters
Reheating the cleaned gas serves three distinct purposes: preventing corrosion, eliminating visible plumes, and improving how pollutants disperse in the atmosphere.
Corrosion is the most immediate concern. When flue gas cools below a certain temperature, sulfuric acid and hydrochloric acid condense out of the gas onto metal surfaces. This “dew point corrosion” attacks steel ductwork, heater tubes, and the stack itself. The threshold varies depending on the gas composition, but operating anywhere near or below the acid dew point accelerates damage rapidly.
Visible plumes require considerably more reheat to prevent. Research from the Electric Power Research Institute found that the temperature needed to avoid a white steam plume is roughly ten times higher than the minimum needed to prevent condensation. On a humid day (95% ambient humidity), stack gas may need to reach 180°F to 222°F to avoid a visible plume, depending on the reheating method used. On drier days (60% humidity), the threshold drops to around 160°F to 183°F. These visible plumes aren’t toxic, but they draw public concern and can violate local regulations.
Beyond the acid dew point, additional reheating has a relatively small effect on ground-level pollutant concentrations. The primary benefit of pushing temperatures higher is plume buoyancy: hotter gas rises faster and disperses more effectively from the stack.
Leakage vs. Non-Leakage Designs
Gas-gas heaters come in two broad categories. Rotary (regenerative) designs use a slowly spinning wheel packed with heat-absorbing material. Hot untreated gas heats one side of the wheel, which rotates into the treated gas stream and releases that heat. These systems are compact and efficient, but small amounts of untreated gas can leak into the clean gas stream as the wheel turns.
Non-leakage designs use separate tube bundles with an intermediate heat-transfer fluid circulating between them. Untreated gas heats the fluid on one side, and the fluid warms the treated gas on the other. Because the two gas streams are physically separated by a closed loop, no cross-contamination occurs. Non-leakage systems are considered more advanced and can achieve lower dust and sulfur oxide emissions at the stack.
Performance and Efficiency
Engineers measure gas-gas heater performance as a ratio of heat transferred to the energy needed to push gas through the system (the pumping power). Pressure drop, the resistance the gas encounters flowing through the heater, is the main efficiency penalty. Higher pressure drop means fans work harder, consuming more electricity.
Optimization studies published in the journal Energies found that redesigning the heating elements in a GGH could reduce pressure drop by about 10% on the untreated gas side and 6% on the treated side. Those changes improved overall system performance by roughly 7.7%. In absolute terms, the untreated side saw pressure drop fall from 220 pascals to 198, while the treated side dropped from 357 to 335 pascals. Temperature changes across each side remained nearly the same, meaning the system moved almost identical heat with less fan energy.
Corrosion and Material Challenges
Dew point corrosion is the leading cause of gas-gas heater failures. When sulfur dioxide, sulfur trioxide, and hydrogen chloride in flue gas combine with condensed water, they form sulfuric acid and hydrochloric acid on metal surfaces. Carbon steel, alloy steel, and stainless steel are all vulnerable. Failures tend to cluster in specific cold spots: the heater tubes themselves, connecting ductwork, and lower sections of the stack where temperatures dip lowest.
Investigations into early heater pipe failures have traced the root cause to a combination of operating at temperatures too close to the acid dew point, frequent shutdowns that allow condensation during cooldown, and steel compositions that lack sufficient corrosion-resistant elements. Adding copper, silicon, nickel, chromium, and molybdenum to the steel at the upper end of their allowable ranges meaningfully improves resistance. For example, specifying 0.4% copper, 0.4% nickel, and 0.4% chromium in the steel order can extend tube life substantially compared to steel at the low end of the same specification.
Some operators also apply enamel or glass-lined coatings to heat exchanger surfaces to create a barrier against acid attack, particularly in the coldest zones where condensation is unavoidable during startup and shutdown cycles.
Where Gas-on-Gas Heating Is Used
The dominant application is in coal-fired power plants with wet flue gas desulfurization systems. Nearly every wet scrubber installation requires some form of gas reheating, and gas-on-gas systems are the most energy-efficient option because they recycle heat already present in the flue gas rather than burning additional fuel.
The same principle appears in other industrial settings. Any process that combusts fuel and produces hot exhaust can benefit from preheating incoming combustion air with outgoing exhaust. This reduces the amount of fuel needed to reach operating temperature. Microchannel heat exchangers, a compact variation, have been used in fuel reforming systems where hot combustion gas preheats and vaporizes fuel with very low pressure drop, enabling rapid startup.
In all these cases, the core concept is the same: two gas streams at different temperatures exchange heat through a shared surface, capturing energy that would otherwise be wasted up the stack.