Nitric acid plant tail gas heater: lifting absorber tail gas from about 25 °C to the inlet temperature the NOx/N2O abatement reactor and the expander turbine need
The gas-to-gas exchanger in a nitric acid plant that takes cold, NOx-laden tail gas from the absorber and heats it with process heat, so that the abatement reactor works and the expander turbine recovers power
In a nitric acid plant the gas leaving the top of the absorption tower is cold, at absorber pressure, and still carries nitrogen oxides and nitrous oxide. Before it reaches the stack it is heated, cleaned of NOx and N2O in a catalytic reactor, and expanded through a turbine that drives the plant's air compressor. The tail gas heater is the exchanger, or train of exchangers, that does the heating, using heat taken from the hot process gas leaving the ammonia oxidation reactor. Its outlet temperature is the number the rest of the tail-gas train depends on: an abatement catalyst has a window it works in, and the expander recovers more power from hotter gas. This page covers the heater by that duty, with operating figures from one plant's published data sheet, the EPA's process description and Uhde's abatement work.
Tail gas heaters (tail gas preheaters, final tail gas heaters) in weak nitric acid plants: gas-to-gas exchangers that heat absorber tail gas against hot process gas ahead of a catalytic NOx/N2O abatement reactor and the tail gas expander, judged by inlet and outlet temperature, pressure, flow and the abatement window they must hit. Industrial heat exchangers as a class are covered on the Heat exchanger page; gasketed plate units on the Gasketed plate-and-frame heat exchanger page; finned-tube cores for gas heaters and economisers on the Heat Exchanger Core page; CO2 gas coolers, food-pack cooling zones and district-heating substations on their own pages.
CNFX editorial research · Sources checked October 13, 2026 · 3 sources
Temperatures, pressures and flow around the tail gas heater in a 180 t/day mono-high plant
Austin Powder Argentina's published data sheet for its mono-high pressure nitric acid plant (Girdler reactor, 9 bar abs, 180 t/day HNO3 design, 56 % acid) lists the tail gas conditions from the absorber to the turbine outlet. The plant reduces NOx with a non-selective catalytic reduction (NSCR) reactor placed before the expander. [1]
Editorial calculation from the data sheet: heating 25,200 kg/h (7.0 kg/s) of tail gas from 25 °C to about 479 °C, with an assumed mean specific heat of 1.05 kJ/kg·K for a nitrogen-rich gas, takes about 7.0 × 1.05 × 454 ≈ 3.3 MW. [1] The specific heat is an assumption, not a value from the source; the real duty follows from the gas composition and whether part of the heating is done in more than one exchanger.
The same data sheet records 5,500 ppm NOx before the NSCR reactor and less than 50 ppm after it, against a 50 ppm limit, with ammonia plant purge gas (360 kg/h) and natural gas (200 kg/h) as reducing agents, and pre- and post-NSCR boilers raising 1,700 kg/h of steam at 15–15.5 bar. [1] Editorial note: AP-42 describes the fuel being oxidised over the catalyst in catalytic reduction, which accounts for the rise from about 480 °C to 710–730 °C across the NSCR reactor; in this plant the 590–610 °C at the expander inlet is therefore reached after the reactor and its boiler, not in the tail gas heater. [2]
Give tail gas flow, composition (NOx, N2O, O2, H2O), pressure and temperature at the absorber outlet, the outlet temperature the abatement reactor needs, and the hot-side gas available.
02
Heater outlet against the abatement catalyst: about 425 °C for N2O decomposition, 200–500 °C for NOx reduction with ammonia
Uhde's paper to the Nitrogen 2006 conference describes its EnviNOx two-bed reactor, placed in the tail gas at the inlet of the tail gas turbine where the temperature is highest, with N2O decomposed over iron zeolite in the first bed and NOx reduced with ammonia added between the beds. [3]
Editorial note: read together, the Uhde figures set the heater outlet a plant has to reach for a chosen abatement route. A plant whose heater delivers about 425 °C or more can use direct N2O decomposition; one that reaches only 350–425 °C needs the reducing-agent variant; NOx removal alone can run from about 200 °C. [3] Raising the heater outlet to move from one window to another changes the heater's area and the heat taken from the process gas, so the heater and the abatement reactor are specified together.
The EPA's AP-42 section on nitric acid describes the catalytic reduction route: tail gases from the absorption tower are heated to ignition temperature, mixed with a fuel such as natural gas, hydrogen, propane, butane, naphtha, carbon monoxide or ammonia, and passed over a catalyst bed. [2]
Uhde finding
Value
Tail gas inlet temperature for the two-bed N2O decomposition process
Iron-zeolite activity for N2O decomposition falls increasingly rapidly [3]
Variant for cooler tail gas
N2O reduction with a hydrocarbon (methane in the tests) plus ammonia for NOx, down to about 350 °C [3]
Effect of 50 ppm NO in the gas
Shifts the N2O decomposition curve 50 K lower; at 450 °C raises the decomposition rate by over 50 percentage points [3]
Effect of pressure
Raising pressure from 1 to 6 bar g increased N2O decomposition by about 24 percentage points [3]
State the abatement process and its required inlet temperature range, and ask the heater supplier for the outlet temperature at minimum and maximum plant load.
03
Where the heat comes from: reheating to about 392 °F against the ammonia oxidation gas in a dual-pressure plant
In AP-42's description of a dual-pressure plant, the absorber tail gas passes an entrainment separator for acid mist removal, is reheated in the ammonia oxidation heat exchanger to approximately 392 °F, and is then expanded in the power-recovery turbine, whose energy can drive the compressor. [2]
The same section gives the pressures that set the heater's design pressure on the tail gas side: ammonia oxidation from slightly negative to about 58 psia, and absorption at 116 to 203 psia in dual-pressure plants; single-pressure plants have operated from atmospheric pressure up to 203 psia. [2] It also states that the small amounts of acid mist that may be present in the absorber exit gas are removed by a separator or collector before the catalytic reduction unit or expander. [2]
Editorial calculation: 392 °F is 200 °C, and 116–203 psia is about 8.0–14.0 bar a. Compared with the 478–480 °C reached before the NSCR reactor in the Austin Powder plant, the AP-42 figure is 278–280 K lower, which is why the outlet temperature, not just the exchanger type, defines the heater. [2][1]
Give the hot-side source (process gas after the waste heat boiler or another stream), its temperature and pressure, and the tail gas design pressure from the absorber pressure.
04
What to send for a nitric acid tail gas heater
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Plant: [mono or dual pressure; capacity t/day HNO3; absorber pressure]
Tail gas: [flow kg/h; NOx, N2O, O2, H2O; inlet temperature]
Outlet: [required temperature; abatement process]
Hot side: [process gas source; temperature; pressure; flow]
Limits: [allowable pressure drop on each side; design code]
Materials: [tube and shell material requested]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What temperature does nitric acid tail gas leave the absorber at?
In Austin Powder's mono-high plant data sheet, 25 °C at 7.97 bar a; it is heated to 478–480 °C before the NSCR reactor. [1]
How hot must tail gas be for N2O decomposition over iron zeolite?
Uhde reports about 425 °C or above for its two-bed process; below 400 °C the catalyst activity for N2O falls increasingly rapidly. [3]
Where does the tail gas heater get its heat?
In AP-42's dual-pressure plant, from the ammonia oxidation heat exchanger, reheating the tail gas to about 392 °F before the power-recovery turbine. [2]
Sources and what still needs a direct check
Sources checked 2026-10-13. Three sources: Austin Powder Argentina's published plant and process data sheet for its nitric acid plant (2024), the US EPA's AP-42 section 8.8 on nitric acid, and Uhde's 2006 Nitrogen conference paper on the EnviNOx process.
